Inthe presence of the acid HBF4, 3-alkenyl-2-phosphoniumindolizines 3aâccan be producedrespectively by adding PhCî¼CCOCH3(2a), CH3OCOCî¼CCOOCH3(2b),and CH3CH2Cî¼CCOCH3(2c) to a mixture of ruthenium complex RuCl2(PPh3)3and the propargyl alcohol (2-Py)CH(OH)Cî¼CH(1). We carefully investigated the mechanism of thisreaction by means of structurally characterizing two key intermediates,ruthenium vinyl (4) and ruthenium carbene (5), and by deuterium-labeling experiments. A plausible mechanism isproposed, which involves addition of a proton to an alkyne carbonand the insertion of an alkyne into the Cα bond of an alkenylcarbenegroup, followed by an α-H elimination and reductiveelimination. [ABSTRACT FROM AUTHOR]