Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Design and Application of New Pyridine-Derived Chiral Ligands in Asymmetric Catalysis

ConspectusThe innovation of chiral ligands has been crucial for the asymmetric synthesis of functional molecules, as demonstrated by several types of widely applied "privileged" ligands. In this context, chiral pyridine-derived ligands, by far some of the oldest and most widely utilized ligands in catalysis, have attracted considerable research interest in the past half-century. However, the development of broadly applicable chiral pyridine units (CPUs) has been plagued by several intertwining challenges, thus delaying advancements in many asymmetric reactions.This Account aims to summarize the recent progress in new CPU-containing ligands, focusing on a rationally designed, modular, and tunable CPU developed in our laboratory. A significant problem thwarting conventional designs is the paradox between broad reactivity and stereoselectivity; that is, while enhanced stereoselectivity may be achieved by introducing chiral elements close to the N atom, the concomitant increase in local steric hindrance often limits catalytic activity and scope. Our newly developed CPU features a rigid [6-5-3] fused-ring framework and a tunable spirocyclic ketal side wall. The well-defined three-dimensional structure minimizes local (inner layer) steric hindrance and tunes the peripheral environment (outer layer) by remote substituents, thus securing reactivity and stereoselectivity. Different chelating ligands were readily assembled using this chiral structural module, with applications in mechanistically diverse transition-metal-catalyzed reactions. Thus, a series of chiral 2,2'-bipyridine ligands were successfully employed in the development of a general, efficient, and highly enantioselective nickel-catalyzed intermolecular reductive addition, Ullmann coupling of ortho-chlorinated aryl aldehydes, and carboxylation of benzylic (pseudo)halides with CO2. Notably, these chiral 2,2'-bipyridine ligands exhibited superior catalytic activity in the reactions compared to common N-based ligands. In addition, highly enantioselective iridium-catalyzed C-H borylation was developed using a CPU-containing N,B-bidentate ligand. Furthermore, mechanistically challenging, additive-free, and broad-scope transfer hydrogenative direct asymmetric reductive amination was achieved using a half-sandwich iridium catalyst supported by a chiral N,C-bidentate ligand. The new ligands demonstrated excellent performance in securing high catalytic activity and stereoselectivity, which, when combined with experimental and computational mechanistic investigations, supported the "double-layer control" design concept.Considering the broad applications of pyridine-derived ligands, the research progress described herein should inspire the creation of novel chiral catalysts and drive the development of many catalytic asymmetric reactions.

Catalytic Regio- and Enantioselective Boracarboxylation of Arylalkenes with CO2 and Diboron

Catalytic asymmetric carboxylation of readily available alkenes with CO2, an abundant and sustainable one-carbon building block, that gives access to value-added α-stereogenic carboxylic acids in an atom- and step-economic manner is highly attractive. However, it has remained a formidable challenge for the synthetic community. Here, the first example of Cu-catalyzed highly regio- and enantioselective boracarboxylation reaction on various arylalkenes with diboron under an atmospheric pressure of CO2 is described, which afforded a variety of chiral β-boron-functionalized α-aryl carboxylic acids with up to 87% yield and 97% ee under mild conditions. Importantly, α-substituted arylalkenes could also be subject to this protocol with excellent enantiopurities, thereby rendering an efficient approach for the generation of enantioenriched carboxylic acids with an α-chiral all-carbon quaternary center. Moreover, high functional group tolerance, scalable synthesis, and facile access to bioactive compounds, like (-)-scopolamine, (-)-anisodamine, and (-)-tropicamide, further demonstrated the synthetic utility of this strategy.

Engineered Biocatalytic Synthesis of β-N-Substituted-α-Amino Acids

Non-canonical amino acids (ncAAs) are useful synthons for the development of new medicines, materials, and probes for bioactivity. Recently, enzyme engineering has been leveraged to produce a suite of highly active enzymes for the synthesis of β-substituted amino acids. However, there are few examples of biocatalytic N-substitution reactions to make α,β-diamino acids. In this study, we used directed evolution to engineer the β-subunit of tryptophan synthase, TrpB, for improved activity with diverse amine nucleophiles. Mechanistic analysis shows that high yields are hindered by product re-entry into the catalytic cycle and subsequent decomposition. Additional equivalents of l-serine can inhibit product reentry through kinetic competition, facilitating preparative scale synthesis. We show β-substitution with a dozen aryl amine nucleophiles, including demonstration on a gram scale. These transformations yield an underexplored class of amino acids that can serve as unique building blocks for chemical biology and medicinal chemistry.

Nickel-Catalyzed Enantioconvergent Carboxylation Enabled by a Chiral 2,2'-Bipyridine Ligand

In contrast to previous approaches to chiral α-aryl carboxylic acids that based on reactions using hazardous gases, pressurized setup and mostly noble metal catalysts, in this work, a nickel-catalyzed general, efficient and highly enantioselective carboxylation reaction of racemic benzylic (pseudo)halides under mild conditions using atmospheric CO₂ has been developed. A unique chiral 2,2'-bipyridine ligand named Me-SBpy featuring compact polycyclic skeleton enabled both high reactivity and stereoselectivity. The utility of this method has been demonstrated by synthesis of various chiral α-aryl carboxylic acids (30 examples, up to 95 % yield and 99 : 1 er), including profen family anti-inflammatory drugs and transformations using the acids as key intermediates. Based on mechanistic experimental results, a plausible catalytic cycle involving Ni-complex/radical equilibrium and Lewis acid-assisted CO₂ activation has been proposed.

Synthesis of bridgehead-azacycles via dual C-N/C-C annulation of α-amino acids, aminals and maleimides

The synthesis of various bridged azacyclic adducts has recently become a reemerging topic due to their bioactive and natural product mimic profiles. Accordingly, herein, we report a method for easy access to succinamide-bridged azacyclic derivatives through the metal-free polarization-controlled dual C-N/C-C annulation of readily available α-amino acids, 2-amino benzaldehydes or pyrrole/indole-2-aldehyde and maleimide substrates. This cascade features a rare dipolarophile-induced diastereo-selective amidative annulation, followed by 3 + 2 cycloaddition as key steps.