Acceleration of Norrish Type I Reaction with Molecular Oxygen and Catalytic CBr₄

Abstract

We report a useful method for facile synthesis of aryl carboxylic acids from aryl ketones by aerobic photooxidation using the inexpensive and easily handled CBr₄ as catalyst. This procedure is applicable to inert compounds under usual photo-irradiation conditions, and appears very attractive from the view point of new method of expansion of Norrish Type I reaction.

Key words

Norrish Type I - photooxidation - carbon tetrabromide - aerobic - carboxylic acid

References and Notes

• 1a Kajigaeshi S. Nakagawa T. Nagasaki N. Fujisaki S. Synthesis  1985,  674 
• 1b Moriarty RM. Prakash I. Penmasta R. J. Chem. Soc., Chem. Commun.  1987,  202 
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• 2 For example, see: Lenoir D. Angew. Chem. Int. Ed.  2006,  45:  3206 ; and references cited therein
• 3a Itoh A. Hashimoto S. Kodama T. Masaki Y. Synlett  2005,  2107 
• 3b Itoh A. Hashimoto S. Masaki Y. Synlett  2005,  2639 
• 3c Hirashima S. Itoh A. Synthesis  2006,  1757 
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5

Typical Procedure
A solution of acetophenone (1a, 0.3 mmol) and CBr₄ (0.06 mmol) in dry EtOAc (5 mL) in a pyrex test tube, purged with an O₂ balloon, was stirred and irradiated externally with a 400 W high-pressure mercury lamp for 12 h. The reaction mixture was concentrated in vacuo, and 10% NaOH aq solution was added. The aqueous solution was washed with Et₂O, and then acidified with 2 N HCl aq solution, which was extracted with Et₂O. The organic layer was washed with brine and dried over MgSO₄, and concentrated in vacuo. The product was pure without further purification.

6

4-Bromobenzoic acid was obtained in 82% yield when the reaction was carried out using p-bromophenacyl bromide as substrate without CBr₄ under photo-irradiation.

7

Benzoic acid was obtained in 76% and 48% yield correspondingly when the reaction was carried out using phenylglyoxal and benzil as substrate under these reaction conditions.