Bicyclo[3.2.2]nonatrienyl-lithium and -potassium and their degenerate rearrangements studied by multiple labelling with 13C and 2H, 13C nuclear magnetic resonance spectroscopy, 13C–13C coupling constants, and isotopic perturbation

Abstract

[¹³C, ²H₇] Bicyclo[3.2.2]nonatrienyl-potassium (8) and -lithium (9) in [²H₁₀]-1,2-dimethoxyethane have been investigated by ¹³C nuclear magnetic resonance spectroscopy. The absence of an observable isotopic perturbation effect on ¹³C chemical shifts indicates a symmetric structure of this (CH)₉^– anion in solution, rather than rapidly equilibrating unsymmetrical homoaromatic ions. One-bond ¹³C–¹³C coupling constants obtained from the [¹³C₂]-labelled ion (10), ¹J_C1C2 38, ¹J_C1C7 40.0, and ¹J_C2C3 62.5 Hz, were compared with those of the corresponding hydrocarbon [¹³C₂]bicyclo[3.2.2]nona-2,6,8-triene (11). The couplings indicate no significant structural change in the bridgehead region and support a symmetrical delocalized structure for the bicyclo[3.2.2]nonatrienyl anion. It is proposed that the controversial stabilization of carbanions is not caused by delocalization and homoaromaticity but is rather due to a previously ignored important interaction with the metal ion. The observed scrambling of ¹³C labels in (8) and (9), from C(3) to C(6)–C(9) and subsequently to C(2), C(4) and C(1), C(5), is consistent with a degenerate rearrangement via the 9-barbaralyl anion. Further support for this mechanism was obtained from the scrambling of two ¹³C labels from C(3) and C(2), C(4) which is three times faster to C(6)–C(9) than to C(1), C(5) in ion (10).