Theoretical studies of the (1,5) sigmatropic hydrogen shift in cyclopentadiene, pyrrole, and phosphole

The chemistry of pyrroles and phosphole are examined computationally. The analytical examination of 2H- and 3H-pyrroles reveal interconversion, which is proved by the close energy levels of the two, with an obstacle of 26.3 kcal mol (-1). A low barrier of 16.0 kcal mol (-1) is observed in the process of transforming 1H-phosphole to a 2H-phosphole. The conformation and electron density diffusions of various transition states, through which the (1,5) hydrogen translocation of cyclopentadiene, pyrrole, and phosphole take place, resemble one another. A concerted (1,5) hydrogen shift is observed as the common mechanism for all the migrations by this deduction.