Synlett 2019; 30(03): 315-318
DOI: 10.1055/s-0037-1611698
letter
© Georg Thieme Verlag Stuttgart · New York

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

Yuta Matsukawa
,
Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan   Email: hirasita@nitech.ac.jp
› Author Affiliations
Further Information

Publication History

Received: 19 October 2018

Accepted after revision: 06 November 2018

Publication Date:
04 January 2019 (online)


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.

Supporting Information

 
  • References and Notes

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  • 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).