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Green Chemistry

Birch reduction runs ammonia-free

Crown ethers simplify the set-up of classic reaction

by Tien Nguyen
June 17, 2018 | APPEARED IN VOLUME 96, ISSUE 25

 

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The new Birch conditions reduce aromatic, heteroaromatic, alkenyl and alkynyl compounds.

To break compounds’ aromaticity, chemists commonly rely on the 74-year-old Birch reduction, which uses sodium metal and ammonia. However, the components’ moisture-sensitive nature—requiring practitioners to trot out potentially hazardous ammonia tanks to freshly distill the reagent—makes running the reaction a chore. A modified Birch reduction developed in Jie An’s lab at China Agricultural University lets scientists skip the arduous set-up (Org. Lett. 2018, DOI: 10.1021/acs.orglett.8b00891). The researchers’ first experiences with the Birch reduction, attempting to reduce graphite to graphene, resulted in long set-up times and a small, but likely still smelly, ammonia leak, spurring them to try to simplify things. The new method replaces ammonia with the commercial crown ether 15-crown-5, which can be recovered after the reaction. The crown ether takes on ammonia’s role for the most part; it mixes with a bench-stable sodium dispersion to form the solvated electrons that reduce the substrates and give the reaction its signature blue hue. However, tertiary amides, typically converted to alcohols under standard Birch conditions, survived in the team’s reaction, suggesting that the crown ether reagent switches the mechanism from an inner- to an outer-sphere electron transfer.

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