Iron Precatalyst Research Articles

A Versatile, Functional Group-Tolerant, and Bench-Stable Iron Precatalyst for Building Arene and Triazine Rings by [2+2+2] Cycloadditions.

We report an efficient iron-catalyzed cycloaddition procedure leading to the construction of (hetero)aromatic rings by alkyne [2+2+2] cycloisomerization. This method relies on the use of an air-stable (N,N)Fe(II) precursor easily prepared from a commercially available ligand derived from 1,10-phenanthroline, reduced in situ into a catalytically active non-innocent (N,N ⋅- )2 Fe(II) species. This system displays a large scope application, operates under mild conditions and at low catalytic charges (25 cycloadducts formed, up to 1.5 mol% catalyst). Moreover, this method also enables access to 29 cycloadducts by cross-cycloisomerization between 1,6- or 1,7-diynes and alkynes in near-equimolar conditions. 1,3,5-Triazines can also be prepared with this procedure starting from the corresponding cyanamides. Scale-up reactions and post-functionalization of several cycloadducts also show that this [2+2+2] cycloaddition can be used in multistep sequences.

Iron-Catalyzed Cross-Coupling of Thioesters and Organomanganese Reagents.

We report a Fukuyama-type coupling of thioesters with aliphatic organomanganese reagents utilizing a cheap and easily available iron(III) precatalyst. The reactions exhibit a wide tolerance of solvents and functional groups, allowing for the conversion of thioesters derived from natural products and pharmaceutical compounds. A strong steric impact from each reaction component (carboxylic moiety, thiol substituent and manganese reagent) was displayed, which enabled regioselective transformation of dithioesters. Mechanistic investigations showed that the released thiolate does not act as a mere spectator ligand, but rather positively influences the stability of intermediate alkyl(II)ferrates.

Modulating Thermostability and Productivity of Benzhydryl‐Substituted Bis(imino)pyridine‐Iron C2H4 Polymerization Catalysts through ortho‐CnH2n−1 (n=5, 6, 8, 12) Ring Size Adjustment

AbstractFour types of benzhydryl‐containing bis(arylimino)pyridine‐iron(II) chloride complex, [2‐{CMeN(2,4,6‐Me3C6H2)}‐6‐{CMeN(Ar)}C5H3N]FeCl2 (Ar=2,4‐(CHPh2)2‐6‐(C5H9)C6H2 Fe1, 2,4‐(CHPh2)2‐6‐(C6H11)C6H2 Fe2, 2,4‐(CHPh2)2‐6‐(C8H15)C6H2 Fe3 and 2,4‐(CHPh2)2‐6‐(C12H23)C6H2 Fe4), discriminable by the ring size of the ortho‐CnH2n−1 (n=5, 6, 8, 12) group, have been prepared in good yield (>85 %) via the reaction of the corresponding N,N,N′‐ligand, L1–L4, with ferrous chloride tetrahydrate. All new complexes were characterized by FT‐IR spectroscopy and elemental analysis. The molecular structure of Fe3 emphasizes the distorted pseudo‐square pyramidal geometry bestowed to the metal center and the steric unevenness provided by the inequivalent N‐mesityl and N‐2,4‐dibenzyhydryl‐6‐cyclooctylphenyl groups. Extremely high activities for ethylene polymerization were achieved at temperatures between 60 °C and 70 °C following pre‐treatment of the iron(II) precatalyst with either MMAO or MAO, with the relative performance being: Fe2>Fe1>Fe3>Fe4. Notably, ortho‐cyclohexyl Fe2, under activation with MMAO, afforded the highest level of performance of this study reaching 2.82×107 g PE per mol (Fe) per h at 70 °C forming strictly linear and narrowly dispersed polyethylene (Mw/Mn=2.0) with moderate molecular weight (11.3 kg per mol). Furthermore, runs performed using Fe1/MMAO at 90 °C saw the catalytic activity drop by around only 40 % highlighting the remarkable thermostability of these catalysts. By comparison, polymerizations performed using MAO as co‐catalyst were less controlled with broader dispersities (Mw/Mn range: 4.8–10.1), while the molecular weights were generally higher (55.7–150.9 kg per mol).

Iron-Catalyzed Regio- and Stereoselective Hydrosilylation of 1,3-Enynes To Access 1,3-Dienylsilanes.

A regio- and stereoselective hydrosilylation of 1,3-enynes with primary and secondary silanes to access 1,3-dienylsilanes is accomplished by employing an iron precatalyst bearing iminopyridine-oxazoline (IPO) ligand. The hydrosilylation proceeds via syn-addition of a Si-H bond to the alkyne group of 1,3-enynes, incorporating the silyl group at the site proximal to the alkene. The reaction features mild conditions, broad substrate scope, and good functional group tolerance. The synthetic utility was demonstrated by gram-scale reactions and further transformations.

Iron-catalysed synthesis and chemical recycling of telechelic 1,3-enchained oligocyclobutanes.

Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n'-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock.

Doubly fused N,N,N-iron ethylene polymerization catalysts appended with fluoride substituents; probing catalytic performance via a combined experimental and MLR study

In agreement with the MLR analysis, the fluorinated iron precatalyst, R1 = F, R2 = R3 = CH(p-FPh)2, proved the most active at 70 °C generating strictly linear polyethylene waxes.

Achieving strictly linear polyethylenes by the NNN‐Fe precatalysts finely tuned with different sizes of ortho‐cycloalkyl substituents

Six examples of newly synthesized α,α’‐bis (aryl)‐2,3:5,6‐bis (pentame thylene)pyridyliron complexes [2,3:5,6‐{C4H8C(NAr)}2C5HN]FeCl2 (Ar = 2‐(c‐C5H9)‐6‐MeC6H3 Fe1, 2‐(c‐C6H11)‐6‐MeC6H3 Fe2, 2‐(c‐C8H15)‐6‐MeC6H3 Fe3, 2‐(c‐C5H9)‐4,6‐Me2C6H2 Fe4, 2‐(c‐C6H11)‐4,6‐Me2C6H2 Fe5, 2‐(c‐C8H15)‐4,6‐Me2C6H2 Fe6; c refers as cyclic), on activation with methylalumoxane (MAO) or modified MAO (MMAO), exhibit high activities towards ethylene polymerization, producing strictly linear polyethylenes with terminal vinyl groups. The catalytic performances are systematically investigated along with various polymerization parameters as well as the microstructures of resultant polyethylenes. The steric hindrances of ortho‐cycloalkyl substituents of Nimino‐aryl groups significantly affect the activities of the corresponding iron precatalysts as well as the microstructures of resultant polyethylenes: higher steric hindrance the ortho‐cycloalkyl substituents, higher activity the iron precatalyst, lower molecular weight the resultant polyethylenes. Experimental observations are additionally supported by the computational study. The resultant polyethylenes exhibited excellent hydrophobicity.

Seeking Heteroatom-Rich Compounds: Synthetic and Mechanistic Studies into Iron Catalyzed Dehydrocoupling of Silanes

A detailed synthetic investigation into the dehydrocoupling of silanes with amines, phosphines, and alcohols using an iron precatalyst (1) is presented. We have furnished over 30 examples of aminos...

Room Temperature Iron-Catalyzed Transfer Hydrogenation and Regioselective Deuteration of Carbon-Carbon Double Bonds.

An iron catalyst has been developed for the transfer hydrogenation of carbon-carbon multiplebonds. Using a well-defined β-diketiminate iron(II) precatalyst, a sacrificial amine and a borane, even simple, unactivated alkenes such as 1-hexene undergo hydrogenation within 1 h at room temperature. Tuning the reagent stoichiometry allows for semi- and complete hydrogenation of terminal alkynes. It is also possible to hydrogenate aminoalkenes and aminoalkynes without poisoning the catalyst through competitive amine ligation. Furthermore, by exploiting the separate protic and hydridic nature of the reagents, it is possible to regioselectively prepare monoisotopically labeled products. DFT calculations define a mechanism for the transfer hydrogenation of propene with nBuNH2 and HBpin that involves the initial formation of an iron(II)-hydride active species, 1,2-insertion of propene, and rate-limiting protonolysis of the resultant alkyl by the amine N-H bond. This mechanism is fully consistent with the selective deuteration studies, although the calculations also highlight alkene hydroboration and amine-borane dehydrocoupling as competitive processes. This was resolved by reassessing the nature of the active transfer hydrogenation agent: experimentally, a gel is observed in catalysis, and calculations suggest this can be formulated as an oligomeric species comprising H-bonded amine-borane adducts. Gel formation serves to reduce the effective concentrations of free HBpin and nBuNH2 and so disfavors both hydroboration and dehydrocoupling while allowing alkene migratory insertion (and hence transfer hydrogenation) to dominate.

Selective [1,4]-Hydrovinylation of 1,3-Dienes with Unactivated Olefins Enabled by Iron Diimine Catalysts.

The selective, intermolecular [1,4]-hydrovinylation of conjugated dienes with unactivated α-olefins catalyzed by α-diimine iron complexes is described. Value-added "skipped" diene products were obtained with exclusive [1,4]-selectivity, and the formation of branched, ( Z)-olefin products was observed with no evidence for alkene isomerization. Mechanistic studies conducted with the well-defined, single-component iron precatalyst (MesDI)Fe(COD) (MesDI = [2,4,6-Me3-C6H2-N═CMe]2); COD = 1,5-cyclooctadiene) provided insights into the origin of the high selectivity. An iron diene complex was identified as the catalyst resting state, and one such isoprene complex, (iPrDI)Fe(η4-C5H8), was isolated and characterized. A combination of single crystal X-ray diffraction, Mößbauer spectroscopy, magnetic measurements, and DFT calculations established that the complex is best described as a high-spin Fe(I) center ( SFe = 3/2) engaged in antiferromagnetic coupling to an α-diimine radical anion ( SDI = -1/2), giving rise to the observed S = 1 ground state. Deuterium-labeling experiments and kinetic analyses of the catalytic reaction provided support for a pathway involving oxidative cyclization of an alkene with the diene complex to generate an iron metallacycle. The observed selectivity can be understood in terms of competing steric interactions in the transition states for oxidative cyclization and subsequent β-hydrogen elimination.

Iron and Single Electron Transfer: All is in the Ligand

AbstractThis account describes some advances we have made in the field of iron catalysis. Two types of reactivity have been uncovered. Based on the use of an iron(II) precatalyst in the presence of NaBH4, the first one consists in a SET which can be useful for the reductive dehalogenation of iodide and bromide derivatives. Switching to the non‐innocent bis‐iminopyridine ligands promotes a previously undescribed Csp2−H activation reaction leading to biaryl derivatives. First clues into the intricate nature of the mechanism were obtained and suggested that the redox‐active bis‐iminopyridine ligand acts as an electron reservoir. The resulting buildup of electron density triggers the C−H bond breaking. All these findings are discussed in light of the existing literature and perspectives are given.

Markovnikov versus anti-Markovnikov Hydrophosphination: Divergent Reactivity Using an Iron(II) β-Diketiminate Pre-Catalyst.

The ability to tune between different regioselectivities using a common pre-catalyst is an unusual yet highly desirable process. Here, we report the use of an iron(II) pre-catalyst that can be used to synthesize vinyl phosphines in a Markovnikov-selective manner in benzene, whereas a simple change to dichloromethane as the reaction solvent leads to the Z-selective anti-Markovnikov product. Preliminary mechanistic studies are reported that suggest Markovnikov selectivity is a radical-mediated process, whereas the anti-Markovnikov selectivity is not radical in nature but is due to a change in oxidation state.

Phosphine- and Amine-Borane Dehydrocoupling Using a Three-Coordinate Iron(II) β-Diketiminate Precatalyst

Dehydrocoupling of phosphine- and amine-boranes is reported using an iron(II) β-diketiminate complex. Dehydrocoupling of amine-boranes is far more facile than the phosphine counterpart, the former proceeding at room temperature with 1 mol% iron precatalyst. This low loading is sufficient to allow in situ kinetic analysis and deuterium labeling studies to be carried out. An iron amido-borane complex has also been isolated, which is believed to be the catalyst resting state. Overall, this has allowed us to postulate a catalytic cycle which proceeds via release of diborazane, iron hydride, and iron amido-borane intermediates.

Efficient Hydrosilylation of Acetophenone with a New Anthraquinonic Amide-Based Iron Precatalyst

A new iron complex based on a noninnocent anthraquinonic ligand has been synthesized and fully characterized through multiple techniques, including NMR, X-ray crystallography, mass spectrometry, and cyclic voltammetry. Exposure of ketone to that complex in the presence of (EtO)2MeSiH affords the corresponding silylated alcohol. Loadings as low as 0.25 mol % afford excellent yields at room temperature. Quantum–chemical analysis of the catalytic mechanism supports activation of the precatalyst to form a Fe–hydride intermediate, followed by ketone reduction and σ-bond metathesis.

Iron(II) catalyzed reductive radical cyclization reactions of bromoacetals in the presence of NaBH4: optimization studies and mechanistic insights

5-Exo-trig radical reductive cyclization reactions of bromoacetals are catalyzed by iron in the presence of the reducing agent NaBH4. Both iron(II) and iron(III) were found to effectively mediate these reactions. As shown by cyclic voltammetry, iron(III) can be reduced to an iron(II) precatalyst before passing through an identical reaction mechanism in which monoelectronic activation of the substrate would occur by an anionic hydridoiron(I) complex. Further studies have established that both the substrate (iodo- vs bromo-derivative) and the precatalytic mixture are decisive in determining the reaction outcome.

A Study of Two Highly Active, Air‐Stable Iron(III)‐μ‐Oxo Precatalysts: Synthetic Scope of Hydrophosphination using Phenyl‐ and Diphenylphosphine

AbstractThe importance of phosphines in synthetic chemistry cannot be underestimated. Catalytic hydrophosphination offers an ideal method to prepare P−C bonds without the need for harsh reaction conditions or stoichiometric amounts of waste by‐product. We herein report our studies into two biocompatible iron(III) complexes in hydrophosphination chemistry using diphenylphosphine under mild and benign reaction conditions (room temperature, solvent‐free) and our extended exploration of hydrophosphination with phenylphosphine, which can be tuned to operate in the absence of catalyst under thermal conditions for single hydrophosphination or solvent‐free with an iron(III) precatalyst to generate the products of double hydrophosphination.magnified image

Details of the Mechanism of the Asymmetric Transfer Hydrogenation of Acetophenone Using the Amine(imine)diphosphine Iron Precatalyst: The Base Effect and The Enantiodetermining Step

The excellent ketone asymmetric transfer hydrogenation (ATH) systems using the precatalysts (S,S)-trans-[FeCl(CO)(PPh2CH2CH2NHCHPhCHPhNCHCH2PAr2)]BPh4 (Ar = Ph (1), p-tolyl (2)) have a fascinating dependence of activity on the base concentration, which is investigated here. The reaction of complex 1 or 2 with 1 equiv of the strong base potassium tert-butoxide in THF for 2–7 days produces the neutral amine(ene-amido) complexes [FeCl(CO)(PPh2CH2CH2NHCHPhCHPhNCHCHPAr2)] (8 and 9). These monodeprotonated complexes have been completely characterized by NMR, EA, and FT-IR spectroscopy and mass spectrometry, and the structure of 9 has been further confirmed by single-crystal X-ray diffraction to reveal a structure with the NH and FeCl bonds parallel and the proton and chloride ligands next to each other. The structures of 8 and 9 and their 1H NMR patterns are similar to those of the active catalyst for the ATH of ketones that is postulated to have NH and FeH bonds parallel with the protonic and hydridic hydrogens adjacent. Identical key nuclear Overhauser effect (NOE) correlations in both 8 and the hydrido complex provide further evidence for the postulated structure of the amine iron hydride intermediate. The catalyst system is not active for the transfer hydrogenation of acetophenone, unless greater than 2 equiv of base is added to activate the precatalyst. The addition of base (up to 8 equiv per iron) increases the reaction rate, while a further increase of the base concentration shows a reduction of activity. The loss of activity with less than 2 equiv of base results from a side reaction of the active amido(ene-amido) complexes with 2-propanol to form an inactive neutral bis(amido) iron complex, which was characterized by NMR spectroscopy. Structural evidence for this was provided by the X-ray crystal structure determination of an analogous bis(amino) iron(II) complex, generated from the reaction of the amine(ene-amido) iron complex with methanol in C6D6 in the presence of BF4–. The presence of excess base prevents this side reaction, thereby favoring the reaction which forms the active amine iron hydride species that is in the catalytic cycle. The structure of the transition state for the reaction of the amine hydrido iron catalyst with acetophenone has been successfully modeled using density functional theory (DFT) calculations. The (R) configuration of the product 1-phenylethanol is induced by the position of the phenyl groups on the catalyst and a π–π stabilizing interaction between the aryl ring on the ketone and the ene-amido moiety on the ligand.

Facile, Catalytic Dehydrocoupling of Phosphines Using β-Diketiminate Iron(II) Complexes.

Catalytic dehydrocoupling of primary and secondary phosphines has been achieved for the first time using an iron pre-catalyst. The reaction proceeds under mild reaction conditions and is successful with a range of diarylphosphines. A proton acceptor is not needed for the transformation to take place, but addition of 1-hexene does allow for turnover at 50 °C. The catalytic system developed also facilitates the dehydrocoupling of phenylphosphane and dicyclohexylphosphane. A change in solvent switches off dehydrocoupling to allow hydrophosphination of alkenes.

Mechanistic Studies of Catalytic Carbon–Carbon Cross-Coupling by Well-Defined Iron NHC Complexes

The mechanism of iron-catalyzed carbon–carbon cross-coupling reactions between Grignard reagents and alkyl halides has been investigated using well-defined N-heterocyclic carbene (NHC) compounds. The iron(II) precatalyst, [Fe2Cl2(μ-Cl)2(IPr)2], was employed in several C–C cross coupling reactions exhibiting the ability to efficiently couple primary and secondary alkyl halides with several aryl and alkyl Grignard reagents. For selected substrates, a 2 mol % catalyst loading (4 mol % Fe) afforded conversions of >99% and were achieved with <8% homocoupling of the electrophile. The mechanism of the coupling reaction was studied by means of radical clock, radical trap, and single-turnover experiments, which support a radical-based cycle involving an Fe(II/III) redox couple. The implications of this mechanism on the efficacy of iron-NHC-catalyzed cross-coupling reactions are discussed.

ChemInform Abstract: Room Temperature Hydrophosphination Using a Simple Iron Salen Pre‐Catalyst.

Phosphines are fundamentally important to the fine chemicals, pharmaceutical and agrochemical industries. Reported is the first example of alkene hydrophosphination using a designed iron pre-catalyst which yields the anti-Markovnikov products in high yield at room temperature. The phosphine products are excellent pro-ligands for Fe-catalyzed Negishi cross-coupling.