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-alpha, C-gamma, and C-delta) have been investigated using MP2/6-31+G*//MP2/6-31+G* ab initio molecular orbital theory. The C-alpha=C(beta)C(gamma)C(delta)X process by the alpha-attack is favored by the strong frontier orbital interactions due to the low pi* orbital, but is disfavored by a lower degree of progress in the developing pi orbital as well as in the ring opening. In contrast, the processes by the gamma-and delta-attacks are disfavored by the high sigma* 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 (pi*, sigma*((1)), and sigma*((2)) levels, exothermicity of the reaction, and steric and entropy effects.