Substituent effects in the hetero-Diels-Alder reaction of thiocarbonyl compounds with butadiene

Structural, kinetic, and thermodynamic parameters for the Diels-Alder reactions of butadiene with different thiocarbonyl compounds have been computed with the 6-31G(d) basis set using Hartree-Fock (HF), post HF (MP2), and a density functional (DF) methods. In the last case, a recent hybrid approach (B3LYP) has been selected, which incorporates gradient corrections and some HF exchange. The general trends provided by the three methods are similar, but some test computations by the more refined MP4 approach indicate that only the B3LYP approach provides reliable absolute values for the activation barriers. Our results show that reactivity is enhanced for electron-deficient thioaldehydes and reduced in electron-rich thioaldehydes. The reactivity order should be driven by the strength of the pi C=S bond which is broken during the reaction. Since the singlet-triplet gap Delta E(ST) in the dienophile is closely related to this quantity, it is a very good reactivity index. This is particularly significant since Delta E(ST) values obtained at the B3LYP level are both reliable and inexpensive. From a more practical point of view, both complete computations and Delta E(ST) gaps suggest that substitution of aminic hydrogens in thioamides by acceptor groups provides effective reagents even in the absence of Lewis acid catalysts.