Peroxidase-catalyzed asymmetric sulfoxidation in organic solvents versus in water

Peroxidase-catalyzed asymmetric sulfoxidations, while synthetically attract ive, suffer from relatively low reaction rates due to poor substrate solubi lities in water and from appreciable spontaneous oxidation of substrates (e specially aryl alkyl sulfides) with H2O2. In this work, we found that both of these shortcomings could be alleviated by switching from aqueous solutio ns. to certain nearly anhydrous (99.7%) organic solvents as sulfoxidation r eaction media. The rates of spontaneous oxidation of the model prochiral su bstrate thioanisole in several organic solvents were observed to be some 10 0-to 1000-fold slower than in water. In addition, the rates of asymmetric s ulfoxidation of thioanisole in isopropyl alcohol and in methanol catalyzed by horseradish peroxidase (HRP) were determined,to be tens to hundreds of t imes faster than in water under otherwise identical conditions. This dramat ic activation is due to a much higher substrate solubility in organic solve nts than in water and occurs even though the intrinsic reactivity of HRP in isopropyl alcohol and in methanol is hundreds of times lower than in water . Sulfoxidation of:thioanisole catalyzed by four other hemoproteins (soybea n peroxidase, myoglobin, hemoglobin, and cytochrome c) is also much faster in isopropyl alcohol than in;water. (C) 2000 John Wiley & Sons, Inc.