Mechanism of Bond Rotation in 9-(1-Naphthyl)methylenecyclooctatrienyl Anions

Article abstract of DOI:10.1021/ja00405a003

The anti isomers of 9-(1-(4-X-naphthyl)-cis-bicyclo<6.1.0>nona-2,4,6-triene (6a-d) (X = OCH3, CH3, H and Cl, respectively) as well as syn isomers 11b and 11d were synthesized by employing three different routes.Treatment of 6a-d with lithium or potassium amide in liquid ammonia or with lithium dimethylamide in hexamethylphosphoramide (HMPA) affords the corresponding 9-naphthylmethylenecyclooctatrienyl anions (2a-d).The latter are considered to be contact ion pairs in liquid ammonia and solvent-separated ion pairs of free ions in HMPA on the basis of the effects of temperature and solvent on the ¹H NMR chemical shifts.The activation energies for rotation around the C₈-C₉ bond have been determined to be > 17.0, 16.4, 14.4, and 12.2 kcal mol^-1 for the lithium salts of 2a, 2b, 2c, and 2d, respectively,>17.0 and 15.1 kcal mol^-1 for the potassium salts of 2a and 2c, respectively, and 16.4 kcal mol^-1 for (9-(p-chlorophenyl)methylenecyclooctatrienyl)lithium (2e), all in liquid ammonia, and 16.2 kcal mol^-1 for the lithium salt of 2b in HMPA.These data, in conjunction with previous evidence that bond rotation in 2c occurs by an unimolecular mechanism, lead to the conclusions that the negative charge is located in the region of C₉ and the naphthyl ring in the transition state (5a-d) and that folding of the eight-membered ring in 5a-d provides a significant driving force for bond rotation by this mechanism