Selective dehydrogenation of tetrahydroisoquinolines in the presence of sulfoxides

1,2,3,4-Tetrahydroisoquinolines undergo selective dehydrogenation in the presence of compounds with a highly polar X=O bond, particularly sulfoxides, under aerobic conditions to give the corresponding 3,4dihydroisoquinolines.The reaction can be carried out in DMSO as solvent, or in toluene in the presence of other sulfoxides, including aryl alkyl sulfoxides.The reaction can also be carried out in toluene in the presence of trimethylphosphine oxide in place of DMSO.NMR spectroscopic studies strongly suggest that the sulfoxide is not consumed during the dehydrogenation reaction.The methodology benefits from simplicity of operation, and the lack of harmful reagents or expensive metal catalysts.