Transition metal-catalyzed epoxidation

Kola (Cola acuminata) extract astringent, skin treatment Zinc oxide astringent, toners Kola (Cola acuminata) extract astringent, topical Aluminum chloride hexahydrate astringent, topical hexahydrate Aluminum chloride anhydrous astringent, veterinary medicine Lead acetate trihydrate asymmetric epoxidation, transition metal catalyzed trans-Stilbene a-terpineol precursor 2-(4-Methyl-3-cyclohexenyl)-2-propanol atmosphere protectant, casting magnesium alloys... 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

Oxidants Available for Selective Transition Metal-catalyzed Epoxidation... 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Table 6.12 Transition metal-catalyzed epoxidation of olefins with H202 as terminal oxidant.

Transition Metal-Catalyzed Epoxidation of Alkenes. Other transition metal oxidants can convert alkenes to epoxides. The most useful procedures involve f-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium or... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

Hi. Cr, Mo, W. In contrast to group IV and V transition metals, the catalytic active oxidant is of another type for group VI transition metal-catalyzed epoxidations The transition-metal-oxo complexes, in which the oxygen that is transferred is bonded to the metal via a double bond, are the active oxidizing species. 

This week s Highlights focuses on three transition metal-catalyzed reactions. Jin-Quan Yu of Cambridge University reports (Organic Lett. 2003,5,4665-4668) that Pd nanoparticles catalyze the hydrogenolysis of benzylic epoxides. The reaction proceeds with inversion of absolute configuration (1 —> 2). 

Jacobsen, E. N. Transition Metal-catalyzed Oxidations Asymmetric Epoxidation. In Comprehensive Organometallic Chemistry IT, Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 1097-1136, and references cited therein. 

This is mainly due to the fact that by means of chiral ligands it is comparatively facile to transfer absolute stereochemical information to a cat-alytically active metal center. However, the success of some of these reactions (e.g. the Sharpless asymmetric epoxidation or the Noyori hydrogenation) must not hide the fact that the number of powerful transition metal-catalyzed C-C coupling reactions, which proceed reliably with high enantioselectivity, is still rather small. 

The selectivity pattern of d° transition metal catalyzed epoxidations is much less readily understood. In the molybdenum-catalyzed epoxidation of (S)-limonene (Table 2, entries 1 and 2) the cis selectivity could perhaps be explained by a directing effect due to -coordinating of the second double bond to molybdenum. Such a selectivity is completely missing in the analogous tungsten-catalyzed reaction of (S)-limonene (Table 2, entry 4) in the absence of a second double bond as, for example, in 3/(-acetoxy-5-cholestene (Table 4) reactions with both metals afford similar diastereomeric ratios. 

Little is known about transition metal catalyzed epoxidation of simple allylic amides. The easily removable trichloroacetyl group is suitable as /V-substituent in the molybdenum-catalyzed epoxidation of (Z)-allylic systems with rm-butyl hydroperoxide, which produces the. sv -com-pound with appreciable selectivity21. 

Recent advances of the preparation of novel optically active organoselenimn compounds, mainly organic diselenides, and their application as chiral ligands to some transition metal-catalyzed reactions and also as procatalysts for asymmetric diethylzinc addition to aldehydes are reviewed. Recent results of catalytic reactions using some organoselenimn compounds such as aUylic oxidation of alkenes and its asymmetric version as well as epoxidation of alkenes are also summarized. 

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