Density functional theory (DFT), at the B3LYP/6-311G(d,p) level of theory, has been employed to study the mechanism, potential energy surface, substituent and solvent effects of the titled reactions. The obtained results indicate that all of these reactions are concerted but nonsynchronous processes, in which the formation of C—Si or C—Ge bond always keeps ahead the formation of C—C bond. For [2＋4] reactions with silabenzenes or germabenzenes as heterodienophiles, the exo approach is less asynchronous and is generally slightly favored. While for [4＋2] reactions with silabenzenes or germabenzenes as heterodienes, the asynchronicity in the bond-formation process is related to the relative position of Si or Ge atom with respect to the substituent at C atom of dienophile in products, and the reaction path with more asynchronicity is kinetically favored. The CCl₃ or NH₂ groups at Si or Ge atom of silabenzene or germabenzene molecules are in favor of both [2＋4] and [4＋2] processes not only thermodynamically but also kinetically, while C(CH₃)₃ as a substituent has opposite effect generally. Compared with corresponding [4＋2] reactions, [2＋4] reactions are decidedly preferred, both thermodynamically and kinetically, which is in full agreement with experimental observations. Benzene or methanol as a solvent has only trivial influence on the potential energy surfaces of the studied reactions.

Key words: silabenzene, germabenzene, Diels-Alder reaction, reaction mechanism, density functional theory