Reaction mechanism intermediate formation

These enzymes, like 4CL, share a common reaction mechanism for formation of adenylate intermediates, with the catalytic processes of adenylation half-reaction and thioesterification half-reaction occurring via a ping-pong mechanism. 

In the depurination of RNA catalyzed by ricin toxin A-chain (RTA), the reaction mechanism involved formation of equihbrium population of an oxocarbenium ion intermediate followed by an isotopicaUy insensitive irreversible step (Fig. 7)." The experimental KlEs were most consistent with the calculated ElEs for oxocarbenium ion formation. This is what would be expected for a mechanism involving freely reversible oxocarbenium ion formation followed by an isotopicaUy insensitive step. 

In contrast to monohydric phenols, also non-alkylated pyiocatechol or hydro-quinone and their monomethyl derivatives are antioxidation effective. During the oxidation in water-alcoholic alkaline medium, 2,5-dihydroxy-l,4-benzoquinone CLII189 190,193 and 2-hydroxy-5-methyl-1,4-benzoquinone19 are formed from pyrocatechol and 4-methylpyrocatechol, respectively. The oxidation of 2-methyl-hydroquinone is more complex and more products are formed. Besides ion radicals CXXXVII and CXLI, also the ion radical CLIII was identified198 in the study of reaction mechanism. Intermediate CLIII corresponds to the formation of dimeric hydroxybenzoquinone CLIV. 

Reaction mechanism of formation of acyl-CoA intermediates by the acyl-CoA ligases... 

The proposed reaction mechanism involves formation of mesoionic l,3-oxazolium-5-olate 22 through the cyclodehydration of 18 by TFAA. The intermediate 22 undergoes trifluoroacetylation followed by decarboxylation to give enol trifluoroacetate (25). Cyclization of 25 leads to oxazolinium salt (26). If the group of 25 is easily removable, the reaction could proceed efficiently. This is the reason why A-benzyl derivatives are the best substrates for this transformation. 

The mechanism of the reaction involves the intermediate formation of an amine cyanate (see previous Section) in aqueous solution urea behaves as an equifi-brium mixture with ammonium cyanate ... 

The mechanism of the reaction probably involves the intermediate formation ofthe covalent dlazo-hydroxide from the diazonium salt the former decomposes... 

This is an example of the Doebner synthesis of quinoline-4-carboxylic acids (cinchoninic acids) the reaction consists in the condensation of an aromatic amine with pyruvic acid and an aldehj de. The mechanism is probably similar to that given for the Doebner-Miller sj nthesis of quinaldiiie (Section V,2), involving the intermediate formation of a dihydroquinoline derivative, which is subsequently dehydrogenated by the Schiff s base derived from the aromatic amine and aldehyde. 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

The main intermediates in the pentaerythritol production reaction have been identified and synthesized (50,51) and the intermediate reaction mechanisms deduced. Without adequate reaction control, by-product formation can easily occur (52,53). Generally mild reaction conditions are favored for optimum results (1,54). However, formation of by-products caimot be entirely eliminated, particularly dipentaerytbritol and the linear formal of pentaerythritol, 2,2 -[meth5lenebis(oxymethylene)]bis(2-hydroxymethyl-1,3-propanediol) [6228-26-8] ... 

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM). 

Ring closure of o-benzoylbenzoic acid to anthraquinone is an unusual reaction in that normally it is not predicted to occur ortho to a keto group. Several theories have been proposed to explain the mechanism whereby this could possibly occur. One involves a complex ionization of o-benzoylbenzoic acid (41), the other favors the intermediate formation of 3-hydroxy-3-phenyl-l(3JT)isobenzofuranone (9) [64693-03-4] and 3-phenylphthaHdyl sulfate (10) (42) ... 

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

The transport of each COg requires the expenditure of two high-energy phosphate bonds. The energy of these bonds is expended in the phosphorylation of pyruvate to PEP (phosphoenolpyruvate) by the plant enzyme pyruvate-Pj dikinase the products are PEP, AMP, and pyrophosphate (PPi). This represents a unique phosphotransferase reaction in that both the /3- and y-phosphates of a single ATP are used to phosphorylate the two substrates, pyruvate and Pj. The reaction mechanism involves an enzyme phosphohistidine intermediate. The y-phosphate of ATP is transferred to Pj, whereas formation of E-His-P occurs by addition of the /3-phosphate from ATP ... 

H2O2 is found to result in formation of 2-ethylindole (75, 90%). Similarly, other trialkylboranes are successfully employed for the synthesis of 2-alkylindoles. A reaction mechanism through 74 as an intermediate is proposed. 

Two reaction mechanisms, such as SN1 and SN2 mechanisms, seem to be possible for explaining formations of 158a-c (Scheme 25). The former requires a resonance-stabilized indolyl cation 165 as an intermediate, while the latter indicates the presence of a transition state like 167. The introduction of a methoxy group into the 5 position of 165 should stabilize the corresponding cation 166, in which nucleophilic substitution on indole nitrogen would become a predominant pathway. 

Highly electron-deficient 1,3,6,8-tetranitronaphthalene 443 was reported to react in ethanol with A -methyl phenylacetamidines, e.g. 444, to give the corresponding benzoquinoline derivatives, e.g. 446 (77JOC435). Though the reaction mechanism of this double nitro group displacement is not known, formation of intermediate 445 and its following cyclization is probably the most reasonable explanation (Scheme 70). 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

The mechanism of the reaction involves initial formation of a tri-organocopper intermediate, followed by coupling and loss of RCu. The coupling is not a typical polar nucleophilic substitution reaction of the sort considered in the next chapter. 

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