Tandem‐Organokatalyse und ‐Photokatalyse: eine Anthrachinon‐ katalysierte sequenzielle Indol‐C3‐Alkylierung/Photooxidation/ 1,2‐Umlagerung

Abstract

Wegen ihrer orthogonalen Reaktivitäten im Grundund angeregten Zustand sind Chinone interessante Organokatalysatoren für die Entwicklung sequenzieller katalytischer Verfahren. Die Entdeckung einer thermischen Chinon-katalysierten C3-Alkylierung von Indolen mit Benzylaminen wird beschrieben, die sequenziell mit einer neuen, durch sichtbares Licht induzierten Photooxidation/1,2-Umlagerung kombiniert wurde. Das Eintopf-Tandemverfahren setzt Indole zu 3-Benzylindolen um, die darauf zu neuen fluoreszierenden 2,2-disubstituierten Indolin-3-onen transformiert werden.

Accelerated Discovery in Photocatalysis using a Mechanism‐Based Screening Method

“…Its excited state redox properties (E 1/2 (*Ir III -Ir II ) =+1.21 Vv s. SCE, E 1/2 (Ir IV -*Ir III ) = À0.89 V vs.SCE. [14,15] Thev alidity of our interpretation of the observed luminescence intensity decreases as genuine dynamic quenching was also evaluated. [11] Alibrary of one hundred potential quenchers was compiled from stock compounds available in our laboratory essentially at random, pre-selecting only on the basis of solubility in acetonitrile.Acuvette containing photocatalyst 1 (10 mm)a nd 2500 equivalents of as ubstrate (25 mm)w as then prepared under oxygen-free conditions in degassed acetonitrile.T he intensity of the luminescence emission of 1 at 472 nm (l ex = 420 nm) under these conditions (I)was measured using astandard fluorescence spectrometer and compared with that of the photocatalyst measured in isolation (I 0 ).…”

“…At this stage,aliterature search was conducted to determine the novelty and potential importance of each discovered substrate.F urther investigations were not carried out with 1-methylindole (Q4)a nd nitrobenzene (Q5)a st hese compounds have been widely shown to act as quenchers in visible-light-mediated reactions. [14,15] Thev alidity of our interpretation of the observed luminescence intensity decreases as genuine dynamic quenching was also evaluated. This led to the removal from consideration of 1-(trifluoromethanesulfonyl)benzotriazole (Q6)a st he UV/Vis spectrum of this compound revealed significant absorption of visible light by this compound (A > 3 at 420 nm).…”

X‐Ray Characterization of an Electron Donor–Acceptor Complex that Drives the Photochemical Alkylation of Indoles

“…Given the electronic similarities with I, 1H-indoles 1 were considered as suitable donors, which could effectively form an EDA complex II upon association with alkyl halides 2 (Figure 1 b). [11] The feasibility of our plan was tested by reacting 3-methyl indole (1 a) with 2,4-dinitrobenzyl bromide (2 a) in MeOH under irradiation by a 23 W CFL lamp [Eq. [9] We hypothesized that visible light irradiation of the transiently generated complex II might induce an electron transfer, affording the contact radical pair III.…”

“…The indole nucleus is a privileged scaffold [10] found in numerous natural products and biologically active compounds, which makes the development of a photochemical indole alkylation strategy synthetically relevant. [11] The feasibility of our plan was tested by reacting 3-methyl indole (1 a) with 2,4-dinitrobenzyl bromide (2 a) in MeOH under irradiation by a 23 W CFL lamp [Eq. (1)].…”

Schnelles Entdecken photokatalytischer Reaktionen durch mechanismusbasiertes Screening

“…Weiterführende Screenings mit 1-Methylindol (Q4)u nd Nitrobenzol (Q5)w urden nicht durchgeführt, da diese bereits als Quencher photokatalytischer Reaktionen mit sichtbarem Licht beschrieben wurden. [14,15] in der Graebe-Ullmann-Reaktion). [17] Hingegen wurden strukturell ähnliche Azide und Diazoniumsalze bereits vielfach als Quencher in Photokatalysen verwendet.…”