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 ΔH≠=21.1±0.8 kcal mol−1 and ΔS≠=−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.