DIELS-ALDER REACTIONS OF AZA-1,3-BUTADIENES AND PHOSPHA-1,3-BUTADIENES WITH ETHYLENE - AN ABINITIO STUDY

The Diels-Alder (DA) reactions of ethylene with 1,3-butadiene, (Z)-l-aza-1,3-butadiene, (E)-l-aza-1,3-butadiene, 2-aza-1,3-butadiene, (Z)-l-phospha-1,3-butadiene, (E)-l-phospha-1,3-butadiene, and 2-phospha-1,3-butadiene were examined using ab initio calculations. All structures, including the transition structures, were completely optimized at HF/6-31G* and MP2/6-31G* with single-point energies evaluated at MP4SDQ. Electron correlation beyond MP2 is needed to obtain reasonable activation energies, but the HF- and MP2-optimized structures are only marginally different. The activation energies for the DA reactions of the phosphabutadienes are small (20-28 kcal mol-1) and these reactions are very exothermic (about -43 kcal mol-1). In contrast, the DA reactions of the azabutadienes are less exothermic and have larger activation barriers. These results are consistent with frontier molecular orbital theory and experiments. The diastereomeric reactions of the (E)- and (Z)-l-aza-1,3-butadienes have activation barriers that differ by 3.27 kcal mol-1, which is attributed to the buildup of lone-pair-pi-bond repulsion during the DA reaction of the Z isomer. This lone-pair effect is smaller in the 1-phospha-l,3-butadiene case because the phosphorus lone pair, having large s-character, is less diffuse than the nitrogen lone pair. The degree of synchronicity of these reactions is discussed with the aid of electron density analysis.