The Intricate Chemistry of Cobalt(II) Halides and Bis(diphenylphosphanyl)ethane

Diastereospecific arylation and cascade deconstructive amidation/thioesterification of readily available lactam-fused bromolactones

An intrinsic goal when designing synthetic methodology is to identify approaches whereby readily accessible precursors are converted into an array of products, which efficiently tap into new 3D-chemical space. In these studies, readily available bicyclic lactam-bromolactones have been interrogated in several fragment growth protocols by utilizing the halogen and lactone motifs as versatile linchpins for strategic construction of C-C, C-N, C-O, and C-S bonds. Diastereospecific C(sp3)-C(sp2) Kumada coupling of sterically imposing [5,5]-bicyclic lactam-bromolactones with several aryl Grignard reagents, under palladium catalysis, furnishes diarylmethane-tethered lactam-lactones in synthetically attractive yields, stereoinvertive fashion, and with a tolerance for many functional groups. When [5,6]-bicyclic lactam-bromolactones, which are prone to β-hydride elimination are employed, efficient arylation is observed only under Co(acac)3-catalyzed conditions. Importantly, these [5,6]-bicyclic lactam-bromolactones undergo retentive arylation, independent of the transition metal catalyst. A base-mediated cascade deconstructive amidation of the [5,6]-bicyclic lactam-bromolactones with primary aliphatic amines proceeds efficiently to afford epoxide-tethered lactam carboxamides, which bear four contiguous stereocenters. Furthermore, an unusual route to homoallylic thioesters has been uncovered through deconstructive contra-thermodynamic thioesterification of the lactam-fused bromolactone precursors.

Cobalt-catalyzed cross-coupling of Umpolung carbonyls with alkyl halides under mild conditions

While classical cross-couplings are dominated by palladium and nickel catalysts, cobalt-based catalysts have shown unique advantages for such cross-coupling reactions in terms of higher catalytic activity and lower toxicity. Herein, we describe a novel cobalt-catalyzed alkyl-alkyl cross-coupling reaction of hydrazone with alkyl halides under mild reaction conditions, where the use of a PNP-type pincer ligand is essential for catalysis. Both aldehyde and ketone hydrazones are compatible with this reaction, giving a series of C(sp³)-C(sp³) coupling products in moderate to good yields.

Activator-free single-component Co(I)-catalysts for regio- and enantioselective heterodimerization and hydroacylation reactions of 1,3-dienes. New reduction procedures for synthesis of [L]Co(I)-complexes and comparison to in situ generated catalysts

Although cobalt(I) bis-phosphine complexes have been implicated in many selective C-C bond-forming reactions, until recently relatively few of these compounds have been fully characterized or have been shown to be intermediates in catalytic reactions. In this paper we present a new practical method for the synthesis and isolation of several cobalt(I)-bis-phosphine complexes and their use in Co(I)-catalyzed reactions. We find that easily prepared (in situ generated or isolated) bis-phosphine and (2,6-N-aryliminoethyl)pyridine (PDI) cobalt(II) halide complexes are readily reduced by 1,4-bis-trimethylsilyl-1,4-dihydropyrazine or commercially available lithium nitride (Li3N), leaving behind only innocuous volatile byproducts. Depending on the structures of the bis-phosphines, the cobalt(I) complex crystallizes as a phosphine-bridged species [(P∼P)(X)CoI[μ-(P∼P)]CoI(X)(P∼P)] or a halide-bridged species [(P∼P)CoI[μ-(X)]2CoI(P∼P)]. Because the side-products are innocuous, these methods can be used for the in situ generation of catalytically competent Co(I) complexes for a variety of low-valent cobalt-catalyzed reactions of even sensitive substrates. These complexes are also useful for the synthesis of rare cationic [(P∼P)CoI-η4-diene]+ X- or [(P∼P)CoI-η6-arene]+ X- complexes, which are shown to be excellent single-component catalysts for the following regioselective reactions of dienes: heterodimerizations with ethylene or methyl acrylate, hydroacylation and hydroboration. The reactivity of the single-component catalysts with the in situ generated species are also documented.

Magnetic anisotropy of two tetrahedral Co(II)-halide complexes with triphenylphosphine ligands

Recently, the choice of ligand and geometric control of mononuclear complexes, which can affect the relaxation pathways and blocking temperature, have received wide attention in the field of single-ion magnets (SIMs). To find out the influence of the coordination environment on SIMs, two four-coordinate mononuclear Co(II) complexes [NEt₄][Co(PPh₃)X₃] (X = Cl^-, 1; Br^-, 2) have been synthesized and studied by X-ray single crystallography, magnetic measurements, high-frequency and -field EPR (HF-EPR) spectroscopy and theoretical calculations. Both complexes are in a cubic space group Pa3̄ (No. 205), containing a slightly distorted tetrahedral moiety with crystallographically imposed C_3v symmetry through the [Co(PPh₃)X₃]^- anion. The direct-current (dc) magnetic data and HF-EPR spectroscopy indicated the anisotropic S = 3/2 spin ground states of the Co(II) ions with the easy-plane anisotropy for 1 and 2. Ab initio calculations were performed to confirm the positive magnetic anisotropies of 1 and 2. Frequency- and temperature-dependent alternating-current (ac) magnetic susceptibility measurements revealed slow magnetic relaxation for 1 and 2 at an applied dc field. Finally, the magnetic properties of 1 and 2 were compared to those of other Co(II) complexes with a [CoAB₃] moiety.

Cobalt-Catalyzed Cross-Couplings between Alkyl Halides and Grignard Reagents

Metal-catalyzed cross-couplings have emerged as essential tools for the construction of C-C bonds. The identification of efficient catalytic systems as well as large substrate scope made these cross-couplings key reactions to access valuable molecules ranging from materials, agrochemicals to active pharmaceutical ingredients. They have been increasingly integrated in retrosynthetic plans, allowing shorter and original route development. Palladium-catalyzed cross-couplings still largely rule the field, with the most popular reactions in industrial processes being the Suzuki and Sonogashira couplings. However, the extensive use of palladium complexes raises several problems such as limited resources, high cost, environmental impact, and frequent need for sophisticated ligands. As a consequence, the use of nonprecious and cheap metal catalysts has appeared as a new horizon in cross-coupling development. Over the last three decades, a growing interest has thus been devoted to Fe-, Co-, Cu-, or Ni-catalyzed cross-couplings. Their natural abundance makes them cost-effective, allowing the conception of more sustainable and less expensive chemical processes, especially for large-scale production of active molecules. In addition to these economical and environmental considerations, the 3d metal catalysts also exhibit complementary reactivity with palladium complexes, facilitating the use of alkyl halide partners due to the decrease of β-elimination side reactions. In particular, by using cobalt catalysts, numerous cross-couplings between alkyl halides and organometallics have been described. However, cobalt catalysis still stays far behind palladium catalysis in terms of popularity and applications, and the expansion of the substrate scope as well as the development of simple and robust catalytic systems remains an important challenge.In 2012, our group entered the cobalt catalysis field by developing a cobalt-catalyzed cross-coupling between C-bromo glycosides and Grignard reagents. The generality of the coupling allowed the preparation of a range of valuable C-aryl and C-vinyl glycoside building blocks. We then focused on the functionalization of saturated N-heterocycles, and a variety of halo-azetidines, -pyrrolidines, and -piperidines were successfully reacted with aryl and alkenyl Grignard reagents under cobalt catalysis. With the objective of preparing valuable α-aryl amides, a cobalt-catalyzed cross-coupling applied to α-bromo amides was studied and then extended to α-bromo lactams. Recently, we also reported an efficient and general cross-coupling involving cyclopropyl- and cyclobutyl-magnesium bromides. This method allows the alkylation of functionalized small strained rings by a range of primary and secondary alkyl halides.

Mechanism of Cobalt-Catalyzed Heterodimerization of Acrylates and 1,3-Dienes. A Potential Role of Cationic Cobalt(I) Intermediates

Coupling reactions of feedstock alkenes are promising, but few of these reactions are practiced industrially. Even though recent advances in the synthetic methodology have led to excellent regio- and enantioselectivies in the dimerization reactions between 1,3-dienes and acrylates, the efficiency as measured by the turnover numbers (TON) in the catalyst has remained modest. Through a combination of reaction progress kinetic analysis (RPKA) of a prototypical dimerization reaction, characterization of isolated low-valent cobalt catalyst precursors involved, several important details of the mechanism of this reaction have emerged. (i) The prototypical reaction has an induction period that requires at least two hours of stir time to generate the competent catalyst. (ii) Reduction of a Co(II) complex to a Co(I) complex, and subsequent generation of a cationic [Co(I)]+ species are responsible for this delay. (iii) Through RPKA using in situ IR spectroscopy, same excess experiments reveal inhibition by the product towards the end of the reaction and no catalyst deactivation is observed as long as diene is present in the medium. The low TON observed is most likely the result of the inherent instability of the putative cationic Co(I)-species that catalyzes the reaction. (iv) Different excess experiments suggest that the reaction is first order in the diene and zero order in the acrylate. (v) Catalyst loading experiments show that the catalyst is first order. The orders in the various regents were further confirmed by Variable Time Normalization Analysis (VTNA). (vi) A mechanism based on oxidative dimerization [via Co(I)/Co(III)-cycle] is proposed. Based on the results of this study, it is possible to increase the TON by a factor of 10 by conducting the reaction at an increased concentration of the starting materials, especially, the diene, which seems to stabilize the catalytic species.

Mechanisms of Cobalt/Phosphine-Catalyzed Cross-Coupling Reactions

>The combination of CoCl₂ with bidentate phosphines is known to catalyze challenging cross-coupling and Heck-type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray-ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low-valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br^- . In cross-coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck-type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single-electron transfer processes and the involvement of radicals in these transformations.

Cobalt-Catalysed Asymmetric Hydrovinylation of 1,3-Dienes

In the presence of bidentate 1,n-bis-diphenylphosphinoalkane-CoCl2 complexes {Cl2Co[P~P]} and Me3Al or methylaluminoxane, acyclic (E)-1,3-dienes react with ethylene (1 atmosphere) to give excellent yields of hydrovinylation products. The regioselectivity (1,4- or 1,2-addition) and the alkene configuration (E- or Z-) of the resulting product depend on the nature of the ligand and temperature at which the reaction is carried out. Cobalt(II)-complexes of 1,1-diphenylphosphinomethane and similar ligands with narrow bite angles give mostly 1,2-addition, retaining the E-geometry of the original diene. Complexes of most other ligands at low temperature (-40 °C) give almost exclusively a single branched product, (Z)-3-alkylhexa-1,4-diene, which arises from a 1,4-hydrovinylation reaction. A minor product is the linear adduct, a 5-alkyl-hexa-1,4-diene, also arising from a 1,4-addition of ethylene. As the temperature is increased, a higher proportion of the major 1,4-adduct appears as the (E)-isomer. The unexpectedly high selectivity seen in the Co-catalysed reaction as compared to the corresponding Ni-catalysed reaction can be rationalized by invoking the intermediacy of an η4-[(diene)[P~P]CoH]+-complex and its subsequent reactions. The enhanced reactivity of terminal E-1,3-dienes over the corresponding Z-dienes can also be explained on the basis of the ease of formation of this η4-complex in the former case. The lack of reactivity of the X2Co(dppb) (X = Cl, Br) complexes in the presence of Zn/ZnI2 makes the Me3Al-mediated reaction different from the previously reported hydroalkenylation of dienes. Electron-rich phospholanes, bis-oxazolines and N-heterocyclic carbenes appear to be poor ligands for the Co(II)-catalysed hydrovinylation of 1,3-dienes. An extensive survey of chiral ligands reveals that complexes of DIOP, BDDP and Josiphos ligands are quite effective for these reactions even at -45 °C and enantioselectivities in the range of 90-99 % ee can be realized for a variety of 1,3-dienes. Cobalt(II)-complex of an electron-deficient Josiphos ligand is especially active, requiring only <1 mol% catalyst to effect the reactions.