Silylene methane

For monosilanes, a metathetical mechanism was postulated by Tilley [24] and an a-elimination mechanism was postulated by Harrod [25]. Neither mechanism was able to explain the experimental results on disilanes. Therefore we have postulated a new mechanism via silylenes, shown in Fig. 5 it seems to be a special P-elimination mechanism called p -elimination [26]. The starting reaction for the generation of the silylene is also shown in Fig. 5. Small amounts of methane should be formed, and we found it experimentally. 

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. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

FIGURE 54. Transition structures and reaction profiles for hydrogen abstraction from methane and silane by triplet silylene and methylene. Bond lengths are in A, bond angles in deg, energy in kcal mol"1. Reprinted with permission from J. Am. Chem. Soc., 106, 4054 (1984). Copyright (1984) American Chemical Society297. 

On the other hand, it is unlikely that methylene is formed from methane or other hydrocarbons in the pyrolysis of coke. In contrast to methylene, its analog silylene is a product of the pyrolysis of silane and disilane (8-17 Purnell and Walsh, 1966 Bowrey and Purnell, 1970). Laser-induced fluorescence was used to study the formation of silylene and its reactivity (see, e.g., Baggott et al., 1988 Jasinski and Chu, 1988), but silylene is not within the scope of this book as there are no diazo compounds involved in its chemistry. Literature in which the reactivity of silylene is compared with that of methylene is reviewed briefly in a publication of Skancke (1993, p. 640). 

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