Ammonium Salts as Convenient Radical Precursors Using Iridium Photoredox Catalysis

Abstract

Ammonium salts are usually considered as highly challenging to reduce into the corresponding radicals because of the strength of their carbon–nitrogen bond. Here, we disclose that several ammonium salts can be readily activated using iridium photoredox catalysis to form radicals and illustrate the synthetic utility of this activation of strong C–N bonds with hydrodeamination reactions and radical couplings. Cyclic voltammetry was exploited to rationalize the reactivity observed for the activation of these ammonium salts.

Visible-light-induced C(sp3)–C(sp3) bond formationviaradical/radical cross-coupling

“…On the basis of previous reports and our observations, 11–13 a plausible mechanism of this radical/radical cross-coupling is proposed in Scheme 5. Under visible-light irradiation, FIrpic is converted to the excited FIrpic*, which is reduced by HEHs to form Ir ( ii ), a benzyl radical, and pyridine through a SET process.…”

“…12 c Hydrogenation or deuteration of benzyl and aryl quaternary ammonium salts via a radical procedure was also successfully realized by Jui, Yao, Wickens, and Theunissen. 13 In addition, cyanation, cyanomethylation and phosphonation of quaternary ammonium salts were disclosed via photo or electrochemical manners. 14 However, the direct radical/radical cross-coupling using quaternary ammonium salts as free radical precursors to approach C(sp 3 )–C(sp 3 ) bonds is underexplored.…”

Recent Advances in Electro- or Photochemical Driven Transformations via Cleavage of the C–N Bond of Quaternary Ammonium Salts

“…Wickens and co-workers have used the electrochemical activation of numerous conventional photocatalysts in the selective cleavage of the C–N bond of quaternary ammonium salts (Scheme 14 ). 36 As discussed previously, the reduction potential of phenyltrimethylammonium salt is very low (–2.5 V vs. SCE), 13 which beyond the potential of most photosensitizers even in their excited state. Wickens and co-workers noticed that after reduction by electricity, some organic photocatalysts such as 4-DPAIPN 19 are reduced to the radical anion species I at the correct voltage (–1.6 V in this case).…”

“…Although further detailed mechanistic studies are necessary, they propose that under the irradia- During their study of the reductive activation of ammonium salts by visible-light-triggered photocatalysis, the Theunissen group noticed that the excited state of Ir(ppy) 3 can be efficiently quenched by aryltrimethylammonium triflates bearing activating groups, such as aryl, ester, nitrile, and trifluoromethyl, on the phenyl rings; their reduction potentials range from -2.13 to -1.77 V vs. SCE which very close to the reduction potential of Ir(ppy) 3 * (E(Ir IV /Ir III *) = -1.73 V vs. SCE). 13 Using this, they realized a hydrodeamination of aryltrimethylammonium triflates by the formation of key aryl radical under irradiation by blue light (Scheme 9). The use of 2.5 mol% of Ir(III) photosensitizer together with a strong light source was necessary for satisfactory efficiency.…”

“…The Theunissen group reported that both simple phenyl- and benzyltrimethylammonium triflate have a very negative reduction potential (–2.35 V vs. SCE). 13 The use of electrosynthesis provides a simple yet effective way for activation because of the tunable nature of the redox ability of such a synthetic tool. In addition, with the renaissance of photosynthesis, 14 recent progress has also shown that quaternary ammonium salts can be directly activated by light 15a or photocatalyst, 15b c enabling the development of C–N bond transformations under mild reaction conditions.…”