Direct and Transfer Hydrosilylation Reactions Catalyzed by Fully or Partially Fluorinated Triarylboranes: A Systematic Study

The present survey serves several purposes. Selected electron-deficient boron Lewis acids catalyze the release of hydrosilanes from cyclohexa-2,5-dien-1-yl-substituted silanes. The two-step process consists of a hydride abstraction to generate a silicon-stabilized Wheland complex and capture of the arene-stabilized silicon cation by the borohydride formed in the previous step. The same boron catalyst will then activate the Si-H bond for the reaction with representative pi- and sigma-donating substrates, alkenes/alkynes and ketones/ketimines, respectively. The net transformation is a transfer hydrosilylation, and the effect that the substitution pattern of the cyclohexa-1,4-diene core and the subsituents at the silicon atom exert on these hydrosilane surrogates is systematically investigated. The results are compared with those obtained employing the hydrosilane directly. Another part of this comprehensive analysis is dedicated to the comparison of literature-known fully or partially fluorinated triarylboranes in both the direct and the transfer hydrosilylation of the aforementioned substrates. The data are tabulated and color-coded, finally providing an overview of promising substrate/reductant/borane combinations. The often different reactivities of pi- and sigma-basic substrates are explained, and it is shown that the Lewis acidity of the boron atom, estimated by the Gutmann-Beckett method, is not the only decisive feature of these boron Lewis acids. Practical mechanistic models are presented to rationalize the interplay between the Lewis acidity and steric situation at the boron and, likewise, the silicon atom as well as the need for fluorination ortho to the boron atom in certain cases.