Promotion of Reductive Elimination Reaction of Diorgano(2,2′-bipyridyl)nickel(II) Complexes by Electron-Accepting Aromatic Compounds, Lewis Acids, and Brønsted Acids.

Abstract

Reductive elimination of R-R from dialkyl(2,2 -bipyridyl)nickel( ), [NiR 2 (bpy)] 1 (R = CH 3 ( 1a ), C 2 H 5 ( 1b ), n -C 3 H 7 ( 1c )), caused by π -coordination of electron-accepting aromatic compounds and reductive elimination of Ar-Ar from [NiAr 2 (bpy)] 2 (Ar = C 6 F 5 ( 2a ) and pyrazolyls ( 2b and 2c )) promoted by electron-accepting aromatic compounds, Lewis acids, and Brønsted acids have been investigated. 1 H-NMR and kinetic data indicate that π -coordination of the electron-accepting aromatic compound to [NiR 2 (bpy)] leads to the reductive elimination of R-R. The rate of the reductive elimination obeys the second-order rate law,Plots of log k vs Σ σ p of the electron-accepting aromatic compound give a line with a slope of 1.8. Brønsted acids cause reductive elimination of Ar-Ar from 2 selectively under several reaction conditions (e.g., 2a with CF 3 COOH in air and 2b with HBr). The reductive elimination reaction of 2a caused by CF 3 COOH obeys the second-order rate law, − 1 at 300.5 K. UV-vis data give information about the electronic states of 2 and the 2b -Brønsted acid adduct. Poly(6-hexylpyridine-2,5-diyl) with a higher molecular weight has been prepared according to the basic information.Reductive elimination is one of the most fundamental reactions in organometallic chemistry.1-4 The reductive elimina-is considered to be a key step in Ni-and Pdcomplex promoted organic synthesis 1,5,6 and polymer synthesis. 7,8 Factors that control the reductive elimination from diorganometallic complexes, [M(Y)(Y )L n ] (M = Ni, Pd; L = neutral ligand such as 2,2 -bipyridyl and tertiary phosphines) have long been investigated. The diaryl nickel complex has been isolated mainly with electron-withdrawing aryl groups that stabilize the Ni-aryl bond.In this paper, we report details of (i) the reductive elimination reaction of [Ni(alkyl) 2 (bpy)] (bpy = 2,2 -bipyridyl) accelerated by the coordination of electron-accepting aromatic compounds such as m-and p-C 6 H 4 (CN) 2 and (ii) the reductive elimination reaction of [Ni(aryl) 2 (bpy)] induced by Lewis acid and protic acids. The Brønsted acids usually induce the cleavage of metal-organic (M-Y) bonds to liberate Y-H compounds; 1,3c,9f,10 however, the present investigation reveals that it can cause selective reductive elimination when the organic group is the electron-accepting aryl group.(2) Analysis of these reductive elimination reactions is expected to improve our understanding of the nickel-complex-promoted synthetic reactions. A part of these results have been published in a communication form. 9c have been determined by X-ray crystallography; they take a square planar structure around Ni, as previously reported for 2c. The molecular structure of 2a is shown in Figure 1 and that of 2b is given in the supporting data.Reductive

Homocoupling of aryl halides promoted by an NiCl2/bpy/Mg system in DMF

“…II‐1, CH 3 COC 6 H 4 Br‐ p with a moderately electron‐withdrawing acetyl group also underwent the C―C coupling reaction. Similar results were reported for the coupling of CH 3 COC 6 H 4 Br‐ p and NCC 6 H 4 Br‐ p promoted by isolated Ni–cod complexes, and the reported results are shown in Nos. II‐4 and 5.…”

“…A part of this suspension was removed and diluted with DMF. The UV–visible spectrum of this diluted DMF suspension showed UV–visible peaks at 428 nm (23 400 cm −1 ) and 600 nm (16 700 cm −1 ), and the energy difference (6700 cm −1 ) between the two absorption peaks suggests that the two peaks are assigned to nickel‐to‐bpy MLCT bands . The reduction of the NiCl 2 ‐bpy mixture with Mg in DMF proceeded much faster than that of the NiCl 2 ‐bpy mixture in DMF with Zn, as judged from the velocity of the change of the color from light green (color of Ni(II)–bpy complex) to deep green (color of Ni(0)–bpy complex).…”

“…Application of this homocoupling to dehalogenative polycondensation of dihaloaromatic compounds to give π‐conjugated polymers will also be reported. Basic reactions of zero‐valent nickel complexes with organic halides have been the subject of many organometallic studies …”

Electron transfer in organonickel complexes of α-diimines: Versatile redox catalysts for C–C or C–P coupling reactions – A review

“…For the sake of completeness the preparative routes to aryl nickel complexes [Ni(Ar)X(bpy)] from [Ni(Et) 2 (bpy)] and aryl halides ArX (Ar = Ph, 2-Tol, 3-Tol, 4-Tol and 2-ClPh, X = I, Br or Cl) [39], or using the oxidative addition reaction of aryl halides to [Ni(COD) 2 ] (COD = 1,5-cyclooctadiene) have to be quoted [30,32,40].…”

Acceleration of Reductive Elimination of [Ar‐Pd‐C] by a Phosphine/Electron‐Deficient Olefin Ligand: A Kinetic Investigation

“…[19][20][21][22][23] They have seldom been used as ligands in catalytic processes. [24][25][26][27][28][29][30][31][32][33] Recently, we reported an efficient C sp 3-involved Negishi-coupling catalyzed by a complex of palladium and the phosphine/electron-deficient olefin ligand 1.…”