Computational, ReactIR-, and NMR-spectroscopic investigations on the chiral formyl anion equivalent N-(alpha-lithiomethylthiomethyl)-4-isopropyl-5,5-diphenyloxazolidin-2-one and related compounds.

Abstract

The 4-isopropyl-3-methylthiomethyl-5,5-diphenyloxazolidin-2-one is readily lithiated in THF on the exocyclic CH2 group (1 --> 2) to give a synthetically useful chiral nucleophilic formylating reagent. We have now studied the lithiation reaction by ReactIR spectroscopy and the structure of the organolithium reagent by computational methods and by NMR-spectroscopic measurements. The lithiation is complete at -78 degrees C within 90 seconds, and it is accompanied by a decrease of the C=O wavenumber by 50 cm(-1). The NMR data (collected in [D8]THF) give no evidence for 13C,6Li coupling or for aggregation; from DPFGSE-ROE spectra the single diastereoisomer of the lithium compound 2 seen in the NMR spectra (NMR-spectroscopic measurements from -105 to -20 degrees C) is assigned like configuration; the 13C=O signal of the oxazolidinone undergoes a 7 ppm downfield shift upon lithiation (1 --> 2); line-shape analyses of the signals from the diastereotopic CH2 and CMe2 protons in lithiated 4,4-dimethyl-3-methylthiomethyloxazolidin-2-one (model compound 7) reveal a deltaH(double dagger) of 8.9 +/- 0.2 kcal mol(-1) for enantiomerization. The theoretical calculations provide an energy-minimum structure for the lithium compound 2 with coordination of the carbonyl oxygen to lithium, with an antiperiplanar arrangement of the C,Li and S,CH3 bonds, and with relative like configuration of the two stereocenters--in perfect agreement with the conclusions from the IR- and NMR-spectroscopic measurements!