Ab initio studies of [1,5]-H shifts: Pentadiene and beyond

Ab initio calculations of the [1,5]-H shift, in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G* basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1, 1',2,3,4,5,5'-heptafluoropenta-1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene II, and (1Z,3E)-1,3,5-trifluaropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the pi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.