Photocatalyzed Triplet Sensitization of Oximes Using Visible Light Provides a Route to Nonclassical Beckmann Rearrangement Products
Xiao Zhang and Tomislav Rovis*
*Department of Chemistry, Columbia University, New York, New York 10027, United States, Email: tr2504columbia.edu
X. Zhang, T. Rovis, J. Am. Chem. Soc., 2021, 143, 21211-21217.
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Syntheses of oximes either provide mixtures of E and Z isomers or the thermodynamically preferred E isomer. Photoisomerization of aryl oximes via visible-light-mediated energy transfer (EnT) catalysis enables a mild and general method to achieve Z isomers. In a one-pot protocol for oxime isomerization and subsequent Beckmann rearrangement, alkyl groups migrate preferentially over aryl groups.
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General Procedure for Photoisomerization of Oximes and Oxime Ethers:
E-oxime (0.1 mmol, 1.0 equiv.), and [Ir(dF-CF3ppy)2(dtbbpy)]PF6 (0.5 mol%) were placed into a 1 dr vial. Ethyl acetate (0.1 M) was added. Reactions were set up under an atmosphere of nitrogen. The resulting solution was irradiated with a Blue LED (Kessil, 40 W, 427 nm) for 2 hours (with a fan set up to maintain room temperature). The solvent was removed by a rotary evaporator (with a water bath set up at a temperature below 40 °C to avoid Z to E isomerization) and afforded Z-oxime product without further purification.
General Procedure for One-Pot Photoisomerization/Beckmann Rearrangement
E-oxime (0.1 mmol, 1.0 equiv.), and [Ir(dF-CF3ppy)2(dtbbpy)]PF6 (1 mol%) were placed into a 1 dr vial. DMF (0.1 M) was added. Reactions were set up under an atmosphere of nitrogen. The resulting solution was irradiated with a Blue LED (Kessil, 40 W, 427 nm) for 2 hours (with a fan set up to maintain room temperature). To a 0.5 dr vial was added cyanuric chloride (TCT, 0.2 mmol, 2.0 equiv) and DMF (0.1 mL). The solution was stirred for 30 minutes and a white solid was formed. Then the reaction mixture in the 1 dr vial was transferred into the 0.5 dr vial. The resultant mixture was wrapped with aluminum foil and stirred at ambient temperature for 20 hours. Upon completion, the reaction was quenched with water and extracted with ethyl acetate twice. The combined organic phase was then washed with brine, dried over anhydrous Na2SO4, filtered and concentrated. Products were purified by flash column chromatography on silica gel (gradient eluent with 5% to 20% ethyl acetate in hexanes unless otherwise noted).
oximes, Beckmann rearrangement, photochemistry