Photogeneration and chemistry of a-hydroxy-substituted arylmethyl carbanions
Date
1990
Authors
Xu, Xigen
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Abstract
The photoretro-aldol reactions of nitrophenyl ethylene glycols and photodecarboxylation of a-hydroxyarylacetate ions and the ir derivatives have been studied. Both reactions are highly efficient only in aqueous solution. There is strong evidence for the generation of a-hydroxyaryl carbanion intermediates in both reactions.
a-Hydroxy nitroaryl carbanions generated in the photoretro-aldol reaction are not protoned at all, but instead lose an electron a nd a hydrogen atom to give the corresponding nitre arylaldehydes. The nitro arylaldehydes have been found to accept the electron released by the initially photogenerated carbanion intermediate, and are reduced to azoxy aldehydes as final secondary products.
It was discovered that not only a-hydroxy nitroary l carbanions, but also other nitrobenzyl carbanions, generated either photochemically or thermally, can also reduce nitro arylaldehydes and other nitroaromatic compounds. The enolate ion of p-ni troacetophenone (43) and derivatives have been found to transfer the electron to unionized 43 and itself are oxidized to p-nitrobenzoic acid (47). The radical anion of 43 formed is further reduced to give p-aminoacetophenone (59) so that the net result is a base-catalyzed disproportionation of 43 to 59 and 47.
a-Hydroxy arylacetate ions (44 and 76-78) were found to have unusually high quantum yields of photodecarboxylation, in comparison with the parent a-arylacetate ions. The photodecarboxylation is highly pH-dependent; the pH not only affects the reactivity, but also controls the product distribution. Various studies show that a-hydroxyaryl carbanions are the photochemically generated primary intermediates in this reaction.