Dynamic Stereochemistry of Monobridged Tetraarylethene Naphthologues. Variations of Bistricyclic Aromatic Enes

Dynamic ¹HNMR experiments on monobridged tetraarylethenes (bistricyclic aromatic enes devoid of one bridge, BAE-1s) revealed the conformational processes in 13-15 (BAE-1s, X=S, Se, and Te) and in their respective thiiranes. The experimental energy barriers ΔG^≠_c were found to be 59.0, 72.2, and 67.9kJmol^-1 for 13, 14, and 15, and 63.4, 64.4, and 61.3kJmol^-1 for 26, 27, and 28, respectively. In contrast to the bis(α-naphthyl) derivatives 13-15, no dynamic processes were detected for the bis(β-naphthyl) derivatives 16-20. These results are in agreement with the mechanisms of enantiomerization of 13-15 derived from DFT calculations. The folded-twisted global minima conformers of 13-15 undergo enantiomerization via inversion of the central ring, accompanied by rotation of the α-naphthyl substituents. The B3LYP/SDD free energy barriers, ΔG₂₉₈, for the enantiomerizations of BAE-1s 13-15 are 57.4, 66.6, and 80.2kJmol^-1, respectively. The minima and transition-state conformations are chiral and the enantiomerization may proceed via chiral pathways.