HINDERED CONFORMATIONAL ISOMERIZATION OF 9,10-DIHYDRO-9,9-DIMETHYL-10-METHYLENEANTHRACENES

Abstract

While the n.m.r. spectrum of 10,10-dimethyl-9-methylene-9,10-dihydroanthracene (I) shows the geminal methyl group absorption as a sharp singlet the spectrum of the dibromo derivative, 10,10-dimethyl-9-dibromomethylene-9,10-dihydroanthracene (II) shows the methyl absorptions as two sharp well-separated peaks at room temperature which coalesce at 91°. 10,10-Dimethyl-9-phenylbromomethylene-9,10-dihydroanthracene (III), and the methyl ester (V) of IV show a broad geminal methyl spectrum at room temperature which separates to a doublet at lower temperatures and sharpens to a singlet at higher temperatures. Rate constants for the first-order processes responsible for the change in spectrum of II, III, and V have been calculated at the coalescence temperatures to be 57 (364 °K), 35 (305 °K), and 61 (300 °K) sec−1, respectively. The ΔH≠'s were used to extrapolate the rates to 305° to give values of 1, 40, and 100 sec−1, respectively. The process being studied is inferred to be the equilibrium between two boat conformations of the center ring in the dihydroanthracene system, rapid interconversion leading to identical environments for the two methyl groups. A comparison with the geometrically similar o,o′-disubstituted biphenyl racemization gives support for this explanation. A number of compounds with a proton and one substituent on the methylene carbon atom of I (substituents: bromine, chlorine, phenyl, carboxy, carbomethoxy, phenylmercapto) and also 10,10-dimethyl-9-phenylcarbomethoxymethylene-9,10-dihydroanthracene (XII) showed a single methyl absorption at room temperature. The methyl spectrum of the mono bromo compound VI did not broaden at temperatures down to 246 °K.