Computational study of C(sp3)-O bond formation at a PdIV centre

Hydroxopalladium(IV) complexes prepared using oxygen or hydrogen peroxide as oxidants

The cycloneophylpalladium(II) complexes [Pd(CH2CMe2C6H4)(κ2-N,N'-L)], 1 or 2, with L = RO(CH2)3N(CH2-2-C5H4N)2, with R = H or Me, respectively, react with either dioxygen or hydrogen peroxide in the presence of NH4[PF6] to give rare examples of the corresponding hydroxopalladium(IV) complexes [Pd(OH)(CH2CMe2C6H4)(κ3-N,N',N''-L)][PF6], 3 or 4. The complexes 3 and 4 are stable at room temperature and have been structurally characterized. On heating a solution of 3 or 4 in moist dimethylsulphoxide, selective reductive elimination with C(sp2)-O bond formation is observed, followed by hydrolysis, to give the corresponding pincer complex [Pd(OH)(κ3-N,N',N''-L)][PF6] and 2-t-butylphenol as major products. A more complex reaction occurs in chloroform solution. The mechanisms of reaction are discussed, supported by DFT calculations.

Experiment and Computational Study on Pd-Catalyzed Methoxyiminoacyl-Directed γ-Alkoxylation of Alkylamides

The direct alkoxylation of amides has been accomplished via methoxyiminoacyl (MIA)-mediated Pd-catalyzed C-H functionalization. A diverse array of alkylamide substrates is amenable to this protocol, providing γ-C(sp3)-alkoxylation of alkylamide derivatives with good to high efficiency. Two aspects of the research were completed to explore the reaction mechanism. On the one hand, the result of the kinetic isotopic effect experiment and control experiment indicated that reductive elimination is a rate-limiting step. On the other hand, density functional theory calculations demonstrated that a concerted Sn2 reductive elimination mechanism pathway is prior. Finally, the MIA group could be efficiently hydrogenated and protected in a one-pot procedure, which provides a short synthetic route to γ-methoxy amino acid derivatives.

Redox-Paired Alkene Difunctionalization Enables Skeletally Divergent Synthesis

Multistep organic synthesis enables conversion of simple chemical feedstocks into a more structurally complex product that serves a particular function. The target compound is forged over several steps, with concomitant generation of byproducts in each step to account for underlying mechanistic features of the reactions (e.g., redox processes). To map structure-function relationships, libraries of molecules are often needed, and these are typically prepared by iterating an established multistep synthetic sequence. An underdeveloped approach is designing organic reactions that generate multiple valuable products with different carbogenic skeletons in a single synthetic operation. Taking inspiration from paired electrosynthesis processes that are widely used in commodity chemical production (e.g., conversion of glucose to sorbitol and gluconic acid), we report a palladium-catalyzed reaction that converts a single alkene starting material into two skeletally distinct products in a single operation through a series of carbon-carbon and carbon-heteroatom bond-forming events enabled by mutual oxidation and reduction, a process that we term redox-paired alkene difunctionalization. We demonstrate the scope of the method in enabling simultaneous access to reductively 1,2-diarylated and oxidatively [3 + 2]-annulated products, and we explore the mechanistic details of this unique catalytic system using a combination of experimental techniques and density functional theory (DFT). The results described herein establish a distinct approach to small-molecule library synthesis that can increase the rate of compound production. Furthermore, these findings demonstrate how a single transition-metal catalyst can mediate a sophisticated redox-paired process through multiple pathway-selective events along the catalytic cycle.

Bismuth Redox Catalysis: An Emerging Main-Group Platform for Organic Synthesis

Bismuth has recently been shown to be able to maneuver between different oxidation states, enabling access to unique redox cycles that can be harnessed in the context of organic synthesis. Indeed, various catalytic Bi redox platforms have been discovered and revealed emerging opportunities in the field of main group redox catalysis. The goal of this perspective is to provide an overview of the synthetic methodologies that have been developed to date, which capitalize on the Bi redox cycling. Recent catalytic methods via low-valent Bi(II)/Bi(III), Bi(I)/Bi(III), and high-valent Bi(III)/Bi(V) redox couples are covered as well as their underlying mechanisms and key intermediates. In addition, we illustrate different design strategies stabilizing low-valent and high-valent bismuth species, and highlight the characteristic reactivity of bismuth complexes, compared to the lighter p-block and d-block elements. Although it is not redox catalysis in nature, we also discuss a recent example of non-Lewis acid, redox-neutral Bi(III) catalysis proceeding through catalytic organometallic steps. We close by discussing opportunities and future directions in this emerging field of catalysis. We hope that this Perspective will provide synthetic chemists with guiding principles for the future development of catalytic transformations employing bismuth.

Unconventional mechanism and selectivity of the Pd-catalyzed C-H bond lactonization in aromatic carboxylic acid

The search for more effective and highly selective C-H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C-H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C-O bond formation step for the Pd(II)-catalyzed C(sp³)-H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C-O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂ oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)-Pd and K⁺-O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)-Pd and η²-(π-phenyl)-Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺ cation in the ortho-C-H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)-H bond lactonization in the presence of C(sp²)-H.

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...

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.

Dialkyl Ether Formation at High-Valent Nickel

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Bismuth-Catalyzed Oxidative Coupling of Arylboronic Acids with Triflate and Nonaflate Salts

Herein we present a Bi-catalyzed cross-coupling of arylboronic acids with perfluoroalkyl sulfonate salts based on a Bi(III)/Bi(V) redox cycle. An electron-deficient sulfone ligand proved to be key for the successful implementation of this protocol, which allows the unusual construction of C(sp²)–O bonds using commercially available NaOTf and KONf as coupling partners. Preliminary mechanistic studies as well as theoretical investigations reveal the intermediacy of a highly electrophilic Bi(V) species, which rapidly eliminates phenyl triflate.

Dynamics of the efficient cyclometalation of the undercoordinated organoplatinum complex [Pt(COD)(neoPh)]+ (neoPh = 2-methyl-2-phenylpropyl)

The cyclometalation reaction of [Pt(COD)(κ¹-neoPh)]⁺ (neoPh = 2-methyl-2-phenylpropyl) to [Pt(COD)(κ²-neoPh)] was studied experimentally and mechanistically using DFT and MD simulations.

Mechanistic investigation into the C(sp3)-H acetoxylation of morpholinones

The study of a selective palladium(ii)-catalyzed C(sp³)–H acetoxylation reaction on a class of cyclic alkyl amines is reported.

The study of a selective palladium(ii)-catalyzed C(sp³)–H acetoxylation reaction on a class of cyclic alkyl amines is reported. Computational modelling and kinetic studies were used to provide support for a mechanism involving selective C–O bond formation from a γ-aminoalkyl-Pd(iv) intermediate. The C–O bond forming step was computed to occur by a dissociative ionization mechanism followed by an S_N2 process involving external acetate attack at the C–Pd(iv) bond. This pathway was computed to be of lowest energy with no competing C–N products observed. Additionally, with a few modifications to reaction conditions, preliminary studies showed that this process could be rendered enantioselective in the presence of a non-racemic BINOL-phosphoric acid.

Controlling Pd(IV) reductive elimination pathways enables Pd(II)-catalysed enantioselective C(sp3)-H fluorination

The development of a Pd(II)-catalyzed enantioselective fluorination of C(sp³)–H bonds would offer a new approach to making chiral organofluorines. However, such a strategy is particularly challenging because of the difficulty in differentiating prochiral C(sp³)–H bonds through Pd(II)-insertion, as well as the sluggish reductive elimination involving Pd–F bonds. Here, we report the development of a Pd(II)-catalyzed enantioselective C(sp³)–H fluorination using a chiral transient directing group strategy. In this work, a bulky, amino amide transient directing group was developed to control the stereochemistry of C–H insertion step and selectively promote C(sp³)–F reductive elimination pathway from Pd(IV)–F intermediate. Stereochemical analysis revealed that while the desired C(sp³)–F formation proceeds via an inner-sphere pathway with retention of configuration, the undesired C(sp³)–O formation occurs through an S_N2-type mechanism. The elucidation of the dual mechanism allows us to rationalize the profound ligand effect on controlling reductive elimination selectivity from high-valent Pd species.

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.

Selective Reductive Elimination at Alkyl Palladium(IV) by Dissociative Ligand Ionization: Catalytic C(sp3 )-H Amination to Azetidines

A palladium(II)-catalyzed γ-C-H amination of cyclic alkyl amines to deliver highly substituted azetidines is reported. The use of a benziodoxole tosylate oxidant in combination with AgOAc was found to be crucial for controlling a selective reductive elimination pathway to the azetidines. The process is tolerant of a range of functional groups, including structural features derived from chiral α-amino alcohols, and leads to the diastereoselective formation of enantiopure azetidines.

Catalytic Intermolecular Carboamination of Unactivated Alkenes via Directed Aminopalladation

An intermolecular 1,2-carboamination of unactivated alkenes proceeding via a Pd(II)/Pd(IV) catalytic cycle has been developed. To realize this transformation, a cleavable bidentate directing group is used to control the regioselectivity of aminopalladation and stabilize the resulting organopalladium(II) intermediate, such that oxidative addition to a carbon electrophile outcompetes potential β-hydride elimination. Under the optimized reaction conditions, a broad range of nitrogen nucleophiles and carbon electrophiles are compatible coupling partners in this reaction, affording moderate to high yields. The products of this reaction can be easily converted to free γ-amino acids and γ-lactams, both of which are common structural motifs found in drug molecules and bioactive compounds. Reaction kinetics and DFT calculations shed light on the mechanism of the reaction and explain empirically observed reactivity trends.