Silylene-based mechanism

Mechanistic aspects of hydrosilylation catalysed by rhodium have been outlined, and the recently introduced silylene-based mechanism has been discussed. ... 

The product 33 (Amax = 345 nm) was formed as a film (estimated by the optical density of the UV spectrum to be several tens of nanometres thick) on a target quartz plate, the surface of which had been treated with 10 wt.% NaOH solution, followed by dichlorodimethylsilane and finally LiAlH4 to afford a surface terminated with -OSiMe2H groups. On the basis of trapping experiments (to confirm the formation of silylenes) and photo-CVD in the presence of styrene (to confirm the absence of radical species), a mechanism based on silylene formation and insertion into the surface Si-H bond was shown to be likely, as outlined in Scheme 19. 

R. Becerra and R. Walsh, Kinetics and mechanisms of silylene reactions a prototype for gas-phase acid/base chemistry, in Research in Chemical Kinetics (Eds. R. G. Compton and G. Hancock), Vol. 3, Elsevier, Amsterdam, 1995, p. 263. 

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

The current view of the mechanism of addition of silylenes to 1,3-dienes is based on the following experimental observations ... 

The reactions between silylene (H2Si ) and the three-membered ring compounds, oxirane, thiirane, and selenirane, which provide possible routes to Si=X (X = O, S, Se) double bonds were studied by ab initio calculations at the MP2 and QCISD correlated levels of theory <2000CJC1496>. Three possible mechanisms have been examined (Scheme 4). Based on the results, the authors have concluded that the silatane route , via the corresponding oxa-, thia-, and selenasilacyclobutanes, is not a likely mechanism for the production of H2Si=X. 

Based on the above ideas on the reaction mechanism, it is expected that the silylene species at higher temperatures display an increased electron acceptor ability compared to the silicide cluster. Evidence for that is provided by investigations of catalyst samples using ferro-magnetic resonance (FMR). 

The action of a catalytic amount of MeONa on methoxymethyldisilane gives o,a>-dimethoxypermethylpolysilanes or cyclic polysilanes, depending on the reaction time (55b). It was concluded that the mechanism involves a concerted nucleophilic substitution with base-assistance or stepwise substitution by silyl anion a silylene mechanism is ruled out. 

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... 

Figure 1. A mechanism for dehydrogenative coupling of silanes, based on a transition-metal silylene complex as an intamediate.

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