Reactant-induced photoactivation of in situ generated organogold intermediates leading to alkynylated indoles via Csp2-Csp cross-coupling

Abstract

Photosensitization of organogold intermediates is an emerging field in catalysis. In this context, an access to 2,3-disubstituted indoles from o-alkynyl aniline and iodoalkyne derivatives via a gold-catalyzed sequence under visible-light irradiation and in the absence of an exogenous photocatalyst was uncovered. A wide scope of the process is observed. Of note, 2-iodo-ynamides can be used as electrophiles in this cross-coupling reaction. The resulting N-alkynyl indoles lend themselves to post-functionalization affording valuable scaffolds, notably benzo[a]carbazoles. Mechanistic studies converge on the fact that a potassium sulfonyl amide generates emissive aggregates in the reaction medium. Static quenching of these aggregates by a vinylgold(I) intermediate yields to an excited state of the latter, which can react with an electrophile via oxidative addition and reductive elimination to forge the key C-C bond. This reactant-induced photoactivation of an organogold intermediate opens rich perspectives in the field of cross-coupling reactions.

3-Sulfonylindoles via Gold- or Silver-Catalyzed Cyclization─1,3-Sulfonyl Migration Sequences under Visible Light Irradiation

“…Based on these mechanistic investigations and our precedent studies, 25 it seems reasonable to assume a preliminary formation of a vinyl metal (gold or silver) intermediate A resulting from the cyclization of the TsN amide anion on the π-activated alkyne by a metal salt (gold/ silver). In the case of gold activation, the stable vinyl gold intermediate A would reach an excited state by direct irradiation, as in Scheme 2a, or by energy transfer (EnT) from the excited state of potassium amide aggregate [1a-K] (cycles (a) and (b) of Scheme 3), as in the alkynylation reaction, 25 and undergo homolytic cleavage of the N−SO 2 Tol bond to generate in situ an indolyl radical and a sulfonyl radical (initiation (d)). 49 Such EnT-driven homolytic cleavage has been, for instance, reported for the photoinduced rearrangement of vinyl tosylates to β-ketosulfones.…”

“…This study stems from the observation of an intriguing faint fluorescent blue spot on the crude product TLCs of some of the recently reported gold(I)catalyzed alkynylation reactions of 2-alkynyl-N-tosyl anilines under visible light irradiation. 25 These spots significantly gained in intensity when the reactions were conducted without an alkyne partner and aroused our interest. Structural identification concluded the formation of 3-sulfonylindole derivatives 2.…”

“… In the course of our studies on gold­(I) catalysis under visible light irradiation, − we now report the reactivity of 2-alkynyl- N -tosyl anilines in the presence of gold­(I) and silver­(I) salts to provide an expedient access to 2-aryl-3-sulfonyl-NH-indoles (Scheme e). This study stems from the observation of an intriguing faint fluorescent blue spot on the crude product TLCs of some of the recently reported gold­(I)-catalyzed alkynylation reactions of 2-alkynyl- N -tosyl anilines under visible light irradiation . These spots significantly gained in intensity when the reactions were conducted without an alkyne partner and aroused our interest.…”

Gold catalysed site-selective cross-coupling of tertiary α-silylamines with 1-iodoalkynes under UVA LED light

“…Gold-catalysed cross-coupling reactions have emerged in recent years as one of the most powerful and versatile synthetic strategies for carbon–carbon bond formation. 1–9 Notable among these are an emerging handful of impressive synthetic methods that have exploited organosilanes as one of the cross-coupling partners (Fig. 1).…”

“…11 We were also directed by recent works showing the utility of 1-iodoalkynes as radical precursors and the scission of a carbon–halogen bond under photoredox conditions assisted by a gold( i , i ) complex to be one of the most efficient. 7,12 In the case of the latter, we reasoned that the oxidative quench pathway that has been proposed to be operative would lead to the formation of the alkynyl radical species. 8 It was anticipated that the reduction of the photocatalyst by a molecule of the tertiary α-silylamine and subsequent β-silyl hyperconjugative stabilisation might then result in its desilylative fragmentation and the site-selective formation of the ensuing radical species.…”