Titanium(III)-induced transformation of hydroxylamines to imines or secondary amines

Article abstract of DOI:10.1246/bcsj.67.2542

N,N-Disubstituted and cyclic hydroxylamines can be converted into the corresponding imines efficiently upon treatment with anhydrons titanium trichloride in THF at room temperature. Similar treatment of N-allylhydroxylamines with anhydrous titanium trichloride gives 1-azadienes, which are versatile synthetie intermediates for aza-Diels-Alder reactions. On the other hand, the same hydroxylamines can be converted into the corresponding secondary amines upon treatment with aqueous titanium trichloride in methanol. It is noteworthy that optically active hydroxylamines, which have chirality at the α-position to nitrogen, can be converted into optically active secondary amines without loss of chirality. Dihydro-2(1H)-quinolinones can be prepared upon treatment of 1-hydroxy-3,4-dihydro-2(1H)-quinolinones with aqueous titanium trichloride. The substrates of N,N-disubstituted and cyclic hydroxylamines can be prepared readily upon treatment of nitrones with nucleophiles. Since nitrones can be prepared by metal-catalyzed oxidations of secondary amines with hydrogen peroxide, the present titanium(III)-promoted reaction of hydroxylamines will provide a convenient method for the synthesis of either α-substituted imines or amines from secondary amines.