Frontier-Orbital Interactions in Mannich Reaction With Polynitromethanes

A Mannich reaction of polymtromethanes takes place at first through addition reaction of amine and aldehyde components to produce the N—methylol derivates which then condense with a psceudo acid component. It is important to study how the intermediate— methylene—amine cationic species as electrophilic reagent react with polynitromethane anions as nucleophilic reagent. The electronic structures of polynitromethane anions （as Lewis, or generalized bases） and methylene—amine cations （as Lewis, or generalized acids） have been calculated by SCF-CNDO/2 method. The energies and phases of the frontier orbitals （HOMO, NHOMO. LUMO and NLUMO） were listed in Table-1 and Table-2. It is seen from tables that the HOMO of nucleophilic reagent localize to C^- reaction center and the LUMO of electrophilic reagent localize to C⁺ reaction center, i.e., the coefficients of corresponding 2p_z—AO’s in the frontier orbitals are larger. Figuresl-4 represent the interactions between the frontier orbitals for the four reaction system which are the soft acid—soft base, hard acid—hard base, soft acid—hard base and hard acid—soft base, respectively; here the concept of Pearson’s hard and soft acids and bases has been used. According to the three principles of bonding, the reactions of "soft—soft" and "hard—hard" acids and bases easily take place and belong to "frontier—controlled" reactions. The reaction of soft base—hard acid can not take place because of the energy difference between the HOMO of the base and LUMO of the acid is larger than 6eV. These results are useful for elucidation of the feasibility of a Mannich reaction for the polynitro—methanes and consistent with those conclusions drawn from Ge and Ae criterions, which had been proposed by us in previous paper. From orbital interaction of hard base—soft acid shown in Fig.4 the experimental fact, which the corresponding Mannich reaction can not take place, is hardly interpreted, which show that the mechanism of Mannish reactions are probably more complicated.