Acid-Mediated Protodeboronation And Synthesis Of Biaryls

Biaryl compounds,important structural motifs in pharmaceuticals and functional materials,are usually formed through Ullmann reaction and Suzuki-Miyaura cross-coupling reaction.The latter is more popular to offer unsymmetric biaryls because of the low toxicity and high stability of the arylboronic acids.The competitive dimerization of two molecular arylboronic acids which is an important method to obtain symmetric biaryl compounds attracts increasing interest.In addition,the deliberate removal of the-B(OH)2 group of the arylboronic acids in several examples,such as asymmetric synthesis,gives the protodeboronation increased value.The homocoupling of arylboronic acids was mainly performed in the presence of a base and no report on the acid-mediated dimerization.This poses a new aspect to the base-mediated coupling reaction.Although several methods have been reported mainly including the use of base and/or metal catalysts,metal-free acid-mediated protodeboronation is more green and environmental.This acid-mediated protodeboronation also poses a new aspect to the base-promoted protodeboronation.In this study,treatment of arylboronic acids with a catalytic amount of Pd(OAc)2 in AcOH leads to symmetrical biaryls through a regioselective oxidative homocoupling reaction.Under the same reaction conditions in the absence of Pd catalyst,protodeboronation was observed.Unlike biaryls normally formed from arylboronic acids in alkaline or occasionally neutral conditions,this coupling reaction is performed in an acidic medium under ligand-free conditions.Both the homocoupling reaction and the protodeboronation are general for a range of arylboronic acids in good to excellent yields under mild conditions.2,2,6,6-tetramethylpiperidin-1-oxyl(TEMPO),as a key additive,facilitates the homocoupling reactions very effectively.The main studies include:1.Investigating of the metal-free acid-mediated protodeboronation.Through optimizing the reaction condition,such as solvents and temperature,a simple,environmental and efficient method of protodeboronation was developed.We also expanded the scope of arylboronic acids,which proved the universality of the protodeboronation reaction.2.Investigating of reaction mechanism of the acid-promoted protodeboronation with the help of density functional theory(DFT)calculations and the natural bond orbital(NBO)analysis.3.Studying the palladium-catalyzed acid-mediated homocoupling of arylboronic acids.We studied the influence of the solvents,temperature,catalysts and various substituents to the homocoupling reaction in the absence of ligand.4.Studying of TEMPO-oxidized synthesis of symmetric biaryls.The influence of different oxidants to the homocoupling of arylboronic acids and the universality of the homocoupling in the presence of TEMPO were studied.