Side chain hydroxylation of aromatic compounds by fungi. Part 4. Influence of the para substituent on kinetic isotope effects during benzylic hydroxylation by Mortierella isabellina

The benzylic hydroxylation of a series of para-substituted toluenes by the fungus Mortierella isabellina has been studied by using CD3, CHD2, and CH2D methyl labelled substrates. Inter- and intra-molecular primary and secondary deuterium kinetic isotope effect ratios have been determined: the intermolecular primary effects are maximal with strongly electron-withdrawing para substituents (R = CN and CF3), while the intramolecular primary effects are minimal for R = H but increase in instances where R is electron donating or withdrawing. These results are interpreted in terms of a dependence of the hydroxylation mechanism on the nature of the para substituent.