Molecular Oxygen as a Redox Catalyst in Intramolecular Photocycloadditions of Coumarins

Abstract

Oxygen as a catalyst! While oxygen is usually excluded in nonoxidative photochemical reactions, the photocycloaddition of 3‐(alk‐4‐en‐1‐yl)‐substituted coumarins greatly benefits from the presence of O2, which was shown to act as a redox catalyst, preferentially in combination with the common antioxidant 3,5‐di‐tert‐butyl‐4‐hydroxytoluene (BHT).

Chromium photocatalysis: accessing structural complements to Diels–Alder adducts with electron-deficient dienophiles

“…Select reports indicate that the excitation of chalcone derivatives and related α,β-unsaturated carbonyl species lead to excimers 33 and/or charge-transfer complexes, 34 which enable single-electron oxidation. 35 The oxidation of enone 1* in this manner would generate radical cation 1˙ + , which would proceed to product 5 as described earlier. Here, we note that enone 1 shows weak emission at 443 nm when excited at 340 nm in acetonitrile, from which we estimate an excited state reduction potential ( 1˙ + /0* ) of –0.80 V, which suggests thermodynamic competency for oxidation of 1* by the photoexcited Cr catalyst.…”

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…In 2012, Schmalz, Griesbeck, and co-workers discovered that the intramolecular [2 + 2] photocycloaddition of coumarin 475 followed a PET pathway ( Scheme 159 ). 816 The authors found that common photosensitizers such as Rose Bengal, tetraphenylporphyrin, or 460 did not improve the yield of rac - 476 . However, under aerobic conditions, the conversion was remarkably increased, particularly in the presence of BHT (3,5-di- tert -butyl-4-hydroxytoluene).…”

Synthese von Indolen mithilfe von sichtbarem Licht: Photoredoxkatalyse für die Palladium‐katalysierte C‐H‐Aktivierung

“…Zuschriften und praktische Alternative [19] zu den gewçhnlich vermiedenen Peroxo-basierten Methoden. Da nur geringe Mengen an Oxidationsmittel erzeugt werden, die direkt verbraucht werden, sind Nebenreaktionen von Startmaterial und Produkt nicht zu beobachten.…”