Radical Chain Mechanism for Alkyl Rearrangement in Organocobalt Complexes

Article abstract of DOI:10.1021/ja00276a017

The molecular rearrangement of a series of 5-hexenylcobalt(III) complexes of various Schiff bases is demonstrated to proceed via an unusual radical chain process.Thus the facility with the 5-hexenyl --> cyclopentylmethyl rearrangement occurs is highly dependent on the presence of trace impurities which can vary from the age of a highly purified sample to the presence of air.We find that the rearrangement of (5-hexenyl)Co^III(salen) I can be deliberately controlled by inhibiting it completely or by promoting it rapidly.For example, the addition of cobalt(II), nitroxide (TEMPO), dioxygen, or dihydroanthracene as well as an electrochemical preduction procedure can effectively squelch the 5-hexenyl rearrangement.Conversely, chemical and electrochemical oxidations with a ferrocenium salt and a platinium anode at 0.4 V, respectively, trigger the rearrangement.In each case, the limited molar amounts of additives (or faradays of charge) are sufficiently small to ensure high kinetic chain lenghts.Inhibition and initiation of the chain process by these techniques relate directly to the destruction and generation, respectively, of alkyl radicals as the prime reactive intermediates.Accordingly, a homolytic displacement (S_H2) of the alkylcobalt(III) complex is proposed, in conjuction with the well-known rearrangement of the hexenyl radical, to constitute the two-step propagation cycle in Scheme III.Such a mechanism accounts for the intermolecular character of the hexenyl rearrangement as established by crossover experiments and the observation of a concurrent alkyl exchange which would otherwise be difficult to explain.The spontaneous rearrangement of a freshly prepared sample of (hexenyl)Co^III(salen) and the dichotomous effect of pyridine as a donor ligand are both readily accommodated within the content of the mechanism in Scheme III.