Shape group studies of molecular similarity and regioselectivity in chemical reactions

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Abstract
The study of the qualitative and quantitative product distribution in a chemical reaction, in particular regioselectivity, is of fundamental importance. Recently, it has been shown that the regioselectivity of some Diels‐Alder cycloadditions can be explained by analyzing the interrelations between electron density contours and molecular electrostatic potentials. This problem is related to a central topic of modern theoretical chemistry and biochemistry: the analysis of molecular shape. This work deals with a rigorous, algebraic method to analyze these surfaces. The procedure is based on the computation of the shape groups (symmetry‐independent homology groups of algebraic topology) of the molecular surface, using either a fully analytical algorithm requiring no visual inspection, or a precise method for processing pictorial information if the latter is available. The method provides a concise description of the molecular contour surface, that can replace the usually intuitive, and somewhat subjective, visual characterization of density and electrostatic potential contours. The method is illustrated for the case of Diels‐Alder reactions by considering a number of monosubstituted dienes. Extensions of the analysis to dienophiles, as well as other types of reactions are also discussed.