Stepwise Chemical Reduction of [4]Cyclo[4]helicenylene: Stereo Transformation and Site-Selective Metal Complexation

A highly strained macrocycle comprising
four [4]helicene panels,
[4]cyclo[4]helicenylene ([4]CH, 1), was synthesized through
a one-pot macrocyclization and chemically reduced by alkali metals
(Na and K), revealing a four-electron reduction process. The resulting
di-, tri-, and tetraanions of compound 1 were isolated
and crystallographically characterized by X-ray diffraction. Owing
to the four axially chiral bi[4]helicenyl fragments, a reversible
stereo transformation of 1 between the (S,R,S,R)- and (S,S,R,R)-configurations was disclosed upon the two-electron uptake, which
was rationally understood by theoretical calculations. The (S,S,R,R)-configuration of 12– was further
stabilized in triply reduced and tetra-reduced states, where structural
deformation led by charges and metal complexation was observed. This
study proposed an approach to alter the configuration of cycloarylenes
in addition to thermal treatment.

由四个[4]螺烯（[4]helicene）单元构成的高张力大环化合物[4]环[4]螺烯亚基（[4]cyclo[4]helicenylene，简称[4]CH，1），通过一锅法大环化反应合成，并经碱金属（钠Na、钾K）进行化学还原，过程展现出四电子还原路径。研究团队成功分离得到化合物1的二、三、四阴离子，并通过X射线衍射（X-ray diffraction）完成晶体学表征。鉴于分子中包含四个轴手性联[4]螺烯片段，化合物1在俘获两电子后，会在(S,R,S,R)与(S,S,R,R)构型之间发生可逆立体构型转变，该过程通过理论计算得到了合理阐释。12–的(S,S,R,R)构型在三还原态与四还原态中进一步得到稳定，此时可观察到由电荷效应与金属配位引发的结构形变。本研究提出了一种除热加工之外，调控环芳类（cycloarylenes）化合物构型的新方法。