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

A chemodivergent mono- and double-dehydrosilylation (DHS) of arylalkenes with hydrosilanes has been realized by bipyridyl-phosphinite (PONN) and bipyridyl-phosphine (PCNN) supported cobalt complexes. A series of phosphinite-containing complexes (PONN)CoCl2 (Co1–Co5) with an O-linker between the P atom and the bipyridyl backbone were synthesized. Among them, the tBu-substituted complex (tBuPONN)CoCl2 (Co4), with NaBEt3H as the activator, exhibits high catalytic efficiency and selectivity for the mono-DHS of arylalkenes with PhMeSiH2 to form (E)-vinylsilane, but it is barely active for the DHS reactions with sterically more hindered secondary hydrosilanes (e.g., Ph2SiH2) and tertiary hydrosilanes. In contrast, the phosphine variant (tBuPCNN)CoCl2 (Co6) with a C-linker and phosphino-tBu substituents catalyzes selective double-DHS with PhMeSiH2, 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 Ph2SiH2), while the iPr-substituted analog (iPrPCNN)CoCl2 (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.