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Abstract
The competitiveness of the BF3 Lewis acid (LA) catalyzed polar Diels–Alder (P-DA) and polar Alder-ene (P-AE) reactions of 2-methyl-1,3-butadiene, a diene possessing an allylic hydrogen, with formaldehyde has been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Coordination of BF3 LA to the oxygen of formaldehyde drastically accelerates both reactions given the high electrophilic character of the BF3:formaldehyde complex. As a consequence, these reactions present a very low activation enthalpy—less than 2.2 kcal·mol−1—thus becoming competitive. In dioxane, the P-AE reaction is slightly favored because of the larger polar character of the corresponding transition state structure (TS). In addition, the Prins reaction between hexahydrophenanthrene and the BF3:formaldehyde complex has also been studied as a computational model of an experimental P-AE reaction. For this LA-catalyzed reaction, the P-DA reaction presents very high activation energy because of the aromatic character of the dienic framework. The present MEDT study allows establishing the similarity of the TSs associated with the formation of the C–C single bond in both reactions, as well as the competitiveness between P-AE and P-DA reactions when the diene substrate possesses at least one allylic hydrogen, thus making it necessary to be considered by experimentalists in highly polar processes. In this work, the term “pseudocyclic selectivity” is suggested to connote the selective formation of structural isomers through stereoisomeric pseudocyclic TSs.
Highlights
IntroductionThe Diels–Alder (DA) reaction between a conjugated diene and an ethylene yielding a cyclohexene derivative, reported for the first time by Otto Diels and Kurt Alder in 1927 [1], is one of the most studied organic reactions from a synthetic as well as theoretical point of view (see Scheme 1) [2,3]
The Diels–Alder (DA) reaction between a conjugated diene and an ethylene yielding a cyclohexene derivative, reported for the first time by Otto Diels and Kurt Alder in 1927 [1], is one of the most studied organic reactions from a synthetic as well as theoretical point of view [2,3].On the other hand, the Alder-ene (AE) reaction, first reported by Kurt Alder in 1943 [4], which involves an alkene with an allylic C–H bond and an ethylene derivative, is one of the simplest ways to achieve the formation of a C–C single bond [5].Molecules 2018, 23, 1913; doi:10.3390/molecules23081913 www.mdpi.com/journal/moleculesMolecules 2018, 23, x FOR PEER REVIEWScheme 1
The present Molecular Electron Density Theory (MEDT) study has been divided into four sections: in Section 3.1, an analysis of the Conceptual DFT (CDFT) reactivity indices at the ground state (GS) of the reagents involved in the competitive polar Diels–Alder (P-DA)
Summary
The Diels–Alder (DA) reaction between a conjugated diene and an ethylene yielding a cyclohexene derivative, reported for the first time by Otto Diels and Kurt Alder in 1927 [1], is one of the most studied organic reactions from a synthetic as well as theoretical point of view (see Scheme 1) [2,3]. The Alder-ene (AE) reaction, first reported by Kurt Alder in 1943 [4], which involves an alkene with an allylic C–H bond and an ethylene derivative, is one of the simplest ways to achieve the formation of a C–C single bond (see Scheme 2) [5]. 1. Diels‐Alder reaction between cyclopentadiene 1 and maleic anhydride 2 yielding the bicyclic adduct 3 [1]
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