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 Li2CuCl4 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 (Δn) 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)