We first compared the parent uncatalyzed reaction involving 2-vinylpyridine ( 1) and trans-1-phenyl-1,3-butadiene ( 2) with the analogous cycloaddition reaction mediated by BF 3.
(18) In addition, we shall also apply the ASM-EDA approach to rationalize the reasons behind the almost complete regio- and diastereoselectivity observed in the transformation, which remains completely unknown so far. To check this, we will apply the combination of the activation strain model (ASM) (16) of reactivity with the energy decomposition analysis (EDA) (17) method, which was proven to provide detailed quantitative insight into the ultimate factors controlling fundamental processes in organic, main group and organometallic chemistry. (15) Therefore, we hypothesized that the Pauli-repulsion lowering and not the proposed LUMO-lowering arguments would constitute the actual factor governing the catalysis in this particular BF 3-mediated cycloaddition reaction involving vinylazaarenes. This so-called Pauli-repulsion lowering concept (10) seems general as it applies also in related cycloadditions where the catalyst establishes noncovalent interactions (hydrogen, (11) halogen, (12) or chalcogen bonds (13)) with the dienophile and even in slightly related catalyzed Michael-addition reactions (14) and iminium-catalyzed cycloadditions. (9) As a result, we found that the reduction of the Pauli repulsion between the key occupied π-molecular orbitals and not the above orbital interactions constitutes the actual physical mechanism behind the acceleration promoted by LAs in Diels–Alder reactions. (7) We have, however, recently demonstrated that this LUMO-lowering concept in slightly related LA-catalyzed Diels–Alder is rather incomplete as it does not consider the impact on the reverse HOMO (dienophile)–LUMO (diene) interaction, which indeed can offset the favorable HOMO (diene)–LUMO (dienophile) interaction.
The observed great acceleration of the cycloaddition was rationalized by invoking the above-mentioned traditional LUMO-lowering concept (4,5) in view of the significant stabilization of the LUMO of the dienophile upon binding to BF 3.