Nickel- and Palladium-Catalyzed Cross-Coupling Reactions at the Bridgehead of Bicyclo1.1.1pentane Derivatives - A Convenient Access to Liquid Crystalline Compounds Containing Bicyclo1.1.1pentane Moieties

Radical addition reactions of organyl iodides 7a-s onto 1.1.1propellane (2) followed by halogen-lithium exchange and transmetallation with zinc chloride, as well as additions of Grignard reagents to 2, have furnished a variety of 3-substituted bicyclo1.1.1pentyl-1-magnesium (14) and -zinc (19) derivatives. The latter have been coupled with various alkenyl, aryl, and biaryl halides and triflates under NiCl2dppe, Pd(PPh3)4, or PdCl2(dppf) catalysis to give a number of 1,3-disubstituted bicyclo1.1.1pentyl derivatives 17, 20, and 23, several of which exhibit liquid crystalline properties, in moderate to very good yields. The coupling products 20ca, 23ab, 23ae, 23ff, and 23fg have been further transformed to yield bicyclo1.1.1pentyl derivatives 32, 24ab, 24ae, 27ff, and 27fg, respectively, bearing alkynyl, cyano, and/or alkenyl groups.