Accelerating the insertion reactions of (NHC)Cu–H via remote ligand functionalization

Most ligand designs for reactions catalyzed by (NHC)Cu–H (NHC = N-heterocyclic carbene ligand) have focused on introducing steric bulk near the Cu center. Here, we evaluate the effect of remote ligand modification in a series of [(NHC)CuH]₂ in which the para substituent (R) on the N-aryl groups of the NHC is Me, Et, ^tBu, OMe or Cl. Although the R group is distant (6 bonds away) from the reactive Cu center, the complexes have different spectroscopic signatures. Kinetics studies of the insertion of ketone, aldimine, alkyne, and unactivated α-olefin substrates reveal that Cu–H complexes with bulky or electron-rich R groups undergo faster substrate insertion. The predominant cause of this phenomenon is destabilization of the [(NHC)CuH]₂ dimer relative to the (NHC)Cu–H monomer, resulting in faster formation of Cu–H monomer. These findings indicate that remote functionalization of NHCs is a compelling strategy for accelerating the rate of substrate insertion with Cu–H species.

Remote modification of an N-heterocyclic carbene ligand with bulky or electron-rich groups in [(NHC)Cu(μ-H)]₂ increases the rate of substrate insertion, which kinetics studies suggest arises from changes in the Cu–H monomer–dimer equilibrium.     

Pd-catalyzed aryl amination mediated by well defined, N-heterocyclic carbene (NHC)-Pd precatalysts, PEPPSI

Pd-N-heterocyclic carbene (NHC)-catalyzed Buchwald-Hartwig amination protocols mediated by Pd-PEPPSI precatalysts is described. These protocols provide access to a range of hindered and functionalized drug-like aryl amines in high yield with both electron-deficient and electron-rich aryl- and heteroaryl chlorides and bromides. Variations in solvent polarity, base and temperature are tolerated, enhancing the scope and utility of this protocol. A mechanistic rationalization for base strength (pKb) requirements is also provided.     

Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

We report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron-rich NHC ligand IPr*^OMe. The COD (1,5-cyclooctadiene) complex [Rh(IPr*^OMe)(COD)]BF₄ adopts a tilted, pseudo-square planar coordination geometry, where bonding to the ipso-carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex [Rh(IPr*^OMe)(H)₂]BF₄ with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H₂ afforded a rhodium complex [Rh(IPr*^OMe)]BF₄ with a single chelating ligand that stabilizes the highly unsaturated metal by two-fold π-face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.     

A computational mapping of the R–NHC coupling pathway – the key process in the evolution of Pd/NHC catalytic systems

C–C coupling reactions are of great importance in metal-catalyzed synthetic transformations. Reductive elimination of two carbon centers is the key stage, which takes place in the metal coordination sphere. In the present study, we provide a detailed analysis of nonclassical R–NHC coupling in the model (NHC)Pdⁱⁱ(Ph)(X)(Solv) complex, which is a representative intermediate of the Mizoroki–Heck and cross-coupling reactions. This C–C bond formation stage proceeds as Ph ligand movement and insertion into the Pd–NHC bond, rather than classical C–C coupling. Based on the analysis by the quantum theory of atoms in molecules (QTAIM) of the reaction path structures, the atomic rearrangements and alterations in the electronic system during the R–NHC coupling process were characterized in detail.     

Relative Lability and Chemoselective Transmetallation of NHC in Hybrid Phosphine‐NHC Ligands: Access to Heterometallic Complexes

The relative lability and transmetallation aptitude of trialkylphosphine and NHC donors, integrated in semi‐rigid hybrid ligands attached to [Ag₄Br₄] pseudo‐cubanes, lies in favor of the NHC and is used to selectively access unprecedented NHC complexes with heterobimetallic cores, such as Ag–Cu ( 4^Cy ) and Ag–Ir ( 5 ^{t Bu} ). These can be viewed as an arrested state before the full transmetallation of both donors, which gives the homodinuclear Cu ( 3^Cy ) and Ir ( 6^Cy ) complexes. The observed NHC transmetallation aptitude and reactivity urges caution in the common notion that views the NHC as a universal spectator.     

Stepwise synthesis of a hydrido, N-heterocyclic dicarbene iridium(III) pincer complex featuring mixed NHC/abnormal NHC ligands

We describe a stepwise synthesis of the hydrido, N-heterocyclic dicarbene iridium(III) pincer complex [Ir(H)I(C(NHC)CC(aNHC))(NCMe)] (3) which features a combination of normal and abnormal NHC ligands. The reaction of the bis(imidazolium) diiodide [(CH(imid)CHCH(imid))]I(2) (1) with [Ir(μ-Cl)(cod)](2) afforded first the mono-NHC Ir(I) complex [IrI(cod)(CH(imid)CHC(NHC))]I (2), which was then reacted with 2 equiv. of Cs(2)CO(3) in acetonitrile at 60 °C for 40 h to yield 3. These observations support our previously proposed mechanism for the formation of hydrido, N-heterocyclic dicarbene iridium(III) pincer complexes from the reaction of bis(imidazolium) salts with weak bases involving a mono-NHC Ir(I) intermediate. We describe the reactivity of the mono-NHC Ir(I) complex 2 under various conditions. By changing the reaction solvent from MeCN to toluene, we observed the cleavage of the imidazol-2-ylidene ring and the formation of an iminoformamide-containing mono-NHC Ir(I) complex [IrI(cod){[NHCH=CHN(Ad)CHO]CHC(NHC)}] (4). Complex 4 was also prepared in high yield from the reaction of 2 with strong bases (potassium tert-butoxide or potassium hexamethyldisilazane), via the initial formation of the complex [IrI(cod)(CH(NHC)CHC(NHC))] (5), which contains a coordinated NHC moiety and a free carbene arm, followed by subsequent hydrolysis of the latter. The bis(imidazolium) salt 1 can be deprotonated by strong bases to form the bis(carbene) ligand C(NHC)CHC(NHC) (6), which readily reacts with [Ir(μ-Cl)(cod)](2) to give the dinuclear complex [{IrI(cod)}(2)(μ-C(NHC)CHC(NHC))] (7), in which the N-heterocyclic bis(carbene) ligand bridges the two metals through the carbene carbon atoms.     

Stable,three-coordinate Ni(CO)(2)(NHC) (NHC = N-heterocyclic carbene) complexes enabling the determination of Ni-NHC bond energies

The synthesis and characterization of three- and four-coordinate Ni(CO)n(NHC) (n = 2, 3; NHC = N-heterocyclic carbene) complexes are reported. Reactions with CO of the Ni(CO)2(NHC) complexes lead to the quantitative formation of Ni(CO)4. Investigation of this reaction under equilibrium conditions allows for the determination of Ni-NHC bond dissociation energies.     

Synthesis and characterization of asymmetric NHC complexes

New chiral and non-chiral rhodium(I)–NHC complexes were synthesized. The first attempt by deprotonation of an imidazolinium salt with KOtBu and reaction with [Rh(COD)Cl]₂ leads to the corresponding rhodium(I) complex. Due to the basic conditions during the reaction a loss of chirality occurs. An alternative transmetallation reaction with a silver(I)–NHC complex yields the desired rhodium(I)–NHC complex under retention of chirality. Both Rh complexes were fully characterized by analytical methods.     

Rhodium(NHC)-catalyzed O-arylation of aryl bromides

The first example of the rhodium-catalyzed O-arylation of aryl bromides is reported. While the right combination of rhodium species and N-heterocyclic carbene (NHC) offered an effective catalytic system enabling the arylation to proceed, the choice of NHC was determined to be most important. The developed O-arylation protocol has a wide range of substrate scope, high functional group tolerance, and flexibility allowing a complementary route to either N- or O-arylation depending on the choice of NHC.     

The Role of Free N‐Heterocyclic Carbene (NHC) in the Catalytic Dehydrogenation of Ammonia–Borane in the Nickel NHC System

Enders' N‐heterocyclic carbene (NHC) dehydrogenates ammonia–borane with a relatively low barrier, producing NH₂BH₂ and NHC–(H)₂. The nickel NHC catalyst present in the reaction media can activate the NHC–(H)₂ produced to regenerate the free NHC and release H₂. The release of free NHC enables further dehydrogenation of ammonia–borane.

     

Synthetic routes to [Au(NHC)(OH)] (NHC = N-heterocyclic carbene) complexes

New procedures for the synthesis of [Au(NHC)(OH)] are reported. Initially, a two-step reaction via the digold complex [{Au(NHC)}(2)(μ-OH)][BF(4)] was probed, enabling the preparation of the novel [Au(SIPr)(OH)] complex and of its previously reported congener [Au(IPr)(OH)]. After further optimization, a one-step procedure was developed.     

Synthesis and coordination chemistry of macrocyclic ligands featuring NHC donor groups

Poly-NHC (NHC = N-heterocyclic carbene) ligands emerged almost immediately after the first stable NHCs had been described. Macrocyclic ligands, featuring NHC donor groups and their metal complexes, however, remained rare until recently. This perspective highlights modern developments in the fields of synthesis and coordination chemistry of macrocyclic poly-NHC ligands. These include the synthesis of tetracarbene ligands which were obtained from complexes of β-functionalized isocyanides followed by cyclization of the coordinated iscocyanide ligands to NH,NH-functionalized NHCs and the subsequent metal template controlled bridging alkylation of the NH,NH-NHCs to yield the macrocycle. The template synthesis of ligands featuring a mixed NHC/phosphine donor set like [11]ane-P(2)C(NHC) and [16]ane-P(2)C(NHC)(2) by linkage of NH,NH-NHCs to different phosphines is also presented. Finally, methods for the preparation of cyclic polyazolium salts, their deprotonation and metalation and the different modes of coordination of such macrocyclic poly-NHC ligands are discussed.     

Continuous Flow Synthesis of [Au(NHC)(Aryl)] (NHC=N-Heterocyclic Carbene) Complexes

The use of weak and inexpensive bases has recently opened promising perspectives towards the simpler and more sustainable synthesis of Au(I)-aryl complexes with valuable applications in catalysis, medicinal chemistry, and materials science. In recent years, continuous manufacturing has shown to be a reliable partner in establishing sustainable and controlled process scalability. Herein, the first continuous flow synthesis of a range of Au(I)-aryl starting from widely available boronic acids and various [Au(NHC)Cl] (NHC=N-heterocyclic carbene) complexes in unprecedentedly short reaction times and high yields is reported. Successful synthesis of previously non- or poorly accessible complexes exposed fascinating reactivity patterns. Via a gram-scale synthesis, convenient process scalability of the developed protocol was showcased.     

Structures and stabilities of group 13 adducts [(NHC)(EX3)] and [(NHC)2(E2X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene) and the search for hydrogen storage systems: a theoretical study

Quantum chemical calculations using density functional theory at the BP86/TZVPP level and ab initio calculations at the SCS-MP2/TZVPP level have been carried out for the group 13 complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX(3) and the bidentate Lewis acids E(2) X(n) bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX(3))] and [(NHC)(2)(E(2)X(n))]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E(2)X(n), which may vary for different atoms E and X. All complexes [(NHC)(2)(E(2)X(4))] possess C(s) symmetry in which the NHC ligands bind in a trans conformation to the group 13 atoms E. The complexes [(NHC)(2)(E(2)H(2))] with E=B, Al, Ga have also C(s) symmetry with a trans arrangement of the NHC ligands and a planar CE(H)E(H)C moiety that has a E=E π bond. In contrast, the indium complex [(NHC)(2)(In(2) H(2))] has C(i) symmetry with pyramidal-coordinated In atoms in which the hydrogen atoms are twisted above and below the CInInC plane. The latter C(i) form is calculated for all chloride systems [(NHC)(2)(E(2)Cl(2))], but the boron complex [(NHC)(2)(B(2)Cl(2))] deviates only slightly from C(s) symmetry. The B(2) fragment in the linear coordinated complex [(NHC)(2)(B(2))] has a highly excited (3)(1)Σ(g)(-) reference state, which gives an effective B≡B triple bond with a very short interatomic distance. The heavier homologues [(NHC)(2)(E(2))] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E(2) fragments have a (1)(1) Δ(g) reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH(3))] and [(NHC)(2)(E(2)H(n))] becomes energetically more favourable when atom E becomes heavier. The indium complexes should therefore be the best candidates of the investigated series for hydrogen-storage systems that could potentially deliver dihydrogen at close to ambient temperature. The hydrogenation reaction of the dimeric magnesium(I) compound [LMgMgL] (L=β-diketiminate) with [(NHC)(EH(3))] becomes increasingly exothermic with the trend B相似文献   

Rh(NHC)-catalyzed direct and selective arylation of quinolines at the 8-position

A new catalytic protocol for the regioselective direct arylation of quinoline derivatives at the 8-position has been developed. The reaction is catalyzed by a Rh(NHC) system, and the choice of the NHC ligand was most important for achieving high reactivity and selectivity.     

(NHC)AuI]-catalyzed rearrangement of allylic acetates

[(NHC)AuCl] complexes (NHC = N-heterocyclic carbene), in conjunction with a silver salt, were found to efficiently catalyze the rearrangement of allylic acetates under both conventional and microwave-assisted heating. The optimization of several reaction parameters (solvent, silver salt, and ligand) as well as a study of the reaction scope are reported. The steric hindrance of the ligand bound to gold was found crucial for the outcome of the reaction as only extremely bulky ligands permitted the isomerization.     

Synthesis of the Cyclic Group 13 Phosphinidenides [(NHC)PMCl2]2 (NHC = SIMes,SIDipp; M = Al,Ga)

The N-heterocyclic carbene stabilized phosphinidenides (SIMes)PK [SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolidine-2-ylidene] and (SIDipp)PK [SIDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolidine-2-ylidene] were used as precursors in salt elimination reactions with MCl₃ (M = Al, Ga) in order to obtain new group 13 phosphinidenide compounds. The new compounds [(NHC)PMCl₂]₂ (NHC = SIMes, SIDipp; M = Al, Ga) exhibit dimerization in solid state as well as in solution and show different shapes of the central M₂P₂ cycle (butterfly or nearly square planar conformation) in solid state, depending on the size of the NHC ligand bound to the phosphorus atom.     

Designing N-heterocyclic carbenes: simultaneous enhancement of reactivity and enantioselectivity in the asymmetric hydroacylation of cyclopropenes

Faster, higher, stronger! The N-heterocyclic carbene (NHC) catalyzed diastereo- and enantioselective hydroacylation of cyclopropenes affords structurally valuable acylcyclopropanes. A new family of electron-rich, 2,6-dimethoxyphenyl-substituted NHCs induces excellent reactivity and enantioselectivity. Preliminary kinetic studies unambiguously demonstrated the superiority of this family of catalysts over known NHCs in this challenging transformation.     

Room-temperature activation of aryl chlorides in Suzuki-Miyaura coupling using a [Pd(micro-Cl)Cl(NHC)]2 complex (NHC = N-heterocyclic carbene)

A straightforwardly synthesised complex, [Pd(micro-Cl)Cl(NHC)](2) (NHC = bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr), has been employed to mediate Suzuki-Miyaura reactions involving aryl chlorides at very low catalyst loadings and at room temperature.     

Electronic nature of N-heterocyclic carbene ligands: effect on the Suzuki reaction

Suzuki reactions of aryl chlorides and arylboronic acids with a range of electronically different N-heterocyclic carbene ligands derived from N,N-diadamantylbenzimidazolium salts are reported. Results indicate that an electron-rich NHC ligand enhances the rate of oxidative addition. However, reductive elimination is unchanged by the electronic nature of the supporting ligand and is primarily affected by the steric environment.