Overcrowded naphthologs of mono-bridged tetraarylethylenes: Analogs of bistricyclic aromatic enes

The naphthalogous mono-bridged tetraarylethylenes 9,9′-di-(1-naphthylmethylene)-9H-fluorene (5) and 9,9′-di-(1-naphthylmethylene)-9H-xanthene (6), analogs of bifluorenylidene (1) and bixanthenylidene (2), have been synthesized and their molecular and crystal structures have been determined. Ene 5 has been prepared by two alternative synthetic routes. The molecular structures of 5 and 6 show that each of these enes has very small twist around the central double bond, but the two naphthalene rings in both 5 and 6 are highly twisted. According to the NMR study, 5 and 6 in solution adopt conformations which are similar to those found by X-ray crystal structure analysis. The notable upfield shifts of H¹ and H⁸ (6.11 and 6.83 ppm, respectively) and H² and H⁷ (6.70 and 6.44 ppm, respectively) in 5 and 6 are due to the shielding caused by the nearly orthogonally twisted naphthalene rings. The B3LYP/6-31G(d) calculations of 5, 6, and their 2-naphthyl and phenyl analogs have been performed. In the 1-naphthyl series, the more efficient conjugation between the naphthyl substituents and the central C=C and the overcrowding due to the peri-hydrogen atoms lead to higher twists of the naphthyl groups and to lower twists of the central C=C. In the 2-naphthyl series, the opposite effects are noted.