Showing papers on "Nucleophile published in 2002"

The chemistry of dimethyl carbonate.

Abstract: Dimethyl carbonate (DMC) is a versatile compound that represents an attractive eco-friendly alternative to both methyl halides (or dimethyl sulfate) and phosgene for methylation and carbonylation processes, respectively. In fact, the reactivity of DMC is tunable: at T = 90 °C, methoxycarbonylations take place, whereas at higher reaction temperatures, methylation reactions are observed with a variety of nucleophiles. In the particular case of substrates susceptible to multiple alkylations (e.g., CH2-active compounds and primary amines), DMC allows unprecedented selectivity toward mono-C- and mono-N-methylation reactions. Nowadays produced by a clean process, DMC possesses properties of nontoxicity and biodegradability which makes it a true green reagent to use in syntheses that prevent pollution at the source. Moreover, DMC-mediated methylations are catalytic reactions that use safe solids (alkaline carbonates or zeolites), thereby avoiding the formation of undesirable inorganic salts as byproducts. The r...

N-tert-butanesulfinyl imines: versatile intermediates for the asymmetric synthesis of amines.

Abstract: N-tert-Butanesulfinyl aldimines 3 and ketimines 4 are exceedingly versatile intermediates for the asymmetric synthesis of amines. The N-tert-butanesulfinyl imines are prepared in high yields by condensing enantiomerically pure tert-butanesulfinamide 1, which is readily available in either configuration, with a wide range of aldehydes and ketones. The tert-butanesulfinyl group activates the imines for the addition of many different classes of nucleophiles, serves as a powerful chiral directing group, and after nucleophilic addition is readily cleaved by treatment of the product with acid. A wide range of highly enantioenriched amines, including α-branched and α,α-dibranched amines, α- and β-amino acids, 1,2- and 1,3-amino alcohols, and α-trifluoromethyl amines, are efficiently synthesized using this methodology. In addition, N-tert-butanesulfinyl imine derivatives provide a new family of ligands for asymmetric catalysis.

Anion−Aromatic Bonding: A Case for Anion Recognition by π-Acidic Rings

Abstract: The basis for unprecedented noncovalent bonding between anions and the aryl centroid of electron-deficient aromatic rings has been demonstrated by an ab initio study of the interaction between 1,3,5-triazine and the fluoride, chloride, and azide ion at the MP2 level of theory. Minima are also located corresponding to C[bond]H...X(-) hydrogen bonding, reactive complexes for nucleophilic attack on the triazine ring, and pi-stacking interactions (with azide). Trifluoro-1,3,5-triazine also participates in aryl centroid complexation and forms nucleophilic reactive complexes with anions. This novel mode of bonding suggests the development of new cyclophane-type receptors for the recognition of anions.

Nucleophilic transition metal based cyclization of allenes

Abstract: Allenes bearing a pro-nucleophile can be cyclized on treatment with a wide variety of transition metal catalysts and reagents: palladium, cobalt, ruthenium, silver, rhodium, lanthanides, gold. The nucleophilic groups can be nitrogen, oxygen or carbon based and can form rings of various sizes, often with good control of stereochemistry. A variety of mechanisms can be proposed for these reactions and the metal complex can be used to introduce a variety of functional groups during cyclization. Several heterocyclic natural products have been prepared using a selection of these reactions.

Fine chemical synthesis with homogeneous palladium catalysts: examples, status and trends

Abstract: During the last decade several palladium-catalyzed reactions have been developed and optimized to a stage that enables application on an industrial scale. The importance of these reactions as novel key steps for fine chemical synthesis is shown. Examples include palladium-catalyzed Heck, Suzuki and Sonogashira coupling reactions, telomerization of 1,3-butadiene with nucleophiles, and carbonylation of aromatic and benzylic halides.

Pd(II) acts simultaneously as a Lewis acid and as a transition-metal catalyst: synthesis of cyclic alkenyl ethers from acetylenic aldehydes.

Abstract: The reaction of carbon-tethered acetylenic aldehydes with alcohols in the presence of a catalytic amount of Pd(OAc)2 in 1,4-dioxane at room temperature gave the 5- or 6-membered acetal products in high yields. The 13C NMR studies suggested that a Pd(II) catalyst exhibited dual roles in the present reaction; the attack of ROH to aldehyde is catalyzed by Lewis acidic Pd(OAc)2, and the nucleophilic oxygen of the resulting hemiacetal reacts with alkyne complexed by Pd(II), giving the alkenyl ethers.

Sequential One-Pot InBr3-Catalyzed 1,4- then 1,2-Nucleophilic Addition to Enones

Abstract: Low sensitivity toward traces of moisture and high tolerance of different functional groups make indium tribromide suitable for carrying out multistep synthetic sequences. In particular, we have realized a 1,4-conjugated addition of indoles/thiols to α,β-unsaturated ketones mediated by a catalytic amount (10 mol %) of InBr3 obtaining the desired β-substituted ketones in good yields. The Lewis acidity of indium salts was not affected by coordinating and acid nucleophiles; therefore, the subsequent catalytic 1,2-addition of Me3SiCN to carbonyl compounds can be performed in one pot. With the optimized atom-efficient protocol, several polyfunctionalized α-silyloxy cyanohydrins were synthesized in good to excellent yields (up to 97%) and a notable level of simple 1,3-diastereoselection (up to 84:16) was recorded in the case of 2-cyclohexen-1-one 2c.

Direct Arylation via Cleavage of Activated and Unactivated C-H Bonds

Abstract: Palladium catalyzed direct coupling of aryl halides not only with various carbon nucleophiles having acidic C-H bonds, but also with aromatic and heteroaromatic compounds accompanied by cleavage of their unactivated C-H bonds, has been significantly developed in recent years In this review the progress of these reactions as effective methods for preparing aromatic fine compounds is summarized A brief description of related direct coupling of arenes with unsaturated compounds is also given

Direct Generation of Nucleophilic Chiral Palladium Enolate from 1,3-Dicarbonyl Compounds: Catalytic Enantioselective Michael Reaction with Enones

Abstract: Generation of chiral palladium enolates from 1,3-dicarbonyl compounds with the palladium aqua complex and its application to the highly efficient catalytic enantioselective Michael reaction with enones are described The palladium aqua complexes are likely to supply Bronsted base and Bronsted acid successively during the reaction The former activates the carbonyl compounds to give chiral palladium enolates, and the latter cooperatively activates enones Using a catalytic amount (2−10 mol %) of the palladium complexes, the various 1,3-dicarbonyl compounds including diketones and β-ketoesters were converted to the desired Michael adducts in good yields (69−92%) with excellent enantiomeric excesses (89−99% ee)

The Vinylogous Intramolecular Morita−Baylis−Hillman Reaction: Synthesis of Functionalized Cyclopentenes and Cyclohexenes with Trialkylphosphines as Nucleophilic Catalysts

Abstract: The development of the vinylogous intramolecular Morita−Baylis−Hillman reaction for the synthesis of functionalized cyclopentenes and cyclohexenes is described. The reaction involves the trialkyphosphine-catalyzed cyclization of 1,6- or 1,7-diactivated 1,5-hexadienes or 1,6-heptadienes, containing carboxyaldehyde, methyl ketone, or methoxycarbonyl as the olefin activating groups. A representative example of this reaction is the Me3P-catalyzed cyclization of 1a in tert-amyl alcohol, which provides the substituted cyclopentene 2a in 95% yield and with 97:3 regioselectivity.

Nucleophilicity in ionic liquids. 2.(1) Cation effects on halide nucleophilicity in a series of bis(trifluoromethylsulfonyl)imide ionic liquids.

Abstract: In this work, the nucleophilicities of chloride, bromide, and iodide have been determined in the ionic liquids [bmim][N(Tf)2], [bm2im][N(Tf)2], and [bmpy][N(Tf)2] (where bmim = 1-butyl-3-methylimidazolium, bm2im = 1-butyl-2,3-dimethylimidazolium, bmpy = 1-butyl-1-methylpyrrolidinium, and N(Tf)2 = bis(trifluoromethylsulfonyl)imide). It was found that in the [bmim]+ ionic liquid, chloride was the least nucleophilic halide, but that changing the cation of the ionic liquid affected the relative nucleophilicities of the halides. The activation parameters ΔH⧧, ΔS⧧, and ΔG⧧ have been estimated for the reaction of chloride in each ionic liquid, and compared to a similar reaction in dichloromethane, where these parameters were found for reaction by both the free ion and the ion pair.

Cyclization reactions of 1,3-bis-silyl enol ethers and related masked dianions

Abstract: Despite their simplicity and synthetic usefulness, reactions of 1,3-dicarbonyl dianions are often problematic, since they represent highly reactive compounds (low reactivity matching) In addition, functionalized electrophiles are often rather labile and reactions with strong nucleophiles can result in polymerisation, decomposition, formation of open-chain products, elimination, or SET-reactions. These intrinsic limitations can be overcome by the use of 1,3-bis-silyl enol ethers, electroneutral 1,3-dicarbonyl dianion equivalents, in Lewis acid catalyzed reactions.

Kinetics and mechanism of cellobiose hydrolysis and retro-aldol condensation in subcritical and supercritical water

Abstract: Reactions of cellobiose in subcritical and supercritical water were studied. Kinetic study on the cellobiose decomposition clarified that the contribution of hydrolysis to the overall cellobiose decomposition rate decreased and that of retro-aldol condensation greatly increased with decreasing pressure in near-critical and supercritical water. It was found that the rate of retro-aldol condensation was expressed as a first-order reaction rate law and the kinetic parameters of this reaction were estimated. With regards to hydrolysis of cellobiose, it was indicated that the rate of hydrolysis was a second-order reaction (first-order reaction of the water concentration) and its activation energy and preexponential factor were determined. Mechanisms of these reactions were discussed based on the experimental findings. It was suggested that hydrolysis of cellobiose mainly took place by the nucleophilic attack of the oxygen atom of the water molecule or by the attack of a proton ion dissociated from supercritica...

Flavonoid oxidation by the radical generator AIBN: a unified mechanism for quercetin radical scavenging.

Abstract: Four oxidized flavonoid derivatives generated from reacting quercetin (a pentahydroxylated flavone) with the peroxyl radical generator 2,2‘-azobis-isobutyronitrile (AIBN) were isolated by chromatographic methods and identified by NMR and MS analyses. Compounds included 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone (2); 1,3,11a-trihydroxy-9-(3,5,7-trihydroxy-4H-1-benzopyran-4-on-2-yl)-5a-(3,4-dihydroxyphenyl)-5,6,11-hexahydro-5,6,11-trioxanaphthacene-12-one (3); 2-(3,4-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid (4); and methyl 3,4-dihydroxyphenylglyoxylate (5). Product ratios under different hydrogen ion concentrations and external nucleophiles revealed that two of the products, namely the substituted benzofuranone (2) and the depside (4), are generated from a common carbocation intermediate. Indirect evidence for the operation of a cyclic concerted mechanism in the formation of the dimeric product (3) is provided. The identification of these products supports the model that the princip...

Remarkably Stable Tetrahedral Intermediates: Carbinols from Nucleophilic Additions to N–Acylpyrroles

Abstract: Asaresultoftheir transient nature, the utilityof these compounds inorganicchemistryremainslimited. Theisolation of tetrahe-dralintermediatescouldprovideinsightintotheacyl-transferprocess as well as useful compounds for organic synthesis.Whiletheadductsformedbytheadditionoforganometallicreagents to N-methoxy-N-methylamides

Nucleophilic catalysis with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for the esterification of carboxylic acids with dimethyl carbonate.

Abstract: 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for carboxylic acid esterification with dimethyl carbonate (DMC). The reaction pathway of this new class of nucleophilic catalysis has been studied. A plausible, multistep mechanism is proposed, which involves an initial N-acylation of DBU with DMC to form a carbamate intermediate. Subsequent O-alkylation of the carboxylate with this intermediate generates the corresponding methyl ester in excellent yield. In the absence of DBU or in the presence of other bases, such as ammonium hydroxide or N-methylmorpholine, the same reaction affords no desired product. This method is particularly valuable for the synthesis of methyl esters that contain acid-sensitive functionality.

Sandmeyer reactions. Part 7. An investigation into the reduction steps of Sandmeyer hydroxylation and chlorination reactions

Abstract: For Sandmeyer hydroxylation and chlorination in aqueous solution, the reduction steps have been investigated by means of correlation analyses of the effects of diazonium ion substitution on the rates of reduction. For simple hydroxylation, a change of behaviour between diazonium ions substituted by electron donor groups and those substituted by electron acceptor groups is interpreted as a change within an inner-sphere process from rate-determining electron transfer to rate-determining association of the reactants. By contrast, for citrate-promoted hydroxylation, a similar change in behaviour may be interpreted as a change between inner- and outer-sphere electron transfers. For chlorination, there is no mechanistic variation within the range of substituents examined but the pattern of behaviour is consistent with an inner-sphere mechanism. The various patterns of behaviour are rationalised in terms of the effects of diazonium ion substitution and catalyst ligation on the reduction potentials and self-exchange rates of the various reacting redox couples. Comparative correlation analyses of reductions and other electrophilic reactions of diazonium ions are used to support the arguments advanced in respect of Sandmeyer reduction steps. It is suggested that the CuI reductants react via a nucleophilic bridging ligand at the diazonium Nβ to give transient Z-adducts which are the precursor complexes and that activation for electron transfer involves rotation about the N–N bond.

Iridium‐Catalysed Allylic Substitution: Stereochemical Aspects and Isolation of IrIII Complexes Related to the Catalytic Cycle

Abstract: Ir-catalysed allylic alkylations of enantiomerically enriched monosubstituted allylic acetates proceed with up to 87% retention of configuration using P(OPh)3 as ligand. High regio- and enantioselectivity of up to 86% ee in asymmetric allylic alkylations of achiral or racemic substrates is achieved with monodentate phosphorus amidites as ligands. Lithium N-tosylbenzylamide was identified as a suitable nucleophile for allylic aminations. Of particular importance is the use of lithium chloride as an additive, generally leading to increased enantioselectivities. Two (π-allyl)IrIII complexes were characterised by X-ray crystal structure analysis and spectroscopic data.

Selective oxidation of aromatic compounds with dioxygen and peroxides catalyzed by phthalocyanine supported catalysts

Abstract: This article summarizes our research in catalytic oxidation on the design and study of supported metallophthalocyanine catalysts. The catalytic properties of these materials were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (Vitamin K3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (precursor of Vitamin E) and in the epoxidation of olefins. Iron tetrasulfophthalocyanine (FePcS) covalently grafted in the dimeric form yielded catalyst more active and selective that those containing monomeric species but suffered from a lack of stability transforming into less selective monomer complexes during catalysis. The stabilization of supported dimer form by covalent link of two adjacent phthalocyanine molecule through appropriate diamine spacer provided more selective and stable catalysts. Trimethyl-1,4-benzoquinone was obtained with 87% yield at 97% conversion of TMP. More demanding oxidation of 2MN afforded 45% yield of VK3. Particular emphasis is placed on the mechanistic aspects of these oxidations using two mechanistic probes, 2-methyl-1-phenylpropan-2-yl hydroperoxide (MPPH) to distinguish between homolytic versus heterolytic cleavage of O–O bond during the formation of active species and thianthrene 5-oxide (SSO) to evaluate nucleophilic versus electrophilic character of formed active species. To illustrate a versatility of the phthalocyanine-based supported catalysts we prepared a novel phthalocyanine complex with eight triethoxysylil substituents which can be directly anchored to the silica without any modification of the silica support. This new catalyst shows good catalytic activity in epoxidation of olefins by dioxygen in the presence of isobutyraldehyde. The same catalytic system was also active in the oxidation of phenols to biphenols with 86% yields. This catalytic system is complementary to previous one that selectively oxidizes phenols to quinones. An appropriate choice of the reaction conditions allows selective oxidation either to quinones or to biaryl compounds.

Well-defined boron-containing polymeric lewis acids.

Abstract: A general new route to well-defined polymeric Lewis acids via borylation of silylated polymers is reported. Trimethylsilylated polystyrene (PS-Si) of controlled molecular weight and low polydispersity (PDI < 1.15) was obtained via atom transfer radical polymerization (ATRP) of 4-(trimethylsilyl)styrene. The functional polymer PS-Si was quantitatively borylated using BBr3 to give poly(4-dibromoborylstyrene) (PS-B), a novel soluble boron-containing polymeric Lewis acid. PS-B readily reacted with nucleophiles serving as a precursor to a family of new polymers with boron centers of variable Lewis acidity. Reaction of PS-B with Cu(C6F5) gave the highly Lewis acidic polymer poly[4-bis(pentafluorophenyl)borylstyrene], the first polymeric analogue of tris(pentafluorophenyl)borane.