Regioselective synthesis of tetrahydroquinolines via syn- and anti-nucleopalladation-initiated cascade processes?

Organic & Biomolecular Chemistry Pub Date: 2020-10-05 DOI: 10.1039/D0OB01840F

Abstract

Palladium(II)-catalyzed regioselective syn-chloropalladation and anti-acetoxypalladation-initiated cascade processes were developed for the synthesis of functionalized tetrahydroquinolines. A series of N-propargyl arylamines tethered with an α,β-unsaturated carbonyl scaffold underwent atom economical cascade reactions to deliver chloro- and acetoxy-substituted tetrahydroquinolines bearing an exocyclic double bond in high yields. A mechanism is proposed for these cascade processes involving a sequential syn-chloropalladation or anti-acetoxypalladation of alkynes followed by intramolecular olefin insertion (6-exo-trig) and protonolysis steps. The reaction was completely regioselective and the terminal aryl/alkyl group of the propargyl moiety dictated the regiochemistry of the initial nucleopalladation. The role of the bidentate nitrogen ligand is crucial to trigger the acetoxypalladation-initiated cascade sequence in contrast to the chloropalladation-initiated process.

Graphical abstract: Regioselective synthesis of tetrahydroquinolines via syn- and anti-nucleopalladation-initiated cascade processes
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