Halogenated metalloporphyrin

Halogenated metalloporphyrins are effective catalysts for selective air oxidation of light alkanes [30] as well as of olefins [31], The postulated mechanism of the reaction (Scheme IX.2) [30c] is similar to those proposed for biological oxidation (by cytochrome P450 and methanemonooxygenase, see Chapter XI). 

The autoxidation of isobutane is now mainly carried out to obtain terf-butyl hydroperoxide [36]. Halogenated metalloporphyrin complexes are reported to be efficient catalysts for the aerobic oxidation of isobutane [18,37]. It was found that the oxidation of isobutane by air (lOatm) catalyzed by NHPI and Co(OAc)2 in benzoni-trile at 100 °C produced tert-butyl alcohol in high yield (81%) along with acetone (14%) (Eq. (6.3)) [38]. 2-Methylbutane was converted into the carbonacetic acid, rather than the alcohols, as prindpal products. These cleaved products seem to be formed via P-sdssion of an alkoxy radical derived from the decomposition of a hydroperoxide by Co ions. The extent of the P Scission is known to depend on the stability of the radicals released from the alkoxy radicals [39]. It is thought that the 3-scission of a terf-butoxy radical to acetone and a methyl radical occurs with more difficulty than that of a 2-methylbutoxy radical to acetone and an ethyl radical. As a result, isobutane produces terf-butyl alcohol as the principal product, while 2-methylbutane affords mainly acetone and acetic acid. 

Hydroxylation of cis-decahydronaphthalene with Fe (TPP)Cl and iodosylbenzene produced mainly c/ -O-decanol (Table 5) [46, 189], indicating that the hydroxylation proceeds with retention of configuration at carbon. Chang and Ebina showed the oxidation of anisole by iodosylbenzene with Fe (TPP)Cl or Fe (TPFPP)Cl (TPFFP tetarkis(pentafluorophenyl)porphyrin) gave methoxyphenols with marked NIH shifts, comparable with those for microsomal systems [190]. At the same time, fluorinated porphyrin was found to be much a more efficient oxidation catalyst. Systematic study on the catalytic efficiency of halogenated metalloporphyrin will be discussed later. 

Alkylidene complexes, for example, (porph)Fe=CQ2 may be formed in reactions of metalloporphyrin with halogenated solvents and such species may be responsible for the toxicity of halogenated insecticides and solvents like CQ4. 

Metalloporphyrins and related compounds continue to attract attention as photosensitizers and as building blocks for construction of artificial lightharvesting arrays. The photophysical properties of several structurally modified porphyrins have been measured with a view to identifying new sensitizers for use in photodynamic therapy. The triplet state properties of porphyrins adsorbed onto the outer surface of vesicles have been describedand the fluorescence spectral properties of some amphiphilic porphyrins have been recorded. Similar studies have been carried out with halogenated tetraar-... 

Calvin continued his studies at the University of Alinnesota, where he investigated the electron affinities of halogen atoms. He received a Ph.D. degree in 1935. As a Rockefeller Foundation fellow at the University of Manchester in England (1935-1937), Calvin worked with Alichael Polanyi, who introduced him to the interdisciplinary approach, on coordination catalysis, the activation of molecular hydrogen, and metalloporphyrins. In 1937 he joined the faculty of the University of California, Berkeley, as an instmctor, and remained there for the balance of his career. 

Single-electron oxidation of the metalloporphyrins 9 (e.g. by halogens or electrochemically) leads to the formation of radical cations. These undergo addition with nucleophiles at a methine bridge and, after acid demetalation, yield monosubstituted porphyrins 10 [18] ... 

The observation that light accelerates these reactions can be rationalized in the framework of the above interpretation. Since light passed through a solution of vanadyl etioporphyrin I without effect on the system, it would appear that photochemically excited species of metalloporphyrins are responsible for the faster reactions as a consequence of their enhanced capability of interaction with halogens and the other reagents. 

The first synthetic metalloporphyrins were found to be oxidatively labile. Few catalytic turnovers were seen, due to the rapid destruction of the porphyrin macrocycle. It has been shown that introduction of halogens onto the aryl groups (of mesotetraarylporphyrins) and... 

Scheme 29.25 Improved methods for the halogenation of sterically hindered metalloporphyrins.

Guengerich FP (1989) Oxidation of halogenated compounds by cytochrome P-450, peroxidases, and model metalloporphyrins. J Biol Chem 264 17198-17205... 

A number of porphyrins with new and/or unusual macrocycles were synthesized in the 1980s and 1990s and the redox properties of these compounds were often compared to potentials for the oxidation or reduction of analogous compounds containing the same metal ion and the well-known TPP or OEP macrocycle. Examples include, but are not limited to, highly halogenated metallopor-phyrins [27-39], metalloporphyrins with highly distorted macrocycles [37, 40-45], and metalloporphyrins linked to one or more other porphyrins or to different redox active molecules [46]. 

Metalloporphyrins and metallocorroles have been shown to acts as alkene aziridina-tion catalysts, and the extent of macrocycle halogenation has been shown to correlate with catalyst performance. Mn(III) complexes of 5,10,15-Tris(pentafluorophenyl) corrole are active in aziridination catalysis [101, 102] electron-withdrawing substituents on the corrole are required to facilitate amide transfer to the alkene. Fluorination of corroles also leads to stable metal imide intermediates during aziridination reactions. 

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