Electrochemical Cyclization of Brominated Allyl Ethers with the Aid of a Chiral Catalyst

Control over the stereochemistry of a desired product in chemical synthesis is an ongoing problem.  Specifically, the selectivity of chiral molecules has been the most challenging problem in organic synthesis.  Often, to overcome this problem, chiral catalysts are employed to induce preference for one chiral product over another.  One catalyst, known as the Jacobsen catalyst, has been utilized for the enantioselective oxidative conversion of alkenes to epoxides in the presence of a large amount of an oxidizer.¹

In the field of electrochemistry, Duñach and co-workers utilized nickel(II) and cobalt(II) complexes with a Jacobsen ligand (a chiral, substituted salen) in constant-current experiments for the electrochemical reductive cyclization of brominated allyl ethers at room temperature, for which they achieved an enantiomeric excess as high as 16%.²  In the present work, we have explored the use of the same cobalt(II) catalyst as Duñach and co-workers, but conducted the experiments under controlled-potential conditions in order to increase the enantiomeric excess.  We expanded the study to more substrates to observe how ther products change as a function of structure.  A generalized reaction is shown in Scheme 1 for the one-electron reductive cyclization of brominated allyl ethers to form the chiral product of interest.   Experiments have been conducted to optimize reaction conditions such as temperature, concentration, and solvent.  Finally, we changed the stereochemistry of the catalyst from the R species to the S species for the purpose of controlling the identity of our products.

 

1.  Jacobsen, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, VCH, New York, 1993 (Chapter 4.2).

2.  Duñach, E.; Esteves, A. P.; Medeiros, M. J.; Pletcher, D.; Olivero, S. J. Electroanal. Chem. 2004, 566, 39–45.