Importance of Ligand Exchanges in Pd(II)-Brønsted Acid Cooperative Catalytic Approach to Spirocyclic Rings

Combinatorial Ligand Assisted Simultaneous Control of Axial and Central Chirality in Highly Stereoselective C-H Allylation

The significance of stereoselective C-H bond functionalization thrives on its direct application potential to pharmaceuticals or complex chiral molecule synthesis. Complication arises when there are multiple stereogenic elements such as a center and an axis of chirality to control. Over the years cooperative assistance of multiple chiral ligands has been applied to control only chiral centers. In this work, we harness the essence of cooperative ligand approach to control two different stereogenic elements in the same molecule by atroposelective allylation to synthesize axially chiral biaryls from its racemic precursor. The crucial roles played by chiral phosphoric acid and chiral amino acid ligand in concert helped us to obtain one major stereoisomer out of four distinct possibilities.

Pd-Catalyzed Tandem Pathway for Stereoselective Synthesis of (E)-1,3-Enyne from β-Nitroalkenes by Using a Sacrificial Directing Group

The involvement of nitroalkenes instead of minimal one alkyne motif for (E)-1,3-enynes synthesis through a palladium catalyzed stereoselective bond forming pathway at room temperature is presented. Implication of nitro group as a sacrificial directing group, formation of magical alkyne on a newly developed Csp 3 -Csp 3 bond with initial palladium-MBH adduct make this methodology distinctive. This protocol features an unprecedented sequential acetate addition, carbon-carbon bond formation, isomerization of double bond and nitromethane degradation in a tandem catalytic walk via dancing hybridization. Mechanistic understanding through identification of intermediates and computational calculations furnishes complete insight into the tandem catalytic pathway. Broad substrates scope and functional groups tolerance make this synthetic methodology magnificent and dynamic. This represents the first example of stereoselective 1,3-enyne synthesis exclusively from alkene substrates by introducing the concept of sacrificial directing group.

Wacker Oxidation of Methylenecyclobutanes: Scope and Selectivity in an Unusual Setting

Methylenecyclobutanes are found to undergo Wacker oxidation via a semi-pinacol-type rearrangement. Key to a successful process is the use of tert-butyl nitrite as oxidant, which not only enables efficient catalyst turn-over but also ensures high Markovnikov-selectivity under mild conditions. Thus, cyclopentanones (26 examples) can be accessed in an overall good yield and excellent selectivity (up to 97 % yield, generally >99 : 1 ketone:aldehyde ratio). Stereochemical analysis of the reaction sequence reveals migration aptitudes in line with related 1,2-shifts. By introducing a pyox ligand to palladium, prochiral methylenecyclobutanes can be desymmetrized, thus realizing the first enantioselective Wacker oxidation.

Room-Temperature Synthesis of 1,3,5-Tri(het)aryl Benzene from Nitroalkenes Using Pd(OAc)2: Complete Mechanistic and Theoretical Studies

Herein, a room-temperature catalytic pathway for 1,3,5-tri(het)aryl derivatives from nitroalkenes using simple Pd(OAc)₂ is presented. This newly developed C-C bond-forming methodology takes place in a cascade manner with the initial pallado-Morita-Baylis-Hillman (MBH) type adduct. The broad substrate scopes, functional group tolerance, and different aryl-substituted benzene derivatives make this methodology more attractive. Furthermore, the mechanistic understanding through isolation of intermediates and DFT studies of the catalytic cycle provide requisite insight into the methodology.

Absence of Intermediates in the BINOL-Derived Mg(II)/Phosphate-Catalyzed Desymmetrizative Ring Expansion of 1-Vinylcyclobutanols

The catalyzed desymmetrizative ring expansion of alkenylcyclobutanols promoted by halofunctionalization of the alkene moiety with N-bromosuccinimide has been experimentally and computationally studied. The reaction yields highly enantioenriched cyclopentanones bearing two all-carbon quaternary stereocenters, one of them being generated in the rearrangement of the cyclobutane ring and the other by enantioselective desymmetrization. The reaction is competitive with the formation of a spiroepoxide, but it turns completely selective toward the cyclopentanone when a chiral bisphosphonium magnesium salt is employed as a catalyst. Mechanistic studies support the formation of an ion pair leading to a complex with only a unit of phosphoric acid, which is the resting state of the catalytic cycle. Calculations reproduce in an excellent way the observed reactivity and predict the effect exerted by the substituents of the aromatic ring linked to the double bond. The computational studies also revealed the reaction as a highly asynchronous concerted process taking place as one kinetic step but in two stages: (i) halogenation of the double bond and (ii) rearrangement of the cyclobutane. No intermediates are present in the reaction as energy minima. The experimental scope of the reaction further confirms the predictions for the observed reactivity and selectivity.

Asymmetric C–H Functionalization Enabled by Pd/Chiral Phosphoric Acid Combined Catalysis

Over the past decade, the combination of chiral phosphoric acid and palladium catalysis has emerged as a robust strategy to accomplish the regio- and stereoselective functionalization of inactive C–H bonds, enabling access to various types of chirality (central, planar, and axial). This review article describes the origin and advances in the asymmetric functionalization of allylic C–H, C(sp2)–H, and C(sp3)–H bonds enabled by chiral phosphoric acid and palladium combined catalysis.

1 Introduction

2.1 Enantioselective Allylic C–H Functionalization

2.2 Enantioselective Non-allylic C(sp3)–H Functionalization

2.3 Enantioselective C(sp2)–H Functionalization

3 Conclusion

Mechanistic insights into rhodium-catalyzed enantioselective allylic alkylation for quaternary stereogenic centers

Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson's catalyst [Rh(Cl)(PPh₃)₃], (R)-BINOL–P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD_(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh₃)₃] by BINOL–P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL–P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)(PPh₃)₃]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C–O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C–C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol⁻¹ more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C–C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol⁻¹), which is in conformity with room temperature reaction conditions.

The origin of high enantioselectivity in the formation of quaternary stereogenic carbon.

Enantioselective Intramolecular Allylic Substitution via Synergistic Palladium/Chiral Phosphoric Acid Catalysis: Insight into Stereoinduction through Statistical Modeling

The mode of asymmetric induction in an enantioselective intramolecular allylic substitution reaction catalyzed by a combination of palladium and a chiral phosphoric acid was investigated by a combined experimental and statistical modeling approach. Experiments to probe nonlinear effects, the reactivity of deuterium-labeled substrates, and control experiments revealed that nucleophilic attack to the π-allylpalladium intermediate is the enantio-determining step, in which the chiral phosphate anion is involved in stereoinduction. Using multivariable linear regression analysis, we determined that multiple noncovalent interactions with the chiral environment of the phosphate anion are integral to enantiocontrol in the transition state. The synthetic protocol to form chiral pyrrolidines was further applied to the asymmetric construction of C-O bonds at fully substituted carbon centers in the synthesis of chiral 2,2-disubstituted benzomorpholines.

Unveiling the Role of Hydrogen Bonding and g-Tensor in the Interaction of Ru-Bis-DMSO with Amino Acid Residue and Human Serum Albumin

Density functional theory calculations have been carried out to observe the role of hydrogen bonding in hydrolysis and the coordination mechanism of three amino acid residues (histidine, cysteine, and alanine) with Ru-bis-DMSO complex via which the complex tends to interact with the HSA protein receptor. The interaction mechanism shows that ruthenium complexes prefer to bind protein receptor through cysteine and histidine residues rather than through alanine, which has been confirmed by DFT evaluated H-bonding and g-tensor analysis. The number of H-bonds plays a major role in stabilizing the intermediates and transition states involved in the Ru-bis-DMSO and amino acid residue interactions. Our theoretical g-tensor values are in good agreement with the available experimental results. Further QM/MM calculation on the Ru-bis-DMSO-HSA adducts reveals that the adduct is more stable when Ru gets coordinated with histidine imidazole rather than cysteine. These investigations helped us in understanding the type of amino acid residue responsible for binding the metal complex Ru-bis-DMSO with the carrier protein HSA.

Is silver a mere terminal oxidant in palladium catalyzed C-H bond activation reactions?

In the contemporary practice of palladium catalysis, a molecular understanding of the role of vital additives used in such reactions continues to remain rather vague. Herein, we disclose an intriguing and a potentially general role for one of the most commonly used silver salt additives, discovered through rigorous computational investigations on four diverse Pd-catalyzed C-H bond activation reactions involving sp2 aryl C-H bonds. The catalytic pathways of different reactions such as phosphorylation, arylation, alkynylation, and oxidative cycloaddition are analyzed, with and without the explicit inclusion of the silver additive in the respective transition states and intermediates. Our results indicate that the pivotal role of silver salts is likely to manifest in the form of a Pd-Ag heterobimetallic species that facilitates intermetallic electronic communication. The Pd-Ag interaction is found to provide a consistently lower energetic span as compared to an analogous pathway devoid of such interaction. Identification of a lower energy pathway as well as enhanced catalytic efficiency due to Pd-Ag interaction could have broad practical implications in the mechanism of transition metal catalysis and the current perceptions on the same.

Recent applications of chiral phosphoric acids in palladium catalysis

Through the combined action of palladium catalysts and chiral phosphoric acids (CPAs) a variety of catalytic asymmetric reactions have been realized during the past decade, including allylation, alkene functionalization, and C-H activation. This review surveys key examples across these various reaction types and examines the different mechanisms by which CPAs can affect stereoinduction in these reaction systems.

A semipinacol rearrangement of vinylogous α-ketol cocatalyzed by a cinchona-based primary amine and N-Boc-phenylglycines: mechanisms, roles of catalysts and the origin of enantioselectivity

A semipinacol rearrangement of vinylogous α-ketol cocatalyzed by a cinchona-based primary amine and Brønsted acids can be achieved with good enantiocontrol, and theoretical investigations have been performed to uncover and understand it in detail.

Cooperativity and serial ligand catalysis in an allylic amination reaction by Pd(ii)-bis-sulfoxide and Brønsted acids

In recent years, transition metal catalysts have been increasingly employed in conjunction with Brønsted acids under one-pot reaction conditions, opening up newer avenues for dual catalytic protocols. Under such dual catalytic conditions, the general premise of holding the native ligands on the catalyst in the same manner throughout the catalytic cycle becomes immediately questionable. We have invoked the likelihood of Serial Ligand Catalysis in an important intramolecular allylic amination of N-Boc (N-tert-butoxycarbonyl) protected homoallylic amine leading to an anti-oxazolidinone product. The reported reaction conditions employed (bis-sulfoxide)Pd(OAc)₂ and dibutyl phosphoric acid (DBPOH) as the catalysts and benzoquinone (BQ) as the oxidant. We used density functional theory computations at the B3LYP-D3 level of theory to examine a comprehensive set of ligand combinations around the Pd center so as to identify the energetically most preferred pathway. The key catalytic events consist of (i) a C-H activation at the allylic position in the catalyst-substrate complex [Pd(L)(L')₂(substrate)], leading to a (L)(L')Pd-π-allyl intermediate, and (ii) an intramolecular C-O bond formation between the carbonyl oxygen of the N-Boc amine and the allyl carbon. Interesting cooperativity between the catalysts in both these steps has been found, wherein the Pd(DBPO^-)₂(BS) species is involved in the C-H activation transition state and Pd(DBPO^-)(BQ) in the C-O bond formation step. The energetic advantage in swapping the bis-sulfoxide ligand on Pd with a benzoquinone upon moving from the first step to the second step confirms the significance of serial ligand catalysis in dual catalytic reactions.

Catalytic Enantioselective House-Meinwald Rearrangement: Efficient Construction of All-Carbon Quaternary Stereocenters

A catalytic asymmetric House-Meinwald rearrangement for the synthesis of both cyclic and acyclic ketones is disclosed. From readily accessible racemic tetrasubstituted epoxides, this approach provides efficient access to chiral ketones bearing α all-carbon quaternary stereocenters with high enantiocontrol. The observation of positive nonlinear effects and nontrivial kinetic feature provided important insights into the mechanism.

Catalytic, Enantioselective α-Alkylation of Azlactones with Nonconjugated Alkenes by Directed Nucleopalladation

A palladium(II)-catalyzed enantioselective α-alkylation of azlactones with nonconjugated alkenes is described. The reaction employs a chiral BINOL-derived phosphoric acid as the source of stereoinduction, and a cleavable bidentate directing group appended to the alkene to control the regioselectivity and stabilize the nucleopalladated alkylpalladium(II) intermediate in the catalytic cycle. A wide range of azlactones were found to be compatible under the optimal reaction conditions to afford products bearing α,α-disubstituted α-amino-acid derivatives with high yields and high enantioselectivity.

Tandem Achmatowicz Rearrangement and Acetalization of 1-[5-(Hydroxyalkyl)-furan-2-yl]-cyclobutanols Leading to Dispiroacetals and Subsequent Ring-Expansion to Form 6,7-Dihydrobenzofuran-4(5 H)-ones

Herein, we report a one-pot protocol for the synthesis of dispiroacetals 4 bearing a cyclobutane motif via tandem Achmatowicz rearrangement and acetalization of 1-[5-(hydroxyalkyl)-furan-2-yl]-cyclobutanols 3 with m-CPBA as the oxidant and AgSbF₆ as an additive to promote the cyclization step in an aqueous medium. Dispiroacetals 4 could subsequently undergo Lewis acid-catalyzed ring expansion and skeletal rearrangement to afford 6,7-dihydro-5 H-benzofuran-4-ones 5.

A computational mechanistic study of Pd(ii)-catalyzed γ-C(sp3)-H olefination/cyclization of amines: the roles of bicarbonate and ligand effect

The detailed mechanism of palladium-catalyzed γ-C(sp3)-H olefination/cyclization of triflyl-protected amines was investigated by density functional theory (DFT) calculations. The olefinated intermediate was initially formed in the first catalytic cycle involving ligand exchange, bicarbonate-assisted C(sp3)-H bond cleavage, alkene insertion and 'reductive β-hydride elimination'. The following syn-addition and reductive elimination furnish the aza-Wacker product. The first step of reductive elimination is the rate-determining step. The mechanism unveils the important roles of bicarbonate: aiding the C-H activation and abstracting the β-proton in the second step of reductive elimination. The parallel bridging mode in the metal-olefin intermediate facilitates the syn-addition, explaining the experimentally observed stereoselectivity. The effect of the monodentate pyridine-based ligands is also discussed.

Palladium-Catalyzed Enantioselective Desymmetrizing Aza-Wacker Reaction: Development and Application to the Total Synthesis of (-)-Mesembrane and (+)-Crinane

Reported is an unprecedented catalytic enantioselective desymmetrizing aza-Wacker reaction. In the presence of a catalytic amount of a newly developed Pd(CPA)₂ (MeCN)₂ catalyst (CPA=chiral phosphoric acid), a pyrox ligand, and molecular oxygen, cyclization of properly functionalized prochiral 3,3-disubstituted cyclohexa-1,4-dienes afforded enantioenriched cis-3a-substituted tetrahydroindoles in good yields with excellent enantioselectivities. A cooperative effect between the phosphoric acid and the pyrox ligand ensured efficient transformation. This reaction was tailor-made for Amaryllidaceae and Sceletium alkaloids as illustrated by its application in the development of the concise and divergent total synthesis of (-)-mesembrane and (+)-crinane.

Chiral phosphoric acid catalysis: from numbers to insights

Chiral phosphoric acids (CPAs) have emerged as powerful organocatalysts for asymmetric reactions, and applications of computational quantum chemistry have revealed important insights into the activity and selectivity of these catalysts.

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO2, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

DFT computations have been performed to gain insight into the mechanisms of formylation/methylation of amines (e.g. methylaniline (1a)/2,2,4,4-tetramethylpiperidine (2a)) with CO2 and hydrosilane ([Si]H2, [Si] = Ph2Si), catalyzed by 1,3,2-diazaphospholene ([NHP]H). Different from the generally proposed sequential mechanism for the methylation of amine with CO2, i.e. methylation proceeds via formylation, followed by further reduction of formamide to give an N-methylated amine, the study characterized a competition mechanism between formylation and methylation. The chemoselectivity originates from the competition between the amine and [NHP]H hydride to attack the formyloxy carbon of [Si](OCHO)2 (the insertion product of CO2 into [Si]H2). When the attack of an amine (e.g.1a) wins, the transformation affords formamide (1b) but would otherwise (e.g.2a) result in an N-methylated amine (2c). The reduction of formamide by [Si]H2 or [NHP]H is highly unfavorable kinetically, thus we call attention to the sequential mechanism for understanding the methylation of amine with CO2. In addition, the study has the following key mechanistic findings. The activation of CO2 by [NHP]H establishes an equilibrium: [NHP]H + CO2 ⇄ [NHP]OCHO ⇄ [NHP]+ + HCO2-. The ions play catalytic roles to promote formylation via HCO2- or methylation via[NHP]+ . In 1a formylation, HCO2- initiates the reaction, giving 1b and silanol byproducts. However, after the initiation, the silanol byproducts acting as hydrogen transfer shuttles are more effective than HCO2- to promote formylation. In 2a methylation, [NHP]+ promotes the generation of the key species, formaldehyde and a carbocation species (IM17+ ). Our experimental study corroborates our computed mechanisms.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Role of Lewis acid additives in a palladium catalyzed directed C-H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl₂ and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl₂ and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl₂ are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl₂·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to C_aryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl₂ and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the C_aryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide

The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.

Asymmetric Cooperative Catalysis in a Three-Component Reaction: Mechanism and Origin of Enantio- and Diastereoselectivities

Mechanistic insights gained through density functional theory (DFT M06 and B3LYP) computations on a three-component cooperative asymmetric catalytic reaction between a diazo ester, a carbamate, and an imine, catalyzed by dirhodium acetate and chiral phosphoric acid (Brønsted acid), are presented. The addition of the dirhodium-bound enol to the imine yielding an α,β-diamino ester is energetically more preferred over a potentially competitive protonation of the same enol leading to an α-amino ester.

Unraveling innate substrate control in site-selective palladium-catalyzed C-H heterocycle functionalization

Understanding the regioselectivity of C-H activation in the absence of directing groups is an important step towards the design of site-selective C-H functionalizations. The Pd(ii)-catalyzed direct arylation of chromones and enaminones provides an intriguing example where a simple substitution leads to a divergence in substrate-controlled site-selectivity. We describe computational and experimental studies which reveal this results from a switch in mechanism and therefore the selectivity-determining step. We present computational results and experimentally measured kinetic isotope effects and labelling studies consistent with this proposal. The C-H activation of these substrates proceeds via a CMD mechanism, which favors more electron rich positions and therefore displays a pronounced kinetic selectivity for the C3-position. However, C2-selective carbopalladation is also a competitive pathway for chromones so that the overall regiochemical outcome depends on which substrate undergoes activation first. Our studies provide insight into the site-selectivity based on the favorability of two competing CMD and carbopalladation processes of the substrates undergoing coupling. This model can be utilized to predict the regioselectivity of coumarins which are proficient substrates for carbopalladation. Furthermore, our model is able to account for the opposite selectivities observed for enaminone and chromone, and explains how a less reactive coupling partner leads to a switch in selectivity.

Transition State Models for Understanding the Origin of Chiral Induction in Asymmetric Catalysis

In asymmetric catalysis, a chiral catalyst bearing chiral center(s) is employed to impart chirality to developing stereogenic center(s). A rich and diverse set of chiral catalysts is now available in the repertoire of synthetic organic chemistry. The most recent trends point to the emergence of axially chiral catalysts based on binaphthyl motifs, in particular, BINOL-derived phosphoric acids and phosphoramidites. More fascinating ideas took shape in the form of cooperative multicatalysis wherein organo- and transition-metal catalysts are made to work in concert. At the heart of all such manifestations of asymmetric catalysis, classical or contemporary, is the stereodetermining transition state, which holds a perennial control over the stereochemical outcome of the catalytic process. Delving one step deeper, one would find that the origin of the stereoselectivity is delicately dependent on the relative stabilization of one transition state, responsible for the formation of the predominant stereoisomer, over the other transition state for the minor stereoisomer. The most frequently used working hypothesis to rationalize the experimentally observed stereoselectivity places an undue emphasis on steric factors and tends to regard the same as the origin of facial discrimination between the prochiral faces of the reacting partners. In light of the increasing number of asymmetric catalysts that rely on hydrogen bonding as well as other weak non-covalent interactions, it is important to take cognizance of the involvement of such interactions in the sterocontrolling transition states. Modern density functional theories offer a pragmatic and effective way to capture non-covalent interactions in transition states. Aided by the availability of such improved computational tools, it is quite timely that the molecular origin of stereoselectivity is subjected to more intelligible analysis. In this Account, we describe interesting molecular insights into the stereocontrolling transition states of five reaction types, three of which provide access to chiral quaternary carbon atoms. While each reaction has its own utility and interest, the focus of our research has been on the mechanism and the origin of the enantio- and diastereoselectivity. In all of the examples, such as asymmetric diamination, sulfoxidation, allylation, and Wacker-type ring expansion, the role played by non-covalent interactions in the stereocontrolling transition states has been identified as crucial. The transfer of the chiral information from the chiral catalyst to the product is identified as taking place through a series of non-covalent interactions between the catalyst and a given position/orientation of the substrate in the chiral environment offered by the axially chiral catalyst. The molecular insights enunciated herein allude to abundant opportunities for rational modifications of the present generation of catalysts and the choice of substrates in these as well as related families of reactions. It is our intent to propose that the domain of asymmetric catalysis could enjoy additional benefits by having knowledge of the vital stereoelectronic interactions in the stereocontrolling transition states.

Insight into the electronic effect of phosphine ligand on Rh catalyzed CO2 hydrogenation by investigating the reaction mechanism

The effect of the outer coordination sphere of the diphosphine ligand on the catalytic efficiency of [Rh(PCH₂X^RCH₂P)₂]⁺ (X^R = CH₂, N–CH₃, CF₂) catalyzed CO₂ hydrogenation was studied. It was found that the hydricity of the metal hydride bond determined the activation energy of the rate determining step of the reaction.

Density functional theory investigation on Pd-catalyzed cross-coupling of azoles with aryl thioethers

The mechanism and the origin of chemoselectivity of Pd-catalyzed C–H/C–S activation have been studied by density functional theory.