Atropisomeric Carboxylic Acids Synthesis via Nickel-Catalyzed Enantioconvergent Carboxylation of Aza-Biaryl Triflates with CO2.

Upgrading CO2 to value-added chiral molecules via catalytic asymmetric C-C bond formation is a highly important yet challenging task. Although great progress on the formation of centrally chiral carboxylic acids has been achieved, catalytic construction of axially chiral carboxylic acids with CO2 has never been reported to date. Herein, we report the first catalytic asymmetric synthesis of axially chiral carboxylic acids with CO2, which is enabled by nickel-catalyzed dynamic kinetic asymmetric reductive carboxylation of racemic aza-biaryl triflates. A variety of important axially chiral carboxylic acids, which are valuable but difficult to obtain via catalysis, are generated in an enantioconvergent version. This new methodology features good functional group tolerance, easy to scale-up, facile transformation and avoids cumbersome steps, handling organometallic reagents and using stoichiometric chiral materials. Mechanistic investigations indicate a dynamic kinetic asymmetric transformation process induced by chiral nickel catalysis.

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2.

Dicarboxylic acids and derivatives are important building blocks in organic synthesis, biochemistry, and the polymer industry. Although catalytic dicarboxylation with CO2 represents a straightforward and sustainable route to dicarboxylic acids, it is still highly challenging and limited to generation of achiral or racemic dicarboxylic acids. To date, catalytic asymmetric dicarboxylation with CO2 to give chiral dicarboxylic acids has not been reported. Herein, we report the first asymmetric dicarboxylation of 1,3-dienes with CO2 via Cu catalysis. This strategy provides an efficient and environmentally benign route to chiral dicarboxylic acids with high regio-, chemo-, and enantioselectivities. The copper self-relay catalysis, that is, Cu-catalyzed boracarboxylation of 1,3-dienes to give carboxylated allyl boronic ester intermediates and subsequent carboxylation of C-B bonds to give dicarboxylates, is key to the success of this dicarboxylation. Moreover, this protocol exhibits broad substrate scope, good functional group tolerance, easy product derivatizations, and facile synthesis of chiral liquid crystalline polyester and drug-like scaffolds.

Nickel-Catalyzed Asymmetric Reductive Carbo-Carboxylation of Alkenes with CO2.

Reductive carboxylation of organo (pseudo)halides with CO2 is a powerful method to provide carboxylic acids quickly. Notably, the catalytic reductive carbo-carboxylation of unsaturated hydrocarbons via CO2 fixation is a highly challenging but desirable approach for structurally diverse carboxylic acids. There are only a few reports and no examples of alkenes via transition metal catalysis. We report the first asymmetric reductive carbo-carboxylation of alkenes with CO2 via nickel catalysis. A variety of aryl (pseudo)halides, such as aryl bromides, aryl triflates and inert aryl chlorides of particular note, undergo the reaction smoothly to give important oxindole-3-acetic acid derivatives bearing a C3-quaternary stereocenter. This transformation features mild reaction conditions, wide substrate scope, facile scalability, good to excellent chemo-, regio- and enantioselectivities. The method highlights the formal synthesis of (-)-Esermethole, (-)-Physostigmine and (-)-Physovenine, and the total synthesis of (-)-Debromoflustramide B, (-)-Debromoflustramine B and (+)-Coixspirolactam A; thereby, opening an avenue for the total synthesis of chiral natural products with CO2 .

α-Amino Acids and Peptides as Bifunctional Reagents: Carbocarboxylation of Activated Alkenes via Recycling CO2.

Carboxylic acids, including amino acids (AAs), have been widely used as reagents for decarboxylative couplings. In contrast to previous decarboxylative couplings that release CO2 as a waste byproduct, herein we report a novel strategy with simultaneous utilization of both the alkyl and carboxyl components from carboxylic acids. Under this unique strategy, carboxylic acids act as bifunctional reagents in the redox-neutral carbocarboxylation of alkenes. Diverse, inexpensive, and readily available α-AAs take part in such difunctionalizations of activated alkenes via visible-light photoredox catalysis, affording a variety of valuable but otherwise difficult to access γ-aminobutyric acid derivatives (GABAs). Additionally, a series of dipeptides and tripeptides also participate in this photocatalytic carbocarboxylation. Although several challenges exist in this system due to the low concentration and quantitative amount of CO2, as well as unproductive side reactions such as hydrodecarboxylation of the carboxylic acids and hydroalkylation of the alkenes, excellent regioselectivity and moderate to high chemoselectivity are achieved. This process features low catalyst loading, mild reaction conditions, high step and atom economy, and good functional group tolerance, and it is readily scalable. The resulting products are subject to efficient derivations, and the overall process is amenable to applications in the late-stage modification of complex compounds. Mechanistic studies indicate that a carbanion is generated catalytically and it acts as the key intermediate to react with CO2, which is also generated catalytically in situ and thus remains in low concentration. The overall transformation represents an efficient and sustainable system for organic synthesis, pharmaceutics, and biochemistry.

CO2 = CO + [O]: recent advances in carbonylation of C-H bonds with CO2.

Carbon dioxide (CO2) is an ideal one-carbon source owing to its nontoxicity, abundance, availability, and recyclability. Although the thermodynamic stability and kinetic inertness of CO2 bring its utilization challenges, many groups have achieved significant progress in the utilization of CO2 to synthesize valuable carbonyl-containing compounds, which could be also generated via oxidative carbonylation of C-H bonds with CO. As CO2 has a higher oxidation state than CO, it could be considered as a combination of CO and oxidant (CO2 = CO + [O]), thus providing a safe, redox-neutral and economic strategy. In this Feature Article, we have summarized the recent advances in carbonylation of C-H bonds with CO2 based on this concept. The plausible mechanisms of such reactions and future of this field are also discussed.

Highly Selective and Catalytic Generation of Acyclic Quaternary Carbon Stereocenters via Functionalization of 1,3-Dienes with CO2.

The catalytic asymmetric functionalization of readily available 1,3-dienes is highly important, but current examples are mostly limited to the construction of tertiary chiral centers. The asymmetric generation of acyclic products containing all-carbon quaternary stereocenters from substituted 1,3-dienes represents a more challenging, but highly desirable, synthetic process for which there are very few examples. Herein, we report the highly selective copper-catalyzed generation of chiral all-carbon acyclic quaternary stereocenters via functionalization of 1,3-dienes with CO2. A variety of readily available 1,1-disubstituted 1,3-dienes, as well as a 1,3,5-triene, undergo reductive hydroxymethylation with high chemo-, regio-, E/Z-, and enantioselectivities. The reported method features good functional group tolerance, is readily scaled up to at least 5 mmol of starting diene, and generates chiral products that are useful building blocks for further derivatization. Systemic mechanistic investigations using density functional theory calculations were performed and provided the first theoretical investigation for an asymmetric transformation involving CO2. These computational results indicate that the 1,2-hydrocupration of 1,3-diene proceeds with high π-facial selectivity to generate an (S)-allylcopper intermediate, which further induces the chirality of the quaternary carbon center in the final product. The 1,4-addition of an internal allylcopper complex, which differs from previous reports involving terminal allylmetallic intermediates, to CO2 kinetically determines the E/Z- and regioselectivity. The rapid reduction of a copper carboxylate intermediate to the corresponding silyl-ether in the presence of Me(MeO)2SiH provides the exergonic impetus and leads to chemoselective hydroxymethylation rather than carboxylation. These results provide new insights for guiding further development of asymmetric C-C bond formations with CO2.

Visible-Light-Driven External-Reductant-Free Cross-Electrophile Couplings of Tetraalkyl Ammonium Salts.

Cross-electrophile couplings between two electrophiles are powerful and economic methods to generate C-C bonds in the presence of stoichiometric external reductants. Herein, we report a novel strategy to realize the first external-reductant-free cross-electrophile coupling via visible-light photoredox catalysis. A variety of tetraalkyl ammonium salts, bearing primary, secondary, and tertiary C-N bonds, undergo selective couplings with aldehydes/ketone and CO2. Notably, the in situ generated byproduct, trimethylamine, is efficiently utilized as the electron donor. Moreover, this protocol exhibits mild reaction conditions, low catalyst loading, broad substrate scope, good functional group tolerance, and facile scalability. Mechanistic studies indicate that benzyl radicals and anions might be generated as the key intermediates via photocatalysis, providing a new direction for cross-electrophile couplings.

Oxy-Difluoroalkylation of Allylamines with CO2 via Visible-Light Photoredox Catalysis.

A selective oxy-difluoroalkylation of allylamines with carbon dioxide (CO2) via visible-light photoredox catalysis is reported. These multicomponent reactions are efficient and environmentally friendly to generate a series of important 2-oxazolidinones with functionalized difluoroalkyl groups. The good functional group tolerance, broad substrate scope, easy scalability, mild reaction conditions, and facile functionalization of products provide great potential for application in organic synthesis and pharmaceutical chemistry.

Highly Regio- and Enantioselective Copper-Catalyzed Reductive Hydroxymethylation of Styrenes and 1,3-Dienes with CO2.

Herein, we report a highly regio- and enantioselective copper-catalyzed reductive hydroxymethylation of styrenes and 1,3-dienes with 1 atm of CO2. Diverse important chiral homobenzylic alcohols were readily prepared from styrenes. Moreover, a variety of 1,3-dienes also were converted to chiral homoallylic alcohols with high yields and excellent regio-, enantio-, and Z/E-selectivities. The utility of this transformation was demonstrated by a broad range of styrenes and 1,3-dienes, facile product modification, and synthesis of bioactive compounds (R)-(-)-curcumene and (S)-(+)-ibuprofen. Mechanistic studies demonstrated the carboxylation of phenylethylcopper complexes with CO2 as one key step.

Visible-Light-Driven Palladium-Catalyzed Radical Alkylation of C-H Bonds with Unactivated Alkyl Bromides.

Reported herein is a novel visible-light photoredox system with Pd(PPh3 )4 as the sole catalyst for the realization of the first direct cross-coupling of C(sp3 )-H bonds in N-aryl tetrahydroisoquinolines with unactivated alkyl bromides. Moreover, intra- and intermolecular alkylations of heteroarenes were also developed under mild reaction conditions. A variety of tertiary, secondary, and primary alkyl bromides undergo reaction to generate C(sp3 )-C(sp3 ) and C(sp2 )-C(sp3 ) bonds in moderate to excellent yields. These redox-neutral reactions feature broad substrate scope (>60 examples), good functional-group tolerance, and facile generation of quaternary centers. Mechanistic studies indicate that the simple palladium complex acts as the visible-light photocatalyst and radicals are involved in the process.

Phosphorylation of Alkenyl and Aryl C-O Bonds via Photoredox/Nickel Dual Catalysis.

A phosphorylation of alkenyl and aryl C-O bonds at room temperature via photoredox/nickel dual catalysis is reported. By starting from easily available and inexpensive sulfonates, a variety of important alkenyl phosphonates and aryl phosphine oxides are generated in moderate to excellent yields. This method features mild reaction conditions, high selectivity, good functional group tolerance, wide substrate scope, and easy scalability.

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.

Coupling of C(sp3)-H bonds with C(sp2)-O electrophiles: mild, general and selective.

Herein is reported the mild and general coupling of amine/ether C(sp3)-H bonds with various kinds of C(sp2)-O electrophiles with high selectivity and efficiency. Valuable allylic/benzylic amines are generated in moderate to excellent yields. The utility of this transformation is demonstrated by a broad substrate scope (>50 examples), good functional group tolerance and facile product modification.

A cinchona alkaloid catalyzed enantioselective sulfa-Michael/aldol cascade reaction of isoindigos: construction of chiral bispirooxindole tetrahydrothiophenes with vicinal quaternary spirocenters.

A cinchona alkaloid catalyzed diastereoselective and enantioselective sulfa-Michael/aldol cascade reaction between 1,4-dithiane-2,5-diol and isoindigos has been successfully developed to afford the highly congested bispirooxindole tetrahydrothiophenes with vicinal quaternary spirocenters in high yields (up to 91%), excellent diastereoselectivities (up to >20 : 1 dr), and good enantioselectivities (up to 98% ee). Some synthetic transformations of the reaction products were also studied.

Asymmetric synthesis of 3,3'-spirooxindoles fused with cyclobutanes through organocatalytic formal [2 + 2] cycloadditions under H-bond-directing dienamine activation.

The first organocatalytic asymmetric synthesis of a spirooxindole skeleton incorporated with a cyclobutane moiety has been successfully developed on the basis of H-bond-directing dienamine activation. Structurally complex spirocyclobutyl oxindoles, which possess four contiguous stereocenters, including one spiro quaternary center, were obtained in good yields (up to 83%) with excellent ß,γ-regioselectivity (>19:1) and stereocontrol (up to >19:1 dr and 97% ee).

Construction of quaternary stereocenters: asymmetric α-amination of branched aldehydes catalyzed by monoimide substituted cyclohexane-1,2-diamines.

A highly efficient enantioselective α-amination of branched aldehydes catalyzed by chiral imide monosubstituted 1,2-diamine derivatives was reported to afford the quaternary stereogenic centers in excellent yields (up to 99%) and enantioselectivities (up to 97% ee). Chirality 25:668-672, 2013. © 2013 Wiley Periodicals, Inc.