NHC Nickel Catalyzed Hiyama‐ and Negishi‐Type Cross‐Coupling of Aryl Fluorides and Investigations on the Stability of Nickel(II) Fluoroaryl Alkyl Complexes

Abstract

The reactivity of [Ni(iPr2Im)4(µ‐COD)] 1 (iPr2Im = 1,3‐diisopropyl‐imidazolin‐2‐ylidene, COD = 1,4‐cyclooctadiene) in Hiyama‐ and Negishi‐type cross‐coupling reactions as well as the synthesis of several novel nickel fluoroaryl alkyl complexes is reported. Hiyama coupling of 1.1 equiv. perfluoroaromatics and 1 equiv. PhSi(OR)3 (R = Me, Et) with 5 mol‐% of 1 as catalyst leads to the C–C coupling product ArF–Ph in good to fair yields. In presence of the additive NMe4F alkoxy transfer from PhSi(OR)3 to the perfluoroarene occurs to yield ArF–OR and PhSiF(OR)2. Negishi cross‐coupling between C6F6 or C7F8 (1 equiv.), diorganozinc reagents [ZnR2] (R = Me, Et) (2.1 equiv.) and 5 mol‐% 1 as the catalyst in toluene at 115 °C leads to ArF–R only in traces. However, NMR experiments revealed that nickel alkyl complexes are readily formed from the reaction of trans‐[Ni(iPr2Im)2(F)(ArF)] with [ZnR2] (R = Me, Et). In course of these investigations, a series of novel nickel alkyl complexes trans‐[Ni(iPr2Im)2(R)(ArF)] (R = Me, ArF = C6F5 2, C7F7 3, C12F9 4; R = Et, ArF = C6F5 5, C7F7 6, C12F9 7) have been synthesized in stoichiometric reactions starting from trans‐[Ni(iPr2Im)2(F)(ArF)] (ArF = C6F5, C7F7, C12F9) and [ZnR2] (R = Me, Et) in thf at –78 °C. As these nickel alkyl complexes 2–7 are stable at room temperature in solution for several days with respect to reductive elimination, their thermal stability was investigated. Heating trans‐[Ni(iPr2Im)2(Me)(C6F5)] 2 for 24 hours at 100 °C leads to 91 % unreacted complex 2 and only traces of reductive elimination product, i.e. C6F5Me, are formed. Furthermore, the nickel ethyl complex trans‐[Ni(iPr2Im)2(Et)(C6F5)] 5 is also very stable, even with respect to β‐hydride elimination. After heating this complex to 100 °C for 24 hours there is still 26 % unreacted 5 left.