Electrostatic Control of Regioselectivity in Au(I)-Catalyzed Hydroarylation

Quantifying effects of second-sphere cationic groups on redox properties of dimolybdenum quadruple bonds

A series of four dimolybdenum paddlewheel complexes supported by anionic N,N-dimethylglycinate (DMG) or zwitterionic N,N,N-trimethylglycine (TMG) ligands was synthesised to examine the effects of charged groups in the second coordination sphere on redox properties of MoMo bonds. An average shift in reduction potential of +35 mV per cationically charged group was measured, which is approximately half of what would be expected for an analogous mononuclear complex.

Palladium-catalyzed cross-coupling of alcohols with olefins by positional tuning of a counteranion

Transition metal-catalyzed cross-couplings have great potential to furnish complex ethers; however, challenges in the C(sp3)-O functionalization step have precluded general methods. Here, we describe computationally guided transition metal-ligand design that positions a hydrogen-bond acceptor anion at the reactive site to promote functionalization. A general cross-coupling of primary, secondary, and tertiary aliphatic alcohols with terminal olefins to furnish >130 ethers is achieved. The mild conditions tolerate functionality that is prone to substitution, elimination, and epimerization and achieve site selectivity in polyol settings. Mechanistic studies support the hypothesis that the ligand's geometry and electronics direct positioning of the phosphate anion at the π-allyl-palladium terminus, facilitating the phosphate's hydrogen-bond acceptor role toward the alcohol. Ligand-directed counteranion positioning in cationic transition metal catalysis has the potential to be a general strategy for promoting challenging bimolecular reactivity.

Structure-based discovery of CFTR potentiators and inhibitors

The cystic fibrosis transmembrane conductance regulator (CFTR) is a crucial ion channel whose loss of function leads to cystic fibrosis, whereas its hyperactivation leads to secretory diarrhea. Small molecules that improve CFTR folding (correctors) or function (potentiators) are clinically available. However, the only potentiator, ivacaftor, has suboptimal pharmacokinetics and inhibitors have yet to be clinically developed. Here, we combine molecular docking, electrophysiology, cryo-EM, and medicinal chemistry to identify CFTR modulators. We docked ∼155 million molecules into the potentiator site on CFTR, synthesized 53 test ligands, and used structure-based optimization to identify candidate modulators. This approach uncovered mid-nanomolar potentiators, as well as inhibitors, that bind to the same allosteric site. These molecules represent potential leads for the development of more effective drugs for cystic fibrosis and secretory diarrhea, demonstrating the feasibility of large-scale docking for ion channel drug discovery.

Dinitrogen Activation within Frustrated Lewis Pairs Is Promoted by Adding External Electric Fields

This study uses computational means to explore the feasibility of N2 cleavage by frustrated Lewis pair (FLPs) species. The employed FLP systems are phosphane/borane (1) and carbene/borane (2). Previous studies show that 1 and 2 react with H2 and CO2 but do not activate N2. The present study demonstrates that N2 is indeed inert, and its activation requires augmentation of the FLPs by an external tool. As we demonstrate here, FLP-mediated N2 activation can be achieved by an external electric field oriented along the reaction axis of the FLP. Additionally, the study demonstrates that FLP -N2 activation generates useful nitrogen compound, e.g., hydrazine (H2N-NH2). In summary, we conclude that FLP effectively activates N2 in tandem with oriented external electric fields (OEEFs), which play a crucial role. This FLP/OEEF combination may serve as a general activator of inert molecules.

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt-porphyrin complexes

A CoII-porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (i.e., Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) bind to the aza-crown ether group of 1. The binding constant for Li+ is comparable to that observed for a free aza-crown ether. LA binding causes an anodic shift in the CoII/CoI couple of between 10 and 40 mV and also impacts the CoIII/CoII couple. The magnitude of the anodic shift of the CoII/CoI couple varies linearly with the strength of the LA as determined by the pKa of the corresponding metal-aqua complex, with dications giving larger shifts than monocations. The extent of the anodic shift of the CoII/CoI couple also increases as the ionic strength of the solution decreases. This is consistent with electric field effects being responsible for the changes in the redox properties of 1 upon LA binding and provides a novel method to tune the reduction potential. Density functional theory calculations indicate that the bound LA is 5.6 to 6.8 Å away from the CoII ion, demonstrating that long-range electrostatic effects, which do not involve changes to the primary coordination sphere, are responsible for the variations in redox chemistry. Compound 1 was investigated as a CO2 reduction electrocatalyst and shows high activity but rapid decomposition.

Manipulation of N-heterocyclic carbene reactivity with practical oriented electric fields

Electric fields can be used to tune the nucleophilicity and electrophilicity of N-heterocyclic carbenes and enhance their catalytic activity.

Sub-Nanometer Mapping of the Interfacial Electric Field Profile Using a Vibrational Stark Shift Ruler

The characterization of electrical double layers is important since the interfacial electric field and electrolyte environment directly affect the reaction mechanisms and catalytic rates of electrochemical processes. In this work, we introduce a spectroscopic method based on a Stark shift ruler that enables mapping the electric field strength across the electric double layer of electrode/electrolyte interfaces. We use the tungsten-pentacarbonyl(1,4-phenelenediisocyanide) complex attached to the gold surface as a molecular ruler. The carbonyl (CO) and isocyanide (NC) groups of the self-assembled monolayer (SAM) provide multiple vibrational reporters situated at different distances from the electrode. Measurements of Stark shifts under operando electrochemical conditions and direct comparisons to density functional theory (DFT) simulations reveal distance-dependent electric field strength from the electrode surface. This electric field profile can be described by the Gouy-Chapman-Stern model with Stern layer thickness of ∼4.5 Å, indicating substantial solvent and electrolyte penetration within the SAM. Significant electro-induction effect is observed on the W center that is ∼1.2 nm away from the surface despite rapid decay of the electric field (∼90%) within 1 nm. The applied methodology and reported findings should be particularly valuable for the characterization of a wide range of microenvironments surrounding molecular electrocatalysts at electrode interfaces and the positioning of electrocatalysts at specific distances from the electrode surface for optimal functionality.

Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

The power of oriented electrostatic fields (ESFs) to influence chemical bonding and reactivity is a phenomenon of rapidly growing interest. The presence of strong ESFs has recently been implicated as one of the most significant contributors to the activity of select enzymes, wherein alignment of a substrate's changing dipole moment with a strong, local electrostatic field has been shown to be responsible for the majority of the enzymatic rate enhancement. Outside of enzymology, researchers have studied the impacts of "internal" electrostatic fields via the addition of ionic salts to reactions and the incorporation of charged functional groups into organic molecules (both experimentally and computationally), and "externally" via the implementation of bulk fields between electrode plates. Incorporation of charged moieties into homogeneous inorganic complexes to generate internal ESFs represents an area of high potential for novel catalyst design. This field has only begun to materialize within the past 10 years but could be an area of significant impact moving forward, since it provides a means for tuning the properties of molecular complexes via a method that is orthogonal to traditional strategies, thereby providing possibilities for improved catalytic conditions and novel reactivity. In this perspective, we highlight recent developments in this area and offer insights, obtained from our own research, on the challenges and future directions of this emerging field of research.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J _P-Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J _P-Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J _P-Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C-F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C-F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

TfOH catalysed domino-double annulation of arenes with propargylic alcohols: a unified approach to indene polycyclic systems

The design and development of a TfOH catalysed domino strategy for the double annulation of arenes with propargylic alcohols for the rapid generation of indene based polycyclic systems is reported. The dehydration, intramolecular 6-endo-dig hydroarylation, and cationic cyclization were consecutively promoted by TfOH. The key features of this strategy are the formation of two C-C bonds, unified access to indene polycyclic systems, excellent yields (up to 95%), high atom economy (>90%), an operationally simple procedure, and water being the only byproduct. By extending this strategy, a two-step synthesis of the pentacyclic systems of hypoxylonol A (43% overall yield from α-tetralone), daldinone A (63% overall yield from β-tetralone) and spiro-tetracyclic framework of incarviatone A has also been achieved.

Gold-Catalyzed Post-Polymerization Modification of Commodity Aromatic Polymers

Synthetic aromatic polymers are ubiquitous and indispensable to modern life, industry, and the global economy. The direct functionalization of these materials remains a considerable challenge on account of their unreactive aromatic C-H bonds and robust physical properties. Here, we demonstrate that homogeneous gold catalysis offers a mild, chemoselective, and practical approach to functionalize high-volume commodity aromatic polymers. Utilizing a gold-catalyzed intermolecular hydroarylation between a methyl ester functionalized alkyne, methyl propiolate, and nucleophilic arenes within polystyrene (PS) results in direct functionalization of phenyl rings with 1,2-substituted methyl acrylate functional groups. The reactivity and functionalization depend on the steric and electronic environment of the catalyst, counterion pairing, and method of activation. The reactivity is broad in scope, enabling the functionalization of arenes within commercial polysulfone (PSU) and waste polyethylene terephthalate (PET). These reactions open new opportunities to chemically transform aromatic polymers and modify their physical properties.

A single-molecule blueprint for synthesis

Chemical reactions that occur at nanostructured electrodes have garnered widespread interest because of their potential applications in fields including nanotechnology, green chemistry and fundamental physical organic chemistry. Much of our present understanding of these reactions comes from probes that interrogate ensembles of molecules undergoing various stages of the transformation concurrently. Exquisite control over single-molecule reactivity lets us construct new molecules and further our understanding of nanoscale chemical phenomena. We can study single molecules using instruments such as the scanning tunnelling microscope, which can additionally be part of a mechanically controlled break junction. These are unique tools that can offer a high level of detail. They probe the electronic conductance of individual molecules and catalyse chemical reactions by establishing environments with reactive metal sites on nanoscale electrodes. This Review describes how chemical reactions involving bond cleavage and formation can be triggered at nanoscale electrodes and studied one molecule at a time.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Vibrational Stark shift spectroscopy of catalysts under the influence of electric fields at electrode-solution interfaces

External control of chemical processes is a subject of widespread interest in chemical research, including control of electrocatalytic processes with significant promise in energy research. The electrochemical double-layer is the nanoscale region next to the electrode/electrolyte interface where chemical reactions typically occur. Understanding the effects of electric fields within the electrochemical double layer requires a combination of synthesis, electrochemistry, spectroscopy, and theory. In particular, vibrational sum frequency generation (VSFG) spectroscopy is a powerful technique to probe the response of molecular catalysts at the electrode interface under bias. Fundamental understanding can be obtained via synthetic tuning of the adsorbed molecular catalysts on the electrode surface and by combining experimental VSFG data with theoretical modelling of the Stark shift response. The resulting insights at the molecular level are particularly valuable for the development of new methodologies to control and characterize catalysts confined to electrode surfaces. This Perspective article is focused on how systematic modifications of molecules anchored to surfaces report information concerning the geometric, energetic, and electronic parameters of catalysts under bias attached to electrode surfaces.

Heterogeneous electrocatalysis: characterization of interfacial electric field within the electrochemical double layer.

Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

A series of tetradentate tris(phosphinimine) ligands (^R3P₃tren) was developed and bound to Cu^I to form the trigonal pyramidal, C_3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F₄] (1PR3) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, 1PMe3, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (−780 mV vs. Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of 1PR3via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy-plane that destabilize 2e (d_xy/d_x²−y²) orbital sets and uniquely stabilized a₁ (d_z²) orbitals. Support is provided that the a₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z-axes of 1PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1PMe3 to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (2PMe3), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo-C_s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal C_s-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' properties via a method orthogonal to classic ligand field tuning.

Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials.

Gold(I)-Catalyzed Intramolecular Hydroarylation of Phenol-Derived Propiolates and Certain Related Ethers as a Route to Selectively Functionalized Coumarins and 2H-Chromenes

Methods are reported for the efficient assembly of a series of phenol-derived propiolates, including the parent system 56, and their Au(I)-catalyzed cyclization (intramolecular hydroarylation) to give the corresponding coumarins (e.g., 1). Simple syntheses of natural products such as ayapin (144) and scoparone (145) have been realized by such means, and the first of these subject to single-crystal X-ray analysis. A related process is described for the conversion of propargyl ethers such as 156 into the isomeric 2H-chromene precocene I (159), a naturally occurring inhibitor of juvenile hormone biosynthesis.

Design of a Functional Chromene-Type Kobayashi Precursor: Gram-Scale Total Synthesis of Natural Xanthones by Highly Regioselective Aryne Annulation

The 2,2-dimethyl-2H-chromene motif is widely found in many bioactive molecules, and is a privileged structure in the pharmaceutical arena. We have developed a concise and regioselective approach to chromenes and chromanes through an aryne-based synthetic strategy. A practical, gram-scale synthetic route to a chromene-type aryne precursor was explored. Subsequently, cyclization under mild conditions afforded tetracyclic xanthone skeletons with excellent regioselectivity. Our approach provides a concise strategy for the gram-scale synthesis of chromene-type xanthones such as 6-deoxyisojacareubin, cylindroxanthone D, staudtiixanthone D, brasilixanthone A and cudracuspixanthone O.

Unifying Conceptual Density Functional and Valence Bond Theory: The Hardness-Softness Conundrum Associated with Protonation Reactions and Uncovering Complementary Reactivity Modes

In this study, we address the long-standing issue-arising prominently from conceptual density functional theory (CDFT)-of the relative importance of electrostatic, i.e., "hard-hard", versus spin-pairing, i.e., "soft-soft", interactions in determining regiochemical preferences. We do so from a valence bond (VB) perspective and demonstrate that VB theory readily enables a clear-cut resolution of both of these contributions to the bond formation/breaking process. Our calculations indicate that appropriate local reactivity descriptors can be used to gauge the magnitude of both interactions individually, e.g., Fukui functions or HOMO/LUMO orbitals for the spin-pairing/(frontier) orbital interactions and molecular electrostatic potentials (and/or partial charges) for the electrostatic interactions. In contrast to previous reports, we find that protonation reactions cannot generally be classified as either charge- or frontier orbital-controlled; instead, our results indicate that these two bonding contributions generally interplay in more subtle patterns, only giving the impression of a clear-cut dichotomy. Finally, we demonstrate that important covalent, i.e., spin pairing, reactivity modes can be missed when only a single spin-pairing/orbital interaction descriptor is considered. This study constitutes an important step in the unification of CDFT and VB theory.

Theoretical Insight into Ni(0)-Catalyzed Hydroarylation of Alkenes and Arylboronic Acids

The density functional theory (ωB97XD functional) is employed to clarify nickel(0)/P^tBu₃-catalyzed hydroarylation of alkenes and arylboronic acids with methanol. The computational results reveal that this reaction goes primarily through the ligand-to-ligand H transfer from the O-H bond to the alkene coordinated with nickel, complexation of arylboronic acid to the nickel-alkyl-methoxyl intermediate, attack of methoxyl on boron, transmetalation, and reductive elimination. The formation of the branched 1,1-diarylalkane, linear 1,1-diarylalkane, and alkene-dimer is also discussed in this work.

Solvent Effect in Gold(I)-Catalyzed Domino Reaction: Access to Furopyrans

We report an efficient synthesis of furopyrans through a gold(I)-catalyzed domino reaction. By starting from the same source and changing the solvent of the reaction, two classes of furopyrans are accessible. During this one-step process, which takes place in DMF, two bonds and two heterocycles are formed. DFT calculations furnish the mechanistic understanding of this transformation. The sequence includes a 5-endo-dig cyclization, a regioselective 8-endo-dig cyclization, and a retro 8π and a 6π electrocyclization.

N-Cyclopropenio-imidazol-2-ylidene: An N-heterocyclic carbene bearing an N-cationic substituent

A cationic NHC 1+ bearing an N-bound 2,3-bis(diisopropylamino)cyclopropenium group is reported. From an easily available dicationic imidazolium precursor, the coordination abilities and stereo-electronic properties of 1+ are evaluated by the formation of Pd(ii), Rh(i) and Au(i) complexes. The cationic gold(i) complex is implemented in representative intramolecular Au(i)-catalyzed cyclizations.

Synthesis of pyrano[2,3-f]chromen-2-ones vs. pyrano[3,2-g]chromen-2-ones through site controlled gold-catalyzed annulations

Regioselective access to 10-substituted-2H,8H-pyrano[2,3-f]chromen-2-ones through the gold-catalyzed intramolecular hydroarylation of readily available 7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivatives at their C-8 congested position was investigated by tuning the electronic and steric properties of the ligand on the gold complex. On the other hand, the combination of the JohnPhosAu(MeCN)SbF6 catalyzed intramolecular hydroarylation of 8-iodo-7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivatives followed by selective palladium/formate C-I reduction allows for the exclusive formation of 2H,8H-pyrano[3,2-g]chromen-2-one regioisomers. The development of these two protocols provides versatile synthetic tools required for exploring the biological activities of these new pyranocoumarin derivatives.

Installation of internal electric fields by non-redox active cations in transition metal complexes

Local electric fields contribute to the high selectivity and catalytic activity in enzyme active sites and confined reaction centers in zeolites by modifying the relative energy of transition states, intermediates and/or products. Proximal charged functionalities can generate equivalent internal electric fields in molecular systems but the magnitude of their effect and impact on electronic structure has been minimally explored. To generate quantitative insight into installing internal fields in synthetic systems, we report an experimental and computational study using transition metal (M1) Schiff base complexes functionalized with a crown ether unit containing a mono- or dicationic alkali or alkaline earth metal ion (M2). The synthesis and characterization of the complexes M1 = Ni(ii) and M2 = Na+ or Ba2+ are reported. The electronic absorption spectra and density functional theory (DFT) calculations establish that the cations generate a robust electric field at the metal, which stabilizes the Ni-based molecular orbitals without significantly changing their relative energies. The stabilization is also reflected in the experimental Ni(ii/i) reduction potentials, which are shifted 0.12 V and 0.34 V positive for M2 = Na+ and Ba2+, respectively, compared to a complex lacking a proximal cation. To compare with the cationic Ni complexes, we also synthesized a series of Ni(salen) complexes modified in the 5' position with electron-donating and -withdrawing functionalities (-CF3, -Cl, -H, -tBu, and -OCH3). Data from this series of compounds provides further evidence that the reduction potential shifts observed in the cationic complexes are not due to inductive ligand effects. DFT studies were also performed on the previously reported monocationic and dicatonic Fe(ii)(CH3CN) and Fe(iii)Cl analogues of this system to analyze the impact of an anionic chloride on the electrostatic potential and electronic structure of the Fe site.

Directing isomerization reactions of cumulenes with electric fields

Electric fields have been proposed as having a distinct ability to catalyze chemical reactions through the stabilization of polar or ionic intermediate transition states. Although field-assisted catalysis is being researched, the ability to catalyze reactions in solution using electric fields remains elusive and the understanding of mechanisms of such catalysis is sparse. Here we show that an electric field can catalyze the cis-to-trans isomerization of [3]cumulene derivatives in solution, in a scanning tunneling microscope. We further show that the external electric field can alter the thermodynamics inhibiting the trans-to-cis reverse reaction, endowing the selectivity toward trans isomer. Using density functional theory-based calculations, we find that the applied electric field promotes a zwitterionic resonance form, which ensures a lower energy transition state for the isomerization reaction. The field also stabilizes the trans form, relative to the cis, dictating the cis/trans thermodynamics, driving the equilibrium product exclusively toward the trans.

Copper-Catalyzed Asymmetric Hydroboration of 2 H-Chromenes Using a Chiral Diphosphine Ligand

A highly regioselective asymmetric hydroboration of 2 H-chromenes catalyzed by the complex of CuCl and diphosphine ligand ( S, R)-DuanPhos has been realized under mild conditions to produce 3-boryl chromans, achieving good yields and excellent enantioselectivities up to 96% ee. This work provides an efficient approach to the synthesis of chiral 3-boryl chromans and derivatives.

Understanding the Reactivity and Selectivity of Fluxional Chiral DMAP-Catalyzed Kinetic Resolutions of Axially Chiral Biaryls

Fluxional chiral DMAP-catalyzed kinetic resolutions of axially chiral biaryls were examined using density functional theory. Computational analyses lead to a revised understanding of this reaction in which the interplay of numerous non-covalent interactions control the conformation and flexibility of the active catalyst, the preferred mechanism, and the stereoselectivity. Notably, while the DMAP catalyst itself is confirmed to be highly fluxional, electrostatically driven π⋅⋅⋅π⁺ interactions render the active, acylated form of the catalyst highly rigid, explaining its pronounced stereoselectivity.

Sub-Femtosecond Stark Control of Molecular Photoexcitation with Near Single-Cycle Pulses

Electric fields can tailor molecular potential energy surfaces by interaction with the electronic state-dependent molecular dipole moment. Recent developments in optics have enabled the creation of ultrashort few-cycle optical pulses with precise control of the carrier envelope phase (CEP) that determines the offset of the maxima in the field and the pulse envelope. This opens news ways of controlling ultrafast molecular dynamics by exploiting the CEP. In this work, we show that the photoabsorption efficiency of oriented H₂CSO (sulfine) can be controlled by tuning the CEP. We further show that this control emanates from a resonance condition related to Stark shifting of the electronic energy levels.

Anion control of tautomeric equilibria: Fe-H vs. N-H influenced by NH···F hydrogen bonding

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(PEtNMePEt)(CO)3 (PEtNMePEt = (Et2PCH2)2NMe) at Fe or N. Protonation of Fe(PEtNMePEt)(CO)3 by [(Et2O)2H]+[B(C6F5)4]- occurs at the metal to give the iron hydride [Fe(PEtNMePEt)(CO)3H]+[B(C6F5)4]-. In contrast, treatment with HBF4·OEt2 gives protonation at the iron and at the pendant amine. Both the FeH and NH tautomers were characterized by single crystal X-ray diffraction. Addition of excess BF4 - to the equilibrium mixture leads to the NH tautomer being exclusively observed, due to NH···F hydrogen bonding. A quantum chemical analysis of the bonding properties of these systems provided a quantification of hydrogen bonding of the NH to BF4 - and to OTf-. Treatment of Fe(PEtNMePEt)(CO)3 with excess HOTf gives a dicationic complex where both the iron and nitrogen are protonated. Isomerization of the dicationic complex was studied by NOESY NMR spectroscopy.

Theoretical Insight into the Au(I)-Catalyzed Intermolecular Condensation of Homopropargyl Alcohols with Terminal Alkynes: Reactant Stoichiometric Ratio-Controlled Chemodivergence

The mechanisms and chemoselectivities on the Au(I)-catalyzed intermolecular condensation between homopropargyl alcohols and terminal alkynes were investigated by performing DFT calculations. The reaction was indicated to involve three stages: transformation of the homopropargyl alcohol (R₁) via intramolecular cyclization to the cyclic vinyl ether (R₁'), formation of the C-2-arylalkynyl cyclic ether (P₁) via hydroalkynylation of R₁' with phenylacetylene (R₂), and conversion from P₁ to 2,3-dihydro-oxepine (P₂). The results revealed the origin of the reaction divergence and rationalized the experimental observations that a 1:3 reactant stoichiometric ratio affords P₁ as the major product, whereas the 1:1.1 ratio results in P₂ in high yield. The reactant stoichiometric ratio-controlled divergent reactivity is attributed to different catalytic activities of the gold catalyst toward different reaction stages. In the 1:3 situation, the excess R₂ induces the Au catalyst toward its dimerization and/or hydration, inhibiting the conversion of P₁ to P₂ and resulting in product P₁. Without excess R₂, the Au catalysis follows a general cascade reaction, leading to product P₂. Theoretical results described a general strategy controlling the reaction divergence by a different reactant stoichiometric ratio. This strategy may be enlightening for chemists who are exploring various synthesis methods with high chemo-, regio-, and enantioselectivities.

The control effects of different scaffolds in chiral phosphoric acids: a case study of enantioselective asymmetric arylation

DFT calculations disclosed that the sign of enantioselectivity in chiral-phosphoric-acid catalyzed reactions can be tuned by BINOL- or SPINOL-derived backbones.

Direct synthesis of aryl-annulated [c]carbazoles by gold(i)-catalysed cascade reaction of azide-diynes and arenes

The gold-catalysed annulation of conjugated alkynes bearing an azido group with arenes gave annulated [c]carbazoles. Using benzene, pyrrole, and indole derivatives as the nucleophiles, benzo[c]-, pyrrolo[2,3-c]-, and indolo[2,3-c]carbazoles were produced, respectively. The reaction proceeded through pyrrole and benzene ring construction accompanied by the formation of two carbon-carbon and one carbon-nitrogen bond and the cleavage of two aromatic C-H bonds. The mechanism of the reaction with pyrrole was investigated by density functional theory calculations. N,N'-dimethylated indolo[2,3-c]carbazole showed dual ultraviolet-visible-near-infrared and fluorescence spectral changes upon electrolysis.

Substantial Impact of Charge on Electrochemical Reactions of Two-Dimensional Materials

Two-dimensional (2D) materials have attracted great interest in catalyzing electrochemical reactions such as water splitting, oxygen reduction, and carbon dioxide reduction. Quantum mechanical simulations have been extensively employed to study the catalytic mechanisms. These calculations typically assume that the catalyst is charge neutral for computational simplicity; however, in reality, the catalyst is usually charged to match its Fermi level with the applied electrode potential. These contradictions urge an evaluation of the charge effects. Here, using the example of hydrogen adsorption on the common 2D electrocatalysts (N-doped graphene and MoS₂) and 3D metal catalysts, and employing the grand canonical density functional theory, we show that the charge on 2D materials can have a much stronger impact on the electrochemical reaction than the charge on 3D metals (the reaction energy can differ by >1 eV after including the charge effects). This arises from the charge-induced change in the occupation of electronic states, which is more significant for 2D materials due to their limited density of states. Our work provides a fundamental understanding of the charge effects in 2D materials, calls for re-evaluation of the previously suggested mechanisms by including the overlooked charge effects, and offers practical guidelines for designing 2D catalysts.

In(III)-TMSBr-Catalyzed Cascade Reaction of Diarylalkynes with Acrylates for the Synthesis of Aryldihydronaphthalene Derivatives

A combined Lewis acid system comprising of two or more Lewis acids occasionally exhibits augmented catalytic activity in organic transformations which are otherwise unrealizable by either of the components exclusively. On the other hand, the efficient construction of multiple new C-C bonds and polycyclic structures in minimal steps remains a subject of great interest in both academia and industry. Herein we report an efficient method to assemble aryldihydronaphthalene derivatives via a cascade reaction of diarylalkynes with acrylates under the catalysis of a combined Lewis acid derived from In(III) salt and TMSBr.

Theoretical Insight into the Mechansim and Origin of Ligand-Controlled Regioselectivity in Homogenous Gold-Catalyzed Intramolecular Hydroarylation of Alkynes

This work aims at understanding the mechanism and regioselectivity in ligand-controlled gold-catalyzed divergent intramolecular hydroarylation of alkynes reported by Jiang et al. ( J. Am. Chem. Soc. 2016 , 138 , 5218 - 5221 ). Focusing on a representative alkyne, N-propargyl-N-tosylaniline, we conducted a detailed computational study on the ortho- and para-position hydroarylation of the alkyne catalyzed by gold(I) catalysts with different ligands. Both the ortho- and para-position hydroarylation reactions are found to follow a similar three-stage mechanism: electrophilic cyclization, proton loss, and protiodeauration. The initial electrophilic cyclization was identified as the rate- and regiochemistry-determining step. With the flexible electron-deficient phosphite ligand, the ortho-position cyclization is identified as the energetically more favorable pathway, while with the rigid electron-abundant phosphine (Xphos) ligand, the dominant pathway turns to the para-position cyclization. The theoretical results are in good agreement with the experimental observations. The π-π interaction between alkynyl phenyl and the directing acylamino group are found to be mainly responsible for the observed ortho-selectivity, while a combination of favorable noncovalent CH···π interaction and steric repulsion between Xphos ligand and alkynyl group contributes to the observed exclusive para-selectivity. The present calculations provide deeper insight into the mechanism and origin of regioselectivity of the title reaction.

N-Heterotricyclic cationic carbene ligands. Synthesis, reactivity and coordination chemistry

Metal complexes based on cationic N-heterotricyclic carbenes have been synthesized and the impact of charge delocalization on their electronic properties has been analysed.