Use of pyridinium bromide perbromide as an analytical reagent

Pyridinium bromide perbromide has been shown to be a satisfactory reagent for the bromination of phenols, unsaturates and aromatic ethers. The color of the reagent permits the spectrophotometric detection of the end-point. Most phenols and unsaturatcs react fast enough, in the presence of a catalyst, to permit titration within 10 min. The aromatic ethers, in general, react very slowly but yield accurate results. The reagent has the advantage over the conventional bromate-bromide that the titrations are fast and the stoichiometry exact. The speed of reaction of PBPB with many functional groups may permit the use of automatic titration procedures with this reagent.     

Synthesis and reactions of perfluoro-n-octylmagnesium bromide

Perfluoro-n-octylmagnesium bromide has been conveniently synthesized through the metal-halogen exchange reaction between perfluoro-n-octyl iodide and ethyl- or phenyl-magnesium bromide. The thermal stability of the Grignard reagent and the products of the thermal decomposition have been estimated in diethyl ether and tetrahydrofuran. Reactions of the Grignard reagent with various substrates such as water, acetone, hexafluoroacetone, trimethylchlorosilane and carbon dioxide indicate the synthetic utility of n-octylmagnesium bromide.     

Sensitive o-toluidine reagent for glucose assay at moderately elevated temperatures

A reagent is developed for increasing the sensitivity of the direct o-toluidine procedure for glucose so that the reaction may be carried out at temperatures as low as 37 °C.The sensitivity is achieved by eliminating all water from the system, except for a minute percentage introduced with the samples and by using a high boric acid concentration. Under these conditions protein does not precipitate and lipids remain dissolved. At 55 °C, interference from bilirubin at a concentration of 20 mg/100 ml is 3.5%.Blood may be collected with sodium fluoride since it does not interfere in the procedure. The rate of color development is slower for aqueous standards than it is for serum at the lower temperatures. This may be corrected by preparing the standards in a 6% albumin solution.Results obtained by applying the reagent with the continuous flow system of analysis (Technicon), and with the discrete sample analyzers, Beckman DSA, Lars Jungberg Autolab (Sweden), Robot Chemist, and RaBA (Japan) are reported.The reagent is useful as a spray reagent for sugar identification on thin-layer chromatography plates. Heating at different temperatures permits the differentiation between certain sugars with similar R_f values.     

Lithium–Bromide Exchange versus Nucleophilic Addition of Schiff's base: Unprecedented Tandem Cyclisation Pathways

By exploring lithium–bromide exchange reactivity of aromatic Schiff's bases with tert-butyllithium (tBuLi), we have revealed unprecedented competitive intermolecular and intramolecular cascade annulation pathways, leading to valuable compounds, such as iso-indolinones and N-substituted anthracene derivatives. A series of reaction parameters were probed, including solvent, stoichiometry, sterics and organolithium reagent choice, in order to understand the influences that limit such ring-closing pathways. With two viable reactivity options for the organolithium on the imine; namely, nucleophilic addition or lithium–bromide exchange, a surprising competitive nature was observed, where nucleophilic addition dominated, even under cryogenic conditions. Considering the most commonly used solvents for lithium–bromide exchange, tetrahydrofuran (THF) and diethyl ether (Et₂O), contrasting reactivity outcomes were revealed with nucleophilic addition promoted in THF, while Et₂O yielded almost double the conversion of cyclic products than in THF.     

(PhCH2PPh3)+Br3 −: A Versatile Reagent for the Chemoselective Oxidation of Sulfides to Sulfoxides

Benzyltriphenylphosphonium tribromide (BTPTB), as a stable solid reagent, is easily prepared by the reaction of benzyltriphenylphosphonium bromide with Br₂. This reagent can be used as an efficient reagent for the chemoselective oxidation of dialkyl and aryl-alkyl sulfides to their corresponding sulfoxides in the presence of diaryl sulfides and primary alcohols. All reactions were performed in a refluxing mixture of methanol and water in very short reaction times.     

Nucleosides and Nucleotides. Part 19. On Detritylation with Zinc Bromide in Oligonucleotide Synthesis

Zinc bromide has been shown by several groups of workers to be a useful reagent in the removal of trityl protecting groups from nucleotides. An attempt is made here to establish optimum conditions for the reaction, a strange observation is reported, and a novel workup via a soluble, lipophilic zinc complex is described.     

Development of a protocol for eight- and nine-membered ring synthesis in the annulation of sp(2),sp(3)-hybridized organic dihalides with keto esters

A protocol has been developed in which annulation reactions of various dihalides with keto esters can be carried out to provide entry to eight- and nine-membered carbocycles. In this process wherein one alkenyl- or aryl bromide and a tethered alkyl chloride comprise the organic dihalide, a selective metal-halogen exchange reaction between the sp(2)-hybridized bromide and an organolithium initiates the process. Transmetalation to an organoytterbium reagent generates a species that undergoes selective carbonyl addition to the ketone of the keto ester, creating a lactone intermediate. Subjection of the resulting chloroalkyl lactone to intramolecular reductive coupling with samarium(II) iodide completes the desired annulation.     

Toward a protecting-group-free halogen-metal exchange reaction: practical, chemoselective metalation of functionalized aromatic halides using dianion-type zincate, tBu4ZnLi2

A versatile preparation method for aromatic zincate compounds through a halogen-zinc exchange reaction using dilithium tetra-tert-butylzincate (tBu4ZnLi2) has been developed. This reagent permits efficient preparation of highly functionalized aromatic zincates, particularly, those with electrophilic functional groups, such as ester, amide, alcohol, and phenol. Halogen-zinc exchange reactions followed by electrophilic trapping (with allyl bromide or benzaldehyde) proved to be a powerful tool for C-C bond formation on functionalized aromatic rings. The functionalized aromatic zincate intermediate was also found to undergo copper- and palladium-catalyzed C-C bond-forming reactions with good yields and high chemoselectivity.     

Use of competition kinetics with fast reactions of grignard reagents

Competition kinetics are useful for estimation of the reactivities of Grignard reagents if the reaction rates do not differ widely and if exact rates are not needed. If the rate of mixing is slower than the rate of reaction, the ratios between the rates of fast and slow reagents are found to be too small. This is concluded from experiments in which results obtained by competition kinetics are compared with results obtained directly by flow stream procedures. A clearer picture of the reactivity ratios is obtained when the highly reactive reagent is highly diluted with its competitor. A fast reagent may account for almost all the product even when present as only 1 part in 100 parts of the competing agent. In this way allylmagnesium bromide is estimated to react with acetone, benzophenone, benzaldehyde, and diethylacetaldehyde ca. 1.5 x 10(5) times faster than does butylmagnesium bromide. The rates found for the four substrates do not differ significantly, and it seems possible that there is a ceiling over the rate of reaction of this reagent, for example, caused by diffusion control. This may explain that competition kinetics using allylmagnesium bromide have failed to show kinetic isotope effects or effects of polar substituents with isotopically or otherwise substituted benzophenones. A recently reported alpha-deuterium secondary kinetic isotope effect for the reaction of benzaldehyde with allylmagnesium bromide was observed at -78 degrees C, but was absent at room temperature. It is suggested that the reaction of benzophenone and benzaldehyde with allylmagnesium bromide has a radical-concerted mechanism since no radical-type products are produced and since no color from an intermediate ketyl is observed even at -78 degrees C.     

Colorimetric determination of osmium (VI) with 1-naphthylamine-3,5,7-trisulfonic acid

An investigation has been made of the organic compound, 1-naphthylamine-3,5,7-trisulfonic acid, for use as a colorimetric reagent for the determination of trace quantities of osniiun(Vl). When this compound is added to an osmium (OsO₄^-2) solution, a violet colored organo-osmium complex is formed which has highest sensitivity at wavelength 560 mμ. The complex reaches maximum color intensity after four hours. The solution should be brought to pH 1.5 and maintained at tins pH with a buffer, after maximum color development. The colored complex conforms to Beer's law over an osmium concentration range of 0 to 6 p.p.m. Its absorbance is not affected by either excess reagent (up to a 7 : 1 ratio) or by temperature variation over the range 15° to 35° C. Many metallic ions interfere with the color reaction and hence require a separation, of osmium from these ions.The nature of the complex in solution was studied by the following three methods: (1) mole ratio, (2) continuous variations and (3) slope ratio. All three methods indicate a ratio of reagent to osmium of 2 to 1.A procedure for the colorimetric determination of osmium(VI) has been developed; it has a sensitivity of 1 part osmium in 15,000,000 parts of solution, with good precision.     

Spectrophotometric evaluation of interferences in three iron reactions for the determination of serum total cholesterol

A spectrophotometric and chemical evaluation of reported interferences for three iron reactions for the determinations of serum cholesterol has been presented. It has been shown that all three reactions are affected by various interfering substances, such as 2-thiouracil, nitrate, azide, bromide, diethylstilbesterol, and steroids. Spectral differences between the reactions are probably due to solvent and anion effects.The incorporation of uranyl acetate as a precipitating agent into the ferric acetate-uranyl acetate procedure did not make the results obtained comparable with the Abell-type extract of a very icteric serum. Incorporation of ferrous sulfate does not noticeably affect the intensity or stability of color development with standards.As proposed, the ferric acetate-uranyl acetate procedure for the determination of cholesterol represents a modified iron reagent, but the reaction mechanism and the procedure described for it is neither new nor direct. The use of the ferric acetate reagent for serum cholesterol as opposed to ferric chloride, ferric ammonium chloride, or ferric perchlorate appears to have no real advantages as a color reaction. In fact the reaction is less sensitive while reagent preparation is more tedious, time consuming, and expensive than the ferric chloride procedure. Although no quantitative studies were performed, the only effects that chloride had on the reactions were in the region of 400 nm, a wavelength sufficiently far enough away from the 560-nm peak that it did not affect determinations. The concept that the ferric acetate-uranyl acetate reagent contains only acetate and sulfate anions is nullified as soon as one adds serum to the reagents.     

Modeling a halogen dance reaction mechanism: A density functional theory study

Since the discovery of the halogen dance (HD) reaction more than 60 years ago, numerous insights into the mechanism have been unveiled. To date however, the reaction has not been investigated from a theoretical perspective. Density functional theory (DFT) was used to model the potential energy surface linking the starting reagents to the lithiated products for each step in the mechanism using a thiophene substrate. It was found that the lithium‐halogen exchange mechanism is critical to understand the HD mechanism in detail and yielded the knowledge that S_N2 transition states (TS) are favored over the four‐center type for the lithium‐bromine exchange steps. The overall driving force for the HD is thermodynamics, while the kinetic factors tightly control the reaction path through temperature. The S_N2 lithium‐bromide TS are barrierless, except the second, which is the limiting step. Finally, the model for the HD is discovered to be a pseudo‐clock type, due to a highly favorable bromide catalysis step and the reformation of 2‐bromothiophene. © 2016 Wiley Periodicals, Inc.     

Cp2TiCl2-catalyzed grignard exchange reactions with 1,3-dienes or styrenes. Preparation of allylic and α-arylethyl grignard reagents by a convenient and quantitative method

The addition of a catalytic amount of Cp₂TiCl₂ to an ether solution of propylmagnesium bromide and 1,3-dienes brings about an exchange reaction forming allylic Grignard reagents.Styrene also undergoes the exchange reaction under the same conditions affording α-phenethylmagnesium bromide.     

Microdetermination of magnesium in biological fluids

A modified procedure for the spectrophotometric determination of serum magnesium is described in which a direct addition of serum is made to a two-piece color reagent system. One solution contains an optimized concentration of the color reagent Magon, 1-azo-2-hydroxy-3-2-(2,4-dimethylcarboxanilido)-naphthalene-1′-(2-hydroxybenzene-5-sulfonate) sodium salt, in a dimethyl sulfoxide medium for binding magnesium and also EGTA, ethylene-bis(oxyethylenenitrilo)-tetraacetic acid, for binding calcium. The other is an alkaline solution of dilute cyanide to bind copper and iron and provide the optimum reaction pH. By means of these ingredients the simple procedure involves a reaction with serum magnesium while masking the interference of calcium and trace metals and allowing the reaction to take place without either separation or deproteinization steps. Optimized characteristics are shown for a linear reaction involving a variable reaction blank in the procedure where calcium is measured against a blank that is constant in its makeup. The linearity of the procedural reagent blank, with respect to its own concentration, helps attain the required linear results for the overall system and this is described by spectral studies.     

The spectrophotometric determination of molybdenum and uranium with quinalizarin

Quinalizarin reacts very sensitively with molybdenum and uranium to form colored chelates having λ_max at 520 and 630 nm, respectively. The molar ratio for both the chelates is 1:1 (metal:reagent). Optimum conditions including the range for adherence to Beer's law, effect of pH on color intensity, effect of excess of the reagent, sensitvity, and interference of the foreign ions has been reported for the photometric determination of these metal ions using quinalizarin, in 50% ethanolic medium and at 30 °C.     

Selectivity under microwave irradiation. Benzylation of 2-pyridone: an experimental and theoretical study

The reaction of 2-pyridone with benzyl bromide in the absence of base and under solvent-free conditions has been studied experimentally and by computational methods. This reaction was one of the first reported examples in which modification of selectivity under microwave irradiation was observed. C- and/or N-alkylations were obtained depending on the benzyl halide and the heating system. N-Alkylation through mechanism A (S_N2 mechanism) is kinetically favoured while C-alkylation through an S_N1-type mechanism is thermodynamically favoured and is observed under microwave irradiation. Two S_N1-type mechanisms (mechanisms B and C) have been calculated, mechanism C being a kind of S_Ni. The influence of the pyridone/benzyl bromide ratio was studied. A second molecule of pyridone stabilizes the transition state and assists the leaving of the bromide ion. The occurrence of C-alkylation under microwave irradiation is explained by the predominance of the thermodynamic control in these conditions. Under microwave irradiation N-alkylation through an S_N1-type mechanism (mechanism C) can also occur. The dependence of the outcome of N-alkylation on the benzyl bromide ratio has been explained by a shift in the mechanism from S_N2 to S_N1 under microwave irradiation. Computational calculations have shown to be a useful tool for determination of the origin of the selectivity under microwave irradiation.     

Thiophosphoryl bromide: a new reagent for the reduction of sulfoxides to sulfides

Thiophosphoryl bromide (PSBr₃) has been found to be a mild, effective reagent for the reduction of sulfoxides of widely differing structural types to the corresponding sulfides.     

Sensitive microdetermination of scandium,yttrium, and lanthanides using Eriochrome azurol B in the presence of a micelle-forming cationic surfactant

The chelation reactions of Sc³⁺, Y³⁺, and some lanthanides with Eriochrome azurol B in the presence and absence of cetyltrimethylammonium bromide has been studied in detail. The complexes show a marked bathochromic shift in their λ_max at pH 6.0. The composition of the complexes in the absence and presence of cetyltrimethylammonium bromide was found to be 1:2 (Metal:ligand). The values of the conditional formation constants have also been evaluated. The studies were further extended to the analytical aspects of the complex equilibria and a suitable procedure has been recommended for the spectrophotometric microdetermiantion of Sc³⁺, Y³⁺, and some lanthanides under study using Eriochrome azurol B in the presence of cetyltrimethylammonium bromide which considerably increases the sensitivity of the color reaction. The use of Eriochrome azurol B in the presence of cetyltrimethylammonium bromide as a complexometric indicator has also been reported.The effect of surfactant concentration and the effect of mineral salts on the absorption spectra of Eriochrome azurol B at pH 11.6 and 6.0 have also been reported.     

Studies on sulfinatodehalogenation XIII. A new sulfinatodehalogenation reagent system: NaHSO3/K3[Fe(CN)6]

The reaction of perfluoroalkyl iodide or perfluoroalkyl bromide with sulfite radical anion (SO₂) generated from sodium bisulfite / potassium ferricyanide in water/ DMF at 70—75°C gave the corresponding perfluoroalkanesulfinate in good yield. Furthermore, it was shown that the reagent system can also initiate the addition of perfluoroalkyl iodide to olefins or alkynes at 50—65°C.     

Sensitive spectrophotometric determination of molybdenum(VI) with Pyrogallol Red and cetyltrimethylammonium ions

Pyrogallol red in the presence of cetyltrimethylammonium bromide is proposed for the spectrophotometric determination of microgram amounts of molybdenum. The sensitivity of the color reaction between molybdenum and Pyrogallol Red has been greatly increased by the sensitizing action of cetyltrimethylammonium bromide (?_{600 nm} = 90,000). Beer's law is obeyed over the range 0.1–0.4 μg/ml of molybdenum. The composition of the complex may therefore be formulated as Mo:PR:CTA = 1:2:4.