Palladium-Catalyzed Tandem Reaction of Yne-Propargylic Carbonates with Boronic Acids:A Simple Method for the Synthesis of Fused Aromatic Rings through Allene-Mediated Electrocyclization

摘要

The palladium-catalyzed tandem reactions of yne-propargylic carbonates with aryl boronic acids,2-furyl boronic acid,and 2-thiophenebor-onic acid,followed by 6pi-electrocycli-zation to give fused ring aromatic products such as naphthalene,benzofuran,and benzothiophene derivatives are realized.Screening of the reaction conditions revealed that the combination of [Pd(PPh_3)_4] in THF gave the best results in terms of reactivity and product yields in the reaction of yne-propargylic carbonates with phenylboronic acid.The reaction is sensitive toward steric hindrance when substituted phenylboronic acids are emplyed.However,when we take 2-furyl boronic acid as the organometallic reagent,most substrates perform very well to give benzofuran derivatives.In addition,2-thio-pheneboronic acid is also a very effective coupling reagent to give bezothio-phenes in high yields.A mechanism is proposed that involves the formation of an allenylpalladium complex from Pd~0 and propargylic carbonate,followed by insertion of an intramolecular triple bond and the Suzuki coupling reaction,and then electrocyclization.