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

摘要

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 l,3-bis(2,6-diisopro-pylphenyl)imidazol-2-ylidene (IPr).A crucial role in all reactions reported is the intermediacy of elec-trophilic 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.