Intermediates reactive, formation

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. 

Besides the increased reactivity, formation of species like 6a may also produce a change in the rate-determing step in substitutions of ortho-derivatives when compared with the para-isomers. For example, it has been recently demonstrated that the formation of 1 (L = F R1 = n-C H7, i-C3H7 R2 = H) is rate-limiting in the reaction of n-propylamine and isopropylamine with o-fluoronitrobenzene in toluene, while it is the decomposition of the corresponding zwitterionic intermediate that is rate-determining in the same reactions... 

The hypothesized transformation pathways of CT and CF to methane are shown in Figure 2. The transformation of CT and CF to methane in the Pd/alumina system, despite the low reactivity of MeCl, indicates that the reactions do not involve sequential dehalogenation of CF to methane (i.e. MeCl is not an intermediate). The formation of C2 and C3 compounds during the transformation of both CT and CF indicates the existence of a radical pathway. However, the production of ethane (12-14%) and CF (18-23%) from CT was much lower than that of methane (51-60%). This implies that the main transformation pathway is a direct reaction of CT to methane, with a secondary pathway involving a trichloromethyl radical which then reacts to form CF and C2 and C3 species. Similarly, the relatively low production of ethane (<1%) from CF indicates that the major pathway for the reaction of CF to ethane occurs through direct transformation to methane, rather than through a dichloromethyl radical species. (Lowry and Reinhard 1999)... 

A Gadow, J Vater, W Schlumbohm, Z Palacz, J Salnikow, H Kleinkauf. Gramicidin S synthetase. Stability of reactive thioester intermediates and formation of 3-amino-2-piperidone. Eur J Biochem 132 229-234, 1983. 

The formation of the cation is the rate-determining step. You can look at this in two ways. Either you could argue that a cation is an unstable species and so it will be formed slowly from a stable neutral organic molecule, or you could argue that the cation is a very reactive species and so all its reactions will be fast, regardless of the nucleophile. Both arguments are correct. In a reaction with an unstable intermediate, the formation of that intermediate is usually the rate-determining step. 

Activation of magnesium. Numerous procedures have been used, whereby the surface of magnesium is activated [A, D] in many cases the procedures are empirical — it is not known whether they work merely by cleaning or etching the metal surface, or whether the formation of some intermediate reactive magnesium compound is involved. A selection of better-known procedures follows examples are described or listed in Section 3.1.1. 

For the evaluation of AOPs in an aqueous phase, it is essential to know the absorption properties of the auxiliary oxidants and of the most important intermediate reactive species. These data are collected in Tab. 6-2. Carbon-centered peroxyl radicals play an important role in AOPs, since free carbon radicals RCH, which are formed for example via hydrogen abstraction by hydroxyl radicals react rapidly with dissolved molecular oxygen with formation of the corresponding peroxyl radicals RCH2O. Two examples of transient absorption characteristics are included in... 

Increase of pH in the range 0-4 decreases the rate. The pH dependence can be accounted for quantitatively by assuming different reactivities for HsIOg and a periodate monoanion. Kaiser and Weidman later studied the oxidation of catechol using a stopped-flow apparatus, and showed that the reaction proceeded via an intermediate. The formation of the latter was second order, and at pH 1.0 the second-order rate coefficient is expressed by (from measurements at 15° and 25 °C)... 

Calvo and Balbuena examined the structure and reactivity of Pd-Pt nanoclusters with 10 atoms in the oxygen reduction reaction. In contrast with what is expected in a periodic slab calculation, they found that mixed states with randomly distributed Pd atoms in a Pt7Pd3 cluster was more stable than an ordered cluster structure due to more eflective charge transfer in the mixed state. They found that increasing the concentration of Pd in the surface favors formation of the OOH species in the first step of the reaction, but Pt atoms were needed to promote the second stage of the oxygen reduction reaction. They report that due to charge transfer eflhcts the Pd atoms have an intermediate reactivity between pure Pd and Pt, and in the mixed cluster the Pd atoms the Pd atoms act more similarly to Pt than in the ordered cluster. 

Tlie cross-coupling of a terminal alkyne 9 with a 1 -bromoalkync 8 in the presence of an aliphatic amine and a catalytic amount of a Cu(I) salt affords unsymmetrically substituted diynes [10, Eq.(5)]. This useful reaction, discovered by Cadiot and Chodkiewicz [8], can be employed advantageously for the synthesis of several polyunsaturated systems. Generally the bromoalkyne is introduced dropwise to a mixture of the alkyne, ethylamine, and MeOH or EtOH in the presence of a catalytic amount of CuCl, and a small amount of NH OH-HCl. The reducing agent, NHjOH-HCl, is used to reduce the copper(TI) ion. The alkynylcopper(I) is assumed to be the reactive intermediate. The formation of the symmetrical diyne can be suppressed by maintaining the concentration of the bromoalkyne. This side reaction is particularly significant in the case of less acidic alkynes such as alkylalkynes [9J. 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

Glutathione contains a nucleophilic -SB group which In many cases detoxifies electrophilic reactive Intermediates through formation of stable glutathione conjugates, as In the case of paracetamol. 

Rh U) and Ir(II) Species. Quite a large number of mononuclear (f Rh(II) and Ir(II) species have been isolated. In fact, formation of these species by either oxidation of the M(I) precursors (Table XV) or reduction of the M(III) precursors (Table XVI) occurs at remarkably accessible redox potentials. This suggests that such species could well play an important role in catalytic reactions mediated by these metals. Regarding their high reactivity, formation of Rh(II) and Ir(II) species will easily lead to catalyst deactivation. The paramagnetic (f M(II) species could even be active intermediates, although proven examples of catalytically active M(ll) species are (stUl) quite limited. 

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