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
Et3SiH 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 H2 and silanes mediated
by HB­(C6F5)2 and heterolytic H2 splitting by boroles, respectively.