Influence of reactant polarity on the course of (4+2) cycloadditions

An ab initio MO-theoretical study (RHF/3-21G* basis set and single point Becke3LYP/6-31G*//RHF/3-21G* calculations) on transition structures (TS) and intermediates in (4 + 2) cycloadditions of (E)-1-amino- and (E)-1-hydroxy-1,3-butadiene, of (E,E)-1,4-diamino- and (E,E)-1,6-dihydroxy-1,3-butadiene, and of 1,1-diamino- and 1,1-dihydroxy-1,3-butadiene to acrylonitrile, fumaronitrile, and 1,1-dicyanoethylene is reported. The amount of dissymmetry in bond formation increases with the capability of the reactants to stabilize a positive or negative charge. A main reason for the concomitant decrease in activation energy derives from polar interactions due to the transfer of-charge from donor diene to acceptor dienophile. Zwitterions are located in the cycloaddition of 1,1-diamino- and 1,1-dihydroxy-1,3-butadiene to 1,1-dicyanoethylene if the solvent is included via the SCRF method. 1,1-Diamino- and 1,1-dihydroxy-1,3-butadiene form molecular complexes of van der Waals nature with 1,1-dicyanoethylene.