Gas phase and condensed phase SNi reactions. The competitive six and seven centre cyclisations of the 5,6-epoxyhexoxide anion. A joint experimental and ab initio study. A comparison with SNi reactions of homologous epoxyalkoxide anions †

Abstract

A. Ab initio calculations [at the MP2 Fc/6-31+G(d) level of theory] indicate that the barriers to the transition states for the competitive six and seven centre SNi cyclisation processes of the 5,6-epoxyhexoxide anion are 35.0 and 39.7 kJ mol–1 respectively. Experimental studies show that (i) in solution, the 5,6-epoxyhexoxide anion cyclises (and at the same time opens the ethylene oxide ring) to give tetrahydropyran-2-methanol as the predominant product on workup, and (ii) collisional activation of the 5,6-epoxyhexoxide anion in the gas phase gives the 2-tetrahydropyranmethoxide anion as the exclusive anionic product. It is proposed that frequency factors (Arrhenius A factors) control the courses of these kinetically controlled gas phase reactions. A comparison of the calculated harmonic vibrational partition functions for the two possible transition states confirms a higher value of QVib for the reaction proceeding through the six-membered transition state.