Suitability of furan, pyrrole and thiophene as dienes for Diels-Alder reactions viewed through their stability and reaction barriers for reactions with acetylene, ethylene and cyclopropene. An AM1 semiempirical and B3LYP hybrid density functional theory study

The uniformity of bond orders in five-membered heterocyclic compounds, Fron tier Molecular Orbital (FMO) energy differences between reactants, and FMO energy changes going from reactants to transition state structures were use d to evaluate the relative reactivity of furan, pyrrole, thiophene and deri vatives of thiophene as dienes for Diels-Alder reactions. To determine the stereochemical outcome of these reactions, Secondary Orbital Interactions ( SOI) between two reactants moieties in the isomeric transition state struct ures were used. All of these qualitative assessments were confirmed through computation of the activation barriers for all possible transition state s tructures by AM1, B3LYP/6-31G(d)//AM1, and in some cases, by B3LYP/6-31G(d) and MP2/6-31 + G(d) methods. All observations made by combinations of vari ous theoretical approaches were in perfect agreement with the experimental results; therefore, they are suggested as a method to be considered by synt hetic organic chemists when plans for the preparation of complex organic mo lecules are being formulated. (C) 1998 Elsevier Science B.V. All rights res erved.