Cationic Arylpalladium Research Articles

Synthesis of γ-Aryl Medium-Sized Cyclic Enones by a Domino 4π-Electrocyclic Reaction/Heck–Matsuda Arylation Sequence at Ambient Temperature

AbstractBicyclo[n.2.0]cyclobutenes were transformed into medium-sized cyclic γ-aryl enones by using a cationic aryl palladium(II) species generated from a diazonium salt. The reaction proceeded at ambient temperature by capturing the cis,trans-cycloalkadiene intermediate generated through a conrotatory 4π-electrocyclic ring-opening reaction, followed by a Heck–Matsuda arylation sequence. Optically pure γ-aryl enones were also synthesized by using a point-to-planar-to-point chirality-transfer process.

Heck arylation of acyclic olefins employing arenediazonium salts and chiral N,N ligands: new mechanistic insights from quantum-chemical calculations

A quantum-chemical study based on the Kohn–Sham density functional theory (DFT) has been performed in order to investigate the asymmetric Heck–Matsuda (HM) reactions involving a chiral N,N ligand pyrimidine-oxazoline. The oxidative addition (OA) of arenediazonium cation was analyzed to provide further insights in the unexpected high reactivity of N,N-ligated palladium(0)-based catalysts observed in these HM processes. DFT calculations suggest that the ionogenic nature of metal center promoted by the arenediazonium cation addition increases the electronic reactivity of palladium(0) toward the donor N,N ligand. The concerted and multi-step mechanisms were computed for the OA step, in which the cationic aryl-palladium(II) complex formation was revealed as the thermodynamic driving force for this process. The importance of conformational analyses was highlighted, particularly when an acyclic alkenol olefin is taken as substrate, since its absence could lead to misleading predictions about the selectivity of these HM systems. While the enantioselectivity is controlled by the combination of steric and dispersion interactions between ligand and substrate, the regioselectivity is mostly oriented by the electronic nature of olefinic carbons. DFT predictions on the selectivity were shown to be in quantitative agreement with experimental results.

Evidence for a Cooperative Mechanism Involving Two Palladium(0) Centers in the Oxidative Addition of Iodoarenes.

Oxidative addition of iodoarenes (ArI) to Pd0 ligated by 1-methyl-1H-imidazole (mim) in polar solvents leads to cationic arylpalladium(II) complexes [ArPd(mim)3 ]+ . Kinetic analyses evidence that this reaction is second order with respect to the concentration of Pd0 , and a mechanism involving the cooperative intervention of two Pd0 centers has been postulated to explain this finding. This unusual behavior is also observed with other nitrogen-containing ligands and it is general for iodobenzenes substituted with electron-donating or weakly electron-withdrawing groups. In contrast, bromoarenes and electron-poor iodoarenes display first-order kinetics with respect to Pd0 . Theoretical calculations performed at the density functional theory (DFT) level suggest the existence of mim-ligated ArI-Pd0 complexes, in which the iodoarene is bound to the metal in a halogen-bond-like fashion. Coordination weakens the C-I bond and facilitates the oxidative insertion of another Pd0 center across this C-I bond. This conceptually novel mechanism, involving the cooperative participation of two distinct metal centers, allows a full explanation of the experimentally observed kinetics.

Pd-Catalyzed Ag(I)-Promoted C3-Arylation of Pyrido[1,2-a]pyrimidin-4-ones with Bromo/Iodo-Arenes.

A regioselective Ag(I)-promoted Pd-catalyzed C3-H activation-arylation of pyrido[1,2-a]pyrimidin-4-ones with bromo/iodo-(hetero)arenes under aqueous conditions has been developed. It affords an efficient access to pharmaceutically important versatile 3-aryl-pyrido[1,2-a]pyrimidin-4-ones. Interestingly, the arylation undergoes via a pathway with an unusual feature involving the formation of cationic arylpalladium species promoted by halo-sequestering Ag salts enabling concerted C3-palladation-deprotonation, as explored by relevant experiments and spectroscopic studies. The present approach is step economical, good yielding, and compatible with various functionalities and applicable to a wide range of starting materials.

Mechanism and Enantioselectivity in Palladium-Catalyzed Conjugate Addition of Arylboronic Acids to β-Substituted Cyclic Enones: Insights from Computation and Experiment

Enantioselective conjugate additions of arylboronic acids to β-substituted cyclic enones have been previously reported from our laboratories. Air- and moisture-tolerant conditions were achieved with a catalyst derived in situ from palladium(II) trifluoroacetate and the chiral ligand (S)-t-BuPyOx. We now report a combined experimental and computational investigation on the mechanism, the nature of the active catalyst, the origins of the enantioselectivity, and the stereoelectronic effects of the ligand and the substrates of this transformation. Enantioselectivity is controlled primarily by steric repulsions between the t-Bu group of the chiral ligand and the α-methylene hydrogens of the enone substrate in the enantiodetermining carbopalladation step. Computations indicate that the reaction occurs via formation of a cationic arylpalladium(II) species, and subsequent carbopalladation of the enone olefin forms the key carbon-carbon bond. Studies of nonlinear effects and stoichiometric and catalytic reactions of isolated (PyOx)Pd(Ph)I complexes show that a monomeric arylpalladium-ligand complex is the active species in the selectivity-determining step. The addition of water and ammonium hexafluorophosphate synergistically increases the rate of the reaction, corroborating the hypothesis that a cationic palladium species is involved in the reaction pathway. These additives also allow the reaction to be performed at 40 °C and facilitate an expanded substrate scope.

Palladium catalysed regioselective arylation of indoles, benzofuran and benzothiophene with aryldiazonium salts

Aryldiazonium tetrafluoroborates have been successfully employed in the direct and regioselective arylation of heteroaromatics such as indole, benzofuran and benzothiophene. The cationic aryl palladium intermediates derived from the aryldiazonium salts act as effective electrophiles in the process. These arylating reactions display operational simplicity and provide the arylated heterocycles regioselectively in moderate to excellent yields.

Palladium-catalyzed oxyarylation of olefins using silver carbonate as the base. Probing the mechanism by electrospray ionization mass spectrometry

The Pd(OAc) 2-catalyzed oxyarylation of electron-rich ( 8 and 12) and electron-poor ( 10) olefins by ortho-iodophenols ( 3a– d) was studied using Ag 2CO 3 as the base, in acetone, and in the presence and absence of PPh 3. The corresponding adducts of oxyarylation were obtained in moderate yields. The reaction mechanism was examined by electrospray ionization mass spectrometry (ESI-MS). Cationic arylpalladium intermediate ( 14), formed by the oxidative insertion of Pd(0) into 3a, and the cationic palladacycles ( 15), obtained by reaction of 14 with olefins 8 and 12, were intercepted by ESI-MS and characterized by ESI-MS/MS.

A mechanistic study on modern palladium catalyst precursors as new gateways to Pd(0) in cationic Heck reactions

Electrospray ionization mass spectrometry (ESI-MS) was used as a means to directly identify catalytic cationic organopalladium species in ligand-controlled Heck reactions involving electron-rich olefins and different Pd-sources. In these high-temperature Heck arylations, the oxidative addition intermediates were observed as bidentate ligand chelated cationic aryl palladium species, suggesting that the used ligand attaches to the metal center at the very beginning of the catalytic cycle. This was also in agreement with the obtained regioisomeric profile of the isolated products. The investigation supports the standard Pd(0)/Pd(II) Heck mechanism and provides further insight regarding the conceivable composition of fundamental Pd(II) intermediates in an ongoing Heck reaction.

Transmetalation of arylpalladium and platinum complexes. Mechanism and factors to control the reaction

This article reviews recent studies on intra- and intermolecular transfer of the aryl ligand bonded to Pd(II) and Pt(II). Cationic arylpalladium complexes with bpy and THF ligands undergo intermolecular aryl group transfer to produce biaryl via a diarylpalladium intermediate. This reaction is applied to cyclization of cationic dinuclear arylpalladium complexes, affording the crown ether derivative with biphenylene units. Analogous arylplatinum complexes do not form diaryl complexes via transmetalation, while they react with CO and phenylallene to cause replacement of the coordinated solvent and insertion of the small molecules into the Pt–C bond, respectively. Conproportionation of PtCl 2(cod) and PtPh 2(cod) produces PtCl(Ph)(cod), which is induced by dissociation of a Cl ligand from the former complex. PtCl 2(cod) reacts also with diarylplatinum complexes with bpy and dppe. Disproportionation of PtPh(CH 2COMe)(cod) and conproportionation of PtPh 2(cod) and Pt(CH 2COMe) 2(cod) take place at 50 °C, but the rates of apparently reversible reactions differ from each other. Addition of OH − to a solution of PtI(Ph)(cod) causes intermolecular phenyl ligand transfer to produce PtPh 2(cod). The dinuclear intermediate complex with bridging OH ligand is prepared from an independent route and fully characterized. The complex causes transmetalation of aryl group of aryl boronic acid.

Neopentyl- and trimethylsilylmethylpalladium chemistry: synthesis of reagents for organopalladium chemistry and the crystal structure of the neopentyl(phenyl)palladium(IV) complex [Pd(mq)(CH 2CMe 3)Ph(bpy)]Br (mq=8-methylquinolinyl, bpy=2,2′-bipyridine)

Synthetic routes to neopentyl and trimethylsilylmethyl complexes of Pd(II) are reported, Pd(CH 2EMe 3)Ph(tmeda) [E=C ( 1), Si ( 3); tmeda= N, N, N′, N′-tetramethylethylenediamine] and Pd(CH 2EMe 3)Ph(bpy) [E=C ( 2), Si ( 4); bpy=2,2′-bipyridine]. Complexes 1 and 3 are formed on the reaction of PdIPh(tmeda) with LiCH 2EMe 3, and they react with bpy to give 2 and 4. Oxidative addition reactions of 8-(bromomethyl)quinoline (mqBr) with Pd(CH 2EMe 3)Ph(bpy) result in the formation of octahedral Pd(IV) complexes [Pd(mq)(CH 2EMe 3)Ph(bpy)]Br [E=C ( 5), Si ( 6)]. An X-ray structural analysis for 5, the first example of a stable cationic arylpalladium(IV) complex, shows a fac-PdC 3N 3 configuration with the neopentyl group trans to the quinoline nitrogen donor atom.

Mechanistic Studies on Oxidative Addition of Aryl Halides and Triflates to Pd(BINAP)2and Structural Characterization of the Product from Aryl Triflate Addition in the Presence of Amine

Kinetic studies of the oxidative addition of phenyl iodide and phenyl triflate to Pd(BINAP)2 (1) are reported. The products of the oxidative addition of phenyl trifluoromethanesulfonate to 1 in the presence of the primary amines n-octylamine and isoamylamine are the cationic aryl palladium(II) complexes [(BINAP)Pd(Ph)(H2NR)]OTf (2a,b), and the product of the oxidative addition of phenyl iodide to 1 is the phenylpalladium iodide {(BINAP)Pd(Ph)I} (3). Structural characterization of the rare, stable σ-aryl Pd(II) triflate complex (2b), in this case coordinated by isoamylamine, is reported. Initial mechanistic studies focused on measuring the rates of reactions containing concentrations of aryl electrophile ranging from 8.94 × 10-7 to 0.2 M. Under these conditions, the oxidative addition of PhOTf and PhI to 1 was inverse first-order in added BINAP ligand when the concentration of ArX was low. The oxidative addition reaction was first-order in ArX when the concentration of ArX was low and was zero order in ArX when the concentration of ArX was high. At these high concentrations, the reaction rate depended solely on the rate for dissociation of BINAP from 1. However, additional experiments using concentrations of aryl iodide and aryl bromide up to 4.0 M led to a measurable increase in observed rate constants and detection of a second concurrent reaction mechanism. This effect was not observed with phenyl triflate. The relevance of this phenomenon to catalytic coupling and oxidative addition of aryl bromides is discussed. Reactions of triflate complex 2a with several bases to form N-octylaniline are also reported.

Cationic Arylpalladium Complexes with Chelating Diamine Ligands, [PdAr(N−N)(solv)]BF4 (N−N = N,N,N‘,N‘-tetramethylethylenediamine, 2,2‘-bipyridine, 4,4‘-dimethyl-2,2‘-bipyridine). Preparation, Intermolecular Coupling of the Aryl Ligands, and Insertion of Alkyne and Allene into the Pd−C Bond

Iodo(aryl)palladium complexes, [PdI{C6H3(CF3)2-3,5}(N−N)] (N−N = tmeda, bpy, 4,4‘-dimethyl-2,2‘-bipyridine (Me2bpy)), react with AgBF4 in CH3CN, acetone, and THF to yield stable cationic arylpalladium complexes [Pd{C6H3(CF3)2-3,5}(N−N)(solv)]BF4. A similar reaction of AgBF4 with [PdI(C6H3Me2-3,5)(bpy)] in CH3CN gives [Pd(C6H3Me2-3,5)(bpy)(CH3CN)]BF4. The complex does not change its NMR spectrum for 1 h at room temperature in CD3CN but undergoes decomposition upon dissolution in acetone to release 3,3‘,5,5‘-tetramethylbiphenyl. Addition of AgBF4 to acetone or THF solutions of [PdI(Ar)(bpy)] (Ar = Ph, C6H3Me2-3,5) and of [PdI(Ar)(Me2bpy)] (Ar = C6H4OMe-4, C6H3Me2-3,5) does not lead to isolation of the cationic arylpalladium complexes and causes intermolecular coupling of the aryl ligands to yield the corresponding biaryls. The reaction of AgBF4 with [PdI(C6H3Me2-3,5)(bpy)] in the presence of an excess amount of dimethyl acetylenedicarboxylate (DMAD) in CH3CN gives [Pd(CZCZ−CZCZ−C6H3Me2-3,5)(bpy)(CH3CN)]BF4 (Z = COOMe) via insertion of two acetylene molecules into the Pd−aryl bond. A similar reaction in acetone or THF causes insertion of three DMAD molecules into the Pd−aryl bond and cyclization of the formed Pd−(CZCZ)3−Ar group to give the product containing a cyclopentadiene structure in the ligand. [PdI(CZCZ−C6H3Me2-3,5)(bpy)] reacts with AgBF4 in CH3CN to form a cationic complex, [Pd(CZCZ−C6H3Me2-3,5)(bpy)(CH3CN)]BF4. A series of cationic Pd complexes, formed through insertion of one, two, and three alkyne molecules into the Pd−aryl bond, are characterized by X-ray crystallography or NMR spectroscopy. Phenylallene reacts with [PdI(C6H3Me2-3,5)(bpy)] in the presence of AgBF4 to give [Pd{η3-CH2C(C6H3Me2-3,5)CHPh}(bpy)]BF4 via insertion of the CC double bond of the allene into the Pd−C bond of the cationic arylpalladium complex. The π-allylpalladium complex crystallizes exclusively in a form with a syn-oriented phenyl substituent but exists in solution as a mixture of the isomers with a syn or anti phenyl substituent.

Evidence for an Equilibrium between Neutral and Cationic Arylpalladium(II) Complexes in DMF. Mechanism of the Reduction of Cationic Arylpalladium(II) Complexes.

Evidence for an Equilibrium between Neutral and Cationic Arylpalladium(II) Complexes in DMF. Mechanism of the Reduction of Cationic Arylpalladium(II) Complexes.

Synthesis and Properties of Cationic Organopalladium Complexes. Remarkable Rate Enhancement in Olefin Insertion into the Palladium–Aryl Bond by the Generation of a Cationic Palladium Complex from trans-[PdBr(Ph)(PMe3)2

Abstract By removing the bromide ligand in trans-[PdBr(Ph)(PMe3)2] (1) by AgBF4 in the presence and absence of pyridine, pyridine-coordinated and solvent-coordinated cationic complexes, trans-[PdPh(py)(PMe3)2]BF4 (2) and trans-[PdPh(solvent)(PMe3)2]BF4 (3), have been obtained. These cationic phenylpalladium complexes show much greater reactivities than do the parent neutral complex 1 toward olefins to give phenylated olefins by insertion followed by β-hydrogen elimination processes. Kinetic studies concerning the insertion of methyl acrylate into the phenyl-palladium bond have indicated that the reactions are first order in the phenylpalladium complexes and that addition of pyridine to a system containing 2 and methyl acrylate inhibits the insertion reaction of the olefin. The results suggest that the generation of a cationic arylpalladium complex with a vacant site is important for the olefins to be inserted.