Pt(II)-Catalyzed Synthesis of Indolines and Tetrahydroquinolines

Significance

Reported is the synthesis of indolines by a platinum(II)-catalyzed tandem reaction of 1,4-aminoalkynes and 1,3-diketones. Under optimal conditions as shown, the reaction proceeds to afford 76-96% yields for R^¹ = EDG aryl or alkyl groups, whereas higher catalyst loading (5 mol%) is required for R^¹ = EWG aryl. Terminal alkynes give better yields than internal alkynes. Unsymmetrical 1,3-diketones afford indolines with moderate to very good regioselectivity, depending on both steric and electronic properties of R⁴, R⁵ and R⁶. A reasonable mechanism was proposed as shown but it is based only on the mass spectral evidence for the formation of the enamine 1. The reaction was extended to 1,5-aminoalkynes to give the corresponding 1,2,3,4-tetrahydroquinolines in good yields, although it was less extensively studied.