Stereochemically Probing the photo-favorskii Rearrangement: A Mechanistic Investigation

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Issue Date
2013-03-01Author
Givens, Richard S.
Rubina, Marina
Stensrud, Kenneth F.
Publisher
American Chemical Society
Type
Article
Article Version
Scholarly/refereed, author accepted manuscript
Rights
This document is the Accepted Manuscript version of a Published Work that appeared in final form in the Journal of Organic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jo301640q.
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Using model (R)-2-acetyl-2-phenyl acetate esters of (S)- or (R)-α-substituted-p-hydroxybutyrophenones (S,R)-12a and (R,R)-12b, we have shown that a highly efficient photo-Favorskii rearrangement proceeds through a series of intermediates to form racemic rearrangement products. The stereogenic methine on the photoproduct, rac-2-(p-hydroxyphenyl)propanoic acid (rac-9), is formed by closure of a phenoxy-allyloxy intermediate 17 collapsing to a cyclopropanone, the “Favorskii” intermediate 18. These results quantify the intermediacy of a racemized triplet biradical 316 on the major rearrangement pathway elusively to the intermediate 18. Thus, intersystem crossing from the triplet biradical surface to the ground state generates a planar zwitterion prior to formation of a Favorskii cyclopropanone that retains no memory of its stereochemical origin. These results parallel the mechanism of Dewar and Bordwell for the ground state formation of cyclopropanone 3 that proceed through an oxyallyl zwitterionic intermediate. The results are not consistent with the stereospecific SN2 ground state Favorskii mechanism observed by Stork, House, and Bernetti. Interconversion of the diastereomeric starting esters of (S,R)-12a and (R,R)-12b during photolysis did not occur thus ruling out leaving group return prior to rearrangement.
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Citation
Givens, R. S., Rubina, M., & Stensrud, K. F. (2013). Stereochemically Probing the photo-Favorskii Rearrangement: A Mechanistic Investigation. The Journal of Organic Chemistry, 78(5), 1709–1717. http://doi.org/10.1021/jo301640q
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