Substitution Of Bromide Research Articles

Solvolysis of chiral cyclohexylidenemethyl triflate. Evidence against a primary vinyl cation intermediate

AbstractSolvolysis of (R)‐4‐methylcyclohexylidenemethyl triflate (6) was examined at 140 °C in various aqueous methanol and some other alcoholic solvents. The main product was (R)‐4‐methylcycloheptanone that maintains the stereochemical purity of 6, with accompanying 4‐methylcyclohexanecarbaldehyde. In the presence of bromide ion, the bromide substitution product was also obtained, mostly with inversion of configuration. It is concluded that the solvolysis does not involve the formation of the primary vinyl cation but proceeds via σ‐bond participation to form the rearranged cycloheptenyl cation as an intermediate. Copyright © 2002 John Wiley & Sons, Ltd.

On the Amination of Tetraazafulvalenes

The palladium catalyzed amination of tetraazafulvalenes is described. Starting from either the tetraalkylated compounds 7 or their vinylogous derivatives 8, substitution of the aryl bromide by different amines was realized. The synthesis of compounds which possess four primary amino groups as well as four hydrazino residues by employing a one-pot, two-step method is also described. Bichromophores are accessible as demonstrated by the introduction of the phenoxazine system into tetraazafulvalenes.

Effects of some imidazolidine alpha2-adrenoceptor agonists in rat isolated atria.

1. The prejunctional alpha2-adrenoceptor agonist activity of four imidazolidines (UK14819, UK14304, UK15121 and UK11957) were compared to that of clonidine in rat isolated atrial preparations. 2. The preparations consisted of spontaneously beating left and right atrial pairs that were incubated with [3H]-noradrenaline. The efflux of radioactivity induced by electrical field stimulation of intramural sympathetic nerves was used as an index of neurotransmitter release and inotropic and chronotropic responses were recorded. 3. Two quinoxalinyl imidazolidines (UK14819 and UK14304 which have chloride and bromide substitution, respectively, in the phenyl moiety) caused decreases in the efflux of radioactivity with stimulation at 2 Hz and 10 Hz but not at 20 Hz. These effects were antagonised by the alpha2-adrenoceptor antagonist idazoxan (0.3 microM) but they were not affected by the alpha1-adrenoceptor antagonist prazosin (0.1 microM). 4. The third halogenated quinoxalinyl imidazolidine analogue (UK15121, which has an iodide substitution in the phenyl ring) and a quinolinyl imidazolidine (UK11957) have actions similar to clonidine. They decreased the efflux of radioactivity with stimulation at 1 Hz (for UK11957) or 2 Hz (for UK15121) and enhanced it at higher frequencies of stimulation. Both the inhibitory and enhancing effects were antagonised by idazoxan but they were not affected by prazosin. 5. Unlike the other three imidazolidines in the present study, the quinolinyl imidazolidine (UK11957), caused a decrease in resting release of radioactivity and this effect was prevented by the monoamine oxidase inhibitor pargyline (30 microM). 6. These findings suggest that the 5-chloro-or 5-bromo substituted quinoxalinyl imidazolidines (UK14819 and UK14304) are full agonists at prejunctional alpha2-adrenoceptors, but the 5-iodo-substituted quinoxalinyl imidazolidine (UK15121) and the quinolinyl analogue of UK14304 (UK11957), like clonidine, appear to be partial agonists at these receptors.

Stabilisation of 2-nitrophenyl-selenosulfide, -diselenide and -thioselenide ions in N,N-dimethylacetamide

The direct addition of sulfur or selenium to 2-nitrophenyl selenide ions (ArSe-) in N,N-dimethylacetamide leads to the formation of ArSeS- or ArSe2- species, which have been characterized by UV-vis spectrophotometry coupled with volammetry. The nucleophilic substitution of benzyl bromide by ArSeS- ions yields selenenyl sulfide ArSeSCH2Ph as the major product.

Catalytic Processes of Oxidation by Hydrogen Peroxide in the Presence of Br2 or HBr. Mechanism and Synthetic Applications

The mechanism and the synthetic applications for the oxidation of alcohols, ethers, and aldehydes by H2O2 catalyzed by Br2 or Br- in a liquid two-phase system (aqueous and organic) are reported. Aliphatic and benzylic primary alcohols and ethers show an opposite behavior, which has been rationalized on the ground of the different electronic configurations of the intermediate alkyl (π-type) and acyl (σ-type) radicals and their influence on enthalpic and polar effects. A two-phase system is particularly useful also for an efficient benzylic bromination by Br2 or Br-; the substitution of the benzyl bromide by OH, OR, and OCOR regenerates Br-, which can be recycled. The evaluation of the relative reactivities of the involved substrates and intermediates has allowed to develop a variety of simple, facile, convenient, and selective syntheses of alcohols, aldehydes, ketones, esters, and benzyl bromides, which fulfill the conditions for practical applications.

Total synthesis of methyl 14-hydroxy-(all- cis)-5,8,11-tetradecatrienoate: A useful intermediate for the synthesis of arachidonic acid analogues

Abstract The first total synthesis of methyl 14-hydroxy-(all-cis)-5,8,11-tetradecatrienoate ( 1a) was accomplished in 11 steps with an overall yield of 14%. Major sequences involve Cu(I) catalyzed propargylic substitution of propargyl bromide 5 by 3-butyn-1-ol, followed by partial reduction of the diyne 6 to the cis,cis-diene 7, and Wittig reaction of the phosphonium iodide 9 with aldehyde 11 to the all-cis skipped triene 10.

Ferrocenophanes with Interannular Boron−Phosphorus Bridges: Synthesis, Structure, and Reactivity toward Nitrogen Bases

Starting from readily available diborylferrocenes 1,1‘-fc[B(Br)R]2 (fc = (C5H4)Fe(C5H4); R: Br (1a), Me (1b), OEt (1d)), ferrocenophanes 1,1‘-fc[B(μ-PPh2)R]2 (R = Br (2a), Me (2b), PPh2 (2d)) and 1,1‘-fc[B(μ-PPh2)OEt][B(μ-PPh2)PPh2] (2c) are obtained upon reaction with LiPPh2. In contrast, 1,1‘-fc[B(Br)pyr]2 (pyr = pyrrolidinyl, 1e) and 2 equiv of LiPPh2 give an open-chain ferrocene 1,1‘-fc[B(PPh2)pyr]2 (2e). The dynamic behavior of the B2P2-bridged species has been investigated using variable-temperature NMR spectroscopy and the Forsén−Hoffman spin saturation transfer method. The reaction of 2a with Lipz (pz = pyrazolide) gives the ansa-ferrocene 1,1‘-fc[B(μ-PPh2)PPh2][B(μ-pz)Br] (3a), featuring an interannular (BNNBP) heterocyclic bridge. When Lipz* (pz* = 4-bromo-3,5-dimethylpyrazolide) was used rather than Lipz, two products could be isolated: 3b, which is analogous to 3a, and 1,1‘-fc[B(μ-PPh2)Br][B(μ-pz)Br] (3c), which bears Br substituents at both bridgehead boron centers. 2d and 3c have been characterized by X-ray crystallography. Treatment of 2a with excess γ-picoline (pic) results in an ansa-bridge opening and bromide substitution with the formation of the ionic species {1,1‘-fc[B(PPh2)pic2]2}Br2 (5a). The presence of three-coordinate phosphorus centers in 5a was confirmed by showing their ability to bind Cr(CO)5 fragments, which afforded the trimetallic complex 5b.

A Practical Procedure for the Synthesis of Alkyl Azides at Ambient Temperature in Dimethyl Sulfoxide in High Purity and Yield

Described is the often neglected, but very efficient, procedure for the preparation of primary and secondary alkyl and cycloalkyl azides in excellent purity and high yields. At ambient temperature conditions, the alkyl azides were readily obtained from alkyl bromides by the nucleophilic substitution of bromide with 1.2 equivalents of sodium azide in dimethyl sulfoxide (DMSO). This facile and practical procedure provides highly pure products and eliminates the hazards associated with distillation of alkyl azides.

Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

The bond dissociation enthalpies in a number of substituted benzyl bromides have been studied in the gas phase, using the toluene carrier technique, and in solution with photoacoustic calorimetry. Gas phase thermolysis with unsubstituted benzyl bromide gives an absolute C–Br bond dissociation enthalpy (Ed) at 298 K of 255 ± 4 kJ mol–1. Competition experiments in the gas phase reveal no substituent effect on the value of Ed(C–Br). Gas phase thermolysis with substituted tert-butylbenzenes also shows no effect on the C–CH3 bond dissociation enthalpy for p-CN, p-OH and p-But substituents, with a Ed(C–CH3) value at 298 K of 299 ± 2 kJ mol–1. In solution, photoacoustic experiments yield no detectable substituent effect for p-CN, p-But and m-CF3 substitution of the benzyl bromide, in contrast with other reports. With photoacoustic calorimetry a bond dissociation enthalpy of 254 ± 4 kJ mol–1 has been found for all benzyl bromides studied. A rationale for the absence of a substituent effect on the benzylic bond dissociation enthalpy is provided.

A new preorganized tridentate ligand bearing three indolethiolate groups. Preparation of 3∶1 subsite-differentiated Fe4S4 clusters

C. Walsdorff, W. Saak and S. Pohl, J. Chem. Soc., Dalton Trans., 1997, 1857 DOI: 10.1039/A608284J

Stopped-flow mechanistic study of bromide substitution by an organonitrile at an iron(II) phosphinic centre; a π-electron driven process

The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr 2(depe) 2] (depe = Et 2PCH 2CH 2PEt 2) by the organonitrile NCCH 2C 6H 4OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br −), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH 2C 6H 4OMe-4)(depe) 2] + and trans-[Fe(NCCH 2C 6H 4OMe-4) 2(depe) 2] 2+, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe) 2] + ( k 1 = 1.52 ± 0.02 s −1) and [Fe(NCR)(depe) 2] 2+ ( k 3 = 0.063 ± 0.008 s −1) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br − for such metal centres has also been investigated and a stronger inhibiting effect of added Br − is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands.

Synthesis and Structure of Mixed Ba12F19ClδBr5–δ Crystals

AbstractIn the structure Ba12F19Cl5 [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba12F19ClδBr5–δ with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF2 and 25 mol% BaX2 with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X‐ray diffraction methods. The refined total Cl−/Br− population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

Kinetics of the nucleophilic substitution of benzyl-tributylammonium bromide with allyl, butyl, and benzyl chlorides and with benzyl acetate and benzyl ether

In this study, we investigated the kinetics of the nucleophilic substitutions, RX + (BzBu3NBr) RBr + (BzBu3NX), where R = allyl, Bu and Bz, when X = Cl; and X = AcO and BzO when R = Bz. The forward and backward rate constants in addition to the activation energies for R = allyl and Bu were also determined. However, only the rate constants at 35°C were determined for the benzyl compounds with toluene as the solvent to reduce the reaction rate. Moreover, the effects of the structures of the groups R and the leaving groups X on the reactivity were compared. Results in this study can provide valuable information for future studies involving the phase transfer catalyzed displacements. © 1996 John Wiley & Sons, Inc.

Deformation of Swollen Erythrocytes Provides a Model of Sickling-Induced Leak Pathways, Including a Novel Bromide-Sensitive Component

AbstractDeoxygenation-induced red blood cell (RBC) sickling probably activates multiple cation leak pathways. In an attempt to model this, we examined the net passive K efflux (“K leak”) from normal and sickle RBCs undergoing elliptical deformation in hypotonic media (200 mOsmol/L). This hypotonic deformation activates two deformation-dependent K leak pathways that are not detectable during the balanced leak (Kefflux = Nainfflux) resulting from deformation of RBCs in isotonic medium. These are (1) a calcium-dependent leak component and (2) a novel leak pathway that is inhibited by substitution of bromide (but not sulfamate) for chloride, which converts the unbalanced K leak (Kefflux > Nainfflux) of hypotonic deformation to a residual balanced leak. This dramatic effect of hypotonic deformation is reversible, is detected in both normal and sickle RBCs, and is inhibited significantly by 4,4’-diisothiocyano-2,2’-stilbene disulfonate. Remarkably, bromide also inhibits by 55% the K leak resulting from authentic deoxygenation-induced RBC sickling and, thereby, blunts the imbalance of accompanying monovalent cation leaks. The unique effect of bromide is not readily explainable on the basis of known behaviors of known ion leak/transport pathways. The mechanical threshold for triggering K leak during hypotonic deformation is at applied shear stress of 164 dyne/cm2, a value similar to the abnormal susceptibility we previously found for oxygenated sickle RBCs during isotonic deformation. These data suggest that membrane stretch accompanying hypotonic deformation activates the same multiple leak pathways that contribute to net K leak during authentic RBC sickling, including a previously unknown bromide-sensitive leak.

Deformation of swollen erythrocytes provides a model of sickling- induced leak pathways, including a novel bromide-sensitive component

Deoxygenation-induced red blood cell (RBC) sickling probably activates multiple cation leak pathways. In an attempt to model this, we examined the net passive K efflux ("K leak") from normal and sickle RBCs undergoing elliptical deformation in hypotonic media (200 mOsmol/L). This hypotonic deformation activates two deformation-dependent K leak pathways that are not detectable during the balanced leak (Kefflux = Nainflux) resulting from deformation of RBCs in isotonic medium. These are (1) a calcium-dependent leak component and (2) a novel leak pathway that is inhibited by substitution of bromide (but not sulfamate) for chloride, which converts the unbalanced K leak (Kefflux > Nainflux) of hypotonic deformation to a residual balanced leak. This dramatic effect of hypotonic deformation is reversible, is detected in both normal and sickle RBCs, and is inhibited significantly by 4,4'-diisothiocyano-2,2'-stilbene disulfonate. Remarkably, bromide also inhibits by 55% the K leak resulting from authentic deoxygenation-induced RBC sickling and, thereby, blunts the imbalance of accompanying monovalent cation leaks. The unique effect of bromide is not readily explainable on the basis of known behaviors of known ion leak/transport pathways. The mechanical threshold for triggering K leak during hypotonic deformation is at applied shear stress of 164 dyne/cm2, a value similar to the abnormal susceptibility we previously found for oxygenated sickle RBCs during isotonic deformation. These data suggest that membrane stretch accompanying hypotonic deformation activates the same multiple leak pathways that contribute to net K leak during authentic RBC sickling, including a previously unknown bromide-sensitive leak.

The separation of tellurium and selenium by polyurethane foam sorbents

Tellurium and selenium can be sorbed from hydrochloric acid and hydrobromic acid solution by both polyether and polyester-based polyurethane foam. Although some acid is needed, the substitution of sodium chloride or sodium bromide increases the extraction significantly. Tellurium is extracted rapidly with > 99% sorbed in 2 min from 1.0/5.0 M and 2.0/4.0 M hydrochloric acid/sodium bromide. Selenium can also be sorbed quantitatively but much more slowly so that a separation is possible based on the relative rates of extraction. The capacity of polyether foam is 3% by weight of tellurium.

Esters are effective co-catalysts in copper-catalyzed methanolysis of aryl bromides

Rate of copper(I)-catalyzed substitution of aryl bromides into methyl-aryl ethers by methoxide ion is greatly enhanced in presence of esters. Soluble copper(I) complexes 2, arising from tetrahedral adducts 1 of methoxide onto esters, are likely responsible for this improvement.

Reactivity of zinc dialkyldithiocarbamates towards halogen‐substituted olefins

AbstractThe reaction of bis(dimethyldithiocarbamato)zinc(II) [Zn(dmtc)2] with halogen‐substituted 2,3‐dimethyl‐2‐butene (e.g., 1‐bromo‐2,3‐dimethyl‐2‐butene; tme‐Br) causes substitution of bromide by the dmtc ligands. Depending on the polarity of the solvent, this results either in a complex ZnBr2 (tme‐dmtc)2, in which the coupling product remains coordinated to the metal, or in the liberation of the ligand, producing ZnBr2 and tme‐dmtc. Crystals of tme‐dmtc have been obtained from methanol solution. An X‐ray structure of this solid has been determined; relevant data: C9H17NS2, Mr = 204.4, monoclinic, P21, a = 8.2087(5), b = 7.3551(5), c = 10.2791(8) Å, β = 106.437(6)°, V = 595.25(8) Å3, Z = 2, Dx = 1.14 g·cm−3, λ(CuKα) = 1.5418 Å, μ(CuKα) = 36.25 cm−1, F(000) = 222, room temperature. Final R = 0.069 for 853 observed reflections.

Calixarene Phosphorylation: Cation Control of Reaction Pathways

The hydroxide ion catalysed phosphorylation of p-t- butylcalix [4] arene by diethyl chlorophosphate under heterogeneous conditions shows a marked sensitivity to the nature of the phase transfer catalyst used. Thus, substitution of tetraethylammonium bromide for the tetrabutylammonium bromide prescribed for the synthesis of the calixarene tetraphosphate leads to a major reaction product becoming a bis (phosphate) in which the calixarene binds as a unidentate ligand to one phosphorus and as a bidentate ligand to the other. An X-ray crystal structure analysis on this new compound shows the calixarene to adopt a somewhat distorted 'cone' conformation with an unusual orientation of one of the t-butyl substituents into the cone cavity. It also reveals that a significant diastereoselectivity must operate in the formation reaction. The only other reaction product detected in this system appears to be the calixarene tetraphosphate in a partial cone conformation.

Techniques for converting Golgi precipitate in CNS neurons into stable electron microscopic markers.

Direct electron microscopy of nervous tissue stained with the Golgi impregnation method is unsatisfactory because the cytoplasm of the cell bodies and processes of the impregnated neurons are completely filled with a compact precipitate of electron dense silver chromate. This precipitate entirely obscures the cytological details of the impregnated neurons. Because of its solidity and instability in aqueous solutions, the silver chromate is also a source of inconvenience during the preparation of the ultrathin sections. This review summarizes methods that have been developed with the aim of replacing the Golgi precipitate in CNS neurons with a more convenient electron dense material--for example, heavy metal salts or metallic particles. Conversion of the precipitate into a stable electron dense marker is done before the material is embedded for electron microscopy. The methods include lead, gold, and bromide substitution, treatment with ammonia, direct chemical reduction into metallic silver, and photoreduction of the silver chromate into silver through irradiation with ultraviolet light.