Flexibility vs rigidity of singly and doubly tethered biphenyls: Structure, dynamic stereochemistry, and resolution of tribenzo[a,c,f]cyclooctane, tetrabenzo[a,de,h,kl]bicycle[6.6.0]tetradecane, and their alkyl derivatives

The barrier for enantiomerization of tribenzo[a,c,f]cyclooctane (1a) as acq uired from dynamic H-1 NNIR experiments was found to be Delta G(double dagg er) = 19.5-20.1 kcal mol(-1), and Delta G(double dagger) = 17.2-17.6 kcal m ol(-1) for its alkylated derivatives (Ib-e) at the coalescence temperatures . Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane (2a) and its tetraethyl de rivative (2b) were separated to their enantiomers by chiral HPLC. The kinet ic values for the enantiomerization were obtained by following the racemiza tion rate of the pure enantiomer. Barriers of Delta G(596)(double dagger) = 49.6 kcal mol(-1) and Delta G(561)(double dagger) = 46.3 kcal mol(-1) for 2a and 2b, respectively, were obtained. Strong solvent and temperature effe cts on the 1H NMR chemical shifts were observed. X-ray crystallography reve aled torsional conformations for la, Ib, and 2a with dihedral angles of 57. 8 degrees, 56.1 degrees/58.2 degrees (two conformations), and 57.7 degrees, respectively. Semiempirical calculations (AM1) reveal higher relative stra in energies and smaller biphenyl dihedral angles for the bulky alkyl substi tuents, which correspond to their lower Delta G(double dagger) values.