Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Am Chem Soc. 2012 Apr 4;134(13):5794-7. doi: 10.1021/ja301612y. Epub 2012 Mar 26.
The ability of two common protected forms of amines (carbamates and sulfonamides) to serve as directing groups in Ni-catalyzed Suzuki reactions has been exploited in the development of catalytic asymmetric methods for cross-coupling unactivated alkyl electrophiles. Racemic secondary bromides and chlorides undergo C-C bond formation in a stereoconvergent process in good ee at room temperature in the presence of a commercially available Ni complex and chiral ligand. Structure-enantioselectivity studies designed to elucidate the site of binding to Ni (the oxygen of the carbamate and of the sulfonamide) led to the discovery that sulfones also serve as useful directing groups for asymmetric Suzuki cross-couplings of racemic alkyl halides. To our knowledge, this investigation provides the first examples of the use of sulfonamides or sulfones as effective directing groups in metal-catalyzed asymmetric C-C bond-forming reactions. A mechanistic study established that transmetalation occurs with retention of stereochemistry and that the resulting Ni-C bond does not undergo homolysis in subsequent stages of the catalytic cycle.
两种常见的保护胺形式（氨基甲酸酯和磺胺类）在镍催化的 Suzuki 反应中作为导向基团的能力已被用于开发用于交叉偶联未活化的烷基亲电试剂的催化不对称方法。外消旋的仲溴化物和氯化物在室温下在商业可得的 Ni 配合物和手性配体的存在下以立体收敛的方式进行 C-C 键形成，ee 值良好。旨在阐明与 Ni 结合部位（氨基甲酸酯和磺胺的氧）的结构对映选择性研究导致发现砜也可用作外消旋烷基卤化物不对称 Suzuki 交叉偶联的有用导向基团。据我们所知，这项研究首次提供了磺酰胺或砜作为金属催化不对称 C-C 键形成反应中有效导向基团的实例。一项机理研究确立了 transmetalation 发生时立体化学保持不变，并且在催化循环的后续阶段中，形成的 Ni-C 键不会发生均裂。
