Cyclometallation of phenylhydrazones: Synthesis, reactivity, crystal structure analysis and novel trinuclear palladium(II) cyclometallated compounds with [C, N, N′] terdentate ligands

Reaction of the ligand C₆H₅N(H)NCMe(C₅H₄N) (a) with palladium(II) acetate in toluene gave the mononuclear cyclometallated complex [Pd{C₆H₄N(H)NCMe(C₅H₄N)}(AcO)] (1a). Reaction of 1a with sodium chloride gave the analogous chlorine compound [Pd{C₆H₄N(H)NCMe(C₅H₄N)}(Cl)] (3a) which could also be prepared by reaction of a with lithium tetrachloropalladate and sodium acetate in methanol for 48 h; whereas shorter reaction times afforded the non-cyclometallated complex [Pd{C₆H₅N(H)NCMe(C₅H₄N)}(Cl)₂] (2a). Reaction of the ligand 2-ClC₆H₄N(H)NCMe(C₅H₄N) · HCl (b), with palladium(II) acetate, or with lithium tetrachloropalladate and sodium acetate, yielded the cyclometallated complex [Pd2-ClC₆H₃N(H)NCMe(C₅H₄N)(Cl)] (1b). Treatment of 3a and 1b with silver trifluoromethanesulphonate (triflate) and triphenylphosphine in acetone gave the mononuclear complexes [Pd{2-RC₆H_nN(H)NCMe(C₅H₄N)}(PPh₃)][CF₃SO₃], (R = H, n = 4, 4a; R = Cl, n = 3, 2b) with the ligand as C,N,N′ terdentate and substitution of chlorine by triphenylphosphine. Reaction of 3a and 1b with silver triflate and the tertiary diphosphine Ph₂P(CH₂)₄PPh₂ (dppb) in a 2:1 molar ratio gave the dinuclear cyclometallated complexes [{Pd[2-RC₆H₃N(H)NCMe(C₅H₄N)]}₂(μ-Ph₂P(CH₂)₄PPh₂)][CF₃SO₃]₂ (R = H, 5a; R = Cl, 3b) with a μ₂-diphosphine bridging ligand. Similarly, treatment of 3a and 1b with silver triflate and the tertiary triphosphines MeC(CH₂PPh₂)₃ (tripod) and (Ph₂PCH₂CH₂)₂PPh (triphos), in 3:1 molar ratio, gave the novel trinuclear complexes [{Pd[C₆H₄N(H)NCMe(C₅H₄N)]}₃{μ₃-MeC(CH₂Ph₂)₃}][CF₃SO₃]₃ (6a) and [{Pd[2-ClC₆H₃N(H)NCMe(C₅H₄N)]}₃{μ₃-(PPh₂CH₂CH₂)₂PPh}][CF₃SO₃] ₃ (4b) regioselectively, with the phosphine as a μ₃-bridging ligand. When the reaction between 3a and triphos was carried out in 1:1 molar ratio the mononuclear complex [Pd{C₆H₄N(H)NCMe(C₅H₄N)}{(PPh₂CH₂CH₂)₂PPh-P,P,P}][ClO₄] (7a) was obtained. The crystal structures of 2b, 3a and 4a have been determined by X-ray crystallography.     

Highly efficient water promoted allylation and propargylation of arylepoxides via rearrangement-carbonyl addition

A simple and highly efficient one-pot procedure for allylation and propargylation of arylepoxides has been developed. A combination of SnCl₂ and catalytic Pd(0) or Pd(II) promotes the reaction of organic halides and epoxides in DMSO with controlled water addition, leading to the regioselective formation of the corresponding homoallyl and homopropargyl alcohols in good yields.     

The first palladium-catalyzed Sonogashira coupling of unactivated secondary alkyl bromides

A palladium-carbene catalyzed Sonogashira coupling of unactivated alkyl bromides with alkyl substituted alkynes is reported. For the first time, unactivated secondary alkyl halides were successfully employed in Sonogashira reactions.     

Synthesis and structural characterization of η-allyl(α-diimine)nickel(II) complexes bearing trimethylsilyl groups

A set of isomeric para- and meta-trimethylsilylphenyl ortho-substituted N,N-phenyl α-diimine ligands [(Ar-NC(Me)-(Me)CN-Ar) Ar=2,6-di(4-trimethylsilylphenyl)phenyl (16); Ar=2,6-di(3-trimethylsilylphenyl)phenyl (17)] have been synthesized through a two-step procedure. The palladium-catalysed cross-coupling reaction between 2,6-dibromophenylamine (7) and 4-trimethylsilylphenylboronic acid (8), 3-trimethylsilylphenylboronic acid (9) was used to prepare 4,4^′-bis(trimethylsilyl)-[1,1^′;3^′,1″]terphenyl-2^′-ylamine (10) and 3,3^′-bis(trimethylsilyl)-[1,1^′;3^′,1″]terphenyl-2^′-ylamine (11). The di-1-adamantylphosphine oxide Ad₂P(O)H (13) and di-tert-butyl-trimethylsilylanylmethylphosphine tert-Bu₂P-CH₂-SiMe₃ (14) were used for the first time as ligands for the Suzuki coupling. The condensation of 2,2,3,3-tetramethoxybutane (15) with anilines 10 and 11 afforded α-diimines 16 and 17. The reaction of π-allylnickel chloride dimer (18), α-diimines (16), (17) and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAF) (19) or silver hexafluoroantimonate (20) led to two sets of isomeric complexes [η³-allyl(Ar-NC(Me)-(Me)CN-Ar)Ni]⁺ X⁻, [Ar=2,6-di(4-trimethylsilylphenyl)phenyl, X=BAF (3), X=SbF₆ (4); Ar=2,6-di(3-trimethylsilylphenyl)phenyl, X=BAF (5), X=SbF₆ (6)]. The steric repulsion of closely positioned trimethylsilyl groups in 4 caused the distortion of the nickel square planar coordination by 17.6° according to X-ray analysis.     

Palladium-catalyzed benzylic-like nucleophilic substitution of benzofuran-, benzothiophene- and indole-based substrates by dimethyl malonate anion

The palladium-catalyzed benzylic-like nucleophilic substitution of acetates derived from benzofuran, benzothiophene and indole was investigated. The asymmetric substitution on racemic 1-(2-benzofuryl)ethyl acetate gave disappointing results, but the substitution product was obtained in 98% ee from (S)-1-(2-benzofuryl)ethyl acetate with overall retention of configuration.     

Design of highly active heterogeneous palladium catalysts for the activation of aryl chlorides in Heck reactions

In situ generation of highly active palladium species by intermediate dissolution of Pd from solid supported catalysts has been demonstrated to be a very successful approach for the activation of aryl chlorides in Heck reactions. The new ‘heterogeneous’ Pd catalysts act as reservoir for molecular Pd species with unsaturated coordination sphere in solution. Crucial Pd leaching and re-deposition onto the support can be controlled by optimization of reaction conditions and by the properties of the catalysts. Pd is re-deposited onto the support at the end of the reaction. The catalysts, palladium supported on activated carbon, on various metal oxides or fluorides and Pd complexes in zeolites, are easy to prepare, though the preparation conditions are crucial. The catalysts convert all aryl bromides completely within minutes (TON 100,000). Aryl chlorides (even deactivated ones) are converted with high yields, within 2-6 h. The catalysts belong to the most active ones in Heck reactions at all (including best homogeneous systems) and fulfill all relevant requirements for practical applications in laboratory and industry.     

Cyano Derivatives of Bis(cyclopentadienyl)titanium(IV). The crystal and molecular structure of μ-Oxo-bis[bis(cyclopentadienyl)cyanotitanium(IV)], [(η5-C5H5)2TiCN]2O

By the reaction of KCN with Cp₂TiCl₂ (Cp = η⁵-C₅H₅) in boiling methanol, bis(cyclopentadienyl)-methoxytitanium(IV) cyanide, Cp₂Ti(OCH₃)CN, is formed which in air is converted into the dinuclear oxygen-bridged derivative (Cp₂TiCN)₂O. By the same procedure, the bis(methylcyclopentadienyl) analogue [MeCp₂TiCN]₂O has been obtained. An X-ray diffraction study of (Cp₂TiCN)₂O has shown that the CN group acts as a unidentate ligand with a Ti? C bond length of 2.158 Å and a Ti? C? N bond angle of 177.7°, very close to linearity. The Ti? O bond distance, 1.836 Å, and the bond angle at the bridging O atom, 174.1°, are normal. The ligands are arranged in a nearly tetrahedral way around the Ti atoms. The structural results are compared to those for similar dinuclear titanium complexes.     

Palladium complexes with ethylene-bridged bis(N-heterocyclic carbene) for C-C coupling reactions

A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)₂ produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs.     

The Methoxycarbonylation of Vinyl Acetate Catalyzed by Palladium Complexes of [1,2‐Phenylenebis(methylene)]bis[di(tert‐butyl)phosphine]

Palladium complexes of [1,2‐phenylenebis(methylene)]bis[di(tert‐butyl)phosphine] ( 1 ) catalyze the methoxycarbonylation of vinyl acetate (= ethenyl acetate) in the presence of methanesulfonic acid (Scheme 1). High selectivities to ester products can be obtained if free phosphine ligand is in excess over the amount of added acid (Table 1). Selectivities to methyl 2‐acetoxypropanoate, a precursor to lactate esters, can be as high as 3.6 : 1 at low temperature and pressure (Table 2). Replacing ^tBu by ⁱPr groups leads to less‐active catalysts and lower selectivities to the branched product. Replacing the phenylene moiety by a naphthalenediyl moiety also gives lower activity, but with similar selectivity to the phenylene‐based analogues. Linear hydrocarbon‐chain linkers as the backbone instead of the phenylenebis(methylene) linker leads to poor catalysis, except for a propane‐1,3‐diyl linker, which gives good rates but poor branched selectivity (Table 5). The effect of different reaction conditions on the catalysis is discussed. The syntheses of the new xylene‐based diphosphines 2 – 5 with one to four ⁱPr groups replacing the ^tBu groups at the P‐atoms of 1 and of the ligands 6 and 7 based on 1,2‐ and 2,3‐dimethylnaphthalene are also described (Schemes 2 and 3).