Formation and Reactivity of an (Alkene)persulfidoiridium(III) Intermediate and Mechanistic and Spectroscopic Investigation of Radical and Nickel(I) Intermediates in Reductive Transformations of Unsaturated Substrates

Abstract

A mechanistic understanding of the activation of substrates and the redox chemistry of active catalysts is important for the development of new catalytic methods. In this work, stoichiometric reactions of well-defined catalysts with substrates of interest were designed and investigated. Key intermediates and active catalysts occurring in these reactions were identified and characterized by a range of methods. In Part I, the activation of elemental sulfur, S8, by an organometallic Ir complex was investigated to gain mechanistic insights. Such studies may provide a basis for the development of new synthetic methods for the introduction of S atoms into organic substrates with C–H or C=C bonds. The reaction of [Ir{PhNC(NMe2)NPh}(cod)] (1), bearing a bidentate guanidinato(1–) ligand, guanPh,Me, with S8 afforded an (alkene)persulfidoiridium(III) intermediate, [Ir(guanPh,Me)(cod)(η2-S2)]. The reaction of this intermediate with 1 produced a dinuclear μ-sulfido intermediate, [{Ir(guanPh,Me)(cod)}2(μ-S)2], while its reaction with S8 afforded a complex with a bidentate catena-S4 ligand, [Ir(guanPh,Me)(cod)(κ2-S4)]. These three organometallic IrSx complexes were characterized by 1H NMR spectroscopy, diffusion-ordered spectroscopy (DOSY), UV–Vis spectroscopy, high-resolution ESI mass spectrometry, IR spectroscopy and elemental analysis. In Part II, organic radical and NiI intermediates occurring in three reductive transformations of unsaturated substrates were investigated and characterized by electron paramagnetic resonance (EPR) spectroscopy. On the basis of stoichiometric reactions of these intermediates, mechanisms for these transformations were proposed. A metal-free C–H bond arylation of pyrazine was achieved by reaction with KOtBu and the π-extension agent Ph2IOTf. In this study, the generation of N-heterocyclic radical anions from the reaction of pyrazine and KOtBu was observed and their involvement in the activation of Ph2IOTf was elucidated. A branched-selective hydroboration method for a range of substituted terminal alkynes was developed using NiII(dpm)2, a substituted terpyridine ligand (L1) and HBpin. In this study, the generation and identity of a precursor complex, NiI(dpm)(L1), and an active catalyst, NiIH(L1), were investigated and established. The involvement of NiIH(L1) in the reaction was indicated by a two-step reaction with an alkyne substrate. A three-component cross-electrophile coupling of 1,3-enyne, alkyl iodide and aryl iodide was developed using a NiII precursor, a nitrogen-donor ligand (L5) and Mn. The identity of the key catalyst, NiI(L5)(Ar), was established and its involvement in the reaction was demonstrated by the stoichiometric reaction with 1,3-enyne and alkyl iodide.