Mechanistic investigations of ruthenium-mediated reactions via vinylidene intermediates

Experimental and computational mechanistic studies are reported for several ruthenium-promoted processes, including the anti-Markovnikov hydration of terminal alkynes, the alkenylation of pyridine and electrophilic fluorination. The synergy between experimental and theoretical results revealed new insights into the previously reported hydration and alkenylation processes of alkynes and allowed the elucidation of a novel outer-sphere electrophilic fluorination mechanism, ‘OSEF’. The mechanism of the anti-Markovnikov hydration of phenylacetylene by [Ru(ƞ5-C5H5)(NCMe)(6-DPPAP)(3-DPICon)]+ (where 3-DPICon = 3-diphenylphosphinoisoquinolone and 6-DPPAP = 6-(diphenylphosphino)-N-pivaloyl-2-aminopyridine) was investigated using density functional theory. The proposed mechanism involves three ligand-assisted processes; alkyne-to-vinylidene tautomerism, attack of water at the vinylidene electrophilic α-carbon and tautomerisation from a hydroxyvinyl to acyl species. A catalyst deactivation product was identified experimentally and a potential mechanism of formation has been explored. The reactivity of [Ru(ƞ5-C5H5)(PR3)(L)2]PF6 (where PR3 = triphenylphosphine, 2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine or diphenyl-2-pyridylphosphine, L = pyridine) in pyridine alkenylation was trialed in order to optimise a previously reported catalytic procedure and eliminate the solvent dependence of the reaction. The synthesis and reactivity of these complexes in both stoichiometric and catalytic reactions have been investigated. A new electrophilic fluorination methodology is reported for the formation of carbon-fluorine bonds in the coordination sphere of ruthenium. A ruthenium-pyridylidene complex, synthesised from non-activated substrates, was observed to undergo rapid and selective monofluorination or deprotio-difluorination under mild reaction conditions. The procedure was then extended to allow the facile preparation of the first mononuclear fluorovinylidene complexes. Retention of the ligands allowed for subsequent investigation of metal-mediated reactivity and revealed unusual carbon-fluorine bond cleavage and migration processes. Experimental and computational mechanistic data suggested that fluorination was taking place via an unprecedented mechanism without metal-fluoride intermediates.