Mechanistic Study of Indolizine Heterocycle Formation
by Ruthenium(II)-Assisted
Three-Component Cross-Coupling<b>/</b>Cyclization
- Publication date
- Publisher
Abstract
In
the presence of the acid HBF<sub>4</sub>, 3-alkenyl-2-phosphonium
indolizines <b>3a</b>–<b>c</b> can be produced
respectively by adding PhCCCOCH<sub>3</sub> (<b>2a</b>), CH<sub>3</sub>OCOCCCOOCH<sub>3</sub> (<b>2b</b>),
and CH<sub>3</sub>CH<sub>2</sub>CCCOCH<sub>3</sub> (<b>2c</b>) to a mixture of ruthenium complex RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> and the propargyl alcohol (2-Py)CH(OH)CCH
(<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