Studies of thermal rearrangements of methyl 3-alkyl-3-methyl-3H-pyrazole- 5-carboxylates. Concerted, stepwise, and unclassified mechanisms

Article abstract of DOI:10.1021/jo00081a028

Studies of the thermal rearrangements of five 3H-pyrazoles are described, in which the migrating groups were methoxymethyl, tert-butyl, 1-adamantyl, p- methoxybenzyl, and benzyl. On the basis of reaction products, rearrangement of the (methoxymethyl)pyrazole in benzene, dichloromethane, and methanol at room temperature occurs by a stepwise mechanism, involving discrete ion-pair intermediates. Observed first-order rate constants, for the rearrangement of four of the 3H-pyrazoles in benzene-d₆ and acetone-d₆ at higher temperatures, were determined. The tert-butyl compound also rearranges by a stepwise mechanism in benzene-d₆, acetone-d₆, and methanol-d₄ based on the fact that isobutene and tert-butyl methyl ether (in methanol) are coproducts of rearrangement. The mechanism of rearrangement of the adamantyl and methoxybenzyl systems, in both benzene-d₆ and acetone-d₆ solvents, is ambiguous, the distinction between stepwise, with tight ion-pair intermediates, and concerted, with some charge separation, being unclear. However, both afforded a methyl ether in low yield during rearrangement in methanol solvent, which suggests a stepwise, ion-pair mechanism for rearrangement in that medium. No methanolysis product could be detected from the rearrangement of the benzyl compound in methanol, which suggests concerted rearrangement. Additional evidence that it rearranges by a concerted mechanism is the fact that it affords only one pyrazole product, whereas the others afford two or more.