Steric and Electronic Control of Dynamic Processes in Aryl-Bridged Dicyclooctatetraenes and Their Dianions

Abstract

The rate constants for ring inversion ( k r.i. ) and bond shift ( k b.s. ) in 1 and 2 were determined by dynamic NMR spectrometry while the rate constants for bond shift and intramolecular charge transfer ( k c.t. ) were determined for 1 2 - /2K + and 2 2 - /2K + . These processes were modeled by HF/3-21G ( * ) ab initio molecular orbital calculations of the ground states and of several transition states for 3, 4, 3 2 -, 4 2 -, 3 2 - /2K +, and 4 2 - /2K + . The results indicate that k r.i. and k b.s. are ca. 2.5 times greater (at 240 and 280 K, respectively) for 2 compared to 1 due to larger steric repulsions in the ground state of 2 . Contrariwise, k b.s. and k c.t. are 1.7 and 166 times greater, respectively, at 280 K for 1 2 - /2K + than for 2 2 - /2K + . These differences are attributed to less twisting and therefore greater π delocalization between the cyclooctatetraenyl rings and the aryl ring in the bond shift and charge-transfer transition states of 1 2 - compared to 2 2 - . The greater difference between 1 2 - and 2 2 - for k c.t. compared to k b.s. is postulated to result from looser ion pairing in the charge-transfer transition state relative to the bond shift transition state.