Photoacid-Catalyzed Friedel–Crafts Arylation of Carbonyls

Abstract

In this report, we demonstrate that visible-light-induced thiourea photoacids catalyze C–C bond-forming reactions. Upon photoirradiation, Schreiner’s thiourea [(N,N′-bis­[3,5-bis­(trifluoromethyl)­phenyl]-thiourea] catalyzes the double Friedel–Crafts addition of indoles to aldehydes and isatins to form the corresponding triarylmethanes and 3,3′-diarylindolin-2-ones. This protocol is amenable to a wide range of aldehyde and isatin electrophiles, as well as a variety of electronically diverse indoles. Mechanistic studies show that light is required for reaction initiation.

Photoinduzierte Protonentransferreaktionen für milde O‐H‐Funktionalisierungsreaktionen unreaktiver Alkohole

“…In der organischen Synthese ist die Anwendung von Photosäuren in PPT‐Reaktionen hingegen stark limitiert und erste Beispiele zeigen Protonierungsreaktionen von Silylenolethern ( 3 ) durch einen UV‐angereten Bi‐Naphthol basierten Katalysators ( 5 ) in geringen Ausbeuten (Schema a) . Im Gegensatz dazu wurden PPT‐Reaktionen zum Zwecke der Knüpfung neuer Kohlenstoff‐Kohlenstoff‐ oder Kohlenstoff‐Heteroatom‐Bindungen und/oder der Aktivierung nicht reaktiver Vorläufer nach unserem Wissen bislang nicht beschrieben.…”

Recent Advances in Photoacid Catalysis for Organic Synthesis

“…Photoirradiation of 10 mol% 4 with 370 nm or blue LEDs facilitates the double addition of indoles 66 with carbonyl compounds 40 to provide access to an array of triarylmethanes and 3,3′-diarylindolin-2-ones 68-73 (Scheme 13). 34 Scheme 11 Acetalization using eosin Y…”

Photoinduced Proton Transfer Reactions for Mild O-H Functionalization Reactions of Unreactive Alcohols

“…We hypothesized that photoexcitation of aryldiazoacetates (6) with visible light should generate a photoexcited state that might act as a photobase in a PPT reaction with mild acids to deliver a diazonium ion and a pendant nucleophilic anion (ion pair 8). The latter should undergo a nucleophilic substitution to deliver the reaction product (9). This concept would now open up a new reactivity pattern of diazoalkanes, which adds to known concepts in carbene transfer reactions or the reaction with strong Brønsted acids (Scheme 1b).…”

“…Control experiments using iPrOH revealed only a very sluggish reaction and poor yield of the corresponding isopropoxy ether, even with a large excess of iPrOH (10 eq.) ( Table 1, entry 9). No background reaction between HFIP and methyl phenyldiazoacetate 6a was observed in the dark even after 24 hours of reaction time (Table 1, entry 10).…”

“…the UV excitation of a binaphthol derived catalyst (5) can be applied in protonation reactions of silyl enol ethers (3) albeit in low yields (Scheme 1a). [9] Contrarily, PPT reactions for the purpose of carboncarbon or carbon-heteroatom bond forming reactions and/or the activation of unreactive precursor has, to the best of our knowledge, not been described. This approach would thus open up a conceptually new approach in organic synthesis, ideally in the presence of mild, low-energy visible light.…”