New findings on the Diels-Alder reactions. An analysis based on the bonding evolution theory

Two Diels-Alder type reactions, i.e., normal electron demand (NED) between 1,3-butadiene (BD) and acrolein (Acr) and inverse electron demand (IED) between 2,4-pentadienal (PDA) and methyl vinyl ether (MVE), have been investigated using the bonding evolution theory ( BET). BET combines topological analysis of the electron localization function ( ELF) and catastrophe theory. Catalyst effect has been incorporated through Lewis acid BH3. The B3LYP hybrid HF/DFT method along with 6-31G(d), 6-311++ G(d, p) basis sets have been used. All reactions yield two-stage mechanism and there is no topological evidence that they might be concerted with two bonds partially formed during transition structure. A formation of six-membered ring requires 10 (or 11) steps separated by two types of catastrophes: fold and cusp. The first "intermolecular" bond (C-1- C-6) is formed at 1.93, 1.92 angstrom (NED) and 1.92, 1.97 angstrom (IED). The six-membered ring is "closed" at 2.11, 2.13 angstrom (NED) and 2.5, 2.6 angstrom (IED) via formation of the second bond C-4-C-5. All reactions begin with "reduction" of CdC bonds to single C-C (cusp catastrophes). Subsequently, the nonbonding electron density is concentrated (fold catastrophes) on terminal C atoms. Finally the new bonds, C-1- C-6 and C-4 - C-5, are established (cusp catastrophes). Both magnitude and regularity of the electron redistribution, happening during reactions enable us to distinguish two effects: (1) the "ring effect", where a large amount of electron density is regularly transferred from double CdC bonds to intermolecular regions and single C - C bonds, (2) the " side chain effect" susually weaker and irregular-involving substituents' bonds. In the transition structure, well formed bonding basin V(C-1, C-6), is observed only for the PDA(...)BH(3)/MVE reaction. For other reactions only the nonbonding basins: V(C-1) and V(C-6), are found in the interaction region (C1C6)-C-....