Novel stereochemistry, reactivity, and stability of an arsenic heterocycle in a metal-promoted asymmetric cycloaddition reaction

The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]phenylene as the chiral auxiliary has been used as the chiral template to promote the asymmetric cycloaddition reaction between diphenylvinylphosphine and 3,4-dimethyl-1-phenylarsole. A diphenylphosphino-substituted asymmetrical heterobidentate arsanorbornene (As-P) ligand was obtained stereoselectively on the chiral palladium template in moderate yield. The chiral benzylamine auxiliary could be removed chemoselectively from the template by treatment with HCl to produce the neutral complex [(As-P)PdCl2]. In contrast to their reported P-P analogue, the arsenic donor in the dichloro complex could be eliminated stereospecifically under mild reaction conditions to generate the corresponding 1-(diphenylphosphino)-3,4-dimethyl-2,4-cyclohexadiene, which remained as a bidentate ligand at the PdCl2 unit via phosphorus and the eta2-C4-C5 double bond. The arsenic-elimination process was found to be influenced by the halo ligand in [(As-P)PdX2]. A similar process was observed with the analogous dibromo complex, but the corresponding diiodo species did not show similar reactivity. All of the novel As-Pd complexes have been characterized by X-ray crystallography.     

Asymmetric synthesis of a chiral diarsine ligand via a cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine

The organopalladium complex containing ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene as a chiral auxiliary has been successfully employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine. In the intramolecular cycloaddition reaction, a pair of separable diastereomeric palladium complexes was obtained in the ratio of 6:1. The chiral naphthylamine auxiliary could be removed chemoselectively from the template by treatment with HCl and subsequently NaI to generate the neutral diiodo complex [(As–As)PdI₂]. Treatment of the diiodo complex with KCN gave the enantiomerically pure As–As bidentate ligand in quantitative yield. In contrast to the reported similar P–P and As–P analogues, both arsenic donors in the diiodo complex could be readily eliminated to produce a structurally novel dimetallic complex.     

Formation of Platinum–Tin Bond by Tin(II)Chloride Insertion

The first ¹¹⁹Sn NMR evidence for the presence of direct platinum–tin bond in solution has been obtained for PtCl(SnCl₃)(bdpp) complex (bdpp = (2S,4S)-2,4-bis(diphenylphosphino)pentane). Various PtCl₂(L₂) complexes (L₂ = heterobidentate P–P, P–O, P–N, P–S chelating ligands) have been reacted with tin(II)chloride resulting in the formation of the corresponding PtCl(SnCl₃)(L₂) derivatives. Tin(II)chloride has been inserted into the Pt–Cl bond transto the harder donor atom of the L₂ ligand.     

Asymmetric hydroarsination reactions toward synthesis of alcohol functionalised C-chiral As–P ligands promoted by chiral cyclometallated complexes

The asymmetric hydroarsination reactions between diphenylarsine and 3-diphenylphosphanyl-but-3-en-1-ol and 2-diphenylphosphanyl-prop-2-en-1-ol have been achieved using the organopalladium complex containing ortho-metallated (R)-[1-(dimethylamino)ethylnaphthalene as the chiral reaction template in high stereoselectivities under mild conditions. Hydroarsination of 3-diphenylphosphanyl-but-3-en-1-ol with diphenylarsine generated only one stereoisomer as five-membered As–P bidentate chelate on chiral naphthylamine palladium template. Using the same chiral metal template, similar hydroarsination reaction was carried out on 2-diphenylphosphanyl-prop-2-en-1-ol which gave two different products in the ratio of 2.6 to 1. The major isomer was identified as the expected five-membered As–P bidentate ligand and the minor isomer was identified as the elimination product. The naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid. Optically pure As–P ligands containing the hydroxy groups at the chiral carbon centres were prepared by ligand displacement. The absolute configuration and coordination properties of the complexes have been established by single crystal X-ray analysis.     

Asymmetric hydroarsination reactions toward synthesis of alcohol functionalised C-chiral As–P ligands promoted by chiral cyclometallated complexes

The asymmetric hydroarsination reactions between diphenylarsine and 3-diphenylphosphanyl-but-3-en-1-ol and 2-diphenylphosphanyl-prop-2-en-1-ol have been achieved using the organopalladium complex containing ortho-metallated (R)-[1-(dimethylamino)ethylnaphthalene as the chiral reaction template in high stereoselectivities under mild conditions. Hydroarsination of 3-diphenylphosphanyl-but-3-en-1-ol with diphenylarsine generated only one stereoisomer as five-membered As–P bidentate chelate on chiral naphthylamine palladium template. Using the same chiral metal template, similar hydroarsination reaction was carried out on 2-diphenylphosphanyl-prop-2-en-1-ol which gave two different products in the ratio of 2.6 to 1. The major isomer was identified as the expected five-membered As–P bidentate ligand and the minor isomer was identified as the elimination product. The naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid. Optically pure As–P ligands containing the hydroxy groups at the chiral carbon centres were prepared by ligand displacement. The absolute configuration and coordination properties of the complexes have been established by single crystal X-ray analysis.     

Readily available phosphine–imine ligands from α-phenylethylamine for highly efficient Pd-catalyzed asymmetric allylic alkylation

A series of novel chiral phosphine–imine ligands have been prepared by a two-step transformation from chiral α-phenylethylamine. The resulting chiral ligands were found to be effective for the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenylprop-2-en-1-yl pivalate with dimethyl malonate, in which up to 94% ee and 99% conversions were obtained. The results demonstrate that the chirality resided on the chelate ring of P–Pd–N complex is more effective for the transfer of the stereochemical information by comparison with the result obtained by Hashimoto and coworkers’ phosphine–imine ligand, in which the chirality lay in the outside of P–Pd–N chelate ring. The effect of solvent, base and substitutent in phosphine–imine ligand on this catalytic reaction is also described.     

Pd complexes of iminophosphine ligands: A homogeneous molecular catalyst for Suzuki–Miyaura cross-coupling reactions under mild conditions

The complexes formed by combining Pd(OAc)₂ and iminophosphine ligands (P^N) are active catalysts in Suzuki–Miyaura cross-coupling reactions under mild conditions. Aryl bromides and iodides, as well as benzyl chlorides give the corresponding coupled products in high yields at low temperatures (25–50 °C) using these catalysts. Iminophosphines containing the most sterically demanding groups attached to the N-imino moiety were the most effective ligands. New divalent Pd complexes of known iminophosphines were synthesised and their activity was compared with the in situ generated catalyst system. The complex resulting from the oxidative addition of 4-bromo anisole [Pd(4-CH₃OC₆H₄)Br(P^N)] was more active than the in situ generated system. However, palladacycles containing the iminophosphine ligand (e.g., {[C₆H₄CH(Me)₂St-Bu]Pd(P^N)}⁺PF₆⁻) were less active than the in situ generated catalyst due to the greater stability of the complexes that involve two bidentate ligands. Poisoning tests demonstrated that homogeneous mononuclear palladium species containing the iminophsophine ligand were responsible for the catalytic activity.     

Origin of enantioselectivity with heterobidentate sulfide-tertiary amine (sp) ligands in palladium-catalyzed allylic substitution

A series of new chiral heterobidentate sulfide-tertiary amine (sp³) ligands 3a-c, 6 were readily prepared from cheap and easily available (R)-cysteine and l-(+)-methionine. A Pd-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate was used as a model reaction to examine the catalytic efficiencies of these aziridine sulfide ligands, and ligand 3b afforded the enantioselectivity of up to 91% ee. The origin of enantioselectivity for heterobidentate sulfide-tertiary amine (sp³) ligands was first rationalized based on X-ray crystallographic data, and NMR spectroscopic data for relevant intermediate palladium allylic complexes. Our results demonstrated that the sulfur atom was a better π-allyl acceptor than the nitrogen atom for heterobidentate sulfide-tertiary amine (sp³) ligands, and the steric as well electronic properties of the palladium allylic complexes were crucial for the enantioselectivity.     

Preparation of “pore-fill” type Pd–YSZ–γ-Al2O3 composite membrane supported on α-Al2O3 tube for hydrogen separation

Mesoporous YSZ–γ-Al₂O₃ membranes were coated on α-Al₂O₃ (Ø2 mm) tube by dipping the α-Al₂O₃ support tube into mixed sol consists of nano-size YSZ and bohemite particles followed by drying and calcination at 600 °C. Addition of bohemite in YSZ sol helped a good adhesion and uniform coating of the membrane film onto α-Al₂O₃ support. The quality of the mesoporous YSZ–γ-Al₂O₃ membranes was evaluated by the gas permeability experiments. The number of defects was minimized when the γ-Al₂O₃ content became more than 40%. Addition of γ-Al₂O₃ inhibited the crystal growth of YSZ, sintering shrinkage and distortion stress. Increase of calcination temperature and time results in the increase of pore size and N₂ permeance. A hydrogen perm-selective membrane was prepared by filling palladium into the nano-pores of YSZ–γ-Al₂O₃ layer by vacuum-assisted electroless plating. Crystal growth of palladium was observed by thermal annealing of the membrane at 600 °C for 40 h. The Pd–YSZ–γ-Al₂O₃ composite membrane revealed improved thermal stability allowing long-term operation at elevated temperature (>500 °C). This has been attributed to the improved fracture toughness of YSZ–γ-Al₂O₃ layer and matching of thermal expansion coefficient between palladium and YSZ. Although fracture of the membrane did not occur, decline of H₂ flux was observed when the membrane was exposed in 600 °C. This has been attributed to the agglomeration of palladium particles by crystal growth and dense packing into the pore networks of YSZ–γ-Al₂O₃ by elevation of temperature.     

A directly ring-to-ring linked ferrocene–pseudotitanocene complex

Addition of excess ferrocenylacetylene (FcCCH) to [η⁵-(C₅H₅)Ti][μ:η²:η²-C₂(SiMe₃)₂]₂[η⁵-(C₅H₅)Mg] (1) affords the novel ferrocene–pseudotitanocene complex [η⁵-1,2,5,6-tetrakis(trimethylsilyl)-4-ferrocenylcyclohexa-1,4-dienyl](η⁵-cyclopentadienyl)titanium(II), [η⁵-(Me₃Si)₄FcC₆H₂]Ti(η⁵-C₅H₅) (2), as the sole isolated titanium-containing product. Its structure was established by EI MS, NMR and UV–vis spectroscopy. The formation of 2 follows the general reaction route of terminal acetylenes with 1.     

Asymmetric synthesis of a chiral arsinophosphine via a metal template promoted asymmetric Diels-Alder reaction between diphenylvinylphosphine and 2-furyldiphenylarsine

The asymmetric Diels-Alder reaction between 2-furyldiphenylarsine and diphenylvinylphosphine was achieved stereospecifically by utilizing an organoplatinum reaction promoter containing the ortho-metalated (R)-(1-(dimethylamino) ethyl)-naphthalene as the chiral auxiliary. The optically pure (+)-As-P heterobidentate cycloadduct could be liberated from the template product by successive treatment with concentrated hydrochloric acid and aqueous potassium cyanide.     

Modeling water exchange on monomeric and dimeric Mn centers

Water exchange on Mn centers in proteins has been modeled with density functional theory using the B3LYP functional. The reaction barrier for dissociative water exchange on [Mn^IV(H₂O)₂(OH)₄] is only 9.6 kcal mol^–1, corresponding to a rate of 6×10⁵ s^–1. It has also been investigated how modifications of the model complex change the exchange rate. Three cases of water exchange on Mn dimers have been modeled. The reaction barrier for dissociative exchange of a terminal water ligand on [(H₂O)₂(OH)₂Mn^IV(-O)₂Mn^IV(H₂O)₂(OH)₂] is 8.6 kcal mol^–1, while the bridging oxo group exchange with a ring-opening mechanism has a barrier of 19.2 kcal mol^–1. These results are intended for interpretations of measurements of water exchange for the oxygen evolving complex of photosystem II. Finally, a tautomerization mechanism for exchange of a terminal oxyl radical has been modeled for the synthetic O₂ catalyst [(terpy)(H₂O)Mn^IV(-O)₂Mn^IV(O)(terpy)]³⁺ (terpy=2,2:6,2-terpyridine). The calculated reaction barrier is 14.7 kcal mol^–1.Contribution to the Björn Roos Honorary Issue     

Photoswitching of the third-order nonlinear optical properties of azobenzene-containing phthalocyanines based on reversible host–guest interactions

Two water soluble azobenzene and phthalocyanine dyads with D–π–A alignment were synthesized. It was found that both compounds showed very large molecular cubic hyperpolarizabilities which are at the order of 10⁻³⁰ esu as the result of their unique chemical structure. The azobenzene moieties of these compounds, upon alternating illumination of UV and visible light, could reversibly associate with α-CD to form inclusion complexes through host–guest interaction in aqueous media, resulting in apparent influences to the 3rd NLO properties of these compounds. This influence is especially significant for the phthalocyanine whose central metal atom is copper (II). The molecular cubic hyperpolarizability γ of the inclusion complex for the copper phthalocyanine is 2.1 × 10⁻³⁰ esu. When the inclusion complex dissociated under the illumination of 365 nm light, γ value increased to 4.2 × 10⁻³⁰ esu, which is a 100% enhancement. Taking account of the large molecular cubic hyperpolarizabilities of these compounds, the present materials are potential as ideal 3rd NLO photoswitching systems.     

Synthesis and catalytic properties of novel ruthenium N-heterocyclic-carbene complexes

The reaction of [RuCl₂(p-cymene)]₂ with 1,3-dialkylimidazolinium salts 1a–f in the presence of a small excess of cesium carbonate yields chelated η⁶-arene, η¹-carbene ruthenium complexes 2a–f. All synthesised compounds were characterized by elemental analysis, NMR spectroscopy. The catalytic activity of RuCl₂(η⁶-arene, η¹-imidazolinylidene) complexes 2a–f was evaluated in the direct arylation of 2-phenylpyridine with chlorobenzene derivatives.     

Interaction of N-(aryl)picolinamides with iridium. N–H and C–H bond activations

Reaction of N-(4-R-phenyl)picolinamide (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords two yellow complexes (1-R and 2-R). The 1-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines, a chloride and a hydride. The 2-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines and two hydrides. Similar reaction of N-(naphthyl)picolinamide with [Ir(PPh₃)₃Cl] affords two organometallic complexes, 3 and 4. In complex 3 the amide ligand is coordinated to the metal center, via C–H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor, along with two triphenylphosphines and one chloride. Complex 4 is similar to complex 3, except a hydride is bonded to iridium instead of the chloride. Structures of the 1-OCH₃, 2-Cl and 4 complexes have been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ir^III–Ir^IV oxidation within 0.50–1.16 V vs. SCE and a reduction of the coordinated amide ligand within −1.02 to −1.25 V vs. SCE.     

A Direct Carbon-13 and Nitrogen-15 NMR Study of Cerium(III)–Isothiocyanate Complexation in Aqueous Solvent Mixtures

A study of lanthanide complexation with isothiocyanate is underway using a multinuclear magnetic resonance technique. For isothiocyanate solutions in water–acetone–Freon mixtures at low temperature, –85––125°C, ligand exchange is slow enough to permit the observation of ¹³C and ¹⁵N NMR signals for coordinated and free anions. For the Ce³⁺–NCS^– system, four coordinated anion signals, displaced from the free anion signal by about +450 to +550 ppm for ¹⁵N and +50 to +80 ppm for ¹³C, are observed. The ¹³C and ¹⁵N spectral data are complementary, showing a signal area concentration dependence and measured coordination numbers consistent with the formation of Ce(NCS)²⁺ through Ce(NCS)^1- ₄. In water–methanol, the extent of complexing is decreased, presumably because of the higher dielectric constant of this medium. In addition, the results of a competitive study of NCS^– and Cl^– ion binding, carried out using ³⁵Cl NMR, is presented.     

Driving forces in substitution reactions of octahedral complexes: The influence of the competitive effect

The reaction of cis-[RuCl₂(P–P)(N–N)] type complexes (P–P = 1,4-bis(diphenylphosphino)butane or (1,1′-diphenylphosphino)ferrocene; N–N = 2,2′-bipyridine or 1,10-phenantroline) with monodentate ligands (L), such as 4-methylpyridine, 4-phenylpyridine and benzonitrile forms [RuCl(L)(P–P)(N–N)]⁺ species. Upon characterization of the isolated compounds by elemental analysis, ³¹P{¹H} NMR and X-ray crystallography it was found out that the type of the L ligand determines its position in relation to the phosphorus atom. While pyridine derivatives like 4-methylpyridine and 4-phenylpyridine coordinate trans to the phosphorus atom, the benzonitrile ligand (bzCN), a good π acceptor, coordinates trans to the nitrogen atom. A ³¹P{¹H} NMR experiment following the reaction of the precursor cis-[RuCl₂(dppb)(phen)] with the benzonitrile ligand shows that the final position of the entering ligand in the complex is better defined as a consequence of the competitive effect between the phosphorus atom and the cyano-group from the benzonitrile moiety and not by the trans effect. In this case, the benzonitrile group is stabilized trans to one of the nitrogen atoms of the N–N ligand. A differential pulse voltammetry experiment confirms this statement. In both experiments the [RuCl(bzCN)(dppb)(phen)]PF₆ species with the bzCN ligand positioned trans to a phosphorus atom of the dppb ligand was detected as an intermediate complex.     

Conformational dependence of the intrinsic acidity of the aspartic acid residue sidechain in N-acetyl- -aspartic acid-N′-methylamide

The sidechain conformational potential energy hypersurfaces (PEHS) for the γ_L, β_L, α_L, and α_D backbone conformations of N-acetyl- -aspartate-N′-methylamide were generated. Of the 81 possible conformers initially expected for the aspartate residue, only seven were found after geometric optimizations at the B3LYP/6-31G(d) level of theory. No stable conformers could be located in the δ_L, _L, γ_D, δ_D, and _D backbone conformations. The ‘adiabatic’ deprotonation energies for the endo and exo forms of N-acetyl- -aspartic acid-N′-methylamide were calculated by comparing their optimized relative energies against those found for the seven stable conformers of N-acetyl- -aspartate-N′-methylamide. Sidechain conformational PEHSs were also generated for the estimation of ‘vertical’ deprotonation energies for both endo and exo forms of N-acetyl- -aspartic acid-N′-methylamide. All backbone–sidechain (N–H⁻O–C) and backbone–backbone (N–HO=C) hydrogen bond interactions were analyzed. A total of two backbone–backbone and four backbone–sidechain interactions were found for N-acetyl- -aspartate-N′-methylamide. The deprotonated sidechain of N-acetyl- -aspartate-N′-methylamide may allow the aspartyl residue to form strong hydrogen bond interactions (since it is negatively charged) which may be significant in such processes as protein–ligand recognition and ligand binding. As a primary example, the molecular geometry of the aspartyl residue may be important in peptide folding, such as that in the RGD tripeptide.     

Voltammetric study of the interaction of lomefloxacin (LMF)–Mg(II) complex with DNA and its analytical application

The voltammetric behavior of the LMF-Mg(II) complex with DNA at a mercury electrode is reported for the first time. In NH₃–NH₄Cl buffer (pH=9.10), the adsorption phenomena of the LMF–Mg(II) complex were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be a reduction of LMF in the complex, and the composition of the LMF–Mg(II) complex is 2:1. In the presence of calf thymus DNA (ctDNA), the peak current of LMF–Mg(II) complex decreased considerably, and a new well-defined adsorptive reduction peak appeared at −1.63 V (vs. SCE). The electrochemical kinetic parameters and the binding number of LMF–Mg(II) with ctDNA were also obtained. Moreover, the new peak currents of LMF–Mg(II)–DNA system increased linearly correlated to the concentration of DNA in the 4.00×10⁻⁷–2.60×10⁻⁶ g ml⁻¹ range when the concentrations of LMF–Mg(II) complex was fixed at 5.00×10⁻⁶ mol l⁻¹, with the detection limits of 2.33×10⁻⁷ g ml⁻¹. An electrostatic interaction was suggested by electrochemical method.     

Some observations on the solvent extraction and spectrophotometric determination of palladium using 3,4-dihydro-4,4,6-trimethyl-2(1H)-pyrimidine-thione as a selective reagent

A highly selective spectrophotometric method is described for the determination of palladium, using 3,4-dihydro-4,4,6-trimethyl-2(1H)-pyridinethione (DTPT). The intense yellow 1:2 complex is extractable in chloroform from aqueous solution of pH 5.5. The maximum absorption occurs at 420 nm, ε = 3.90 × 10⁴ liter/mol⁻¹ cm⁻¹ and the sensitivity of the determination is 0.023 μg/ml. Palladium can be determined over a range of 0.4–24.6 ppm. CN⁻ interferes in this determination and should be absent. The method is applied to the determination of palladium in hydrogenation catalysts.