Recent advances in homogeneous base-metal-catalyzed transfer hydrogenation reactions
Graphical abstract
Introduction
The catalytic transfer hydrogenation (TH) of unsaturated organic compounds, such as alkenes, alkynes, carbonyl compounds, imines, nitriles, N-heterocycles, etc., has emerged as an effective alternative methodology for the wasteful stoichiometric reduction reactions using metal hydride reagents and direct catalytic hydrogenations using molecular hydrogen [1,2]. Despite direct hydrogenations with compressed hydrogen gas are considered as the most atom economical transformations, the reactions typically require rather harsh experimental conditions, such as high temperatures and H2 pressure [2]. This causes safety and selectivity concerns for such transformations and necessitates the use of rather expensive high-pressure reactors. In contrast to direct hydrogenation, catalytic TH reactions do not require addition of H2 gas, utilize sacrificial hydrogen donors (mainly formic acid and alcohols, which serve as both H2 sources and reaction media) and generally proceed under relatively mild conditions (Scheme 1) [[1], [2], [3]]. All this results in operational simplicity, reduced cost, enhanced safety, and improved selectivity of TH reactions, compared to classical stoichiometric reduction methods and catalytic hydrogenations with molecular hydrogen. For these reasons, TH and asymmetric transfer hydrogenation (ATH) reactions have become a powerful synthetic tool in preparation of many specialty chemicals, including the synthesis of natural products, biologically active molecules, and pharmaceuticals [[1], [2], [3], [4], [5]].
Conventionally, homogeneous catalytic systems for TH and ATH of unsaturated organic compounds are mostly based on heavy (2nd and 3rd row) late transition metals, such as Ru, Rh, Ir, etc. [[1], [2], [3],6], with Ru playing the major role in the design of highly stereoselective ATH catalysts [1](a), [1](i), [1](j), [6]. In 2001, the importance of these developments was recognized with the Nobel Prize to Noyori [6c]. Despite their high catalytic activity, the second and third-row late transition metals have low natural abundance and are therefore expensive. Due to this, during the past decades, an important venue of research in this area has been the development of “greener” TH and ATH catalysts based on more economical, more environmentally benign and generally less toxic 1st row transition metals [1](i), [4], [7]. Not surprisingly, since iron is the most abundant transition metal and Fe compounds in many aspects resemble the properties of ruthenium ones [8], as these elements share the same group, among 3 d base-metals (Mn, Fe, Co, and Ni) iron complexes have attracted utmost attention in the development of sustainable catalytic systems for TH and ATH reactions [1](i), [4](a), [7]. Other base-metal TH systems, such as those based on Mn, Co and Ni, have been developed to a lesser extent and generally showed lower catalytic activities [1](i), [4](a). Nonetheless, significant achievements have been reported in recent years for such TH catalysts with novel ligand platforms, including asymmetric reductions and the reactions with more sustainable hydrogen sources, such as H2O, NH3·BH3 [9], etc. This review aims to highlight these recent developments and summarize the advances and challenges in the field of catalytic TH and ATH reactions with abundant Mn, Fe, Co, and Ni complexes. Special attention is paid to the design of catalysts and the mechanisms of TH reactions. The review is limited to the last seven-year period (since 2015). Several recent achievements in TH and ATH catalysis with base-metals were reviewed in 2018; however, these accounts are rather brief and very little mechanistic considerations for TH reactions were discussed [10]. Moreover, several more narrow reviews limited to certain base-metals, hydrogen sources, employed ligands, classes of organic substrates and/or applications of TH methodology to different types of organic transformations were published since 2015 [11]. Prior achievements have been comprehensively discussed elsewhere [1](i), [4](a), [12].
Section snippets
Fe-catalyzed transfer hydrogenation reactions
Hydrogenation – an extensively studied field [2]. Probably, most of the scientific community would agree with such a statement. However, it would totally be incorrect, if we were to claim the same regarding iron-mediated TH. Iron has been, and still is, a rising star in this field since milestone achievements by the groups of Morris and Beller [13]. Current chapter would probably provide as much information as Co-, Ni-, and Mn-based sections of this review combined. Although, owing to its high
Co-catalyzed transfer hydrogenation reactions
Pioneering works on Co-catalyzed TH reactions appeared in the literature back in 2013. Hanson reported a (PNP)Co-catalyzed TH of ketones and imines [49] and Peters published a (PBP)Co pincer dinitrogen complex that was able to activate H2 and Me2NH–BH3, catalyze hydrogenation of terminal alkenes, and perform TH of terminal alkenes using amine-boranes [50]. These findings inspired many others, and several highly active and selective Co-based TH systems were reported recently.
Transfer hydrogenation of unsaturated C–C bonds
Examples for Ni-catalyzed TH of unsaturated hydrocarbons are scarce [1](i), [62], [64]. Recently, only a couple of nickel systems have been shown to reduce non-conjugated alkenes and alkynes using formic acid and water as hydrogen sources [62]. Both of these systems employed simple commercially available nickel halide pre-catalysts (NiBr2, NiCl2(DME) and NiCl2·6H2O; 5–20 mol%); however, they also required the addition of excess Zn as a reducing agent (2–5 equiv. to the substrate). Thus, Moran
Mn-catalyzed transfer hydrogenation reactions
Manganese-catalyzed TH reactions have appeared in the literature only recently and these examples are not nearly as extensive as other earth-abundant metals covered in this review. Since TH chemistry of manganese is dominated by the reduction of carbonyl compounds, no substrate classification will be made in this section – all achievements shall rather appear in chronological order.
In 2017, the Beller group pioneered manganese-mediated TH of ketones employing complexes 5.1–5.6 (prepared by
Conclusions
Despite a few decades of research in homogeneous catalytic TH reactions, this field is still vastly expanding, shifting towards the design of more sustainable catalytic systems based on abundant and benign first-row transition metals. In this respect, for many years, examples of base-metal catalysts for TH reactions were dominated by iron complexes and mostly concerned TH and ATH of aldehydes and ketones. Efforts focused on the design of novel ligand platforms unravel new mechanistic pathways,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
D.H. and A.Y.K. would like to thank Nazarbayev University for financial support via FDCRG grants (Nr. 240919FD3905 to D. H. and Nr. 240919FD3911 to A.Y.K.).
Dr. Daler Baidilov was born in 1993 in Pavlodar, Kazakhstan, where he received his pre-university education. In April 2016, he obtained his B·Sc. degree in Chemistry at Nazarbayev University (Astana, Kazakhstan). In January 2017, Daler was given an opportunity to pursue his graduate studies at Brock University (St. Catharines, Canada) under the supervision of Professor Tomas Hudlicky in the field of synthetic organic chemistry. After defending his Ph.D. thesis in December 2019, Daler accepted
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Dr. Daler Baidilov was born in 1993 in Pavlodar, Kazakhstan, where he received his pre-university education. In April 2016, he obtained his B·Sc. degree in Chemistry at Nazarbayev University (Astana, Kazakhstan). In January 2017, Daler was given an opportunity to pursue his graduate studies at Brock University (St. Catharines, Canada) under the supervision of Professor Tomas Hudlicky in the field of synthetic organic chemistry. After defending his Ph.D. thesis in December 2019, Daler accepted the offer from Paraza Pharma, Inc. (Montreal, Canada) and holds a Scientist position with Paraza since then. Shortly, he will join the research group of Professor Viresh Rawal (the University of Chicago, USA) as a postdoctoral fellow, and undertake a total synthesis project.
Dr. Davit Hayrapetyan was born 1987 in Armenia. He received his Diploma degree in Chemistry in 2009 from Lomonosov Moscow State University (Russia) and a Ph.D. degree in organic and organometallic chemistry in 2012 from the same University. In 2014 he Moved to Japan to join Prof. Shu Kobayashi's group at the University of Tokyo as a Postdoctoral Fellow. In 2015 he joined Prof. Kevin Lam's laboratory at Nazarbayev University (Kazakhstan), and in 2016 he replenished the research group of Prof. Lukas Gooβen at Ruhr-Universität Bochum (Germany). Since 2019 he holds the position of Independent Postdoctoral Scholar under The Office of the Provost at Nazarbayev University. His main research interests are in the field of Organic Synthesis, Catalysis and Organic Electrochemistry.
Dr. Andrey Y. Khalimon was born and raised in Russia. He received Dipl. Chem. degree in organic chemistry (majoring organometallic chemistry) from Lomonosov Moscow State University (Russia) in 2004 and Ph.D. in inorganic chemistry from Brock University (Canada) in 2010 under the supervision of Prof. Georgii I. Nikonov. After postdoctoral appointents at the University of Calgary (Canada) with Prof. Warren E. Piers during 2010–2013 and at the Catalysis Research Laboratory (CaRLa/BASF, Germany) during 2013–2014, he joined Nazarbayev University (Kazakhstan) in 2015 as an Assistant Professor of Chemistry at the School of Sciences and Humanities. His research interests mainly include homogeneous catalysis for small molecules activation, development of new abundant transition metal catalysts for selective transformations of organic substrates into value-added products and catalytic reduction of unsaturated organic molecules, including mechanistic studies of such reactions in order to develop systems with predictable properties and reactivity patterns.