Reactivity in eliminative cleavage of activated four-membered rings. The behaviour of 3-hydroxythietane derivatives

Abstract

3-Hydroxythietane derivatives undergo retro-aldol ring fission in aqueous sodium hydroxide at a rate determined by the substituent (H or Ph) at the 3 position and by the oxidation state of sulfur. Reactions occur between 4 × 10⁴ and 5 × 10⁵ times faster than for the open chain analogues. These rate accelerations correlate with an expression of strain energy between 41% and 33% compared with approximately 26% for alkene-forming eliminations of cyclobutanes. These differences, together with the differences in relative sensitivities to α-phenyl substitution and uniformly positive values of entropies of activation for 3-hydroxythietane derivatives, suggest a greater degree of ring cleavage in the transition structure for thietanes than for cyclobutanes. Ring fission is, however, accompanied by concomitant protonation of the leaving carbon group as evidenced by the isotope discrimination value of 1.7 for reaction of 3e. 3-Hydroxythietane itself does not undergo retro-aldol ring cleavage but rather anionic polymerisation to yield polymer 9a.