Manganese acyl complex

A very recent result that vinylmanganese complex was isolated by treatment of manganese acyl complex with triethylsilane gives further support for the formation of enolic species in the catalytic reaction. Carbonylation of the siloxyvinyl derivatives followed by protonation gives a-ketoacylmanganese complexes. 

Insertion takes place between a Jt-bonded fragment and a a-bonded fragment in mutual cis-positions, as was described above. The de-insertion reaction can only proceed if there is a vacant site cis to the acyl group. The experiment outlined in Figure 2.12 proves this point. A manganese acetyl complex which is labelled with 13C at the acyl carbonyl group was synthesised and heated to give de-insertion of CO. The result was that the only product formed contained the methyl substituent in a position cis to the labelled 13C. 

The mechanism of the reaction of the alcohol (or water) with the acyl complex to produce ester (or acid) and regenerate the cobalt hydride complex is not known. Because the reaction of the analogous manganese complex with alcohols is known to proceed through a hemiacetal-like complex, this mechanism has been written for the carboxylation reaction (equation 42). 

The manganese esters (+)- and (-)-C5H5Mn(NO)(COOR)P(CeH5)3 ra-cemize in solution by change of configuration of the Mn atom in much the same way as the acyl complexes 17 and 18. However, there is no... 

The electrophilicity of the alkylidyne carbon in cationic complexes of manganese and rhenium such as 154 or 157 is well established. A study by Chen et al. established that the carbonyl ligands are also potential sites for nucleophilic attack (157,158). Reaction of 154 with the bulky carborane anion LiCjBioH, results in the formation of two products the carbene complex 156, resulting from attack at the alkylidyne carbon, and the alkylidyne acyl complex 155, resulting from attack at a carbonyl ligand [Eq. (135)]. At room temperature complex 155 transforms into complex 156. 

Iron manganese complex (159,160). The isolated products are the a-keto-acyl manganese complex 158 [Eq. (136)] and the rhenium acyl complex 160 [Eq. (137)]. Low temperature spectroscopic studies revealed the initial... 

In the case of manganese(I), with an electronic configuration of 3d , the four-electron donor group RCO— would meet the EAN rule. Indirect support to the suggestion of a small steady state concentration of this type of intermediate along the reaction profile comes from isolation of stable rj -coordinated acyl complexes of ruthenium(II) , of group IVA transition elements and of thorium , all in the highest oxidation state. Also > -acyl complexes of vanadium(I) ° and niobium(V) have been reported. Most of the molybdenum acyl complexes are of the type . 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

The corresponding manganese(I) complexes (Tr-C5HsMn-7r-C7H7R) (R Me, Ph) can also be obtained by acylation of 7r-C5H5Mn-7r-C6H(, but they cannot be reduced to uncharged Mn(0) species 49). 

Further studies on the reaction of CH3Mn(CO)s with MnfCOls indicated that these species are in equilibrium with an anionic acyl complex which can be trapped as a carbene complex by O-alkylation (Anderson and Casey, 1971). The corresponding reaction of Re(CO)5 with CH3Mn(CO)s did not produce the expected manganese-carbene complex (VIII), but gave the rhenium-... 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

The closely related manganese complex 86, resulting from the treatment of the acylate derivative 85 with [MesSiJlOTf] [89]... 

The modified cephalosporin ceftobiprole (31-8), yet another compound that contains a double bond at the ring carbon, though in this case with a rather complex extended side chain, has shown activity in the clinic against some strains of multidrug resistant bacteria. The synthesis starts with the weU-precedented acylation of the cephalosporin (31-2), available in several steps from the commercially available 7-acetoxy cephalosporanic acid, with the activated thiadiazole carboxylic acid (31-1). The hydroxyl group in the product (31-3) is then oxidized with manganese dioxide to afford the corresponding aldehyde (31-4). This product is then condensed with the fcw-pyrrolidyl phosphonium salt (31-5), itself protected with the... 

N-Acylation, reduction of nitro to amino, and condensation produce dihydrotriazine 5.31. This system is readily dehydrogenated with manganese dioxide to afford the fully aromatic heterocycle 5.32. Note how relatively simple chemistry can be used to form a quite complex heterocycle. 

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