Narrowly-Distributed Conjugated Polymers Synthesized through Suzuki Polymerization with Palladium(II) N-Heterocyclic Carbene Complex Confined in Dendritic Mesoporous Silica Nanoparticles

A catalytic heterogenous Suzuki polymerization method was developed by confining the Pd(II)-catalyzed cross coupling reactions to take place exclusively in the nanochannels of dendritic mesoporous silica nanoparticles. Conjugated polymers with various monomer combinations, including donor-acceptor structures, were obtained in high yields. The molecular weights of the obtained polymers were well controlled with narrow molecular weight distributions (PDI value down to 1.13). All the polymeric products were highly soluble in common organic solvents, granting them with high processability. All the features of this confined Suzuki polymerization method endow the conjugated polymers great potential in optoelectronic applications.

Combined Experimental and Computational Mechanistic Investigation of the Palladium-Catalyzed Decarboxylative Cross-Coupling of Sodium Benzoates with Chloroarenes

Reported herein is a mechanistic investigation into the palladium-catalyzed decarboxylative cross-coupling of sodium benzoates and chloroarenes. The reaction was found to be first-order in Pd. A minimal substituent effect was observed with respect to chloroarene, and the reaction was zero-order with respect to chloroarene. Palladium-mediated decarboxylation was assigned as the turnover-limiting step based on an Eyring plot and density functional theory computations. Catalyst performance was found to vary based on the electrophile, which is best explained by catalyst decomposition at Pd(0). The 1,5-cyclooctadiene (COD) ligand contained in the precatalyst CODPd(CH₂TMS)₂ (Pd1) was shown to be a beneficial additive. The bench-stable Buchwald complex XPhosPdG2 could be used with exogenous COD and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (XPhos) instead of complex Pd1. Adding exogenous XPhos significantly increased the catalyst turnover number and enhanced reproducibility.

The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Azine-containing biaryls are ubiquitous scaffolds in many areas of chemistry, and efficient methods for their synthesis are continually desired. Pyridine rings are prominent amongst these motifs. Transition-metal-catalysed cross-coupling reactions have been widely used for their synthesis and functionalisation as they often provide a swift and tuneable route to related biaryl scaffolds. However, 2-pyridine organometallics are capricious coupling partners and 2-pyridyl boron reagents in particular are notorious for their instability and poor reactivity in Suzuki-Miyaura cross-coupling reactions. The synthesis of pyridine-containing biaryls is therefore limited, and methods for the formation of unsymmetrical 2,2'-bis-pyridines are scarce. This Review focuses on the methods developed for the challenging coupling of 2-pyridine nucleophiles with (hetero)aryl electrophiles, and ranges from traditional cross-coupling processes to alternative nucleophilic reagents and novel main group approaches.

Pyrrole-Protected β-Aminoalkylzinc Reagents for the Enantioselective Synthesis of Amino-Derivatives

Chiral β-aminoalkylzinc halides were prepared starting from optically pure commercial β-amino-alcohols. These amino-alcohols were converted to the corresponding N-pyrrolyl-protected alkyl iodides which undergo a zinc insertion in the presence of LiCl (THF, 25 °C, 10-90 min). Subsequent Negishi cross-coupling or acylation reactions with acid chlorides produced amino-derivatives with retention of chirality. Diastereoselective CBS-reductions of some prepared N-pyrrolyl-ketones provided 1,3-subsituted N-pyrrolyl-alcohols with high diastereoselectivity. Additionally, a deprotection procedure involving an ozonolysis allowed the conversion of the pyrrole-ring into a formamide without loss of optical purity.

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II (OAc)₂ (XPhos). This Pd^II center is then reduced to the Pd⁰ (XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I -Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰ (XPhos) as the reactive species in equilibrium with the unreactive Pd⁰ (XPhos)₂ . Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.

Late-Stage Functionalization of Peptides and Cyclopeptides Using Organozinc Reagents

We report a new late-stage functionalization of small peptides and cyclopeptides relying on the Negishi cross-coupling of readily prepared iodotyrosine- or iodophenylalanine-containing peptides with aryl-, heteroaryl-, and alkylzinc pivalates or halides. In silico and in vitro determinations of membrane permeability parameters of the modified cyclopeptides showed that in most cases, the solubility was improved by the introduction of polar pyridyl units while the cell-membrane permeability was maintained.

Preparation and Reactivation of Heterogeneous Palladium Catalysts and Applications in Sonogashira, Suzuki, and Heck Reactions in Aqueous Media

A new type of heterogeneous palladium catalyst, PdMgAl-LDH, was facilely prepared by the immobilization of Pd2+ species in the layers of a Mg-Al layered double hydroxide (LDH) with co-precipitation, and then fully characterized by using powder XRD, thermogravimetric differential thermal analysis, TEM, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy techniques. These catalysts can efficiently catalyze copper-free Sonogashira, Suzuki and Heck coupling reactions of various aryl iodides, bromides, and chlorides in aqueous media under phosphine-ligand- and organic-base-free conditions. These catalysts feature easy recovery through simple filtration and could be reused at least six times without a marked loss in activity. Notably, they can be facilely reactivated by a combination of nitrolysis with co-precipitation. The basic LDH skeletons could effectively stabilize the Pd0 species created in situ and donate electron density to the Pd0 center to facilitate the oxidative addition of aryl halides, thus the PdMgAl-LDH catalysts are stable during catalysis.

Continuous-Flow Synthesis of Biaryls by Negishi Cross-Coupling of Fluoro- and Trifluoromethyl-Substituted (Hetero)arenes

A continuous-flow method for the regioselective arylation of fluoroarenes and fluoropyridines has been developed. The telescoped procedure reported here consists of a three-step metalation, zincation, and Negishi cross-coupling sequence, providing efficient access to a variety of functionalized 2-fluorobiaryl products. Precise temperature control of the metalation step, made possible by continuous-flow technology, allowed for the efficient preparation of the arylated products in high yields and short residence times. Additionally, several examples of the regioselective arylation of benzotrifluoride derivatives are also provided.

A General and Practical Palladium-Catalyzed Direct α-Arylation of Amides with Aryl Halides

An efficient system for the direct catalytic intermolecular α-arylation of acetamide derivatives with aryl bromides and chlorides is presented. The palladium catalyst is supported by Kwong's indole-based phosphine ligand and provides monoarylated amides in up to 95% yield. Excellent chemoselectivities (>10:1) in the mono- and diarylation with aryl bromides were achieved by careful selection of bases, solvents, and stoichiometry. Under the coupling conditions, the weakly acidic α-protons of amides (pKa up to 35) were reversibly depotonated by LiO t Bu, NaO t Bu, or NaN(SiMe3)2.