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.
Staley
S W
SW
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Kehlbeck
J D
JD