Reprinted from the Journal of the American Chemical Society, 114, 5005-5010 (1992).
©American Chemical Society, 1992.
William J. Leigh* and D. Scott Mitchell
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: The addition reactions of N-phenyl, N-para-biphenyl, and N,N'-para,para'-terphenyl maleimide with 7-dehydrocholesteryl acetate have been carried out in the isotropic and cholesteric liquid crystalline phases of a series of steroidal mesogens, and in the isotropic, smectic A, and smectic B phases of 4,4'-dialkylbiphenyl mesogens at temperatures between 180 and 240C. In isotropic solvents, mixtures of four adducts are obtained, in relative yields that are essentially independent of the maleimide substituent. The three major products (two ene-adducts and one Diels-Alder adduct) are formed via transition states in which the long molecular axes of the reactants are oriented perpendicular to one another, while the fourth (minor) product is an ene-adduct formed via a transition state with a parallel relative orientation of the reactants' long molecular axes. The relative yield of the latter product is enhanced when the reaction is carried out in cholesteric or smectic liquid crystalline solvents, to an extent which correlates with both the degree of order possessed by the liquid crystal and the molecular length of the N-arylmaleimide employed. For example, this adduct is the major product of reaction of N-para,para'-terphenylmaleimide with 1 in the cholesteric phase of cholesteryl para-chlorobenzoate at 200C. Studies of the temperature dependence of the product distributions afford estimates of the difference in enthalpy and entropy between parallel and perpendicular transition states in the cholesteric phase. The effect of the smectic B phase on the activation parameters appears to be smaller than that of the cholesteric phase; it is suggested that this is due to poor solubility of the reactants in the smectic phase, which leads to complex variations in reactant solubilization and reactivity as a function of temperature. The smectic A phase of this mesogen exerts much greater control on reactivity than the cholesteric phases at 240C.