PUB - Publikationen an der Universität Bielefeld

The hydride transfer and proton transfer occurring between the constituents of ion/neutral complexes of tert-butyl cations and electron-rich and electron-poor 1,3-diphenylpropanes, [(CH3)(3)C+ C6H5CH2CH2CH2C6H4X] were investigated by use of CI/MIKE spectrometry of nine 1-(tert-butylphenyl)-3-arylpropanes and 16 site-specific deuterium-labeled isotopomers. The competition between H- abstraction by the (CH3)(3)C+ ion from the neutral arene and H+ transfer to it was found to be strongly affected by the electron-donating substituents, in particular by X = OCH3, on the one hand, and X = F and CF3, on the other, suggesting that the 1,3-diphenylpropane molecule within the I/N complex acts and reacts as a bidentate solvating partner to the carbocation. The effect of the substituents X on the regioselectivity of the intra-complex hydride abstraction from the two benzylic CH2 groups, k(gamma-H)/k(alpha-H), and their influence on the kinetic isotope effect (k(H)/k(D))(gamma), operating during the abstraction from the substituted benzylic moiety, were determined in a semi-quantitative approach by assuming (k(H)/k(D))(alpha) = 1.60, the generally observed value for the unsubstituted benzylic moiety. The regioselectivity range was found to span almost three orders of magnitude, from k(gamma-H)/k(alpha-H) >= 11.2 for the complex [(CH3)(3)C+ (C6H5CH2CH2CH2C6H4)-H-alpha-H-gamma(p-OCH3)] to k(gamma-H)/k(alpha-H) <= 0.04 for the complex [(CH3)(3)C+ (C6H5CH2CH2CH2C6H4)-H-alpha-H-gamma(p-CF3)]. (C) 2006 Elsevier B.V. All rights reserved.