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

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

Synthesis, Structure, and Reactivity of an Osmacyclopentene Complex

Treatment of OsCl₂(PPh₃)₂(CHC­(PPh₃)­CH­(OH)-η²-CCH) (<b>1</b>) with PPh₃ and Bu₄NCl in CH₂Cl₂ under air gave paramagnetic osmacyclopentene [OsCl₂(PPh₃)₂(CHC­(PPh₃)­CH­(OH)­C­(CH­(PPh₃)))]Cl (<b>5</b>). Heating the suspension of <b>5</b> in CH₂Cl₂ under a nitrogen atmosphere led to the formation of η²-allene-coordinated osmacycle OsCl₂(PPh₃)­(CHC­(PPh₃)­CHCCH­(P­(C₆H₄)­Ph₂)) (<b>6</b>) and chloro-osmabenzene OsCl₂(PPh₃)₂­(CHC­(PPh₃)­CHCClCH) (<b>7</b>) via disproportionation reaction. Oxidative transformation of <b>5</b> under an oxygen atmosphere yielded osmafuran [OsCl­(PPh₃)₂­(CHC­(PPh₃)­CHO)­(CC­(PPh₃))]Cl (<b>10</b>). In addtion, complexes <b>6</b>, <b>7</b>, and <b>10</b> can also undergo ligand substitution reactions with NaSCN to afford more stable analogues