Zur Lewis-Acidität von Nickel(0), X. Diorganylmagnesium-Komplexe von Nickel(0): (TMEDA)MgCH3(μ-CH3)Ni(C2H4)2

Alkali-metal nickelates: catalytic cross-coupling, clusters and coordination complexes

Alkali-metal nickelates are a class of highly reactive heterobimetallic complexes derived from Ni(0)-olefins and polar organo-alkali-metal reagents. First reported over 50 years ago, it is only in recent years that these overlooked complexes have found formidable roles in sustainable catalysis and beyond. In this article, we will showcase the emerging catalytic applications of lithium nickelates and discuss the mechanisms by which these heterobimetallic complexes facilitate challenging cross-coupling reactions. We will also review the unique structure and bonding of alkali-metal nickelates, as interrogated by X-ray crystallography and complementary bonding analysis, and finally explore the diverse coordination and co-complexation chemistry of these heterobimetallic complexes.

Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications

Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li₄(solv)_n(Ar)₄Ni₂{μ₂:η²,η²-Ph-CC-Ph} (where solv = Et₂O, THF; Ar = Ph, o-Tol, naphthyl, 4-^tBu-C₆H₄) which can be accessed through the combination of Ni(COD)₂, Ph-CC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMO^tBu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li₂(solv)_n(Ar)₂Ni{η²-Ph-CC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.

A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation

The nature of transition-metal-olefin bonding has been explained by the Dewar-Chatt-Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal-ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal-bis-ethylene complex forming an unprecedented planar metalla-bis-cyclopropane structure with magnesium-based metalloligands. Treatment of LMgEt (L = [(DippNCMe)₂CH]^-, Dipp = 2,6-ⁱPr₂C₆H₃) with Ni(cod)₂ (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂, which features a linear Mg-Ni-Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni(C₂H₄)₂ moiety as a nickelaspiropentane. The exposure of (LMg)₂Ni(C₂H₄)₂ to 1 bar H₂ at room temperature produced a four-hydride-bridged complex (LMg)₂Ni(μ-H)₄. The profile of H₂ activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂ catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.

Mechanisms of the Nickel-Catalysed Hydrogenolysis and Cross-Coupling of Aryl Ethers

The Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers has emerged as a powerful synthetic tool to transform inert phenol-derived electrophiles into functionalised aromatic molecules. This has attracted significant interest due to its potential to convert the lignin fraction of biomass into chemical feedstocks, or to enable orthogonal reactivity and late-stage synthetic modification. Although the scope of nucleophiles employed, and hence the C–C and C–heteroatom bonds that can be forged, has expanded significantly since Wenkert’s seminal work in 1979, mechanistic understanding on how these reactions operate is still uncertain since the comparatively inert Caryl–O bond of aryl ethers challenge the involvement of classical mechanisms involving direct oxidative addition to Ni(0). In this review, we document the different mechanisms that have been proposed in the Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers. These include: (i) direct oxidative addition; (ii) Lewis acid assisted C–O bond cleavage; (iii) anionic nickelates, and; (iv) Ni(I) intermediates. Experimental and theoretical investigations by numerous research groups have generated a pool of knowledge that will undoubtedly facilitate future discoveries in the development of novel Ni-catalysed transformations of aryl ethers.

1 Introduction

2 Direct Oxidative Addition

3 Hydrogenolysis of Aryl Ethers

4 Lewis Acid Assisted C–O Bond Cleavage

5 Anionic Nickelates

6 Ni(I) Intermediates

7 The ‘Naphthalene Problem’

8 Conclusions and Outlook

Synthesis and reactivity of heterometallic complexes containing Mg- or Zn-metalloligands

Heteronuclear metal complexes comprising main group metals and transition metals have attracted widespread attention from researchers due to their applications in stoichiometric and catalytic activation of small molecules with possible cooperative effects. Herein, the advances of heterometallic complexes containing Mg- or Zn-metalloligands over the past ten years are reviewed. They consist of two parts: (i) synthetic approaches to heterometallic complexes. Only a brief discussion is made on the different Mg/Zn precursors since they have been summarized before. (ii) Stoichiometric and catalytic reactivities of heterometallic complexes containing Mg/Zn metalloligands. The exploration of the cooperative catalytic reaction of heterometallic complexes is still in its infancy, promising but challenging; thus, further investigations are required in the future.

Supported σ-Complexes of Li-C Bonds from Coordination of Monomeric Molecules of LiCH3, LiCH2CH3 and LiC6H5 to MoMo Bonds

LiCH₃ and LiCH₂CH₃ react with the complex [Mo₂(H)₂(μ‐Ad^Dipp2)₂(thf)₂] (1⋅thf) with coordination of two molecules of LiCH₂R (R=H, CH₃) and formation of complexes [Mo₂{μ‐HLi(thf)CH₂R}₂(Ad^Dipp2)₂], 5⋅LiCH₃ and 5⋅LiCH₂CH₃, respectively (Ad^Dipp2=HC(NDipp)₂; Dipp=2,6‐ⁱPr₂C₆H₃; thf=C₄H₈O). Due to steric hindrance, only one molecule of LiC₆H₅ adds to 1⋅thf generating the complex [Mo₂(H){μ‐HLi(thf)C₆H₅}(μ‐Ad^Dipp2)₂], (4⋅LiC₆H₅). Computational studies disclose the existence of five‐center six‐electron bonding within the H−Mo≣Mo−C−Li metallacycles, with a mostly covalent H−Mo≣Mo−C group and predominantly ionic Li−C and Li−H interactions. However, the latter bonds exhibit non‐negligible covalency, as indicated by X‐ray, computational data and the large one‐bond ^6,7Li,¹H and ^6,7Li,¹³C NMR coupling constants found for the three‐atom H−Li−C chains. By contrast, the phenyl group in 4⋅LiC₆H₅ coordinates in an η² fashion to the lithium atom through the ipso and one of the ortho carbon atoms.

The Anionic Pathway in the Nickel-Catalysed Cross-Coupling of Aryl Ethers

The Ni‐catalysed cross‐coupling of aryl ethers is a powerful method to forge new C−C and C−heteroatom bonds. However, the inert C(sp²)−O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni⁰ centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron‐rich hetero‐bimetallic nickelates by adding organometallic nucleophiles to a Ni⁰ centre. Assessing the rich co‐complexation chemistry between Ni(COD)₂ and PhLi has led to the structures and solution‐state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni⁰‐ate complexes plays a key role in the catalytic cycle.

Nature of the Short Rh-Li Contact between Lithium and the Rhodium ω-Alkenyl Complex [Rh(CH2CMe2CH2CH═CH2)2]. Inorg Chem 2021;60:8790-8801. [PMID: 34097392 DOI: 10.1021/acs.inorgchem.1c00737] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We describe the preparation of the cis-bis(η¹,η²-2,2-dimethylpent-4-en-1-yl)rhodate(I) anion, cis-[Rh(CH₂CMe₂CH₂CH═CH₂)₂]^-, and the interaction of this species with Li⁺ both in solution and in the solid state. For the lithium(diethyl ether) salt [Li(Et₂O)][Rh(CH₂CMe₂CH₂CH═CH₂)₂], VT-NMR and ¹H{⁷Li} NOE NMR studies in toluene-d₈ show that the Li⁺ cation is in close proximity to the d_z² orbital of rhodium. In the solid-state structure of the lithium(12-crown-4) salt [Li(12-crown-4)₂][Li{Rh(CH₂CMe₂CH₂CH═CH₂)₂}₂], one lithium atom is surrounded by two [Rh(CH₂CMe₂CH₂CH═CH₂)₂]^- anions, and in this assembly there are two unusually short Rh-Li distances of 2.48 Å. DFT calculations, natural energy decomposition, and ETS-NOCV analysis suggest that there is a weak dative interaction between the 4d_z² orbitals on the Rh centers and the 2p_z orbital of the Li⁺ cation. The charge-transfer term between Rh and Li⁺ contributes only about the 1/5 of the total interaction energy, however, and the principal driving force for the proximity of Rh and Li in compounds 1 and 2 is that Li⁺ is electrostatically attracted to negative charges on the dialkylrhodiate anions.

Metal-only Lewis Pairs of Rhodium with s, p and d-Block Metals

Metal-only Lewis pairs (MOLPs) in which the two metal fragments are solely connected by a dative M→M bond represent privileged architectures to acquire fundamental understanding of bimetallic bonding. This has important implications in many catalytic processes or supramolecular systems that rely on synergistic effects between two metals. However, a systematic experimental/computational approach on a well-defined class of compounds is lacking. Here we report a family of MOLPs constructed around the RhI precursor [(η5 -C5 Me5 )Rh(PMe3 )2 ] (1) with a series of s, p and d-block metals, mostly from the main group elements, and investigate their bonding by computational means. Among the new MOLPs, we have structurally characterized those formed by dative bonding between 1 and MgMeBr, AlMe3 , GeCl2 , SnCl2 , ZnMe2 and Zn(C6 F5 )2, as well as spectroscopically identified the ones resulting from coordination to MBArF (M=Na, Li; BArF - =[B(C6 H2 -3,5-(CF3 )2 )4 ]- ) and CuCl. Some of these compounds represent unique examples of bimetallic structures, such as the first unambiguous cases of Rh→Mg dative bonding or base-free rhodium bound germylene and stannylene species. Multinuclear NMR spectroscopy, including 103 Rh NMR, is used to probe the formation of Rh→M bonds. A comprehensive theoretical analysis of those provides clear trends. As anticipated, greater bond covalency is found for the more electronegative acids, whereas ionic character dominates for the least electronegative nuclei, though some degree of electron sharing is identified in all cases.

Nickel(0)-Induced β-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate β-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)₂ (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. β-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C₃-symmetric complexes with three heteroleptic magnesium mixed β-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

Nickel-Catalyzed Anionic Cross-Coupling Reaction of Lithium Sulfonimidoyl Alkylidene Carbenoids With Organolithiums

The mechanistic platform for a novel nickel0 -catalyzed anionic cross-coupling reaction (ACCR) of lithium sulfonimidoyl alkylidene carbenoids (metalloalkenyl sulfoximines) with organometallic reagents is reported herein, affording substituted alkenylmetals and lithium sulfinamides. The Ni0 -catalyzed ACCR of three different types of metalloalkenyl sulfoximines, including acyclic, axially chiral and exocyclic derivatives, with sp2 organolithiums and sp2 and sp3 Grignard reagents has been studied. The ACCR of metalloalkenyl sulfoximines with PhLi in the presence of the Ni0 -catalyst and precatalyst Ni(PPh3 )2 Cl2 afforded alkenyllithiums, under inversion of configuration at the C atom and complete retention at the S atom. In a combination of experimental and DFT studies, we propose a catalytic cycle of the Ni0 -catalyzed ACCR of lithioalkenyl sulfoximines. Computational studies reveal two distinctive pathways of the ACCR, depending on whether a phosphine or 1,5-cyclooctadiene (COD) is the ligand of the Ni atom. They rectify the underlying importance of forming the key Ni0 -vinylidene intermediate through an indispensable electron-rich Ni0 -center coordinated by phosphine ligands. Fundamentally, we present a mechanistic study in controlling the diastereoselectivity of the alkenyllithium formation via the key lithium sulfinamide coordinated Ni0 -vinylidene complex, which consequently avoids an unselective formation of an alkylidene carbene Ni-complex and ultimately racemic alkenyllithium.

Nickel-catalyzed coupling reaction of alkyl halides with aryl Grignard reagents in the presence of 1,3-butadiene: mechanistic studies of four-component coupling and competing cross-coupling reactions

We describe the mechanism, substituent effects, and origins of the selectivity of the nickel-catalyzed four-component coupling reactions of alkyl fluorides, aryl Grignard reagents, and two molecules of 1,3-butadiene that affords a 1,6-octadiene carbon framework bearing alkyl and aryl groups at the 3- and 8-positions, respectively, and the competing cross-coupling reaction. Both the four-component coupling reaction and the cross-coupling reaction are triggered by the formation of anionic nickel complexes, which are generated by the oxidative dimerization of two molecules of 1,3-butadiene on Ni(0) and the subsequent complexation with the aryl Grignard reagents. The C-C bond formation of the alkyl fluorides with the γ-carbon of the anionic nickel complexes leads to the four-component coupling product, whereas the cross-coupling product is yielded via nucleophilic attack of the Ni center toward the alkyl fluorides. These steps are found to be the rate-determining and selectivity-determining steps of the whole catalytic cycle, in which the C-F bond of the alkyl fluorides is activated by the Mg cation rather than a Li or Zn cation. ortho-Substituents of the aryl Grignard reagents suppressed the cross-coupling reaction leading to the selective formation of the four-component products. Such steric effects of the ortho-substituents were clearly demonstrated by crystal structure characterizations of ate complexes and DFT calculations. The electronic effects of the para-substituent of the aryl Grignard reagents on both the selectivity and reaction rates are thoroughly discussed. The present mechanistic study offers new insight into anionic complexes, which are proposed as the key intermediates in catalytic transformations even though detailed mechanisms are not established in many cases, and demonstrates their synthetic utility as promising intermediates for C-C bond forming reactions, providing useful information for developing efficient and straightforward multicomponent reactions.

Ethereal C-O Bond Cleavage Mediated by Ni(0)-Ate Complex: A DFT Study

Density functional theory calculations were performed to explore the mechanism of Ni-catalyzed cross-coupling reactions involving organo-lithium and -zinc reagents through ethereal C-O bond cleavage. Based on this work, together with our previous mechanistic study on etheric Kumada-Tamao reaction, we identify and characterize a novel catalytic cycle for cross-coupling mediated by Ni(0)-ate complex.

A Mild and Ligand-Free Ni-Catalyzed Silylation via C-OMe Cleavage

Metal-catalyzed transformations that forge carbon-heteroatom bonds are of central importance in organic synthesis. Despite the formidable potential of aryl methyl ethers as coupling partners, the scarcity of metal-catalyzed C-heteroatom bond formations via C-OMe cleavage is striking, with isolated precedents requiring specialized, yet expensive, ligands, high temperatures, and π-extended backbones. We report an unprecedented catalytic ipso-silylation of aryl methyl ethers under mild conditions and without recourse to external ligands. The method is distinguished by its wide scope, which includes the use of benzyl methyl ethers, vinyl methyl ethers, and unbiased anisole derivatives, thus representing a significant step forward for designing new C-heteroatom bond formations via C-OMe scission. Applications of this transformation in orthogonal silylation techniques as well as in further derivatizations are also described. Preliminary mechanistic experiments suggest the intermediacy of Ni(0)-ate complexes, leaving some doubt that a canonical catalytic cycle consisting of an initial oxidative addition of the C-OMe bond to Ni(0) species comes into play.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

An unexpected transmetalation intermediate: isolation and structural characterization of a solely CH3 bridged di-copper(i) complex

Structural characterization of unsupported, two metal centres bridging methyl groups is rare. They have been proposed as transmetalation intermediates in cuprate chemistry, but as yet no structural evidence has been presented. We have isolated a di-copper(i) complex with solely a methyl ligand bridging two Cu(i) atoms, representing a new bonding mode of CH3.

Cross-Couplings Using Aryl Ethers via C-O Bond Activation Enabled by Nickel Catalysts

Arene synthesis has been revolutionized by the invention of catalytic cross-coupling reactions, wherein aryl halides can be coupled with organometallic and organic nucleophiles. Although the replacement of aryl halides with phenol derivatives would lead to more economical and ecological methods, success has been primarily limited to activated phenol derivatives such as triflates. Aryl ethers arguably represent one of the most ideal substrates in terms of availability, cost, safety, and atom efficiency. However, the robust nature of the C(aryl)-O bonds of aryl ethers renders it extremely difficult to use them in catalytic reactions among the phenol derivatives. In 1979, Wenkert reported a seminal work on the nickel-catalyzed cross-coupling of aryl ethers with Grignard reagents. However, it was not until 2004 that the unique ability of a low-valent nickel species to activate otherwise unreactive C(aryl)-O bonds was appreciated with Dankwardt's identification of the Ni(0)/PCy3 system, which significantly expanded the efficiency of the Wenkert reaction. Application of the nickel catalyst to cross-couplings with other nucleophiles was first accomplished in 2008 by our group using organoboron reagents. Later on, several other nucleophiles, including organozinc reagents, amines, hydrosilane, and hydrogen were shown to be coupled with aryl ethers under nickel catalysis. Despite these advances, progress in this field is relatively slow because of the low reactivity of benzene derivatives (e.g., anisole) compared with polyaromatic substrates (e.g., methoxynaphthalene), particularly when less reactive and synthetically useful nucleophiles are used. The "naphthalene problem" has been overcome by the use of N-heterocyclic carbene (NHC) ligands bearing bulky N-alkyl substituents, which enables a wide range of aryl ethers to be coupled with organoboron nucleophiles. Moreover, the use of N-alkyl-substituted NHC ligands allows the use of alkynylmagnesium reagents, thereby realizing the first Sonogashira-type reaction of anisoles. From a mechanistic perspective, nickel-catalyzed cross-couplings of aryl ethers are at a nascent stage, in particular regarding the mode of activation of C(aryl)-O bonds. Oxidative addition is one plausible pathway, although such a process has not been fully verified experimentally. Nickel-catalyzed reductive cleavage of aryl ethers in the absence of an external reducing agent provides strong support for this oxidative addition process. Several other mechanisms have also been proposed. For example, Martin demonstrated a new possibility of the involvement of a Ni(I) species, which could mediate the cleavage of the C(aryl)-O bond via a redox-neutral pathway. The tolerance of aryl ethers under commonly used synthetic conditions enables alkoxy groups to serve as a platform for late-stage elaboration of complex molecules without any tedious protecting group manipulations. Aryl ethers are therefore not mere economical alternatives to aryl halides but also enable nonclassical synthetic strategies.

Cross-coupling reaction of alkyl halides with grignard reagents catalyzed by Ni, Pd, or Cu complexes with pi-carbon ligand(s)

Transition metal-catalyzed cross-coupling reactions of organic halides and pseudo-halides containing a C-X bond (X = I, Br, Cl, OTf, OTs, etc.) with organometallic reagents are among the most important transformations for carbon-carbon bond formation between a variety of sp, sp(2), and sp(3)-hybridized carbon atoms. In particular, researchers have widely employed Ni- and Pd-catalyzed cross-coupling to synthesize complex organic structures from readily available components. The catalytic cycle of this process comprises oxidative addition, transmetalation, and reductive elimination steps. In these reactions, various organometallic reagents could bear a variety of R groups (alkyl, vinyl, aryl, or allyl), but the coupling partner has been primarily limited to sp and sp(2) carbon compounds: alkynes, alkenes, and arenes. With alkyl coupling partners, these reactions typically run into two problems within the catalytic cycle. First, oxidative addition of alkyl halides to a metal catalyst is generally less efficient than that of aryl or alkenyl compounds. Second, the alkylmetal intermediates formed tend to undergo intramolecular beta-hydrogen elimination. In this Account, we describe our efforts to overcome these problems for Ni and Pd chemistry. We have developed new catalytic systems that do not involve M(0) species but proceed via an anionic complex as the key intermediate. For example, we developed a unique cross-coupling reaction of alkyl halides with organomagnesium or organozinc reagents catalyzed by using a 1,3-butadiene as the additive. This reaction follows a new catalytic pathway: the Ni or Pd catalyst reacts first with R-MgX to form an anionic complex, which then reacts with alkyl halides. Bis-dienes were also effective additives for the Ni-catalyzed cross-coupling reaction of organozinc reagents with alkyl halides. This catalytic system tolerates a wide variety of functional groups, including nitriles, ketones, amides, and esters. In addition, we have extended the utility of Cu-catalyzed cross-coupling reactions. With 1-phenylpropyne as an additive, Cu-catalyzed reactions of alkyl chlorides, fluorides, and mesylates with Grignard reagents proceed efficiently. These new catalytic reactions use pi-carbon ligands such as pi-allyl units or alkynes instead of heteroatom ligands such as phosphines or amines. Overall, these reactions provide new methodology for introducing alkyl moieties into organic molecules.

Carbon-carbon bond-forming reactions using alkyl fluorides

This account reviews C-C bond formation reactions using alkyl fluorides mostly focusing on the transition-metal-catalyzed reactions. These reactions proceed efficiently under mild conditions by the combined use of Grignard reagents and transition-metal catalysts, such as Ni, Cu, and Zr. It is proposed that ate complex intermediates formed by the reaction of these transition metals with Grignard reagents play important roles as the active catalytic species. Organoaluminun reagents react directly with alkyl fluorides in nonpolar solvents at room temperature to form C-C bonds. These studies demonstrate the practical usefulness of alkyl fluorides in C-C bond formation reactions and provide a promising method for the construction of carbon frameworks employing alkyl fluorides. The scope and limitations, as well as reaction pathways, are discussed.

Diastereoselective cobalt-catalyzed reductive aldol cyclizations using diethylzinc as the stoichiometric reductant

[reaction: see text] Cobalt catalysis enables a new method for the generation of zinc enolates using diethylzinc to reduce alpha,beta-unsaturated amides. This method has been applied to a high-yielding diastereoselective reductive aldol cyclization.

Syntheses and Structures of Alkaline Earth Metal Bis(diphenylamides)

Various preparative procedures are employed in order to synthesize alkaline earth metal bis(diphenylamides) such as (i) metalation of HNPh2 with the alkaline earth metal M, (ii) metalation of HNPh2 with MPh2, (iii) metathesis reaction of MI2 with KNPh2, (iv) metalation of HNPh2 with PhMI in THF, and (v) metathesis reaction of PhMI with KNPh2 followed by a dismutation reaction yielding MPh2 and M(NPh2)2. The magnesium compounds [(diox)MgPh2]infinity (1) and (thf)2Mg(NPh2)2 (2) show tetracoordinate metal atoms, whereas in (dme)2Ca(NPh2)2 (3), (thf)4Sr(NPh2)2 (4), and (thf)4Ba(NPh2)2 (5) the metals are 6-fold coordinated. Additional agostic interactions between an ipso-carbon of one of the phenyl groups of the amide ligand and the alkaline earth metal atom lead to unsymmetric coordination of the NPh2 anions with two strongly different M-N-C angles in 3-5.

Surprising role of aryl halides in nickel-catalyzed reductive aldol reactions

[reaction: see text] A new nickel-catalyzed method for the reductive aldol addition of acrylates and aldehydes has been developed. An unexpected requirement for an aryl iodide additive was found in the process, and the effect was shown to be linked to an initiation step.

A structurally characterized Ni–Al methyl-bridged complex with catalytic ethylene oligomerization activity

A bimetallic Ni-Al (2.5087(15) A) complex with an agostic alpha-C-H, [eta-C5H4CH=N(C6F5)]Fe[eta-C5H4PPh2]Ni(AlMe3), has been isolated and crystallographically established. The complex is active towards ethylene oligomerization/polymerization under moderate conditions.