Transfer Semihydrogenation Research Articles

Transfer Semihydrogenation of Alkynes Catalyzed by Imidazo[1,5-a]pyrid-3-ylidene–Pd Complexes: Positive Effects of Electronic and Steric Features on N-Heterocyclic Carbene Ligands

Abstract To investigate the catalytic utility of the imidazo[1,5-a]pyrid-3-ylidene (IPC) ligand, Pd-catalyzed transfer semihydrogenation of alkynes with formic acid as a hydrogen source was conducted. The steric bulkiness of the substituent on N2 affected the configuration of the π-allyl moiety of the precatalyst of IPC-Pd-π-allyl complexes and the robustness of the catalytic process. The catalytic activities of IPC-Pd complexes were clearly higher than those of conventional NHC-Pd complexes.

Highly stereoselective heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes to access cis-alkenes

A highly efficient heterogeneous palladium-catalyzed transfer semihydrogenation of internal alkynes was achieved in dimethylformamide at 145 ºC by using an MCM-41-immobilized bidentate nitrogen palladium(II) complex [MCM-41-2N-Pd(OAc)2] as catalyst and dimethylformamide/KOH as hydrogen source, yielding a variety of cis-alkenes in good to high yields with excellent stereoselectivity. This supported palladium catalyst can be easily recovered by filtration of the reaction solution and recycled up to seven times with almost consistent activity.

Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes.

Palladium-catalyzed transfer semihydrogenation of alkynes using H2O as the hydrogen source and Mn as the reducing reagent is developed, affording cis- and trans-alkenes selectively under mild conditions. In addition, this method provides an efficient way to access various cis-1,2-dideuterioalkenes and trans-1,2-dideuterioalkenes by using D2O instead of H2O.

Using alcohols as simple H2-equivalents for copper-catalysed transfer semihydrogenations of alkynes.

Copper(i)/N-heterocyclic carbene complexes enable a transfer semihydrogenation of alkynes employing simple and readily available alcohols such as isopropanol. The practical overall protocol circumvents the use of commonly employed high pressure equipment when using dihydrogen (H2) on the one hand, and avoids the generation of stoichiometric silicon-based waste on the other hand, when hydrosilanes are used as terminal reductants.

Highly Chemo- and Stereoselective Transfer Semihydrogenation of Alkynes Catalyzed by a Stable, Well-Defined Manganese(II) Complex

Herein we report unprecedented manganese-catalyzed semihydrogenation of internal alkynes to (Z)-alkenes using ammonia borane as a hydrogen donor. The reaction is catalyzed by a pincer complex of the earth-abundant manganese(II) salt in the absence of any additives, base, or superhydride. The ammonia borane smoothly reduces the manganese precatalyst [Mn(II)–PNP][Cl]2 to the catalytically active species [Mn(I)–PNP]–hydride in the triplet spin state. This manganese hydride is highly stabilized by complexation with the alkyne substrate. Computational density functional theory (DFT) analysis studies of the reaction mechanism rationalize the origin of stereoselectivity toward formation of (Z)-alkenes.

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

The synthesis and structures of the first SiII -donor supported manganese(II) complexes [L1]MnCl2 , [L2]MnCl2 , and [L3]2 MnCl2 are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1 mol % loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl2 shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98 %) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH2 , NO2 , and OH groups at the phenyl group of 1-phenyl substituted alkynes.

E- and Z-Selective Transfer Semihydrogenation of Alkynes Catalyzed by Standard Ruthenium Olefin Metathesis Catalysts.

Selective transfer semihydrogenation of alkynes to yield alkenes was achieved with commercial first and second generation Hoveyda-Grubbs catalysts and formic acid as a hydrogen donor. This catalytic system is distinguished by its selectivity and compatibility with many functional groups (halogens, cyano, nitro, sulfide, alkenes). The metathetic activity of the ruthenium catalysts may be utilized in tandem sequences of olefin metathesis plus alkyne reduction.

ChemInform Abstract: Efficient E‐Selective Transfer Semihydrogenation of Alkynes by Means of Ligand‐Metal Cooperating Ruthenium Catalyst.

AbstractA practical and synthetically flexible protocol for the efficient E‐selective semihydrogenation of alkynes employing a ruthenium complex bearing a bifunctional PC(sp3)P pincer ligand is developed.

Efficient E‐Selective Transfer Semihydrogenation of Alkynes by Means of Ligand‐Metal Cooperating Ruthenium Catalyst

AbstractThe manuscript describes an efficient protocol for the E‐selective transfer semihydrogenation of internal alkynes employing a previously described bifunctional ruthenium‐based PC(sp3)P complex, 20 mol% of sodium formate and nearly stoichiometric formic acid. Semihydrogenation of the terminal alkynes results in the formation of the corresponding styrenes. The mechanism of the reaction includes stepwise transfer of both the hydride and the proton of the formic acid to the substrate. This fact allowed for the development of a facile protocol for the synthesis of monodeuterated alkenes by simply applying formic acid/sodium formate/D2O mixture as a hydrogen source. High yields, selectivity and functional group compatibility have been generally achieved.magnified image

Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

AbstractA protocol was developed to distinguish between well‐defined molecular and nanoparticle‐based catalysts for the Pd‐catalyzed semihydrogenation reaction of alkynes to Z‐alkenes. The protocol applies quantitative partial poisoning and dynamic light scattering methods, which allow the institution of additional validation experiments. For the quantitative partial poisoning method, tetramethylthiourea (TMTU) was developed as an alternative for the standard poison ligand CS2, and was found to be superior in its applicability. The protocol and the TMTU poison ligand were validated using the well‐described [PdII(phenanthroline)]‐catalyzed copolymerization of styrene and CO, confirming that this system is clearly operating as a well‐defined molecular catalyst. The protocol was subsequently applied to three catalyst systems used for the semihydrogenation of alkynes. The first was proposed to be a molecular [Pd0(IMes)] catalyst that uses molecular hydrogen, but the data gathered for this system, following the new protocol, clearly showed that nanoparticles (NPs) are catalytically active. The second catalyst system studied was an N‐heterocyclic carbene (NHC) Pd system for transfer semihydrogenation using formic acid as the hydrogen source, which was proposed to operate through an in situ generated molecular [Pd0(IMes)] catalyst in earlier studies. The investigations showed that only a small fraction of the Pd added becomes active in the catalytic reaction and that NPs are formed. However, despite these findings, a clear distinction between catalytic activity of NPs versus a molecular catalyst could not be made. The third investigated system is based on a [PdII(IMes)(η3‐allyl)Cl] precatalyst with additive ligands. The combined data gathered for this system are multi‐interpretable, but suggest that a partially deactivated molecular catalyst dominates in this reaction.

Selective Transfer Semihydrogenation of Alkynes Using AuNPore

Selective Transfer Semihydrogenation of Alkynes Using AuNPore

Allylpalladium(II) Histidylidene Complexes and Their Application in Z ‐Selective Transfer Semihydrogenation of Alkynes

We have studied the use of amino acid histidine as a precursor for N-heterocyclic carbene (NHC) ligands. This natural amino acid possesses an imidazole substituent, which makes it an interesting NHC precursor that contains both an acid and an amino functionality. These functionalities may be used for further tuning of NHC complexes. We have developed routes for the synthesis of symmetric and dissymmetric alkyl, benzyl, and aryl-substituted histidinium salts. Subsequently, the corresponding Ag and Pd histidylidenes were synthesized and the palladium complexes were tested in the Z-selective transfer semihydrogenation of alkynes. Histidylidene palladium complexes that contain additional donor functionalities were found to display good selectivities. The best catalytic results were obtained with a Pd-histidylidene complex that contains two picolyl functional groups.

Selective transfer semihydrogenation of alkynes with nanoporous gold catalysts.

A facile, highly chemo- and stereoselective transfer semihydrogenation of alkynes to Z-olefins has been achieved by use of unsupported nanoporous gold (AuNPore) as a heterogeneous catalyst together with formic acid as a hydrogen donor. A variety of terminal/internal and aromatic/aliphatic alkynes were reduced to the corresponding alkenes in high chemical yields with good functional-group tolerance. The catalyst is robust enough to be reused without leaching.

Convenient Transfer Semihydrogenation Methodology for Alkynes Using a PdII-NHC Precatalyst

A convenient and easy-to-use protocol for the Z-selective transfer semihydrogenation of alkynes was developed, using ammonium formate as the hydrogen source and the easily prepared and commercially available, highly stable complex PdCl(η3-C3H5)(IMes) (1) as the (pre)catalyst. Combined with triphenyl posphine as an additional ligand, this system provides a robust catalytic synthetic method that shows little to no over-reduction or isomerization after full substrate conversion. The system allows the direct use of solvents and reagents, as received from the supplier without drying or purification, thus providing a practical method for semihydrogenation of a broad range of alkynes. The mechanism behind these high and enhanced selectivities was determined through a set of kinetic experiments.

Improved Practical Synthesis of the Sex Pheromone of Female Sweet Potato Weevil, Cylas Formicarius

An improved synthesis of (Z)-dodec-3-en-1-yl (E)-2-butenoate (1), the female sex pheromone of sweet potato weevil, Cylas Formicarius, was reported. The synthesis bypassed the utilization of carcinogenic ethylene oxide, highly toxic HMPA, highly hydroscopic and unrecyclable DMPU, ethylene diamine, and liquid ammonia, for the alkylation of terminal alkyne. Instead, NaI was used to serve as a good nucleophile for alkyl bromide and a good leaving group for 1-lithiated alkyne. A safer and stereoselective palladium-catalyzed transfer semihydrogenation, using KOH/DMF as the hydrogen source, was employed for alkyne cis-hydrogenation, without using molecular hydrogen. The total yield was 62% over 5 steps, and the sex pheromone 1 obtained was verified spectroscopically. Field test results showed that the synthetic sex pheromone 1 was as effective as the commercial source or the sample provided by other research group.

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

A transmetallation route, using silver(I) precursors, to several zero- and di-valent palladium complexes with chelating bis(N-heterocyclic carbene) ligands bearing various N-substituents has been established. The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the structure of a representative compound, [Pd(0)(bis-(Mes)NHC)(η(2)-ma)] (3a), was confirmed by X-ray crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity was observed, complex 3a showed activity (TOF = 49 mol(sub) mol(cat)(-1) h(-1)) and selectivity comparable to its monodentate counterparts in the semihydrogenation of 1-phenyl-1-propyne with molecular hydrogen.

Nickel-Catalyzed Transfer Semihydrogenation and Hydroamination of Aromatic Alkynes Using Amines As Hydrogen Donors

The transfer hydrogenation of diphenylacetylene to yield cis- and trans-stilbenes was achieved using a variety of amines as hydrogen donors and the complex 1 ([(dippe)Ni(μ-H)]2) in catalytic amounts (0.5% mol). The use of nucleophilic amines such as pyrrolidine in neat conditions afforded the hydroamination of diphenylacetylene, in moderate to high yields. Cyclization of 2-ethynylaniline also was carried out under similar conditions, with 1 in catalytic amounts, but in low yield, mainly due to the formation of homocoupling products of the starting material. The hydrogenation of diphenylacetylene by using other nitrogenated compounds such as aromatic N-heterocycles was addressed to give a metal-mediated process, using 1 in stoichiometric amounts.

Stereodivergent Ruthenium‐Catalyzed Transfer Semihydrogenation of Diaryl Alkynes

[Ru(3)(CO)(12)]-catalyzed transfer semihydrogenation of various functionalized diaryl alkynes with N,N-dimethylformamide (DMF) and water as hydrogen source affords cis- and trans-stilbenes. The stereodivergent approach can be switched by the use of acetic (HOAc) or trifluoroacetic (TFA) acid as additives. The catalytic processes can be applied to the synthesis of analogues of natural products such as cis-combretastatin A-4 and trans-resveratrol.

Rigid pyridyl substituted NHC ligands, their Pd(0) complexes and their application in selective transfer semihydrogenation of alkynes

AbstractThe synthesis of an air‐stable series of Pd0 complexes with unsymmetric bidentate N‐pyridine N‐heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, followed by transmetallation of the C‐N ligands to obtain the target electron‐rich zerovalent palladium compounds. The bidentate coordination behaviour of the ligands was confirmed by 1H, 13C NMR and X‐ray spectroscopy. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z‐alkenes, with selectivities up to 99%. Copyright © 2011 John Wiley & Sons, Ltd.

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo- and stereoselectivity when using a zerovalent palladium(NHC)(maleic anhydride)-complex as precatalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuterium-labeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR, we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations, we propose a mechanism in which hydrogen transfer from coordinated formate anion to zerovalent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.