Study of the mechanism for the hydrolysis of alkoxy(aryl)(phenyl)-λ6- sulfanenitriles, ArPhS(OR)(?N)

Article abstract of DOI:10.1246/bcsj.73.957

The hydrolysis of alkoxy(aryl)(phenyl)-λ⁶-sulfanenitriles in several buffer solutions was found to follow a good pseudo-first-order kinetic equation, giving the corresponding sulfoximides and alcohols (for the case of the hydrolysis of neopentyloxy-λ⁶-sulfanenitrile, giving a rearranged product, 2-methyl-2-butanol). The dependence of the rate of hydrolysis on the structure of the alkyl group showed the opposite trend to the usual S(N)2 character, i.e. Me < Pr < Bu ? Et < i-Pr, except for neopentyl. The pH-rate profiles indicated that the rate of hydrolysis is also first order in [H⁺] at pH more than 6.08, and trends to saturate at low pH. According to these kinetic results, a two-step reaction mechanism was proposed which involves a pre-equilibrium protonation on the nitrogen atom of the alkoxy-λ⁶- sulfanenitriles, followed by a rate-determining C-O bond cleavage via an S(N)2 or S(N)1 mechanism on the alkyl carbon atom depending on the structure of the alkyl group. From a double-reciprocal plot of 1/k(obs) vs. 1/[H⁺], the pK(a) value and the rate constant of the second reaction of neopentyloxy(diphenyl)-λ⁶-sulfanenitrile were estimated to be 5.02 and 7.02x10^-3 s^-1, respectively. The substituent effects on the phenyl group of neopentyloxy(diphenyl)-λ⁶-sulfanenitrile afforded a large negative p- value (-1.88) for pK(a) and positive one (+1.66) for the second reaction at 25.2 °C. The small negative p-values observed at pH 6.27 for diphenyl(propoxy)-λ⁶-sulfanenitrile (-0.42) and neopentyloxy(diphenyl)- λ⁶-sulfanenitrile (-0.26) were found to be the results of a cancellation of those for the opposite trend of the reactions of the pre-equilibrium and the second step. The activation parameters for both the pre-equilibrium and the subsequent reactions were also estimated based on the parameters for the hydrolysis of neopentyloxy(diphenyl)-λ⁶-sulfanenitrile at pH 6.22 and 2.99. The buffer effect is due to a nucleophilic attack of the buffer base to the alkyl carbon atom of the protonated alkoxy-λ⁶-sulfanenitriles. The sulfoximide moiety in the protonated λ⁶-sulfanenitrile is revealed to be a very good leaving group.