Metal-Free Borylation Of Aryl Carbon-Hydrogen Bonds

Arylboronic acids and their derivatives are important intermediates and usefulbuilding blocks in modern organic synthesis because they are readily transformed intothe desired products, so approaches to the diversely functionalized arylboronate acidsand their derivatives have gained increasing attention. Recently, the directfunctionalization of unreactive C-H bonds has emerged as a very active field in organicsynthesis. The borylation of aryl C-H bonds has also been developed under catalysis oftransition-metals. Although there is great progress in transition-metal such as palladium,iridium or rhodium-catalyzed synthesis of arylboronate acids and their derivatives, somedrawbacks such as the use of expensive, toxic catalysts and ligands or harsh reactionconditions. Therefore, it is very desirable to develop new, efficient and practicalmethods for synthesis of arylboronic acids and their derivatives. In this dissertation, weinvestigated borylation of aryl C-H bonds, and some important results were obtained asfollows:A convenient and efficient method for C-H borylation of substituted2-arylpyridines has been developed. The borylation was easily performed with BBr3inthe presence of i-Pr2NEt in CH2Cl2. The borylation products were transformed into thecorresponding aryl halides, sulfones, azides and arylamines in good yields undercopper-catalyzed aerobic oxidation by using inorganic salts (potassium iodide,ammonium bromide, sodium alkylsulfinates, sodium azide) as the functional groupsources. This method made functionalizations of aryl C-H bonds easy.A simple and efficient metal-free ortho C-H borylation of2-aryloxypyridines hasbeen developed. The borylation protocol used readily available2-aryloxypyridines asthe starting materials, inexpensive BBr3as the borylating reagent, and cheap pinacol asthe esterifying agent, THF as the solvent, and the corresponding aryl boronates wereobtained in good yields. The present method made reaction-handling andpost-processing easy and was of good tolerance towards various functional groups inthe substrates.An easy and economical metal-free ortho C-H bis-borylation of substituted diarylethers has been developed, and the synthesized arylboronic agents were readilytransformed into various dibenzofurans. The bis-borylation protocol used readily available substituted diaryl ethers as the starting materials, inexpensivetetramethylethylenediamine as the ligand, n-butyl lithium as the base, cheap trimethylborate as the borylating reagent, and the corresponding arylboronic agents weresynthesized in good to excellent yields. Further, the obtained arylboronic agents weretransformed into dibenzofurans in high yields by using I2as the reactant, Pd(OAc)2asthe catalyst, K2CO3as the base in CH3CN under nitrogen atmosphere.