Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Abstract

Metal-free transfer hydrogenation of polarized olefins (RR′CCEE′: R, R′ = H or organyl, E, E′ = CN or CO₂Me) using amine borane adducts RR′NH–BH₃ (R = R′ = H, AB; R = Me, R′ = H, MAB; R = ^tBu, R′ = H, tBAB; R = R′ = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H_N transfer is in agreement with a concerted transition state. The very reactive intermediate [NH₂BH₂] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy₂BNH₂. The final product borazine (BHNH)₃ is assumed to be formed by dehydrocoupling of [NH₂BH₂] or its solvent stabilized derivative [NH₂BH₂]–(solvent), rather than by dehydrogenation of cyclotriborazane (BH₂NH₂)₃ which is the trimerization product of [NH₂BH₂].