Missing-link macrocycles: hybrid heterocalixarene analogues formed from several different building blocks

The synthesis and structural characterization of hybrid heterocalix[4]arene analogues containing pyrrole, benzene, methoxy-substituted benzene, and pyridine subunits is described. Macrocycles 1 and 2, examples of calix[2]benzene[2]pyrrole and calix[1]benzene[3]pyrrole systems, respectively, are synthesized by the condensation of pyrrole and an appropriate phenylbis(carbinol). Macrocycles 3 and 7, examples of calix[2]benzene[1]pyridine[1]pyrrole and calix[1]pyridine[3]pyrrole, respectively, are synthesized by the use of a carbene-based pyrrole-to-pyridine ring-expansion procedure. Single-crystal X-ray analysis reveals that compounds 1a, 1b, and 2b adopt 1,3-alternate conformations in the solid state, whereas compounds 3 and 7 display structures that are best described as "flattened partial cones" in terms of their conformation. (Series a refers to pure benzene-derived systems, whereas series b indicates macrocycles containing 5-methoxyphenyl subunits). In the solid state, the methoxy-functionalized macrocycles 1b and 2b, and the chloropyridine-containing macrocycle 7 exist as dimers. In the case of 1a and 7, these compounds interact with neutral solvent in the solid state. The conformations of the macrocycles in solution were explored by temperature-dependent proton NMR and NOESY spectral analysis. At 188 K, macrocycles 1a and 2a adopt flattened 1,3-alternate conformations, whereas macrocycles 3 and 7 exist in the form of flattened partial-cone conformations. Standard proton NMR titration analyses were carried out in the case of macrocycles 1a and 2a, and reveal that at least the second of these systems is capable of binding fluoride and chloride anions in CD(2)Cl(2) solution at room temperature (K(a)=571 and 17M(-1) in the case of 2a and F(-) and Cl(-), respectively).     

Metal complexes of porphycene,corrphycene, and hemiporphycene: stability and coordination chemistry

Porphyrin (P), porphycene (Pc), corrphycene (Cn), and hemiporphycene (Hpc) represent a series of well defined "4-N in" constitutional porphyrin isomers. These isomers, in the form of their octaethyl derivatives, represent a congruent set of porphyrinoids whose properties can be compared. In this study we report how variations in electronic structure and nitrogen-core size in the free-base forms of these four systems are reflected in the properties of their corresponding metal complexes. Specifically, the effects that these differences have on the axial ligation properties of the Zn(II), Mg(II), Ni(II), and Co(II) complexes of P, Pc, Cn, and Hpc in toluene using pyridine as the axial ligand are detailed. Also reported are the relative stabilities of these complexes under acidic conditions. It is shown that for the zinc, magnesium, and cobalt complexes, there are distinct differences in the ability to maintain four-, five-, or six-coordinate geometries in the presence of similar concentrations of pyridine. By contrast, no apparent differences in axial ligand binding affinity are seen for the four nickel complexes. Little difference in stability was likewise seen when these same complexes were subject to acid-mediated demetallation, with all four falling into stability class II, according to the accepted porphyrin stability ranking system. High stabilities were also seen in the case of the cobalt complexes, with the Pc and Cn complexes being of stability class III and the P and Hpc derivatives falling into stability class II. The Zn(II) and Mg(II) complexes were all far less stable than the corresponding Ni(II) and Co(II) complexes. In this case, semiquantitative analyses of the rate of acid-induced decomposition revealed the following stability sequence P>Cn>Hpc>Pc for both the Zn(II) and Mg(II) complexes. Single-crystal X-ray diffraction structures were solved for the Zn(II), Mg(II), and Ni(II) complexes of the octaethyl derivatives of Hpc, Cn, and Pc as well as a Co(II) octamethylcorrphycene and are reported as part of this study. These solid-state structures confirm four-coordinate species for the Ni(II) complexes, four- and five-coordinate species for the Mg(II) and Zn(II) complexes, and a six-coordinate species for the lone Co(II) complex.     

Quinoxaline-oligopyrroles: improved pyrrole-based anion receptors

Novel quinoxaline derivatives bearing dipyrromethane or tripyrromethane substituents act as improved anion receptors as compared to the unsubstituted dipyrrolylquinoxaline core from which they are derived.     

Classroom note: Compact,convex, and symmetric sets are discs

Define the centre of a parallelogram to be the intersection of its diagonals. It was shown in an earlier paper that the intersection of arbitrarily many parallelograms with the same centre is the unit disc about that centre in a metric defined using ideas from Linear Algebra. In this note, it is shown that this characterizes compact, convex sets, which are symmetric about a point.     

Combining selection valve and mixing chamber for nanoflow gradient generation: Toward developing a liquid chromatography cartridge coupled with mass spectrometer for protein and peptide analysis

Toward developing a micro HPLC cartridge, we have recently built a high-pressure electroosmotic pump (EOP). However, we do not recommend people to use this pump to deliver an organic solvent directly, because it often makes the pump rate unstable. We have experimented several approaches to address this issue, but none of them are satisfactory. Here, we develop an innovative approach to address this issue. We first create an abruption (a dead-volume) within a fluid conduit. We then utilize an EOP to withdraw, via a selection valve, a train of eluent solutions having decreasing eluting power into the fluid conduit. When these solutions are further aspirated through the dead-volume, these solutions are partially mixed, smoothening concentration transitions between two adjacent eluent solutions. As these solutions are pushed back, through the dead-volume again, a smooth gradient profile is formed. In this work, we characterize this scheme for gradient formation, and we incorporate this approach with a high-pressure EOP, a nanoliter injection valve, and a capillary column, yielding a micro HPLC system. We then couple this micro HPLC with an electrospray ionization – mass spectrometer for peptide and protein separations and identifications.     

A Convenient Synthesis Of 2-Methyl-5-(Methylthio)Benzothiazole

A convenient and efficient four step synthesis of 2-methyl-5-(methylthio)benzothiazole is presented.     

Redox‐Switchable Ring‐Closing Metathesis: Catalyst Design,Synthesis, and Study

High yielding syntheses of 1‐(ferrocenylmethyl)‐3‐mesitylimidazolium iodide ( 1 ) and 1‐(ferrocenylmethyl)‐3‐mesitylimidazol‐2‐ylidene ( 2 ) were developed. Complexation of 2 to [{Ir(cod)Cl}₂] (cod=cis,cis‐1,5‐cyclooctadiene) or [Ru(PCy₃)Cl₂(?CH‐o‐O‐iPrC₆H₄)] (Cy=cyclohexyl) afforded 3 ([Ir( 2 )(cod)Cl]) and 5 ([Ru( 2 )Cl₂(?CH‐o‐O‐iPrC₆H₄)]), respectively. Complex 4 ([Ir( 2 )(CO)₂Cl]) was obtained by bubbling carbon monoxide through a solution of 3 in CH₂Cl₂. Spectroelectrochemical IR analysis of 4 revealed that the oxidation of the ferrocene moiety in 2 significantly reduced the electron‐donating ability of the N‐heterocyclic carbene ligand (ΔTEP=9 cm^?1; TEP=Tolman electronic parameter). The oxidation of 5 with [Fe(η⁵‐C₅H₄COMe)Cp][BF₄] as well as the subsequent reduction of the corresponding product [ 5 ][BF₄] with decamethylferrocene (Fc*) each proceeded in greater than 95 % yield. Mössbauer, UV/Vis and EPR spectroscopy analysis confirmed that [ 5 ][BF₄] contained a ferrocenium species, indicating that the iron center was selectively oxidized over the ruthenium center. Complexes 5 and [ 5 ][BF₄] were found to catalyze the ring‐closing metathesis (RCM) of diethyl diallylmalonate with observed pseudo‐first‐order rate constants (k_obs) of 3.1×10^?4 and 1.2×10^?5 s^?1, respectively. By adding suitable oxidants or reductants over the course of a RCM reaction, complex 5 was switched between different states of catalytic activity. A second‐generation N‐heterocyclic carbene that featured a 1′,2′,3′,4′,5′‐ pentamethylferrocenyl moiety ( 10 ) was also prepared and metal complexes containing this ligand were found to undergo iron‐centered oxidations at lower potentials than analogous complexes supported by 2 (0.30–0.36 V vs. 0.56–0.62 V, respectively). Redox switching experiments using [Ru( 10 )Cl₂(?CH‐o‐O‐iPrC₆H₄)] revealed that greater than 94 % of the initial catalytic activity was restored after an oxidation–reduction cycle.     

Measurement of the muon capture rate in hydrogen gas and determination of the proton's pseudoscalar coupling gP

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation. The capture rate from the hyperfine singlet ground state of the microp atom was obtained from the difference between the micro(-) disappearance rate in hydrogen and the world average for the micro(+) decay rate, yielding Lambda(S)=725.0+/-17.4 s(-1), from which the induced pseudoscalar coupling of the nucleon, g(P)(q(2)=-0.88m(2)(micro))=7.3+/-1.1, is extracted.     

Observation of B-->eta'K* and evidence for B+-->eta'rho+

We present an observation of B-->eta'K*. The data sample corresponds to 232x10(6) BB[over ] pairs collected with the BABAR detector at the PEP-II asymmetric-energy B factory at the Stanford Linear Accelerator Center. We measure the branching fractions (in units of 10(-6)) B(B(0)-->eta'K*0)=3.8+/-1.1+/-0.5 and B(B+-->eta'K*+)=4.9(1.7)(+1.9)+/-0.8, where the first error is statistical and the second systematic. A simultaneous fit results in the observation of B-->eta'K* with B(B-->eta'K*)=4.1(-0.9)(+1.0)+/-0.5. We also search for B-->eta'rho and eta'f(0)(980)(f(0)-->pi+pi-) with results and 90% confidence level upper limits B(B+-->eta'rho+)=8.7(-2.8-1.3)(+3.1+2.3) (<14), B(B(0)-->eta'rho0)<3.7, and B(B(0)-->eta'f(0)(980)(f(0)-->pi+pi-))<1.5. Charge asymmetries in the channels with significant yields are consistent with zero.