Promoting Difficult Carbon-Carbon Couplings: Which Ligand Does Best?

Mechanistic study on the reductive elimination of (aryl)(fluoroaryl)palladium complexes: a key step in regiospecific dehydrogenative cross-coupling

Cross-dehydrogenative coupling (CDC) reactions have attracted attention as short-step synthetic methods for C-C bond formation. Recently, we have developed CDC reactions between naphthalene and fluorobenzene. Rather than exhibiting general regioselectivity, this reaction proceeds selectively at the β-position of naphthalene. In this study, investigation using model complexes as reaction intermediates revealed that the origin of the unique selectivity is the exclusive occurrence of reductive elimination at the β-position. Detailed studies on the reductive elimination showed that the steric hindrance of the naphthyl group and the electron-withdrawing properties of fluorobenzene determine the position at which the reductive elimination reaction proceeds. These results show that the selectivity of the C-H functionalisation of polycyclic aromatic hydrocarbons (PAHs) is determined not by the C-H cleavage step, but by the subsequent reductive elimination step. The regioselective CDC reaction was adaptable to various PAHs but was less selective for pyrene with extended π-conjugation. In fluorobenzene substrates, the F atoms at the two ortho positions of the C-H moiety are necessary for high selectivity. The substrate ranges are in good agreement with the proposed mechanism, in which the reductive elimination step determines the regioselectivity.

Pd-Catalyzed Oxidative C-H Arylation of (Poly)fluoroarenes with Aryl Pinacol Boronates and Experimental and Theoretical Studies of its Reaction Mechanism

We report the synergistic combination of Pd(OAc)2 and Ag2O for the oxidative C-H arylation of (poly)fluoroarenes with aryl pinacol boronates (Ar-Bpin) in DMF as the solvent. This procedure can be conducted easily in air, and without using additional ligands, to afford the fluorinated unsymmetrical biaryl products in up to 98 % yield. Experimental studies suggest that the formation of [PdL2(C6F5)2] in DMF as coordinating solvent does not take place under the reaction conditions as it is stable to reductive elimination and thus would deactivate the catalyst. Thus, the intermediate [Pd(DMF)2(ArF)(Ar)] must be formed selectively to give desired arylation products. DFT calculations predict a low barrier (5.87 kcal/mol) for the concerted metalation deprotonation (CMD) process between C6F5H and the Pd(II) species formed after transmetalation between the Pd(II)X2 complex and aryl-Bpin which forms a Pd-Arrich species. Thus a Pd(Arrich)(Arpoor) complex is generated selectively which undergoes reductive elimination to generate the unsymmetrical biaryl product.

Palladium mono-N-protected amino acid complexes: experimental validation of the ligand cooperation model in C-H activation

Mechanistic proposals for the C-H activation reaction enabled by mono-N-protected amino acid ligands (MPAAs) have been supported by DFT calculations. The direct experimental observation of the ligand-assisted C-H activation has not yet been reported due to the lack of well-defined isolated palladium complexes with MPAAs that can serve as models. In this work, palladium complexes bearing chelating MPAAs (NBu₄)[Pd(κ²-N,O-AcN-CHR-COO)(C₆F₅)py] (Ac = MeC(O); R = H, Me) and [Pd(κ²-N,O-MeNH-CH₂-COO)(C₆F₅)py] have been isolated and characterized. Their evolution in a solution containing toluene leads to the C-H activation of the arene and the formation of the C₆F₅-C₆H₄Me coupling products. This process takes place only for the ligands with an acyl protecting group, showing the cooperating role of this group in a complex with a chelating MPAA, therefore experimentally validating this working model. The carboxylate group is inefficient in this C-H activation.

Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX2 complexes: metal-promoted phosphorescence enhancement

Complexes trans-[PdX₂L₂] (X = Cl and Br), where L is 1-(PR₂),2-(CHCH-C(O)Ph)-C₆F₄ (R = Ph, Cy, and iPr), display phosphorescent emission in the solid state, whereas due to their substantially lower lifetimes, the free ligands exhibit fluorescent behaviour. Alternatively, structurally identical derivatives with halide replaced by CN^- or Pd replaced by Pt are non-emissive. DFT calculations explain this diverse behaviour, showing that the hybridization of orbitals of the MX₂ moiety with those of the chalcone fragment of ligands is significant only for the LUMO of the emissive compounds. In other words, in our complexes, only ML_MCT processes (L_M = Metal-perturbed Ligand-centered orbital) lead to observable luminescence.

Intimate relationship between C-I reductive elimination, aryl scrambling and isomerization processes in Au(III) complexes

¹⁹F NMR monitoring shows that heating trans-[Au^IIIRf₂I₂]^- solutions (Rf = C₆F₃Cl₂-3,5) leads to formation of cis-[AuRf₂I₂]^-, [AuRf₃I]^- and [AuRfI₃]^-via kinetic competition between isomerization and Rf/I scrambling. The system evolution is driven by the easy Rf-I reductive elimination from [AuRfI₃]^- (forming also [AuI₂]^-), which is faster than any of the Rf-Rf couplings from the coexisting species, hindering the commonly desired and thermodynamically preferred C-C coupling. A kinetic model where I^- dissociation triggers both isomerization and transmetalation steps is proposed, which fits well the experimental data. DFT calculations support that the lower bond strength of Au^III-I compared to other halides produces a pathway switch that makes C-I coupling kinetically preferred. Consequently, it is better avoided in reactions looking for C-C coupling.

Problematic ArF-Alkynyl Coupling with Fluorinated Aryls. From Partial Success with Alkynyl Stannanes to Efficient Solutions via Mechanistic Understanding of the Hidden Complexity

The synthesis of aryl-alkynyl compounds is usually achieved via Sonogashira catalysis, but this is inefficient for fluorinated aryls. An alternative method reported by Shirakawa and Hiyama, using alkynylstannanes and hemilabile PN ligands, works apparently fine for conventional aryls, but it is also poor for fluorinated aryls. The revision of the unusual literature cycle reveals the existence and nature of unreported byproducts and uncovers coexisting cycles and other aspects that explain the reasons for the conflict. This knowledge provides a full understanding of the real complexity of these aryl/alkynylstannane systems and the deviations of their evolution from that of a classic Stille process, providing the clues to design several very efficient alternatives for the catalytic synthesis of the desired Ar^F-alkynyl compounds in almost quantitative yield. The same protocols are also very efficient for the catalytic synthesis of alkynyl-alkynyl' hetero- and homocoupling.

Palladium-Catalyzed Defluorinative Coupling of Difluoroalkenes and Aryl Boronic Acids for Ketone Synthesis

The transition-metal-catalyzed carbonylation reaction is a useful approach for ketone synthesis. However, it is often problematic to use exogenous carbonyl reagents, such as gaseous carbon monoxide. In this manuscript, we report a novel palladium-catalyzed coupling reaction of gem-difluoroalkenes and aryl boronic acids that yields bioactive indane-type ketones with an all-carbon α-quaternary center. Characterization and stoichiometric reactions of the key intermediates RCF₂ Pd^II support a water-induced defluorination and cross-coupling cascade mechanism. The vinyl difluoromethylene motif serves as an in situ carbonyl precursor which is unprecedented in transition-metal-catalyzed coupling reactions. It is expected to raise broad research interest from the perspectives of ketone synthesis, fluoroalkene functionalization, and rational design of new synthetic protocols based on the unique reactivity of difluoroalkyl palladium(II) species.

Pd/Cu bimetallic catalysis to access highly fluorinated biaryls from aryl halides and fluorinated arenes

An efficient Pd/Cu bimetallic cross-coupling catalysis of fluoroaryl halides and fluoroarenes is reported. In situ generation of the Cu nucleophile by rate determining C-H activation of highly fluorinated aryls (≥4 F atoms) leads to high cross-coupling selectivity with little formation of homocoupling products.

Palladium-Catalyzed Direct γ-C(sp3)-H Arylation of β-Alkoxy Cyclohexenones: Reaction Scope and Mechanistic Insights

Direct γ-C(sp3)-arylation of unactivated electron-rich enones is a long-standing challenge. Herein, we report a mild Pd-catalyzed method for direct γ-C(sp3)-arylation of various unactivated β-alkoxy cyclohexenones. The method is not only...

Selective synthesis of fluorinated biaryls by [MCl2(PhPEWO-F)] (M = Ni, Pd) catalysed Negishi cross-coupling

Highly selective cross-couplings to polyfluorinated assymmetric biaryls, including the symmetric biaryl C₆F₅-C₆F₅, are achieved at relatively low temperature (80 °C) and in short times using [MCl₂(PhPEWO-F)] catalysts (M = Ni, Pd; PhPEWO-F = 1-(PPh₂), 2-(CHCH-C(O)Ph)-C₆F₄), Ar^FI, and Zn(C₆F₅)₂ as example of highly fluorinated nucleophile.

Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2 Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands

The Ni^II literature complex cis-[Ni(C₆F₅)₂(THF)₂] is a synthon of cis-Ni(C₆F₅)₂ that allows us to establish a protocol to measure and compare the ligand effect on the Ni^II → Ni⁰ reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate phosphines, PR₂(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C₆F₅-C₆F₅ coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 °C, were found for the different ligands tested. The PR₂(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for Ni^II complexes; they yield up to 96% C₆F₅-C₆F₅ coupling in 5 min at 25 °C (the rest is C₆F₅H) and 100% coupling with no hydrolysis in 8 h at -22 to -53 °C.

Palladium-Catalyzed Homocoupling of Highly Fluorinated Arylboronates: Studies of the Influence of Strongly vs Weakly Coordinating Solvents on the Reductive Elimination Process

C-C reductive elimination from [PdL₂(C₆F₅)₂] to form polyfluorinated biaryls has been a challenge for over 50 years. Thus, palladium-catalyzed homocoupling of arylboronates (Ar_F-Bpin) containing two ortho-fluorine substituents is very difficult, as the reaction typically stops at the [PdL₂(Ar_F)₂] stage after two transmetalation steps. The transmetalated complexes cis-[Pd(MeCN)₂(C₆F₅)₂] (3a), cis-[Pd(MeCN)₂(2,4,6-C₆F₃H₂)₂] (3b), and cis-[Pd(MeCN)₂(2,6-C₆F₂H₃)₂] (3e) have been isolated from the reaction of Ar_F-Bpin with Pd(OAc)₂ in acetonitrile solvent, with no homocoupling observed. However, catalytic homocoupling proceeds smoothly in a "weakly coordinating" arene solvent as long as no ancillary ligands or coordinating solvents are present. DFT computations reveal that the active catalyst formed by arene solvent coordination leads to an overall reduced barrier for the reductive elimination step compared to the formation of stable [PdL₂(Ar_F)₂] complexes in the presence of a donor ligand or solvent L.

Pd-Catalyzed Alkene Diamination Reactions of Nitrogen Electrophiles: Synthesis of Cyclic Guanidines and Ureas Bearing Dialkylaminomethyl Groups

The Pd-catalyzed coupling of N-allylguanidines or N-allylureas with O-benzoylhydroxylamine derivatives affords cyclic guanidines or cyclic ureas bearing dialkylaminomethyl groups. The desired products are obtained in good yield, and substrates bearing substituents at the allylic position are transformed with moderate diastereoselectivity. The mechanism of these reactions appears to involve anti-aminopalladation of the alkene, followed by a rare sp3C-sp3N bond-forming reductive elimination from an alkylpalladium complex that contains β-hydrogen atoms.

Electrochemical instability of highly fluorinated tetraphenyl borates and syntheses of their respective biphenyls

Highly fluorinated tetraphenyl borate anions are of importance as weakly coordinating anions in metalorganic reactions. However, at high positive potentials their electrochemical stability in organic solvents is not sufficient. This was investigated by a comprehensive cyclic voltammetry study and can be used synthetically to generate highly fluorinated biphenyls.

Mechanistic Insight Leads to a Ligand Which Facilitates the Palladium-Catalyzed Formation of 2-(Hetero)Arylaminooxazoles and 4-(Hetero)Arylaminothiazoles

By using mechanistic insight, a new ligand (EPhos) for the palladium-catalyzed C-N cross-coupling between primary amines and aryl halides has been developed. Employing an isopropoxy group at the C3-position favors the C-bound isomer of the ligand-supported palladium(II) complexes and leads to significantly improved reactivity. The use of a catalyst system based on EPhos with NaOPh as a mild homogeneous base proved to be very effective in the formation of 4-arylaminothiazoles and highly functionalized 2-arylaminooxazoles. Previously, these were not readily accessible using palladium catalysis.