Allylpalladium Complexes Research Articles

Molecular Library Obtained by Allene Insertion into the Pd−C Bond of Cyclopalladated Complexes: Biological and Pharmacological Evaluation

AbstractA minilibrary of cationic N‐heterocycles has been prepared and evaluated. The potential for the preparation was a result of the high versatility of palladium‐mediated chemistry. The synthesis of the novel molecules was based on intramolecular quaternization of tertiary amine attached allylpalladium complexes. The steric and electronic factors of the reaction are discussed. The structures of the synthesized molecules made them candidates for precise biological and pharmacological evaluations. Of the various N‐heterocyclic compounds, 2,2‐dimethyl‐3‐methylenenaphtho[def]quinolizinium showed antibacterial activity at micromolar concentrations. This compound also proved to be a nanomolar competitive antagonist for the channel site of the nicotinic acetylcholine receptor. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Reactions of free radicals with η 3-allylpalladium(II) complexes: cyclohexyl radicals

Allyl palladium complexes of the types [(η 3-allyl)PdCl] 2, (η 3-allyl)PdCl(PPh 3) and [(η 3-allyl)Pd(PPh 3) 2]Cl (allyl=C 3H 5, 1-MeC 3H 4, 2-MeC 3H 4, 1-PhC 3H 4, 2-PhC 3H 4) react with cyclohexyl radicals derived from the visible light photolysis of (c-hex)Co(DMG) 2(py). The reactions proceed via initial attack of the free radical at the metal center, followed by β-hydrogen elimination and subsequent reductive elimination of propene, 1-butene, isobutene, 3-phenylpropene and 2-phenylpropene, respectively. The 3-phenylpropene can be catalytically isomerized to the thermodynamically more stable 1-phenylpropene by either palladium metal or palladium(0) products, but the formation of 1-butene and 3-phenylpropene as primary products is unusual. A mechanism, differing in many ways from that proposed previously for analogous reactions of phenyl and trityl radicals, is proposed for the overall reaction and supported by use of the labeled cobaloxime, (2,2,6,6-D 4-c-hex)Co(DMG) 2(py).

Mechanistic aspects of C–C bond formation involving allylpalladium complexes: the role of computational studies

Computational studies have proven to be a useful tool for the study of mechanistic aspects of reactions involving allylpalladium complexes as intermediates. Previous work on the mechanisms of reactions involving such intermediates is discussed, as well as our contributions in this field. Thus, we report on the study of the mechanism of the insertion of alkenes into allylpalladium intermediates involved in the Oppolzer carbocyclization, the possible reaction pathways for the allylstannylation of alkynes, and the carbon-carbon reductive elimination from bis(allyl)palladium complexes. Structures for elusive intermediates, transition states, and even a novel mechanism for the reductive elimination in allyl-allyl cross-coupling reactions have been proposed.

A New Access to 3,5‐Disubstituted Piperazinones via Pd(0)‐Catalyzed Amination.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Diastereomerism in palladium(II) allyl complexes of P,P-, P,S- and S,S-donor ligands, Ph 2P(E)N(R)P(E′)Ph 2 [R=CHMe 2 or ( S)-*CHMePh; E=E′=lone pair or S]: solution behaviour, X-ray crystal structure and catalytic allylic alkylation reactions

The reactions of achiral homodonor diphosphazane ligands, Ph 2P(E)N(CHMe 2)P(E′)Ph 2 [E=E′=lone pair ( 1) or S ( 2)] with the chloro bridged palladium dimers, [Pd(η 3-1,3-R′-R″C 3H 3)(μ-Cl)] 2 (R′, R″=H, Me or Ph) in the presence of NH 4PF 6 give cationic η 3-allyl palladium complexes, [Pd(η 3-1,3-R′-R″C 3H 3)(L-L′)]. Each of these complexes exists as single species in solution except the 1,3-dimethyl allyl complex, which exists as two isomers ( syn/ syn- and syn/ anti-) in solution. Analogous allyl palladium complexes derived from the chiral homodonor diphosphazane ligand, Ph 2PN(( S)-*CHMePh)PPh 2 ( 3) exist as a single species when the allyl moiety is symmetrical (R′=R″=H or Ph) but a 1:1 mixture of two different face-coordinated diastereomers if the allyl moiety is unsymmetrically substituted (R′≠R″). The 1,3-dimethyl–allyl complex, [Pd(η 3-1,3-Me 2C 3H 3){Ph 2PN(( S)-*CHMePh)PPh 2- k 2 P, P}]PF 6 ( 20) exists as a mixture of two isomers. The major isomer ( 20a) has the syn/ syn-allylic arrangement while the minor isomer ( 20b) has the syn/ anti configuration. These isomers ( 20a and 20b) equilibrate in solution via a syn– anti isomerisation pathway. Achiral and chiral heterodonor monosulphide ligands, Ph 2P(S)N(R)PPh 2 [R=CHMe 2 ( 4), ( S)-*CHMePh( 5)] give rise to allyl palladium complexes which exist as several isomers in solution. The presence of a chiral centre in the ligand 5 enables the observation of two NMR distinguishable face-coordinated diastereomers for its allyl complexes. However, the solid state structure of [Pd(η 3-C 3H 5){Ph 2P(S)N(( S)-*CHMePh)PPh 2- k 2 P, S}]PF 6 ( 27) shows the presence of only one diastereomer. Two-dimensional (2D) phase sensitive 1H 1H NOESY and ROESY measurements indicate that the monosulphide complexes undergo exchange by the opening of the η 3-allyl group selectively at the trans position with respect to the greater π-acceptor phosphorus centre to generate a η 1-bonded intermediate. Preliminary studies on the use of the ligands 1– 5 for catalytic allylic alkylation reactions of an unsymmetrically substituted substrate, ( E)-1-phenyl-2-propenyl-acetate are reported.

New direction in organopalladium chemistry: structure and reactivity of unsaturated hydrocarbon ligands bound to multipalladium units.

Examination of the manner of interaction between Pd(0) and allylpalladium(II) complexes, both being involved as key intermediates in Pd-catalyzed allylic coupling, led us to discover a new role for such combinations in affecting the stereochemistry of the transformations. A similar investigation of the system involving Pd(0) and allenyl/propargyl complexes of Pd(II) led to the discovery of dinuclear Pd(I)bond;Pd(I) complexes containing bridging allenyl/propargyl ligands, which exhibited novel structural and reactivity aspects of great synthetic significance. A systematic comparison was made between the structure, stability, and reactivity of allyl and allenyl/propargyl ligands in dinuclear complexes and those in mononuclear counterparts. On the basis of MO calculations, coordination behavior specific to the ligands of the dinuclear complex is attributed to the occurrence of the back-donating interaction from filled Pdbond;Pd bonding orbitals to vacant ligand pi* orbitals. Similar bonding features are the origin of the ready synthesis of novel one-dimensional sandwich complexes composed of conjugated polyene ligands and linear polypalladium chains. A substitutionally labile dipalladium complex reacts with an equimolar amount of trienes or alkynes to give formal [4pi + 2sigma] or [2pi + 2sigma] adducts, respectively, which undergo further unique transformations with additional unsaturated substrates.

Backbone Effect of MAP Ligands on Their Coordination Patterns with Palladium(II)

The allylpalladium complexes (R)-4 and (R)-5 of chiral P, N auxiliaries (R)-1 and (R)-3 have been prepared. The X-ray structures of the complexes have been determined, and the results clearly show that two different chelating modes (P, N vs P, Cσ) are involved. The coordination patterns of donating atoms were dependent on the binaphthyl backbone of the chiral ligands. The isodesymmetrization revealed that the reaction occurs without regiochemical memory effect, which demonstrates a full dynamic equilibrium has been attained before nucleophilic attack. These results, combined with the solution behaviors of the complexes revealed by NMR studies, provided a further understanding of their asymmetric induction in allylic substitution of 1,3-diphenylprop-2-en-1-yl acetate with dimethyl malonate. The dramatic enhancement of enantioselectivity with (R)-3 as a chiral inducer can be related to the different chelating mode of catalyst (R)-5 from that of (R)-4. The change of bite angles of the two donors with palladium in the complexes (R)-4 and (R)-5 was considered as a direct reason for their dramatically different enanatioselectivities in the catalysis.

A Mild and Efficient Method for the Stereoselective Formation of C—O Bonds: Palladium‐Catalyzed Allylic Etherification Using Zinc(II) Alkoxides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Palladium(ii) allyl complexes of chiral diphosphazane ligands: ambident coordination behaviour and stereodynamic studies in solutionOrganometallic chemistry of diphosphazanes. Part 17.11g.Electronic supplementary information (ESI) available: NMR and crystallographic data, stereodynamic behaviour schemes. See http://www.rsc.org/suppdata/dt/b2/b207447h/

The chemistry of η3-allyl palladium complexes of pyrazolyl substituted diphosphazane ligands, Ph2P(E)N(R)PPh(N2C3HMe2-3,5) [E = lone pair, R = CHMe2 (1); E = lone pair, R = (S)-*CHMePh (2); E = S, R = CHMe2 (3)] bearing a stereogenic phosphorus centre has been investigated and the complexes, [Pd(η3-1,3-R′2-C3H3){κ2-Ph2P(E)N(R)PPh(N2C3HMe2-3,5)}](PF6) [E = lone pair or sulfur; R = CHMe2 or (S)-*CHMePh; R′ = H, Me or Ph; 4–13], have been isolated. Detailed NMR studies reveal that these complexes exist as a mixture of isomers in solution. The structures of four complexes have been determined by X-ray crystallography and only one isomer is observed in the solid state in each case. The allyl complexes formed by ligands 1 and 2 display a P,N-coordination mode except the 1,3-diphenyl allyl complex, [Pd(η3-1,3-Ph2-C3H3){κ2-Ph2PN(CHMe2)PPh(N2C3HMe2-3,5)}](PF6) (8), which shows the presence of both P,N- and P,P-coordinated isomers in solution with the former predominating. On the other hand, the complexes bearing the diphosphazane monosulfide ligand 3 display P,S-coordination. Two-dimensional phase sensitive 1H–1H NOESY and ROESY measurements indicate that several of the above allyl palladium complexes undergo syn–anti and/or cis–trans isomerization in solution through an η1-intermediate formed by the opening of the η3-allyl group selectively at the trans position with respect to the phosphorus centre. Preliminary investigations show that the diphosphazanes (1 and (SR)-2) function as efficient auxiliary ligands for catalytic allylic alkylation reactions but lead to only low levels of enantiomeric excess.

Reactions of free radicals with η3-allylpalladium(ii) complexes: phenyl and trityl radicals

The compounds (η3-allyl)PdCl(PPh3) and [(η3-allyl)Pd(PPh3)2]Cl react with phenyl and trityl radicals generated from the thermal decomposition of phenylazotriphenylmethane (PhNNCPh3, PAT) in benzene at 60 °C. The products are the palladium phenyl compounds, [PdPhCl(PPh3)]2 and trans-PdPhCl(PPh3)2, respectively, and 4,4,4-triphenyl-1-butene, the latter being the result of coupling of the trityl radical with the allyl ligands. In contrast, [(η3-allyl)PdCl]2 reacts with phenyl and trityl radicals under the same conditions to form palladium metal, trityl chloride and 3-phenylpropene, which is subsequently catalytically isomerized to 1-phenylpropene. These disparate results are interpreted in all cases in terms of initial attack by phenyl radicals on the palladium(II) to give phenyl–palladium(III) intermediates, and it is the secondary reactions, influenced by the presence or absence of coordinated PPh3 ligands, which provide variety in the products. The reaction of [(4-methoxy-1,3-η3-cyclohexenyl)PdCl]2 gives a mixture of trans-3-methoxy-6-phenylcyclohexene and trans-4-methoxy-3-phenylcyclohexene, consistent with initial formation of a phenyl–palladium(III) intermediate followed by phenyl migration to the η3-cyclohexenyl ligand (reductive elimination).

The final steps of the Oppolzer cyclization: mechanism of the insertion of alkenes into allylpalladium(II) complexes.

We report here the results of a computational study on the the mechanism of the Oppolzer cyclization. These results lead us to conclude that the insertion of olefins in Pd-allyl complexes probably takes place directly from the eta(3)-allyl species. The presence of a phosphane ligand in the reagents plays the role of enhancing the electron density on the Pd atom; this makes the alkene moiety more reactive towards insertion by back-donation from the metal. The results also indicate that the configuration of the new stereogenic centers is fixed in the insertion of the alkene into the (eta(3)-allyl)palladium species.

Role of Planar Chirality of S,N‐ and P,N‐Ferrocene Ligands in Palladium‐Catalyzed Allylic Substitutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reaction chemistry of [Pd 2(μ-OH) 2L 4] 2+ with aryl amines. : Structures of [Pd 2(μ-OH){μ-NH( p-tol)}(PPh 3) 4](BF 4) 2, [Pd(η 3-CH 2C(NH( p-tol)CH 2)(PPh 3) 2](BF 4), and [Pd(PMe 2Ph) 2(μ-PF 2O 2)] 2(PF 6) 2

The reaction of [Pd 2(μ-OH) 2(PPh 3) 4](BF 4) 2 ( 1) with aromatic amines (aniline and p-toluidine) in dichloromethane yields the dinuclear complexes [Pd 2(μ-OH)(μ-NHAr)(PPh 3) 4](BF 4) 2 ( 2) (Ar=Ph, p-tol). The same reaction in acetone gives the palladium(II) allyl complex, [Pd(η 3-CH 2C(NH p-tol)CH 2)(PPh 3) 2](BF 4) ( 3). An attempt to prepare the PMe 2Ph analog of 1 as the PF 6 − salt yielded instead [Pd(PMe 2Ph) 2(μ-PF 2O 2)] 2(PF 6) 2 ( 7).The structures of 2 (Ar= p-tol), 3, and 7 have been determined by X-ray diffraction methods.

Use of the Pauson-Khand reaction products as monomers in the palladium(II)-catalyzed homopolymerization and copolymerization of norbornene derivatives

New norbornene derivatives synthesized from Pauson-Khand reaction products were homopolymerized and copolymerized with norbornene with an allyl-palladium complex as a catalyst. The ketone group was tolerated by the polymerization reaction. Monomers bearing protected alcohols were easily homopolymerized. Most of the homopolymers were soluble in tetrahydrofuran, CH 2 Cl 2 , toluene, and cyclohexane. As the steric bulkiness of the substituent increased, the chain length of the homopolymer decreased. Copolymers with a molecular weight of up to 153,800 were formed and were soluble in tetrahydrofuran, CH 2 Cl 2 , toluene, and diethyl ether.

Activation and Reactivity of (NHC)Pd(allyl)Cl (NHC = N-Heterocyclic Carbene) Complexes in Cross-Coupling Reactions

Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

A mild and efficient method for the stereoselective formation of C-O bonds: palladium-catalyzed allylic etherification using zinc(II) alkoxides.

[reaction: see text] A highly chemo- and stereoselective palladium-catalyzed allylic etherification reaction is described. The use of zinc(II) alkoxides proved effective in promoting the addition of the oxygen nucleophile derived from aliphatic alcohols to eta(3)-allylpalladium complexes. Using diethylzinc (0.5 equiv), 5 mol % of Pd(OAc)(2), and 7.5 mol % of 2-di(tert-butyl)phosphinobiphenyl in THF, the cross-coupling reaction between various aliphatic alcohols and allylic acetates proceeded at ambient temperature to furnish allylic ethers with high stereoselectivity.

Role of planar chirality of S,N- and P,N-ferrocene ligands in palladium-catalyzed allylic substitutions.

Palladium-catalyzed asymmetric allylic substitutions using thioether and phosphino derivatives of ferrocenyloxazoline as ligands have been investigated with a focus on studying the role of planar chirality. In allylic alkylation, up to 98% ee and 95% ee were achieved with S,N- and P,N-ligands, respectively. In allylic amination, 97% ee was realized with P,N-ligands in the presence of TBAF. Several palladium allylic complexes were characterized by X-ray diffraction and/or solution NMR. Thioether derivatives of ferrocenyloxazolines with only planar chirality showed lower enantioselectivity in the allylic alkylation except 5c because of the formation of a new chirality on sulfur atom during the coordination of sulfur with palladium. On the other hand, in the planar chiral P,N-ligands without central chirality, (Sp)-11a-c there was no such disturbance and comparatively higher enantioselectivity in both palladium-catalyzed allylic alkylation and amination was provided.

Allyl–palladium complexes with fluorinated benzene thiolate ligands. Examination of the electronic effects in the Pd-catalyzed allylic alkylation reaction with the catalytic system [(η 3-C 3H 5)Pd(μ-SR f)] 2/PR 3

The bimetallic complexes [(η 3-C 3H 5)Pd(μ-SR f)] 2 [R f=C 6F 5, ( 1); C 6F 4H-4, ( 2); C 6H 4F-2, ( 3); C 6H 4F-3, ( 4) and C 6H 4F-4, ( 5)] in the presence of para-substituted phosphines P(C 6H 4X-4) 3 [X=OCH 3, CH 3, H, F, Cl, and CF 3] are efficient catalytic systems in the allylic alkylation couplings of ( E)-3-acetoxy-1,3-diphenyl-1-propene and dimethyl malonate. Results concerning the electronic effects of both sulfur and phosphorus substituents in this reaction are discussed.

Effect of chloride ions on allylic diphosphine palladium (II) complexes: NMR characteristics and chemical reactivity

A systematic study of the effect of chloride ions on the NMR characteristics of several cationic diphosphine η 3-allyl palladium complexes has revealed in some instances a deshielding of the protons. Chloride ions also enhance the reactivity of the complexes. Thus 〚(Me-Duphos)(1,3-diphenylallyl)palladium〛chloride leads mostly to 〚(Me-Duphos)dichloropalladium〛 and to chalcone at room temperature, whereas at low temperature a pair of cationic and anionic allyl palladium complexes is obtained.

Origin of Enantioselectivity in Palladium-Catalyzed Asymmetric Allylic Alkylation Reactions Using Aminophosphine Ligands

The asymmetric induction observed in palladium-catalyzed allylic alkylation reactions with typical substrates was investigated using three structurally related chiral PN ligands, which all contain a binaphthyl unit and either a benzyl (1) or ferrocenylmethyl (2, 3) fragment bridging phosphorus and nitrogen complexation sites. The intermediate allyl palladium(II) complexes of ligands 1−3 have been isolated and spectroscopically characterized. For two of them, 1K and 3K, the crystal structure has been determined. A rationalization of the experimental findings on a molecular level is proposed based on the results of DFT (ADF) and molecular mechanics (CERIUS2) calculations. Studies on both the entire molecules and simpler models indicate that the stereochemical outcome is not determined by electronic effects, but mostly by a delicate balance of steric repulsions. In particular, the presence of a benzyl or a ferrocenylmethyl group in the chelate chain has a strong effect on its arrangement, thereby directly influencing asymmetric induction.