AN INVESTIGATION OF SUBSTITUENT EFFECTS ON THE CONFORMATION OF 9,10-DIHYDROANTHRACENES BY MOLECULAR MECHANICS CALCULATIONS AND X-RAY STRUCTURE-ANALYSIS

MM2 calculations for a series of 9-R-9,10-dihydroanthracenes (DHAs) and cis-9-R-10-R'-9,10-dihydroanthracenes were performed to investigate the effect of substituents on the geometry of the central ring in 9,10-dihydroanthracenes. In general, the observed variation in central ring folding as compared to 9,10-dihydroanthracene itself is rather small for the monosubstituted compounds, but is quite pronounced for the cis-9-R-10-R' disubstituted deriviatives. These variations in conformational preferences are rationalized in terms of the 1,4 transannular effect of substituents across the ring, and by the peri interaction between the substituents and the neighboring aromatic hydrogens. X-ray structure determinations on two monosubstituted compounds, 9-Me-DHA and 9-SiMe3-DHA, and two disubstituted compounds, cis-9-SiMe3-10-Me-DHA and 9-SiMe3-10-iPr-DHA, provide strong support for the validity of the calculations in predicting optimum structures for monosubstituted and cis-disubstituted DHAs.