Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(CF)-Mediated Formation of P-Si Bonds.

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (RRPH, R = H or R) with hydrosilanes (RRRSiH, R, R = H or R) to produce synthetically useful silylphosphines (RRP-SiRRR) has been achieved using B(CF) as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(CF) but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a RRHP·B(CF) adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(CF) catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (RRP-SiRRR). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(CF) at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H-acceptor.