Thiocarbonyl elimination from sulphenic systems. Part I. Elimination of hydrogen cyanide from substituted diphenylmethyl thiocyanates

Abstract

Diphenylmethyl thiocyanates undergo elimination of hydrogen cyanide on treatment with sodium isopropoxide in isopropyl alcohol. The yield of thiobenzophenones varies with the nature of the substituents, the elimination fraction being unity in the case of 4-chloro- and 3-methoxy-derivatives. The second-order rate constants measured at 20° fit the Hammett equation giving ρ=+3·5 (log k₀= 0·785). Optically active 4-chlorodiphenylmethyl thiocyanate does not racemize appreciably during the elimination. Tritium exchange experiments carried out on 4-phenyldiphenylmethyl thiocyanate resulted in a very small, if any, tritium intake by the unchanged material. On the basis of these results and of the kinetic isotope effect, k_H/k_D= 3·0, a concerted E1cB-like mechanism is suggested. The occurrence of the thiocarbonyl elimination from other diphenylmethyl sulphenic systems and the factors governing such reaction in the case of diphenylmethyl thiocyanates are discussed.