Stereochemically Probing the Photo-Favorskii Rearrangement: A Mechanistic Investigation

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• 作者：Richard S. Givens ; Marina Rubina ; Kenneth F. Stensrud
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：March 1, 2013
• 年：2013
• 卷：78
• 期：5
• 页码：1709-1717
• 全文大小：611K
• 年卷期：v.78,no.5(March 1, 2013)
• ISSN：1520-6904

文摘

Using model (R)-2-acetyl-2-phenyl acetate esters of (S)- or (R)-伪-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 鈥淔avorskii鈥?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 proceeds through an oxyallyl zwitterionic intermediate. The results are not consistent with the stereospecific S_N2 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.