Alkenes, epoxidation catalyzed

Scheme 11. The mechanism of alkene epoxidation catalyzed by chromium-salen complexes...

S. Banfi, M. Dragoni, F. Montanari, G. Pozzi, S. Quid, Alkene epoxidations catalyzed by chemically robust Mn(III) porphyrins and promoted by HOCl under aqueous-organic 2-phase... 

E. T. Farinas, M. Alcalde, F. Arnold, Alkene epoxidation catalyzed by cytochrome P450 BM-3 139-3, Tetrahedron 60 (2004) 525. 

T. G. Traylor, A. R. Miksztal, Alkene epoxidations catalyzed by iron(III), manganese(ni), and chromium(III) porphyrins. Effects of metal and porphyrin substituents on selectivity and regiochemistry of epoxidation, ]. Am. Chem. Soc. 11 (1989) 7443. 

T.G. Traylor and A.R. Miksztal, Alkene epoxidations catalyzed by iron(III),... 

Figure 12. Scheme for alkene epoxidation catalyzed by Ru(poipXO)2/pyNO systems (adapted... 

Figure 21.3. Aerobic alkene epoxidation catalyzed by (THA)6[p-Fe4(H20)io(SbW9033)2]-...

Clarke R, Gahagan M, Mackie RK, et al. Alkene epoxidation catalyzed by camphor-derived beta-ketophosphonate complexes ofmolybdenum(VI). J Chem Soc Dalton Trans. 1995 (7) 1221-1226. 

Figure 11. Alkene epoxidations catalyzed by palladium nitro complexes.

Metalloporphyrin complexes have been shown to catalyze epoxidation of alkenes (Fig. 12), and in this case metal oxo intermediates have been implicated [19-21]. In most cases strong oxidants are required however, it has recently been reported that a ruthenium trans-dioxo complex catalyses direct epoxidation of an alkene with molecular oxygen as the oxidant [22]. It is believed that porphyrinato metal complex-catalyzed epoxidations are related to alkene epoxidations catalyzed by cytochrome P-A50 or other active enzymatic systems. It has recently been shown that creating site-isolation by anchoring (tetra-phenylporphinato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of epoxidation of cyclohexene by sodium hypochlorite [23]. 

Compounds lb and 2b were the Urst fluorinated ligands tested in Mn-catalyzed alkene epoxidation [5,6]. The biphasic Uquid system perfluorooc-tane/dichloromethane led to excellent activity and enantioselectivity (90% ee) in the epoxidation of indene with oxygen and pivalaldehyde (Scheme 1, Table 1). In addition, the fluorous solution of the catalyst was reused once and showed the same activity and selectivity. This represents a considerable improvement over the behavior in the homogeneous phase, where the used catalyst was bleached and reuse was impossible. Unfortunately, indene was the only suitable substrate for this system, which failed to epoxidize other alkenes (such as styrene or 1,2-dihydronaphthalene) with high enantioselectivity. The system was also strongly dependent on the oxidant and only 71% ee was obtained in the epoxidation of indene with mCPBA at - 50 °C. 

Table 4 Results of the alkene epoxidation reactions with m complexes immobilized on MCM-41 by cationic exchange -CPBA catalyzed by (salen)Mn ...

Prichanont, S., Leak, D.J. and Stuckey, D.C. (1998) Alkene monooxygenase-catalyzed whole cell epoxidation in a two-liquid phase system. Enzyme and Microbial Technology, 22 (6), 471 479. 

Fig. 10.3. Mechanism of the acid-catalyzed proton shift in the rearrangement of some alkene epoxides (10.10) to aldehydes (10.11)...

We also note that some 2,2-disubstituted oxiranes have toxicological significance, as exemplified by 2,2-dimethyloxirane (2-methyl-l, 2-epoxypropane, 10.43, R = Me). This compound is the toxic metabolite of 2-methyl-prop-1-ene (isobutene), a gaseous alkene widely used as a monomer in the industrial production of adhesives, plastics, and other polymers. Interestingly, detoxification of this epoxide catalyzed by liver epoxide hydrolase was high in the human, intermediate in the rat, and low in the mouse [125], These activities were inversely correlated with the epoxide levels measured in vitro in liver tissue of these species. 

Other examples of oxidant-iron(III) adducts as intermediates in iron porphyrin-catalyzed reactions have been published as listed in references 54a-k. Competitive alkene epoxidation experiments catalyzed by iron porphyrins with peroxy acids, RC(0)00F1, or idosylarenes as oxidants have been proposed to have various intermediates such as [(porphyrin)Fe (0-0-C(0)R] or [(porphyrin)Fe (0-I-Ar)]. Alkane hydroxylation experiments catalyzed by iron porphyrins with oxidant 3-chloroperoxybenzoic acid, m-CPBA, have been proposed to operate through the [(porphyrin)Fe (0-0-C(0)R] intermediate. J. P. CoUman and co-workers postulated multiple oxidizing species, [(TPFPP )Fe =0] and/or [(TPFPP)Fe (0-I-Ar)] in alkane hydroxylations carried out with various iodosylarenes in the presence of Fe(TPFPP)Cl, where TPFPP is the dianion of me50-tetrakis(pentafluorophenyl)porphyrin. ... 

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

We report here three studies that address three separate but significant issues in the emerging area of selective catalytic oxidation by TMSP-type complexes. The first study establishes for the first time that some TMSP complexes are compatible with basic oxidants and basic conditions. The second study reports the first oxidation, in this case selective alkene epoxidation, by the economically and environmentally desirable oxidant, aqueous hydrogen peroxide, catalyzed by TMSP complexes. The third study demonstrates that redox active polyoxometalates can be derivatized with alcohols in a manner that should prove useful for fabricating future generations of more sophisticated and selective TMSP catalysts. 

Alkene Epoxidation by H2O2 Catalyzed by TMSP Complexes. The... 

Alkenes. Alkenes are, in general, metaholically stable. The majority of alkene-containing drugs do not exhibit significant rapid metabolism at the double bond. There are some isolated examples of alkene-containing compounds that undergo epoxidation, catalyzed by mixed-function oxidase, or that add water across the double bond to give an alcohol. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel- complexes (e.g., LNiin-0, LNi=0, LNiin-0-... 

Many complexes written as MoCVI or M0O5L (L = bidentate ligand) undoubtedly contain the MoO(02)2 unit. For example, L may be Me3NO, Bu"PO, Pr"AsO, Ph3AsO, CsH5NO, etc.298 The analytical data and the observation of v(Mo—O) at 950-970 and v(0—O) at —850 cm-1 leave little doubt about the formulation of these complexes.298 The complexes stoichiometrically epoxidize alkenes or catalyze alkene epoxidation by t-butylhydro-peroxide.298... 

Catalysts. Acrolein and methacrolein 1,4-addition polymerization is catalyzed by lithium complexes of quinoline. The pcracctic acid epoxidation of a wide range of alkenes is catalyzed by 8-hydroxyquinoline. 

Some attempts to use simple Cu and Ag salts as catalyst for alkene epoxidation have been performed. Both silver nitrate and triflate in acetonitrile as the solvent were unsuccessful (ref. 9). However, we have found that Ag20 as well as AgN03, the latter in the presence of tertiary amines, can catalyze the oxidation of alkenes with iodosylbenzene as oxygen donor (ref. 10). The results for the epoxidation of different alkenes catalyzed by Ag20 and with iodosylbenzene as oxygen donor at 60 0 C in CHCI3 are given in Table 3. 

Independent rate studies by Bruice and Castellino143 and by the Bauld group144 concluded that any alkene radical cation must have a lifetime less than about 10 12 s, ruling out any meaningful role in the reaction for the free species. More recent work has generally concluded that no alkene cation radical is involved in epoxidations catalyzed by iron(III)... 

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