Synthesis of Amides and Lactams in Supercritical Carbon Dioxide

Abstract

Supercritical carbon dioxide can be employed as an environmentally friendly alternative to conventional organic solvents for the synthesis of a variety of carboxylic amides. The addition of amines to ketenes generated in situ via the retro-ene reaction of alkynyl ethers provides amides in good yield, in many cases with ethylene or isobutylene as the only byproducts of the reaction. Reactions with ethoxy alkynes are performed at 120−130 °C, whereas tert-butoxy derivatives undergo the retro-ene reaction at 90 °C. With the exception of primary, unbranched amines, potential side reactions involving addition of the amines to carbon dioxide are not competitive with the desired C−N bond-forming reaction. The amide synthesis is applicable to the preparation of β-hydroxy and β-amino amide derivatives, as well as amides bearing isolated carbon−carbon double bonds. Preliminary experiments aimed at developing an intramolecular variant of this process to afford macrolactams suggest that the application of CO2/co-solvent mixtures may offer advantages for the synthesis of large-ring compounds.