Cleavage of diphenylmethyl-, triphenylmethyl-, fluoren-9-yl-, and substituted benzyl-trialkylstannanes by methanolic sodium methoxide. Evidence for a duality of mechanism

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Abstract
Product isotope effects (p.i.e.) given by the product ratio RH : RD obtained in 1 : 1 MeOH–MeOH have been measured for cleavage of RSnMe3 compounds by methanolic sodium methoxide; values (with estimated uncertainties of ± 20%) are: (R =)p-NCC6H4CH2, m-NCC6H4CH2, or 3,5-Cl2C6H3CH2, 2.1; Ph2CH, 1.5; Ph3C, 1.3; fluoren-9-yl, 8.4; 9-methylfluoren-9-yl, 8.4. (For the compounds with R = fluoren-9-yl, change to Et3Sn or Pri 3Sn groups has no significant effect on the p.i.e. value, but for R = Ph2CH this change may be accompanied by a small fall in the p.i.e. value.) Values of the ratio of the rate in MeOH to that in MeOH are: p-NCC6H4CH2, 0.57 (25 °C); m-NCC6H4CH2, 0.85 (25 °C); 3,5-Cl2C6H3CH2, 0.82 (50 °C); Ph2CH, 0.59; Ph3C, 0.47; fluoren-9-yl, 0.46; 9-methylfluoren-9-yl, 0.50. Approximate ratios of the rates for tin to those of corresponding silicon compounds are: C6H6CH2, 27.5 (50 °C); m-ClC6H4CH2, 13.5 (50 °C); m-CF3C6H4CH2, 9 (50 °C); 3,5-Cl2C6H3CH2, 10.5 (50 °C); m-NCC6H4CH2, 9.5 (25 °C); p-NCC6H4CH2, 24 (25 °C); Ph2CH, 190 (25 °C); Ph3C, 40 000 (25 °C); fluoren-9-yl, 19 000 (25 °C). It is suggested that for the benzyl compounds, except perhaps the p-cyano derivative, the rate-determining step involves proton transfer to the separating R group, but for the triphenylmethyl and fluoren-9-yl compounds it involves separation of the carbanion R–; the diphenylmethyl and p-cyanobenzyl compounds may react by a mixture of the two mechanisms.

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