Ruthenium-mediated C–C coupling reactions of alkynes – mechanistic investigations based on DFT calculations

Abstract

This article describes the highlights of the reactions of the substitutionally labile neutral pseudo 14VE complexes RuCp(COD)Cl (COD = 1,5-cyclooctadiene) and the cationic complexes [RuCp(CH3CN)2L]+ with alkynes. The ligand L is a co-ligand, whose nature turns out to be critical to the reaction’s outcome being tertiary phosphines (PR3) and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr). A crucial role in all reactions reported is the intermediacy of electrophilic ruthenacyclopentatriene complexes, an entity featuring a biscarbene functionality. While with RuCp(COD)Cl the catalytic cylcotrimerization of alkynes is achieved, [RuCp(CH3CN)2(L)]+ complexes are catalytically inactive for this process. These compounds undergo selective head-to-tail coupling of two alkynes. However, addition of a third alkyne is precluded due to an unusual migratory insertion of the PR3 and the N-heterocyclic carbene, respectively, into the Ru–C carbon double bond of a ruthenacyclopentatriene intermediate to afford stable allyl carbenes. With parent acetylene, on the other hand, an unusual C–C coupling process takes place involving three acetylene molecules and migration of the NHC ligand to give formal [2 + 2 + 1] cycloaddition products. Conceivable mechanisms for all these reactions are established by means of DFT/B3LYP calculations.