Synthesis of Pd(II) and Pt(II) complexes possessing bicyclo[3.2.0]heptanyl phosphinite ligands: Identification of a novel Pd(II) precatalyst for 1,6-diene cycloisomerisation

The synthesis of monodentate and bidentate phosphinite ligands, possessing the unusual bicyclo[3.2.0]heptane framework, are reported. A convenient, tin-free synthesis of a key intermediate, namely 3-endo-6-endo-dihydroxybicyclo[3.2.0]heptane, is described. The air-sensitive phosphinite ligand 1 was either protected as the borane adduct, which is air-stable, or reacted directly with bis(acetonitrile)palladium(II)chloride to give the novel air and moisture stable palladium(II) complex 11. A platinum(II) relative 12 has also been synthesised by reaction of phosphinite 1 with bis(benzonitrile)platinum(II)chloride. Each complex has been thoroughly characterised and their molecular structures confirmed by X-ray diffraction studies. In catalytic applications, such as cross-coupling reactions of organometallic reagents with organohalides, an unexpectedly poor activity has been established for 11. For example, Suzuki-Miyaura cross-coupling of activated and deactivated aryl bromides with aryl boronic acids, in the presence of catalytic quantities of 11, proceed in low yield, accompanied by substantial homocoupling. Palladium agglomeration, to produce catalytically inactive Pd black, is rapid in these reactions, under both aqueous and non-aqueous conditions. The poor reactivity is proposed to arise through an unfavourable near tetrahedral ‘(PP′)Pd(0)’ geometry, which slows the oxidative addition step in the catalytic cycle with either activated or deactivated aryl halides. The steric bulk of the ligand and the associated large P-M-P′ bite angle, particularly at the palladium zero oxidation state, is proposed to account for the poor reactivity. However, we have established that cationic derivatives of 11 promote the cycloisomerisation of diallylmalonate in a regioselective fashion.     

Synthesis,characterization and first application of chiral C2‐symmetric bis(phosphinite)–Pd(II) complexes as catalysts in asymmetric intermolecular Heck reactions

A series of new chiral C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes have been synthesized and for the first time used as catalysts in the palladium‐catalysed asymmetric intermolecular Heck coupling reactions of 2,3‐dihydrofuran with iodobenzene or aryl triflate. Under optimized conditions, products were obtained with high conversions and moderate to good enantioselectivities. The new C₂‐symmetric bis(phosphinite) ligands and their palladium(II) complexes were characterized using multinuclear NMR and Fourier transform infrared spectroscopies and elemental analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

First use of a palladium complex with a thiosemicarbazone ligand as catalyst precursor for the Heck reaction

A novel palladium complex with salicylaldehyde N(4)-ethylthiosemicarbazone has been synthesized and according to its crystal structure the ligand is bound to the metal in an O, N, S-terdentate coordination mode. This phosphorus-free system efficiently catalyzes the Heck reaction of aryl bromides (from electron-rich to electron-poor) with styrene under argon, with turnover numbers of up to 43,000, at 150 °C after 24 h, and with a selectivity toward trans-stilbenes ranging from 92 to 96%. In air, for activated aryl bromides and for a palladium concentration of 1 mM, the yields are essentially the same as those obtained when the reaction was performed under argon.     

Use of tetrahydropyrimidinium salts for highly efficient palladium-catalyzed cross-coupling reactions of aryl bromides and chlorides

New, sterically demanding 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (2) as NHC precursors have been synthesized and characterized. These salts, in combination with palladium acetate, provided active catalysts for the cross-coupling of aryl chlorides and bromides under mild conditions. The catalytic system was applied to the Heck, Suzuki and benzaldehyde (Kumada) coupling reactions. Catalyst activity was found to be influenced by the presence of a methoxy group on the ring of the p-position of benzyl substituent of the ligand precursor.     

Pd2dba3/P(i-BuNCH2CH2)3N: a highly efficient catalyst for the one-pot synthesis of trans-4-N,N-diarylaminostilbenes and N,N-diarylaminostyrenes

A Pd₂dba₃/P(i-BuNCH₂CH₂)₃N catalyzed one-pot synthesis of unsymmetrically substituted trans-4-N,N-diarylaminostilbenes and both symmetrically and unsymmetrically substituted N,N-diarylaminostyrene derivatives is reported. The procedure involves two or more palladium catalyzed sequential coupling reactions (an amination and an inter-molecular Heck reaction) in one-pot using the same catalyst system with two different aryl halides, including aryl chlorides and hetero aryl halides as the coupling partners.     

A highly active two six-membered phosphinite palladium PCP pincer complex [PdCl{C6H3(CH2OPPr)2-2,6}]

The diphosphinite 1,3-bis[(di-isopropylphosphinite)methyl]benzene (2) has been synthesized and its complexation in palladium chemistry investigated. Complex 3 represents the first example of a two six-member PCP pincer bis(phosphinite) species. A catalytic study of 3 in the Heck reaction, revealed this specie to have an outstanding activity in the olefination of iodo-, bromo- and chloro-benzenes. When compared with other PCP pincer complexes, the results show that both by increasing the ring size from five to six-membered and using phosphinite instead of phosphine groups lead to a more active catalyst.     

Synthesis of a new bidentate ferrocenyl N-heterocyclic carbene ligand precursor and the palladium (II) complex trans-[PdCl2(CfcC)], where (CfcC) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene

A new ferrocenyl-N-heterocyclic carbene ligand precursor 1,1′-bis[(1-tert-butylimidazolium)-3-methyl]ferrocene dichloride has been synthesised and structurally characterised. The imidazolium salt was readily deprotonated in situ with KN(SiMe₃)₂ and reacted with [PdCl₂ (cod)] to afford the structurally characterised palladium (II) complex trans-[PdCl₂(C^∧fc^∧C)], where (cod) = 1,5-cyclooctadiene and (C^∧fc^∧C) = 1,1′-di-tert-butyl-3,3′-(1,1′-dimethyleneferrocenyl)-diimidazol-2-ylidene.     

Palladium(II) complexes with S-benzyl dithiocarbazate and S-benzyl-N-isopropylidenedithiocarbazate: Synthesis,spectroscopic properties and X-ray crystal structures

Two new palladium(II) bis(NS) chelates, bis(S-benzyl dithiocarbazato)palladium(II) (1) and bis(S-benzyl-N-isopropylidenedithiocarbazato)palladium(II) (2), have been prepared and characterized using single-crystal X-ray diffraction and spectroscopic (electronic, IR and NMR) techniques. Complex 1 has a perfectly square planar trans configuration (point group C_i), while complex 2 has a distorted square planar cis configuration.     

Effect of N1-substituted pyrazolic hybrid ligands on palladium catalysts for the Heck reaction

In this paper we have explored the influence of several linkers present on the [PdCl₂(L)] complexes, where L is 3,5-dimethylpyrazolic hybrid ligand N1-substituted by polyether chains and/or phenyl groups. These complexes have been used as pre-catalysts in the Heck reaction between phenyl halides and tert-butyl acrylate. The corresponding complexes efficiently catalyze the Heck olefination and provide good yields under phosphine-free conditions, even for aryl chlorides. Different reaction conditions were investigated and it was found that the nature of the ligand has an important influence on the effectiveness of the catalytic system. Ligand 1,8-bis(3,5-dimethyl-1H-pyrazol-1-yl)-3,6-dioxaoctane (L1), which has previously shown to be the most flexible and versatile, achieved high turnover numbers within very short reaction times and low catalyst loadings.     

Synthesis of barettin

The indole alkaloid barettin (with bromine in 6-position), isolated from the marine sponge Geodia Barretti, has been synthesised via a Horner-Wadsworth-Emmons type reaction from 6-bromoindole-3-carboxaldehyde to introduce the dehydro-functionality. Subsequent deprotection and cyclisation afforded the natural product in Z-conformation.     

DFT study on the preactivation reaction of a palladium catalyst precursor in phosphine-free Heck reactions

Abstract  The mechanism of the preactivation process of trans-dichlorobis(diethanolamine-N)palladium(II) complex is investigated using density functional theory. The role of diethanolamine (a solvent for the reaction in the absence of a strong base) and acetonitrile (solvent for the reaction in the presence of a strong base) is analyzed by using a discrete model. The Onsager model is applied to assess the effect of the bulk medium. Both models show that diethanolamine activates the complex and thus is a better suited solvent for the Heck reactions of the investigated complex. Graphical abstract        

Synthesis of all-cis-3-(2-diphenylphosphinoethyl)-1,2,4-tris(diphenylphosphinomethyl)cyclopentane (Ditricyp) from dicyclopentadiene

A new tetraphosphine, all-cis-3-(2-diphenylphosphinoethyl)-1,2,4-tris(diphenylphosphinomethyl)cyclopentane (Ditricyp), has been synthesised in seven steps from commercially available dicyclopentadiene. The ozonolysis of dicyclopentadiene occurred first on the double bond of the bicycloheptene moiety. A very high chemoselective ozonolysis was observed at −60 °C leading to the diol after reductive treatment. From this diol, cis,cis,cis-3-(2-hydroxyethyl)-1,2,4-tri(hydroxymethyl)cyclopentane was obtained after a second ozonolysis. Mesylation and substitution with Ph₂PLi led to the title tetradiphenylphosphine Ditricyp. The efficiency of this new tetraphosphine ligand for palladium-catalysed coupling reactions has been studied. Satisfactory results in terms of substrate/catalyst ratio have been obtained for Suzuki, Negishi and Sonogashira couplings and also for Heck vinylation reaction. After chromatographic separation, one enantiomer of this ligand associated to palladium was able to induce enantioselective allylic alkylation with modest enantiomeric excess.     

trans-Bis-[(−)ephedrinate]-palladium complex: synthesis, molecular modeling and use as catalyst

The reaction between palladium acetate, (−)-ephedrine and potassium acetate led to bis-chelate complex Pd[OCH(Ph)NH(Me)]₂ whose the trans-structure is obtained from calculations. The use of this complex to catalyze either the 1,4-hydrogenation of (E)-2-benzyliden-1-tetralone or Heck reaction of phenyl iodide with 3-methyl-3-buten-2-ol led to a low enantiomeric excess.     

Efficient and fast Heck vinylation of 2-bromo-6-methyl pyridines with methylacrylate. Application to the synthesis of 6-methyl cyclopenta[b]pyridinone

Heck vinylation of 2-bromo-6-methyl-3-substituted pyridines using η³-allylpalladium chloride dimer/P(o-Tol)₃ complex/toluene and dimethylacetamide (DMA) as co-solvent with methyl acrylate is reported. Electronic and steric effects were investigated engaging diversely 2-bromo-3,6-disubstituted pyridines. As application, a new synthesis of the 6-methyl cyclopenta[b]pyridinone building-block connecting Heck vinylation, alkene reduction and Dieckmann condensation is described.     

Synthesis of a diverse set of phosphorus ligands on solid support and their screening in the Heck reaction

Representative members from five different families of supported phosphine and phosphinite ligands have been prepared from common templates—polymer-bound aminoalcohols. These include β-aminophosphines, N,β-diphosphinoamines, α,β-diphosphinoamines, β-aminophosphinites and N-phosphino-β-aminophosphinites. Representatives of each family of ligands were complexed with Pd(OAc)₂ and screened in the Heck reaction of iodo- and bromobenzene. While the reaction of iodoarenes is ligand-independent, the reaction of bromoarenes is ligand-sensitive. The rationale for this behavior is suggested. Lead ligands for the reaction of bromoarenes were determined.     

A novel P,S-heterodonor ligand and palladium(0) complex catalyzed Suzuki cross-coupling reaction

Axially dissymmetric P,S-heterodonor ligand L3 synthesized from BINOL is an effective promoter in the palladium(0)-catalyzed Suzuki cross-coupling reaction of phenylboronic acid with aryl bromides and iodide at 60-80 °C. On the basis of ¹³C and ³¹P NMR spectroscopic investigation and X-ray diffraction, it was revealed that N,N-dimethylthiocarbamate-phosphine ligand L3 might be a P,S-heterodonor bidentate ligand to palladium(0) center.     

Synthesis of dendrimeric N-glyoxylamide peptide mimics

A range of peptidomimetic dendrimers based on 1,3,5-benzenetricarbonyl trichloride and tris(2-aminoethyl)amine central cores has been synthesised through the facile ring-opening of N-acylisatins with amino acids, alcohols and other amines.     

Efficient access to disubstituted exo-glycals. Application to the preparation of C-glycosyl compounds

Efficient methods of preparation of disubstituted exo-glycals by palladium cross-coupling reaction on the readily available dibromo exo-glycal and methoxycarbonyl exo-glycal have been developed. Hydrogenation of these new monosubstituted and disubstituted exo-glycals proceeded with a high stereocontrol and led to original C-glycosyl compounds.     

Synthesis and structure of C2-symmetric N-heterocyclic carbene complexes of palladium

The first 9- and 11-membered cis and trans C₂-symmetric benzimidazol-2-ylidene palladium(II) complexes based on a trans-2,2-dimethyl-1,3-dioxalane backbone were synthesised and the configuration of the complexes was elucidated via NMR and X-ray crystallography.     

Assembled catalyst of palladium and non-cross-linked amphiphilic polymer ligand for the efficient heterogeneous Heck reaction

The efficient heterogeneous Heck reaction was achieved by a new networked and supramolecular catalyst PdAS-V (1b). Employing of PdAS-V in 5.0×10⁻⁵ mol equiv. efficiently progressed the heterogeneous Heck reaction of a series of aryl iodides with acrylates, styrenes and acrylic acid. PdAS-V was successfully recycled five times without any decrease in its activity, and showed good stability in toluene and water, and hence the Heck reaction was efficiently performed in both reaction media. The use of 8.0×10⁻⁷ mol equiv. of PdAS-V resulted in the coupling product in 92% yield with the turnover number (TON) and the turnover frequency (TOF) of PdAS-V reached up to 1,150,000 and 12,000, respectively. The efficient synthesis of resveratrol was achieved via the PdAS-V-promoted Heck reaction.