Preface
The field of nickel catalysis has made tremendous advances in the past decade. There are several key properties of nickel that have allowed for a broad range of innovative reaction development, such as facile oxidative addition and ready access to multiple oxidation states. In recent years, these properties have been increasingly understood and leveraged to perform transformations long considered exceptionally challenging. Herein, we discuss some of the most recent and significant developments in homogeneous nickel catalysis with an emphasis on both synthetic outcome and mechanism.
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. A case study of their use as precatalysts for the nickel-catalyzed
carbonyl-ene reaction is presented, showing superior reactivity in
comparison to reactions using Ni(cod)2. Furthermore, as
the precatalysts are all stable to air, no glovebox or inert-atmosphere
techniques are required to make use of these complexes for nickel-catalyzed
reactions.
The synthesis and characterization of the air-stable nickel(II) complex trans-(PCy2Ph)2Ni(o-tolyl)Cl is described in conjunction with an investigation of its use for 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 bench-top 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.
Natural products are a continual source of inspiration for chemists, particularly for organic chemists engaged in reaction development and methodology. In the early stages of our research program, we were drawn to macrocyclic natural products containing allylic alcohol moieties, such as (−)-terpestacin (1, Figure 1). We envisioned, in an ideal case, an intramolecular reductive coupling (a field still in its infancy at the time) could be developed to join an alkyne and an aldehyde to yield this allylic alcohol, simultaneously closing the macrocycle. For this reason we began studying reductive coupling as a tool for C–C bond formation. Additionally, as our program developed, it became clear that a number of other natural products, such as amphidinolide T1 (2), which, although they do not contain allylic alcohols, could be produced in an analogous fashion after modification of the allylic alcohol formed from such a macrocyclization.
Herein, we report the first ligand-controlled, nickel-catalyzed cross-coupling of aliphatic N-tosylaziridines with aliphatic organozinc reagents. 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. Moreover, we have developed an air-stable nickel(II) chloride/ligand precatalyst that can be handled and stored outside a glovebox. 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), transmetalation (retention), and reductive elimination (retention).
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