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]bicyclo[6.6.0]tetradecane, and Their Alkyl Derivatives.

Abstract

The barrier for enantiomerization of tribenzo[a,c,f]cyclooctane (1a) as acquired from dynamic (1)H NMR experiments was found to be DeltaG() = 19.5-20.1 kcal mol(-)(1), and DeltaG() = 17.2-17.6 kcal mol(-)(1) for its alkylated derivatives (1b-e) at the coalescence temperatures. Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane (2a) and its tetraethyl derivative (2b) were separated to their enantiomers by chiral HPLC. The kinetic values for the enantiomerization were obtained by following the racemization rate of the pure enantiomer. Barriers of DeltaG()(596) = 49.6 kcal mol(-)(1) and DeltaG()(561) = 46.3 kcal mol(-)(1) for 2a and 2b, respectively, were obtained. Strong solvent and temperature effects on the (1)H NMR chemical shifts were observed. X-ray crystallography revealed torsional conformations for 1a, 1b, 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 strain energies and smaller biphenyl dihedral angles for the bulky alkyl substituents, which correspond to their lower DeltaG() values.