Nucleophile activated

Thiazolium derivatives unsubstituted at the 2-position (35) are potentially interesting precursors of A-4-thiazoline-2-thiones and A-4-thiazoline-2-ones. Compound 35 in basic medium undergoes proton abstraction leading to the very active nucleophilic species 36a and 36b (Scheme 16) (43-46). Special interest has been focused upon the reactivity of 36a and 36b because they are considered as the reactive species of the thiamine action in some biochemical reaction, and as catalysts for several condensation reactions (47-50). 

Xenon difluoride [4, 5, 7, 8,10] is a white crystalline material obtained through the combination of fluorine and xenon m the presence of light The reagent is commercially available and possesses a relatively long shelf-life when stored cold (freezer) Xenon difluoride is very effective for small-scale fluormation of alkenes and activated nucleophilic substrates. The reactions are usually conducted between 0 °C and room temperature in chloroform or methylene chloride solutions Hydrogen fluoride catalysis is sometimes helpful Xenon difluoride reacts in a manner that usually involves some complexation between the substrate and reagent followed by the formation of radical and radical cation intermediates... 

The Ullman reaction has long been known as a method for the synthesis of aromatic ethers by the reaction of a phenol with an aromatic halide in the presence of a copper compound as a catalyst. It is a variation on the nucleophilic substitution reaction since a phenolic salt reacts with the halide. Nonactivated aromatic halides can be used in the synthesis of poly(arylene edier)s, dius providing a way of obtaining structures not available by the conventional nucleophilic route. The ease of halogen displacement was found to be the reverse of that observed for activated nucleophilic substitution reaction, that is, I > Br > Cl F. The polymerizations are conducted in benzophenone with a cuprous chloride-pyridine complex as a catalyst. Bromine compounds are the favored reactants.53,124 127 Poly(arylene ether)s have been prepared by Ullman coupling of bisphenols and... 

Increasing the pH accelerated the reactions, which indicates that the thiolate RS is the active nucleophile. 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

The stereoselectivity can be enhanced by addition of Ti(0-i-Pr)4. The active nucleophile under these conditions is expected to be an ate complex in which a much larger Ti(0- -Pr)4 group replaces Li+ as the Lewis acid.313 Under these conditions, the syn anti ratio is dependent on the stereochemistry of the enolate. 

O-Si bond can also be cleaved in the presence of Si-active nucleophiles. Hence, eliminations play the major role in reactions of SENAs. 

Cationic intermediates A can react with active nucleophiles Y- to give coupling products (399) in high yields, with the trans configuration of the nucleophile and the substituent R predominating. (As in the case of the [3+ 2]-cycloaddition reactions with six-membered cyclic nitronates (162), this stereoselectivity can be attributed to the favorable approach of the nucleophile to the plane of the cation which is distal with respect to the C-6 atom. It should be noted that this... 

Electrochemical formation of allylnickel species and their addition to aldehydes were reported. a Allylnickel(i) species generated via one-electron reduction of 3-allylnickel(ii) intermediates are considered as active nucleophilic species. 

The hydroxide ion is not the most active nucleophile with which the dye-fibre bonds in reactive dyeings have to contend. Many commercial detergent formulations contain sodium... 

Although acidic pulping methods have largely been displaced over the past 50 years by neutral and alkaline processes, there is still a significant amount carried out. Acid sulfite pulping uses combinations of sulfur dioxide and water at high temperatures and pressures. An appropriate base is used to control the pH and, although usually acidic, it is possible to perform these reactions at neutral or even alkaline pH. The most active nucleophile present is the bisulfite ion,... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

In its simplest form, the phase-transfer catalysed nucleophilic substitution reaction, RX + Y -4 RY + X", in which the active nucleophile Y" is transferred from the aqueous into the organic phase, can be depicted by Scheme 1.5. The mechanism requires the extraction of the nucleophilic anion by the quaternary ammonium cation Q+ as the ion-pair [Q+Y ] into the organic phase, where the nucleophilic reaction can take place. Subsequent to the reaction the spent catalyst forms an ion pair with the released anion X- and equilibration of [Q+X-] between the two phases establishes a... 

Our group has very recently reported that, when temperatures are 130-140°C, the reaction of Scheme 4.21 can proceed efficiently (yields up to 83% in methyl carbamates), without any catalyst in the presence of SCCO2 at 90 bar." ° SCCO2 plays a double role in this reaction (1) it acts as a catalyst in the formation of the active nucleophiles (Scheme 4.21), and (2) at a pressure over 90 bar, it inhibits the formation of A-methylated carbamates [RN(Me)C02Me], which are possible by-products. 

Dicarbonyl compounds are widely used in organic synthesis as activated nucleophiles. Because of the relatively high acidity of the methylenic C—H of 1,3-dicarbonyl compounds, most reactions involving 1,3-dicarbonyl compounds are considered to be nucleophilic additions or substitutions of enolates. However, some experimental evidence showed that 1,3-dicarbonyl compounds could react via C—H activations. Although this concept is still controversial, it opens a novel idea to consider the reactions of activated C H bonds. The chiral bifunctional Ru catalysts were used in enantioselective C C bonds formation by Michael addition of 1,3-dicarbonyl compounds with high yields and enantiomeric excesses. ... 

In addition to metal catalysts, organocatalysts could also be used in asymmetric cyanation reactions. Chiral Lewis bases, modified cinchona alkaloids, catalyzed asymmetric cyanation of ketones by using ethyl cyanoformate as the cyanide source (Scheme 5.34)." Similar to metal-catalyzed reactions, ethyl cyanoformate was first activated by chiral Lewis bases to form active nucleophiles. Various acyclic and cyclic dialkyl ketones were transformed into the desired products. Because of using... 

This chemistry was extended to a catalytic enantioselective alkenylation and phenylation of aldehydes and a-ketoesters. Using CuF-DTBM-SEGPHOS complex, products were obtained with excellent enantioselectivity from a wide range of aldehydes including aromatic and aliphatic aldehdyes, [Eq. (13.26)]. Previously catalytic enantioselective vinylation and phenylation are restricted using the corresponding zinc reagents. The active nucleophile is proposed to be an alkenyl or phenyl copper, based on NMR studies. The chiral CuF catalyst can also be applied to a catalytic enantioselective aldol reaction to ketones... 

The tetrahedral intermediate is generated by a nucleophilic attack on the carbonyl carbon atom of the activated nucleophile, which in the case of chymotrypsin and trypsin is the catalyhc Serl95 (Table 4.3). It is important to note that a small structural rearrangement occurs when the planar carbonyl moiety is converted into the tetrahedral intermediate (Scheme 4.4). 

The catalysis of CO2 hydration by carbonic anhydrase II occurs via the two chemically independent steps outlined in Scheme 2 a general mechanistic profile is found in Fig. 23. The first step involves the association of substrate with enzyme and the chemical conversion of substrate into product. The second step is product dissociation and the regeneration of the catalytically active nucleophile zinc hydroxide (Coleman, 1967). Below, we address the structural aspects of zinc coordination in each of these steps. 

The voltammetric reduction of a series of dialkyl and arylalkyl disulfides has recently been studied in detail, in DMF/0.1 M TBAP at the glassy carbon electrode The ET kinetics was analyzed after addition of 1 equivalent of acetic acid to avoid father-son reactions, such as self-protonation or nucleophilic attack on the starting disulfide by the most reactive RS anion. Father-son reactions have the consequence of lowering the electron consumption from the expected two-electron stoichiometry. Addition of a suitable acid results in the protonation of active nucleophiles or bases. The peak potentials for the irreversible voltammetric reduction of disulfides are strongly dependent on the nature of the groups bonded to the sulfur atoms. Table 11 summarizes some relevant electrochemical data. These results indicate that the initial ET controls the electrode kinetics. In addition, the decrease of the normalized peak current and the corresponding increase of the peak width when v increases, point to a potential dependence of a, as discussed thoroughly in Section 2. 

A comparison of NA hydrolysis by 8 and 9 shows for the first time that Znu-bound alkoxides may lead to better nucleophiles than Zn11-bound hydroxides in an aqueous environment. Unless the alkoxide anion is bound to zinc(II) ion, it may rather work as a general base to yield hydroxide, which then yields the active nucleophile. Hence, zinc(II) ion may be viewed as an alkoxide-protecting agent. It is of... 

See R. Bruckner, Advanced Organic Chemistry, Harcourt/Academic Press, San Diego, 2002, pp 419-422, for an interesting discussion of the active nucleophile in this reaction. 

The substitution of aldehydes as electrophiles into activated nucleophilic positions of aromatic rings, such as the ortho or para positions in a phenol, is well known from the commercial importance of phenol-formal-... 

Reaction of coordinated CO with less active nucleophiles can take place when the carbonyl ligand is sufficiently activated. The manner in which this activation occurs is by a reduction in the backbonding interaction (60) that may be achieved when the CO-bound metal ion is in a higher oxidation state. 

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