Revisiting hydrogen [1,5] shifts in cyclopentadiene and cycloheptatriene as birnolecular reactions

Hydrogen [1,5] shifts are pericyclic reactions and take place typically in 1,3-pentadiene. However, because of structure restriction, the symmetry-allowed thermal reactions of 1,3-cyclopentadiene (CPD) and 1,3,5-cycloheptatriene (CHT) suffer large energy barriers, Delta U-double dagger = +26.9 kcal/mol and Delta L-double dagger = +37.5 kcal/mol by density-functional theory (B3LYP/6-31G*) calculations, respectively. This theoretical study has shown that exo [4+2] and [6+4] cycloadduct dimers involve novel hydrogen-shift channels. After hydrogen migration, one-center adducts are obtained, which undergo Cope rearrangements leading to the second one-center adducts. From the intermediates, reverse routes lead to CPD and CHT with [1,5] migrated hydrogens. A correlation between cycloadditions and [1,5] and [3,3] sigmatropic rearrangements in pericyclic reactions is also discussed.