Nickel precatalysts as enabling tools for catalytic coupling reactions

Abstract

[Chemical formula] A series of air-stable nickel complexes of the form L₂Ni(aryl)X (L = monodentate phosphine, X = Cl, Br) and LNi(aryl)X (L = bis-phosphine) have been synthesized and are presented as a library of precatalysts suitable for a wide variety of nickel-catalyzed transformations. These complexes are easily synthesized from low-cost NiCl₂-6H₂O or NiBr₂-3H₂O and the desired ligand followed by addition of 1 equiv of Grignard reagent. A selection of these complexes were characterized by single-crystal X-ray diffraction, and an analysis of their structural features is provided. [Chemical formula] The air-stable nickel(II) complex trans-(PCy₂Ph)₂Ni(o-tolyl)Cl was employed as a precatalyst for the Mizoroki-Heck-type, room temperature, internally selective coupling of substituted benzyl chlorides with terminal alkenes. This reaction, which employs a terminal alkene as an alkenylmetal equivalent, provides rapid, convergent access to substituted allylbenzene derivatives in high yield and with regioselectivity greater than 95:5 in nearly all cases. The reaction is operationally simple, can be carried out on the benchtop with no purification or degassing of solvents or reagents, and requires no exclusion of air or water during setup. Synthesis of the precatalyst is accomplished through a straightforward procedure that employs inexpensive, commercially available reagents, requires no purification steps, and proceeds in high yield. [Chemical formula] The nickel-catalyzed cross-coupling of aliphatic N-tosylaziridines with aliphatic organozinc reagents is described. The reaction protocol displays complete regioselectivity for reaction at the less hindered C-N bond, and the products are furnished in good to excellent yield for a broad selection of substrates. An air-stable nickel(II) chloride/ligand precatalyst was also developed and employed for the reaction. In addition to increasing the activity of this catalyst system, this also greatly improves the practicality of this reaction, as the use of the very air-sensitive Ni(cod)₂ is avoided. Finally, mechanistic investigations, including deuterium-labeling studies, show that the reaction proceeds with overall inversion of configuration at the terminal position of the aziridine by way of aziridine ring opening by Ni (inversion), transmetallation (retention), and reductive elimination (retention).