Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring‐Expansion Sequence with Indoles

Abstract

Lanthanide photocatalysts are much less investigated in synthetic chemistry than rare and expensive late transition metals. We herein introduce GdIII photocatalysis of a highly regioselective, intermolecular [2+2] photocycloaddition/ring‐expansion sequence with indoles, which could provide divergent access to cyclopenta[b]indoles and indolines. A simple and commercially available Gd(OTf)3 salt is sufficient for this visible‐violet‐light‐induced transformation. The reaction proceeds either through a transient or start‐to‐end dearomatization cascade and shows excellent regioselectivity (usually >95:5 r.r.), broad scope (59 examples), good functional group tolerance and facile scale‐up under mild, direct visible‐light‐excitation conditions. Mechanistic investigations reveal that direct excitation of the Gd(OTf)3/indole mixture gives an excited state intermediate, which undergoes the subsequent [2+2] cycloaddition and cyclobutane‐expansion cascade.

Lanthanide–tetrapyrrole complexes: synthesis, redox chemistry, photophysical properties, and photonic applications

“…68 In organic asymmetric synthesis, the concept of photoredox catalysis, operating through singleelectron transfer, energy transfer and radical processes, has long become a research hotspot and gives rise to unprecedented reactivities and products. 383 Given that eye-catching photocatalyst applications of lanthanides have recently come under the limelight, 384,385 and that lanthanide-tetrapyrrole complexes possess unique photophysical and redox properties, we believe that new photocatalytic potentials of lanthanidetetrapyrrole systems will soon be unleashed. One aspect is asymmetric synthesis, which is currently and actively pursued in our laboratory; on the other hand, further real-life photocatalytic applications of lanthanide-tetrapyrroles for environmental organic waste treatment and biomass conversion are also foreseeable.…”

Deal;Photoredox Catalysis for the Cycloaddition Reactions

“…Recently, group of Glorius introduced a gadolinium-catalyzed intermolecular [2 + 2] cycloaddition/ring-expansion sequence with protecting-group-free indoles under the irradiation of violet light for the generation of diverse cyclopenta[b]indoles and indolines. [59] The indole substrates 71 with traceless directing group were firstly coordinated with Gd(III)-catalyst (72), followed by the cycloaddition with styrenes or dienes to generate cyclobutane-containing products 73. Subsequently, the four-membered rings were expanded to cyclopentanestructures (74 and 77) via semi-pinacol rearrangement.…”

Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring‐Expansion Sequence with Indoles

“…We next extended the scope of the structurally novel spirocyclicc yclopenta[b]indoles (5a-i)a nd the dearomatization product cyclopenta[b]indolines (7a-h), which were afforded in generally excellent yields of 52-90 %and 58-85 %, respectively,a nd each as as ingle regioisomer.T he polycyclic structures of 4ar, 5a and 7fwere confirmed by Xray crystallographic analysis. [28]…”