Intramolecular effects in the cycloaddition of three ethylenes vs. the Diels–Alder reaction

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Abstract
The Diels–Alder (D–A) reaction possesses a much smaller barrier than the cycloaddition of three ethylenes (C-3E) despite the opposite effect of the corresponding reaction exothermicities. These reactions have been studied by means of ab initio computations coupled with analysis by the curvecrossing model. It is shown that the intramolecular advantage, exhibited by the D–A reaction, has both electronic and entropic components. The electronic advantage is found to be much more dominat, approximately 26 kcal mol–1 at the MP4(FC)/6-31G*//HF/3-21G level of theory. The relationship of the electronic advantage and the notions of ‘reactant deformations’ and critical distances are explored and quantitated. A simple mechanism which elucidates the origins of the inramolecular electronic effect is presented. The effect is shown to be exerted through a stabilization of the reactant excited state that possesses the bonding features of the product: so called ‘the prepared excited state’. The excitation energy gap between the ground state and the prepared excited state is the promotion energy of the reaction and is the root cause of the barrier according to the curve crossing model (ref. 4). The reduction of the promotion energy gap for the Diels–Alder reaction by 124 kcal mol–1 results, therefore, in a corresponding reduction of the barrier by ca. 26 kcal mol–1. Considerations of the promotion energy gap allow for a semiquantitative estimation of the intramolecular electronic advantage at different distances. It is concluded the electronic advantage of proximity will be smaller than the entropic component unless it is possible to bring the π-bonds into a critical distance which is shorter than ca. 2.7 Å.