Carboxylation Reactions with Carbon Dioxide Using N‐Heterocyclic Carbene‐Copper Catalysts

The development of versatile catalyst systems and new transformations for the utilization of carbon dioxide (CO₂) is of great interest and significance. This Personal Account reviews our studies on the exploration of the reactions of CO₂ with various substrates by the use of N‐heterocyclic carbene (NHC)‐copper catalysts. The carboxylation of organoboron compounds gave access to a wide range of carboxylic acids with excellent functional group tolerance. The C?H bond carboxylation with CO₂ emerged as a straightforward protocol for the preparation of a series of aromatic carboxylic esters and butenoates from simple substrates. The hydrosilylation of CO₂ with hydrosilanes provided an efficient method for the synthesis of silyl formate on gram scale. The hydrogenative or alkylative carboxylation of alkynes, ynamides and allenamides yielded useful α,β‐unsaturated carboxylic acids and α,β‐dehydro amino acid esters. The boracarboxylation of alkynes or aldehydes afforded the novel lithium cyclic boralactone or boracarbonate products, respectively. The NHC‐copper catalysts generally featured excellent functional group compatibility, broad substrate scope, high efficiency, and high regio‐ and stereoselectivity. The unique electronic and steric properties of the NHC‐copper units also enabled the isolation and structural characterization of some key intermediates for better understanding of the catalytic reaction mechanisms.     

Poly(ionic liquid)s based on imidazolium hydrogen carbonate monomer units as recyclable polymer‐supported N‐heterocyclic carbenes: Use in organocatalysis

Synthesis of novel poly(ionic liquid)s, namely, poly(1‐vinyl‐3‐alkylimidazolium hydrogen carbonate)s, denoted as poly([NHC(H)][HCO₃])s or PVRImHCO₃, where R is an alkyl group (R = ethyl, butyl, phenylethyl, dodecyl), is described. Two distinct synthetic routes were explored. The first method is based on the free‐radical polymerization (FRP) of 1‐vinyl‐3‐alkylimidazolium monomers featuring a hydrogen carbonate counter anion (HCO₃^?), denoted as VRImHCO₃. The latter monomers were readily synthesized by alkylation of 1‐vinylimidazole (VIm), followed by direct anion exchange of 1‐vinyl‐3‐alkylimidazolium bromide monomers (VRImBr), using potassium hydrogen carbonate (KHCO₃) in methanol at room temperature. Alternatively, the same anion exchange method could be applied onto FRP‐derived poly(1‐vinyl‐3‐alkylimidazolium bromide) precursors (PVRImBr). All PVRImHCO₃ salts proved air stable and could be manipulated without any particular precautions. They could serve as polymer‐supported precatalysts to generate polymer‐supported N‐heterocyclic carbenes, referred to as poly(NHC)s, formally by a loss of “H₂CO₃” (H₂O +CO₂) in solution. This was demonstrated through selected organocatalyzed reactions of molecular chemistry, known as being efficiently mediated by molecular NHC catalysts, including benzoin condensation, transesterification and cyanosilylation of aldehyde. Of particular interest, recycling of the polymer‐supported precatalysts was possible by re‐carboxylation of in situ generated poly(NHC)s. Organocatalyzed reactions could be performed with excellent yields, even after five catalytic cycles. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4530–4540     

Rhodium(I) and Silver(I) Complexes of 4, 5‐Dicyano‐1, 3‐dimesityl‐ and 4, 5‐Dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene

The new N‐heterocyclic carbene (NHC) precursors 4, ‐dicyano‐1, ‐dimesityl‐ ( 9 ) and 4, 5‐dicyano‐1, 3‐dineopentyl‐2‐(pentafluorophenyl)imidazoline ( 14 ) were synthesized. The structure of 9 could be determined by X‐ray crystallography. With the 2‐pentafluorophenyl‐substituted imidazolines 9 and 14 , the [AgCl(NHC)], [RhCl(COD)(NHC)], and [RhCl(CO)₂(NHC)] complexes [NHC = 4, 5‐dicyano‐1, 3‐dimesitylimidazol‐2‐ylidene ( 3 ) and 4, 5‐dicyano‐1, 3‐dineopentylimidazol‐2‐ylidene ( 4 )] were obtained. Crystal structures of [AgCl( 3 )] ( 15 ), [RhCl(COD)( 3 )] ( 17 ), [RhCl(COD)( 4 )] ( 18 ), and [RhCl(CO)₂( 3 )] ( 19 ) were solved and with the crystal data of 19 , the percent buried volume ( %V_bur) of 31.8(±0.1) % was determined for NHC 3 . Infrared spectra of the imidazolines 9 and 14 and of the complexes 15 – 20 were recorded and the CO stretching frequencies of complexes 19 and 20 were used to determine the Tolman electronic parameters of the newly obtained NHCs 3 (TEP: 2060 cm^–1) and 4 (TEP: 2061 cm^–1), thus proving that 1, 3‐substitution of maleonitrile‐NHCs does not have a significant effect for the high π‐acceptor strength of these carbenes.     

Effective Formylation of Amines with Carbon Dioxide and Diphenylsilane Catalyzed by Chelating bis(tzNHC) Rhodium Complexes

The reductive formylation of amines using CO₂ and hydrosilanes is an attractive method for incorporating CO₂ into valuable organic compounds. However, previous systems required either high catalyst loadings or high temperatures to achieve high efficiency, and the substrate scope was mostly limited to simple amines. To address these problems, a series of alkyl bridged chelating bis(NHC) rhodium complexes (NHC=N‐heterocyclic carbene) have been synthesized and applied to the reductive formylation of amines using CO₂ and Ph₂SiH₂. A rhodium‐based bis(tzNHC) complex (tz=1,2,3‐triazol‐5‐ylidene) was identified to be highly effective at a low catalyst loading and ambient temperature, and a wide substrate scope, including amines with reducible functional groups, were compatible.     

An Abnormal N‐Heterocyclic Carbene–Carbon Dioxide Adduct from Imidazolium Acetate Ionic Liquids: The Importance of Basicity

In the reaction of 1‐ethyl‐3‐methylimidazolium acetate [C₂C₁Im][OAc] ionic liquid with carbon dioxide at 125 °C and 10 MPa, not only the known N‐heterocyclic carbene (NHC)–CO₂ adduct I , but also isomeric aNHC‐CO₂ adducts II and III were obtained. The abnormal NHC‐CO₂ adducts are stabilized by the presence of the polarizing basic acetate anion, according to static DFT calculations and ab initio molecular dynamics studies. A further possible reaction pathway is facilitated by the high basicity of the system, deprotonating the initially formed NHC‐CO₂ adduct I , which can then be converted in the presence of the excess of CO₂ to the more stable 2‐deprotonated anionic abnormal NHC–CO₂ adduct via the anionic imidazolium‐2,4‐dicarboxylate according to DFT calculations on model compounds. This suggests a generalizable pathway to abnormal NHC complex formation.     

From an Isolable Acyclic Phosphinosilylene Adduct to Donor‐Stabilized SiE Compounds (E=O,S, Se)

Reaction of the arylchlorosilylene‐NHC adduct ArSi(NHC)Cl [Ar=2,6‐Trip₂C₆H₃; NHC=(MeC)₂(NMe)₂C:] 1 with one molar equiv of lithium diphenylphosphanide affords the first stable NHC‐stabilized acyclic phosphinosilylene adduct 2 (ArSi(NHC)PPh₂), which could be structurally characterized. Compound 2 , when reacted with one molar equiv selenium and sulfur, affords the silanechalcogenones 4 a and 4 b (ArSi(NHC)(?E)PPh₂, 4 a : E=Se, 4 b : E=S), respectively. Conversion of 2 with an excess of Se and S, through additional insertion of one chalcogen atom into the Si?P bond, leads to 3 a and 3 b (ArSi(NHC)(?E)‐E‐P(?E)Ph₂, 3 a : E=Se, 3 b : E=S), respectively. Additionally, the exposure of 2 to N₂O or CO₂ yielded the isolable NHC‐stabilized silanone 4 c , Ar(NHC)(Ph₂P)Si?O.     

Facile Syntheses of N‐Heterocyclic Carbene Precursors through I2‐ or NIS‐Promoted Amidiniumation of N‐Alkenyl Formamidines

We have developed I₂‐ or N‐iodosuccinimide (NIS)‐mediated amidiniumation of N‐alkenyl formamidines for the syntheses of cyclic formamidinium salts, some of which could be directly used as N‐heterocyclic carbene (NHC) precursors. Treatment of iodine‐containing formamidinium salts with Al₂O₃ led to the formation of cyclic formamidinium salts with an unsaturated backbone. A rhodium(I) complex ligated by a representative NHC was prepared by the reaction of [Rh(cod)Cl]₂ (cod=1,5‐cyclooctadiene) with the free carbene obtained in situ from deprotonation of the corresponding formamidinium salts. The NHCs prepared in situ can also react with S₈ to afford the corresponding thiones.     

NHC‐Coordinated Diphosphene‐Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2

An NHC‐coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon‐soluble monomeric Au^I hydride, which readily adds CO₂ at room temperature yielding the corresponding Au^I formate. The reversible reaction can be expedited by the addition of NHC, which induces β‐hydride shift and the removal of CO₂ from equilibrium through the formation of an NHC‐CO₂ adduct. The Au^I formate is alternatively formed by dehydrogenative coupling of the Au^I hydride with formic acid (HCO₂H), thus in total establishing a reaction sequence for the Au^I hydride mediated dehydrogenation of HCO₂H as chemical hydrogen storage material.     

Expanded‐Ring N‐Heterocyclic Carbenes for the Stabilization of Highly Electrophilic Gold(I) Cations

Strategies for the synthesis of highly electrophilic Au^I complexes from either hydride‐ or chloride‐containing precursors have been investigated by employing sterically encumbered Dipp‐substituted expanded‐ring NHCs (Dipp=2,6‐iPr₂C₆H₃). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6‐Dipp or 7‐Dipp) and shown to feature significantly more electron‐rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ‐donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au]⁺ fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C₆F₅)₃, by contrast, generates a species (at low temperatures) featuring a [HB(C₆F₅)₃]^? fragment with spectroscopic signatures similar to the “free” borate anion. Subsequent rearrangement involves B?C bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr^f₄] (Ar^f=C₆H₃(CF₃)₂‐3,5), systems of the type [(NHC)AuCl] (NHC=6‐Dipp or 7‐Dipp) generate dinuclear complexes [{(NHC)Au}₂(μ‐Cl)]⁺ that are still electrophilic enough at gold to induce aryl abstraction from the [BAr^f₄]^? counterion.     

Prediction on the Origin of Selectivities in Base‐controlled Switchable NHC‐catalyzed Transformations

The base‐controlled mechanisms for N‐heterocyclic carbene (NHC)‐catalyzed divergent [3+3] and [3+2] annulation reactions were examined by using the DFT method. The reaction initiates with the complexation of NHC and enal to give the Breslow intermediate, which diverges afterward. Then, the azomethine imine can either react with the Breslow intermediate to give the six‐membered ring product or the β‐carbon protonation occurs for forming the enolate intermediate controlled by different bases. The formed enolate intermediate reacts with azomethine imine to afford the five‐membered ring product. The calculated results show that only the base K₂CO₃ can facilitate the structural transformation between homoenolate and enolate to switch the chemoselectivity; therefore, the [3+3] annulation happens preferentially in the presence of base DBU while the other situation occurs with K₂CO₃ as base. The NCI analysis results reveal that the stereoselectivity is predominately determined by the π???π, C?H???O, and C?H???N interactions. The obtained mechanistic insights should provide valuable clues for the rational design of these kinds of divergent reactions.     

N‐Heterocyclic Carbene–Ytterbium Amide as a Recyclable Homogeneous Precatalyst for Hydrophosphination of Alkenes and Alkynes

The N‐heterocyclic carbene–ytterbium(II) amides (NHC)₂Yb[N(SiMe₃)₂]₂ ( 1 : NHC: 1,3,4,5‐tetramethylimidazo‐2‐ylidene (IMe₄); 2 : NHC: 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene (IiPr)) and the NHC‐stabilized rare‐earth phosphide (IMe₄)₃Yb(PPh₂)₂ ( 3 ) have been synthesized and fully characterized. Complexes 1 – 3 are active precatalysts for the hydrophosphination of alkenes, alkynes, and dienes and exhibited much superior catalytic activity to that of the NHC‐free amide (THF)₂Yb[N(SiMe)₂]₂. Complex 1 is the most active precursor among the three complexes. In particular, complex 1 can be recycled and recovered from the reaction media after the catalytic reactions. Furthermore, it was found that complex 3 could catalyze the polymerization of styrene to yield atactic polystyrenes with low molecular weights. To the best of our knowledge, complex 1 represents the first rare‐earth complex that can be recovered after catalytic reactions.     

SiSi Double Bonds: Synthesis of an NHC‐Stabilized Disilavinylidene

An efficient two‐step synthesis of the first NHC‐stabilized disilavinylidene (Z)‐(SIdipp)SiSi(Br)Tbb ( 2 ; SIdipp=C[N(C₆H₃‐2,6‐iPr₂)CH₂]₂, Tbb=C₆H₂‐2,6‐[CH(SiMe₃)₂]₂‐4‐tBu, NHC=N‐heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr₂(SIdipp) with the 1,2‐dibromodisilene (E)‐Tbb(Br)SiSi(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC‐stabilized bromo(silyl)silylene, namely SiBr(SiBr₂Tbb)(SIdipp) ( 1 ). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr₂(SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C₈K to give the NHC‐stabilized disilavinylidene 2 . Both low‐valent silicon compounds were comprehensively characterized by X‐ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum‐chemical methods.     

Comprehensive Basicity Scales for N‐Heterocyclic Carbenes in DMSO: Implications on the Stabilities of N‐Heterocyclic Carbene and CO2 Adducts

A very broad acidity scale (≈40 pK units) for about 400 N‐heterocyclic carbene precursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4‐triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by a well examined computational method. Varying the backbone structure or flanking N‐substituents can have different extent of acidifying effects, depending on both the nature and number of substituent(s). The Gibbs energies (ΔG_rs) for the reactions between the corresponding NHCs and CO₂ were also calculated. There is a good linear correlation between the pK_as of most NHCPs and ΔG_rs, suggesting that a greater basicity of NHC leads to a more stable NHC‐CO₂ adduct. Interestingly, the nearby asymmetric environment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the ΔG_r values.     

Versatile Micropatterns of N‐Heterocyclic Carbenes on Gold Surfaces: Increased Thermal and Pattern Stability with Enhanced Conductivity

Patterned monolayers of N‐heterocyclic carbenes (NHCs) on gold surfaces were obtained by microcontact printing of NHC–CO₂ adducts and NHC(H)[HCO₃] salts. The NHC‐modified areas showed an increased conductivity compared to unmodified gold surface areas. Furthermore, the remaining surface areas could be modified with a second, azide‐functionalized carbene, facilitating further applications and post‐printing modifications. Thorough elucidation by a variety of analytical methods offers comprehensive evidence for the viability of the methodology reported here. The protocol enables facile access to versatile, microstructured NHC‐modified gold surfaces with highly stable patterns, enhanced conductivity, and the option for further modification.     

An efficient and recyclable silica‐supported carbene–Cu(II) catalyst for the oxidative coupling reaction of terminal alkynes with H‐phosphonates under base‐free reaction conditions

The oxidative coupling reactions of terminal alkynes with H‐phosphonates were explored using SiO₂−NHC−Cu(II) (5.0 mol%) as catalyst at room temperature under base‐free reaction conditions. The reactions of a variety of terminal alkynes with H‐phosphonates generated the corresponding alkynylphosphonate products in good to excellent yields. In addition, SiO₂−NHC−Cu(II) could be recovered and recycled for six consecutive trials without significant loss of its reactivity. Copyright © 2011 John Wiley & Sons, Ltd.     

Cu‐Catalyzed Formal Methylative and Hydrogenative Carboxylation of Alkynes with Carbon Dioxide: Efficient Synthesis of α,β‐Unsaturated Carboxylic Acids

The sequential hydroalumination or methylalumination of various alkynes catalyzed by different catalyst systems, such those based on Sc, Zr, and Ni complexes, and the subsequent carboxylation of the resulting alkenylaluminum species with CO₂ catalyzed by an N‐heterocyclic carbene (NHC)–copper catalyst have been examined in detail. The regio‐ and stereoselectivity of the overall reaction relied largely on the hydroalumination or methylalumination reactions, which significantly depended on the catalyst and alkyne substrates. The subsequent Cu‐catalyzed carboxylation proceeded with retention of the stereoconfiguration of the alkenylaluminum species. All the reactions could be carried out in one‐pot to afford efficiently a variety of α,β‐unsaturated carboxylic acids with well‐controlled configurations, which are difficult to construct by previously reported methods. This protocol could be practically useful and attractive because of its high regio‐ and stereoselectivity, simple one‐pot reaction operation, and the use of CO₂ as a starting material.     

Oxidative Coupling of Anionic Abnormal N‐Heterocyclic Carbenes: Efficient Access to Janus‐Type 4,4′‐Bis(2H‐imidazol‐2‐ylidene)s

The oxidative coupling of anionic imidazol‐4‐ylidenes protected at the C2 position with [MnCp(CO)₂] or BH₃ led to the corresponding 4,4′‐bis(2H‐imidazol‐2‐ylidene) complexes or adducts, in which the two carbene moieties are connected through a single C?C bond. Subsequent acidic treatment of the later species led to the corresponding 4,4′‐bis(imidazolium) salts in good yields. The overall procedure offers practical access to a novel class of Janus‐type bis(NHC)s. Strikingly, the coplanarity of the two NHC rings within the mesityl derivative 4,4′‐bis(IMes), favored by steric hindrance along with stabilizing intramolecular C?H???π aryl interactions, allows the alignment of the π‐systems and, as a direct consequence, significant electron communication through the bis(carbene) scaffold.     

Reversible Uptake of COS,CS2, and SO2: Ionic Liquids with O‐Alkylxanthate,O‐Alkylthiocarbonyl,and O‐Alkylsulfite Anions

CO₂‐binding organic liquids (CO₂BOLs) are mixtures of a base (typically an amidine or guanidine) and an alcohol, and have been shown to reversibly capture and release CO₂ with low reaction energies and high gravimetric CO₂ capacity. We now report the ability of such liquid blends to chemically bind and release other acid gases such as CS₂, COS, and SO₂ analogously to CO₂. These systems bind with sulfur‐containing acid gases to form colored ionic liquids with new O‐alkylxanthate, O‐alkylthiocarbonyl, and O‐alkylsulfite anions. The capture and thermal stripping of each acid gas from these systems and their applicability towards flue gas desulfurization is discussed.     

A Persistent 1,2‐Dihydrophosphasilene Adduct

The reaction of the arylchlorosilylene–NHC adduct ArSi(NHC)Cl [Ar=2,6‐Trip₂‐C₆H₃; NHC=(MeC)₂(NMe)₂C] 1 with one molar equiv of LiPH₂^.dme (dme=1,2‐dimethoxyethane) affords the first 1,2‐dihydrophosphasilene adduct 2 (ArSi(NHC)(H)?PH). The latter is labile in solution and can undergo head‐to‐tail dimerization to give [ArSi(H)PH]₂ 3 and “free” NHC. Further stabilization of 2 by complexation with {W(CO)₅} affords the isolable 1,2‐dihydrophosphasilene–tungsten complex 4 [ArSi(NHC)(H)?P(H)W(CO)₅]. Additionally, the new 1‐silyl‐2‐hydrophosphasilene ArSi(NHC)(H)?PSiMe₃ 5 could be synthesized and structurally characterized. DFT studies confirmed that the Si?P bond in 2 and 4 is mostly zwitterionic with drastically decreased double‐bond character.     

Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated Rh^I species [RhCl(NHC)(η²‐HC?CCH₂Ph)(py)] ( 3 ) and [RhCl(NHC){η²‐C(tBu)?C(E)CH?CHtBu}(py)] ( 4 ) and the Rh^III–hydride–alkynyl species [RhClH{? C?CSi(Me)₃}(IPr)(py)₂] ( 5 ). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C? H oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity.