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