Shi Asymmetric Epoxidation Reaction

Shi Asymmetric Epoxidation Reaction - Myers Reviews: Shi...

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R 1 R 2 O O O O O O CH 3 CH 3 H 3 C H 3 C O O O O O CH 3 H 3 C CH 3 H 3 C O O R 1 R 2 R 3 O O O O O CH 3 H 3 C CH 3 H 3 C O O R 3 R 2 R 1 R 1 R 2 O R 1 R 2 H 3 C Ph CH 3 H 3 C Ph CH 3 CH 3 CH 3 H 3 C SO 4 2– O O O O O CH 3 CH 3 H 3 C H 3 C O O R 1 R 2 O H 3 C C 10 H 21 CH 3 Ph CH 3 CH 3 CH 3 O O O O O CH 3 CH 3 H 3 C H 3 C O OO SO 3 O O O O O O CH 3 CH 3 H 3 C H 3 C H 3 C H 3 C CH 3 CH 3 H 3 C CH 3 HSO 5 Ph O O O O O CH 3 CH 3 H 3 C H 3 C OH SO 3 Soojin Kwon Chem 2 15 Shi Asymmetric Epoxidation Reaction Reviews: Wang, Z.-X.; Tu, W; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem. Soc. 1997 , 119 , 11224-11235. Frohn, M.; Shi, Y. Synthesis 2000 , 14 , 1979-2000. It is proposed that the Shi epoxidation proceeds through a dioxirane intermediate and a spiro transition state and that a so-called planar transition state is a main competing pathway. The spiro transition state is believed to be electronically favored as a result of a stabilizing interaction between an oxygen lone pair of the dioxirane with the π * orbital of the olefin. General Transformation: oxone, pH 10.5, base H 2 O, CH 3 CN Myers Ketone 1 can be readily prepared from D -fructose ($15/kg) by ketalization (acetone, HClO 4 , 0 °C, 53%) and oxidation (PCC, 23 °C, 93%). L -Fructose can be prepared in 3 steps from readily available L -sorbose. Ketone 1 can be used catalytically (20–30 mol %). Oxone (a commercial mixture of 2:1:1 KHSO 5 :KHSO 4 :K 2 SO 4 ) is used as the stoichiometric oxidant but H 2 O 2 /CH 3 CN can also be used (peroxyimidic acid is the proposed oxidant). Generally, the optimum pH for dioxirane epoxidation is 7–8. At higher pH, Oxone tends to decompose. However, at pH 7–8 the Shi catalyst decomposes due to to competing Baeyer-Villiger reaction. By increasing the pH to 10.5 (by addition of K 2 CO 3 ), the amount of ketone used can be reduced to a catalytic amount (30 mol %) and the amount of Oxone can be reduced to a stoichiometric amount (1.5 equiv), suggesting that at this pH the ketone is sufficiently reactive to compete with Oxone decomposition.
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This note was uploaded on 04/07/2008 for the course CHEM 215 taught by Professor Shair during the Fall '07 term at Harvard.

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Shi Asymmetric Epoxidation Reaction - Myers Reviews: Shi...

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