Stereoselective Synthesis of trans-Tetrahydroindolines Promoted by a Tungsten π Base

Indoline and tetrahydroquinoline derivatives form η2-bound complexes with the dearomatization agent {TpW(NO)(PMe3)} that can be isolated as triflic acid salts. Protonation occurs selectively in the aromatic ring, either ortho or para to the nitrogen. With the tetrahydroquinoline complexes, para protonation dominates. For the indoline ligands, alkylation of the nitrogen dramatically affects the ortho/para isomer ratio present after protonation. In the case of the N-ethylindoline and N-isopropylindoline ligands, the bridgehead-protonated isomer (ortho) is formed with >10/1 selectivity. This indolinium isomer is found to undergo acid-catalyzed hydroarylation or hydroamination with various heterocycles. Oxidative decomplexation is demonstrated for the pyrazole and 2-methylfuran derivatives using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The resulting tetrahydroindolines feature three new stereocenters determined by the configuration of the tungsten complex.