Popescu Mihai V, Mekereeya Aroonroj, Alegre-Requena Juan V, Paton Robert S, Smith Martin D
Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, UK.
Department of Chemistry, Colorado State University, 1301 Center Ave, Ft. Collins, CO, 80523-1872, USA.
Angew Chem Int Ed Engl. 2020 Dec 14;59(51):23020-23024. doi: 10.1002/anie.202009704. Epub 2020 Oct 8.
The [2+2] photocycloaddition is the most valuable and intensively investigated photochemical process. Here we demonstrate that irradiation of N-acryloyl heterocycles with blue LED light (440 nm) in the presence of an Ir complex leads to efficient and high yielding fused γ-lactam formation across a range of substituted heterocycles. Quantum calculations show that the reaction proceeds via cyclization in the triplet excited state to yield a 1,4-diradical; intersystem crossing leads preferentially to the closed shell singlet zwitterion. This is geometrically restricted from undergoing recombination to yield a cyclobutane by the planarity of the amide substituent. A prototropic shift leads to the observed bicyclic products in what can be viewed as an interrupted [2+2] cycloaddition.
[2+2]光环加成反应是最有价值且研究最为深入的光化学过程。在此我们证明，在铱配合物存在的情况下，用蓝色发光二极管光（440纳米）照射N-丙烯酰基杂环化合物，会导致一系列取代杂环化合物高效且高产率地形成稠合γ-内酰胺。量子计算表明，该反应通过三重态激发态的环化反应生成1,4-双自由基；系间窜越优先导致闭壳单重两性离子。由于酰胺取代基的平面性，这在几何上限制了其发生重组以生成环丁烷。质子转移导致了可视为中断的[2+2]环加成反应中所观察到的双环产物。
