μ-Oxo-bridged iron(iii) complexes for the selective reduction of aromatic ketones catalyzed through base promoted in situ nanoparticle formation

Selective TH of aromatic ketones catalyzed by μ-oxo-diiron complexes and in situ NP formation in the presence of a base gave excellent yields (∼99–100%).

Ruthenium(ii)-catalyzed reductive N–O bond cleavage of N-OR (R = H, alkyl, or acyl) substituted amides and sulfonamides

A convenient method for the reductive cleavage of the N–O bonds of amide derivatives was developed using ruthenium(ii)-catalyzed transfer hydrogenation reaction.

An iron variant of the Noyori hydrogenation catalyst for the asymmetric transfer hydrogenation of ketones

We report the design of a new iron catalyst for the asymmetric transfer hydrogenation of ketones. This type of iron catalyst combines the structural characteristics of the Noyori hydrogenation catalyst (an axially chiral 2,2'-bis(phosphino)-1,1'-binaphthyl fragment and the metal-ligand bifunctional motif) and an ene(amido) group that can activate the iron center. After activation by 8 equivalents of potassium tert-butoxide, (S_A,R_P,SS)-7a and (S_A,R_P,SS)-7b are active but nonenantioselective catalysts for the transfer hydrogenation of acetophenone and α,β-unsaturated aldehydes at room temperature in isopropanol. A maximum turnover number of 14480 was observed for (S_A,R_P,SS)-7a in the reduction of acetophenone. The right combination of the stereochemistry of the axially chiral 2,2'-bis(phosphino)-1,1'-binaphthyl group and the carbon-centered chiral amine-imine moiety in (S_A,R_P,RR)-7b' afforded an enantioselective catalyst for the preparation of chiral alcohols with moderate to good yields and a broad functional group tolerance.

Pyridine N-oxide promoted hydrosilylation of carbonyl compounds catalyzed by [PSiP]-pincer iron hydrides

Five [PSiP]-pincer iron hydrides 1–5 were used as catalysts to study the effects of pyridine N-oxide and the electronic properties of [PSiP]-ligands on the catalytic hydrosilylation of carbonyl compounds.

Iron N-heterocyclic carbene complexes in homogeneous catalysis

This review article summarizes recent development of homogeneous iron N-heterocyclic carbene catalysts.

The origin of different driving forces between O–H/N–H functional groups in metal ligand cooperation: mechanistic insight into Mn(i) catalysed transfer hydrogenation

The origin of different catalytic activity between two structurally similar Lewis basic bifunctional catalysts.

Transfer hydrogenation of aldehydes and ketones catalyzed using an aminophosphinite POCNH pincer complex of Ni(ii)

(POCN^H)NiBr catalyzes the transfer hydrogenation of aldehydes and ketones in iPrOH. The reactions tolerate alkenes, esters, amides, nitriles, and heterocycles and proceed via the metal–ligand cooperative mechanism through (POCN)Ni^II species.

Chiral cis-iron(ii) complexes with metal- and ligand-centered chirality for highly regio- and enantioselective alkylation of N-heteroaromatics

Iron-catalyzed highly regio- and enantioselective organic transformations with generality and broad substrate scope have profound applications in modern synthetic chemistry; an example is herein described based on cis-Fe^II complexes having metal- and ligand-centered chirality. The cis-β Fe^II(N₄) complex [Fe^II(L)(OTf)₂] (L = N,N′-bis(2,3-dihydro-1H-cyclopenta-[b]quinoline-5-yl)-N,N′-dimethylcyclohexane-1,2-diamine) is an effective chiral catalyst for highly regio- and enantioselective alkylation of N-heteroaromatics with α,β-unsaturated 2-acyl imidazoles, including asymmetric N1, C2, C3 alkylations of a broad range of indoles (34 examples) and alkylation of pyrroles and anilines (14 examples), all with high product yields (up to 98%), high enantioselectivity (up to >99% ee) and high regioselectivity. DFT calculations revealed that the “chiral-at-metal” cis-β configuration of the iron complex and a secondary π–π interaction are responsible for the high enantioselectivity.

A cis-β Fe^II complex having metal- and ligand-centered chirality catalyzes highly regio- and enantioselective alkylation of indoles (at the N1, C2, or C3 position), pyrroles and anilines with α,β-unsaturated 2-acyl imidazoles (48 examples, up to 99% ee).

Synthesis and Catalytic Application of Knölker-Type Iron Complexes with a Novel Asymmetric Cyclopentadienone Ligand Design

Asymmetric catalysis is an essential tool in modern chemistry, but increasing environmental concerns demand the development of new catalysts based on cheap, abundant, and less toxic iron. As a result, Knölker-type catalysts have emerged as a promising class of iron catalysts for various chemical transformations, notably the hydrogenation of carbonyls and imines, while asymmetric versions are still under exploration to achieve optimal enantio-selectivities. In this work, we report a novel asymmetric design of a Knölker-type catalyst, in which the C2-rotational symmetric cyclopentadienone ligand possesses chiral substituents on the 2- and 5-positions near the active site. Four examples of the highly modular catalyst design were synthesized via standard organic procedures, and their structures were confirmed with NMR, IR, MS, and polarimetry analysis. Density functional theory (DFT) calculations were conducted to elucidate the spatial conformation of the catalysts, and therewith to rationalize the influence of structural alterations. Transfer- and H2-mediated hydrogenations were successfully established, leading to appreciable enantiomeric excesses (ee) values up to 70%. Amongst all reported Knölker-type catalysts, our catalyst design achieves one of the highest ee values for hydrogenation of acetophenone and related compounds.

TiO₂ Photocatalysis for Transfer Hydrogenation

Catalytic transfer hydrogenation reactions, based on hydrogen sources other than gaseous H₂, are important processes that are preferential in both laboratories and factories. However, harsh conditions, such as high temperature, are usually required for most transition-metal catalytic and organocatalytic systems. Moreover, non-volatile hydrogen donors such as dihydropyridinedicarboxylate and formic acid are often required in these processes which increase the difficulty in separating products and lowered the whole atom economy. Recently, TiO₂ photocatalysis provides mild and facile access for transfer hydrogenation of C=C, C=O, N=O and C-X bonds by using volatile alcohols and amines as hydrogen sources. Upon light excitation, TiO₂ photo-induced holes have the ability to oxidatively take two hydrogen atoms off alcohols and amines under room temperature. Simultaneously, photo-induced conduction band electrons would combine with these two hydrogen atoms and smoothly hydrogenate multiple bonds and/or C-X bonds. It is heartening that practices and principles in the transfer hydrogenations of substrates containing C=C, C=O, N=O and C-X bond based on TiO₂ photocatalysis have overcome a lot of the traditional thermocatalysis' limitations and flaws which usually originate from high temperature operations. In this review, we will introduce the recent paragon examples of TiO₂ photocatalytic transfer hydrogenations used in (1) C=C and C≡C (2) C=O and C=N (3) N=O substrates and in-depth discuss basic principle, status, challenges and future directions of transfer hydrogenation mediated by TiO₂ photocatalysis.

Versatile Coordination and C-C Coupling of Diphosphine-Tethered Imine Ligands with Ni(II) and Ni(0)

Ligands that can adapt their coordination mode to the electronic properties of a metal center are of interest to support catalysis or small molecule activation processes. In this context, the ability of imine moieties to bind in either an η¹(N)-fashion via σ-donation of the lone pair or, less commonly, in an η²(C,N)-fashion via π-coordination is potentially attractive for the design of new metal–ligand cooperative systems. Herein, the coordination chemistry of chelating ligands with a diphosphine imine framework (PCNP) to nickel is investigated. The imine moiety binds in an η¹(N)-fashion in a Ni(II)Cl₂ complex. The uncommon η²(C,N)-interaction is obtained in Ni(0) complexes in the presence of a PPh₃ coligand. Increasing the bulk on the phosphine side-arms in the Ni(0) complexes, by substituting phenyl for o-tolyl groups, leads to a distinct binding mode in which only one of the phosphorus atoms is coordinated. In the absence of a coligand, a mixture of two different dimeric Ni(0) complexes is formed. In one of them, the imine adopts an uncommon η¹(N)η²(C,N) bridging mode of the ligand to nickel, while the second one may involve reactivity on the ligand by the formation of a new C–C bond by oxidative coupling. The latter is supported by the isolation and structural characterization of a crystalline bis-CO derivative featuring a C–C bond formed by oxidative coupling of two imine moieties.

The coordination chemistry of a series of diphosphine-imine ligands (PCNP) to Ni is investigated, with the purpose of developing systems that present metal−ligand cooperativity. The ligands bind in versatile ways, adapting to the oxidation state and coordination environment of the metal center. Additionally, a dimeric derivative undergoes oxidative C−C coupling in the presence of CO, resulting in an unusual mixed valence Ni(II)/Ni(0) dinuclear complex.

Accessible Bifunctional Oxy-Tethered Ruthenium(II) Catalysts for Asymmetric Transfer Hydrogenation

A concise synthesis of new oxy-tethered ruthenium complexes effective for the asymmetric transfer hydrogenation of aromatic ketones is described. The oxy-tether was constructed via a defluorinative etherification arising from an intramolecular nucleophilic substitution of a perfluorinated phenylsulfonyl substituent. The obtained tethered complexes exhibited desirable catalytic activity and selectivity, especially in the asymmetric transfer hydrogenation of functionalized aromatic ketones. The robustness and rigidity of the tether contribute to their superior catalytic performance relative to the nontethered prototype complex.

Transfer hydrogenation of aldehydes catalyzed by silyl hydrido iron complexes bearing a [PSiP] pincer ligand

The synthesis and characterization of a series of silyl hydrido iron complexes bearing a pincer-type [PSiP] ligand (2-R₂PC₆H₄)₂SiH₂ (R = Ph (1) and ⁱPr (5)) or (2-Ph₂PC₆H₄)₂SiMeH (2) were reported. Preligand 1 reacted with Fe(PMe₃)₄ to afford complex ((2-Ph₂PC₆H₄)SiH)Fe(H)(PMe₃)₂ (3) in toluene, which was structurally characterized by X-ray diffraction. ((2-ⁱPr₂PC₆H₄)SiH)Fe(H)(PMe₃) (6) could be obtained from the reaction of preligand 5 with Fe(PMe₃)₄ in toluene. Furthermore, complex ((2-ⁱPr₂PC₆H₄)Si(OMe))Fe(H)(PMe₃) (7) was isolated by the reaction of complex 6 with 2 equiv. MeOH in THF. The molecular structure of complex 7 was also determined by single-crystal X-ray analysis. Complexes 3, 4, 6 and 7 showed good to excellent catalytic activity for transfer hydrogenation of aldehydes under mild conditions, using 2-propanol as both solvent and hydrogen donor. α,β-Unsaturated aldehydes could be selectively reduced to corresponding α,β-unsaturated alcohols. The catalytic activity of penta-coordinate complex 6 or 7 is stronger than that of hexa-coordinate complex 3 or 4.

The synthesis and characterization of a series of silyl hydrido iron complexes bearing a [PSiP] pincer ligand were reported. These complexes showed good to excellent catalytic activity for transfer hydrogenation of aldehydes under mild conditions.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Mechanisms of the H2- and transfer hydrogenation of polar bonds catalyzed by iron group hydrides

This Perspective describes the mechanism-based development of iron-group catalysts for the asymmetric hydrogenation of ketones and imines.

Highly Efficient and Selective Hydrogenation of Aldehydes: A Well-Defined Fe(II) Catalyst Exhibits Noble-Metal Activity

The synthesis and application of [Fe(PNPMe-iPr)(CO)(H)(Br)] and [Fe(PNPMe-iPr)(H)2(CO)] as catalysts for the homogeneous hydrogenation of aldehydes is described. These systems were found to be among the most efficient catalysts for this process reported to date and constitute rare examples of a catalytic process which allows selective reduction of aldehydes in the presence of ketones and other reducible functionalities. In some cases, TONs and TOFs of up to 80000 and 20000 h-1, respectively, were reached. On the basis of stoichiometric experiments and computational studies, a mechanism which proceeds via a trans-dihydride intermediate is proposed. The structure of the hydride complexes was also confirmed by X-ray crystallography.

Synthesis and reactivity of BINEPINE-based chiral Fe(II) PNP pincer complexes

ABSTRACT

A new asymmetric chiral PNP ligand based on the 2,6-diaminopyridine scaffold featuring a R-BINEPINE moiety was prepared. Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv of PNP-iPr,BIN at room temperature afforded the coordinatively unsaturated paramagnetic complexes [Fe(PNP-iPr,BIN)X2]. The structure of [Fe(PNP-iPr,BIN)Cl2] is described. Both complexes react readily with the strong π-acceptor ligand CO in solution to afford selectively the diamagnetic complexes trans-[Fe(PNP-iPr,BIN)(CO)X2] in quantitative yield. Due the lability of the CO ligand, these complexes are only stable under a CO atmosphere and isolation in pure form was not possible. The preparation of the carbonyl hydride complex [Fe(PNP-iPr,BIN)(H)(CO)Br] was achieved albeit in low yields via a one pot procedure by treatment of [Fe(PNP-iPr,BINEP)Br2] with CO and subsequent reaction with Na[HBEt3]. This complex was obtained as an inseparable mixture of two diastereomers in a ca. 1:1 ratio and was tested as catalyst for the hydrogenation of ketones. The catalyst showed acceptable activity under mild conditions (5 bar H2, room temperature) with yields up to >99 % within 18 h.

GRAPHICAL ABSTRACT