Asymmetric epoxidation radicals

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

Asymmetric cyclopropanation, 57, 1 Asymmetric dihydroxylation, 66 2 Asymmetric epoxidation, 48, 1 61, 2 Asymmetric reduction, 71, 1 Asymmetric Strecker reaction, 70, 1 Atom transfer preparation of radicals, 48, 2... 

On the other hand, the addition of a quaternary ammonium salt to the reaction medium accelerates the isomerization of the radical intermediate [36]. Thus, the epoxidation of c/j-stilbene in the presence of A -benzylquinine salt gives rranr-stilbene oxide with 90% ee as major product (Table 6B.1, entry 24). This protocol provides an effective method for the synthesis of trans-epoxides. In contrast to the epoxidation of c/s-di- and tri-substituted olefins for which complexes 11-13 are the catalysts of choice, the best catalyst for the epoxidation of tetra-substituted conjugated olefins varies with substrates (Table 6B.1, entries 27 and 28) [37]. The asymmetric epoxidation of 6-bromo-2,2,3,4-tetramethylchromene is well-promoted by complex 14 and that of 2-methyl-3-phenylindene, by complex 12a. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

Since 2000 a few catalysts for asymmetric epoxidation based on polymer-anchored chiral l,l -bi-2-naphthol (BINOL) have been developed. Polystyrene-supported BINOL was prepared by radical copolymerization of styrene with BINOL, bearing 4-vinylbenzyloxy groups in the 3- or 6-position [96]. Immobilization of lanthanum or ytterbium was accomplished by treatment of the polymers... 

Manganese has a rich history within the field of organic synthesis (363, 364). For example, the permanganate anion has long been used as an oxidant to produce a variety of products (363). Manganese111 acetate also has been extensive explored over the years for the initiation of free radical reactions that lead to carbon-carbon bond formation. These topics have been reviewed and will not be presented further here (363, 364). Manganese chemistry, however, has made an impact in other areas as well, notably the asymmetric epoxidation of alkenes. 

Fu H, Newcomb M, Wong CH. 1991. Pseudomonas oleovorans monooxygenase catalyzed asymmetric epoxidation of allyl alcohol derivatives and hydroxylation of a hypersensitive radical probe with the radical ring-opening rate exceeding the oxygen rebound rate. JAm Chem Soc 113 5878-5880. 

Benzylic C-H bonds undergo oxidative esterification with TBHP in the presence of tetrabutylammonium iodide as catalyst and carboxylic acids in good to excellent yields. A free radical process has been proposed. Asymmetric epoxidation of electron-poor terminal alkenes bearing different carbonyl groups has been achieved with a cinchona thiourea/TBHP system. The corresponding epoxides, containing a quaternary stereocentre, were isolated in yields up to 98% and enantioselectivity up to 99%. A direct oxidative CDC of indole with A-aryltetrahydroisoquinolines in the 0 presence of a gold catalyst and TBHP resulted in the formation of a variety of alkylated heteroarenes (Scheme 24). ... 

With chiral catalyst 47d regioselective reductive ring opening reactions of racemic epoxides can be achieved, For alkynyl epoxy ethers it leads to effective asymmetric 5-exo cyclization of one radical, but reduction of the other [161, 162]. 

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