Aromaticity of transition states

Evans/Dewar Concept of Aromaticity of Transition States in Pericyclic Reactions... 

Aromaticity of transition state decides if the reaction is thermally allowed or forbidden on the basis of Evan s rule. 

In this section, we illustrate the applicability of the model to some important special cases, and summarize the relationship between aromaticity and chemical reactivity, expressed in the properties of transition states. 

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

The effect of solvation of transition states has been discussed in relation to aromatic nucleophilic substitution. 

The aromatic-antiaromatic transition state rules are. another formulation of the Woodward-Hoffmann type rules (Table 8.3). 

Aromaticity is one of the fundamental principles of organic chemistry, used to predict products from chemical reactions based on the stability of the possible products, as well as to rationalize the stability of transition states, such as the transition state of the Diels Alder reaction (/). Aromatic species have An + 2n electrons in a cyclic system that allows complete delocalization of the electrons. 

To provide an example of a reaction that is very different to electrophilic aromatic substitution, the oxidation of formic acid by bromine was also studied. This reaction, which involves electrophilic attack on the formate anion (15) (Cox and McTigue, 1964 Smith, 1972 Herbine et al., 1980 Brusa and Colussi, 1980), is catalysed by a-CD (/c /k2u = 11) (Tee et al., 1990a), and the degree of transition state stabilization (Xts = 0.18 mM) is similar to that for phenols (Table A4.2) and most of the other substrates (Table A4.4). 

Bovine pancreatic chymotrypsin (Mr 25,191) is a protease, an enzyme that catalyzes the hydrolytic cleavage of peptide bonds. This protease is specific for peptide bonds adjacent to aromatic amino acid residues (Trp, Phe, Tyr). The three-dimensional structure of chymotrypsin is shown in Figure 6-18, with functional groups in the active site emphasized. The reaction catalyzed by this enzyme illustrates the principle of transition-state stabilization and also provides a classic example of general acid-base catalysis and covalent catalysis. 

There appears to be much interest in the mechanism of various pericyclic transfer-mations, particularly of the Cope rearrangement. A pair of interacting allyl radicals, an aromatic species, or a 1,4-cyclohexanediyl diradical are the possible intermediates and transition states for the rearrangement represented here as resonance hybrids in the transformation of 1,5-hexadiene (Scheme 4.16). Two high-order theoretical studies indicate that the Cope rearrangement is concerted and proceeds via an aromatic chair transition state (33).362,364... 

Possible reasons why transition state aromaticity is able to develop early while resonance development lags behind proton transfer at the transition state, and why anti-aromaticity lags behind proton transfer, will be discussed in the section on ab initio calculations. These calculations have provided important additional insights because they allow a direct probe of transition state aromaticity or anti-aromaticity. 

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