Comparison of gas phase and condensed phase SNi reactions. The competitive four- and five-centre cyclisations of the 3,4-epoxybutoxide anion. A joint experimental and theoretical study
Ab initio calculations [at the MP2-Fc/6-31+G(d) level] indicate that the 3,4-epoxybutoxide anion should undergo competitive SNi cyclisations (through four- and five-membered transition states) to yield the (M – H)– ions of oxetan-2-ylmethanol and tetrahydrofuran-3-ol respectively. The barriers to the transition states are comparable (ca. 70 kJ mol–1) for each process, and the latter product is the more stable by 82 kJ mol–1 at the level of theory indicated. Gas phase studies of the 3,4-epoxybutoxide anion excited by collisional activation are in accord with this scenario, and show, in addition, that deprotonated 2-oxetanylmethanol can convert to the starting material. Base treatment of 2-(oxiran-2-yl)ethan-1-ol (3,4-epoxybutan-1-ol) in two different solvent systems [10% aqueous sodium hydroxide and sodium hydride–tetrahydrofuran (both at reflux)] yields the same two products observed in the gas phase studies. However, deprotonated tetrahydrofuran-3-ol is the kinetic product in both solvent systems.