The Use of (−)‐Sparteine/Organolithium Reagents for the Enantioselective Lithiation of 7,8‐Dipropyltetrathia[7]helicene: Single and Double Kinetic Resolution Procedures

The effect of organolithium reagent (RLi: R= n Bu, i Pr, s Bu, t Bu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (−)‐sparteine‐mediated silylation of 7,8‐dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 equivalent of RLi (R= i Pr, s Bu, t Bu) and a large excess of (−)‐sparteine (R= n Bu, s Bu) is often needed to achieve substantial conversions and good ee values. With n BuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient t BuLi/(−)‐sparteine adduct with which the need for a high (−)‐sparteine/RLi ratio can be avoided. Single‐ and double‐kinetic resolution (KR) procedures give enantiopure samples of 2‐trimethylsilyl‐ and 2,13‐di(trimethylsilyl)‐7,8‐dipropyltetrathia[7]helicene and two‐step double‐KR combining (−)‐sparteine/ s BuLi and chiral formamides affords the synthetically valuable 2‐formyl‐7,8‐dipropyltetrathia[7]helicene. This is the first use of (−)‐sparteine for the enantioselective lithiation of helicenes and the first report of t BuLi outperforming s BuLi in a (−)‐sparteine‐mediated procedure.