Base catalysed rearrangements involving ylide intermediates. Part 18. Competing [1,2], [1,3], and [1,4] rearrangements of ammonium ylides

Abstract

The [1,2] rearrangements of acyl stabilised ammonium ylides are, in several cases, accompanied by competing [1,3] rearrangements and in one case by a [1,4] rearrangement. For examples involving migrating benzyl or phenylethyl groups the mechanism of these rearrangements has been studied. Thus the competing [1,2], [1,3], and [1,4] rearrangements of the ylide (12) are largely intramolecular, but the intermolecularity is as high as 28% for the [1,2] and [1,3] rearrangements and 14% for the [1,4] rearrangement in methanol at 55 °C. The competing [1,2] and [1,3] rearrangements of the optically active ylide (29a) give products with predominant retention of the configuration of the migrating phenylethyl group, but the intramolecular stereoselectivity of the [1,2] rearrangement is significantly greater than that of the [1,3] rearrangement. These results are consistent with a radical pair pathway for all three modes of rearrangement. Suitably substituted acyl stabilised allylammonium ylides (55) rearrange by competing [1,2], [1,3], [3,2], and [3,3] processes.