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Synthesis

Tropylium cation now serves as an organocatalyst

Delocalized seven-carbon ring system facilitates metal-free batch and flow acetalization reactions

July 3, 2017 | APPEARED IN VOLUME 95, ISSUE 27

The tropylium ion is a curious seven-membered aromatic ring (C7H7+) used in organic synthesis and as a ligand for metal complexes. Chemists led by T. Vinh Nguyen of the University of New South Wales Sydney have shown for the first time that tropylium salts can act as organic Lewis acid catalysts (Green Chem. 2017, DOI: 10.1039/c7gc01519d). Carbonyl compounds used in multistep reactions are often temporarily functionalized via acetalization reactions to mask their reactivity. This protection step typically relies on a metal salt Lewis acid catalyst. But residual metal can pose problems in purifying pharmaceutical products, leading to excessive use of solvents and generating waste. Organocatalysts that avoid metals are often a greener option. Building on their prior research on tropylium chemistry, Nguyen and coworkers reasoned that the carbocation with one positive charge delocalized over the conjugated seven-membered ring could serve as a “soft Lewis catalyst” for protective acetalizations of aldehydes with alcohols and esters. The team also tested the method in a flow reactor system to save time and facilitate catalyst recycling, further reducing the environmental impact of the reaction. “This is one of the most interesting papers I have read in a long time,” says James H. Davis Jr. of the University of South Alabama, who studies ionic liquids and molecular salts. “I find the simplicity of the catalyst and its broad, metal-free scope of activity especially exciting.”

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