[Pd(IPr*)(cinnamyl)Cl]: an efficient pre-catalyst for the preparation of tetra-ortho-substituted biaryls by Suzuki-Miyaura cross-coupling

IPr*Thia - wingtip-flexible, sterically hindered, modular, N,C/S,C-chelating thiazole-donor N-heterocyclic carbene ligands

N-Heterocyclic carbenes (NHCs) represent a pivotal class of ligands in coordination chemistry owing to their unique electronic properties. In particular, hemilabile N-heterocyclic carbenes have garnered significant attention over the past decade due to their capacity to transiently coordinate to metals and open coordination sites. However, hemilabile NHC ligands have been predominantly limited to N, O and P donors, while NHC ligands bearing versatile S-donors have been severely underdeveloped. Herein, we report wingtip-flexible, sterically hindered NHC ligands that feature N,C/S,C-chelating thiazole donors in combination with the powerful IPr* (IPr* = (2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) scaffold. These ligands are prepared using a highly modular SNAr arylation of thiazole derivatives. Full structural and electronic characterization is reported. The ligands feature a high barrier to rotation around the N-thiazole axis (10 kcal mol-1). The ligands are evaluated for their steric, electron-donating and π-accepting properties as well as coordination chemistry to Ag(I), Pd(II), Rh(I) and Se. Preliminary studies on Ag, Pd and Rh catalysis are presented. The efficiency of the approach is highlighted by preparing a library of unsymmetrical imidazolium precursors. The mono-IPr* wingtip provides a highly hindered yet sterically flexible environment adjusting to metal centers, while the N-thiazolyl wingtip displays a fluxional behavior that interchanges from the hard/soft N,C to soft/soft S,C coordination. Considering the importance of hemilabile N-heterocyclic carbene ligands in metal stabilization in inorganic and organometallic chemistry, we expect that this class of ligands will be of broad interest.

N-Indole-Substituted Imidazolylidene Pd-PEPPSI Precatalysts: Enhanced Performance with a 3,5-Diisopropyl-4-indolyl Moiety

Pd-PEPPSI complexes of N-(4-indolyl)-N'-phenylimidazol-2-ylidene (IIn) ligands with a 5-isopropyl-4-indolyl moiety are synthesized and evaluated in heteroarene C-H arylation, Suzuki-Miyaura cross-coupling, and Buchwald-Hartwig amination reactions. The IIn-Pd complex bearing a 3,5-diisopropyl-4-indolyl substituent (C5) exhibits the best catalytic activity in this series and substantially outperforms commercial precatalyst PEPPSI-Pd-IPr. The results also suggest that the alkyl group at position 3 of the 4-indolyl moiety shows stronger impacts than that at position 5.

[(NHC)Pd(OAc)2]: Highly Active Carboxylate Pd(II)-NHC (NHC = N-Heterocyclic Carbene) Precatalysts for Suzuki-Miyaura and Buchwald-Hartwig Cross-Coupling of Amides by N-C(O) Activation

In the past eight years, the selective cross-coupling of amides by N-C(O) bond activation has emerged as a highly attractive manifold for the manipulation of traditionally unreactive amide bonds. In this Special Issue on Next-Generation Cross-Coupling Chemistry, we report the Suzuki-Miyaura and Buchwald-Hartwig cross-coupling of amides by selective N-C(O) cleavage catalyzed by bench-stable, well-defined carboxylate Pd(II)-NHC (NHC = N-heterocyclic carbene) catalysts {[(NHC)Pd(O2CR)2]}. This class of Pd(II)-NHCs promotes cross-coupling under exceedingly mild room-temperature conditions owing to the facile dissociation of the carboxylate ligands to form the active complex. These readily accessible Pd(II)-NHC precatalysts show excellent functional group tolerance and are compatible with a broad range of amide activating groups. Considering the mild conditions for the cross-coupling and the facile access to carboxylate Pd(II)-NHC complexes, we anticipate that this class of bench-stable complexes will find wide application in the activation of amide N-C(O) and related acyl X-C(O) bonds.

%VBur index and steric maps: from predictive catalysis to machine learning

Steric indices are parameters used in chemistry to describe the spatial arrangement of atoms or groups of atoms in molecules. They are important in determining the reactivity, stability, and physical properties of chemical compounds. One commonly used steric index is the steric hindrance, which refers to the obstruction or hindrance of movement in a molecule caused by bulky substituents or functional groups. Steric hindrance can affect the reactivity of a molecule by altering the accessibility of its reactive sites and influencing the geometry of its transition states. Notably, the Tolman cone angle and %VBur are prominent among these indices. Actually, steric effects can also be described using the concept of steric bulk, which refers to the space occupied by a molecule or functional group. Steric bulk can affect the solubility, melting point, boiling point, and viscosity of a substance. Even though electronic indices are more widely used, they have certain drawbacks that might shift preferences towards others. They present a higher computational cost, and often, the weight of electronics in correlation with chemical properties, e.g. binding energies, falls short in comparison to %VBur. However, it is worth noting that this may be because the steric index inherently captures part of the electronic content. Overall, steric indices play an important role in understanding the behaviour of chemical compounds and can be used to predict their reactivity, stability, and physical properties. Predictive chemistry is an approach to chemical research that uses computational methods to anticipate the properties and behaviour of these compounds and reactions, facilitating the design of new compounds and reactivities. Within this domain, predictive catalysis specifically targets the prediction of the performance and behaviour of catalysts. Ultimately, the goal is to identify new catalysts with optimal properties, leading to chemical processes that are both more efficient and sustainable. In this framework, %VBur can be a key metric for deepening our understanding of catalysis, emphasizing predictive catalysis and sustainability. Those latter concepts are needed to direct our efforts toward identifying the optimal catalyst for any reaction, minimizing waste, and reducing experimental efforts while maximizing the efficacy of the computational methods.

Plausible PEPPSI catalysts for direct C-H functionalization of five-membered heterocyclic bioactive motifs: synthesis, spectral, X-ray crystallographic characterizations and catalytic activity

In this study, a series of benzimidazolium salts were synthesized as asymmetric N-heterocyclic carbene (NHC) precursors. Nine novel palladium complexes with the general formula [PdX2(NHC)(pyridine)] were synthesized using benzimidazolium salts in the PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) theme. All synthesized Pd(ii) complexes are stable. The synthesized compounds were thoroughly characterized by respective spectroscopic techniques, such as 1HNMR, 13C NMR, FTIR spectroscopy, X-ray crystallography and elemental analysis. The geometric structure of the palladium N-heterocyclic carbene has been optimized in the framework of density functional theory (DFT) using the B3LYP-D3 dispersion functional with LANL2DZ as a basis set. The on/off mechanism of pyridine assisted Pd-NHC complexes made them the best C-H functionalized catalysts for regioselective C-5 arylated products. Five membered heterocyclic compounds such as 2-acetyl furan, furfuryl acetate 2-acetylthiophene and N-methylpyrrole-2-carboxaldehyde were treated with numerous aryl bromides and arylchlorides under optimal catalytic reaction conditions. Interestingly, all the prepared catalysts possessed essential structural features that facilitated the formation of desired coupling products in quantitative yield with excellent selectivity. The arylation reaction of bromoacetophenone was highly catalytically active with only 1 mol% catalyst loading at 150 °C for 2 hours. To check the efficiency of the synthesized complexes, three different five member heterocyclic substrates (2-acetylfuran, 2-acetylthiophen, 2-propylthaizole) were tested with a number of aryl bromides bearing both electron-donating and electron-withdrawing groups on para position. The data in Tables 2-4. Indicated that electron-donating groups on the para position of aryl halide decreased the catalytic conversion while electron-withdrawing groups increased the catalytic conversion this was due to the high nucleophilicity of the electron-donating substituents.

Emerging Trends in Cross-Coupling: Twelve-Electron-Based L1Pd(0) Catalysts, Their Mechanism of Action, and Selected Applications

Monoligated palladium(0) species, L1Pd(0), have emerged as the most active catalytic species in the cross-coupling cycle. Today, there are methods available to generate the highly active but unstable L1Pd(0) catalysts from stable precatalysts. While the size of the ligand plays an important role in the formation of L1Pd(0) during in situ catalysis, the latter can be precisely generated from the precatalyst by various technologies. Computational, kinetic, and experimental studies indicate that all three steps in the catalytic cycle─oxidative addition, transmetalation, and reductive elimination─contain monoligated Pd. The synthesis of precatalysts, their mode of activation, application studies in model systems, as well as in industry are discussed. Ligand parametrization and AI based data science can potentially help predict the facile formation of L1Pd(0) species.

NHC-BIAN-Cu(I)-Catalyzed Friedländer-Type Annulation of 2-Amino-3-(per)fluoroacetylpyridines with Alkynes on Water

The direct catalytic alkynylation/dehydrative cyclization of 2-amino-3-trifluoroacetyl-pyridines on water was developed for the efficient synthesis of a broad range of fluorinated 1,8-naphthyridines from terminal alkynes. A novel N-heterocyclic carbene (NHC) ligand system that combines a π-extended acenaphthylene backbone with sterically bulky pentiptycene pendant groups was successfully utilized in a copper- or silver-mediated cyclization. Computational analysis of the reaction pathway supports our explanation of the different experimental conversions and yields for the set of copper and silver catalysts. The impact of steric hindrance at the metal center and the flexibility of substituents on the imidazole ring of the NHC on catalytic performance are also discussed.

Halogen-Bridged Methylnaphthyl Palladium Dimers as Versatile Catalyst Precursors in Coupling Reactions

Halogen‐bridged methylnaphthyl (MeNAP) palladium dimers are presented as multipurpose Pd‐precursors, ideally suited for catalytic method development and preparative organic synthesis. By simply mixing with phosphine or carbene ligands, they are in situ converted into well‐defined monoligated complexes. Their catalytic performance was benchmarked against state‐of‐the‐art systems in challenging Buchwald–Hartwig, Heck, Suzuki and Negishi couplings, and ketone arylations. Their use enabled record‐setting activities, beyond those achievable by optimization of the ligand alone. The MeNAP catalysts permit syntheses of tetra‐ortho‐substituted arenes and bulky anilines in near‐quantitative yields at room temperature, allow mono‐arylations of small ketones, and enable so far elusive cross‐couplings of secondary alkyl boronic acids with aryl chlorides.

Mechanistic Aspects of the Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling Reaction

The story of C-C bond formation includes several reactions, and surely Suzuki-Miyaura is among the most outstanding ones. Herein, a brief historical overview of insights regarding the reaction mechanism is provided. In particular, the formation of the catalytically active species is probably the main concern, thus the preactivation is in competition with, or even assumes the role of the rate determining step (rds) of the overall reaction. Computational chemistry is key in identifying the rds and thus leading to milder conditions on an experimental level by means of predictive catalysis.

IPr# - highly hindered, broadly applicable N-heterocyclic carbenes

IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represents the most important NHC (NHC = N-heterocyclic carbene) ligand throughout the field of homogeneous catalysis. Herein, we report the synthesis, catalytic activity, and full structural and electronic characterization of novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept, including IPr#, Np# and BIAN-IPr#. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420, enabling broad access of the academic and industrial researchers to new ligands for reaction optimization and screening. In particular, the synthesis of IPr# hinges upon cost-effective, modular alkylation of aniline, an industrial chemical that is available in bulk. The generality of this approach in ligand design is demonstrated through facile synthesis of BIAN-IPr# and Np#, two ligands that differ in steric properties and N-wingtip arrangement. The broad activity in various cross-coupling reactions in an array of N–C, O–C, C–Cl, C–Br, C–S and C–H bond cross-couplings is demonstrated. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(i), Rh(i) and Pd(ii) is presented. Given the tremendous importance of NHC ligands in homogenous catalysis, we expect that this new class of NHCs will find rapid and widespread application.

We report novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420.

Synthesis and catalytic activity of palladium complexes bearing N-heterocyclic carbenes (NHCs) and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) ligands

The synthesis and characterization of novel palladium complexes bearing N-heterocyclic carbenes (NHCs) and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) are reported. These organometallic complexes can be easily obtained using two different synthetic strategies that involve either the substitution of the pyridine ligand from trans-[Pd(NHC)(Py)Cl2] or by simple addition of the CAP ligand to dimeric species [Pd(NHC)Cl2]2. The mixed NHC/CAP complexes were tested as pre-catalysts in the Buchwald-Hartwig aryl amination coupling, showing good catalytic activity, especially in the case of cis-[Pd(IPr)(CAP)Cl2].

Differences in the Performance of Allyl Based Palladium Precatalysts for Suzuki-Miyaura Reactions

Palladium(II) precatalysts are used extensively to facilitate cross-coupling reactions because they are bench stable and give high activity. As a result, precatalysts such as Buchwald's palladacycles, Organ's PEPPSI species, Nolan's allyl-based complexes, and Yale's 1-tert-butylindenyl containing complexes, are all commercially available. Comparing the performance of the different classes of precatalysts is challenging because they are typically used under different conditions, in part because they are reduced to the active species via different pathways. However, within a particular class of precatalyst, it is easier to compare performance because they activate via similar pathways and are used under the same conditions. Here, we evaluate the activity of different allyl-based precatalysts, such as (η³-allyl)PdCl(L), (η³-crotyl)PdCl(L), (η³-cinnamyl)PdCl(L), and (η³-1-tert-butylindenyl)PdCl(L) in Suzuki-Miyaura reactions. Specifically, we evaluate precatalyst performance as the ancillary ligand (NHC or phosphine), reaction conditions, and substrates are varied. In some cases, we connect relative activity to both the mechanism of activation and the prevalence of the formation of inactive palladium(I) dimers. Additionally, we compare the performance of in situ generated precatalysts with commonly used palladium sources such as tris(dibenzylideneacetone)dipalladium(0) (Pd₂dba₃), bis(acetonitrile)dichloropalladium(II) (Pd(CH₃CN)₂Cl₂), and palladium acetate. Our results provide information about which precatalyst to use under different conditions.

Ligand Steric Effects of α-Diimine Nickel(II) and Palladium(II) Complexes in the Suzuki-Miyaura Cross-Coupling Reaction

Nickel catalysts represent a low cost and environmentally friendly alternative to palladium-based catalytic systems for Suzuki-Miyaura cross-coupling (SMC) reactions. However, nickel catalysts have suffered from poor air, moisture, and thermal stabilities, especially at high catalyst loading, requiring controlled reaction conditions. In this report, we examine a family of mono- and dinuclear Ni(II) and Pd(II) complexes with a diverse and versatile α-diimine ligand environment for SMC reactions. To evaluate the ligand steric effects, including the bite angle in the reaction outcomes, the structural variation of the complexes was achieved by incorporating iminopyridine- and acenaphthene-based ligands. Moreover, the impact of substrate bulkiness was investigated by reacting various aryl bromides with phenylboronic acid, 2-naphthylboronic acid, and 9-phenanthracenylboronic acid. Yields were the best with the dinuclear complex, being nearly quantitative (93-99%), followed by the mononuclear complexes, giving yields of 78-98%. Consequently, α-diimine-based ligands have the potential to deliver Ni-based systems as sustainable catalysts in SMC.

Pd(NHC)(cinnamyl)Cl-catalyzed Suzuki cross-coupling reaction of aryl sulfonates with arylboronic acids

A series of N-heterocyclic carbene (NHC)-palladium catalysts have been synthesized and applied to catalyze the Suzuki coupling reaction efficiently between aryl sulfonates and arylboronic acids with the potassium phosphate heptahydrate as a base. The desired yields are obtained even with less reactive aryl tosylates or aryl mesylates as substrates. This method was applied successfully to the synthesis of the (R)-2-(t-butoxycarbonylamino)-3-(biphenyl-4-yl)-propan-1-ol which is the key intermediate of sacubitril, a component of the newly approved antihypertensive drug "Entresto."

Bulky-Yet-Flexible Carbene Ligands and Their Use in Palladium Cross-Coupling

In recent years, several classes of new N-heterocyclic carbene (NHC) ligands were developed around the concept of “flexible steric bulk”. The steric hindrance of these ligands brings stability to the active species, while ligand flexibility still allows for the approach of the substrate. In this review, the synthesis of several types of new classes, such as IBiox, cyclic alkyl amino carbenes (CAAC), ITent, and IPr* are discussed, as well as how they move the state-of-the-art in palladium catalyzed cross-coupling forward.

Well-Defined Pre-Catalysts in Amide and Ester Bond Activation

Over the past few decades, transition metal catalysis has witnessed a rapid and extensive development. The discovery and development of cross-coupling reactions is considered to be one of the most important advancements in the field of organic synthesis. The design and synthesis of well-defined and bench-stable transition metal pre-catalysts provide a significant improvement over the current catalytic systems in cross-coupling reactions, avoiding excess use of expensive ligands and harsh conditions for the synthesis of pharmaceuticals, agrochemicals and materials. Among various well-defined pre-catalysts, the use of Pd(II)-NHC, particularly, provided new avenues to expand the scope of cross-coupling reactions incorporating unreactive electrophiles, such as amides and esters. The strong σ-donation and tunable steric bulk of NHC ligands in Pd-NHC complexes facilitate oxidative addition and reductive elimination steps enabling the cross-coupling of broad range of amides and esters using facile conditions contrary to the arduous conditions employed under traditional catalytic conditions. Owing to the favorable catalytic activity of Pd-NHC catalysts, a tremendous progress was made in their utilization for cross-coupling reactions via selective acyl C⁻X (X=N, O) bond cleavage. This review highlights the recent advances made in the utilization of well-defined pre-catalysts for C⁻C and C⁻N bond forming reactions via selective amide and ester bond cleavage.

Pd-catalyzed enantioselective cyclopropanation of nitriles with mono substituted allyl carbonates enabled by the bulky N-heterocyclic carbene ligand

Enantioselective cyclopropanation of nitriles with allyl reagents was realized using Pd/NHC as a catalyst and the reasons for the cyclopropanation were investigated.

Computationally Assisted Mechanistic Investigation and Development of Pd-Catalyzed Asymmetric Suzuki-Miyaura and Negishi Cross-Coupling Reactions for Tetra-ortho-Substituted Biaryl Synthesis

Metal-catalyzed cross-coupling reactions are extensively employed in both academia and industry for the synthesis of biaryl derivatives for applications to both medicine and material science. Application of these methods to prepare tetra-ortho-substituted biaryls leads to chiral atropisomeric products that introduces the opportunity to use catalyst-control to develop asymmetric cross-coupling procedures to access these important compounds. Asymmetric Pd-catalyzed Suzuki-Miyaura and Negishi cross-coupling reactions to form tetra-ortho-substituted biaryls were studied employing a collection of P-chiral dihydrobenzooxaphosphole (BOP) and dihydrobenzoazaphosphole (BAP) ligands. Enantioselectivities of up to 95:5 and 85:15 er were identified for the Suzuki-Miyaura and Negishi cross-coupling reactions, respectively. Unique ligands for the Suzuki-Miyaura reaction vs the Negishi reaction were identified. A computational study on these Suzuki-Miyaura and Negishi cross-coupling reactions enabled an understanding in the differences between the enantiodiscriminating events between these two cross-coupling reactions. These results support that enantioselectivity in the Negishi reaction results from the reductive elimination step, whereas all steps in the Suzuki-Miyaura catalytic cycle contribute to the overall enantioselection with transmetalation and reductive elimination providing the most contribution to the observed selectivities.