syn, Anti isomerization of 5,5′-Bis-5 H-dibenzo[a,d]cyclohepten-5-ylidene

syn-5,5′-Bis-5 H-dibenzo[a,d]cyclohepten-5-ylidene (2syn), an overcrowded bistricyclic aromatic ene with central seven-membered rings, undergoes a thermal isomerization to the anti stereoisomer (2anti) at elevated temperatures, with ΔG_inv^≯ (174°C)=36.4 kcal/mole. The high-energy barrier for syn → anti isomerization of 2 is compared with the corresponding inversion barrier of 5 H-dibenzo[a,d]cycloheptene derivatives and interpreted in terms of a lower degree of overcrowding in 2 relative to bistricyclic enes (1) with central five- and six-membered rings. The overcrowding features of 2syn, 2anti, and their anti-10,11,10′,11′-tetrahydro derivative (3) is analyzed, and a mechanism for the syn → anti isomerization of 2 is proposed.