Manganese-oxo complexes

Kurz P (2009) Oxygen evolving reactions catalysed by manganese-oxo-complexes adsorbed on clays. Dalton Trans 31 6103-6108... 

When the reactant is cyclohexene, in the first step of Scheme 26, the direct hydrogen abstraction for the allylic oxidation (path 1) competes with the electron transfer (from the alkene to the M-oxo complex) for the epoxidation (path 2). Because the manganese complex is more readily reduced than the chromium... 

Discussions to this point rely on the hypothesis that the ligands of oxo(salen)manganese(V) complexes have planar structures by analogy to metalloporphyrin complexes and Mn(III)-salen complexes 11 and 12, the structures of which were determined by the X-ray crystallographic analysis [29a,47]. However, the assumption that the ligand of the oxo-Mn-salen species is planar failed to give a satisfactory explanation for the following stereochemistry observed in the recent study. Trans-cis selectivity in the epoxidation of 1-alkylindenes usually improves as the steric bulk of the catalyst increases. However, the epoxidation of 1-methylindene with the smallest... 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

The use of Mn-salen catalysts for asymmetric epoxidation has been reviewed.30 Oxo(salen)manganese(V) complexes, generated by the action of PhIO on the corresponding Mn(III) complexes, have been used to oxidize aryl methyl sulfides to sulfoxides.31 The first example of C—H bond oxidation by a (/i-oxo)mangancsc complex has been reported.32 The rate constants for the abstraction of H from dihydroanthracene correlate roughly with O—H bond strengths. 

The use of metal oxo-complexes for the oxidation of aldehydes to carboxylic acids is also well-known (Fig. 9-36), although, once again, the isolation of intermediate complexes is relatively rare. In particular, high oxidation state manganese or chromium complexes are commonly used for this process. 

Cyclopentadienylmetallaboranes, with iridium, 7, 380 Z /MYZ-Cyclopentadienyl—oxo complexes, with Zr(IV), 4, 866 Cyclopentadienyl phenoxides, with manganese, 5, 827-828 Cyclopentadienyl—phenoxo dichloro complexes, with Ti(IV), 4, 498-499... 

The proposed mechanism for epoxidation with 9.38B as the catalyst is shown in Fig. 9.8. The oxidation state of manganese in 9.38A and 9.38B is three. The oxygen donor NaOCl or PhIO oxidizes Mn3+ to Mn5+, and an oxo complex such as 9.39 is produced. Reaction of 9.39 with the alkene produces the chiral epoxide and regenerates 9.38B. While there is enough evidence for the basic mechanism and the involvement of a catalytic intermediate such as 9.39, there is some controversy about the details of the oxygen atom transfer from 9.39 to the alkene. 

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