Manganese porphyrins oxidation with iodosylbenzene

In a reaction modeled on the use of cytodirtxne P-450 to catalyze oxidations with iodosylbenzene, iron or manganese porphyrins have been used to catalyze aziridinations with iodinanes (Scheme 22). In this early report cis- or rranr-stilbene each gave the rrans-aziridine, but stereoselectivity has since been achieved for the sulfonylaziridines (Section 3.5.2.6). 

Finally in this section, the oxidation of alkanes using porphyrin manganese(lil) complexes with iodosylbenzene as biological oxidation mimics has been reported. In the conversion of cyclohexane into bromocyclohexane, for example, a yield of 90% based on manganese bromide complex is quoted. 

The first reaction in the mechanism is the oxidation of the Mn-porphyrin with iodosylbenzene. The electronic energy as a fimction of the reaction coordinate for the quintet spin state is shovm in Fig. 19.5, starting and ending with the weakly interacting complexes. The reaction coordinate was taken to be the distance between the manganese and oxygen atoms. This reaction is highly exothermic, with a AEreaction of -170 kj/mol, and proceeds without a barrier. The results for this step on the triplet spin surface were similar. 

The oxidation of saturated hydrocarbons in the presence of iron- or manganese-containing catalysts can be achieved by using a variety of oxidants including alkyl hydroperoxides, peroxycarboxylic acids, iodosyl-benzene, dihydrogen peroxide, and dioxygen (9-11). It has been shown that chiral iron- and manganese-porphyrin complexes catalyze the asymmetric epoxidation of unfunctionalized alkenes (75). Except for a number of experiments in which up to 96 % enantiomeric excess (ee) has been reported (16,17), in most epoxidation reactions with chiral porphyrins only a low to moderate enantiomeric excess of the product is obtained (18,19). In association with these catalysts, alkyl hydroperoxides and iodosylbenzene are often used as primary oxidants (18,19). 

Iron(III) and manganese(III) porphyrins have been found to be effective catalysts for the oxidation of nitroso to nitro compounds, as in, for example, the conversion of nitrosobenzene to nitrobenzene with iodosylbenzene as the oxygen source. 5)... 

Figure 6.34 A synthetic porphyrin containing manganese (V) which can hydroxylate saturated hydrocarbons through a free-radical mechanism at room temperature. Manganese (III) is oxidized to manganese (V) with iodosylbenzene...

Historically, the interest of using manganese complexes as catalysts for the epox-idation of alkenes comes from biologically relevant oxidative manganese porphyrins. The terminal oxidants compatible with manganese porphyrins were initially restricted to iodosylbenzene, sodium hypochlorite, alkyl peroxides and hydroperoxides, JV-oxides, KHSO5, and oxaziridines. Molecular oxygen can also be used in the... 

Compared with chiral manganese porphyrin complexes [50], the use of the Mn-salen catalysts results generally in ee s higher than 90% and yields exceeding 80% [39, 58]. A wide range of oxidants including hypochlorite [58], iodosylbenzene [58], or m-chloroperbenzoic acid (m-CPBA) can be applied [59]. Excellent ee s are observed for epoxidation reactions of cix-disubstituted alkenes and trisubstituted alkenes catalyzed by the Mn-salen complexes 11 and 12, employing iodosylbenzene as the oxidant. In sharp contrast, the epoxidation of trans-olefins showed moderate selectiv-... 

These red compounds contain a Mn—N triple bond, are diamagnetic, five-coordinate and show considerable chemical stability. Thus while the isoelectronic oxochromium(IV) or oxomolyb-denum(IV) porphyrins behave, respectively, as weak oxidants and reductants, the corresponding manganese(V)nitrido species exhibits enhanced redox stability. [MnN(TPP)] and related porphyrin species were also independently prepared by the reaction of iodosylbenzene with [MnmX(por-phyrin)] (X = Cl, Br, OAc) in dichloromethane itt the presence of excess ammonia.758... 

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... 

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