Active Role of Hydrogen Bonds in Rupe and Meyer−Schuster Rearrangements

Rupe and Meyer−Schuster rearrangements for the R₂C(OH)C⋮CH + H₃O⁺ and (H₂O)₉ model (R = methyl and phenyl groups) have been investigated by the use of density functional theory calculations. In the substrate R₂C(OH)C⋮CH catalyzed by H₃O⁺(H₂O), three reaction channels, the two rearrangements and S_N (nucleophilic substitution), were predicted by the frontier molecular orbital theory. The S_N (the OH-group exchange) path was found to have a large activation energy. For 2-methylbut-3-yn-2-ol (R = Me), the Rupe rearrangement has been found to be much more favorable than the Meyer−Schuster rearrangement. For 1,1-diphenylprop-2-yn-1-ol (R = Ph), the occurrence of Meyer−Schuster rearrangement is very likely with the small activation energy. Both rearrangements do not involve the carbonium ion intermediates. However, the calculated geometries of the first transition state are carbonium-ion-like. Dehydration and hydration may occur via the intermolecular proton relay along the hydrogen-bond chains. Minimal models were proposed to represent reaction mechanisms of both rearrangements.