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'C=CEE': R, R' = H or organyl, E, E' = CN or CO(2)Me) using amine borane adducts RR'NH-BH(3) (R = R' = H, AB; R = Me, R' = H, MAB; R = (t)Bu, 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 NH(2)=BH(2)] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy(2)BNH(2). The final product borazine (BHNH)(3) is assumed to be formed by dehydrocoupling of NH(2)=BH(2)] or its solvent stabilized derivative NH(2)=BH(2)]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH(2)NH(2))(3) which is the trimerization product of NH (2) Metal-free transfer hydrogenation of polarized olefins (RR¢C CEE¢: R, R¢ = H or organyl, E, E¢ = CN or CO 2 Me) using amine borane adducts RR¢NH-BH 3 (R = R¢ = H, AB; R = Me, R¢ = H, MAB; R = t Bu, 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.

Metal‐Free Dehydrogenation of Amine‐Boranes by Tunable N‐Heterocyclic Iminoboranes

“…The reactivity of i Pr 2 N=BH 2 with Me 2 NH ⋅ BH 3 was also investigated and an equilibrium was observed between i Pr 2 N=BH 2 , Me 2 NH ⋅ BH 3 , i Pr 2 NH ⋅ BH 3 , and Me 2 N=BH 2 (Scheme ) . Berke and coworkers also demonstrated that polarized olefins could act as hydrogen acceptors in the presence of various amine‐boranes (Scheme ) . Although the solvent‐free reaction of 2‐cyclohexylidenemalonitrile with ammonia borane H 3 N ⋅ BH 3 is rapid (at room temperature: <10 min in [D 8 ]THF, 1 h in CD 3 CN, 20 h in C 6 D 6 ), substantially longer reaction times of up to five days were noted with alkyl‐substituted amine‐boranes.…”

“…Mechanistic studies of hydrogen transfer from amine‐boranes to Berke's polarized olefin and imines, as well as to the aminoborane i Pr 2 N=BH 2 have been reported. In the case of the polarized olefin (H 2 C) 5 C=C(CN) 2 , a step‐wise mechanism was postulated where fast hydride addition was followed by slower proton transfer . In contrast, the H + /H − transfer to imines (RCH=N−Ph, R=electron‐withdrawing group) was proposed to occur through a concerted six‐membered transition state on the basis of DFT calculations .…”

Dehydrogenation of Amine–Boranes Using p‐Block Compounds

“…Interestingly, labelling studies using NH 3 ⋅BD 3 and ND 3 ⋅BH 3 revealed that the hydric B−H hydrogen is transferred to the most nucleophilic carbon of the C=C double bond through hydroboration, which is in contrast to the expected concerted double hydrogen transfer and suggests that a different mechanism is operative . Kinetic isotope effect studies and intermediate trapping revealed that the olefin hydrogenation occurs in a two‐step process, in which first the hydrogen is transferred by hydroboration, and then a rate‐determining proton transfer from the amine takes place ( 13 , Scheme ) . In addition, it was hypothesized that the generated (solvent‐stabilized) aminoborane NH 2 =BH 2 intermediate is capable of a second double hydrogen transfer to the olefin through transition state 15 , which explains the formation of borazine and polyborazylene.…”

“…[21] Kinetic isotope effect studies and intermediate trapping revealed that the olefin hydrogenation occurs in at wo-step process, in which first the hydrogen is transferred by hydroboration, and then ar ate-determining proton transfer from the amine takes place (13,S cheme 10). [36] In addition, it was hypothesized that the generated (solventstabilized) aminoborane NH 2 =BH 2 intermediate is capable of a second double hydrogent ransfer to the olefin throught ransition state 15,w hich explainst he formation of borazine and polyborazylene.…”

Designing an Effective Metal‐Free Lewis Acid Catalyst for Ammonia‐Borane Dehydrogenation: A DFT Investigation on Triarylboranes

“…Next, we probed the fate of 1 theoretically in the presence of Lewis acids B(C 6 F 5 ) 3 and B(C 6 H 5 ) 3 in nondonor solvents such as benzene. There are experimental studies in which the dehydrogenation of 1 is performed in nonpolar solvents such as benzene and toluene . In benzene, both B(C 6 F 5 ) 3 and B(C 6 H 5 ) 3 exist as free Lewis acids as benzene has no donor atom to form a Lewis acid–base adduct.…”