Alkenes oxidation with iodosylbenzene

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). 

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. 

Diederich et al. had postulated that the highly reactive iron-oxo species, arising from oxygen transfer from the oxidant to the Fem site [87], should be greatly stabilised by enclosure within a dendritic superstructure. The catalytic potential of the dendrimers 6 a-c was determined in the epoxidation of alkenes [83 a, 88] (1-octene and cyclooctene) and the oxidation of sulphides [83 a] ((methylsulphanyl)benzene and diphenyl sulphide) to sulphoxides - in dichloro-methane with iodosylbenzene as oxidising agent. Compared to the known metal-porphyrin catalysts, 6a-c exhibit only low TON (7 and 28, respectively, for... 

To demonstrate the flexibility of the approach to catalyst design that we set out in this paper, the epoxidation of alkenes using iodosylbenzene has also been studied. Initial studies focused on MnHY salen catalysts for the epoxidation of styrene, however, the reaction was slow, and low yields of styrene oxide were observed. Analysis of the reaction mixture revealed the breakdown of the salen ligand within a few turnovers. Subsequently Mn-Al-MCM-41 was used with iodosylbenzene as the oxygen donor and cis -stilbene was used as substrate, and the results, together with those of control experiments, are shown in Table 3. 

Oxidation of ]V-MeTTPFenCl (46, 52). Catalytic alkene oxidation by iron N-alkylporphyrins requires that the modified heme center can form an active oxidant, presumably at the HRP compound I level of oxidation. To show that iron N-alkyl porphyrins could form highly oxidized complexes, these reactive species were generated by chemical oxidation and examined by NMR spectroscopy. Reaction of the (N-MeTTP)FenCl with chlorine or bromine at low temperatures results in formation of the corresponding iron(III)-halide complex. Addition of ethyl- or t-butyl-hydroperoxide, or iodosylbenzene, to a solution of N-MeTTPFenCl at low temperatures has no effect on the NMR spectrum. However, addition of m-chloroperoxybenzoic acid (m-CPBA) results in the formation of iron(III) and iron(IV) products as well as porphyrin radical compounds that retain the N-substituent. 

In pursuit of biomimetic catalysts, metaUoporphyrins have been extensively studied in attempts to mimic the active site of cytochrome P450, which is an enzyme that catalyzes oxidation reactions in organisms. In recent decades, catalysis of alkene epoxidation with metaUoporphyrins has received considerable attention. It has been found that iron [1-3], manganese [4,5], chromium [6], and cobalt porphyrins can be used as model compounds for the active site of cytochrome P450, and oxidants such as iodosylbenzene, sodium hypochlorite [7,8], hydrogen peroxide [9], and peracetic acid [10] have been shown to work for these systems at ambient temperature and pressure. While researchers have learned a great deal about these catalysts, several practical issues limit their applicability, especially deactivation. 

The mechanism in Fig. 19.4 is stepwise, in which the radical intermediate is formed before the concerted one. Step 1 involves formation of the metal-oxo intermediate via oxygen transfer to the manganese center from iodosylbenzene. In Step 2, the alkene reacts with the oxidized porphyrin to form the radical intermediate. From this intermediate, the concerted intermediate is formed (Step 3), and finally styrene oxide desorbs in Step 4. The radical intermediate can also... 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

Various oxidations with [bis(acyloxy)iodo]arenes are also effectively catalyzed by transition metal salts and complexes [726]. (Diacetoxyiodo)benzene is occasionally used instead of iodosylbenzene as the terminal oxidant in biomimetic oxygenations catalyzed by metalloporphyrins and other transition metal complexes [727-729]. Primary and secondary alcohols can be selectively oxidized to the corresponding carbonyl compounds by PhI(OAc)2 in the presence of transition metal catalysts, such as RuCls [730-732], Ru(Pybox)(Pydic) complex [733], polymer-micelle incarcerated ruthenium catalysts [734], chiral-Mn(salen)-complexes [735,736], Mn(TPP)CN/Im catalytic system [737] and (salen)Cr(III) complexes [738]. The epox-idation of alkenes, such as stilbenes, indene and 1-methylcyclohexene, using (diacetoxyiodo)benzene in the presence of chiral binaphthyl ruthenium(III) catalysts (5 mol%) has also been reported however, the enantioselectivity of this reaction was low (4% ee) [739]. 

In the MOF PIZA-3 (PIZA, porphyrinic Illinois zeolite analog), Mn(III) is found both in the porphyrin struts and as a structural metal node. The framework is structurally stable and is used for the oxidation of cycUc alkanes and alkenes with iodosylbenzene or peracetic acid as the oxidant [117]. Reaction is found to take place at the outer surface, which is justified by the authors by the unfavorable hydrophilic properties of the pore interior. Yields were similar to those obtained with homogeneous Mn(III) porphyrin systems or those immobilized inside inorganic supports as heterogeneous catalysts. Less than 0.1 mM of metalloporphyrin or degradation products were observed in the reaction mixtures, with no loss of oxidation activity observed in a second run when peracetic acid was used. 

The Mn(III) complex 31b was tested as a catalyst for the epoxidation of various alkenes using sodium hypochlorite or iodosylbenzene as oxidants. Although oxidation took place, no selectivity was observed. For example, allylresorcinol was not epoxidized with rates higher than that of allylbenzene. Presumably, the substrate is not bound in the cleft of 31b because the latter is occluded by methoxy groups. It is possible that the reaction occurs on the outside of the metalloclip, which cannot discriminate between guest molecules. 

Epoxidation of alkenes and hydroxylation of alkanes can be achieved under mild conditions with iron porphyrin catalysts and iodosylbenzene as the oxidant.497 499,488... 

Cobalt-catalyzed epoxidation of alkenes has been carried out with the cobalt derivative of (174), employing iodosylbenzene as the oxidant. Epoxidation of cfa- -methylstyrene furnishes exclusively the cis-epoxide (equation 62). The reaction proceeds through an active oxo-cobalt(IV) species, and is mote selective than reactions proceeding through oxo-chromium or oxo-manganese species. The catalyst can be recovered unchanged by simple filtration. 

The oxidative decarboxylation of aliphatic carboxylic acids is best achieved by treatment of the acid with LTA in benzene, in the presence of a catalytic amount of copper(II) acetate. The latter serves to trap the radical intermediate and so bring about elimination, possibly through a six-membered transition state. Primary carboxylic acids lead to terminal alkenes, indicating that carbocations are probably not involved. The reaction has been reviewed. The synthesis of an optically pure derivative of L-vinylglycine from L-aspartic acid (equation 14) is illustrative. The same transformation has also been effected with sodium persulfate and catalytic quantities of silver nitrate and copper(II) sulfate, and with the combination of iodosylbenzene diacetate and copper(II) acetate. ... 

Traylor et al. reached different conclusions in studies of alkene epoxidation. In homogeneous solutions in which iodosylbenzene was solubilized in dichloro-methane/alcohol/water, very rapid epoxidation (300 turnovers/sec) was found to be independent of substrates or concentration of alkane (124). No dependence on alkene concentrations was reported by Dicken et al. when they employed /7-cyanodimethylaniline 7V-oxide (725). Meanwhile, Traylor et al. found an accumulation of 7V-alkyliron porphyrin complexes (35) during the course of epoxidation of norbornene (126), and showed that it is the only intermediate accumulated in either the homogeneous system or the heterogeneous system 35 was found to form with a bimolecular rate constant of 450 M sec and decomposes at a rate of 0.07 sec. Epoxide formation occurs with a rate constant of at least 10 Af" sec in these conditions. Thus 35 is not an intermediate in the production of epoxide however, it could account for at least a small percentage of the products, since 35 was found to catalyze alkene epoxidation by utilizing PhIO (Scheme XIX). 

Another approach is the nucleophilic displacement of chloride from chloro-propyl-silica with a pyridine-substituted porphyrin. These materials are active in the epoxidation of alkenes, where iodosylbenzene is the preferred oxidant, and in the oxidation of alkanes to alcohols and ketones. The copolymerisation of a porphyrin containing four attached trimethoxysilane groups with tetra-ethoxysilane, leading to an active hybrid silica-porphyrin, offers another route to these important catalysts. 

---