Arylamine‐Catalyzed Enamine Formation: Cooperative Catalysis with Arylamines and Acids

Abstract

The explosive growth of organocatalysis has had a huge impact on asymmetric catalysis in the past decade. Transition-metal catalysis, on the other hand, has been established for a long time as one of the most powerful methods in organic synthesis. Aminocatalysis is a major field in organocatalysis. The combination of organocatalysis with the more traditional metal Lewis acid catalysis has emerged, aiming to achieve organic transformations that cannot be accomplished by organocatalysis or metal catalysis independently. Although it promises huge potential, this research area has grown only slowly. The major challenge lies in the incompatibility of the catalysts, in particular, the combination of enamine catalysis with harder metal Lewis acid is very difficult. The circumvention of this problem would represent an important breakthrough, given the huge number of substrates that can be activated by the large variety of metal Lewis acids. Herein, we present the solution to this longstanding problem by using arylamines as the catalysts in enamine catalysis. Very importantly, we demonstrate that arylamines can serve as efficient amine catalysts in direct asymmetric aldol reactions. Furthermore, we have developed a highly chemoand enantioselective three-component azaDiels–Alder reaction by combining arylamines with metal Lewis acids. The combination of enamine catalysis with metal Lewis acid catalysis was first reported by Ibrahem and Codava in 2006. Since then, considerable progress has been made in this area, leading to a series of exciting discoveries. However, these combinations were limited to soft metals, such as Cu, Ag, Au, Ir, and Pd or Pd, activating either p-allyl electrophiles or alkynes (Scheme 1,A and B). Combining enamine catalysis with harder metal Lewis acid (Scheme 1, C) turned out to be very challenging because of acid–base self-quenching reactions, which render the catalysts inactive. In asymmetric aminocatalysis involving either an enamine or an iminium intermediate, a chiral aliphatic secondary or primary amine serves as the catalyst. Aliphatic amines are hard bases, and thus likely to be compatible with softer metals based on the soft/hard approach, but less likely to be compatible with harder metals. We hoped to find an amine catalyst that is compatible with a large variety of metal Lewis acids to significantly extend the scope of enamine/ metal Lewis acid catalysis, and to facilitate the development of a new research area of iminium/metal Lewis acid catalysis. We considered to use arylamines, such as aniline, because they have a much lower pKa value (4–6) than aliphatic amines (9–11), and should be much softer because of the delocalization of the lone pair to the aromatic p system. It appeared to us that arylamines are ideal candidates for combination with harder metal Lewis acids. Despite their ubiquity in organic chemistry, arylamines have never been used in enamine catalysis. This may be mainly due to the general understanding that the nucleophilicity of arylamines is much lower compared to aliphatic amines. However, List and co-workers suggested the formation of enamine intermediates from arylamines as a step in organocatalytic cascade reactions. In a recent report, Gong and co-workers also suggested that an achiral arylamine played a crucial role in controlling the stereochemistry of a Friedl nder condensation by forming an enamine intermediate. We speculate that arylamines might be suitable to serve as an efficient amine catalyst in enamine catalysis in conjunction with a stronger metal Lewis acid. The lower nucleophilicity of enamines can be compensated by the following factors: 1) facilitated formation of enamine in the presence of a metal Lewis acid; 2) higher activation of the electrophiles by a metal Lewis acid. The asymmetric aza-Diels–Alder reaction (ADAR) is the most convenient and powerful method to form nitrogencontaining heterocycles, which are one of the most important structural motifs in natural products, pharmaceuticals, and biosystems. While the recent progress on normal-electrondemand ADARs based on dienamines and imine dienophiles Scheme 1. Combination of enamine catalysis with metal catalysis.