Mechanistic Studies on the Activation of Heteroatom-Functionalized Alkynes and Allenes by cis-[Ru(II)/Os(II) (dppm)₂Cl₂]

Systematic investigations on the reactions between cis-[M(dppm)₂Cl₂] (M = Ru/Os; dppm = 1,1-bis(diphenylphosphino)methane) and pyridine/quinoline substituted homopropargylic alcohols uncovered the diverse Ru(II)/Os(II)-induced alkyne activation pathways. The alkynes underwent cyclization on M via a “non-vinylidene” pathway at lower temperatures, resulting in alkenyl intermediates which might further metallacyclize to give metallapyrroloindolizines. Conversely, reactions at higher temperatures induced alkyne cyclization on M via a “vinylidene” pathway, affording cyclic oxacarbene complexes. Additionally, a rare decyclization mechanism was observed during the transformation of a metallacyclization-resistant alkenyl complex into a cyclic oxacarbene complex. DFT calculations were employed to validate the experimental findings. Overall, these results not only provide insights into controlling alkyne activation pathways, but also offer new strategies for preparing metalated heterocyclic and metallacyclic complexes.  

Os-oxazolopyridylidenyl complex, derived from “non-vinylidene-involving” reaction between cis-[Os(dppm)₂Cl₂] (dppm = 1,1-bis(diphenylphosphino)methane) and 2-propargyloxypyridine, was reacted with a range of nucleophiles, revealing the unconventional reactivities that leads to the formation of N-alkenyl 2-pyridonyl ethers or Os-propenylidenyl ammonium complexes. The catalytic reactions between cis-[Os(II)(dppm)₂Cl₂] and 2-propargyloxypyridine HC≡CCH₂O(2-py) (L1) in CH₂Cl₂ with alcohols were therefore examined, demonstrating exceptional selectivity in the formation of N-alkenyl 2-pyridonyl ethers. In addition, an unexpected formation of osmapyrido[2,1-b]oxazinium complexes was detected when alcohols were used exclusively as solvents, leading to a deactivation pathway that significantly hampered the catalytic performance. Overall, these findings provide valuable insights into the post-transformation of metalated heterocyclic complexes by controlling the reaction conditions and highlight the potential of cis-[Os(dppm)₂Cl₂] for generating useful organic compounds.

Osmafuran complexes in the form of cis-[Os(L^{^}L)2(C^{^}O)]+ (L^{^}L = 1,1-bis(diphenylphosphino)methane (dppm) or 2,2’-bipyridine (bpy); C^{^}O = [C(CH₃)C(CH₃)CRO]^-, coordinating atoms in italics) were prepared from reactions between dichloro-Os(II) complexes cis-[Os(L^{^}L)₂Cl₂] and allenols H₂C=C=C(CH₃)(CH(R)(OH)) in alcoholic solvents. Rearrangement of allenolates H₂C=C=C(CH₃)(CH(R)(O^-)) involving deprotonation of the secondary alcoholic carbon and protonation of the terminal allenic carbon was evident from the X-ray crystal structures of the resulting complexes. DFT calculations suggested a rearrangement mechanism involving Os-alkene intermediates cis-[Os(L^{^}L)₂(H₂C=CHC(CH₃)(=C(R)(O^-)))]⁺ but not Os-hydride intermediates. Overall, these osmafurans are not directly derived from common metal-allene reaction patterns.