Effect of Para Substituents on the Rate of Bond Shift in Arylcyclooctatetraenes

• Published in: Journal of organic chemistry 2001-08, Vol.66 (16), p.5572-5579
• Main Authors: Staley, Stuart W, Kehlbeck, Joanne D
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo010526h

The rate constants for bond shift (k BS) in phenylcyclooctatetraene (1b) and its p-nitro and p-methoxy analogues (1a and 1c, respectively) in THF-d 8 were determined by dynamic NMR spectrometry to be identical, but k BS is eight times greater at 280 K relative to 1b when the para substituent is cyclooctatetraenyldipotassium (2 2-/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 k BS 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 2 2-/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 k BS in 2 2-/2K+. The effect of substituents on a possible contribution of heavy atom tunneling to the reaction mechanism is also discussed.