Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity

We herein report the first light-driven
selective monoderivatization
(desymmetrization) of two chemically equivalent carbonyl groups in
a single chromophore. By comparing of four symmetric regioisomers,
featuring two equivalent ortho-methylbenzaldehyde
units, we identify dimethyl­therephtalaldehydes (DMTAs) which
can be activated in a dual wavelength-selective fashion. Under visible
light and UV-light irradiation, DMTAs undergo two consecutive Diels–Alder
reactions exhibiting near-quantitative endo-selectivity
(>99%) and provide excellent yields (96–98%). The influence
of the regioisomerism of the dialdehydes on their photochemical behavior
is profound, evidenced by an in-depth investigation of their photochemical
performance. We exemplify the capability of the photosystems via the
synthesis of complex Diels–Alder adducts with various dienophiles,
including alkynes.

本文首次报道了单发色团（chromophore）内两个化学等价羰基的光驱动选择性单衍生化（去对称化）反应。通过对四种带有两个等价邻甲基苯甲醛（ortho-methylbenzaldehyde）单元的对称区域异构体进行比较，我们筛选出可通过双波长选择性方式激活的二甲基对苯二甲醛（dimethyl terephthalaldehydes, DMTAs）。在可见光与紫外光辐照下，DMTAs可发生两步连续的狄尔斯-阿尔德（Diels–Alder）反应，该反应具有接近定量的内型选择性（>99%），产物收率可达96%~98%。二醛的区域异构现象对其光化学行为具有显著影响，该结论通过对其光化学性能的深入研究得到证实。我们通过与包括炔烃在内的多种亲双烯体（dienophile）合成复杂的狄尔斯-阿尔德加合物，展示了该光系统的应用能力。