Catalytic Effect of Organoaluminium Chloride Reagents on the Dienophilic Reactivity of Indolizine and 2-Phosphaindolizine towards [2+4] Cycloaddition: A DFT Investigation

The catalytic effect of the aluminium chloride reagents R1AlCl2 (R1 = Cl, Me, Et) on the dienophilic reactivity of the >C=C< functionality of indolizine and >C=P- functionality of 2-phosphaindolizine in their Diels-Alder (DA) reaction with 1,3-butadiene has been investigated theoretically at the DFT (B3LYP/6-31+G**) level. The activation barriers of the DA reactions of the uncomplexed dienophiles with 1,3-butadiene are quite high. Co-ordination of the aluminium reagent to the carbonyl group of indolizine or 2-phosphaindolizine raises the activation barriers further. On the other hand, co-ordination of the aluminium catalyst to the σ2, λ3-P atom of 2-phosphaindolizine lowers the activation barrier making the DA reaction possible. Electronic structure calculations of indolizine, 2-phosphaindolizine and their complexes to the aluminium catalyst indicate, that this behavior can be explained by a combination of energetic and stereoelectronic influences of the catalyst on the lowest unoccupied molecular orbital (LUMO) of the dienophiles. Co-ordination of the aluminium catalyst to the dienophile lowers the orbital energy of the LUMO in any case making it energetically more feasible for the normal electron demand DA-reactions. However, O co-ordination polarizes the spatial distribution of the LUMO away from the dienophilic center while P co-ordination increases the orbital contribution of the >C=P fragment to the LUMO.