Stabilization of reactive intermediates

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

Figure 14.6 Resonance stabilization of reactive intermediates from (i) bis(chloromethyl) ether, (ii) aliphatic nitrogen mustard, (iii) allyl chloride, (iv) benzyl chloride, (v) bis(morpholino)methane, (vi) benzoyl chloride, and (vii) dimethylcarbamyl chloride.

Gerlt, J. A. (1999) Stabilization of reactive intermediates and transition states in enzyme active sites by hydrogen bonding, Comprehensive Natural Products Chemistry 5, 5-29. 

This article will describe the different chemical strategies used by enzymes to achieve rate acceleration in the reactions that they catalyze. The concept of transition state stabilization applies to all types of catalysts. Because enzyme-catalyzed reactions are contained within an active site of a protein, proximity effects caused by the high effective concentrations of reactive groups are important for enzyme-catalyzed reactions, and, depending on how solvent-exposed the active site is, substrate desolvation may be important also. Examples of acid-base catalysis and covalent (nucleophilic) catalysis will be illustrated as well as examples of "strain" or substrate destabilization, which is a type of catalysis observed rarely in chemical catalysis. Some more advanced topics then will be mentioned briefly the stabilization of reactive intermediates in enzyme active sites and the possible involvement of protein dynamics and hydrogen tunneling in enzyme catalysis. 

An understanding of interaction diagrams is not absolutely necessary for using the principle of electron flow to predict organic reaction products. However, it is useful for understanding reactivity trends and the stability of reactive intermediates. This section relies on the principles discussed in Section 1.6, An Orbital View of Bonding. 

Inner-phase stabilization of reactive intermediates (macroheterocycles as molecular containers, carceplexes) 01EJO423. 

Inner Phase Stabilization of Reactive Intermediates Concept... 

Both Figures 5.2 and 5.3 provide a starting point for the consideration of reactive intermediates and fiieir roles in organic reactions. In the current discussion, we want to focus on the structural features that affect the stability of reactive intermediates and then relate this understanding to the chemical course and kinetics of a reaction. In most cases these reactive intermediates will be ions in which a carbon atom formally bears the positive or negative charge or radicals and carbenes in which the nonbonded electrons are formally localized on a carbon atom. Certainly there are other carbon-centered reactive intermediates (such as carbynes and atomic carbon ), and... 

The hydrophobic cavity of M4L5 cage and its strong affinity for cationic guests can be exploited for encapsulation and stabilization of reactive intermediates [18]. 

For all the basic classes of reactive intermediates, thermodynamic data that allow valuable comparisons of relative stabilities are available BDEs for radicals, HI As for cations, and pKa values for anions. The trends in relative stabilities of reactive intermediates are generally well treated by the bonding model developed in Chapter 1. 

Remember, a mechanism can never be completely proven. However, after having read Chapters 7 and 8, we are ready to present the experiments that are used to support a proposed mechanism. Furthermore, with the analysis of the structure and stability of reactive intermediates covered in Chapters 1 and 2, we are ready to explain their reactivity. Moreover, after having read Chapter 9, we have a foundation in catalysis. Therefore, Chapters 10 and 11 represent a culmination of much of what you have learned so far. 

The behaviors of molecules in dilute solutions are very different when isolated from bulk media in the confined spaces inside molecular capsules. The molecular capsules are novel tools of modern physical organic chemistry and have been used as reaction chambers, chiral receptors, stabilizers of reactive intermediates, " transition states, and modifiers of reactions in limited quarters. ... 

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