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

Abstract

Dynamic 1 HNMR experiments on monobridged tetraarylethenes (bistricyclic aromatic enes devoid of one bridge, BAE-1s) revealed the conformational processes in 13-15 (BAE-1 s, X = S, Se, and Te )a nd in their respective thiiranes. The experimental energy barriers DG ¼ 6 c were found to be 59.0, 72.2, and 67.9 kJ mol À1 for 13, 14,a nd 15,a nd 63.4, 64.4, and 61.3 kJ mol À1 for 26, 27,a nd 28,r espectively. In contrast to the bis(a-naphthyl) derivatives 13-15,n od ynamic processes were detected for the bis(b-naphthyl) derivatives 16-20.T hese results are in agreement with the mech-anismso fe nantiomerization of 13-15 derived from DFT calculations. The folded-twisted global minima conformers of 13-15 undergo enantiomerization via inversiono ft he central ring, accompanied by rotationo ft he a-naphthyl substituents. The B3LYP/SDD free energy barriers, DG 298 ,f or the enantiomerizationso fB AE-1s 13-15 are 57.4, 66.6, and 80.2 kJ mol À1 ,r espectively.T he minimaa nd transition-state conformations are chiral and the enantiomerization may proceed via chiral pathways.

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs**

“…The reported twist angle of the substance is only 7.4°(Figure 3b) and relationships between its conformation and properties, especially optical, have not been well studied. [7] In the current effort, we designed two MAEs that contain a 10-methyleneanthrone moiety and two appended bulky aromatic (9-anthryl or mesityl) rings. The array of groups in these substances cause steric repulsion between the tricyclic anthrone unit and the two bulky aromatic rings, which leads to structural frustration of the C=C bond associated with the formation of conformations having large fold and twist angles (Figure 3c).…”