Radical C−N Borylation of Aromatic Amines Enabled by a Pyrylium Reagent

Abstract

Herein, we report a radical borylation of aromatic amines through a homolytic C(sp2)−N bond cleavage. This method capitalizes on a simple and mild activation via a pyrylium reagent (ScPyry‐OTf) thus priming the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp2)−N bond and forges a new C(sp2)−B bond. The unique non‐planar structure of the pyridinium intermediate, provides the necessary driving force for the aryl radical formation. The method permits borylation of a wide variety of aromatic amines indistinctively of the electronic environment.

Organic Borylation Reactions via Radical Mechanism

“…反应以 Ir 与 2-苯基吡啶及 NHC 的配合物为光催 化剂, 将苯并三唑还原开环, 并脱去氮气形成邻位酰胺 基芳基自由基, 最后与联硼试剂反应生成硼酯化产物. 图 6 苯并三唑的光催化开环-硼酯化反应 Scheme 6 Ring-opening and borylation of benzotriazoles via photocatalysis [24] . 机理研究认为吡啶盐中间体 可以被单电子还原, 进而得到芳基自由基, 实现硼酯化 过程.…”

N-Heterocyclic Carbene-Catalyzed Truce–Smiles Rearrangement of N-Arylacrylamides via the Cleavage of Unactivated C(aryl)–N Bonds

“…103 kcal/mol) . In fact, only a limited number of reports on the catalytic transformation of unactivated aniline derivatives have been appeared to date. , One major approach to this type of catalytic transformation of C­(aryl)–N bonds involves the use of transition metal catalysts, which allows for cross-coupling type reactions, although the scope of both the aniline derivatives and the nucleophiles is significantly limited compared with that for the cross-coupling using aryl halides (Scheme A). , The Truce–Smiles rearrangement of aniline derivatives is a unique alternative approach for converting an unactivated C­(aryl)–N bond to a C­(aryl)–C bond via an intramolecular S N Ar reaction. , Although the Truce–Smiles rearrangement enables an otherwise difficult bond disconnection, the need for the use of a stoichiometric amount of a strong base (e.g., BuLi, LDA etc.) diminishes its synthetic utility, and therefore catalytic variants would be highly desirable.…”

Photoinduced Deaminative Borylation of Unreactive Aromatic Amines Enhanced by CO2

“…To overcome this problem, several strategies for C–N bond transformation have been developed. Typically, highly reactive species, such as quaternary ammonium salts ([Ar-NR 3 ] + ) and diazonium salts ([Ar-N 2 ] + X – ), are prepared from arylamines in advance or in situ, and their C–N bonds are consequently converted to various functional groups by coupling reactions or radical reactions (Scheme , a-1). − The C–N bonds of arylpyridinium salts, which are readily prepared from anilines, are also transformed by a photoredox or Lewis base catalyst (Scheme , a-1). − For other methods, nickel-catalyzed reduction and borylation of N -aryl amides and coupling reactions of N -arylsulfonamides and N -aryl amides via C–N bond cleavage were developed (Scheme , a-2). In these reactions, the C–N bonds are more reactive than those of the corresponding amino groups because of the electron-withdrawing effect of the acyl or sulfonyl groups.…”

“…The resulting C–B bonds can be transformed into diverse substituents and functional groups . Metal-free radical borylation of aromatic amines, , including photoinduced reactions, ,− have recently been developed. However, the reactions generally require stoichiometric amounts of additional reagents to prepare the corresponding salts (Scheme , a-1).…”