A Radical Chain: Mononuclear “Gold Only” Photocatalysis

Abstract

We herein report an unprecedented reactivity of mononuclear gold catalysts acting as classical catalysts and at the same time as active partners in a radical chain mechanism, whereby gold is the only catalyst. The mechanism of the photo‐induced photosensitizer‐free “gold only”‐catalyzed cross coupling was studied in detail – experimentally and theoretically – based on the reaction between arylboronic acids and aryldiazonium salts. With a systematic set of stoichiometric experiments under various conditions and analytic methods, we could show that this mechanism is initiated by an aryl radical formed in the presence of blue LEDs and MeOH. This aryl radical enters the catalytic cycle to oxidize gold(I) to gold(II). Single electron transfer from gold(II) to the aryldiazonium salt then gives the cationic gold(III) complex by formation of a chain‐supporting aryl radical which then is enabled to start a new cycle by oxidation of gold(I) without the need for irradiation. At least every 432 cycles of the radical chain, a new chain‐initiating radical has to restart the radical chain. Eventually, the boronic acid is activated by the BF4− anion to transmetalate to gold(III), and reductive elimination delivers the desired product.

Site Isolation in Metal–Organic Layers Enhances Photoredox Gold Catalysis

“…On the basis of these results and literature precedents, ,,− we propose a plausible mechanism for Hf–Ru–Au catalyzed cross-coupling in Figure b. Aryldiazonium salt 1c is reduced by excited Ru-PS or initiated by a light-mediated radical chain process to generate an aryl radical, which adds to P -AuCl and forms an Au­(II)-aryl complex.…”

Visible Light-Driven, Gold(I)-Catalyzed Preparation of Symmetrical (Hetero)biaryls by Homocoupling of Arylazo Sulfones

“…Currently, two strategies are mainly exploited to achieve these targets, namely, the use of a colored photocatalyst able to activate the substrates via electron or atom (hydrogen or halogen) transfer and the use of commercially available or properly designed organic molecules that absorb in the visible region . However, although the synthesis of asymmetric biphenyls under either photochemical or photocatalyzed conditions has been largely documented, the related photoinduced homocoupling processes are still largely underdeveloped . These include the photocatalyzed Ullmann reaction of aryl halides using KNb 3 O 8 @AuNP, the [Au­(I)]-photoredox-catalyzed coupling of aryl iodides developed by Barriault and co-workers (Scheme a) or the dual photoredox/nickel-catalyzed dimerization of aryl bromides .…”

Biaryl Coupling of Aryldiazonium Salts and Arylboronic Acids Catalysed by Gold

“…More recently, in a reaction catalysed by gold under irradiation in the absence of ruthenium, it was pointed out that the first step of the coupling was oxidative addition. 17e f Overall, all the studies carried out so far agree that the key species involved in the oxidative addition step are aryl radicals. In prior work, we demonstrated by EPR and electrochemistry 14a that aryl diazonium salts lead to radicals in the presence of ascorbic acid.…”

“…The mechanism of the reaction promoted only by light may involve alternative activation modes of the di­azonium salt and may proceed via the formation of π–π complexes between the aryldiazonium salt and the gold complex. 17f…”