Indirect electroreductive cyclization and electrohydrocyclization using catalytic reduced nickel(II) salen.

Abstract

[Chemical reaction: See text] We describe efforts to achieve the electroreductive cyclization (ERC) and the electrohydrocyclization (EHC) reactions using catalytic nickel(II) salen as a mediator. While nickel(II) salen proved effective, the analogous cobalt complex as well as nickel(II) cyclam were not. The transformations were achieved in yields ranging from 60 to 94% using either a mercury pool or an environmentally preferable reticulated vitreous carbon (RVC) cathode. These examples represent the first instances wherein a nickel salen complex has been used in this manner. Clear differences between the voltammetric behavior of the ERC and EHC substrates were observed. The bisenoate 14, for example, displays a substantially larger catalytic current. When the structurally modified mediator 31 was used, the electron-transfer pathway shuts down. Instead, the reduced form of 31 behaves as an electrogenerated base, leading to the formation of the intramolecular Michael adduct 23. Presumably, the methyl groups of the modified ligand diminish the ability of the reduced form of the complex to serve as a nucleophile but not as a base. Aldehyde 23 was also characterized as a side product of the nickel(II) salen mediated electroreductive cyclization of 11. Given that it is absent from nonmediated processes, its formation is linked to the presence of the mediator. To account for the results, we favor the existence of a mechanistic continuum involving an equilibrium between nickel(II) salen (15) and two reduced forms, one being the metal-centered species 16, the other being a ligand-centered species 17. We postulate that one form may be more prominently involved with the chemistry than another, depending upon the electronic properties/requirements of the substrate, and suggest that the equilibrium will shift to accommodate the need. Thus, for a hard electrophile like an alkyl halide, the properties of 16 ought to dominate, whereas 17 ought to predominate as the reactive species accounting for the chemistry described herein since it properly matches a soft ligand-centered nucleophile with a soft electron deficient alkene.