Copper-catalyzed ortho C-H Azidation of Anilines Using Molecular Oxygen as Terminal Oxidant

doi:10.6023/A19020070

Abstract

Organic azides are widely used in chemical synthesis, drug discovery, bioconjugation, and material science, owing to their flexible transformations to useful chemicals such as amines, amides, isocyanates and heterocycles. In light of the diverse value of azide-containing compounds, numerous synthetic methods have been established to access this significant functionality. Among them, direct C—H azidation reactions have attracted particular attention due to their cost- and atom-efficiency. Previous methods for the preparation of azido-substituted anilines require the employment of stoichiometric amount of harsh oxidants such as hyperoxides and hypervalent iodine reagents. To synthesize these valuable compounds in an economical and environmentally benign manner, a simple and efficient copper-catalyzed ortho C—H azidation of anilines using molecular oxygen as terminal oxidant has been developed. The reaction proceeded smoothly with the assistance of pyridine at room temperature, and afforded the synthetically useful azido-substituted anilines in moderate to good yields. Notably, the process of dehydrogenation coupling of anilines to azo compounds was significantly suppressed in this protocol. This method allows for the highly regioselective formation of C—N₃ bonds under mild reaction conditions, and exhibits good functional group and substrate scope compatibility. A general procedure for the azidation of anilines is as follows: a mixture of aniline (0.4 mmol) and CuBr (5.7 mg, 0.04 mmol) is loaded in a 20 mL Schlenk tube, which is equipped with a magnetic stir bar and subjected to evacuation/flushing with oxygen three times. Subsequently, DCM (4.0 mL), pyridine (6.3 mg, 0.08 mmol) and TMSN₃ (92.2 mg, 0.8 mmol) are added to the Schlenk tube via syringe, and the formed mixture is stirred at room temperature until the amount of target product no longer increases, which is monitored by TLC. After completion of the reaction, the solution is concentrated under vacuum and further purified by column chromatography on silica gel to give the desired product (eluent: petroleum ether/ethyl acetate).

Key words: molecular oxygen, aniline, azidation, C—H bond activation;, copper catalysis

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Lin, Fengguirong, Liang, Yujie, Li, Xinyao, Song, Song, Jiao, Ning. Copper-catalyzed ortho C-H Azidation of Anilines Using Molecular Oxygen as Terminal Oxidant[J]. Acta Chimica Sinica, 2019, 77(9): 906-910.

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Fig. & Tab. 6

Scheme 1. Direct C—H azidation of aniline derivatives

Table 1. Optimization of reaction conditions a

Table 2. Optimization of reaction conditions a

Table 3. Azidation of non-ortho-substituted anilines a,b

Scheme 2. Transformation of 2a

Scheme 3. Proposed mechanism

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