Benzylic olefins, epoxidation using

Dihydronaphthalene is often used as a model olefin in the study of epoxidation catalysts, and very often gives product epoxides in unusually high ee s. In 1994, Jacobsen discovered in his study on the epoxidation of 1,2-dihydronaphthalene that the ee of the epoxide increases at the expense of the minor enantiomeric epoxide.Further investigation led to the finding that certain epoxides, especially cyclic aromatically conjugated epoxides, undergo kinetic resolution via benzylic hydroxylation up to a krei of 28 (Scheme 1.4.9). 

The method is quite useful for particulary active alkyl halides such as allylic, benzylic, and propargylic halides, and for a-halo ethers and esters, but is not very serviceable for ordinary primary and secondary halides. Tertiary halides do not give the reaction at all since, with respect to the halide, this is nucleophilic substitution and elimination predominates. The reaction can also be applied to activated aryl halides (such as 2,4-dinitrochlo-robenzene see Chapter 13), to epoxides,217 and to activated olefins such as acrylonitrile, e.g.,... 

Precursor of Useful Chiral Ligands. OPEN is widely used for the preparation of chiral ligands. Organometallic compounds with these ligands act as useful reagents or catalysts in asymmetric induction reactions such as dihydroxylation of olefins, transfer hydrogenation of ketones and imines, Diels-Alder and aldol reactions, desymmetrization of meso-diols to produce chiral oxazolidinones, epoxidation of simple olefins, benzylic hydroxylation, and borohydride reduction of ketones, imines, and a,p-unsaturated carboxylates. ... 

A general synthesis for all diastereomeric L-hexoses, as an example for monosaccharides that often do not occur in the chiral pool, has been worked out. The epoxidation of allylic alcohols with tertiary butyl hydroperoxide in presence of titanyl tartaric ester catalysts converts the carbon-carbon double bond stereose-lectively to a diol and is thus ideally suited for the preparation of carbohydrates. The procedure is particularly useful as a repetitive two-carbon homologiza-tion in total syntheses of higher monosaccharides and other poly hydroxy compounds. It starts with a Wittig reaction of a benzylated a-hydroxy aldehyde with (triphenylphosphoran-ylidene)acetaldehyde to produce the olefinic double bond needed for epoxidation. Reduction with sodium-borohydride... 

Epoxidation of stilbenes and other alkenes occupies a great deal of attention. Asymmetric epoxidation is an industrially important method for synthesizing epoxides from readily available olefins. In particular, the use of coordination complexes of transition metals as catalysts is of abiding importance, as it proffers an effective possibility for the synthesis of enantiomericaHy pure compounds ((19, 20] references therein). The manganese(111) complex with a diamide ligand was found to catalyze both the epoxidation of (Z)- and ( )-sti]benes with high conversion and the oxidation of benzyl alcohol to benzaldehyde (Figure 2.3) [21]. 

Lynch and co-workers from the Merck Research Laboratories made elegant use of the Jacobsen system in their synthesis of CDP840 (Scheme 14.53). Epoxidation of tri-substituted alkene 134 with catalyst 135 furnished 136 in 89% ee and 58% yield. The stereo-outcome is dictated by formation of the lower energy Z-olefin intermediate. Epoxide 136 was further elaborated to secondary alcohol 137 in a particularly unusual gin-selective reduction of the epoxide with LiBH4, under the influence of Lewis acidic reductant borane. While the mechanism of this reduction is unknown, it may be due to carbocation formation at the doubly benzylic centre, followed by internal delivery of hydride directed by the in situ generated alkoxide. 

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