Abstract:

Based on the principle of conservation of orbital symmetry(PCOS), it can be concluded that it is the interaction between the most stable valent orbitals of diene and dienophile that determines the regioselectivity of the Diels-Alder reaction, which produces the more stable σ1 bond of the two formed σ bonds.The regioisomer which possesses more stable σ1 bond in transition state will be the main product of the Diels-Alder reaction, which is called σ1 rule in this paper.The correctness to predict the main product of Diels-Alder reaction by using σ1 rule has been confirmed by quantum-chemical calculation, and the calculation also proves that the σ1 bond forms between the most stable valent orbital of the individual reactants, diene and dienophile.

Key words: Diels-Alder reaction, Principle of conservation of orbital symmetry, Regioselectivity, Orbital interaction