2-Lithio-3,3-dimethyl-2-oxazolinyloxirane: Carbanion or Azaenolate? Structure, Configurational Stability, and Stereodynamics in Solution

Chemical studies have shown that, in ethereal solvents, oxiranyllithium Li-1 is configurationally unstable either on the macroscopic (Hoffmann's text) or microscopic time scale at low temperature (175 K). An optically pure sample of oxazolinyloxirane 1, once deprotonated, racemized within 1 min at -130°C in THF/Et2O (3:2) (t1/2 = 6.05 s); the application of the Eyring equation suggested a barrier to inversion for Li-1 of 8.8 kcal/mol at -130°C. Despite this, Li-1 exhibited an unusual thermal stability undergoing a successfully deuterium incorporation (>98%) also at 25°C with a little decomposition. The structure, configurational stability, and stereodynamics in solution of α-lithiated oxazolinyloxirane Li-1 have been also synergically investigated by means of in situ IR and NMR spectroscopy. IR spectroscopic studies showed that lithiation of 1 is complete at -98°C within 1 min and is accompanied by a decrease of the C=N wavenumber by only 60 cm-1, so supporting the idea that the structure of Li-1 may be more similar to that of an "organolithium" rather than an "azaenolate". In addition to this, multinuclear magnetic resonance studies suggested that at least in a range of concentration of 0.08-0.3 M, Li-1 mainly exists in THF as a monomelic η3-aza-allyl coordinated species rapidly equilibrating, on the NMR time scale, with a complex mixture of diastereomeric oxazoline-bridged dimeric species variously intraaggregated. An exchange mechanism by which monomers would interchange their Li atoms via one of the above dimeric species and which may be responsible for the fast racemization Li-1 undergoes as soon as is generated has been proposed.