Journal of the Chemical Society. Perkin transactions I p. 1953 - 1962 (1981)
Update date:2022-08-03
Topics:
Mageswaran, Sivapathasuntharam
Ollis, W. David
Sutherland, Ian O.
The allylic ammonium ylides (22) and (47) and sulphonium ylide (53) are isolable, usually as crystalline solids, because their <3,2> sigmatropic rearrangements are inhibited by the strain associated with the bicyclic transition state of a concerted reaction mechanism.The tetrahydropyridinium (22) and dihydrothiopyranium (29) ylides undergo a thermal <3,2> rearrangement, but the pyrrolinium ylide (47) rearranges by a <1,2>, rather than a <3,2>, sigmatropic pathway.The dihydrothiophenium ylide (53) does not undergo either a <1,2> or <3,2> sigmatropic rearrangement but instead reacts in a bimolecular fashion to give eventually buta-1,3-diene, 2,5-dihydrothiophen, and the heterocycle (55).The <3,2> rearrangements of the ylides (22) and (29) follow predominantly an endo-pathway leading to a single diastereomer of the products (23) and (31); this strong endo preference is not shown by analogous acyclic ammonium ylides.
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