Interpreting Oxidative Addition of Ph-X (X = CH3, F, Cl, and Br) to Monoligated Pd(0) Catalysts Using Molecular Electrostatic Potential

Unveiling the Intermolecular Interactions between Drug 5-Fluorouracil and Watson-Crick/Hoogsteen Base Pairs: A Computational Analysis

The adsorption of 5-fluorouracil (5FU) on Watson-Crick (WC) base pairs and Hoogsteen (HT) base pairs has been studied using the dispersion-corrected density functional theory (DFT). The adsorption, binding energy, and thermochemistry for the drug 5FU on the WC and HT base pairs were determined. The most stable geometries were near planar geometry, and 5FU has a higher preference for WC than HT base pairs. The adsorption energies of 5FU on nucleobase pairs are consistently higher than pristine nucleobase pairs, indicating that nucleobase pair cleavage is less likely during the adsorption of the 5FU drug. The enthalpy change for the formation of 5FU-DNA base pairs is higher than that for the formation of 5FU-nucleobases and is enthalpy-driven. The E gap of AT base pairs is higher, suggesting that their chemical reactivity toward further reaction would be less than that of GC base pairs. The electron density difference (EDD) analysis shows a significant decrease in electron density in aromatic regions on the purine bases (adenine/guanine) compared to the pyrimidine bases. The MESP diagram of the stable 5FU-nucleobase pair complexes shows a directional interaction, with the positive regions in a molecule interacting with the negative region of other molecules. The atoms in molecule analysis show that the ρ(r) values of C=O···H-N are higher than those of N···H/N-H···O. The N···H intermolecular bonds between the base pair/drug and nucleobases are weak, closed shell interactions and are electrostatic in nature. The noncovalent interaction analysis shows that several new spikes are engendered along with an increase in their strength, which indicates that the H-bonding interactions are stronger and play a dominant role in stabilizing the complexes. Energy decomposition analysis shows that the drug-nucleobase pair complex has a marginal increase in the electrostatic contributions compared to nucleobase pair complexes.

Nickel-Based Catalysts for the Selective Monoarylation of Dichloropyridines: Ligand Effects and Mechanistic Insights

This report describes a detailed study of Ni phosphine catalysts for the Suzuki-Miyaura coupling of dichloropyridines with halogen-containing (hetero)aryl boronic acids. With most phosphine ligands these transformations afford mixtures of mono- and diarylated cross-coupling products as well as competing oligomerization of the boronic acid. However, a ligand screen revealed that PPh2Me and PPh3 afford high yield and selectivity for monoarylation over diarylation as well as minimal competing oligomerization of the boronic acid. Several key observations were made regarding the selectivity of these reactions, including: (1) phosphine ligands that afford high selectivity for monoarylation fall within a narrow range of Tolman cone angles (between 136° and 157°); (2) more electron-rich trialkylphosphines afford predominantly diarylated products, while less-electron rich di- and triarylphosphines favor monoarylation; (3) diarylation proceeds via intramolecular oxidative addition; and (4) the solvent (MeCN) plays a crucial role in achieving high monoarylation selectivity. Experimental and DFT studies suggest that all these data can be explained based on the reactivity of a key intermediate: a Ni0-π complex of the monoarylated product. With larger, more electron-rich trialkylphosphine ligands, this π complex undergoes intramolecular oxidative addition faster than ligand substitution by the MeCN solvent, leading to selective diarylation. In contrast, with relatively small di- and triarylphosphine ligands, associative ligand substitution by MeCN is competitive with oxidative addition, resulting in selective formation of monoarylated products. The generality of this method is demonstrated with a variety of dichloropyridines and chloro-substituted aryl boronic acids. Furthermore, the optimal ligand (PPh2Me) and solvent (MeCN) are leveraged to achieve the Ni-catalyzed monoarylation of a broader set of dichloroarene substrates.

Ni-Catalyzed Enantioselective Intramolecular Mizoroki-Heck Reaction for the Synthesis of Phenanthridinone Derivatives

A Ni-catalyzed enantioselective intramolecular Mizoroki-Heck reaction has been developed to transform symmetrical 1,4-cyclohexadienes with attached aryl halides into phenanthridinone analogues containing quaternary stereocenters. Herein, we report important advances in reaction optimization enabling control of unwanted proto-dehalogenation and alkene reduction side products. Moreover, this approach provides direct access to six-membered ring heterocyclic systems bearing all-carbon quaternary stereocenters, which have been much more challenging to form enantioselectively with nickel-catalyzed Heck reactions. A wide range of substrates were demonstrated to work in good to excellent yields. Good enantioselectivity was demonstrated using a new synthesized chiral iQuinox-type bidentate ligand (L27). The sustainability, low price of nickel catalysts, and significantly faster reaction rate (1 h) versus that of a recently reported palladium-catalyzed reaction (20 h) make this process an attractive alternative.

Synthesis of tetracyclic 4H-benzo[5,6]chromeno[3,4-d]oxazoles via palladium-catalyzed intramolecular direct heteroarylation

We report a palladium-catalyzed intramolecular direct heteroarylation of oxazole tethered β-naphthols to access corresponding tetracyclic 4H-benzo[5,6]chromeno[3,4-d]oxazoles. Various functional groups are well tolerated and furnished the desired products in good to excellent yields under the present reaction conditions. The scale-up reaction and synthetic utility of the resulting molecules have been demonstrated. Moreover, UV/vis absorption and fluorescence emission properties have been evaluated for these polyheterocyclic compounds.

Guided discovery of chemical reaction pathways with imposed activation

Computational power and quantum chemical methods have improved immensely since computers were first applied to the study of reactivity, but the de novo prediction of chemical reactions has remained challenging. We show that complex reaction pathways can be efficiently predicted in a guided manner using chemical activation imposed by geometrical constraints of specific reactive modes, which we term imposed activation (IACTA). Our approach is demonstrated on realistic and challenging chemistry, such as a triple cyclization cascade involved in the total synthesis of a natural product, a water-mediated Michael addition, and several oxidative addition reactions of complex drug-like molecules. Notably and in contrast with traditional hand-guided computational chemistry calculations, our method requires minimal human involvement and no prior knowledge of the products or the associated mechanisms. We believe that IACTA will be a transformational tool to screen for chemical reactivity and to study both by-product formation and decomposition pathways in a guided way.

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...

A reactivity model for oxidative addition to palladium enables quantitative predictions for catalytic cross-coupling reactions

Making accurate, quantitative predictions of chemical reactivity based on molecular structure is an unsolved problem in chemical synthesis, particularly for complex molecules. We report an approach to reactivity prediction for...

Electrostatic Potential Topology for Probing Molecular Structure, Bonding and Reactivity

Following the pioneering investigations of Bader on the topology of molecular electron density, the topology analysis of its sister field viz. molecular electrostatic potential (MESP) was taken up by the authors’ groups. Through these studies, MESP topology emerged as a powerful tool for exploring molecular bonding and reactivity patterns. The MESP topology features are mapped in terms of its critical points (CPs), such as bond critical points (BCPs), while the minima identify electron-rich locations, such as lone pairs and π-bonds. The gradient paths of MESP vividly bring out the atoms-in-molecule picture of neutral molecules and anions. The MESP-based characterization of a molecule in terms of electron-rich and -deficient regions provides a robust prediction about its interaction with other molecules. This leads to a clear picture of molecular aggregation, hydrogen bonding, lone pair–π interactions, π-conjugation, aromaticity and reaction mechanisms. This review summarizes the contributions of the authors’ groups over the last three decades and those of the other active groups towards understanding chemical bonding, molecular recognition, and reactivity through topology analysis of MESP.

Design and DFT study of nitrogen-rich donor systems for improved photovoltaic performance in dye-sensitized solar cells

Donor modifications, especially through N-annulation, for enhancing the structure–performance relationship of D–π–A systems for DSSC applications.

Comparative study of aryl halides in Pd-mediated reactions: key factors beyond the oxidative addition step

The comparative experimental study of Ar–X (X = Cl, Br, I) reactivity and analysis reported herein suggest that oxidative addition cannot be considered the sole reason of the observed low reactivity of aryl chlorides.

Absorption and emission properties of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (M = Al, Ga, In) and correlations with molecular electrostatic potential

Substituent effect for a series of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (Mq3) of aluminum, gallium, and indium are investigated using density functional theory (DFT) for the ground state properties and the time-dependent version of DFT (TDDFT) for their absorption and emission properties. A comparison between the ground state energy of mer and fac isomers of all the complexes revealed that the mer configuration is always more stable than fac. The substituent effect is significantly reflected at the fluorescence maximum (λ_F ) values whereas the effect is moderate at the absorption maximum (λ_abs ) values. The molecular electrostatic potential (MESP) at the metal center (V_M ) and the most electron rich region indicated by MESP minimum (V_min ), located at the oxygen of phenoxide ring exhibit excellent correlations with the λ_F and Stokes shift (λ_F -λ_abs ) values. The study suggests the use of Stokes shift as an experimental quantity to measure the excited state substituent effect while the V_min or V_M emerge as theoretical quantities to measure the same.

Utilization of BozPhos as an Effective Ligand in Enantioselective C-H Functionalization of Cyclopropanes: Synthesis of Dihydroisoquinolones and Dihydroquinolones

The bisphosphine monoxide ( R, R)-BozPhos enables enantioselective C-H functionalization of cyclopropanes in a palladium-catalyzed cyclization. The synthesis of a broad spectrum of dihydroisoquinolones and dihydroquinolones in good yields and high enantiomeric excess was achieved through the use of this hemilabile ligand. Furthermore, the isolation of an intermediary palladium(II)-BozPhos complex after oxidative addition was successful and a second complex provided further insight into bond length and angles through a crystal structure.

Computational Ligand Descriptors for Catalyst Design

Ligands, especially phosphines and carbenes, can play a key role in modifying and controlling homogeneous organometallic catalysts, and they often provide a convenient approach to fine-tuning the performance of known catalysts. The measurable outcomes of such catalyst modifications (yields, rates, selectivity) can be set into context by establishing their relationship to steric and electronic descriptors of ligand properties, and such models can guide the discovery, optimization, and design of catalysts. In this review we present a survey of calculated ligand descriptors, with a particular focus on homogeneous organometallic catalysis. A range of different approaches to calculating steric and electronic parameters are set out and compared, and we have collected descriptors for a range of representative ligand sets, including 30 monodentate phosphorus(III) donor ligands, 23 bidentate P,P-donor ligands, and 30 carbenes, with a view to providing a useful resource for analysis to practitioners. In addition, several case studies of applications of such descriptors, covering both maps and models, have been reviewed, illustrating how descriptor-led studies of catalysis can inform experiments and highlighting good practice for model comparison and evaluation.

Switching the nature of catalytic centers in Pd/NHC systems by solvent effect driven non-classical R-NHC Coupling

A well-established oxidative addition of organic halides (R-X) to N-heterocyclic carbene (NHC) complexes of palladium(0) leads to formation of (NHC)(R)Pd^II (X)L species, the key intermediates in a large variety of synthetically useful cross-coupling reactions. Typical consideration of the cross-coupling catalytic cycle is based on the assumption of intrinsic stability of these species, where the subsequent steps involve coordination of the second reacting partner. Thus, high stability of the intermediate (NHC)(R)Pd^II (X)L species is usually taken for granted. In the present study it is discussed that such intermediates are prone to non-classical R-NHC intramolecular coupling process (R = Me, Ph, Vinyl, Ethynyl) that results in removal of NHC ligand and generation of another type of Pd catalytic system. DFT calculations (BP86, TPSS, PBE1PBE, B3LYP, M06, wB97X-D) clearly show that outcome of R-NHC coupling process is not only determined by chemical nature of the organic substituent R, but also strongly depends on the type of solvent. The reaction is most favorable in polar solvents, whereas the non-polar solvents render the products less stable. © 2019 Wiley Periodicals, Inc.

Hydrogen elimination reactivity of ruthenium pincer hydride complexes: a DFT study

The pincer effect is explained for various pincer hydride complexes, differing in the donor atoms, using activation barriers, and MESP parameters.

Predicting reduction potentials of 1,3,6-triphenyl fulvenes using molecular electrostatic potential analysis of substituent effects

The influence of mono- and multiple substituent effect on the reduction potential (E⁰ ) of 1,3,6-triphenyl fulvenes is investigated using B3LYP-SMD/6-311+G(d,p) level density functional theory. The molecular electrostatic potential (MESP) minimum at the fulvene π-system (V_min ) and the change in MESP at any of the fulvene carbon atoms (ΔV_C ) for both neutral and reduced forms are used as excellent measures of substituent effect from the para and meta positions of the 1,3 and 6-phenyl moieties. Substitution at 6-phenyl para position has led to significant change in E⁰ than any other positions. By applying the additivity rule of substituent effects, an equation in ΔV_C is derived to predict E⁰ for multiply substituted fulvenes. Further, E⁰ is predicted for a set of 2000 hexa-substituted fulvene derivatives where the substituents and their positions in the system are chosen in a random way. The calculated E⁰ agreed very well with the experimental E⁰ reported by Godman et al. Predicting E⁰ solely by substituent effect offers a simple and powerful way to select suitable combinations of substituents on fulvene system for light harvesting applications. © 2018 Wiley Periodicals, Inc.

Electronic effect of ligands vs. reduction potentials of Fischer carbene complexes of chromium: a molecular electrostatic potential analysis

Molecular electrostatic potential at the chromium centre (V_Cr) emerges as a powerful predictor of reduction potential (E⁰).

Halide Abstraction Competes with Oxidative Addition in the Reactions of Aryl Halides with [Ni(PMen Ph(3-n) )4 ]

Density functional theory (DFT) calculations have been used to study the oxidative addition of aryl halides to complexes of the type [Ni(PMen Ph(3-n) )4 ], revealing the crucial role of an open-shell singlet transition state for halide abstraction. The formation of NiI versus NiII has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMen Ph(3-n) )3 ], via an open-shell singlet transition state; (ii) SN 2-type oxidative addition to [Ni(PMen Ph(3-n) )3 ], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMen Ph(3-n) )2 ]. For the overall reaction between [Ni(PMe3 )4 ], PhCl, and PhI, a microkinetic model was used to show that our results are consistent with the experimentally observed ratios of NiI and NiII when the PEt3 complex is used. Importantly, [Ni(PMen Ph(3-n) )2 ] complexes often have little, if any, role in oxidative addition reactions because they are relatively high in energy. The behaviour of [Ni(PR3 )4 ] complexes in catalysis is therefore likely to differ considerably from those based on diphosphine ligands in which two coordinate Ni0 complexes are the key species undergoing oxidative addition.