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Although aromatic hydrocarbons are not as tough to functionalize as alkanes, they are still a challenge. Only a few catalytic systems that can activate aromatic carbon-hydrogen bonds are known. Author have found a remarkably efficient catalytic method for regioselectively adding aromatic compounds to acetylenes. Even sterically bulky arenes add to alkynes when catalyzed by palladium acetate in trifluoroacetic acid (TFA), the group reports. The addition occurs in a trans fashion, so the products are cis alkenes, which can be hard to prepare by other methods. The active catalytic species is a Pd(II) cationic species—[PdO2CCF3]+—formed from TFA, the researchers suggest. Author who has been investigating CH activation by palladium for more than 30 years, asked research fellow Chengguo Jia to investigate the coupling of acetylenes with aromatic compounds. Unexpectedly, Jia discovered that with TFA as solvent the aromatics add to acetylenes instead.
"It is surprising that the reaction occurs under mild conditions such as room temperature, "and that not only inter- but also intramolecular reactions take place. More surprising, we found that heteroaryls such as methylfurane, pyrroles, and indoles can undergo this reaction in acetic acid in lieu of TFA at room temperature." Heteroaromatic compounds, such as the coumarins, are of special interest to the pharmaceutical industry, he notes.
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