Concerted transition state

The other possible mechanism is the removal of a proton from the transition state concerted with the electron transfer. Overall, the equations can be combined and expressed as a hydrogen atom transfer reaction (equation 41) °. ... 

Ionization of allylic systems, generating stabilized ions, is one of the most successful methods of inducing cyclization. On treating chiral linalool with protic acids, a 6-exo reaction takes place via a chairlike transition state (concerted mechanism) but the configuration is inverted on building chiral limonene (10)14. 

Enzyme catalyzed mechanisms represent fundamentally familiar reactions from organic chemistry (Figure 2.17). Acid-base catalysis is associated with the donation or subtraction of protons. Acid catalysis is a process in which partial proton transfer from an acid lowers the free energy of the reaction transition state, while base catalysis is a process in which partial proton subtraction by a base lowers the free energy of the reaction transition state. Concerted acid-base catalysis, where both processes occur simultaneously, is a common enzymatic mechanism. 

Do we expect this model to be accurate for a dynamics dictated by Tsallis statistics A jump diffusion process that randomly samples the equilibrium canonical Tsallis distribution has been shown to lead to anomalous diffusion and Levy flights in the 5/3 < q < 3 regime. [3] Due to the delocalized nature of the equilibrium distributions, we might find that the microstates of our master equation are not well defined. Even at low temperatures, it may be difficult to identify distinct microstates of the system. The same delocalization can lead to large transition probabilities for states that are not adjacent ill configuration space. This would be a violation of the assumptions of the transition state theory - that once the system crosses the transition state from the reactant microstate it will be deactivated and equilibrated in the product state. Concerted transitions between spatially far-separated states may be common. This would lead to a highly connected master equation where each state is connected to a significant fraction of all other microstates of the system. [9, 10]... 

Sigmatropic rearrangements are normally classified as concerted processes with relatively nonpolar transition states. However, the Fischer cyclization involves rearrangement of a charged intermediate and ring substituents have a significant effect on the rate of the rearrangement. The overall cyclization rate... 

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... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

HOMO of one ethylene mol ecule and the LUMO of an other do not have the proper symmetry to permit two O bonds to be formed in the same transition state for concerted cycloaddition... 

Reactions of this type are rather rare and seem to proceed in a stepwise fashion rather than by way of a concerted mechanism involving a single transition state... 

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

Compound (122) is also obtained by decarboxylative ring-opening of l,2-benzisoxazole-3-carboxylic acid. It has also been concluded that the reaction involves an intermediateless, concerted loss of carbon dioxide via a transition state in which the negative charge is spread over the carboxyl group and the isox azole ring. 

The rate constants and ko were equal to 3 x 10 and lO s respectively [Shian et al. 1980 Bratan and Strohbusch 1980]. There are two equivalent ways of stepwise transfer, and, therefore, the transition state and MEP are two-fold, if the stepwise transfer is energetically preferable. On the other hand, there is only one way of concerted transfer, which lies between the saddle points. Based on this reasoning, de la Vega et al. [1982] have found that the barrier for stepwise transfer (25kcal/mol) is 3.1 kcal/mol lower than that for concerted transfer. These authors have proposed a model two-dimensional PES,... 

The ionization and direct displacement mechanisms can be viewed as the extremes of a mechanistic continuum. At the 8 1 extreme, there is no covalent interaction between the reactant and the nucleophile in the transition state for cleavage of the bond to the leaving group. At the 8 2 extreme, the bond formation to the nucleophile is concerted with the bondbreaking step. In between these two limiting cases lies the borderline area, in which the degree of covalent interaction between the nucleophile and the reactant is intermediate between the two limiting cases. The concept of ion pairs is important in the consideration of... 

For many secondary sulfonates, nucleophilic substitution seems to be best explained by a concerted mechanism with a high degree of carbocation character at the transition state. This has been described as an exploded transition state. Both the breaking and forming bonds are relatively weak so that the carbon has a substantial positive charge. However, the carbocation per se has no lifetime because bond breaking and fonnadon occur concurrently."... 

A stronger bond between the nucleophilic atom and carbon is reflected in a more stable transition state and therefore a reduced activation energy. Since the 8 2 process is concerted, the strength of the partially formed new bond is reflected in the energy of the transition state. 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of... 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Fig. 8.2. Contour plot showing a favOTed concerted mechanism for the first step in acetal hydrolysis, in which proton transfer is more complete in the transition state than C—O bond breaking.

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