Supercritical carbon dioxide as solvent and temporary protecting group forrhodium-catalyzed hydroaminomethylation

Supercritical carbon dioxide (scCO(2)) acts simultaneously as solvent and t emporary protecting group during homogeneously rhodium-catalyzed hydroamino methylation of ethyl methallylic amine. Cyclic amines are formed as the maj or products in scCO(2), whereas the cyclic amide is formed preferentially i n conventional solvents. Multinuclear high-pressure NMR spectroscopy reveal ed that this selectivity switch is mainly due to reversible formation of th e carbamic acid in the solvent CO,, which reduces the tendency for intramol ecular ring closure at the Rh-acyl intermediate. These results substantiate the general concept of using scCO(2) as a protective medium for amines in homogeneous catalysis and demonstrate for the first time its application fo r selectivity control.