In Situ Generated Bulky Palladium Hydride Complexes as Catalysts for the Efficient Isomerization of Olefins. Selective Transformation of Terminal Alkenes to 2-Alkenes

Application of an in situ generated bulky palladium(II) hydride catalyst obtained from a 1:1:1 mixture of Pd(dba)2, P(tBu)3, and isobutyryl chloride provides an efficient protocol for the isomerization and migration of a variety of olefins. In addition to the isomerization of (Z)- to (E)-olefins, the conjugative migration of allylbenzenes, allyl ethers, and amines was effectively achieved in near-quantitative yields and with excellent functional group tolerance. Catalyst loadings in the range of 0.5−1.0 mol % were typically applied, but even loadings as low as 0.25 mol % could be achieved when the reactions were performed under neat conditions. More interestingly, the investigated catalyst proved to be selective for converting terminal alkenes to 2-alkenes. This one-carbon migration process for monosubstituted olefins provides an alternative catalyst, which bridges the gap between the allylation and propenylation/vinylation protocols. Several substrates, including homoallylic alcohols and amines, were selectively transformed into their corresponding 2-alkenes, and examples using enantiomerically enriched substrates provided products without epimerization at the allylic stereogenic carbon centers. Finally, some mechanistic investigations were undertaken to understand the nature of the active in situ generated Pd−H catalyst. These studies revealed that the catalytic system is highly dependent on the large steric demand of the P(tBu)3 ligand. The use of an alternative ligand, cataCXium PinCy, also proved effective for generating an active catalyst, and it was demonstrated in some cases to display better selectivity for the one-carbon shifts of terminal olefins. A possible intermediate involved in the preparation of the active catalyst was characterized by its single-crystal X-ray structure, which revealed a monomeric tricoordinated palladium(II) acyl complex, bearing a chloride ligand.

由二(二亚苄基丙酮)合钯(0)（Pd(dba)2）、三叔丁基膦（P(tBu)3）与异丁酰氯（isobutyryl chloride）按1:1:1摩尔比混合原位制得的大位阻钯(II)氢催化剂，可高效实现多种烯烃的异构化与迁移转化。除可实现(Z)-烯烃向(E)-烯烃的异构化外，该催化体系还能有效完成烯丙基苯、烯丙基醚及烯丙基胺类的共轭迁移，反应产率接近定量，且具有优异的官能团耐受性。常规催化剂用量为0.5~1.0 mol%，若在无溶剂（纯相）条件下进行反应，催化剂用量甚至可低至0.25 mol%。更值得关注的是，该催化剂可选择性地将末端烯烃转化为2-烯烃。针对单取代烯烃的单碳迁移工艺，可提供一种替代催化体系，填补了烯丙基化与丙烯基化/乙烯基化方案之间的技术空白。多种底物（包括高烯丙基醇与高烯丙基胺）均可被选择性转化为对应的2-烯烃；使用对映体富集底物的反应实例表明，产物在烯丙基手性碳中心未发生差向异构化。最后，本研究开展了部分机理探究，以阐明原位生成的活性钯(II)氢催化剂的本质。相关研究揭示，该催化体系高度依赖于P(tBu)3配体的大位阻效应。换用另一种配体cataCXium PinCy也可制得活性催化剂，且在部分案例中展现出更优异的末端烯烃单碳迁移选择性。研究还对活性催化剂制备过程中可能存在的中间体进行了单晶X射线结构表征，结果显示该中间体为带有氯离子配体的单核三配位钯(II)酰基配合物。