CBS-QB3 computational examination of substituent effects on the interconversion of 1,3,5-cyclooctatriene and bicyclo[4.2.0]-2,4-octadiene

The position of equilibrium between 7-substituted and 7,8-disubstituted 1,3,5-cyclooctatrienes (1) and the corresponding valence isomeric bicyclo[4.2.0]-2,4-octadienes (2) is highly sensitive to the nature of substituents. In particular, the difference between trans and cis-7,8 isomers in this regard has never been explained. These differences have been clarified by computational means. The CBS-QB3 hybrid quantum chemical computational method reproduces the experimental free energy differences between a series of cyclooctatrienes (1) and the corresponding valence isomers (2) with excellent accuracy; the MAD and root mean square (rms) differences are 0.54 kcal/mol and 0.58 kcal/mol, respectively. The energy barriers between several derivatives of 1 and 2 were computed with good accuracy (MAD=1.3 kcal/mol and rms=1.5 kcal/mol) by the same procedure. The dihedral angle between the substituents increases upon electrocyclic conversion of trans-7,8-disubstituted cyclooctatrienes to the bicyclic isomer and decreases for the corresponding cis isomers. This differential effect explains both the higher bicyclic proportion at equilibrium and the faster rate of cyclization in the trans series compared to cis.