Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors
- 12 October 2011
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Organic & Biomolecular Chemistry
- Vol. 10 (4), 852-860
- https://doi.org/10.1039/c1ob06381b
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 HN 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].This publication has 50 references indexed in Scilit:
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