Stereoselective Transfer Semi-Hydrogenation of Alkynes to E -Olefins with N -Heterocyclic Silylene-Manganese Catalysts

Polydentate Amidinato-Silylenes, -Germylenes and -Stannylenes

This review article focuses on amidinatotetrylenes that potentially can (or have already shown to) behave as bi- or tridentate ligands because they contain at least one amidinatotetrylene moiety (silylene, germylene or stannylene) and one (or more) additional coordinable fragment(s). Currently, they are being widely used as ligands in coordination chemistry, small molecule activation and catalysis. This review classifies those that have been isolated as transition metal-free compounds into five families that differ in the position(s) of the donor group(s) (D) on the amidinatotetrylene moiety, namely: ED{R1NC(R2)NR1}, EX{DNC(R2)NR1}, EX{R1NC(D)NR1}, EX{DNC(R2)ND} and E{R1NC(R2)ND}2 (E=Si, Ge or Sn). Those that do not exist as transition metal-free compounds but have been observed as ligands in transition metal complexes are cyclometallated and ring-opened amidinatotetrylene ligands. This article presents schematic descriptions of their structures, the approaches used for their syntheses and a quick overview of their involvement (as ligands) in transition metal-catalysed reactions. The literature is covered up to the end of 2023.

Amidinatotetrylenes Donor Functionalized on Both N Atoms: Structures and Coordination Chemistry

E(hmds)(bqfam) (E = Ge (1a), Sn (1b); hmds = N(SiMe3)2, bqfam = N,N'-bis(quinol-8-yl)formamidinate), which are amidinatotetrylenes equipped with quinol-8-yl fragments on the amidinate N atoms, have been synthesized from the formamidine Hbqfam and Ge(hmds)2 or SnCl(hmds). Both 1a and 1b are fluxional in solution at room temperature, as the E atom oscillates from being attached to the two amidinate N atoms to being chelated by an amidinate N atom and its closest quinolyl N atom (both situations are similarly stable according to density functional theory calculations). The hmds group of 1a and 1b is still reactive and the deprotonation of another equivalent of Hbqfam can be achieved, allowing the formation of the homoleptic derivatives E(bqfam)2 (E = Ge, Sn). The reactions of 1a and 1b with [AuCl(tht)] (tht = tetrahydrothiophene), [PdCl2(MeCN)2], [PtCl2(cod)] (cod = cycloocta-1,5-diene), [Ru3(CO)12] and [Co2(CO)8] have been investigated. The gold(I) complexes [AuCl{κE-E(hmds)(bqfam)}] (E = Ge, Sn) have a monodentate κE-tetrylene ligand and display fluxional behavior in solution the same as that of 1a and 1b. However, the palladium(II) and platinum(II) complexes [MCl{κ3E,N,N'-ECl(hmds)(bqfam)}] (M = Pd, Pt; E = Ge, Sn) contain a κ3E,N,N'-chloridotetryl ligand that arises from the insertion of the tetrylene E atom into an M-Cl bond and the coordination of an amidinate N atom and its closest quinolyl N atom to the metal center. Finally, the binuclear ruthenium(0) and cobalt(0) complexes [Ru2{μE-κ3E,N,N'-E(hmds)(bqfam)}(CO)6] and [Co2{μE-κ3E,N,N'-E(hmds)(bqfam)}(μ-CO)(CO)4] (E = Ge, Sn) have a related κ3E,N,N'-tetrylene ligand that bridges two metal atoms through the E atom. For the κ3E,N,N'-metal complexes, the quinolyl fragment not attached to the metal is pendant in all the germanium compounds but, for the tin derivatives, is attached to (in the Pd and Pt complexes) or may interact with (in the Ru2 and Co2 complexes) the tin atom.

Tandem manganese catalysis for the chemo-, regio-, and stereoselective hydroboration of terminal alkynes: in situ precatalyst activation as a key to enhanced chemoselectivity

The manganese(ii) complex [Mn(iPrPNP)Cl2] (iPrPNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) was found to catalyze the stereo- and regioselective hydroboration of terminal alkynes employing HBPin (pinacolborane). In the absence of in situ activators, mixtures of alkynylboronate and E-alkenylboronate esters were formed, whereas when NaHBEt3 was employed as the in situ activator, E-alkenylboronate esters were exclusively accessed. Mechanistic studies revealed a tandem C-H borylation/semihydrogenation pathway accounting for the formation of the products. Stoichiometric reactions hint toward reaction of a Mn-H active species with the terminal alkyne as the catalyst entry pathway to the cycle, whereas reaction with HBPin led to catalyst deactivation.

Recent advances in the selective semi-hydrogenation of alkyne to (E)-olefins

Considering the potential importance and upsurge in demand, the selective semi-hydrogenation of alkynes to (E)-olefins has attracted significant interest. This article highlights the recent advances in newer technologies and important methodologies directed to (E)-olefins from alkynes developed from 2015 to 2023. Notable features summarised include the catalyst or ligand design and control of product selectivity based on precious and nonprecious metal catalysts for semi-hydrogenation to (E)-olefins. Mechanistic studies for various catalytic transformations, including synthetic application to bioactive compounds, are summarised.

Structural snapshots of an Al-Cu bond-mediated transformation of terminal acetylenes

The copper(i) alumanyl derivative, [{SiN^Dipp}Al-Cu(NHC^iPr)] (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-di-isopropylphenyl; NHC^iPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), reacts in a stepwise fashion with up to three equivalents of various terminal alkynes. This reactivity results in the sequential formation of cuprous (hydrido)(alkynyl)aluminate, (alkenyl)(alkynyl)aluminate and bis(alkynyl)aluminate derivatives, examples of which have been fully characterised. The process of alkene liberation resulting from the latter reaction step constitutes a unique case of alkyne transfer semi-hydrogenation in which the C-H acidic alkyne itself acts as a source of proton, with the Cu-Al bond providing the requisite electrons to effect reduction. This reaction sequence is validated by DFT calculations, which rationalise the variable stability of the initially formed heterobimetallic hydrides.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Stereoselective Semi-Hydrogenations of Alkynes by First-Row (3d) Transition Metal Catalysts

The chemo- and stereoselective semi-hydrogenation of alkynes to alkenes is a fundamental transformation in synthetic chemistry, for which the use of precious 4d or 5d metal catalysts is well-established. In mankind's unwavering quest for sustainability, research focus has considerably veered towards the 3d metals. Given their high abundancy and availability as well as lower toxicity and noxiousness, they are undoubtedly attractive from both an economic and an environmental perspective. Herein, we wish to present noteworthy and groundbreaking examples for the use of 3d metal catalysts for diastereoselective alkyne semi-hydrogenation as we embark on a journey through the first-row transition metals.

Bench-Stable Dinuclear Mn(I) Catalysts in E-Selective Alkyne Semihydrogenation: A Mechanistic Investigation

Dinuclear manganese hydride complexes of the form [Mn2 (CO)8 (μ-H)(μ-PR2 )] (R=Ph, 1; R=iPr, 2) were used in E-selective alkyne semi-hydrogenation (E-SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observed E-selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating 2 with (cyclopropylethynyl)benzene contains a cis-alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H2 affords exclusively trans-(2-cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non-radical mechanism for E-SASH, which is highly unusual for Mn-H catalysts. The catalytically active species are virtually inactive towards cis to trans alkene isomerization indicating that the E-selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations.

Dipyrromethane‐Based PGeP Pincer Germyl Rhodium Complexes

A family of germyl rhodium complexes derived from the PGeP germylene 2,2’‐bis(di‐isopropylphosphanylmethyl)‐5,5’‐dimethyldipyrromethane‐1,1’‐diylgermanium(II), Ge(pyrmPⁱPr₂)₂CMe₂ (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh₃)₃] and [RhCl(cod)(PPh₃)] (cod=1,5‐cyclooctadiene) to give, in both cases, the PGeP‐pincer chloridogermyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(CO)] (6), with HCl, HSnPh₃ and Ph₂S₂ rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (X=Cl, 7; SnPh₃, 8) and [Rh(SPh)₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (9), respectively, and with H₂ to give the hexacoordinate derivative [RhH₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4‐vinyltoluene and 4‐vinylfluorobenzene with catecholborane under mild conditions.

C(sp2)–H Activation with Bis(silylene)pyridine Cobalt(III) Complexes: Catalytic Hydrogen Isotope Exchange of Sterically Hindered C–H Bonds

The bis(silylene)pyridine cobalt(III) dihydride boryl, trans-[ ptol SiNSi]Co(H)2BPin (ptolSiNSi = 2,6-[EtNSi(NtBu)2CAr]2 C5H3N, where Ar = C6H5CH3, and Pin =pinacolato) has been used as a precatalyst for the hydrogen isotope exchange (HIE) of arenes and heteroarenes using benzene-d 6 as the deuterium source. Use of D2 as the source of the isotope produced modest levels of deuterium incorporation and stoichiometric studies established modification of the pincer ligand through irreversible addition of H2 across the silylene leading to catalyst deactivation. High levels of deuterium incorporation were observed with benzene-d 6 as the isotope source and enabled low (0.5 - 5 mol%) loadings of the cobalt precursor. The resulting high activity for C-H activation enabled deuterium incorporation at sterically encumbered sites previously inaccessible with first-row metal HIE catalysts. The cobalt-catalyzed method was also compatible with aryl halides, demonstrating a kinetic preference for chemoselective C(sp2)-H activation over C(sp2)-X (X = Cl, Br) bonds. Monitoring the catalytic reaction by NMR spectroscopy established cobalt(III) resting states at both low and high conversions of substrate and the overall performance was inhibited by the addition of HBPin. Studies on precatalyst activation with cis-[ ptol SiNSi]Co(Bf)2H and cis-[ ptol SiNSi]Co(H)2Bf (where Bf = 2-benzofuranyl), support the intermediacy of bis(hydride)aryl cobalt intermediates as opposed to bis(aryl)hydride cobalt complexes in the catalytic HIE method. Mechanistic insights resulted in an improved protocol using [ ptol SiNSi]Co(H)3 NaBHEt3 as the precatalyst, ultimately translating onto higher levels of isotopic incorporation.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

The Rise of Manganese-Catalyzed Reduction Reactions

Recent developments in manganese-catalyzed reducing transformations—hydrosilylation, hydroboration, hydrogenation, and transfer hydrogenation—are reviewed herein. Over the past half a decade (i.e., 2016 to the present), more than 115 research publications have been reported in these fields. Novel organometallic compounds and new reduction transformations have been discovered and further developed. Significant challenges that had historically acted as barriers for the use of manganese catalysts in reduction reactions are slowly being broken down. This review will hopefully assist in developing this research area, by presenting a clear and concise overview of the catalyst structures and substrate transformations published so far.

1 Introduction

2 Hydrosilylation

3 Hydroboration

4 Hydrogenation

5 Transfer Hydrogenation

6 Conclusion and Perspective

A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0) complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical

Using the potentially tridentate N,N′-bis(N-heterocyclic silylene)pyridine [SiNSi] pincer-type ligand, 2,6-N,N′-diethyl-bis[N,N′-di-tert-butyl(phenylamidinato)silylene] diaminopyridine, led to the first isolable bis(silylene)pyridine-stabilized manganese(0) complex, {κ³-[SiNSi]Mn(dmpe)} 4 (dmpe = (Me₂P)₂C₂H₄), which represents an isolobal 17 VE analogue of the elusive Mn(CO)₅ radical. The compound is accessible through the reductive dehalogenation of the corresponding dihalido (SiNSi)Mn(ii) complexes 1 (Cl) and 2 (Br) with potassium graphite. Exposing 4 towards the stronger π-acceptor ligands CO and 2,6-dimethylphenyl isocyanide afforded the related Mn(0) complexes κ²-[SiNSi]Mn(CO)₃ (5) and κ³-[SiNSi]Mn(CNXylyl)₂(κ¹-dmpe) (6), respectively. Remarkably, the stabilization of Mn(0) in the coordination sphere of the [SiNSi] ligand favors the d⁷ low-spin electronic configuration, as suggested by EPR spectroscopy, SQUID measurements and DFT calculations. The suitability of 4 acting as a superior pre-catalyst in regioselective hydroboration of quinolines has also been demonstrated.

An isolable bis(silylene)pyridine stabilized manganese(0) complex {κ³-[SiNSi]Mn(dmpe)}, isolobal to elusive Mn(CO)₅ radical has been synthesized and fully characterised.

Manganese-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes with iPrOH as hydrogen source

The Mn-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes is reported herein, along with Mn-catalyzed hydration of α-keto alkynes. Mechanistic studies displayed an asymmetrical Mn-hydride species performing the catalytic turnover.

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Heterobimetallic Pd/Mn and Pd/Co complexes as efficient and stereoselective catalysts for sequential Cu-free Sonogashira coupling-alkyne semi-hydrogenation reactions

A series of heterobimetallic Pd^II/M^II complexes (M^II = Mn, Co) were synthesised and tested as precatalysts for sequential Sonogashira coupling-alkyne semi-hydrogenation reactions to form Z-aryl alkenes. The carbometalated heterobimetallic Pd^II/Co^II complex CoPdL3' demonstrated an apparent cooperative effect compared to the corresponding monometallic counterparts. This compound was identified as a potent single-molecule catalyst for the one-pot Cu-free Sonogashira coupling of aryl bromides with terminal alkynes followed by chemo- and stereoselective semi-hydrogenation of the alkyne intermediate using NH₃·BH₃ as a hydrogen source. Furthermore, different aromatic substrates have been tested to show the generality of the reaction for the synthesis of Z-alkenes, including biologically active combretastatin A-4. In addition, the homogeneous nature of the catalytically active species was demonstrated.

Stannylenes based on pyrrole-phosphane and dipyrromethane-diphosphane scaffolds: syntheses and behavior as precursors to PSnP pincer palladium(II), palladium(0) and gold(I) complexes

2-Ditertbutylphosphanylmethylpyrrole (H₂pyrmP^tBu₂) and 2,2'-bis(diisopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane ((HpyrmPⁱPr₂)₂CMe₂) have been used to synthesize new P-donor-stabilized stannylenes in which the Sn atom is attached to one, SnCl(HpyrmP^tBu₂) (1) and Sn{N(SiMe₃)₂}(HpyrmP^tBu₂) (2), or two pyrrolyl-phosphane scaffolds, Sn(HpyrmP^tBu₂)₂ (3), or to a dipyrromethane-diphosphane scaffold, Sn(pyrmPⁱPr₂)₂CMe₂ (4). It has been found that stannylenes 3 and 4 are excellent precursors to transition metal complexes containing PSnP pincer-type ligands. Their reactions with chlorido transition metal complexes have afforded [PdCl{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (6), [PdCl{κ³P,Sn,P-SnCl(pyrmPⁱPr₂)₂CMe₂}] (7) and [Au{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (8), which contain a PSnP pincer-type chloridostannyl ligand. While complexes 6 and 7 are square-planar palladium(II) complexes, compound 8 is an uncommon gold(I) complex having a T-shaped coordination geometry with a very long Sn-Au bond (3.120 Å). The T-shaped palladium(0) complex [Pd{κ³P,Sn,P-Sn(pyrmPⁱPr₂)₂CMe₂}] (9), which contains an unprecedented PSnP pincer-type stannylene that behaves as a Z-type (σ-acceptor) ligand, has been prepared from 4 and [Pd(η³-C₃H₅)(η⁵-C₅H₅)].

Advancements in multifunctional manganese complexes for catalytic hydrogen transfer reactions

Catalytic hydrogen transfer reactions have enormous academic and industrial applications for the production of diverse molecular scaffolds. Over the past few decades, precious late transition-metal catalysts were employed for these reactions. The early transition metals have recently gained much attention due to their lower cost, less toxicity, and overall sustainability. In this regard, manganese, which is the third most abundant transition metal in the Earth's crust, has emerged as a viable alternative. However, the key to the success of such manganese-based complexes lies in the multifunctional ligand design and choice of appropriate ancillary ligands, which helps them mimic and, even in some cases, supersede noble metals' activities. The metal-ligand bifunctionality, achieved via deprotonation of the acidic C-H or N-H bonds, is one of the powerful strategies employed for this purpose. Alongside, the ligand hemilability in which a weakly chelating group tunes in between the coordinated and uncoordinated stages could effectively stabilize the reactive intermediates, thereby facilitating substrate activation and catalysis. Redox non-innocent ligands acting as an electron sink, thereby helping the metal center in steps gaining or losing electrons, and non-classical metal-ligand cooperativity has also played a significant role in the ligand design for manganese catalysis. The strategies were not only employed for the chemoselective hydrogenation of different reducible functionalities but also for the C-X (X = C/N) coupling reactions via HT and downstream cascade processes. This article features multifunctional ligand-based manganese complexes, highlighting the importance of ligand design and choice of ancillary ligands for achieving the desired catalytic activity and selectivity for HT reactions. We have also discussed the detailed reaction pathways for metal complexes involving bifunctionality, hemilability, redox activity, and indirect metal-ligand cooperativity. The synthetic utilization of those complexes in different organic transformations has also been detailed.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

Progress in the preparation and characterization of silylene iron, cobalt and nickel complexes

The synthesis and characterization of Fe, Co and Ni complexes supported by silylene ligands in the past ten years are summarized. Due to the decrease of the electron cloud density on the Si atom after coordination, the downfield shift of the 29Si chemical shift is accompanied by the coordination between the free silylene ligand and metal. The strong electron-donating ability of silylene makes the metal center more electron-rich, which is conducive to the oxidative addition reaction in the metal center. In some cases, the coordination ability of silylene is stronger than those of phosphine and carbene ligands. Therefore, silylene transition metal complexes have better catalytic activity. The further challenges in this field are to develop new polydentate silylene ligands, synthesize chelate silylene-phosphine and silylene-carbene ligands, and design new silylene transition metal complexes for more catalytic research.

Catalytic Transfer Hydrogenation of Arenes and Heteroarenes

Transfer hydrogenation reactions are of great interest to reduce diverse molecules under mild reaction conditions. To date, this type of reaction has only been successfully applied to alkenes, alkynes and polarized unsaturated compounds such as ketones, imines, pyridines, etc. The reduction of benzene derivatives by transfer hydrogenation has never been described, which is likely due to the high energy barrier required to dearomatize these compounds. In this context, we have developed a catalytic transfer hydrogenation reaction for the reduction of benzene derivatives and heteroarenes to form complex 3-dimensional scaffolds bearing various functional groups at room temperature without needing compressed hydrogen gas.

Aminolysis of bis[bis(trimethylsilyl)amido]-manganese, -iron, and -cobalt for the synthesis of mono- and bis-silylene complexes

We report a method for the synthesis of amidinate stabilized silylene (NHSi) metal-halide complexes from M{N(SiMe3)2}2 (M = Mn, Fe, Co). The reported reactions can be used to make mono-silylene or bis-silylene complexes and the resulting products can be easily controlled by the reaction stoichiometry. Additionally, we apply this new synthetic protocol for the synthesis of a bis-iron complex derived from a bis-amidinate ligand having a terphenyl backbone.

C(sp2)-H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp2)-H Activation and Catalytically Relevant Intermediates

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp²)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( ^Ar SiNSi)CoH₃]·NaHBEt₃ ( ^Ar SiNSi = 2,6-[EtNSi(N^tBu)₂CAr]₂C₅H₃N, Ar = C₆H₅ (1-H ₃ ·NaHBEt ₃ ), 4-MeC₆H₄ (2-H ₃ ·NaHBEt ₃ )) and trans-[( ^Ar SiNSi)Co(H)₂BPin] (Ar = C₆H₅ (1-(H) ₂ BPin), 4-MeC₆H₄ (2-(H) ₂ BPin), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp²)-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, 2-H ₃ ·NaHBEt ₃ and 2-(H) ₂ BPin also promoted C(sp²)-H activation of benzofuran, yielding [(^ArSiNSi)CoH(Bf)₂] (Ar = 4-MeC₆H₄, 2-H(Bf) ₂ , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by ¹H NMR spectroscopy established the trans-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Phosphane-functionalized heavier tetrylenes: synthesis of silylene- and germylene-decorated phosphanes and their reactions with Group 10 metal complexes

The stable phosphane-functionalized heavier tetrylenes E(tBu2bzam)pyrmPtBu2 (E = Si (1Si), Ge (1Ge); tBu2bzam = N,N'-ditertbutylbenzamidinate; HpyrmPtBu2 = ditertbutyl(2-pyrrolylmethyl)phosphane) have been prepared by reacting the amidinatotetrylenes E(tBu2bzam)Cl (E = Si, Ge) with LipyrmPtBu2. The reactions of 1Si and 1Ge with selected M0 and MII (M = Ni, Pd, Pt) metal precursors have allowed the synthesis of square-planar [MCl2{κ2E,P-E(tBu2bzam)pyrmPtBu2}] (M = Ni, Pd, Pt; E = Si, Ge), tetrahedral [Ni{κ2E,P-E(tBu2bzam)pyrmPtBu2}(cod)] (E = Si, Ge; cod = 1,5-cyclooctadiene) and triangular [M{κ2E,P-E(tBu2bzam)pyrmPtBu2}(PPh3)] (M = Pd, Pt; E = Si, Ge) complexes, showing that 1Si and 1Ge are excellent Si,P- and Ge,P-chelating ligands that, due to their large steric bulk, are able to stabilize three-coordinate Pd0 and Pt0 complexes.

Ruthenium-Catalyzed E-Selective Alkyne Semihydrogenation with Alcohols as Hydrogen Donors

Selective direct ruthenium-catalyzed semihydrogenation of diaryl alkynes to the corresponding E-alkenes has been achieved using alcohols as the hydrogen source. The method employs a simple ruthenium catalyst, does not require external ligands, and affords the desired products in > 99% NMR yield in most cases (up to 93% isolated yield). Best results were obtained using benzyl alcohol as the hydrogen donor, although biorenewable alcohols such as furfuryl alcohol could also be applied. In addition, tandem semihydrogenation–alkylation reactions were demonstrated, with potential applications in the synthesis of resveratrol derivatives.

Homogeneous catalytic transfer semihydrogenation of alkynes – an overview of hydrogen sources, catalysts and reaction mechanisms

Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks.

Transfer hydrogenation of alkynes into alkenes by ammonia borane over Pd-MOF catalysts

Ammonia borane with both hydridic and protic hydrogens in its structure acted as an efficient transfer hydrogenation agent for selective transformation of alkynes into alkenes in non-protic solvents.