Reductive decomplexation of pi-allyltricarbonyliron lactone complexes; a new route to stereo-defined 1,7-diols and 2,3-diene-1,7-diols

Treatment of pi-allyltricarbonyliron lactone complexes bearing an adjacent leaving group, with lithium naphthalenide causes decomplexation to acyclic dienols in excellent yield and without any scrambling of the allylic centre.     

Manganese η2-complexes as auxiliaries for stereoselective aldol synthesis of allenyl carbinols

A convenient and robust manganese auxiliary was linked via an η(2)-bond to alkynyl esters and ketones using a mild complexation reaction with methylcyclopentadienyl manganese tricarbonyl. This complex readily underwent aldol reactions with exclusive α-substitution and in good diastereoselectivities especially with aryl ketone substrates. This selectivity has been rationalized using a cyclic transition state model in which the manganese auxiliary plays a critical role in promoting E(O)-geometry of the cumulenolate intermediate.     

Stereoselective synthesis of the octahydronaphthalene unit of integramycin via an intramolecular Diels-Alder reaction

[reaction: see text] The racemic cis-decalin core fragment 30 of integramycin was synthesized by a sequence involving a highly diastereoselective intramolecular Diels-Alder reaction of triene 24. A remarkable switch in stereoselectivity occurred upon changing the dienophile unit of 24 from (Z)- to (E)-geometry.     

Dinuclear platinum complexes with biological relevance based on the 1,2-diaminocyclohexane carrier ligand

The synthesis of bifunctional dinuclear platinum complexes, [{PtCl(dach)}(2)-mu-Y](n+)Cl(n) (1-3; Y = H(2)N(CH(2))(3)NH(2)(CH(2))(4)NH(2), H(2)N(CH(2))(6)NH(2)(CH(2))(6)NH(2), and H(2)N(CH(2))(6)NH(2)(CH(2))(2)NH(2)(CH(2))(6)NH(2), respectively; Figure 1) is reported. There was no labilization of the polyamine linker groups of the cis-1,2-diaminocyclohexane complexes in the presence of sulfur-containing species at physiological pH, in contrast to previous studies preformed on trans complexes. Metabolism reactions are somewhat dependent on the nature of the polyamine: at physiological pH, the spermidine complex 1 forms an inert (tetraamine)platinum species in which one platinum is chelated by a central and terminal amino group. The stability of cis-geometry complexes may make them viable second-generation polynuclear platinum clinical candidates.     

Efficient hydrogenation of biomass-derived cyclic di-esters to 1,2-diols

The unprecedented homogeneous hydrogenation of cyclic di-esters, in particular biomass-derived glycolide and lactide, to the corresponding 1,2-diols is catalyzed by Ru(II) PNN (1) and Ru(II) CNN (2) pincer complexes under mild hydrogen pressure and (in the case of 1) neutral conditions. No racemization was observed when a chiral di-ester was used.     

Some benzimidazole amide complexes of ruthenium(II)

The synthesis and characterisation of ruthenium(II) complexes with 2-amidobenzimidazoles are reported. The complexes RuCl₂(DMSO)₄ and RuCl₂(PPh₃) react with 2-(acetamido)benzimidazole (AB) and 2-(benzamido)benzimidazole (BB) it acetone to give products of the type [Ru(L)₂(N−O)₂]Cl₂ [L=DMSO, PPh₃, N−O=AB, BB). The displacement reactions are faster in the case of methyl (AB) than phenyl (BB) substituted ligands. The ligands are bifunctional chelating agents coordinating through the tertiary nitrogen of benzimidazole ring and amide oxygen. The complexes are characterised based on their elemental analysis, conductivity data, infrared,¹H and³¹P nmr spectra. Acis-geometry is proposed for all the complexes reported.     

Metal complexes of mercaptoamines—II: New assignment of the structure of bis(β-mercaptoethylamine)nickel(II) and related compounds

A cis-geometry was proposed for the green complex bis(β-mercaptoethylamine) nickel(II) on the basis of its reactivity, IR spectroscopy and orbital geometry considerations. The crystal and molecular structure of this complex has been determined by X-ray diffraction and shows that the complex has a trans geometry. The crystal structure consists of trans-NiL₂ molecules strongly linked through NH-S hydrogen bonds. Electronic and IR spectroscopy, and powder X-ray analysis are in agreement with the same trans geometry for the complexes bis(β-mercaptoethylamine)palladium(II) and bis-(γ-mercaptopropylamine)nickel(II). As a result of the new assignment of the structure of these complexes some concepts, such as its reactivity with nickel(II), are reconsidered.     

The complexation of a novel squaric bis(thiosemicarbazone); 3,4-bis{[(aminothioxomethyl)amino]azamethylene}cyclobut-ene-1,2-diol

A novel chelating agent (Sqtsc; H(4)L) bearing both hard and soft donor atoms was synthesized by the condensation of squaric acid with thiosemicarbazide. The ligand has two symmetrical sets of donor atoms (SNO), therefore, it was allowed to react with the metal ions at the mole ratio 2:1 (M:L). Mono- and bi-nuclear chelates were obtained in which the ligand showed a variety of modes of bonding viz. (OO)(2-), (SNNS)(2-) and (SNO)(2-) per each metal ion supporting the ambidentate and flexidentate characters of the ligand. The mode of bonding and basicity of the ligand depend mainly on the type of the metal cation and its counter anion. All the obtained complexes have the preferable O(h)-geometry except the VO(II)-complex (7) which has also the preferable square pyramid geometry. Structural elucidation was achieved via elemental and spectral data.     

De novo synthesis of 2-substituted syn-1,3-diols via an iterative asymmetric hydration strategy

The enantioselective syntheses of several protected 4-substituted syn-3,5-dihydroxy carboxylic esters have been achieved from the corresponding achiral (E,E)- or (E,Z)-1,3-dienoates. The route relies upon an enantio- and regioselective Sharpless dihydroxylation and a palladium-catalyzed reduction to form gamma-substituted delta-hydroxy-1-enoates. The resulting delta-hydroxy-1-enoates are subsequently converted into benzylidene-protected 4-substituted syn-3,5-dihydroxy carboxylic esters in one step. The benzylidene-protected 3,5-dihydroxy carboxylic esters are produced in good overall yields (20-54%) and high enantiomeric excess (73-97% ee).     

Unprecedented long-range 1,7-bromination in gold complexes of N-(aryl)imino functionalized N-heterocyclic carbenes

An unique long-range 1,7-bromination reaction is observed in gold(iii) complexes of N-(aryl)imino functionalized N-heterocyclic carbene with the bromination occurring at two different carbon (sp(2) and sp(3)) centers spatially separated by ca. 6.4 A but existing in extended conjugation to each other. In particular, the unusual distant 1,7-brominated gold(iii) complexes [1-R-3-{N-(p-bromo-2,6-di-i-propylphenylimino)-2-phenyl-1-bromoethyl}imidazol-2-ylidene]AuBr(3) [R = Me (), i-Pr (), t-Bu (), -CH(2)Ph ()] were synthesized cleanly at room temperature under ambient conditions from the reactions of molecular bromine with the gold(i) complexes [1-R-3-{N-(2,6-di-i-propylphenylimino)-2-phenylethyl}imidazol-2-ylidene]AuCl [R = Me (), i-Pr (), t-Bu (), -CH(2)Ph ()]. All of the 1,7-bromination products (, , and ) have been structurally verified by X-ray diffraction studies.     

Molecular orbital calculations on η3- and η5-hexadienyl complexes of the platinum metals; an analysis of their conformational preferences and fluxional characteristics

Extended Hückel molecular orbital calculations are reported for the η³- and η⁵-platinum metal complexes [Pt(η³-C₆H₇)(PH₃)₂]⁺ and [Pt(η⁵-C₆H₇)(PH₃)₂]⁺. The η³-geometry is found to be only 0.56 eV more stable than the η⁵-geometry. This leads to a low energy fluxional process in these molecules. The stereochemistry of this fluxional process is rationalised in terms of the conformational preferences of the [Pt(η⁵-C₆H₇)(PH₃)₂]⁺ ions.     

Crystal and molecular structure of [N N′-(3,3′-dipropylamine)-bis(salicylideneaminato)monoacetate]cobalt(III) complex

The crystal and molecular structure of [N,N′-(3,3′-dipropylamine)bis(salicyclideneaminato)-monoacetate]cobalt(III) complex has been determined by a three-dimensional X-ray diffraction study. In the complex, the cobalt ion has an octahedral coordination environment with cis-geometry for the two salicyclideneaminato moieties. An oxygen atom of the acetate ion is coordinated to cobalt ion and another oxygen atom is hydrogen-bonded to the secondary amine nitrogen atom.     

Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A)

The [H(+)]-catalyzed dissociation rate constants of several trivalent lanthanide (Ln) complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (LnDO2A(+), Ln = La, Pr, Eu, Er and Lu) have been determined in two pH ranges: 3.73-5.11 and 1.75-2.65 at four different temperatures (19-41.0 °C) in aqueous media at a constant ionic strength of 0.1 mol dm(-3) (LiClO(4)). For the study in the higher pH range, i.e. pH 3.73-5.11, copper(II) ion was used as the scavenger for the free ligand DO2A in acetate/acetic acid buffer medium. The rates of Ln(III) complex dissociation have been found to be independent of [Cu(2+)] and all the Ln(III) complexes studied show [H(+)]-dependence at low acid concentrations but become [H(+)]-independent at high acid concentrations. Influence of the acetate ion content in the buffer on the dissociation rate has also been investigated and all the complexes exhibit a first-order dependence on [Acetate]. The dissociation reactions follow the rate law: k(obs) = k(Ac)[Acetate] + K'k(lim)[H(+)]/(1 + K'[H(+)]) where k(AC) is the dissociation rate constant for the [Acetate]-dependent pathway, k(lim) is the limiting rate constant, and K' is the equilibrium constant for the reaction LnDO2A(+) + H(+) ? LnDO2AH(2+). In the lower pH range, i.e. pH 1.75-2.65, the dye indicator, cresol red, was used to monitor the dissociation rate, and all the Ln(III) complexes also show [H(+)]-dependence dissociation pathways but without the rate saturation observed at higher pH range. The dissociation reactions follow the simple rate law: k(obs) = k(H)[H(+)], where k(H) is the dissociation rate constant for the pathway involving monoprotonated species. The absence of an [H(+)]-independent pathway in both pH ranges indicates that LnDO2A(+) complexes are kinetically rather inert. The obtained k(AC) values follow the order: LaDO2A(+) > PrDO2A(+) > EuDO2A(+) > ErDO2A(+) > LuDO2A(+), whereas the k(lim) and k(H) values follow the order: LaDO2A(+) > PrDO2A(+) > ErDO2A(+) > EuDO2A(+) > LuDO2A(+), mostly consistent with their thermodynamic stability order, i.e. the more thermodynamically stable the more kinetically inert. In both pH ranges, activation parameters, ΔH*, ΔS* and ΔG*, for both acetate-dependent and proton-catalyzed dissociation pathways have been obtained for most of the La(III), Pr(III), Eu(III), Er(III) and Lu(III) complexes, from the temperature dependence measurements of the rate constants in the 19-41 °C range. An isokinetic (linear) relationship is found between ΔH* and ΔS* values, which supports a common reaction mechanism.     

Iron complexes of (e)- and (z)-1,2-dichlorodisilenes

Novel disilene-iron complexes [(E)- (1E) and (Z)-(eta2-R3SiClSi=SiClSiR3)Fe(CO)4 (1Z), SiR3 = tBu2MeSi] were synthesized by the reaction of the corresponding tetrachlorodisilane with an excess amount of K2Fe(CO)4, and the structures of 1E and 1Z were determined by X-ray crystallography. These complexes constitute not only the first transition-metal complexes with E,Z-isomerism but also the first complexes with halogen-substituted disilene ligands. The initial formation of 1Z during the synthetic reaction and the slow one-way isomerization of 1Z to 1E are rationalized by the intervention of the corresponding silylene complex (R3SiCl2Si)(R3Si)Si=Fe(CO)4.     

Synthesis and DNA-binding properties of dinuclear platinum(II)-amine complexes of 1,7-dicarba-closo-dodecaborane(12)

The first examples of platinum(II)-amine complexes containing a dicarba-closo-dodecaborane(12) (carborane) moiety are described; preliminary in vitro DNA-binding experiments indicate that the complexes are capable of targetting plasmid DNA.     

Representation of Zn(II) complexes in polarizable molecular mechanics. Further refinements of the electrostatic and short-range contributions. Comparisons with parallel ab initio computations

We present refinements of the SIBFA molecular mechanics procedure to represent the intermolecular interaction energies of Zn(II). The two first-order contributions, electrostatic (E(MTP)), and short-range repulsion (E(rep)), are refined following the recent developments due to Piquemal et al. (Piquemal et al. J Phys Chem A 2003, 107, 9800; and Piquemal et al., submitted). Thus, E(MTP) is augmented with a penetration component, E(pen), which accounts for the effects of reduction in electronic density of a given molecular fragment sensed by another interacting fragment upon mutual overlap. E(pen) is fit in a limited number of selected Zn(II)-mono-ligated complexes so that the sum of E(MTP) and E(pen) reproduces the Coulomb contribution E(c) from an ab initio Hartree-Fock energy decomposition procedure. Denoting by S, the overlap matrix between localized orbitals on the interacting monomers, and by R, the distance between their centroids, E(rep) is expressed by a S(2)/R term now augmented with an S(2)/R(2) one. It is calibrated in selected monoligated Zn(II) complexes to fit the corresponding exchange repulsion E(exch) from ab initio energy decomposition, and no longer as previously the difference between (E(c) + E(exch)) and E(MTP). Along with the reformulation of the first-order contributions, a limited recalibration of the second-order contributions was carried out. As in our original formulation (Gresh, J Comput Chem 1995, 16, 856), the Zn(II) parameters for each energy contribution were calibrated to reproduce the radial behavior of its ab initio HF counterpart in monoligated complexes with N, O, and S ligands. The SIBFA procedure was subsequently validated by comparisons with parallel ab initio computations on several Zn(II) polyligated complexes, including binuclear Zn(II) complexes as in models for the Gal4 and beta-lactamase metalloproteins. The largest relative error with respect to the RVS computations is 3%, and the ordering in relative energies of competing structures reproduced even though the absolute numerical values of the ab initio interaction energies can be as large as 1220 kcal/mol. A term-to-term identification of the SIBFA contributions to their ab initio counterparts remained possible even for the largest sized complexes.     

Stereodivergent approach to beta-hydroxy alpha-amino acids from C(2)-symmetrical alk-2-yne-1,4-diols

[reaction: see text] A new stereodivergent route to erythro- and threo-beta-substituted serines from a common C(2)-symmetrical alk-2-yne-1,4-diol is described. Stereocontrol in such an acyclic system is achieved by taking advantage of symmetry. Stereoselective alkyne reduction to either (Z)- or (E)-olefin allows selection of the stereochemistry of alpha-carbon in the final amino acid by using a Pd(0)-catalyzed process. This strategy has been applied to the synthesis of (2S,3S)-3-hydroxyleucine.     

带恶唑啉衍生物侧链的1，7-二氮-12-冠-4的合成及其应用

用手性α-氨基醇和原氯乙酸三乙酯作用得到的手性恶唑啉2a-2c和1，7-二氯- 12-冠-4反应，合成了带手性侧链的1，7-二氯-12-冠-4衍生物1a-1c，并将其铜配 合物运用于重氮醋酸酯对烯烃的不对称环丙烷化反应．     

Stereoselective synthesis of vinylic chalcogenides through vinylic substitution by lithium organylchalcogenolates

Enol phosphates and enol tosilates of β-dicarbonyl compounds react with lithium organoselenolates to give β-organoseleno (Z)-α,β-unsaturated carbonyl compounds. Tetrasubstituted vinylic vic-bis(organylchalcogenides) of (E)-geometry have been prepared by this method.     

Dielectric relaxation studies of monoalcohol solutions with 1,2-diols

Dielectric relaxation processes in solutions of monoalcohols with ethanediol, 1,2-propanediol, and 1,2-butanediol have been studied using the dielectric time domain spectroscopy technique. The dielectric spectrum has been analyzed in terms of two dispersions. The influence of varying content of diol, and of chainlength of the alcohol, on the static dielectric constant _s and on the main dispersion is reported. For the long-chain alcohols _s is lowered and the main relaxation time successively shortened with the addition of diol to the solution. The results are compared to these found for aqueous alcohol solutions and discussed in terms of a structural model with rupturing of predominantly linear complexes on addition of water, diols, or on increasing the temperature.