Understanding the Reactivity of Captodative Ethylenes in Polar Cycloaddition Reactions. A Theoretical Study † Luis R. Domingo,* ,‡ Eduardo Chamorro, § and Patricia Pe ´rez § Departamento de Quı ´mica Orga ´nica, UniVersidad de Valencia, Dr. Moliner 50, E-46100 Burjassot, Valencia, Spain, and Departamento de Ciencias Quı ´micas, Laboratorio de Quı ´mica Teo ´rica, Facultad de Ecologı ´a y Recursos Naturales, UniVersidad Andre ´s Bello, AV. Repu ´blica 275, 8370146 Santiago, Chile domingo@utopia.uV.es ReceiVed March 13, 2008 The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for the reactivity toward nucleophiles, it is shown that a simple index chosen for the nucleophilicity, Ν, based on the HOMO energy is useful explaining the reactivity of these CD ethylenes toward electrophiles. Introduction Cycloaddition reactions are one of the most important processes with both synthetic and mechanistic interest in organic chemistry. 1 In this type of reaction, two new σ bonds are formed at the ends of two interacting π systems along with the formation of a carbocycle system. These reactions have been mechanisti- cally classified in the class of the pericyclic reactions. 2 The feasibility of these processes has been related to the Woodward- Hoffmann rules. 3 For unsaturated hydrocarbon reagents, the activation energies associated with the concerted bond-formation processes show very high values. It is well-known, however, that electron-withdrawing (EW) and (or) electron-releasing (ER) sub- stituents on one or both reagents notably increase the cycloaddition reaction rate. This chemical substitution affords to change the † Dedicated to Prof. K. N. Houk on the occasion of his 65th birthday. ‡ Universidad de Valencia. § Universidad Andre ´s Bello. (1) (a) Carruthers, W. Some Modern Methods of Organic Synthesis; 2nd ed.; Cambridge University Press: Cambridge, 1978. (b) Carruthers, W. Cycloaddition Reactions in Organic Synthesis; Pergamon: Oxford, 1990. (2) (a) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; John Wiley and Sons: New York, 1976. (b) Fleming, I. Pericyclic Reaction; Oxford University Press, Oxford, 1999. (3) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital Symmetry; Verlag Chemie: Weinheim, 1970. 10.1021/jo800572a CCC: $40.75  2008 American Chemical Society J. Org. Chem. 2008, 73, 4615–4624 4615 Published on Web 05/17/2008