5.2 Oxidation: C–O Bond Formation by C–H Activation

• Published in: Comprehensive Chirality pp. 36 - 68
• Main Author: Irie, R.
• Format: Book Chapter
• Language: English; Japanese
• Published: Elsevier Ltd 2012
• Subjects: Allylic oxidation; Asymmetric desymmetrization; Benzylic oxidation; Enantioselective CH oxidation; Enantiotopos-differentiating reaction; Site- and stereoselective C > H activation; Enantioselective CH oxidation; Benzylic oxidation; Allylic oxidation; Asymmetric desymmetrization; Enantiotopos-differentiating reaction; Site- and stereoselective C > H activation
• ISBN: 9780080951676; 0080951686; 9780080951683; 0080951678
• DOI: 10.1016/B978-0-08-095167-6.00503-6

This chapter focuses on catalytic enantioselective direct CH oxygenations. Recent great progress in this area has relied on activated CH bonds of benzylic, allylic, and heterocyclic substrates. For enantioselective benzylic oxidation, chiral porphyrin, salen, and other metal complexes inspired by biological oxidation serve as effective catalysts. The strategy of asymmetric desymmetrization enables enantioselective transformations of meso-cyclic ethers and amines by catalytic α CH oxidation. Enantioselective allylic oxidation is feasible dominantly with chiral copper catalysts (the Kharasch–Sosnovsky reaction), but palladium systems work as well albeit yet with limited applications. The enantioselectivity reaches very high levels in several cases, but there is still great room for improvement in catalyst turnovers, scope of substrates, stoichiometric balance of substrate and cooxidant, atom-efficiency of cooxidants. However, enantioselective oxidation of unactivated CH bonds is a formidable challenge in asymmetric synthesis. Some promising studies related to this subject are also highlighted.