Benchtop Nickel Catalysis Invigorated by Electron-Deficient Diene Ligands

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Due to the rarity of precious metals like palladium, nickel catalysis is becoming an increasingly important player in organic synthesis, especially for the formation of bonds with sp 3 -hybridized carbon centers. Traditionally, catalytic processes involving active Ni( 0) species have relied on Ni(COD) 2 or in situ reduction of Ni(II) salts. However, Ni(COD) 2 is an air-and temperature-sensitive material that requires use in an inertatmosphere glovebox, and in situ reduction protocols of Ni(II) salts using metallic or organometallic reductants add additional complications to reaction development. This Account chronicles the development of air-stable Ni(0) precursors as replacements for Ni(COD) 2 or in situ reduction. Based on Schrauzer's seminal discovery of Ni(COD)(DQ) as an air-stable zerovalent organonickel complex, our research laboratories at Scripps Research and Bristol Myers Squibb have developed a class of precatalysts based on the Ni(COD)(EDD) (EDD = electron-deficient diene) framework, relying on the steric and electronic properties of the supporting diene to render the metal center stable to air, moisture, and even silica gel but reactive to ligand substitution and redox changes. The stable Ni(0) complexes can be accessed through ligand exchange with Ni(COD) 2 , through reduction of Ni(acac) 2 using DIBAL-H, or electrochemically via cathodic reduction of Ni(acac) 2 to Ni(COD) 2 , followed by addition of an EDD ligand in one pot. As a toolkit, the complexes demonstrate reactivity that is equivalent or enhanced compared to Ni(COD) 2 , catalyzing C−C and C−N cross-couplings, Miyaura borylations, C−H activations, and other transformations. Since the initial report on Ni(COD)(DQ), its reactivity in C(sp 2 )−CN activation, metallophotoredox, and electric field-induced cross-coupling have also been demonstrated. By incorporating the precatalyst toolkit into reaction discovery campaigns, our laboratories have been able to perform C(sp 3 )− S(alkyl) couplings and metallonitrenoid carboamination, both of which represent challenging transformations that were inaccessible with traditional phosphine, nitrogen, or electron-deficient olefin ligands. Computational and experimental studies demonstrate how the quinone ligands are hemilabile, adopting η 1 (O)-bound geometries to relieve steric strain or stabilize transition states and intermediates; redox-active, able to transiently oxidize the metal center; and electron-withdrawing or -donating, depending on metal oxidation state and coordination geometry. These studies show how the ligands enable key steps in catalysis beyond imparting airstability. Since our report documenting the catalytic activity of Ni(COD)(DQ), many other laboratories have also observed unique reactivity with this precatalyst. Ni(COD)(DQ) was found to offer superior reactivity to Ni(COD) 2 in C−N cross coupling to form N,N-diaryl sulfonamides and in preparation of biaryls from aryl halides and benzene through a Ni-mediated, base-assisted homolytic a...

Electrosynthesis of iminophosphoranes and applications in nickel catalysis

“…To gain insight into the viability of using iminophosphorane ligands in high energy photochemical conditions (365 nm) with open shell species generated via hydrogen atom transfer (HAT) catalysis, we explored the use of MacMillan's metallaphotoredox method 62 for direct arylation of C(sp 3 )–H bonds ( Scheme 6C ). Gratifyingly, irradiation of mixtures containing 2-trifluoromethyl-5-bromopyridine (58) and cyclohexane (59) in the presence of tetrakis(tetrabutylammonium)decatungstate (( n -Bu 4 N) 4 [W 10 O 32 ], TTBADT), Ni(COD)(DQ) 63 and iminophosphorane 31 with 365 nm provided the desired product 60 in useful yields.…”

A Ni0(cod)(dq) (COD: 1,5-cycloctadiene; DQ: duroquinone) complex as a catalyst precursor for oligothiophene and polythiophene synthesis

“…Recently, Engle and co-workers reported that Ni(cod)(dq) (DQ: duroquinone) ( 2 ) was available as a catalyst precursor for several nickel(0)-catalyzed synthetic reactions as a surrogate of Ni(cod) 2 because of its much improved stability to oxidation and thermal decomposition. 12,18,19 It was also shown that the employment of Ni(cod)(dq) ( 2 ) along with an appropriate ligand showed comparable reactivities to Ni(cod) 2 ( 1 ) in several coupling reactions such as Suzuki–Miyaura coupling, Buchwald–Hartwig amination, etc . However, despite the remarkable breakthroughs in nickel catalysts, there are surprisingly few reports of their subsequent utilization in catalytic synthetic reactions (Chart 1).…”