Synthesis and ring size effect of macrocyclic ethynylhelicene oligomers

[reaction: see text] The cyclization of acyclic ethynylhelicene oligomers with decyl 3,5-diiodobenzoate under optimized conditions gave the corresponding optically active [n+n]cycloalkynes (n = 4-8) in high yields. Their structures were compared in terms of ring size by using (1)H NMR, UV-vis, and CD spectroscopies and vapor pressure osmometry (VPO). The UV-vis spectra exhibited an increase in absorbance in proportion to n. In contrast, the CD spectra of the macrocycles exhibited a large ring size effect, comparable Deltaepsilon values despite the increase in n and temperature-dependent properties of the [8+8]cycloalkyne. It was concluded that [4+4]cycloalkyne, [5+5]cycloalkyne, [6+6]cycloalkyne, and [7+7]cycloalkyne have rigid structures, while [8+8]cycloalkyne has a flexible structure.     

Functionalized [3 + 3]cycloalkynes: substituent effect on self-aggregation by nonplanar pi-pi interactions

[reaction: see text] Optically active (M)-2,11-dihydroxy-1,12-dimethylbenzo[c]phenanthrene-5,8-dicarbonitrile was synthesized from (M)-1,12-dimethyl-2,11-dinitrobenzo[c]phenanthrene-5,8-dicarbonitrile by the reduction and hydroxylation of nitro groups. The compound was converted to several oxygen-functionalized [3 + 3]cycloalkynes with -OH, -OSiMe2-t-Bu, -OAc, -OTf, or -ONf groups, which are chiral arylene ethynylene macrocycles containing three helicenes. The aggregation behaviors of these [3 + 3]cycloalkynes were examined in CHCl3, THF, and acetone using 1H NMR, CD, and vapor pressure osmometry (VPO) studies and were compared with that of the parent [3 + 3]cycloalkyne. An increasing strength of aggregation in CHCl3 was observed in the following order of the substituted derivatives: -H > -ONf > -OTf > -OAc > -OSiMe2-t-Bu. In THF the following strength of aggregation was observed: -OTf > -ONf > -OAc > -H > -OSiMe(2)-t-Bu > -OH. The aggregation of the functionalized [3 + 3]cycloalkynes is stronger for the compounds with electron-withdrawing substituents than for those with electron-donating substituents. (M)-1,12-dimethylbenzo[c]phenanthrene-2,5,8,11-tetraol was also synthesized from the same intermediate. This electron-rich helicene was readily oxidized to 5,6-quinone in air, and the quinone was suggested to form a self-charge-transfer complex in solid state.     

3+3]Cycloalkyne dimers linked by an azo group: a stable cis-azo compound forms polymeric aggregates by nonplanar pi-pi interactions

We previously reported that the structure of the linker moiety controlled intramolecular and bimolecular aggregation of [3+3]cycloalkyne oligomers, which are cyclic acetylene derivatives containing helicenes. Here, novel [3+3]cycloalkyne dimers linked by the azo group are synthesized, and aggregation behaviors are studied, which turned out to be considerably different. The trans- and cis-azo compounds were synthesized by the oxidative coupling of a [3+3]cycloalkyne amine derivative, and the stereochemistry was determined by UV-vis spectroscopy. 1H NMR, CD, gel permeation chromatography analysis, and vapor pressure osmometry in chloroform revealed that the trans-isomer forms a strong and selective bimolecular aggregate. The cis-isomer forms a trimolecular aggregate at a concentration below 1 mM and a polymeric aggregate at above 1 mM. Unlike known diaryl azo compounds, these azo isomers do not interconvert when subjected to heating or irradiation. In contrast, a model compound lacking the cyclic helicene structure isomerizes readily.     

The Cycloaddition Reactions of Angle Strained Cycloalkynes. A Theoretical Study

The potential energy surfaces for the cycloaddition reactions of angle strained cycloalkynes to ethylene have been studied using ab initio methods. All the stationary points were determined with the MP2/6–311G (d,p) method with some calculations performed at the CCSD(T)/6–311G++G(d,p)//MP2/6–311G (d,p) level. Three kinds of cycloalkyne species, including monocyclic alkynes, bridged bicyclic alkynes, and heterocyclic alkynes, have been chosen in this work as model reactants. Two different reaction pathways have been proposed: (A) 1,2‐carbon shift and (B) 1,2‐hydrogen shift. That is, reactants → [2+1]‐TS‐1 → spiro‐carbene intermediate → (A) TS‐A → Pro‐A or (B) TS‐B → Pro‐B. As a result, it is found that ground‐state cycloalkyne appears to react more like a monocarbene than like an alkyne or a vicinal dicarbene as conventionally proposed. Our theoretical investigations also suggest that a cycloalkyne with a small C‐C≡C bond angle should be a good candidate for cycloaddition to an olefin. Moreover, in the cycloaddition reaction of a small (≤ six‐membered ring six‐membered) ring cycloalkyne, both 1,2‐carbon and 1,2‐hydrogen migrations will compete with each other. On the other hand, reactions involving larger (≥ seven‐membered) ring cycloalkynes should proceed with a 1,2‐carbon shift, leading to the major [2+2] cycloadduct. Furthermore, a configuration mixing model has been used to rationalize the computational results and to develop an explanation for the barrier heights. The results obtained allow a number of predictions to be made.     

Structural Distortion of Cycloalkynes Influences Cycloaddition Rates both by Strain and Interaction Energies

The reactivities of 2-butyne, cycloheptyne, cyclooctyne, and cyclononyne in the 1,3-dipolar cycloaddition reaction with methyl azide were evaluated through DFT calculations at the M06-2X/6-311++G(d)//M06-2X/6-31+G(d) level of theory. Computed activation free energies for the cycloadditions of cycloalkynes are 16.5–22.0 kcal mol⁻¹ lower in energy than that of the acyclic 2-butyne. The strained or predistorted nature of cycloalkynes is often solely used to rationalize this significant rate enhancement. Our distortion/interaction–activation strain analysis has been revealed that the degree of geometrical predistortion of the cycloalkyne ground-state geometries acts to enhance reactivity compared with that of acyclic alkynes through three distinct mechanisms, not only due to (i) a reduced strain or distortion energy, but also to (ii) a smaller HOMO–LUMO gap, and (iii) an enhanced orbital overlap, which both contribute to more stabilizing orbital interactions.     

Mobile phase variations in thermospray liquid chromatography-mass spectrometry of pesticides

The effect of four different mobile phase compositions with reversed-phase methanol-water (50:50) + 0.05 M ammonium acetate, methanol-water (50:50) + 0.05 M ammonium formate, acetonitrile-water (50:50) + 0.05 M ammonium acetate and acetonitrile-water (50:50) + 0.05 M ammonium formate were compared in filament-on thermospray liquid chromatography-mass spectrometry for the determination of carbamate and chlorotriazine pesticides. In the positive-ion mode, [M + H]+ and [M + NH4]+ were generally the base peaks for the chlorotriazines and the carbamates, respectively. Depending on the mobile phase used, other adduct ions obtained corresponded to [M + CH3CN + H]+, [M + CH3OH + NH4]+, [M + CH3COONH4 + NH4 - 2H2O]+, [M + CH3CN + NH4]+, [M + CH3COONH4 + H - H2O]+ and the dimer [2M + H]+. In the negative-ion mode, [M - H]- and adducts with the ionizing additive [M + CH3COO]- or [M + HCOO]- were obtained. Other ions for the carbamates carbaryl and oxamyl corresponded to [M - CONHCH3 + CH3COOH]- and [M - CON(CH3)2 + HCOO]-, respectively. The variation of mobile phase composition provides additional structural information in thermospray liquid chromatography-mass spectrometry with no appreciable loss of sensitivity. Applications are reported for the determination of carbamate and chlorotriazine pesticides at the ng/g level in spiked and real soil samples, respectively.     

Low valent phosphorus in the molecular anions [P5Se12]5- and beta-[P6Se12]4-: phase change behavior and near infrared second harmonic generation

The caesium salts of the novel molecular anions [P5Se12]5- and [P6Se12]4- are phase change materials and exhibit near infrared, non-linear optical second harmonic generation; [P5Se12]5- is a coordination complex with an octahedral P3+ center chelated by two [P2Se6]4- ligands whereas [P6Se12]4- features a [P2]4+ dimer chelated by two [P2Se6]4- ligands.     

A non-covalent dimer formed in electrospray ionisation mass spectrometry behaving as a precursor for fragmentations

A non-covalent-bonded dimer was detected in the positive ion electrospray ionisation (ESI) mass spectra of a synthetic impurity. In tandem mass spectrometry (MS/MS) experiments using collision-induced dissociation (CID), the ion was found to behave as a [M+H]+-type precursor ion for fragmentation until MS5. The dimer was probably formed through multi-hydrogen bonds over a proton bridge. When the fragmentation occurred at the center of the bridge, the dimer was broken apart to give monomer fragments at MS6. However, no corresponding deprotonated dimer [2M-H]- was found in the negative ion ESI spectra. The dimer was extremely stable, and it could still be observed when a fragmentation voltage of up to 50 V was applied in the ionisation source. The formation of the non-covalent dimer was also found to be instrument-dependent, but independent of sample concentration. Accurate mass measurements of the [2M+H]+ and [M+H]+ ions, and their MSn product ions, provided the basis for assessing the fragmentation mechanism proposed for [2M+H]+. The fragmentation pathway was also illustrated for the deprotonated molecule [M-H]-.     

Heteroatom‐embedded Medium‐Sized Cycloalkynes: Concise Synthesis,Structural Analysis,and Reactions

A variety of medium‐sized cycloalkynes were efficiently synthesized by the double Nicholas reaction of cobalt complex and bis(hetero)substituted acyclic compound. The alkyne moiety within the ring has a unique bent structure and high reactivity toward cycloaddition reactions. Furthermore, preparation of multifunctionalized alkynes was achieved by embedding the cycloalkyne within a peptide chain.     

Heteroatom‐embedded Medium‐Sized Cycloalkynes: Concise Synthesis,Structural Analysis,and Reactions

A variety of medium‐sized cycloalkynes were efficiently synthesized by the double Nicholas reaction of cobalt complex and bis(hetero)substituted acyclic compound. The alkyne moiety within the ring has a unique bent structure and high reactivity toward cycloaddition reactions. Furthermore, preparation of multifunctionalized alkynes was achieved by embedding the cycloalkyne within a peptide chain.     

The first enantiomerically pure [n]triangulanes and analogues: sigma-[n]helicenes with remarkable features

(M)-(-)- and (P)-(+)-Trispiro[2.0.0.2.1.1]nonanes [(M)- and (P)-3] as well as (M)-(-)- and (P)-(+)-tetraspiro[2.0.0.0.2.1.1.1]undecanes [(M)- and (P)-4]-enantiomerically pure unbranched [4]- and [5]triangulanes-have been prepared starting from racemic bicyclopropylidenecarboxylic [(1RS)-12] and exo-dispiro[2.0.2.1]heptane-1-carboxylic [(1RS,3SR)-13] acids. The optical resolutions of rac-12 and rac-13 furnished enantiomerically pure acids (S)-(+)-12, (R)-(-)-12, (1R,3S)-(-)-13, and (1S,3R)-(+)-13. The ethyl ester (R)-25 of the acid (R)-(-)-12 was cyclopropanated to give carboxylates (1R,3R)-26 and (1R,3S)-26. The ester (1R,3S)-26 and acids (1R,3S)-13 and (1S,3R)-13 were converted into enantiomerically pure methylene[3]triangulanes (S)-(-)- and (R)-(+)-28. An alternative approach consisted of an enzymatic deracemization of endo-[(1SR,3SR)-dispiro[2.0.2.1]heptyl]methanol (rac-20) or anti-[(1SR,3RS)-4-methylenespiropentyl]methanol (rac-18). This afforded (S)-(-)- and (R)-(+)-28 (starting from rac-20), as well as enantiomerically pure (M)-(-)- and (P)-(+)-1,4-dimethylenespiropentanes [(M)- and (P)-23] starting from rac-18. The methylenetriangulanes (S)-(-)- and (R)-(+)-28 were cyclopropanated furnishing (M)- and (P)-3. The rhodium-catalyzed cycloaddition of ethyl diazoacetate onto (S)-(-)- and (R)-(+)-28 yielded four diastereomeric ethyl trispiro[2.0.0.2.1.1]nonane-1-carboxylates in approximately equal proportions. The enantiomerically pure esters (1R,3S,4S)- and (1S,3R,4R)-30 were isolated by careful distillation and then transformed into [5]triangulanes (M)- and (P)-4 using the same sequence of reactions as applied for (M)- and (P)-3. The structures of the key intermediates (R)-12 and rac-31 were confirmed by X-ray analyses. Although [4]- and [5]triangulanes have no chromophore which would lead to any significant absorption above 200 nm, they have remarkably high specific rotations even at 589 nm with [alpha](20)D=-192.7 [(M)-3, c=1.18, CHCl(3))] or +373.0 [(P)-4, c=1.18, CHCl(3))]. This remarkable optical rotatation is in line with their helical arrangement of sigma bonds, as confirmed by a full valence space single excitation configuration interaction treatment (SCI) in conjunction with DFT computations at the B3LYP/TZVP//B3LYP/6-31+G(d,p) level of theory which reproduce the ORD very well. Thus, it is appropriate to call the helically shaped unbranched [n]triangulanes the "sigma-[n]helicenes", representing the sigma-bond analogues of the aromatic [n]helicenes.     

Amide-silyl ligand exchanges and equilibria among group 4 amide and silyl complexes

M(NMe2)4 (M = Zr, 1a; Hf, 1b) and the silyl anion (SiButPh2)- (2) in Li(THF)2SiButPh2 (2-Li) were found to undergo a ligand exchange to give [M(NMe2)3(SiButPh2)2]- (M = Zr, 3a; Hf, 3b) and [M(NMe2)5]- (M = Zr, 4a; Hf, 4b) in THF. The reaction is reversible, leading to equilibria: 2 1a (or 1b) + 2 2 <--> 3a (or 3b) + 4a (or 4b). In toluene, the reaction of 1a with 2 yields [(Me2N)3Zr(SiButPh2)2]-[Zr(NMe2)5Li2(THF)4]+ (5) as an ionic pair. The silyl anion 2 selectively attacks the -N(SiMe3)2 ligand in (Me2N)3Zr-N(SiMe3)2 (6a) to give 3a and [N(SiMe3)2]- (7) in reversible reaction: 6a + 2 2 <--> 3a + 7. The following equilibria have also been observed and studied: 2M(NMe2)4 (1a; 1b) + [Si(SiMe3)3]- (8) <--> (Me2N)3M-Si(SiMe3)3 (M = Zr, 9a; Hf, 9b) + [M(NMe2)5]- (M = Zr, 4a; Hf, 4b); 6a (or 6b) + 8 <--> 9a (or 9b) + [N(SiMe3)2]- (7). The current study represents rare, direct observations of reversible amide-silyl exchanges and their equilibria. Crystal structures of 5, (Me2N)3Hf-Si(SiMe3)3 (9b), and [Hf(NMe2)4]2 (dimer of 1b), as well as the preparation of (Me2N)3M-N(SiMe3)2 (6a; 6b) are also reported.     

Syntheses and properties of enantiomerically pure higher (n > or = 7) [n-2]triangulanedimethanols and sigma-[n]helicenes

(P)-(+)-Hexaspiro[2.0.0.0. 0.0.2.1.1.1.1.1]pentadecane [(P)-17] as well as (M)-(-)- and (P)-(+)-octaspiro[2.0.0.0.0.0.0.0.2.1.1.1.1.1.1.1]nonadecanes [(M)- and (P)-25]-enantiomerically pure unbranched [7]- and [9]triangulanes-have been prepared starting from racemic THP-protected (methylenecyclopropyl)methanol 6. The relative configurations of all important intermediates as well as the absolute configurations of the key intermediates were established by X-ray crystal structure analyses. This new convergent approach to enantiomerically pure linear [n]triangulanes for n=7, 9 was also tested in two variants towards [15]triangulane. Some of the most prominent and unexpected features of the newly prepared compounds are the remarkable modes of self-assembly of the diols (P)-14, (E)-(3S,3'S,4S,4'S,5R,5'R)-21, (P)-(+)-22, and (E)-31 in the solid state through frameworks of intermolecular hydrogen bonds leading to, depending on the respective structure, nanotube- [(P)-14, (P)-(+)-22, and (E)-31], honeycomb-like structures [(E)-(3S,3'S,4S,4'S,5R,5'R)-21] or a supramolecular double helix [(P)-(+)- and (M)-(-)-22]. Liquid crystalline properties of the esters and ethers of the diols (P)-14, (P)-, and (M)-22 have also been tested. Although all of these [n]triangulanes have no chromophore which would lead to significant absorptions above 200 nm, they exhibit surprisingly high specific rotations even at 589 nm with [alpha](20)(D)=+672.9 (c=0.814 in CHCl(3)) for (P)-(+)-17, +909.9 (c=0.96 in CHCl(3)) for (P)-(+)-25, -890.5 (c=1.01 in CHCl(3)) for (M)-(-)-25, and -1302.5 (c=0.36 in CHCl(3)) for (M)-(-)-39, and the specific rotations increase drastically on going to shorter wavelengths. This outstanding rotatory power is in line with their rather rigid helical arrangement of sigma bonds, and accordingly these helically shaped unbranched [n]triangulanes may be termed "sigma-[n]helicenes", as they represent the sigma-bond analogues of the aromatic pi-[n]helicenes. Density functional theory (DFT) computations at the B3 LYP/6-31+G(d,p) level of theory for the geometry optimization and time-dependent DFT for determining optical rotations with a triplet-zeta basis set (B3 LYP/TZVP) reproduce the optical rotatory dispersions (ORD) very well for the lower members (n=4, 5) of the sigma-[n]helicenes. For the higher ones (n=7, 9, 15) the computed specific rotations turn out increasingly larger than the experimental values. The remarkable increase of the specific rotation with an increasing number of three-membered rings is proportional neither to the molecular weight nor to the number of cyclopropane rings in these sigma-[n]helicenes.     

Self-assembled hemicapsules with inherent functionalities: modeling of a supramolecular electrostatic self-assembly

An approach to functional self-assembled hemicapsules is described consisting of the use of multivalent (valency > or = 4) and divalent components, the functional groups of which have a relatively weak binding affinity. Electrostatic self-assembly of tetrakis(pyridiniummethyl)cavitand hemispheres (H) and doubly charged anions (A) in polar media gives rise to an equilibrium mixture that consists, as detected with ESI-MS, of hemicapsule H2A3, capsule H2A4, and other ion-pair associates. Fitting 1H NMR data with a model that includes (hemi)capsules and ion-pair associates gave an effective molarity (EM) for the intramolecular assembly of the host (H) with sulfate (A) of 0.19 +/- 0.02 M and binding constants of the functional [2 + 3] hemicapsules H2A3 and the [2 + 4] capsules H(2)A(4) in methanol of 3.25 x 10(12) M(-4) and 3.45 x 10(15) M(-5), respectively. A substantial amount of the functional [2 + 3] hemicapsules H2A3 with respect to [2 + 4] capsules H2A4 is present in solution, with ratios of H2A3 to H2A4 of 5.67-0.43 in the studied concentration range (0.1-25 mM of [H]tot). The [2 + 3] hemicapsules H2A3 built with sulfate linkers incorporate guests between the closely positioned pyridinium planes.     

Supramolecular motifs and solvatomorphism within the compounds [M(bpy)3]2[NbO(C2O4)(3)]Cl.nH2O (M = Fe2+, Co2+, Ni2+, Cu2+ and Zn2+; n = 11, 12). Syntheses, structures and magnetic properties

Solvatomorphism has been found between two series of complexes of the composition [M(bpy)3]2[NbO(C2O4)3]Cl.nH2O [M = Fe2+ (1, 2), Co2+ (3, 4), Ni2+ (5, 6), Cu2+ (7) and Zn2+ (8, 9); bpy = 2,2'-bipyridine)], crystallizing in the monoclinic space group P2 1/c [3, 5, 8 (n = 11)] or in the orthorhombic space group P21 21 21 [2, 4, 6, 7 (n = 12)]. All the structures contain two symmetry independent [M(bpy)3]2+ cations, one [NbO(C2O4)3]3- anion, one Cl(-) anion, and crystal water molecules. The cations possess a trigonally distorted octahedral geometry, with an additional tetragonal distortion in 7. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [M(bpy)3]2+ cations--quadruple aryl embrace (QAE) contact. The contact is realized by the alignment of two molecular two-fold rotation axes, preserving the parallel orientation of the molecular three-fold rotation axes. The resulting two-dimensional honeycomb lattices of [M(bpy)3]2+ cations are placed between the hydrogen bonding layers made of [NbO(C2O4)3]3- and Cl(-) anions and the majority of the crystal water molecules. The temperature-dependent magnetic susceptibility measurements (1.8-300 K) show a significant orbital angular momentum contribution for 3 and 4 (high-spin Co2+), the influence of zero-field splitting for 5 and 6(Ni2+) and a substantially paramagnetic Curie behaviour for the Cu2+ compound (7).     

Strain-promoted azide-alkyne cycloadditions of benzocyclononynes

Preliminary studies related to the design and development of new cycloalkyne reagents for metal-free click coupling are reported. Cyclononynes are more stable than cyclooctynes, and the robust benzocyclononyne platform offers spontaneous reactivity toward azides at rates competitive with other azidophiles that have been employed for metal-free click coupling. Benzocyclononynes (e.g., 1) provide valuable insight into the design of new cycloalkynes for strain-promoted azide-alkyne cycloaddition (SPAAC) couplings for applications in which side reactions and decomposition of the reagent must be kept to a minimum.     

An equilibrium analysis of the aqueous H+-MoO4(2-)-(HP)O(3)2- and H+-MoO4(2-)-(HP)O(3)2--HPO4(2-) systems

The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0相似文献   

Direct oxidation of L-cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH<9.00), 10.0 mM EDTA (9.010.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, L-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and micro=0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt=k[cysteine]tot[Fe(III)], with k={k2Ka1[H+]2+k3Ka1Ka2[H+]+k4Ka1Ka2Ka3{/}[H+]3+Ka1[H+]2+Ka1Ka2[H+]+Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2=3.4+/-0.6 M(-1) s(-1) and k3=(1.18+/-0.02)x10(6) M(-1) s(-1) (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3=(2.13+/-0.08)x10(3) M(-1) s(-1) and k4=(1.01+/-0.06)x10(4) M(-1) s(-1) (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)4]-, with k11=4x10(5) M(-1) s(-1). The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6x10(6) M(-1) s(-1) for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.     

Analysis of pyridoquinoline derivatives by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry

A method using liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (LC/APCI-MS) has been developed for the characterization and determination of pyridoquinoline derivatives 4,6-bis(dimethylaminoethylamino)-2,8,10-trimethylpyrido[3,2-g]quinoline, 4,6-bis(dimethylaminoethoxy)-2,8,10-trimethylpyrido[3,2-g]quinoline and 4,6-bis[(dimethylaminoethyl)thio]-2,8,10-trimethylpyrido[3,2-g] quinoline, all with potential antitumor properties. LC separation was performed on a conventional C18 column using a binary mobile phase composed of acetonitrile and 50 mM aqueous ammonium formate at pH 3. The APCI mass spectra obtained showed that proton addition giving [M + H]+ was the common mode of ionization to the amino- and thiopyridoquinolines, whereas the alkoxypyridoquinoline was identified by the main formation of the [M - (C2H3)N(CH3)2 + H]+, followed by the [M + H]+ ion. The LC separation conditions and MS detection parameters were optimized for the determination. The analytical method was also applied to the determination of these pyridoquinoline derivatives in fetal calf serum using liquid-liquid extraction with dichloromethane. Acceptable recovery values were obtained, ranging between 45 and 98%.     

Preparation of cyclophanes by room-temperature ring-closing alkyne metathesis with imidazolin-2-iminato tungsten alkylidyne complexes

Room-temperature ring-closing alkyne metathesis of 1,2-, 1,3-, and 1,4-bis(3-pentynyloxymethyl)benzenes has been investigated in the presence of catalytic amounts of an imidazolin-2-iminato tungsten alkylidyne complex. The m- and p-diynes selectively form the respective [10]metacyclophane or [10.10]paracyclophane, respectively, whereas a mixture of monomeric and dimeric cycloalkynes is obtained in the case of the o-diyne. DFT calculations reveal that the different selectivities can be attributed to the relative thermodynamic stability of the emerging cyclophanes.