Copper-catalyzed epoxidation

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Starting with a copper catalyzed epoxide ring opening, using phenyl magnesium chloride, a secondary alcohol was generated, which in turn was transformed into an electrophile by tosylation (Scheme 29). The tosylate 117... 

An unusual example of copper-catalyzed epoxidation using substoichiometric amounts of soluble Cu(ll) compounds in methylene chloride uses MCPBA as a terminal oxidant. As a mixture of cis- and /ra j -epoxides can be obtained from a single stereoisomeric alkene, a stepwise radical mechanism is suggested <2000TL1013>. 

Figure 9 (A) In situ generation of dimethyldioxirane and subsequent epoxidation of c/s-l,4-PBD and (B) copper-catalyzed epoxidation of hydroxyl-terminated PBD, which leaves hydroxyl end groups intact.

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

The alkynyl epoxide 73 undergoes a copper-catalyzed reductive metallation by wBuLi [81, 83]. The resultant allenyllithium compound 74 is a versatile intermediate and reacts with various electrophiles (Scheme 3.37). 

The use of vinyl epoxides as substrates in enantioselective copper-catalyzed reactions, on the other hand, has met with more success. An interesting chiral ligand effect on Cu(OTf)2-catalyzed reactions between cyclic vinyloxiranes and dialkylzinc reagents was noted by Feringa et al. [51]. The 2,2 -binaphthyl phosphorus amidite ligands 32 and 43 (Fig. 8.5), which have been successfully used in copper-catalyzed enantioselective conjugate additions to enones [37], allowed kinetic resolution of racemic cyclic vinyloxiranes (Scheme 8.26). 

Introduction of the silylmethyl group into organic halides, tosylates, and epoxides is achieved by nickel, palladium-, or copper-catalyzed cross-coupling reactions. 

Copper-catalyzed asymmetric allylic alkylations with organometalUc reagents used for small-ring openings, such as epoxides and aziridines, have gained interest. Illustrating... 

Addition of 10% Cul to the reaction of allylic Grignard reagents with epoxides exerts a marked efiect on the regioselectivity. The catalyst also increases the rate of reaction. The a-products are also the main products when sufficient Cul is added to form the stoichiometric dialkyl cuprate of the Grignard reagent, but yields are lower than in the copper-catalyzed reaction. ... 

Condensation with carbonyl compounds. Formation of epoxides from aldehydes by reaction with sulfonium ylides is subject to asymmetric induction. The latter species have been generated from 91, 92, and 93, and also those derived from monoterpenes, e.g., 94 " and 95.- Of course the ylides can be obtained in situ by deprotonation of sulfonium salts or copper-catalyzed decomposition of diazoalkanes (with the carbenoids trapped by the sulfides). 

Copper, titanium, cobalt and iron substituted mesoporous silicas (Cu-, Ti-, Co-, and Fe-HMS) were synthesized with dodecylamine surfactant as templating reagent. Three assembled pathways were used to bond Ti tartrate complex over mesoporous silicas (HMS). The above described catalysts were characterized by XRD and FT-IR, their metal loadings were measured by chemical analysis method. In catalytic testing, Cu-HMS and especially Fe-HMS show the best catalytic activity for hydroxylation of phenol with H2O2 in the presence of water. Ti-HMS and especially Ti tartrate complex assembled HMS catalysts exhibit the best epoxidative activity for catalyzing epoxidation of styrene with rcrt-butyl hydroperoxide. 

---