Experimental and Computational Assessment of Reactivity and Mechanism in C(sp(3))-N Bond-Forming Reductive Elimination from Palladium(IV)

High Selectivity in Csp2-Csp2 versus Csp3-O Reductive Elimination from Cycloplatinated(IV) Complexes

The cycloplatinated(IV) complexes trans-[Pt(p-MeC6H4)(C∧N)(OAc)2(H2O)] (C∧N = benzo[h]quinolate, bhq, 2a, and 2-phenylpyridinate, ppy, 2b) were prepared by reacting the corresponding [Pt(p-MeC6H4)(C∧N)(SMe2)] precursors with PhI(OAc)2 through an oxidative addition (OA) reaction. Thermolysis of 2a at 65 °C generates cis-[Pt(κ1N-10-(p-MeC6H4)-bhq)(OAc)2(H2O)], 3a, which is the product of a Csp2Ar-Csp2bhq reductive elimination (RE). The observed coupling reaction is significantly different from the previously reported analogous thermolysis of trans-[PtMe(C∧N)(OAc)2(H2O)] (C∧N = bhq, 2c, and ppy, 2d) that selectively releases Me-OAc (C-O RE). The density functional theory (DFT) calculations and experimental observations reveal that the Csp2Ar-Csp2bhq coupling reaction occurs through the dissociation of a coordinated water ligand. This in turn is followed by the concomitant bond forming and bond breaking process via a three-center ring transition state, in contrast to the Csp3Me-OAc coupling, which had taken place by an outer sphere SN2 type RE reaction in methyl complexes.

Transition-Metal-Catalyzed 1,2-Diaminations of Olefins: Synthetic Methodologies and Mechanistic Studies

1,2-Diamines are synthetically important motifs in organo-catalysis, natural products, and drug research. Continuous utilization of transition-metal based catalyst in direct 1,2-diamination of olefines, in contrast to metal-free transformations, with numerous impressive advances made in recent years (2015-2023). This review summarized contemporary research on the transition-metal catalyzed/mediated [e. g., Cu(II), Pd(II), Fe(II), Rh(III), Ir(III), and Co(II)] 1,2-diamination (asymmetric and non-asymmetric) especially emphasizing the recent synthetic methodologies and mechanistic understandings. Moreover, up-to-date discussion on (i) paramount role of oxidant and catalyst (ii) key achievements (iii) generality and uniqueness, (iv) synthetic limitations or future challenges, and (v) future opportunities are summarized related to this potential area.

Diverting the 5-exo-Trig Oxypalladation to Formally 6-endo-Trig Fluorocycloetherification Product through 1,2-O/Pd(IV) Dyotropic Rearrangement

Pd-catalyzed cyclizative functionalization of γ-hydroxyalkenes affords tetrahydrofuran derivatives via a key 5-exo-trig oxypalladation step. Herein, we report a palladium(II)-catalyzed, Selectfluor-mediated formal 6-endo-trig fluorocycloetherification of γ-hydroxyalkenes for the synthesis of functionalized tetrahydropyrans. Mechanistically, an σ-alkyl-Pd(II) intermediate resulting from the 5-exo-trig oxypalladation process is isolated and characterized by X-ray crystallographic analysis. Its oxidation with Selectfluor to Pd(IV) triggers the chemoselective 1,2-O/Pd(IV) dyotropic rearrangement affording, after C-F bond-forming reductive elimination, the tetrahydropyrans with concurrent generation of a tertiary carbon-fluorine bond. The occurrence of this 1,2-positional interchange is further evidenced by trapping the rearranged quaternary C(sp³)-Pd bond by an internal nucleophile that is materialized by the development of a Pd(II)-catalyzed oxidative bis-heterocyclization of alkenes.

Rethinking Catalyst Trapping in Ni-Catalyzed Thieno[3,2-b]thiophene Polymerization

Catalyst-transfer polymerization (CTP) is a chain-growth method used to synthesize conjugated polymers. Although CTP works well for most donor-type monomers, the polymerization stalls with thieno[3,2-b]thiophene when using Ni catalysts. Previous reports have rationalized this result by suggesting that the catalyst is trapped in a Ni0 π-complex with the highly electron-rich arene. In this study, evidence is provided that the catalyst trap is more likely a NiII complex that arises from oxidative insertion of Ni0 into the C-S bonds of thieno[3,2-b]thiophene. This result is consistent with the known reactivity of Ni0 complexes toward S-heteroarenes and is supported herein by 31P nuclear magnetic resonance spectra acquired in situ, as well as data collected from small-molecule model reactions and density-functional theory simulations of the polymerization. We propose that this C-S insertion pathway and related off-cycle reactions may be relevant to understanding or enabling the CTP of other monomers with fused thiophenes.

ReNeGate: A Reaction Network Graph-Theoretical Tool for Automated Mechanistic Studies in Computational Homogeneous Catalysis

Exploration of the chemical reaction space of chemical transformations in multicomponent mixtures is one of the main challenges in contemporary computational chemistry. To remove expert bias from mechanistic studies and to discover new chemistries, an automated graph-theoretical methodology is proposed, which puts forward a network formalism of homogeneous catalysis reactions and utilizes a network analysis tool for mechanistic studies. The method can be used for analyzing trajectories with single and multiple catalytic species and can provide unique conformers of catalysts including multinuclear catalyst clusters along with other catalytic mixture components. The presented three-step approach has the integrated ability to handle multicomponent catalytic systems of arbitrary complexity (mixtures of reactants, catalyst precursors, ligands, additives, and solvents). It is not limited to predefined chemical rules, does not require prealignment of reaction mixture components consistent with a reaction coordinate, and is not agnostic to the chemical nature of transformations. Conformer exploration, reactive event identification, and reaction network analysis are the main steps taken for identifying the pathways in catalytic systems given the starting precatalytic reaction mixture as the input. Such a methodology allows us to efficiently explore catalytic systems in realistic conditions for either previously observed or completely unknown reactive events in the context of a network representing different intermediates. Our workflow for the catalytic reaction space exploration exclusively focuses on the identification of thermodynamically feasible conversion channels, representative of the (secondary) catalyst deactivation or inhibition paths, which are usually most difficult to anticipate based solely on expert chemical knowledge. Thus, the expert bias is sought to be removed at all steps, and the chemical intuition is limited to the choice of the thermodynamic constraint imposed by the applicable experimental conditions in terms of threshold energy values for allowed transformations. The capabilities of the proposed methodology have been tested by exploring the reactivity of Mn complexes relevant for catalytic hydrogenation chemistry to verify previously postulated activation mechanisms and unravel unexpected reaction channels relevant to rare deactivation events.

Migrative Carbofluorination of Saturated Amides Enabled by Pd-Based Dyotropic Rearrangement

Directly editing an all-carbon quaternary carbon itself of nonstrained acyclic molecules remains underexploited despite the recent advances in the fields of both C-H and C-C bond activation. Herein, we report a palladium-catalyzed migrative carbofluorination of saturated amides enabled by the activation of both the C(sp³)-H and the C_quaternary-Cσ bonds. In this transformation, the α-quaternary carbon of Weinreb amides is converted to α-tertiary fluoride with concurrent migration of an aryl or an amido group from the α- to β-carbon. DFT calculations indicate that the dyotropic rearrangement proceeds through an unusual anti-selective [2.1.0] bicyclic transition state. The reaction, compatible with a broad range of functional groups, is stereospecific and is applicable to the synthesis of enantioenriched products.

Modular Synthesis of Benzocyclobutenes via Pd(II)-Catalyzed Oxidative [2+2] Annulation of Arylboronic Acids with Alkenes

Benzocyclobutenes (BCBs) are highly valuable compounds in organic synthesis, medicinal chemistry, and materials science. However, catalytic modular synthesis of functionalized BCBs from easily accessible starting materials remains limited. We report herein an efficient synthesis of diversely functionalized BCBs by a Pd(II)-catalyzed formal [2+2] annulation between arylboronic acids and alkenes in the presence of N-fluorobenzenesulfonimide (NFSI). An intermolecular carbopalladation followed by palladium oxidation, intramolecular C(sp²)-H activation by a transient C(sp³)-Pd(IV) species, and selective carbon-carbon (C-C) bond-forming reductive elimination from a high-valent five-membered palladacycle is proposed to account for the reaction outcome. Kinetically competent oxidation of alkylPd(II) to alkylPd(IV) species is important to avoid the formation of a Heck adduct. The reaction forges two C-C bonds of the cyclobutene core and is compatible with a wide range of functional groups. No chelating bidentate directing group in the alkene part is needed for this transformation.

Glycosyl Exchange of Unactivated Glycosidic Bonds: Suppressing or Embracing Side Reactivity in Catalytic Glycosylations

While developing boron-catalyzed glycosylations using glycosyl fluoride donors and trialkylsilyl ether acceptors, competing pathways involving productive glycosylation or glycosyl exchange were observed. Experimental and computational mechanistic studies suggest a novel mode of reactivity where a dioxolenium ion is a key intermediate that promotes both pathways through addition to either a silyl ether or to the acetal of an existing glycosidic linkage. Modifications in catalyst structure enable either pathway to be favored, and with this understanding, improved multicomponent iterative couplings and glycosyl exchange processes were demonstrated.

Palladacycle-Catalyzed Regioselective Heck Reaction Using Diaryliodonium Triflates and Aryl Iodides

We describe a P-containing palladacycle-catalyzed regioselective Heck reaction of 2,3-dihydrofuran with diaryliodonium salts and aryl iodides to afford 2-aryl-2,5-dihydrofurans and 2-aryl-2,3-dihydrofurans, respectively, in good yields. Mechanistic studies revealed that the oxidative addition of diaryliodonium salts to palladacycles to form Pd(IV) species showed high chemoselectivity and that electron-rich aryl moieties were preferentially transferred to the Heck product. DFT calculations indicated that the regioselectivity-determining step is the reductive elimination reaction rather than the isomerization and reinsertion of Pd(IV)-hydride intermediates into the double bond.

Organometallic Ni(II), Ni(III), and Ni(IV) Complexes Relevant to Carbon-Carbon and Carbon-Oxygen Bond Formation Reactions

The synthesis and spectroscopic and structural characterization of well-defined organometallic Ni(II) and Ni(III) complexes bearing the PyNMe3 ligand - a tetradentate N-based macrocyclic ligand which coordinates to the metal center...

Pd(II)-Catalyzed Synthesis of Benzocyclobutenes by β-Methylene-Selective C(sp3)-H Arylation with a Transient Directing Group

Methylene-selective C-H functionalization is a significant hurdle that remains to be addressed in the field of Pd(II) catalysis. We report a Pd(II)-catalyzed synthesis of benzocyclobutenes by methylene-selective C(sp³)-H arylation of ketones. The reaction utilizes glycine as a transient directing group and a 2-pyridone ligand, which may govern the methylene selectivity by making intimate molecular associations with the substrate during concerted metalation-deprotonation. This reaction is shown to be highly selective for intramolecular methylene C(sp³)-H arylation, thus enabling sequential C(sp³)-H functionalization.

Multiple Reaction Pathways of Eight-Membered Rhodacycles in Rh-Catalyzed Annulations of 2-Alkenyl Phenols/Anilides with Alkynes

Density functional theory calculations were performed to study the competing pathways of rhodacycle intermediates generated in Rh(III)-catalyzed annulations of 2-alkenyl phenols and 2-alkenyl anilides with alkynes. The results show that the multiple pathways of eight-membered rhodacycles can be subtly tuned to give specific cyclic products. The seven-membered oxacyclic and spirocyclic products from 2-alkenyl phenols are formed by favoring the pathway of dissociating the Rh-O bond of O-contained rhodacycles, which are followed by antarafacial nucleophilic attack. The indoline product from 2-alkenyl anilides is generated through the pathway of intramolecular olefin migratory insertion of the N-contained rhodacycle.

Ligand Rearrangement Leads to Tetrahydrothiophene-Functionalized N,S-Heterocyclic Carbene Palladium(II) Complexes

Tetrahydrothiophene-functionalized N,S-heterocyclic carbene palladium(II) complexes are synthesized through an unexpected rearrangement that proceeds with palladium(II) trifluoroacetate and not with palladium(II) acetate, palladium(II) bromide, or palladium(II) chloride. A series of these complexes were isolated and characterized by X-ray crystallography. The mechanism of formation of these [3.2.1]-palladabicycles was explored, and the catalytic capabilities of these complexes were demonstrated in representative C-C coupling reactions.

The role of hypervalent iodine(iii) reagents in promoting alkoxylation of unactivated C(sp3)-H bonds catalyzed by palladium(ii) complexes

Although Pd(OAc)2-catalysed alkoxylation of the C(sp3)-H bonds mediated by hypervalent iodine(iii) reagents (ArIX2) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations. In this study, we shed light on this important issue with the aid of the density functional theory (DFT) calculations for alkoxylation of butyramide derivatives. We found that the previously proposed mechanism in the literature is not consistent with the experimental observations and thus cannot be operating. The calculations allowed us to discover an unprecedented mechanism composed of four main steps as follows: (i) activation of the C(sp3)-H bond, (ii) oxidative addition, (iii) reductive elimination and (iv) regeneration of the active catalyst. After completion of step (i) via the CMD mechanism, the oxidative addition commences with an X ligand transfer from the iodine(iii) reagent (ArIX2) to Pd(ii) to form a square pyramidal complex in which an iodonium occupies the apical position. Interestingly, a simple isomerization of the resultant five-coordinate complex triggers the Pd(ii) oxidation. Accordingly, the movement of the ligand trans to the Pd-C(sp3) bond to the apical position promotes the electron transfer from Pd(ii) to iodine(iii), resulting in the reduction of iodine(iii) concomitant with the ejection of the second X ligand as a free anion. The ensuing Pd(iv) complex then undergoes the C-O reductive elimination by nucleophilic attack of the solvent (alcohol) on the sp3 carbon via an outer-sphere SN2 mechanism assisted by the X- anion. Noteworthy, starting from the five coordinate complex, the oxidative addition and reductive elimination processes occur with a very low activation barrier (ΔG ‡ 0-6 kcal mol-1). The strong coordination of the alkoxylated product to the Pd(ii) centre causes the regeneration of the active catalyst, i.e. step (iv), to be considerably endergonic, leading to subsequent catalytic cycles to proceed with a much higher activation barrier than the first cycle. We also found that although, in most cases, the alkoxylation reactions proceed via a Pd(ii)-Pd(iv)-Pd(ii) catalytic cycle, the other alternative in which the oxidation state of the Pd(ii) centre remains unchanged during the catalysis could be operative, depending on the nature of the organic substrate.

Synthesis and Characterization of Bidentate (P^N)Gold(III) Fluoride Complexes: Reactivity Platforms for Reductive Elimination Studies

A new family of cationic, bidentate (P^N)gold(III) fluoride complexes has been prepared and a detailed characterization of the gold-fluoride bond has been carried out. Our results correlate with the observed reactivity of the fluoro ligand, which undergoes facile exchange with both cyano and acetylene nucleophiles. The resulting (P^N)arylgold(III)C(sp) complexes have enabled the first study of reductive elimination on (P^N)gold(III) systems, which demonstrated that C(sp² )-C(sp) bond formation occurs at higher rates than those reported for analogous phosphine-based monodentate systems.

Aryl C(sp2)-X Coupling (X = C, N, O, Cl) and Facile Control of N-Mono- and N,N-Diarylation of Primary Alkylamines at a Pt(IV) Center

We present the first example of an unprecedented and fast aryl C(sp²)–X reductive elimination from a series of isolated Pt(IV) aryl complexes (Ar = p-FC₆H₄) LPt^IVF(py)(Ar)X (X = CN, Cl, 4-OC₆H₄NO₂) and LPt^IVF₂(Ar)(HX) (X = NHAlk; Alk = n-Bu, PhCH₂, cyclo-C₆H₁₁, t-Bu, cyclopropylmethyl) bearing a bulky bidentate 2-[bis(adamant-1-yl)phosphino]phenoxide ligand (L). The C(sp²)–X reductive elimination reactions of all isolated Pt(IV) complexes follow first-order kinetics and were modeled using density functional theory (DFT) calculations. When a difluoro complex LPt^IVF₂(Ar)(py) is treated with TMS–X (TMS = trimethylsilyl; X= NMe₂, SPh, OPh, CCPh) it also gives the corresponding products of the Ar–X coupling but without observable LPt^IVF(py)(Ar)X intermediates. Remarkably, the LPt^IVF₂(Ar)(HX) complexes with alkylamine ligands (HX = NH₂Alk) form selectively either mono- (ArNHAlk) or diarylated (Ar₂NAlk) products in the presence or absence of an added Et₃N, respectively. This method allows for a one-pot preparation of diarylalkylamine bearing different aryl groups. These findings were also applied in unprecedented mono- and di-N-arylation of amino acid derivatives (lysine and tryptophan) under very mild conditions.

Synthesis and Reactivity of Stable Alkyl-Pd(IV) Complexes Relevant to Monodentate N-Directed C(sp3)-H Functionalization Processes

Alkyl-Pd(IV) complexes are frequently invoked in the proposed mechanisms of Pd-catalyzed C(sp³)-H functionalization reactions, though few examples of Pd(IV) complexes containing cyclopalladated substrates have been isolated due to the instability of the high-valent Pd(IV) center. Herein, we report the synthesis of stable and isolable OCO pincer-supported alkyl-Pd(IV) complexes containing cyclopalladated alkylamine and oxime frameworks, which represent the first examples of alkyl-Pd(IV) complexes derived from the oxidation of cyclopalladated monodentate N-donor substrates. The aminoalkyl-Pd(IV) complexes reacted efficiently with O- and N-nucleophiles to afford γ-functionalized alkylamine products. A mechanistic study of the nucleophile-mediated reductive elimination was conducted using an oxime-derived Pd(IV) complex, which revealed the intermediacy of a previously unexplored anionic Pd(IV) species.

What Does the Machine Learn? Knowledge Representations of Chemical Reactivity

In a departure from conventional chemical approaches, data-driven models of chemical reactions have recently been shown to be statistically successful using machine learning. These models, however, are largely black box in character and have not provided the kind of chemical insights that historically advanced the field of chemistry. To examine the knowledgebase of machine-learning models-what does the machine learn-this article deconstructs black-box machine-learning models of a diverse chemical reaction data set. Through experimentation with chemical representations and modeling techniques, the analysis provides insights into the nature of how statistical accuracy can arise, even when the model lacks informative physical principles. By peeling back the layers of these complicated models we arrive at a minimal, chemically intuitive model (and no machine learning involved). This model is based on systematic reaction-type classification and Evans-Polanyi relationships within reaction types which are easily visualized and interpreted. Through exploring this simple model, we gain deeper understanding of the data set and uncover a means for expert interactions to improve the model's reliability.

Selective Cross-Dehydrogenative C(sp3)-H Arylation with Arenes

Selective C(sp³)-C(sp²) bond construction is of central interest in chemical synthesis. Despite the success of classic cross-coupling reactions, the cross-dehydrogenative coupling between inert C(sp³)-H and C(sp²)-H bonds represents an attractive alternative toward new C(sp³)-C(sp²) bonds. Herein, we establish a selective inter- and intramolecular C(sp³)-H arylation of alcohols with nondirected arenes that thereby provides a general pathway to access a wide range of β-arylated alcohols, including tetrahydronaphthalen-2-ols and benzopyran-3-ols, with high to excellent chemo- and regioselectivity.

Reactivity of a formal Cu(iii)-alkyl species toward aniline: a DFT investigation

In this work, through theoretical investigation of the reactivity between a formal Cu(iii)-alkyl species and aniline, we demonstrated the possibility of a S_N2-like C–N coupling mechanism.

Car-Parrinello Monitor for More Robust Born-Oppenheimer Molecular Dynamics

Born-Oppenheimer molecular dynamics (BOMD) is a promising simulation method for exploring the possible reaction pathways of a chemical system, but one significant challenge is the increased difficulty of converging the self-consistent field (SCF) calculation that often accompanies the breaking and forming of chemical bonds. To address this challenge, we developed an enhancement to the BOMD simulation method called the Car-Parrinello monitor (CPMonitor) that uses Car-Parrinello molecular dynamics (CPMD) to recover from SCF convergence failures. CPMonitor works by detecting SCF convergence failures in BOMD and switching to a CPMD Hamiltonian to propagate through the region of configuration space where the SCF calculation is unable to converge, then switching back to BOMD when good convergence behavior is re-established. We present a series of simulation studies that use CPMonitor, including detailed studies of the thermodynamic and dynamical properties of simple systems, as well as ab initio nanoreactor simulations containing transition metal atoms that were previously not possible to simulate using standard BOMD methods. Our studies show that CPMonitor can make BOMD simulations robust to SCF convergence difficulties and improve simulation performance and stability in reaction discovery applications.

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³ )-C and C(sp³ )-O bond-forming reactions at high-valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³ )-C(sp² ) bond-forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300-fold faster than the Ni^IV , depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^- ), the Ni^IV complex exclusively undergoes C(sp³ )-OAc bond formation, while the Ni^III analogue forms the C(sp³ )-C(sp² ) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M-C(sp³ ) bonds, and thus their relative reactivity towards outer-sphere S_N 2-type bond-forming reactions.

Catalytic Carbonyl-Olefin Metathesis of Aliphatic Ketones: Iron(III) Homo-Dimers as Lewis Acidic Superelectrophiles

Catalytic carbonyl-olefin metathesis reactions have recently been developed as a powerful tool for carbon-carbon bond formation. However, currently available synthetic protocols rely exclusively on aryl ketone substrates while the corresponding aliphatic analogs remain elusive. We herein report the development of Lewis acid-catalyzed carbonyl-olefin ring-closing metathesis reactions for aliphatic ketones. Mechanistic investigations are consistent with a distinct mode of activation relying on the in situ formation of a homobimetallic singly bridged iron(III)-dimer as the postulated active catalytic species. These "superelectrophiles" function as more powerful Lewis acid catalysts that form upon association of individual iron(III)-monomers. While this mode of Lewis acid activation has previously been postulated to exist, it has not yet been applied in a catalytic setting. The insights presented are expected to enable further advancement in Lewis acid catalysis by building upon the activation principle of "superelectrophiles" and to broaden the current scope of catalytic carbonyl-olefin metathesis reactions.

Anion ligand promoted selective C–F bond reductive elimination enables C(sp2)–H fluorination

A detailed mechanism study on the anion ligand promoted selective C–H bond fluorination is reported.

Discovery of conical intersection mediated photochemistry with growing string methods

Conical intersections (CIs) are important features of photochemistry that determine yields and selectivity. Traditional CI optimizers require significant human effort and chemical intuition, which typically restricts searching to only a small region of the CI space. Herein, a systematic approach utilizing the growing string method is introduced to locate multiple CIs. Unintuitive MECI are found using driving coordinates that can be generated using a combinatorial search, and subsequent optimization allows reaction pathways, transition states, products, and seam-space pathways to be located. These capabilities are demonstrated by application to two prototypical photoisomerization reactions and the dimerization of butadiene. In total, many reaction pathways were uncovered, including the elusive stilbene hula-twist mechanism, and a previously unidentified product in butadiene dimerization. Overall, these results suggest that growing string methods provide a predictive strategy for exploring photochemistry.

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes

Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe₂ complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe₂ complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

Electrostatics Generated by a Supramolecular Capsule Stabilizes the Transition State for Carbon-Carbon Reductive Elimination from Gold(III) Complex

Tetrahedral assemblies of stoichiometry M₄L₆ have been proven to catalyze a range of chemical reactions including the carbon-carbon reductive elimination reaction from transition metals such as gold. Here, we perform quantum chemical calculations of Gold(III) transition metal complexes in vacuum, and encapsulated in Ga₄L₆^12- or Si₄L₆^8- assemblies within both a reaction field continuum solvent and in an aqueous molecular environment with counterions, to rationalize the rate enhancements observed experimentally for the reductive elimination reaction. We find that the Ga₄L₆^12- assembly lowers the energy barrier of the reaction compared to Si₄L₆^8-, which is consistent with kinetic trends observed experimentally. We have determined that the primary factor for catalytic rate acceleration stems from the electrostatic environment emanating from the Ga₄L₆^12- capsule as opposed to the water or counterions.

Reductive Elimination from Phosphine-Ligated Alkylpalladium(II) Amido Complexes To Form sp3 Carbon-Nitrogen Bonds

We report the formation of phosphine-ligated alkylpalladium(II) amido complexes that undergo reductive elimination to form alkyl-nitrogen bonds and a combined experimental and computational investigation of the factors controlling the rates of these reactions. The free-energy barriers to reductive elimination from t-Bu₃P-ligated complexes were significantly lower (ca. 3 kcal/mol) than those previously reported from NHC-ligated complexes. The rates of reactions from complexes containing a series of electronically and sterically varied anilido ligands showed that the reductive elimination is slower from complexes of less electron-rich or more sterically hindered anilido ligands than from those containing more electron-rich and less hindered anilido ligands. Reductive elimination of alkylamines also occurred from complexes bearing bidentate P,O ligands. The rates of reactions of these four-coordinate complexes were slower than those for reactions of the three-coordinate, t-Bu₃P-ligated complexes. The calculated pathway for reductive elimination from rigid, 2-methoxyarylphosphine-ligated complexes does not involve initial dissociation of the oxygen. Instead, reductive elimination is calculated to occur directly from the four-coordinate complex in concert with a lengthening of the Pd-O bond. To investigate this effect experimentally, a four-coordinate Pd(II) anilido complex containing a flexible, aliphatic linker between the P and O atoms was synthesized. Reductive elimination from this complex was faster than that from the analogous complex containing the more rigid, aryl linker. The flexible linker enables full dissociation of the ether ligand during reductive elimination, leading to the faster reaction of this complex.

PdIV Species Mediation in PdII-Catalyzed Direct Alkylation of Arenes with Oxiranes: A DFT Study

The reaction mechanisms of Pd(OAc)₂-catalyzed dehydrogenative alkylation of 2-phenylpyridine with oxirane were investigated using DFT calculations. The most plausible reaction pathway was confirmed as a Pd^II/IV/II catalytic cycle consisting of four processes: C-H activation, ring-opening oxidative addition of oxirane, reductive elimination, and recovery of the catalyst. According to the B2PLYP/DGDZVP computational data, the oxidative addition of oxirane for converting Pd^II to Pd^IV was assigned to be the rate-determining step with a free-energy barrier of 28.1 kcal·mol^-1. For comparison, we also studied the alternative Pd^II-only pathway without a change of oxidation state and found that it was hindered kinetically by a high free-energy barrier of 75.1 kcal·mol^-1 occurring for the ring-opening migratory insertion of oxirane. In addition, the small-ring strain of oxirane should be responsible for the feasible C-O bond-cleavage and subsequent Pd^II → Pd^IV conversion, because the designed four-, five-, and six-membered-ring reagents did not display such an oxidative addition reactivity. Lastly, an extended reactivity order among oxirane, PhI, PhBr, and PhCl toward oxidative addition onto Pd^II to form Pd^IV was proposed in this article based on the computed kinetic parameters.