A Complementary Approach to Dialkylsilylenes via Thermodynamically Stable Boron-Ate Silylenoids Derived from Chlorosilylboranes.

Silylenes are highly reactive divalent silicon species that enable diverse transformations via σ-insertion and π-addition to silylene acceptors, and thus represent key intermediates for the incorporation of silicon moieties into various chemical bonds. However, the practical use of silylenes as useful silicon synthons remains limited, as specific stabilizing substituents are currently required in order to suppress self-condensation and oligomerization. Herein, we report a novel strategy to generate diversely substituted silylenes through a reaction with an electron-deficient aryllithium to form boron-ate silylenoids derived from isolable chlorosilylborane precursors. Several chlorosilylboranes were readily prepared via the Si-H borylation of dialkylanisylsilanes and subsequent chlorination of the anisyl group, or via the monoborylation of dihydrosilanes followed by chlorination of the remaining hydride group. The generated silylenes exhibit the inherent reactivity associated with free silylenes, i.e., σ-insertions into Si-H bonds and π-additions to alkene and carbonyl compounds. Mechanistic studies revealed that the concentrations of the lithium silylenoid and the subsequent silylene remained lower than that of the boron-ate silylenoid due to the thermodynamic and kinetic unfavorability of the boron-lithium exchange. As a result of the low concentration of the generated silylene, it selectively reacts with the silylene acceptor rather than participating in side reactions such as self-condensation and oligomerization, which are problematic in the reduction of dichlorosilanes. Consequently, this work represents a reliable strategy to harness silylenes as versatile silicon synthons for transformations that are difficult to realize via the more conventional alkali-metal-mediated reduction of dihalosilanes.