To Bind or Not to Bind: Mechanistic Insights into C–CO2 Bond Formation with Late Transition Metals

Catalytic Regio- and Enantioselective Remote Hydrocarboxylation of Unactivated Alkenes with CO2

The catalytic regio- and enantioselective hydrocarboxylation of alkenes with carbon dioxide is a straightforward strategy to construct enantioenriched α-chiral carboxylic acids but remains a big challenge. Herein we report the first example of catalytic highly enantio- and site-selective remote hydrocarboxylation of a wide range of readily available unactivated alkenes with abundant and renewable CO2 under mild conditions enabled by the SaBOX/Ni catalyst. The key to this success is utilizing the chiral SaBOX ligand, which combines with nickel to simultaneously control both chain-walking and the enantioselectivity of carboxylation. This process directly furnishes a range of different alkyl-chain-substituted or benzo-fused α-chiral carboxylic acids bearing various functional groups in high yields and regio- and enantioselectivities. Furthermore, the synthetic utility of this methodology was demonstrated by the concise synthesis of the antiplatelet aggregation drug (R)-indobufen from commercial starting materials.

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.

Observation of inserted oxocarbonyl species in the tantalum cation-mediated activation of carbon dioxide dictated by two-state reactivity

Reductive activation of carbon dioxide (CO2) has drawn increasing attention as an effective and convenient method to unlock this stable molecule, especially via transition metal-catalyzed reactions. Taking the [TaC4O8]+ ion-molecule complex formed in the laser ablation source as a representative, the reactivity of the tantalum metal cation towards CO2 molecules is explored using infrared photodissociation spectroscopy combined with quantum chemical calculations. The strong absorption in the carbonyl stretching region provides solid evidence for the insertion reactions into CO bonds by the tantalum cation. Two inserted oxocarbonyl products are identified based on the great agreement between the experimental results and simulated infrared spectra of energetically low-lying structures in the singlet and triplet states. The pivotal role of two-state reactivity in driving CO2 activation among three different spin states is rationalized by potential energy surface analysis. Our conclusion provides valuable insight into the intrinsic mechanisms of CO2 activation by the tantalum metal cation, highlighting the affinity of tantalum for CO bond insertion in addition to typical "end-on" binding configurations.

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO₂ on an industrial scale. The high kinetic and thermodynamic stabilities of CO₂, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO₂ molecule over the last decade or so. This review focuses on the advancements in CO₂ transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO₂ transformations.

Factors Governing the Catalytic Insertion of CO 2 into Arenes – A DFT Case Study for Pd and Pt Phosphane Sulfonamido Complexes

The potential of Pd/Pt complexes for catalytic carboxylation of arenes with CO₂ is investigated by means of computational chemistry. Recently we reported that the bis[(2‐methoxyphenyl)phosphino]‐benzenesulfonamido palladium complex 1 inserts CO₂ reversibly in its Pd−C(aryl) bond generating carboxylato complex 2. In the present work we study how geometric and electronic factors of various ligands and substrates influence the overall activation barrier (energy span, ES) of a potential catalytic cycle for arene carboxylation comprising this elementary step. The tendency of the key intermediates to dimerize and thus deactivating the potential catalysts is examined as well as the role of the base, which inevitably is needed to stabilize the reaction product. We show that Pd and Pt complexes I(Pd)‐L16‐S1 and I(Pt)‐L16‐S1 do not dimerize, enable the computation of complete catalytic cycles, and show interestingly low ES values of 26.8 and 24.5 kcal/mol, respectively.

Mechanistic Studies of the Catalytic Reduction of CO2 to CO: Efficient CO-Releasing Si- and Ge-Based Catalysts

Besides its significant challenges, efficient catalytic conversion of CO₂ to value-added chemicals is highly desired. Herein, we report efficient silicon- and germanium-based catalysts for CO₂ activation and its reduction to CO studied using B3LYP-GD3/6-31++G(d,p)/tetrahydrofuran (THF) and M06-2X/6-311++g(d,p)/THF density functional theory methods. The catalysts were systematically designed based on the previously reported silicon- and germanium-based compounds. The germanium-based catalysts are reported for the first time in this study. The calculated transition state energy barriers (5.7-15.8 kcal/mol) indicate that all the catalysts can easily activate CO₂. Among all the B3LYP-GD3-calculated transition-state energy barriers, the highest energy barrier found (27.2-28.3 kcal/mol) is for the protonation of the carboxylic acid group of the silacarboxylic and germacarboxylic acids. Once the silacarboxylic and germacarboxylic acids are protonated, the water molecule can easily dehydrate and leave the catalysts with CO. The electrochemical reduction of the M-CO (M = Si and Ge) complexes further enhances the complexes to easily release CO, with all transition state energy barriers being lower than 10 kcal/mol. The results show that both CO₂ activation and its reduction to CO using the studied catalysts are thermodynamically and kinetically favorable. This work provides an important insight for CO₂ activation and its reduction to CO using earth-abundant and nontoxic main group element-based catalysts.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The kinetics of carbon dioxide insertion into a pincer-supported palladium methyl complex are studied. The complex inserts carbon dioxide at room temperature, and we explore both solvent and Lewis acid effects on carbon dioxide insertion.

DFT insights into the Ni-catalyzed regioselective hydrocarboxylation of unsaturated alkenes with CO2

The nickel-catalyzed hydrocarboxylation of alkenes using carbon dioxide has recently become an appealing method to prepare functionalized carboxylic acids with high efficiency and regioselectivity. Herein, density functional theory (DFT) calculations were conducted on the Ni-catalyzed hydrocarboxylation of aryl-/alkyl-substituted alkenes with CO₂. The α- and β-carboxylation of aromatic and aliphatic olefins originate from distinct catalytic cycles: H-transfer-carboxylation and carboxylation-H-transfer pathways. The typical hydrometallation-carboxylation mechanism is unlikely because water/carbonic acid (H-resource) are inferior hydride donors.

Ni(I)-Alkyl Complexes Bearing Phenanthroline Ligands: Experimental Evidence for CO2 Insertion at Ni(I) Centers

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species-complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions-has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO₂, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.