Palladium‐Catalyzed Oxygenative Cross‐Coupling of Ynamides and Benzyl Bromides by Carbene Migratory Insertion

A palladium‐catalyzed oxygenative cross‐coupling of ynamides and benzyl bromides has been developed. After subsequent hydrogenation, α,α‐disubstituted amide derivatives were obtained in good yields. Migratory insertion of α‐oxo palladium carbene species, generated by intermolecular oxidation, is proposed as the key step in this reaction. The study demonstrates the potential of ynamides to serve as carbene precursors in palladium‐catalyzed C?C bond‐forming cross‐coupling reactions.     

Palladium‐Catalyzed Intramolecular Carbene Insertion into C(sp3)−H Bonds

A palladium‐catalyzed carbene insertion into C(sp³)?H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium‐catalyzed C(sp³)?C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)?H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.     

CH–π and CF–π Interactions Lead to Structural Changes of N‐Heterocyclic Carbene Palladium Complexes

The role of CH–π and CF–π interactions in determining the structure of N‐heterocyclic carbene (NHC) palladium complexes were studied using ¹H NMR spectroscopy, X‐ray crystallography, and DFT calculations. The CH–π interactions led to the formation of the cis‐anti isomers in 1‐aryl‐3‐isopropylimidazol‐2‐ylidene‐based [(NHC)₂PdX₂] complexes, while CF–π interactions led to the exclusive formation of the cis‐syn isomer of diiodobis(3‐isopropyl‐1‐pentafluorophenylimidazol‐2‐ylidene) palladium(II).     

Enantioselective Palladium‐Catalyzed Carbene Insertion into the N−H Bonds of Aromatic Heterocycles

C3‐substituted indoles and carbazoles react with α‐aryl‐α‐diazoesters under palladium catalysis to form α‐(N‐indolyl)‐α‐arylesters and α‐(N‐carbazolyl)‐α‐arylesters. The products result from insertion of a palladium‐carbene ligand into the N−H bond of the aromatic N‐heterocycles. Enantioselection was achieved using a chiral bis(oxazoline) ligand, in many cases with high enantioselectivity (up to 99 % ee). The method was applied to synthesize the core of a bioactive carbazole derivative in a concise manner.     

A Chiral Macrocyclic Tetra-N-Heterocyclic Carbene Yields an “All Carbene” Iron Alkylidene Complex

The first chiral macrocyclic tetra-N-heterocyclic carbene (NHC) ligand has been synthesized. The macrocycle, prepared in high yield and large scale, was ligated onto palladium and iron to give divalent C₂-symmetric square planar complexes. Multinuclear NMR and single crystal X-ray diffraction demonstrated that there are two distinct NHCs on each ligand, due to the bridging chiral cyclohexane. Oxidation of the iron(II) complex with trimethylamine N-oxide yielded a bridging oxo complex. Diazodiphenylmethane reacted with the iron(II) complex at room temperature to give a paramagnetic diazoalkane complex; the same reaction yielded the “all carbene” complex at elevated temperature. Electrochemical measurements support the assignment of the “all carbene” complex being an alkylidene. Notably, the diazoalkane complex can be directly transformed into the alkylidene complex, which had not been previously demonstrated on iron. Finally, a test catalytic reaction with a diazoalkane on the iron(II) complex does not yield the expected cyclopropane, but actually the azine compound.     

On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition‐reductive elimination sequences. These reactivities as well as analytical investigations using X‐ray diffraction and X‐ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4‐bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H₂ activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.     

Synergistic N‐Heterocyclic Carbene/Palladium‐Catalyzed Reactions of Aldehyde Acyl Anions with either Diarylmethyl or Allylic Carbonates

Benzylation and allylation of aldehyde acyl anions were enabled by the merger of a thiazolium N‐heterocyclic carbene (NHC) catalyst and a palladium/bisphosphine catalyst in a synergistic manner. Owing to the mildness of the reaction conditions, various functional groups were tolerated in the substrates.     

N-杂环卡宾Pd络合物的合成及其在Buchwald-Hartwig反应中的应用

合成鉴定了二个N-杂环卡宾环金属Pd络合物[1,3-N,N’-双{2,6-二取代基苯基}咪唑基-2-卡宾][N,N-二甲基卞胺-2-C,N]氯化钯(Ⅱ)(3a：取代基 = 异丙基；和3b：取代基 = 甲基)，评价了它们在氯代芳烃和各种胺偶联反应中的催化性能。发现NaOBu-t是效果较好的碱，及Pd周围有较大的立体位阻的3a在4-甲基氯苯与吗啡啉反应中较3b有更好的催化性能.     

Robust and Electron‐Rich cis‐Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

A new imidazolinium ligand precursor [L²H]Cl ( 2 ) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L¹H]Cl ( 1 ), 2 is very sensitive to moisture and can undergo ring‐opening reactions very readily. Palladium complexes with the ring‐opened products from imidazolinium salts were isolated and characterized by X‐ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring‐opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis‐[PdCl₂(L)(PR₃)] (L=L¹ and L²; R=Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable‐temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes on varying the carbene group (imidazol‐2‐ylidene group (unsaturated carbene) vs. imidazolin‐2‐ylidene (saturated carbene)) and the phosphine group (PPh₃ vs. PCy₃) were studied in detail by X‐ray crystallography, X‐ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis‐[PdCl₂(L²)(PCy₃)] is a competent Pd^II precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.     

C(sp3)−H Functionalizations Promoted by the Gold Carbene Generated from Vinylidenecyclopropanes

The gold carbene generated from vinylidenecyclopropanes (VDCPs) can smoothly perform a C(sp³)?H bond insertion reaction, stereoselectively affording the intramolecular C(sp³)?H bond functionalized product, benzoxepine, with syn‐configuration in moderate to good yields under mild conditions. The KIE investigation on this bond functionalization partially revealed that the carbene insertion step might be rate‐determining. Using a chiral gold(I) catalyst, the first example on the asymmetric variant of gold carbene insertion into C(sp³)?H bond has been disclosed, giving the desired products with excellent results.     

Carbene tetrel-bonded complexes

An ab initio calculation has been carried for the carbene tetrel bonded complexes CH₃Y???CH₂ (Y = F, CN, NC, and NO₂), CH₃F???CZ₂ (Z = Cl and CH₃), XH₃F???CF₂ (X = C, Si, Ge, and Sn), SiF₄???CF₂, and XH₃F???NHC (N-heterocyclic carbene), where carbene is treated as a Lewis base and XH₃Y is a Lewis acid. Formation of the tetrel bond is mainly attributed to charge transfer from the lone pair on the C atom in the carbene toward the σ* X–Y orbital and also the σ* X–H one in the strong tetrel bond. The carbene tetrel bond is strengthened/weakened by the electron-withdrawing group in the tetrel donor/acceptor and enhanced by the methyl group in C(CH₃)₂. NHC forms a stronger carbene tetrel bond in XH₃F???NHC (X = Si, Ge, and Sn) where it exceeds that of the majority of H-bonds. Interestingly, the tetrel bond becomes stronger in the order of X = C < Ge < Sn < Si in XH₃F???NHC and the largest interaction energy occurs in SiH₃F???NHC, amounting to ?103 kJ/mol. The carbene tetrel bond can be strengthened by cooperative effect with the N???M interaction in trimers H₂C???CH₃CN???M (M = CH₃CN, HCN, ICN, SbH₂F, LiCN, and BeH₂) and has doubled in H₂C???CH₃CN???BeH₂.

     

Rational Exploration of N‐Heterocyclic Carbene (NHC) Palladacycle Diversity: A Highly Active and Versatile Precatalyst for Suzuki–Miyaura Coupling Reactions of Deactivated Aryl and Alkyl Substrates

As less attention has been focussed on the design of highly efficient palladium precatalysts to ensure the smooth formation of the active catalyst for metal‐mediated cross coupling reactions, we herein demonstrate that combining the bulky N‐heterocyclic carbene (NHC) 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with cyclopalladated acetanilide as the optimal palladium precatalyst leads to superior catalytic activity compared with the state‐of‐the‐art NHC–Pd catalysts. The complex was discovered through the evaluation of a small, rationally designed library of NHC–palladacycles prepared by a novel, practical and atom‐economic method, the direct reaction of IPr?HCl with palladacycle acetate dimers.     

Direct C—H Bond Activation of Benzoxazole and Benzothiazole with Aryl Bromides Catalyzed by Palladium(II)‐N‐heterocyclic Carbene Complexes

Herein, we report that a series of novel palladium(II)‐NHC complexes (NHC=N‐heterocyclic carbene) were synthesized. The structures of all novel complexes were characterized by ¹H NMR, ¹³C NMR, FT‐IR spectroscopy and elemental analysis techniques. These palladium(II)‐NHC complexes were tested as efficient catalysts in the direct C—H bond activation of benzoxazole and benzothiazole with aryl bromides in the presence of 1 mol% catalyst loading at 150 °C for 4 h. Under the given conditions, various aryl bromides were successfully applied as the arylating reagents to achieve the 2‐arylbenzoxazoles and 2‐arylbenzothiazoles in acceptable to high yields.     

Pd-N-杂环卡宾化合物催化的Heck反应、Suzuki反应进展

Pd催化的C—C键偶联反应是形成碳—碳单键的重要反应之一.传统上,使用膦化合物为配体来调整催化活性及选择性.但大多数Pd-膦化合物对空气稳定性差,容易被氧化;在溶液中易于解离出膦配体而降低催化剂稳定性,通常需要给反应体系中加入较多的膦配体以保持催化剂的稳定性和活性.1991年发现的稳定N-杂环卡宾(NHC)类配体具有富含电子、给电子能力强,对金属配位能力强,结构易修饰等特点,使得金属-NHC化合物成为金属有机化学、催化等领域研究新的焦点.Pd-NHC化合物已经可催化多类有机反应,是继传统Pd-膦催化剂外的又一类高效催化剂.综述了近年来不同结构的NHC如单齿简单NHC、双齿NHC、含其它配位原子的NHC等配体与Pd的配合物在Heck反应、Suzuki反应等偶联反应中的应用.     

Isolation of a Reactive Tricoordinate α‐Oxo Gold Carbene Complex

The [(P,P)Au=C(Ph)CO₂Et]⁺ complex 3 [where (P,P) is an o‐carboranyl diphosphine ligand] was prepared by diazo decomposition at ?40 °C. It is the first α‐oxo gold carbene complex to be characterized. Its crystallographic structure was determined and DFT calculations have been performed, unraveling the key influence of the chelating (P,P) ligand. The gold center is tricoordinate and the electrophilicity of the carbene center is decreased. Complex 3 mimics transient α‐oxo gold carbenes in a series of catalytic transformations, and provides support for the critical role of electrophilicity in the chemoselectivity of phenol functionalization (O?H vs. C?H insertion).     

Diamino‐Substituted Carbene Transfer between Transition Metal Ions

Diamino‐carbene ligand transfer between various metalions is studied, particularly with Pd (II) to Rh(I), Rh(I) to Au(I). Reactions of various carbene complexes with AgPF₆ result in the cleavage of the M=C bond to give the protonated carbene species, imidazolidin‐2‐ylidinium salt, indicating the presence of free carbene ligand in the reaction medium. When the carbene transfer process was carried out in tetrahydrofuran, the polymerization of tetrahydrofuran occurred.     

Palladium(II)‐N‐Heterocyclic Carbene Complexes: Efficient Catalysts for the Direct C‐H Bond Arylation of Furans with Aryl Halides

This paper contains the synthesis and characterization of the seven new benzimidazolium salts and their corresponding new palladium(II)‐NHC complexes with the general formula [PdX₂(NHC)₂], (NHC = N‐heterocyclic carbene, X = Cl or Br), and also their catalytic activity in direct C‐H bond arylation of 2‐substituted furan derivatives with aryl bromides and aryl chlorides. Under the optimal conditions, these palladium(II)‐NHC complexes showed the good catalytic performance for the direct C‐H bond arylation of 2‐substituted furans with (hetero)aryl bromides, and with readily available and inexpensive aryl chlorides. The C‐H bond arylation regioselectively produced C5‐arylated furans by using 1 mol% of the palladium(II)‐NHC catalysts in moderate to high yields.     

Cross-Coupling of Diarylborinic Acids and Anhydrides with Arylhalides Catalyzed by a Phosphite/N-Heterocyclic Carbene Co-supported Palladium Catalyst System

A highly efficient cross-coupling of diarylborinic acids and anhydrides with aryl chlorides and bromides has been effected by using a palladium catalyst system co-supported by a strong σ-donor N-heterocyclic carbene (NHC), N,N'-bis(2,6-diisopropylphenyl) imidazol-2-ylidene, and a strong π-acceptor phosphite, triphenylphosphite, in tert-BuOH in the present of K(3)PO(4)·3H(2)O. Unsymmetrical biaryls with a variety of functional groups could be obtained in good to excellent yields using as low as 0.01, 0.2-0.5, and 1 mol % palladium loadings for aryl bromides and activated and deactivated aryl chlorides, respectively, under mild conditions. A ligand synergy between the σ-donor NHC and the π-acceptor phosphite in the Pd/NHC/P(OPh)(3) catalytic system has been proposed to be responsible for the high efficacy to arylchlorides in the cross-coupling. A scalable and economical process has therefore been developed for synthesis of Sartan biphenyl from the Pd/NHC/P(OPh)(3) catalyzed cross-coupling of di(4-methylphenyl)borinic acid with 2-chlorobenzonitrile.     

N‐Heterocyclic Carbene Adducts of Aluminium Triiodide

N‐Heterocyclic carbene adducts of aluminium triiodide, IMes · AlI₃ ( 1 ) and IPr · AlI₃ ( 2 ) (IMes = 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene and IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) are reported. These adducts are available by the reaction of aluminium triiodide with the correspondingN‐heterocyclic carbene. Compounds 1 and 2 are soluble in hydrocarbon solvents, stable in inert atmosphere, and have been characterised by elemental analysis, NMR spectroscopy and single‐crystal X‐ray diffraction studies.     

Triazole‐Functionalized N‐Heterocyclic Carbene Complexes of Palladium and Platinum and Efficient Aqueous Suzuki–Miyaura Coupling Reaction

Imidazolium salts bearing triazole groups are synthesized via a copper catalyzed click reaction, and the silver, palladium, and platinum complexes of their N‐heterocyclic carbenes are studied. [Ag₄(L1)₄](PF₆)₄, [Pd(L1)Cl](PF₆), [Pt(L1)Cl](PF₆) (L1=3‐((1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl)‐1‐(pyrimidin‐2‐yl)‐1H‐imidazolylidene), [Pd₂(L2)₂Cl₂](PF₆)₂, and [Pd(L2)₂](PF₆)₂ (L2=1‐butyl‐3‐((1‐(pyridin‐2‐yl)‐1H‐1,2,3‐triazol‐4‐yl)methyl)imidazolylidene) have been synthesized and fully characterized by NMR, elemental analysis, and X‐ray crystallography. The silver complex [Ag₄(L1)₄](PF₆)₄ consists of a Ag₄ zigzag chain. The complexes [Pd(L1)Cl](PF₆) and [Pt(L1)Cl](PF₆), containing a nonsymmetrical NCN ′ pincer ligand, are square planar with a chloride trans to the carbene donor. [Pd₂(L2)₂Cl₂](PF₆)₂ consists of two palladium centers with CN₂Cl coordination mode, whereas the palladium in [Pd(L2)₂](PF₆)₂ is surrounded by two carbene and two triazole groups with two uncoordinated pyridines. The palladium compounds are highly active for Suzuki–Miyaura cross coupling reactions of aryl bromides and 1,1‐dibromo‐1‐alkenes in neat water under an air atmosphere.