The Anionic Oxy-Cope Rearrangement:  Using Chemical Reactivity to Reveal the Facile Isomerization of the Parent Substrates in the Gas Phase

The rearrangements of 1,5-hexadiene-3-oxide and 3-methyl-1,5-hexadiene-3-oxide have been studied in the gas phase, using both Fourier transform mass spectrometry (FTMS) and the flowing afterglow (FA) technique. Gas-phase studies of ionic rearrangements can be limited by analysis techniques such as collision-induced dissociation, which have the potential of driving the rearrangement prior to fragmentation. In the studies reported here, we have utilized methanol-O-d, methyl nitrite, and dimethyl disulfide as chemical reactivity probes to discern whether rearrangement of either of the alkoxides to their corresponding enolates occurs. Of the three structural probe reagents, dimethyl disulfide has been found to be most ideal, since it reacts efficiently with both alkoxides and enolates to produce a unique product from each. On the basis of the reactions observed between dimethyl disulfide and anions generated from 1,5-hexadien-3-ol and 3-methyl-1,5-hexadien-3-ol, we have found that the gas-phase Cope rearrangement of both tertiary and secondary alkoxides occurs under both FTMS and FA conditions. Use of dimethyl disulfide in the FTMS and evaluation of ion residence time in the FA lead to the establishment of an upper limit on the ΔH ‡ of the rearrangement of both the parent secondary and tertiary substrates as ∼11 kcal mol-1 at 298 K. This value is consistent with our B3LYP/6-31+G* prediction. The rearrangement is also faster in the gas phase than in solution, in accord with theoretical predictions.