Theoretical Studies on the Gas-Phase Nucleophilic Ring Opening of 3,4-Epimino-, 3,4-Epoxy-, and 3,4-Epithio-1-butene

The ring opening reaction of 3,4-epimino-1-butene (EIP), 3,4-epoxy-1-butene (EOB) and 3,4-epithio-1-butene (ETB) initiated by the attack of hydroxide ion at three carbons (C α, C γ, and C δ ) have been investigated using MP2/6-31+G*//MP2/6-31+G* ab initio molecular orbital theory. The C α C β C γ C δ X process by the α-attack is favored by the strong frontier orbital interactions due to the low π* orbital, but is disfavored by a lower degree of progress in the developing π orbital as well as in the ring opening. In contrast, the processes by the γ-and δ-attacks are disfavored by the high σ* orbitals, but are favored by a greater strain energy release in a greater degree of ring opening in the transition state. The barriers to the ring opening processes are found to depend on the strain energy of the three-membered heterocycles, bond strength of the C−X bonds, frontier orbital (π*, σ* (1), and σ* (2) ) levels, exothermicity of the reaction, and steric and entropy effects.