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

Metal-free transfer hydrogenation of polarized olefins (RR'C=CEE': 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 = Bu-t, 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 [NH2=BH2] 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 [NH2=BH2] or its solvent stabilized derivative [NH2=BH2]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH2NH2)(3) which is the trimerization product of [NH2=BH2].