Diarylbis(acylamino)spiro-λ⁴-sulfanes and cyclic acylaminosulfonium salts. A kinetic study of equilibrium and hydrolysis reactions

Abstract

Kinetics for the hydrolysis of diarylbis(acylamino)spiro-λ⁴-sulfanes 3a–e and 9 and precursor acylaminosulfonium salts 5a–e and 8 both leading to sulfoxides 6a–e and 10, respectively, have been studied under pseudo-first-order conditions in dioxane–water mixtures. Medium, substituent (ρ⁺ –0.44) and solvent isotope effects (k_H2O/k_D2O ≈2) indicate that the cleavage of one of the S–N bonds and a simultaneous proton-transfer from H₂O to the leaving carbamoyl group take place in the rate-determining step of the hydrolysis of spiro-λ⁴-sulfanes 3a–e. The acylaminosulfonium and OH^– ion intermediates formed together are converted to sulfoxides 6a–e in fast steps. The hydrolysis of acylaminosulfonium salts shows deviation from first-order kinetics at the starting period of the reaction, which may be ascribed to an equilibrium formation of spiro-λ⁴-sulfanes. The equilibrium between 3a and 5a has been studied by spectroscopic methods, and K₃₅ 5 × 10^–7 mol dm^–3 was obtained in 70 : 30 (v/v) dioxane–water, at 25 °C. The equilibrium is shifted towards the acylaminosulfonium salts as the acidity of the solvent increases with the progress of the hydrolysis, or when acid is added to the mixture. In the case of hydrolysis of 5a–e and 8 the medium, substituent and deuterium solvent isotope effects (k_H2O/k_D2O ≈4.3) as well as the general base catalytic effect of acetate ions suggest that a nucleophilic attack of water on the sulfonium centre occurs in the rate-determining step with the cleavage of one of the OH bonds in water and with the formation of a hydroxy(acylamino)-λ⁴-sulfane. Acylaminosulfonium salt 12, having a sterically hindered reaction centre, undergoes hydrolysis only with more nucleophilic OH^– ions. Relative reactivities and different ring-size effects observed for bis(acyloxy)-, (acyloxy)(acylamino)- and bis(acylamino)-spiro-λ⁴-sulfanes and cyclic acylaminosulfonium salts are compared and interpreted.

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