Experimental and Theoretical Investigations of Lithio-Indenyl Carbamate/(−)-Sparteine and (−)-α-Isosparteine Complexes

1-Carbamoyloxy-1-lithio-indene complexes of (-)-sparteine and (-)-alpha-isosparteine 5/epi-5 and 6/epi-6 were prepared and found to be configurationally unstable on the macroscopic time scale. Ratios of epimers and the rates of interconversion, including some activation parameters, were determined by temperature-dependent line shape analysis in the H-1 NMR spectrum. The highest barrier for interconversion (Delta Delta H-not equal > 25 kcal mol(-1)) was recorded for the 2-methylindenide/(-)-sparteine complexes 5b/epi-5b, whereas the activation energy for the epimerisation of the indenide/(-)-a-isosparteine complex 6a/epi-6a is too low to be recorded by this method. Trapping of the lithium compounds 5/epi-5 and 6/epi-6 proceeds with methyl iodide in an anti-S-E' process. The obtained enantiomeric ratios correlate roughly with the observed epimeric ratios in the lithium intermediates. The absolute configurations of the methylation products 7c and 7d have been established by chemical correlation with the known (+)-(S)3-methyl-1H-indan-1-one. The ratios correspond well to those predicted by semiempirical PM3 calculations on the relative stability of the diastereomeric complexes.