Electrophilic aromatic substitution theoretical calculations

By theoretical calculations (B3LYP/6-31G ) four reaction pathways were investigated formation of endo or egzo product with initial bond formation to C2 or C3 in indole. For each mechanism theoretical 13C KIE were analysed and the best agreement of theoretical and experimental KIEs was found for the reaction involving the intermediacy of the radical cation 11, resulting from electrophilic aromatic substitution of indole at C3 by cyclohexadiene in the rate-limiting step ... 

Eq. (5.11)). Further, when diaryl sulfoxide was used (Eq. (5.12)), it was expected that Friedel-Crafts type reaction would yield a mixture of products originating from electrophilic aromatic substitution of intermediates 60. However, again, the alkylation occurred only at the positions adjacent to the thioaryl group (Eq. (5.12)). To gain insight at this unexpected regioselectivity, the authors performed theoretical calculations on a model system [43]. 

Carboxylation of aromatics with carbon dioxide with AI2CI,/AI has been studied by Olah, Prakash, and co-workers425 and shown to be a chemoselective process to give aromatic carboxylic acids in good to excellent yields (20-80°C, CO pressure = 57 atm). Two possible mechanistic pathways with the involment of organoaluminium intermediates and complexes of C02 with AICI3 were postulated. On the basis of extensive experimental studies and theoretical calculations, the authors concluded that the most feasible mechanism involves CO2 activated with superelectrophilic aluminum chloride. Complex 116 reacts with aromatics in a typical electrophilic substitution. 

The normal mode of reaction of cycloproparenes with electrophiles involves opening of the three-membered ring and leads to benzylic derivatives (see Section 3.B.3.). The bis(triiso-propylsilyl) group offers efficient protection of the cyclopropene moiety, so that electrophilic attack of the protected benzocyclopropene occurs at the aromatic ring. Reaction of 1,1-bis(triisopropylsilyl)benzocyclopropene with 67% nitric acid gave the 3-nitro derivative 1 in 58% yield. Electrophilic attack on the cycloproparene occurs at C3 and this is consistent with theoretical calculations. A variety of substituted derivatives of benzocyclopropene are available from the nitro compound (see Section 3.5.). 

Considerable advances have been made in recent years in the understanding of the aromatic substitution reactions of oxazoles. Molecular orbital calculations (Section III, B) predict that electrophilic attack should occur preferentially at position 5, and indeed this is observed. The relative order of reactivity calculated theoretically is not in complete accord with the experimentally observed order (5 > 4 > 2) therefore it is evident that the electrophilic substitution reactions are rather more complex than the present theoretical calculations would predict. 

Proton transfer reactions play very important role in chemistry and biochemistry [1-3]. Considerable attention has been focused on the gas phase reactions in the last decades, since they are free of the solvent pollution thus being related to the intrinsic reactivity [4 6]. In particular, investigations of gas-phase acidities and basicities were some of the major undertakings in the field [7,8]. The proton affinity (PA), on the other hand is an interesting thermodynamic property by itself. It gives useful information on the electronic structure of base in question and serves as an indicator of the electrophilic substitution susceptibility of aromatic compounds [9]. It is the aim of this article to describe some recent advances in theoretical calculations of the proton affinities of substituted aromatics. We shall particularly dwell in more detail on the additivity rules, which enable simple and quick estimates of PAs in heavily substituted benzenes and naphthalenes. Some prospects for future developements will be briefly discussed too. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

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