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

摘要

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.