INTERNAL-ROTATION INVOLVING A PREEQUILIBRIUM RING INVERSION

3-Mesitylnaphthopyrans exhibit slow or locked internal rotation on the NMR time scale at ambient temperatures. Conformational analysis demonstrates that ground-state mesityl preferentially occupies the equatorial position of the nonplanar pyran ring and that rotation is blocked in this state. However, rotation is relatively unhindered in the higher energy axial state. A transannular substituent effect on the barrier height suggests that mesityl site exchange is a multistep process involving a preequilibrium inversion of the pyran ring followed by rotation of mesityl in the axial state and final relaxation to the site-exchanged equatorial ground state. © 1979, American Chemical Society. All rights reserved.