Trifluoromethylation of a Well-Defined Square-Planar Aryl-NiII Complex involving NiIII/CF3. and NiIV−CF3 Intermediate Species

5 years ago

Trifluoromethylation of a Well-Defined Square-Planar Aryl-NiII Complex involving NiIII/CF3. and NiIV−CF3 Intermediate Species

Xavi Ribas, Steven Roldán-Gómez, Anna Company, Mireia Rovira, Christopher J. Whiteoak, Vlad Martin-Diaconescu, Josep M. Luis

Ni-mediated trifluoromethylation of an aryl−Br bond in model macrocyclic ligands (Ln−Br) has been thoroughly studied, starting with an oxidative addition at Ni0 to obtain well-defined aryl-NiII-Br complexes ([Ln−NiII]Br). Abstraction of the halide with AgX (X=OTf− or ClO4−) thereafter provides [Ln−NiII](OTf). The nitrate analogue has been obtained through a direct C−H activation of an aryl−H bond using NiII salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl−NiII complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [Ln−NiII](OTf) with CF3+ sources afforded Ln−CF3 products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L1−NiIII/CF3. adduct, which rapidly recombines to form a [L1-NiIV-CF3](X)2 intermediate species. A final facile reductive elimination affords L1−CF3. The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies. NiIV−CF3 intermediates:. 1OTf and 5OTf complexes with CF3+ sources (TDTT or Togni reagent) afforded the quantitative formation of L1−CF3 and L5−CF3 products. Mechanistic and theoretical investigations point towards the involvement of a feasible SET step followed by CF3. radical and aryl−NiIII recombination to form an aryl-NiIV-CF3 intermediate species. The rich Ni redox chemistry gathered in this square-planar system is sought to facilitate the development of new Ni-based trifluoromethylation methodologies.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201702168