23697-18-9Relevant academic research and scientific papers
Effect of para substituents on the rate of bond shift in arylcyclooctatetraenes
Staley,Kehlbeck
, p. 5572 - 5579 (2001)
The rate constants for bond shift (kBS) in phenylcyclooctatetraene (1b) and its p-nitro and p-methoxy analogues (1a and 1c, respectively) in THF-d8 were determined by dynamic NMR spectrometry to be identical, but kBS is eight times greater at 280 K relative to 1b when the para substituent is cyclooctatetraenyldipotassium (22-/2K+). These results are discussed in the context of (a) possible, intrinsically small substituent effects (as determined by 13C chemical shifts in the ground state (GS)) for 1a-c and (b) differences in steric interactions and resonance stabilization between the ground and BS transition state (TS). The latter factor was modeled by employing HF/3-21G(*) ab initio molecular orbital calculations of the GS and ring inversion TS. It is concluded that kBS is unchanged in 1a-c because the potentially greater π interaction in the BS TS is counterbalanced by a greater degree of twist between the aryl and COT rings resulting from increased steric hindrance relative to the GS. However, π interaction assumes a greater importance in the TS of 22-/2K+ owing to a decreased HOMO-LUMO energy gap compared to 1a-c, particularly when the counterions are solvated. This causes a decrease in the inter-ring twist angle and, together, these changes are responsible for the observed increase in kBS in 22-/2K+. The effect of substituents on a possible contribution of heavy atom tunneling to the reaction mechanism is also discussed.
