Abstract
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 IrIII 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.
Highlights
The [2+2] photocycloaddition is the most valuable and intensively investigated photochemical process
With a working substrate synthesis and cyclization procedure in hand, we examined the scope and limitations of this visible light mediated process (Table 2)
We discovered an alternative lower energy pathway that could operate in the presence of water (5.7 kcal molÀ1, Figure 1 C)
Summary
The [2+2] photocycloaddition is the most valuable and intensively investigated photochemical process. The parent substrate in which indole is N-functionalized with 2-phenyl acrylic acid cyclized smoothly to afford bicyclic lactam 3 in 98 % isolated yield.
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