With the objective to understand how the pattern and degree of chlorination influence on the properties of the title molecules, a computational study on biphenyl and all the chlorinated biphenyls (from 1 to 10 chlorine atoms, 209 congeners) has been undertaken. The study includes conformational searches (and further refinement by molecular dynamics simulations) and the ab initio calculation of the molecular electrostatic potential (MEP) and the dipole moments for all the congeners. The most significant property is the MEP, finding a good correlation between the MEPs and the substitution pattern on chlorinated biphenyls. The most toxic congeners possess highly positive values of electrostatic potential on the aromatic rings and highly negative values of electrostatic potential on the chlorine atoms. Additionally, we have found that the toxic congeners possess conformations with low dipole moments, a fact that may be linked to the ready accumulation on the adipose tissue. The results on the geometry and electrostatic properties of chlorinated biphenyls can be useful to rationalize their selective toxicities.

The X‐ray structures of fifteen 1, 3‐imidazolidine, 1, 3‐oxazolidine, 1, 3‐dioxan‐4‐one, and hydropyrimidine‐4(1H)‐one derivatives are described (Table 2) and compared with known structures of similar compounds (Figs. 1–20). The differences between structures containing exocyclic N‐acyl groups and those lacking this structural element arise from the A1,3 effect of the amide moieties. Even t‐Bu groups are forced into axial positions of six‐ring half‐chair or into flag‐pole positions of six‐ring twist‐boat conformers by this effect (Figs. 16–20). In the N‐acylated five‐membered heterocycles, a combination of ring strain and A1, 3 strain leads to strong pyramidalizations of the amide N‐atoms (Table 1) such that the acyl groups wind up on one side and the other substituents on the opposite side of the rings (Figs. 4–9 and Scheme 3). Thus, the acyl (protecting!) groups strongly contribute to the steric bias between the two faces of the rings. Observed, at first glance surprizing stereoselectivities of reactions of these heterocycles (Schemes 1 and 2) are interpreted (Scheme 3) as an indirect consequence of the amide A1, 3 strain effect. The conclusions drawn are considered relvant for a better understanding of the ever increasing role which amide groups play in stereoselective syntheses.

Aromaticity is an essential concept in chemistry, invented to account for the stability, reactivity, molecular structure, and properties of many organic and inorganic compounds. In recent years, numerous methods to quantify aromaticity based on the energetic, magnetic, structural, and electronic properties of molecules have been proposed but none of them is universal. The inability of establishing a universal scale of aromaticity based on a single parameter is due to the multidimensional character of this phenomenon. Consequently, aromaticity analyses should be carried out by employing a set of aromaticity descriptors on the basis of different physical manifestations of aromaticity. Here, we report a universal scale of aromaticity for pi-organic compounds based on the Euclidean distance between neurons in a self-organizing map. The most widely used aromaticity indicators have been used as molecular descriptors, and so our approach provides the first scale of aromaticity which contains the energetic, magnetic, and structural aspects of this property. The method is applicable to a wide variety of unsaturated organic compounds and allows quantification of both aromaticity and antiaromaticity. Additionally, the position of a compound on the bidimensional map determinates immediately the following: (a) the group (aromatic, nonaromatic, or antiaromatic) to which the system belongs, (b) their degree of pi-electronic delocalization, and (c) the similarity in aromaticity/antiaromaticity between different compounds. This new scale of aromaticity is able to indicate the expected order of aromaticity of analogues of fulvene and heptafulvene, heteroaromatic species, substituted benzenes, and functionalized cyclopentadienyl compounds.

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