Crystal structure, computational study, and Hirshfeld analysis of <i>exo</i>-1,2,3,5-tetraphenyl-1a',9b'-dihydrospiro[bicyclo[3.1.0]hexane-6,1′-cyclopropa[<i>l</i>]phenanthren]-2-en-4-one

The reaction of dibenzonorcarynyliden(e/oid) with phencyclone was recently reported to give a congested spiro-pentane with <i>endo</i> 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 <i>exo</i> 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 <i>P</i>2₁/<i>n</i> (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 <i>endo</i> spiro-pentane diastereomers from phencyclone and tetra-cyclone are both more stable than the corresponding <i>exo</i> 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 <i>endo</i> 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 <i>endo</i> diastereomer in favor of the less thermodynamically stable <i>exo</i> isomer.