Metal silylenoids

Reactivity and Attributes of Metal Silylenoids and Silylmetal Complexes... 

Transition Metal Silylenoid Complex-Catalyzed Hydrosilation Reactions... 

Transition Metal Silylenoid-Catalyzed Atom Transfer Reactions... 

This chapter examines the synthesis and reactivity of transition metal silylenoids and silylmetal complexes to provide context for the focus of this chapter silver-mediated silylene transfer reactions. 

REACTIVITY AND ATTRIBUTES OF METAL SILYLENOIDS AND SILYLMETAL COMPLEXES... 

Late transition metal silylenoid complexes were first synthesized by exploiting the lability of silicon-triflate bonds (Scheme 7.2).34 37 Tilley and coworkers reported that... 

Scheme 7.1. Light-induced 1,2-silyl shift to synthesize early transition metal silylenoid complexes.

Tilley and coworkers reported a more direct procedure for the synthesis of late transition metal silylenoid complexes by reaction with silanes [equation (7.1)].36,37 Exposing iridium complex 14 to dimesitylsilane afforded iridium silylenoid 15.36 In addition to dimesitylsilane, other silanes could be used, including diphenyl-, diethyl-, or dimethylsilane. Primary silanes such as mesitylsilane and 2,4,6-triisopropylphe-nylsilane were also tolerated as substrates.37... 

Scheme 7.3. Generation of transition metal silylenoid complexes from stable silylenes.

The coordination of stable silylenes to metal complexes was also reported to produce transition metal silylenoids.40,41 Exposure of unsaturated silylene 16 to Ni(CO)4 resulted in the formation of the disilylene-substituted tetrahedral nickel complex 17 (Scheme 7.3).42,43 Similarly, mixing Ph PAuCI with decamethylsilico-cene 18 produced the silylgold complex 19.44 The 29 Si H NMR spectrum of 19 (8 78 ppm) revealed its silylenoid character. In addition to nickel and gold, other metals, including tungsten,41 platinum,45 iron,40 and ruthenium,46 have been utilized to form silylmetal complexes of stable silylenes. 

Metal-catalyzed silylene extrusion from hexamethylsilirane 20 was also used to generate transition metal silylenoids [equation (7.2)] 47 In the presence of a metallo-porphyrin, [Os(TTP)]2 (where TTP = the dianion of meso-tetra-p-tolylporphyrin), hexamethylsilirane 20 extruded dimethylsilylene to produce the THF-stabilized osmium silylenoid complex 21. Despite the coordinated THF molecule, crystallographic analysis of the silylosmium complex 21 revealed the sp2 nature of the silicon atom (Os-C-Si bond angles 121° and 117°). Osmium silylenoid 21 was also synthesized from the reaction of [Os(TPP)]2- and dimethyldichlorosilane.47... 

The structure and bonding of metal silylene complexes varied from those of their carbon analogs. While Fischer-type metal carbene complexes without solvent adducts have been extensively characterized,48 most metal silylenoid complexes contain a bound solvent molecule or counterion on the silicon atom. The bond energy for donor silicon complex 22 was determined to be between 15 and 20kcal/mol 49,50... 

The reactivity of early transition metal silylenoid complexes is still emerging. An example of the chemistry that these complexes can participate in is the sila-Wittig reaction [equation (7.4)].53 In this transformation, a metathesis occurred between the chromium silylenoid 24 and the dimethyl carbonate to afford a new Schrock carbene, 25, and the trimerized product 26. This methodology allowed access a new carbene complex that eluded previous synthetic efforts. 

A catalytically active silylsilver intermediate was observed using low-temperature 29Si NMR and IR spectroscopy (Scheme 7.18).83 Exposure of cyclohexene silacyclopropane 58 to the silver complex produced cyclohexene as well as a new species, which exhibited two doublets at 97 ppm (./Agsi = 260 and 225 Hz) in the 29 Si 1H NMR spectmm. The 29 Si H NMR spectra of this species are consistent with a Lewis base-stabilized metal silylenoid. Tilley and coworkers have reported that (Et3P)2Pt(H)-Si(Sf-Bu)2(OTf) appears at 52 ppm,39 and Jiitzi et al. observed... 

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