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Scientists Overcome Basicity Barrier Imparted by Aliphatic Amines in Palladium-catalyzed Alkenes Hydroaminocarbonylation

Aug 21, 2015

Transitional metal catalyzed hydroaminocarbonylation of alkenes is an important method to synthesize amides. However, only a few catalytic systems dealing with hydroaminocarbonylation of alkenes have been reported. The effective catalytic reactor in this catalytic system is triggered by [M-H] which can only be formed under relatively acid condition. But aliphatic amines which are more basic may inhibit the generation of [M-H], and then prevent the form of N-alkylsubstituted amides. Therefore, solutions to overcome the inherent side effect of the strong basicity of aliphatic amines would be desirable to increase the range of amides.

The research group led by Prof. HUANG Hanmin at the Lanzhou Institute of Chemical Physics (LICP) of Chinese Academy of Sciences has developed a new and efficient protocol to implementate the palladium-catalyzed hydroaminocarbonylation of alkenes with a variety of aminals by C-N bond cleavage. It allows synthesis of a variety of N-alkylsubstituted amides under mild reaction conditions.

Based on the progress made in using aminals as electrophiles for palladium-catalyzed coupling reactions and aminals’ reaction, the researchers postulated that the palladium hydride species might be generated in the presence of a catalytic amount of acid and aminal since the basicity of the aminal is lower, and would allow subsequent migratory insertion of the alkene and CO into the palladium hydride species to generate the acyl palladium species A. The interaction of A with an aminal would allow generation of the key intermediate B together with the iminium C through C-N bond cleavage. The iminium C would quickly react with another molecule of A, facilitated by water, to afford the second molecule of B together with paraformaldehyde and acid (HX). Reductive elimination of B leads to the final hydroaminocarbonylation products.

To validate their hypothesis, the researchers initially investigated the hydroaminocarbonylation reaction with styrene and aminal in the presence of a catalytic amount of Pd (TFA)2, acid, and H2O (0.55 equiv) under 10 atm of CO in anisole at 120℃. The results showed that two amino moieties of the aminal were successfully incorporated into the desired amide, thus suggesting that their strategy is feasible. They then established a cooperative catalytic system through the synergistic combination of palladium, paraformaldehyde, and acid for promoting the hydroaminocarbonylation of alkenes with both aliphatic and aromatic amines.

This study aimed at gaining a detailed mechanistic understanding of this reaction and the application of this strategy in other reactions are currently in progress. It paved the way for establishing new C-N bondformation reactions by using this cooperative catalysis. The findings have been published in Angew. Chem. Int. Ed. 2015, 54, 7657 –7661.

The work was support by the National Natural Science Foundation of China and the “135” Planning of LICP.

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