Effect of Para Substituents on the Rate of Bond Shift in Arylcyclooctatetraenes
| Autor: | Joanne D. Kehlbeck, Stuart W. Staley |
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| Rok vydání: | 2001 |
| Předmět: | |
| Zdroj: | The Journal of Organic Chemistry. 66:5572-5579 |
| ISSN: | 1520-6904 0022-3263 |
| DOI: | 10.1021/jo010526h |
| Popis: | 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 (13)C 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 pi 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, pi 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. |
| Databáze: | OpenAIRE |
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