Catalytic reactions involving silanes

The proposed mechanism of the catalytic reaction involves the formation of the Cu(l) alkoxide 68 by displacement of either the chloride or the NHC from 65-67, followed by conversion to the hydride 69 by metathetical exchange of the tert-butoxide by the H of the silane (Fig. 2.10). 

Many catalytic reactions involve the activation of Si—H bonds and consequently the reaction of silanes with platinum(0) has been studied extensively. The reactions of hydrosilanes with transition-metal complexes, including platinum, has been extensively reviewed by Corey and Braddock-Wilking.58... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

The structures of the functional catalyst or relevant intermediates in the proposed catalytic cycle in solution are presently unknown however, the available spectroscopic experiments are informative [78f]. In CD2CI2 at -78 C, H-NMR spectra of a 1 1 mixture of the silyl enol ether and Sn(OTf)2-diamine 102 did not show evidence of metallation of the enol silane to give the corresponding Sn(ll) enolate. Although the catalytic reactions were conducted in propionitrile and not dichloromethane, these experiments support the contention that a metalloe-nolate is not involved as an intermediate in the catalytic cycle. 

However, several catalytic reactions had been proposed to occur by a mechanism involving metal-silicon double bonds. - Kumada reported oligomerization of disilanes (Equation 13.33), and Ojima reported the scrambling and oligomerization of silanes in the presence of Wilkinson s catalyst (Equation 13.34), and both processes were proposed to occur by silylene intermediates. The formation of a silylene complex of iron tetracarbonyl was claimed in 1977 (Equation 13.35), but this compoimd was never isolated. Since the first structural data reported in 1987, a variety of metal-sUylene complexes have been prepared that are stabilized by coordination of a Lewis base to the silicon of the silylene complex, as in Equation 13.35. The first base-free silylene complex was reported by Tilley... 

Allylic trimethoxysilanes are activated by a catalytic combination of CuCl and TBAF.118 The mechanism of this reaction is not entirely clear, but it seems to involve fluoride activation of the silane. These reactions are stereoconvergent for the isomeric 2-butenyl silanes, indicating that reaction occurs through an acyclic TS. 

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

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