Determination of ring inversion barriers for 1H,3H-naphtho[1,8-cd]-pyran and its methyl substituted derivatives by molecular mechanics calculations and dynamic nuclear magnetic resonance spectroscopy

Molecular mechanics calculations with Allinger's MMPI program indicate that substitution of the methylene hydrogens in 1H,3H-naphtho[1,8-cd]pyran with methyl groups lowers the barrier to ring inversion. This seems to arise from increasing steric interactions in the ground state, whereas the energy of the transition state is less affected. The transition state for the inversion process has been calculated to possess C_2υ, symmetry. Increased steric interactions on the introduction of methyl groups result in a more planar (i.e less puckered) ground state with an increased C-O-C bond angle. Dynamic ¹H n.m.r. spectroscopy of 2,2-dimethyl-1H,3H-naphtho[1,8-cd]pyran reveals a △G^† value for ring inversion of 6.9 kcal mol^-1 at -133°. For the four other naphthopyrans studied by dynamic n.m.r. only upper limits for the inversion barrier are obtained.