Novel Liquid Crystalline Compounds Containing Bicyclo[3.1.0]hexane Core Units

Additions of ethyl or tert ‐butyl diazoacetates to 4‐substituted cyclopentenes 6 and 17 under dirhodium tetraacetate/tetraoctanoate catalysis led to mixtures of tert ‐butyl endo , exo ‐ and exo , exo ‐3‐carboxyl(aryl)bicyclo[3.1.0]hexane‐6‐carboxylates 7 and 18 in yields of 54−90% from which exo , exo ‐diastereomers were isolated in yields of 39−63%. Diester exo , exo ‐ 7 was saponified and converted into diaryl diesters exo , exo ‐ 9a , b in overall yields of 42 and 46%, respectively. The esters exo , exo ‐ 18 were reduced to the corresponding hydroxymethyl derivatives, these were transformed to the iodomethyl compounds which in turn were coupled with various alkylmagnesium halides, via Li 2 CuCl 4 catalysis, to give 3‐aryl‐6‐alkylbicyclo[3.1.0]hexyl derivatives exo , exo ‐ 21 in overall yields of 72−83%. Fluorinated 3‐(2‐arylethyl)‐6‐pentylbicyclo[3.1.0]hexane exo , exo ‐ 32 could be prepared in five steps from 4‐ethoxy‐2,3‐difluorobenzaldehyde 26a adopting essentially the same synthetic strategy, but in an overall yield of only 8%, and 6‐(4‐cyanophenyl)‐3‐pentylbicyclo[3.1.0]hexane exo , exo ‐ 38b was obtained by Pd(OAc) 2 catalyzed cyclopropanation of 4‐pentylcyclopentene 34b with (4‐cyanophenyl)diazomethane 36b in 29% yield. A comparison of the liquid crystalline properties of these newly prepared compounds containing a bicyclo[3.1.0]hexane core with those of the known analogous compounds with a cyclohexane fragment shows that as a rule, a bicyclo[3.1.0]hexane moiety decreases the transition temperature, while the dielectric (Δϵ) and optical (Δ n ) anisotropies are comparable. However, the bicyclo[3.1.0]hexane unit has a poorer mesogenic potential. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)