Cycloaddition Reactions of Silyldiazoalkanes Involving Rearrangements from Carbon to Nitrogen

The products obtained from the 1,3 cycloadditions of a variety of silyldiazoalkanes, R3SiCN2R′, to several alkenes were studied: the data are consistent with the initial formation of the expected Δ1-pyrazoline. However, when R′ = Ph, COOEt, but not Me, a thermal 1,3 rearrangement of the silyl group from carbon to nitrogen subsequently occurs. Base-catalyzed isomerizations of the Δ1 to Δ2-pyrazolines were noted, as was the desilylation of the Δ1-pyrazolines in refluxing methanol. Mechanisms are proposed for the various reactions.