Conformation selective catalytic oxidation

A study of the catalytic oxidation of six stereoisomers of 5-cyclohexene-l,2,3,4-tetrol revealed448 that, in all cases, an allylic hydroxyl group is selectively attacked, and a consideration of their favored conformations suggested that quasi-axial groups are selectively dehydrogenated before those that are quasi-equatorial. 

Another interesting aspect of this work is the relation between conformation and catalytic activity of the polymeric catalyst. The authors showed that the main chain of PLL— Cu(II) has a helical conformation and that the selectivity ratio in the oxidation of L versus D-DOPA is proportional to the a-helix content of the polymer. However, a highly helical catalyst containing a relatively low Cu " -content (see last entry of Table X) exibited almost no selectivity - which was taken to indicate that the a-helical structure may be necessary, but not sufficient for the selectivity. (Data of Table X alone are anyway not sufficient to demonstrate per se the importance of the a-helix in catalysis, since the various helical contents have been obtained by varying temperature, and this independent parameter could in principle affect die selectivity ratio). To further substantiate the importance of the helical conformation in catalysis, Nozawa and Hataro investigated the behaviour of poly (L-a-7-diaminobutyric acid). This forms a complex with cupper, but has no helical content, and it does not catalyze the oxidation of DOPA selectivity [42]. 

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

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