Active Precatalysts Research Articles

DFT Investigation of Suzuki-Miyaura Reactions with Aryl Sulfamates Using a Dialkylbiarylphosphine-Ligated Palladium Catalyst.

Aryl sulfamates are valuable electrophiles for cross-coupling reactions because they can easily be synthesized from phenols and can act as directing groups for C-H bond functionalization prior to cross-coupling. Recently, it was demonstrated that (1-tBu-Indenyl)Pd(XPhos)Cl (XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl) is a highly active precatalyst for room-temperature Suzuki-Miyaura couplings of a variety of aryl sulfamates. Herein, we report an in-depth computational investigation into the mechanism of Suzuki-Miyaura reactions with aryl sulfamates using an XPhos-ligated palladium catalyst. Particular emphasis is placed on the turnover-limiting oxidative addition of the aryl sulfamate C-O bond, which has not been studied in detail previously. We show that bidentate coordination of the XPhos ligand via an additional interaction between the biaryl ring and palladium plays a key role in lowering the barrier to oxidative addition. This result is supported by NBO and NCI-Plot analysis on the transition states for oxidative addition. After oxidative addition, the catalytic cycle is completed by transmetalation and reductive elimination, which are both calculated to be facile processes. Our computational findings explain a number of experimental results, including why elevated temperatures are required for the coupling of phenyl sulfamates without electron-withdrawing groups and why aryl carbamate electrophiles are not reactive with this catalyst.

A thiosemicarbazone–palladium(II)–imidazole complex as an efficient pre-catalyst for Suzuki–Miyaura cross-coupling reactions at room temperature in aqueous media

A Pd(II) complex of two sterically crowded ligands, specifically an N,S-donor thiosemicarbazone and an N-donor imidazole, has been synthesized and characterized by physicochemical and spectroscopic methods. X-ray single-crystal analysis revealed that the coordination geometry around the palladium center is distorted square planar, and the chloride ligand is involved in intermolecular bifurcated X–H···Y-type (where X = C, N and Y = Cl) hydrogen bonding. This complex proved to be a highly active and retrievable pre-catalyst for additive-free Suzuki–Miyaura cross-coupling reactions of arylboronic acids with aryl bromides or chlorides at room temperature and 60 °C, respectively. The reactions require a low catalyst loading and the complex is converted to ~1.5–2.0 nm-sized Pd nanoparticles (probably the real catalyst). The catalyst can be reused up to seven times without significant loss in activity. Since the reaction proceeds under mild conditions in aqueous medium and the catalyst is recoverable, it provides an environmentally benign alternative to the existing protocols for Suzuki–Miyaura reactions. Biaryls can be synthesized in high yield under greener reaction conditions in the presence of efficient pre-catalyst, thiosemicarbazone–palladium(II)–imidazole.

Cobalt-Catalyzed Stereoretentive Hydrogen Isotope Exchange of C(sp3)-H Bonds.

Cobalt dialkyl complexes bearing α-diimine ligands proved to be active precatalysts for the nondirected, C(sp3)-H selective hydrogen isotope exchange (HIE) of alkylarenes using D2 gas as the deuterium source. Alkylarenes with a variety of substitution patterns and heteroatom substituents on the arene ring were successfully labeled, enabling high levels of incorporation into primary, secondary, and tertiary benzylic C(sp3)-H bonds. In some cases, the HIE proceeded with high diastereoselectivity and application of the cobalt-catalyzed method to enantioenriched substrates with benzylic stereocenters provided enantioretentive hydrogen isotope exchange at tertiary carbons.

Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions.

The reduction of Pd(II) intermediates to Pd(0) is a key elementary step in a vast number of Pd-catalyzed processes, ranging from cross-coupling, C-H activation, to Wacker chemistry. For one of the most powerful new generation phosphine ligands, PtBu3, oxidation state Pd(I), and not Pd(0), is generated upon reduction from Pd(II). The mechanism of the reduction of Pd(II) to Pd(I) has been investigated by means of experimental and computational studies for the formation of the highly active precatalyst {Pd(μ-Br)(PtBu3)}2. The formation of dinuclear Pd(I), as opposed to the Pd(0) complex, (tBu3P)2Pd was shown to depend on the stoichiometry of Pd to phosphine ligand, the order of addition of the reagents, and, most importantly, the nature of the palladium precursor and the choice of the phosphine ligand utilized. In addition, through experiments on gram scale in palladium, mechanistically important additional Pd- and phosphine-containing species were detected. An ionic Pd(II)Br3 dimer side product was isolated, characterized, and identified as the crucial driving force in the mechanism of formation of the Pd(I) bromide dimer. The potential impact of the presence of these side species for in situ formed Pd complexes in catalysis was investigated in Buchwald-Hartwig, α-arylation, and Suzuki-Miyaura reactions. The use of preformed and isolated Pd(I) bromide dimer as a precatalyst provided superior results, in terms of catalytic activity, in comparison to catalysts generated in situ.

Cross-Coupling Catalysis by an Anionic Palladium Complex

Recent studies have shown that anionic palladium complexes are viable catalysts for a range of catalytic cross-coupling reactions. We present a one-step synthesis of the anionic “ligandless” palladium complex [NBu4][Pd(DMSO)Cl3] together with its crystal structure. This compound has been shown to be an active precatalyst in the Mizoroki–Heck reaction. Under Jeffery conditions, activated aryl chlorides can be coupled in yields of up to 94% without the need of an additional ligand. The presence of a small amount of water was necessary for product formation. An Amatore–Jutand-type catalytic cycle is consistent with the results presented herein. For comparison with the known mixed complex [(pym-Im-Me)2PdCl][Pd(DMSO)Cl3], the cationic complex [(pym-Im-Me)2PdCl]PF6 (pym = pyrimidyl, Im = imidazolin-2-ylidene) has been synthesized and characterized using standard techniques.

Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation.

We recently reported a bis(imino)pyridine (or pyridine diimine, PDI) manganese precatalyst, (Ph2PPrPDI)Mn (1), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for 1-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min-1 have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, 1 has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min-1. Under stoichiometric and catalytic conditions, addition of PhSiH3 to (Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (Ph2PPrPDI)MnH (2) was achieved upon adding NaEt3BH to (Ph2PPrPDI)MnCl2 and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone was added to 1, radical transfer yielded (Ph2PPrPDI·)Mn(OC·(Ph)(CF3)) (3), which undergoes intermolecular C-C bond formation to produce the respective Mn(II) dimer, [(μ-O,Npy-4-OC(CF3)(Ph)-4-H-Ph2PPrPDI)Mn]2 (4). Upon finding 3 to be inefficient and 4 to be inactive, kinetic trials were conducted to elucidate the mechanisms of 1- and 2-mediated hydrosilylation. Varying the concentration of 1, substrate, and PhSiH3 revealed a first order dependence on each reagent. Furthermore, a kinetic isotope effect (KIE) of 2.2 ± 0.1 was observed for 1-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using 2, suggesting 1 and 2 operate through different mechanisms. Although kinetic trials reveal 1 to be the more active precatalyst for carbonyl hydrosilylation, a concurrent 2-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, 1-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while 2-mediated hydrosilylation occurs via insertion.

Ken Tanaka

Ken Tanaka

Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst.

The well-defined silica-supported molybdenum oxo alkyl species (≡SiO-)MoO(CH2tBu)3 was selectively prepared by grafting of MoO(CH2tBu)3Cl onto partially dehydroxylated silica (silica700) using the surface organometallic chemistry approach. This surface species was fully characterized by elemental analysis and DRIFT, solid-state NMR, and EXAFS spectroscopy. This new material is related to the active species of industrial supported MoO3/SiO2 olefin metathesis catalysts. It displays very high activity in propene self-metathesis at mild (turnover number = 90 000 after 25 h). Remarkably, its catalytic performance outpaces those of the parent imido derivative and its tungsten oxo analogue.

SOMC-Designed Silica Supported Tungsten Oxo Imidazolin-2-iminato Methyl Precatalyst for Olefin Metathesis Reactions

Synthesis, structure, and olefin metathesis activity of a surface complex [(≡Si-O-)W(═O)(CH3)2-ImDippN] (4) (ImDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) supported on silica by a surface organometallic chemistry (SOMC) approach are reported. The reaction of N-silylated 2-iminoimidazoline with tungsten(VI) oxytetrachloride generated the tungsten oxo imidazolin-2-iminato chloride complex [ImDippNW(═O)Cl3] (2). This was grafted on partially dehydroxylated silica pretreated at 700 °C (SiO2-700) to afford a well-defined monopodal surface complex [(≡Si-O-)W(═O)Cl2-ImDippN] (3). 3 underwent alkylation by ZnMe2 to produce [(≡Si-O-)W(═O)(CH3)2-ImDippN] (4). The alkylated surface complex was thoroughly characterized by solid-state NMR, elemental microanalysis, Raman, FT-IR spectroscopies, and XAS analysis. 4 proved to be an active precatalyst for self-metathesis of terminal olefins such as propylene and 1-hexene.

Investigation of N-Heterocyclic Carbene-Supported Group 12 Triflates as Pre-catalysts for Hydrosilylation/Borylation.

N-Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]+ complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd-H and Hg-H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr⋅Cd(μ-OTf)2 ]2 (IPr=[(HCNDipp)2 C:]; Dipp=2,6-iPr2 C6 H3 ) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation.

Amino Ether–Phenolato Precatalysts of Divalent Rare Earths and Alkaline Earths for the Single and Double Hydrophosphination of Activated Alkenes

A range of stable ytterbium(II) amino ether–phenolato amido complexes of the type {LONxOy}Yb{N(SiMe3)2}, together with a congeneric samarium(II) and two calcium and strontium amido and alkyl derivatives, have been synthesized. They constitute very active, fully regioselective (anti-Markovnikov), and chemoselective precatalysts for the intermolecular hydrophosphination of styrene derivatives with PhPH2, achieving TONs up to 2150, TOFs up to 30 h–1 and chemoselectivities in the region of 95–99%. The ytterbium(II) precatalysts, among which is most prominently {LONO4}Yb{N(SiMe3)2} (3), where the ligand possesses a 15-c-5-aza-crown ether side arm, outperform their related calcium analogues, and the activity of the catalyst increases substantially with the denticity of the ligand. The rate law rate = k[p-tBu-styrene][3] was established following kinetic monitoring of the hydrophosphination of p-tBu-styrene and PhPH2 catalyzed by 3. The kinetic studies also revealed that the reactions are entropically driven and...

Sterically Encumbered Tetraarylimidazolium Carbene Pd-PEPPSI Complexes: Highly Efficient Direct Arylation of Imidazoles with Aryl Bromides under Aerobic Conditions

A series of sterically encumbered tetraarylimidazolium carbene Pd-PEPPSI complexes were conveniently prepared and fully characterized. These sterically encumbered Pd-PEPPSI complexes act as active precatalysts for the direct arylation of imidazoles with aryl bromides under aerobic conditions. The catalytic performance of Pd-PEPPSI complexes in cross-coupling processes is investigated. Under the optimal protocols, the cross-coupling reactions regioselectively produced C5-arylation products in moderate to excellent yields, which could tolerate a wide range of functional aryl bromides.

ChemInform Abstract: Iron Chemistry Made Easy: Chiral N2P2 Ligands for Asymmetric Catalysis

AbstractReview: [47 refs.

Active O,Npy,N-Titanium(IV) Fluoride Precatalysts for Ethylene Polymerization: Exploring “Fluoride Effects” on Polymer Properties and Catalytic Performance

Three examples of discrete six-coordinate O,Npy,N-bearing titanium(IV) trifluoride complexes, [{2-(C5H4N)-6-(CMe2O)C5H3N}TiF3] (1), [{2-(C6H4-2′-O),6-(CMe═N(2,6-i-Pr2C6H3))C5H3N}TiF3] (2a), and [{2-(3-C12H8-2-O)-6-(CMe═N(2,6-i-Pr2C6H3))C5H3N}TiF3] (3a), were prepared using a one-step HF elimination protocol from cis-[(THF)2TiF4] and the corresponding pincer ligand precursor (HL1, HL2H, and HL2Ph). A mer configuration of the three fluoride ligands is adopted in their solid-state structures, which is mirrored in solution, as shown by mutual two-bond F–F coupling in their 19F{1H} NMR spectra. For purposes of comparison, the chloride counterparts of 2a and 3a, [{2-(C6H4-2′-O),6-(CMe═N(2,6-i-Pr2C6H3)C5H3N}TiCl3] (2b) and [{2-(3-C12H8-2-O)-6-(CMe═N(2,6-i-Pr2C6H3))C5H3N}TiCl3] (3b), are also reported. On treatment with excess MAO, 1 is scarcely active in ethylene polymerization, 2a is more active, and the most sterically protected system, 3a, represents the most active nonmetallocene metal-fluoride precatalyst r...

N-Heterocyclic Carbene-Ytterbium Amide as a Recyclable Homogeneous Precatalyst for Hydrophosphination of Alkenes and Alkynes.

The N-heterocyclic carbene-ytterbium(II) amides (NHC)2Yb[N(SiMe3)2]2 (1: NHC: 1,3,4,5-tetramethylimidazo-2-ylidene (IMe4); 2: NHC: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr)) and the NHC-stabilized rare-earth phosphide (IMe4)3Yb(PPh2)2 (3) have been synthesized and fully characterized. Complexes 1-3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC-free amide (THF)2Yb[N(SiMe)2]2. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare-earth complex that can be recovered after catalytic reactions.

Iron Chemistry Made Easy: Chiral N2P2 Ligands for Asymmetric Catalysis

In this account, we summarize our recent work on the preparation of C 2-symmetric macrocyclic N2P2 and open-chain NPPN iron(II) complexes. The investigation of these new ligand classes is enabled by the development of a synthetic route to the key enantiopure, P-stereogenic dialdehyde. Macrocyclization with a diamine affords N2P2 ligands, the iron(II) complexes of which are highly active and selective precatalysts for the asymmetric transfer hydrogenation of ketones. Condensation with monoamines affords open-chain NPPN ligands, of which the highly azaphilic iron(II) complexes are tested in the asymmetric Strecker reaction. 1 Introduction 1.1 Iron Complexes: The Issue of Stability 1.2 Open-Chain PNNP Ligands 1.3 Goals and Motivation 2 N2P2 Macrocycles 2.1 The Key C 2-Symmetric Synthon 2.2 C 2-Symmetric N2P2 Macrocycles 3 Macrocyclic Iron(II) Complexes 3.1 Bis(acetonitrile) N2P2 Complexes 3.2 Bis(acetonitrile) (NH)2P2 Complexes 3.3 Enlarging the Toolbox: Isonitrile (CNR) Ligands 3.4 Asymmetric Transfer Hydrogenation Fe/N2P2 4 Chiral Open-Chain NPPN Ligands 4.1 Synthesis of Bis(acetonitrile) Complexes 4.2 Complex Derivatization 4.3 Asymmetric Transfer Hydrogenation Fe/NPPN 4.4 Strecker Reaction of Azomethine Imines 5 Conclusion and Outlook

Controlling the molecular weights of polyethylene waxes using the highly active precatalysts of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides: synthesis, characterization, and catalytic behavior.

A series of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides were synthesized and characterized using FT-IR and elemental analysis, and the molecular structures of complexes , and were confirmed to present a pseudo-square-pyramidal or trigonal-bipyramidal geometry around the cobalt center using single-crystal X-ray diffraction. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all cobalt precatalysts gave high activities up to the level of 10(7) gPE mol(-1) (Co) h(-1) toward ethylene polymerization, being one of most active cobalt-based precatalysts. In comparison with cobalt analogues, the title precatalysts generally possessed longer lifetime along with good thermo-stability; moreover, the resultant polyethylenes were highly linear and unimodal in most cases.

Thermally stable and highly active cobalt precatalysts for vinyl-polyethylenes with narrow polydispersities: integrating fused-ring and imino-carbon protection into ligand design

Finely tuned cobalt complexes, upon activation with either MAO or MMAO, exhibited high activities up to 80 °C and produced highly linear vinyl-polyethylenes.

Modification of silica-supported tungsten neosilyl oxo precatalysts: impact of substituted phenol on activity and stability in olefin metathesis

Development of a novel and convenient strategy to access a large variety of highly active tungsten oxo olefin metathesis pre-catalysts.

Copper(II) Coordination Polymers Self-Assembled from Aminoalcohols and Pyromellitic Acid: Highly Active Precatalysts for the Mild Water-Promoted Oxidation of Alkanes.

Three novel water-soluble 2D copper(II) coordination polymers-[{Cu2(μ2-dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1), [{Cu2(μ2-Hedea)2}2(μ4-pma)]n·4nH2O (2), and [{Cu(bea)(Hbea)}4(μ4-pma)]n·2nH2O (3)-were generated by an aqueous medium self-assembly method from copper(II) nitrate, pyromellitic acid (H4pma), and different aminoalcohols [N,N-dimethylethanolamine (Hdmea), N-ethyldiethanolamine (H2edea), and N-benzylethanolamine (Hbea)]. Compounds 2 and 3 represent the first coordination polymers derived from H2edea and Hbea. All the products were characterized by infrared (IR), electron paramagnetic resonance (EPR), and ultraviolet-visible light (UV-vis) spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS(±)), thermogravimetric and elemental analysis, and single-crystal X-ray diffraction (XRD), which revealed that their two-dimensional (2D) metal-organic networks are composed of distinct dicopper(II) or monocopper(II) aminoalcoholate units and μ4-pyromellitate spacers. From the topological viewpoint, the underlying 2D nets of 1-3 can be classified as uninodal 4-connected layers with the sql topology. The structures of 1 and 2 are further extended by multiple intermolecular hydrogen bonds, resulting in three-dimensional (3D) hydrogen-bonded networks with rare or unique topologies. The obtained compounds also act as highly efficient precatalysts for the mild homogeneous oxidation, by aqueous H2O2 in acidic MeCN/H2O medium, of various cycloalkanes to the corresponding alcohols and ketones. Overall product yields up to 45% (based on cycloalkane) were attained and the effects of various reaction parameters were investigated, including the type of precatalyst and acid promoter, influence of water, and substrate scope. Although water usually strongly inhibits the alkane oxidations, a very pronounced promoting behavior of H2O was detected when using the precatalyst 1, resulting in a 15-fold growth of an initial reaction rate in the cyclohexane oxidation on increasing the amount of H2O from ∼4 M to 17 M in the reaction mixture, followed by a 2-fold product yield growth.