Synthesis and Reactivity of o-Benzylphosphino- and o-α-Methylbenzyl(N,N-dimethyl)amine-Boranes

The series of o-benzylphosphino-boranes, o-(R2B)C6H4CH2PtBu2 (R = Cl 3, Ph 4, Cy 6, C6F5 7, Mes 8) and o-(BBN)C6H4CH2PtBu2 (5), were synthesized from reactions of the respective chloroboranes with the lithiated benzylphosphine. In an analogous fashion, the alpha-methylbenzyl(N,N-dimethyl)amine-boranes o-(R2B)C6H4CH(Me)NMe2 (R = Cl 10, Ph 11, Cy 12, C6F5 13, Mes 14) were prepared. While these species were inactive in the catalytic hydrogenation of tBuN=CHPh, compounds 7, 8, and 14 did react with H-2 at elevated temperatures (100 degrees C), resulting in the elimination of HC6F5 and mesitylene, respectively. In the latter case, the species o-((Mes)HB)C6H4CH(Me)NMe2 15 was isolated. 14 was also shown to react with H2O to give the species o-((Mes)(HO)B)C6H4CH(Me)NMe2 16 with the loss of mesitylene. The structure of these compounds and the nature of these reactions were also probed spectroscopically, crystallographically, and computationally. The energies for the products of hydrogenation, the phosphonium and ammonium hydridoborates, were computed. In all cases, these products were endothermic with respect to the precursor phosphine-boranes and amine-boranes and H-2. The barriers to H-2 activation were found to be in the range of 24-38 kcal/mol. These theoretical studies also demonstrate that the steric bulk around the boron center dramatically affects the activation barrier for H-2 activation, while the Lewis acidity of the borane has the largest effect on the stabilization of the resulting onium-borohydride. In the case of the elimination reactions, the driving forces appear to be the loss of arene byproduct and formation of a strong donor-acceptor bond.