Synthetic Development and Chemical Reactivity of Transition-Metal Silylene Complexes

Abstract

A variety of transition-metal complexes with terminal silylene ligands have become available in recent years, because of the discovery of several preparative methods. In particular, three general synthetic routes to these complexes have emerged, on the basis of anionic group abstraction, coordination of a free silylene, and alpha-hydrogen migration. The direct transformation of organosilanes to silylene ligands at a metal center (silylene extrusion) has also been observed, and this has further spurred the exploration of silylenes as ligands. This Account describes the synthetic development of silylene ligands in our laboratory and resulting investigations of stoichiometric and catalytic chemistry for these species.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…For example, transition metal silylene complexes bind and activate substrates at the metal-bound silicon center, which has Lewis acidic character (Scheme 2, B). [15][16] In these reactions, a metal-silicon bond is preserved upon binding of the substrate, but the resulting adduct possesses a silicon center that is 4-coordinate rather than 5-coordinate. However, more recently reported hydrosilation mechanisms do invoke the formation of intermediates in which the substrate binds to a 5-or 6-coordinate silicon center (Scheme 2, C).…”

“…Terminal silylene ligands usually exhibit Lewis acidity at silicon (Scheme 45), and this property features heavily in the reactivity of most silylene complexes. [15,16,82] A wide range of nucleophilic substrates (e.g. water, [83] alcohols, [84] ketones, [85] isocyanates, [86] olefins, [87] nitriles, [88] isocyanides [89] ) undergo activation by the highly electrophilic silicon center of these complexes.…”

“…Additionally, when the σ-complex could be directly observed, it was formed relatively easily by mixing a silane with a simple metal complex precursor. In contrast, silylene complexes are very challenging to prepare, [15] and studies of these species were not initially focused on hydrosilation reactivity. [82] Instead, interest in silylene ligands stemmed from their possible involvement in other catalytic reactions (e.g.…”

“…Rochow's Direct Process, [90] Si-Si bond formation [91] ), as well as from fundamental interest in examining heavy analogues of carbene complexes. A variety of methods have been developed for preparing isolable silylene complexes, [15,16] and it has subsequently been discovered that many of these species participate in stoichiometric and catalytic hydrosilations. [85e,f;87a,c,d] The focus on studying isolated and well-characterized silylene complexes may contribute to the tendency to observe their involvement in stoichiometric transformations rather than catalytic reactions.…”

“…This review will examine hydrosilation catalysts for which the generation of an electrophilic silicon center is key to the activation of the unsaturated substrate. [13][14][15][16] These types of mechanisms were first proposed in the 1990's and a range of…”