Concerted reaction mechanism, step

In the case of addition of fluorine to double bonds (Table II), note that the corresponding initiation step (lb) is probably exothermic by 5-36 kcal and thus plays an important role. A second important possibility is the concerted reaction mechanism 2, which is exothermic by 50-68.4 kcal per carbon atom. 

Two pathways are observed for nucleophile addition to 48 in water (Scheme 49) (i) uncatalyzed nucleophile addition to form the oxygen anion 48 that undergoes rapid protonation (ii) specific acid-catalyzed nucleophile addition. The SDIE on the specific acid-catalyzed addition of solvent and bromide anion to 48 are kH/kD = 0.68 and 1.0, respectively, for reactions in 50/50 (v/v) water trifluoroethanol,67 but a smaller SDIE of kH/kD = 0.41 is observed for the specific acid-catalyzed addition of an aqueous solvent to l.52 The larger SDIE for acid-catalyzed addition of Br to 48 is consistent with a concerted reaction mechanism, in which protonation of oxygen and carbon-bromine bond formation occur in a single step with a rate constant kHBr (Scheme 49). 

Each equation m Figure 4 6 represents a single elementary step An elementary step IS one that involves only one transition state A particular reaction might proceed by way of a single elementary step m which case it is described as a concerted reaction, or by a series of elementary steps as m Figure 4 6 To be valid a proposed mechanism must meet a number of criteria one of which is that the sum of the equations for the elementary steps must correspond to the equation for the overall reaction Before we examine each step m detail you should verify that the mechanism m Figure 4 6 satisfies this requirement... 

The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

The reaction mechanisms of organic electrode reactions are thus composed of at least one ET step at the electrode as well as preceding and follow-up bond-breaking, bond-forming, or structural rearrangement steps. These chemical steps may be concerted with the electron transfer [15, 16]. The instrumental techniques described in this chapter allow the investigation of the course of the reaction accompanying the overall electrolysis. 

Contrary to the normal Michael reaction of the chloro ester 1-Me with (di-phenylmethylene)amine (DPMA-H) in methanol (Scheme 29), reaction of DMPA-H with 1-Me in methylene chloride or tetrachloromethane containing sodium hydride proceeded much more slowly and gave the isoquinoline 126 in 26 and 39% yield, respectively (Scheme 42) [60 a]. The structure of 126 was ascertained by X-ray crystallography [60a]. Taking into consideration the stability of the Michael adducts 94 in refluxing CCI4, the reaction mechanism with intermediacy of a formal [4-1-2] cycloadduct 125 which may be formed in either a concerted or, more probably, a two-step domino Michael reaction, has been postulated [60a]. 

Second-order kinetics for oxidation of adamantane and other hydrocarbons by stoich. RuO /aq. acetone or /aq. CCyCHjCN were demonstrated there is a large deuterium isotope effect, a small enthalpy and a large negative entropy of activation. A two-step mechanism was proposed a pre-equilibrium reaction in which (for the alkene RH), RH + RuO gives RH.RuO which, in a rate-determining concerted reaction, yields R-0-Ru02(0H) which is then aquated to form R(OH) [388] (Fig. 1.10) [389, 390],... 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

The reaction is similar to the Diels-Alder (in action, not in scope), and if dienes are involved, the latter reaction may compete, though most olefins react with a diene either entirely by 1,2 or entirely by 1,4 addition. Three mechanisms can be proposed938 analogous to those proposed for the Diels-Alder reaction. Mechanism a is a concerted pericyclic process, and mechanisms b and c are two-step reactions involving, respectively, a diradical (99) and a diion (100) intermediate. As in 5-47, a diradical intermediate must be a singlet. 

It was originally reported that irradiation of 2-methylcyclohexa-none gave solely franj-5-heptenal,321 in accord with the then-postulated concerted singlet mechanism. Now, however, it has been reported that 2,6-dimethylcyclohexanone yields both cis- and /rstep process.328 The reaction seems to resemble type I cleavage very closely, with the difference that disproportionation in the biradical primary product is much faster than decarbonylation, except in high vibrational levels of the excited singlet state, or when both ends of the biradical so produced are resonance stabilized.329... 

If the concerted four-center mechanism for formation of chloromethane and hydrogen chloride from chlorine and methane is discarded, all the remaining possibilities are stepwise reaction mechanisms. A slow stepwise reaction is dynamically analogous to the flow of sand through a succession of funnels with different stem diameters. The funnel with the smallest stem will be the most important bottleneck and, if its stem diameter is much smaller than the others, it alone will determine the flow rate. Generally, a multistep chemical reaction will have a slow rate-determining step (analogous to the funnel with the small stem) and other relatively fast steps, which may occur either before or after the slow step. 

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