Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling

Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/ L1 ( L1 =trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni^I–Ni^III catalytic cycle with three main steps: transmetalation of [Ni^I( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R₁ to produce [Ni^I( L1 )(R₁)], oxidative addition of R₂X with [Ni^I( L1 )(R₁)] to produce [Ni^III( L1 )(R₁)(R₂)X] through a radical pathway, and C? C reductive elimination to generate the product and [Ni^I( L1 )X]. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol^?1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/ L2 ( L2 =trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1 . Importantly, the Ni⁰–Ni^II catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni^II( L1 )(R₁)(R₂)] is very difficult.     

Mechanism of Ligand‐Controlled Regioselectivity‐Switchable Copper‐Catalyzed Alkylboration of Alkenes

Cu‐catalyzed alkylboration of alkenes with bis(pinacolato)diboron ((Bpin)₂) and alkyl halides provides a ligand‐controlled regioselectivity‐switchable method for the construction of complex boron‐containing compounds. Here, we employed DFT methods to elucidate the mechanistic details of this reaction and the origin of the different regioselectivity induced by Xantphos and Cy‐Xantphos. The calculation results reveal that the catalytic cycle mainly proceeds through the migratory insertion of alkenes on Cu‐Bpin complex, the oxidative addition of alkyl halides, and the reductive elimination of a C?C bond. Meanwhile, the rate‐ determining step is the oxidative addition of alkyl halides and the regioselectivity‐determining step is the migratory insertion of alkenes. The bulky cyclohexyl group of Cy‐Xantphos facilitates the approach of the substituents of alkenes to Bpin in the migratory insertion step and thus leads to the Markovnikov products. The less bulky phenyl group on Xantphos prefers keeping the substituents of alkenes away from the Bpin moiety in the migratory insertion step and thus results in anti‐Markovnikov products.     

Nickel‐Catalyzed Reductive Allylation of Tertiary Alkyl Halides with Allylic Carbonates

The construction of all C(sp³) quaternary centers has been successfully achieved under Ni‐catalyzed cross‐electrophile coupling of allylic carbonates with unactivated tertiary alkyl halides. For allylic carbonates bearing C1 or C3 substituents, the reaction affords excellent regioselectivity through the addition of alkyl groups to the unsubstituted allylic carbon terminus. The allylic alkylation method also exhibits excellent functional‐group compatibility, and delivers the products with high E selectivity.     

In situ generation of highly active bis(N‐heterocyclic)carbene palladium as an efficient catalyst in direct S‐arylation of methylphenyl sulfoxide and the Heck reaction: Ligand steric effects in product selectivity

The use of 1,3‐bis(N‐heterocyclic)carbene ligands with different alkyl wingtip groups (alkyl = methyl, isopropyl and tert ‐butyl) is an effective method for the palladium‐catalysed direct S ‐arylation of methylphenyl sulfoxide and C–C coupling of various of aryl halides with alkenes. The reactions proceed in moderate to good yields. Interestingly, it is shown experimentally that, by using bulkier bidentate N‐heterocyclic carbene ligands, more selective catalytic systems towards cis products in Heck coupling reactions can be achieved.     

Room‐temperature Suzuki–Miyaura cross‐coupling reaction with α‐diimine Pd(II) catalysts

An α‐diimine Pd(II) complex containing chiral sec‐phenethyl groups, {bis[N,N′‐(4‐methyl‐2‐sec‐phenethylphenyl)imino]‐2,3‐butadiene}dichloropalladium (rac‐ C1 ), was synthesized and characterized. rac‐ C1 was applied as an efficient catalyst for the Suzuki–Miyaura cross‐coupling reaction between various aniline halides and arylboronic acid in PEG‐400–H₂O at room temperature. Among a series of aniline halides, rac‐ C1 did not catalyze the cross‐coupling of aniline chlorides and fluorides but efficiently catalyzed the cross‐coupling of aniline bromides and iodides with phenylboronic acid. The catalytic activity reduced slightly with increasing steric hindrance of the aniline bromides. The complexes {bis[N,N′‐(4‐fluoro‐2,6‐diphenylphenyl)imino]‐2,3‐butadiene}dichloropalladium and {bis[N,N′‐(4‐fluoro‐2,6‐diphenylphenyl)imino]acenaphthene}dichloropalladium were also found to be efficient catalysts for the reaction. Copyright © 2015 John Wiley & Sons, Ltd.     

PEPPSI‐Type Palladium Complexes Containing Basic 1,2,3‐Triazolylidene Ligands and Their Role in Suzuki–Miyaura Catalysis

A series of PEPPSI‐type palladium(II) complexes was synthesized that contain 3‐chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki–Miyaura cross‐coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol‐2‐ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N‐heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.     

Expedient Mechanosynthesis of N,N‐Dialkyl Imidazoliums and Silver(I)–Carbene Complexes in a Ball‐Mill

The absence of solvent, associated with intensive mechanical agitation, allowed the first mechanosynthesis of high‐value silver(I)–carbene complexes and the corresponding N,N‐dialkylimidazolium precursors. This procedure gave outstanding results in terms of yield and reaction time, when compared to solution‐based conditions previously described in literature, and was generalized to unprecedented compounds. Silver(I)–carbene complexes could either be obtained from N,N‐dialkylimidazolium salts or directly from imidazole and alkyl halides in a one‐pot two‐step procedure without isolating the imidazolium intermediate. Additionally, an efficient one‐pot three‐step sequence, including imidazole alkylation, silver metalation, and transmetalation is reported.     

Synthesis and characterization of Ni(II) complexes bearing of 2‐(1H–benzimidazol‐2‐yl)‐phenol derivatives as highly active catalysts for ethylene oligomerization

Novel Ni(II) complexes of 2‐(1H–benzimidazol‐2‐yl)‐phenol derivatives (HL_x: x  =  1–5; C1–C5 ) have been synthesized and characterized. In the mononuclear complexes, the ligands were coordinated as bidentate, via one imine nitrogen and the phenolate oxygen atoms. The structures of the compounds were confirmed on the basis of FT‐IR, UV–Vis, ¹H‐, ¹³C–NMR, inductively coupled plasma and elemental analyses (C, H and N). The purity of these compounds was ascertained by melting point (m.p.) and thin‐layer chromatography. The geometry optimization and vibrational frequency calculations of the compounds were performed using Gaussian 09 program with B3LYP/TZVP level of theory. All Ni(II) complexes were activated with diethylaluminum chloride (Et₂AlCl), so that C2 showed the highest activity [6600 kg mol^?1 (Ni) h^?1], where the ligand contains a chlorine substituent. Oligomers obtained from the complexes consist mainly of dimer and trimer, and also exhibit high selectivity for linear 1‐butene and 1‐hexene. Both the steric and electronic effects of coordinative ligands affect the catalytic activity and the properties of the catalytic products.     

Dichlorovanadium (IV) complexes with salen‐type ligands for ethylene polymerization

Vanadium complexes with tetradentate salen‐type ligands were first time explored in ethylene polymerizations. The effects of the vanadium complex structure, the alkyl aluminum cocatalysts type (EtAlCl₂, Et₂AlCl, Et₃Al, and MAO), and the polymerization conditions (Al/V molar ratio, temperature) on polyethylene yield were explored. It was found that EtAlCl₂ in conjunction with investigated vanadium complexes produced the most efficient catalytic systems. It was shown, moreover, that the structural changes of the tetradentate salen ligand (type of bridge which bond donor nitrogen atoms and type of substituent on aryl rings) affected activity of the catalytic system. The complexes containing ligands with cyclohexylene bridges were more active than those with ethylene bridges. Furthermore, the presence of electron‐withdrawing groups at the para position and electron‐donating substituents at the ortho position on the aryl rings of the ligands resulted in improved activity in relation to the systems with no substituents (with the exception of bulky t‐Bu group). The results presented also revealed that all vanadium complexes activated by common organoaluminum compounds gave linear polyethylenes with high melting points (134.8–137.6 °C), high molecular weights, and broad molecular weight distribution. The polymer produced in the presence of MAO possesses clearly lower melting point (131.4 °C) and some side groups (around 9/1000 C). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6940–6949, 2008     

Computational Insight into Nickel‐Catalyzed Carbon–Carbon versus Carbon–Boron Coupling Reactions of Primary,Secondary, and Tertiary Alkyl Bromides

The nickel‐catalyzed alkyl–alkyl cross‐coupling (C?C bond formation) and borylation (C?B bond formation) of unactivated alkyl halides reported in the literature show completely opposite reactivity orders in the reactions of primary, secondary, and tertiary alkyl bromides. The proposed Ni^I/Ni^III catalytic cycles for these two types of bond‐formation reactions were studied computationally by means of DFT calculations at the B3LYP level. These calculations indicate that the rate‐determining step for alkyl–alkyl cross‐coupling is the reductive elimination step, whereas for borylation the rate is determined mainly by the atom‐transfer step. In borylation reactions, the boryl ligand involved has an empty p orbital, which strongly facilitates the reductive elimination step. The inability of unactivated tertiary alkyl halides to undergo alkyl–alkyl cross‐coupling is mainly due to the moderately high reductive elimination barrier.     

C−H Bond Activation/Arylation Catalyzed by Arene–Ruthenium–Aniline Complexes in Water

Water‐soluble arene–ruthenium complexes coordinated with readily available aniline‐based ligands were successfully employed as highly active catalysts in the C?H bond activation and arylation of 2‐phenylpyridine with aryl halides in water. A variety of (hetero)aryl halides were also used for the ortho‐C?H bond arylation of 2‐phenylpyridine to afford the corresponding ortho‐ monoarylated products as major products in moderate to good yields. Our investigations, including time‐scaled NMR spectroscopy and mass spectrometry studies, evidenced that the coordinating aniline‐based ligands, having varying electronic and steric properties, had a significant influence on the catalytic activity of the resulting arene–ruthenium–aniline‐based complexes. Moreover, mass spectrometry identification of the cycloruthenated species, {(η⁶‐arene)Ru(κ²‐C,N‐phenylpyridine)}⁺, and several ligand‐coordinated cycloruthenated species, such as [(η⁶‐arene)Ru(4‐methylaniline)(κ²‐C,N‐phenylpyridine)]⁺, found during the reaction of 2‐phenylpyridine with the arene–ruthenium–aniline complexes further authenticated the crucial roles of these species in the observed highly active and tuned catalyst. At last, the structures of a few of the active catalysts were also confirmed by single‐crystal X‐ray diffraction studies.     

Sulfonated salicylidene thiadiazole complexes with Co (II) and Ni (II) ions as sustainable corrosion inhibitors and catalysts for cross coupling reaction

Condensation of 2,5‐dihydrazinyl thiadiazole with 5‐sodium sulfonate salicylaldehyde afforded dibasic tetradentate pincer N,O,O,N‐salicyldiene thiadiazole ligand (H₂Sanp). The novel dipolar ligand formed para‐magnetic pincer complexes within Co (II) and Ni (II) ions (Co‐Sanp and Ni‐Sanp) under sustainable conditions. The water‐soluble ligand and its metal‐complexes were estimated by mass, IR and UV–Visible spectroscopy, EA (elemental analyses), TGA (Thermogravimetric analyses), magnetic susceptibility, and conductivity measurements. The catalytic reactivity of Co‐Sanp and Ni‐Sanp were evaluated in the Suzuki and Buchwald‐Hartwig cross coupling reaction in aqueous‐methanol binary mixtures. Both reactions of boronic acid or aryl amines with aryl halides gave high chemoselective yield of C―C or C―N product. The inhibition characteristics of H₂Sanp and its Ni‐ and Co‐complexes were performed for the C‐steel corrosion in 1.0 M HCl using electrochemical measurements and surface analysis methods. These methods indicated that the synthesized compounds have served as efficient mixed‐type corrosion inhibitors and their adsorption on the steel surface obeyed isotherm model of Langmuir. Co‐Sanp inhibitor displays the best corrosion inhibition efficiency, and the capacity is up to 97.11% at of 250 mg L^?1. Surface analysis confirms formation of protective layer on the C‐steel surface.     

Mononuclear Carbene Dithiolate [Ni(NHX)(′S2C ′)] Complexes with HNX = HNPiPr3 or HNSPh2 Coligands (′S2C ′ 2– = 1,3‐ImidazolidinylN,N′‐bis(2‐benzenethiolate)(2–))

Cleavage reactions of the dinuclear [{Ni(′S₂C ′)}₂] · DMF (′S₂C ′ ^2– = 1,3‐imidazolidinyl‐N,N′‐bis(2‐benzenethiolate)(2–)) with HNPiPr₃ or HNSPh₂ yielded the mononuclear complexes [Ni(NHPiPr₃)(′S₂C ′)] ( 1 ) and [Ni(NHSPh₂)(′S₂C ′)] ( 2 ) which have been completely characterized. The nickel‐carbene‐dithiolate [Ni(′S₂C ′)] moiety is one of the very rare complex fragments that are able to coordinate both HNPR₃ or HNSR₂. IR spectra and X‐ray structure determinations show that 1 and 2 exhibit intramolecular N–H…S(thiolate) hydrogen bonds. Geometric parameters and NMR spectroscopic data of 1 and 2 are compatible with N–X single bonds and ylidic structures of the HNPiPr₃ and HNSPh₂ ligands. Comparison of Ni–N distances in diamagnetic and paramagnetic [Ni(NHSPh₂)] complexes was rendered possible through the X‐ray structure determination of the homoleptic [Ni(NHSPh₂)₆]Cl₂ ( 3 ) which formed as minor by‐product in the synthesis of 2 .     

Nickel-catalyzed reductive cross-coupling of polyfluoroarenes with alkyl electrophiles by site-selective C–F bond activation

A nickel-catalyzed reductive cross-coupling reactions between polyfluoroarenes and alkyl electrophiles is reported to access substituted fluoroarenes through chelation-assisted C–F activation. Diverse primary and secondary alkyl (pseudo)halides can be employed to couple with polyfluoroarenes, showing excellent regioselectivity. Furthermore, the nickel-catalyzed asymmetric cross-coupling of polyfluoroarenes with racemic alkyl halides is preliminarily explored. In addition, the practicability of the title transformation is also demonstrated by total synthesis of losmapimod and an analog as key steps. The developed method exhibits many advantages, including economic catalytic systems, commercially available alkyl electrophiles, and lack of sensitive organometallic reagents.     

Insertion of CO2 Mediated by a (Xantphos)NiI–Alkyl Species

The incorporation of CO₂ into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni^II to Ni^I by either Zn or Mn, followed by CO₂ insertion into Ni^I‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)Ni^IIBr₂ by Zn affords Ni^I species. (tBu‐Xantphos)Ni^I‐Me and (tBu‐Xantphos)Ni^I‐Et complexes undergo fast insertion of CO₂ at 22 °C. The substantially faster rate, relative to that of Ni^II complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.     

C–C bridged Ni(II) complexes bearing β‐keto‐9‐fluorenyliminato ligands prepared by different in situ bonding mechanisms and their use in catalytic (co)polymerization of norbornene and styrene

Two C–C bridged Ni(II) complexes bearing β‐keto‐9‐fluorenyliminato ligands with electron‐withdrawing groups (─CF₃), Ni{PhC(O)CHC[N(9‐fluorenyl)]CF₂}₂ (Ni 1 ) and Ni{CF₃C(O)CHC[N(9‐fluorenyl)]Ph}₂ (Ni 2 ), were synthesized by metal coordination reaction and different in situ bonding mechanisms. The C–C bridged bonds of Ni 1 were formed by in situ intramolecular trifluoromethyl and 9‐fluorenyl carbon–carbon cross‐coupling reaction and those of Ni 2 were formed by in situ intramolecular 9‐fluorenyl carbon–carbon radical coupling reaction mechanism. The obtained complexes were characterized using ¹H NMR spectroscopy and elemental analyses. The crystal and molecular structures of Ni 1 and Ni 2 with C–C bridged configuration were determined using X‐ray diffraction. Ni 1 and Ni 2 were used as catalysts for norbornene (NB) polymerization after activation with B(C₆F₅)₃ and the catalytic activities reached 10⁶ g_polymer mol_Ni^?1 h^?1. The copolymerization of NB and styrene catalyzed by the Ni 1 /B(C₆F₅)₃ system showed high activity (10⁵ g_polymer mol_Ni^?1 h^?1) and the catalytic activities decreased with increasing feed content of styrene. All vinyl‐type copolymers exhibited high molecular weight (10⁴ g mol^?1), narrow molecular weight distribution (M_w/M_n = 1.71–2.80), high styrene insertion ratios (11.13–50.81%) and high thermal stability (T_d > 380°C) and could be made into thin films with high transparency in the visible region (400–800 nm).     

N‐heterocyclic carbene–palladium complexes for Suzuki–Miyaura coupling reaction with benzyl chloride and aromatic boronic acid leading to diarylmethanes

A family of N‐heterocyclic carbene–palladium(II)–N,N‐dimethylbenzylamine complexes ((NHC)LPdCl₂; L = N,N‐dimethylbenzylamine) were synthesized as well as characterized using single‐crystal X‐ray diffraction and spectroscopic data. These complexes exhibited higher catalytic activities for the Suzuki reaction of benzyl chlorides to afford diarylmethanes under milder conditions than other efficient (NHC)LPdCl₂ complexes. Using the optimum conditions, the expected coupling products were obtained in moderate to high yields. All reactions were carried out in air and all starting materials were used as supplied without purification.     

Potential N‐Heterocyclic Carbene Precursors in the Palladium‐Catalyzed Heck Reaction

A novel 1‐(cyclobutylmethyl)‐substi‐tuted imidazolidinium/benzimidazolium salts as N‐heterocyclic carbene (NHC) precursors were successfully synthesized and characterized by ¹H NMR, ¹³C NMR, IR, and elemental analysis techniques. These compounds were easily prepared from the reaction of N‐alkyl imidazoline/N‐alkyl benzimidazole with bromomethylcyclobutane in high yields. The in situ formed catalytic system derived from the NHC precursor and Pd(OAc)₂ was used in the Heck reaction between aryl halides and styrene with potassium hydroxide in water. The corresponding Heck products were obtained in good yields. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 24:77–83, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21065     

Cyclopentadienyl Ruthenium–Nickel Catalysts for Biomimetic Hydrogen Evolution: Electrocatalytic Properties and Mechanistic DFT Studies

The new dinuclear nickel–ruthenium complexes [Ni(xbsms)RuCp(L)][PF₆] (H₂xbsms=1,2‐bis(4‐mercapto‐3,3‐dimethyl‐2‐thiabutyl)benzene; Cp^?=cyclopentadienyl; L=DMSO, CO, PPh₃, and PCy₃) are reported and are bioinspired mimics of NiFe hydrogenases. These compounds were characterized by X‐ray diffraction techniques and display novel structural motifs. Interestingly, [Ni(xbsms)RuCpCO][PF₆] is stereochemically nonrigid in solution and an isomerization mechanism was derived with the help of density functional theory (DFT) calculations. Because of an increased electron density on the metal centers [Eur. J. Inorg. Chem. 2007 , 18 , 2613–2626] with respect to the previously described [Ni(xbsms)Ru(CO)₂Cl₂] and [Ni(xbsms)Ru(p‐cymene)Cl]⁺ complexes, [Ni(xbsms)RuCp(dmso)][PF₆] catalyzes hydrogen evolution from Et₃NH⁺ in DMF with an overpotential reduced by 180 mV and thus represents the most efficient NiFe hydrogenase functional mimic. DFT calculations were carried out with several methods to investigate the catalytic cycle and, coupled with electrochemical measurements, allowed a mechanism to be proposed. A terminal or bridging hydride derivative was identified as the active intermediate, with the structure of the bridging form similar to that of the Ni? C active state of NiFe hydrogenases.     

Chelatkomplexe LM/n von Übergangsmetallen mit Iminophosphin-säureamidato-Liganden R2P(NR′)2– (= L)

Chelate Complexes LM/n of Transition Metals with Phosphinoimidic Amidato Ligands R₂P(NR′)₂^– (= L) Reaction of LLi with metal halides or metal halide complexes affords chelate complexes LM/n (L = R₂P(NR′)₂^–; M = Cr⁺⁺⁺, Co⁺⁺, Ni⁺⁺, Zn⁺⁺). With the bulky ligand t-Bu₂P(NSiMe₃)₂^– and Ni(PPh₃)₂Cl₂ or Ni(dme)Br₂ (dme = dimethoxyethane) only halide bridged chelates [LNiHal]₂ (Hal = Cl, Br) containing tetrahedral chromophors NiN₂Hal₂ were obtained. Main objects of investigation were the bischelates L₂Ni 2 . 2 a (R = i-Pr, R′ = Me) and 2 c (R = Ph, R′ = Et) are planar, 2 b (R = i-Pr, R′ = Et) and 2 d–g (R, R′ = i-Pr, i-Pr; Ph, i-Pr; Et, SiMe₃; Ph, SiMe₃) tetrahedral. In solutions of 2 b and 2 c a conformational equilibrium planar (diamagnetic) tetrahedral (paramagnetic) exists that is shifted to the right with increasing temperature and is dominated by the tetrahedral ( 2 b ) or planar conformer ( 2 c ) at room temperature. As is the case with the isovalence electronic compounds [R₂P(S)NR′]₂Ni small substituents R′ apparently favour the planar state and in contrast to some complexes [R₂P(O)NR′]₂Ni no paramagnetic planar species 2 have yet been observed. These findings that are derived from the results of magnetic measurements and of UV/VIS as well as NMR spectroscopy are confirmed by crystal structure determinations: 2 a was found to be planar (orthorhombic; a = 3382.8(11), b = 1124.0(4), c = 8874(3); P2₁2₁2; Z = 6), and 2 g to be tetrahedral (monocline; a = 1268.4(2), b = 1806.8(2), c = 1971.6(2), P2₁/n; Z = 4). The bite angle NNiN of the chelate ligand in 2 a (ca. 77°) is similar to those in paramagnetic planar complexes [R₂P(O)NR′]₂Ni (NNiO 74–77°) and shows that a small chelate bite does not necessarily imply paramagnetism of planar Ni(II) complexes.