Boryl Radical Mediated Hydro(gem-diboryl)alkylation of Alkenes with Sterically Hindered NHC Boranes

Abstract

NHC boryl radical mediated halogen atom transfer (XAT) is useful in organic synthesis. Yet, most of the reaction ends only with reducing the halogen to hydrogen, that is, the C−X to C−H. This is especially dominant for electron-deficient alkyl halides, where the formed electrophilic radical reacts rapidly with NHC boranes. Herein, by employing a sterically hindered NHC borane as the boryl radical precursor (IPr•BH 3 ), we were able to use the electrophilic-deficient alkyl halide (α-Iodide gemdi(B(pin))methane) in the C−C bond formation reaction. Mono-, disubstituted styrene, aliphatic alkenes, and heteroatomsubstituted alkenes were used as reaction partners. Forty hydro(gem-diboryl)methylation products were obtained at room temperature in moderate to good yields. Detailed mechanistic studies revealed that the reaction mainly involved the radical process.B oryl radical chemistry has been getting increasing attention in the past several decades. 1 Due to the electron-deficient nature of the boron, the neutral, 3-center-5-electron boryl radicals are extremely unstable. 2 In the 1980s, Roberts and co-workers found that the Lewis base (LB) ligation could stabilize the boryl radical, and the thus formed LB-ligated boryl radicals are a more stable 4-center-7-electron species than the nonligated ones. 3 Since 2008, Curran and coworkers have extensively studied a new type of boryl radicals ligated by N-heterocyclic carbenes (NHCs), which showed versatile reactivities in various reactions. 4 Among the many reactions, boryl radical mediated halogen atom transfer (XAT) is characteristic. 5 Yet, most of the reaction ends only with reducing the halogen to hydrogen, that is, the C−X to C−H. This is especially dominant for electron-deficient alkyl halides, where the formed electrophilic radical reacts rapidly with boranes. For electron-neutral/rich alkyl halide, the obtained radical from XAT is nucleophilic and reacts with boranes more slowly. 6 This gives chances for interrupted boryl radicalmediated reduction, which allows the transformation from C− X to C−C. In 2023, Noel and co-workers reported the photocatalyzed NHC boryl radical-mediated C(sp 3 )-C(sp 3 ) bond formation reaction, where the in situ formed radical was trapped with highly electron-deficient alkenes (Scheme 1a). 7 Later, the reaction was found to occur without the photocatalyst under UV-A light irradiation. 7 Recently, Leonori and co-workers have reported the photocatalyzed trimethyl amine boryl radical-mediated C(sp 3 )-C(sp 2 ) reaction, where the in situ formed radical was trapped with Aryl-Ni(II) species (Scheme 1b). 8 As electrophilic radicals could add to unactivated alkenes, it would be highly desirable to trap the radical derived from the process and achieve the hydro-