Silylene mechanism

The discussion about the mechanism of the dehydrogenative polymerization reaction has not yet been completed. However, the reaction mechanism seems to be strongly influenced by the specific random conditions that apply for each particular system. Presumably with late transition metals a silylene mechanism is more appropriate. It may be a matter of the steric constraints of the system to shift the reaction towards a-bond metathesis. 

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. 

K. L. Bobbitt, Photochemical Generation of Germylenes and Silylenes Mechanism of Germylene and Silylene Addition to 1,3-Dienes, Doctoral Dissertation, Washington University, St. Louis, December 1990. 

If the activation energy for the formation of silylenes is smaller than the activation energy for the formation of radicals, a silylene mechanism occurs ... 

Since an insertion reaction into alkyl or aryl silicon bonds seems to be difficult, a radical mechanism usually takes place in the pyrolysis of such compounds. On the other hand, at Si—H, Si—Hal, Si—OR and Si—N bonds, a silylene mechanism occurs because the insertion reaction has only a small activation energy. 

As seen in an earlier chapter, the silylene mechanism is preferred in presence of a Si—H bond. However, observations on pyrolysis need not apply to reactions, induced by electrical discharge. 

The proposed silylene mechanism gives an explanation for the high selectivity of (CH3)2SiCl2 formation in the "Direct Synthesis" of methylchlorosilanes (Miiller-Rochow process). Via an oxidative addition of CH3CI to methylsilylenes on the surface of a Cu/Si catalyst, (CH3)3SiCl2 is produced in a kinetically controlled process (Scheme 2). 

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. 

Summary The Direct Process discovered by Rochow and Muller around 1940 is the basic reaction used to produce methylchlorosilanes, which are the monomeric intermediates used for production of silicones. An understanding of the elementary reactions, the nature of active sites and the action of promotors does not nearly come close to the performance level of the industrial process and the economic importance. The silylene-mechanism is a useful model to understand the complex product mixture from the reaction of silicon with chloromethane. 

The reaction mechanism is not clarified yet. Older proposals by Hurd and Rochow, Bazant, Klebansky, Fikhtengolts and Voorhoeve, or Golubtsov are today replaced by the silylene mechanism [6], which provides a good explanation for the products obtained in the Direct Process. 

Insertion of silylenes into C-Cl bond forms methylchlorosilanes Scheme 1. The silylene-mechanism for the formation of methylchlorosilanes. 

The silylene-mechanism alone does not explain the formation of Si-H bonds, higher aliphatic groups attached to silicon (e.g., ethyl and propyl) and the formation of hydrocarbons, mainly methane or elemental carbon. 

Figure 15.5 Silylene mechanism of the Rh- addition of a Si-H bond. Left hydride catalyzed hydrosilylation of ketones. The transfer from rhodium to the carbonyl C...

Copper is an essential catalyst in the direct reaction, usually added in the form of CusSi. Lewis and coworkers, in a review of the direct reaction, have suggested that a divalent silicon atom, perhaps attached to two copper atoms, reacts with methyl chloride to insert into the C—Cl bond, a typical silylene reaction (equation 107). The resulting intermediate (53) could then react with a second methyl chloride molecule by insertion into the C—Cl bond to yield Me2SiCl2. The other major products of the direct reaction, namely MeSiCls, MeSiHCh, HSiCls and MesSiCl, can be accounted for by assuming the intermediacy of similar silylene-like species. In another book chapter, Ono and coworkers have presented kinetic evidence consistent with the silylene mechanism for the direct reaction . At this point the evidence for silylene intermediates in the methyl chloride-silicon reaction is suggestive but not truly compelling. 

Harrod has recently described theoretical work on a metal-silylene mechanism.In the former paper Tilley proposed that the mechanism for catalysis by the early transition metals differs from that of the late transition metals. With the early transition metals a sigma metathesis mechanism was proposed in which no formal change in the oxidation state of the metal occurs during the catalytic cycle. Tilley also suggested that oxidative-addition reductive-elimination mechanisms are the most consistent with results obtained from the late transition metals. This is an application of the general mechanisms... 

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