Preparation of Axially Chiral N,N′‐Diarylimidazolium and N‐Arylthiazolium Salts and Evaluation of Their Catalytic Potential in the Benzoin and in the Intramolecular Stetter Reactions

N ‐Aryl‐substituted imidazoles 14b , 15b and thiazoles 17 − 22 , 25 were prepared which contain a stereogenic axis and which can occur as atropisomers. The imidazolium salts 15b could be obtained from 2‐isopropylaniline ( 12b ) and diacetyl ( 11 ) in three steps (19% yield) whereas the synthesis of their tert ‐butyl analogues failed. The meso ‐isomer meso ‐ 15b prevailed ( dr = 90/10). The chiral thiazolium salts rac ‐ 17 (53% yield) and rac ‐ 22 (49% yield) were prepared in two steps from 2‐ tert ‐butylaniline ( 12a ). The enantiomerically pure thiazolium salt 19 was obtained from α‐bromomenthone ( 23 ) and the aniline 12a (27% overall yield). In contrast to the imidazolium salts 15b , the thiazolium salts proved to be suitable catalysts in the benzoin condensation of benzaldehyde ( 1 → 2 ) and in the intramolecular Stetter reaction of the α,β‐unsaturated ester 9a . The best results obtained with catalyst 19 (20 mol %) were 85% yield of product 2 (40% ee ) and 75% yield of product 10a (50% ee ). The stereogenic axis of catalyst 19 is not configurationally stable in the catalytically active carbene intermediate 28 . The catalyst is recovered as a mixture of diastereomeric atropisomers 19 and 26 in a ratio of 70:30 to 75:25. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)