Presence Of Nucleophiles Research Articles

Reaction of Carbenes with Cyclic Ethers in the Presence of Nucleophiles. A Three-Component Coupling Reaction.

Reaction of Carbenes with Cyclic Ethers in the Presence of Nucleophiles. A Three-Component Coupling Reaction.

Enol cation radicals in solution, 3[1] Reaction of enol cation radicals in the presence of nucleophiles

AbstractThe one‐electron oxidation chemistry of three diaryl‐substituted enols, 1,1‐dimesityl‐1‐propen‐2‐ol (7), 1,2‐dimesityl‐1‐propen‐1‐ol (8), and 2‐mesityl‐2‐phenylethenol (9), has been examined. As a consequence of the pronounced steric shielding around the β‐carbon, 7 does not react with nucleophiles upon addition of 2 eq. of oxidant, but cyclizes to yield benzofuran 18. Under identical conditions, the less shielded enols 8 and 9 react with methanol and other nucleophiles. Several mechanistic hypotheses for nucleophile incorporation are tested, thus allowing the second part of the α‐umpolung reaction of ketones via enol cation radicals to be elucidated mechanistically. Fast‐scan cyclic voltammetry studies (2000 V · s‐1) show no reversible wave of the enol cation radicals. The corresponding methyl enol ether cation radicals, however, have a lifetime of several seconds at ‐10°C, and the kinetics of their reactions with acetonitrile and methanol was probed. The results obtained are consistent with fast deprotonation being the primary reaction of enol cation radicals. Under the given oxidative conditions the radicals formed are further oxidized. Consequently one‐electron oxidation of enols provides an interesting entry to α‐carbonyl carbocation chemistry.

Photochemical Behaviour of 1,2,5-Oxadiazoles — Irradiation of Some 3-Acylamino-1,2,5-oxadiazoles in the Presence of Nucleophiles

The photochemical behaviour of some 3-acylamino-1,2,5-oxadiazoles (furazans) has been investigated. On irradiation at 254 nm in the presence of nucleophiles (ammonia, primary or secondary amines), the photoreaction produced 3-substituted 1,2,4-oxadiazoles in which the substituent at C(3) arises from the used reagent. Some mechanistic considerations are reported

β-Substitution de la méso-tétraphénylporphyrine de zinc par voie électrochimique

The electrochemical oxidation of the zinc tetraphenylporphyrin complex in a mixed acetonitrile–dichloroethane solution in the presence of nucleophiles (Py, 3-Pic, [Formula: see text], SCN−) leads to the formation of the corresponding monosubstituted metalloporphyrin. For each of these nucleophiles the substitution occurs at a pyrrole carbon atom (β-substitution). The electrochemical conditions of these substitutions are discussed and an overall reaction is proposed. Key words: porphyrins, electrochemical reactions, nucleophilic substitution.

Multiple bonds between main-group elements and transition metals. 77. Condensation reactions of methyltrioxorhenium(VII) with catechols and aromatic thiols

The title compound methyltrioxorhenium (VII) reacts with catechols to yield complexes of the formula CH 3 Re(O) 2 (1,2-O 2 C 6 R 4 ) ( ) that are fully characterized as pyridine adducts 3. Conducting these reactions in the presence of nucleophiles, e.g; halides, gives the hexacoordinate anions [CH 3 Re(O) 2 (1,2-O 2 C 6 R 4 )(X)] - (X=Cl,Br,I). The rhenium (V) species «CH 3 ReO 2 », prepared in situ from 1 by reduction with (polymer bound) triphenylphosphane, reacts with phenanthrenequinone in the presence of pyridine to give the neutral hexacoordinate complex CH 3 Re(O) 2 (9,10,O 2 C 14 H 8 )(NC 5 H 5 ) (5). Reaction of 1 with thiophenol gives initially the rhenium (VII) complex CH 3 ReO(SC 6 H 5 ) 4 (6), which upon heating undergoes intramolecular elimination of diphenyl disulfide, C 6 H 5 SSC 6 H 5 , to give the dinuclear rhenium (V) complex [CH 3 ReO(μ-SC 6 H 5 )(SC 6 H 5 )] 2 (7;X-ray diffraction study). Condensation of 1 with benzene-1,2-dithiol gives the rhenium (VII) complex CH 3 ReO(1,2-S 2 C 6 H 4 ) 2 (8)

The effect of the structure of the substituent in position ten on the voltammetric behaviour of phenothiazin derivatives

The electrochemical oxidation of seven derivatives of phenothiazin substituted in position 10 on the phenothiazin skeleton has been studied in acetonitrile, using a platinum disc electrode. The electron donor properties of the derivatives that depend on the substituent structure and the presence of nucleophiles in acetonitrile were followed. The diffusion coefficients for the formation of the radical cations were from 10-5 to 10-6 cm2 s-1 and depended on the concentration of the acid in acetonitrile medium. A determination of these substances has been proposed, using differential pulse voltammetry, with a detection limit of 1 . 10-7 mol l-1.

Stereoselective construction of three- and four-contiguous asymmetric centers: Novel possibility by alkylative rearrangement of epoxy silyl ethers

Stereospecific 1,2-rearrangement of epoxy silyl ethers in the presence of nucleophiles is described. The reaction affords 1,3-diols or corresponding mono-silylated ethers, which provides a route to establish three- or four-contiguous asymmetric centers in one step.

Preparation of 1,3-Functionalized Adamantanes by the Lewis Acid Catalyzed Electrophilic Cyclization of 7-Methylenebicyclo[3.3.1]nonan-3-one in the Presence of π- andN-Nucleophiles

Preparation of 1,3-difunctionalized adamantane derivatives from 7-methylenebicyclo[3.3.1] nonane-3-one (1) in the presence of nucleophiles under Lewis acid catalysis is described. Reaction of 1 with benzene using a variety of Lewis acid catalysts gave 1,3-diphenyladamantane (4) as the major product. Trimethylsilyl cyanide and azide efficiently reacted with 1 in the presence of zinc iodide as the catalyst to give good yields of the corresponding 3-trimethylsiloxyadamantyl isocyanide and azide, respectively. Trimethylsilyl isothiocyanate gave a 1:1 mixture of the thiocyanate and isothiocyanate derivatives. Silyl enol ethers, such as those of acetophenone and cyclopentanone, and allyltrimethylsilane also reacted with 1 to afford the corresponding 1,3-difunctionalized adamantanes.

ChemInform Abstract: Anodic Oxidation of (Trimethylsilyl)methanes with π‐Electron Substituents in the Presence of Nucleophiles.

AbstractThe oxidation potentials of methanes (I) with π‐electron substituents are decreased by introduction of a trimethylsilyl group.

Factors relevant in bacterial pyrroloquinoline quinone production

Quinoprotein content and levels of external pyrroloquinoline quinone (PQQ) were determined for several bacteria under a variety of growth conditions. From these data and those from the literature, a number of factors can be indicated which are relevant for PQQ production. Synthesis of PQQ is only started if synthesis of a quinoprotein occurs, but quinoprotein synthesis does not depend on PQQ synthesis. The presence of quinoprotein substrates is not necessary for quinoprotein and PQQ syntheses. Although the extent of PQQ production was determined by the type of organism and quinoprotein produced, coordination between quinoprotein and PQQ syntheses is loose, since underproduction and overproduction of PQQ with respect to quinoprotein were observed. The results can be interpreted to indicate that quinoprotein synthesis depends on the growth rate whereas PQQ synthesis does not. In that view, the highest PQQ production can be achieved under limiting growth conditions, as was shown indeed by the much higher levels of PQQ produced in fed-batch cultures compared with those produced in batch experiments. The presence of nucleophiles, especially amino acids, in culture media may cause losses of PQQ due to transformation into biologically inactive compounds. Some organisms continued to synthesize PQQ de novo when this cofactor was administered exogenously. Most probably PQQ cannot be taken up by either passive diffusion or active transport mechanisms and is therefore not able to exert feedback regulation on its biosynthesis in these organisms.

β-Substitutions on meso-tetraphenylporphyrin by direct electrochemical oxidation in the presence of nucleophiles

The electrochemical oxidation of meso-tetraphenylporphyrin in chloroform–acetonitrile in the presence of various nucleophiles leads to the formation of the corresponding β-substituted salt.

Anodic Oxidation of (Trimethylsilyl)methanes with π-Electron Substituents in the Presence of Nucleophiles

Abstract It was found that oxidation potentials of methanes with π-electron substituents were decreased by introduction of a trimethylsilyl group. The anodic oxidation of benzyl-, allyl-, aryl(or alkyl)thiomethyl-, and aryloxymethyl-substituted trimethylsilanes smoothly proceeded in the presence of nucleophiles, e.g. alcohols and carboxylic acids, to eliminate the trimethylsilyl groups giving the corresponding alkoxylated and carboxylated products in moderate or high yields without full optimization of electrolytic conditions, while aminomethylsilanes did not undergo such a kind of anodic oxidation.

ChemInform Abstract: Novel Molecular Rearrangements of 4‐Hydroxy‐2‐cyclopentenones.

AbstractRearrangement of hydroxycyclopentenone derivatives in the presence of nucleophiles is investigated.

Organic Electrochemistry in the Solid State‐Overoxidation of Polypyrrole

AbstractAnodic overoxidation of polypyrrole in the presence of nucleophiles such as H2O, OH−, CH3OH, CH3O−, Br− and CN− has been studied in detail. At potentials up to 2.1 V (vs. SCE), irreversible anodic peaks are observed with 200 nm layers on Pt. In most of the cases, a total of z = 4 – 6 F/mol monomer unit initially oxidized in the reversible potential region to the radical cation has been evaluated. Only every third ring is involved by this way. Characteristic products in aqueous systems are pyrrolinones (with a keto group in 3‐position), z = 4, and the product of further hydroxylation in 4‐position (z = 6). In the presence of the other nucleophiles, corresponding substitution products seem to be generated. The central reactive intermediate seems to be the dication. Pronounced deviations from this scheme, which is confirmed by IRRAS results, seem to proceed in the presence of strong nucleophiles as OH−, which add initially to the radical cation, and upon strong polarization, which may lead to an attack of the other rings and/or polymer destruction. Practical implications are due to polymer modification and battery material optimization.

Semiquinone anion radicals from addition of amino acids, peptides, and proteins to quinones derived from oxidation of catechols and catecholamines. An ESR spin stabilization study.

Either chemical or enzymatic oxidation of catechols or catecholamines in the presence of nucleophiles (amino acids, peptides, and proteins) leads to the production of ring-substituted o-semiquinones which have been detected by ESR spin stabilization techniques. In many cases, radicals have been completely characterized and structures assigned. Chemical considerations point to a mechanism involving addition of nucleophile to o-quinone, followed by oxidation of product to o-semiquinone. These results confirm that addition occurs in oxidizing polyhydroxy aromatic systems, probably via o-quinone, in a reaction considered to account for much of the toxicity found for catechols and catecholamines.

ChemInform Abstract: Photochemistry of Phenyl Azide: The Role of Singlet and Triplet Phenylnitrene as Transient Intermediates.

AbstractCombining several techniques such as UV/VIS, fluorescence, ESR, and quantitative product analysis, irradiation of the azide (I) at ambient temp. is shown to result in the formation of azepine (III) in the presence of nucleophiles.

Kinetics and mechanism of the nitrosation of N-acetyltryptophan and of the denitrosation of N-acetyl-N1-nitrosotryptophan

Both the formation and the denitrosation of N-acetyl-N1-nitrosotryptophan have been studied kinetically in aqueous solution at 25 °C at acidities between 1 M-HClO4 and pH 4. A value of 850 l mol–1 has been obtained for the equilibrium constant for the formation of N-acetyl-N1-nitrosotryptophan. At acidities ([H+]) greater than 0.1 M the rate constants for both nitrosation and denitrosation increase linearly with the concentration of acid, and the reaction rates are unaffected by the addition of nucleophiles (Br– and SCN–). The results are consistent with a mechanism for nitrosation where the rate-limiting step is the proton transfer from the protonated N-nitroso species to the medium. For denitrosation the corresponding protonation of the nitrosamine is rate-limiting. These conclusions are confirmed by the results obtained when the reactions are carried out in heavy water. However, in the pH range 1–4 the rates of both nitrosation and denitrosation are independent of the acidity of the medium and are again unaffected by the presence of nucleophiles or buffers. It is suggested that in this region nitrosation occurs at C-3. This is followed by deprotonation and an internal NO migration from C to N which is rate-limiting. This mechanism also accounts for earlier results on the denitrosation reaction at even lower acidities (pH 4–7), where acid catalysis and nucleophilic catalysis are found. Results of experiments in heavy water are compatible with this mechanism.

Fluorination with caesium fluoroxysulphate. Room temperuture flourination of phenyl substituted olefins

Room-temperature fluorination of 1,1-diphenylethene with caesium fluoroxysulphate in methylene chloride resulted in an addition elimination process, thus yielding 1, 1-diphenyl-2-fluoroethene, while in the presence of nucleophile containing species, i.e., hydrogen fluoride, methanol, or acetic acid, either vicinal difluorides, methoxy fluorides or acetoxy fluorides were formed. Reaction with norbornene in the absence of a nucleophile gave only 7-fluoronortricyclane and 7-syn-fluoronorborn-2-ene, while similar reaction with 2-phenylprop-1-ene gave 3-fluoro and 3,3-difluoro-2-phenylprop-1-ene. 1,1-diphenylprop-1-ene, 1,1-diphenyl-2-fluoroethene and 1,1-diphenyl-2-bromoethene were converted with caesium fluoroxysulphate in the presence of various nucleophile donating species, i.e. hydrogen fluoride, methanol, or acetic acid, to adducts with nucleophile bonding following Markovnikov type regioselectlvity.

Conversion of 3-imidazoline-3-oxide nitroxyl radicals into nitronylnitroxyl radicals

1. 1-Hydroxy- or 1-oxyl-3-imidazoline 3-oxides with acyl, hydroxyalkyl, or oxime groups, or a hydrogen atom, in the 2-position of the heterocycle undergo oxidation in the presence of nucleophiles with the intermediate formation of 4H-imidazole di-N-oxides, to give nitronylnitroxyl radicals. 2. A new series of stable nitronylnitroxyl radicals with a functional group in the 4-position have been obtained. 3. The electronic effects of the nitronylnitroxyl group have been determined from measurements of the pKa values of the OH and NH2 groups in 4-hydroxy- and 4-amino-5,5-dimethyl-4-phenyl-3-imidazoline 3-oxide 1-oxyl using EPR spectroscopy.

ChemInform Abstract: Acid Decomposition of Tris(dimethylglyoximato)nickelate(IV) in the Presence of Nucleophiles. Evidence for Substitution Inertness of the Nickel(IV) Complex.

AbstractDie Kinetik der Zersetzung des Titelkomplexes in wäßriger Lösung ist nur vom pH‐ Wert des Mediums und nicht von der Art oder Menge der vorhandenen Nucleophilen (‐N3, ‐SCN, ‐CN, NH3, Py, Gly, Imidazol, L‐His, Nitrilotriacetat, Ethylendiamintetraacetat, 2,2′‐Bipyridyl, 1,10‐Phenanthrolin) abhängig.