Si–H Bond Activation at the Boron Center of Pentaphenylborole

Si–H bond activation is usually considered a domain of transition-metal complexes, and only few metal-free systems have proven suitable for this task. We have now found that Et 3 SiH readily reacts with pentaphenylborole to afford 1-bora-3-cyclopentenes as the syn and anti addition products. Here, Si–H bond cleavage is accomplished at a single boron center, a reactivity that is facilitated by a combination of high electrophilicity and loss of antiaromaticity. The mechanism of this transformation most likely involves a sequence of adduct formation, σ-bond metathesis, and conrotatory ring closure, similar to that observed for H/D exchange between H 2 and silanes mediated by HB(C 6 F 5 ) 2 and heterolytic H 2 splitting by boroles, respectively.