Catalytic Allylic Oxidation of Cyclic Enamides and 3,4‑Dihydro‑2H‑Pyrans by TBHP

• Published in: Journal of organic chemistry Vol. 82; no. 16; pp. 8506 - 8513
• Main Authors: Yu, Yang, Humeidi, Ranad, Alleyn, James R, Doyle, Michael P
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 18.08.2017; Amer Chemical Soc
• Subjects: alkenes; Allyl Compounds - chemistry; Amides - chemistry; Catalysis; catalysts; chemical structure; Chemistry; Chemistry, Organic; cleavage (chemistry); free radicals; Ketones - chemical synthesis; Ketones - chemistry; Molecular Structure; organic chemistry; oxidation; Oxidation-Reduction; Physical Sciences; Pyrans - chemistry; Science & Technology; tert-Butylhydroperoxide - chemistry; CENTERED RADICALS; ASYMMETRIC 1,4-ADDITION; LEWIS-ACID; DIRHODIUM CAPROLACTAMATE; BETA-SCISSION REACTION; COPPER; TERT-BUTYL HYDROPEROXIDE; CONJUGATE ADDITION; STEREOCONTROLLED ACCESS; MOLECULAR-OXYGEN
• ISSN: 0022-3263; 1520-6904; 1520-6904
• DOI: 10.1021/acs.joc.7b01163

Allylic oxidation of heteroatom substituted cyclic alkenes by tert-butyl hydroperoxide (70% TBHP in water) using catalytic dirhodium caprolactamate [Rh2(cap)4] forms enone products with a variety of 2-substituted cyclic enamides and 3,4-dihyro-2H-pyrans. These reactions occur under mild reaction conditions, are operationally convenient to execute, and are effective for product formation with as low as 0.25 mol% catalyst loading. With heteroatom stabilization of the intermediate allylic free radical two sites for oxidative product formation are possible, and the selectivity of the oxidative process varies with the heteroatom when R = H. Cyclic enamides produce 4-piperidones in good yields when R = alkyl or aryl, but oxidation of 2H-pyrans also gives alkyl cleavage products. Alternative catalysts for TBHP oxidations show comparable selectivities but give lower product yields.