Computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes. Evidence that n–π orbital mixing in the HOMO is not the source of the syn π-facial selectivity in Diels–Alder reactions

Abstract

A series of C5 monomethyl (1a–1f) and pentamethylcyclopentadienes (2a–2f) bearing stereogenic C5 heteroatom substituents (-NH2, -OH, -OCH3, and -SCH3) have been studied computationally (abinitio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds 1d–1f and 2d–2f, the conformations 3 and 4 of Cs symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. On the other hand, the twisted-anti conformers 5a and 5b are the most stable geometries of 1c and 2c. Analysis of the computed MO eigenvalues (orbital energies), MO eigenvectors (orbital coefficients), and the pe spectra of cyclopentadienes 2a–2f established that n–π orbital mixing is not important in the HOMO's of 1c–1d or 2c–2d. That the ionization energy of the HOMO is found to be virtually independent of the substituent at C5 in the series 2a–2f, provides support for the computational results. Because 2c–2d undergo Diels–Alder reactions selectively, syn to the heteroatom substituent, n–π orbital mixing in the HOMO cannot be the source of the π-facial selectivity observed for these compounds.