Umpolung rearrangement

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

In this mode, an umpolung of reactivity for the nucleophile occurs, leading to many useful transformations. More complex pathways are not unusual these include addition to multiple bonds, vinylic and acetylenic substitution, rearrangements - some involving ring-expansion or ring-contraction - generation of reactive intermediates, etc. Solvent effects, especially solvent participation, may add new dimensions to reactivity. 

Umpolung reactivity is also evident in the addition of an organolithium to a vinyl carbamate 56.34 The product 57 can be quenched with electrophiles at low temperature, but rearranges to 58 by a [l,2]-acyl transfer on warming. 

Sigmatropic rearrangements of allyl sulphonium allyl ylides (75) provide the basis of a repetitive one-pot ring-expansion sequence" for 2-vinyl thiocyclic compounds (Scheme 50). A paper" and a section in a review article" have described the utilization of allyl thiol dianions and allyl sulphide anions in carbonyl group umpolung . 

The attempted formation of (127) by NHC-catalysed annulation of chromene-3-carboxaldehydes (128) was unsuccessful since this umpolung activation proved to favour a rearrangement process giving 3-methyl coumarins (129) in moderate to good yield (Scheme Ib). " ... 

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