Redox-responsive catalysis: fine tuning of chemoselectivity in the intramolecular reaction of diazo compounds catalysed by ferrocene-functionalised dirhodium(ii) complexes

Abstract

A series of heteroleptic dirhodium(II) complexes (1a–k) of formula [Rh₂(OAc)₃(L)], where L is a redox-active ferrocenecarboxylate ligand, have been evaluated in the redox-responsive, dirhodium(II)-catalysed decomposition of several diazo substrates prone to chemoselectivity issues. The influence of the counter-anion of chemically oxidised ferrocenyl-containing complexes on the chemoselectivity of the decomposition of the diazo compound 2 has been investigated with complexes 1a–d: bulky, weakly coordinating SbF₆⁻ and BAr^F₄⁻ ([(3,5-(CF₃)₂C₆H₃)₄B]⁻) result in a greater chemoselectivity difference between the reduced and oxidised versions of the catalyst than BF₄⁻. The correlation between the electronic nature of the ferrocenyl substituents and the extent of the chemoselectivity switch is not obvious, as complex 1a with unsubstituted ferrocene gave the best results upon oxidation among complexes 1a–k. The introduction of a saturated tether in 1j–k drastically decreases the chemoselectivity difference between the reduced and oxidised species. Finally, two additional diazo compounds with various chemoselectivity issues (9 and 10) were evaluated in the presence of complexes 1a and 1a⁺·SbF₆⁻: the decomposition of diazo complex 9, producing an aromatic C–H insertion product 11 and a cyclopropanation product 12, led to a 42% chemoselectivity difference between 1a and 1a⁺·SbF₆⁻. This demonstrates that changing the electronic properties of only one ligand on dirhodium, by oxidation of its ferrocenyl part, can have a marked influence on the reaction selectivity.