Osmium-Acyl Decarbonylation Promoted by Tp-Mediated Allenylidene Abstraction: A New Role of the Tp Ligand

Phenylacetylene and 1,1-diphenyl-2-propyn-1-ol displace the pyridyl and ethylene ligands of OsTp­{κ¹-<i>C</i>[NC₅H₃Me]}­(η²-CH₂CH₂)­(PⁱPr₃) (<b>1</b>) to afford the alkynyl-vinylidene derivatives OsTp­(CCR)­(CCHR)­(PⁱPr₃) (Tp = hydridotris­(pyrazolyl)­borate; R = Ph, (<b>2</b>), C­(OH)­Ph₂ (<b>3</b>)). Stirring of toluene solutions of <b>3</b> in the presence of neutral aluminum oxide initially produces the dehydration of the vinylidene ligand to give the alkynyl-allenylidene OsTp­{CCC­(OH)­Ph₂}­(CCCPh₂)­(PⁱPr₃) (<b>4</b>), which subsequently undergoes a Meyer-Schuster rearrangement to yield the acyl-allenylidene OsTp­{C­(O)­CHCPh₂}­(CCCPh₂)­(PⁱPr₃) (<b>5</b>). In toluene, at 60 °C, the B–H bond of the Tp ligand of <b>5</b> abstracts the allene from the osmium coordination sphere by means of a 1,1-hydroboration of the Os–C_α double bond. As a result, the acyl decarbonylation takes place to form Os­(^{Ph2CCCH}Tp)­(CHCPh₂)­(CO)­(PⁱPr₃) (<b>6</b>). In acetonitrile at 60 °C, the allene tautomerizes into the alkyne to afford Os­(^Ph2HCCCTp)­(CHCPh₂)­(CO)­(PⁱPr₃) (<b>7</b>). The abstraction modifies the reactivity of the system toward HBF₄. Thus, while the protonation of <b>5</b> leads to the alkylidyne [OsTp­{C­(O)­CHCPh₂}­(CCHCPh₂)­(PⁱPr₃)]­BF₄ (<b>8</b>), the protonation of <b>6</b> and <b>7</b> gives the alkylidenes [Os­(^{Ph2CCCH}Tp)­(CHCHPh₂)­(CO)­(PⁱPr₃)]­BF₄ (<b>9</b>) and [Os­(^Ph2HCCCTp)­(CHCHPh₂)­(CO)­(PⁱPr₃)]­BF₄ (<b>10</b>). Both <b>9</b> and <b>10</b> are unstable in solution. In dichloromethane, at room temperature, they evolve into the corresponding olefin derivatives [Os­(^{Ph2CCCH}Tp)­(η²-CH₂CPh₂)­(CO)­(PⁱPr₃)]­BF₄ (<b>11</b>) and [Os­(^Ph2HCCCTp)­(η²-CH₂CPh₂)­(CO)­(PⁱPr₃)]­BF₄ (<b>12</b>) by means of a C_β,C_α-hydrogen shift on the alkylidene ligand