Nickel Catalysis has important properties, such as easy oxidative addition and the ability to obtain many oxidation states, which have been understood more in the past decade and has led to advances in its chemistry.1 It is a group 10 metal positioned above palladium on the periodic table. Palladium is a chemical that won the Nobel Prize in Chemistry in 2010 for a palladium catalyzed cross coupling and was thought to be similar to nickel. One of the first important uses of a nickel won Sabatier a Nobel Prize in 1912 for his work using nickel to hydrogenate an ethylene complex.
2 G?nther Wilke made one of the most important contributions to organonickel chemistry, in which he completed oxidative addition of aryl halides to a nickel(0) complex to give a nickel(2) complex, with a slight by-product of nickel(1).3 More recently, since the 1970s, nickel has served many different organic reactions including cross-coupling, nucleophilic allylation, oligomerization, cycloisomeration, and reductive coupling.1
Oxidative addition with nickel readily occurs because of the nature of the electropositive transition metal4, which allows cross-coupling electrophiles to be used, and it is more reactive than palladium, which, as mentioned before, nickel was previously thought to be eerily similar to.
5 Nickel has many properties including oxidation states of -1, 0, +1, +2, +3, and +4, completing facile oxidative addition, completing facile ?-migratory insertion, having radical pathways that are accessible; and having a market price far lower than the rest of the group 10 metals.1 These properties make nickel good for catalysis. It has many uses, but this paper will focus on its uses in cross-coupling reactions, Heck reaction, reductive coupling reactions, and future work in nickel catalysis.
Cross-coupling has many variations including, cross-coupling of aryl halides, cross-coupling of phenol derivatives, benzylic cross-coupling, cross-coupling of aziridines, cross-coupling of sp3 electrophiles, and reductive cross-coupling.1 Nickel-catalyzed direct arylation of electron rich heteroatoms is an important advancement in the functionalization of C-H bonds. This development was discovered separately by groups Itami and Miura.6 Itami’s group found that heteroatoms could by arylated by N,N-ligands with cheap nickel catalyst with aryl halides and triflates.7 The group said Ni(OAc)2/bippy worked best with aryl bromides and aryl iodides, while Ni(OAc)2/dppf worked most effectively with aryl chlorates/triflates. Itami’s group found the heteroarenes which worked well as coupling partners were thiazole, benzothiazole, oxazole, benzoxazole, and benzimidazole as shown in Scheme 107.7
Miura’s group developed a catalyst system that is cheap and provided the direct C2 arylation of oxazoles and thiazoles. The nickel catalyst system developed by this group worked with various aryl bromides in the C-H arylation of azoles.8 Miura’s group used NiBr2 as the catalyst precursor and 2,9-dimethyl-1,10-phenanthroline hydrate as the ligand.6,8 To ensure Ni(0) was formed, adding zinc powder was found to be the best option.6,8 The proposed mechanism, as shown in Scheme 108, for this was the oxidative addition of the aryl halide to the nickel(0), which is catalytically active. This addition then generated a nickel(II) which undergoes transmetalation with organolithium reagent generated in situ. The desired arylated product is formed by an elimination reduction.6,8 Since these reactions did not work with simple arenes, but only electron rich heteroatoms, another reaction was proposed. For benzene and naphthalene, as shown in Scheme 109, a [Cp2Ni] catalyst and BEt3 was used to complete arylation and was also applied to pyridine except using PPh3 instead of BEt3.6,9
An example of the cross-coupling of phenol derivatives was developed using the ideas of Nickel-catalyzed Suzuki-Miyaura Coupling.10 The reaction conditions to perform the coupling of aryl pivalate esters rely on the use of nickel(II) precatalyst. The precatalyst used was NiCl2(PCy3)2 with the advantage over a nickel(0) catalyst due to it not needing glovebox handling. Powdered K3PO4 was used as the base, and toluene was used as the solvent.10 One of the attractive features of this method is the potential to convert phenols to biaryls in a one-pot synthesis.10-11 The method was able to use fused aromatics, non-fused aromatics, and heterocycles, and substrates that had electron-withdrawing and electron donating groups also underwent the coupling.10
Benzylic cross-coupling using nickel catalysis is shown in the enantiospecific cross coupling of benzylic ammonium triflates with boronic acids. This method uses Ni(cod)2 as the catalyst. This catalyst is used without ancillary phosphine or N-heterocyclic carbene ligands.12 These conditions allow the use of heteroaromatic and vinyl boronic acids as coupling partners as well as ammonium triflates with different substituents at the benzylic stereocenter. This catalyst is interesting because it expands the use of enantiospecific cross coupling of the amine-derived substrates and produces highly enantioenriched products.12
Aziridines are three membered N-heterocycles that undergo ring opening reactions and produce valuable amine products. Metal-mediated cross-coupling of aziridines using a nickel catalyst and organic reagents was first discovered in 2012.13 One application that has been produced is nickel-catalyzed enantioselective cross coupling of aziridines. This method can be used to couple styrenyl aziridines with aryl iodines using reductive conditions. It can be done using complete regioselectivity for the benzylic position and with high enantioselectivity. This method can be used to isolate enantiopure phenethylamines from a racemic mixture with simple preparation, and it is applicable because these structures are common in the drug world.13
One example of cross-coupling of sp3 electrophiles using nickel catalysis was done in 2017. The researchers created an electrochemical method for reducing the nickel catalyst complexes using sp2-sp3 cross-coupling between aryl and alkyl bromides.14 They also found that the current passed during this reaction was just as effective if not more effective in reduction as using a zinc powdered metal.14 This advantage broadens the use of electrochemistry in the area of nickel-catalysis reductive cross-coupled reactions.
A reductive cross-coupling has also been developed, coupling vinyl bromides and benzyl chlorides. The conditions are mild, and a number of functional groups can be used.15 A NiCl2(dme) catalyst is used along with bis(oxazoline) and gives products that are enantioenriched and have aryl-substituted tertiary allylic stereogenic centers. It used Mn0 to reduce the nickel-catalyst.15 Reductive cross-coupling using a nickel catalyst reaction have been useful in creating carbon-carbon bonds formed by two organic electrophiles.16
The Heck reaction is interesting because it forms a carbon-carbon bond. This process of forming this carbon-carbon bond is powerful and unique because it can couple alkenes with various aryl, alkenyl, and alkyl halides.17 Matsubara describes how nickel catalysis can be used to form allylbenzene derivatives. He shows these derivatives being formed using a nickel catalyzed intermolecular benzylation and heterobenzylation. In this process, he proves that a wide range of benzyl chlorides and alkene coupling partners can be used in this process.17 The group also compared the nickel catalyzed reactions to palladium catalyzed reactions and found a contrast in the two. For the nickel catalyzed reactions, the group found that the electronically unbiased aliphatic olefins can proceed at room temperature. They also discovered the reactions provided a 1,1-disubstituted olefins and had a very high selectivity for the 1,1-disubstituted olefins. This finding was in contrast to the usually more common 1,2-disubstitute olefins.17 The pathway of this reaction showed shorter nickel-ligand bond length make it better for the steric differentiation between the H and alkyl substituents of the alkene.1,17 The same group also created an air-stable precatalyst which contained nickel. This precatalyst was found to increase reaction rates as well as decrease the need for the Ni(cod)2 ligand.1,18 Nickel’s properties can also be seen in a branch-selecting Heck reaction, in which the nickel’s oxidative addition property can be used on electrophiles that are cheap, stable, and usually unreactive.1,19
Reductive Coupling Reactions are another type of reactions that can use nickel catalysis. This reaction combines two ?-components with a reducing agent which forms a new ?-bond between the two coupling partners, as well as forming a new C-H-? bond, which comes from the reducing agent.1 This reductive coupling reaction usually follows a pathway using an oxidative cyclization, which is followed by coordination of the reducing agent to nickel or by ?-bond metathesis, which ultimately gives a nickel hydride. This reaction is terminated by reductive elimination.1,20 Reductive Coupling Reactions have had a couple of key developments in the past decade. The first development is potentially finding a milder reducing agent, which was solved by a couple of groups.1 The first group’s research concluded the use of methanol to form the appropriate hydride.1,21 Another group made a similar discovery a few years later using alcohols as the reducing agents along with nickel(II) salts as the catalyst to perform a reductive coupling reaction of alkynes and epoxides.1,22 The second challenging development with this type of reaction is regioselectivity.1 There have been a few studies on this challenge, but I will only mention a couple of the more recent ones. Montgomery and his group found a way to control regioselectivity of aldehyde-alkene reductive coupling using a nickel catalyst.23 The same group also found a way to reverse regioselectivity of a nickel-catalyzed alkyne-aldehyde reductive coupling reaction by reducing the steric bulkiness of the N-heterocyclic carbene ligands. Montgomery’s group found that the regioselectivity is affected by the orientation of the N-substituents on the ligand.24
Nickel Catalysis has become a more well-known field over the past decade as seen in the studies described above. The knowledge of the elementary steps and oxidation states of nickel in the many reaction mechanisms is understood because of the mechanism studies.1 More research needs to be done to be more knowledgeable about nickel catalysis. One of the developments expected to be researched more in the future is Csp3-Csp3 bond formation.1 More promising research is also expected to be done on the development of sources of nickel, which can be used for catalysis, that is low-cost, air-stable, and easier to handle.1 Research on nickel catalysis for the production of energy from redox reactions is also being researched to try to solve the energy crisis.25 Nickel is an inexpensive metal that has many properties that work well for catalysis and is a promising metal for future research in catalysis due to these properties.
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