Activation of Aryl Halides by Nickel(I) Pincer Complexes: Reaction Pathways of Stoichiometric and Catalytic Dehalogenations

Abstract

Homolytic C-X bond cleavage of organohalides by the T-shaped nickel(I) complexes [LigNi(I)] 1 bearing the iso-PyrrMeBox ligand had been found previously to be the crucial activation step in the asymmetric hydrodehalogenation of geminal dihalides. Here, this mechanistic investigation is extended to aryl halides, which allowed a systematic study of the activation process by a combination of experimental data and density functional theory modeling. While the activation of both aryl chlorides and geminal dichlorides appears to proceed via an analogous transition state, the generation of a highly stabile nickel(II)aryl species in the reaction of the aryl chlorides for the former represents a major difference in the reactive behavior. This difference was found to have a crucial impact on the activity of these nickel pincer systems as catalysts in the dehalogenation of aryl chlorides compared to geminal dichlorides and highlights the importance of the regulatory pathways controlling the nickel(I) concentration throughout the catalysis. These results along with the identification and characterization of novel nickel(II)aryl species are presented.

Expeditious and Solvent‐Free Nickel‐Catalyzed C−H Arylation of Arenes and Indoles

“…On the basis of the above experimental observations and previous reports, [4j, 6a, e, 17] ap roposed mechanism for the arylation is shown in Scheme6.Most likely, catalyst 1 is transformed into the active complex A,which reacts with 2a throughaturnover-limiting CÀHn ickelation to deliver the nickelacycle B.A lternatively, 1 would react directly with 2a in the presence of LiHMDS to generate intermediate B.S pecies B would trigger the radicalg eneration through iodine atom transfer (IAT) to form C,f ollowed by the radical rebound to give the Ni IV species D.R eductive elimination will then give the product 4aa and regenerate the catalyst 1.A lternately, B would activate 4iodotoluenei nabimolecular oxidative fashion to deliver [Ni III ] (E)a nd Ni III I( C). [18] Reductive elimination of 4aa from E would produce Ni I species F,w hich upon disproportionation with C regenerates 1 and B to complete the cycle. [19] Scheme3.Nickel-catalyzed CÀHa rylation of indoles.…”

Nickel-catalysed retro-hydroamidocarbonylation of aliphatic amides to olefins

“…2. Because the C–N bond activation and decarbonylative process have been confirmed in our previous work3031, we suspected that the formed intermediate C′ in aliphatic amides could occur homolytic cleavage reversibly49. Nonetheless, details for the mechanism of this reaction are not clear at present and more studies are required to fully elucidate the reaction mechanism.…”

The Electronic Properties of Ni(PNN) Pincer Complexes Modulate Activity in Catalytic Hydrodehalogenation Reactions

“…[1][2][3][4][5] Among the numerous HDH methods that have been developed, [6][7][8][9][10][11][12][13][14][15] nickel pincer complexes have recently emerged as attractive, stable, homogeneous catalysts for such reactions. [16][17][18][19][20][21] In 2012, the Hu group reported that "Nickamine" [16,22] (complex A in Figure 1) could be used as a precatalyst for hydrodehalogenation reactions of both aryl and alkyl halides by employing diethyloxymethylsilane as reductant and sodium methoxide as base. Rettenmeier, Wadepohl and Gade found that a chiral Ni(NNN) complex, B, (Figure 1) can be transformed to a stable Ni(I) derivative via reaction with LiBEt 3 H and could be used to effect stereoselective HDH of various alkyl halides via a radical mechanism.…”