Emergent Atropisomerism Through Diels–Alder Reactions of Vinylallenes

Atropisomeric styrenes remain comparatively underdeveloped because configurational stability about the C(sp 2 )C(sp 2 ) bond is difficult to achieve and often relies on substantial preinstalled steric congestion. We report a conceptually distinct approach in which atropisomeric styrenes emerge through Diels–Alder reactions of substituted vinylallenes with N ‐methylmaleimide. In these reactions, cycloaddition‐induced geometric and hybridization changes generate a sterically hindered styrenyl atrop‐axis as a direct consequence of bond formation rather than through direct construction of a congested C(sp 2 )C(sp 2 ) bond. Simultaneously, the reactions generate stereogenic ring junction centers that convert the two orientations about the atrop‐axis into diastereomeric relationships, giving rise directly to distinguishable atropisomeric cycloadducts from achiral precursors. Experimental and computational studies reveal that atropisomerization barriers are governed not simply by steric congestion about the atrop‐axis, but by conformational flexibility of the fused bicyclic framework. Unexpectedly, gem ‐dimethyl substitution at the exocyclic alkene lowers the atropisomerization barrier through ground‐state destabilization arising from enhanced cyclohexene puckering and reduced distortion required to reach the rotational transition structure. Extension of this design principle enabled the preparation of configurationally stable atropisomeric styrenes that could be separated chromatographically and analyzed independently at ambient temperature.