Reaction selectivity, transition state

The Lewis acid-catalyzed reaction of nitrone 21 with ethyl vinyl ether 22 (Scheme 8.8) was also investigated for BH3 and AlMe3 coordinated to 21 [32]. The presence of BH3 decreases the activation energy for the formation of 23 by 3.1 and 4.5 kcal mol to 9.6 kcal mol for the exoselective reaction and 11.6 kcal-mol for the endo-selective reaction, respectively, while the activation energy for the formation of 24 increases by >1.4 kcal mol , compared to those for the uncatalyzed reaction. The transition-state structure for the BH3-exo-selective 1,3-dipolar cycloaddition reaction of nitrone 21 with ethyl vinyl ether 22 is shown in Fig. 8.19. 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

Conducting reactions in nanospace where the dimensions of the reaction vessel are comparable to those of the reactants provides a new tool that can be used to control the selectivity of chemical transformations.1 This dimensional aspect of nano-vessels has been referred to as shape selectivity.2 The effect of spatial confinement can potentially be exerted at all points on the reaction surface but its influence on three stationary points along the reaction coordinate (reactants, transition states, and products) deserve special attention.3,4 (1) Molecular sieving of the reactants, excluding substrates of the incorrect dimension from the reaction site can occur (reactant selectivity). (2) Enzyme-like size selection or shape stabilization of transition states can dramatically influence reaction pathways (transition state selectivity). (3) Finally, products can be selectively retained that are too large to be removed via the nano-vessel openings/pores (product selectivity). 

The first model was very simple. It is symmetrical with respect to the reactants and products and it violates the so called cut-off rules of Wolfsberg and Stem which state that atoms positioned two bonds away from the isotopic site should be included in the calculations (Wolfsberg and Stem, 1964 Stem and Wolfsberg, 1966). Ibis model was chosen so that a large number of calculations could be made rapidly. The second, more realistic, model using the reactants shown in reaction (45) was used to calculate the KIEs for certain selected transition state structures suggested by the simpler model. 

Referring to a kinetically controlled reaction in which selectivity parallels the relative thermodynamic stabilities of the products. Often in such reactions, the transition state occurs late along the reaction coordinate. 

Phase space theory can be thought of as, in effect, considering a loose or, as it is sometimes called, orbiting [333] transition state regardless of the nature of the reaction. The need to select transition state properties for each individual reaction considered is avoided and it has been argued that a virtue of the theory is that it gives definite predictions [452]. 

FIGURE 12.8 Fully optimized geometry of selected transition states for 2-pentanone + OH reaction, corresponding to secondary beta (A) and secondary alpha (B) abstractions. 

When pubhshed reports of the diffusivity of paraffins in ZSM-5 catalysts obtained from uptake rate measurements appeared grossly inconsistent with catalytic behavior. Werner participated in resolving the problem by determining diffusivities from catalytic behavior of catalysts of very different particle sizes. The analysis not only confirmed the many orders of magnitude higher true diffusivities but also allowed Werner to extend the technique to demonstrate that shape selectivity could occur due to lack of fit of a reactant (e.g., diffusion of a dimethyl paraffin) in the structure or lack of fit of a reaction complex (transition state) that must be created on the active site (e.g., the methyl paraffin/propyl cation complex). 

Enthalpies of formation were determined using different methods. Intermediates, products and some selected transition states were calculated at B3LYP/6-31 lG(d,p) level of calculation, and by use of the Group Additivity method. Barrier values from Hadad et al. [32], from Mebel et al. [33] and from other reaction systems are also used. The groups developed in chapter 4 were used to aid in the evaluation of the enthalpies of formation of large size species. The enthalpies of the reference species used in the working reactions are listed in Appendix A. 

For an example of such interaction in the TS of non-catalyzed alkyne/azide cycloaddition (click reaction), see Gold, B., Shevchenko, N. E., Bonus, N., Dudley, G. B., Alabugin, 1. V. (2012). Selective Transition State Stabihzation via Hyperconjugative and Conjugative Assistance Stereoelectronic Concept for Copper-Free Click Chemistry. The Journal of Organic Chemistry, 77(1), 75-89. 

In a chemical reaction, a more stable transition state, measured by the magnitude of the activation energy, implies an easier chemical reaction. Aromatic transition states are also known to facilitate the chemical reaction. Zhou and Parr defined the activation hardness as the hardness difference of the products and the transition state and found, in the case of electrophilic aromatic substitution, that the smaller the activation hardness, the faster the reaction is. For this specific reaction they also found a correlation of the activation hardness and Wheland s cation localization energy, also proposed as an indicator of aromaticity. This finding can indeed be interpreted as a manifestation of the maximum hardness principle. A transition state with a high hardness is more stable than one with a smaller hardness and is therefore easier to reach energetically. The same can be said about two transition states with different aromaticity. Again, hardness and aromaticity parallel each other. The activation hardness has been used in numerous applications for the prediction of site selectivity in chemical reac-... 

In the all carbon and hydrogen instance, the Alder-Ene reaction is considered to be concerted and is thermally allowed based on Woodward-Hoffman rules. Thus, the Alder-Ene reaction is proposed to be a six-electron process, like the Diels-Alder reaction, having transition states endo and exo) analogous to the Diels-Alder reaction. However, the Alder-Ene reaction is easily modulated by steric effects as secondary electronic stabilizing effects have yet to be clearly identified. For example, Berson reported c/5-2-butene reacted with maleic anhydride to provide about a 4 1 ratio of endo. exo adducts 5 6, while trans-l-hvAeae provided little selectivity at 43 57 ratio of 5 6. In the reaction of maleic anhydride with tra 5-2-butene, the e.xo-TS encounters a steric interaction that the endo-TS does not. Steric effects are... 

Thiazolidinethione-derived chiral auxiliaries have similar reactivity and selectivity, as shown in Table 2.15. Because of the increased nucleophilicity of the thiazolidinethione ring, chelation-controlled reaction through transition state 85 enabled preferential formation of the non-Evans syn product 101. One equivalent of i-Pr2NEt, TMEDA, or (—)-sparteine and 1 equiv. TiCl4 provided 101 diastereoselectively. Interestingly, when the reaction was performed with 2 equiv. of TMEDA or (—)-sparteine, Evans syn aldol adduct 100 was obtained diastereoselectively. These aldol products can be converted to a variety of other functionality under mild conditions. Other oxazolidinethiones and thiazolidinethiones have resulted in comparable diastereoselectivity and yields. 

How does the degree of bond breaking in the transition state affect the selectivity of the reagent In the less reactive bromination reaction, the transition state for the first propagation step is more product-like and resembles the radical product. Thus, the ease with which the C—H bond is broken reflects the effect of alkyl groups on the stability of the radical. The reaction then shows a selectivity that reflects the stabihty of the radical product. In the chlorination reaction, the transition state is more reactant-like, and the alkyl group has not developed much radical character. Hence, the type of C—H bond—primary, secondary, or tertiary— has little effect on the reaction, and low selectivity is the result. 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

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