Concerted versus Stepwise Mechanism

Reaction Path Calculations and Enei Barriers for Proton Transfer 

The excited-state energies are given in kcalmol . (b) Structure and energy of the 1 2 solute-lHjOjj complexes calculated for the stationary points along the triple proton transfer path. 

It can therefore be noticed [68] that the complete tautomerization in this type of a solute-solvent complex may be blocked, since large solvent reorganization of solute-solvent hydrogen bonds is often accompanied by radiationless deactivation of an excited fluorophore. 

---

One of the best tests for DFT is probably the Diels-Alder reaction (see Figure 3) of butadiene and ethylene. It has only been recently that the controversy over whether the reaction undergoes a concerted or stepwise mechanism has led to consensus, The reaction mechanism of butadiene and ethylene is concerted, i,e, the reaction involves a transition state with two equal C-C distances of approach between the two reactants. However, the activation barrier for the stepwise mechanism is only a few kcal mol higher, Goldstein et al, have shown that the B3-LPY method is capable of direct comparison of concerted versus stepwise mechanisms. Table 18 illustrates the DFT and ab initio predictions of the geometrical parameters d, R, and Ri, as shown in Figure 3, and the activation energy for the transition state involved in... 

Figure 3.19 Resolution of the concerted versus stepwise mechanism of excited-state double proton transfer in doubly hydrogen-bonded dimer of 7-azaindole [90, 91]. Schematic presentations of (a) the concerted, one-step, synchronous double proton transfer in 7-azaindole dimer and (b) the stepwise proton transfer following formation...

When a reaction involves multiple bonding changes, a question may arise whether the bonding changes occur by a stepwise or concerted pathway. An answer to such a question based on the classical reaction theory is that the reaction proceeds by a concerted pathway, by a stepwise pathway, or by a mixture of the two separate pathways. However, if one takes into account dynamic effects, the answer to the question of concerted versus stepwise may be much more complex. It is interesting to point out here that the case reported by Singleton for the ene reaction affords a case, where stepwise mechanism can dynamically operate on a concerted PES. This contrasts with the reactions described in section Nonstatistical Product Distribution , in which the... 

A kinetic study of the nucleophilic substitution reaction of substituted phenyl diphenylphosphinates (530a) and diphenylphosphinothioates (530b) with alkali-metal ethoxides (MOEt M = Li, Na, K) in anhydrous ethanol has been reported (Scheme 178). Experimental studies revealed the wide-ranging and important effects of structural change P=0 to P=S bond on reactivity, mechanisms (concerted versus stepwise), and on... 

As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

FUNCTIONALIZATION OF SEMICONDUCTOR SURFACES BY ORGANIC LAYERS CONCERTED CYCLOADDITION VERSUS STEPWISE FREE-RADICAL REACTION MECHANISMS... 

Functionalization of Semiconductor Surfaces by Organic Layers Concerted Cycloaddition versus Stepwise Free-Radical Reaction Mechanisms 333... 

Scheme 5. The concerted oxenoid versus the stepwise oxygen-rebound mechanism for the C-H oxidation by dioxi ranes.

For a discussion of a diradical stepwise versus concerted mechanism for reactions with chalcogens, see Orlova, G. Goddard, J.D. J. Org. Chem. 2001, 66, 4026. 

A.H.M. Renfrew et al.. Stepwise versus Concerted Mechanisms at Trigonal Carbon Transfer of the 1,3,5-Triazinyl Group Between Aryl Oxide Ions in Aqueous Solution, J. Am. Chem. Soc., 1995,117, 5484. 

---