Ozone is a clean oxidant which reacts with a large variety of organic molecules. Studies have been carried out using ozone as an oxygen atom donor (OAD) in metalloporphyrin catalysed olefin epoxidation. The most striking feature of these reactions is the catalytic formation of epoxide with yields ranging from 5 to 42%, which corresponds to catalytic activities from 5 to 39 turnovers. Cis olefins are more reactive than their trans isomers and terminal olefins are only slowly epoxidised. The key factor reducing selectivity is the noncatalysed direct ozonolysis of the olefin.
The ionic halogenation of tetramesitylporphyrin (TMP) by N-bromosuccinimide or N-chlorosuccinimide gives meso-tetramesityl-beta-octabromoporphyrin (H2Br8TMP) and meso-tetrakis(3-chloro-2,4,6-trimethylphenyl)-beta-octachloroporp hyrin (H2Cl12TMP) respectively. In both cases, the principal halogenation sites are the beta-pyrrole positions of the macrocycle and not the benzylic positions of the mesityl substituents.
Both manganese and iron derivatives of these 2 robust porphyrins are efficient catalysts for olefin epoxidation and alkane hydroxylation. The kinetic isotope effects of hydroxylation reactions were determined using different porphyrin catalysts and various oxygen atom donors. The kH/kD values ranged from 2 +/- 0.8 to 9 +/- 3 and were found to be more dependent on the central metal and the oxidant than on the porphyrin ligand. Iodosylbenzene is the only oxygen atom donor which, in association with iron porphyrins, is able to give high kinetic isotope effects close to the primary isotope effects reported for cytochrome P-450 itself.
A simple model system was selected for a mechanistic study of the catalytic behaviour of manganese(III) porphyrins in alkene epoxidation. Tetra-n-butylammonium monopersulphate ((CnBU)4NHSO5) and meta-chloroperbenzoic acid (MCPBA) were chosen as oxygen sources because of their solubility in low polarity solvents. This allowed the alkene epoxidation catalysed by manganese(III) porphyrins to be carried out under homogeneous conditions in dichloroethane.
Investigations have been directed towards the elucidation of some critical aspects of the reaction, namely: the effect of the added axial ligand; the kinetic behaviour of the epoxidation and the role of the chloride ion (the counter ion of the catalyst) which is expected to be relevant in a low polarity medium where ion paris are likely to form.
The present project is in the field of catalytic homogeneous oxidations at the border chemical and biochemical fields. The goal was to find new selective reactions for both hydroxylation and epoxidation of hydrocarbons, out of the well-developed domain of autoxidation (free radical chemistry), and to understand at the molecular level the mechanism of the discrimination between oxygen versus electron transfer in heme-containing enzymes, lyke cytochrome P-450, horseadish peroxidase, chloroperoxidase or catalase.
Funding SchemeCSC - Cost-sharing contracts