Porphyrins carbon bond cleavage

Most of the studies on organo-gallium, indium, and thallium porphyrin complexes have focused on synthesis and properties of the complexes, and rather little attention has been devoted to reaction chemistry. Two areas which have received some attention are the insertion of small molecules (SO2 or CO2) into the metal—carbon bonds and photochemical metal—carbon bond cleavage. The... 

Okamoto, T., K. Sasaki, and S. Oka (1988). Biomimetic oxidation with molecular oxygen. Selective carbon-carbon bond cleavage of 1,2-diols by molecular oxygen and dihydropyridine in the presence of iron-porphyrin catalysts. J. Am. Chem. Soc. 110, 1187-1196. 

For example, <7-bonded porphyrins with Rh, P, As, or Sb central ions undergo reversible oxidations and reductions at the 7T-conjugated macrocycle and there is little effect of the cr-bonded axial ligand on the porphyrin electrochemical behavior [12, 21]. However, derivatives with Al, Ga, In or T1 central metals all undergo a rapid cleavage of the metal-carbon bond after a one-electron oxidation of the compound. In contrast, the Fe, Ru, and Co cr-bonded metalloporphyrins (see further sections of the review) will undergo a metal-centered... 

The electrochemistry of (P)Ge(R)2 and (P)Ge(R)X are characterized by one or two reversible single-electron additions leading to porphyrin r-anion radicals and dianions. The compounds also undergo two oxidations, the first of which is always reversible in the case of the (P)Ge(R)2 complexes [58]. A rapid cleavage of the Ge-carbon bond was proposed to follow electron transfer and the final product of the oxidation was formulated as (P)Ge(R)(Cl04) [58]. 

There are other organic cofactors such as thiamine pyrophosphate and biotin that participate in carbon-carbon bond formation and cleavage, cofactors that participate in reduction/oxidation, or redox, reactions such as nicotinamide and flavin moieties discussed in some of the earlier examples, and still others that are metal based such as vitamin B12 and porphyrin, which is our next topic. 

A polypyrrole film electrochemically deposited on gold electrodes from an MeCN-liCl04/Co(OAc)2 solution shows electrocatalytic activity in dioxygen reduction [404]. The catalytic electroreduction of dithio dipropionic acid (PSSP) with the water-soluhle cohalt(II I)tetrakis(4-trimethyl-ammonium phenyl) porphyrin (CoTMAP) has heen studied. The Co catalyst adsorbed on the glassy carbon electrode plays a major role in the electroreductive cleavage of the S—S bond [405]. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

Aoyama Y, Yoshida T, Sakurai K-i, Ogoshi H (1986) Activation of arene carbon-hydrogen bonds. Highly regioselective aromatic metalation with rhodium(III) porphyrin and subsequent cleavage of carbon-rhodium bond. Organometallics 5 168-173... 

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