Overcrowded twisted bi-4H-cyclopenta[def]-phenanthren-4-ylidene versus bi-9H-fluorenylidene, syn pyramidalization

A crystal structure determination for bi-4 H-cyclopenta[def]-phenanthren-4-ylidene, 1, reveals a molecular conformation rather like that reported earlier for bifluorenylidene, 2. However, unexpectedly, the twist angle about the ethylenic linkage is substantially smaller in 1 (average values are 30.8 and 40.7°, respectively). Comparison of bonding geometry, combined with the results of molecular mechanics calculations for 1, demonstrates that syn pyramidalization can play an important role in determining a stable conformation for overcrowded, twisted aromatic enes. The average out-of-plane bending angles for the ethylenic carbon atoms of 1 and 2 are 8.6 and 4.4°, respectively.