An isosparteine derivative for stereochemical assignment of stereogenic (chiral) methyl groups using tritium NMR: Theory and experiment

(N-CHDT)-(alpha)-isosparteinium ditosylamide can be used in conjunction with tritium NMR spectroscopy to assign the configuration of an intact stereogenic (chiral) methyl group. The S-CHDT group has a H-3 chemical shift that is 49 ppb downfield of the R-CHDT resonance. The sign and magnitude of this chemical shift difference of these diastereotopic tritium nuclei are found to be in agreement with predictions made via a purely ab initio computational approach. The chemical shift difference is due to an equilibrium isotope effect originating from a novel (CH3N)-N-... hydrogen bond. Despite the improved tritium chemical shift dispersion, this method is not useful for determining the enantiopurity of CHDTN(TOS)(2) due to partial racemization that occurs during the derivatization step. Milder methylation conditions are described for reactions using methyl rho-toluenesulfonate or methyl-d(3) triflate. These studies suggest that (-)-(alpha)- isosparteine is a potential new reagent for chirality analysis of methyl groups originating from suitably reactive electrophiles.