Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

Selective Manipulation of Well-Defined Trinuclear Pd(II)-Complexes

Several strategies are available to design well-defined multimetallic molecular entities bearing functional ligands. Substoichiometric exchange reactions in the coordination sphere of pre-existing multinuclear precursors are relatively underexploited in this context. Palladium(II) acetate is not a mononuclear compound in the solid state but rather exists as a trimer, i. e. [Pd3(OAc)6]. Although this material is ubiquitously used to synthesize mononuclear Pd species, it may principally also lend itself to selective exchange of some of the edge-sharing acetate units in its triangular motif, whilst keeping the overall multinuclear architecture intact. Strikingly, little is known about the controlled manipulation and substoichiometric substitution chemistry of this well-defined conglomerate. We herein conclusively demonstrate that, for the first time, the targeted exchange of two or four acetate units from the Pd3(acetate)6 platform is possible, thereby installing either one or two new tridentate ligands onto this trinuclear architecture. Follow-up exchange and substitution chemistry is available without disrupting the multimetallic nature of the core structure. New complexes 2-7 are all conclusively characterized using multinuclear NMR spectroscopy, UV-vis and IR spectroscopy as well as X-ray diffraction analysis.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me₂ daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂ daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂ daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂ daf, reminiscent of behavior displayed by the workhorse 2,2'-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9'-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.

Transition-Metal-Catalyzed Divergent C-H Functionalization of Five-Membered Heteroarenes

Conversion of common reactants to diverse products is a key objective of organic syntheses. Recent developments in transition-metal-catalyzed C-H functionalization have increased the interest in such conversions. Both the position of functionalization and the type of the substituent can be varied, allowing systematic diversification of common structural cores. Because five-membered heteroarenes (pyrazole, imidazole, thiazole, pyrrole, and thiophene) are ubiquitous in pharmaceuticals and organic functional materials, the selective C-H functionalization of these heterocyclic cores facilitates both the optimization of their physicochemical properties and streamlining of their preparation. In addition, the parent forms of these heterocycles are more readily available and inexpensive than any other derivatives of their families. Hence, their nondirected C-H functionalization is highly desirable. Although various regioselective reactions have been developed, many of them target the most reactive site; hence, except for some extensively studied arylation reactions, regiodivergent functionalization of two or more sites has been limited.This Account summarizes our work on the regiodivergent, nondirected C-H functionalization of five-membered heteroarenes with alkenes and alkynes. These unsaturated hydrocarbons are readily available, and all the composing atoms can be incorporated into products with high atom efficiency. Furthermore, the installed alkenyl groups can be transformed to other useful functional groups. To achieve comparable selectivity to that observed in the traditional reactions of these heteroarenes with highly electrophilic reagents and strong bases, a transition metal catalytic system was carefully devised with a more streamlined synthesis. A judicious choice of metals, ligands, acid and base additives, and solvents orchestrates divergent transformations using electronic and steric effects of the heteroarenes. Although C-H cleavage is a rate- and site-selectivity-determining step in most cases, the subsequent steps involving the formation of C-C bonds are often more critical than the other steps. For the C-H cleavage step, modulating the electronic properties of catalysts to make them electrophilic allows preferential alkenylation at the nucleophilic position. In addition, the presence of an internal base that can be exploited for concerted metalation-deprotonation of the acidic C-H bond offers alternative regioselectivity. Furthermore, we developed our own ligand system based on a conformationally rigid pyrazolonaphthyridine scaffold that enables aerobic C-H alkenylation reactions with steric control. We showed that the electronic and steric effects of heteroarenes can be further extended to chemodivergent reactions with norbornene derivatives. Depending on whether the palladacycle is formed, heteroarenes selectively undergo 1:2 annulation with norbornene derivatives and three-component reactions with other azoles through the Pd-norbornene adducts or Catellani and 2:1 annulation reactions through the palladacycle intermediates.Other research groups have also contributed to the development of divergent reactions, in investigations ranging from the pioneering studies in the early days of research on C-H functionalization to recent studies with new ligands. We have also discussed these studies in context. These approaches provide access to many heteroarenes with systematically varied substituents. We believe that new ligand systems and mechanistic insights gained through these studies will enrich fields beyond C-H functionalization of five-membered heteroarenes.

Sterically controlled C-H alkenylation of pyrroles and thiophenes

Pd-catalyzed C-H alkenylations targeting the least hindered position of N-alkyl pyrroles and 3-substituted thiophenes, as opposed to electronically controlled approaches, are developed. The steric demand and stable bidentate binding mode of the pyrazolonaphthyridine ligand are key to the success of these sterically controlled alkenylations using oxygen as an oxidant.

Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc)2-Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O₂) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd⁰ during catalytic turnover. We previously showed that Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O₂ in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)₂ catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)₂ catalyst systems under three different oxidation conditions: (1) O₂ alone, (2) O₂ with cocatalytic BQ, and (3) O₂ with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to synergistic roles of O₂ and BQ, both of which are capable of oxidizing Pd⁰ to Pd^II The reaction of O₂ generates H₂O₂ as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd^II NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout the reaction, resulting in better reaction performance.

Catalyst-Controlled Regioselectivity in Pd-Catalyzed Aerobic Oxidative Arylation of Indoles

Pd-catalyzed C-H arylation of heteorarenes is an important and widely studied synthetic transformation; however, the regioselectivity is often substrate-controlled. Here, we report catalyst-controlled regioselectivity in the Pd-catalyzed oxidative coupling of N-(phenylsulfonyl)indoles and aryl boronic acids using O₂ as the oxidant. Both C2- and C3-arylated indoles are obtained in good yield with >10:1 selectivity. A switch from C2 to C3 regioselectivity is achieved by including 4,5-diazafluoren-9-one or 2,2'-bipyrimidine as an ancillary ligand to a "ligand-free" Pd(OTs)₂ catalyst system. Density functional theory calculations indicate that the switch in selectivity arises from a change in the mechanism, from a C2-selective oxidative-Heck pathway to a C3-selective C-H activation/reductive elimination pathway.

Can Donor Ligands Make Pd(OAc)2 a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria

Palladium(II)-catalyzed oxidation reactions represent an important class of methods for selective modification and functionalization of organic molecules. This field has benefitted greatly from the discovery of ancillary ligands that expand the scope, reactivity, and selectivity in these reactions; however, ancillary ligands also commonly poison these reactions. The different influences of ligands in these reactions remain poorly understood. For example, over the 60-year history of this field, the Pd^II/0 redox potentials for catalytically relevant Pd complexes have never been determined. Here, we report the unexpected discovery of (L)Pd^II(OAc)₂-mediated oxidation of hydroquinones, the microscopic reverse of quinone-mediated oxidation of Pd⁰ commonly employed in Pd^II-catalyzed oxidation reactions. Analysis of redox equilibria arising from the reaction of (L)Pd(OAc)₂ and hydroquinones (L = bathocuproine, 4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones, provides the basis for determination of (L)Pd^II(OAc)₂ reduction potentials. Experimental results are complemented by density functional theory calculations to show how a series of nitrogen-based ligands modulate the (L)Pd^II(OAc)₂ reduction potential, thereby tuning the ability of Pd^II to serve as an effective oxidant of organic molecules in catalytic reactions.

4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

4,5-diazafluorene (daf) and 9,9’-dimethyl-4,5-diazafluorene (Me₂daf) are structurally similar to the important ligand 2,2’-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Me₂daf as a ligand than those containing bpy. New complexes Mn(CO)₃Br(daf) (2), Mn(CO)₃Br(Me₂daf) (3), and [Ru(Me₂daf)₃](PF₆)₂ (5) have been prepared and fully characterized to understand the influence of the Me₂daf framework on their chemical and electrochemical properties. Structural data for 2, 3, and 5 from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Me₂daf chelate angles in comparison to the analogous Mn(CO)₃(bpy)Br (1) and [Ru(bpy)₃]²⁺ (4) complexes. Electronic absorption data for these complexes confirm the electronic similarity of daf, Me₂daf, and bpy, as spectra are dominated in each case by metal-to-ligand charge transfer bands in the visible region. However, the electrochemical properties of 2, 3, and 5 reveal that the redox-active Me₂daf framework in 3 and 5 undergoes reduction at a slightly more negative potential than that of bpy in 1 and 4. Taken together, the results indicate that Me₂daf could be useful for preparation of a variety of new redox-active compounds, as it retains the useful redox-active nature of bpy but lacks the acidic, benzylic C–H bonds that can induce secondary reactivity in complexes bearing daf.

Stereoselective gridization and polygridization with centrosymmetric molecular packing

The gridarenes, with well-defined edges and vertices, represent versatile nanoscale building blocks for the installation of frameworks and architectures but suffer from difficulty in stereoselective control during their synthesis. Here we report a diastereoselective gridization of superelectrophilic diazafluorene-containing substrates (A_mB_n) with crescent shapes into Drawing Hands grids (DHGs). The meso-selectivity reaches 75.6% diastereomeric excess (de) during the gridization of A₁B₁-type substrates and maintains ~80% de during the polygridization of A₂B₂-type monomers. Such stereocontrol originates from the centrosymmetric molecular packing of two charge-delocalized superelectrophiles with synergistically π–π stacking attractions and coulombic repulsions. As meso-stereoregular structures show 20∼30 nm in length, the rigid ring/chain-alternating polygrids have a Mark–Houwink exponent of 1.651 and a molecular weight (M) dependence of the hydrodynamic radius R_h ∼ M^1.13. Via the simulation of chain collapse, meso-configured polygridarenes still adopt rod-like conformations that facilitate the high rigidity of organic nanopolymers, distinguished from toroid backbones of rac-type polygrids.

Gridarenes with well-defined edges and vertices represent versatile nanoscale building blocks for installating frameworks but suffer from lack of stereoselective control during their synthesis. Here, the authors report a diastereoselective gridization of superelectrophilic diazafluorene-containing substrates with crescent shapes into Drawing Hands grids (DHGs).

Aerobic Acyloxylation of Allylic C-H Bonds Initiated by a Pd0 Precatalyst with 4,5-Diazafluoren-9-one as an Ancillary Ligand

Palladium-catalyzed allylic C-H oxidation has been widely studied, but most precedents use acetic acid as the coupling partner. In this study, a method compatible with diverse carboxylic acid partners has been developed. Use of a Pd⁰ precatalyst under aerobic reaction conditions leads to oxidation of Pd⁰ by O₂ in the presence of the desired carboxylic acid to generate a Pd^II dicarboxylate that promotes acyloxylation of the allylic C-H bond. Good-to-excellent yields are obtained with a roughly 1:1 ratio of the alkene and carboxylic acid reagents. Optimized reaction conditions employ 4,5-diazafluoren-9-one (DAF) as a ligand, in combination with a quinone/iron phthalocyanine cocatalyst system to support aerobic catalytic turnover.

Operando Spectroscopic and Kinetic Characterization of Aerobic Allylic C-H Acetoxylation Catalyzed by Pd(OAc)2/4,5-Diazafluoren-9-one

Allylic C-H acetoxylations are among the most widely studied palladium(II)-catalyzed C-H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd^I complex [Pd^I(DAF)(OAc)]₂, followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd^I dimer into a π-allyl-Pd^II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O₂ as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.

Templating metastable Pd2 carboxylate aggregates

Evaluation of the potential for metal-metal (M-M) cooperation to enable catalysis requires access to specific polynuclear aggregates that display appropriate geometry and size. In many cases, exerting synthetic control over the aggregation of simple metal salts is a challenge. For example, Pd(ii) acetate self assembles as a trimer (i.e. Pd3(OAc)6) both in the solid state and in solution and does not feature close Pd-Pd interactions. Related carboxylate-supported Pd2 aggregates (i.e. Pd2(OAc)4), which would feature close Pd-Pd interactions, are thermodynamically metastable in solution phase and thus largely unavailable. Here we demonstrate ion metathesis within pre-formed metal-organic frameworks (MOFs) to prepare metastable Pd2 tetracarboxylates sites. The newly synthesized materials are characterized by elemental analysis, PXRD, SCXRD, EXAFS, XANES, and gas adsorption analysis. In addition, the critical role of network solvation on the kinetics of ion metathesis was revealed by coupled TGA-MS and ICP-MS experiments. The demonstration of templated ion metathesis to generate specific metastable coordination sites that are inaccessible in solution phase chemistry represents a new opportunity to interrogate the chemistry of specific polynuclear metal aggregates.

Unraveling the Role of a Flexible Tetradentate Ligand in the Aerobic Oxidative Carbon-Carbon Bond Formation with Palladium Complexes: A Computational Mechanistic Study

Mechanistic details of the aerobic oxidative coupling of methyl groups by a novel (^MeL)Pd^II(Me)₂ complex with the tetradentate ligand, ^MeL = N, N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane, has been explored by density functional theory calculations. The calculated mechanism sheds light on the role of this ligand's flexibility in several stages of the reaction, especially as the oxidation state of the Pd changes. Ligand flexibility leads to diverse axial coordination modes, and it controls the availability of electrons by modulating the energies of high-lying molecular orbitals, particularly those with major d _z² character. Solvent molecules, particularly water, appear essential in the aerobic oxidation of Pd^II by lowering the energy of the oxygen molecule's unoccupied molecular orbital and stabilizing the Pd^X-O₂ complex. Ligand flexibility and solvent coordination to oxygen are essential to the required spin-crossover for the transformation of high-valent Pd^X-O₂ complexes. A methyl cation pathway has been predicted by our calculations in transmetalation between Pd^II and Pd^IV intermediates to be preferred over methyl radical or methyl anion pathways. Combining an axial and equatorial methyl group is preferred in the reductive elimination pathway where roles are played by the ligand's flexibility and the fluxionality of trimethyl groups.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Dual Role of H2O2 in Palladium-Catalyzed Dioxygenation of Terminal Alkenes

A palladium-catalyzed, environmentally friendly dioxygenation reaction of simple alkenes has been developed that enabled rapid assembly of valuable α-hydroxy ketones with high atom economy. Notably, control experiments and ¹⁸O isotope-labeling experiments established that H₂O₂ played a dominant dual role in this transformation.

Detection of Palladium(I) in Aerobic Oxidation Catalysis

Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric PdI species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C-H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric PdI complex, [PdI (μ-DAF)(OAc)]2 , which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this PdI complex and its implications for catalytic reactivity are discussed.

Oxidation of Hindered Allylic C-H Bonds with Applications to the Functionalization of Complex Molecules

We report the palladium-catalyzed oxidation of hindered alkenes to form linear allylic esters. The combination of palladium(II) benzoate, 4,5-diazafluoren-9-one, and benzoquinone catalyzes the mild oxidation of terminal alkenes with tert-butyl benzoyl peroxide as an oxidant in the presence of diverse functional groups. Selective oxidation of terminal alkenes in the presence of trisubstituted and disubstituted alkenes has been achieved, and the ability to conduct the reaction on a gram scale has been demonstrated. The mild conditions and high tolerance for auxiliary functionality make this method suitable for the synthesis and derivatization of complex molecules.

The development and mechanistic investigation of a palladium-catalyzed 1,3-arylfluorination of chromenes

A mild palladium-catalyzed ligand-controlled regioselective 1,3-arylfluorination of 2[H]-chromenes has been developed.

A mild palladium-catalyzed ligand-controlled regioselective 1,3-arylfluorination of 2[H]-chromenes has been developed. The products with a syn-1,3 substitution pattern were obtained with high enantiomeric excess using a PyrOx ligand, wherein the utility of these pyranyl-fluorides was further demonstrated through their participation in a diastereoselective C–C bond forming reaction. Ligand dependent divergent formation of both the 1,3- and 1,2- alkene difunctionalization products was observed. The nature of this bifurcation was investigated through experimental studies in combination with computational and statistical analysis tools. Ultimately, the site selectivity was found to rely on ligand denticity and metal electrophilicity, the electronics of the boronic acid, and the donor ability of the directing group in the substrate.

Pd(ii)-Catalyzed oxidative dearomatization of indoles: substrate-controlled synthesis of indolines and indolones

A Pd(ii)-catalyzed oxidative dearomatization of indoles provides a straightforward access to diverse fused indolines and indolones bearing C2-oxygenated quaternary stereocenters.

Diazafluorenone-Promoted Oxidation Catalysis: Insights into the Role of Bidentate Ligands in Pd-Catalyzed Aerobic Aza-Wacker Reactions

2,2'-Bipyridine (bpy), 1,10-phenanthroline (phen) and related bidentate ligands often inhibit homogeneous Pd-catalyzed aerobic oxidation reactions; however, certain derivatives, such as 4,5-diazafluoren-9-one (DAF), can promote catalysis. In order to gain insight into this divergent ligand behavior, eight different bpy- and phen-derived chelating ligands have been evaluated in Pd(OAc)₂-catalyzed oxidative cyclization of (E)-4-hexenyltosylamide. Two of the ligands, DAF and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me₂bpy), support efficient catalytic turnover, while the others strongly inhibit the reaction. DAF is especially effective and is the only ligand that exhibits "ligand-accelerated catalysis". Evidence suggests that the utility of DAF and 6,6'-Me₂bpy originates from the ability of these ligands to access κ¹-coordination modes via dissociation of one of the pyridyl rings. This hemilabile character is directly observed by NMR spectroscopy upon adding one equivalent of pyridine to solutions of 1:1 L/Pd(OAc)₂ (L = DAF and 6,6'-Me₂bpy), and is further supported by an X-ray crystal structure of Pd(py)(κ¹-DAF)OAc₂. DFT computational studies illuminate the influence of three different chelating ligands [DAF, 6,6'-Me₂bpy, and 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂phen)] on the energetics of the aza-Wacker reaction pathway. The results show that DAF and 6,6'-Me₂bpy destabilize the corresponding ground-state Pd(N~N)(OAc)₂ complexes, while stabilizing the rate-limiting transition state for alkene insertion into a Pd-N bond. Interconversion between κ²- and κ¹-coordination modes facilitate access to open coordination sites at the Pd^II center. The insights from these studies introduce new ligand concepts that could promote numerous other classes of Pd-catalyzed aerobic oxidation reaction.

Platinum nanoparticles supported on polymeric ionic liquid functionalized magnetic silica: effective and reusable heterogeneous catalysts for the selective oxidation of alcohols in water

In this manuscript, we demonstrate that functionalized magnetic silica nanoparticles are efficient and recyclable catalysts for the selective, aerobic oxidation of various primary and secondary alcohols.