The 4-dimethylaminophenyl substituted thiazolidine 1 is subject to pH-dependent ring-opening and ring-closing equilibria yielding the resonance stabilised thiol iminium ion 2 and diprotonated thiazolidine 3 shown in Scheme 1.

Scheme 1

Rate constants for ring-opening or ring-closing can be measured spectrophotometrically using stopped flow or temperature jump fast reaction techniques taking advantage of the absorption of the iminium ion at λ_max= 400nm. In the ring-opening direction the reaction is acid-catalysed at low pH but becomes inversely dependent upon acid concentration above pH 1.5 and independent of acid concentration above pH 3.5.

The inverse acid dependence is consistent with a rapid deprotonation of the thiol group of the iminium ion 2 followed by rate-determining ring-closure, as shown in Scheme 2. The pH-independent reaction implies that at low acid concentrations the deprotonation step becomes rate-determining. This is a rare example of rate-determining proton transfer between electronegative atoms (S and H₂O) and is confirmed by pronounced catalysis by buffer acids in carboxylic, cacodylic, imidazole and lutidine buffers.

Scheme 2

Assignment of a rate constant k_-1 = 3 x 10¹⁰ M^-1s^-1 for protonation of the thiolate anion by H₃O⁺ implies a rate constant k₂ = 10⁷ s^-1 for closing of the thiazoldine ring. This is a large rate constant for a 5-endo trig transformation, which is formally disfavoured by Baldwin's rules. Interestingly, the rate constant is reduced by factors of up to 10⁸-fold by replacing the N-H of the thiazoldine ring by N-butyl or N-phenyl groups. The interpretation of these results will be discussed.

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