Electron-Catalyzed Cross-Coupling of Arylboron Compounds with Aryl Iodides

Purple Light-Promoted Coupling of Bromopyridines with Grignard Reagents via SET

Alkyl- and arylpyridines and 2,2'-bipyridines are conventionally prepared by Minisci reactions of pyridines and transition metal-catalyzed coupling reactions of halopyridines. Herein, purple light-promoted radical coupling reactions of 2- or 4-bromopyridines with Grignard reagents in Et2O or a mixture of Et2O and tetrahydrofuran in regular glassware without the need for a transition metal catalyst were disclosed for the first time. Methyl, primary and secondary alkyl, cycloalkyl, aryl, heteroaryl, pyridyl, and alkynyl Grignard reagents were compatible with the protocol. As a result, alkyl- and arylpyridines and 2,2'-bipyridines were easily prepared. Single electron transfer from the Grignard reagent to bromopyridine was stimulated by purple light. An electron extruded from the dimerization of the Grignard reagent worked as the catalyst. Light on/off experiments indicated that constant irradiation was required for product formation. Studies of radical clock substrates verified the involvement of a pyridyl radical from bromopyridine and the noninvolvement of an alkyl or aryl radical from the Grignard reagent. The available proof supports a photoinduced SRN mechanism for the new coupling reactions.

Manipulation of an electron by photoirradiation in the electron-catalyzed cross-coupling reaction

An electron has recently been shown to catalyze the cross-coupling reaction of organometallic compounds with aryl halides. In terms of green and sustainable chemistry, the electron catalysis is much more desirable than the inevitably used transition metal catalysis but a high temperature of more than 100°C is required to achieve it. Here, we disclose that visible light photoirradiation accelerates the electron-catalyzed reaction of arylzinc reagents with aryl halides with the aid of a photoredox catalysis. Photoexcitation of a photoredox catalyst and an anion radical intermediate respectively affects the supply and transfer of the electron catalyst, promoting the cross-coupling reaction to proceed at room temperature. The supply of the electron catalyst by the photoredox catalysis makes the scope of aryl halides wider.

Denitrative Hydroxylation of Unactivated Nitroarenes

A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalization protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that nitroarenes may be substituted by an electron-catalysed radical-nucleophilic substitution (S_RN 1) chain mechanism.

Recent Synthesis Developments of Organoboron Compounds via Metal-Free Catalytic Borylation of Alkynes and Alkenes

Diboron reagents have been traditionally regarded as "Lewis acids", which can react with simple Lewis base to create a significant nucleophilic character in one of boryl moieties. In particular, bis(pinacolato)diboron (B₂pin₂) reacts with simple Lewis bases, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through metal-free catalytic diboration and the β-boration reactions of alkynes and alkenes.