Conformational space and dynamic stereochemistry of overcrowded homomerous bistricyclic aromatic enes - A theoretical study

The conformational spaces and dynamic stereochemistry of representative overcrowded homomerous bistricyclic aromatic enes (1, X = Y) are investigated, applying the semiempirical PM3 method. The experimental energy barriers for E,Z isomerizations, enantiomerizations, and conformational inversions of 1 and related compounds, derived from DNMR and other kinetic studies, are reviewed. This study focuses on the analysis of the minima, transition states, and dynamic mechanisms of the conformational isomerizations of bifluorenylidene (2), dixanthylene (3), dithioxanthylene (9), and bi-5H-dibenzo[a,d]cyclohepten-5-ylidene (11). The four representative bistricyclic enes differ in the sizes of their central rings and in their bridging groups. The mechanisms of the interconversions of the twisted, anti-folded, and syn-folded conformations and of thermal E,Z isomerizations (topomerizations), enantiomerizations, and conformational inversions (including combinations) are elucidated. The calculated energy barriers for E,Z topomerizations of 2, 3, 9, and 11 are 25.3, 16.4, 24.3, and 39.3 kcal/mol, respectively. The corresponding barriers for enantiomerizations or conformational inversions are 4.9, 15.9, 24.3, and 37.6 kcal/mol, respectively. In most cases, the agreement with experimentally determined values is within 1-3 kcal/mol. New mechanisms are proposed for the E,Z isomerizations and conformational inversions of anti-folded 3, 9, and 11, involving low-symmetry folded/twisted transition states and the respective syn-folded intermediates.