Influence of Reactant Polarity on the Course of (4 + 2) Cycloadditions

Abstract

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 )-1,4-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.