A Computational Mechanistic Study of an Unprecedented Heck-Type Relay Reaction: Insight into the Origins of Regio- and Enantioselectivities

Pd(II)-Catalyzed Asymmetric [2+2] Annulation for the Construction of Chiral Benzocyclobutenes

Asymmetric de novo synthesis of benzocyclobutenes (BCBs) via catalytic intermolecular reaction is highly desired for efficient access to this important class of compounds, yet such a strategy remains unmet challenge. Here, we report a Pd/Pyrox-catalyzed asymmetric [2+2] annulation between arylboronic acids and functionalized alkenes, providing an unprecedented efficient protocol to access various enantio-enriched BCBs in a modular and versatile manner under mild conditions. A broad substrate scope with excellent enantioselectivity has been achieved under the current protocol. The isolation and characterization of the key chiral palladacycle intermediate, together with DFT calculations, provides strong evidence for the catalytic pathway including an enantiodetermining arylpalladation step.

Facile construction of distal and diversified tertiary and quaternary stereocenters

Exploration of novel chiral pharmaceutical candidates is motivation to immersive efforts among synthetic chemists. Achieving skeletal construction and chiral diversity in a highly efficient manner is a momentous goal in the chemical society. Unfortunately, current methods for chiral induction focus primarily on a specific site. Herein, we realized the asymmetric chain-walking arylation of alkenyl alcohols for the construction of multisite tertiary and quaternary stereocenters in high yields and enantioselectivity. This new operation-friendly strategy carries substantial potential for future industrialization.

A Cascade C(sp3)-H Annulation Involving C(alkyl),C(alkyl)-Palladacycle Intermediates

C-H bond functionalization involving C,C-palladacycle intermediates provides a unique platform for developing novel reactions. However, the vast majority of studies have been limited to the transformations of C(aryl),C-palladacycles. In sharp contrast, catalytic reactions involving C(alkyl),C(alkyl)-palladacycles have rarely been reported. Herein, we disclose an unprecedented cascade C(sp3)-H annulation involving C(alkyl),C(alkyl)-palladacycles. In this protocol, alkene-tethered cycloalkenyl bromides undergo intramolecular Heck/C(sp3)-H activation to generate C(alkyl),C(alkyl)-palladacycles, which can be captured by α-bromoacrylic acids to afford tricyclic fused pyridinediones. In addition, this strategy can also be applied to indole-tethered cycloalkenyl bromides to construct pentacyclic fused pyridinediones via suquential Heck dearomatization/C(sp3)-H activation/decarboxylative cyclization. Notably, the removal of α-bromoacrylic acids in the reaction of alkene-tethered cycloalkenyl bromides can build an interesting tricyclic skeleton containing a four-membered ring. Preliminary mechanistic experiments indicate that five-membered C(alkyl),C(alkyl)-palladacycles serve as the key intermediates. Meanwhile, density functional theory (DFT) calculations have provided insights into the reaction pathway.

Remote Functionalization by Pd-Catalyzed Isomerization of Alkynyl Alcohols

In recent years, progress has been made in the development of catalytic methods that allow remote functionalizations based on alkene isomerization. In contrast, protocols based on alkyne isomerization are comparatively rare. Herein, we report a general Pd-catalyzed long-range isomerization of alkynyl alcohols. Starting from aryl-, heteroaryl-, or alkyl-substituted precursors, the optimized system provides access preferentially to the thermodynamically more stable α,β-unsaturated aldehydes and is compatible with potentially sensitive functional groups. We showed that the migration of both π-components of the carbon-carbon triple bond can be sustained over several methylene units. Computational investigations served to shed light on the key elementary steps responsible for the reactivity and selectivity. These include an unorthodox phosphine-assisted deprotonation rather than a more conventional β-hydride elimination in the final tautomerization event.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

A substrate descriptor based approach for the prediction and understanding of the regioselectivity in caged catalyzed hydroformylation

The use of data driven tools to predict the selectivity of homogeneous catalysts has received considerable attention in the past years. In these studies often the catalyst structure is varied, but the use of substrate descriptors to rationalize the catalytic outcome is relatively unexplored. To study whether this may be an effective tool, we investigated both an encapsulated and a non-encapsulated rhodium based catalyst in the hydroformylation reaction of 41 terminal alkenes. For the non-encapsulated catalyst, CAT2, the regioselectivity of the acquired substrate scope could be predicted with high accuracy using the Δ13C NMR shift of the alkene carbon atoms as a descriptor (R2 = 0.74) and when combined with a computed intensity of the CC stretch vibration (ICC stretch) the accuracy increased further (R2 = 0.86). In contrast, a substrate descriptor approach with an encapsulated catalyst, CAT1, appeared more challenging indicating a confined space effect. We investigated Sterimol parameters of the substrates as well as computer-aided drug design descriptors of the substrates, but these parameters did not result in a predictive formula. The most accurate substrate descriptor based prediction was made with the Δ13C NMR shift and ICC stretch (R2 = 0.52), suggestive of the involvement of CH-π interactions. To further understand the confined space effect of CAT1, we focused on the subset of 21 allylbenzene derivatives to investigate predictive parameters unique for this subset. These results showed the inclusion of a charge parameter of the aryl ring improved the regioselectivity predictions, which is in agreement with our assessment that noncovalent interactions between the phenyl ring of the cage and the aryl ring of the substrate are relevant for the regioselectivity outcome. However, the correlation is still weak (R2 = 0.36) and as such we are investigating novel parameters that should improve the overall regioselectivity outcome.

Regio- and enantioselective remote dioxygenation of internal alkenes

Methods for the enantioselective direct oxygenation of internal alkenes have provided chemists with versatile and powerful toolboxes for the synthesis of optically pure alcohols, one of the most privileged structural motifs. Regioselectivity, however, remains a formidable challenge in the functionalization of internal alkenes. Here we report a palladium-catalysed highly regio- and enantioselective remote 1,n-dioxygenation (n ≥ 4) of internal alkenes with engineered pyridine-oxazoline (Pyox) ligands. The reactions proceed efficiently and exhibit a broad substrate scope with excellent regio- and enantioselectivity, affording optically pure 1,n-diol acetates as the key synthons for important bioactive molecules. Experimental studies and density functional theory calculations provide evidence that the regioselectivity is governed by the reactivity disparity of two allylic C-H bonds, where the oxypalladation is reversible and the first palladium migration step proves to be the regioselectivity-determining step, enabled by the modified phenyl-substituted Pyox ligands.

Mechanism and Origins of Diastereo- and Regioselectivities of Palladium-Catalyzed Remote Diborylative Cyclization of Dienes via Chain-Walking Strategy

Density functional theory calculations have been performed to investigate the palladium-catalyzed remote diborylative cyclization of dienes. The computations reveal that the reaction proceeds through a rarely explored Pd(II)/Pd(IV) catalytic cycle, and the formal σ-bond metathesis between the alkylpalladium intermediate and B₂ pin₂ occurs via the pathway of the B-B oxidative addition/C-B reductive elimination involving the high-valent Pd(IV) species. The diastereoselectivity is determined by the migratory insertion into the Pd-C bond, which is mainly due to the combination of the torsional strain effect, steric repulsion and C-H-O hydrogen-bonding interaction. The steric hindrance around the reacting carbon group in the C-B reductive elimination turns out to be a key factor to provide the driving force of the chain walking of the Pd center to the terminal primary carbon position, enabling the experimentally observed remote regioselectivity.

Density Functional Theory Analysis of the Copolymerization of Cyclopropenone with Ethylene Using a Palladium Catalyst

Density functional theory has been used to elucidate the mechanism of Pd copolymerization of cyclopropenone with ethylene. The results reveal that introducing ethylene and cyclopropenone to Pd catalyst is thermodynamically feasible and generates the α,β-unsaturated ketone unit (UnitA). Cis-mode insertion and Path A_1a are the most favorable reaction routes for ethylene and cyclopropenone, respectively. Moreover, cyclopropenone decomposition can generate CO in situ without a catalyst or with a Pd catalyst. The Pd-catalyzed decomposition of cyclopropenone exhibits a lower reaction barrier (22.7 kcal/mol) than its direct decomposition. Our study demonstrates that incorporating CO into the Pd catalyst can generate the isolated ketone unit (UnitB). CO is formed first; thereafter, UnitB is generated. Therefore, the total energy barrier of UnitB generation, accounting for the CO barrier, is 22.7 kcal/mol, which is slightly lower than that of UnitA generation (24.0 kcal/mol). Additionally, the possibility of copolymerizing ethylene, cyclopropenone, and allyl acetate (AAc) has been investigated. The free energy and global reactivity index analyses indicate that the cyclopropenone introduction reaction is more favorable than the AAc insertion, which is consistent with the experimental results. Investigating the copolymerization mechanism will help to develop of a functionalization strategy for polyethylene polymers.

Three-Component Asymmetric Ni-Catalyzed 1,2-Dicarbofunctionalization of Unactivated Alkenes via Stereoselective Migratory Insertion

An asymmetric 1,2-dicarbofunctionalization of unactivated alkenes with aryl iodides and aryl/alkenylboronic esters under nickel/bioxazoline catalysis is disclosed. A wide array of aryl and alkenyl nucleophiles are tolerated, furnishing the products in good yield and with high enantioselectivity. In addition to terminal alkenes, 1,2-disubstituted internal alkenes participate in the reaction, establishing two contiguous stereocenters with high diastereoselectivity and moderate enantioselectivity. A combination of experimental and computational techniques shed light on the mechanism of the catalytic transformation, pointing to a closed-shell pathway with an enantiodetermining migratory insertion step, where stereoinduction arises from synergistic interactions between the sterically bulky achiral sulfonamide directing group and the hemilabile bidentate ligand.

Density Functional Theory Study of the Regioselectivity in Copolymerization of bis-Styrenic Molecules with Propylene Using Zirconocene Catalyst

Density functional theory (DFT) was used to study the regioselectivity of the copolymerization of propylene and the bis-styrenic molecules (DVB and BVPE) using a zirconocene catalyst. This study reveals the following: when hydrogen is introduced to reactivate the catalyst on the vinyl bonds containing DVB or BVPE, the second vinyl bond is inserted into the polymer in a regio-irregular 1,2-way. (I) The 1,2-insertion mode forms more thermodynamically stable products. (II) The 2,1 insertion, DVB-PP1, or BVPE-PP1 needs to rotate 180° along the Zr-C1 bond to complete the process; thus, it is easier to accomplish the 1,2 insertion. (III) The analysis of the local electrophilicity/nucleophilicity index and the Fukui functions also indicate that the 1,2-insertion mode is the optimal insertion mode. Investigating the mechanism of this experimental phenomenon is important in the development of a functionalization strategy for polypropylene (PP) polymers.

Machine learning studies on asymmetric relay Heck reaction—Potential avenues for reaction development

The integration of machine learning (ML) methods into chemical catalysis is evolving as a new paradigm for cost and time economic reaction development in recent times. Although there have been several successful applications of ML in catalysis, the prediction of enantioselectivity ( ee) remains challenging. Herein, we describe a ML workflow to predict ee of an important class of catalytic asymmetric transformation, namely, the relay Heck (RH) reaction. A random forest ML model, built using quantum chemically derived mechanistically relevant physical organic descriptors as features, is found to predict the ee remarkably well with a low root mean square error of 8.0 ± 1.3. Importantly, the model is effective in predicting the unseen variants of an asymmetric RH reaction. Furthermore, we predicted the ee for thousands of unexplored complementary reactions, including those leading to a good number of bioactive frameworks, by engaging different combinations of catalysts and substrates drawn from the original dataset. Our ML model developed on the available examples would be able to assist in exploiting the fuller potential of asymmetric RH reactions through a priori predictions before the actual experimentation, which would thus help surpass the trial and error loop to a larger degree.

Bulky selenium ligand stabilized trans-palladium dichloride complexes as catalyst for silver-free decarboxylative coupling of coumarin-3-carboxylic acids

This report describes synthesis of three new trans -palladium dichloride complexes of bulky selenium ligands. These complexes possess a Cl-Pd-Cl rotor spoke attached to a Se-Pd-Se axle. The new ligands and palladium complexes ( C1 - C3 ) were characterized with the help of NMR, HRMS, UV-Vis., IR, and elemental analysis. The single crystal structure of metal complex C2 confirmed a square planer geometry of complex with trans -orientation. The X-ray structure revealed intramolecular secondary interactions (SeCH---Cl) between chlorine of PdCl 2 and CH 2 proton of selenium ligand. Variable temperature NMR data shows coalescence of diastereotopic protons, which indicates pyramidal inversion of selenium atom at elevated temperature. The relaxed potential energy scan of C2 suggests a rotational barrier of ~12.5 kcal/mol for rotation of chlorine atom through Cl-Pd-Cl rotor. The complex C3 possess dual intramolecular secondary interactions (OCH 2 ---Cl and SeCH 2 ---Cl) with stator ligand. Molecular rotor C2 was found to be most efficient catalyst for the decarboxylative Heck-coupling under mild reaction conditions. The protocol is applicable to a broad range of substrates with large functional group tolerance and low catalyst loading (2.5 mol %). The mechanism of decarboxylative Heck-coupling reaction was investigated through experimental and computational studies. Importantly the reaction works under silver-free conditions which reduces the cost of overall protocol. Further, the catalyst also worked for decarboxylative arylation and decarboxylative Suzuki-Miyaura coupling reactions with good yields of the coupled products.

Mechanism of Pd-catalysed C(sp3)-H arylation of thioethers with Ag(I) additives

Mechanistic studies reveal that Pd-catalyzed C(sp3)-H arylation of thioethers with silver(i) additives takes place via C(sp3)-H activation, oxidative addition and reductive elimination, wherein all steps proceed via the heterodimeric Pd-Ag pathway. Besides, the active heterodimeric Pd-Ag species are detected by mass spectrometry via control experiments.

Oxidation of Pd(II) with disilane in a palladium-catalyzed disilylation of aryl halides: a theoretical view

Density functional theory (DFT) calculation has been used to reveal the mechanism of the Pd-catalyzed disilylation reaction of aryl halides. The DFT calculations indicate that the reaction starts with the oxidative addition of the C-I bond to the Pd(0) catalyst. Concerted metalation-deprotonation (CMD) can then generate a five-membered palladacycle. Insertion of Pd(ii) into the Si-Si bond in disilane followed by two sequential steps of reductive eliminations yields the disilylation product and regenerates the Pd(0) catalyst. According to the NPA charge analysis along the reaction coordinates, the formal oxidative addition of the Si-Si bond to palladium could be considered as the insertion of palladium into the Si-Si bond. However, the conventional oxidative addition of the C-I bond to palladium is exactly an oxidation process with the electron transfer from the palladium atom to the C-I bond. Therefore, electron rich Pd(0) is beneficial for the oxidation process, and Pd(ii) prone to acquire electrons is beneficial for the insertion process.

Base-Free Cross-Couplings of Aryl Diazonium Salts in Methanol: PdII -Alkoxy as Reactivity-Controlling Intermediate

Pd-catalyzed cross-coupling reactions of aryl diazonium salts are generally assumed to proceed via cationic Pd^II intermediates which in turn would be highly reactive in the subsequent transmetalation step. Contrary to this belief, we herein report our observation and rationalization of opposing reactivities of ArN₂ ⁺ in Suzuki (=effective) and Stille (=ineffective) cross-couplings in MeOH. Our systematic experimental and computational studies on the roles of transmetalating agent, solvent, base and the likely involvement of in situ formed diazoether derivatives challenge the currently accepted mechanism. Our data suggest that the observed solvent dichotomy is primarily due to Pd^II -methoxy intermediates being formed, which are unreactive with arylstannanes, but highly reactive with arylboronic acids, complementing the Suzuki "Pd-oxy" mechanism with the direct demonstration of transmetalation of a Pd^II -alkoxy complex. Lewis acids were found to circumvent this reactivity divergence, promoting efficient couplings regardless of the employed conditions or coupling partners.

Stereoselective Remote Functionalization via Palladium-Catalyzed Redox-Relay Heck Methodologies

Exploration of novel, three-dimensional chemical space is of growing interest in the drug discovery community and with this comes the challenge for synthetic chemists to devise new stereoselective methods to introduce chirality in a rapid and efficient manner. This Minireview provides a timely summary of the development of palladium-catalyzed asymmetric redox-relay Heck-type processes. These reactions represent an important class of transformation for the selective introduction of remote stereocenters, and have risen to prominence over the past decade. Within this Minireview, the vast scope of these transformations will be showcased, alongside applications to pharmaceutically relevant chiral building blocks and drug substances. To complement this overview, a mechanistic summary and discussion of the current limitations of the transformation are presented, followed by an outlook on future areas of investigation.

Mechanism of nickel-catalyzed direct carbonyl-Heck coupling reaction: the crucial role of second-sphere interactions

We present a detailed DFT mechanistic study on the first Ni-catalyzed direct carbonyl-Heck coupling of aryl triflates and aldehydes to afford ketones. The precatalyst Ni(COD)2 is activated with the phosphine (phos) ligand, followed by coordination of the substrate PhOTf, to form [Ni(phos)(PhOTf)] for intramolecular PhOTf to Ni(0) oxidative addition. The ensuing phenyl-Ni(ii) triflate complex substitutes benzaldehyde for triflate by an interchange mechanism, leaving the triflate anion in the second coordination sphere held by Coulomb attraction. The Ni(ii) complex cation undergoes benzaldehyde C[double bond, length as m-dash]O insertion into the Ni-Ph bond, followed by β-hydride elimination, to produce Ni(ii)-bound benzophenone, which is released by interchange with triflate. The resulting neutral Ni(ii) hydride complex leads to regeneration of the active catalyst following base-mediated deprotonation/reduction. The benzaldehyde C[double bond, length as m-dash]O insertion is the rate-determining step. The triflate anion, while remaining in the second sphere, engages in electrostatic interactions with the first sphere, thereby stabilizing the intermediate/transition state and enabling the desired reactivity. This is the first time that such second-sphere interaction and its impact on cross-coupling reactivity has been elucidated. The new insights gained from this study can help better understand and improve Heck-type reactions.

Mechanistic Study on Palladium-Catalyzed Regioselective Oxidative Amination: Roles of Ammonium Salts

Anti-Markovnikov selective oxidative amination reaction with simple alkenes is particularly promising but challenging because of the inherent electronic effect of the alkene substrate which is in favor of the Markovnikov product. In a recently reported Pd-catalyzed anti-Markovnikov oxidative amination reaction, the addition of quaternary ammonium salts is shown to be critical. We performed a comprehensive DFT study to elucidate the reaction mechanism and the origin of the regioselectivity, as well as the roles of the ammonium salts. Our results show that without and with the ammonium salts the reaction mechanisms are different. Detailed analyses indicate that the steric effects account for the switch of regioselectivity. The roles of the quaternary ammonium salts have been elucidated: (1) Me₄NOAc plays the role of base in deprotonating the phthalimide and allows the reaction to proceed through a trans-aminopalladation mechanism; (2) Me₄NCl facilitates the thermodynamically favorable transformation of Pd(OAc)₂ to the palladate ([Pd(AcO)₂Cl₂]^2-), which lessens the polarity of the carbon-carbon double bond, minimizes the inherent electronic effects, and leads to a steric-effect-controlled reaction; (3) Me₄NCl is essential in decreasing the activation barrier in the rate-determining ligand exchange step by Cl^- acting as a better leaving group (compared to AcO^-).

Palladium Catalyzed Stereoselective Arylation of Biocatalytically Derived Cyclic 1,3-Dienes: Chirality Transfer via a Heck-Type Mechanism

Microbial arene oxidation of benzoic acid with Ralstonia eutropha B9 provides a chiral highly functionalized cyclohexadiene, suitable for further structural diversification. Subjecting this scaffold to a Pd-catalyzed Heck reaction effects a regio- and stereoselective arylation of the cyclohexadiene ring, with 1,3-chirality transfer of stereogenic information installed in the microbial arene oxidation. Quantum chemical calculations explain the selectivity both by a kinetic preference for the observed arylation position and by reversible carbopalladation in competing positions. Further product transformation allowed the formation of a tricyclic ketone possessing four stereogenic centers. This demonstrates the capability of the method to introduce stereochemical complexity from planar nonchiral benzoic acid in just a few steps.

Mechanistic Study of Unprecedented Highly Regioselective Hydrocyanation of Terminal Alkynes: Insight into the Origins of the Regioselectivity and Ligand Effects

Density functional theory (DFT) calculations were performed to gain insight into the mechanism of the nickel-catalyzed hydrocyanation of terminal alkynes with Zn(CN)₂ and water to exclusively generate the branched nitrile with excellent Markovnikov selectivity. After precatalyst activation to give the LNi(0) active species, the transformation proceeds via the following steps: (1) oxidative addition of H₂ O to the LNi(0) provides the intermediate LNi(II)H(OH); (2) ligand exchange of LNi(II)H(OH) with Zn(CN)₂ gives the intermediate LNi(II)H(CN); (3) alkyne insertion to the LNi(II)H(CN) forms the alkenyl nickel complex, followed by the reductive elimination step reaching the final product. This mechanism is kinetically and thermodynamically more favorable than that of the experimental proposed ones. On the basis of the experimental observations, more water molecules cannot further improve the reaction as it has also been rationalized. Furthermore, the origin of the high regioselectivity of the product, the variable effectiveness of the metal mediator as function of ligands, as well as the high yield of the alkyl-substituted alkynes substrates, is analyzed in detail. © 2019 Wiley Periodicals, Inc.

The origin of regioselectivity in Cu-catalyzed hydrocarbonylative coupling of alkynes with alkyl halides

Steric interactions mediate a switch between a ketone and allylic alcohol in Cu-catalyzed hydrocarbonylative coupling of alkynes with alkyl halides.

Metal-catalyzed alkyne oxidation/CH functionalization: Effects of oxidant, temperature, and metal catalyst on chemoselectivity

Gold-catalyzed intermolecular alkyne oxidation has attracted much synthetic attention, but mostly suffering undesired over-oxidation. Recent experiments demonstrated that over-oxidation could be dramatically suppressed in zinc(II)-catalyzed intermolecular alkyne oxidation/CH functionalization. By means of first-principle density functional theory calculations, we explored the mechanism of the M-catalyzed intermolecular alkyne oxidations (M = Zn(OTf)₂ and Au⁺ PR₃ ) as well as the effects of oxidants, temperature, and metal catalysts on chemoselectivity, in an effort to disclose the origin of the extraordinary chemoselectivity pertaining to zinc catalysis. Our calculations indicate that the Zn-catalyzed intermolecular alkyne oxidation/CH functionalization proceeds by a Friedel-Crafts alkylation mechanism rather than metal carbene insertion mechanism. The chemoselectivity of CH functionalization against over-oxidation in Zn catalysis, in comparison with gold catalysis, can be jointly controlled by four factors: (1) the use of less nucleophilic N-oxide, (2) the enhanced electrophilicity and carbocationic nature of the carbenic site in the α-oxo metal carbenoid intermediate, (3) enhanced steric repulsion to incoming oxidant exerted by bulky ancillary ligand in the close nearby of the carbenic site to disfavor intermolecular over-oxidation and (4) the large negative value of activation entropy in the intermolecular over-oxidation pathway, that jointly give rise to lower activation free energy for the intramolecular cyclization/CH functionalization pathway than for the intermolecular over-oxidation pathway. © 2018 Wiley Periodicals, Inc.

Theoretical Study of Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition of Cyclohexadiene and Norbornadiene with 1,2-Diols to Form Bridged Carbocycles

The recent success of Krische et al. ( Angew. Chem., Int. Ed. 2017 , 56 , 14667 -14671 ) in achieving a ruthenium(0)-catalyzed transfer hydrogenative cycloaddition of 1,2-diols with cyclohexadiene and norbornadiene in excellent yield with exo- and diastereoselectivity represents an exciting development in the synthesis of bridged carbocycles. In the present work, the possible catalytic mechanisms and origin of the exo- and diastereoselectivity for cyclohexadiene and norbornadiene were studied in detail by density functional theory calculations. The theoretical results indicate that the exoselective pathway for the cyclohexadiene substrate proceeds by a novel two-step successive C-C coupling, while the endoselective pathway undergoes the C-C coupling reaction in a conventional concerted manner. The origin of the preferential chemoselectivity of dione-cyclohexadiene C-C coupling over aromatization to benzene was investigated. Aromatization to benzene is unfavorable because of the large distortion energy of the three-membered ring in the transition state of hydrogen migration. From distortion/interaction analysis, for norbornadiene, the distortion energy plays the main role in determining the exoselectivity.

A DFT study of Co(i) and Ni(ii) pincer complex-catalyzed hydrogenation of ketones: intriguing mechanism dichotomy by ligand field variation

Ligand field variation governs the mechanism dichotomy for isoelectronic catalysts.

A mechanism exploration of stereodivergent coupling of aldehydes and alkynes catalyzed synergistically by rhodium and amine

The stereodivergent coupling of alkynes and aldehydes with a synergistic catalyst approach using rhodium and amine.

Unveiling the mechanism and regioselectivity of iron-dipyrrinato-catalyzed intramolecular C(sp3)–H amination of alkyl azides

The mechanism of iron-catalyzed C(sp³)–H amination was established, in which regioselectivity arose from both radical stability and ring strain.

Harnessing Noncovalent Interactions in Dual-Catalytic Enantioselective Heck-Matsuda Arylation

The use of more than one catalyst in one-pot reaction conditions has become a rapidly evolving protocol in the development of asymmetric catalysis. The lack of molecular insights on the mechanism and enantioselectivity in dual-catalytic reactions motivated the present study focusing on an important catalytic asymmetric Heck-Matsuda cross-coupling. A comprehensive density functional theory (M06 and B3LYP-D3) investigation of the coupling between a spirocyclic cyclopentene and 4-fluorophenyl diazonium species under a dual-catalytic condition involving Pd₂(dba)₃ (dba = trans, trans-dibenzylideneacetone) and chiral 2,2'-binaphthyl diamine (BINAM)-derived phosphoric acids (BDPA, 2,2'-binaphthyl diamine-derived phosphoric acids) is presented. Among various mechanistic possibilities examined, the pathway with explicit inclusion of the base (in situ generated sodium bicarbonate/sodium biphosphate) is found to be energetically more preferred over the analogous base-free routes. The chiral phosphate generated by the action of sodium carbonate on BDPA is found to remain associated with the reaction site as a counterion. The initial oxidative addition of Pd(0) to the aryl diazonium bond gives rise to a Pd-aryl intermediate, which then goes through the enantiocontrolling migratory insertion to the cyclic alkene, leading to an arylated cycloalkene intermediate. Insights on how a series of noncovalent interactions, such as C-H···O, C-H···N, C-H···F, C-H···π, lp···π, O-H···π, and C-F···π, in the enantiocontrolling transition state (TS) render the migration of the Pd-aryl to the si prochiral face of the cyclic alkene more preferred over that to the re face are utilized for modulating the enantioselectivity. Aided by molecular insights on the enantiocontrolling transition states, we predicted improved enantioselectivity from 37% to 89% by changes in the N-aryl substituents of the catalyst. Subsequent experiments in our laboratory offered very good agreement with the predicted enantioselectivities.

A Diverted Aerobic Heck Reaction Enables Selective 1,3-Diene and 1,3,5-Triene Synthesis through C-C Bond Scission

Substituted 1,3-dienes are valuable synthetic intermediates used in myriad catalytic transformations, yet modern catalytic methods for their preparation in a highly modular fashion using simple precursors are relatively few. We report here an aerobic boron Heck reaction with cyclobutene that forms exclusively linear 1-aryl-1,3-dienes using (hetero)arylboronic acids, or 1,3,5-trienes using alkenylboronic acids, rather than typical Heck products (i.e., substituted cyclobutenes). Experimental and computational mechanistic data support a pericyclic mechanism for C-C bond cleavage that enables the cycloalkene to circumvent established limitations associated with diene reagents in Heck-type reactions.

Efficient and stereodivergent synthesis of unsaturated acyclic fragments bearing contiguous stereogenic elements

Synthetic organic strategies that enable the catalytic and rapid assembly of a large array of organic compounds possessing multiple stereocenters in acyclic systems are somewhat rare, especially when it comes to reaching today’s high standards of efficiency and selectivity. In particular, the catalytic preparation of a three-dimensional molecular layout of a simple acyclic hydrocarbon skeleton possessing several stereocenters from simple and readily available reagents still represents a vastly uncharted domain. Here, we report a rapid, modular, stereodivergent and diversity-oriented unified strategy to construct acyclic molecular frameworks bearing up to four contiguous and congested stereogenic elements, with remarkably high levels of stereocontrol and in only three catalytic steps from commercially available alkynes. A regio- and diastereoselective catalytic Heck migratory insertion reaction of alkenylcyclopropyl carbinols merging selective C–C bond cleavage of a cyclopropane represents the key step.

Chiral Brønsted acid-catalyzed intramolecular SN2' reaction for enantioselective construction of a quaternary stereogenic center

Construction of a quaternary stereogenic center was accomplished through the enantioselective intramolecular allylic substitution reaction of bis-trichloroacetimidate catalyzed by a chiral phosphoramide derivative.

An enantioselective intramolecular anti-S_N2′ cyclization reaction for the construction of a quaternary stereogenic center was accomplished through the activation of the leaving group using a binaphthol-derived phosphoramide as the chiral Brønsted acid catalyst. The present allylic substitution reaction is beneficial not only for the regioselective nucleophilic substitution at the multi-substituted site of the double bond but also for controlling the stereochemical outcome because of using a geometrically defined double bond. Indeed, the reaction afforded synthetically useful amino alcohol derivatives having a tetra-substituted carbon center in a highly enantioselective manner in most cases, in which the modification of the sulfonamide unit of the phosphoramide catalyst was demonstrated to improve the enantioselectivity. Experimental and theoretical elucidation of the reaction mechanism suggested that the reaction proceeds through a synchronous anti-S_N2′ pathway, although NMR monitoring of the reaction indicated the formation of the phosphorimidate ester via the S_N2 reaction of the catalyst with the substrate, which results in catalyst deactivation. Further theoretical studies of the origin of the stereochemical outcome at the generated quaternary stereogenic center were performed. Structural analysis of the transition states at the enantio-determining step revealed that the distinct discrimination of the substituents attached to the geometrically defined double bond is achieved by the anthryl and sulfonamide substituents of the catalyst through the three-point hydrogen bonding interactions and the T-shaped C–H···π interactions.

Strong Preference of the Redox-Neutral Mechanism over the Redox Mechanism for the TiIV Catalysis Involved in the Carboamination of Alkyne with Alkene and Diazene

Titanium catalysis generally prefers redox-neutral mechanisms. Yet it has been reported that titanium could promote bond formations in a way similar to reductive elimination. Accordingly, redox catalytic cycles involving Ti^IV /Ti^II cycling have been considered. By studying, as an example, the carboamination of alkynes with alkenes and azobenzene catalyzed by the [Ti^IV ]=NPh imido complex, we performed DFT computations to gain an understanding of how the "abnormal" catalysis takes place, thereby allowing us to clarify whether the catalysis really follows Ti^IV /Ti^II redox mechanisms. The reaction first forms an azatitanacyclohexene by alkyne addition to the [Ti^IV ]=NPh bond, followed by alkene insertion. The azatitanacyclohexene can either undergo C^α -C^γ coupling, to afford bicyclo[3.1.0]imine, or β-H elimination, to yield a [Ti^IV ]-H hydride, which then undergoes C^α =C^β or C^γ =C^δ insertion to give an α,β- or β,γ-unsaturated imine, respectively. Both the geometric and electronic structures indicate that the catalytic cycles proceed through redox-neutral mechanisms. The alternative redox mechanisms (e.g., by N-H or C-H reductive elimination) are substantially less favorable. We concluded that electronically, the Ti^IV catalysis intrinsically favors the redox-neutral mechanism, because a redox pathway would involve Ti^II structures either in the triplet ground state or in the high-lying open-shell singlet state, but the involvement of triplet Ti^II structures is spin-forbidden and that of singlet Ti^II structures is energetically disadvantageous.

A theoretical study on the oxidation of alkenes to aldehydes catalyzed by ruthenium porphyrins using O2 as the sole oxidant

Density functional theory (DFT) calculations were used to study the ruthenium porphyrin-catalyzed oxidation of styrene to generate an aldehyde. The results indicate that two reactive oxidants, dioxoruthenium and monooxoruthenium-superoxo porphyrins, participate in the catalytic oxidation. In the mechanism, the resultant monooxoruthenium porphyrin acts in the tandem epoxide isomerization (E-I) to selectively yield an aldehyde and generate a dioxoruthenium porphyrin, thereby triggering new oxidation reaction cycles. In this calculation, several key elements responsible for the observed oxidative ability have been established by using Frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, etc., which include the reaction energy, the spin exchange effect, the spin-state conversion process, and the energy level of the lowest unoccupied molecular orbitals (LUMOs) of the reactive oxidants. The comparative oxidative abilities of the ruthenium-oxo/superoxo compounds with different axial ligands are also investigated. The results suggest that the ruthenium-oxo/superoxo species featuring a chlorine axial ligand is more reactive than that substituted with oxygen. This tuneable reactivity can be understood when considering the different electronic characters of the two ligands and the effective atomic number rule (EAN).