A highly selective and general photocatalytic C–F borylation protocol that employs a rhodium biphenyl complex as a triplet sensitizer and the nickel catalyst [Ni­(IMes)2] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the C–F bond activation and defluoroborylation process is reported. This tandem catalyst system operates with visible (blue, 400 nm) light and achieves borylation of a wide range of fluoroarenes with B2pin2 at room temperature in excellent yields and with high selectivity. Direct irradiation of the intermediary C–F bond oxidative addition product trans-[NiF­(ArF)­(IMes)2] leads to very fast decomposition when B2pin2 is present. This destructive pathway can be bypassed by indirect excitation of the triplet states of the nickel­(II) complex via the photoexcited rhodium biphenyl complex. Mechanistic studies suggest that the exceptionally long-lived triplet excited state of the Rh biphenyl complex used as the photosensitizer allows for efficient triplet energy transfer to trans-[NiF­(ArF)­(IMes)2], which leads to dissociation of one of the NHC ligands. This contrasts with the majority of current photocatalytic transformations, which employ transition metals as excited state single electron transfer agents. We have previously reported that C­(arene)–F bond activation with [Ni­(IMes)2] is facile at room temperature, but that the transmetalation step with B2pin2 is associated with a high energy barrier. Thus, this triplet energy transfer ultimately leads to a greatly enhanced rate constant for the transmetalation step and thus for the whole borylation process. While addition of a fluoride source such as CsF enhances the yield, it is not absolutely required. We attribute this yield-enhancing effect to (i) formation of an anionic adduct of B2pin2, i.e., FB2pin2 –, as an efficient, much more nucleophilic {Bpin–} transfer reagent for the borylation/transmetalation process, and/or (ii) trapping of the Lewis acidic side product FBpin by formation of [F2Bpin]− to avoid the formation of a significant amount of NHC-FBpin and consequently decomposition of {Ni­(NHC)2} species in the reaction mixture.

Organoboron compounds have important synthetic value and can be applied in numerous transformations. The development of practical and convenient ways to synthesize boronate esters has thus attracted significant interest. Photoinduced borylations originated from stoichiometric reactions of alkanes and arenes with well-defined metal–boryl complexes. Now, photoredox-initiated borylations, catalyzed by either transition metal or organic photocatalysts, and photochemical borylations with high efficiency have become a burgeoning area of research. In this Focus Review, we summarize research on photoinduced borylations, especially emphasizing recent developments and trends. This includes the photoinduced borylation of arenes, alkanes, aryl/alkyl halides, activated carboxylic acids, amines, alcohols, and so on based on transition metal catalysis, metal-free organocatalysis, and direct photochemical activation. We focus on reaction mechanisms involving single-electron transfer, triplet-energy transfer, and other radical processes.

A highly selective and general photoinduced C−Cl borylation protocol that employs [Ni(IMes) 2 ] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes is reported. This photoinduced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B 2 pin 2 at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes) 2 ] undergoes very fast chlorine atom abstraction from aryl chlorides to give [Ni I (IMes) 2 Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [Ni I (IMes) 2 Cl] with aryl chlorides generating additional aryl radicals and [Ni II (IMes) 2 Cl 2 ]. The aryl radicals react with an anionic sp 2 −sp 3 diborane [B 2 pin 2 (OMe)] − formed from B 2 pin 2 and KOMe to yield the corresponding borylation product and the [Bpin(OMe)] •− radical anion, which reduces [Ni II (IMes) 2 Cl 2 ] under irradiation to regenerate [Ni I (IMes) 2 Cl] and [Ni(IMes) 2 ] for the next catalytic cycle.

A highly efficient and general protocol for traceless, directed C3-selective C–H borylation of indoles with [Ni­(IMes)2] as the catalyst is reported. Activation and borylation of N–H bonds by [Ni­(IMes)2] is essential to install a Bpin moiety at the N-position as a traceless directing group, which enables the C3-selective borylation of C–H bonds. The N-Bpin group which is formed is easily converted in situ back to an N–H group by the oxidative addition product of [Ni­(IMes)2] and in situ-generated HBpin. The catalytic reactions are operationally simple, allowing borylation of a variety of substituted indoles with B2pin2 in excellent yields and with high selectivity. The C–H borylation can be followed by Suzuki–Miyaura cross-coupling of the C-borylated indoles in an overall two-step, one-pot process providing an efficient method for synthesizing C3-functionalized heteroarenes.

An unusual multiple isocyanide insertion reaction with methyleneindolinone using indium(III) chloride as the catalyst has been disclosed. This strategy allows for the rapid construction of structurally complex spirooxindole in an efficient manner. The present protocol features mild conditions, atom economy, and broad substrate scope.