Catalyst-Controlled Mono- and Double-Dehydrosilylation of Arylalkenes: Divergent Synthesis of (E)-Vinylsilanes and (E)-Divinylsilanes

A chemodivergent mono- and double-dehydrosilylation (DHS) of arylalkenes with hydrosilanes has been realized by bipyridyl-phosphinite (P O NN) and bipyridyl-phosphine (P C NN) supported cobalt complexes. A series of phosphinite-containing complexes (P O NN)CoCl 2 ( Co1 – Co5 ) with an O-linker between the P atom and the bipyridyl backbone were synthesized. Among them, the t Bu-substituted complex ( t Bu P O NN)CoCl 2 ( Co4 ), with NaBEt 3 H as the activator, exhibits high catalytic efficiency and selectivity for the mono-DHS of arylalkenes with PhMeSiH 2 to form ( E )-vinylsilane, but it is barely active for the DHS reactions with sterically more hindered secondary hydrosilanes (e.g., Ph 2 SiH 2 ) and tertiary hydrosilanes. In contrast, the phosphine variant ( t Bu P C NN)CoCl 2 ( Co6 ) with a C-linker and phosphino- t Bu substituents catalyzes selective double-DHS with PhMeSiH 2, furnishing symmetric quaternary ( E )-divinylsilanes with two vinyl groups attached to the same Si-center. Moreover, Co6 was found to be highly efficient for the mono-DHS of arylalkenes with various hindered secondary hydrosilanes (TOF up to 21,000 h –1 when using Ph 2 SiH 2 ), while the i Pr-substituted analog ( i Pr P C NN)CoCl 2 ( Co7 ) proved effective and selective for the mono-DHS with bulky tertiary hydrosilanes. Thus, a combination of Co6 -catalyzed mono-DHS with secondary hydrosilane and Co7 -catalyzed mono-DHS with the resulting tertiary vinyl hydrosilane enables the sequential double-DHS, allowing for the synthesis of unsymmetric quaternary ( E )-divinylsilanes from two different arylalkenes. The observation of Co(I) silyl and Co(III) bis(silyl) hydride complexes suggests that the DHS reactions probably proceed through a Co(I)/Co(III) redox process with a Co(III) bis(silyl) hydride species as the catalyst resting state.