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

Abstract

In the presence of the acid HBF₄, 3-alkenyl-2-phosphonium indolizines <b>3a</b>–<b>c</b> can be produced respectively by adding PhCCCOCH₃ (<b>2a</b>), CH₃OCOCCCOOCH₃ (<b>2b</b>), and CH₃CH₂CCCOCH₃ (<b>2c</b>) to a mixture of ruthenium complex RuCl₂(PPh₃)₃ and the propargyl alcohol (2-Py)­CH­(OH)­CCH (<b>1</b>). We carefully investigated the mechanism of this reaction by means of structurally characterizing two key intermediates, ruthenium vinyl (<b>4</b>) and ruthenium carbene (<b>5</b>), 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<i>-</i>H elimination and reductive elimination