Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.     

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

We describe herein computational studies on the unusual ability of Pd(PtBu₃)₂ to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu₃ that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR₃)Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu₃ favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium‐catalyzed carbonylation reactions with simple phosphine ligands, including the key rate‐determining steps and palladium(0) catalyst resting state in carbonylations.     

Density functional study of the complete pathway for the Heck reaction with palladium diphosphines

The reaction mechanism for the complete catalytic cycle of the Heck reaction (between phenyl bromide, C₆H₅Br, and ethylene, C₂H₄, in the presence of the base, NEt₃ to form the product styrene, C₆H₅–C₂H₃), catalyzed by diphosphinopalladium complexes, Pd(PR₃)₂ {R = H, Me, Ph}, was investigated by using density functional theory (DFT). The relative free energies of the fully-optimized species in gas phase at 298.15 K and 1 atm were corrected for solvation in DMSO at 1 mol/L by using conductor-like polarizable continuum model (CPCM). The calculations indicate a four-step mechanism for the catalysis, including oxidative addition of C₆H₅Br, migratory insertion of C₆H₅ to C₂H₄, β-hydride transfer/olefin elimination of product, and catalyst regeneration by removal of HBr. Our calculations demonstrate that Pd π-complexes can be formed with phenyl bromide and ethylene before the oxidative addition occurs. Subsequently, various reaction paths were studied for the oxidative addition of phenyl bromide to palladium complexes, coordinated by phosphine(s) and/or ethylene. Interestingly, all pathways lead to palladium monophosphine as the active catalyst. Careful exploration was made on two possible pathways for the migratory insertion and β-hydride-transfer/olefin elimination: (1) the neutral path with bromide bound to Pd and (2) the cationic path with prior bromide ion dissociation. The neutral path is preferred to the cationic path, especially when the more bulky phosphines such as triphenylphosphine are involved.     

Synthesis and characterization of mono- and dinuclear aryl palladium(II) complexes: oxidative additions of 1,4-dihalogenated benzene or 4,4′-dibromobiphenyl to Pd(PR3)4

Mononuclear palladium(II) complexes 1–12, (C₆H₄X-4)PdX?(PR₃)₂ (X?=?I, Br, or Cl; X??=?I or Br; R?=?Ph, Cy, Et, or Me), were synthesized by oxidative addition of 1,4-dihalogenated benzene to Pd(PR₃)₄; dinuclear palladium(II) complexes 13–15, (Me₃P)₂XPd(C₆H₄-1,4)PdX?(PMe₃)₂ (X, X??=?I or Br), could be obtained only using trimethylphosphine. Another method to prepare 13–15 is via re-oxidative addition of the corresponding mononuclear palladium(II) complexes and Pd(PMe₃)₄. Using 4,4′-dibromobiphenyl as the starting material, the mononuclear palladium(II) complexes [C₆H₄(C₆H₄Br-4)-4]PdBr(PPh₃)₂ (16) and [C₆H₄(C₆H₄Br-4)-4]PdBr(PCy₃)₂ (17) with bulky phosphines could be synthesized at relative low temperature, while dinuclear 18, (Cy₃P)₂BrPd(C₆H₄C₆H₄-4,4?)PdBr(PCy₃)₂, was prepared by bis-oxidative addition at higher temperature. The re-oxidative addition of 16 and Pd(PMe₃)₄ gave dinuclear 19, (Me₃P)₂BrPd(C₆H₄C₆H₄-4,4?)PdBr(PMe₃)₂, accompanying phosphine exchange. X-ray diffraction analysis revealed that formation of dinuclear palladium(II) complexes depends on the reaction temperature, phosphine ligands, and bridging groups.     

The influence of the nature of phosphine ligand on palladium catalysts for cross-coupling of weakly nucleophilic potassium pentafluorophenyltrifluoroborate with ArHal and PhCH2Hal (Hal=Br,Cl)

The influence of the ligand nature on catalytic activity of palladium catalysts for cross-coupling of weakly nucleophilic potassium pentafluorophenyltrifluoroborate, which imitates the behavior of electron-deficient organoboron reagents, with aryl halides, ArHal (Hal=Br, Cl) was studied. The activity of the catalysts generated in situ from Pd(OAc)₂ and appropriate phosphorous containing ligands and the reaction selectivity was found to depend on the nature of bulky phosphines used as ligands. As a result, conditions for involving the electron-deficient organoboron reagent—potassium pentafluorophenyltrifluoroborate—in the palladium-catalyzed cross-coupling with aryl bromides and aryl chlorides were identified. It was demonstrated that the chosen conditions are appropriate for the reaction of K[C₆F₅BF₃] with benzyl chloride and benzyl bromide deriving pentafluorophenylarylmethanes, C₆F₅CH₂Ar.     

Oxidative addition of aryl tosylates to palladium(0) and coupling of unactivated aryl tosylates at room temperature

Aryl tosylates are attractive substrates for Pd-catalyzed cross-coupling reactions, but they are much less reactive than the more commonly used aryl triflates. We report the oxidative addition of aryl tosylates to Pd(PPF-t-Bu)[P(o-tolyl)3] and to Pd(CyPF-t-Bu)[P(o-tolyl)3] at room temperature to produce the corresponding palladium(II) aryl tosylate complexes. In the presence of added bromide ions, arylpalladium(II) bromide complexes were formed. The rate of oxidative addition was accelerated by addition of either coordinating or weakly coordinating anions, and the reactions were faster in more polar solvents. The mild conditions for oxidative addition allowed for the development of Pd-catalyzed Kumada couplings and amination reactions of unactivated aryl tosylates at room temperature. The catalysts for these mild couplings of aryl tosylates were generated from palladium precursors and the sterically hindered Josiphos-type ligands that induced oxidative addition of aryl tosylates to Pd(0) at room temperature.     

Oxidative addition of aryl chlorides to palladium N-heterocyclic carbene complexes and their role in catalytic arylamination

Density functional theory has been used to investigate various solvated species that may be formed from palladium bis N-heterocyclic carbene complexes, [Pd(cyclo-C{NRCH}₂)₂], (PdL₂) in benzene solution. Formation of an η²-arene complex is shown to stabilise a monocarbene species, PdL(η²-C₆H₅X), where the arene is either the solvent or a reacting aryl halide. Oxidative addition of an aryl chloride has been modelled, and the most likely transition state has been established as a PdL(arylchloride) species, with just one carbene ligand coordinated to the palladium. The catalytic cycle for aryl amination has been investigated and the oxidative addition of the aryl halide shown to be the rate determining step. Reductive elimination of the aryl amine has a lower activation energy. Oxidative addition of alkyl halides has been shown to be less favourable because of the absence of an unsaturated group, such as the aryl ring, to bond to the palladium.     

THEORETICAL INVESTIGATION OF KETENE BONDING MODES IN BIS(PHOSPHINE) PALLADIUM KETENE COMPLEXES

Abstract

Theoretical studies were carried out on a series of bis(phosphine) palladium ketene complexes (PR₃)₂Pd(CH₂=C=O), and on the related CH₂=C=O and Pd(PR₃)₂ molecular fragments in order to investigate the electronic structure and the bonding of the ketene ligand to the metal fragment in these complexes. An analysis of the frontier MOs has been performed in order to understand the interactions between the ketene and the metal fragments. The calculated results have shown that the η²-(C,C) mode is preferred over the η²-(C,O) mode by 10–15 kcal/mol in bis(phosphine) palladium ketene complexes. The basicity and bulkiness of the phosphine ligands PR₃ have little effect on the bonding mode in (PR₃)₂Pd(CH₂=C=O) complexes. The most stable structure was calculated to be the η²-(C,C) square planar geometry with the CH₂ group of ketene out of the molecular plane. Comparison and discussion between the two bonding modes were also presented in this paper.     

On the mechanism of the palladium-catalyzed β-arylation of ester enolates

The palladium‐catalyzed β‐arylation of ester enolates with aryl bromides was studied both experimentally and computationally. First, the effect of the ligand on the selectivity of the α/β‐arylation reactions of ortho‐ and meta‐fluorobromobenzene was described. Selective β‐arylation was observed for the reaction of o‐fluorobromobenzene with a range of biarylphosphine ligands, whereas α‐arylation was predominantly observed with m‐fluorobromobenzene for all ligands except DavePhos, which gave an approximate 1:1 mixture of α‐/β‐arylated products. Next, the effect of the substitution pattern of the aryl bromide reactant was studied with DavePhos as the ligand. We showed that electronic factors played a major role in the α/β‐arylation selectivity, with electron‐withdrawing substituents favoring β‐arylation. Kinetic and deuterium‐labeling experiments suggested that the rate‐limiting step of β‐arylation with DavePhos as the ligand was the palladium–enolate‐to‐homoenolate isomerization, which occurs by a β? H‐elimination, olefin‐rotation, and olefin‐insertion sequence. A dimeric oxidative‐addition complex, which was shown to be catalytically competent, was isolated and structurally characterized. A common mechanism for α‐ and β‐arylation was described by DFT calculations. With DavePhos as the ligand, the pathway leading to β‐arylation was kinetically favored over the pathway leading to α‐arylation, with the palladium–enolate‐to‐homoenolate isomerization being the rate‐limiting step of the β‐arylation pathway and the transition state for olefin insertion its highest point. The nature of the rate‐limiting step changed with PCy₃ and PtBu₃ ligands, and with the latter, α‐arylation became kinetically favored. The trend in selectivity observed experimentally with differently substituted aryl bromides agreed well with that observed from the calculations. The presence of electron‐withdrawing groups on these bromides mainly affected the α‐arylation pathway by disfavoring C? C reductive elimination. The higher activity of the ligands of the biaryldialkylphosphine ligands compared to their corresponding trialkylphosphines could be attributed to stabilizing interactions between the biaryl backbone of the ligands and the metal center, thereby preventing deactivation of the β‐arylation pathway.     

Oxidative addition of N-halosuccinimides to palladium(0): the discovery of neutral palladium(II) imidate complexes, which enhance Stille coupling of allylic and benzylic halides

The Stille coupling of organostannanes and organohalides, mediated by air and moisture stable palladium(II) phosphine complexes containing succinimide or phthalimide (imidate) ligands, has been investigated. An efficient synthetic route to several palladium(II) complexes containing succinimide and phthalimide ligands, has been developed. cis-Bromobis(triphenylphosphine)(N-succinimide)palladium(II) [(Ph₃P)₂Pd(N-Succ)Br] is shown to mediate the Stille coupling of allylic and benzylic halides with alkenyl, aryl and allyl stannanes. In competition experiments between 4-nitrobromobenzene and benzyl bromide with a cis-stannylvinyl ester, (Ph₃P)₂Pd(N-Succ)Br preferentially cross-couples benzyl bromide, whereas with other commonly employed precatalysts 4-nitrobromobenzene undergoes preferential cross-coupling. Furthermore, preferential reaction of deactivated benzyl bromides over activated benzyl bromides is observed for the first time. The type of halide and presence of a succinimide ligand are essential for effective Stille coupling. The type of phosphine ligand is also shown to alter the catalytic activity of palladium(II) succinimide complexes.     

Studies of the oxidation of palladium complexes by the advanced oxidation process Pretreatment of model catalysts for precious metal analysis

The advanced oxidation process (AOP) for the pretreatment of model palladium catalysts has been studied. Most standard metal analysis techniques are for metal ions free of organic ligands. Spent palladium catalysts contain organic ligands that need to be removed prior to analysis. AOP uses a combination of hydrogen peroxide and UV light to generate radicals that decompose such ligands, freeing up metals for further analysis. Palladium acetate Pd(OAc)₂, palladium acetylacetonate Pd(acac)₂, and tris(dibenzylideneacetone)dipalladium (Pd₂(dba)₃) were chosen as model precious metal catalysts for investigation. AOP was found to decompose ligands in Pd(OAc)₂, Pd(acac)₂ and give accurate Pd(II) quantification, while ligand decomposition and oxidation of Pd(0) to Pd(II) were demonstrated in treatments involving Pd₂(dba)₃. The effects of solubility of the palladium complexes, continuous addition of H₂O₂ during AOP treatments, sample pH, concentration of H₂O₂, and length of UV irradiation are reported.     

N2Phos – an easily made,highly effective ligand designed for ppm level Pd-catalyzed Suzuki–Miyaura cross couplings in water

A new biaryl phosphine-containing ligand from an active palladium catalyst for ppm level Suzuki–Miyaura couplings, enabled by an aqueous micellar reaction medium. A wide array of functionalized substrates including aryl/heteroaryl bromides are amenable, as are, notably, chlorides. The catalytic system is both general and highly effective at low palladium loadings (1000–2500 ppm or 0.10–0.25 mol%). Density functional theory calculations suggest that greater steric congestion in N₂Phos induces increased steric crowding around the Pd center, helping to destabilize the 2 : 1 ligand–Pd(0) complex more for N₂Phos than for EvanPhos (and less bulky ligands), and thereby favoring formation of the 1 : 1 ligand–Pd^o complex that is more reactive in oxidative addition to aryl chlorides.

A new, biaryl phosphine-containing ligand, N₂Phos, forms a 1 : 1 complex with Pd resulting in an active catalyst at the ppm level for Suzuki–Miyaura couplings in water, enabled by an aqueous micellar medium. Notably, aryl chlorides are shown to be amenable substrates.     

Pd‐Catalyzed Carbonylative α‐Arylation of Aryl Bromides: Scope and Mechanistic Studies

Reaction conditions for the three‐component synthesis of aryl 1,3‐diketones are reported applying the palladium‐catalyzed carbonylative α‐arylation of ketones with aryl bromides. The optimal conditions were found by using a catalytic system derived from [Pd(dba)₂] (dba=dibenzylideneacetone) as the palladium source and 1,3‐bis(diphenylphosphino)propane (DPPP) as the bidentate ligand. These transformations were run in the two‐chamber reactor, COware, applying only 1.5 equivalents of carbon monoxide generated from the CO‐releasing compound, 9‐methylfluorene‐9‐carbonyl chloride (COgen). The methodology proved adaptable to a wide variety of aryl and heteroaryl bromides leading to a diverse range of aryl 1,3‐diketones. A mechanistic investigation of this transformation relying on ³¹P and ¹³C NMR spectroscopy was undertaken to determine the possible catalytic pathway. Our results revealed that the combination of [Pd(dba)₂] and DPPP was only reactive towards 4‐bromoanisole in the presence of the sodium enolate of propiophenone suggesting that a [Pd(dppp)(enolate)] anion was initially generated before the oxidative‐addition step. Subsequent CO insertion into an [Pd(Ar)(dppp)(enolate)] species provided the 1,3‐diketone. These results indicate that a catalytic cycle, different from the classical carbonylation mechanism proposed by Heck, is operating. To investigate the effect of the dba ligand, the Pd⁰ precursor, [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)], was examined. In the presence of DPPP, and in contrast to [Pd(dba)₂], its oxidative addition with 4‐bromoanisole occurred smoothly providing the [PdBr(Ar)(dppp)] complex. After treatment with CO, the acyl complex [Pd(CO)Br(Ar)(dppp)] was generated, however, its treatment with the sodium enolate led exclusively to the acylated enol in high yield. Nevertheless, the carbonylative α‐arylation of 4‐bromoanisole with either catalytic or stoichiometric [Pd(η³‐1‐PhC₃H₄)(η⁵‐C₅H₅)] over a short reaction time, led to the 1,3‐diketone product. Because none of the acylated enol was detected, this implied that a similar mechanistic pathway is operating as that observed for the same transformation with [Pd(dba)₂] as the Pd source.     

Mechanism of the formation of palladium complexes serving as catalysts in hydrogenation reactions : II. Reactivity of palladium phosphine halides towards molecular hydrogen

[(PPh₃)₃(PPh₂)₂Pd₃Cl] Cl, benzene and aniline hydrochloride were isolated as products of the reactions of (PPh₃)₂PdCl₂]₂ or [(PPh₃)PdCl₂]₂ with H₂ in organic amines (Am). Similar products were obtained when (Ph₃P)₂Pd(Ph)Br was treated with H2₃ Both in amines and aromatic solvents. The reaction between H₂ and [(PBu₃)PdCl₂]₂ resulted in the formation of [(PBu₃(PBu_2)PdCl2 ·. 2 Am The kinetic data for H₂ absorption by solutions of palladium(II) complexes are consistent with the heterolytic mechanism of cleavage fo hte HH bond in the coordination sphere of palladium(II); the function of the H⁺ acceptor being performed by the bases (e.g. Am or Ph). The reaction between the palladium complexes and H₂ is autocatalytic. Reduction of the initial Pd^II complexes leads to lower oxidation state palladium complexes, which catalyse the reduction of Pd^II complexes. In the coordination sphere of the lower oxidation state palladium complexes, the oxidative addition of PR₃ to Pd takes place with formation of compounds containing a Pd-R bond. It is the reaction between these complexes and H₂ that yields palladium compounds with PR₂ ligands.     

Dinuclear Pd(II) and Pt(II) compounds bearing a bridging π-conjugated moiety: synthesis and reactivity toward alkynes and isocyanides

Abstract

Dinuclear Pd(II) halides that contain bridging π-conjugated groups, trans,trans-[(PR₃)₂(X)Pd–Y–Pd(X)(PR₃)₂] (X?=?Br; YH₂ = terpyridine, fluorenone, benzil, benzthiadiazole), were prepared by the oxidative addition of corresponding dihalo π-conjugated reagents to [Pd(styrene)(PR₃)₂]. Similar reactions involving dihalobenzil, dihalobithiophene, or dihaloterthiophene afforded dinuclear Pt(II) halides containing bridging π-conjugated groups. Additionally, when the dihalosilole derivatives {2,5-dibromo-1,1-dimethyl (or diphenyl)-3,4-diphenylsilole} reacted with [Pd(styrene)(PR₃)₂], mono or dinuclear Pd(II) complexes bearing a dimethyl (or diphenyl)-3,4-diphenylsilole group were obtained. π-Conjugation extension reactions of dinuclear bithiophene-bridged Pd(II) halides with HC≡C–R {R?=?SiPh₃, C(O)OMe} in the presence of CuI and HNEt₂ led to the unexpected formation of bis(acetylide) Pd(II) complexes of the form, [Pd(C≡C–R)₂(PR₃)₂] and bithiophene. In contrast, treatment of the dinuclear Pd(II) halides with two equiv of organic isocyanide resulted in isocyanide insertion into the Pd???C bonds to afford π-conjugation-extended dinuclear Pd(II) compounds bearing a π-conjugated moiety.     

Gold/Palladium Alloy for Carbon–Halogen Bond Activation: An Unprecedented Halide Dependence

New catalytic activity of gold/palladium alloy nanoclusters (NCs) for carbon–halogen bond activation is demonstrated. In the case of an aryl chloride, the inclusion of gold in a bimetallic catalyst is indispensable to achieve the coupling reactions. Gold has the unique effect of stabilizing palladium, such that Pd²⁺ leached from clusters by means of spillover of chloride during oxidative addition. The thus‐formed spillover intermediate further reacts heterogeneously in both Ullmann and Suzuki‐type coupling reactions through a new type of mechanism. In the case of an aryl bromide, Ullmann coupling occurs through the spillover of bromide, similar to that of aryl chloride. However, a significant fraction of palladium also leached, which diminished the Ullmann coupling activity of the aryl bromide and, as a result, the order of reactivity was ArCl>ArBr. With regard to the activation of the C?Br bond towards a Suzuki‐type reaction, the inclusion of a higher gold content in gold/palladium clusters stabilized palladium to prevent the leaching of Pd²⁺ from the clusters by means of spillover of bromide. The spillover intermediate reacts heterogeneously with PhB(OH)₂, palladium‐rich gold/palladium, or pure palladium clusters; the oxidative addition of ArBr favors the extraction of palladium from the clusters, yielding Pd²⁺ intermediates. The extracted intermediates react homogenously (Pd^2+/Pd⁰ catalysis) with PhB(OH)₂, which results in the higher selectivity of the cross‐coupling product. An initial step to observe such unprecedented halide dependency, together with the dynamic behavior of palladium on the surface of gold is the oxidative addition of Ar?X. A detailed insight into the first oxidative addition process was also examined by quantum chemical calculations.     

Palladium Chemistry Related to Benzyl Bromide Carbonylation: Mechanistic Studies

Summary.  Palladium(II) complexes of the general formula PdCl₂ (PR₃)₂ with PR₃ = { P(OPh)₃}, P(O-4-MeC₆H₄)₃, P(O-2-MeC₆H₄)₃, and PPh₂(OBu) were reduced by NEt₃ in chloroform or benzene to Pd(0) complexes Pd(PR₃)₄ and Pd(PR₃)_x(NEt₃) _4−x . The same reaction performed in the presence of air gave CH₃CHO or CH₃CH₂CHO when NPr₃ was used instead of NEt₃. Pd(P(OPh)₃)₄ reacted with benzyl bromide affording the oxidative addition product cis-PdBr(CH₂Ph)(P(OPh)₃)₂. The reaction of PdCl₂(P(OPh)₃)₂ with benzyl bromide was observed only in the presence of NEt₃, and a dimeric complex of [PdBr(CH₂Ph)(P(OPh)₃)]₂ was identified as the reaction product. Both benzyl complexes reacted fast with CO (1 atm) to form acyl complexes exhibiting ν(CO) bands at 1709 and 1650 cm⁻¹.     

Catalytic activity of η-(olefin)palladium(0) complexes with iminophosphine ligands in the Suzuki-Miyaura reaction. Role of the olefin in the catalyst stabilization

The catalytic activity of η²-(olefin)palladium(0)(iminophosphine) complexes in the Suzuki-Miyaura coupling is strongly dependent on the reaction conditions and on the nature of the ligands. The reaction is at the best carried out in aromatic solvents in the presence of K₂CO₃ at 90-110 °C. Higher reaction rates are obtained when the R substituent on the N-imino group is an aromatic group of low steric hindrance and the olefin is a moderate π-accepting ligand such as dimethyl fumarate. At temperatures lower than 90 °C, a self-catalyzed process leading to catalyst deactivation becomes predominant. Preliminary mechanistic investigations indicate that the oxidative addition of the aryl bromide to a Pd(0) species is the rate determining step in the catalytic cycle and that the olefin plays a key role in catalyst stabilization. Systems in situ prepared by mixing Pd(OAc)₂ or Pd(dba)₂ with 1 equiv of iminophosphine appear substantially less active than the preformed catalysts.     

Palladium Chemistry Related to Benzyl Bromide Carbonylation: Mechanistic Studies

 Palladium(II) complexes of the general formula PdCl₂ (PR₃)₂ with PR₃ = { P(OPh)₃}, P(O-4-MeC₆H₄)₃, P(O-2-MeC₆H₄)₃, and PPh₂(OBu) were reduced by NEt₃ in chloroform or benzene to Pd(0) complexes Pd(PR₃)₄ and Pd(PR₃)_x(NEt₃) _4−x . The same reaction performed in the presence of air gave CH₃CHO or CH₃CH₂CHO when NPr₃ was used instead of NEt₃. Pd(P(OPh)₃)₄ reacted with benzyl bromide affording the oxidative addition product cis-PdBr(CH₂Ph)(P(OPh)₃)₂. The reaction of PdCl₂(P(OPh)₃)₂ with benzyl bromide was observed only in the presence of NEt₃, and a dimeric complex of [PdBr(CH₂Ph)(P(OPh)₃)]₂ was identified as the reaction product. Both benzyl complexes reacted fast with CO (1 atm) to form acyl complexes exhibiting ν(CO) bands at 1709 and 1650 cm⁻¹.     

Di-t-butyl(ferrocenylmethyl)phosphine: air-stability, structural characterization, coordination chemistry, and application to palladium-catalyzed cross-coupling reactions

Di-t-butyl(ferrocenylmethyl)phosphine (1) has been isolated and structurally characterized. This ligand was found to be reasonably air stable as a solid and it has been shown to possess electron donating ability similar to that of tri-i-propylphosphine. A palladium catalyst bearing this ligand performed room temperature Suzuki-Miyaura coupling reactions with aryl bromides. Modest Heck coupling reactivity with aryl bromides was also observed at 100 °C. Complexation of 1 with Pd₂(dba)₃ led to formation of (1)₂Pd⁰. Addition of 4-bromoanisole to solutions containing both 1 and Pd₂(dba)₃ led to formation of an oxidative addition product when 1:Pd ratios were ?1. With a 2:1 ratio of 1:Pd, monophosphine complex formation and oxidative addition were significantly inhibited.