Abstract
A recent computational analysis of the stabilizing intramolecular OH⋯O contact in 1,2-dialkyl-2,3-epoxycyclopentanol diastereomers has been extended to thiiriane, aziridine and phosphirane analogues. Density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2) and CCSD(T) coupled-cluster computations with simple methyl and ethyl substituents indicate that electronic energies of the isomers are lowered by roughly 3 to 4 kcal mol−1 when the OH group of these cyclopentanol systems forms an intramolecular contact with the O, S, N or P atom on the adjacent carbon. The results also suggest that S and P can participate in these stabilizing intramolecular interactions as effectively as O and N in constrained molecular environments. The stabilizing intramolecular OH⋯O, OH⋯S, OH⋯N and OH⋯P contacts also increase the covalent OH bond length and significantly decrease the OH stretching vibrational frequency in every system with shifts typically on the order of −41 cm−1.
Highlights
Known as the epoxy alcohol-aldol rearrangement [1], the Type III semipinacol rearrangement reaction [2,3] is the Lewis acid-mediated conversion of 2,3-epoxyalcohols to the corresponding β-hydroxycarbonyl (Figure 1)
Full geometry optimizations were performed on all 36 structures with the M06-2X [51] global hybrid density functional theory (DFT) method and two sets of correlation consistent triple zeta basis sets, one without and one with diffuse functions on all atoms
M06-2X harmonic vibrational frequencies were computed with the TZ and aTZ basis sets for every optimized structure to ensure they are minima with no imaginary frequencies
Summary
Known as the epoxy alcohol-aldol rearrangement [1], the Type III semipinacol rearrangement reaction [2,3] is the Lewis acid-mediated conversion of 2,3-epoxyalcohols to the corresponding β-hydroxycarbonyl (Figure 1). Additional, proposed mechanistic details for Type III semipinacol rearrangements [17,18,19,20] are not supported by experimental or theoretical evidence. This apparent void sparked an initial interest in investigating why 2,3-epoxyalcohols rearrange to the corresponding ketols
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