Reversible Stereodivergent Cycloaddition of Racemic Helicenes to [60]Fullerene: A Chiral Resolution Strategy

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Reversible Stereodivergent Cycloaddition of Racemic Helicenes to [60]Fullerene: A Chiral Resolution Strategy

Rosa M. Giron, Jiangkun Ouyang, Ludovic Favereau, Nicolas Vanthuyne, Jérôme Crassous, Salvatore Filippone, Nazario Martin

To cite this version:

Rosa M. Giron, Jiangkun Ouyang, Ludovic Favereau, Nicolas Vanthuyne, Jérôme Crassous, et al.. Reversible Stereodivergent Cycloaddition of Racemic Helicenes to [60]Fullerene: A Chiral Resolution Strategy. Organic Letters, American Chemical Society, 2018, 20 (7), pp.1764-1767.

�10.1021/acs.orglett.8b00256�. �hal-01767767�

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Reversi [60]fulle

Rosa M. Gi tore Filippo

a Departamen Spain.

b Institut des S

cAix Marseill

d)IMDEA–Nan

ABSTRACT:

for the chiral r of racemic hel conventional c rials in high op

Asymmetric chromatograph tion methods, ly active comp of these strateg order to face the constantly In this rega mixtures (RRM enantiopure co which both k failed.¹ In this cally interact w vent) giving eventually be al isomers (re (chemodiverge for both enant tion, nor stoic divergent RRM products.

On the othe interest in new particular, car

ible stere erene: a

irón,

^a

Jianku one,

^a

* Nazar

to de Química Sciences Chim

e University, C nociencia, C/ F

[60]Fullerene resolution of ra

licenes onto [6 chromatograph ptical purity.

c (chemical o hic separation,

are the main m pounds. Far fro gies are still de new challengi high demand o ard, the use o

M) constitutes ompounds to t kinetic resolu strategy, both with a chiral s rise to non-e diastereoisome giodivergent) ent). Although tiomers are ne chiometric am M require high er hand, chirali w fields such rbon-based na

eodiverg a chiral r

un Ouyang,

^b

rio Martín.

^a

a Orgánica, U miques de Renn CNRS, Centra Faraday, 9, C

e and its scarce acemates. By u 60]fullerene w hy and, afterwa

or enzymatic) , kinetic resolu methodologies t om being out o eveloping new ng chemical p of enantiopure f divergent re s a suitable a the large numb ution and asy

enantiomers o substrate (reag nantiomeric p ers (stereodiver

or different ch h, neither diffe ecessary, as in mount of chiral h selectivity a ity has experie as, for instanc anostructures s

gent cyc resolutio

b

Ludovic F

a,d

*

Universidad Co nes UMR 6226 ale Marseille, Campus de Can

ely-explored re using only cata were carried ou ards, a simple c

) synthesis, ution or crysta

to access to op of interest, eac w alternative to problems as w chiral compou eactions on rac lternative to a mber of process ymmetric synt of a racemate ch gent, catalyst o

products that rgent), constitu hemical compo erent reaction n the kinetic re l reagents, eff and easily sepa

enced an incre ce, nanoscienc such as helice

cloadditi on strat

Favereau,

^b

N

omplutense de 6, Campus Bea

iSm2, Marseil ntoblanco, E-2

eversible coval alytic amounts ut. The formed catalyzed 1,3-d

chiral alliza- ptical- h one ols in well as unds.

cemic afford ses in

thesis hemi- or sol- could ution- ounds

rates esolu- ficient

arable easing e.² In enes,³

ful new or me fun tion com

I exp tion che cha [60 hav use chi Ho bee mo ods (as usi han

ion of ra egy

Nicolas Vant

e Madrid, Avda aulieu, 35042 lle, France 28049 Madrid

ent chemistry of chiral agen d helicene/fulle dipolar retro-cy

llerenes,⁴ or c w optoelectron chiroptical pro ent. Actually, f nctional group

n is the only mpounds.⁶ In the search f pensive and tim

ns, we focused emistry and, i aracter. Actu 0]fullerene by ve been reporte ed, followed by iral fullerenes owever, to the en previously ore, from one h

s for the ena symmetric cata

ing only mino nd, the availab

acemic h

tuyne,

^c

Jean

a. Complutens Rennes Cedex d, Spain.

has been harne nts, stereodiver erene diastereo ycloaddition af

curved polyaro nic properties s operties, stemm for these type s, the enantiom available path for alternative me consuming

d our attention in particular, o ually, stereos using chiral re ed.⁷ In this rega y retro-function such as, for best of our kn

used for the hand, the intro antioselective

alysis) allows r amounts of bility of new sy

helicene

nne Crassou

se s/n, E-2804 x, France.

essed as an eff rgent 1,3-dipol omers were eas

fforded helicen

omatic hydroc such as charge ming from thei s of compoun meric chromat hway to obtain racemate reso chiral chromat n on the [60]fu on its less-exp selective cyc eagents (asymm ard, RRM have nalization reac

instance, D₂-C nowledge, fulle

racemates res oduction of new

functionalizati their applicat chiral material ynthetic tools t

es to

s,

^b

* Salva-

40 Madrid,

ficient alternati lar cycloadditio sily separated nes starting ma

carbons,⁵ featu e-carrier mobil

ir chiral arrang ds, often lacki tographic sepa n optically acti olutions avoidi tographic sepa fullerene covale

ploited reversib cloadditions

metric inductio e previously be ctions, to separ

C₇₆ and D₂-C erenes have nev solution. Furth w catalytic me on of fulleren tion in RRM ls.⁹ On the oth o carry out ret

ive ons by ate-

ure lity ge- ing ara-

ive ing ara-

ent ble on on) een rate

84.⁸ ver her- eth- nes

by her tro-

(3)

cycloaddition processes¹⁰ would enable the recovery of the former racemic starting materials in an enantioenriched form.

Thus, in order to probe the usefulness of this approach, we decided to carry out the enantiodivergent resolution of racemic helicenes, followed by a subsequent retro-cycloaddition reaction, affording the respective isolated enantiomers in an effi- cient manner, alternative to the use of highly expensive and time consuming chiral HPLC.

Helicenes are non-planar polycyclic aromatic compounds whose helical backbone is formed of ortho-fused aromatic rings. Their extended π-conjugation combined with their helical topology provide them with high chiroptical properties (huge optical rotations, strong circular dichroic responses and substantial circularly polarized luminescence) and good elec- tronic properties (conductivity and redox properties).^{3, 11} For these reasons, they are of great interest for many optoelectron- ic applications such as chiral organic light-emitting diodes (OLEDs),¹² transistors,¹³ photovoltaic devices,¹⁴ chiroptical switches,¹⁵ or non-linear optical (NLO) materials.¹⁶ Further- more, due to their chemical structure based only on aromatic sp² carbon atoms and without functional groups, we consid- ered helicenes a useful racemic benchmark to probe the efficiency of stereodivergent reversible reactions by means of [60]fullerene. In particular, we used 2-formylhexahelicene racemate 1 as starting material to carry out the synthesis of the respective helicene-iminoester racemate 2.

In a first step, racemic 2-hexalicene-iminoesters underwent an enantioselective azomethine ylide 1,3-dipolar cycloaddition reaction to [60]fullerene. Among the available catalytic sys- tems, we chose the pair Cu(II) acetate/(R_p)-Fesulphos which revealed its efficiency in previous studies directed to the for- mation of the (2S,5S) pyrrolidino[3,4:1,2][60]fullerene with ee values up to 92%.^9aThanks to the high enantiocontrol on the two new formed stereocenters, two diastereomeric helicene- pyrrolidino[3,4:1,2][60]fullerenes were formed in good enantiomeric excesses, which were easily separated by conventional silica-gel chromatography. The chiroptical properties of these derivatives, resulting from the curved Csp² helicene and [60]fullerene moieties, were studied. Finally, the use of a metal-catalyzed retro-cycloaddition reaction under mild conditions^{10a, 10e} afforded both starting 1,3-dipoles which, after in- situ hydrolysis, yielded both 2-formylhexahelicene enantiomers which were obtained in good yield and optical purity (Scheme 1).

Scheme 1: Resolution strategy carried out for 2- formylhexahelicene racemate 1.

HN CO2tBu

4 R +

1: R = CHO

2: R = CH=N-CH2-CO2tBu C60

1,3-dipolar cycloaddition

H N CO2tBu

Diastereomeric mixture

Retro 1,3-dipolar cycloaddition

Conventional Chromatografic column 3

Racemate

OHC CHO

P-1 M-1

Isolated enantiomers tBuO2C NH2·HCl

Retro 1,3-dipolar cycloaddition

3 4

C60

s s s s

P-Helicene

M-Helicene

It is important to note that the use of only 10% of chiral catalyst (Cu(II) acetate/(Rp)-Fesulphos) directed efficiently the cycloaddition of the racemic helicenes 2 onto [60]fullerene affording two diastereoisomers, 3 and 4, as result of the high enantiocontrol in the new formed asymmetric carbons of pyr- rolidine (the two diastereoisomers differ in the helicenes moiety configuration, while both C-2 and C-5 feature the same configuration, almost completely a S,S configuration, see below). Remarkably, diastereoisomers 3 and 4, obtained in a 1:1 ratio and in 80% yield (with respect to helicenes 2) (see Supp. Information, Figure S3), could be easily separated by conventional silica-gel chromatography, thus fulfilling the key requirement for the separation of racemates by stereodivergent RRM.

Each separated diastereoisomer, analysed by chiral HPLC and circular dichroism (CD), showed a very high optical purity: diastereoisomer 4 was obtained in a high enantioenriched form, being ee > 99% between (M,S,S)-4 and (P,R,R)-4, (see Supp. Information, Figure S4) while for diastereoisomer 3 the enantiomeric ratio, (P,S,S)-3 to (M,R,R)-3, was 99/1 (see Supp. Information, Figure S5).

In order to confirm the stereochemical outcome in the racemic series, we carried out the same synthetic study on each of both pure enantiomers (P and M) of helicene 1.¹⁷ This study should allow: i) a more easy characterization of the products, and ii) the assessment of the catalytic system to maintain the sign of the asymmetric induction in the presence of another chiral element.

Thus, since 3 is the main product when optically pure P-2 underwent the cycloaddition reaction onto [60]fullerene in the presence of 10% of Cu(II) acetate/(R_p)-Fesulphos, it can be deduced the configuration (P,S,S) for this diastereoisomer.

Diastereoisomer 4, formed in a minor amount as result of the unfavorable (R,R) asymmetric induction sense of the catalytic system, features a (P,R,R) configuration. On the contrary and consistently, (M,S,S)-4 is the main product and (M,R,R)-3 is the minor one when M-2 is used.

Therefore, the catalytic system proved to maintain a high level of stereoselectivity even in the presence of a helicoidal chiral element. It was also observed a slight different behavior with each helicene enantiomers. Indeed, the diastereomeric ratio between (P,S,S)-3 and (P,R,R)-4 (92/8), observed in the

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M

reaction with S1) was highe Information, F ing of chiralit tate/(R_p)-FeSul (Scheme 2).

Scheme 2: Cy M-2 to C₆₀.

tBuO₂C N

N

M-2

The CD spe characteristic p duct and they Thus, at 330 n peak in the ci positive or neg or M, respectiv of the [60]full show a lower the absolute st Once proved carry out the s such functiona covalent chem out the racema interest on t dino[3,4:1,2][6 Prato reaction) –in a quantitat hyde starting m the experimen chiral aldehyde

Figure 1. CD s corresponding t

iminoester P-2 er than (M,S,S Figure S2). Thi ty P and chir lphos and a m ycloaddition re

1) (R_p)-FeS Cu(OA dry Toluen

2) C60(2 e 92%

CO₂tBu

1) (Rp)-FeSul Cu(OAc dry Toluene

2) C60(2 equ 83%

P-2

ectra of the fo peaks of helice y also confirm nm the helicen ircular dichrois gative sign cor vely, while the erene monoadd intensity (Figu ereochemistry d the efficienc stereodivergent alization in ord mistry of fullere

ate resolution. I the retro-cycl 60]fullerenes d ).^{10a, 10e} Howev tive manner– t materials in ref ntal conditions

es 1.

spectra (2 x 10 to [60]fullerene

2, (see Supp. I S)-4/(M,R,R)-3

is is a consequ rality of the c mismatching w eaction of imin

SulPhos, Ac)2 ne, Ar, 1h

equiv.), rt

%

H N

(P,S,S

lPhos, )2 , Ar, 1h uiv.), rt

(M,R,R)-3

our stereoisome ene and of [60 m the aforeme ne moiety give sm spectrum ( rresponding to e peak at 427 n ducts chirality ure 1, inset), an

of each stereoi cy of the chiral

t RRM, we ex der to assess the

enes as a usefu In particular, w loaddition rea developed by ver, despite this to pristine [60]

fluxing toluene in order to av

0⁻⁴ M in CH2Cl e chirality.

Information, F (83/17) (see uence of the m catalyst Cu(II) with the chirali noesters P-2 a

CO₂tBu

S)-3

+ (P,R,R

HN C

+

(M,S,S)-4

ers feature bot 0]fullerene mon entioned asign es rise to an in (Figure 1), wit o the enantiom nm is the finge , which appear nd is used to a isomer. ¹⁸ l catalytic syste xtended the sco

e singular reve ful platform to we have focuse action of py some of us ( s reaction give ]fullerene and e, we had to ch void racemizati

l2, 25 °C). Inset Figure

Supp.

match- ) ace- ity M and

R)-4

CO2tBu

4

th the noad- nment.

ntense th the mers P erprint red to assign em to ope of ersible carry ed our yrroli- (retro- es rise

alde- hange ion of

t peak

W TH tion exc 78%

[99 pur retr M (se Sch

I les as spe of wit ver cen chi pyr exc dia cyc hyd in rep ful eff

AU Co

* E

AC NM Co mía 520

We eventually HF (70^oC) the n reaction in g cess. Thus, hel

% yield and ee 9% ee, (P,S,S) rity (see Supp ro-cycloadditio with 82% yiel ee Supp. Inform

heme 3. Enan

H N

(P,S,S)- 99% ee

(M,S,S 97% e H N

In summary, w ss-explored “re

a useful altern ecies. In the re the racemate o th helicenes is rgent 1,3-dipo nes onto [60]fu iral agents–

rrolidino[3,4:1 cesses. Subseq astereoisomers cloaddition rea drolyses lead good yields an presents a gen llerenes for rac ficiency in the r

UTHOR INF orresponding A Email: salvatore Email:jeanne.cr Email: nazmar@

CKNOWLED M acknowledge ouncil (ERC-32 a y Competitiv 045-R) and

found in the p suitable condi good yields an licene starting e = 99 % from )-3], thus main p. Information,

on of 4 [92% e ld although the mation, Figure

tiospecific ret

Cu(C

70 CO₂tBu

-3 e

S)-4 ee

CO₂tBu

Cu(C

70

we report for t eversible” cova native to carry epresentative s of a pyrrolidino described. It i olar cycloaddit ullerene –emplo

affording tw ,2][60]fulleren quent conventio

separation action yields t

to the starting nd excellent opt neral and effi cemates resolut

resolution of th

FORMATIO Author e.filippone@ucm

rassous@univ-r

@ucm.es

DGMENT es financial sup

0441-Chirallca vidad (MINECO

the Comuni

pair Cu(ClO4)2/ itions to revers

d maintaining material P-1 w the separated d ntaining uncha , Figure S8). A ee, (M,S,S)] aff e ee was slight S9) (Scheme 3 rocycloadditio

ClO4)2/ dppe

THF 0^oC / 3 days

N CH2O2tBu H 2

N CO₂tBu H H

H2

ClO4)2/ dppe

THF 0^oC / 3 days

the first time o alent chemistry y out the resol studied exampl o[3,4:1,2][60]fu involves the rev tion reaction

oying only cata wo diastereom nes in excelle

onal silica-gel and metal-c the starting 1, g 2-formylhelic tical purity. Th cient method tion and, in pa hese singular h

N

m.es;

rennes1.fr

pport by the Eu arbon), the Min

O) of Spain (p idad Autónom

/dppe in refluxi se the cycload the enantiome was obtained in diastereoisome anged the opti Analogously, t fords the helice ly lower, 80%

3).

on reaction.

O

P-1 99% ee

O

M-1 80% ee H

2O

H2O

on the use of t y of [60]fullere lution of racem

le, the resoluti ullerene endow versible stereo of racemic he alytic amounts meric helicen ent enantiome

chromatograp catalyzed ret ,3-dipoles who cene enantiom he above protoc

which valida articular, for th helicene system

uropean Resear nisterio de Econ project CTQ20 ma de Mad

ing ddi- eric

n a er 3 cal the ene ee

the ene mic ion wed odi-

eli- s of

ne- eric phy tro- ose mers col ates heir ms.

rch no- 14- drid

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(PHOTOCARBON project S2013/MIT-2841). R.M.G. thanks the Spanish MECD for a FPU grant (FPU13/03677). JO thanks the Chinese Scholarship Council for financial support. LF and JC thank the CNRS and the University of Rennes 1.

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