Kinetic hydrogen isotope effects in intermolecular hydride transfer from arylalkanes to 9-arylfluoren-9-yl cations

Abstract

Rates of hydride-ion transfer to a series of 9-phenylfluoren-9-yl cations, substituted in the phenyl group, from triphenylmethane and 4,4′-dimethoxydiphenylmethane have been measured at 30°C in trifluoroacetic acid solution containing 6% v/v acetic acid. Deuterium kinetic isotope effects have been measured directly and show evidence of a broad maximum, the peak value occurring when the difference in pK_R⁺ values between the product and reactant carbenium ions is around zero. Application of Marcus theory suggests that the intrinsic barrier to hydride transfer in these systems is substantially larger than for hydride transfer from formate ion to a series of triphenylmethyl cations. A limited series of measurements has also been made of tritium kinetic isotope effects on hydride transfer from 4,4′-dimethoxydiphenylmethane to 9-arylfluoren-9-yl cations and these have been used to derive deuterium–tritium isotope effects. The relevance of the results to the question of the origin of isotope-effect maxima is briefly discussed.