Doulcet Julien, Stephenson G Richard
School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ (UK).
Chemistry. 2015 Dec 14;21(51):18677-89. doi: 10.1002/chem.201502958. Epub 2015 Nov 12.
The effect of organolithium reagent (RLi: R=nBu, iPr, sBu, tBu), 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=iPr, sBu, tBu) and a large excess of (-)-sparteine (R=nBu, sBu) is often needed to achieve substantial conversions and good ee values. With nBuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient tBuLi/(-)-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/sBuLi 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 tBuLi outperforming sBuLi in a (-)-sparteine-mediated procedure.
有机锂试剂（RLi：R = 正丁基、异丙基、仲丁基、叔丁基）、溶剂（二乙醚、二乙醚/四氢呋喃和甲基叔丁基醚）以及化学计量比对（-）-斯巴丁介导的7,8-二丙基四硫杂[7]螺旋烯硅烷化反应的影响表明，不同寻常的是，通常需要大大超过0.5当量的RLi（R = 异丙基、仲丁基、叔丁基）和大量过量的（-）-斯巴丁（R = 正丁基、仲丁基）才能实现较高的转化率和良好的对映体过量值。然而，对于正丁基锂，仅一当量的有机锂试剂就足以获得高转化率。使用方便的叔丁基锂/（-）-斯巴丁加合物可得到最佳结果，这样可以避免对高（-）-斯巴丁/RLi比例的需求。单动力学拆分和双动力学拆分（KR）方法可得到对映体纯的2-三甲基硅基-和2,13-二（三甲基硅基）-7,8-二丙基四硫杂[7]螺旋烯样品，并且将（-）-斯巴丁/仲丁基锂与手性甲酰胺相结合的两步双KR法可得到具有合成价值的2-甲酰基-7,8-二丙基四硫杂[7]螺旋烯。这是（-）-斯巴丁首次用于螺旋烯的对映选择性锂化反应，也是叔丁基锂在（-）-斯巴丁介导的反应中优于仲丁基锂的首次报道。
