Discrete iron complexes for the selective catalytic reduction of aromatic, aliphatic, and α,β-unsaturated aldehydes under water-gas shift conditions

Copper-catalysed low-temperature water-gas shift reaction for selective deuteration of aryl halides

The introduction of deuterium atoms into organic compounds is of importance for basic chemistry, material sciences, and the development of drugs in the pharmaceutical industry, specifically for identification and quantification of metabolites. Hence, methodologies for the synthesis of selectively labelled compounds continue to be a major area of interest for many scientists. Herein, we present a practical and stable heterogeneous copper catalyst, which permits for dehalogenative deuteration via water–gas shift reaction at comparably low temperature. This novel approach allows deuteration of diverse (hetero)aryl halides with good functional group tolerance, and no reduction of the aromatic rings or other easily reducible formyl and cyano groups. Multi-gram experiments show the potential of this method in organic synthesis and medicinal chemistry.

A practical and stable heterogeneous copper catalyst has been developed for dehalogenative deuteration via water–gas shift reaction at low temperature, allowing deuteration of diverse (hetero)aryl halides with good functional group tolerance.

(Cyclopentadienone)iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air-stable that are extensively studied as an active pre-catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non-innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer-type oxidation of alcohols), C-C and C-N bond formation (e. g., reductive aminations, α-alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.

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.

Novel synthesis of a four-electron-reduced ruthenium(ii) NADH-type complex under water-gas-shift reaction conditions

A four-electron-reduced ruthenium(ii) NADH-type complex, [Ru(bbnpH₄)(CO)₂Cl](PF₆) (bbnpH₄ = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(5,10-dihydrobenzo[b][1,5]naphthyridine)), has been successfully synthesized by mixing an NAD⁺-type ligand, bbnp (bbnp = 2,2'-(4-(tert-butyl)pyridine-2,6-diyl)bis(benzo[b][1,5]naphthyridine)), and [Ru(CO)₂Cl₂] under moderate water-gas-shift reaction conditions, which has been fully characterized by single-crystal X-ray structure analysis, ESI-TOF mass spectrometry, and NMR spectroscopy.

Synthesis and applications to catalysis of novel cyclopentadienone iron tricarbonyl complexes

New approaches to enantiomerically-pure cyclopentadienone iron complexes, and their applications to ketone reductions reactions, are described.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Harnessing the Power of the Water-Gas Shift Reaction for Organic Synthesis

Since its original discovery over a century ago, the water-gas shift reaction (WGSR) has played a crucial role in industrial chemistry, providing a source of H2 to feed fundamental industrial transformations such as the Haber-Bosch synthesis of ammonia. Although the production of hydrogen remains nowadays the major application of the WGSR, the advent of homogeneous catalysis in the 1970s marked the beginning of a synergy between WGSR and organic chemistry. Thus, the reducing power provided by the CO/H2 O couple has been exploited in the synthesis of fine chemicals; not only hydrogenation-type reactions, but also catalytic processes that require a reductive step for the turnover of the catalytic cycle. Despite the potential and unique features of the WGSR, its applications in organic synthesis remain largely underdeveloped. The topic will be critically reviewed herein, with the expectation that an increased awareness may stimulate new, creative work in the area.

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

Iron cyclopentadienone complexes derived from C2-symmetric bis-propargylic alcohols; preparation and applications to catalysis

The following complexes were prepared through cyclisation of a bis-propargylic alcohol and were tested as redox catalysts for hydrogen transfer reactions of alcohols and ketones.

Efficient Hydrogenation of Ketones and Aldehydes Catalyzed by Well-Defined Iron(II) PNP Pincer Complexes: Evidence for an Insertion Mechanism

We have prepared and structurally characterized a new class of Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)] n (L = Br-, CH3CN, pyridine, PMe3, SCN-, CO, BH4-; n = 0, +1) based on the 2,6-diaminopyridine scaffold where the PiPr2 moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)] n with labile ligands (L = Br-, CH3CN, BH4-) and NH spacers are efficient catalysts for the hydrogenation of both ketones and aldehydes to alcohols under mild conditions, while those containing inert ligands (L = pyridine, PMe3, SCN-, CO) are catalytically inactive. Interestingly, complex [Fe(PNPMe-iPr)(H)(CO)(Br)], featuring NMe spacers, is an efficient catalyst for the chemoselective hydrogenation of aldehydes. The first type of complexes involves deprotonation of the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide cooperation, but no reversible aromatization/deprotonation of the PNP ligand. In the case of the N-methylated complex the mechanism remains unclear, but obviously does not allow bifunctional activation of dihydrogen. The experimental results complemented by DFT calculations strongly support an insertion of the C=O bond of the carbonyl compound into the Fe-H bond.