Auto‐Tandem Cooperative Catalysis Using Phosphine/Palladium: Reaction of Morita–Baylis–Hillman Carbonates and Allylic Alcohols

Auto‐tandem catalysis (ATC), in which a single catalyst promotes two or more mechanistically different reactions in a cascade pattern, provides a powerful strategy to prepare complex products from simple starting materials. Reported here is an unprecedented auto‐tandem cooperative catalysis (ATCC) for Morita–Baylis–Hillman carbonates from isatins and allylic carbonates using a simple Pd(PPh₃)₄ precursor. Dissociated phosphine generates phosphorus ylides and the Pd leads to π‐allylpalladium complexes, and they undergo a γ‐regioselective allylic–allylic alkylation reaction. Importantly, a cascade intramolecular Heck‐type coupling proceeds to finally furnish spirooxindoles incorporating a 4‐methylene‐2‐cyclopentene motif. Experimental results indicate that both Pd and phosphine play crucial roles in the catalytic Heck reaction. In addition, the asymmetric versions with either a chiral phosphine or chiral auxiliary are explored, and moderate results are obtained.     

Dimerization of terminal alkynes catalyzed by chloro(η-pentadienyl) bis(triphenylphosphine)ruthenium(II) and kinetics of phosphine substitution

Exchange of PMe₂Ph for PPh₃ in (η⁵-pentadienyl)ruthenium{bis(triphenylphosphine)}chloride, (η⁵-C₅H₇)Ru(PPh₃)₂Cl (1) under first order conditions proceeds rapidly in THF at room temperature. A pseudo-first order rate constant of 17 ± 2 × 10⁻⁴ s⁻¹ is obtained for the reaction at 21 °C. The rate constant is essentially independent of the phosphine concentration. The activation parameters, ΔH^† = 16.1 ± 0.4 kcal mol⁻¹ and ΔS^† = −16 ± 1 cal K⁻¹ mol⁻¹ differ from those reported for phosphine exchange in CpRu(PPh₃)₂Cl (2) and (η⁵-indenyl)Ru(PPh₃)₂Cl (3). The reaction of 1 with PMe₂Ph is about 70 times faster than the reaction of 2 at 30 °C and some 40 times faster than the reaction of 3 at 20 °C. (η⁵-C₅H₇)Ru(PPh₃)₂Cl(1) is more active than the ruthenium(II) complexes 2, 3, and TpRu(PPh₃)₂Cl (4) in the catalytic dimerization of terminal alkynes with nearly quantitative conversion of PhCCH and FcCCH at ambient temperature in 24 h. The enhanced substitution rate is accompanied by >50% conversion of phenylacetylene to oligomeric products. Reaction of 1 with NaPF₆ in acetonitrile yields the cationic ruthenium(II) complex [(η⁵-C₅H₇)Ru(PPh₃)₂(CH₃CN)][PF₆] (7). The latter complex is much less active in reactions with phenylacetylene than 1 but avoids the formation of oligomeric products.     

N-Heterocyclic carbenes as ligands in palladium-catalyzed Tsuji–Trost allylic substitution

A Pd(0)-catalyzed allylic substitution (i.e., Tsuji–Trost reaction) using N-heterocyclic carbene as a ligand was investigated. It has been proven that an imidazolium salt 2d having bulky aromatic rings attached to the nitrogens in its imidazol-2-ylidene skeleton is suitable as a ligand precursor and that a Pd₂dba₃–imidazolium salt 2d–Cs₂CO₃ system is highly efficient for producing a Pd–NHC catalyst in this reaction. Allylic substitution using a Pd–NHC complex differed from that using a Pd–phosphine complex as follows: (1) the reaction using a Pd–NHC complex required elevated temperature (50 °C or reflux in THF), (2) allylic carbonates were inert to a Pd–NHC complex, and (3) nitrogen nucleophiles such as sulfonamide and amine did not react with allylic acetate. It was also found that allylic substitution with a soft nucleophile using a Pd–NHC catalyst proceeds via overall retention of configuration to give the product in a stereospecific manner, the stereochemical reaction course obviously being the same as that of the reaction using a Pd–phosphine complex.     

Stepwise phosphine sulfide formation and metal-bridging reaction of tetradentate and tridentate phosphine ligands on palladium(II)

Kinetic studies on the stepwise phosphine sulfide formation reaction of the five-coordinate trigonal-bipyramidal Pd(II) complexes with the tripodal tetradentate phosphine ligand, [PdCl(pp₃)]Cl and [Pd(4-Cltp)(pp₃)](BF₄) (pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine; 4-Cltp = 4-chlorothiophenolate), were carried out, and it was revealed that the reactions proceeded via the intermediate with a pendant dissociated phosphino group. Formation of the intermediate was utilized for the bridging reaction onto Pt(II) to form the phosphine-bridged linear trinuclear and cyclic tetranuclear mixed-metal complexes. Difference in the steric conversion mechanism in the phosphine-bridging reaction between the linear tridentate phosphine (bis[2-(diphenylphosphino)ethyl]phenylphosphine) and pp₃ is also reported.     

Allyl palladium dithiocarbamates and related dithiolate complexes as precursors to palladium sulfides

Allyl-palladium dithiocarbamate complexes, [Pd(allyl)(S₂CNR₂)], have been prepared from the addition of dithiocarbamate salts to [Pd(allyl)(μ-Cl)]₂ and TGA and DSC studies have been carried out in order to assess their potential as MOCVD precursors to palladium sulfides. For comparison [(η³-C₄H₇)Pd(S₂PPh₂)] and [Pd(S₂CNMeR)₂] (R = Bu, Hex) have also been prepared and tested as precursors. The unsymmetrical dithiocarbamate complex, [(η³-C₃H₅)Pd(S₂CNMeHex)], which has a melting point of 65 °C was selected as the best single source precursor and thin films of predominantly Pd_2.8S were deposited on glass slides. The crystal structures of [(η³-C₄H₇)Pd(S₂CNMe₂)], [(η³-C₄H₇)Pd(S₂CNPr₂)], [(η³-C₄H₇)Pd(S₂PPh₂)] and [Pd(S₂CNMeBu)₂] are reported. All except [(η³-C₄H₇)Pd(S₂CNPr₂)] show weak intermolecular S?H or Pd?H interactions.     

Novel neutral arylnickel(II) phosphine catalysts containing 2-oxazolinylphenolato N-O chelate ligands for ethylene oligomerization and propylene dimerization

A series of new neutral arylnickel(II) phosphine complexes 1 bearing 2-oxazolinylphenolato ligands [2-(4-R¹-5-R²-C₃H₂NO)-C₆H₄O]Ni(2-R⁴-4-R³-C₆H₃)(PPh₃) were synthesized by reactions of sodium salts of 2-(4,5-dihydro-2-oxazolyl)phenol derivatives with trans-Ni(Ar)(Cl)(PPh₃)₂ or by direct reactions of the ligands with trans-Ni(Ar)(Cl)(PPh₃)₂ in the presence of NEt₃. These neutral Ni(II) complexes 1 exhibited high activities and selectivities in ethylene oligomerization and propylene dimerization. The catalytic activities and the product distributions were dependent on the selection of various organoaluminum cocatalysts and phosphine scavenger (Ni(COD)₂). The effects of various reaction conditions on ethylene oligomerization were also examined. The highest activity of 5.51 × 10⁵ g oligomers/(mol Ni · h) and 83% selectivity of C₆ internal olefins were obtained in 1a/MAO catalytic system in ethylene oligomerization. The oligomers consisted mainly of lower carbon olefins in the range of C₄-C₈. Complexes 1 showed the moderate tolerance of polar additives in ethylene oligomerization. The highest activity of 1a/MAO in propylene dimerization reached to 1.32 × 10⁵ g oligomers/(mol Ni · h).     

Synthesis and characterization of three nickel(ii) carborane complexes containing nido- or closo-carborane diphosphine ligand

Three nickel(II) carborane complexes, [Ni₂(μ-Cl)₂{7,8-(PPh₂)₂-7,8-C₂B₉H₁₀}₂] (1), [Ni{7-(OPPh₂)-8-(PPh₂)-7,8-C₂B₉H₁₀}{7,8-(PPh₂)₂-7,8-C₂B₉H₁₀}] (2) and [NiBr₂{1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀}] · CH₂Cl₂ (3), have been synthesized by the reactions of 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane with NiCl₂ · 6H₂O or NiBr₂ · 6H₂O in ethanol under different conditions, respectively. For complex 1, it could also be obtained under the solvothermal condition. All the three complexes were characterized by elemental analysis, FT-IR, ¹H and ¹³C NMR spectroscopy and X-ray structure determination. Single crystal analysis shows that the molecular symmetry of complex 1 is centrosymmetric, containing two same structure units - Ni(7,8-(PPh₂)₂-7,8-C₂B₉H₁₀) linked by two bridged-Cl atoms. The central square plane formed by the [Ni₂Cl₂] unit is almost parallel to the two side NiPP planes. For complex 2, the coordination environment of the Ni atom is a seriously distorted square-planar, in which two positions come from the chelating diphosphine ligand [7,8-(PPh₂)₂-7,8-C₂B₉H₁₀]⁻ degraded from the closo species, while the other two are occupied by an unsymmetrical chelating phosphine oxide ligand [7-(OPPh₂)-8-(PPh₂)-7,8-C₂B₉H₁₀]⁻. As for complex 3, the geometry at the Ni atom is a slightly distorted square-planar. The closo carborane diphosphine ligand 1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀ was coordinated bidentately to the metal ion through the two phosphorus atoms, and the two Br atoms are at cis position which can fulfill the four coordination mode of the metal.     

MCM-41-supported bidentate phosphine palladium(0) complex as an efficient catalyst for the heterogeneous Stille reaction

A Stille coupling reaction of organostannanes with organic halides has been developed in the presence of a catalytic amount of MCM-41-supported bidentate phosphine palladium(0) complex (0.5 mol %) in DMF/H₂O (9:1) under air atmosphere in high yields. This polymeric palladium catalyst exhibits higher activity than Pd(PPh₃)₄ and can be reused at least 10 times without any decrease in activity.     

Three novel bismetallacyclopropa[60]fullerene complexes formed via intermediate monometallacyclopropa[60]fullerene diphosphine ligands

The homodinuclear bismetallacyclopropa[60]fullerene complexes (η²-C₆₀)M(μ-η¹,η¹-trans-Ph₂PCHCH PPh₂)₂M(η²-C₆₀) (1, M = Pt; 2, M = Pd) were prepared by reaction of C₆₀ with M(dba)₂ (dba = dibenzylideneacetone) and trans-1,1′-bis(diphenylphosphino)ethylene in 82% and 92% yield, whereas reaction of C₆₀ with Pd(dba)₂ and trans-dppet followed by treatment with C₆₀ and Pt₂(dba)₃ gave rise to the heterodinuclear complex (η²-C₆₀) Pd(μ-η¹,η¹-trans-Ph₂PCHCH PPh₂)₂Pt(η²-C₆₀) (3) in 65% yield. Mechanistic study showed that these reactions involve the intermediates of monometallacyclopropa[60]fullerene diphosphine ligands (η²-C₆₀)M(η¹-trans-Ph₂PCHCHPPh₂)₂ (4, M = Pt; 5, M = Pd). All the mono- and bismetallacyclopropa[60]fullerene complexes 1-5 have been fully characterized by elemental analysis and spectroscopy, as well as for 2 by X-ray crystallography.     

1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): Electronic effects on cross-coupling reactions and thermal decompositions

The synthesis of 1,3-diarylimidazolidin-2-ylidene (NHC) precursor, 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, (3b) has been extended to the electronically and sterically modified NHC precursors 3a (X = H), 3c (X = Br) and 3e (X = Cl) in order to investigate the electronic effect of a p-substituent (X) on cross-coupling catalysts. Complexes of the type PdCl₂(NHC)₂ (5), PdCl₂(NHC)(PPh₃) (6) and [RhCl(NHC)(cod)] (7) were prepared from 3 or 4d (1,3-bis(2,4-dimethylphenyl)-2-trichloromethylimidazolidin). Initial decomposition temperatures of the complexes 5 and 6 were determined by TGA. In situ formed complexes from Pd(OAc)₂ and 3 as well as the preformed complexes 5 and 6 have been tested as catalysts in coupling of phenylboronic acid with 4-haloacetophenones. The electron donating ability of NHCs derived from 3 was assessed by measuring C-O frequencies in the respective [RhCl(NHC)(CO)₂] complex 8 which was prepared by replacement of cod ligand of 7 with CO. An interesting correlation between the electron-donating nature of the aryl substituent and catalytic activity and also initial decomposition temperature of the complexes 5 and 6 was observed.     

Study of half-sandwich platinum group metal complexes bearing dpt-NH2 ligand

A quite general approach for the preparation of η⁵-and η⁶-cyclichydrocarbon platinum group metal complexes is reported. The dinuclear arene ruthenium complexes [(η⁶-arene)Ru(μ-Cl)Cl]₂ (arene = C₆H₆, C₁₀H₁₄ and C₆Me₆) and η⁵-pentamethylcyclopentadienyl rhodium and iridium complexes [(η⁶-C₅Me₅)M(μ-Cl)Cl]₂ (M = Rh, Ir) react with 2 equiv. of 4-amino-3,5-di-pyridyltriazole (dpt-NH₂) in presence of NH₄PF₆ to afford the corresponding mononuclear complexes of the type [(η⁶-arene)Ru(dpt-NH₂)Cl]PF₆ {arene = C₁₀H₁₄ (1), C₆H₆ (2) and C₆Me₆ (3)} and [(η⁶-C₅Me₅)M(dpt-NH₂)Cl]PF₆ {M = Rh (4), Ir (5)}. However, the mononuclear η⁵-cyclopentadienyl analogues such as [(η⁵-C₅H₅)Ru(PPh₃)₂Cl], [(η⁵-C₅H₅)Os(PPh₃)₂Br], [(η⁵-C₅Me₅)Ru(PPh₃)₂Cl] and [(η⁵-C₉H₇)Ru(PPh₃)₂Cl] complexes react in presence of 1 equiv. of dpt-NH₂ and 1 equiv. of NH₄PF₆ in methanol yielded mononuclear complexes [(η⁵-C₅H₅)Ru(PPh₃)(dpt-NH₂)]PF₆ (6), [(η⁵-C₅H₅)Os(PPh₃)(dpt-NH₂)]PF₆ (7), [(η⁵-C₅Me₅)Ru(PPh₃)(dpt-NH₂)]PF₆ (8) and [(η⁵-C₉H₇)Ru(PPh₃)(dpt-NH₂)]PF₆ (9), respectively. These compounds have been totally characterized by IR, NMR and mass spectrometry. The molecular structures of 4 and 6 have been established by single crystal X-ray diffraction and some of the representative complexes have also been studied by UV–Vis spectroscopy.     

A new tris(ferrocenylamine) ditertiary phosphine: Synthesis and co-ordination studies

The new tris(ferrocenylamine) ditertiary phosphine 1,1′-{FcCH₂N(CH₂PPh₂)CH₂(η⁵-C₅H₄)}₂Fe [Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄)] has been prepared along with two coordination complexes. All compounds have been characterised by a combination of spectroscopic and analytical methods. The single crystal X-ray structure of the pentametallic Ru₂Fe₃ complex 5 has been determined.     

Cyclopalladation of N-phenyl-4-ferrocenylmethylpyrazoles: Crystal structure of [Pd{κ-C,N-C6H4-1-[(3,5-Me2-C3N2)-CH2-(η-C5H4)Fe(η-C5H5)]}Cl(PPh3)] · CH2Cl2

The synthesis and characterization of pyrazole derivatives of general formula [C₆H₄-4-R-1-{(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)}] [R = OMe (1a) or H (1b)] with a ferrocenylmethyl substituent are described.The study of the reactivity of compounds 1 with palladium(II) acetate has allowed the isolation of complexes (μ-AcO)₂[Pd{κ²-C,N-C₆H₃-4-R-1-[(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)]}]₂ (2) [R = OMe (2a) or H (2b)] that contain a bidentate [C(sp², phenyl), N]⁻ ligand and a central “Pd(μ-AcO)₂Pd” unit.Furthermore, treatment of 2 with LiCl produced complexes (μ-Cl)₂[Pd{κ²-C,N-C₆H₃-4-R-1-[(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)]}]₂ (3) [R = OMe (3a) or H (3b)] that arise from the replacement of the acetato ligands by the Cl⁻.Compounds 2 and 3 also react with PPh₃ giving the monomeric complexes [Pd{κ²-C,N-C₆H₃-4-R-1-[(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)]}X(PPh₃)] {X⁻ = AcO⁻ and R = OMe (5a) or H (5b) or X⁻ = Cl⁻ and R = OMe (6a) or H (6b)}, where the phosphine is in a cis-arrangement to the metallated carbon atom. Treatment of 3 with thallium(I) acetylacetonate produced [Pd{κ²-C,N-C₆H₃-4-R-1-[(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)]}(acac)] (7) [R = OMe (7a) or H (7b)]. Electrochemical studies of the free ligands and the cyclopalladated complexes are also reported. The dimeric complexes 3 also react with MeO₂C-CC-CO₂Me (in a 1:4 molar ratio) giving [Pd{(MeO₂C-CC-CO₂Me)₂C₆H₃-4-R-1-[(3,5-Me₂-C₃N₂)-CH₂-(η⁵-C₅H₄)Fe(η⁵-C₅H₅)]}Cl] (8) [R = OMe (8a) or H (8b)], which arise from the bis(insertion) of the alkyne into the σ{Pd-C(sp², phenyl)} bond of 3.     

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh₃)Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh₃)Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.     

Syntheses and characterization of cyano-bridged homo and hetero bimetallic complexes containing η and η-cyclic hydrocarbons

The reactions of [(ind)Ru(PPh₃)₂CN] (ind = η⁵-C₉H₇) (1) and [CpRu(PPh₃)₂CN] (Cp = η⁵-C₅H₅) (2) with [(η⁶-p-cymene)Ru(bipy)Cl]Cl (bipy = 2,2′-bipyridine) (3) in the presence of AgNO₃/NH₄BF₄ in methanol, respectively, yielded dicationic cyano-bridged complexes of the type [(ind)(PPh₃)₂Ru(μ-CN)Ru(bipy)(η⁶-p-cymene)](BF₄)₂ (4) and [Cp(PPh₃)₂Ru(μ-CN)Ru(bipy)(η⁶-p-cymene)](BF₄)₂ (5). The reaction of [CpRu(PPh₃)₂CN] (2), [CpOs(PPh₃)₂CN] (6) and [CpRu(dppe)CN] (7) with the corresponding halide complexes and [(η⁶-p-cymene)RuCl₂]₂ formed the monocationic cyano-bridge complexes [Cp(PPh₃)₂Ru(μ-CN)Os(PPh₃)₂Cp](BF₄) (8), [Cp(PPh₃)₂Os(μ- CN)Ru(PPh₃)₂Cp](BF₄) (9) and [Cp(dppe)Ru(μ-CN)Os(PPh₃)₂Cp](BF₄) (10) along with the neutral complexes [Cp(PPh₃)₂Ru(μ-CN)Ru (η⁶-p-cymene)Cl₂] (11), [Cp(PPh₃)₂Os(μ-CN)Ru(η⁶-p-cymene)Cl₂] (12), and [Cp(dppe) Ru(μ-CN)Ru(η⁶-p-cymene)Cl₂] (13). These complexes were characterized by FT IR, ¹H NMR, ³¹P{¹H} NMR spectroscopy and the molecular structures of complexes 4, 8 and 11 were solved by X-ray diffraction studies.     

Recyclable N-heterocyclic carbene/palladium catalyst on graphene oxide for the aqueous-phase Suzuki reaction

Graphene oxide (GO) was functionalized with a N-heterocyclic carbene (NHC) precursor, 3-(3-aminopropyl)-1-methylimidazolium bromide ([APMIm][Br]) for the immobilization of palladium catalyst. The GO-supported NHC precursor (IMGO) formed a stable complex with Pd(OAc)₂ (GO–NHC–Pd), which showed excellent catalytic activity and fast reaction kinetics in the aqueous-phase Suzuki reaction of aryl bromides and chlorides at relatively mild conditions (1 h at 50 °C). The GO–NHC–Pd catalyst was reused several times without any loss of its catalytic activity in the Suzuki reaction of aryl bromide.     

Synthesis and crystal structure of four Pd(II) complexes containing nido or closo carborane diphosphine ligands

Three Pd(II) complexes [Pd₂(μ-Cl)₂{7,8-(PPh₂)₂-7,8-C₂B₉H₁₀}₂] · 0.25CH₂Cl₂ (1), [Pd{7,8-(PPh₂)₂-7,8-C₂B₉H₁₀}₂] · 4CHCl₃ (2) and [PdCl₂(1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀)] (3) have been synthesized by the reactions of 1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀ with PdCl₂ in acetonitrile, cyanophenyl and dichloromethane, respectively. A fourth complex, [PdI₂(1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀)] (4), was obtained by a ligand exchange reaction through the substitution of the Cl⁻ of complex 3 with I⁻. All four complexes have been characterized by elemental analysis, FT-IR, ¹H and ¹³C NMR spectroscopy and X-ray structure determination. Single crystal X-ray determination showed that the carborane cage, nido for 1, 2 and closo for 3, 4, was coordinated bidentately to the Pd atom through the two P atoms, and the geometry at the Pd atom was square-planar in all the complexes.     

Studies of reactions of o-xylylene-α,α′-dihalides with palladium complexes and the catalytic synthesis of 3-isochromanone

Homogeneous catalysis by palladium complexes with phosphorus(III) ligands of the carbonylation of o-xylylene dihalides in the presence of water to form 3-isochromanone has been studied. Triphenylphosphine was found to provide the most effective catalyst, and by-products and intermediates of systems containing this ligand have been investigated. 2-Indanone is one by-product but is unstable to decomposition under catalytic conditions. Excess PPh₃ is necessary to prolong activity of the catalyst but is also transformed to bis-phosphonium compound [o-C₆H₄(CH₂PPh₃)₂]X₂ (X = Cl or Br); this quaternization has been investigated and the structure of the bromide salt determined by X-ray diffraction. An unstable oxidative addition product of Pd(PPh₃)₄ was detected as a probable intermediate and related to the previously reported but catalytically-inactive complex trans-Pd(o-CH₂C₆H₄CH₂Cl)Cl(PMe₃)₂, which has been structurally characterized by X-ray diffraction in this work.     

Synthesis and characterization of amide-functionalized N-heterocyclic carbene-Pd complexes

Amide-functionalized N-heterocyclic carbene (NHC) precursors such as azolium compounds have been designed and synthesized. Reaction of PdCl₂(CH₃CN)₂ with the NHC-Ag complex derived from the azolium salt gave [(NHC)PdCl₂]₂ or (NHC)₂PdCl₂, whereas PdCl₂(PPh₃)₂ reacted with the Ag complex to afford a mixed carbene/phosphine complex such as (NHC)(PPh₃)PdCl₂ together with a cationic [(NHC)(PPh₃)₂PdCl]⁺Cl⁻ whose structure was characterized by X-ray crystallographic studies. Thus, the library of NHC-Pd complexes with a tethered amide group has been successfully expanded.     

Formation of anionic palladium(0) complexes ligated by the trifluoroacetate ion and their reactivity in oxidative addition

As established previously for Pd(OAc)₂, Pd⁰ complexes are formed in situ from Pd(OCOCF₃)₂ and n equiv. triarylphosphines (4-Z-C₆H₄)₃P (Z = CF₃, F, Cl, H, CH₃; n ? 3). The phosphines are the intramolecular reducing agents and are oxidized to triarylphosphine oxides. The generated Pd⁰ complexes are anionic species ligated by the trifluoroacetate anion: Pd⁰(PAr₃)_n(OCOCF₃)⁻ (n = 2 or 3). Pd⁰(PAr₃)₂(OCOCF₃)⁻ is the reactive species involved in the oxidative addition to PhI. This leads to trans-PhPd(OCOCF₃)(PPh₃)₂, involved in equilibrium with the cationic complex trans-[PhPd(PPh₃)₂(DMF)]⁺, instead of the expected trans-PhPdI(PPh₃)₂ complex. The existence of anionic Pd⁰ complexes ligated by the acetate or trifluoroacetate ions delivered by the precursors Pd(OAc)₂ or Pd(OCOCF₃)₂, respectively, as well as their comparative reactivity in oxidative additions are consistent with theoretical DFT calculations.