Metal-Free Photochemical Borylation of Aryl HalidesAuthor(s): Charlo William
Arylboronic acids and esters have a wide range of uses in the chemical, pharmaceutical, and materials sciences. They are flexible synthons for the creation of carbon–carbon or carbon–heteroatom bonds in synthetic organic chemistry. Reactions of arylmetallic intermediates with trialkyl borates, followed by transesterication or hydrolysis, are the traditional procedures for producing arylboron compounds. These reactions have several significant limitations, including restricted functional group tolerance and the requirement of strict anhydrous conditions. Transition metal-catalyzed borylation processes including palladium, nickel, copper, and zinc have developed as very helpful ways for converting C–X bonds to C-B bonds in recent decades . Direct C–H borylation techniques based on transition-metal catalysts have been developed more recently. Several transition-metal-free approaches for C–B bond synthesis have been developed to reduce costs and the amount of heavy metal residue in final products. Ito and colleagues showed that an alkali alkoxide-mediated borylation of aryl halides can be achieved using a silylborane as the only borylating reagent [1,4]. Zhang and colleagues found that aryl iodides could be borylated in reuxing methanol with 4.0 equivalents of bis(pinacolato) diboron and 2.0 equivalents of Ce2CO3 as the promoter. The reaction time ranged from a few hours to several days, with moderate yields [2,3]. Under mild circumstances, Fernandes and Muniz converted diaryliodonium acetates to arylboronates.