Synergistic Dimetallic Effects in Propargylic Substitution Reaction Catalyzed by Thiolate-Bridged Diruthenium Complex

Abstract

The origin of unique catalytic activity of a thiolate-bridged diruthenium complex in nucleophilic substitution reactions of propargylic alcohols, which features a diruthenium-allenylidene complex as a key intermediate, was studied with the aid of density functional calculations (B3LYP). Comparison of mono- and diruthenium systems has shown that the rigid but reasonably flexible Ru-Ru core structure plays a critical role in the catalyst turnover step (i.e., dissociative ligand exchange of the product pi-complex with the starting propargyl alcohol that goes through a coordinatively unsaturated Ru complex). In the diruthenium system, the energy loss due to coordinative unsaturation can be compensated by reinforcement of the Ru-Ru bond, while such an effect is unavailable in the monoruthenium counterpart. Weaker back-donation ability of the diruthenium complex is also advantageous for dissociation of the pi-complex. Thus, ligand exchange takes place smoothly in the diruthenium system to regenerate the reactive species, while the monoruthenium reaction stops at a dead-end Ru product pi-complex. The present studies have also shown the important role of protic molecules (e.g., MeOH) that mediate smooth proton transfer in the propargyl alcohol-allenylidene transformation.

Ruthenium‐Catalyzed Enantioselective Propargylation of Aromatic Compounds with Propargylic Alcohols via Allenylidene Intermediates

“…This vinylidene complex is then transformed into a η 2 ‐coordinated alkyne complex, which liberates the propargylated furan 3 a by reaction with a propargylic alcohol 1 a, and regenerates the starting complex. We proposed a similar catalytic cycle for the sulfur‐bridged diruthenium‐catalyzed propargylic substitution reactions of propargylic alcohols with nucleophiles in our previous studies, along with a DFT calculation 4a,c. It has already been proposed that the synergistic effect in the diruthenium complex is quite important for the promotion of the catalytic reaction 4a,c…”

Direct Nucleophilic SN1‐Type Reactions of Alcohols

“…Nakamura carried out theoretical investigations of the reaction developed by Nishibayashi. [103,104] Thiolatebridged diruthenium complexes produce, by the reaction with the propargylic alcohols, diruthenium-allenylidene complexes like 86 ( Figure 5). The flexible Ru-Ru core structure plays a critical role in the catalyst turnover step.…”

Ruthenium‐Catalyzed Enantioselective Propargylic Phosphinylation of Propargylic Alcohols with Phosphine Oxides

“…We consider that the reaction pathway of the present propargylic phosphinylation with diarylphosphine oxides is almost the same with that with other nucleophiles. [15] However, detailed investigations on the reaction pathway of the propargylic phosphinylation with diarylphosphine oxides have not yet been reported until now.…”