Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: carbene ligand rotation and application to the Stille reaction

Reaction of [Pd(PPh(3))(4)] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl(2)(PPh(3))(C(3)(Ar)(2))] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)(2)] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl(2)(C(3)(Ar)(2))](2) (Ar = Ph 7, p-(OMe)C(6)H(4)8, p-(F)C(6)H(4)9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl(2)(NCMe)(C(3)(Ar)(2))] (Ar = Ph 10, p-(OMe)C(6)H(4)11 and p-(F)C(6)H(4)) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl(2)(PR(3))(C(3)(Ar)(2))] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C(6)H(4), R = Ph 14, Ar = p-(F)C(6)H(4), R = Ph 15). Crystal structures of complexes 6·3.25CHCl(3), 10, 11·H(2)O and 12-15 are reported. DFT calculations of complexes 10-12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5-9 were tested for catalytic activity in C-C coupling (Mizoroki-Heck, Suzuki-Miyaura and, for the first time, Stille reactions) and C-N coupling (Buchwald-Hartwig amination) showing excellent conversion with moderate to high selectivity.     

Computational study of a new heck reaction mechanism catalyzed by palladium(II/IV) species

In this theoretical study on the Heck reaction we explore the feasibility of an alternative pathway that involves a PdII/PdIV redox system. Usually, the catalytic cycle is formulated based on a Pd0/PdII mechanism. We performed quantum chemical calculations using density functional theory on a model system that consisted of diphosphinoethane (DPE) as a bidentate ligand and the substrates ethylene and phenyl iodide to compare both mechanisms. Accordingly, the PdII/PdIV mechanism is most likely to occur in the equatorial plane of an octahedral PdIV complex. The energy profiles of both reaction pathways under consideration are largely parallel. A major difference is found for the oxidative addition of the C-I bond to the palladium centre. This is a rate-determining step of the PdII/PdIV mechanism, while it is facile for a Pd0 catalyst. The calculations show that intermediate ligand detachment and reattachment is necessary in the course of the oxidative addition to PdII. Therefore, we expect the PdII/PdIV mechanism to be only feasible if a weakly coordinating ligand is present.     

Efficient Stille cross-coupling reaction catalyzed by the Pd(OAc)2/Dabco catalytic system

[reaction: see text] An efficient Pd(OAc)2/Dabco-catalyzed Stille cross-coupling reaction procedure has been developed. In the presence of Pd(OAc)2 and Dabco (triethylenediamine), various aryl halides including aryl iodides, aryl bromides, and activated aryl chlorides were coupled efficiently with organotin compounds to afford the corresponding biaryls, alkene, and alkynes in good to excellent yields. Furthermore, high TONs [turnover numbers, up to 980,000 TONs for the coupling reaction of 1-bromo-4-nitrobenzene and furan-2-yltributyltin] for the Stille cross-coupling reaction were observed.     

Rate and mechanism of the reaction of alkenes with aryl palladium complexes ligated by a bidentate P,P ligand in Heck reactions

The regioselectivity of the Heck reaction is supposed to be highly affected by the electronic properties of the alkene and the ionic or neutral character of the aryl palladium(II) complexes involved in the reaction with alkenes. In Heck reactions performed in dmf, [Pd(dppp){dppp(O)}Ph](+) (dppp=1,2-bis(diphenylphosphino)propane) is generated in the oxidative addition of PhI with [Pd(0)(dppp)(OAc)](-) formed in situ from Pd(OAc)(2) associated to two equivalents of dppp. [Pd(dppp){dppp(O)}Ph](+) is not very reactive with alkenes (styrene or methyl acrylate); however, it reacts with iodide ions (released in the catalytic reactions) to give [Pd(dppp)IPh] and with acetate ions (used as base) to give [Pd(dppp)(OAc)Ph]. [Pd(dppp)(OAc)Ph] reacts with styrene and methyl acrylate exclusively by an ionic mechanism, that is, via the cationic complex [Pd(dppp)(dmf)Ph](+) formed by dissociation of the acetate ion. The reaction of [Pd(dppp)IPh] is more complex and substrate dependent. It reacts with styrene exclusively by the ionic mechanism via [Pd(dppp)(dmf)Ph](+). [Pd(dppp)IPh] (neutral mechanism) and [Pd(dppp)(dmf)Ph](+) (ionic mechanism) react in parallel with methyl acrylate. [Pd(dppp)(dmf)Ph](+) is more reactive than [Pd(dppp)IPh] but is always generated at lower concentration.     

Photoinduced magnetization of the cyano-bridged 3d-4f heterobimetallic assembly Nd(DMF)(4)(H(2)O)(3)(micro-CN)Fe(CN)(5).H(2)O (DMF = N,N-dimethylformamide)

Photoinduced magnetization of the cyano-bridged 3d-4f heterobimetallic assembly Nd(DMF)4(H2O)3(mu-CN)Fe(CN)5.H2O (1) (DMF = N,N-dimethylformamide) is described in this paper. The chiMT values are enhanced by about 45% after UV light illumination in the temperature range of 5-50 K. We propose that UV light illumination induces a structural distortion in 1. This small structural change is propagated by molecular interactions in the inorganic network. Furthermore, the cooperativity resulting from the molecular interaction functions to increase the activation energy of the relaxation processes, which makes observation of the photoexcited state possible. The flexible network structure through the hydrogen bonds in 1 plays an essential role for the photoinduced phenomenon. This finding may open up a new domain for developing the molecule-based magnetic materials.     

Mechanistic aspects of hydrosilylation catalyzed by (ArN=)Mo(H)(Cl)(PMe3)3

The reaction of (ArN=)MoCl(2)(PMe(3))(3) (Ar = 2,6-diisopropylphenyl) with L-Selectride gives the hydrido-chloride complex (ArN=)Mo(H)(Cl)(PMe(3))(3) (2). Complex 2 was found to catalyze the hydrosilylation of carbonyls and nitriles as well as the dehydrogenative silylation of alcohols and water. Compound 2 does not show any productive reaction with PhSiH(3); however, a slow H/D exchange and formation of (ArN=)Mo(D)(Cl)(PMe(3))(3) (2(D)) was observed upon addition of PhSiD(3). Reactivity of 2 toward organic substrates was studied. Stoichiometric reactions of 2 with benzaldehyde and cyclohexanone start with dissociation of the trans-to-hydride PMe(3) ligand followed by coordination and insertion of carbonyls into the Mo-H bond to form alkoxy derivatives (ArN=)Mo(Cl)(OR)(PMe(2))L(2) (3: R = OCH(2)Ph, L(2) = 2 PMe(3); 5: R = OCH(2)Ph, L(2) = η(2)-PhC(O)H; 6: R = OCy, L(2) = 2 PMe(3)). The latter species reacts with PhSiH(3) to furnish the corresponding silyl ethers and to recover the hydride 2. An analogous mechanism was suggested for the dehydrogenative ethanolysis with PhSiH(3), with the key intermediate being the ethoxy complex (ArN=)Mo(Cl)(OEt)(PMe(3))(3) (7). In the case of hydrosilylation of acetophenone, a D-labeling experiment, i.e., a reaction of 2 with acetophenone and PhSiD(3) in the 1:1:1 ratio, suggests an alternative mechanism that does not involve the intermediacy of an alkoxy complex. In this particular case, the reaction presumably proceeds via Lewis acid catalysis. Similar to the case of benzaldehyde, treatment of 2 with styrene gives trans-(ArN=)Mo(H)(η(2)-CH(2)═CHPh)(PMe(3))(2) (8). Complex 8 slowly decomposes via the release of ethylbenzene, indicating only a slow insertion of styrene ligand into the Mo-H bond of 8.     

Palladium catalyzed reaction in aqueous DMF: synthesis of 3-alkynyl substituted flavones in the presence of prolinol

(S)-Prolinol facilitated the coupling reaction of terminal alkynes with 3-iodoflavone under palladium-copper catalysis in aqueous DMF affording a mild and convenient method for the synthesis of 3-alkynyl substituted flavones of potential biological interest.     

The use of chloroalkyldichloroarsines in hybrid ligand synthesis. The preparation of but-3-enylbis(3-dimethylarsinopropyl)arsine and 3-dimethylarsinopropylbis(but-3-enyl)arsine,and their reaction with nickel(II) salts to form pentacoordinate complexes. Novel nickel(II)—olefin bonds

Routes have been developed to the hitherto unobtainable arsine-olefin ligands (CH₂CHCH₂CH₂)_nAs(CH₂CH₂CH₂AsMe₂)_3-n (n = 1, tasol, but-3-enylbis(3-dimethylarsinopropyl)arsine; n = 2, dasdol, 3-dimethylarsinopropylbis(but-3-enyl)arsine) by making use of the difference in reactivity between the ClC and AsCl bonds in the precursor Cl(CH₂)_mAsCl₂ (m = 2,3) molecules. Thus, the triarsine obtained by reaction of 2-chloroethyldichloroarsine with the Grignard reagent of 3-chloropropyldimethylarsine yields 2-chloroethylbis(3-dimethylarsinopropyl)arsine, from which tasol is obtainable by subsequent reaction with either the Grignard reagent of vinyl bromide or, preferably, with vinyllithium. Similarly, 3-chloropropyldichloroarsine reacts with the Grignard reagent of 4-chlorobut-1-ene to form 3-chloropropylbis(but-3-enyl)arsine which, on reaction with sodium dimethylarsenide yields dasdol. The tasol ligand reacts with nickel(II) salts to form [NiX(tasol)]⁺ (X = Cl, Br) and [NiI₂(tasol)], the former are trigonal bipyramidal and contain a nickel(II)—olefin bond, and the latter are square pyramidal containing a [NiI₂As₃] coordination sphere. In addition, tasol forms a number of polynuclear complexes with nickel(II). The dasdol ligand acts as a bidentate arsine to form only [NiX(dasdol)₂)]⁺ The formation of novel nickel(II)—olefin bonds in the [NiX(tasol)]⁺ cations is discussed.     

Ethene/norbornene copolymerization with palladium(II) alpha-diimine catalysts: from ligand screening to discrete catalyst species

Sixteen palladium(II) alpha-diimine catalysts were investigated in a screening-like procedure for the copolymerization of ethene with norbornene. The resulting copolymers were characterized by (13)C NMR spectroscopy, differential scanning calorimetry, gel permeation chromatography, and viscosimetry. The degree of incorporation of norbornene in the polymer chain is very high for most of the catalysts. To validate the results achieved in the screening, two catalysts, [[ArN=CHCH=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (1 b'; Ar=2,6-Me(2)C(6)H(3), BAr(f) (4)=B[3,5-C(6)H(3)(CF(3))(2)](4)) and [[ArN=C(CH(3))C(CH(3))=NAr]Pd(Me)(CH(3)CN)]BAr(f) (4) (2 c'; Ar=2,6-iPr(2)C(6)H(3)), were synthesized as discrete catalytically active species, and their copolymerization behavior was investigated in detail. In agreement with the screening results, 1 b' incorporates norbornene much better in the polymer chain than ethene, a property that has no analogue in metallocene catalysts.     

Preparation of palladium membranes from the reaction of Pd(C3H3)(C5H5) with H2: wet-impregnated deposition

A new technique to prepare a palladium membrane for high-temperature hydrogen permeation was developed: Pd(C₃H₃)(C₅H₅) an organometallic precursor reacted with hydrogen at room temperature to decompose into Pd crystallites. This reaction together with sintering treatment under hydrogen and nitrogen in sequence resulted in the formation of dense films of pure palladium on the surface of the mesoporous stainless steel (SUS) support. Under H₂ atmosphere the palladium membrane could be sintered at 823 K to form a skin layer inside the support pores. The hydrogen permeance was 5.16×10⁻² cm³ cm⁻² cm Hg⁻¹ s⁻¹ at 723 K. H₂/N₂ selectivity was 1600 at 723 K.     

Polymerization of norbornene with a pendent sulfonyl fluoride group catalyzed by palladium complex bearing tBu3P ligand

The combination of palladium complex (^tBu₃P)Pd(Me)Cl ( 1 ) and NaB[3,5‐(CF₃)₂C₆H₃]₄ (NaBAr₄) catalyzed homopolymerization of a novel monomer, norbornene (NB) with a pendent 2‐fluorosulfonyltetrafluoroethoxymethyl chain (NBSF). Catalytic activities of 1 /NaBAr₄ were higher than those of previously reported palladium or nickel catalysts, probably, because the palladium center with electron donative ^tBu₃P ligand was barely poisoned by the sulfonyl fluoride coordination. Thus, 1 /NaBAr₄ is the current best catalyst system for NBSF polymerization. The catalyst system also gave copolymers of NB with NBSF. The obtained copolymers have high sulfonyl‐fluoride incorporation and a narrow molecular weight distribution. Present catalyst system could control incorporation ratio of NBSF by changing a feed monomer ratio with slow addition of NB solution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5133–5141, 2008     

Umpolung in allylic phosphonates. Regioselective reaction of acetoxy allylic phosphonates with nucleophiles catalyzed by palladium(0) complex

Vinylic phosphonates with functional groups can be synthesized regioselectively by the reaction of acetoxy allylic phosphonates with soft carbon nucleophiles in the presence of bis(trimethylsilyl)acetamide (BSA) under palladium(0) catalysis with moderate to good yields.     

Aldol reaction of enol esters catalyzed by cationic species paired with tetrakis(pentafluorophenyl)borate

The crossed aldol reaction of enol esters, which are weak carbon nucleophiles, with aldehydes was effectively carried out under mild conditions by using a catalytic amount of several cationic species paired with tetrakis(pentafluorophenyl)borate.     

Preparation of a palladium dimer with a cobalt-containing bulky phosphine ligand: Its application in palladium catalyzed Suzuki reactions

A palladium dimer with a cobalt-containing phosphine ligand, {(μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-(tBu)₂PCCC₆H₄-κC¹)Pd(μ-Cl)}₂ (3), was prepared from the reaction of its monomer precursor, (μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-(tBu)₂PCCC₆H₄-κC¹)Pd(μ-OAc) (2), with LiCl. The crystal structure of 3, determined by X-ray diffraction methods, revealed a doubly chloride-bridged palladium dimeric conformation. Suzuki coupling reactions of bromobenzene with phenylboronic acid were carried out catalytically using these two novel palladium complexes 2 and 3 as catalyst precursors. Factors such as the molar ratio of substrate/catalyst, reaction temperature, base and solvent that might affect the catalytic efficiencies were investigated. As a general rule, the performance is much better by employing 3 than 2 as the catalyst precursor.