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PubMed2001

Atom-efficient metal-catalyzed cross-coupling reaction of indium organometallics with organic electrophiles.

The novel metal-catalyzed cross-coupling reaction of indium organometallics with organic electrophiles is described. Triorganoindium compounds (R(3)In) containing alkyl, vinyl, aryl, and alkynyl groups are efficiently prepared from the corresponding lithium or magnesium organometallics by reaction with indium trichloride. The cross-coupling reaction of R(3)In with aryl halides and pseudohalides (iodide 2, bromide 5, and triflate 4), vinyl triflates, benzyl bromides, and acid chlorides proceeds under palladium catalysis in excellent yields and with high chemoselectivity. Indium organometallics also react with aryl chlorides as under nickel catalysis. In the cross-coupling reaction the triorganoindium compounds transfer, in a clear example of atom economy, all three of the organic groups attached to the metal, as shown by the necessity of using only 34 mol % of indium. The feasibility of using R(3)In in reactions with different electrophiles, along with the high yields and chemoselectivities obtained, reveals indium organometallics to be useful alternatives to other organometallics in cross-coupling reactions.

Journal Article↗
PubMed2004

Li(3+delta)V6O13: a short-range-ordered lithium insertion mechanism.

The structures of Li3V6O13 and Li(3+delta)V6O13, delta approximately 0.3, have been determined by single-crystal X-ray diffraction. Both compounds have the space group C2/m, with very similar cell parameters. In Li3V6O13, the Li atoms are found in the Wyckoff positions 4(i) and 2(b) with multiplicities of four and two, respectively. Since Li3V6O13 exhibits no superstructure reflections, it is concluded that Li3V6O13 contains one disordered lithium ion in an otherwise ordered centrosymmetric structure. On inserting more lithium into the structure, the Li(3+delta)V6O13 phase is formed with the homogeneity range 0 < delta < 1. It is concluded that the site for the extra inserted lithium ion is closely coupled to the position of the disordered lithium ion in Li3V6O13. A mechanism for this behaviour and for the further formation of the Li6V6O13 end-phase in the LixV6O13 system is proposed.

Journal Article↗
PubMed2002

Formation of metallaboranes from the group IV transition metals and pentaborane(9): crystal and molecular structure of [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)].

The reactions between [(C(5)H(5))(2)MCl(2)] (where M = Ti, Zr, Hf) and Li[B(5)H(8)] in a variety of solvents have been investigated. In the case of Zr, a pale orange solid, mu-(Cp(2)ClZr)B(5)H(8) (1), is formed in 70% yield. Compound 1 exists as a B(5)H(9) cage with a Cp(2)ClZr moiety replacing a bridging H atom. The variable temperature NMR spectra of 1 reveal two fluxional processes, one (DeltaG(++) = 54 kJ mol(-1)) which renders a plane of symmetry in the molecule and a higher temperature one (DeltaG(++) = 48 kJ mol(-1)) which renders all the basal B atoms equivalent. Dynamic processes are suggested to account for these observations. Passage of a CH(2)Cl(2) solution of 1 through silica gel affords 2, [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)], a yellow, air-stable, crystalline solid, in 14% yield. The cation in 2, [(Cp(2)Zr)(2)B(5)H(8)](+), consists of a distorted spiro[2.2]pentane-like B(5) moiety comprising two B(3) triangles sharing a naked boron vertex. The two triangles are twisted 73 degrees with respect to each other, and the two [Cp(2)Zr] groups bond in a trihapto arrangement to the two opposite B-B-B edges. Each exterior B-Zr edge is H-bridged, and the B atoms possess terminal hydrogens. Reactions of Cp(2)HfCl(2) with Li[B(5)H(8)] lead to the formation of the analogue of 2, [(Cp(2)Hf)(2)B(5)H(8)][B(11)H(14)] (3). The precursor to 3, that is, the Hf analogue of 1, is not observed. Reaction between Li[B(5)H(8)] and Cp(2)TiCl(2) afforded no identifiable products, but reaction with CpTiCl(3) resulted in cage coupling and the formation of B(10)H(14).

Boranes↗
PubMed2006

Tandem nucleophilic addition/fragmentation reactions and synthetic versatility of vinylogous acyl triflates.

A thorough analysis of the chemistry of vinylogous acyl triflates provides insight into important chemical processes and opens new directions in synthetic technology. Tandem nucleophilic addition/C-C bond cleaving fragmentation reactions of cyclic vinylogous acyl triflates 1 yield a variety of acyclic acetylenic compounds. Full details are disclosed herein. A wide array of nucleophiles, such as organolithium and Grignard reagents, lithium enolates and their analogues, hydride reagents, and lithium amides, are applied. The respective reactions produce ketones 2, 1,3-diketones and their analogues 3, alcohols 4, and amides 5. The present reactions are proposed to proceed through a 1,2-addition of the nucleophile to the carbonyl group of starting triflates 1 to form tetrahedral alkoxide intermediates C, followed by Grob-type fragmentation, which effects C-C bond cleavage to yield acyclic acetylenic compounds 2-5 and 7. The potent nucleofugacity of the triflate moiety is channeled through the sigma-bond framework of 1, providing direct access to the fragmentation pathway without denying other typical reactions of cyclic vinylogous esters. The synthetic versatility of vinylogous acyl triflates, including functionalization reactions of the cyclic enone core (1 --> 6 or 8), is also illustrated.

Chemistry, Organic↗
PubMed1987

Differential effects of pertussis toxin and lithium on muscarinic responses in the atria and ileum: evidence for receptor heterogeneity.

Pretreatment with pertussin toxin was shown to inhibit selectively muscarinic responses in the atria, but have no effect on responses in the ileum. The converse was true when the animals were pretreated with lithium. These data are consistent with the hypothesis that muscarinic receptors in the atria are coupled to an inhibition of adenylate cyclase, whilst those in the ileum are coupled to inositol phospholipid hydrolysis and indicate differences in the muscarinic receptors in these two tissues.

Adenylate Cyclase Toxin↗
PubMed1988

Acute effects of lithium on dopaminergic responses: iontophoretic studies in the rat visual cortex.

The interactions between lithium and cortical dopaminergic receptors were investigated using the iontophoretic technique to record and apply dopaminergic compounds, GABA, acetylcholine and LiCl on neurons in the primary visual cortex of the rat. The main responses to dopamine (DA) or to the D1 agonist (+/- )SKF38393 on spontaneously-active (SA) or visually-driven (VD) units was a prolonged decrease in firing and a reduction in the responsiveness to pulses of acetylcholine. The D1 antagonist SCH23390, applied iontophoretically or intravenously, blocked or attenuated the inhibitory responses to both DA and (+/- )SKF38393. The D2 agonist quinpirole (LY171555) either produced only slight excitations or had no effects on both VD and SA units. The concomitant application of lithium blocked the inhibitory responses to DA and to (+/- )SKF38393 but did not modify the responsiveness to LY171555. In addition, the DA- and (+/- )SKF38393-induced decreases in responsiveness to acetylcholine were also suppressed by lithium. These effects were on dopaminergic mechanisms, since the excitatory responses to acetylcholine alone as well as the inhibitions caused by GABA were unchanged by the application of lithium. These results imply that the modifications in sensitivity to dopaminergic agents induced by lithium are mediated by dopamine D1 receptors and are discussed in relation to adenylate-cyclase.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed2005

Method for analysis of polar volatile trace components in aqueous samples by gas chromatography.

A new method has been developed for direct analysis of volatile polar trace compounds in aqueous samples by gas chromatography. Water samples are injected onto a short packed precolumn containing anhydrous lithium chloride. A capillary column is coupled in series with the prefractionation column for final separation of the analytes. The enrichment principle of the salt precolumn is reverse to the principles employed in conventional methods such as SPE or SPME in which a sorbent or adsorbent is utilized to trap or concentrate the analytes. Such methods are not efficient for highly polar compounds. In the LiCl precolumn concept, the water matrix is strongly retained on the hygroscopic salt, whereas polar as well as nonpolar volatile organic compounds show very low retention and are eluted ahead of the water. After transfer of the analytes to the capillary column, the retained bulk water is removed by backflushing the precolumn at elevated temperature. For direct injections of 120 microL of aqueous samples, the combined time for injection and preseparation is only 3.5 min. With this procedure, direct repetitive automated analyses of highly volatile polar compounds such as methanol or tetrahydrofuran can be performed, and a limit of quantification in the low parts-per-billion region utilizing a flame ionization detector is demonstrated.

Journal Article↗
PubMed1977

Synthesis and PMR and mass spectra of potential metabolites and other derivatives of bis(2-ethylhexyl) phthalate.

The 3- and 4-hydroxylated derivatives of the plasticizer bis(2-ethylhexyl) phthalate and the monoesters derived from these hydroxylated derivatives were synthesized. The structures of the two 4-hydroxy monoesters were confirmed by lithium borohydride reduction to their corresponding hydroxymethylbenzoic acids, followed by cyclization of the reduced compounds to known phthalides. All compounds were characterized by PMR and mass spectra. The PMR data reiterate the difficulty in predicting chemical shifts in multisubstituted benzene rings.

Diethylhexyl Phthalate↗
PubMed1995

A calorimetric characterization of the salt dependence of the stability of the GCN4 leucine zipper.

The effects of different salts (LiCl, NaCl, ChoCl, KF, KCl, and KBr) on the structural stability of a 33-residue peptide corresponding to the leucine zipper region of GCN4 have been studied by high-sensitivity differential scanning calorimetry. These experiments have allowed an estimation of the salt dependence of the thermodynamic parameters that define the stability of the coiled coil. Independent of the nature of the salt, a destabilization of the coiled coil is always observed upon increasing salt concentration up to a maximum of approximately 0.5 M, depending on the specific cation or anion. At higher salt concentrations, this effect is reversed and a stabilization of the leucine zipper is observed. The effect of salt concentration is primarily entropic, judging from the lack of a significant salt dependence of the transition enthalpy. The salt dependence of the stability of the peptide is complex, suggesting the presence of specific salt effects at high salt concentrations in addition to the nonspecific electrostatic effects that are prevalent at lower salt concentrations. The data is consistent with the existence of specific interactions between anions and peptide with an affinity that follows a reverse size order (F- > Cl- > Br-). Under all conditions studied, the coiled coil undergoes reversible thermal unfolding that can be well represented by a reaction of the form N2<==>2U, indicating that the unfolding is a two-state process in which the helices are only stable when they are in the coiled coil conformation.

Anions↗
PubMed1998

Two-dimensional nuclear-Zeeman-resolved electron spin echo envelope modulation (NZ-ESEEM) spectroscopy.

A two-dimensional electron spin echo envelope modulation (ESEEM) experiment, called nuclear-Zeeman-resolved ESEEM (NZ-ESEEM), that correlates nuclear transition frequencies with nuclear Zeeman frequencies is introduced. NZ-ESEEM is basically a three-pulse ESEEM experiment complemented by a magnetic-field pulse applied during part of the free evolution period between the second and third microwave pulse. The inner working of the new approach is explained and the instrumentation is discussed. The capacity of the method is illustrated by two examples of applications.

Algorithms↗
PubMed1977

Cation permeability ratios of sodium channels in normal and grayanotoxin-treated squid axon membranes.

Permeabilities of squid axon membranes to various cations at rest and during activity have been measured by voltage clamp before and during internal perfusion of 4 X 10(-5) M grayanotoxin I. The resting sodium and potassium permeabilities were estimated to be 6.85 X 10(-8) cm/sec and 2.84 X 10(-6) cm/sec, respectively. Grayanotoxin I increased the resting sodium permeability to 7.38 10(-7) cm/sec representing an 11-fold increase. The potassium permeability was increased only by a factor of 1.24. The resting permeability ratios as estimated by the voltage clamp method before application of grayanotoxin I were Na (1): Li (0.83): formamidine (1.34): guanidine (1.49): Cs (0.87): methylguanidine (0.86): methylamine (0.78). Grayanotoxin I did not drastically the resting permeability ratios with a result of Na (1): Li (0.95): formamidine (1.27): guanidine (1.16): Cs (0.47): methylguanidine (0.72): methylamine (0.46). The membrane potential method gave essentially the same resting permability ratios before and during application of grayanotoxin I if corrections were made for permeability to choline as the cation substitute and for changes in potassium permeability caused by test cations. The permeability ration choline/Na was estimated to be 0.72 by the voltage clamp method and 0.65 by the membrane potential method. Grayanotoxin I decreased the ration to 0.43. The permeability ratios during peak transient current were estimated to be Na (1): Li (1.12): formamidine (0.20): guanidine (0.20): Cs (0.085): methylguanidine (0.061): methylamine (0.036). Thus the sodium channels for the peak current are much more selective to cation than the resting sodium channels. It appears that the resting sodium channels in normal and grayanotoixn I-treated axons are operationally different from the sodium channels that undergo a conductance increase upon stimulation.

Animals↗
PubMed1999

Ion permeation and block of P2X(2) purinoceptors: single channel recordings.

P2X(2) purinoceptors are cation-selective channels activated by ATP and its analogues. Using single channel measurements we studied the channel's selectivity for the alkali metal ions and organic monovalent cations NMDG(+), Tris(+), TMA(+), and TEA(+). The selectivity sequence for currents carried by alkali metal ions is: K(+) > Rb(+) > Cs(+) > Na(+) > Li(+), which is Eisenman sequence IV. This is different from the mobility sequence of the ions in free solution suggesting there is weak interaction between the ions and the channel interior. The relative conductance for alkali ions increases linearly in relation to the Stokes radius. The organic ions NMDG(+), Tris(+), TMA(+) and TEA(+) were virtually impermeant. The divalent ions (Mn(2+), Mg(2+), Ca(2+) and Ba(2+)) induced a fast block visible as a reduction in amplitude of the unitary currents. Using a single-site binding model, the divalent ions exhibited an equilibrium affinity sequence of Mn(2+) > Mg(2+) > Ca(2+) > Ba(2+).

Adenosine Triphosphate↗
PubMed1976

Chromatin proteins from normal, vegetalized, and animalized sea urchin embryos.

The chemical composition of the chromatin, the fractional content of histones and nonhistone chromatin proteins (NHP), and the biosynthesis of these proteins in normal, vegetalized, and animalized embryos of the sea urchin Strongylocentrotus droebachiensis at the blastula, mesenchyme blastula, and gastrula stages have been studied. The amount of the NHP in the chromatin from normal and vegetalized embryos increases during early embryonic development while that in animalized embryos remains without change at the mesenchyme blastula stage and then decreases. During development the histone content in all three cases slightly decreases. Polyacrylamide gel electrophoresis reveals that both fractional composition of histones and their biosynthesis in normal, vegetalized, and animalized embryos display no differences. During development, however, some changes occur, so that the relative amount of histones F1 and F2a2 increases, F2b decreases, while F3 and F2a1 remains constant. Histone F1 at the blastula stage consists of two subfractions while at the gastrula stage it consists of three subfractions. The histone F2a1 consists of one and two, respectively. Histone F3 at all stages is made up of three subfractions; histone F2b is made up of two; and the histone F2a2 is electrophoretically homogeneous. Specific radioactivity of the arginine-rich histones F3 and F2a1 tends to increase during development, while that of moderately lysine-rich histones F2b and F2a2 does not change, and that of the lysine-rich histone F1 decreases. The NHP in normal, vegetalized, and animalized embryos at different developmental stages consist of 17 fractions that can be separated by isoelectrofocusing within the 4.5-8.8 pH range. Quantitative changes have been observed in the fractions focused at pH 4.5-6.1 during development and in normal and modified embryos at the gastrula stage.

Animals↗