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

A series of C5 monomethyl (1a–1f) and pentamethylcyclopentadienes (2a–2f) bearing stereogenic C5 heteroatom substituents (-NH 2 , -OH, -OCH 3 , and -SCH 3 ) 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 C s symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. …