Metalloporphyrins, alkane oxidation

Murahashi, S.-l., T. Naota, and N. Komiya (1995). Metalloporphyrin-catalyzed oxidations of alkanes with molecular oxygen in the presence of acetaldehyde. Tetrahedron Lett. 36, 8059-8062. 

It is important to note that alkane oxidation reactions by various oxygen atom donors in cases when a metal-complex catalyst is a metalloporphyrin (or even any other complex) can he considered to he a model for biological hydrocarbon oxidation. The next chapter will be fully devoted to the oxidations of alkanes and arenes in living cells and modeling these processes using metal complexes. 

Some recent examples of metalloporphyrin catalyzed alkane oxidations are listed in Table V. 

Metalloporphyrin catalyzed oxidation of alkanes and cycloalkanes with 0... 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

The metalloporphyrin-PhIO system catalyzes the oxidation of alkanes mainly to alcohols under mild conditions. High selectivity for the hydroxylation at the tertiary carbon is observed.132 Yields up to 40% based on the oxidant consumed are obtained. Acyclic alkanes usually exhibit very poor reactivity. A large isotope effect and retention of configuration in the oxidation of ds-decalin are additional important characteristics of the process. 

The extremely hindered manganese(III) tetra(2,4,6-triphenylphenyl)porphyrin acetate [Mn(TTPPP)OAc], in contrast, gives the less hindered 3,3-dimethylbutane-l-ol with exceptionally high selectivity. Hindered metalloporphyrins also exhibit an increasing reactivity in the oxidation of the -1 CH2 site in straight-chain alkanes.148... 

Epoxidation Catalyzed by Metalloporphyrins. Metalloporphyrins, which have thoroughly studied as catalysts in alkane oxygenations, have also been tested as epoxidation catalysts.119,122,244,245,307 Iodosylbenzene (PhIO), sodium hypochlorite, alkyl hydroperoxides, potassium hydrogen persulfate, and molecular oxygen are the oxygen sources used most frequently in these oxidations.119... 

Among the oxidation catalyzed by heme enzymes, alkane hydroxylation is the hardest process to mimic with synthetic metalloporphyrins (7). This finding is also true for myoglobin mutants, even though the mutants are able to afford... 

Metalloporphyrins and some related metal complexes are effective catalysts in IOB oxidations, as already discussed for alkenes, and acids (Sections 5.1.1.1 and 5.2.4). Also, sulphides have been oxidized to sulphoxides [58]. Some other substrates of various types underwent such catalysed oxidations, because these systems mimic the natural oxidant cytochrome P-450 [2]. From a synthetic point of view, only a few reactions are of importance alkanes were mainly used which underwent regio- and stereo-specific hydroxylation, for instance the methyl group of a pyrrole derivative was converted into hydroxymethyl, leading to one-pot preparation of dipyrro-methanes [59], The preparation of elaborated catalysts is, however, very demanding and precludes a wider use. 

Whereas important progress has been made regarding the use of metalloporphyrins as catalysts for alkene epoxidations and alkane hydroxyla-tions, work concerning the mechanism of hydroxylation of aromatic hydrocarbons has received only limited attention. In fact, the main problem encountered with the design of systems capable of performing such oxidative reactions is in the preparation of superstructured porphyrins for the selective complexation of aromatic compounds. 

D. Mansuy, J. F. Bartoli, M. Momenteau, Alkane hydroxylation catalyzed by metalloporphyrins-evidence for different active oxygen species with aUcylhydroperoxides and lodosobenzene as oxidants. Tetrahedron Lett. 23 (1982) 2781. 

Murahashi and coworkers have studied the aerobic oxidation of alkanes catalyzed by poly-fluorinated metalloporphyrins including Ru" (TPFPP)(CO) in the presence of acetaldehyde (Scheme 1.13) . ... 

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). 

Scheme 1X.2. The mechanism postulated for the air oxidation of alkanes catalyzed by halogeiimed metalloporphyrins.

Many chemical model systems based on metalloporphyrin catalysts and mimicking cytochrome P450-dependent monooxygenases have been described during these last decade. Several review articles have been devoted to these systems 2-10. in that context, very recent results about the preparation and catalytic properties of new homogeneous and supported catalysts will be described in a first chapter. In the second chapter, some preliminary results showing that the oxidation of alkanes by a dioxygenase-like mechanism could occur in the presence of iron porphyrin catalysts activated either photochemically or thermally, will be reported. 

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