Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines ((RRPH)-R-1-P-2, R-2 = H or R-1) with hydrosilanes ((RRRSiH)-R-3-R-4-Si-5, R-4, R-5 = H or R-3) to produce synthetically useful silylphosphines ((RRP)-R-1-P-2-(SiRRR5)-R-3-R-4) has been achieved using B(C6F5)(3) as the catalyst (10 mol %, 100 degrees C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C6F5)(3) but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a (RRHP)-R-1-H-2 center dot B(C6F5)(3) 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(C6F5)(3) catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a nonmetal -catalyzed hydrosilylation of P-P bonds to produce silylphosphines ((RRP)-R-1-P-2 (SiRRR5)-R-3-R-4). 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(C6F5)(3) at 25 degrees 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-2-acceptor.