Peroxidase-catalyzed asymmetric sulfoxidation in organic solvents versus in water.

Peroxidase-catalyzed asymmetric sulfoxidations, while synthetically attractive, suffer from relatively low reaction rates due to poor substrate solubilities in water and from appreciable spontaneous oxidation of substrates (especially aryl alkyl sulfides) with H(2)O(2). In this work, we found that both of these shortcomings could be alleviated by switching from aqueous solutions to certain nearly anhydrous (99.7%) organic solvents as sulfoxidation reaction media. The rates of spontaneous oxidation of the model prochiral substrate thioanisole in several organic solvents were observed to be some 100- to 1000-fold slower than in water. In addition, the rates of asymmetric sulfoxidation of thioanisole in isopropyl alcohol and in methanol catalyzed by horseradish peroxidase (HRP) were determined to be tens to hundreds of times faster than in water under otherwise identical conditions. This dramatic activation is due to a much higher substrate solubility in organic solvents 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 (soybean peroxidase, myoglobin, hemoglobin, and cytochrome c) is also much faster in isopropyl alcohol than in water.