SYN, ANTI ISOMERIZATION OF 5,5'-BIS-5H-DIBENZO[A,D]CYCLOHEPTEN-5-YLIDENE

syn-5,5'-Bis-5H-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 DELTAG(inv)# (174-degrees-C) = 36.4 kcal/mole. The high-energy barrier for syn --> anti isomerization of 2 is compared with the corresponding inversion barrier of 5H-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.