Kinetic Stabilization of the [1.1]Paracyclophane System: Isolation and X‐ray Structural Analysis of a [1.1]Paracyclophane Derivative and Its Interconversion with the Transannular Adduct

Abstract Intentionally designed kinetic stabilization of the [1.1]paracyclophane skeleton has been achieved by multiple substitution of the aromatic rings with trimethylsilylmethyl and N , N ‐dimethylcarbamoyl groups, which serve to shield the proximate bridgehead carbon atoms sterically from access by other reagents. The bis(Dewar benzene) precursor ( 1 a ) has been prepared in essentially the same manner as previous derivatives—starting from the photocycloaddition of 1,4‐bis(trimethylsilyl)‐2‐butyne to octahydroindacene‐1,5‐dione—except for a few critical modifications described in the text. Substituted [1.1]paracyclophane ( 2 a ), photochemically generated from the precursor, is indefinitely stable at 50 °C and suffers decomposition only by 8 % after 2 h at 100 °C in degassed n ‐decane, demonstrating its greatly improved kinetic stability compared to previous [1.1]paracyclophanes. Since 2 a undergoes efficient photochemical transformation into the transannular addition product 3 a , irradiation of 1 a tends to produce a mixture of products consisting mainly of 3 a . Compound 3 a , however, reverts thermally to 2 a in a process of half life 40 min at 55 °C; the activation parameters for this process are Δ H ≠ =21.1±0.8 kcal mol −1 and Δ S ≠ =−10.5±2.6 cal K −1  mol −1 . Thus, on heating 3 a in benzene and cooling the resultant solution, 2 a is obtained as orange‐red crystals. X‐ray crystallographic analysis of 2 a reveals benzene rings bent to the highest degree ever reported for a paracyclophane, with their face‐to‐face arrangement in unusually close proximity. The shortest nonbonding interatomic distance is 2.376 Å; less than the sum of the van der Waals radii by more than 1.0 Å. The generation of related substituted [1.1]paracyclophanes and their kinetic stabilities are also reported.