缺电子的硼烷催化的硅烷对CO的还原遵循一种反常理的机制,其中Si在羰基氧原子对硅原子的亲核攻击之前,H键被硼路易斯酸激活。如此形成的硼氢化物是实际的还原剂。这些步骤是通过使用硅立体异构硅烷进行阐明的,但是由于硅原子的消旋作用,将相同的技术应用于相关的CN基团的还原尚无定论。本研究现在通过我们的轴向手性硼烷催化剂和轴向手性硅烷试剂(两种对映体形式)的有意结合证明,这些氢甲硅烷基化的机理基本相同。硼烷/硅烷对的明确立体化学结果表明,两者均参与确定对映选择性的氢化物转移步骤。在发现我们的轴向手性碳原子团之后,这些实验成为可能6 F 5取代的硼烷在亚胺氢化硅烷化中诱导相当水平的对映体诱导。
Steric effects in palladium-catalysed amination of aryl triflates and nonaflates with the primary amines PhCH(R)NH2 (R=H, Me)
作者:Rebecca E. Meadows、Simon Woodward
DOI:10.1016/j.tet.2007.11.074
日期:2008.2
A systematic study of the effects of aryl triflate and nonaflate structure on the yield of amination with the primary amines PhCH(R)NH2 (R=H, Me) under palladium catalysis has been carried out. High throughput screening indicated that a catalyst composed of X-Phos/Pd-2(dba)(3)/1.4-dioxane was optimal based on a model reaction of Ar(ORf) [R-f-T-f (SO2CF3) Nf (SO2(CF2)(3)CF3)] with PhCH2NH2. Comparisons of the reactivity of various ArOTf and ArONf [Ar=4-MePh, 2-naphthyl, 1-naphthyl, 2-PhC6H4] indicated that both ortho substitution in the aryl electrophile and at the alpha-position on the amine are detrimental to the coupling particularly when they occur in combination. Despite being formally a monodentate ligand use of X-Phos leads to only small degrees of racemisation when using (R)-PhCH(Me)NH2 (typically resulting in a reduction from 97 to 86-94% ee for the amine stereocentre). (c) 2007 Elsevier Ltd. All rights reserved.