STERIC AND ELECTRONIC CONTROL OF DYNAMIC PROCESSES IN ARYL-BRIDGED DICYCLOOCTATETRAENES AND THEIR DIANIONS

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 ra te constants for bond shift and intramolecular charge transfer (k(c.t. )) were determined for 1(2-)/2K(+) and 2(2-)/2K(+). These processes we re modeled by HF/3-21G(()) ab initio molecular orbital calculations o f 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 st ate of 2. Contrariwise, k(b.s.) and k(c.t.) are 1.7 and 166 times grea ter, respectively, at 280 K for 1(2-)/2K(+) than for 2(2-)/2K(+). Thes e differences are attributed to less twisting and therefore greater pi delocalization between the cyclooctatetraenyl rings and the aryl ring in the bond shift and charge-transfer transition states of 1(2-) comp ared to 2(2-). The greater difference between 12- and 22- 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 trans ition state.