Compared with the well-explored enamine catalysis with secondary amines, the development of efficient enamine-based primary amine catalysts has remained as an elusive goal until recently. We present herein that a simple chiral primary−tertiary diamine 1d in combination with TfOH acts as an efficient enamine-based primary amine catalyst. The 1d−TfOH catalytic system could effectively catalyze the asymmetric direct aldol reaction with broad substrate scope as well as high regio- and diastereoselectivity and enantioselectivity under ambient temperature. Significantly, the reactions accommodate the synthetically important but challenging substrates, such as linear aliphatic ketones, with high regioselectivity and unprecedented syn diastereoselectivity. These results are in sharp contrast with the secondary amine mediated similar reactions wherein anti diastereoselectivity was normally observed.

Acidity effect: Correlations of pKa with catalytic activity and stereoselectivity were determined and linear free energy relationships (LFERs) were observed for both pKa−log (k) and pKa−log (R/S) correlations in meta‐ and/or para‐substituted aromatic thioureas (see figure). These results provided a basis for new catalyst development and several improved catalysts were identified in our initial attempts.

A new approach of asymmetric supramolecular catalysis has been developed by combining the supramolecular recognition of beta-cyclodextrin (beta-CD) and the superior property of a chiral primary amine catalyst. The resulted beta-CD enamine catalysts could effectively promote asymmetric direct aldol reactions with excellent enantioselectivity in an aqueous buffer solution (pH = 4.80). The identified optimal catalyst CD-1 shows interesting characteristics of supramolecular catalysis with selective recognition of aldol acceptors and donors. A detailed mechanistic investigation on such supramolecular catalysis was conducted with the aid of NMR, fluorescence, circular dichroism, and ESI-MS analysis. It is revealed that the reaction is initialized first by binding substrates into the cyclodextrin cavity via a synergistic action of hydrophobic interaction and noncovalent interaction with the CD-1 side chain. A rate-limiting enamine forming step is then involved which is followed by the product-generating C-C bond formation. A subsequent product release from the cavity completes the catalytic cycle. The possible connections between molecular recognition and asymmetric catalysis as well as their relevance to enamine catalysis in both natural enzymes and organocatalysts are discussed based on rational analysis.

The first primary aminocatalytic direct cross-aldol reaction of acetaldehyde is presented. Among the various vicinal diamines screened, the L-tert-leucine derivative 1c in conjunction with (H 4 SiW 12 O 40 ) 0.25 was identified as the optimal catalyst; good catalytic activity (up to 99 % yield in 4 h), and high

Magnetic polyoxometalates (POMs) are obtained by a simple sonication between functionalized magnetic nanoparticles and polyoxometalates. This material can be used not only as a highly active acid catalyst, but also as a catalyst support for chiral amines.

Enamine protonation: A chiral diamine catalyzes an asymmetric Friedel–Crafts reaction through catalytic enantioselective protonation of an enamine. This process can be applied to a range of α‐substituted acroleins and indoles with high yields of products and high enantioselectivity (up to 94 % ee). An OH/π interaction between H2O and the indole ring was found to play an important role in the transition state (see scheme).

A new strategy for the immobilization of asymmetric organocatalysts by combining polystyrene (PS)/sulfonic acids and chiral amines in situ through acid-base interactions is presented. The PS/sulfonic acids play a dual role as catalyst anchors and modulators for activity and stereoselectivity. Different types of polymeric sulfonic acids were examined and 1% divinylbenzene (DVB) cross-linked PS/sulfonic acid 1 e with a medium loading of sulfonic acid moieties was found to be the optimal support. Furthermore, the noncovalency of this system allows combinatorial screening of optimal catalysts for the targeted reactions. In this regard, highly efficient and enantioselective heterogeneous catalysts were identified for the asymmetric direct aldol and Michael addition reactions. The catalysts could be easily recovered by filtration and reused for six cycles with similar stereoselectivity but slightly decreased activity. Significantly, the deactivated catalysts could be regenerated following an acidic washing/amine recharging procedure.

A series of primary-tertiary diamine catalysts were designed and synthesized from primary natural amino acids. Application of these new chiral catalysts in direct aldol reactions of alpha-hydroxyketones showed very good catalytic activity (up to 97% yield) and high syn selectivity (up to syn/ anti = 30:1, 99% ee).