Modular and Selective Arylation of Aryl Germanes (C−GeEt₃) over C−Bpin, C−SiR₃ and Halogens Enabled by Light‐Activated Gold Catalysis

Abstract

Selective Csp2 –Csp2 couplings are powerful strategies for the rapid and programmable construction of bi‐ or multiaryls. To this end, the next frontier of synthetic modularity will likely arise from harnessing the coupling space that is orthogonal to the powerful Pd‐catalyzed coupling regime. This report details the realization of this concept and presents the fully selective arylation of aryl germanes (which are inert under Pd0/PdII catalysis) in the presence of the valuable functionalities C−BPin, C−SiMe3, C−I, C−Br, C−Cl, which in turn offer versatile opportunities for diversification. The protocol makes use of visible light activation combined with gold catalysis, which facilitates the selective coupling of C−Ge with aryl diazonium salts. Contrary to previous light‐/gold‐catalyzed couplings of Ar–N2+, which were specialized in Ar–N2+ scope, we present conditions to efficiently couple electron‐rich, electron‐poor, heterocyclic and sterically hindered aryl diazonium salts. Our computational data suggest that while electron‐poor Ar–N2+ salts are readily activated by gold under blue‐light irradiation, there is a competing dissociative deactivation pathway for excited electron‐rich Ar–N2+, which requires an alternative photo‐redox approach to enable productive couplings.

A Radical Chain: Mononuclear “Gold Only” Photocatalysis

“…we propose that the photo‐induced “gold only” catalytic cycle is initiated by a visible light‐mediated formation of an aryl radical (as we described recently [68] ) which oxidizes gold(I) to gold(II) [70] (Figure 4). [71] In the radical chain, a subsequent SET from gold(II) to the aryldiazonium salt then gives the cationic gold(III) complex and an aryldiazo radical, which extrudes dinitrogen to form the chain‐propagating aryl radical. In average, this radical chain operates for the formation of at least 432 equivalents of the cross‐coupled product per initiating radical, before a light‐generated aryl radical restarts the next radical chain.…”

Recent Advances in Gold(III) Chemistry: Structure, Bonding, Reactivity, and Role in Homogeneous Catalysis

“…This would suggest that the oxidation of Au(I) is fast and the activation of the C−Si bond by the oxidized Au (III) is the rate limiting step. Moreover, the formation of arylphosphonium salts was observed by 31 499 By using Ph 3 PAuCl under blue LED irradiation, a broad range of biaryl derivatives was obtained under mild conditions. Notably, arylgermanes were found to be more reactive than arylsilanes and arylboronic esters and chemoselective coupling of aryls containing BPin and SiEt 3 substituents was achieved.…”

“…Very recently, Schoenebeck and co-workers have used aryl-germanes as coupling partners in photoactivated C–C coupling with aryldiazonium salts (Scheme ). By using Ph 3 PAuCl under blue LED irradiation, a broad range of biaryl derivatives was obtained under mild conditions. Notably, arylgermanes were found to be more reactive than arylsilanes and arylboronic esters and chemoselective coupling of aryls containing BPin and SiEt 3 substituents was achieved.…”

Light in Gold Catalysis

“…Very recently, Schoenebeck et al developed a selective arylation of aryl germanes (183a) in the presence of C-BPin, C-TMS, C−I, C−Br, and C−Cl, which offered a versatile tool for a rapid diversification (Scheme 51). 137 For a successful coupling of electron-poor aryldiazonium salts, the sole goldcatalyzed method was utilized, while the reaction with electron-rich diazo species required a photocatalyst for an efficient coupling. At first, the authors focused on the coupling of electron-poor groups.…”