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Canada
Synthesis aDd Chemistry of Highly Distorted Polycyclic Aromatic Hydrocarbons
by
MichaelR.Mannion (B.Se., Hons.), Memorial University ofNewfoundland
St.John's. Newfoundland, 1994 A thesis submitted to the School of Graduate Studiesin panialfulfillment of the requirements for the degree of
Doctor of Philosophy.
Department of Chemistry Memorial University ofNewfoundland
St. John's, Newfoundland, Canada luly21, 1999
Abstnd
A significant feature of smaller cyclopbanes and buckminsterful..lerenesisthe presence ofnonplanar aromatic rings.Suchcompounds are of considerable interest due to boththe synthetic cballe:nge they poseandto their unusual conformational, spectroscopic, and chemical behavior. Agreatdealofworkbasfocussed on determining the extent to which an aromaticringcanbedistorted from planarity while remaining isolablc under ambient conditions. Although this questionhasbeen examinedindetail for isolatedbenzene rings (for example,throughinvestigations of [n]paracyclophanes), analogous stUdies of polycyclic aromatic hydrocarbon(PAR)frameworkshave oever beenpunucd.
Herethe firstsystematic examination of the distortion from planarity of a PAH moiety is reported. The synthesis of a Dumber of [n](2..1)pyrenopbanes from [3.3]dithiacyclophaDe precursorsi5descnDed. Some physical, spectroscopic,and chemicalproperties of these molecules are also described. and a number of X·ray strUctures ofmarkedly nonplanar aromatic moieties are reported. From this data, it is concluded that the cnd-to-eod bend of the most strained pyrenophane prepared is greater thanthe average end-to-end bend ofthepyreoe moiety. However, POAV analysis of the pyramidalization of pyrenophane sp2 carbon atoms reveals markedly lower pyramidalizationsthanare observedinDSb Clll. Attemptsatthe functionalization of {n}(2.7)pyrenopbanes in the hope of using them. as precursors for larger oonplanar PAlls were made. However. suitable conditions for functionalization of pyrenopbancs were not found.Asynthetic opproach to • C,.dtiralI.6-[n]pyttnOphane isalsodoseribcd.
Anancmpted synthesisof a derivative of the buckybowl pinakene using a tandem Bergmancycloaromat:imionlfrecradicalconjugate addition is presented.
The experimentalwork.ispreceded byreviewsof the literature concerning the concept of aromaticity. nonplanar aromatic molecules (especially [n]paracyclophanes) andfullcrene fragments.
iii
AclatOWledgf:IDf:Db
I wouldfirstliketoexpressmy deep appreciationtoDr.GrahamJ.Bod~llfor his valuableadvice,patient guidance,andeve:r<nthusiastic encouragement throughout my timeingraduate school.
I should also mention all members of the Bodwell group from 1994 - 1999: Tom Houghton. ZullO Pi. Alan Swinamer. Bala Yarlagadda, Jason Kennedy, Carl Kennedy, Barb Pike, Heather Davis, CrystalLynch.Cherie Noble, Cyril DeSilva, Jeff Harris, Sbu- LinChen. Rolf Vermeij, Jan Pattie, Jonathan Langille. JiangLi,Diane Burke, Jim Fleming, Paul Mugfonl, Paul Hurley, Wendy Lines, SlepbeoLear.Man McGettigan,aod Scott Ruttgaizer, all of whom itbasbeenapleasuretowork. with. Specialthankstoall
!bose whobelped proofreadIbistbesis.
I would like tothankmy supervisory committee. Or. Paris GeorgbiouandOr.
Murray Brooker,for their helpful criticism and eocouragemenLI mustalsothankalarge number of faculty. staff. and fellow graduate students who have assisted mewithhelpful discussions and advice. They includeDr.Hugh Anderson, Dr. John Bridson.Dr.D. Jean Burnell, Dr. Brian Gregory.Dr.Peter Golding,Dr.Bob Helleur, Dr. Chet Jablonski, C.
Robert Lucas, Dr. Ray Poirier,Dr.Peter Pickup,Dr.AllanStein,Colin Cameron.
Sheldon Crane, Ed Hudson, James Xides, and Pat Hannon.
Iamalsograteful toDr.ChetJablonski, Nathalie Brunet, Ed Vessey, and Dave Millerfor assistancewith NMRspectra;toDr.Brian GregoryandMarionBaggsfor massspectral ana.Iyses;andtoDr.John BridsonandDave Millerfor their excellent X-ray crystaIlograpbic work.
Fmally, [extendspecialthankstotheentire MUN Organic groUP. thestaffof the Chemistry Department, to all MUN Chemistry students. and all my mendsandespecially my family for their support, help and friendship.
Generous financial support from the Natw'a1 Sciences and Engineering Research Council.theSchool of Graduate Studies. and Memorial University of Newfoundlandis gratdWlyocknowledgcd.
Table of Contents
Title .
Abstract... ...•..•...
Acknowledgements•...
Table ofConteDts . ListofFigures . List of Schemes .
. i
.. ii
... iii ... iv
. ix
. xi Listof Tables...•.••..
ListofSymbots, Abbreviations,andAcronyms ..•. .. xvi
Introduction .
...
\.. 9 .. 10
..12 17
. \9
..20
. 3
... 7
I 23.3 •OtherEnergetic Criteria...
1.2.3.4 - Conclusion-Energetics.
1.2.4 - Aromatici.ty - Magnetic Criteria
1 2.4.1 - Magnetos:bemistrv - A BriefInqoduction 21 1 2 4.2 MagneticSusceptibility and Aromaticitv 22 1.24.3 _DiamagneticSupptibilityExaltati0Q 24
I 2.4.4 -NMJlMethods... . 25
1.2.M.l-a....ical Shifb ....•.••••... ... 26
1.2.4.4.2 -Coupling ConstQ1l/s 26
1.2.4.4.3 -S<JhenlShiftMethods... ....•.•.•••..••••.••••... 27 Cbapter 1 - Benzene and Aromaticity
1.1 - B!gzeneaDdits Proosrties 1.1.1- Eoriy History...•...
1.1.2· The Pando][ 01 Beaune _ ..
1.2 - Aromatieity
1.2.1 - Aromatieity - Early Dtvelopments...•.•..
1.2.2 - Aroaaaticity - Reactivity Criteria .•...
1.2.3 - Aromaticity - EneJlUk: Criteria I 2.3.1 A Miscellany ofRgonance Energies ..
1.2.3 2 •qVI 1"1:Energeticsin Benzene .
J2.4.5 -Nucleus~IndependentChemical Shifts 28
1.2.4.6 - Magnetic AnisotroPY Assorted SpectroscopicMethods.. 29
1.2.4.6.1 -MagnetQ-Oplical Methods:TheFaraday andCouon-MouJonEffects... . 29
1.2.4.6.2-MolecuJarZeemanEfficl... 30
1.2.4.6.3 -High Field Deuterium NMR 30 1.2.4.6.4 -Mitchell'sDMDHP "Localization Probe" 31 1.2.4.7 - Magnetic Criteria Conclusion.... . 33
1.2.5 - Arom.ticity- Geometric Criteria... .33
1.2.5 I-Julg', IndexAI... ...34
1.2,52 - POzhuskii's Index 6R. .. ... 35
1.2.5.3· Bird's IndexIA... .. ..35
1.2.5.4-Jursic'sIndex;0... . 37
1.2.5.5 • Krvgowski's HOMA. EN QED and Related Indices 37 1.2.5 6 -Bird'sCE Index... . 42
1.2.6 - Multidimcnsion.lity of Arom.ticity? . 43 1.2.7-Arom.ticity-OtberCriteri... ,,4$
1.2.8 - Arom.ticity - Conclusion 46 Ch.pter 2 - Noppl.aAr Arom.tic Molecules... .... .... ... .. .. 49
2.1~IDtToductiOD to StrainiDOrg.nic Chcmistry... . 49
2.2 - C.rbon Pynmid.lizatioa U.l-DellDitio.ofPynalidallzlltio•... " """""'''''''''''"" 52 2.2.2~QuantifiClldoa of Pyramicbli:zatioa... 54
2.3 - Cydopbagg 2.3.1 -lntrodudioD .ad Nomcnclaturc... 56
2.3.2-P.ncyclopbancs 58 2.3.2.1-Synthesis CreperalRemarks... .. 59
2.3.2.2 -Synfuesis ofPara.cycloPhanes 60
2.3.23 _Physical and SpectroscopicProperties 67 2.3.2.3.1 -Ultraviolet Spectroscopy 68 2.J.2.J.2 -NMRSpectroscopy"." """""" """. """" "" 71
23.2.4 -BendAngles... .. 72
2.3.2.5 -ChemistryofParacyclophang 74
1.3.1.5./ -'T'MmaJ andPhotOCM",icol Reactions 75 1.3.1.11-Jlydrogenation... ...•.•... 75 2.3.1.5.3 -Reactions wirh EJecrrophi/u 76
2.3.2.6 - Are Paracydophanes Aromatic? 77
2.3.2.6./ -Reacrivity... . 78
1.3.1.6.1-EMrptics 78
2.3.1.6.3 -Magnetic Crireria.. 80
2.3.2.6.4 -Geometric and Other Criteria... . 81
2.3.3· Paratyclopbanes - Conclusion 82
Cbapter 3 - Buckyballs IDd Buckyb01'VIs .. 83
3.1 -Carbon and its Allotropes 83
3.2 -BuckmiDsterfullengg
3.2.1· DiscoveryoftbeFuUermes 84
3.2.2 - Stndure and Pb)'Jica.l Properties. ... 85 3.2.3-Chomistry... . 88
3.2 3.1 -EndobeckalAdducts 89
3.2.3.2-Rcduc1ion... 9Q
3.2.3.3.AdditionProducts ...90
3.3 -BaIDbowis. ..90
3.3.1- ConDDuleDe.... 91
3.3.2·SumaDene Ind Related C, Fngmeatl 96
3.3.3 -ConDDulene-ContaiDinc PAlb... . t03
3.3.4 - C1Buc:kybowls... . 107
3.3,s - C70Buckybowls... .. 108
3.4 -Aromatic Belts 109
3-$ CondusioDS 113
151 . 157
. 158
.•... 115 . .... 116
.. 164 165 ... 169 . ...•... 148 ..149 .... 151 Chapter~AlkrgmuJRaditai Cyc:lizatiOD Appro.ch to Fulkrenc Frumcnts 4.1 - Thc lk!'lDlaD Cydo.rom.tizatioD
...I.I-Introduction ..
5.4 2,7-Dimctbo!VlJY!'!De .
S.s·
Inlll,6)Pyruopbyel5.5.1-Introdumoa ·PAHs WithaTwilL 110
5.5.2 • (9)(1,6]Pyrcnopb.ac - Synthesis 172
5.6 - CODclasions 175
5.7 Experimental 176
5.1.2 - Synthetic Methods For 12.2]CyclopblDC!J ..
5.1.3 -PAH -Containing Cyclopbancs
5.2 - Syntbesis .ad Cbcmistry of 1 D-Dion(nJ<2,DpyunoDb.ae5 5.2,1 - The Projttt....
5.2,2 -1,u.Diou[al(2,7)pyrmopbua: Rttrosynthesis ..
5.2.3 ·ltD·Dion[a)(2,7)pynnopbua: Synthais..
5.3 - Synthesis .nd Chcmistry of laJ<2,DPyrcnonb.nes 5.3,1- [n](2,7)Pyrenopbaues-Rctrosyntbesis 5.3.2· [n](2,7)Pyruopbua-Syutbesis
".1.2 - EnediyDa Ind tbc Bergm.n Reaction.
4.2 - Attcmpted Synthesis of Bowl-Shaped Molecules Theory
4.2.1- Thc Idea.... .. 120
4.2.2· Retrosynlbmc AD.lysis... . 121 4.3-Attcmpted Synlbais of Bowl-Sbaped Molccules·Results Ind Discuuion.123
4.4-E:lperimental... .. 134
Chapter 5 - Syntbesis of (nJPyrtDopbana 5.1 Nonpiapir PARs
5.1.1-latrodumoD .
papter 6 Pyrmopbaac Spectroscopy. ChnaW »Dd Applications
6.J-latroductioD 216
Y-PImical
rro..
I'lit! 2166.3 - X-Ray Crystallomphy 217
. 232
. 222
.222
. 224
..225
. 127
viii
6.4 -
NMR
SDtd1'!RoOY ..6.4.1- lUNMR-Assipmeat .
6.4.2 - IUNMR. -Trends .
6.4.3 - DC AuignmeDt .nd Patterns .
6.4.4 - Other NMR Experiments . 6.5 -Ultnyjolet-Visible SpectroSl:ODy . 6.6-Cydil: Volt.mmetry
6.6.1-latroduction to CV... . 234
6.6.1- Cyc=lie Voltammetry - Edperimeatal.. . 235 6.6.3· Cyclic Voitalllmetry - Results ud Discussion•.•... ,. . 238 6.7 -ChemistryofPyrenopb.ag... .. ..240
6.7.1-Diels-AlderRndioDJ -Iatrodudion ..241
6.7.1 • Diels-AlderRuctioas -Resulb andDistusslou 244 6.7.3 - Elec:tropbilie SubtlitulioD - H.lagea.lioD.ndFriedel
Craft!AcylatiOD... . ...•..•••... .... 247
6.73 I Bromination... .. 247
6.7.3.2 - Friedel Crafts Acylation... ...253
6.7.3.3 - ReactionwithLewis Acids. . 254
6.7.3.4 - Protonatiog... . 255
6.7.4·Readioas with Organometallk Compouads 257
6.7.5-Hydrogea.tioD... . 259 6.8 -CoU.borativeWork... .260
6.9 -CoaduliaM... . 263
6.10 -E:a:oerimental , 264
~- X-RayCrystal Structures of Selected Compounds. . 271
~- Selected NMR Spectra from Cb.pter 4 280
~• Sc:leded NMRSpectnfrom Cb.pter! 311
~-Se... NMR Spcctnfro.Chap'.r6.••.•.•..•.••.•...•...•.. 444
ListofFipra
Fipre 1-1: Proposed Stn1ctures of8e:nzene... . 4 Tagare 1-2: Ladenburg's Objection to K.ekuli's Structure:
Two posstble o-isomers.... . 5
Figure 1-3: The Paradox of Benzene: Unsaturated,yetInert... . 7 Figure 1-4: Conversion of Cyclobexatriene to Benzene... . 13 Figure 1-5: Delocalizatioo Energy Sc:heme for Benzene... . 16 Fipre 1-6: Partitioning ofrl:-anda·EnergiesinBenzene 18 Figure 1·7: GEO, EN,andHOMA Analyses ofBenzene
inSelected Conformations... .... 40
Figure 2-1:BondAngle Strain, Cyclopropane vs. Methane .. 50 TlgUre 2-2: Torsional Straininn·butane... .51 Figure 2-3: Non-boDdedstraininl,2-di-terl-butylethylene SI Figure 2-4: Effect of Pyramidalizatioo Without Rehybridization.. . 52 Figure 2-5: Effect ofPyramidalization With Rebybridization 53
Figure 2-6: Definition of Anglestand 9.. 55
Figure 2·7: Approaches to the Synthesis of [n]Patacyclophanes 59 Figure 2-8: MOEnergyLevels of Planar and Nonplanar Benzene
xOrl>itals(Allinger.R.f.S4) ....70
1'"1p" %-9: Anglesaand~... ...73 1'"1p" 3-1:
c...
DisplayingSingleandDoubleBoods g6Figure 3-2: Corannuleoe. Wustrating Bowl-ta-Bowl Inversion... ... 92 Figure 3-3: IncreaseinStrainEnergy00 Fonnation of Successive
BridgesinSumancne... ... 96
Figure 3-4: Possible Mechanism for Rearrangement of 42 to 44.. . 105 Figure 4-1: Retrosynthetic Anaylsis ofEnediynophane lb 122 Figure 5-1: RettIlsyntbetic Analysis of l,a-Diou(0J(2,7)pyreoopbane, 31. .. .. 157 Figve S-2: Possible Mechanism for FonnatiOD of41 from 31 161 1'"11"" 5-3: RettosyntbeticAnalysis of[n](2,7)py=opbane, 48 164 1'"11""5-4: Views nf(I.6)Pyrenopbane64 170
FigloR6-I: Planes For BendM entnfPyrenopbanes._ 219
235 ... 222
. 237
... 240
. 241
. 236
226 ...230 ... 231
Figure 6-9: Annellation of 5-membered Rings onto Pyrenopbanes.
Figure 6-10: Diels-Alder Adducts ofCyclophanes...
Figure 6-2: Proton Nomenclature for NMR Assignment. ..
Figure 6-3: Carbon Nomenclature for NMR Assignment Figure6-4:DioxapyrenophaneUV SpectrainEther Figure6-5:PyrenopbaneUVSpectrainEther FIp.. 6-6: Styliud CV .
F'lgDre 6-7: Dioxapyrenophane Cyclic Voltammetty, Normalized to1mM••...
Figure6-8: HydrocarbonPyrenophaneCyclicVo!tammetry,
Normal.izcdtolmM .
,i
ListofSdlellHS
&beme 2-1: Diels-AlderSyntbc:sis ofParacyclopbanes... ._. 60
Scheme 2-2: AcyloinRouteto Paracyclophanes. 60
5memt2-3:~XylyleneRouteto [8]Paracyclopbane 61
Scheme 24: WolffReamngement af20... . 62
Scheme 2-5: Synthesis of [7]Paracyclophane.. . 62
Scheme 2-6: Possible Intermediatesinthe Formation of lld from 24 63
Scbeme 2-7: Interconversion of 11e and 27 64
Scheme2--8:Route to Paracyclophane 30 from Enone 28... ..64 Scheme 2-9: Mechanism for Formation of 32 and 33 from 31 65 Scheme 2-10: SynthesisandReactions of [4IParacyciophane 66 Scheme 2-11: [)jels-Alder Reaction of Paracyclopbanes. ... .. 75 Scheme2-1~:Hydrogenation of[n]Paracyclophane... . 76 Sdacme 2-13: Acid-eatalyzed Rearrangement ofParacyclophanes _, 76 Scheme 2-14: Products ofParacyclophane Bromination 77 Scheme 3-1: Examples ofExohedral Fullerene Derivatives 88 Scheme 3-2: 1,2· and 1,4-addition to C60... . 89 Scheme 3-3: Failed Corannulene Synthesis Ancmpts.. . 92
Scbeme 3-4: Synthetic Approaches to Corannuleoc 93
Scheme 3-5: Nonpyrolytic Synthesis ofConnnuicne 94
Scbane 3-6: Another Pyrolytic CorannuleneSynthesis... . 94 Schaue )..7: Corannulcne from Pyrolysis ofSilyl Enol Ether 11 95 Scbaae 3-8: Siegel's Synthesis ofCorannulenopbane 14.... 95 Scheme )..9: Mehta's Corannulene Syntheses... . 96 Scheme].10:Dehydrogcnativc Cyclization of18... 97 Scheme].11:Pyrolytic Dehydrobromination of21... 98
Scheme ].oU: Dehydroge:native Pyrolysis of26 99
Sch....3-13, Dchydrogcnativc Pyrolysis of27...•... 100 Scheme ),,14: Rabideau's Synthesis ofTriiodeootripbenylcnen 101
Scbem.e )..15: Scott's Synlhesis of34 102
Scheme 3--16: Synthesisof22byPyrolysis ofTribromoarene 35... 102
Sclleme3--17: Rabideau's Synthesisof39 _ 104
SdIom. 3-18: Pyrolysis of40 Genera... Pi 1... ...104 Scheme 3--19: PyrolysisProducts from Bis-Fluoranthrylidene 42 lOS
Scheme 3-20: 39 from Pyrolysis of 48 106
Scheme 3·21: Rabideau's Nonpyrolytic Synthesis of39... 106
Scheme 3-22: Ienneske:ns' Synthesis nrsL... 107
Sc:heme 3-23: SynthesisandRearrangement of53... ...107 Scheme 3-24: Agranat's Pd-Catalyzed Synthesis of56 108 Scheme 3--15: Synthetic Approaches to Cyclopenta[c,d]pyrene60 ..109 SdIem.3-26: Stoddart's Synthesis of [12]COU...70... . 110 Scheme 3--27: Cory's Diels-Alder Route to CyclaceneDerivative 73 III Scheme 3--28: Herges'RingExpansion of78 to Picotube79 113
Scheme 4·1: The Bergman Cycloaromatization.. .116
Schtmt 4-2: Syntheses ofEnediyne Moieties... .... 117 Sc:htme 4-3: More Syntheses ofEnecliyne Moieties.... ..118 Scheme4-4: Grissom'sTandemEnediyne--Radical Cyclization ... 119
Scheme 4-5:Bergman·Radica1Cyclization Cascade 120
Scbtme 4-6:Enediyne-Cyclization Route to Dibenzopinakene Skeleton 121 Schtme.J.7: SynthesisofDiester18... .. 123 Scheme 4-8: Synthesis ofVinylogous Diester 37... .. 123
Scheme 4-9: Synthesis of 1,2-Diethynylbenzene. 38 124
SchaDe 4-10: Products ofCoupling Reaction 125
Sc.btme "'11:Synthesis ofEnediyne Precursors 46and47 126 Scheme 4-12: Approaches to Expanded Thiacyclopbanes... . 127
Scheme 4-13: Failed Arbuzov Reaction... . 128
SeIl 4-U: Failed McMurry Coupling... . 12g
_.4-1S: ApproochestoPa!h
C... 129
SCb 4-16: SynthesisofSSltodS9... . 129Sell 4-17: AttemptedMcMurryCouplingofS9 130
Sdtaae.J.18: Transformations ofPropargylic Bromide 58 130
Scheae 19: Synthesis ofEoediynes 61and65. .. 131
Scheme 20: Results ofCycloaromatization Experiments... ... 132
Sl:heme 4-11: .Competing 5-Exo and 6-Endo Modes ofCyclization 133 Scheme 5--1: [2.2]Paracyclophane Synthesis by Hofmann Elimination. 149 Schem. 5-1, [2.2]Motacyclophane Synthesis by WurtzCoupling... ... ISO Scheme 5--3: SyntheticPathwaysfrom Dithia[3.3]metac:yclophane. 7... 151
Sl:heme 5-4: Two Methods for DithiametacyclophaDe Synthesis... 154
Scheme 5-5: Possible Transformations of Tethered Dithiametaeyclophanes, 26 .. 155
Scheme5-6:Conformational Properties ofCyclophaneu.. 156
Scheme 5--7: Possible Photochromism of27 and 29.. 156
Scheme 5-8: Esterificationof37 158 Scheme 5--9: Synthesis ofTetrabromidcs 34fromDiester 36... 158
Scheme 5-10: Synthesis ofDithiacyclopbanes 33 1S9 Scheme 5-11: Stevens Rearrangement of33... .. 160
Scheme So12: Cyclophanediene 32 by Hofmann Elimination.... .. 160
Scbeme 5-13: Products of Hofmann EliminatiOD of Longer-tether Cyc10pbaoes 40... . 161
Scheme So14: Dehydrogenation ofMixturetoProduce Pyrenophane 162 Scheme 5--15: Dehydrogenation 0£32 to Pyrenopbane 31... . .. .. 162
Scheme 5--16: Synthesis of Aryl Triflate 52... 165
Scheme 5--17: Sonogashira Coupling of 52 to Afford Diynetetraesters54... 165
Scheme 5-18: Synthesis ofTctrabromides56... . 166
Scheme 5-19: Synthesis ofThiacyclophanesso... 166
Scheme 5020: Stevens Rearrangement of SO... .. 167
Scheme 5-21: Synthesis ofPyrenopbanes 48 167 Scheme5on:Synthesis ofDimethoxythiacyclopbane 62... .. ...•... 169
Scbem.e5-13:Synthesis ofDimetboxypyreoe63... 169
Sob 5-24, Po••ibleRouteto Pyreoophanes64aod6ll...•.•...• 171
Sob e5-25'SynthesisofDi_n 172 Scheme 5-26: Preparat:ionofTetrabromidc 66 172 Scheme 5-27: Preparation ofThiacyclophanes 65and67... •... 173
Scheme S-U: Synthesis ofPyrenophanes 64 and 68 173 Sdleme~1:Productsof Addition to (l,4)[nINapbtbalenopbane ....•.... 243 Scbeme~2:Productsof DieIs-Alder Reaction of22 andTCNE ..244 Scbeme 6--3: Diels-Alder Reaction oflaandPTAD. ...•. 245 Scbeme6-4:Matsumoto's Bis-Adduct of8and PTAD... . 245 Scheme 6--5: niels-Alder Adducts ofPyrenopbanes and TCNE. . 246 Scbeme6-6:Bromination of(l.)Napbthalenopbane... . 248 Scheme 6--7: Addition ofBI2to[61P8l8CyclopbaneDerivati~. 248
Scheme6-8:Route to Brominated Pyreoophane 39 249
Scheme~9;Mechanism for Tether Cleavage of Ib by Br: 251 Scheme 6--10: Friedel-Crafts Acetylation of[lO]Paracyclophane... . . 253 Scbeme 6-11: ElectJ'Opbilic Telomerization ofNaphtbalenophane 18. . 254 Scheme 6-12: Acid-Catalyzed Rearrangement of[6]Paracyclophane 256 Sdleme~13:Possible Structure of Protonated Pyrenopbane la.... . 256 Sclleme 6-14: Directed Ortbometallation ofPyrenopbanela... . 257 Scheme 6-15: Metalation of Strained Metacyclophane 52.. . 257 Scbeme 6--16: Product of Reaction ofPyrenophanelawithfBuLL... . 258 Scbeme 6--17: Mechanism ofRiDg-opening by fBuLL... 259
Scbeme 6-18: Hydrogenation ofParacyclophane 259
List of Tables Table 1-1: HOMADataorSel~edMo!ecules . Table 1-1: POAV Angles of Distorted Aromatics Table 2-2: UV Spectroscopic Data on [n]Paracyclophanes . Table 2-3: IH NMR Data on [n]Paracyclophanes . Table 2-4: StrainEnergyandBendAngles of[n]Paracyclopbanes Table 2-5: HOMA. QEO,andEN Indices for Selected Aromatics..
Table 6-1: Tether Bond Angles
Table 6-2: Angles Between Least Squares Planes (') Table 6-3: POAV Pyramidalization Angles (') for 1a and
D,,,
C10Table 6-4:IH Chemical Shifts ...
Table 6-5: IlC Chemical Shifts ofPyrene Moiety...••..
Table 6-6: IJC Chemical Shifts ofTetbcrs . Table 6-7: IJC~Hcoupling ConstantsandCalculated Hybridization
<II;%, char.) .
Table 6-8: Major Pyrenophane UVNis Absorbance Bands ....
Table 6-9: Cyclic Voltammetry Oxidation Potentials
. .41
... 56 . ... 69 . .. 71 .73
. 81
.218 ... 219 ... 221 . ..•..224 .226
. 227
. 229
. 232
238
A
A
•
A A Ae Add.AO ARE Av BAC BB BE bp Cat CE CHD
CV d DABCO
List ofSymbols,AbbreviJltiollJ, aDd AcroDyms
Molar Magnetic Susceptibility
"Alpha" band (inaromatic::UV Spectrum) Bend angle (in [n]paracyclophanes)
"Beta" band (in aromatic UV Spectrum) Bend angle (in [n]paracyclophanes) Heat
(inNMR)ChemicalShift Extinction Coefficient MagneticSusceptibilityExaltation Wavelength
PyramidalizatiODangle Angstroms Amperes Ae<tyl. CH,C(O)- Addition Atomic Orbital Adiabatic Resuuance Energy Avenge
BondAlternation Coefficient BroadBand[131NMR) BondEnergyCoefficient Boiling Point Catalyst ConjugationEnergy Cyclobexadie:ne Cyolie Voltammetry
Deuterium[mNMRsolvents, e.g. TIIF-da 1.4-DiaDbicyclo[2.2.2]octaDe
DBU DDQ DMDIlP DMSO DMF DRE E EJ-MS EN ENDOR Eq.
ESR Et Fig.
FVP FVT GEO HETCORR IIMPA HOMA HOMO HOSE HSE HSRE
xvii
1,8-Diazabicyclo{S.4.0]undec-7-eDe 1;J.-Dichloro-5.6-.dicyanobcnzoquinone IS,l6-Dimcthyl- LS,t6-Dihydropyrene Dimethylsulfoxide
Dimethylfonnamide DewarResooancc Energy Electrophilic group Electron ImpactMassSpectrum Enc<gcticIndex
Electron-Nuclear Double Resonance Equivalents
Electron Spin Resonance Spectroscopy Ethyl,C1HS·
Figwe
Flash Vacuum Pyrolysis Flash Vacuwn Thermolysis Geometric Index Heteronuclear Condation Hcxame1hylpbosphoramidc Harmonic Oscillator Model of Aromaticity Highest Occupied Molecular Orbital Harmonic Oscillator Stabilization Energy Homodesmic StabilizationEnergy Hcss-ScbaadResonanceE=gy
h HPLC hv IPR IR ISE
"1 K kJ KDE LilIMDS lit LUMO COSY m-CPBA M' Max.
M.
min MM MO
mp NICS NMR NOED [0]
P P.
PAR P.
PCA
Hour
HighPressureLiquid Chromatography Light
Isolated Pentagon Rule Inlil=I
Isodesmic StabilizationEnergy (inNMR) Coupling Constant (Hz) Kelvin
Kilojoule
KekuIe Defonnation Energy Lithium Hexamethyldisilmde Li~
LowestUnoccupied MolecularOrbital Correlation Spectroscopy m~/~oroperoxybenzoicacid Masspeak
Maximwn MethyI.CH]- Minute -or- minimum Molecular Mechanics Molecular Orbital Melting Point
Nucleus Independent Chemical Shift Nuclear Magnetic Resonance NuclearOverhauserEffect Difference Oxidation
"'Para"band
em
aromaticUV Spectrum) AnodicPeakPotentialPolycyclic Aromatic Hydrocarbon CathodicPeakPotc:ntial PrincipalCompo_Analysis
""'
Ph Pbenyl, 4Hs·
PPP Pariser-Parr-Pople
POAV n-Orbital Axis Vector
ppm Parts per Million
PTAD N-Phenyl-l,3,4-triazoline-2,5-dione
pyr. Pyridine.C,HsN
RA Relative Aromaticity
RBF Round Bottom Flask
RBFA Relative Bond-Fixing Ability
RE Resonance Energy
REPE Resonance Energy Pcr Electron
,gt. Reagent
RT Room Temperature
SCE Saturated Calomel Electrode
SCF Self-Consistent Field
SSCE Saturated Sodium Chloride/Calomel
STO Slater~Type Orbital
TBAl Tetrabutylammonium iodide
'Bu /l!'rt-Butyl.(CH3hC-
'BuOK Potassium tert-butoxide, (CH3hCOK
TCNE Tetraeyanoethylene
Tf Trifluoromethanesulfonyl, CF3SOr
THF Tetrabydrofuran
tic ThinLayer Chromatography
TMEDA Tetramethylethylenediamine
Ts p-Toluenesulfonyl
UV Ultraviolet
Vis Visible (1igbl)
V Volts
VRE VerticalResonanceEnergy
VT Variable Temperature
IDtroduetiOD
Since thediscoveryofber:lu:ne by Michael Faradayin1825, the study ofbenze:ne anditspropertiesbasp~a central rolein thedevelopment of modem chemistry.
Such concepts as valence, resonance,andmolecular orbital theory arose,atleast in part, asattemptstoexplain thelUlusualstabilityandspectroscopic properties associated with benzene's "aromatic sextet." Indeed. accounting for these unusual properties, usually referred to collectivcl)' as "uomaticity," has become ooe of the:DKlstfascl.uating and frustrating problems to face theoretical and cxperimental chemists during the20"
century, and,as thisthesiswilldemonstrate, itseemsunlikely that any simple resolution to this issue willbepresentedinthe nearfirture.
Among the many questions posed about benzene, one concerns the effect that bending the (normallyflat) six-membered ring out of planarity would have on its
"aromatic" properties,Duringthe seton<!halfof the twentieth century,thisproblem was investigated extensively by the prepantion and study of [n]meta- and [n]paracyciopbaDes, molecules whose benzene rings an: forced to bend by a short tether attached to two ends of the "aromatic" moiety. Ascomputational chemistry became more sophisticatedandreliableinthe 19805 and 199Os, agreatnumber of theoretical stlJdjes oftbese compounds was conducted as well.
The discoveryof bucJcminsterfulerenes \fullercnesj inthemid-1980slidded new significance totheearlier, curiosity-drivenresearchinto the effects of nonplanarity on aromaticity. ManypracticalapplicationsYl'C:repostulatedforfulIetenCSandrelated compounds, includingtheirpotential uses as molecularwiresand high temperature supen::onductors. This made a detailed understanding of electron delocalizationand 'aromaticity'infuUerc:nes very important, and considerable workbasbeen and is currently being conducted to investigatethisproblem.
The work described in this thesisspans allthese topics. At the outset of our research into the properties of pyrenopbanes, no systematic investigation into the effects of bending a polycyclic aromatic hydrocarbon(PAR)out of planarityhadeverbeen pursued. Many questions could beansweredby such studies: How does significant oonplanarityaffectthe "'aromatic" properties of polycyclic aromatic hydrocarbons -the dcIoc:a1i2atioD, the diatropicrinScurrent,the stability? How far canthe.aromatic DKliety
bebentbeforethecompounds fail to fonn orbecometoounstable tobeisolable under ambient conditions?Whatsortof reactivity patterns do these compounds display? How do these properties compare withtheknown properties ofbuckminsterful.lerenes?
Itisbard.toknowiftheanswerstothese questionswillhave any practical applications or profound influence on the topics described above. However, to appreciate the significance they do have, a detailed understanding of aromaticity and cyclopbane chemistr)' is essential. This work will therefore begin with an overview of these topics.
Itwill then describeourresearchintocwved PAHs, and discuss the significance of this workinthe contexts of aromaticity, cyclopbane chemistry, and fullerene chemistry.
Chapter 1 - 8AAuDe aDd Aromaticitv 1.1BauDe aad jb Properties
1.1.1 -Early Hiltory
OnJune 16, 1825, Michael Faraday presented a paper to the Royal Societyin London entitled "On New Compounds of Carbon and Hydrogen, and on Certain Other Products Obtained During the Decomposition of Oil by Heat...1At the time, Faraday was working as a laboratory assistant to Prof. Humphry Davy at !he Royal Institution, and mucboftheirworkinvolvedtheisolation and condensation ofgases. In thiscase., Faradaybad beengiven several cylinders of compressediUuminatinggas,whicb was manufacturedbythe"Portable GasCompanrbydecomposing whale oil atredheat.
Faraday collected the condensed liquid from !hese cylinders and from it, using fractional crystallization, he isolated a compound we now know as benzene, 1. Faraday detennined the density and melting point of this new compoWld, and demonstrated that it burned with a smoky yellow flame. Due to the uncertain atomic weight values at the time, Faraday determined the empirical formula ofthisnewcompound to be C1H, andbetherefore namedit "Bicarburetof Hydrogen."
A decade later, by dry-distilling the acid isolated from benzoinresin(benzoic acid, 2) with lime, EilhardMitseb~lichobtained a volatile liquidwhichhe called
"Benzin.nJ He recognized that this liquidwasidentical with that described earlier by Faraday. He conducted the first chemical studies of benzene, synthesizing nitrobenzene,
o (}>«),
3
( } So,H
I'"
h S
I l)fndly,M.PM. »-.RD.JoaISoc.II15, llS,440"..
m.
b)Hafuer,K.htgnIeJ-.bfLUE1fgl.1979,18,641-651.
1F"lDdIay,A.dHpuJrrJr.,.sfCI¥mtrtrrDuc:bror1:holCo..LoDdoD:1937, p.1)!.
3, azobenzene, 4, and benzenesulfonic acid, 5, experiments that laid the groundwork for the development of the dyestuffs industry laterinthe century.
Justus von Liebig considered the name "Benzin" to imply a relationship with stIychnine and quinine, so he renamed this compoundbenzol (the suffix -61 refers to "oil"
inGerman). Ironically, this early attempt at systematic nomenclature resultedin confusioninEngland andFrance due to its similarity to the systematic names of alcohols, soin these COWltriesthe compoundwas finally renamed "'benzene.'>'! Auguste Laurent,S the French chemist who proposed this --ene ending, also recommended changing the compound's name entirely to "phene" (etymologically related to the Greek~(mlW,''to shine'') to indicate the compound's original discovery from illuminating gas. Although this nomenclature never replaced the tenn benzene itself, the benzeneringas a substituent isstill referred to as ·'phenyl.'t6
By the 186Os, the chemical formula of benzene had been conclusively determined to bec~. The structure, or arrangement of these atoms in space, was unknown.
However,in1858, Friederich August Kekule7andArchibald Coupers had independently proposed atheory on the structure of organic compounds, known asthevalencetheory, that suggested that carbon atoms always attach themselves to four other groups - in modern terms, carbon is tetravalent. Although obvious to modem chemists, at the time this proposalwas (at risk of using an unintentional pun) a radical9one. Most chemists hadresigned themselves to the belief that the arrangement of carbons and hydrogens in
o
ladenburg Claus
rn
Annstron,
&Baeyer Figure I-I: ProposedStructures of Benzene
, Wmderlich, R. J. CMm. Ed.I!M~,26, 358-361.
4Badger,O.M.tfrpmqti£chqrqctvgnddromgticinlcambridge UniversityPress., 1969, p. I.
SdeMilt,C.J. Chem.Ed.1951,28,191-204•
• Thorpe,T.E.E.umjnffltlpricqlChpmBooktforLibrariesPress,189-4 (l972reprint).
7 a)KekulC,.F.A., Speech atBerlin City Hall, 1890. Translated byO.T. Benfey,inJ.Churl.Ed.1958, 35, 21·23. b)Kauffinan,G.B.J. Chen!.Ed. 1971,49,813·811.
IBenfey,O.T.J.Chem.Ed.199,J6, 319-320.
Figun 1-1: Ladenburg's Objection to KekuJe's Structure. Two possible o·isomen
organic compounds was too complex to determine,andsuch compounds could only be characterizedandclassifiedby,forinstance,the number of carbonstheycontained.
Kekuleapplied his controversial theorytobenzene in 1865, proposing the now familiar cyclobexatriene or 'hexagon' structure (Fig. I-I). Whetherthere is anytruthto the legend that Kekule conceived this idea after adreamin which he saw a snake eating its own tail is debatable. Some historians suggest that this imaginative storywaspromoted byKekule himself to discourage rumors that he bad stolen his idea from someone else, perhapsCouper.Inany event, his proposal immediately afforded a simple explanation to hitherto intractable problems concerning isomeric derivatives of benzene - for instance, whythere was only one isomer of phenol, 6, and toluene, 7. butthreeisomers of xylene and cresol, of which the respective o--isomers (8&9) are shown.10
Someobjections'Wereraised to Kelculc's theory. Ladenbw'g, for instance, demonstrated bow,ifa~ticstructure~postulated. there should be fOIlT,DOtthree, isomers of clisubstitutedbenzenes.sucb as xylene (Fig. 1·2).11 K.ekuJi.rathervaguely, proposeda "mechanical motion" or oscillation of the double bonds aroundthering, thereby rendering the two 1;1.-di.substitutedstructuIesequivalent.Thiswas considered by many a rather desperate device to save his hexagon theory, and other proposals for the
str'\)CtUI'C:of benzene were subsequently advanced (Fig. 1-1). However, the idea that
0(CH,
V
7
ACH' V
8
'lathe1~century, tbetcnD'"rIdic:arrefural.toorgmiclfOUPS,often ofllDCeftlinsttuenn:,whidlcouJd becmicdIIJrw&b;reectiooswidlout:cbIDpatbeirc:ompositioa,Co&-metbyllWiic:al,bccmylrwtical .. Brock.W.H.F.cpwHmqryp'Q_dey FOIIWII.Pn:ss.Loadoa.l991.p.261.
organic stnK:tures were inherently unknowablewas rapidlydiscan1ed. By the18705, effectivelyallorganiccbem.istswerestructtD"8list,12dedicated to exploring a substance's propertiesbydetermining itschemical structure. Kekule's proposed structure of benzene, which initiatedthisrevolutioninthinking..wasappropriately describedin1898 as "the most brilliant piece ofscientific productioninthe whole of organic chemistry...l l
Thediscovery of the elearonin1897 by 1. J. Thompson,aodhis subsequent interpretation ofbondingas electron transfer -the polartheory of valence - onceagain allowed benzene to playa roleinthe development of structural chemistry. While the polar theory worked excellently with polar molecules such asHel,invoking a polar bond between two carbon atoms ora carbon and a hydrogen atom seemed less likely.In1916, GilbertLewisproposedtheconceptofthe'sharedpair,.14 Electronswerenottransferred from one atom to another. butwereshared between them.Carbon, forinstance,would arrange itself suchthatitwas sharing eight electrons with other atoms. This 'octet rule' wasaccepted due to its utilityinexplaining regioselectivityinthe substitution of benzene and its derivatives.Thedistortion of the octets of the benzene ring carbons by electron withdrawing or electron donating substituents,andtheir resultant respeaive meta- or ortho.poro-directing behaviOW'. couldbeexplained by the octet rule,.butnotbythe earlier polar theory of valence.I! Once again.benzenebadbeen centralinthe development of a fundamental theory of modem chemistry.
In this section,abrief historical overview of the important role benzene has playedinthe evolution ofmodem chemistry hasbeenpresented. However, the most puzzl.ing property ofbenzene,and the one most relevant to thework thatthisthesiswill ultimately describe,hasoot yetbeenconside:ed. Itisthisproperty -commonly known as aromaticity -thatwill now be examined indetail.
IIGamtt,PJ.~WileyIOdSoas.NewYork.19l6.
11Ref.IO.p.261.
IJRef. 10. p. 269.
.. Lewis,u.N.JAm.. Chem.Soc. 1916, 18, 762.
uSaItmwI,M.D.J CJ:mt. Ed.197",51.491-502.
~NOReaction
O I - . -
KMnO.. No Reaction h"zIP_
~NOReaction
1.1.2 -The Plnldox orBeauGe
Lloydl6bassuggestedthatchemistshave become "desensitized"tojusthow unusual thechemistry ofbenzeneis.when comparedtothat of other unsatUrated compounds. He goes on to describethefollowing thought experiment (Fig. 1.3):
Consider a chemistwho basbeentaught only thechemistryof aliphatic compounds.
saturatedandunsaturated. Whenpresentedwith thestructUralfonnula of benzeneand asked to predict itsproputi~suchan individual wouldalmOstcenaioly suggest that benzenewould:
-undergo addition of brominetoyielda vicinal dibrontide;
-decoloriz.ean aqueouspermanganate solution, to afford oxidized products;
-react rapidly with hydrogenanda catalyst to yield cyclohexane.
Many more items could be added tothis list, but the point is already clear. "Nonnal"
unsaturatedcompounds, such as cyclohexene, 10, will undergo addition reactions like thoseillustrated below. The misinfonned chemist would no doubt be puzzled to leam thatallthese predictions are wrong. Benzene does notreactwith bromine without a catalyst. andwhen it does, it undergoes a substitution ratherthanan addition, regeneratingtheunsaturated system. Nor does it decolorize permanganate., oor add hydrogen, except under extremely forcing conditions. Thispairof apparently contradictory properties -unsaturated,yetinert -iswhatP.J.Garratt referred to as the
"Paradox ofBcnzene...1I
Next, supposethechemistwas presented with thestructure:of cyclobutadieoe, 11.
Ifobservant, he or she might have concluded that therewassomething special about
r-Y
B'7~'~''Br
o ~rY'"
~-~"
o
-,- Flpn---'-_l_-3_'_1'he Pandox ofll=e: Unsaturaled, yetInert.
NUoyd,D. lkCb-mgfCqnhwIUrt!QdifCq!gzqa!r!k-To RcPCNqtTqlkUkBmrnwJobD Wiley&:SoDs,C1idIesrJ:r.619.
cyclic conjugatedsystemSlikebe:ozcne. Not wishingtoappearignoranttwice. beor she may predictthatthiscompound wouldbeinerttothe previously mentioned conditions, or reactwithelectrophiles such as brominetoafford substitution products. Of course, be or shewould be, onceagain,totally incorrect. Not only does cyclobutadiene not display
o []
10 11
unusual stability, it isinfactso unstable that it cannotbeisolated or even observed except under special conditions at exceedingly low temperatures, or when stabilized by incarcerationina car<:eplex.16,17
Some other examples of atypical behavior of cyclic, conjugated compounds should be mentionedatthis point. Insteadof displaying alternating double and single bondsanda~ksymmetry(as suggested bytheKekuJe structure),allexperimental data suggestthatbenzene bas a bond-equalizedstructurewithsixC.cbondsof equal length.
IntheNMR..benzene's protons resonate much further downfieidthanthoseofconjugated polyencs, while hydrogens held above or belowtheplane ofthebenzene ring are shifted upfield. Finally,as willbediscussed later, benzene's bebavior when placedinstrong magnetic fields is decidedly differentfromthat expected ofa 'nonnal' polyene.
Thisbizarre and complex behavior of cyclic conjugated systemsisone of the most thoroughly investigated pbeoomenainchemistry,and yetsimple, universally acceptedexplanationsandclassificationsremainelusive." Such behaviorisgenerally denoted by the term. "aromaticity," but, aswill beseen,thistermbasberome almost as confusing and intractable asthephenomenon (or phenomena? - vide infra) thatit describes.
n Cnm,DJ.;TIDIlCI',M.E.;Tbomas.,R.AItflrW.a-..lnt.£d.EngI.1991,JO,1024-1027.
IIForGalaalRefaalcessee:a) Agrmat, liDBerza-mE.D.;PuUmm,B.(Eds.)AI'!!!f!tjdtyplf!tkt damptiritrAIltiA'J"'l'idtv YoL3, IsnldAc.d.OfScieDcesaDdlluawlitics,Ierusalem,I;7I. b) Po2!lInkii,AJ. a...HIUrrXyf:.. Ce-p.1-'2I.717-749.c)Cook,MJ.iK.a!:ritzky,A..R.;Uadon.P.
Aa,.H~~a-.19'74.11.VS-lS6. d)MiDkia,V1.; GIukhovtsev, MoN.; Simkin, B.Y.dlsl!!IJlJim.
fl1K!dlflltr9l!tJtir:iIyJolmWiley&;SoDs, New York, 1994. SeeatsoRefs.4,II,16, aDd 30.
1.2 - Aromatidtv
1.2.1 - Aromatic:ity - Early Developments
Asearly as the 1820's, hydrocarbons with distinctive, generally pleasant odors and ahighC:H ratio (compared to other, "aliphatic" organic. compounds) were denoted by the term "aromatic hydrocarbons.,,19 Among these were oil of wintergreen (methyl salicylate, 12) and oil of cinnamon (cinnamaldehyde, 13), as well as certain compounds such as camphor, 14,andlimonene, 15, that would notbeconsidered "aromatic" now.to
o
N12 13 14 15 16 17
The term was gradually limited to substances that displayed the abnormal chemical reactivity described earlier. Many of these substances, it was noted, contained a benzene ring moiety, but others, such as pyridine, 16, and furan, 17, did not.In1890, Bamberger suggested that all aromatics bore a bexacentric system of ''potential valenccs," but this proposal did not become popular.21 With the advent of the electronic theory ofvalence, it wassuggested that, for some reason, the arrangement of the n:.-electroos in benzene led to enhanced stability. Tnis theory of the "aromatic sextet," proposed in 1925 by Annit and Robinson,22was completely empirical and offered no theoretical rationale. However, it wasone of the first serious attempts to define aromatic compounds: a compound is aromaticifit contains a cyclic, conjugated system. of 6 n:-electroos. In1931, Hockel23 proposed a far more elaborate model, based on quantum theory, that demonstrated that any compound with a cyclic, conjugated n:-system containing 40+2 electrons, where 0 is any integer,willdisplay enhanced stability (like benzene), while any compoundwitha 1'Ref.IO,p.263.
a Sinclair,J.P.(Ed.). Ooomit:Chem'UryMotlWgpJat-NqnbmmrojtfArpmatjg AcademicPress, New York, 1969.
:u Kolb, D. J. Chuf..Ed. 1m,56, 334-337.
ztArmit,J.W.;Robinson,R.J. CMm.Soc.I!n5,127,l604-1618.
:u I) H6tkc1,E. ZPhyJik 1931,7",201. b) HGckeI, E. ZPhysLU932,76,628. c)Dewar,MJ.S.11K Afglg;u!arOrhitp!T1w;nqfOrgqntsChcmitJry McGraw-Hill, New York, It69.pp. 92·100.
10
cyclic, conjugated 1t-system containing 40 electrons will displayreducedstability (like cydobutadiene). 1l:lissystc:m,knownas Huckel's Nle, is familiartomost chemists,and is still frequently invoked by nonspecjalists as the definition of aromaticity.
Hfickel'srule.however, suffers from some shortcomings. Oneisthatitis not quantifiable. According to Hilckel'srole.a molecule is either aromatic or itisnot. and there isDOwaytodetmnine. for example, how aromatic pyridineisrelative to benzene.
Secondly, Huckel's rule is~on an extremelysimplified view of quantum theory, and is theoretically deeply flawed. Finally, Hfickel's ruledoesnotaccoWltforthebehavior of large annulenes or polycyclic aromatic hydrocarbons.
For six decades, theoreticians and experimentalists have struggled to come up witha simple, reliable method to determineqUQflfitolfve/ythe aromaticity of any molecule of interest. The body of relevant literature is vast, confusing, and encompasses diverse areas such as quantum physics, computational chemistry, various Conns of molecular spectroscopy. crystallography,andstatistics. A comprehensive review of this topic is well outsidethescope ofthisthesiS.24However. a detailed exposition of some of the approaches to this problem will nowbepresented.
1.2.2 -Aromaticity -RactivityCriteria
The phenomenon that most people intuitively associate witharomaticmolecules is their relativeunreactivity,andtheirtendencytoundergo substitution instead of addition reactions. This tendency to'"retainthetype," as described by Annitand Robinson.basbeen termed "menedeism,n25anda numerical quantification of this tendency(e.g.by measuring therateofa Diels-AJderreaction of thearomaticmoiety ofa molecule)hasbeen proposed as a suitable criterion for the measurement of aromaticity.26 Other suggestionsinclude examining a compound's thermalstabilityor itsreactivity towards nucleophiles or electrophiles.lib These reactivity definitions of aromaticity amounttostating that aromaticity involves "having achemisuy likethat of benzene."
One problem with proposals involving the measurement of reaction rates, besides such practical difficulties as compensating for differences in solvationand.other factors
II1bemostrec:clltmajor'examiDationofddsUlpicisthatofMinkindcd.;re( 17d ISLJoyd, D.;ManbaIl,D.L.vrg-.o-:.,/IILEd.EIrgI.1m. 11,-404-401.
\I
unrelatedtoaromaticity,isthataromaticityisgenerally considered to be a ground state property. Reactivity (at least kinetic reactivity). on the otherband.is determinedbythe differencein energy betweenthegroundstateand the transitionstaleof whatever reaction is beingconsidered. So reactivity is oot dinctly relatedtothegrotmdstate,andkinetic measurements of reactivity do not provide unambiguous infonnation about the ground state energy of a molecule.27Perhaps a bener definition than "baving a chemistry like benune," one that does not involve excited (or transition) states, is "having a low ground state enthalpy." Alow ground state enthalpy of, for example, an aromatic sextet of electrons, would disfavor addition reactions and favor SUbstitutiODS. So. by measuring thethermodynamic equiJibritDDbetweenan aromatic compoundanditsaddition reaction product, a measure of the compouod's aromaticity couldbeobtained.Theobjection to lhis proposal canbeillutratedbytheconsideration of 'aromatic' molecules like the cyclopentadienide anion 18. or astrainedmolecule like (S)paracyelophanc. 19, substances which have pronounced tendencies to undergo addition reactions, and therefore most emphatically do not "have a chemistry like benzene." However, as willbe discussedinmore detail later, their ground state energies are well below those predicted for 'nonaromatic' molecules. Consider, for example, the reduced basicity of cyclopentadienide 18 when comparedtothatof • linear polyene-anion such as pentadienide, 20. and 18 and 19 display other properties normally associatedwith
"aromaticity." such as a diatropic ringCWl"CDl It was to account fortheproperties of compoUDdslike18and19 thatthereactivitydefinition of aromaticity was discarded.in favor of energetic criteria, whichwillnowbedisc:ussed.
18
• DWm,W.T. J. a.-..SoeB.1J'7o.612-616.
l '~O.J.a.-.Soe 1t6O, 1274-1279.
19 20
12
1.2.3 - Aromatidty - EDtf'ldk Criteria 1.2.3 1 - A MisceUanyofResonance Energies
Undergraduate textbooksusuallystatethattheenthalpy ofhydrogenation of benzene is36 kcalImol lessthanthreetimes the enthalpy of hydrogenation of cyclobexene.andthisvalueis therefore quoted as the resonance energy of beozene.2S This extremely naive experiment, which neglects such fundamental factors&$changesin bond lengths, hybridization, and nonbonded repulsion, is. an attempt to demonstrate quantitatively the energetic stabilization obtained by the cyclic deloca.lization of1t- electronsinbenzene.Themagnitude of this stabilization,calledthe resonance ener&?'.
can be defined formally asthedi.fferenceinenergybetweenreal, delocalized benzene and anhypothetical, localizedqclobexatriene. Becausethe latter does not exist,theenergy ofthisreference molecule cannotbedetermined experimentally. Theenergy mustbe calculated. eitherbyaddingupempiricalbondenergies. or by using sophisticated computational tecbniques.Insimplelerms,the resonance energy(RE)of benzene canbe expressed by the equation:
~)=All...,- 6E(C·1I) - lE(C·C) - lE(C=C) (1) where.6H.(baaaIc)isthe enthalpy of atomization of benzene, and E represents the bond energies ofC-H,
c-e
(single)andC=C(double)bonds.respectively. The energy ofthe realmolecule, lUI.(baqac)ocanbedetermined experimentally orcomputationally.Although apparently simple.thevalues determined for the resonance energy of benzene rangefrom5to 64kca1lmol.)I))1Thisastonishinglywiderangeistheresultof a lack of agreement onwhattheexact structureof "cyclohexatriene" reallyis.andbowthebond entbalpiestuedinequation1 shouldbecalculated.l1
Although manypapm andtextbooks refer confidentlytothe"resonance energy"
(or stabilization energy. or delocalization energy) of benzene and other aromatic compounds. most authorsappearunaware. or atleastfailtomention.thatthere are
13
several differenttypesof resonance energy.33 Thetypesdifferinthe geometric and energetic assumptions madeincalculating the energy of the reference molecule.
."'- 01
BoodLengIhDeloceli:auon./ B ""...Equalization /(KekuleBenzene)"'"
o 0
(C~~ ~~)
BoodLenglh
0 / . ..i.-
Equalization
1.&
DeIocalhalion C(Bond-Equalized Cydohexatrfene)
Figure 1-4:Conversion ofCyclohexatrienc to Benzene To simplify this discussion, consider the thennocbcmicaI cycle in Fig. 1-4.lo4 It shows the conversion of the model compound, cyclohexatriene (A) to benzene (D). The exact geometry(bondlengths) of cyclohexatriene A varies according to the assumptions made(vide infra), but it is always assumed to be 'bond localized' - displayingDJh symmetry with alternating long and short single and double bonds. The1telectronsinA arealsoassumed tobecompletely localized,inother words,Don-interacting. Benzene (D).as described before, is both bond-equalized and1telectron delocalized.
Now let us consider the changes requiredtotransform A into D. There aretwo ways of doing this. First,ODecan allow the electrons of A to delocal.ize fully, while maintainingthe geometry of A constant. This gives B. astructurewe shall call '"Kekule Benzene." The energy for the delocalization step A -+ 8 shallbecalled EA.~B.
K.ekule benzene is simply benzene whose bonds havebeenstretchedandcompressed, as ina molecular vibration. The molecule can now relaxtothe lowest energyD6bgeometry
DGeorge.,P.;Bodt,C.W.;TraclmDan,M.J.C1rem.Ed. 1984,61,225-227.
DOlukhovtsev, M.J.Chem.Ed 1m, 74, 132-136.
ua)Mulliken, R.S.;Parr, R.O.J.Clwm.P1ry;r. 1951, /9,1271-1278_ b) CouIson.C.A.; Altmann. SL.
7hm!.FtIT'tldtlySoc.I952, 48, 293-302.c)Coulson,. CAinC!tpricpJS9cfm Sympwie-SD«fg{
~Chemica1Society,LoncIon.I95tl. p.9S.
14
ofbenzene,D.The energy for the bond length equalization step, B-+D. shallbetermed EB .... D·The other waytoconvert cyclohexatriene to benzene simply involves reversing the order of the operations. Cyclohcxatrienc A canfirstbe distorted to abondequalized but n-electron-localized structW'e C, that can be called "Bond-equalized Cyclohexatrienc," whose geometry is the same as that of benzene. The energy for A -+
Cis EA-+c.Finally, the electronsinstrUcture C canbe allowed to be fully delocalized, transforming C into benzene. D, with an energy EC...D.The total energy of the cycle. E A-+D.willbeequaI to(EA ... B+E1-+ D)or(EA ....C+EC.... D).Depending on the context, Eli. ....D.E ...0,and E C....Dhave all been termed the "resonance energy" of benzene. Now the terminology referring to these various changesinenergy shallbe considered.
Theisodesmic stabilization energy (1SE) measures the energy gained by delocalization of electrons relative to a reference structurewith completely isolated (noninteracting) double bonds (E ...D). Thus, the bond energies and geometriesusedin calculating the energy of the reference strUctureAwouldbedetermined from ethylene (C~).ethane (C-C), and methane(C-H). Examples of ISE include the HOckel resonance energy(as originally proposed), and empirical resonance energy methodslS sometimes applied to heterocycles. It shouldbenoted that even nonaromatic, conjugated moleculeswilldisplay considerable ISE. For example, the ISE for butadiene (relative to 2 moles of ethylene) hasbeenestimated at6-8kcallmol.~The64 kcallmol value for the resonance of benzene isan ISE value, corresponding to the energy calculated for the reaction (vide infra for more OD this):
Benzene+6C~ ~J CHJCHl+J CH2CH2 (2)32 Aromatic compoundsare assumed to display stabilization energyin excessof that alreadyfoundinacyclic, conjugated polyenessuchasbutadiene. ThelfoltlOiksmic stabilizJltion tntrgy (HSE)measures only the energy gained by the cyclic delocalizatioD of electrons (alsoE A...,O). The difference from the ISEisthat thereference structure is calculatedusing the bondlengths and energies of an acyclic polyene, not those of isolated singlean4 double bonds asinethylenean4 ethane, sothatitlacks only cyclic delocalizatioD. TheHSEistherefore lower than the corresponding ISEinaromatic uCooketa£jrd:171:.
IS
compounds. Sincethehood energies of anacyclic, conjugated polyene canbecalculated very accurately, theHSEis frequentlyIIKIrereliablethan theISE.TheDewar Resonance Energy(DRE)l7andthe Hess-SchaadResonanceEnergy
(HsREY*
are examples of HSEs. HSEs like the ORE are usefultootsfor obtaining numericalestimatesof aromatic stabilization.Ifpositive.themolec:uleis considered tobearomatic (ORE of benzene - 21.5 kcallmol).l4tI Ifclose to zero(+I.2 kca1lmol) the moleculeis taken tobe nonaromatic.and ifnegath"c, the moleculeis considered antiaromatic. Therefore, the sign and magnitude of HSEs canbe usedtodescribe quantitatively aromatieity in molecules. Frequently,theHSE is divided bythenumber of electronsinthe aromatic systemtoafford a Resonance Energy Per Electron (REPE)J' value., which allows the comparison of, for example, polycyclic aromatic hydrocarbonswithbenzene. The REPE is therefore.intheory, a Wliversally applicable, quantitative measure of aromaticity.The ISEs and HSEsboth involve comparing a real molecule (benzene) with a hypothetical molecule in its optimized geometry (A~D). Thus the bond lengthsand energies of imaginary cyclobexatrieneAareequaltothose of ethane/ethylene(inthe case of15E)or those of an acyclic polyene(inthecase of HSE). Thepointisthatthe geometry of thereal molecule is diffiTtnJ fromthatof the reference molecule,anda changeingeometry accompanies the delocalizatioD of the electrons. Thestabilization energies thus determined are calledtUliIJboJit::TtSO"tuIce ellngUa(AREs). However, computational techniques allow the comparison of areal molecule (benzene)withan 'olefinic', nondelocalized reference molecule (Bond-equalized Cyclohexatricne, C) in 1M same geomelry as benum. In suchcalculations, the atoms are heldinthe same positions, and. interactionsbetweencertain orbitalscanbeturned off by 'fictitiouswalls' that electrons cannot penetrate.40 Such methods, wherethegeometry of thetwo structures being compared is identical, determine the vel'1ical resonance energies (YREsll as in EA-+aand Ec-+o.42Anexample ofVRE is the Jug~nanceenergy.21
MRd34e,p.I01.
" .) Oewm', MJ.S.1'bfMolgi£Orbjtql17wryqfOrggnlcclrmrm.McGl1lw,H.iUBookCo., New Yen: 1969. b) Baird, N.C. Can. J. 0.-. U69, 41, 3535-3531.
" Hcss,B.A.;SCbMd, U.J.hLChan.Soc.lJ71,9J,lOS-310.
"Has,B.A.;Sc:hMd,U.J.Orr-Qar.lm,J6,34I3-3423.
• 8dln:Ds,8.;xtlsIcr,A.M.;Ju&,K.J.~a-r..1994.59,2546-2551.
'ITbe1mD~'isusaf intbesemc impliedbytbeFruck-CondoD.priDdpIe. Thedlanp:in eIedroDic$lnICUftocanwitbout. •dIu&einiar.mmdar1lisl:IDces.
16
Thesignificance of theVRE isthat itdescn1les the aromatic stabilization attributable to thedeloealization ofthex-system. only, becausethegeometry(andhence the energy) of the(Jsystem. does ootchange.TheARE,OIltheotherband,consists of changesinboth the1tandCJenergy components. ThegraphinFig. 1-5 demonstrates qualitatively the changes inenergy (at the PPP..sCF-MO level of calculation) that OCCW" as 'cyclobexatricnc' andbenzene are distorted from their optimal geometries (Aand0, respectively). Notethatelectron deloca1ization stabilizes benzene at all geometries, but most orallin thebond-equalizedDQgeometry.41
I~Ci"""""""r""""-'-'--"""""
i
EM: •1
(D1mrtlonEnergy)1
A··l--···.·---·--·.···----··.·.·T·.··.· l
lit-localized: Eeo . . . , : : ...
·r.~~~···T·~l...···
: (KHLMDistortion :! ~-:-
i _-j i .
Figure 1-5: Delocalization Energy Scheme for Benzene.
ClAilma,J.-LBvJLCJwM.Soc.JaptIIf.. U90.6J.1956-1960.
17
1.2.3.2 -gvs.11:Energeticsin Beqzene
"The issueof the energy of the11:andaframeworkson going from a localizedtoa delocalizedstructurerequiresa brief digression. Itwasalwaysassumed that the properties ofbenzeneresultedfrom a stabilization ofthe1t-system. Itwasalsoalways assumedthattheequalization of bond lengths observedinbenzenewasa product ofTt-
deloc:alization as well -inother'WOrds.the'It-systempreferstheD6b geometrytothe localized1»". However,in1985 Sbaik andHibertyreleased thefirstof a series of papersothat questionedthis'conventional wisdom'. Their conclusionswere surprising:
the asystem.notthe'It.wasresponsible for bond equalizationinbenzene. The x-system prefers tobebood·a1temant.butthe presence of the a-system forces the 'It-system to assume an otherwise unstable, bond-equalized conformation. The basis for this rather heretical assertionwas their (computational) demonstration of the tendency of atomsthat formstrongbonds,withlow triplet excitation energies, to prefer a localized rather than a delocalizedstate.SinceC~x-bonds are relatively strong, the 1t-system should prefer to begeometrically localizedintoa~hgeometry. A1thougb some of their conclwions have beencaJled into question.44their conclusionthatthe x-systemin benzene isdistortive at DQIgeometry (i.e..wouldprefertorelax.toa~geometry) basbeenconfirmed by other groups' computatiooat'andcxperimentat'evidence. Theirwork.,andthework of others, basallowed a detailed partitioning of energy of the xandaelectroasinboth real,
"aromatic..benzene and olefinic cyclohexatrieoe at both alternatingandnoaaltemating geometries. ThisisillustatedinFig. 1-6.
d.)Hiberty, P.C.; Shaile, 5.5.; Lefour, J.·M.; Ohanessian, G.J.Orr.Chem. 1985, SO, 4657-4659. b) Shaik,5.S.;Hibcrty,P.C.J. Am. Chcm.Soc.19";107,3089-3095. c:)Hibmy,P.C.; Shaik, 5.5.;
0h&Dessian,G.;Lefollt,I.-M.J.Otg.Clrrm. 1986,SI,3908-3909. d) Hiben:y,P.C.;5baik,5.5.;
0haDesa;ian,G.; Lefour, I.·M.J. Am. ChuL Soc. 1917, 109,363·374. e) Hiberty, P.C.; Sbaik, 5.S.;
0balIessian,G.; Lefour,I.-M.J. PIrp. ChDrr.19l8, 92,SO~'094.f)Hiberty, P.C.;DanoviclJ.D.;Sbwti, A.;Sbaik,s.s.J.Alit.Chat.Soc.1995,I17,TI60-7761.
.. • )Baird,N.C.J.Orz.CItat.19I6i,Jl,3907·3908. b) GIeDda1izJ&, E.D.;Faust,R.;Slreitwicscr-, A.;
VoUbardt.K.P.c.;WeiDboId,F.J. ha.
a-..
Soc.1993,IU,109S2·I09S7.IS.)Ju&K.;Kosce-,AMJ.bt.Cital.Soc. 1990,1/2,6'772.-6777. b)Gobbi,A.; Yamaguc:hi.Y.;
FrmIda&G.;SCbaefa",K.F.
a.-.
PIry$.LdL1995,2#,27-31.18
l'l:-localized
...
(VertlcaJRE)
1t-delocallzed
D",
Figure 1-6 : Partitioning of'ft-and0-EnergiesinBenzene.
Several conclusions canbededuced from this partition scheme:
Benzene (with deloca1ized electrons) is stabilized relative to an olefinic, n- localizedreference at all geometries, but the greatest VRE is obtained at the D6Ilgeometry.
The o-frameworkis most stable attheD6hgeometIy.
The 'It-framework. whether localized or delocalized, prefers theOJIIgeometry.
Without the a-framework's tendency to prefer theD6hgeometry, benzene would have astructurewithalternating single and double bonds, like acyclic polyolefins.
Sowhat isthesignificance of these studies? Besides demonstrating that the
"aromatic" properties ofbenzene are extIemely complex. it callsinto question most ofthe workthatimplicitly assumed that aromaticityandaromatic stabilizationwere solely an effect of the x-system. Bydemonstrating the role of the a-frame inaromatic
• •) Haas, Y.;Zilberg, S.J.Am. CkIrt. Soc. 1995, 117.S387-S3&8. b)Shaik, 5.5.; Zilberg, S.; Hus. Y.
Ace. CMm.. Re:. 1996, 29, 21 1-218. c)Shaik.5.S.; Shurki, A.; Danovich. D.; Hiberty, P.e.J.Am. Chnn..
19
stabilizatio~anew variableisaddedtothe confused concept of'resonance energy'. One must now consider the vertical resonance energy, which involves the 'It-system alone,and adiabatic resonance energy,\JIhicb results from energychangesof both the0-and1t- systems.
1.2.3.3 -Other Energetic Criteria
Not all methods of determining the stabilizationenergyof benzenerequire comparison to animaginarysuucture. The enthalpy of reactions. such as:
c.H.+6 CH.~3 CH,CH,+3 CH,CH, (1) and
c.H.+3 H,CCH,~3 H,CCHCHCH, (3)
canbecalculated using computational techniques.Theadvantage is that no imaginary, ill-defined structure needs to be invoked. However, depending on the basis sets and other variablesused.theenergy calculated for these reactions can still vary. Reaction 2, already mentionedinthesection describing isodesmic stabilization energies, produces a Mrof642kcalImoI.JIpossiblythehighest value ever proposed for theRE ofbcnzeoe.
Reaction 2isan isodumic reaction,inwhichthereareequalnumbers of formal single anddoubleboodsbetweenC atomsinthereactantsandproducts. Reaction 3, on the otherhand, isa homodumotic reaction,in which thereactants and products havethe sameDlUJ1berandtypeof C-CandC-Hbonds. Thisbas been evaluatedusing a number of computational methods,withresults ranging from 20.6 (AMI) to 28 (SCF-3-210·) kcaUmol.11dJO Because the product ofthisreaction (butadiene) contains conjugated double bonds, the results of this calculation refer to homodesmic stabilization energies (HSEs).
Another method of avoiding the problematic imaginary reference molecule is to consider benzene alone. For instance. consider the energyrequiredto distortbenzene from itspreferredD61 geometry toan olefinic.lJ)bgeometry(ED-+.).Thisenergyhu beencalledthe"compression energy," ho'WeVCrthisoameismisleading, as bonds are SDel996,//8. 666-671.d)Sbmki,A.;S1Iaik,.s.s.Ane-.
a....
/JrLEd.EJwl.1997,36, 22QS..2207.20
stretchedasweUas compressed. We shall therefore refer to it as theKekuleDistortion Energy(KOE). The energiesdeterminedforthe KDEusing quantum mechanical calculations ranges from 4.2 to 6.0 kca1Imol.JO.41 Using a harmonic oscillator model, a value of 12.3 kca1lmolwasobtained(vid~infra).14However, simply distortingbem:eoe toa bond-altemant. olefinic geometry doesnotremove the stabilization due tothen- electronic delocalization.andit isthelatter quaDtity thatisgenerally understood tobe associated withthearomaricity of benzene. Given the recent findings coocem.i.Dg1tanda energeticsinaromatic compounds.itisdebatable whether the KDE is neeessarily proportional to adiabatic resonance energies or any other value connally associated with aromaticity.Amore recent approach along these lines willbeconsidered later.
1.2.3.4 - Conclusion - Energetics
The preceding paragraphs have described a nwnher of different methods of determining the 'stabilization energy' that differOD.thebasis of the characteristics of the reference molecule, or the lack ofODe.AJthough the examples of the methods described here all involved benzene., most of these methods can alsobeappliedtoheterocycles and PARs.Isitthereforeapplicable to a quantitative,univenalrnea.suremenl of atomatieity?
Intheory, theansweris~. Thestabilizationenergy,as long asit is specifically and rigorously defined, shouldbedeterminable for any molecule,andthevalue could be compared to· that ofberszene,whichis genen1lyacceptedas the"'paradigm"of aromaticity.4lr PnK:tic:al problemsremain,however. Even forbenzene, the recently computedabinitio resoaance energi.esrange from 23.4 (6·310·)49 to 36 (MP3/6-310·)so kca1Imol,valueswhichdiffer by about 50%. Nor is it always clear whether an adiabatic or a vertical resonancecoergyis beingdescribedbysuch calculations. adding tothe confusion. Systems such as heterocycles and PAHs are more complex, and pose a greater cballenge for the accurate calculation of resonance energies. With the constant improvement ofab initio tccbniques, a reliable, consistent method for the accurate determination of resonance energiesmightbedevelopedin thefUture. Untilthen,the
nJEOIIChdc.R.Anpfo.a....JIlLEd.Uwl.tm.26.1291.
• Kryaowsld.ToM.;ADulewicz, R.; Kru.vE:wJki,J. Aeta o,.,t.I98J. 8J9. 132-139.
... Hcu,B.AJ.; Scbud,L. J. bLC/MM.. Soc. 1983. JOJ. 7500-7SOS•
• Wiha&KB.;NItaji,D.;8reamlaD,eM.J.bL
