Showing papers on "Nucleophile published in 1989"

Coupling agents and sterically hindered disulfide linked conjugates prepared therefrom.

Abstract: This invention discloses heterobifunctional coupling agents for making a wide array of molecular conjugates. The agents contain a sterically hindered thiol, linked through a spacer arm to a second group reactive toward nucleophiles such as 1° and 2° amines or reactive thiols that are present on biological and organic materials. The coupling agents are useful for making conjugates containing a sterically hindered linkage.

Nucleophile-assisted cleavage of silane cation radicals

Abstract: La scission des radicaux cationiques (benzyl trialkyl) silanes dans l'acetonitrile est en faveur d'un mecanisme mettant en jeu une participation nucleophile du solvant

Reactions and Syntheses in the Organic Chemistry Laboratory

Abstract: C-C BOND FORMATION Nucleophilic Addition to Aldehydes, Ketones, Carboxylic Acid Derivatives (Esters, Anhydrides) and alpha,beta-Unsaturated Carbonyl Compounds Carbonyl Olefination Alkylation of Aldehydes/Ketones, Carboxylic Acids, and beta-Dicarbonyl Compounds Reactions of the Aldol and Mannich Type Electrophilic and Nucleophilic Acylation Reactions of Alkenes via Carbenium Ions Transition-Metal-Catalyzed Reactions Pericyclic Reactions Radical Reactions OXIDATION AND REDUCTION Epoxidation of C=C Bonds Dihydroxylation of C=C Bonds Oxidation of Alcohols to Carbonyl Compounds Enantioselective Reduction of Ketones Biomimetic Reductive Amination Enantioselective Wacker HETEROCYCLIC COMPOUNDS Three- and Four-Membered Heterocycles Five-Membered Heterocycles Six-Membered Heterocycles Condensed Heterocycles Other Heterocyclic Systems Heterocyclic Dyes SELECTED NATURAL PRODUCTS Isoprenoids Carbohydrates Amino Acids and Peptides Nucleotides and Oligonucleotides DOMINO REACTIONS Domino Reactions in Synthetic Methodology Domino Reactions in The Synthesis of Alkaloids Domino Reactions in The Synthesis of Isoprenoids Domino Reactions for The Synthesis of Chromans And Dioxins Domino Reactions in The Synthesis of Chiroptical Switches INDEX OF REACTIONS INDEX OF PRODUCTS SUBJECT INDEX

Evidence that metaphosphate monoanion is not an intermediate in solvolysis reactions in aqueous solution

Abstract: The slope of plots of log k2 against the pKa of the leaving group for reactions of uncharged, monoanionic, and dianionic oxygen nucleophiles with three phosphorylated pyridine monoanions changes from PI, = -098 to -079 as the pK, of the oxygen nucleophile increases from 36 to 157 (aqueous solution; 25 "C; ionic strength, 15) This coupling between the strength of the nucleophile and the measure of bond cleavage is described by an interaction coefficient, pxy = dp,,/dpK,,, = dp,,,/dpK,, = 0013, and provides evidence for concerted phosphoryl transfer between pyridine and oxygen bases The solvolysis of phosphorylated pyridines and of acetyl phosphate in aqueous solution also shows the behavior expected for a bimolecular substitution reaction with no metaphosphate intermediate: (1 ) The values of log k2 for reactions of oxygen nucleophiles, including water, with phosphorylated y-picoline monoanion follow a single line of slope 051 in a plot against log k2 for the corresponding bimolecular reactions with methyl 2,4-dinitrophenyl phosphate monoanion (Kirby, A J; Younas, M J Chem SOC B 1970, 1165) (2) Water behaves as expected for a nucleophile of its pKa in a Br~nsted-type plot of log k2 for reaction with phosphorylated y-picoline against the pK, of oxygen nucleophiles (3) The value of PI, = -102 for the hydrolysis of phosphorylated pyridines is less negative than the value of Ow' = -125 for complete breaking of the P-N bond (4) The value of Pi, = -102 for the reaction of water with phosphorylated pyridines fits the correlation of PI, against the pKa of oxygen nucleophiles for concerted, bimolecular reactions, with a slope ofp, = a~l,/dpKn,, = 0013 (5) The reactions of acetyl phosphate monoanion and dianion with aqueous alcohols show selectivity that depends on the pKa of the alcohol, with a larger selectivity for the less reactive dianion [50% aqueous alcohol (v/v), 55 "C] Metaphosphate monoanion has been discussed as a possible reaction intermediate since 1955 when eq 1 was proposed in order to explain the rapid hydrolysis of phosphate monoester monoanScheme I

Nucleophilic opening of chiral bis(aziridines): a route to enantiomerically pure .alpha.-amino aldehydes or acids and polysubstituted piperidines

Abstract: La reaction regiospecifique de decyclisation d'aziridines derivees du mannitol se fait par des nucleophiles organometalliques

Phosphoryl Transfer to Anionic Oxygen Nucleophiles. Nature of the Transition State and Electrostatic Repulsion

Abstract: Bimolecular phosphoryl transfer to anionic oxygen nucleophiles from phosphorylated pyridine monoanions is observed in aqueous solution at 25 OC and ionic strength 1.5. The dependence on ionic strength of several reactions suggests that screening of charge repulsion with an increase in ionic strength from 0 to 1.5 causes only - 5-fold increases in rate for these reactions between monoanions. Comparisons with reactions of neutral substrates suggest that the electrostatic repulsion which remains at ionic strength 1.5 causes a decrease in rate of - 10-fold or less. The small values of p,,, = 0.3 and the large negative values of PI, = -( 1 .O-0.8) for reactions of phosphorylated pyridines with anionic oxygen nucleophiles, and the small value of p,,, = 0.10 for the reverse reaction of substituted pyridines with acetyl phosphate dianion, suggest that there is little bond formation to the oxygen nucleophile and much bond cleavage to the pyridine leaving group in the dissociative, metaphosphate-like transition state of a concerted displacement reaction. The absence of an effect of Mg2+ binding on the value of p = 1.05-1.1 for the formation of phosphorylated pyridines shows that there is no significant charge/dipole interaction between Mg% and the substituent on the pyridine and that the effect of Mgz+ binding is less than the effect of protonation. The values of p,,, for phosphoryl transfer to nitrogen nucleophiles are not significantly different with oxygen and nitrogen leaving groups of the same pK,. Phosphoryl transfer to anionic oxygen nucleophiles is an im- portant biochemical reaction that usually involves a magnesium ion. Examples include the phosphorylation of AMPZ-, ADP3-, acetate ion, and 3-phosphoglycerate ionz Despite the biochemical importance of phosphoryl transfer to anionic nucleophiles, the transition state for the nonenzymatic reaction has not been characterized. One reason that these reactions have seldom been examined may be a widely held belief that electrostatic repulsion is large in phosphoryl transfer to anionic oxygen nucleophiles; in fact, the possibility has been raised that enzymes can catalyze phosphoryl transfer by overcoming electrostatic repulsion.z4 However, the arguments that have been presented to support a large effect of electrostatic repulsion on the rate of phosphoryl transfer are

The reactions of phosphazenes with difunctional and polyfunctional nucleophilic reagents

Abstract: After a brief introduction of the reaction patterns of N3P3Cl6 and N4P4Cl8 with monofunctional nucleophilic reagents, the reactions of these two chlorocyclophosphazenes with difunctional, trifunctional and tetrafunctional nucleophilic reagents are surveyed. These reagents contain aliphatic and aromatic hydroxyl and primary and secondary amino groups. Spiro, ansa, dangling and bridging units have been observed as well as a combination of these. The stability of the various ring systems is discussed. Crystal structures and 1H, 13C and 31P n.m.r. spectroscopic studies are reported.

Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. I. Catalysis by metal ions in the reaction of p-nitrophenyl diphenylphosphinate with ethoxide

Abstract: The effect of macrocyclic crown ether and cryptand complexing agents on the rate of the nucleophilic displacement reaction of p-nitrophenyl diphenylphosphinate by alkali metal ethoxides in ethanol at 25 °C has been studied by spectrophotometric techniques. For the reactions of potassium ethoxide, sodium ethoxide, and lithium ethoxide, the observed rate constant increased in the order KOEt < NaOEt < LiOEt. Crown ether and cryptand cation-complexing agents have a retarding effect on the rate. Increasing the ratio of complexing agent to base results in a decrease in kobs to a minimum value corresponding to the rate of reaction of free ethoxide ion. In complementary experiments, alkali metal ions were added to these reaction systems in the form of unreactive salts, causing an increase in reaction rate. The kinetic data were analysed in terms of ion-pairing treatments, which allowed evaluation of rate coefficients due to free ethoxide ions and metal ion – ethoxide ion pairs. Possible roles of the metal cations...

The pentafluorophenylxenon(II) cation: [C6F5Xe]+; the first stable system with a xenon–carbon bond

Abstract: Pentafluorophenylxenon(II) pentafluorophenyltrifluoroborate, [C6F5Xe]+[C6F5BF3]–, can be prepared analogously to pentafluorophenylhalogen(III) and (V) fluorides, C6F5HalF2 and C6F5HalF4(with Hal = Br or I), by nucleophilic displacement of fluorine in XeF2 using B(C6F5)3 as an aryl-transfer reagent; the resulting colourless solid with a stable xenon–carbon bond is characterised in solution by 129Xe and 19F n.m.r. and chemically as an electrophilic transfer reagent for pentafluorophenyl groups.

Ethylene Biosynthesis part 10. Synthesis and study of racemic, (1R, 2S)-, and (1S, 2R)-1-Amino-2-(hydroxymethyl)cyclopropanecarboxylic Acid

Abstract: The preparation of optically active 1-amino-2-(hydroxymethyl)cyclopropanecarboxylic acid has been achieved by a route involving cycloalkylation of dimethyl malonate with epichlorohydrin and subsequent Hofmann rearrangement. The reaction of epichlorohydrin with nucleophiles may occur al either electrophilic site, epoxide or halide. Based on the absolute configuration of the starting materials and the lactones obtained, it has been shown that the initial step of the cycloalkylation occurs at the epoxide moiety. The 1-amino-2-(hydroxymethyl)-cyclopropanecarboxylic acid, an analogue of the precursor to the plant growth hormone ethylene, is to be used in affinity purification techniques and in generation of antibodies.

Stereoselective nucleophilic additions to the carbon-nitrogen double bond. 2. Chiral iminium ions derived from "second generation" chiral amines

Abstract: Les iminiums derives des chloro-2'- et dichloro-2',6' phenyl-1 ethylamines reagissent selon une addition nucleophile avec des hydrures; cette reaction est un exemple d'induction asymetrique 1,3

Palladium-catalyzed reactions in the synthesis of 3- and 4-substituted indoles. 4

Abstract: 4-Bromo-1-tosylindole (1) was converted to tricyclic indole enone 11, a potential intermediate in the synthesis of tetracyclic ergot alkaloids, by a series of palladium-catalyzed processes. Attempts to construct the ergot D ring by the hetero-Diels-Alder reaction of enone 11 and 1-azabutadiene 12 produced not the expected (4 + 2) adduct 13 but the benz(cd)indoline derivative 14 resulting from attack of the aza diene at the indole 2-position. The thermodynamic stability of the naphthol nucleus makes enone 11 generally susceptible to attack at the indole 2-position, as evidenced by the attack of hydride and methyl cuprate nucleophiles at this portion forming indolines 16 and 17, respectively.

Perfluoroalkylations of nitrogen-containing heteroaromatic compounds with bis(perfluoroalkanoyl) peroxides

Abstract: Perfluoroalkylations of nitrogen-containing heteroaromatic compounds with bis(perfluoroalkanoyl) peroxides were studied. Bis(trifluoroacetyl) peroxide, bis(heptafluorobutyryl) peroxide, and bis(pentadecafluoro-octanoyl) peroxide were found to be useful and effective reagents for perfluoromethylations, perfluoropropylations, or perfluoroheptylations of pyrrole and its derivatives, while these peroxides could not be applied for the perfluoroalkylations of pyridine or imidazole. For the perfluoroalkylations with the peroxides, electron transfer from the substrate to the peroxide, which affords a perfluoroalkyl radical and a cation radical of the substrate in a solvent cage, is proposed. In pyrroles, since the delocalization of the N lone pair to π-systems lowers the nucleophilicity of the N lone pair and increases the electron density at the π-orbital, the electron transfer readily occurred and perfluoroalkylated pyrroles were obtained in good yield and regioselectively. However, nucleophilic attack of the N lone pair to the O–O bond of the peroxide was superior to the electron transfer in pyridine or imidazole of which the N lone pairs are very nucleophilic.

Acylphosphonic acids and methyl hydrogen acylphosphonates: physical and chemical properties and theoretical calculations

Abstract: Acylphosphonic acids (5) and methyl hydrogen acylphosphonates (3) were synthesized by di- and mono-demethylation of dimethyl acylphosphonates (1). Spectroscopic data (i.r., 31P and 1H n.mr.) are reported for these types of compounds for the first time. Examination of their hydrolytic stability under acidic and basic conditions revealed that except for methyl hydrogen acylphosphonates (3) that are unstable under highly alkaline pH conditions, the C–P bond in these types of compounds is stable in most cases. Nucleophilic reagents, e.g. amines, borohydride, or hydroxylamine react with the carbonyl group of ionized acylphosphonates with the preservation of the C–P bond, to yield α-imino-, α-hydroxy-, or α-oxyimino-alkylphosphononate anions, respectively. Semi-empirical quantum mechanical (MNDO/H) calculations were performed on benzoylphosphonic acid (5c) and on the esters and anions derived from it, as representatives of their classes, in order to assess bond lengths and preferred conformations, and to estimate charges on the carbonyl and phosphoryl groups. Calculations show that for both neutral and ionized (anions) compounds free rotation around the C–P bond is expected due to the low energy barriers.