Olefin complexes epoxidations

A number of alkylperoxy Co111 complexes including mainly polyimine ligands have been prepared, exemplified by (223).979 Hydroxylation of saturated hydrocarbons, preferentially at the more nucleophilic C—H bonds, yields alcohols, ketones, and t-butylperoxo products, whereas olefins form epoxides if they carry no allylic H atoms. 

Sajus et al. [243,244] synthesized the peroxo complex of molybdenum(VI) and studied its reaction with a series of olefins. This peroxo complex M0O5 was proved to react with olefins with epoxide formation. The selectivity of the reaction increases with a decrease in the complex concentration. It was found to be as much as 95% at epoxidation of cyclohexene by M0O3 in a concentration 0.06 mol L-1 at 288 K in dichloroethylene [244], The rate of the reaction was found to be... 

A broad range of olefins, acetals, epoxides, alcohols, and chlorides were demonstrated effective alternative starting materials. Cobalt and rhodium carbonyls and bimetallic complexes were shown to catalyze the domino hydro-... 

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

The mesityl complex [(mes)Os(C2H4)(CH3CN)] results in addition of benzene when irradiated with ultraviolet light in benzene solution (Fig. 19) [103]. Photolysis of the complex in perdeuterotoluene in frozen matrices results in D atom abstraction. Photolysis of Os(VI) dioxo complexes such as [0s(0)2(Bu-salch)] (Scheme 1) with olefins yields epoxides, in one instance with some degree of chiral selectivity [104],... 

As with cytochrome P-450 oxidations of alkanes, the mechanism of P-450-catalyzed formation of oxiranes has been extensively studied, but certain details remain in dispute272. Examples of mechanistic schemes include formation of a transient [FeIV—0]+2-olefin complex followed by collapse to an [Fe—Oj-olefin adduct that collapses further to the epoxide and other products264. An alternative mechanism involves a 2a + 2s addition of olefin to the Fe=0 double bond to give an intermediate metallocene, followed by rearrangement to products273-275. 

Coordination catalysis via alkyl hydroperoxides is well documented (4, 31). Selective oxidations of olefins to epoxides (Reaction 16), using especially Group IV, V, and VI transition-metal complexes, can occur possibly via oxygen-transfer processes of the type... 

Arnone et al. studied the epoxidation of various olefins 220 with perfluorinated oxaziridine 80 (Equation 10) <1996JOC8805>. Alkyl-substituted olefins are epoxidized with this oxaziridine under particularly mild conditions. Electron-deficient substrates can also be epoxidized, and the more electron deficient the double bond is, the more severe the reaction conditions become. The reaction is chemoselective and stereoselective, with air-alkenes affording air-epoxides. Various complex and polyfunctionalized substrates of natural origin (monoterpenes, sesquiterpenes, and steroids) have been epoxidized effectively with this reagent (Table 18). 

Selective epoxidation of olefins by vanadium(V) alkyl peroxo complexes has also been reported (76). These complexes are very effective stereo-selective reagents for the transformation of olefins into epoxides. The mechanism consists of binding of the olefin to the metal to displace one of the peroxo-oxygen atoms, nucleophilic attack of the bound oxygen atom on the coordinated electron-deficient olefin, dissociation of the epoxide, and reaction of the remaining vanadium intermediate with... 

Another possibility for enhaneing the selectivity toward epoxides is use of the urea-H202 (UHP) adduct. This enables the oxidation to be carried out in water-free solutions, thus avoiding formation of any diols and other side reactions. In the case of the oxidation of chiral allylic alcohols (see below) high diastereo-selectivities have been achieved [3]. The ability to transform olefins to epoxides diastereoselectively seems to indicate that the reaction proceeds through a peracid-like transition state. However, a drawback of the urea-H202 system is the insolubility of the polymeric complex. 

G. Yin, M. Buchalova, A. M. Danby, C. M. Perkins, D. Kitko, J. D. Carter, W. M. Scheper, D. H. Busch, Olefin oxygenation by the hydroperoxide adduct of a nonheme Manganese(lV) complex Epoxidations by a metallo-peracid produces gentle selective oxidations, /. Am. Chem. Soc. 127 (2005) 17170. 

The use of polypyrazolylborate copper(I) complexes as catalysts in the conversion of olefins into epoxides 01JOM(617-618)110. 

The catalytic activity of various complex compounds of technetium was tested in the metathesis of olefins [17], epoxide ring opening reactions [18], epoxide ring formation by reaction of cyclohexene with fe/7-butylhydroperoxide [19,20], and the preferred production of tram-epoxides in the liquid phase oxidation of cw/tram-n-alkenes [21 j. 

The above scheme includes the formation of the oxoiron(IV) porphyrin cation radical via 0-atom transfer from PhIO within an intermediate complex. The double bond approaches from the side and parallel to the porphyrin ring. This geometry is clearly favorable for the cis-olefins. The epoxidation step consists in oxygen atom transfer directly from the oxoiron(IV) to the olefin within an intermediate that has been detected with mCPBA as oxidant [46]. According to... 

Cationic olefin complexes of dicarbonyl(> -cyclopentadienyl) iron have been of wide interest in syntheses for a number of years. These complexes, generally isolated as their tetrafluoroborate or hexafluorophosphate salts, have been prepared by the reaction of Fe(f -CsHs)(CO)2Br with simple olefins in the presence of Lewis acid catalysts, by protonation of allyl ligands in Fe(t/ -CsHjXCO)2[(allyl)KC ] complexes, or by treatment of these with cationic electrophiles, by hydride abstraction from Fe(f) -CsHjXCO)2(alkyI) complexes, through reaction of epoxides with Fe(fi -CsHjXCO)2 anion followed by protonation, or by thermally induced ligand exchange between [Fe( i -CjHsXCO)2(ij -2-methyH-propene)][BF4] " or [Fe( -C,HsXCO)2(tetrahydrofuran)][BF4] and excess olefin. 

---