Complexation between reagents formation

Mercurous fluoride [13967-25 ] Hg2p2, is less effective than Hgp2. The addition of chlorine or iodine to the reagent increases its reactivity owing to the formation of a complex between Hgp2 and HgX2 (4,12). 

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 equatorial selectivity observed with organolithium reagents is enhanced in diethyl ether as the reaction solvent by the addition of lithium perchlorate (Table l)12. I3C-NMR studies47 indicate that the formation of a complex between lithium perchlorate and the carbonyl group, which also leads to a dramatic enhancement of the rate of the addition reaction, accounts for the increased diastereoselectivity. 

The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

A standard Lowry-based protein assay has been adjusted to the special conditions encountered with skin [126], Basically, proteins reduce an alkaline solution of Cu(II)-tartrate to Cu(I) in a concentration-dependent manner. Then, the formation of a blue complex between Folin-Ciocalteau reagent (a solution of complex polymeric ions formed from phosphomolybdic and phosphotungstic heteropoly acids) and Cu(I) can be measured spectrophotometrically at 750 nm. A calibration curve can be obtained by dissolving known amounts of stratum corneum in 1 M sodium hydroxide. A piece of tape that has not been in contact with skin is subjected to an identical procedure and serves as negative control. The method was recently adapted to a 96-well plate format, notably reducing analysis times [129],... 

Case 2 The rate-determining step of the extraction reaction is the interfacial formation of the complex between the metal ion and the interfacially adsorbed extracting reagent. Here, the rate-determining step of the extraction reaction can be written as... 

Case 3 There are two interfacial rate-determining steps, consisting of 1) formation of an interfacial complex between the interfacially adsorbed molecules of the extractant and the metal ion and (2) transfer of the interfacial complex from the interface to the bulk organic phase and simultaneous replacement of the interfacial vacancy with bulk organic molecules of the extractant. For this mechanism, we distinguish two possibilities. The first (case 3.1) describes the reaction with the dissociated anion of the extracting reagent, B"(ad). The second (case 3.2) describes the reation with the undissociated extractant, BH(ad). 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

Little is known of the details of these processes. The most extensive investigations have been done with NaBH4. Studiesof the kinetics31,32,48,95 show that the reaction is first order in ketone and in hydride, that transfer of the first hydride is the slowest step, and that the alkoxyborohydrides formed in in the first step react very quickly.69 The kinetics are compatible with direct reaction between ketone and hydride or with reversible formation of a complex between the reactants folllowed by a slow hydride transfer. Reduction rates do not depend on pH,90,95 provided the solution is sufficiently alkaline for the reagent to be stable. 

Corey has made a study of the relative reactivities of various olefins to attack by excited cyclohexenone and has concluded that the excited state is an electrophilic reagent.420 This fact has led him to postulate formation of a charge-transfer complex between excited... 

There is a further aspect of polar additions to alkenes that we should consider, namely, that electrophilic reagents form loose complexes with the 77 electrons of the double bonds of alkenes prior to reaction by addition. Complexes of this type are called charge-transfer complexes (or ir complexes). Formation of a complex between iodine and cyclohexene is demonstrated by the fact that iodine dissolves in cyclohexene to give a brown solution, whereas its solutions in cyclohexane are violet. The brown solution of iodine in cyclohexene slowly fades as addition occurs to give colorless trims-1,2-diiodocyclohexane. 

An example of transfer of electron from nucleophile to substrate is seen in the formation of the radical anions (observable by ESR) of 5-halo-2A/,3W-benzotf>]thiophene-2,3-diones on treatment with nucleophiles.20 It has been proposed in some cases, that this single-electron transfer step takes place through a charge transfer complex between the nucleophile and the aromatic substrate.21-22 Some reactions occur spontaneously, i.e. without any catalysts or reagents other than the substrate and the nucleophile, but the initiation process is usually, although not invariably, photostimulated (near-ultraviolet radiation, 300-... 

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