Reduction products in copper(I)-promoted diazonium ion reactions. Hydrogen abstraction from amines coordinated to copper(I), from water, and from transient radicals

Abstract

Reduction products are observed in amine-copper(I) perchlorate promoted decomposition of aryldiazonium salts when the amine ligands are capable of hydrogen atom donation. The effective reducing agents evidently are the complexed rather than the free ligands, i.e., the tris(amine)copperd) cation in the case of tetraand tricoordinated salts and the bis(amine)copper(I) cation in the dicoordinated systems, in the presence of excess ligand a new product is obtained, which is probably an ammonium salt formed from the amine and the incipient aryl cation. In the absence of hydrogen-donating ligands it has been shown that the small but consistent amount of benzophenone formed in the copper®-promoted decomposition of 2-diazobenzophenone tetrafluoroborate arises from (a) the precursor to 9-fluorenone and (b) the solvent, water. Hydrogen atom abstraction from both these sources is rate determining and subject to a kinetic isotope effect. Since water is a notoriously poor hydrogen atom donor, transfer of a hydrogen atom from the equated coordination sphere of copper® via a bridged intermediate is postulated.

Homolytic Reduction of Onium Salts

“…Amine copper(i) perchlorates proved to be effective catalysts for this homolytic process (Scheme 6). [24,25] 2-Diazobenzophenone tetrafluoroborate was used as model substrate. Depending on the aromatic amine ligand, it was possible to promote the aryl radical cyclization to afford 9-fluorenone among a number of by-products.…”