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 S_Ni 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 Q_Vib for the reaction proceeding through the six-membered transition state.

B. A comparison is made of the reported competitive S_Ni reactions for 2,3-epoxypropoxide, 3,4-epoxybutoxide, 4,5-epoxypentoxide and 5,6-epoxyhexoxide anions. For all but the 3,4-epoxybutoxide system, the exclusive or major product is that which contains the smaller of the two ring systems for both gas phase and condensed phase reactions. In the case of the 3,4-epoxybutoxide system: (i) in the gas phase, both four and five membered ring S_Ni products are formed in comparable yield, and (ii) in the condensed phase, the major product is that with the larger ring.