Si—O bond cleavage

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Anodic oxidation of hydroquinone disilyl ethers also takes place easily at around 1 V vs SCE, as shown in Table 1044. It was proposed that an initial one-electron oxidation generates a cation radical which decomposes by a Si—O bond cleavage to form quinones (equation 40)44. 

The reactions of (Fl3Si)20 fall into two general classes those that proceed with Si-O bond cleavage, usually promoted by Lewis acids, and those that lead to condensation reactions, usually promoted by Lewis bases. Typical reactions are shown in Scheme 37. ... 

With acetonitrile as solvent, the esters discussed above gave exclusively the product of Si-O bond cleavage with KSCN, KOCN, CsF and NaN3153. 

The Ic2 values (M s ) of the solvolysis of various RsSiOPh in aqueous ethanol at 25 °C were determined under acidic and basic conditions . The size of the silyl groups influences the rate and follows the order listed in entries 55 and 56 of Table 1. The correlation between the Si—O—C angle and the oxygen basicities may be completely different, or even reversed, in comparison with trimethylsilyl and rert-butyldimethylsilyl ethers. The relative hydrolysis rates may be simply controlled by steric hindrance to solvent assistance for the Si—O bond cleavage following the protonation step. 

Schmittel, M., KeUer, M., and Burghart, A., Silyl enol ether cation radicals in solution nucleophile assisted Si-O bond cleavage, /. Chem. Soc., Perkin Trans. 2, 2327, 1995. 

Hevia E, Kennedy AR, Klett J, McCall MD (2009) Direct lateral metallation using alkali-metal mediated zincation (AMMZn) SiC-H vs.Si-O bond cleavage. Chem Commun 3240-3242. doi 10.1039/b903592c... 

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