Base-catalysed ring openings of 1,2-diphenylcycloalkanols having five-, six-, seven- and eight-membered rings

Abstract

trans-Isomers of 1,2-diphenylcyclopentanol, 5, 1,2-diphenylcyclohexanol, 6, 1,2-diphenylcycloheptanol, 7, and 1,2-diphenylcyclooctanol, 8, have been prepared as have their acyclic analogues, threo- and erythro-3,4-diphenylhexan-3-ol, 9. All structural assignments are confirmed by X-ray crystal structure determinations and experimentally determined structures are compared with the results of empirical force field calculations which also yield strain energies for each of the compounds. With alkali metal dimsyl–dimethyl sulfoxide or 1,3-diaminopropane with its potassium salt as base, the cycloalkanols are isomerised to enolates of corresponding 1,n-diphenylalkan-1-ones, and the acyclic alkanols cleaved to propylbenzene and the enolate of propiophenone. The products are consistent with a polar mechanism involving collapse of alkoxide to expel a benzylic carbanion, followed by one or more proton transfers to yield the observed products. Rates increase in the order, 6 < 5 7 < 9 < 8, with a spread in reactivities of ca. 10⁶. Logarithms of relative rates correlate poorly with estimates of strain release in the reactions. Correlations are improved by incorporation of estimates of entropy changes associated with ring opening or cleavage, but remain poor. The fate of isotopic labels in the reactions of 2,n,n-trideuterio-1,2-diphenylcycloalkanols. [²H₃]-5, [²H₃]-7 and [²H₃]-8, shows that protonation of the benzylic carbanion is by solvent DMSO for the cyclooctanol, [²H₃]-8, and that competing intramolecular proton transfer occurs in the cycloheptanol, [²H₃]-7, and cyclopentanol, [²H₃]-5. Kinetic isotope effects associated with the labelling patterns are consistent with a change in rate-limiting step from the initial carbon–carbon bond cleavage in the case of 8, to rate-limiting proton transfer in the case of 5.