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Synlett 2011(10): 1435-1438  
DOI: 10.1055/s-0030-1260760
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Direct Benzylic Oxidation with Sodium Hypochlorite Using a New Efficient Catalytic System: TEMPO/Co(OAc)2

Can Jin, Li Zhang, Weike Su*
Key Laboratory of Pharmaceutical Engineering of Ministry of Education, College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310014, P. R. of China
Fax: +86(571)88320752; e-Mail: pharmlab@zjut.edu.cn;
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Publication History

Received 5 March 2011
Publication Date:
26 May 2011 (online)

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Abstract

Direct benzylic oxidation of arenes was achieved using NaClO/TEMPO/Co(OAc)2. Various aromatic aldehydes and ketones were obtained from alkylarenes directly by benzylic oxidation in good to excellent yields. The reaction reactivity, selectivity, and scope of the reaction were investigated.

Key words

benzylic oxidation - arenes - transition metals - TEMPO - sodium hypochlorite

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General Procedure for the Oxidation of 1a Using Ca(ClO) 2 To a mortar were added 1a (1 mmol), TEMPO (0.05 mmol), Co(OAc)2 (0.01 mmol), Ca(ClO)2 (2.5 mmol), and silica gel (0.3 g). After 0.5 h under solid grinding at r.t., the reaction was complete (TLC control). The reaction mixture was dissolved in CH2Cl2 (3 mL). After filtration, the solvent was evaporated off. The remaining mixture was passed through a silica gel column to give 2a. White solid; mp 47.8-49.4 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 7.74-7.84 (m, 4 H), 7.53-7.63 (m, 2 H), 7.41-7.52 (m, 4 H). ¹³C NMR (100 MHz, CDCl3): δ = 196.7, 137.5, 132.3, 130.0, 128.2. MS (EI): m/z (%) = 182 (100) [M+], 105 (15), 77 (14), 51 (8).

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General Procedures
To a solution of alkylarenes (1 mmol) in CH2Cl2 (3 mL) at 0-5 ˚C, TEMPO (0.05 mmol) and Co(OAc)2 (0.01 mmol) were added followed by the quick addition of a sample containing 3 mmol of aq NaClO at pH 8.3. After 6 h under magnetic stirring, the reaction was complete (TLC control). The organic phase is separated, washed with H2O, and dried over Na2SO4. After filtration, the solvent was evaporated off. The remaining mixture was passed through a silica gel column to obtain the pure products.
Compound 2o: pink solid; mp 160.9-162.1 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 8.21 (d, J = 7.8 Hz, 2 H), 7.39-7.68 (m, 6 H), 2.13 (s, 3 H). ¹³C NMR (100 MHz, CDCl3): δ = 181.4, 169.9, 141.8, 140.6, 134.4, 133.1, 133.0, 131.5, 130.3, 130.0, 129.0, 128.6, 127.7, 126.3, 89.6, 23.6. MS (EI): m/z (%) = 323 (3) [M+ + 4], 321 (18) [M+ + 2], 319 (27) [M+], 284 (64), 242 (100), 213 (95), 178 (71). ESI-HRMS: m/z calcd for C16H12Cl2NO2: 320.0245; found: 320.0226.

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Gramscale Preparation of 2a
To a solution of 1a (8.4 g,50 mmol) in CH2Cl2 (150 mL) at 0-5 ˚C, TEMPO (0.39 g, 2.5 mmol) and Co(OAc)2 (0.12 g, 0.5mmol) were added followed by the quick addition of a sample containing 150 mmol of aq NaClO at pH 8.3. The mixture was vigorously stirred for 6 h. The organic phase is separated and washed with H2O. The solvent was evaporated off. Purification of the residue by recrystallization gave 2a (7.98 g, 47.5 mmol, mp 47-49 ˚C) in 95% yield.