Thermal rearrangements of 3H-pyrazoles, 4H-pyrazoles, and cyclopentadienes

Abstract

<p>3H-Pyrazoles generally undergo thermal rearrangement by concerted [1,5]-sigmatropic migrations. It has been discovered that 3H-pyrazoles 111a-e undergo thermal rearrangement by an alterative mechanism. A stepwise, ion-pair, mechanism operates in these rearrangements, in benzene/toluene, chloroform, and methanol solvents. This mechanisms operates because the R sustituents can form relatively stable cations. Large rate enhancements for these rearrangements combined with ion-pair trapping by methanol support this mechanism</p> <p>[Figure Removed]</p> <p>Since, 3H-pyrazoles 111b and 111e rearrange to afford 4H-pyrazoles 120b and 120e, respectively, the thermal rearrangements of these more stable isomers were investigated. Ion-pair trapping experiments with methanol suggest that these 4H-pyrazoles also rearrange by a stepwise mechanism and that different ion-pair intermediates are formed from these 4H-pyrazoles compared to the isomeric 3H-pyrazoles.</p> <p>The sensitivity of the stepwise mechanism to the stabilization of the pyrazoles nucleus was explored in the study of the thermal rearrangements of 128b-e. Based on cation trapping experiments with methanol it appears that 3H-pyrazoles 128b-e rearrange by a stepwise mechanism in methanol, but with "tighter" ion pair intermediates. A concerted mechanism, with some charge separation, seems to operated in the thermal rearrangements of 128f.</p> <p>The generality of rearrangement by a stepwise mechanism was explored by studying the thermal rearrangements of cyclopentadienes 148 a-e. Cyclopentadienes 148 a-d undergo stepwise rearrangement in methanol solvent, based on ion-pair trapping experiments, while cyclopentadiene 148e rearranges by concerted [1,5]-sigmatropic migrations of carbomethoxy.</p> <p>This work has shown that thermal rearrangements, where a concerted sigmatropic mechanism can in theory operate, do not necessarily proceed by this mechanism. Furthermore, transition states for such rearrangements seem to run from those with little or no charge seperation, through transition states with increasing charge separation, to the two-step, ion-pair extreme.</p>