Proton transfer reactions in dihydrogen bonded systems.

摘要

As possible substrates for one-photon infrared-pumped reaction (IRPR), the structures and decomposition paths of complexes between AlH₄ − and three proton donors, H₂O, HF, and HCl, have been studied by ab initio methods at the MP2//6-311++G** level of theory. Dihydrogen bonded complexes, [AlH₄…HA]− (A=OH, F, Cl), were obtained as minima on the potential energy surface (PES). In each case only one transition state was found for the proton transfer and H₂ loss process. For each cluster, the geometry and energy characteristics of reactants, complex, transition state, and products were analyzed with [AlH ₄…HCl]− emerging as the best IRPR candidate. The calculated intrinsic reaction coordinate (IRC) confirmed the one-step proton transfer and H₂ loss with no intermediate. Classical trajectories for the vibrationally activated process were calculated on the ab initio potential energy surface, beginning with varying degrees of excitation of the modes associated with proton transfer, 1411 and 1973 cm−1. Proton transfer from HCl and loss of H₂ were calculated to occur on the femtosecond time scale for several of the initial conditions, including the first vibrationally excited level.; The influence of the dihydrogen bonding on the activation parameters of ketone reduction by tetrabutyl ammonium borohydride, NBu₄BH ₄, has been studied. 2-Hydroxycyclopentanone was chosen as substrate because it was the most thoroughly studied in the previous works in this group. The reaction kinetics were found to be first order in ketone and in borohydride with a rate constant of k = 2.20 ± 0.160 × 10−3 l·mol−1·s−1 at room temperature. Formation of a dihydrogen bonding complex between the hydroxyl substituent and the BH₄− lowered the activation enthalpy by 6.6 kcal/. Comparison of the activation entropy for the 2-hydroxycyclopentanone and cyclopentanone reductions, −29.6 and −22.9 e.u. respectively, pointed to a higher organization degree in the transition state when the substrate bears an α-hydroxyl group. When a hydrogen bonding solvent was used the activation parameters were lowered even more as expected, due to increase number of proton donor partners. Direct analysis of the products ratio showed a smaller stereochemical control than reported previously. The influences of different factors on the stereochemical outcome are discussed.