Ligand-Controlled Reactivity, Selectivity, and Mechanism of Cationic Ruthenium-Catalyzed Hydrosilylations of Alkynes, Ketones, and Nitriles: A Theoretical Study

Density functional theory calculations with the M06 functional have been performed on the reactivity, selectivity, and mechanism of hydrosilylations of alkynes, ketones, and nitriles catalyzed by cationic ruthenium complexes [CpRu­(L)­(MeCN)₂]⁺, with L = P^<i>i</i>Pr₃ or MeCN. The hydrosilylation of alkynes with L = P^<i>i</i>Pr₃ involves an initial silyl migration mechanism to generate the anti-Markovnikov product, in contrast to the Markovnikov product obtained with L = MeCN. The bulky phosphine ligand directs the silyl group to migrate to C_β of the alkyne. This explains the anti-Markovnikov selectivity of the catalyst with L = P^<i>i</i>Pr₃. By contrast, the silane additions to either ketone or nitrile proceed through an ionic S_N2-Si outer-sphere mechanism, in which the substrate attacks the Si center. The P^<i>i</i>Pr₃ ligand facilitates the activation of the Si–H bond to furnish a η²-silane complex, whereas a η¹-silane complex is formed for the MeCN ligand. This property of the phosphine ligand enables the catalytic hydrosilylation of ketones and nitriles in addition to that of alkynes