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

The conformational spaces and dynamic stereochemistry of representative ove rcrowded homomerous bistricyclic aromatic enes (1, X = Y) are investigated, applying the semiempirical PM3 method. The experimental energy barriers fo r E,Z isomerizations, enantiomerizations, and conformational inversions of 1 and related compounds, derived from DNMR and other kinetic studies, are r eviewed, This study focuses on the analysis of the minima, transition state s, and dynamic mechanisms of the conformational isomerizations of bifluoren ylidene (2), dixanthylene (3), dithioxanthylene (9), and bi-5H-dibenzo[a,d] cyclohepten-5-ylidene (11). The four representative bistricyclic enes diffe r in the sizes of their central rings and in their bridging groups. The mec hanisms of the interconversions of the twisted, anti-folded, and syn-folded conformations and of thermal E,Z isomerizations (topomerizations), enantio merizations, and conformational inversions (including combinations) are elu cidated. The calculated energy barriers for E,Z topomer izations of 2, 3, 9 , and 11 are 25.3, 16.4, 24,3, and 39.3 kcaL/mel, respectively. The corresp onding 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 mechanis ms 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.