Epoxidation with metal

Metallic halide salts. The present state of knowkdgt < nu-oeming reactions of epoxides with metallic halide salts is due in Jhj g< measure to the work of MoorwcJn and oo-workers1131-1UI in the fielil of oxonium compounds. 

Pradilla et al. have shown that simple p-tolyl vinyl sulfoxides undergo nucleophilic epoxidation with metal alkyl peroxides to give enantiopure sulfinyl oxiranes.138 This process takes place with fair to excellent diastereoselectivities. The same group recently reported the epoxidation of diastereomeric hydroxy vinyl sulfoxides, bearing an additional stereocenter adjacent to the reactive carbon-carbon double bond. Hydroxy vinyl sulfoxides 256 and 258 underwent epoxidation with lithium ferf-butyl peroxide with high anti selectivity. However, when potassium ferf-butyl peroxide was used, only hydroxy vinyl sulfoxide 256 showed anti... 

Presumably, a PET is involved also in the cationic photopolymerization of epoxides with metal arene complexes [181, 182]. 

Chemical analysis of the lower molecular weight polymers produced by polymerization of epoxides with metal alkoxides in DMSO shows that sulfur is present [5] in low concentrations (< 0.4 percent) and this would arise from initiation involving DMSO, that is, pre-initiation [Eq. (10.17)] followed by ... 

Whilst the Sharpless epoxidation with titanium catalysts and the Jacobsen-Katsuki epoxidation with manganese(salen) complexes are at the forefront of enantioselec-tive epoxidation with metal catalysts, there are alternative systems available. Ruthenium pyridinebisoxazoline (PYBOX) complexes have been independently reported, using either phenyliodinium diacetate or sodium periodate as... 

S. S. Balula, A. C. Coelho, S. S. Braga, A. HazeU, A. A. Valente, M. Pillinger, J. D. Seixas, C. C. Romao, I. S. Gongalves, Influence of cyclodextrins on catalytic olefin epoxidation with metal-carbonyl componnds, crystal structure of the TRIMEB complex with CpFefCOl Cl, Organometallics, 2007, 26, 6857-6863. 

Several research groups reported that various ILs could be effective cocatalysts in the copolymerization of CO2 and epoxides with metal salen or metal porphyrin complexes [126-130]. In some cases, it was shown that the activities of theses metal complexes were drastically enhanced by the co-presence of IL, although they had no or a very low activity for the coplymerization in the absence of IL. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Displacement of activated chlorine atoms also proceeds with certain types of organic compounds, but only in the presence of Lewis acid catalysts. Particular examples include epoxides, polyhydric alcohols, trialkylphosphites (12), and P-aminocrotonates (13). These additives are commonly used in conjunction with metallic stabilizers to provide complete, high performance, commercial stabilizer packages. 

The chemistry of a-metalated epoxides and aziridines (the a prefix will from now on not be included but should be assumed) has been reviewed previously [1], but in this chapter it is our intention to focus on those reactions involving them that are useful in synthesis, rather than just of pedagogical interest. Beginning with metalated epoxides, since the greater amount of work has involved them, we intend to present carefully chosen examples of their behavior that delineate the diverse nature of their chemistry. We will then move on to metalated aziridines, the chemistry of which, it will become apparent, closely mirrors that of their epoxide cousins. 

The (3-elimination of epoxides to allylic alcohols on treatment with strong base is a well studied reaction [la]. Metalated epoxides can also rearrange to allylic alcohols via (3-C-H insertion, but this is not a synthetically useful process since it is usually accompanied by competing a-C-H insertion, resulting in ketone enolates. In contrast, aziridine 277 gave allylic amine 279 on treatment with s-BuLi/(-)-spar-teine (Scheme 5.71) [97]. By analogy with what is known about reactions of epoxides with organolithiums, this presumably proceeds via the a-metalated aziridine 278 [101]. 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

Transition-metal atoms have been shown to deoxygenate epoxides to alkenes (36). Chromium and titanium atoms emerged as the most effective species in this regard, abstracting over two equivalents of oxygen. By studying the reaction of a wide range of epoxides with chromium atoms, the reaction... 

Hdft, E. Enantioselectivc Epoxidation with Peroxidic Oxygen. 164, 63-77 (1993). Hoggard, P. E. Sharp-Line Electronic Spectra and Metal-Ligand Geometry. 171, 113-142... 

Complex (17) of Class 3 has no chiral auxiliary, but is endowed with facial chirality by the presence of a bridging strap (Figure 4).65 Treatment of (17) with oxidant generates metal oxo bonds, preferentially on the sterically less hindered (nonbridged) side of the complex, and epoxidation with (17) is low in enantioselectivity (Scheme 10). However, the enantioselectivity is considerably improved by the addition of imidazole. The imidazole has been considered to coordinate the metal center from the nonbridged side and to force the formation of metal oxo bonds on the bridged (chiral) side, thus enhancing enantioselectivity. 

Another example of this cooperative catalysis has been presented by Konsler et al.101 in the course of their asymmetric ring-opening (ARO) study. They found that the ARO of mew-epoxides with TMS-N3, catalyzed by Cr salen compound 132, showed a second-order kinetic dependence on the catalyst.102 They then proposed that there might be cooperative, intramolecular bimetallic catalysis taking place, with one metal activating the substrate mew-epoxide and... 

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