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{κ 1 - C [NC 5 H 3 Me]}(η 2 -CH 2 ═CH 2 )(P i Pr 3 ) ( 1 ) to afford the alkynyl-vinylidene derivatives OsTp(C≡CR)(═C═CHR)(P i Pr 3 ) (Tp = hydridotris(pyrazolyl)borate; R = Ph, ( 2 ), C(OH)Ph 2 ( 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)Ph 2 }(═C═C═CPh 2 )(P i Pr 3 ) ( 4 ), which subsequently undergoes a Meyer-Schuster rearrangement to yield the acyl-allenylidene OsTp{C(O)CH═CPh 2 }(═C═C═CPh 2 )(P i Pr 3 ) ( 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–C α double bond. As a result, the acyl decarbonylation takes place to form Os( Ph2C═C═CH Tp)(CH═CPh 2 )(CO)(P i Pr 3 ) ( 6 ). In acetonitrile at 60 °C, the allene tautomerizes into the alkyne to afford Os( Ph2HCC≡C Tp)(CH═CPh 2 )(CO)(P i Pr 3 ) ( 7 ). The abstraction modifies the reactivity of the system toward HBF 4 . Thus, while the protonation of 5 leads to the alkylidyne [OsTp{C(O)CH═CPh 2 }(≡CCH═CPh 2 )(P i Pr 3 )]BF 4 ( 8 ), the protonation of 6 and 7 gives the alkylidenes [Os( Ph2C═C═CH Tp)(═CHCHPh 2 )(CO)(P i Pr 3 )]BF 4 ( 9 ) and [Os( Ph2HCC≡C Tp)(═CHCHPh 2 )(CO)(P i Pr 3 )]BF 4 ( 10 ). Both 9 and 10 are unstable in solution. In dichloromethane, at room temperature, they evolve into the corresponding olefin derivatives [Os( Ph2C═C═CH Tp)(η 2 -CH 2 ═CPh 2 )(CO)(P i Pr 3 )]BF 4 ( 11 ) and [Os( Ph2HCC≡C Tp)(η 2 -CH 2 ═CPh 2 )(CO)(P i Pr 3 )]BF 4 ( 12 ) by means of a C β,C α -hydrogen shift on the alkylidene ligand.