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.

Ligand Exchange Reactions of [(tmeda)NiR2] (R = CH2CMe3, CH2SiMe3, CH2CMe2Ph) with N-Heterocyclic Carbenes and Bidentate Phosphines

“…1.952(2) Å), they differ significantly in the trans isomer 2a- cis (Ni–C­(NHC) 1.905 Å vs Ni–C­(neosilyl) 2.032 Å). These metric values are in good agreement with other structurally characterized Ni-NHC complexes ,,− and can be attributed to the different trans-influence of alkyl and NHC ligands, respectively.…”

NHC‐Stabilized Nickel Olefin, Dialkyl, and Dicyanido Complexes

“…Both complexes adopt in the solid state square planar geometries for which the nickel atom is coordinated with two NHC and two alkyl ligands in a trans ‐configuration. The bonding parameter are unexceptional, the Ni–C bond lengths to the NHC ligand ( 9 : 1.879(2) Å, 11 : 1.887(3) Å) are close to those of other trans ‐[Ni II (NHC) 2 X 2 ] complexes, and the bond lengths Ni–C to the alkyl ligand ( 9 : 2.006(2) Å, 11 : 2.017(3) Å) are in the expected range. In comparison to Danopoulos' cis ‐configured complex cis ‐[Ni( i Pr 2 Im Me ) 2 (CH 3 ) 2 ][4d] are the Ni–C distances to the NHC carbene carbon atoms of 1.8790(16) Å in 9 slightly shorter (in cis ‐[Ni( i Pr 2 Im Me ) 2 (CH 3 ) 2 ]: 1.930(2) Å, 1.910(2) Å) and the Ni–CH 3 distances of 2.0060(15) Å slightly longer (in cis ‐[Ni( i Pr 2 Im Me ) 2 (CH 3 ) 2 ]: 1.966(2) Å, 1.975(3) Å).…”

“…We reported recently on the reactivity of the dinuclear complex [{Ni( i Pr 2 Im) 2 } 2 ( µ ‐(η 2 :η 2 )‐COD)] ( i Pr 2 Im = 1,3‐diisopropyl‐imidazolin‐2‐ylidene, COD = 1,4‐cyclooctadiene) in Hiyama‐ and Negishi‐type cross‐coupling reactions as well as on the synthesis of several novel nickel fluoroaryl alkyl complexes . We demonstrated that the nickel‐catalyzed Negishi cross‐coupling of C 6 F 6 or C 7 F 8 and zinc diorganyl reagents [ZnR 2 ] (R = Me, Et) leads only in traces to the desired product Ar F ‐R, although the corresponding nickel alkyl complexes are readily formed from the reaction of trans ‐[Ni( i Pr 2 Im) 2 (F)(Ar F )] with [ZnR 2 ] (R = Me, Et), as exemplified by the synthesis and characterization of a series of complexes trans ‐[Ni( i Pr 2 Im) 2 (R)(Ar F )] (R = CH 3 , Ar F = C 6 F 5 , C 7 F 7 , C 12 F 9 ; R = C 2 H 5 , Ar F = C 6 F 5 , C 7 F 7 , C 12 F 9 ) . We also have shown that these nickel alkyl complexes are stable even at elevated temperatures in solution for several days with respect to reductive elimination.…”

C−F Bond Activation and Functionalizations Enabled by Metal‐free NHCs and Their Metal Complexes

“…This is due to the formation of nickel alkyl complexes trans ‐[Ni( i Pr 2 Im) 2 (alkyl)(ArF)] (alkyl=Me, Et) 43 , which do not undergo controlled reductive elimination even at high temperature (Scheme 22). [67] …”