Theoretical investigations of [4πS+2πS] cyclodimerization and stereoselectivity of phthalazin derivatives

Semiempirical molecular orbital calculations using atom superposition and electron delocalization molecular orbital (ASED-MO) method are performed to rationalize the stereoselectivity and cyclodimerization of the cycloaddition processes. The calculations indicate that the reactivity of 1-chlorophthalazin and 1-(phthalazin-1-yl)pyridinium-3-olate as 4 pi /2 pi pericyclic components depend upon the energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels. Increasing of the ability of the [4 pi + 2 pi] cyclodimerization of the investigated compounds,is a function of a small HOMO-LUMO energy gap. The molecules with small energy gap are able to dimerize. Also, the calculations show the importance of the interactions between the frontier molecular orbitals to rationalize the stereoselectivity of the cycloaddition processes. (C) 2001 Elsevier Science B.V. All rights reserved.