Pd-NHC catalysed regioselective activation of B(3,6)-H of o-carborane - a synergy between experiment and theory

Regioselective B-H activation of o-carboranes is an effective way for constructing o-carborane derivatives, which have broad applications in medicine, catalysis and the wider chemical industry. However, the mechanistic basis for the observed selectivities remains unresolved. Herein, a series of density functional theory (DFT) calculations were employed to characterise the palladium N-heterocyclic carbene (Pd-NHC) catalysed regioselective B(3,6)-diarylation of o-carboranes. Computational results at the IDSCRF(ether)-LC-ωPBE/BS1 and IDSCRF(ether)-LC-ωPBE/BS2 levels showed that the reaction undergoes a Pd(0) → Pd(II) → Pd(0) oxidation/reduction cycle, with the regioselective B(3)-H activation being the rate-determining step (RDS) for the full reaction profile. The computed RDS free energy barrier of 24.3 kcal mol-1 agrees well with the 82% yield of B(3,6)-diphenyl-o-carborane in ether solution at 298 K after 24 hours of reaction. The Ag2CO3 additive was shown to play a crucial role in lowering the RDS free energy barrier and facilitating the reaction. Natural charge population (NPA) and molecular surface electrostatic potential (ESP) analyses successfully predicted the experimentally observed regioselectivities, with electronic effects being revealed to be the dominant contributors to product selectivity. Steric hindrance was also shown to impact the reaction rate, as revealed by experimental and computational characterisation studies of substituents and ligand effects. Furthermore, computational predictions aligned with the experimental findings that NHC ligands outperform the phosphine ones for this particular reaction. Overall, the observed trends reported in this work are expected to assist in the rational optimisation of the efficiency and regioselectivity of this and related reactions.

Manipulations of phenylnorbornyl palladium species for multicomponent construction of a bridged polycyclic privileged scaffold

Hexahydromethanocarbazole is a privileged scaffold in the discovery of new drugs and photoactive organic materials due to its good balance between structural complexity and minimized entropy penalty upon receptor binding. To address the difficulty of synthesizing this highly desirable bridged polycyclic scaffold, we designed a convenient multicomponent reaction cascade as intercepted Heck addition/C-H activation/C-palladacycle formation/electrophilic attack of ANP/N-palladacycle formation/Buchwald amination. A distinguishing feature of this sophisticated strategy is the successive generation of two key phenylnorbornyl palladium species to control the reaction flow towards desired products. DFT calculations further reveal the crucial roles of Cs₂CO₃ and 5,6-diester substitutions on the norbornene reactant in preventing multiple side-reactions. This innovative method exhibits a broad scope with good yields, and therefore will enable the construction of natural-product-like compound libraries based on hexahydromethanocarbazole.

Mechanistic insight into construction of axially chiral biaryls via palladium/chiral norbornene cooperative catalysis: a DFT-based computational study

DFT calculations rationalize the enantioselectivity and clarify the reaction sequence of two aryl halides as well as the substantial role of the ortho-ester group in the aryl bromide.

Compatibility Score for Rational Electrophile Selection in Pd/NBE Cooperative Catalysis

A mechanistically guided approach is developed to predict electrophile compatibility in the palladium/norbornene (Pd/NBE) cooperative catalysis for the ipso/ortho difunctionalization of aryl halides. A key challenge in these reactions is to identify orthogonal electrophile and aryl hali de starting materials that react selectively with different transition metal complexes in separate oxidative addition events in the catalytic cycle. We performed detailed experimental and computational mechanistic studies to identify the catalytically active Pd(II) intermediate and the substrate-dependent mechanisms in reactions with various types of carbon and nitrogen electrophiles. We introduced the concept of electrophile compatibility score (ECS) to rationally select electrophiles based on the orthogonal reactivity of electrophile and aryl halide towards the Pd(0) and Pd(II) complexes. This approach was applied to predict electrophile compatibility in the Pd/NBE cooperative catalysis with a variety of electrophilic coupling partners used in alkylation, arylation, amination, and acylation reactions.

DMAP and PivOH-promoted amination/allenization reaction

This report described the first DMAP and PivOH-promoted ortho-C-H amination and ipso-allenization reaction of iodobenzenes realized by Pd/norbornene cooperative catalysis. Based on control experiments and DFT calculations, we speculated that the three ligands have different functions and mechanism paths in the reaction.

Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene cooperative catalysis has emerged as a distinct approach to construct polyfunctionalized arenes from readily available starting materials. This Review provides a comprehensive overview of this field, including the early stoichiometric investigations, catalytic reaction developments, as well as the applications in the syntheses of bioactive compounds and polymers. The section of catalytic reactions is divided into two parts according to the reaction initiation mode: Pd(0)-initiated reactions and Pd(II)-initiated reactions.

Norbornene, norbornadiene and their derivatives: promising semi-products for organic synthesis and production of polymeric materials

The methods for synthesis of promising norbornene monomers from norbornadiene and quadricyclane are summarized. A strategy for their synthesis is discussed, combining theoretical and experimental approaches to the selection of catalysts and the conditions for carrying out stereoselective reactions. The mechanisms of catalytic reactions of synthesis of norbornene monomers, as well as the progress in the macromolecular design of functional polymeric materials based on them, are considered. The data on industrial processes of production of polynorbornenes and areas of their use are presented.

The bibliography includes 297 references.

Theoretical studies on the mechanism of Pd2+-catalyzed regioselective C-H acylation of azoxybenzenes with α-oxocarboxylic acids

The reaction mechanism of the Pd2+-catalyzed regioselective C(sp2)-H acylation of azoxybenzenes with α-oxocarboxylic acids has been studied by density functional theory (DFT) calculations. This reaction mechanism involves five major steps: C-H activation, deprotonation, decarboxylation, reductive elimination and oxidation. Our calculation results indicate that the N-coordinated pathway is better than the O-coordinated pathway, which can be interpreted by distortion-interaction analysis of the C-H bond activation transition states. Furthermore, we also suggest that the C-H bond acylation of aryl 1 is more favorable than that of aryl 2, which can be attributed to the fact that five-membered ring transition states are more favorable than four-membered ring transition states and the ON-group has positive charge.

Insights into the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone

A computational study on the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone was performed using the DFT method.