Electrochemical Approach to Copper-Catalyzed Reversed Atom Transfer Radical Cyclization

Electrochemically mediated atom transfer radical cyclization (eATRC) has been developed as an easy and clean method allowing the synthesis of halogenated cyclic compounds. This method has been successfully applied to the copper-catalyzed atom transfer radical cyclization of some N-allyl-alpha,alpha-dichloroamides in acetonitrile (CH3CN) using a copper complex with tris(2-pyridylmethyl)amine (TPMA) with a metal loading of 1%. The catalyst is introduced as [Cu(II)TPMA](2+) and is activated and continuously regenerated to its active copper(I) form by reduction at a platinum (Pt) electrode. During the ATRC process a new copper(II) complex, namely, [ClCu(II)TPMA](+), whose reduction potential is ca. 0.350 V more negative than that of the starting [Cu(II)TPMA](2+), is formed. Therefore, the choice of the applied potential is critical and should be done taking care that all copper(II) species are reduced to copper(I). The compounds undergo very high conversions (79-100%) in a few hours of electrolysis, producing a cyclic gamma-lactam (yield 60-98%) as a mixture of two isomers, with a good cis-diastereoselectivity [dr (cis/trans) = 59/41-83/17]. [Cu(II)PMDE-TA](2+) (PMDETA= N, N, N',N '',N ''-pentamethyldiethylenetriamine) which is much cheaper, albeit less reactive than [Cu(II)TPMA](2+), was also investigated but the results were not satisfactory.