[15430-62-3] · C5H7BrN2O · 2-Bromo-2-cyano-N,N-dimethylacetamide · (MW 191.03)
(monobrominating agent selective for a-carbon of enolizable ketones2)
Physical Data: mp 53-54 °C; bp 134-135 °C/0.3 mmHg.
Solubility: sol ethanol, benzene, chloroform, ethyl acetate, and THF; insol petroleum ether and hexane; slightly sol hot ether, isopropyl alcohol, and H2O. It is stable at rt in neutral or acidic solution, but unstable in basic media.
Form Supplied in: colorless solid.
Preparative Method: via bromination of 2-cyano-N,N-dimethylacetamide with Br2 in AcOH/Ac2O at 8-10 °C.3
Handling, Storage, and Precautions: the reagent is fairly stable at rt. For long storage, keep at 0-10 °C, shielded from light. This reagent should be handled in a fume hood.
This reagent is effective for the introduction of one bromine atom into ketones. Monobromination occurs regioselectively at the a-carbon of ketones on the side favorable to enolization. The reaction proceeds on refluxing the ketone substrate with an equimolar amount of the reagent, usually in benzene (eqs 1-8).2 As the reaction most likely involves a radical mechanism,2 nonpolar solvents are preferred. During the course of the reaction, the reagent is debrominated to 2-cyano-N,N-dimethylacetamide, which can be easily isolated and recycled to the original reagent through bromination.2
This highly selective reagent does not react with esters and does not add to carbon-carbon double bonds. Allylic bromination is not observed with this reagent. Bromomalononitrile shows reactivity similar to the reagent, but reacts with difficulty and in lower yield in some cases.2 In the literature only a few examples of brominating reagents have appeared bearing bromine-carbon bonds, such as 2,2-dibromomalononitrile,4 2,2,2-tribromoacetophenone,5 and 5,5-Dibromo-2,2-dimethyl-1,3-dioxane-4,6-dione.6
Hokkaido Institute of Pharmaceutical Sciences, Otaru, Japan
University of Shizuoka, Japan