Design und Synthese neuer flüssigkristalliner Substanzen mit zentraler Bicyclo[3.1.0]hexaneinheit

Abstract

Starting from the first observations of the phenomenon of liquid crystallinity by Reinitzer in 1888 and by Lehmann in 1889, the design and preparation of molecules possessing liquid crystalline properties has been of interest to physical-organic chemists for an extended time. Among the liquid crystalline compounds, cyclopropane derivatives, especially 1,2-disubstituted cyclopropanes, provide more rigid conformations than those with simole alkyl groups that are widely used as fragments in liquid crystalline compounds.No general synthetic approach to 3,6-disubstituted bicyclo[3.1.0]hexanes has been reported, except for several low-yielding non-selective reactions. The first liquid crystalline compound with such a skeleton was an azaanalogue - a Schiff's base of 3-substituted (3-azabicyclo[3.1.0]hexyl)amine - which was prepared in 2000 applying the Kulinkovich-de Meijere reductive aminocyclopropanation to N-arylpyrroline. Indeed, this compound did demonstrate better liquid crystalline properties than its analogues with a 1,4-disubstituted cyclohexane instead of the 3-azabicyclo[3.1.0]hexane moiety.According to MOPAC/AM1 calculations such compounds have similar molecular shapes as analogues with a 1,4-disubstituted cyclohexane moiety and, because the former being slimmer, they might have superior properties. These predictions prompted us to embark on a program to prepare such compounds and test their properties.The first model compound with a bicyclo[3.1.0]hexane skeleton was prepared applying a dirhodium tetraacetate-catalyzed cyclopropanation with ethyl diazoacetate to the known tert-butyl cyclopent-3-ene-1-carboxylate as a key step.The second model compounds were synthesized by Cyclopropanation of 4-arylcyclopentenes with tert-butyl diazoazetate under dirhodium tetraoctanoate catalysis followed by saponification, reduction, iodination and cross-coupling with various alkylmagnesium bromides under Li2CuCl4 catalysis adopting a protocol of Nicolaou et al. The comparison of the physical properties discloses that a bicyclo[3.1.0]hexane moiety as a rule decreases the transition temperature. The dielectric and optical anisotropies are comparable.