Copper(I)- and Mesoionic-Hydroxyamide-Catalyzed Chemoselective Aerobic Oxidation of Primary Benzylic Alcohols

Abstract

A new aerobic oxidation system consisting of Cu(I)I, 2,2'-bipyridine, N-methyl imidazole, and a mesoionic hydroxyamide was developed, with which selective oxidation of a broad range of benzylic alcohols was achieved.

Key words

mesoionic compounds - aerobic oxidation - copper - benzylic alcohols - aldehydes

• References and Notes

• 1 Wertz S, Studer A. Green Chem. 2013; 15: 3116
• 2 Brackman W, Gaasbeek CJ, Smit PJ. Rec. Trav. Chim. 1966; 85: 437
• 3 Semmelhack MF, Schmid CR, Cortes DA, Chou CS. J. Am. Chem. Soc. 1984; 106: 3374
• 4a Gamez P, Arends IW. C. E, Reedijk J, Sheldon RA. Chem. Commun. 2003; 2414
• 4b Gamez P, Arends IW. C. E, Sheldon RA, Reedijk J. Adv. Synth. Catal. 2004; 346: 805
• 5 Betzemeier B, Cavazzini M, Quici S, Knochel P. Tetrahedron Lett. 2000; 41: 4343
• 6 Kumpulainen ET. T, Koskinen AM. P. Chem. Eur. J. 2009; 15: 10901
• 7 Hoover JM, Stahl SS. J. Am. Chem. Soc. 2011; 133: 16901
• 8 Hickey DP, Schiedler DA, Matanovic I, Doan V, Atanassov P, Minteer SD, Sigman MS. J. Am. Chem. Soc. 2015; 137: 16179
• 9 Araki S, Yamamoto K, Yagi M, Inoue T, Fukagawa H, Hattori H, Yamamura H, Kawai M, Butsugan Y. Eur. J. Org. Chem. 1998; 121
• 10a Matsukawa Y, Hirashita T, Araki S. Tetrahedron 2017; 73: 6052
• 10b Matsukawa Y, Hirashita T, Araki S. Eur. J. Org. Chem. 2018; 1359
• 11a Badalyan A, Stahl SS. Nature 2016; 535: 406
• 11b The redox potential of TEMPO (0.31 V vs. Ag/Ag⁺) was observed in acetonitrile.
• 12a Hoover JM, Ryland BL, Stahl SS. J. Am. Chem. Soc. 2013; 135: 2357
• 12b Ryland BL, McCann SD, Brunold TC, Stahl SS. J. Am. Chem. Soc. 2014; 136: 12166
• 13 Improved Synthetic Procedure for 1,3-Diphenyltetrazolium-5-hydroxyamide (1): To a suspension of NH₂OH•HCl (3.48 g, 50.0 mmol) in MeCN (200 mL) was added Et₃N (6.93 mL, 50.0 mmol), and the mixture was stirred for 2 h at room temperature, followed by addition of 5-chloro-1,3-diphenyltetrazolium tetrafluoroborate (3.44 g, 10.0 mmol). The mixture was stirred for a further 3 h and then the solvent was evaporated in vacuo. To the residue was added saturated NaHCO₃ (12.0 g, 143 mmol) solution. The resulting mixture was extracted with CH₂Cl₂ and shaken with 1m HCl (100 mL). The aqueous phase was washed with CH₂Cl₂ and basified with saturated NaHCO₃. The resulting precipitate was extracted with CH₂Cl₂ and dried with anhydrous Na₂SO₄, and the resulting solution was evaporated in vacuo to give brown crystals of 1 (1.65 g, 65%).
• 14 General Procedure for the Optimization of the Cu–1-Catalyzed Aerobic Oxidation of Benzyl Alcohol (Table [1]): A mixture of benzyl alcohol 2a (0.4 mmol), Cu salt (0.02 mmol), ligand (0.02 mmol), base (0.04 mmol), and 1 (0.02 mmol) was vigorously stirred in MeCN (4.0 mL) at room temperature for 24 h in the presence of PhCN (0.2 mmol) as an internal standard. At intervals, aliquots were analyzed by GC after being passed through a SiO₂ column eluted with CH₂Cl₂. The yield of benzaldehyde 3a (t _R = 4.3 min) and the recovery of 2a (t _R = 9.4 min) were calculated on the basis of calibration curves by using authentic samples.
• 15 Representative Procedure for the Cu–1-Catalyzed Aerobic Oxidation of Benzylic Alcohols (Scheme 1, Benzhydrol 2j): A mixture of benzhydrol 2j (74 mg, 0.40 mmol), CuI (3.7 mg, 0.019 mmol), 2,2'-bipyridine (3.1 mg, 0.020 mmol), N-methyl imidazole (3.5 mg, 0.043 mmol), and 1 (5.1 mg, 0.020 mmol) was vigorously stirred in MeCN (4.0 mL) at room temperature for 3 h. The solvent was evaporated under reduced pressure and the residue was passed through a SiO₂ column eluted with CH₂Cl₂ to give 3j (73 mg, 99%). ¹H NMR (300 MHz, CDCl₃): δ 7.49 (t, J = 7.5 Hz, 4 H), 7.60 (t, J = 7.2 Hz, 2 H), 7.81 (d, J = 6.9 Hz, 4 H).