PdII Complexes Research Articles

Palladium‐Catalyzed Diorganozinc Conjugate Additions to Enones: Preparative and Computational Studies

AbstractThe Pd‐catalyzed conjugate addition of diorganozinc reagents to α,β‐unsaturated carbonyl compounds has been applied to a variety of enone substrates, both cyclic and acyclic, encompassing various combinations of aryl and alkyl substitution, in combination with diorganozinc reagents incorporating alkyl (Me, linear and branched) or Ph groups. While both Pd0 and PdII complexes have been found to be competent precatalysts, reaction yields are strongly dependent on the relative amounts of Pd to phosphane ligand. Thus, a 1:1 Pd/P ratio is well tolerated, but going to Pd/P = 1:2 leads to poor conversions and yields. A calculated Pd0‐mediated pathway involving an unusual oxidative addition step with simultaneous transmetalation, followed by reductive elimination, has been found to be compatible with the observed phosphane effect.

Mechanistic Studies on the SCS-Pincer Palladium(II)-Catalyzed Tandem Stannylation/Electrophilic Allylic Substitution of Allyl Chlorides with Hexamethylditin and Benzaldehydes

This paper describes a mechanistic study of the SCS-pincer Pd(II)-catalyzed auto-tandem reaction consisting of the stannylation of cinnamyl chloride with hexamethylditin, followed by an electrophilic allylic substitution of the primary tandem-reaction product with 4-nitrobenzaldehyde to yield homoallylic alcohols as the secondary tandem products. As it turned out, the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction. These studies have provided insight in the catalytic behavior of SCS-pincer Pd(II) complexes in the auto-tandem reaction and on the formation and possible involvement of Pd(0) species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS-pincer Pd(II) -mediated catalysis, that is, true auto-tandem catalysis. Accordingly, this study has provided the appropriate reaction conditions that allow the pincer catalysts to be recycled and reused.

2.2]Paracyclophane–Triazolyl Monophosphane Ligands: Synthesis and Their Copper and Palladium Complexes

AbstractTriazole‐based phosphanes have recently been shown to be interesting ligands in organometallic catalysis. The synthesis of novel [2.2]paracyclophane‐substituted triazole‐based phosphane ligands was achieved. The possibility to synthesize these molecules in a highly enantioenriched form is also reported. Moreover, the ability of the novel ligands to coordinate with metal centers was explored. Besides a dinuclear copper complex, a PdII complex could also be isolated, the structure of which was characterized by X‐ray analysis. PdII complexes derived from these ligands provide highly active catalysts for the Suzuki–Miyaura coupling of aryl chlorides.

Synthesis and Metal Complexes of Thiourea Ligands Containing Carbohydrate‐Derived Substituents

AbstractTwo glucose‐derived thiourea derivatives, 2a and 2b, were prepared by addition of the corresponding amino sugars to a solution of 4‐nitrobenzoyl isothiocyanate (Scheme 1). The thioureas were isolated as colorless solids in good yields and were fully characterized by NMR spectroscopy, optical rotation, elemental analysis, and also by single‐crystal X‐ray diffraction. Attempts to obtain CuII and NiII bis(chelate) complexes with these thioureas failed. However, the C(1)‐protected thiourea derivative 2a reacted with orthopalladated acetato‐bridged dimers to afford the corresponding monomeric PdII complexes 3 and 4 (Scheme 2). In these compounds, the thiourea coordinates to the metal as monoanionic O,S chelate ligand, which was confirmed by X‐ray crystallography.

NHC PdII Complex Bearing 1,6‐Hexylene Linker: Synthesis and Catalytic Activity in the Suzuki–Miyaura and Heck–Mizoroki Reactions

AbstractA new functionalized N‐heterocyclic carbene PdII complex, [1,1′‐(hexylene‐1,6‐diyl)bis(3‐n‐butylbenzimidazol‐2‐ylidene)][PdCl2(CH3CN)]2 (2) has been synthesized by reaction of NHC‐Ag transmetalation reagent with PdCl2(CH3CN)2. The molecular structure of 2 adopts an open conformation. Complex 2 was tested as a catalyst for Suzuki–Miyaura couplings of aryl halides with arylboronic acids in water and air. The results of these efforts show that 2 is an efficient precatalyst for such reactions. In addition, 2 is a good catalyst in Heck–Mizoroki reactions of aryl bromides with styrene.

Making Carbon–Chlorine Bonds by Dipalladium Electrocatalysis

Abstract[(Benzo[h]quinolinyl)PdII(μ‐OAc)]2 and [(2‐phenylpyridinyl)PdII(μ‐OAc)]2 undergo one‐electron oxidation to afford mixed‐valent (PdII–PdIII) species. Electrochemical oxidation of the PdII–PdII complexes in the presence of chloride at the PdIII–PdII/PdII–PdII potential results in a two‐electron loss with addition of two chlorides to form [(benzo[h]quinolinyl)PdIIICl(μ‐OAc)]2 and [(2‐phenylpyridinyl)PdIIICl(μ‐OAc)]2, respectively. When both excess benzo[h]quinoline and chloride are present, [(benzo[h]quinolinyl)PdII(μ‐OAc)]2 electrocatalyzes the chlorination of the substrate to afford 10‐chlorobenzo[h]quinoline with high chemical and Faradaic yields.

Cyclization–carbonylation–cyclization coupling reaction of α,β-alkynic hydrazones with palladium(ii)-bisoxazoline catalyst

A cyclization-carbonylation-cyclization coupling reaction (CCC-coupling reaction) of α,β-alkynic hydrazones, catalyzed by (box)Pd(II) complexes, afforded symmetrical ketones bearing two pyrazole groups in good to excellent yields. This method is applicable to a broad range of substrates.

Palladium-catalyzed intermolecular fluoroesterification of styrenes: exploration and mechanistic insight

A novel palladium-catalyzed intermolecular oxidative fluoroesterification of vinylarenes has been developed using NFSI, one of the mildest electrophilic fluorinating reagents. The reaction presents an efficient synthetic pathway to afford a series of α-monofluoromethylbenzyl carboxylates in good to excellent yields. Rather than following an electrophilic fluorination pathway, the reaction is initiated through oxidation of Pd(0) to a Pd(II) fluoride complex by NFSI, followed by fluoropalladation of a styrene to generate an α-monofluoromethylbenzyl–Pd intermediate. Generally, reductive elimination of benzyl–PdII complexes is favored with relatively strong oxy-nucleophiles to afford C–O bonds. This reaction, however, exhibited the opposite reactivity: strong acids with weak nucleophilicity, such as CF3CO2H and CCl3CO2H, were prone to afford the fluoroesterification product, while weak acids with strong nucleophilicity, such as HOAc and BzOH, did not deliver the C–O bond product. Further mechanistic studies determined that Csp3–Pd(O2CR), a key intermediate, was generated through ionic ligand exchange between benzyl–Pd(NZ2) and CF3CO2H, and the final C–O bond was possibly formed through reductive elimination of a high-valent Csp3–Pd(O2CR) complex via an SN2-type nucleophilic attack pathway.

Kinetics and mechanism of chloramine-T oxidation of glycolic acid catalysed by PdII complex in alkaline medium

Kinetics and mechanism of chloramine-T oxidation of glycolic acid catalysed by PdII complex in alkaline medium

Cis-Dichloro complexes of NiII, PdII, and PtII derived from 2-pyridylmethyl-N-substituted imines

Synthesis and structural characterization of cis-dichlorocomplexes derived from 2-pyridylmethyl-N-methylimine (1), 2-pyridylmethyl-N-ethylimine (2), and 2-pyridylmethyl-N-(2-bromophenyl)imine (3) of general formula [MII{(C5H4N)–C(H)NR}nCl2] {M=Ni, Pd, Pt; R=Me, Et, –(2-Br–C6H4); n=1, 2} are described. Paramagnetic complexes of NiII (1a–3a) were identified by elemental analyses, vibrational spectroscopy and for 2a and 3a, by electronic spectroscopy; the complexes of PdII (1b–3b) and PtII (1c–3c) were characterized by elemental analyses, vibrational spectroscopy and NMR. X-ray molecular structures of 1a, 2a, 1b–3b, 2c and 3c displayed a bidentate coordination mode of the N-substituted pyridylimines (1–3) towards the metallic atoms with the formation of five-membered chelate rings and the chlorine atoms in a cis coordination. In 1a and 2a a distorted octahedral local geometry around the nickel atom was observed, whereas a distorted square planar local geometry around the palladium and platinum atoms in the compounds 1b–3b, 2c and 3c was displayed.

Detection of Pd(III) and Pd(IV) Intermediates during the Aerobic Oxidative C–C Bond Formation from a Pd(II) Dimethyl Complex

The dimethyl PdII complex (MeN4)PdIIMe2 (MeN4 = N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) is readily oxidized by dioxygen in the presence of protic solvents to selectively eliminate ethane. UV–vis, EPR, ESI-MS, and NMR studies reveal the formation of several PdIII and PdIV intermediates during the aerobically induced C–C bond formation reaction, including the key intermediate [(κ3-MeN4)PdIVMe3]+, which leads to ethane elimination. The latter complex was also synthesized independently and structurally characterized to reveal a distorted octahedral geometry that is proposed to promote facile reductive elimination. Overall, this study represents a rare example of aerobic oxidation of an organometallic PdII precursor that leads to a well-defined PdIV species, which undergoes selective C–C bond formation under ambient conditions.

ChemInform Abstract: Cyclization—Carbonylation—Cyclization Coupling Reaction of γ‐Propynyl‐1,3‐diketones with Palladium(II)‐Bisoxazoline Catalyst.

Cyclization–carbonylation–cyclization coupling reaction (CCC-coupling reaction) of γ-propynyl-1,3-diketones catalyzed by (box)PdII complexes afforded symmetrical ketones bearing two oxabicyclic groups in moderate to excellent yields.

Cyclization-carbonylation-cyclization coupling reaction of propargyl ureas with palladium(II)-bisoxazoline catalyst.

The cyclization-carbonylation-cyclization coupling reaction (CCC-coupling reaction) of propargyl ureas catalyzed by PdII(box) complexes afforded symmetrical ketones bearing two 2-amino-2-oxazoline groups in good to moderate yields.

Structural and dynamic NMR characterization of [Pd(bipy)(R-thiourea)2]2+ and [Pd(phen)(R-thiourea)2]2+ cations

The synthesis and characterization of 10 complexes, [Pd(bipy)(R-TU)2]Cl2 and [Pd(phen)(R-TU)2]Cl2 (bipy=2,2′-bipyridyl; phen=1,10-phenanthroline; R-TU=N-alkyl substituted thioureas), is presented. The conformational and dynamic behavior in solution, analyzed by several NMR techniques, is compared to that of the free thiourea ligands. Spectra at variable temperatures of the free thioureas are consistent with hampered rotation around the C–N bonds. Mono-alkyl derivatives, in methanol, show equilibria between syn and anti conformers, whereas di-alkyl thioureas show equilibria between the syn–anti and syn–syn conformers (syn and anti indicate the position of the alkyl chain with respect to the S atom over the two amino-branches). Syn protons of the alkyl-chains are converted into the corresponding anti protons by C–N rotation, which also exchanges external (close to the S atom) to internal N–H (on the opposite side with respect to the S atom) within the NMR timescale. This is also observed for the corresponding PdII complexes, which, according to the enhancement of the double C–N bond character, present slower syn/anti exchanges. Moreover, metal coordination selects anti conformers for the mono-alkyl thioureas, and syn–anti conformers for the di-alkyl thioureas.

Palladium‐mediated Cleavage of S–C Bonds: Preparation and Characterization of Palladium(II) Complexes of 2, 6‐Diformyl‐4‐tert‐butylthiophenol Dioxime

AbstractThe synthesis of air‐sensitive 2, 6‐diformyl‐4‐tert‐butylthiophenol dioxime H3L3 was achieved by a Pd‐mediated S–C cleavage of the corresponding S‐tert‐butyl protected thioether. The novel ligand forms a dinuclear, neutral PdII2 complex, which is stabilized by two N···HO hydrogen bonds to give a pseudo‐macrocyclic structure. The crystal structure of a PdII complex of an oxidized isothiazole derivative of H3L3 is also reported.

Formation of the Palladium(IV) Complex [(Me3tacn)PdIVMe3]+ through Aerobic Oxidation of (Me3tacn)PdIIMe2 (Me3tacn = N,N′,N″-Trimethyl-1,4,7-triazacyclononane)

The dimethyl PdII complex (Me3tacn)PdIIMe2 (Me3tacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane) undergoes facile aerobic oxidation to yield the stable species [(Me3tacn)PdIVMe3]+. EPR, UV–vis, and ESI-MS studies suggest an inner-sphere mechanism for the oxidation of (Me3tacn)PdMe2 by O2 and formation of PdIV species. In addition, the structurally characterized complex [(Me3tacn)PdIVMe3]I undergoes selective elimination of ethane at elevated temperatures. Overall, this system represents one of the first examples of aerobic oxidation of a PdII organometallic precursor to yield a well-defined PdIV product, supporting the role of PdIV species as viable intermediates in Pd-mediated catalytic or stoichiometric aerobic oxidative transformations.

Polymeric 2,2′-biquinolyl-containing NiII complexes as catalysts for the Suzuki reaction

The Suzuki reaction with polymeric Ni catalysts based on the biquinolyl-containing polyamic acids can be carried out under conditions close to the conditions used for the catalytic transformations in the presence of PdII complexes with the same polymeric ligands. However, the yields of the cross-coupling products on the Ni catalysts are somewhat lower than those obtained in the presence of palladium complexes. The yield of the cross-coupling product increases with the increase in the conformational mobility of the polymeric ligand. Unlike catalysis by palladium complexes, in catalytic transformations by nickel complexes activation of the catalyst is required. Such an activation can be carried out either by addition of chemical reducing agents or by applying the corresponding cathodic potential (−0.8 V relative to Ag/AgCl/KCl). The electrochemical activation was shown to lead to higher yields of the cross-coupling product than the use of chemical reducing agents.

N‐Methyl‐2, 2'‐Dipicolylamine Complexes as Potential Models for Metal‐Mediated Base Pairs

AbstractMetal‐mediated base pairs can be used to insert metal ions into nucleic acids at precisely defined positions. As structural data on the resulting metal‐modified DNA are scarce, appropriate model complexes need to be synthesized and structurally characterized. Accordingly, the molecular structures of nine transition metal complexes of N‐methyl‐2, 2'‐dipicolylamine (dipic) are reported. In combination with an azole‐containing artificial nucleoside, this tridentate ligand had recently been used to generate metal‐mediated base pairs (Chem. Commun. 2011, 47, 11041–11043). The PdII and PtII complexes reported here confirm that the formation of planar complexes (as required for a metal‐mediated base pair) comprising N‐methyl‐2, 2'‐dipicolylamine is possible. Two HgII complexes with differing stoichiometry indicate that a planar structure might also be formed with this metal ion, even though it is not favored. In the complex [Ag2(dipic)2](ClO4)2, the two AgI ions are located close to one another with an Ag···Ag distance of 2.9152(3) Å, suggesting the presence of a strong argentophilic interaction.

Pyridine-Assisted Chlorinations and Oxidations by Palladium(IV)

The reactivity of the bis-NHC complex LPdIVCl4 (L = κ2-[R-NHCCH2NHC-R] with R = C14H29) in chlorinations and oxidations of organic substrates was considerably increased in the presence of pyridine. For alkene chlorinations, this effect was due to the in situ formation of highly reactive LPdIVCl3(py)+, which was able to transfer Cl+ to the C═C bond in a ligand-mediated process (devoid of π complexation), which did not require py dissociation. The enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild conditions in a reaction where py served as a base, broadening the known scope of reactivity for PdIV complexes. LPdIVCl3(py)+ could be formed from Cl–/py exchange or from the oxidation of LPdIICl(py)+ by Cl2. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both PdII and PdIV complexes allowed for the catalytic chlorination of styrene with LPdIVCl4 as a sacrificial oxidant, thereby establishing the princi...

Efficient catalytic oxidation of internal alkenes, aromatic hydrocarbons and alcohols by using orthometallated and non-orthometallated PdII complexes

Efficient catalytic oxidation of internal alkenes, aromatic hydrocarbons and alcohols by using orthometallated and non-orthometallated PdII complexes