C−H Bond Activation of Terminal Allenes: Formation of Hydride-Alkenylcarbyne-Osmium and Disubstituted Vinylidene-Ruthenium Derivatives

Abstract

The reactivity of the dihydrides MH 2 Cl 2 (P i Pr 3 ) 2 (M = Os (1), Ru (2)) toward allenes has been studied. Complex 1 reacts with 2 equiv of 3-methyl-1,2-butadiene and 1-methyl-1-(trimethylsilyl)allene to give 1 equiv of olefin and the π-allene derivatives OsCl 2 (η 2 -CH 2 dCdCRMe)(P i Pr 3 ) 2 (R = Me (3), Me 3 Si (4)). The X-ray structure of 4 proves the coordination to the metal center of the carbon-carbon double bond of the allene with the lowest steric hindrance. In toluene, complexes 3 and 4 are unstable and evolve into the hydride-alkenylcarbyne derivatives OsHCl 2 (tCCHdCRMe)(P i Pr 3 ) 2 (R = Me (5), Me 3 Si ( 6)). DFT calculations on the model compound OsCl 2 (η 2 -CH 2 dCdCMe 2 )(PMe 3 ) 2 (3t) suggest that the π-allene to hydride-alkenylcarbyne transformation involves the migration of both hydrogen atoms of the CH 2 group of the allene. The first of them occurs between the terminal and central carbon atoms and takes place throught the metal center. The second one is a 1,2-hydrogen shift from the allene terminal carbon to osmium. The reactions of the ruthenium complex 2 with the previously mentioned allenes give olefins and RuCl 2 (η 2 -CH 2 dCdCRMe)(P i Pr 3 ) 2 (R = Me (7), Me 3 Si ( 8)), which in dichloromethane and in the presence of allene afford the disubstituted vinylidene complexes RuCl 2 (dCdCRMe)(P i Pr 3 ) 2 (R = Me (9), Me 3 Si ( 10)). The structure of 10 in the solid state has been determined by X-ray diffraction analysis. DFT calculations show that the formation of 9 and 10 can be rationalized in terms of the initial isomerization of 7 and 8 to alkenylcarbene species, which subsequently undergo metathesis reactions with a second allene molecule.

Recent Mechanistic and Synthetic Developments in the Chemistry of Transition‐Metal Vinylidene Complexes

“…Indeed, it is thought that the vinylidene complex is formed by a 1,2acyl migration from this p complex. Also, it has also been shown that both allenes [60] and alkylidenecyclopropanes [61] may be employed to prepare ruthenium and osmium vinylidene complexes respectively. In the latter case, the metal deconstructs the cyclopropane to from both the vinylidene ligand and ethene.…”

Dehydrative Cyclization of Alkynals: Vinylidene Complexes with the Cβ Incorporated into Unsaturated Five‐ or Six‐Membered Rings

“…[6] Ishii and coworkers have demonstrated that under appropriate conditions it is also possible to form dicarbon-disubstituted vinylidenes from unfunctionalized internal alkynes. [7] Both allenes and alkylidenecyclopropanes have been used to prepare ruthenium [8] and osmium vinylidenes, [9] respectively. Vinylidene ligands having the C b atom incorporated into a ring are extremely rare and have been prepared by multistep procedures from allenylidene compounds.…”

Reactions of Ruthenium–Allenylidene Complexes Tethering a Cyclopropyl Group

“…However, transformation of metal allenylidene complexes into disubstituted vinylidene ones, accompanied with a ring formation at Cβ, is rare 6a–b. Disubstituted metal vinylidene complexes were prepared by migration of carbon substituents of internal alkynes,6c isomerization of internal alkynones,6d and CH bond activation of terminal allenes 6e. Recently, some ruthenium–allenylidene complexes were employed for the synthesis of functionalized propargylic compounds 7.…”