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

Abstract

1-Carbamoyloxy-1-lithio-indene complexes of (À)-sparteine and (À)-a-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 1 H NMR spectrum. The highest barrier for interconversion (DDH = b 25 kcal mol À1 ) was recorded for the 2-methylindenide/(À)-sparteine complexes 5 b/epi-5 b, whereas the activation energy for the epimerisation of the indenide/(À)-a-isosparteine complex 6 a/epi-6 a 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 7 c and 7 d 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.

Highly Enantioselective Reactions of Configurationally Labile Epimeric Diamine Complexes of Lithiated S‐Benzyl Thiocarbamates

“…If the epimeric complexes are configurationally stable at low temperatures (−78 °C) but equilibrate at higher temperatures (−30 °C), it should be possible to follow this epimerization process by means of NMR spectroscopy 31. 32 We expected to see signals for two species at lower temperatures, an increase in intensity for one group of signals with a corresponding decrease in intensity for the other group upon warming the sample, and finally, at higher temperatures, a single set of signals belonging to one single epimeric complex within the sample.…”

Nonracemic, Chiral Homoenolate Reagents Derived from (Cycloalk-1-enyl)methyl Carbamates and Evaluation of Their Configurational Stabilities

“…The mechanism of the epimerisation, which requires the formal migration of the lithium cation from one face to the other, is still unknown, although attempts to achieve better insight have been performed. [34] The semiquantitative relationships observed in this paper may be explained by an anti-S E Ј substitution reaction at the γ-position of the ion pairs 17·4 by ''vagabonding'' lithium cation in the solution; its rate should be decreased with increasing steric shielding of this position. Further efforts are necessary to uncover the mechanism.…”

Soft Microporous Green Materials from Natural Soybean Oil

“…The large majority of the hitherto known dynamic resolutions are mediated by enzymes,2c,e and their scope was enhanced in the 1990s by combining enzymatic conversions with transition‐metal catalysis 2e. 4 The relatively rare nonenzymatic methods rely partly on stoichiometric amounts of chiral auxiliaries5 or co‐solvents3, 6 and partly on transition‐metal catalysts 7…”