Supercritical Carbon Dioxide as Solvent and Temporary Protecting Group for Rhodium-Catalyzed Hydroaminomethylation

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Abstract
Supercritical carbon dioxide (scCO2) acts simultaneously as solvent and temporary protecting group during homogeneously rhodium‐catalyzed hydroaminomethylation of ethyl methallylic amine. Cyclic amines are formed as the major products in scCO2, whereas the cyclic amide is formed preferentially in conventional solvents. Multinuclear high‐pressure NMR spectroscopy revealed that this selectivity switch is mainly due to reversible formation of the carbamic acid in the solvent CO2, which reduces the tendency for intramolecular ring closure at the Rh–acyl intermediate. These results substantiate the general concept of using scCO2 as a protective medium for amines in homogeneous catalysis and demonstrate for the first time its application for selectivity control.