A Molecular Rotor That Probes the Helical Inversion of Stiff-Stilbene

Abstract

Probing the inversion kinetics of a molecular helix is inherently a challenging task. We demonstrate herein that a fastrotating pentiptycene component could function as an external NMR probe to afford the kinetic information on the inversion of a neighboring helical stiff-stilbene unit.

Push‐Pull Stiff‐Stilbene: Proton‐Gated Visible‐Light Photoswitching and Acid‐Catalyzed Isomerization

“…Desymmetrization of stiff‐stilbene has previously been achieved through mixed McMurry[ 4d , 4f ] and Barton‐Kellogg [19] reactions. In 1992, Lapouyade and co‐workers first described compound ( E )‐ 1 , which was obtained via a mixed coupling of 5‐bromo‐1‐indanone and 5‐dimethylamino‐1‐indanone followed by cyanation using copper cyanide.…”

On–off switching of the correlated motion in a rotation‐inversion dual‐mode molecular system

“…The previously reported ortho ‐Br‐substituted stiff ‐stilbene ( E )‐ 2 was reacted with the amine 3 via Pd‐catalyzed CN coupling followed by deprotection of the hydroxyl group with tetra‐ n ‐butylammonium fluoride to afford ( E )‐ 4 . [ 21 ] The amine 3 was in turn prepared by reacting the commercially available 3‐amino‐2,2‐dimethyl‐1‐propanol with triisopropylsilyl (TIPS) trifluoromethanesulfonate. When ( E )‐ 4 was treated with trifluoromethanesulfonic anhydride (Tf 2 O) in a basic condition at −78°C, the DMAZ group is formed, giving rise to the target compound ( E )‐ 1 .…”

“…The design of 1 follows our recent works on the stereodynamic NMR chemistry of stiff-stilbene-based rotation-inversion dual-motion molecular systems. [21,22] In principle, the four diastereotopic NMR signals of an H-shaped rotator could sequentially afford the kinetic information of the rotation and the inversion as long as the kinetics among k R , k I , and k nmr can be tuned by temperature such that the condition of types A and B or C and D in Figure 1b can be achieved. According to our recent work, [22] systems with a dialkylamino rotator are good candidates for having both the C N rotation and the stiff-stilbene inversion timescales suitable for VT NMR experiments.…”

“…In contrast, a planar C 2 ‐symmetrical rotator could work as an NMR probe only when both the rotation and the inversion are slower than the NMR timescale (types F and H), and under this condition, the faster motion mode is probed. Our previous works illustrated the types A, F, and H by using a pentiptycene‐acetylene (H‐shaped) [ 21 ] or dimethylamino (C 2 ‐planar) [ 22 ] rotator. We envisaged that an amino‐based rotator having both the H‐shaped and C 2 ‐planar protons would not only afford the kinetics of the two motion modes but be also able to identify whether the CN rotation and the CC inversion are correlated.…”

“…We recently employed ortho-substituted stiff-stilbenes to construct molecular systems with the feature of rotation-inversion dual-motion mode (Figure 1a). [21,22] Because of the steric hindrance caused by the ortho-substituent, the stiff-stilbene scaffold is twisted in both the E and Z isomers with a helical sense denoted as (M, M) or (P, P). The barrier for the thermal helical inversion between the (M, M) and the (P, P) forms depends on the nature of the ortho-substituent, which functions as a rotator.…”