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Cooperative effect of carborane and pyridine in the reaction of carboranyl alcohols with SOCl2 :
halogenation versus oxidation
Vincent Terrasson, Jose Giner Planas, Damien Prim, Clara Vinas, Francesc Teixidor, Mark E Light, Michael B Hursthouse
To cite this version:
Vincent Terrasson, Jose Giner Planas, Damien Prim, Clara Vinas, Francesc Teixidor, et al.. Coopera-
tive effect of carborane and pyridine in the reaction of carboranyl alcohols with SOCl2 : halogenation
versus oxidation. IMEBoron, 2008, Platja d’Aro, Spain. �hal-02913342�
Cooperative effect of carborane and pyridine in the reaction of carboranyl alcohols with SOCl 2 : halogenation versus oxidation.
aInstitut de Ciència de Materials de Barcelona (CSIC), Campus de la U.A.B, 08193 Bellaterra, Spain.
bInstitut Lavoisier UMR CNRS 8180, Université de Versailles-Saint Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles, France.
cSchool of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
References
[1]R. Bohlmann In Comprehensive Organic Synthesis; B.M. Trost, I. Fleming, Eds.; Pergamon: Oxford, 1991; Vol. 6, p 203.
[2]a) R.A. Moss, X. Fu, R.R. Sauers, J. Phys. Org. Chem. 2007, 20, 1. b) S.P. Marsden, Contemp. Org.
Synth. 1997, 4, 118.
[3]a) J.G. Planas, F. Teixidor, C. Viñas, M.E. Light, M.B. Hursthouse, Chem. Eur. J. 2007, 13, 2493. b) J.G. Planas, C. Viñas, F. Teixidor, M.E. Light, M.B. Hursthouse, CrystEngComm 2007, 9, 888. c) V.
Terrasson, S. Marque, M. Georgy, J.-M. Campagne, D. Prim, Adv. Synth. Catal. 2006, 348, 2063. d) V.
Terrasson, D. Prim, J. Marrot, Eur. J. Inorg. Chem. 2008, 2739.
V. Terrasson,a,b J. G. Planas,aD. Prim,bC. Viñas,aF. Teixidor,a M.E. Light,cM.B. Hursthouse.c
Organic chlorides are key compounds in organic synthesis since they serve as intermediates in a wide variety of reactions.[1]These molecules are commonly obtained by the reaction of an alcohol with thionyl chloride, one of the most employed halogenating reagents.[2]With the aim to synthesize new o-carborane derivatives for material science and catalytic applications,[3]we have prepared new carboranyl alcohols and carried out their reactions with thionyl chloride. If alcohols bearing a phenyl substituent gave the expected chlorinated compounds in good yields, those substituted with a pyridine ring surprisingly afforded the ketones as the only products under the same reaction conditions.[4]
Following an adaptation to known procedures,[5]we have prepared the series of four new carboranyl methylalcohols bearing a phenyl 1a-b or a pyridine substituent 2a-b (Scheme 1).
We thank CICYT (Project MAT2006-05339), Generalitat de Catalunya (2005/SGR/00709), Spanish Government (RyC to J.G.P.), MENRT-France (grant to V.T.), CNRS, Université de Versailles and UK EPSRC for financial support.
Institut Lavoisier de Versailles
UMR 8180
Molecular structures for 1a and 2a have been determined by X-ray structure analysis (Figure 1).
Figure 1
X-ray structure of 2a X-ray structure of 1a
Reactions of the phenylmethylalcohol derivatives 1 a-b with an excess of SOCl2 under reflux conditions afforded the expected chlorides 3 a-b in excellent yields (Scheme 2).
Scheme 3
R OH
SOCl2 CH2Cl2 reflux, 16 h
R O
N N
4a, R = Me, 88 % 4b, R = Ph, 91 % 2 a-b
Surprisingly, the pyridylmethyl alcohols 2 a-b did not afford the expected chlorides under the same conditions but gave the ketones 4 a-b (Scheme 3).
R OH
SOCl2 CH2Cl2 reflux, 16 h
R Cl
3a, R = Me, 97 % 3b, R = Ph, 90 % 1 a-b
Scheme 2
Chlorides 3 a-b and ketones 4 a-b have been unambiguously characterized by spectroscopic methods. Molecular structures for 3a and 4a have been determined by X-ray structure
analysis (Figure 2). X-ray structure of 3a X-ray structure of 4a Figure 2
Oxidation of the alcohols 2 a-b to the ketones 4 a-b is clearly affected by both the pyridine and carborane moities in the same molecule since either 1 a-b or the organic counterpart (2-pyridyl)phenylmethanol 5 exclusively afford the chloride products (Schemes 2 and 4).
Scheme 4
N OH
SOCl2
CH2Cl2 reflux, 16 h
N Cl
5 99 %
Halogenation versus oxidation in these compounds can be interpreted as a competition between nucleophilic addition of Cl-versus proton abstraction at the benzylic position (Schemes 5 and 6). NMR data of the intermediate II clearly show that the benzylic proton is more acidic than that for the starting alcohol 2a (δ= 7.29 ppm compared to 5.11 ppm). This is probably due to the positive charge at the pyridine ring in II. This charge effect (making the benzylic proton more acidic) combined with the bulkiness of the carboranyl fragment hindering the nucleophilic attack of Cl-could explain this unusual oxidation reaction.
[4]V. Terrasson, J.G. Planas, D. Prim, C. Viñas, F. Teixidor, M.E. Light, M.B. Husrthouse, J. Org.
Chem. 2008, accepted.
[5]H. Nakamura, K. Aoyagi, Y. Yamamoto, J. Org. Chem. 1997, 62, 780.
When the reaction of 2a with SOCl2 was monitored by NMR, the complete conversion of the alcohol to the intermediate II was observed within 5 min at room temperature.
Further transformation according to Scheme 6 was only obtained by heating the reaction medium.
Scheme 6 Scheme 1
R Ar OH
R Li
+
THF -78 C, 1h Ar-CHO
1a, R = Me, Ar = Ph, 95 % 1b, R = Ph, Ar = Ph, 90 %
2a, R = Me, Ar = 2-Pyridyl, 87 % 2b, R = Ph, Ar = 2-Pyridyl, 81 %
These alcohols were obtained in good to excellent yields (Table 1) and fully characterized by spectroscopic methods.
Whereas no intramolecular contacts are found in 1a, there is a clear intramolecular O-H…N hydrogen bond in 2a.
The usual reaction pathway of the halogenation using SOCl2 is the formation of the intermediate I. The latter affords the corresponding chloride by a nucleophilic addition of Cl- (Scheme 5).
Scheme 5
1a-b C2B10H10 3a-b
O ClOS
H
I
∆
∆
∆
∆
Cl Nucleophilic Attack SOCl2
SOCl2
2a-b C2B10H10 4a-b
O HN ClOS
H
II
rt fast
∆
∆∆
∆
Base Proton Abstraction
