Ni‐Catalyzed Amination Reactions: An Overview

Nitrogen‐containing organic compounds are valuable in many fields of science and industry. The most reliable method for the construction of C(sp²)–N bonds is undoubtedly palladium‐catalyzed amination. In spite of the great achievements made in this area, the use of expensive Pd‐based catalysts constitutes an important limitation for large‐scale applications. Since nickel is the least expensive and most abundant among the group 10 metals, the interest in Ni‐based catalysts for processes typically catalyzed by palladium has grown considerably over the last few years. Herein, we revise the development of Ni‐catalyzed amination reactions, emphasizing the most relevant and recent advances in the field.     

General and selective synthesis of primary amines using Ni-based homogeneous catalysts

The development of base metal catalysts for industrially relevant amination and hydrogenation reactions by applying abundant and atom economical reagents continues to be important for the cost-effective and sustainable synthesis of amines which represent highly essential chemicals. In particular, the synthesis of primary amines is of central importance because these compounds serve as key precursors and central intermediates to produce value-added fine and bulk chemicals as well as pharmaceuticals, agrochemicals and materials. Here we report a Ni-triphos complex as the first Ni-based homogeneous catalyst for both reductive amination of carbonyl compounds with ammonia and hydrogenation of nitroarenes to prepare all kinds of primary amines. Remarkably, this Ni-complex enabled the synthesis of functionalized and structurally diverse benzylic, heterocyclic and aliphatic linear and branched primary amines as well as aromatic primary amines starting from inexpensive and easily accessible carbonyl compounds (aldehydes and ketones) and nitroarenes using ammonia and molecular hydrogen. This Ni-catalyzed reductive amination methodology has been applied for the amination of more complex pharmaceuticals and steroid derivatives. Detailed DFT computations have been performed for the Ni-triphos based reductive amination reaction, and they revealed that the overall reaction has an inner-sphere mechanism with H₂ metathesis as the rate-determining step.

A Ni-triphos based homogeneous catalyst enabled the synthesis of all kinds of primary amines by reductive amination of carbonyl compounds with ammonia and hydrogenation of nitroarenes.     

Synthesis,Characterization, and DNA Binding Studies of a Nickel(II) Complex Containing the 1,3‐Bis(1‐propylbenzimidazol‐2‐yl)‐2‐oxapropane

A complex of formula [Ni(pobb)₂](pic)₂, (pobb = 1,3‐bis(1‐propylbenzimidazol‐2‐yl)‐2‐oxapropane, pic = 2,4,6‐trinitrophenol), has been synthesized and structurally characterized by physico‐chemical and spectroscopic methods. The crystals crystallize in the monoclinic system, space group C2/c, a = 25.766(11) Å, b = 14.943(7) Å, c = 19.543(14) Å, α = 90°, β = 129.722(4)°, γ = 90°, Z = 4. The coordination environment around nickel(II) atom can be described as a distorted octahedral geometry. The interactions of the ligand pobb and the nickel (II) complex with calf thymus DNA (CT‐DNA) are investigated by using electronic absorption titration, ethidium bromide‐DNA displacement experiments and viscosity measurements. The experimental evidence indicated the compounds interact with calf thymus DNA through intercalation.     

CH Activation of Benzene by a Photoactivated NiII(azide): Formation of a Transient Nickel Nitrido Complex

Photochemical activation of nickel‐azido complex 2 [Ni(N₃)(PNP)] (PN^HP=2,2′‐di(isopropylphosphino)‐4,4′‐ditolylamine) in neat benzene produces diamagnetic complex 3 [Ni(Ph)(PN^PN^H)], which is crystallographically characterized. DFT calculations support photoinitiated N₂‐loss of the azido complex to generate a rare, transient Ni^IV nitrido species, which bears significant nitridyl radical character. Subsequent trapping of this nitrido through insertion into the Ni? P bond generates a coordinatively unsaturated Ni^II imidophosphorane P?N donor. This species shows unprecedented reactivity toward 1,2‐addition of a C? H bond of benzene to form 3 . The structurally characterized chlorido complex 4 [Ni(Cl)(PN^PN^H)] is generated by reaction of 3 with HCl or by direct photolysis of 2 in chlorobenzene. This is the first report of aromatic C? H bond activation by a trapped transient nitrido species of a late transition metal.     

C?H Activation of Benzene by a Photoactivated NiII(azide): Formation of a Transient Nickel Nitrido Complex

Photochemical activation of nickel‐azido complex 2 [Ni(N₃)(PNP)] (PN^HP=2,2′‐di(isopropylphosphino)‐4,4′‐ditolylamine) in neat benzene produces diamagnetic complex 3 [Ni(Ph)(PN^PN^H)], which is crystallographically characterized. DFT calculations support photoinitiated N₂‐loss of the azido complex to generate a rare, transient Ni^IV nitrido species, which bears significant nitridyl radical character. Subsequent trapping of this nitrido through insertion into the Ni P bond generates a coordinatively unsaturated Ni^II imidophosphorane PN donor. This species shows unprecedented reactivity toward 1,2‐addition of a C H bond of benzene to form 3 . The structurally characterized chlorido complex 4 [Ni(Cl)(PN^PN^H)] is generated by reaction of 3 with HCl or by direct photolysis of 2 in chlorobenzene. This is the first report of aromatic C H bond activation by a trapped transient nitrido species of a late transition metal.     

Synthesis,Structure and Magnetic Properties of a Novel Nickel(II) Radical Heterospin Complex with Salicylaldoxime

A novel heterospin complex containing both Ni^II and nitroxide radical ligands: [Ni(salox)₂(NIT4Py)₂] ( 1 ) (salox = salicylaldoxime, NIT4Py = 2‐(4′‐pyridyl)‐4,4,5,5‐ tetramethyl‐imidazoline‐1‐oxyl‐3‐oxide) has been synthesized and structurally characterized. The structure consists of neutral Ni(salox)₂(NIT4Py)₂ moieties bridged by intermolecular hydrogen bonds, forming a one‐dimensional chain structure. Magnetic measurements show intramolecular antiferromagnetic interactions between NIT4Py and Ni²⁺ ion.     

First Reaction of a Chiral Gly-Ni(Ⅱ) Complex in Water

The amination of a chiral Ni(II) complex as a model reaction was studied to develop a highly practical, efficient, and stereospecific approach for Ni(II) complex reactions in an aqueous phase. A convenient and clean water‐mediated reaction of the Ni(II) complex has been reached, and a broad spectrum of substrates could participate in the process effectively to produce desired products in good yields and excellent diastereoselectivities. Significantly, it is the first report of chiral gly‐Ni(II) complex reaction in aqueous media.     

Catalytic Cyclization and Dimerization of 1.5‐Hexadiene by Transition Metals: The Reactivity of NiII and PdII Complexes containing Bi‐ or Tridendate Ligands and of Bis(alkyne)nickel(0) Compounds

The novel nickel(II)‐ and palladium(II) complexes (box)Ni(acac)₂ ( 1 ), (box)Ni(mes)(Br) ( 2 ), (box)Pd(mes)₂ ( 3 ), [(PEMA)NiBr]Br ( 4 a ) and [(PEMA)PdCl]Cl ( 4 b ) were synthesized and investigated by IR‐, MS‐ and (except the paramagnetic compound 1 ) by NMR spectroscopy (box: 4,4,4′,4′‐tetramethyl‐2,2′‐bisoxazoline, PEMA: N‐diphenylphosphinoethyl‐N‐(N′,N′‐dimethylaminoethyl)‐methylamine). According to the X‐ray analyses the metal atoms in 1 are in a distorted‐octahedral surrounding, in the organometallic compounds 2 and 3 they have square‐planar coordination, in the ionic complex 4 a is one halogen anion uncoordinated. According to the X‐ray analyses the analogous complexes [(DEPA)NiCl]Cl ( 5 a ) and [(DEPA)PdCl]Cl ( 5 b ) (DEPA: N,N‐bis(diphenylphosphinoethyl)‐amin)) display the same structure in the solid state as found in 4 a . The ni compounds catalyze at ambient temperature in the presence of Et₃Al the selective cyclization of 1.5‐hexadiene to form methylene‐cyclopentane (MeCP) in high yields. The combination of the Ni compounds with methylalumoxanes (MAO) leads to the catalytic formation of isomeric dimers containing one cyclopentyl ring as main products. The binuclear Ni(0) complex [(alkyne)Ni]₂ ( 6 ) (alkyne: bis(t‐butylalkynyl)‐bis‐phenylsilan) is more reactive in both catalytic reactions.     

Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d–4f} Single‐Molecule Magnets: A Theoretical Perspective

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni₃^IIILn^III} star‐type complexes [Ln=Gd ( 1 ), Dy ( 2 )] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2 . DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1 . Acute ?Ni?O?Gd bond angles present in 1 instigate a significant interaction between the 4f_xyz orbital of the Gd^III ion and 3d orbital of the Ni^II ions, leading to rare and strong antiferromagnetic Ni???Gd interactions. Calculations reveal the presence of a strong next‐nearest‐neighbour Ni???Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI‐SO calculations performed on complex 2 suggest that the octahedral environment around the Dy^III ion is neither strong enough to stabilize the m_J |±15/2〉 as the ground state nor able to achieve a large ground‐state–first‐excited‐state gap. The ground‐state Kramers doublet for the Dy^III ion is found to be the m_J |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the J_NiDy interaction as ?1.45 cm^?1. The strong Ni???Dy and next‐nearest‐neighbour Ni???Ni interactions are found to quench the QTM to a certain extent, resulting in zero‐field SMM behavior for complex 2 . The absence of any ac signals at zero field for the structurally similar [Dy(AlMe₄)₃] highlights the importance of both the Ni???Dy and the Ni???Ni interactions in the magnetization relaxation of complex 2 . To the best of our knowledge, this is the first time that the roles of both the Ni???Dy and Ni???Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established.     

Pentacoordinated Carboxylate π‐Allyl Nickel Complexes as Key Intermediates for the Ni‐Catalyzed Direct Amination of Allylic Alcohols

Direct amination of allylic alcohols with primary and secondary amines catalyzed by a system made of [Ni(1,5‐cyclooctadiene)₂] and 1,1′‐bis(diphenylphosphino)ferrocene was effectively enhanced by adding nBu₄NOAc and molecular sieves, affording the corresponding allyl amines in high yield with high monoallylation selectivity for primary amines and high regioselectivity for monosubstituted allylic alcohols. Such remarkable additive effects of nBu₄NOAc were elucidated by isolating and characterizing some nickel complexes, manifesting the key role of a charge neutral pentacoordinated η³‐allyl acetate complex in the present system, in contrast to usual cationic tetracoordinated complexes earlier reported in allylic substitution reactions.     

Chemo‐ and Enantioselective Oxidative α‐Azidation of Carbonyl Compounds

We report high‐performance I⁺/H₂O₂ catalysis for the oxidative or decarboxylative oxidative α‐azidation of carbonyl compounds by using sodium azide under biphasic neutral phase‐transfer conditions. To induce higher reactivity especially for the α‐azidation of 1,3‐dicarbonyl compounds, we designed a structurally compact isoindoline‐derived quaternary ammonium iodide catalyst bearing electron‐withdrawing groups. The nonproductive decomposition pathways of I⁺/H₂O₂ catalysis could be suppressed by the use of a catalytic amount of a radical‐trapping agent. This oxidative coupling tolerates a variety of functional groups and could be readily applied to the late‐stage α‐azidation of structurally diverse complex molecules. Moreover, we achieved the enantioselective α‐azidation of 1,3‐dicarbonyl compounds as the first successful example of enantioselective intermolecular oxidative coupling with a chiral hypoiodite catalyst.     

A Mononuclear η2‐Bonded Phosphaalkyne Nickel(0) Complex

The synthesis and characterization of the new complex [Ni(Im^iPr)₂(η²‐P≡C‐tBu)] ( 1 ) is reported. Compound 1 represents the first structurally characterized example of a mononuclear nickel(0) complex with a side on coordinated phophaalkyne ligand.     

Nucleophilic Iron Complexes in Proton‐Transfer Catalysis: An Iron‐Catalyzed Dimroth Cyclocondensation

The nucleophilic iron complex Bu₄N[Fe(CO)₃(NO)] (TBA[Fe]) is an active catalyst in C?H‐amination but also in proton‐transfer catalysis. Herein, we describe the successful use of this complex as a proton‐transfer catalyst in the cyclocondensation reaction between azides and ketones to the corresponding 1,2,3‐triazoles. Cross‐experiments indicate that the proton‐transfer catalysis is significantly faster than the nitrene‐transfer catalysis, which would lead to the C?H amination product. An example of a successful sequential Dimroth triazole–indoline synthesis to the corresponding triazole‐substituted indolines is presented.     

Effects of Ni particle size on amination of monoethanolamine over Ni-Re/SiO2 catalysts

Ni-Re/SiO₂ catalysts with controllable Ni particle sizes (4.5–18.0 nm) were synthesized to investigate the effects of the particle size on the amination of monoethanolamine (MEA). The catalysts were characterized by various techniques and evaluated for the amination reaction in a trickle bed reactor at 170°C, 8.0 MPa, and 0.5 h^?1 liquid hourly space velocity of MEA (LHSV_MEA) in NH₃/H₂ atmosphere. The Ni-Re/SiO₂ catalyst with the lowest Ni particle size (4.5 nm) exhibited the highest yield (66.4%) of the desired amines (ethylenediamine (EDA) and piperazine (PIP)). The results of the analysis show that the turnover frequency of MEA increased slightly (from 193 to 253 h^?1) as the Ni particle sizes of the Ni-Re/SiO₂ catalysts increased from 4.5 to 18.0 nm. Moreover, the product distribution could be adjusted by varying the Ni particle size. The ratio of primary to secondary amines increased from 1.0 to 2.0 upon increasing the Ni particle size from 4.5 to 18.0 nm. Further analyses reveal that the Ni particle size influenced the electronic properties of surface Ni, which in turn affected the adsorption of MEA and the reaction pathway of MEA amination. Compared to those of small Ni particles, large particles possessed a higher proportion of high-coordinated terrace Ni sites and a higher surface electron density, which favored the amination of MEA and NH₃ to form EDA.     

Research on the direct amination of benzene to aniline by NiAlPO4–5 catalyst

A series of NiAlPO₄–5 molecular sieves have been hydrothermally synthesized and shown high and stable activity in the direct amination of benzene to aniline with hydroxylamine hydrochloride as aminating agent. The as‐synthesized and calcined samples were investigated with XRD, SEM, TG, BET, NH₃‐TPD and FT‐IR to explore the crystalline, coordination and location of the incorporated transition metal ions. The results indicated that nickel ions were incorporated into the framework of AlPO₄–5 and the as‐synthesized catalysts were highly crystalline, and possessed good thermal stability. Among them, Ni(0.3)AlPO₄–5 showed the highest catalytic activity for the direct amination of benzene with the highest aniline yield of 73.2% and 100.0% selectivity to aniline. After the catalyst was reused for 5 times, the activity remained little change.     

A Novel Macrocyclic Nickel(II) and Ferricyanide 1D Zigzag Chain: Synthesis,Crystal Structure,and Magnetic Properties

A novel cyan‐bridged macrocyclic nickel complex [{NiL¹}{Fe(bipy)(CN)₄}]2·5H₂O 1 (L¹ = 3,10‐dimethyl‐1,3,6,8,10,12‐hexaazacyclotetradecane, bipy = 2,2‐bipyridine) was synthesized and structurally characterized. The complex exhibits one‐dimensional zigzag chain structures. Each ferrous(II) ion connects two nickel(II) ions using two trans CN^? groups, and the remaining CN^? groups are terminal. Magnetic measurement shows weak ferromagnetic interaction between the nearest Ni(II) ions through the diamagnetic Fe(II) ion.     

Synthesis,Structures, and Magnetic Properties of New Thiocyanato Coordination Polymers with 4‐(3‐Phenylpropyl)pyridine as Ligand

The reaction of different metal salts with 4‐(3‐phenylpropyl)pyridine (ppp) lead to the formation of compounds of composition M(NCS)₂(ppp)₄ [M = Mn ( Mn‐1 ); Fe ( Fe‐1 ), Ni ( Ni‐1 ); Cd ( Cd‐1 )], M(NCS)₂(ppp)₂(H₂O)₂ [M = Mn ( Mn‐2 ); Ni ( Ni‐2 )] and [M(NCS)₂(ppp)₂]_n [M = Mn ( Mn‐3 ); Ni ( Ni‐3 ); Cd ( Cd‐3 )]. On heating compounds M‐1 decompose without the formation of any ppp deficient intermediate. In contrast, on heating, Ni‐2 transforms into a compound of composition M(NCS)₂(ppp)₂ that does not correspond to Ni‐3 . Unfortunately, this compound is of low crystallinity and therefore, its structure cannot be determined. The crystal structures of compounds M‐1 and M‐2 consist of discrete complexes, in which the metal cations are octahedrally coordinated. In compounds M‐3 the metal cations are linked by pairs of μ‐1,3‐bridging anions into chains. IR spectroscopic investigations show, that the value of the asymmetric CN stretching vibration depend on the coordination mode of the anionic ligand as well as on the nature of the metal cation. Magnetic measurements reveal that Ni‐3 shows only Curie‐Weiss behavior without any magnetic anomaly. A similar behavior is also found for Ni‐3 . Comparison of the magnetic properties of Ni‐3 with those of similar compounds indicates that the magnetic properties are only minor influenced by the Co‐ligand.     

Nickel‐catalyzed Buchwald–Hartwig amination of pyrimidin‐2‐yl tosylates with indole,benzimidazole and 1,2,4‐triazole

Nickel‐catalyzed Buchwald–Hartwig amination of pyrimidin‐2‐yl tosylates with indole and benzimidazole was achieved using Ni(dppp)Cl₂ as catalyst, yielding a variety of novel C2‐substituted pyrimidine derivatives in good yields. This reaction proved to be tolerant of various pyrimidin‐2‐yl tosylates bearing either electron‐donating or electron‐withdrawing groups as well as nucleophiles including indole, benzimidazole and 1,2,4‐triazole. Copyright © 2016 John Wiley & Sons, Ltd.     

Copper‐Catalyzed Oxidative Amination and Allylic Amination of Alkenes

Enamines and enamides are useful synthetic intermediates and common components of bioactive compounds. A new protocol for their direct synthesis by a net alkene C? H amination and allylic amination by using catalytic Cu^II in the presence of MnO₂ is reported. Reactions between N‐aryl sulfonamides and vinyl arenes furnish enamides, allylic amines, indoles, benzothiazine dioxides, and dibenzazepines directly and efficiently. Control experiments further showed that MnO₂ alone can promote the reaction in the absence of a copper salt, albeit with lower efficiency. Mechanistic probes support the involvement of nitrogen‐radical intermediates. This method is ideal for the synthesis of enamides from 1,1‐disubstituted vinyl arenes, which are uncommon substrates in existing oxidative amination protocols.     

Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand

The first 16 valence electron [bis(NHC)](silylene)Ni⁰ complex 1 , [(^TMSL)ClSi:→Ni(NHC)₂], bearing the acyclic amido‐chlorosilylene (^TMSL)ClSi: (^TMSL=N(SiMe₃)Dipp; Dipp=2,6‐Prⁱ₂C₆H₄) and two NHC ligands (N‐heterocyclic carbene=:C[(Prⁱ)NC(Me)]₂) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi‐NiH complex 2 . Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene‐coordinated (chloro)(silyl)nickel(II) complex 3 , {[cat(^TMSL)Si](Cl)Ni←:BH(NHC)₂}, via the cleavage of two B−O bonds and simultaneous formation of two Si−O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B−O bonds.