Aromatic reactivity. Part LVI. Solvent isotope effects in cleavage of aryltrimethylstannanes by acetic acid and aryltrimethylsilanes by trifluoroacetic acid

Abstract

Values of the product isotope effect, corresponding with the product ratio XC₆H₅:XC₆H₄D obtained on cleavage in a 1 : 1 SH (ordinary acid)–SD (O-deuteriated acid) medium, have been determined for cleavage of (a) XC₆H₄SnMe₃ compounds by acetic acid at 50°, and (b) XC₆H₄SiMe₃ compounds by trifluoroacetic acid at 21°. For system (a) the values range from 5·9 for X =p-OMe to 4·9 for X =m-CF₃, and for (b) all the values lie between 6·0 and 6·5. In both systems, for compounds for which the overall solvent isotope effects, given by the ratios of the separate rates in SH and SD media, have also been measured, the overall solvent isotope effects are the same within experimental error as the product isotope effects, indicating that there is no significant secondary solvent isotope effect for the proton transfers from the acids. It is suggested that the proton transfer to form the Wheland intermediate is rather more than half-complete in both cases. In the cleavage of the m-CF₃C₆H₄MMe₃(M = Si or Sn) compounds by trifluoroacetic acid, on average significantly more than one deuterium atom per molecule is incorporated into the benzotrifluoride product, and this anomaly is discussed.