Toward a switchable molecular rotor. Unexpected dynamic behavior of functionalized overcrowded alkenes

In an approach toward a photochemically bistable molecular rotor the synthesis of cis-1a and trans-1b isomers of 2-(2,6-dimethylphenyl)-9-(2',3'-dihydro-1'H-naphtho[2,1-b]thiopyran-1'-ylidene)-9H-thioxanthene (1), being sterically overcrowded alkenes functionalized with an o-xylyl group as a rotor, is described. The key steps in the synthesis are a Suzuki coupling to attach the xylyl moiety and a diazo-thioketone coupling with subsequent desulfurization to introduce the central olefinic bond in 1. The X-ray structure of cis-1a revealed an anti-folded helical conformation. Dynamic NMR studies on both isomers were performed, to elucidate the kinetics of their rotation processes and to investigate the possibility to control the biaryl rotation by photochemical cis-trans isomerization. A rotation barrier was found by coalescence spectroscopy for cis-1a: ΔGc‡ = 22 ± 1 kcal mol-1 in DMSO-d6. 2D exchange spectroscopy (EXSY) showed barriers for cis-1a: ΔG303‡ 19.0 ± 0.2 kcal mol-1 and trans-1b: ΔG303‡ = 19.7 ± 0.2 kcal mol-1 in DMSO-d6, respectively. Molecular mechanics and semiempirical calculations support the unexpected higher barrier for trans-1b. Since the rotation barriers are different for the cis and trans isomer, it can be concluded that control of a second mechanical effect, e.g. the rate of rotation of an attached biaryl rotor, is feasible in a photochemically switchable system