Synthesis of substituted cyclooctatetraenide dianions

Mechanism of a novel exchange process in alkali metal salts of 1,5-dicyclooctatetraenylnaphthalene dianion

A novel low-temperature intramolecular exchange was detected by (13)C NMR spectrometry in the Na(+) and K(+) salts of the title compound. The process causes the pairwise exchange in the dianion ring of C(2"), C(3"), and C(4") with C(8"), C(7"), and C(6"), respectively. The free energy of activation (DeltaG()(exch)) for the dipotassium salt (2(2-)/2K(+)) in THF-d(8) at 230 K is 12.6 kcal mol(-1). Two key questions are addressed: (1) Why are these carbons anisochronous and (2) what is the mechanism of exchange? NMR data for 1-cyclooctatetraenylnaphthalenedipotassium (3(2-)/2K(+)) as well as ab initio HF/3-21G(++) calculations for 3, 3(2-), and 3(2-)/2K(+) indicate that the nonequivalence is due to both slow rotation across a barrier at which the naphthalene and COT(2)(-) rings are approximately coplanar and slow inversion of the neutral COT ring. This results in the noteworthy circumstance of diastereotopic carbons being observed in a molecule that does not possess either a stereogenic or a prostereogenic center. Comparison of DeltaG()(exch) and DeltaG(++)(BS) for 2(2-)/2K(+) with the corresponding values for 2(2-)/2Na(+) and 2(2-)/2Li(+) and of DeltaG(++)(exch) with DeltaG(++) for ring inversion in 1,4-dicyclooctatetraenylnaphthalene leads to the conclusion that COT(2-) ring rotation and COT ring inversion both contribute to exchange in 2(2-)/2K(+) in a 3:1 ratio, but that exchange occurs exclusively by ring rotation in 2(2-)/2Li(+). The latter result is attributed to looser ion pairing in the dilithium (and disodium) salts.

Effect of para substituents on the rate of bond shift in arylcyclooctatetraenes

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.