Six-membered Palladacycle Research Articles

Mononuclear palladacycles of N,N′-diaryl-2-iminoisoindolines

Abstract Reaction of acetato-bridged dinuclear palladacycles, [Pd(iminoisoindoline)(μ-OAc)]2, with stoichiometric amounts of PR3 (where R = Ph or Cy) resulted in formation of the corresponding mononuclear phosphine-ligated, six-membered palladacycles with the general formula [Pd(iminoisoindoline)(OAc)PR3]. The analogous chloride complexes were synthesized by reaction of [Pd(iminoisoindoline)(μ-OAc)]2 with LiCl in acetone followed by addition of phosphine to afford the monomeric derivatives [Pd(iminoisoindoline)(Cl)PR3]. Representative crystal structures of both types of mononuclear palladacycles confirmed the mononuclear nature of the complexes and showed a trans-arrangement of the phosphine ligand to the heterocyclic imine-nitrogen of the palladacycles.

Synthesis and structural characterization of enantiopure exo and endo six-membered oxazoline-derived palladacycles

Direct palladation of ( S)-4-benzyl-2-methyl-2-oxazoline ( 1) and ( S)-2-benzyl-4- tert-butyl-2-oxazoline ( 2) using Pd(OAc) 2 in MeCN afforded the corresponding μ-acetato-dimeric complexes with six-membered exo and endo palladacycles, respectively. The same complexes were obtained by reacting coordination complexes Pd( 1) 2(OAc) 2 and Pd( 2) 2(OAc) 2 with Pd(OAc) 2 in MeCN. Metalation of ( S)-2,4-dibenzyl-2-oxazoline ( 3) with Pd(OAc) 2 in AcOH, MeCN or CH 2Cl 2 resulted in the regiospecific formation of the six-membered endo palladacycle. The obtained μ-acetato-dimeric complexes were converted to the corresponding μ-chloro-dimeric derivatives 7, 11 and 13 by treatment with LiCl in acetone. The mononuclear PPh 3 adducts 8, 12 and 14 were obtained by reacting dimers 7, 11 and 13 with PPh 3 in benzene. NMR spectroscopy data supported the proposed structures of all complexes and suggested that exo and endo palladacycles in 8 and 12 have rigid boat conformations in CHCl 3. The X-ray crystal structures of the μ-acetato dimer 6 with the exo palladacycle and the PPh 3 adduct 14 with the endo metalacycle revealed boat conformation of both palladacycles and chiral twisted conformations δ( S) and λ( S), respectively, of the oxazoline rings in the solid state.

Palladium complexes of N-aryl-2-pyridylamines

Palladium complexes of N-phenyl-2-pyridylamine ( 4) and dipyridylamine substrates ( 7, 11) have been studied. Due to the coordination ability of the pyridine-nitrogen atoms, the pyridyl substrates, 4, 7, 11 were subjected to Pd(OAc) 2 complexations and a number of N-aryl-2-pyridylamine Pd complexes ( 13– 17) were isolated and characterised, in particular by NMR and ESI-MS. A new method for the preparation of the acetato-bridged six-membered ring palladacycle complex ( 13) of 4 is reported. The dipyridyl amines 7, 11 formed cis/trans bis-dentate acetato-bridged dimeric Pd 2Lig 2(OAc) 2 ( 14a, b/ 16a, b) and Pd 3Lig 2(OAc) 4 complexes ( 15a, b/ 17a, b). The N-aryl-2-pyridylamine substrates ( 4, 7, 11) were prepared by oxidative nucleophilic substitution, by 1,3-cycloaddition reaction or by Buchwald amination.

Nitrogen-Assisted Ortho Lithiation: One-Pot Synthesis of New Classes of Bidentate and Tetradentate Mixed P∼N Ligands

Metalation of one aryl group of Ph3PNR (R = methyl, isopropyl) affords an easy access to the corresponding lithium salt, which in turn reacts with chlorophosphines R2PCl (R = Ph, i-Pr) to yield a new class of bidentate ligand featuring one iminophosphorane and one phosphino group. Reaction of the phenyl derivative with [Pd(COD)Cl2] yielded the expected six-membered palladacycle complex, which was structurally characterized. Extension of this new synthetic methodology toward the elaboration of enantiomerically pure tetradentate ligands has also been achieved.

Reactivity of Ortho-Palladated Phenol Derivatives with Unsaturated Molecules. Carbodiimide Insertion into a C−Pd Bond and/or O−H Phenol Addition to a Carbodiimide

The first examples of the insertion of carbodiimides into a late-transition-metal−carbon bond and the unprecedented intramolecular addition of a phenol to a carbodiimide have allowed the synthesis of O,N and O,C six-membered palladacycles.

Palladium-Catalyzed Heteroannulation of Cyclic Alkenes by Functionally Substituted Aryl Iodides.

Indolines and 2,3-dihydrobenzofurans are produced in good yields by the Pd(0)-catalyzed heteroannulation of cyclic and bicyclic alkenes by o-amino- and o-hydroxyaryl iodides. These processes are only successful with cyclic olefins in which the key alkylpalladium intermediate cannot undergo facile palladium β-hydride elimination. These reactions appear to involve: (1) oxidative addition of the aryl iodide to the palladium catalyst, (2) arylpalladation of the olefin, (3) possible coordination of the internal nucleophile to the palladium, (4) formation of a six-membered palladacycle, and (5) reductive elimination of the organopalladium intermediate to give the heteroannulation product and regenerate Pd(0).

Enantioselective Synthesis of Six-Membered Palladacycles Having Metal-Bound Stereogenic Carbons: Isolation and Reactivity of Palladacycles Containing Readily Accessible β-Hydrogens

Five enantiopure palladacycles containing palladium bonded to a stereogenic carbon and an N-coordinated oxindole were synthesized by the reaction of alkenyl aryl triflates 2 and 9 with Pd(0) bisphosphine complexes. Two palladacyclic complexes, 3beta and 10alpha, were characterized by single-crystal X-ray crystallography. The reactivity of neutral palladacycles 3beta and 10beta was studied in detail. These unusual palladium alkyls, which have three accessible beta-hydrogens, are thermally stable at temperatures as high as 120 degrees C. At higher temperature, or at room temperature in the presence of weak acids, these complexes epimerize at the stereogenic carbon bonded to palladium. The mechanism of the acid-promoted epimerization was studied in detail. During this epimerization, cationic palladium alkyls 13/14 and 33 and cationic palladium hydride alkene complexes 31 and 32 are in rapid equilibrium.

Palladium-catalyzed heteroannulation of cyclic alkenes by functionally substituted aryl iodides

Indolines and 2,3-dihydrobenzofurans are produced in good yields by the Pd(0)-catalyzed heteroannulation of cyclic and bicyclic alkenes by o-amino- and o-hydroxyaryl iodides. These processes are only successful with cyclic olefins in which the key alkylpalladium intermediate cannot undergo facile palladium β-hydride elimination. These reactions appear to involve: (1) oxidative addition of the aryl iodide to the palladium catalyst, (2) arylpalladation of the olefin, (3) possible coordination of the internal nucleophile to the palladium, (4) formation of a six-membered palladacycle, and (5) reductive elimination of the organopalladium intermediate to give the heteroannulation product and regenerate Pd(0).

Ortho-Metalated Primary Amines. 6.1 The First Synthesis of Six-Membered Palladacycles from Primary Amines Containing Electron-Withdrawing Substituents: End of the Limiting Rules of Cope and Friedrich on Cyclopalladation of Benzyl- and Phenethylamines

When H2N(CH2)2C6H4OMe-4 (RNH2) and Pd(OAc)2 are reacted in a 2:1 molar ratio, the complex [Pd(OAc)2(NH2R)2] (1) is obtained. Complex 1 reacts with 1 equiv of Pd(OAc)2 to give the dinuclear complex [Pd(OAc)(μ-OAc)(NH2R)]2 (2). When complex 2 is heated in acetonitrile at 80 °C, the ortho-metalated complex [Pd{C6H3(CH2)2NH2-2-OMe-5-κ2C,N}(μ-OAc)]2 (3a) is obtained. Complex 3a is also prepared by refluxing RNH2 and Pd(OAc)2 in a 1:1 molar ratio in acetonitrile. Complex 3a reacts with NaBr or LiCl to afford the complexes [Pd{C6H3(CH2)2NH2-2-OMe-5-κ2C,N}(μ-X)]2 (X = Cl (3b), Br (3c)). PPh3 splits the acetate or halide bridge in complex 3a or 3b to give [Pd{C6H3(CH2)2NH2-2-OMe-5-κ2C,N}X(PPh3)] (X = OAc (4a), Cl (4b)). With other 2-(phenyl)ethylamines or the corresponding chlorhydrates, Pd(OAc)2, and PPh3 as starting materials (molar ratio 1:1:1), the complexes [Pd{C6H3(CH2)2NH2-2-X-5-κ2C,N}Y(PPh3)] (Y = OAc, X = Cl (5), F (6); Y = Cl, X = NO2 (7)) have been isolated and fully characterized. These compounds include the first six-membered palladacycles synthesized from primary phenethylamines with electron-withdrawing substituents on the aryl ring. The crystal structures of 3a and 5 have been determined by X-ray diffraction.

Novel unsymmetrical PCP′ pincer ligands and their palladium(II) complexes

Synthetic routes towards novel PCP′ pincer ligands were devised. Ligand 1-(Pr 2 i POCH 2)-3-(Bu 2 t PCH 2)(C 6H 4) is prepared in a three step synthesis from 1,3-benzenedimethanol and 1-(Pr 2 i PO)-3-(Bu 2 t PCH 2)(C 6H 4) is accessible in three steps from 3-hydroxybenzylalcohol. Both their palladium(II) complexes are prepared in good yields but are distinctly different since [PdCl{(C 6H 3)(OPPr 2 i )-2-(CH 2PBu 2 t )-6}] possesses two five-membered palladacycles, whereas [PdCl{(C 6H 3)(CH 2PBu 2 t )-2-(CH 2OPPr 2 i )-6}] is unusual for a pincer complex in that it contains both five- and six-membered palladacycles. Both compounds also represent the first examples of pincer complexes where one donor is a phosphinite and the other is a phosphine. The X-ray structures of these complexes were solved and are discussed. The data reveal that an increase in the metallacycle ring-size leads to changes in bond lengths, but more importantly to significant increases in the bond angles.

NC-palladacycles as highly effective cheap precursors for the phosphine-free Heck reactions

Eight cyclopalladated complexes of the formula [Pd 2(μ-L) 2(NC) 2] (L=OAc, Cl; NC=cyclometalled N donor: o-(2-pyridyl)phenyl, o-(2-pyridyloxy)phenyl, o-(2-pyridylmethyl)phenyl, o-( N, N-dimethylaminomethyl)phenyl, 8-quinolylmethyl and others) and a six-membered palladacycle with OC coordination (ligand related to 2-acetoamido-4-nitrophenyl), are highly efficient catalysts for the Heck arylation of olefins (styrene, ethyl acrylate) by aryl halides (iodobenzene, bromobenzene, 4-bromoacetophenone). These catalysts are air stable, easy to obtain from a vast number of readily available nitrogen containing molecules, are generally much cheaper than phosphine-ligated palladacycles, but as or more efficient than the latter. Turnover numbers ( ton) of up to 4 100 000 and turnover frequencies ( tof) up to 530 000 are achieved in the reaction of iodobenzene with ethyl acrylate. Bromobenzene undergoes the Heck reaction ( ton=400–700; tof=5–30) in the presence of the promoter additive Bu 4NBr. The palladacycles are likely to operate in a common phosphine-free Pd(0)/Pd(II) catalytic cycle, while the differences between various types of palladacycle precursors are accounted for by the kinetics of the catalyst preactivation step.

CH activation at the 3-position of pentane chains to form [NC(sp 3)N] − complexes incorporating six-membered pallada(II)cyclic rings and pyridine, pyrazole and N-methylimidazole donor groups. Structural studies and comparison with [NC(sp 2)N] − complexes

Alkylpalladium complexes bearing the [NC(sp 3)N] − donor motif and two six-membered palladacycles are generated on activation of C(sp 3)H bonds by palladium(II) acetate. Cyclopalladation of the new reagents 1,5-bis(pyridin-2-yl)pentane [CH 2(CH 2CH 2py) 2] ( 1), 1,5-bis(pyrazol-1-yl)pentane [CH 2(CH 2CH 2pz) 2] ( 2) and 1,5-bis( N-methylimidazol-2-yl)pentane [CH 2(CH 2CH 2mim) 2] ( 3), followed by reaction with lithium chloride afford the palladium(II) complexes Pd{CH(CH 2CH 2py) 2- N, C, N′}Cl ( 5), Pd{CH(CH 2CH 2pz) 2- N, C, N′}Cl ( 6) and Pd{CH(CH 2CH 2mim) 2- N, C, N′}Cl ( 7), respectively. Abstraction of chloride from 6 with AgBF 4 in acetone generates the cationic acetone complex [Pd{CH(CH 2CH 2pz) 2- N, C, N′}(OCMe 2)][BF 4] ( 8). X-ray crystal structures of 5, 6 and 8 reveal tridentate [NC(sp 3)N′] − intramolecular coordination of the ligands. These structures are compared with that of a closely related [NC(sp 2)N] − system in [Pd{2,6-(pzCH 2) 2C 6H 3- N, C, N′}(OH 2)][BF 4] ( 10), obtained on cyclopalladation of 1,3-bis(pyrazol-1-ylmethyl)benzene followed by derivatisation to form the aqua complex.

Five- and six-membered palladacycles derived from [(η 5-C 5H 5)Fe{(η 5-C 5H 4)-CH=N-(C 6H 4-2-C 6H 5)}

The reaction of the novel ferrocenyl Schiff base: [(η 5-C 5H 5)Fe{(η 5-C 5H 4)-CH=N-(C 6H 4-2-C 6H 5)}] ( 1) with Na 2[PdCl 4] and Na(CH 3COO)·3H 2O in a 1:1:1 molar ratio in methanol is reported. In this reaction two different di-μ-chloro-bridged cyclopalladated complexes: [Pd{[(η 5-C 5H 3)-CH=N-(C 6H 4-2-C 6H 5)]Fe(η 5-C 5H 5)}(μ-Cl)] 2 ( 2a) and [Pd{[(C 6H 4-2-C 6H 4)-N=CH-(η 5-C 5H 4)]Fe(η 5-C 5H 5)}(μ-Cl)] 2 ( 2b) can be formed depending on the experimental conditions. Compounds 2a and 2b, which differ in the nature of the metallated carbon atom (C sp 2,ferrocene or C sp 2,biphenyl , respectively), undergo cleavage of the ‘Pd(μ-Cl) 2Pd’ bridges in the presence of thallium (I) acetylacetonate, deuterated pyridine or triphenylphosphine giving the monomeric derivatives: [Pd(C ∧N)(acac)] ( 3a, 3b) and [Pd(C ∧N)Cl(L)] {with L=py- d 5 ( 4a, 4b), PPh 3( 5a, 5b)}. The reactions of 2 with 1,2-bis(diphenylphosphino)ethane (dppe) reveal that the two isomers ( 2a and 2b) exhibit different reactivity versus dppe. These results have been interpreted on the basis of steric effects.

Aliphatic cyclopalladation. Crystal structure of

Cyclopalladation of 2,6-dimethyl- N-(2′,4′,6′-trimethyl-benzylidene) aniline ( 1) in refluxing acetic acid followed by reaction with excess LiX affords the dinuclear ▪ ( X = Br ( 2 a ), I ( 2 b )), which contain six-membered palladacycles. Compound 2a crystallizes in the monoclinic space group C2/ c, with a = 18.879(3), b = 19.044(3), c = 10.169(2) A ̊ , β = 117.04(2)° and Z = 4 . The overall geometry of 2a includes a planar Pd 2Br 2 rhomboid and a syn arrangement of the free phenyl rings of cyclometallated ligands.

Cyclometallated compounds of Pd(II) with pyrazole derivatives. Unusual double palladation of diarylbis( N-pyrazolyl)methanes

Reaction between palladium(II) acetate and bis(arylpyrazolyl)methanes with two aryl groups 3-RC 6H 4, 2a–c or the 3-RC 6H 4 and one 5-RC 6H 4, 3a–c (R = H, 3′-OCH 3, or 2′-Br) 3a–c in refluxing acetic acid, affords metallacycles of stoichiometry LPd 2, with two different five- and six-membered palladacycles, one containing an aromatic carbon-metal bond and the other a pyrazole carbon-metal bond, repectively. Treatment of acetate-bridged palladacycles 5a–c and 6–c with Tl(acac) yields monomeric products 7a–c and 8a–c of formula [LPd 2(acac) 2]. The products have been characterized by elemental analysis (C, H, N) and NMR spectroscopy. The preferential formation of a six-membered metallacycle in the first step of the reaction is discussed.

Six-membered palladacycles formed by cyclopalladation of 2-anilinopyridine derivatives

Reactions of palladium(II) chloride with 2-substituted pyridines (HL), 2-(p-R-C6H4-Y)-C5H4N (R = H, CH3, Cl; Y= NH, NCH3, O, S, CH2) form 1∶2 complexestrans-[PdCl2(HL)2], HL being coordinated through a pyridine-N atom. When the ratio PdCl2/HL = 1/1, the pyridine derivatives with Y = NH are cyclopalladated to form another type of complexes [PdClL]2. In [PdClL]2 the deprotonated ligand L is chelated through pyridine-N and phenylortho-C atoms forming an unusual six-membered palladacycle. Like other cyclopalladated complexes containing a five-membered palladacycle, [PdClL]2 reacts with pyridine (py) to form adducts [PdClL(py)]. [Pd(acac)L] and [Pd(dtc)L] were also prepared and characterized (acac=acetylacetonate and dtc =N,Ndimethyldithiocarbamate ion).