Ab initio studies of strained ring molecules

Abstract

The work reported in this thesis concerns the acid catalysed rearrangement of epoxides in the presence and absence of intramolecular nucleophiles. The potential energy surface (MP2/6-31G*//MP2/6-31G* and B3LYP/6-31G*) for rearrangement of protonated propene oxide to protonated propanal has been established. The rearrangement exhibits a 2 kcal/mol preference for rotation of oxygen away from the more hindered face of the oxirane plane containing the methyl. The rearrangement pathway involves two distinct steps; first, rupture of the oxirane and second, hydride migration. The latter does not commence until rupture of the C-O bond is complete. The combination of these two steps defines a concerted asynchronous rearrangement pathway and exhibits a 20 : 1 preference for migration of the proton trans to the methyl over the cis. Kinetic isotope effects of the acid and BF₃-catalysed rearrangement of methyl propene oxide to methylpropanal are consistent with a 1,2 hydride shift to a carbocation intermediate (B3LYP/6-31G*). Inverse secondary kinetic isotope effects for hydride migration reflect changes in Cl-H(D) stretching and out of plane bending frequencies. A calculated correction applied to the experimentally observed migration of hydrogen/deuterium (MH/MD = 1.92) results in a primary kinetic isotope effect for the reaction (kH/kD = 1.557) close to the theoretically calculated value (kH/kD = 1.677). The inversion and retention transition structures for intramolecular reaction of protonated cis- and trans-3,4-epoxypentan-1-ol which give protonated cis- and trans-2-methylfuran-3-ols have been determined at the ab initio MP2/6-31G* and hybrid density functional B3LYP/6-31G* levels of theory. Intrinsic reaction coordinate calculations for the lower energy inversion pathways for formation of the 2-methylfuran-3-ols show that intramolecular attack occurs in concert with ring opening. A complex of the 5-membered transition structure from the trans-epoxide with the Houk theozyme 3.5 kcal/mol lower in energy than the complex previously reported as a model for the antibody IgG26D9 catalysed intramolecular cyclisation of trans-4,5-epoxyhexan-1-ol reverses the preference of that theozyme to favour furan formation. This negates the theozyme as a model for the antibody reaction. A new theozyme is reported which favours pyran formation over furan formation consistent with the antibody result. The potential energy surface for the acid and BF₃ catalysed rearrangement of cis- and trans-4,5-epoxyhexan-1-ol involving inversion and retention of configuration at the reaction centre at the HF/6-31G* and B3LYP/6-31G* levels are reported. The preference for furan formation over pyran is attributed to the more favourable O-Cep-O bond angles at the transition structures for furan formation.