Driven by the inherent synthetic potential of CO2 as an abundant, inexpensive and renewable C1 chemical feedstock, the recent years have witnessed renewed interest in devising catalytic CO2 fixations into organic matter. Although the formation of C-C bonds via catalytic CO2 fixation remained rather limited for a long period of time, a close look into the recent literature data indicates that catalytic carboxylation reactions have entered a new era of exponential growth, evolving into a mature discipline that allows for streamlining the synthesis of carboxylic acids, building blocks of utmost relevance in industrial endeavors. These strategies have generally proven broadly applicability and convenient to perform. However, substantial challenges still need to be addressed reinforcing the need to cover metal-catalyzed carboxylation area in a conceptual and concise manner, delineating the underlying new principles that are slowly emerging in this vibrant area of expertise.

Transition-Metal-Catalyzed Carboxylation Reactions with Carbon Dioxide.

ConspectusDifluoroalkylated compounds play a remarkably important role in life and materials sciences because of the unique characteristics of the difluoromethylene (CF2) group. In particular, precise introduction of a CF2 group at the benzylic position can dramatically improve the biological properties of the corresponding molecules. As a consequence, difluoroalkylation of aromatic compounds has become a powerful strategy in modulating the bioactivities of organic molecules. However, efficient strategies to selectively synthesize difluoroalkylated arenes had been very limited before 2012. Traditional synthetic methods in this regard suffer from either harsh reaction conditions or narrow substrate scope, significantly restricting their widespread applications, particularly for late-stage difluoroalkylation of bioactive molecules. To overcome these limitations, a straightforward route to access these valuable difluoroalkylated skeletons is the direct introduction of the difluoroalkylated group (CF2R) onto ...

Transition-Metal (Cu, Pd, Ni)-Catalyzed Difluoroalkylation via Cross-Coupling with Difluoroalkyl Halides.

The recent years have witnessed the development of metal-catalyzed reductive carboxylation of organic (pseudo)halides with CO2 as C1 source, representing potential powerful alternatives to existing methodologies for preparing carboxylic acids, privileged motifs in a myriad of pharmaceuticals and molecules displaying significant biological properties. While originally visualized as exotic cross-coupling reactions, a close look into the literature data indicates that these processes have become a fertile ground, allowing for the utilization of a variety of coupling partners, even with particularly challenging substrate combinations. As for other related cross-electrophile scenarios, the vast majority of reductive carboxylation of organic (pseudo)halides are characterized by their simplicity, mild conditions, and a broad functional group compatibility, suggesting that these processes could be implemented in late-stage diversification. This perspective describes the evolution of metal-catalyzed reductive carb...

http://pubs.acs.org/doi/pdf/10.1021/acscatal.6b02124

Metal-Catalyzed Carboxylation of Organic (Pseudo)halides with CO2

Nickel-catalyzed reductive cross-coupling reactions have emerged as powerful methods to join two electrophiles These reactions have proven particularly useful for the coupling of sec-alkyl electrophiles to form stereogenic centers; however, the development of enantioselective variants remains challenging In this Perspective, we summarize the progress that has been made toward Ni-catalyzed enantioselective reductive cross-coupling reactions

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Nickel-Catalyzed Enantioselective Reductive Cross-Coupling Reactions.

An intermolecular, three-component reductive dicarbofunctionalization of alkenes is presented here. The combination of Ni catalysis with TDAE as final reductant enables the direct formation of Csp3–Csp3 and Csp3–Csp2 bonds across a variety of π-systems using two different electrophiles that are sequentially activated with exquisite selectivity under mild reaction conditions.

Nickel-Catalyzed Reductive Dicarbofunctionalization of Alkenes.

The formation of C–C bonds directly by catalytic reductive cross-coupling of two different electrophiles represents one of the practical synthetic protocols that differs from the conventional nucleophile/electrophile coupling methods. Particularly the reductive coupling of alkyl electrophiles with other electrophiles is still a challenge. This report summarizes the advances in the formation of C(sp3)–C(sp3) bonds between two alkyl electrophiles, with emphasis on the control of chemoselectivity that is exceedingly challenging to achieve due to similar structures and reactivities of two unactivated alkyl halides. The coupling of alkyl halides with aryl or acyl electrophiles was also discussed based on the chemical approach developed by our group, followed by a brief overview of the reactions of tertiary alkyl halides. In the end, a brief overview of the challenges in this exciting field was illustrated. Whereas the reaction mechanisms generating alkyl–alkyl products are proposed to involve reactions of Ni(I) species with alkyl halides to generate Ralkyl–Ni(III)–Ralkyl intermediates through a radical/Ni cage-rebound process, the obtained evidence seemingly supports that the radical chain mechanism governs the acylation and arylation of alkyl halides. The latter features a cage-escaped alkyl radical.

https://pubs.rsc.org/en/content/articlepdf/2015/qo/c5qo90045j

Nickel-catalyzed reductive coupling of alkyl halides with other electrophiles: concept and mechanistic considerations

The use of geminal di(guanidinium) and acridin-9(10H)-one-derived di(carboxylate) derivatives (1a-c and 2a-e, respectively) allows stabilization of heterodimers characterized by high binding affinities in water (maximum ΔG 105 M-1) as inferred from UV-vis spectroscopic titrations and ITC measurements, therefore rivaling or surpassing the interaction energy between the strongest DNA or RNA triplet pairs. These duplexes are readily accessible and are structurally modifiable, rendering them attractive as building blocks for creating heteroduplex constructs. Incorporating poly(ethylene glycol)-decorated benzyl groups into the dicarboxylate, allows formation of hydrogels in the case of 1b-2c.

AAAA-DDDD Quadruple H-Bond-Assisted Ionic Interactions: Robust Bis(guanidinium)/Dicarboxylate Heteroduplexes in Water.

The use of pyridine as the sole ligand for the reductive vinylation of unactivated secondary alkyl halides under Ni-catalyzed conditions has been developed. Both alkyl- and aryl-substituted vinyl bromides are suitable, in which alkyl-decorated α-alkenyl bromides resulted in the α-products in good results.

Jun Gu

Papers

Nickel-catalyzed reductive coupling of alkyl halides with other electrophiles: concept and mechanistic considerations

AAAA-DDDD Quadruple H-Bond-Assisted Ionic Interactions: Robust Bis(guanidinium)/Dicarboxylate Heteroduplexes in Water.

Nickel-Catalyzed Reductive Cross-Coupling of Vinyl Bromides with Unactivated Alkyl Halides