THEORETICAL-STUDIES OF THE [1,5] SIGMATROPIC HYDROGEN SHIFT IN CYCLOPENTADIENE, PYRROLE, AND PHOSPHOLE

The [1,5] sigmatropic hydrogen shifts in cyclopentadiene, pyrrole, and phosphole were examined using ab initio calculations at the MP2/6-31G* level. The activation enthalpies (corrected for 298 K) for these shifts were computed to be 26.4 kcal mol-1 for cyclopentadiene, 44.5 kcal mol-I for 1H-pyrrole to 2H-pyrrole, 26.3 kcal mol-1 for 2H-pyrrole to 3H-pyrrole, 26.2 kcal mol-1 for 3H-pyrrole to 3H-pyrrole, 16.0 kcal mol-1 for 1H-phosphole to 2H-phosphole, 26.9 kcal mol-1 for 2H-phosphole to 3H-phosphole, and 24.7 kcal mol-1 for 3H-phosphole to 3H-phosphole. These barrier heights are interpreted in terms of relative stabilities of the isomers, their aromatic character, and their electronic distributions. The calculated barriers are used to understand the reactions of the pyrroles and phospholes.