Carbonylation of Olefins under Mild Temperature Conditions in the Presence of Palladium Complexes

During a search for catalysts that allow carbonylation reactions on olefins to proceed below 100 °C, complex palladium(II ) compounds having the formula L_mPdX_n were found to be catalytically active. L denotes a ligand such as a phosphine, nitrile, amine, or olefin, X is an acid residue, and m+n is 3 or 4. The catalysts permit the carbonylation of heat-sensitive compounds, as well as selective carbonylation of polyunsaturated olefins. The new process can also be carried out on the industrial scale, as is shown by the carbonylation of cyclododecatriene     

Imidazol(in)ium-2-Carboxylates as N-Heterocyclic Carbene Ligand Precursors for Ruthenium Metathesis Initiators

Summary: Imidazol(in)ium-2-carboxylates were used as N-heterocyclic carbene (NHC) ligand precursors to convert the [RuCl₂(p-cymene)]₂ dimer into three ruthenium-arene complexes of the [RuCl₂(p-cymene)(NHC)] type. The decarboxylation of NHC · CO₂ betaines also provided a convenient synthetic path to prepare five well-known ruthenium-NHC catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, via ligand exchange with phosphine-containing, first generation ruthenium-benzylidene or indenylidene complexes. Both procedures are particularly attractive from a practical point of view, because NHC · CO₂ adducts are stable zwitterionic compounds that can be stored and handled with no particular precautions.     

Recyclable and eco-friendly Pd-complexed graphene oxide/N-heterocyclic carbene catalyst for various coupling reactions in aqueous phase

In this study, we report recyclable Pd-complexed graphene oxide (GO) catalysts with a bulky N-heterocyclic carbene (NHC) ligand (GO/NHC-Pd) for various cross-coupling reactions in the aqueous phase. To prepare GO/NHC-Pd, the NHC precursor and a trimethoxy-silane linker were combined on the GO surface via condensation, followed by the Pd chelation over GO/NHC. The GO-based catalysts were applied to three types of CC cross-coupling reactions including Suzuki, Heck and Sonogashira reactions in the aqueous phase, to evaluate their catalytic performance. The reusability of the catalysts was tested by performing five consequent cross-coupling reactions.     

On the way to biodegradable poly(hydroxy butyrate) from propylene oxide and carbon monoxide via β-butyrolactone: Multisite catalysis with newly designed chiral indole-imino chromium(III) complexes

Enantioenriched poly(hydroxy butyrate) (PHB) is a biodegradable polyester of significant commercial interest as an environmentally benign substitute of commodity polyolefines. We report on the design and development of new chiral indole-based ligand families and on their chromium(III) complexes as enantioselective catalysts for the conversion of propylene oxide and carbon monoxide to enantioenriched β-butyrolactone, the key monomer for the production of PHB by ring-opening polymerization. The enantioselective carbonylation catalysts are based on new chiral tri- and tetradentate [N₂O] and [N₄] chromium(III) complexes containing chiral indolaldimine ligand scaffolds. The conceptual design of these ligands is inspired by Jacobsen’s salicylaldimine lead structure; the key difference is an exchange of the salicyl-O-donor against an indole-N-donor, allowing additional structural diversity and stereoelectronic tuning by the indole substitution pattern. Synthetically, chiral indolealdimines are easily accessible from 7-formylindoles by standard Schiff base condensation with chiral amine building blocks; the 7-formylindoles in turn are synthesized from the corresponding 7-bromoindoles by the Rapoport synthesis, and the starting 7-bromoindoles are accessible from 2-bromoaniline by the classical Fischer indole synthesis. Three generations of chiral [N₂O] and [N₄] chromium(III) catalysts have been developed and evaluated in the enantioselective carbonylation of racemic propylene oxide with carbon monoxide using tetracarbonylcobaltate as the nucleophilic reagent for the insertion of carbon monoxide into the activated propylene oxide/chiral Lewis acid complex. The best catalyst out of 10 candidates showed at a temperature of 80 °C an activity of 37% conversion, 100% chemoselectivity, and 19% stereoselectivity.     

A Three‐Membered Cyclic Phosphasilene

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N‐heterocyclic carbene (NHC). First reactivity studies show a) that the PSi₂ ring is a competent ligand to the Fe(CO)₄ fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh₃ results in the rapid head‐to‐head or head‐to‐tail dimerization of the PSi₂ unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.     

Predictive Catalysis in Olefin Metathesis with Ru-based Catalysts with Annulated C60 Fullerenes in the N-heterocyclic Carbenes

Predictive catalysis must be the tool that does not replace experiments, but acts as a selective agent, so that synthetic strategies of maximum profitability are used in the laboratory in a surgical way. Here, nanotechnology has been used in olefin metathesis from homogeneous Ru-NHC catalysts, specifically annulating a C₆₀ fullerene to the NHC ligand. Based on results with the C₆₀ in the backbone, a sterile change with respect to the catalysis of the metal center, an attempt has been made to bring C₆₀ closer to the metal, by attaching it to one of the two C−N bonds of the imidazole group of the SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) ligand (reference NHC ligand of the 2^nd generation Grubbs catalysts) to increase the steric pressure of C₆₀ in the first sphere of reactivity of the metal. The DFT calculated thermodynamics and the kinetics of SIMes-derived systems show that they are efficient catalysts for olefin metathesis.     

[(NHC)(NHCewg)RuCl2(CHPh)] Complexes with Modified NHCewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Imidazolium salts (NHC_ewg ? HCl) with electronically variable substituents in the 4,5‐position (H,H or Cl,Cl or H,NO₂ or CN,CN) and sterically variable substituents in the 1,3‐position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)_ewg] complexes. The reactions of [(NHC)RuCl₂(CHPh)(py)₂] with the [AgI(NHC_ewg)] complexes provide the respective [(NHC)(NHC_ewg)RuCl₂(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring‐closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2–0.5 mol % at 80 °C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC_ewg group with 1,3‐Me,iPr and 4,5‐Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL‐NHC)(NHC_ewg)RuCl₂(CHPh)] and [(FL‐NHC_ewg)(NHC)RuCl₂(CHPh)], with a dansyl fluorophore (FL)‐tagged electron‐rich NHC ligand (FL‐NHC) and an electron‐deficient NHC ligand (FL‐NHC_ewg) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore‐tagged ligand. In this manner, it was shown for ring‐opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC_ewg ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.     

Direct in situ synthesis of cationic N-heterocyclic carbene iridium and rhodium complexes from neat ionic liquid: Application in catalytic dehydrogenation of cyclooctadiene

A direct synthetic route to cationic N-heterocyclic carbene (NHC) complexes of rhodium and iridium from neat dialkyl-imidazolium ionic liquids (ILs) has been found. The method uses complexes bearing basic anionic ligands, [M(COD)(PPh₃)X], X = OEt, MeCO₂, which react with the inactivated imidazolium cation in the absence of external bases yielding one M-NHC moiety and the free protonated base. This new one-pot synthesis leaving pure, catalytically active IL solutions is faster, cleaner and more efficient than traditional syntheses of such NHC complexes. The observed reactivity also gives insight into NHC incorporation of rhodium and iridium catalyzed reactions performed in common dialkyl-imidazolium ILs.The complexes synthesised in this manner are compared with their bis-phosphine analogues in terms of activity for catalytic dehydrogenation of 1,5-cyclooctadiene and 1,3-cyclooctadiene in neat [BMIM][NTf₂] as solvent. Even at high temperature, no ligand exchange reaction is observed with [(COD)M(PPh₃)₂] [NTf₂] catalysts. As expected, the yields of all the reactions were low, iridium was much more active in C-H activation than rhodium and the NHC ligands were more stable than triphenylphosphine. For all catalysts, the isomerisation of 1,5-cyclooctadiene is the major reaction. However, the phosphine-NHC complex of iridium seems to be more selective for dehydrogenation than its bis-phosphine counterpart, which is more active in transfer-hydrogenation and less stable under the applied conditions. Different reaction conditions were tried in order to optimise selectivity for dehydrogenation over isomerisation and transfer-hydrogenation. Surprisingly, with 1,3-cyclooctadiene as substrate selectivity for dehydrogenation is much higher than with 1,5-cyclooctadiene for all catalysts.     

Polyisobutylene-supported N-heterocyclic carbene palladium catalysts

This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an N,N′-bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N′-dialkyl heterocyclic carbene ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The reactivity, recycling and reusability of these catalysts has been examined.     

Olefin metathesis catalyst: stabilization effect of backbone substitutions of N-heterocyclic carbene

Ruthenium olefin metathesis catalysts bearing an N-phenyl-substituted N-heterocyclic carbene (NHC) ligand that are resistant to decomposition through C-H activation have been prepared and tested in ring closing metathesis (RCM), cross metathesis (CM), and ROMP reactions. The N, N'-diphenyl-substituted NHC complex proved to be one of the most efficient catalysts in RCM to form tetrasubstituted olefins.     

Use of tetrahydropyrimidinium salts for highly efficient palladium-catalyzed cross-coupling reactions of aryl bromides and chlorides

New, sterically demanding 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (2) as NHC precursors have been synthesized and characterized. These salts, in combination with palladium acetate, provided active catalysts for the cross-coupling of aryl chlorides and bromides under mild conditions. The catalytic system was applied to the Heck, Suzuki and benzaldehyde (Kumada) coupling reactions. Catalyst activity was found to be influenced by the presence of a methoxy group on the ring of the p-position of benzyl substituent of the ligand precursor.     

1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): Electronic effects on cross-coupling reactions and thermal decompositions

The synthesis of 1,3-diarylimidazolidin-2-ylidene (NHC) precursor, 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, (3b) has been extended to the electronically and sterically modified NHC precursors 3a (X = H), 3c (X = Br) and 3e (X = Cl) in order to investigate the electronic effect of a p-substituent (X) on cross-coupling catalysts. Complexes of the type PdCl₂(NHC)₂ (5), PdCl₂(NHC)(PPh₃) (6) and [RhCl(NHC)(cod)] (7) were prepared from 3 or 4d (1,3-bis(2,4-dimethylphenyl)-2-trichloromethylimidazolidin). Initial decomposition temperatures of the complexes 5 and 6 were determined by TGA. In situ formed complexes from Pd(OAc)₂ and 3 as well as the preformed complexes 5 and 6 have been tested as catalysts in coupling of phenylboronic acid with 4-haloacetophenones. The electron donating ability of NHCs derived from 3 was assessed by measuring C-O frequencies in the respective [RhCl(NHC)(CO)₂] complex 8 which was prepared by replacement of cod ligand of 7 with CO. An interesting correlation between the electron-donating nature of the aryl substituent and catalytic activity and also initial decomposition temperature of the complexes 5 and 6 was observed.     

Synthesis of Pd complexes bearing an enantiomerically resolved seven-membered N-heterocyclic carbene ligand and initial studies of their use in asymmetric Wacker-type oxidative cyclization reactions

The development of enantiomerically resolved, axially-chiral seven-membered N-heterocyclic carbene (⁷NHC) ligands for palladium is described. These ⁷NHC ligands are derived from enatiomerically pure 2,2′-diamino-6,6′-dimethylbiphenyl, which is transformed via a synthetic sequence consisting of ortho-arylation, N-alkylation, and cyclization to afford seven-membered-ring amidinium salts. Synthesis of the seven-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields ⁷NHC-Pd^II complexes. The chiral ⁷NHC-Pd complexes were examined as chiral catalysts under aerobic conditions in two intramolecular oxidative amination reactions of alkenes. In one case, enantioselectivities up to 63% ee were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a Pd^II catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands.     

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt₃ adducts. By applying complexes 3 with KHBEt₃ as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt₃ as cocatalyst.     

Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt–terpyridine complexes

Understanding the activity and selectivity of molecular catalysts for CO₂ reduction to fuels is an important scientific endeavour in addressing the growing global energy demand. Cobalt–terpyridine compounds have been shown to be catalysts for CO₂ reduction to CO while simultaneously producing H₂ from the requisite proton source. To investigate the parameters governing the competition for H⁺ reduction versus CO₂ reduction, the cobalt bisterpyridine class of compounds is first evaluated as H⁺ reduction catalysts. We report that electronic tuning of the ancillary ligand sphere can result in a wide range of second-order rate constants for H⁺ reduction. When this class of compounds is next submitted to CO₂ reduction conditions, a trend is found in which the less active catalysts for H⁺ reduction are the more selective towards CO₂ reduction to CO. This represents the first report of the selectivity of a molecular system for CO₂ reduction being controlled through turning off one of the competing reactions. The activities of the series of catalysts are evaluated through foot-of-the-wave analysis and a catalytic Tafel plot is provided.     

Synthesis and Application of Water‐Soluble NHC Transition‐Metal Complexes

We provide an overview on the state‐of‐the‐art in transition‐metal complexes formed with water‐soluble NHC ligands. Paths to introducing water solubility by ligand design are elucidated and some general properties of water‐soluble NHC complexes are highlighted. The enhanced hydrophilicity of water‐soluble catalysts offers advantages in applications. While studies based on C? C coupling reactions still dominate the field, recent reports show water‐soluble NHC complexes can be applied in metathesis and hydrogenation reactions and turn out to be among the best performing catalysts known. Nevertheless, wide areas of this young field remain to be investigated, offering great potential for future research.     

Phosphine‐NHC Manganese Hydrogenation Catalyst Exhibiting a Non‐Classical Metal‐Ligand Cooperative H2 Activation Mode

Deprotonation of the Mn^I NHC‐phosphine complex fac‐[MnBr(CO)₃(κ²P,C‐Ph₂PCH₂NHC)] ( 2 ) under a H₂ atmosphere readily gives the hydride fac‐[MnH(CO)₃(κ²P,C‐Ph₂PCH₂NHC)] ( 3 ) via the intermediacy of the highly reactive 18‐e NHC‐phosphinomethanide complex fac‐[Mn(CO)₃(κ³P,C,C‐Ph₂PCHNHC)] ( 6 a ). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16‐e mangana‐substituted phosphonium ylide complex fac‐[Mn(CO)₃(κ²P,C‐Ph₂P=CHNHC)] ( 6 b ) as key intermediate able to activate H₂ via a non‐classical mode of metal‐ligand cooperation implying a formal λ⁵‐P–λ³‐P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre‐catalysts for ketone hydrogenation in the Mn^I series reported to date (TON up to 6200).     

Chelated ruthenium catalysts for Z-selective olefin metathesis

We report the development of ruthenium-based metathesis catalysts with chelating N-heterocyclic carbene (NHC) ligands that catalyze highly Z-selective olefin metathesis. A very simple and convenient procedure for the synthesis of such catalysts has been developed. Intramolecular C-H bond activation of the NHC ligand, promoted by anion ligand substitution, forms the appropriate chelate for stereocontrolled olefin metathesis.     

Synthetic and structural studies of NHC–Pt(dvtms) complexes and their application as alkene hydrosilylation catalysts (NHC = N-heterocyclic carbene,dvtms = divinyltetramethylsiloxane)

The synthesis and structural characterization of a series of platinum complexes, bearing N-heterocyclic carbenes (NHC) and divinyltetramethylsiloxane (dvtms) as supporting ligands, are described. The reaction of commercially available Karstedt’s catalyst (Pt₂{(η²-ViSiMe₂)₂O}₃) with in situ generated NHC leads to monomeric platinum(0) complexes in which one NHC is bound to the metal center, as indicated by spectroscopic analysis and single-crystal X-ray diffraction studies. The relative reactivity trend for these complexes as catalysts for the hydrosilylation of alkenes is discussed in terms of NHC ligand steric properties.     

N-Heterocyclic Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

A study on the reactivity of the N-heterocyclic silylene Dipp₂NHSi (1,3-bis(diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-yliden) with the transition metal complexes [Ni(CO)₄], [M(CO)₆] (M=Cr, Mo, W), [Mn(CO)₅(Br)] and [(η⁵-C₅H₅)Fe(CO)₂(I)] is reported. We demonstrate that N-heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi-bridged complex [{Ni(CO)₂(μ-Dipp₂NHSi)}₂] ( 2 ), complexes [M(CO)₅(Dipp₂NHSi)] (M=Cr 3 , Mo 4 , W 5 ), [Mn(CO)₃(Dipp₂NHSi)₂(Br)] ( 9 ) and [(η⁵-C₅H₅)Fe(CO)₂(Dipp₂NHSi-I)] ( 10 ). DFT calculations on several model systems [Ni(L)], [Ni(CO)₃(L)], and [W(CO)₅(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ- and π-interaction depends significantly on the system under investigation. The σ-interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π-interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ-donor ligand, and contributions of π-symmetry play only a minor role for the NHC and NHSi co-ligands.