General Direct Transformation of Secondary Amides to Ketones via Amide Activation

Carbon-carbon bond formation and functional group transformation are two cornerstones of organic synthesis. Generally, most of the established methods involve the transformation of a more reactive molecule to a more stable one, while the inverse transformation from a more stable molecule to a more reactive one is challenging. Secondary amides are a class of quite stable compounds, and ketones are a class of extremely versatile compounds enabling a number of fundamental transformations. The direct transformation of secondary amides into ketones is of high relevance in organic synthesis. Nonetheless, no such method has been reported when this work was disclosed. Here, we report in detail the general direct transformation of secondary amides into ketones by Tf2O-mediated deaminative alkylation with organocerium reagents. The influence of the base additive and the organometallic reagent on this transformation was investigated. It was found that 2-fluoropyridine gave the best results of the bases screened, and both organocerium reagents (RCeCl2) generated in situ from RLi and CeCl3 and cerium complexes generated in situ from RMgX and CeCl3 are superior to organomagnesium, organolithium and organozinc reagents. The optimum reaction condition was thus determined as successive treatment of a dichloromethane (0.25 mol/L) solution of secondary amide (1.0 equiv.) and 2-fluoropyridine (1.2 equiv.) with 1.1 molar equivalents of Tf2O (-78 degrees C, then 0 degrees C), and 3.0 molar equivalents of RM/CeCl3 (-78 degrees C), followed by hydrolysis with 2 mol/L aqueous HCl solution. This protocol shows wide substrate applicability. Using the developed method, a variety of ketones including alkyl-alkyl ketones, alkyl-aryl ketones, aryl-aryl ketones, alpha,beta-unsaturated ketones, and beta-chloroenones have been synthesized in 65%similar to 90% yields. It should be noted that the reaction of alpha,beta-unsaturated amides led to a highly selective 1,2-addition and the use of alkynyl cerium reagents generated from lithium acetylides yielded beta-chloroenones. Moreover, weakly basic allcynylborane reagents and allyltrimethyl silane, and even weakly nucleophilic styrene can also be applied as nucleophiles for the deaminative alkylation of secondary amides. On the basis of the experimental results, a plausible mechanism involving nitrilium ion intermediate and ketimine is proposed. As one significance of these transformations, secondary amides successfully serve as effective acylating reagents in the intermolecular reactions with organometallic reagents, allyltrimethyl slime and styrene (reductive acylation).National Basic Research Program (973 Program) of China [2010CB833200]; National Natural Science Foundation of China [21072160, 20832005]; Scholarship Award for Excellent Doctoral Student, Ministry of Education of Chin