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

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 Prⁱ₃Sn 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.