Oxidation of 1,2,3,4-Tetrahydroisoquinolines to 3,4-Dihydroisoquinolines with Molecular Oxygen Catalyzed by Copper(Ö†) Chloride

-A catalytic oxidation system, a CuC12-02 system, was efficient for dehydrogenation of 1,2,3,4-tetrahydroisoquinolines to 3.4-dihydroisoquinolines. Oxidation of 1,2,3.4-tetrahydroquinoline was also carried out. The oxidation of amines to imines can be only carried out under severe reaction conditions genera1ly.l 1.2,3,4-Tetrahydroisoquinoiines are one of the cyclic amines and only a few oxidation methods to the 3,4-dihydroisoquinolines have been r e p ~ r t e d . ~ The 3,4-dihydro-isoquinoline derivatives become useful intermediates for the synthesis of isoquinoline a~kaloids.~ However, the oxidation of 1,2.3,4-tetrahydroisoquinolines sometimes proceeds until undesirable overoxidized imine N-oxides.4 Especially, there has been no report for dehydrogenation of the tetrahydroisoquinolines with molecular oxygen. As dehydrogenation of heterocyclic compounds, oxidation of indoiines to indoles was reported with a CuCI-pyridine-02 systern.5 We have investigated a CuC12-amine-02 catalytic oxidation system, and effective synthetic methods of p-benzoquinones6 and p-hydroxybenzaldehyde7 from phenols were reported i n the previous papers. We report herein the catalytic