Organic cyclisations of propargyl and allyl bromoesters in microemulsions catalysed by electrogenerated nickel(I) tetramethylcyclam

Abstract

While the reductive intramolecular cyclisation of propargyl and allyl bromoesters catalysed by [Ni(tmc)] + gives good yields of the desired products using N,N-dimethylformamide as the solvent, the use of this aprotic solvent presents practical, safety and environmental issues. This paper therefore reports the search for non-toxic alternatives, in particular the study of microemulsions prepared from water, hydrocarbons, surfactant and alcohol co-surfactant. It is shown that the [Ni(tmc)] 2+ /[Ni(tmc)] + couple is reversible in such media and that [Ni(tmc)] + reacts rapidly with propargyl and allyl bromoesters to give excellent yields of cyclic products. Indeed, these microemulsions are convenient media for such syntheses.

Electrocarboxylation and related radical coupling processes of aryl and benzyl halides in microemulsion

“…Rusling et al have reported free radical generation and coupling processes in bicontinuous microemulsion [17][18][19]. Similar electro generated free radical reactions have also been reported [20,21]. Electrochemical coupling of benzyl free radicals generated from benzyl bromide with active methylene compounds under galvanostatic experimental conditions in cationic microemulsions was reported from this laboratory [21].…”

Electroreductive Remediation of Halogenated Environmental Pollutants

“…Electrogenerated nickel­(I) cyclam and nickel­(I) tetramethyl cyclam have been employed to carry out a variety of reductive cyclization reactions involving allyl 2-halophenyl ethers, propargyl and allyl bromoesters, − and N -allyl-α-halo­amides …”

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

“…bisphosphine, proceed via an inner-sphere pathway of metal-centered halogen atom abstraction. 32 Similarly, Ni(I) cyclam or tetramethylcyclam, a widely utilized redox mediator for RX reduction 30,[33][34][35][36] bearing no redox-active ligand, has been shown to catalyze the electroreduction of RX at potential values ~ 1 V more positive than the direct reduction of the substrate, rendering an OSET mechanism unlikely 37 . Yet, the nature of the oxidative addition adduct, i.e.…”

“…It has been shown that RX oxidative addition processes at Ni­(I) complexes bearing a strongly sigma-donating and redox-inactive ligand, i.e., bisphosphine, proceed via an inner-sphere pathway of metal-centered halogen atom abstraction . Similarly, Ni­(I) cyclam or tetramethylcyclam, a widely utilized redox mediator for RX reduction ,− bearing no redox-active ligand, has been shown to catalyze the electroreduction of RX at potential values ∼1 V more positive than the direct reduction of the substrate, rendering an OSET mechanism unlikely . Yet, the nature of the oxidative addition adduct, i.e., halogen atom transfer vs two-electron oxidative insertion into the RX bond, , remains unresolved.…”