Mechanistic Study of Indolizine Heterocycle Formation by Ruthenium(II)-Assisted Three-Component Cross-Coupling<b>/</b>Cyclization

Abstract

In the presence of the acid HBF 4, 3-alkenyl-2-phosphonium indolizines 3a – c can be produced respectively by adding PhC≡CCOCH 3 ( 2a ), CH 3 OCOC≡CCOOCH 3 ( 2b ), and CH 3 CH 2 C≡CCOCH 3 ( 2c ) to a mixture of ruthenium complex RuCl 2 (PPh 3 ) 3 and the propargyl alcohol (2-Py)CH(OH)C≡CH ( 1 ). We carefully investigated the mechanism of this reaction by means of structurally characterizing two key intermediates, ruthenium vinyl ( 4 ) and ruthenium carbene ( 5 ), and by deuterium-labeling experiments. A plausible mechanism is proposed, which involves addition of a proton to an alkyne carbon and the insertion of an alkyne into the Cα bond of an alkenylcarbene group, followed by an α - H elimination and reductive elimination.