Manifestations of Bridgehead-Bridgehead Interactions in the Bicyclo[1.1.1]pentane Ring System.

A series of 3-halo-substituted bicyclo[1.1.1]pentane-1-carboxylic acids 1 (Y = COOH; X = F, Cl, Br, I, and CF(3)) as well as the parent compound 1 (Y = COOH, X = H) have been prepared, and a study of some of their properties have been made. It was found that their reactions with xenon difluoride cover a wide range of reactivities. On one hand, the fluoro acid 1 (Y = COOH, X = F) displayed no apparent reaction at all while, on the other, the bromo acid 1 (Y = COOH, X = Br) and parent compound 1 (Y = COOH, X = H) underwent ready reaction with complete disintegration of the ring system. A possible explanation is advanced based on polar kinetic and thermodynamic effects governing the lifetime of an intermediate acyloxy radical species. The relative ease of oxidation of the carboxylates 1 (Y = COO(-); X = H, F, Cl, Br, I, CF(3), and COOCH(3)), as mirrored by their peak oxidation potential values (E(p)) determined by cyclic voltammetry, also covers a wide range. These data coupled with the dissociation constants (pK(a)) of some of the acids 1 (Y = COOH; X = H, F, Cl, and CF(3)) reflect significantly on the modes of transmission of electronic effects acting through the bicyclo[1.1.1]pentane ring system.

1-Aryl-F-1,3-butadienes and Unsymmetrical alpha,omega-Diaryl-F-polyenes.

Para-substituted 1-aryl-F-1,3-butadiene derivatives p-R-C(6)H(4)-CF=CFCF=CF(2) (2a-g), where R = H, CH(3), OCH(3), OC(6)H(11), N(CH(3))(2), Br, and CF(3), have been prepared by reaction of appropriate arylmagnesium bromides with F-1,3-butadiene. (19)F NMR chemical shifts correlate with sigma(p) Hammett substituent constants. Compounds 2a-e react on their terminal perfluorovinyl group with C-nucleophiles and were used as intermediates in subsequent reactions with suitable organolithium compounds for the synthesis of a new series of unsymmetrically substituted alpha,omega-diaryl-F-polyenes RC(6)H(4)(CF=CF)(n)()C(6)H(4)R', where n = 2 or 3 and R' is one of the substituents specified above. A variety of new synthons containing alpha,omega-diaryl-F-1,3-butadiene skeleton and reactive Br, OH, CH=O, and COOH functionalities in para-positions on aromatic rings has been also prepared. Structural aspects, NMR spectra, and mesogenic properties of the title compounds are discussed.