Oxidation of the C—H bond

Abstract The development of methodology for asymmetric functionalization of the C—H bond offers a challenge in the strive for acquiring optically active building-blocks from simple starting materials. Asymmetric synthesis has emerged as an exciting area of chemistry by judiciously employing the principles of organic synthesis, molecular recognition, metal coordination chemistry, and catalysis. Among the various strategies available for exploiting the pools of chiral compounds, catalytic asymmetric induction offers a distinct advantage in achieving a high level of enantioselectivity during bond formation mainly due to the influence of asymmetry possessed by the ligands in the catalyst. The titanium-catalysed epoxidation1−3 of allylic alcohols, the osmium-catalysed dihydroxylation4−7 of alkenes, the copper-catalysed cyclopropanation8−10 of alkenes, and the ruthenium-catalysed asymmetric hydrogenation11,12 of alkenes are outstanding achievements in the catalytic asymmetric induction approach. Another approach is inspired by the analogy derived from nature, where the monooxygenase enzymes are, for example, known to effect stereospecific oxidation of organic compounds. This biomimetic approach has led to widespread research activity in this area and as a result an impressive start has been made in achieving good levels of selectivity in the oxidation of organic substrates by employing well-crafted small molecular catalysts. The possibility of mimicking the enantio-selective reaction of monooxygenases is a highly viable proposition from the point of view of synthetic organic chemists. Indeed, the design and development of synthetic monooxygenase mimics that catalyse enantioselective epoxidation of unfunctionalized alkenes have already made an impressive impact on stereoselective synthesis mainly due to the pioneering efforts of the research groups of Groves, Jacobsen,15,17 and Katsuki,18−22 respectively. On the other hand, the enantioselective oxidation of the saturated C—H bond has proven more difficult to accomplish with synthetic catalysts. Consequently, there has been continual effort to achieve a high level of enantioselectivity in the oxidation of the saturated C—H bond over the past few years. The following sections deal with the results achieved so far on the enantioselective oxidation of the C—H bond by employing chiral catalysts derived from ligands easily prepared from simple and easily accessible starting materials. The emphasis is on the experimental procedures of those reactions which are operationally simple and easy to perform on a synthetically useful scale.