Si-H bond cleavage

Dichlorobis(triethylphosphine)nickel(II) is an effective catalyst for the reaction of symmetrical tetramethyldisilane with unsaturated hydrocarbons (237). A hydronickel complex was postulated as being involved in the reaction, although the mechanism may involve the intermediacy of a dimethylsilylene compound formed by Si—Si bond cleavage rather than the silyl-nickel bond being formed by Si—H bond cleavage. The complex was an effective catalyst for formation of the... 

Example The molecular ion of l,2-bis(trimethylsiloxy)benzene, m/z 254, undergoes methyl loss by Si-C bond cleavage as typically observed for silanes (Fig. 6.48). Rearrangement of the [M-CHb] ion then yields [Si(Me)3], m/z 73 (base peak). This is not an ortho elimination with concomitant H transfer as defined in the strict sense, but the observed reaction is still specific for the orthoisomer. [190,203] In the spectra of the meta- and para-isomers the [Si(Me)3] ion is of lower abundance, the [M-CHb] ion representing the base peak in their spectra. Moreover, the m/z 73 ion is then generated directly from the molecular ion which is clearly different from the two-step pathway of the ortho-isomer. 

The stereochemical behavior of Ph2CHLi, i.e., cleavage of the Si—H bond with predominant inversion, can be explained as follows. We are faced with a very soft nucleophile of low level (Scheme 10). The substrate superjacent MO-nucleophile HOMO interaction prevails. The big lobe of the Si hybrid orbital points to the rear in the superjacent MO (Scheme 11), and the inversion is therefore favored. However, this reaction is quite slow (10). 

Highly reactive halogen-substituted silanes were understandably avoided in earlier mechanism studies, and attention was devoted instead to the hydrolytic and solvolytic cleavage of the more stable Si-H bond. The reaction, most simply represented by the expression, SiH -)- H20 - SiOH -f- H2, is strongly base catalyzed. However, as Price (55) pointed out, the stoichiometry is represented better by the equation SiH + ROH -j- OH - - SiOH + OR- -)- H2. Price found that the rate of hydrogen evolution is given by the expression kx t = In [VF[VF— V]... 

The photolytic cleavage of the Si—Si bond in 9-(trimethylsilyl)-9-silafluorenes in the presence of acetone afforded a Si—O(C) bond via a Si=C double bond94. The same reactions performed in neat alcohol (MeOH or EtOH) give rise to direct formation of a Si—H bond (Scheme 14). 

When disilane, SizHg, is used in this reaction, the Si-H linkage is broken in preference to the Si-Si bond (165). Silicon-carbon bonds are not usually susceptible to cleavage, although Si-alkynyl bonds provide an exception (entry 47). If both Si-H and Si-alkynyl bonds are present, however, it is the former that cleaves (entry 56). The special case of metal insertion into Si-C bonds of strained heterocycles is dealt with separately in Section II,G,2, while an isolated but interesting example of Si-Hg bond cleavage appears in entry 36. 

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