Methanolysis of the benzylideneaniline derivatives ArC(MMe3)NAr' (M = Si or Sn)

Abstract

Rates of cleavage of compounds (I) and (II) of the type p-XC6H4C(MMe3)NC6H4Y-p, X = H, Y = OMe, Me, or Cl, or Y = H, X = H, OMe, Me, or Cl, in MeOH (to give the benzylideneanilines XC6H4CHNC6H4Y) have been measured spectrophotometrically at 25 °C. The cleavages occur readily in methanol alone, but are catalysed by NaOMe and, at least for the tin compounds, by MeCO2H. In the neutral cleavage the tin compound (IIa) is ca. 49 times as reactive as the corresponding silicon compound (Ia). The rate isotope effect (r.i.e.), the ratio of the rate constant in MeOH to that in MeOD, is ca. 1.3 for (II), and the product isotope effect (p.i.e.), given by the product ratio RH : RD on cleavage in 1 : 1 MeOH–MeOD is 1.1, while for the silicon compounds the r.i.e. is ca. 1.8. Electron release by X or Y increases the ease of reaction. The results are interpreted in terms of a cyclic mechanism in which transfer of a proton from an MeOH molecule to the nitrogen of the CN bond is synchronous with formation of the M–OMe bond, and probably also with the breaking of the C–M bond, the proton transfer to the imine being considerably advanced in the transition state. In the base cleavage the tin compound (IIa) gives normal values for the r.i.e., p.i.e., and r.i.e. : p.i.e. ratio (ca. 0.96, 1.65, and 0.58, respectively), consistent with an electrophilically-assisted mechanism in which a proton is transferred from the solvent to the carbon atom of the imino-group as the C–Sn bond breaks. For the silicon compounds, the r.i.e. values, of ca. 1, are wholly abnormal, as is the fact that the silicon compound (Ia) is more reactive, by a factor of 2, than its tin analogue (IIa), and it is suggested that there is electrophilic assistance even for the silicon compounds, but involving proton transfer to nitrogen. For both tin and silicon compounds electron withdrawal in Y facilitates the base cleavage. Cleavage of (II) is catalysed 70 times as effectively by MeCO2H as by MeONa, and in the media used oxonium ion does not compete with MeCO2H as the catalyst.