摘要
多元催化已在多种新反应中得以成功应用,尤其是通过双金属中心实现金属转移化步骤的二元催化已经引起人们广泛关注~([1])。考虑到金属镍对交叉偶联反应的高催化活性,含镍的双金属协同催化作用值得期待。最近,D.J.Weix等人通过镍/钯协同催化成功实现了Ullman交叉偶联反应~([2]),结果显示只有在镍/钯同时存在时此反应才具有高效的偶联选择性,这突破了经典Ullman反应低选择性的难题。因此,我们对镍,钯以及镍/钯同时存在下的所有可能的反应路径进行了理论探索,目的是揭示该催化反应机理,探究双金属协同催化作用。计算结果表明,反应经过氧化加成,双金属转移化和还原消除三个主要步骤。通过对比不同催化剂的反应势能面:钯催化反应具有最高的活化能垒,而镍催化较钯催化反应能垒有所降低,但在实验条件下上述两种反应皆无法顺利进行;只有当镍/钯同时催化此反应时活化能垒最低。此外,我们通过对比其它过渡金属(Co,Cu,Ag,Au,Pt)的还原能,预测钴和铂有望替代双金属催化剂中的钯成为更高效的催化剂。
Multi-catalysis has delivered numerous reactions. Particularly, dual catalysis involving two metal centers transmetallation, has attracted many attentions.~([1]) Moerover, Ni-catalyzed cross-couplings have been applicied in organic synthesis and dual metal catalytic effect containing nickel is worth looking forward. Remarkably, D.J. Weix and co-workers has made significant breakthroughs in Ni/Pd-dual catalyzed Ullman-type reaction.~([2]) Despite of an aforementioned mechanistic pathway, details still remain ambiguous. Thus, taking it as an example, we have conducted DFT calculations on the possible pathways of this cross-coupling reaction in the presence of isolated Ni(bpy)_2, isolated Pd(dppp)_2 as well as Ni(bpy)_2/Pd(dppp)_2 dual catalysts. As a result, the catalytic cycle consists of three fundamental steps: oxidative addition, two metal centers transmetalation, and reductive eliminationthe. The possible pathways could be ruled out in both isolated Ni(bpy)_2 and Pd(dppp)_2 catalytic cycles. In addition, we have predicted Co(0) and Pt(0) may instead of Pd(0) and successfully apply in this Ullman-type cross-coupling.
引文
[1]Allen,A.E.;Mac Millan D.W.C.Chem.Sci.2012,3:633.
[2]Ackerman1,L.K.G.;Lovell1,M.M.;Weix,D.J.Nature 2015,524:454.