Group 14 p-Effect

The stabilizing effect of group 14 elements when placed in the p-position to a carbocation centre is known as P-effed. This stabilization has been widely employed in synthetic transformations and, recently, it has allowed the isolation and X-ray structure determination of a secondary aUcyl carbenium ion by Bochmann et al. 

As suggested by Mo and Schleyer [7a], a reference molecule with the same number of diene conjugations is a better choice than a molecule with the same number of ji-electrons, because the ASE values (computed using the block-localized wave function, BLW, method) exhibit a better correlation with the nuclear-independent chemical shift (NICS) values. We followed their suggestion and adopted the former approach for the calculation of the ASEs using the AE values derived from the EDA. 

As can be readily seen from the data in Table 12.5, the homoaromatic cyclobutene 

The reliability of the EDA method has been proved in the study of the aromaticity of systems where the appHcation of the very popular methods for quantifying aromaticity of a molecule (i.e., NICS and magnetic susceptibility anisotropy. Ay, calculations) fails or has severe Hmitations. Three different families of compounds with controversial aromatic nature have been considered (i) neutral exocyclic substituted cyclopropenes (HC)2C=E, (ii) group 14 homologs of the cyclopropenyhum cation, and (iii) metaUabenzenes. 

The focus of this study is to find out to what extent different exocyclic (C=X) substituents induce aromaticity and antiaromaticity in cyclopropene derivatives (HC)2C=X [32]. Although the consensus opinion suggests that parent methylenecy-clopropene, (HC)2C=CH2, the simplest cross-conjugated cyclic hydrocarbon, is nonaromatic, it is admitted that cyclopropenone is at least modesdy aromatic [33-35]. However, quantitative assessments of the aromaticity of other (HC)2C=X derivatives have not led to satisfactory agreement. Thus, this family of compounds represents a paramount opportunity to use the EDA-aromaticity method to give a definitive answer to a question that has been controversially discussed in the literature for several decades [33-35]. 

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