STUDIES OF THERMAL REARRANGEMENTS OF METHYL 3-ALKYL-3-METHYL-3H-PYRAZOLE-5-CARBOXYLATES - CONCERTED, STEPWISE, AND UNCLASSIFIED MECHANISM

Studies of the thermal rearrangements of five 3H-pyrazoles are describ ed, in which the migrating groups were methoxymethyl, tert-butyl, 1-ad amantyl, p-methoxybenzyl, and benzyl. On the basis of reaction product s, rearrangement of the (methoxymethyl)pyrazole in benzene, dichlorome thane, 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 benze ne-d(6) and acetone-d(6) at higher temperatures, were determined The t ert-butyl compound also rearranges by a stepwise mechanism in benzene d(6), acetone-d(6), and methanol-d(4) based on the fact that isobutene and tert-butyl methyl ether (in methanol) are coproducts of rearrange ment. The mechanism of rearrangement of the adamantyl and methoxybenzy l systems, in both benzene d(6) and acetone-d(6) solvents, is ambiguou s, 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 meth anol solvent, which suggests a stepwise, ion-pair mechanism for rearra ngement 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 c oncerted mechanism is the fact that it affords only one pyrazole produ ct, whereas the others afford two or more.