Practical Synthesis of 1,12-Difunctionalized o-Carborane for the Investigation of Polar Liquid Crystals

Two isomerically pure 1,12-difunctionalized derivatives of o-carborane, 12-iodo-1-vinyl (1b) and 12-iodo-1-phenyl (1c), are conveniently obtained on a practical scale in yields of 22% and 32%, respectively, by monoiodination of the corresponding o-carborane derivatives (4b and 4c) followed by separation of the regioisomers by crystallization (1b) and chromatography (1c). Subsequent functional group transformations gave access to other derivatives, including two liquid-crystalline compounds, in which o-carborane is a linear structural element. Regioselectivity of substitution on the carborane cage and on the benzene ring correlates with the inductive effect parameter of the substituent. The preparation of analogous derivatives of m-carborane was also investigated.