Crystal structure, computational study, and Hirshfeld analysis of...

The reaction of dibenzonorcarynyliden(e/oid) with phencyclone was recently reported to give a congested spiro-pentane with endo stereochemistry. Herein we report that, in sharp contrast, an analogous reaction using tetra-cyclone, instead of phencyclone, gives the highly crowded title spiro-pentane but with exo stereochemistry as determined by X-ray crystallography. This new tetra-cyclone adduct (C₄₄H₃₀O) crystallizes upon slow evaporation from hexa-nes/ethyl acetate in the monoclinic crystal system and P2₁/n (No. 14) space group. It has one mol-ecule in the asymmetric unit and four mol-ecules per unit cell. DLPNO-CCSD(T)/def2-TZVP//B3LYP/def2-SVP calculations indicate that the endo spiro-pentane diastereomers from phencyclone and tetra-cyclone are both more stable than the corresponding exo forms by 6.68 and 5.35 kcal mol^-1, respectively. As noted previously in the phencyclone system, favorable π-stacking inter-actions between the two flat biphenyl moieties in the product and transition state may lead to the preferential formation of the endo diastereomer. However, the ability of the phenyl rings in the 3,4-position of the tetra-cyclone component to rotate could introduce destabilizing steric inter-actions in the transition state that hinder formation of the endo diastereomer in favor of the less thermodynamically stable exo isomer.