Showing papers by "Martin Lutz published in 2009"

Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for molecular hydrogen evolution

Abstract: The large-scale production of clean energy is one of the major challenges society is currently facing. Molecular hydrogen is envisaged as a key green fuel for the future, but it becomes a sustainable alternative for classical fuels only if it is also produced in a clean fashion. Here, we report a supramolecular biomimetic approach to form a catalyst that produces molecular hydrogen using light as the energy source. It is composed of an assembly of chromophores to a bis(thiolate)-bridged diiron ([2Fe2S]) based hydrogenase catalyst. The supramolecular building block approach introduced in this article enabled the easy formation of a series of complexes, which are all thoroughly characterized, revealing that the photoactivity of the catalyst assembly strongly depends on its nature. The active species, formed from different complexes, appears to be the [Fe2(μ-pdt)(CO)4{PPh2(4-py)}2] (3) with 2 different types of porphyrins (5a and 5b) coordinated to it. The modular supramolecular approach was important in this study as with a limited number of building blocks several different complexes were generated.

Consequences of N,C,N'- and C,N,N'-coordination modes on electronic and photophysical properties of cyclometalated aryl ruthenium(II) complexes.

Abstract: The effects of isoelectronic replacement of a neutral nitrogen donor atom by an anionic carbon atom in terpyridine ruthenium(II) complexes on the electronic and photophysical properties of the resulting N,C,N'- and C,N,N'-cyclometalated aryl ruthenium(II) complexes were investigated. To this end, a series of complexes was prepared either with ligands containing exclusively nitrogen donor atoms, that is, [Ru(R-1-tpy)(R-2-tpy)](2+) (R-1, R-2 = H, CO2Et), or bearing either one N,C,N'- or C,N,N'-cyclometalated ligand and one tpy ligand, that is, [Ru(R-1-(NCN)-C-Lambda-N-Lambda)(R-2-tpy)](+) and [Ru(R-1-(CNN)-N-Lambda-N-Lambda)(R-2-tpy)](+), respectively. Single-crystal X-ray structure determinations showed that cyclometalation does not significantly alter the overall geometry of the complexes but does change the bond lengths around the ruthenium(II) center, especially the nitrogen-to-ruthenium bond length trans to the carbanion. Substitution of either of the ligands with electron-withdrawing ester functionalities fine-tuned the electronic properties and resulted in the presence of an IR probe. Using trends obtained from redox potentials, emission energies, IR spectroelectrochemical responses, and the character of the lowest unoccupied molecular orbitals from DFT studies, it is shown that the first reduction process and luminescence are associated with the ester-substituted C,N,N'-cyclometalated ligand in [Ru(EtO2C-(CNN)-N-Lambda-N-Lambda)(tpy)](+). Cyclometalation in an N,C,N'-bonding motif changed the energetic order of the ruthenium d(zx), d(yz), and d(xy) orbitals. The red-shifted absorption in the N,C,N'-cyclometalated complexes is assigned to MLCT transitions to the tpy ligand. The red shift observed upon introduction of the ester moiety is associated with an increase in intensity of low-energy transitions, rather than a red shift of the main transition. Cyclometalation in the C,N,N'-binding motif also red-shifts the absorption, but the corresponding transition is associated with both ligand types. Luminescence of the cyclometalated complexes is relatively independent of the mode of cyclometalation, obeying the energy gap law within each individual series.

G-quadruplex self-assembly regulated by Coulombic interactions

Abstract: Self-assembly offers the possibility to organize molecules in a given architecture through a subtle interplay between different noncovalent interactions. Although the kind of molecular association can often be predicted from information present in the individual molecules, the synthesis of supramolecular assemblies having a perfectly defined size and shape remains challenging. Here, we introduce the use of Coulombic interactions to control the supramolecular synthesis of finite, well-defined nanostructures. In particular, we demonstrate that the energy associated with the separation of ion pairs can regulate very precisely guanosine self-assembly into discrete G-quadruplexes. Assemblies comprising 8, 12, 16 or 24 guanosine molecules can be selectively and quantitatively obtained simply by tuning the stabilization of the dissociated anions in the solvent environment. Thus, factors such as solvent polarity, the nature of the anion and the cation-anion distance are shown to have a decisive role in the growth of G-quadruplexes.

CuI Complexes with a Noninnocent PNP Ligand: Selective Dearomatization and Electrophilic Addition Reactivity

Abstract: The neutral, T-shaped complex CuI(PN−PtBu) (2), featuring a dearomatized 2,6-bis(diphosphino)pyridine (PNP)-pincer ligand, is shown to interact rapidly with electrophiles. This has enabled the synthesis of acetato complex 3. Furthermore, C−C bond formation onto the deprotonated methylene-bridgehead carbon is observed with MeOTf as the electrophile. This represents the first case of selective modification of the lutidine-based backbone of such noninnocent PNP ligands. Theoretical calculations support the formation of monomeric complex 2 and indicate the high reactivity of the methylene fragment in this CuI complex.

Cationic and neutral Ni(II) complexes containing a non-innocent PNP ligand: formation of alkyl and thiolate species.

Abstract: The synthesis and characterization of a series of cationic and neutral Ni-complexes with the non-innocent PNPtBu pincer ligand is discussed. Starting with the dicationic complex 1, [Ni(PNPtBu)(NCMe)](BF4)2, a small series of dicationic and monocationic NiII complexes has been prepared. Substitution with tert-butyl isocyanide and azide occurs readily in MeCN solution. IR spectroscopy provided a practical handle to access the formal valence state of the ligand. For the mono- and dicationic tert-butyl isocyanide species 3 and 6 the main vibrational bands in the IR spectra were reproduced quantitatively by DFT theoretical calculations, showing good agreement with the experimentally observed Δν upon dearomatization of the PNPtBu backbone. Using a selective dearomatization-reprotonation methodology the mononuclear Ni-thiolate species 7 and 8 are cleanly generated and their structures have been determined by X-ray crystal structure determination. Alternatively, starting from the monocationic species [Ni(PNPtBu)Cl]BF4, neutral alkyl derivatives are easily available in a two-step procedure, and these species have been spectroscopically characterized.

Highly Selective Cobalt-Catalyzed Hydrovinylation of Styrene

Abstract: Phosphine complexes of cobalt halide salts activated by diethylaluminum chloride are shown to yield highly active catalysts in the hydrovinylation of styrene, with unprecedented high selectivity to the desired product 3-phenyl-1-butene (3P1B). Double-bond isomerization, a common problem in codimerization reactions, only occurs after full conversion with these catalyst systems, even at elevated temperature. The most active catalysts are based on cobalt halide species combined with either C1- or C2-bridged diphosphines, heterodonor P,N or P,O ligands, flexible bidentate phosphine ligands or monodentate phosphine ligands. Kinetic investigations show an order >1 in catalyst, which indicates either the involvement of dinuclear species in the catalytic cycle or partial catalyst decomposition via a bimolecular pathway.

N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling

Abstract: The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin, square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while the benzimidazolato-functionalized complex crystallizes as the trans isomer. The activity of these novel complexes in the Kumada cross-coupling of phenylmagnesium chloride with 4-chloroanisole and 4-fluoroanisole was investigated. One of the benzimidazolato-functionalized complexes shows the highest activity in this reaction reported to date, yielding the desired product in quantitative yields within 30 min (4-chloroanisole), or 150 min (4-fluoroanisole) with only 1 mol% catalyst.

Redox chemistry and electronic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine-bridged diruthenium complexes controlled by N,C,N'-biscyclometalated ligands.

Abstract: To investigate the consequences of cyclometalation for electronic communication in dinuclear ruthenium complexes, a series of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridged diruthenium complexes was prepared and studied. These complexes have a central tppz ligand bridging via nitrogen-to-ruthenium coordination bonds, while each ruthenium atom also binds either a monoanionic, N,C,N'-terdentate 2,6-bis(2'-pyridyl)phenyl (R-N boolean AND C boolean AND N) ligand or a 2,2':6',2 ''-terpyridine (tpy) ligand. The N,C,N'-, that is, biscyclometalation, instead of the latter N,N', N ''-bonding motif significantly changes the electronic properties of the resulting complexes. Starting from well-known [{Ru(tpy)}(2)(mu-tppz)](4+) (tpy = 2,2':2 '',6-terpyridine) ([3](4+)) as a model compound, the complexes [{Ru(R-N boolean AND C boolean AND N)}(mu-tppz){Ru(tpy)}](3+) (R-N boolean AND C(H)boolean AND N = 4-R-1,3-dipyridylbenzene, R = H ([4a](3+)), CO2Me ([4b](3+))), and [{Ru(R-N boolean AND C boolean AND N)}(2)(mu-tppz)](2+), (R = H ([5a](2+)), CO2Me ([5b](2+))) were prepared with one or two N,C,N'-cyclometalated terminal ligands. The oxidation and reduction potentials of cyclometalated [4](3+) and [5](2+) are shifted negatively compared to non-cyclometalated [3](4+), the oxidation processes being affected more significantly. Compared to [3](4+), the electronic spectra of [5](2+) display large bathochromic shifts of the main MLCT transitions in the visible spectral region with low-energy absorptions tailing down to the NIR region. One-electron oxidation of [3](4+) and [5](2+) gives rise to low-energy absorption bands. The comproportionation constants and NIR band shape correspond to delocalized Robin-Day class III compounds. Complexes [4a](3+) (R = H) and [4b](3+) (R = CO2Me) also exhibit strong electronic communication, and notwithstanding the large redox-asymmetry the visible metal-to-ligand charge-transfer absorption is assigned to originate from both metal centers. The potential of the first, ruthenium-based, reversible oxidation process is strongly negatively shifted. On the contrary, the second oxidation is irreversible and cyclometalated ligand-based. Upon one-electron oxidation, a weak and low-energy absorption arises.

DNA cleavage and antitumour activity of platinum(II) and copper(II) compounds derived from 4-methyl-2-N-(2-pyridylmethyl)aminophenol: spectroscopic, electrochemical and biological investigation

Abstract: The reaction of the redox-active ligand, Hpyramol (4-methyl-2-N-(2-pyridylmethyl)aminophenol) with K2PtCl4 yields monofunctional square-planar [Pt(pyrimol)Cl], PtL-Cl, which was structurally characterised by single-crystal X-ray diffraction and NMR spectroscopy. This compound unexpectedly cleaves supercoiled double-stranded DNA stoichiometrically and oxidatively, in a non-specific manner without any external reductant added, under physiological conditions. Spectro-electrochemical investigations of PtL-Cl were carried out in comparison with the analogue CuL-Cl as a reference compound. The results support a phenolate oxidation, generating a phenoxyl radical responsible for the ligand-based DNA cleavage property of the title compounds. Time-dependent in vitro cytotoxicity assays were performed with both PtL-Cl and CuL-Cl in various cancer cell lines. The compound CuL-Cl overcomes cisplatin-resistance in ovarian carcinoma and mouse leukaemia cell lines, with additional activity in some other cells. The platinum analogue, PtL-Cl also inhibits cell-proliferation selectively. Additionally, cellular-uptake studies performed for both compounds in ovarian carcinoma cell lines showed that significant amounts of Pt and Cu were accumulated in the A2780 and A2780R cancer cells. The conformational and structural changes induced by PtL-Cl and CuL-Cl on calf thymus DNA and ϕX174 supercoiled phage DNA at ambient conditions were followed by electrophoretic mobility assay and circular dichroism spectroscopy. The compounds induce extensive DNA degradation and unwinding, along with formation of a monoadduct at the DNA minor groove. Thus, hybrid effects of metal-centre variation, multiple DNA-binding modes and ligand-based redox activity towards cancer cell-growth inhibition have been demonstrated. Finally, reactions of PtL-Cl with DNA model bases (9-Ethylguanine and 5′-GMP) followed by NMR and MS showed slow binding at Guanine-N7 and for the double stranded self complimentary oligonucleotide d(GTCGAC)2 in the minor groove.

Ni(salen): a System That Forms Many Solvates with Interacting Ni Atoms

Abstract: Recrystallization of [N,N’-Ethylene-bis(salicylideneiminato)]-nickel(II) [Ni(salen)] has been carried out from a large selection of solvents. Crystals can be either solvent free or solvates. This study is based on X-ray crystal structure determinations, which include the redetermination of Ni(salen) at low-temperature, the finding of three new solvates of Ni(salen), that is, Ni(salen)·CH2Cl2, Ni(salen)·AcOH, and Ni(salen)·1.5MeOH, and the reexamination of the known solvate Ni(salen)·chloroform at low- and room-temperature. The crystal structures of Ni(salen) and its solvates are stabilized by salen···salen and/or solvent···salen and/or solvent···solvent intermolecular interactions. A special case is the crystal structure of Ni(salen)·1.5MeOH where the salen/solvent ratio is 2:3 rather than 1:1. For all solvated structures, the solvent and Ni(salen) molecules always interact via Dsolvent−H···Osalen (Dsolvent = C, O) hydrogen bond interactions, the strengths of which depend on the solvent of recrystallizati...

A phosphorus analogue of Bis(η4-cyclobutadiene)iron(0)

Abstract: P makes it possible: The convenient oxidative synthesis of the 16-electron organophosphorus iron sandwich complex [Fe(eta(4)-P(2)C(2)tBu(2))(2)] (see structure) suggests that the elusive all-carbon complex [Fe(eta(4)-C(4)H(4))(2)] is a viable synthetic target.

N-heterocyclic carbene-functionalized ruthenium phosphinidenes: what a difference a twist makes.

Abstract: Catalyst tuning by changing ligands is a well-established protocol in transition-metal chemistry. N-Heterocyclic carbenes (NHCs) and tertiary phosphines (R3P) are the ubiquitous ligand actors. Here we demonstrate that the relative σ-donor/π-acceptor ability of the NHC ligand itself can be influenced by a simple substituent-controlled conformational change, thereby directly impacting the reactivity of the transition-metal complex.

Calcium carbene complexes with boranophosphorano side-arms: CaC[P(Ph)2BH3]2

Abstract: The ligand H2C(PPh2BH3)2 (4-H2) reacted with one or a half equivalent of (para-tBu-C6H4CH2)2Ca·(THF)4 to form the calcium complexes Ca[HC(PPh2BH3)2]2 (4-H)2Ca and CaC(PPh2BH3)2 (4-Ca), respectively. The crystal structures of their THF adducts (4-H)2Ca·THF and [4-Ca·(THF)]2 follow the same trends as observed for the corresponding iminophosphorano substituted complexes Ca[HC(PPh2NR)2]2 and [CaC(PPh2NR)2]2. The P–C bonds shorten upon gradual deprotonation, whereas the P–B and P–N bonds elongate. The geometries of DFT-optimized model systems and complete molecular structures show similar trends. Also the charge distribution within the boranophosphorano complexes is similar to that in the iminophosphorano complexes. The high positive charges on Ca (1.74–1.75) indicate a predominantly ionic ligand–Ca bonding. High negative charges on the central carbon atom (−1.103 in Ca[HC(PH2BH3)2]2 and −1.775 in [CaC(PH2BH3)2]2) compare well to those calculated for analogous iminophosphorano complexes (−1.126 Ca[HC(PH2NH)2]2 and −1.847 [CaC(PH2NH)2]2, respectively). Thus, in both types of complexes delocalization of electron density over the boranophosphorano or iminophosphorano substituents does not play a major role. Complex [4-Ca·(THF)]2 does not react with adamantyl cyanide. Instead the adduct (4-Ca)2·(THF)·(adamantyl-CN) formed (a crystal structure has been determined).

Cationic Ruthenium‐Cyclopentadienyl‐Diphosphine Complexes as Catalysts for the Allylation of Phenols with Allyl Alcohol; Relation between Structure and Catalytic Performance in O‐ vs. C‐Allylation

Abstract: A new catalytic method has been investigated to obtain either O- or C-allylated phenolic products using allyl alcohol or diallyl ether as the allyl donor. With the use of new cationic ruthenium(II) complexes as catalyst, both reactions can be performed with good selectivity. Active cationic Ru(II) complexes, having cyclopentadienyl and bidentate phosphine ligands are generated from the corresponding Ru(II) chloride complexes with a silver salt. The structures of three novel (diphosphine)Ru(II)CpCl catalyst precursor complexes are reported. It appears that the structure of the bidentate ligand has a major influence on catalytic activity as well as chemoselectivity. In addition, a strong cocatalytic effect of small amounts of acid is revealed. Model experiments are described that have been used to build a reaction network that explains the origin and evolution in time of both O-allylated and C-allylated phenolic products. Some mechanistic implications of the observed structure vs. performance relation of the [(diphosphine)RuCp]+ complexes and the cocatalytic role of added protons are discussed.

Solid-State Structural Characterization of Cutinase-ECE-Pincer-Metal Hybrids

Abstract: The first crystal structures of lipases that have been covalently modified through site-selective inhibition by different organometallic phosphonatepincer– metal complexes are described. Two ECE-pincer-type d8-metal complexes, that is, platinum (1) or palladium (2) with phosphonate esters (ECE=[(EtO)-(O=)PACHTUNGTRENUNG(-O-C6H4-ACHTUNGTRENUNG(NO2)- 4)ACHTUNGTRENUNG(-C3H6-4-ACHTUNGTRENUNG(C6H2-ACHTUNGTRENUNG(CH2E)2)] ; E= NMe2 or SMe) were introduced prior to crystallization and have been shown to bind selectively to the Ser120 residue in the active site of the lipase cutinase to give cut-1 (platinum) or cut-2 (palladium) hybrids. For all five presented crystal structures, the ECE-pincer–platplatinum or –palladium head group sticks out of the cutinase molecule and is exposed to the solvent. Depending on the nature of the ECE-pincer–metal head group, the ECE-pincer–platinum and – palladium guests occupy different pockets in the active site of cutinase, with concomitant different stereochemistries on the phosphorous atom for the cut-1 (SP) and cut-2 (RP) structures. When cut-1 was crystallized under halide-poor conditions, a novel metalinduced dimeric structure was formed between two cutinase-bound pincer– platinum head groups, which are interconnected through a single m-Cl bridge. This halide-bridged metal dimer shows that coordination chemistry is possible with protein-modified pincer–metal complexes. Furthermore, we could use NCN-pincer–platinum complex 1 as site-selective tool for the phasing of raw protein diffraction data, which shows the potential use of pincer–platinum complex 1 as a heavy-atom derivative in protein crystallography.

Synthesis of conformationally constrained peptidomimetics using multicomponent reactions.

Abstract: A novel modular synthetic approach toward constrained peptidomimetics is reported. The approach involves a highly efficient three-step sequence including two multicomponent reactions, thus allowing unprecedented diversification of both the peptide moieties and the turn-inducing scaffold. The turn-inducing properties of the dihydropyridone scaffold were evaluated by molecular modeling, X-ray crystallography, and NMR studies of a resulting peptidomimetic. Although modeling studies point toward a type IV β-turn-like structure, the X-ray crystal structure and NMR studies indicate an open turn structure.

Sulfato-bridged ECE-pincer palladium(II) complexes: structures in the solid-state and in solution, and catalytic properties

Abstract: ECE-pincer sulfato palladium complexes (pincer = [C6H3(CH2E)2-2,6]−; E = SPh (1), SMe (2), StBu (3), NMe2 (4)) were synthesized and characterized In the solid-state (X-ray determinations) 3 and 4 exist as neutral ECE-pincer palladium sulfato complexes with a μ2-O,O′ bridging sulfato ligand IR and Raman spectroscopic studies revealed that in the solid-state the complexes can be present as either solely neutral or as a mixture of neutral and ionic species, depending on the preparation and morphology of the solids In water, ionic complexes with non-coordinating sulfate ions prevail Preliminary studies of the catalytic activity of 2–4 in the Suzuki–Miyaura C–C cross-coupling reaction of 3-iodobenzoic acid and sodium tetraphenylborate in water reveal that the C–C cross-coupling product is efficiently formed in good yields at room temperature

Synthesis and characterization of Pd(II)-methyl complexes with N-heterocyclic carbene-amine ligands.

Abstract: A number of palladium(II) complexes with a heteroditopic NHC-amine ligand and their precursor silver(I) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.

A Multicomponent Reaction Towards N‐(Cyanomethyl)amides

Abstract: Multicomponent reactions (MCRs)[1] have emerged as useful tools in diversity-oriented synthesis (DOS).[2] Although a number of substituents can be varied over a broad range, MCRs often access products of one specific scaffold, limiting the overall chemical diversity. To overcome this, MCRs have been combined with other types of reactions such as Diels–Alder reactions,[3] click chemistry,[4] and cyclization strategies[5] to generate structural diversity and a high degree of complexity in a minimal number of reaction steps. Our group contributed in this field by trapping 1-azadienes,[ 6] generated by the three-component reaction (3-CR) between phosphonates, nitriles and aldehydes,[7] with various reactants resulting in novel 4-CRs for a variety of heterocyclic scaffolds.[8]

One-pot synthesis and immobilisation of sulfonate-tethered N-heterocyclic carbene complexes on polycationic dendrimers.

Abstract: Easy does it! Anion-tethered N-heterocyclic carbene metal complexes (metal=AuI, RhI) were efficiently synthesised and immobilised on a polycationic dendrimer by an in situ transmetallation-immobilisation reaction, leading to discrete non-covalent metallodendritic assemblies

A novel hemilabile calix[4],quinoline-based P,N-ligand: coordination chemistry and complex characterisation

Abstract: The synthesis of the calix[4]arene-based P,N-ligand 3 (5,11,17,23-tetra-tert-butyl-25-[(2-quinolylmethyl)oxy]-26,27,28-(μ3-phosphorustrioxy)calix[4]arene), in which the nitrogen atom-containing moiety has been introduced at the lower rim of the cavity prior to P-functionalisation, is described and its coordination properties investigated. In the crystal structure, the calix[4]-cavity adopts a cone conformation with an exo orientation of the phosphorus lone pair enabling P-N chelation. 1H, 13C, 31P and 1H{15N} HMQC NMR spectra indicated that, in complexes [PdCl(CH3)(3)] (4) and [Rh(CO)Cl(3)] (5), ligand 3 coordinates in a chelating fashion, while in cis-[PtCl2(3)2] (6) and [Rh(acac)(CO)(3)] (7) it behaves as a monodentate ligand, coordinating via the phosphorus atom only. X-Ray crystal structure determinations were performed for [PdCl(CH3)(3)] (4) and cis-[PtCl2(3)2] (6). The cationic Pd complex [Pd(CH3)(CH3CN)(3)][PF6] (8) was found to be active in a CO/ethylene copolymerisation reaction. Good selectivities were observed for the Pd-catalysed allylic alkylation of cinnamyl acetate with in situ prepared catalysts. [Rh(acac)(CO)2] modified with ligand 3 catalyses the hydroformylation of 1-octene with low selectivities towards linear aldehydes. High-pressure NMR experiments on the hydrido carbonyl rhodium(3) were inconclusive, different species were formed.

Iridium phosphinidene complexes: a comparison with iridium imido complexes in their reaction with isocyanides.

Abstract: 18-Electron nucleophilic, Schrock-type phosphinidene complexes 3 [Cp*(Xy—N≡C)Ir═PAr] (Ar = Mes*, Dmp, Mes) are capable of unprecedented [1 + 2]-cycloadditions with 1 equiv of isocyanide RNC (R = Xy, Ph) to give novel iridaphosphirane complexes [Cp*(Xy—N≡C)IrPArC═NR]. Their structures were ascertained by X-ray diffraction. Density functional theory investigations on model structures revealed that the iridaphosphirane complexes are formed from the addition of the isocyanide to 16-electron species [Cp*Ir═PAr] forming first complex 3 that subsequently reacts with another isocyanide to give the products following a different pathway than its nitrogen analogue [Cp*Ir≡Nt-Bu] 1.

Molecular-weight-enlarged multiple-pincer ligands: synthesis and application in palladium-catalyzed allylic substitution reactions

Abstract: Three different pincer ligand systems are synthesized via nucleophilic substitution reactions of polyaromatic benzyl bromides as support molecules and phenol derivatives as ligand precursors. Retention tests using a polymeric nanofiltration membrane show moderate to good retention in THF and CH2Cl2. Concentration-dependent NMR spectroscopy gives no indication for the formation of aggregates in solution. The three ligand systems are active in both the allylic alkylation and allylic amination reactions and show high selectivity towards the linear trans products. An investigation of the kinetic parameters of the allylic amination reaction show that the reaction of cinnamyl acetate with morpholine is of zero order in cinnamyl acetate and of first order in morpholine. The order in catalyst is found to be one, and the rate constant k’ is determined for a reaction under standard conditions. Moreover, one of the tri- ACHTUNGTRENUNG(pincer)-palladium complexes is used as soluble, molecularweight- enlarged homogeneous catalyst in continuous allylic alkylation and amination reactions. The conversion of the allylic alkylation reaches a maximum of 30%, while a maximum conversion of 80% is reached in the allylic amination reaction. No palladium black was formed.