Stable 1,4-diaza-1,3-butadiene(α-diimine)-zinc and -aluminium radicals formed in single electron transfer reactions: their consequences for organic syntheses

A discussion is given of the formation of complexes of the reaction of 1,4-di-t-butyl-1,4-diaza-1,3-butadiene t-Bu-NCHCHN-t-Bu (t-Bu-DAB) or of t-Bu-NCH-2-C₅H₄N (t-Bu-Pyca) with Et₂Zn or Et₃Al. It is shown that after the initial formation of four-coordinate complexes Et₂Zn(t-Bu-DAB)_and Et₂Zn(t-Bu-Pyca) with chelated diimine groups and of Et₃Al(t-Bu-DAB) and Et₃Al(t-Bu-Pyca) with monodentate bonded diimine groups a variety of reactions may occur. These reactions involve ethyl transfer to either the C- or the N-atoms of the ligand NCCN skeleton, and the formation of persistent organo-zinc and -aluminium radical complexes was also observed. The latter radical complexes have been studied by ESR spectroscopy. In the case of zinc the radical complex [EtZn(t-Bu-DAB)^.] is in equilibrium with a dinuclear complex in which the two t-Bu-DAB ligands are coupled via a CC bond. The mechanistic aspects of the reaction are briefly discussed.