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.
Highlights
In light of our recent studies that identified organogermanes as highly reactive functionality in gold-catalyzed couplings with ArH,[12g] we envisioned that we might be able to widen the scope in ArN2+ electronics as well as add another dimension of modularity in biaryl construction by developing a selective arylation of CÀGeEt3 under light activation that tolerates all other widely used and typical coupling functionalities, that is, CÀBPin, CÀSiMe3 as well as halogens (CÀI, CÀ Br and CÀCl)
We subsequently explored the scope of the couplings and investigated especially those aryl germanes that would, as the corresponding aryl boronic acids, be unstable and sluggishly reactive or incompatible in the widely employed Pd-catalyzed Suzuki cross coupling,[21] that is, highly fluorinated, heterocyclic, sterically hindered as well as halogenated aryl germanes.[7]
The use of [(Me2S)AuCl]/dppe in conjunction with the photocatalyst strongly favored the productive coupling for the electron-rich para-methoxyaryl diazonium salt
Summary
In light of our recent studies that identified organogermanes as highly reactive functionality in gold-catalyzed couplings with ArH,[12g] we envisioned that we might be able to widen the scope in ArN2+ electronics as well as add another dimension of modularity in biaryl construction by developing a selective arylation of CÀGeEt3 under light activation that tolerates all other widely used and typical coupling functionalities, that is, CÀBPin, CÀSiMe3 as well as halogens (CÀI, CÀ Br and CÀCl). We initially set out to investigate the feasibility of the arylation of triethyl(4-fluorophenyl)germane with 4-nitrobenzenediazonium tetrafluoroborate and examined various commonly employed AuI catalysts under blue LED irradiation in MeCN (see Supporting Information for additional details).[20] Pleasingly, the relatively cheap, commercially available and air-stable [(PPh3)AuCl] proved to be effective and gave the corresponding coupling product in 96 % yield after 2 h reaction time at room temperature.
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