Preparation and Detection of Enantiomerically Enriched and Configurationally Stable alpha-Thioalkyllithium Compounds

摘要

Chiral a-alkylthioalkvilithium compounds are, in contrast to their a-alkoxy analogues, eonfigurationally instable even at -80 deg C. The causes and mechanisms of the configurational conversions have been thoroughly investigated. For example, using racemic l-acylthioaikyl!ithium ccnipounds. Hoffmann et al. showed That sterically demanding aryl groups on the sulfur atom raise the barrier to configurational inversion and that the torsion about the C-S bond is rate determining. Enuntiomeri-cally enrichedx-thioalkyllithiurn compounds, whose chiraiity is determined solely by the stereogenic carbanionic center. have never been prepared to the best of our knowledge. We now report a method for forming enantiomerically enriched, configure-lionally labile ionpairs, and the synthesis of a configurationally stable 2-thioa!kyllithium compound. As we have shown. several alkyl carbamates (2 with O instead of S) can be lithiated under the influence of the chiral additive (- )-sparteine with effective differentiation between the enantiotopic protons pro-5-H and pro-R-H; the complexes (3 with O instead of S) are configurationally stable- and electrophilic substitution kads to products (4 with O instead of S) with retention of configuration and enantiomeric excesses of >=95% ee. The corresponding S-alkylthiocarbamates 2 formed from 1, 3-oxazoiidine-3-curbonyi chloride (1) and sodium thiolatcs were deprotonated under the same conditions in diethyl ether and trapped with various electrophiles to give the substitution products 4-6 (Table t ). The enantiomeric excesses were between 40 ana 60 % te and are almost totally independent of the electrophiie used.