Melcher Michaela-Christina, Rolim Alves da Silva Bianca, Ivšić Trpimir, Strand Daniel
Department of Chemistry, Centre for Analysis and Synthesis, Lund University, Naturvetarvägen 14, 221 00 Lund, Sweden.
ACS Omega. 2018 Mar 30;3(3):3622-3630. doi: 10.1021/acsomega.8b00127. eCollection 2018 Mar 31.
Chiral dienes are useful ligands in a number of asymmetric transition-metal-catalyzed reactions. Here, we evaluate the efficiency of 2,5-disubstituted 1,3,4,6-tetrahydropentalenes as ligands to rhodium(I). 2,5-Dibenzyl and diphenyl tetrahydropentalenes were synthesized in two steps and resolved, either chromatographically, or through fractional crystallization of diastereomeric rhodium(I) salts. When evaluated in a 1,4-arylation reaction, the 2,5-dibenzyl ligand gave up to 99% ee. The use of a well-defined rhodium complex as catalyst, CsCO as the base, and toluene/water as solvent was found to have a pronounced beneficial effect on the selectivity of the reaction. The homologous 2,5-diphenyl ligand on the other hand proved to be highly prone to racemization/loss of chirality during catalysis. Control experiments reveal that this rearrangement proceeds via a rhodium-mediated 1,3-hydride shift. Implications for ligand design and catalysis are discussed.
手性二烯在许多不对称过渡金属催化反应中是有用的配体。在此，我们评估了2,5-二取代的1,3,4,6-四氢并五苯作为铑（I）配体的效率。2,5-二苄基和二苯基四氢并五苯通过两步合成并拆分，要么通过色谱法，要么通过非对映体铑（I）盐的分步结晶。当在1,4-芳基化反应中进行评估时，2,5-二苄基配体的对映体过量（ee）高达99%。结果发现，使用结构明确的铑配合物作为催化剂、碳酸铯作为碱以及甲苯/水作为溶剂对反应的选择性有显著的有益影响。另一方面，同源的2,5-二苯基配体在催化过程中被证明极易发生外消旋化/手性丧失。对照实验表明，这种重排是通过铑介导的1,3-氢迁移进行的。文中讨论了对配体设计和催化的影响。
