Porphyrins as catalyst

D. Cobalt Porphyrins as Catalysts for the Polymeri/.ation of Alkenes. . . 2X9... 

YuasaM, Nishfliara R, Shi C, Anson EC. 2001. A comparison of several meso-tetraalkyl cobalt porphyrins as catalysts for the electroreduction of dioxygen. Pol) Adv Technol 12 266. 

The catalyst can also have a significant influence on the stereoselectivity of cyclopropanation reactions [1023], For instance, cyclopropanation of styrene with ethyl diazoacetate and copper or rhodium catalysts normally proceeds with low diastereoselectivity. With ruthenium porphyrines as catalysts, however, up to 92% de can be achieved [1041,1042]. 

On the other hand, in two other papers, the formation of hydrogen gas was not mentioned, whereas carbon monoxide and formic acid were both observed as products. In studies carried out by Ogura and coworkers [123], electrogenerated [Co(PPh3)2L] (where L is a substituted quinoline, bipyridine, or phenan-throline moiety) was employed as a catalyst for the reduction of CO2 in anhydrous organic solvents, conditions for which the current efficiency for production of CO (the main product) was 83%. Similarly, in an investigation done by Behar et al. [124], who used cobalt porphyrins as catalysts in an acetonitrile medium, the formation of both carbon monoxide and formic acid was noted however, the catalytic species did not appear to contain cobalt(I), but rather a cobalt(O) species complexed with carbon dioxide. 

Substituted cobalt porphyrins as catalysts in sulfuric acid. Further very active chelates for the reduction of oxygen in acids discovered in the early 70s were CoTPP by Sandstede and co-workers 9,13-15) and CoTAA and FeACC by Beck and co-workers 8 12>. Sandstede et al. determined the activities of their chelate catalysts by the suspension method briefly explained in Section 2.2.2.3. 

Cydooctene has been epoxidized with oxygen using a manganese porphyrin as catalyst (equation... 

Finally, mention should be made of the use of iron porphyrins as catalysts in the Polonovski reaction. An added feature of this approach is that the yV-oxide can act as an oxygen source for other synthetically useful, iron porphyrin catalyzed transformations. 

Mukaiyama and coworkers [1378] have performed such oxidations with molecular oxygen in the presence of pivaldehyde under catalysis with related salen 3.70 Mn(lH) complexes however, lower enantioselectivities were obtained. The use of various manganese or iron porphyrins as catalysts also gives lower enantioselectivities [971, 1379, 1379a], as do the oxidations with chiral oxaziridines in stoichiometric amounts [741],... 

Benzoquinones were derived from the oxidation of electron-rich methoxyarenes by magnesium monoperoxyphthalate with a water-soluble iron porphyrin as catalyst (ref.45). 

Because of their extended 7r-system, porphyrins and related compounds absorb visible light. Therefore, their potential application as chromophores for harvesting solar energy, as a means to mimic the natural role of chlorophyll, has been under intense study. In addition, porphyrins can act as mediators of electron transfer, both as oxidants and as reductants. Therefore, the role of porphyrins as catalysts for various reductions and oxidations also has been studied by many authors. 

Examples involving osmium [168,169] and iron [170] porphyrins as catalysts have also been reported but the catalysts mainly provide a trans product. A mechanism for iron porphyrin-catalyzed cyclopropanation was proposed by Kodadek and coworkers [170]. A transition state for carbene transfer which is reached later than in the rhodium porphyrin catalyzed reaction is suggested. In this case, the olefin is parallel to the metallocarbene and significant bonding has occurred. This geometry explains why 1,2 disubstituted alkenes are poor substrates since there is a steric interaction between the porphyrin ring and the alkene [170]. 

The catalytic production of olefins, diethyl maleate and fumarate, from ethyl diazoacetate has been reported with osmium [ 149] and ruthenium [ 128] porphyrins. Despite the periodic relationship of ruthenium to iron and osmium and the syntheses of different carbene complexes of ruthenium porphyrins, developed by Collman et al. [125-127], it is only very recently that cyclopropanation [135,171] and ethyl diazoacetate insertion into heteroatom bond reactions [172] were observed using ruthenium porphyrins as catalysts. The details of the catalytic reaction of diazo esters with simple olefins catalyzed with ruthenium porphyrins have been reported [173]. Product yields. 

The first ruthenium porphyrin-catalyzed intramolecular carbenoid C - H insertion to afford selectively cis-2,3-disubstituted-2,3-dihydroergocornine using tosylhydrazone salts as the carbene source was reported by Zheng et al. [192]. This general strategy was applied in natural product synthesis to provide a route to the total synthesis of racemic epi-conocarpan. Enantio-selective synthesis of 2,3-dihydrobenzofurans was also achieved by a similar route using chiral ruthenium porphyrins as catalysts for this interesting carbon-carbon bond formation [193]. Recently, it was found that dinuclear fx-oxo osmium porphyrins are able to catalyze intermolecular carbene insertion into C - H bonds in cyclohexene [153]. 

Kostas, I.D., Coutsolelos, A.G., Charalambidis, G. and Skondra, A. (2007) The first use of porphyrins as catalysts in cross-couphng reactions a water-soluble palladium complex with a porphyrin ligand as an efficient catalyst precursor for the Suzuki-Miyaura reaction in aqueous media under aerobic conditions. Tetrahedron Lett., 48, 6688-91. 

From these results, it is clear that chlorophyll adsorbed at an interface can catalyze redox reactions between solutes in the two liquid phases, and these reactions are accompanied by injection of negative charges into the low dielectric membrane interior. Metallo-complexes of porphyrins also have similar catalytic properties at the oil/water interface without illumination [13,14,43]. An advantage of porphyrins as catalysts in such experiments is their chemical stability in different media. 

In addition, carbenoid insertion of aliphatic C-H bond can be achieved in the presence of iron(III) porphyrin as catalyst. A benzylic C-H insertion product was obtained along with the arene C-H insertion product in a ratio of about 5 1 when p-xylene was used as substrate and solvent (eq 57). 

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