Rearrangement of Protonated Propene Oxide to Protonated Propanal

Abstract

Calculations at the MP2/6-31G*//MP2/6-31G* level show there are concerted asynchronous pathways connecting protonated propene oxide and protonated propanal. With cleavage of the C−O bond of protonated propene oxide, the preference for rotation of oxygen away from the more hindered face of the oxirane plane containing the methyl group is quantified as 2 kcal/mol. This 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. The reaction is predicted to show a 20:1 preference for migration of the proton trans to the methyl over the cis.

Molecular Orbital Studies of the Intramolecular Reaction of Protonated cis- and trans-3,4-Epoxypentan-1-ol

“…These two sequential process are preceded by an initial rotation of the methyl to bring an adjacent CH of the methyl into alignment with the electrondeficient center that subsequently develops. 12 Rotation of the methyl group is not observed in the inversion transition structure for intramolecular ether formation for protonated cis-3,4-epoxypentan-1-ol 13 since the hydroxyl attack at carbon occurs early in the reaction and in concert with epoxide CO bond rupture. Carbonyl formation by hydride migration does not compete with furan formation, consistent with the higher calculated activation barrier for hydride migration.…”

“…We now report results of ab initio and density functional calculations of the transition structures for the intramolecular rearrangement of the protonated form of trans-and cis-3,4-epoxypentan-1-ol (9) and (10) involving inversion of configuration at the site of intramolecular nucleophilic attack that result in formation of the protonated form of trans-and cis-2-methylfuran-3-ols (11) and (12) and the corresponding 1-(oxacyclobutan-2′-yl)ethanols. We also report transition structures for formation of 11 and 12 with retention of configuration.…”

“…11 Protonation can occur from either face of the epoxide ring to give the stereoisomeric syn and anti oxonium ions. 11 Calculations carried out on the proton-catalyzed rearrangement of propene oxide 12 show the syn-protonated oxonium ion is marginally higher in energy than the anti diastereomer. Therefore, if hydrogen bonding effects are neglected, protonation of cis-3,4-epoxypentan-1-ol (10) would be expected to be favored on the less hindered face of the epoxide (i.e., 13, Figure 5), while for the trans-3,4-epoxypentan-1-ol (9), the faces are hindered more equally.…”

“…A comparison with corresponding bonds for the transition structure (TS 19-20, Figure 7) for the rearrangement of protonated propene oxide (19) to protonated propanal (20) show that in the latter a suitably disposed CH adjacent to the developing site of electron deficiency increases to a maxima at the transition geometry of 1.13 Å. 12 This bond length extension, indicative of hyperconjugative stabilization, is not observed for either the intramolecular four-or five-membered transition structures ( Figure 5) for inversion of configuration, which is consistent with intramolecular hydroxyl attack being earlier in the reaction coordinate than hydride migrationin the propene oxide rearrangement. Furthermore, the rearrangement of propene oxide exhibits a reaction profile that could be described as asynchronous with two distinct and independent processes that are not linked by the formation of a discrete intermediate ( Figure 7).…”

“…Furthermore, some imaginative calculations have been used to show the effect of a proximate acid source and counterion in influencing transition-state structure, thereby mimicking the regiochemistry dictated by the catalytic antibody. 5 The boron trifluoride-catalyzed reaction of trans-and cis-3,4-epoxypentan-1-ol (9 and 10) in diethyl ether results in the formation of trans-and cis-2-methylfuran-3-ols (11) and (12) (Figure 4). There is no evidence in either reaction for the formation of the more strained four-membered ring ether.…”