TOWARD A SWITCHABLE MOLECULAR ROTOR - UNEXPECTED DYNAMIC BEHAVIOR OF FUNCTIONALIZED OVERCROWDED ALKENES

In an approach toward a photochemically bistable molecular rotor the S ynthesis of cis-la and trans-1b isomers of aphtho[2,1-b]thiopyran-1'-y lidene)-9H-thioxanthene (1), being sterically overcrowded alkenes func tionalized with an o-xylyl group as a rotor, is described. The key ste ps in the synthesis are a Suzuki coupling to attach the xylyl moiety a nd a diazo-thioketone coupling with subsequent desulfurization to intr oduce the central olefinic bond in 1. The X-ray structure of cis-la re vealed an anti-folded helical conformation. Dynamic NMR studies on bot h isomers were performed, to elucidate the kinetics of their rotation processes and to investigate the possibility to control the biaryl rot ation by photochemical cis-trans isomerization. A rotation barrier was found by coalescence spectroscopy for cis-la: Delta G(c)(double dagge r)=22 +/- 1 kcal mol(-1) in DMSO-d(6). 2D exchange spectroscopy (EXSY) showed barriers for cis-la: Delta G(303)(double dagger) 19.0 +/- 0.2 kcal mol(-1) and trans-1b: Delta G(303)(double dagger)=19.7 +/- 0.2 kc al mol(-1) in DMSO-d(6), respectively. Molecular mechanics and semiemp irical calculations support the unexpected higher barrier for trans-lb . Since the rotation barriers are different for the cis and trans isom er, 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.