Improvements in Diels–Alder cycloadditions with some acetylenic compounds under solvent-free microwave-assisted conditions: experimental results and theoretical approaches

The Diels–Alder irreversible cycloadditions of 1,3-cyclohexadiene , 3-carbomethoxy-2-pyrone and 2-methoxythiophene with acetylenic dienophiles under solvent-free conditions are described. By strict comparisons with conventional heating under similar conditions, important specific microwave effects are revealed in the two last cases whereas they are absent in the first one. They are discussed in terms of asynchronous mechanisms in agreement with ab initio calculations at the HF/6-31G(d) level indicating dissymmetries in transition states. Specific MW effects can be understood by considering the enhancements in dipole moments from ground states to transition states.