Concerted mechanism/reaction relationships

These treatments have been also applied to S/yAr. For example, for a neutral nucleophile, all the classical pathways identified at present are represented by the general reaction mechanism shown by Scheme 2. A concerted mechanism, indicated by the diagonal path in Scheme 2, had not been discussed until lately, but was observed, among other systems, in the hydrolysis of l-chloro-2,4,6-trinitrobenzene and 1-picrylimidazole. The study was then extended to other related substrates and structure-reactivity relationships could be obtained78. 

Homoallylic type alcohols (67), on the other hand, give predominantly cyclopentenones independent of substitution (equation 37). In Ae 3-hydroxyalkyl-substituted systems, presumably allene oxide (68) is the intermediate. Thus it would appear Aat the initial site of allene oxidation is not critical to the success of the reaction. Either precursor (58) or (59) is expected to give the observed stereochemical relationships of the newly formed stereocenters by the concerted mechanism. Finally, Cha has noted that the two intermediates may lead to different stereochemical relationships by the zwitterionic mechanism. This assumes a specific pathway for breakdown of (58) or (59). That stereochemical information is preserved in the reaction is shown by the selective transformations in equation (38). 

In the following discussion, we appraise tlie next two production mechanisms of tire compounds liaving the structural isomers on tire basis of the yield relationships produced by the shock experiments. (1) The shock reaction is a nidical addition reaction, and (2) it is a concerted cycloaddition reaction controlled by the Woodward-Hoffinaim rules [146,147]. However, only toluene is assumed to be formed by the radical reaction. 

Linear free-energy relationships (LFER) with monoanionic phosphorylated pyr-idines indicate a loose transition state in which metaphosphate is not an intermediate.16 The hydrolysis of the monoanion of 2,4-dinitrophenyl phosphate is thought to be concerted,39 but the possibility of a metaphosphate intermediate has not been ruled out with esters having less activated leaving groups. A stereochemical study of the hydrolysis of phenyl phosphate monoanion indicates that the reaction proceeds with inversion.21 This result implies either a concerted mechanism, or a discrete metaphosphate intermediate in a pre-associative mechanism. 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

The anti-coplanar relationship is not possible with the (E) isomer, however, so an Elcb mechanism involving formation of a vinyl carbanion and subsequent elimination of the bromide ion was proposed (equation 10.29). The ratio of the rate constant for reaction of (Z)-p-nitro-j8-bromostyrene to that of the (E) isomer was an order of magnitude smaller than the ratio of rate constants for (Z)- and (E)-)8-bromostilbenes. This difference was attributed to the stabilization of the carbanion intermediate by the p-nitro group. With a less acidic H proton, the carbanion mechanism in equation 10.29 is slower, but the concerted mechanism in equation 10.28 is not as significantly affected. Therefore, the ratio of rate constants is much greater for elimination of HCl from isomeric chloroalkenes. 

The arrow formalism lets us map out the formation of the ozonide. The 7t bond of the alkene reacts with ozone to produce the five-membered ring. These primary ozonides are extremely difficult to handle, and it took great experimental skill on the part of Rudolf Criegee (1902-1975) and his co-workers at the University of Karlsruhe in Germany to isolate the ozonide produced in the reaction between ozone and frarar-di-fcrf-butylethylene. A concerted mechanism predicts that the stereochemical relationship of the alkyl groups in the original alkene will be preserved in the ozonide, and this is what happens (Fig. 10.51). 

A firm understanding of concerted cycloaddition reactions developed as a result of the formulation of the mechanism within the framework of molecular orbital theory. Consideration of the molecular orbitals of reactants and products revealed that in some cases a smooth transformation of the orbitals of the reactants to those of products is possible. In other cases, reactions that appear feasible if no consideration is given to the symmetry and spatial orientation of the orbitals are found to require high-energy transition states when the orbitals are considered in detail. (Review Section 11.3 of Part A for a discussion of the orbital symmetry analysis of cycloaddition reactions.) These considerations permit description of various types of cycloaddition reactions as allowed or forbidden and permit conclusions as to whether specific reactions are likely to be energetically feasible. In this chapter, the synthetic applications of cycloaddition reactions will be emphasized. The same orbital symmetry relationships that are informative as to the feasibility of a reaction are often predictive of the regiochemistry and stereochemistry of the process. This predictability is an important feature for synthetic purposes. Another attractive feature of cycloaddition reactions is the fact that two new bonds are formed in a single reaction. This can enhance the efficiency of a synthetic process. 

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. 

H, Cl, Br, NO2, Me, MeO) by bromamine-B, catalysed in the presence of HCl in 30% aqueous methanol by RuCls have been smdied and a biphasic Hammett a-relationship derived. A kinetic study of the ruthenium(in)-catalysed oxidation of aliphatic primary amines by sodium A-bromo-j -toluenesulfonamide (bromamine-T, BAT) in hydrochloric acid medium has been undertaken and the mechanism of the reaction discussed. A concerted hydrogen-atom transfer one-electron transfer mechanism is proposed for the ruthenium(in)-catalysed oxidation of 2-methylpentane-2,4-diol by alkaline hexacyanoferrate(III). The kinetics of the oxidation of propane-... 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

A. Williams, Concerted Organic and Bioorganic Mechanisms, CRC Press, New York, 2000. W. P. Jencks, How Does a Reaction Choose Its Mechanism , Chem. Soc. Rev. 1981,10, 345. J. P. Richard, Simple Relationships between Carbocation Lifetime and the Mechanism for Nucleophilic Substitution at Saturated Carbon, Adv. Carbocation Chem. 1989, 1, 122. T. W. Bentley and G. Llewellyn, Scales of Solvent Ionizing Power, Prog. Phys. Org. Chem. 1990, 17, 121. 

The cyclic mechanism is probably seldom a fully concerted (E2) process, and the different timing of individual electron shifts results in a transition towards the El or ElcB mechanisms (cf. Sect. 2.1.1). The choice of the mechanism depends on the reactant structure as well as on the catalyst nature. As an indicator of the mechanism, either the degree of stereoselectivity (see refs. 68, 121, 132 and 141) or the value of the reaction parameter of a linear free energy relationship, e.g. p or p constants of the Hammett and Taft equations (cf. ref. 55), may be used. 

Frequently enzymes act in concert with small molecules, coenzymes or cofactors, which are essential to the function of the amino acid side chains of the enzyme. Coenzymes or cofactors are distinguished from substrates by the fact that they function as catalysts. They are also distinguishable from inhibitors or activators in that they participate directly in the catalyzed reaction. Chapter 10, Vitamins and Coenzymes, starts with a description of the relationship of water-soluble vitamins to their coenzymes. Next, the functions and mechanisms of action of coenzymes are explained. In the concluding sections of this chapter, the roles of metal cofactors and lipid-soluble vitamins in enzymatic catalysis are briefly discussed. 

Methanolysis of die sulfonates (175)141 and the reaction of the sulfonate ester (102) with hydroxylamine (103)88 were looked at earlier. Yoh and co-workers have looked at die reactions of (Z)-phenylediyl (X)-benzenesulfonates with (Y)-pyridines in acetonidile under pressure and die structure-reactivity relationships established show that as die pressure is increased die mechanism moves from a dissociative 5n2 to early-type concerted 5k2.276 In otiier sdtdies also under pressure the same group found that a mechanistic change from associative, k 2 to late-type 5n2 occurs as the pressure is increased in die reaction of (Z)-phenacyl (X)-benzenesvdfonates with (Y)-pyridines in acetone.277... 

Kinetic data indicate that the hydrolysis of 5-2,4-dinitrophenyl 4,-hydroxythioben-zoate (61) in mild alkaline solutions (pH 8-11) most likely follows a dissociative, ElcB pathway, through a p-oxoketene intermediate, whereas at higher pH values an associative mechanism carries the reaction flux (Scheme 18). LFER relationships obtained from a kinetic study on the alkaline hydrolyses of substituted 5-aryl 4 -hydroxythiobenzoates seem to suggest that the associative pathway is a concerted, one-step process, rather than the classical mechanism via a tetrahedral intermediate.51... 

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