Halogen-bonding-promoted Photo-induced C–X Borylation of Aryl Halide using Phenol Derivatives

This study investigates the photo-induced C–X borylation reaction of aryl halides by forming a halogen-bonding complex. The method employs 2-naphthol as a halogen-bonding acceptor and proceeds under mild conditions without a photoredox catalyst under 420 nm blue light irradiation. The method is highly chemoselective, broadly functional group tolerant, and provides concise access to corresponding boronate esters. Mechanistic studies reveal that forming the halogen-bonding complex between aryl halide and naphthol acts as an electron donor-acceptor complex to furnish aryl radicals through photo-induced electron transfer. INTRODUCTION Visible light-mediated photoexcitation of small molecules has been used for various useful reactions to specific reactivity derived from electrically excited open-shell chemical species.1 On the contrary, direct photochemical activation of small molecules requires strong energy such as ultraviolet light. Moreover, indirect methods via activation of expensive transition metal-based photocatalysts with visible light response are frequently used.2 Photochemical reactions using electron donor-acceptor (EDA) complexes, a new type of non-covalent bonding proposed by Mulliken, have recently attracted much attention as an alternative methodology to photocatalysis.3 EDA complexes are formed by the coordination between two molecules, such as electron donors and electron acceptors. In many cases, the energy of this transition is in the visible light region, and one electron is transferred from the electron donor to the electron acceptor, leading to a reactive open-shell intermediate (Figure 1a).4 Various aggregation modes of EDA complexes are reported, and p-p interactions and hydrogen bonds are frequently used in the organic photochemistry, but recently n-s interactions such as halogen-bonding interactions have been attracting attention.5