Oxidative Kinetic Resolution of Alkyl Aryl Sulfoxides

Abstract

Kinetic resolution is observed in the oxidation of racemic alkyl aryl sulfoxides using a combination of VO(acac)₂ and ligand 1 in chloroform at 0 °C, conditions previously described for asymmetric oxidation of prochiral sulfides. Kinetic resolution is also ­observed in toluene as the solvent, although higher temperatures are required.

Key words

asymmetric catalysis - oxidation - kinetic resolution - sulfoxides - Schiff bases

References and Notes

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37

The ratios of response factors for each sulfoxide and sulfone at 252 nm were determined as follows: R = Me; Ar = Ph (9.9), p-BrC₆H₄ (7.5), p-ClC₆H₄ (16.5), p-Tol (12.7), p-MeOC₆H₄ (1.8), p-NO₂C₆H₄ (0.56), 2-Naphth (4.9): R = Et, Ar = Ph (10.0).

40

HPLC-grade chloroform was purchased from Fisher Scientific, stabilized with amylene (100 ppm). Since CHCl₃ can also be stabilized with a small amount of alcohols, and may contain HCl, we investigated the effects of these additives in the kinetic resolution reaction. Using dried CHCl₃ gave very similar results to those observed using either unpurified CHCl₃ or dried CHCl₃ with 1 mol% of 2 M HCl. Interestingly, however, the addition of 0.5 mol% of i-PrOH gave moderate conversion (35%) to the sulfone and racemic sulfoxide.

43

In our experience, use of 23 mL of solvent per mmol of substrate as originally reported results in slow reactions and much poorer yields, a problem that can be overcome by drastically reducing the amount of solvent. Alonso and Nájera have made similar observations when conducting larger-scale reactions: Alonso D. A. and Nájera, C., personal communication from Alonso, D. A., August 2006.