Mechanistic Characterization of Aerobic Alcohol Oxidation Catalyzed by Pd(OAc)2/Pyridine Including Identification of the Catalyst Resting State and the Origin of Nonlinear [Catalyst] Dependence

Palladium(II)-catalyzed direct alkoxylation of arenes: evidence for solvent-assisted concerted metalation deprotonation

Density functional theory investigations on the mechanism of palladium acetate catalyzed direct alkoxylation of N-methoxybenzamide in methanol reveal that the key steps involve solvent-assisted N-H as well as C-H bond activations. The transition state for the critical palladium-carbon bond formation through a concerted metalation deprotonation (CMD) process leading to a palladacycle intermediate has been found to be more stable in the methanol-assisted pathway as compared to an unassisted route.

Fluorenone synthesis by palladacycle-catalyzed sequential reactions of 2-bromobenzaldehydes with arylboronic acids

A new, anionic four-electron donor-based (type I) palladacycle-catalyzed sequential reaction of 2-bromobenzaldehydes with arylboronic acids based on the addition reaction, cyclization via C-H activation-oxidation sequence is described. Our study provided an efficient access to a variety of substituted fluorenones/indenofluorenediones from readily available arylboronic acids and 2-bromobenzaldehydes.

Mechanistic studies of Wacker-type intramolecular aerobic oxidative amination of alkenes catalyzed by Pd(OAc)2/pyridine

Wacker-type oxidative cyclization reactions have been the subject of extensive research for several decades, but few systematic mechanistic studies of these reactions have been reported. The present study features experimental and DFT computational studies of Pd(OAc)(2)/pyridine-catalyzed intramolecular aerobic oxidative amination of alkenes. The data support a stepwise catalytic mechanism that consists of (1) steady-state formation of a Pd(II)-amidate-alkene chelate with release of 1 equiv of pyridine and AcOH from the catalyst center, (2) alkene insertion into a Pd-N bond, (3) reversible β-hydride elimination, (4) irreversible reductive elimination of AcOH, and (5) aerobic oxidation of palladium(0) to regenerate the active trans-Pd(OAc)(2)(py)(2) catalyst. Evidence is obtained for two energetically viable pathways for the key C-N bond-forming step, featuring a pyridine-ligated and a pyridine-dissociated Pd(II) species. Analysis of natural charges and bond lengths of the alkene-insertion transition state suggest that this reaction is best described as an intramolecular nucleophilic attack of the amidate ligand on the coordinated alkene.

Aerobic Oxidative Coupling of o-Xylene: Discovery of 2-Fluoropyridine as a Ligand to Support Selective Pd-Catalyzed C-H Functionalization

An improved method for direct oxidative coupling of o-xylene could provide streamlined access to an important monomer used in polyimide resins. The use of 2-fluoropyridine as a ligand has been found to enable unprecedented levels of chemo- and regioselectivity in this Pd-catalyzed aerobic oxidative coupling reaction. Preliminary insights have been obtained into the origin of the effectiveness of 2-fluoropyridine as a ligand.

Synthetic and mechanistic studies on Pd(0)-catalyzed diamination of conjugated dienes

Various dienes and a triene can be regioselectively diaminated at the internal double bond with good yields and high diastereoselectivity using di-tert-butyldiaziridinone (5) as the nitrogen source and Pd(PPh(3))(4) (1-10 mol %) as the catalyst. Kinetic studies with (1)H NMR spectroscopy show that the diamination is first-order in total Pd catalyst and inverse first-order in PPh(3). For reactive dienes, such as 1-methoxybutadiene (6g) and alkyl 1,3-butadienes (6a, 6j), the diamination is first-order in di-tert-butyldiaziridinone (5) and zero-order in the olefin. For olefins with relatively low reactivity, such as (E)-1-phenylbutadiene (6b) and (3E,5E)-1,3,5-decatriene (6i), similar diamination rates were observed when 3.5 equiv of olefins were used. Pd(PPh(3))(2) is likely to be the active species for the insertion of Pd(0) into the N-N bond of di-tert-butyldiaziridinone (5) to form a four-membered Pd(II) complex (A), which can be detected by NMR spectroscopy. The olefin complex (B), formed from intermediate A via ligand exchange between the olefin substrate and the PPh(3), undergoes migratory insertion and reductive elimination to give the diamination product and regenerate the Pd(0) catalyst.

Development of Safe and Scalable Continuous-Flow Methods for Palladium-Catalyzed Aerobic Oxidation Reactions

The synthetic scope and utility of Pd-catalyzed aerobic oxidation reactions has advanced significantly over the past decade, and these reactions have potential to address important green-chemistry challenges in the pharmaceutical industry. This potential has been unrealized, however, because safety concerns and process constraints hinder large-scale applications of this chemistry. These limitations are addressed by the development of a continuous-flow tube reactor, which has been demonstrated on several scales in the aerobic oxidation of alcohols. Use of a dilute oxygen gas source (8% O(2) in N(2)) ensures that the oxygen/organic mixture never enters the explosive regime, and efficient gas-liquid mixing in the reactor minimizes decomposition of the homogeneous catalyst into inactive Pd metal. These results provide the basis for large-scale implementation of palladium-catalyzed (and other) aerobic oxidation reactions for pharmaceutical synthesis.

A catalytic, Brønsted base strategy for intermolecular allylic C-H amination

A Brønsted base activation mode for oxidative, Pd(II)/sulfoxide-catalyzed, intermolecular C-H allylic amination is reported. N,N-diisopropylethylamine was found to promote amination of unactivated terminal olefins, forming the corresponding linear allylic amine products with high levels of stereo-, regio-, and chemoselectivity. The predictable and high selectivity of this C-H oxidation method enables late-stage incorporation of nitrogen into advanced synthetic intermediates and natural products.

The palladium-catalyzed aerobic kinetic resolution of secondary alcohols: reaction development, scope, and applications

The first palladium-catalyzed enantioselective oxidation of secondary alcohols has been developed, utilizing the readily available diamine (-)-sparteine as a chiral ligand and molecular oxygen as the stoichiometric oxidant. Mechanistic insights regarding the role of the base and hydrogen-bond donors have resulted in several improvements to the original system. Namely, addition of cesium carbonate and tert-butyl alcohol greatly enhances reaction rates, promoting rapid resolutions. The use of chloroform as solvent allows the use of ambient air as the terminal oxidant at 23 degrees C, resulting in enhanced catalyst selectivity. These improved reaction conditions have permitted the successful kinetic resolution of benzylic, allylic, and cyclopropyl secondary alcohols to high enantiomeric excess with good-to-excellent selectivity factors. This catalyst system has also been applied to the desymmetrization of meso-diols, providing high yields of enantioenriched hydroxyketones.

Recent advancements and challenges of palladium(II)-catalyzed oxidation reactions with molecular oxygen as the sole oxidant

During the past 10 years there have been significant advances in Pd(II)-catalyzed oxidation reactions where the use of ligands has led to the development of catalytic systems capable of achieving high turnover numbers, which employ molecular oxygen as the sole stoichiometric oxidant. This Feature article will highlight some of the recent developments in direct molecular oxygen-coupled Pd(II)-catalyzed oxidation reactions with an emphasis on enhanced catalytic systems and new reactions. Additionally, limitations of current catalytic systems, such as ligand oxidation, are presented and their implications for the development of new reactions are discussed.

Mechanistic investigations of the iridium(III)-catalyzed aerobic oxidation of primary and secondary alcohols

The commercially available catalysts [(Cp*IrCl2)2] is employed with O2 as the terminal oxidant in the presence of catalytic amounts of Et3N for the aerobic oxidation of primary and secondary alcohols. A new mechanism for the Ir-catalyzed aerobic oxidation is also presented that suggests that the transition metal maintains its +3 oxidation state throughout the entire catalytic cycle.

Palladium-catalyzed aerobic alcohol oxidation supported by a new type of α-diimine ligands

A series of new ionic sulfonated α-diimine ligands were prepared and employed as supporting ligands for the palladium-catalyzed aerobic alcohol oxidation. The electronic properties, rigidity, and steric hindrance of ligands have remarkable influences upon the catalytic activity of the palladium complexes. The best catalytic result could be achieved using ligand 9 with an electron-withdrawing sulfonyl substituent together with bulky groups (iso-propyl in 2,6-positions). This diimine/palladium-catalyzed system is highly efficient for the oxidation of various benzylic alcohols, and showed moderate yields for the secondary aliphatic alcohols and cyclic alcohols.Key words: α-diimine, palladium, aerobic oxidation, alcohol.

TEMPO/HCl/NaNO2 catalyst: a transition-metal-free approach to efficient aerobic oxidation of alcohols to aldehydes and ketones under mild conditions

Hydrochloric acid, a very inexpensive and readily available inorganic acid, has been found to cooperate exquisitely with NaNO(2)/TEMPO in catalyzing the molecular-oxygen-driven oxidation of a broad range of alcohol substrates to the corresponding aldehydes and ketones. This transition-metal-free catalytic oxidative conversion is novel and represents an interesting alternative route to the corresponding carbonyl compounds to the metal-catalyzed aerobic oxidation of alcohols. The reaction is highly selective with respect to the desired product when carried out at room temperature in air at atmospheric pressure. Notably, the use of very inexpensive NaNO(2) and HCl in combination with TEMPO for this highly selective aerobic oxidation of alcohols in air at ambient temperature makes the reaction operationally and economically very attractive. The results of mechanistic studies, performed with the aid of electrospray ionization mass spectrometry (ESI-MS), are presented and discussed. TEMPO, TEMPOH, and TEMPO(+) were observed in the redox cycle by means of ESI-MS. On the basis of these observations, a mechanism is proposed that may provide an insight into the newly developed aerobic alcohol oxidation.

Mechanism of the aerobic oxidation of alcohols by palladium complexes of N-heterocyclic carbenes

Quantum mechanics (B3LYP density functional theory) combined with solvation (Poisson-Boltzmann polarizable continuum solvent model) was used to investigate six mechanisms for the aerobic oxidation of alcohols catalyzed by (NHC)Pd(carboxylate)(2)(H(2)O) complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Of these, we find that "reductive beta-hydride elimination", in which the beta-hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provides the lowest-energy route and explains the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pK(a). The traditional beta-hydride elimination mechanism cannot be responsible for the experimentally observed kinetic parameters, which we find could result from the subsequent reductive elimination of acetic acid, which yields a slightly higher calculated activation barrier. Reversible beta-hydride elimination may provide a mechanism for the racemization of chiral alcohols, which would undermine attempts at an enantioselective oxidation. Competition among these pathways can be influenced by changing the electronic properties of the carboxylate and substrate.

Palladium-Catalyzed Oxidative Amination of Alkenes: Improved Catalyst Reoxidation Enables the Use of Alkene as the Limiting Reagent

Palladium-catalyzed methods for intermolecular aerobic oxidative amination of alkenes have been identified that are compatible with the use of alkene as the limiting reagent. These procedures, which enhance the utility of this reaction with alkenes that are not commercially available, are demonstrated with substrates bearing dialkyl ether, carboxyester, epoxide, and silyl ether groups.

Palladium-catalyzed aerobic oxidative amination of alkenes: development of intra- and intermolecular aza-Wacker reactions

Palladium-catalyzed methods for the aerobic oxidative coupling of alkenes and oxygen nucleophiles (e.g., water and carboxylic acids) have been known for nearly 50 years. The present account summarizes our development of analogous aerobic oxidative amination reactions, including the first intermolecular aza-Wacker reactions compatible with the use of unactivated alkenes. The reactions are initiated by intra- or intermolecular aminopalladation of the alkene. The resulting alkylpalladium(II) intermediate generally undergoes beta-hydride elimination to produce enamides or allylic amide products, but in certain cases, the Pd-C bond can be trapped to achieve 1,2-difunctionalization of the alkene, including carboamination and aminoacetoxylation. Mechanistic studies have provided a variety of fundamental insights into the reactions, including the effect of ancillary ligands on palladium catalysts, the origin of the Brønsted-base-induced switch in regioselectivity in the oxidative amination of styrene, and evidence that both cis- and trans-aminopalladations of alkenes are possible. Overall, these reactions highlight the potential utility of an "organometallic oxidase" strategy for the selective aerobic oxidation of organic molecules.