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37.2.1.4.2 Method 2: Manganese-Mediated Asymmetric Epoxidation

DOI: 10.1055/sos-SD-037-00140

Adolfsson, H.Science of Synthesis, (200837251.

A breakthrough in the area of asymmetric epoxidation came in the beginning of the 1990s when the groups of Jacobsen and Katsuki more or less simultaneously discovered that chiral manganesesalen complexe 23 catalyzed the enantioselective formation of epoxides (Scheme 28).[‌87‌‌89‌] The discovery that simple nonchiral manganesesalen complexes could be used as catalysts for alkene epoxidation was already established about five years earlier, and the typical terminal oxidants used with these catalysts closely resemble those of the porphyrin systems.[‌54‌] In contrast to the titanium-catalyzed asymmetric epoxidation discovered by Sharpless, the manganesesalen system does not require precoordination of an allylic or homoallylic alcohol to the catalyst, hence unfunctionalized alkenes can be efficiently and selectively oxidized. The enantioselectivity has been shown to be highly sensitive toward the substitution pattern of the alkene substrate. Excellent selectivity (>90% ee) is obtained for aryl- or alkynyl-substituted terminal, Z-disubstituted, and trisubstituted alkenes, especially using the second-generation catalysts (here exemplified by complex 24),[‌90‌] whereas E-disubstituted alkenes are epoxidized with low rates and low enantiomeric excesses (<40%) (Scheme 29).

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Meeeeeee (eee%) Meeeeee (eeeee) Meeeeeeee (eee%) Meeeeeeeee ee (%) Meeee (%) Mee
88 (8.8) MeMMe 8-MMMM (88) MeMe, 8°M 88 88 [‌888‌]
88 (8.8) MeMMe (8) 8-MMMM (88) MM8Me8, 8°M 88 88 [‌88‌]
88 (88) MeMMe (8) MM8Me8, ee 88 88 [‌88‌]
88 (88) 88% M8M8 (88) MM8Me8/M8M (8:8), 8°M 88 88 [‌88‌]
88 (88) 8% M8M8 (88) MM8Me8/M8M (8:8), 8°M 88 88 [‌88‌]
88 (8) 88% M8M8 (8.88) MMM (8.8) MM8Me8/MeMM (8:8), 8°M 88 88 [‌88‌]
88 (8.88.8) MMMe (8.8) MM (8) MM8Me8/MMM (88:8), 88°M 88 88 [‌888‌]
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References