Formate and CO2 Enable Reductive Carboxylation of Imines: Synthesis of Unnatural α-Amino Acids

Herein, a photocatalytic umpolung strategy for reductive carboxylation of imines for the synthesis of α-amino acids was disclosed. Carbon dioxide radical anion (CO2•-) generated from formate is the key single electron reductant in the reactions. An unprecedentedly broad substrate scope of imines with excellent reaction yields was obtained with carbon dioxide (CO2) and formate salt as carbon sources.

Synthesis of gem-Difluorohomoallyl Amines via a Transition-Metal-Free Defluorinative Alkylation of Benzyl Amines with Trifluoromethyl Alkenes

A mild and transition-metal-free defluorinative alkylation of benzyl amines with trifluoromethyl alkenes is reported. The features of this protocol are easy-to-obtain starting materials, a wide range of substrates, and functional group tolerance as well as high atom economy, thus offering a strategy to access a variety of gem-difluorohomoallyl amines, which are extensively distributed in pharmaceuticals and bioactive agents, with excellent chemoselectivity. The primary products can be further transformed to a diversity of 2-fluorinated pyrroline compounds.

Stable 2-Azaallyl Salts as a Bridge between 2H-Azirines and Densely Functionalized 2H-Pyrroles

An efficient protocol for the synthesis of stable 2-azaallyl anion salts by the reaction of alkyl 2-bromo-2H-azirine-2-carboxylates with trimethylsilyl cyanide/Bu4NF has been developed. The domino reaction proceeds in four steps via the cleavage of the azirine C-C bond to provide the tetrabutylammonium salts of stereochemically pure 2-azaallyl anions having U-configuration relative to the cyano groups. The anions with an ortho-substituted aryl group or styryl group exist as a mixture of two geometrical isomers across the N2-C3 bond. 2-Azaallyl anion salts have been shown to be convenient substrates for the one-pot synthesis of densely functionalized 2H-pyrroles by the alkylation-cyclization sequence.

Base-mediated carboxylation of C-nucleophiles with CO2

Carbon dioxide (CO2) is an available, abundant, and renewable C1 resource, which could be converted into value-added chemicals. Due to its inherent thermodynamic stability and kinetic inertness, it is difficult to realize its efficient utilization. Nevertheless, many elegant strategies for the utilization of CO2 have been developed using Lewis bases, frustrated Lewis pairs, hydroxyl-containing compounds, amino-group-containing compounds or transition metal catalysis. Among them, base-mediated carboxylation of C-nucleophiles is an environmentally friendly strategy for CO2 conversion, which is operationally simple, using low-toxicity bases and economical available promoters, without the use of complex ligands or cocatalysts. This review summarizes related work on the base-mediated carboxylation of C-nucleophiles with CO2, based on the effects of nucleophiles, promoters, additives, and solvents. The types of pronucleophile are categorized as follows: hydrocarbon with C(sp3)-H, C(sp2)-H or C(sp)-H bonds, organosilanes, organotin, organoboron, and N-tosylhydrazones. Typical mechanisms and applications of these carboxylation reactions are also depicted. Moreover, mechanistic comprehension of CO2 activation and conversion at a molecular level aims to further expand the repertoire of carboxylation transformations mediated by bases.

Aldehyde-catalysed carboxylate exchange in α-amino acids with isotopically labelled CO2

The isotopic labelling of small molecules is integral to drug development and for understanding biochemical processes. The preparation of carbon-labelled α-amino acids remains difficult and time consuming, with established methods involving label incorporation at an early stage of synthesis. This explains the high cost and scarcity of C-labelled products and presents a major challenge in 11C applications (11C t1/2 = 20 min). Here we report that aldehydes catalyse the isotopic carboxylate exchange of native α-amino acids with *CO2 (* = 14, 13, 11). Proteinogenic α-amino acids and many non-natural variants containing diverse functional groups undergo labelling. The reaction probably proceeds via the trapping of *CO2 by imine-carboxylate intermediates to generate iminomalonates that are prone to monodecarboxylation. Tempering catalyst electrophilicity was key to preventing irreversible aldehyde consumption. The pre-generation of the imine carboxylate intermediate allows for the rapid and late-stage 11C-radiolabelling of α-amino acids in the presence of [11C]CO2.

Cobalt-Catalyzed Umpolung Alkylation of Imines To Generate α-Branched Aliphatic Amines

Here we report a general and mild approach to prepare α-branched aliphatic amines from imines. This method capitalizes on a cobalt-catalyzed umpolung alkylation of imines, employs easily available reaction partners, and demonstrates a broad substrate scope. Mechanistic studies suggest this transformation occurs by a radical pathway.

Light-Promoted Organic Transformations Utilizing Carbon-Based Gas Molecules as Feedstocks

Carbon-based gas molecules are readily available feedstocks and are widely used in industry as building blocks or fuels. However, their application in the synthesis of fine chemicals has been hampered due to operational complexity, poor reaction efficiency and selectivity. Recent development of photoredox-promoted transformations using such gaseous reagents has received considerable attention from the synthetic community. In this review, efforts in developing light-promoted organic transformations using carbon-based natural gases as C1 or C2 feedstocks and to overcome the associated challenges are briefly summarized.

Synthesis of Unprotected 2-Arylglycines by Transamination of Arylglyoxylic Acids with 2-(2-Chlorophenyl)glycine

The transamination of α-keto acids with 2-phenylglycine is an effective methodology for directly synthesizing unprotected α-amino acids. However, the synthesis of 2-arylglycines by transamination is problematic because the corresponding products, 2-arylglycines, transaminate the starting arylglyoxylic acids. Herein, we demonstrate the use of commercially available l-2-(2-chlorophenyl)glycine as the nitrogen source in the transamination of arylglyoxylic acids, producing the corresponding 2-arylglycines without interference from the undesired self-transamination process.

Reductive Coupling of Carbon Dioxide and an Aldehyde Mediated by a Copper(I) Complex toward the Synthesis of α-Hydroxycarboxylic Acids

Copper-mediated reductive coupling between CO₂ and an aldehyde to form α-hydroxycarboxylic acid was achieved using silylborane as a reductant. CO₂ cleanly inserted into a copper-carbon bond that was formed by the reaction between a silylcopper-NHC complex and an aldehyde. A series of reactions that regenerate the silylcopper complex were developed for the synthesis of an α-hydroxycarboxylic acid. After repeating each step iteratively for two cycles, 0.62 equiv of α-hydroxycarboxylic acid based on the copper complex was obtained.

Toward ideal carbon dioxide functionalization

This Perspective recapitulates recent developments of carbon dioxide utilization in carbon-carbon bond formation reactions, with an intention of paving a way toward sustainable CO2-functionalization and its tangible applications in synthetic chemistry. CO2 functionalization reactions possess intrinsic drawbacks: the high kinetic inertness and thermodynamic stability of CO2. Numerous procedures for CO2 utilization depend on energy-intensive processes (i.e. high pressure and/or temperature), often solely relying on reactive substrates, hampering its general applications. Recent efforts thus have been dedicated to catalytic CO2-utilization under ambient reaction conditions, however, it is still limited to a few activation modes and the use of reactive substrates. Herein, ideal CO2-functionalization with particular emphasis on sustainability will be discussed based on the following sub-categories; (1) metal-catalyzed 'reductive' carboxylation reaction of halides, olefins and allyl alcohols, (2) photochemical CO2-utilization, (3) redox-neutral CO2-functionalization, and (4) enantioselective catalysis incorporating CO2 to form C-CO2 bonds (excluding strain mediated reactions with epoxide- and aziridine-based substrates). Recent progress in these fields will be discussed with the proposed reaction mechanisms and selected examples, highlighting redox-neutral, umpolung, and asymmetric carboxylation to postulate ideal CO2 functionalization reactions to be developed in the near future.

Recent advances of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in photocatalytic transformations

In this review, the recent advances of the application of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a photoredox catalyst in the past three years (2016–2018) for various organic reactions are summarized.

Visible light-promoted CO2 fixation with imines to synthesize diaryl α-amino acids

Light-mediated transformations with CO₂ have recently attracted great attention, with the focus on CO₂ incorporation into C–C double and triple bonds, organohalides and amines. Herein is demonstrated visible light -mediated umpolung imine reactivity capable of engaging CO₂ to afford α-amino acid derivatives. By employing benzophenone ketimine derivatives, CO₂ fixation by hydrocarboxylation of C=N double bonds is achieved. Good to excellent yields of a broad range of α,α–disubstituted α-amino acid derivatives are obtained under mild conditions (rt, atmospheric pressure of CO₂, visible light). A procedure that avoids tedious chromatographic purification and uses sustainable sunlight is developed to highlight the simplicity of this method.

Fixation of CO₂ in organic molecules is an area of great interest due to the implications in sustainable chemistry. Here, the authors show a visible light-mediated hydrocarboxylation of ketimines with atmospheric CO₂ to afford a number of α,α–diaryl α-amino acid derivatives.

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

This review covers the use of 2-azaallyl anions, 2-azaallyl cations, and 2-azaallyl radicals in organic synthesis up through June 2018. Particular attention is paid to both foundational studies and recent advances over the past decade involving semistabilized and nonstabilized 2-azaallyl anions as key intermediates in various carbon-carbon and carbon-heteroatom bond-forming processes. Both transition-metal-catalyzed and transition-metal-free transformations are covered. Azomethine ylides, which have received significant attention elsewhere, are discussed briefly with the primary focus on critical comparisons with 2-azaallyl anions in regard to generation and use.

Selective and Catalytic Hydrocarboxylation of Enamides and Imines with CO2 to Generate α,α-Disubstituted α-Amino Acids

The first catalytic hydrocarboxylation of enamides and imines with CO₂ to generate valuable α,α-disubstituted α-amino acids is reported. Notably, excellent chemo- and regio-selectivity are achieved, significantly different from previous reports on β-carboxylation of enamides, homocoupling or reduction of imines. Moreover, this transition-metal-free procedure exhibits low loading of an inexpensive catalyst, easily available substrates, mild reaction conditions, high efficiency, facile scalability and easy product derivatization, providing great potential for application in organic synthesis, pharmaceutical chemistry, and biochemistry.

Iron(III) N,N-Dialkylcarbamate-Catalyzed Formation of Cyclic Carbonates from CO2 and Epoxides under Ambient Conditions by Dynamic CO2 Trapping as Carbamato Ligands

Easily available and inexpensive Fe^III carbamates were employed in the solvent-free synthesis of a series of cyclic carbonates from epoxides and CO₂ at room temperature and atmospheric pressure, in the presence of a cocatalyst. Different experimental conditions (type and concentration of catalyst and cocatalyst, as well as reaction time) were investigated: Fe(O₂ CNEt₂ )₃ and NBu₄ Br acted as the best catalyst/cocatalyst combination, allowing the formation of propylene carbonate and 1,2-butylene carbonate with quantitative yield and selectivity in 24 h. According to NMR and DFT studies, the reaction proceeds with the dynamic trapping of carbon dioxide as a carbamato ligand.

Umpolung Synthesis of Vicinal Diamines: Diastereoselective Addition of 2-Azaallyl Anions to Davis-Ellman's Imines

A highly regioselective and diastereoselective addition of 2-azaallyl anions to N- tert-butanesulfinylimines is reported. This methodology affords the preparation of enantiomerically and diastereomerically pure vicinal diamines bearing two adjacent stereocenters. Reactions proceed efficiently (yield up to 94%), diastereoselectively ( dr values up to 98:2:0:0), and site-selectively to deliver products with differentiated amino groups.

Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

The mechanism of the Ni⁰ -catalyzed reductive carboxylation reaction of C(sp² )-O and C(sp³ )-O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp² )-O and C(sp³ )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.

Ruthenium-catalyzed umpolung carboxylation of hydrazones with CO2

The first ruthenium-catalyzed umpolung carboxylation of hydrazones with CO₂ to generate important aryl acetic acids is reported.

The first ruthenium-catalyzed umpolung carboxylation of hydrazones with CO₂ to generate important aryl acetic acids is reported. Besides aldehyde hydrazones, a variety of ketone hydrazones, which have not been successfully applied in previous umpolung reactions with other reactive electrophiles, also show high reactivity and selectivity under mild conditions. Moreover, this operationally simple protocol features good functional group tolerance, is readily scalable, and offers easy derivation of important structures, including bioactive felbinac and adiphenine. Computational studies reveal that this umpolung reaction proceeds through the generation of a Ru-nitrenoid followed by concerted [4 + 2] cycloaddition with CO₂.

Palladium-Catalyzed Four-Component Cascade Reaction for the Synthesis of Highly Functionalized Acyclic O,O-Acetals

A palladium-catalyzed four-component cascade reaction of carbon dioxide, amines, allenyl ethers, and aryl iodides has been developed for the first time. The novel reaction allows simultaneous construction of three different new bonds (C-N, C-O, and C-C) in a single step, affording an efficient method for the synthesis of a variety of highly functionalized acyclic O,O-acetals. Excellent chemo- and regioselectivity, wide substrate scope, and good functional group tolerance are features of the method.

Umpolung Synthesis of 1,3-Amino Alcohols: Stereoselective Addition of 2-Azaallyl Anions to Epoxides

We report the direct preparation of 1,3-amino alcohols that contain up to three contiguous stereogenic centers by the umpolung coupling of imines and epoxides. Nucleophilic 2-azaallyl anions, generated from imines, are stereoselectively added to epoxides to furnish 1,3-amino alcohols after hydrolysis of the product imine. Transformations afford amino alcohols with >98% site selectivity with respect to both reaction partners and in up to >98% yield and >20:1 dr.

Photochemical Carboxylation of Activated C(sp3 )-H Bonds with CO2

From ugly duckling to beautiful C1: Although CO₂ may represent an ideal C1 source, it is challenging to use it as a raw material and direct carboxylation with CO₂ has mainly been confined to highly reactive species. However, recent significant breakthroughs have been made in photochemical carboxylation of challenging, un-acidic, C(sp³ )-H bonds, including benzylic, allylic and amine C-H bonds.

Chemical reduction of CO2facilitated by C-nucleophiles

This feature article describes recent advances in chemical reduction of CO₂facilitated by carbon-based molecular nucleophiles.