Department of Chemistry , University of California , Berkeley , California 94720 , United States.
Department of Chemistry and Center for Advanced Scientific Computing and Modeling , University of North Texas , Denton , Texas 76203 , United States.
J Am Chem Soc. 2018 Apr 11;140(14):4893-4904. doi: 10.1021/jacs.8b00928. Epub 2018 Apr 2.
We report the formation of phosphine-ligated alkylpalladium(II) amido complexes that undergo reductive elimination to form alkyl-nitrogen bonds and a combined experimental and computational investigation of the factors controlling the rates of these reactions. The free-energy barriers to reductive elimination from t-BuP-ligated complexes were significantly lower (ca. 3 kcal/mol) than those previously reported from NHC-ligated complexes. The rates of reactions from complexes containing a series of electronically and sterically varied anilido ligands showed that the reductive elimination is slower from complexes of less electron-rich or more sterically hindered anilido ligands than from those containing more electron-rich and less hindered anilido ligands. Reductive elimination of alkylamines also occurred from complexes bearing bidentate P,O ligands. The rates of reactions of these four-coordinate complexes were slower than those for reactions of the three-coordinate, t-BuP-ligated complexes. The calculated pathway for reductive elimination from rigid, 2-methoxyarylphosphine-ligated complexes does not involve initial dissociation of the oxygen. Instead, reductive elimination is calculated to occur directly from the four-coordinate complex in concert with a lengthening of the Pd-O bond. To investigate this effect experimentally, a four-coordinate Pd(II) anilido complex containing a flexible, aliphatic linker between the P and O atoms was synthesized. Reductive elimination from this complex was faster than that from the analogous complex containing the more rigid, aryl linker. The flexible linker enables full dissociation of the ether ligand during reductive elimination, leading to the faster reaction of this complex.
我们报告了膦配位的烷基钯（II）酰胺配合物的形成，这些配合物经历还原消除形成烷基-氮键，并对控制这些反应速率的因素进行了实验和计算研究。与以前报道的 NHC 配位的配合物相比，t-BuP 配位的配合物的还原消除的自由能势垒要低得多（约 3 kcal/mol）。一系列电子和空间位阻不同的苯胺基配体的配合物的反应速率表明，还原消除从电子密度较低或空间位阻较大的苯胺基配体的配合物比从电子密度较高和空间位阻较小的配体的配合物更慢。含有双齿 P,O 配体的配合物也能发生烷基胺的还原消除。这些四配位配合物的反应速率比三配位、t-BuP 配位的配合物的反应速率慢。从刚性、2-甲氧基芳基膦配位的配合物进行还原消除的计算途径不涉及氧的初始解离。相反，还原消除被计算为直接从四配位配合物发生，同时伴随着 Pd-O 键的延长。为了在实验上研究这种效应，合成了一种含有 P 和 O 原子之间的柔性脂肪族连接体的四配位 Pd(II) 苯胺基配合物。该配合物的还原消除速度比含有刚性芳基连接体的类似配合物更快。柔性连接体允许醚配体在还原消除过程中完全解离，从而导致该配合物更快的反应。
