Enantioselective Photochemical Generation of a Short‐Lived, Twisted Cycloheptenone Isomer: Catalytic Formation, Detection, and Consecutive Chemistry

Abstract Cyclohept‐2‐enone‐3‐carboxylic acid undergoes a photochemical isomerization from its cis ‐ to its trans ‐form either upon direct irradiation ( λ = 366 nm) or in the presence of a triplet sensitizer ( λ = 459 nm). The intermediate chiral trans ‐isomer was detected by step‐scan FTIR, displaying a lifetime of 130 µs (r.t., CH 2 Cl 2 ). Ensuing Diels–Alder reactions of the trans ‐isomer occurred smoothly and produced chiral trans ‐fused cycloaddition products (14 examples, 24%–98% yield). Benzylation led to esters, which were separated by chiral HPLC and which were employed to evaluate a possible enantioselective reaction course. It was discovered that a chiral phosphoric acid with a pendant sensitizing group induces a notable enantioselectivity in the photoisomerization step. The planar chirality of the trans ‐cycloheptene translates into point chirality in the Diels–Alder reaction (seven examples, up to 38% ee ). Computational studies suggest that the chiral conformation of the cis ‐isomer adopted within the assembly to the chiral phosphoric acid induces the enantioselectivity in a one‐bond flip (OBF) toward the trans ‐isomer. Trajectory surface hopping (TSH) simulations showed exemplarily how a chiral trans ‐cyclohept‐2‐enone is formed from a chiral cis ‐conformer. For the Diels–Alder reaction, a weak ground state selectivity was found to attenuate the enantioselectivity achieved in the photochemical step.