Metal-catalyzed reactions epoxidation

The introduction of various metal-catalyzed reactions, however, remarkably expanded the scope of the epoxidation of Q,.3-unsaturatcd ketones. Enders et al. have reported that a combination of diethylzinc and A-methyl-pseudoephedrine epoxidizes various o,. j-unsaturatcd ketones, under an oxygen atmosphere, with good to high enantioselectivity (Scheme 23).126 In this reaction, diethylzinc first reacts with the chiral alcohol, and the resulting ethylzinc alkoxide is converted by oxygen to an ethylperoxo-zinc species that epoxidizes the a,/3-unsaturated ketones enantioselectively. Although a stoichiometric chiral auxiliary is needed for this reaction, it can be recovered in almost quantitative yield. 

Compared to metal-catalyzed asymmetric epoxidation reactions, asymmetric versions of this reaction without the need of a catalyst (apart from a base) are rarely known. In 2000 Adam and coworkers reported a method for the asymmetric Weitz-Scheffer epoxidation of substituted enones 91 by the secondary, optically active hydroperoxide (5 )-(l-phenyl)ethyl hydroperoxide (equation 27, Table 10). ... 

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). 

Metal-catalyzed reactions of C02 and epoxides that give polycarbonates and/or carbonates have been extensively investigated as a potentially effective C02 fixation (Beckman, 1999 Inoue, 1987). The possible reaction mechanism is illustrated in Figure 3.8 (Darensbourg et al., 1999). The repetition of the reaction sequence in which C02 inserts into a metal-alkoxide bond, followed by ring-opening of the epoxide with the metal carbonate forms the alternating copolymer. In 1969, this copolymerization was first reported by Inoue and Tsuruta who used a Zn catalyst derived from... 

Alkenylboron compounds cyclopropanations, 9, 181 haloetherification, 9, 182 hydrogenation and epoxidation, 9, 182 metal-catalyzed reactions, 9, 183 metallic reagent additions, 9, 182 via radical addition reactions, 9, 183 5-Alkenylboron compounds, cross-coupling reactions, 9, 208 Alkenyl complexes with cobalt, 7, 51 with copper, 2, 160, 2, 174 with Cp Re(CO) (alkene)3 , 5, 915-916 with dicarbonyl(cyclopentadienyl)hydridoirons, 6, 175 with gold, 2, 255... 

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. 

A further interesting extension of the allyl alcohol epoxidation reaction is the metal-catalyzed direct hydroxy-epoxidation of alkenes. In such a reaction an ene reaction with singlet oxygen, 02, or Schenk reaction forming an ally lie hydroperoxide is combined with metal-catalyzed intramolecular epoxidation to form epoxy alcohols For example, with 2,5-dimethyl-2,4-hexadiene (83) two different isomers, 84 and 85, were obtained with Ti(OPr-/)4 and VO(acac)2, respectively (equation 16). Similar reactions were also car-... 

Oligoethylene glycols with symmetrically substituted terminal alkyl substituents have been prepared by the sodium-metal-catalyzed reaction of a lower oligoethylene glycol with 2 eq of a substituted epoxide (Bradshaw and Jolley, 1979 Jolley and Bradshaw, 1980 McDaniel et al., 1989). The reactions... 

Table 3.5. Alkene Epoxidation by Transition Metal Catalyzed Reaction with Alkyl Hydroperoxides SS...

An example of using deuterated reactants for detailed kinetic studies of transition metal catalyzed reactions in ionic hquids vras contributed by Abu-Omar and coworkers. They studied the Rh-catalyzed epoxidation of olefins at ambient temperatures using [Dglstyrene and [Dio]-cylcohexene [71]. They also applied H-NMR experiments of [D3]-diperoxorhenium, formed in situ by reaction of [D3]-methyltrioxorhenium and urea hydrogen peroxide (UHP) to determine rate constants in single turnover experiments. 

There has not been much research in the area of metal-catalyzed reactions of thiiranes, which are sulfur analogues of epoxides. It has been shown that chlorodicarbonylrhodium(I) dimer can catalyze the homogeneous desulfurization of thiiranes to olefins [35]. This reaction is stereospecific and proceeds for thiiranes having aliphatic, aromatic, ether, and ester substituents (Equation (6.8)). This chemistry contrasts with that observed with cobalt. At room temperature, when 2-phenylthiirane, was treated with CO (latm), CH3I, KOH, CgHg, tetrabutylam-monium bromide, and a catalytic amount of cobalt carbonyl, fl-mercapto acid, was... 

The metal complexes [MCl(CO)(Ph3P)2] (M = Rh, Ir) catalyze epoxidation of tetramethylethylene with tert-hutyl hydroperoxide in good yield and the selectivity was better 90%) with rhodium than iridium, equation (285). In this case a reasonable mechanism for epoxide formation involves epoxidation of unreacted olefin with the intermediate aUylic hydroperoxide, XXXIX. The allylic hydroperoxide was found to reach levels in excess of 11% during the course of the metal catalyzed reactions of tetramethylethylene with oxygen [470], equation (284). 

Previous reviews have dealt with metal-catalyzed [93] and stoichiometric [94] oxidation of amines in a broad sense. This section will be limited to the selective oxidation of tertiary amines to N-oxides. Amine N-oxides are synthetically useful compounds [95, 96] and are frequently used as stoichiometric oxidants in osmium-[97-99] manganese- [100] and ruthenium-catalyzed [101,102] oxidations, as well as in other organic transformations [103-105]. Aliphatic tert-amine N-oxides are usefid surfactants [96] and are essential components in hair conditioners, shampoos, toothpaste, cosmetics, and so on [106]. Chiral N-oxides have been used in asymmetric catalysis involving metal-free catalytic transformations [107] as well as metal-catalyzed reactions where the N-oxide serves as a ligand [107, 108]. Chiral tertiary amine N-oxides were recently used as reagents in asymmetric epoxidation of a,(3-unsaturated ketones [109]. 

In this chapter, we discuss theoretical studies of some selected transition metal-catalyzed reactions of carbon dioxide to illustrate how important concepts and insights can be derived as a result of these studies. These selected reactions include hydrogenation of CO2 with Hj, coupling reactions of COj and epoxides, reduction of CO2 with organoborons, carboxylation of olefins with COj, and hydrocarboxy-lation of olefins with CO2 and Hj. They are fundamentally important reactions of carbon dioxide and have been intensively investigated experimentally and theoretically. This chapter is not intended to be a comprehensive review. Instead, we discuss the above-mentioned selected examples that we believe to be representative and important in the area of homogeneous catalysis of COj by transition metals from our own perspective. 

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