Diels−Alder Active-Template Synthesis of Rotaxanes and Metal-Ion-Switchable Molecular Shuttles

A synthesis of [2]rotaxanes in which Zn(II) or Cu(II) Lewis acids catalyze a Diels−Alder cycloaddition to form the axle while simultaneously acting as the template for the assembly of the interlocked molecules is described. Coordination of the Lewis acid to a multidentate endotopic 2,6-di(methyleneoxymethyl)pyridyl- or bipyridine-containing macrocycle orients a chelated dienophile through the macrocycle cavity. Lewis acid activation of the double bond causes it to react with an incoming “stoppered” diene, affording the [2]rotaxane in up to 91% yield. Unusually for an active-template synthesis, the metal binding site “lives on” in these rotaxanes. This was exploited in the synthesis of a molecular shuttle containing two different ligating sites in which the position of the macrocycle could be switched by complexation with metal ions [Zn(II) and Pd(II)] with different preferred coordination geometries.

本文报道了一类[2]轮烷（[2]rotaxanes）的合成方法：其中Zn(II)与Cu(II)路易斯酸（Lewis acids）可催化狄尔斯-阿尔德（Diels−Alder）环加成反应以构建轮烷轴，同时作为模板介导互锁分子的组装。路易斯酸与多齿内向型（endotopic）含2,6-二(亚甲基氧甲基)吡啶基或联吡啶基的大环（macrocycle）配位后，可将螯合的亲双烯体（dienophile）定位于大环空腔之中。路易斯酸活化双键后，可使其与加入的“封端”二烯发生反应，最终以最高91%的产率得到[2]轮烷。与常规活性模板合成不同的是，此类轮烷中金属结合位点得以保留。利用这一特性，我们合成了一种包含两种不同配位位点的分子梭（molecular shuttle），通过与具有不同偏好配位几何构型的金属离子[Zn(II)与Pd(II)]配位，即可实现大环在分子梭上的位置切换。