A novel approach to systematic classification of organic reactions. Hierarchical subgraphs of imaginary transition structures

Abstract

A new systematic classification of organic reactions is presented in terms of imaginary transition structures (ITS) and their hierarchical n-nodal subgraphs. The ITS corresponding to an individual organic reaction is a structural formula which has par-bonds (invariant bonds during the reaction), out-bonds (bonds appearing in the starting stage), and in-bonds (bonds appearing only in the product stage). The n-nodal subgraph indicates the corresponding reaction type and is divided into two parts: reaction kernels (RK) and terminal descriptors (TD). The RK is a set of adjacent carbon reaction centres to which out-and in-bonds are incident. The RK affords information on the changes of carbon skeletons, e.g., ‘substitution’, ‘construction (C–C bond formation)’, ‘cleavage’, ‘addition’, and ‘elimination’. The TD is a set of terminal non-carbon atoms and imaginary bonds incident to the terminals. Odd- and even-nodal subgraphs have common TDs, respectively, which afford information on the changes of oxidation states, e.g., ‘oxidative’, ‘reductive’, and ‘isohypsic’. This fact stems from the alternant character of reaction strings.