The Study of Indirect Electroreductive Cyclisation of Propargyl Derivatives Using [Ni(tmc)]Br2 As Catalyst in Ionic Liquids

Five-membered heterocyclic compounds are among the most important natural biologically active compounds.  X-heterocycles (X = O, N) are usually synthesised by radical cyclisation methodology [1] based on organotin hydrides which are toxic and also troublesome to separate from the reaction products.

A convenient alternative to synthetic methods involving organometallic hydrides is the use of electrogenerated nickel(I) complexes as mediators for cathodic intramolecular cyclisations [2].  This methodology proved to be successful in the formation of carbocyclic products and substituted tetrahydrofurans. Such work has largely used aprotic organic solvents such as DMF which presents some toxicity.

Due to an increasingly “green”-conscious industrial community, researchers have started to examine more eco-friendly and sustainable chemical processes. Ionic liquids (ILs) represent a class of alternative solvents currently receiving serious consideration with the promise of both environmental and technological benefits [3].

            A number of room-temperature ILs has been synthesized, characterized and applied in organic synthesis and catalyzed processes. Their characteristics render them promising replacements for volatile organic solvents (VOCs), which are a source of environmental problems.

            In the present work, the reductive intramolecular cyclisation of bromopropargyloxy esters catalysed by electrogenerated (1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane)nickel(I), [Ni(tmc)]⁺ as the catalysts in ILs, N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, [N_{1 1 1 2(OH)}][NTf₂] and 1-ethyl-3-methylimidazolium ethylsulfate, [C₂mim][C₂SO₄] has been investigated by cyclic voltammetry and controlled-potential electrolysis.  During controlled-potential electrolyses of solutions of Ni(II) complexes in the presence of bromopropargyloxy and bromoallyl derivatives, catalytic reduction of the latter proceeds via cleavage of the carbon–bromine bond to form a radical intermediate that undergoes cyclization to afford the substituted tetrahydrofurans [4].

 

ACKNOWLEDGEMENTS

 

We are grateful to the Fundação para a Ciência e Tecnologia (PEst-C/QUI/UI0686/2011) for financial support of this work.

 

REFERENCES

 

[1] B. Giese, “Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds”, Pergamon Press, Oxford, 1986.

[2] E. Dunach, M.J. Medeiros, S. Olivero, New J. Chem., 30, 1534 (2006).

[3] Ionic liquids IIIB: Fundamentals, Process, Challenges, and Opportunities, ACS Symp. Ser. (Eds.: R. D. Rogers and K. R. Seddon), ACS, Washington DC, 2005.

[4] 5. P.K. Mandal, G. Maiti, S.C. Roy, J. Org. Chem., 63, 2829 (1998).