Nickel has historically been surrounded by a cloud of mystery. While trying to extract
copper from a mineral deposit, miners found in the eighteenth century that it was
not only impossible to do, but also they became seriously ill. Convinced that the
deposit was the mischievous work of the devil, miners named it ‘Kupfernickel’ or ‘Old
Nick’s Copper’, a colloquial name for the devil according to Saxon mythology. Years
later, Axel Fredrik Cronstedt managed to identify arsenic as being responsible for
the miners’ illnesses, together with a new metal that he called nickel. It was not
until the twentieth century that the scientific community recognized the potential
of nickel as a component in catalysts for synthetic organic endeavors. In 1922, Nobel
Laureate Paul Sabatier had already noticed the outstanding catalytic activity of nickel
together with its Achilles’ heel, which was nothing but the difficulty to control
its promiscuous reactivity, suggesting that Ni should not be suited for synthetic
methods.
However, such an observation was probably premature, as the genesis of cross-coupling
reactions can be traced back to the seminal work of Kumada and Corriu in 1972,[1] who independently described the ability of nickel catalysts to enable the coupling
of Grignard reagents with alkenyl and aryl halides. Although Ni-catalyzed reactions
were overshadowed by the impressive applications of Pd-based technologies discovered
shortly thereafter, recent years have witnessed tremendous activity in the area of
nickel catalysis. The reasons for such a renaissance cannot be merely attributed to
Ni being a cheaper substitute of Pd. Indeed, Ni is more electropositive than Pd; therefore,
oxidative addition reactions occur more rapidly at Ni centers, an observation that
has turned into a strategic advantage for activating particularly strong σ-bonds that
cannot be accessed with Pd catalysts. The binding of alkenes to Ni complexes is exceptionally
strong when compared to Pd, and it comes as no surprise that the latter have become
privileged catalysts for the mono- and difunctionalization of olefins. Unlike classical
polar Pd(0)/Pd(II) mechanisms, multiple catalytic regimes are viable with Ni catalysis.
This observation is related to their propensity to trigger single-electron transfer
processes, an aspect of utmost synthetic relevance in the context of photoredox catalysis.
In addition, the low bond dissociation energy of the C–Ni bond provides the fundamental
basis for tackling uphill transformations that would otherwise be beyond reach.
In this SYNLETT cluster, we have contributions from authoritative experts in the area
of Ni catalysis, ranging from technologies occurring via two-electron manifolds to
photoredox endeavors that operate by the intermediacy of open-shell species. Specifically,
Murakami illustrates the ability of a Ni catalyst to streamline the preparation of
1,3-dienes from simple carbonyl groups.[2] Stradiotto shows the potential of DalPhos ligands for effecting a general N-arylation
of amides with (hetero)aryl electrophiles,[3] while Liu,[4] Yin[5] and Shi[6] offer innovative protocols for the rapid preparation of biaryls via C–O bond functionalization,
Ullman cross-coupling reactions with two sp2 hybridized organic halides, and Hiyama-type technologies for accessing vinylated
arenes, respectively. Yorimitsu describes the development of a general Negishi-type
cross-coupling reaction of easily accessible trialkylsulfonium salts,[7] while Fleischer and Berkefeld report the utilization of a Ni catalyst to cleave
simple allyl ethers chemoselectivity via formal sp3 C–O functionalization.[8] The groups of Mei and Fang[9] and Wang[10] demonstrate that Favorskii rearrangements and reductive allylic defluorinative coupling
reactions are not only within reach, but also allow rapid access to cyclopropanes
decorated with nitriles or aliphatic alcohols possessing a difluoroalkene isostere
on the alkyl side chain. Ogoshi[11] and Kimura,[12] respectively, describe the development of carbonylation and carboxylation methods
for rapidly and reliably accessing γ-lactams and carboxylic acids possessing alkenes
on the side chain.
Meanwhile, Nishihara[13] and Yamaguchi[14] demonstrate remarkable decarbonylation events for incorporating alkyne motifs and
nitriles into arenes via C–F and C–O bond cleavage. Zhang,[15] Engle,[16] and Peng and Qiu[17] continue their studies on Ni-catalyzed difunctionalization of olefins by using fluorinated
congeners, heteroleptic complexes or dihalogenated building blocks, accessing molecules
that would be beyond reach otherwise. Fu and Wang describe the utilization of N-acylsaccharins in a rather intriguing oxidative transamidation of tertiary aromatic
amines,[18] while Gong[19] and Newhouse[20] report the utilization of Ni catalysts in broadly applicable hydrodeoxygenation
of alkyl oxalates and benzylic dehydrogenation events.
Following up their ongoing interest in the field, Jarvo describes a new protocol for
streamlining the preparation of fluorinated cyclopropanes,[21] whereas Zhu and He describe the ability of ancillary ligands to dictate the site-selectivity
in a Ni-catalyzed reductive hydroarylation of styrene motifs.[22] Chu,[23] Fensterbank,[24] Martin[25] and Amgoune[26] merge photoredox catalysis and Ni catalysts to enable dual functionalization of
vinyl boranes, monoalkylation of dichlorostyrenes, photodehalogenation of organic
halides, and sp3 C–H acylation events with N-acyl imides. Last but not least, Diao offers a versatile tool for synthetic organic
and organometallic chemists by tabulating experimentally measured redox potentials
of Ni complexes supported by commonly employed ligands.[27]
As judged by the wealth of recent literature data on Ni catalysis, it is evident that
this area of expertise has become an indispensable tool for the ever-expanding repertoire
of our synthetic arsenal when forging C–C and C–heteroatom bonds. Undoubtedly, the
meteoric development of metallaphotoredox catalysis and the ability to modulate the
properties of Ni catalysts by suitable ligand modulation have offered conceptually
new pathways for molecular assembly and ‘top-down’ strategies to explore currently inaccessible chemical space. Indeed, Ni catalysis
has recently been adopted in late-stage functionalization as a strategy to boost lead
generation approaches in the early phases of drug discovery programs. Therefore, it
is inevitable to predict that Ni catalysis is in the midst of a transition that might
well impact the practice of organic chemistry for years to come.
