Base catalysed rearrangements involving ylide intermediates. Part 2. The Stevens [1,2] and [3,2] sigmatropic rearrangements of allylic ammonium ylides
The base catalysed rearrangement of the quaternary ammonium salts (9) usually gives the [3,2] sigmatropic rearrangement product (10) rather than the Stevens [1,2] rearrangement product (11). An earlier claim has been corrected : the corresponding reaction of the acetylenic ammonium chloride (16) gives the allene (18). These results are discussed in terms of a competition between the symmetry-allowed [3,2] sigmatropic rearrangement and the non-allowed Stevens [1,2] rearrangement which is formulated either as proceeding via a radical pair intermediate or as being a concerted symmetry-forbidden process.