Diels-Alder reaction: A theoretical comprehensive study of substituent effects using the 'H* method'

The use of fictitious hydrogen atoms (H*) with a variable non-integer nuclear charge Z* allows to mimic a substituents (including field effects) whose the electron attracting power can be continuously varied with Z*. This way, we study the effects of these substituents, in various positions of the reactants, on the activation energy E. of the Diels-Alder reaction. In a first part, a semi-quantitative exploration is carried out with one H*-substituted reactant, the other remaining unsubstituted. For ethylene substitution, E, is not a monotonous function of Z*, with a maximum indicating a change in the electron demand as confirmed by NBO analysis. This change occurs nearby Z* = 1 (unsubstituted ethylene), in agreement with the change in the HOMO-LUMO leading interaction. By contrast, diene monosubstitution results in a quasi-linear decreasing function of Ea on Z* though a change in electron demand and in the leading HOMO-LUMO interaction already occurs nearby Z* = 1. It reveals that the reaction is no longer controlled by the frontier interactions when butadiene bears donor substituents. In a second part, we define and calculate indices of reactivity, which characterize the effects of a second substitution in a given position of monosubstituted ethylene or diene. We briefly discuss their consequences for reactivity, regioselectivity and stereoselectivity. Finally, we investigate the possibility of computing a partition of sigma and pi effects. We conclude that for butadiene substitution, sigma effects are almost the same in positions 1 and 2 whereas Tu effects should be stronger in position 2. (c) 2005 Elsevier B.V. All rights reserved.