Diamidocarbene Research Articles

Cleavage of Carbodicarbenes with N2O for Accessing Stable Diazoalkenes: Two-Fold Ligand Exchange at a C(0)-Atom.

The cleavage of carbophosphinocarbenes and carbodicarbenes with nitrous oxide (N2O) leads to the formation of room-temperature stable diazoalkenes. The utility of Ph3P/N2 and NHC/N2 ligand exchange reactions were demonstrated by accessing novel benzimidazole- and benzothiazole derived diazoalkenes, which are not accessible by the current state-of-the-art methods. The stable diazoalkenes subsequently allow further ligand exchange reactions at C(0) with carbon monoxide, isocyanide, or a diamidocarbene (DAC). Overall, the combination of hitherto unknown NHC/N2 and N2/L (L = DAC, CO, R-NC) ligand exchange reactions at a C(0) center allow the selective functionalization of the carbodicarbene ligand structure which represents a new methodology to rapidly assemble novel carbodicarbenes or cumulenic compounds.

Excited-state properties of Cu-TADF complexes: A density functional theory study

Organometallic complexes, including copper atom, have attracted great interest as thermally activated delayed fluorescence (TADF) emitters for light emitting diode (LED) applications. This is ascribed to the potential low-cost, abundant availability of copper and most importantly to the ability of copper to enhance the spin–orbit couplings and, consequently, increase the reverse intersystem crossing rates. In this article, we use density functional theory (DFT) to investigate the excited state properties of six copper complexes based on N-heterocyclic carbene ligand, monoamido-amino carbene and diamido carbene, and carbazole ligand. The DFT calculations show that the lowest excited states consist of three groups, i.e., (i) local carbazole excitations, (ii) carbazole-to-carbene intramolecular charge transfer states, and (iii) metal-to-ligand charge transfer states. Only the latter states are characterized with large spin–orbit couplings. The DFT calculations show that the surrounding medium could have a major effect on electronic spectrum by reordering the states. Our results suggest that the TADF properties of the investigated complexes can be affected by the chemical structure of the ligands as well as by the dielectric properties of the LED device active layer.

The Dynamic Lewis Acid-Carbene Hybrid: Pushing the Electrophilicity of Carbenes to the Limit.

This work describes a Lewis-acid-coordination strategy to efficiently enhance the electrophilicity of a carbene beyond structural modification. A hybrid BCF-DAC is formed by the coordination of a Lewis acid, B(C6F5)3 (BCF), to an N,N'-diamidocarbene (DAC), possessing superior low LUMO energy that is indicated by theoretical calculation. This endows the hybridized carbene with a unique reactivity that speeds up the activation of the sp3-hybridized C-H bond of toluene and the [2+1] cycloaddition with C2H2. More strikingly, the hybrid readily undergoes [2+1] cycloaddition with C2H4 under ambient conditions, which is the first example of a stable carbene reacting with ethylene. The Lewis acid approach also features dynamic behavior and electrophilicity tunability.

Highly phosphorescent carbene-metal-carboranyl complexes of copper(I) and gold(I).

New phosphorescent "carbene-metal-carboranyl" (CMC) Cu(I) and Au(I) complexes based on the diamidocarbene (DAC) ligand show up to 68% photoluminescence quantum yield and microsecond range lifetimes. CMC organic light emitting diodes (OLEDs) emit sky-blue and warm white electroluminescence.

Diamidocarbene-Based Thiele and Tschitschibabin Hydrocarbons: Carbonyl Functionalized Kekulé Diradicaloids.

Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.

New Outcomes of Beryllium Chemistry: Lewis Base Adducts for Salt Elimination Reactions.

A range of mono- and bis-adducts of beryllium dichloride with cyclic (alkyl)(amino)carbenes (CAAC), cyclic diamidocarbene (DAC), and carbodiphosphorane (CDP) are prepared, and their reactions with nucleophiles are studied. Salt elimination with Yamashita and Nozaki's lithium diazaborolide reagent led to the isolation of an unsymmetrical beryllium diazaborolyl complex and a base-stabilized diazaborolyl beryllium chloride. From structural and spectroscopic analyses, the Be-B bonding in these compounds was determined to be polar covalent in character. In addition, the nucleophilic addition of magnesium anthracenediyl to one of the adducts resulted in the isolation of an interesting tetracyclic beryllium-bridged molecule.

Metal‐Free Activation of Enthalpically Strong Bonds: Unraveling the Potential of Hitherto Unexplored Singlet Carbenes

AbstractDensity functional theory calculations have been carried out on a number of ambiphilic and electrophilic carbenes (both experimentally known and computationally designed) to understand their reactivity profile towards the activation of thermally robust bonds. Based on the calculated values of activation energy barriers, it was found that for ammonia (NH3), most of the molecules favor an electrophilic mode of activation. On the other hand, even though the electrophilic pathway is found to be more favorable than the nucleophilic one for activation of phosphine (PH2Ph) by majority of the carbenes, the barrier heights for both pathways are calculated to be comparable for diamidocarbene (DAC), cyclic amino aryl carbenes (CAArCs) and bicyclic alkyl amino carbene (BICAAC). In agreement with their better electron donation and acceptance abilities, the hitherto unexplored BICAAC, CAArCs and cyclic alkyl amido carbenes (CAAmC) are predicted to be more effective than cyclic alkyl amino carbenes (CAACs) towards the activation of both ammonia and phosphine. For the activation of silane (SiH3Ph), a hydride transfer pathway is found to be more favorable than the proton transfer pathway. Further, the calculated values of total Gibbs free energies and activation energy barriers for the splitting of N−H, P−H and Si−H bonds by a majority of the molecules are found to be comparable to those of the experimentally evaluated ones, implying that these known‐yet unexplored‐carbenes may be considered as suitable candidates for the activation of such enthalpically strong bonds.magnified image

Selective one- and two-electron reductions of a haloborane enabled by a π-withdrawing carbene ligand.

A carbene-stabilised neutral boryl radical and a boryl anion are isolated via selective one- and two-electron reduction of a diamidocarbene (DAC) adduct of dibromo(pentafluorophenyl)borane. Both the radical and the anion have been characterised by various spectroscopic techniques in solution, while the structures have been ascertained by single-crystal X-ray diffraction. In contrast, the reduction of the analogous cyclic (alkyl)(amino) carbene (CAAC) adduct yields a C-H activation product.

Olefin Hydroborations with Diamidocarbene–BH3 Adducts at Room Temperature

An isolable N,N’-diamidocarbene (DAC) was previously shown to promote the B–H bond activation of various BH3 complexes. The resultant DAC–BH3 adducts facilitated olefin hydroborations under mild conditions and in the absence of exogenous initiators. The substrate scope for such transformations was further explored and is described herein. While organoboranes were obtained in quantitative yields from various terminal and internal olefins, use of the latter substrates resulted in intramolecular ring-expansion of the newly formed DAC–borane adducts.

Diamidocarbene Induced B–H Activation: A New Class of Initiator-Free Olefin Hydroboration Reagents

Enabled by its enhanced electrophilicity and attenuated nucleophilicity, an N,N′-diamidocarbene (DAC) was found to promote the B–H bond activation of various BH3 complexes and the B–B bond of bis(pinacolato)diboron, constituting the first such examples for an isolable carbene. The resultant DAC-BH3 adducts datively coordinated to Lewis bases (i.e., dimethyl sulfide, trimethylamine, or pyridine) and facilitated the hydroboration of various olefins under mild conditions and in the absence of exogenous initiators.

Reduction of a diamidocarbene-supported borenium cation: isolation of a neutral boryl-substituted radical and a carbene-stabilized aminoborylene.

We have synthesized the first diamidocarbene (DAC)-supported borenium salt, [][OTf], which was found to readily undergo two sequential 1-electron reductions. The first reduction forms a thermally robust, crystalline boryl-substituted DAC-centred radical () which has been fully characterized by XRD, EPR spectroscopy, and DFT analyses. The 1-electron reduction of resulted in the formation of a DAC-supported aminoborylene, , which has been characterized computationally and by multinuclear NMR spectroscopy.

Assessing the reactivity of the N,N′‐diamidocarbenes toward compounds containing early p‐block elements

The ability of an isolable N,N′‐diamidocarbene (DAC) to activate compounds containing early p‐block elements was explored. At ambient temperature, the DAC formally inserted into the Si–H bonds of various silanes to afford the corresponding DAC–silane adducts. The aforementioned adducts did not undergo intramolecular ring expansion as observed with other carbene–silane analogues. The DAC was also found to form a coordination complex with aluminum chloride, and a structurally rich, tris(aluminum) species was obtained upon exposure of the DAC to trimethylaluminum. The results gleaned from these studies are expected to facilitate the development of novel organometallic species as well as of new types of organocatalyzed silylations and aluminations. Copyright © 2014 John Wiley & Sons, Ltd.

Exploring the Chemistry of N,N′-Diamidocarbenes with Organophosphorus Compounds

A readily available N,N'-diamidocarbene (DAC) was found to insert into the P-H bonds of primary and secondary phosphines as well as a phosphonate ester. In contrast, tertiary phosphines catalyzed the ring contraction of the DAC to an iminopyrrolidinedione. Treating the DAC with trimethyl phosphite afforded the corresponding diamidophosphonate ester and olefinic products expected from an Arbuzov-type reaction.

Elucidation of Carbene Ambiphilicity Leading to the Discovery of Reversible Ammonia Activation

An N,N'-diamidocarbene (DAC) was found to activate a broad range of primary as well as secondary aliphatic and aromatic amines. The relative rates measured for the insertion of the DAC into the primary amines were consistent with an electrophilic activation mechanism; in contrast, the DAC functioned as a nucleophile upon treatment with secondary aryl amines. Collectively, these results constituted the first ambiphilic process for an isolable carbene. By comparison, an analogous diaminocarbene was found to serve exclusively as a nucleophile under similar conditions and led to the discovery of the first organic reagent to reversibly activate ammonia.

Elucidating the Mechanism of Reversible Oxiranations via Magnetization Transfer Spectroscopy

The reversible [2 + 1] cycloadditions between an N,N'-diamidocarbene (DAC) and eight aldehydes were examined using NMR spectroscopy. Variable temperature magnetization transfer experiments revealed higher exchange rates and lower activation barriers when electron-deficient aldehydes were employed. Likewise, competitive equilibrium studies indicated a thermodynamic preference for electron-deficient aryl and sterically unhindered alkyl aldehydes compared to more electron-rich or bulkier substrates. Collectively, these and other data collected were consistent with the oxiranation process proceeding in an asynchronous manner.

Exploring the nucleophilicity of N,N′‐diamidocarbenes: Heteroallenes and related compounds as coupling reagents

The propensity of a stable N,N′‐diamidocarbene (DAC) to react with various heteroallenes was explored. The DAC condensed with 4‐nitrophenyl azide as well as 4‐nitrophenyl isothiocyanate to afford the respective acyclic triazene or zwitterionic dihydropyrimidinium imidothiolate products. Treating the DAC with two equivalents of 3‐nitrophenyl isocyanate afforded an iminooxazolidinone rather than the expected hydantoin. Similarly, the addition of diphenylketene to the DAC afforded a dioxolane product, whereas analogous reactions involving N‐heterocyclic carbenes (NHCs) resulted in betaine adducts. Although the DAC as well as various diaminocarbenes condensed with 2,6‐dimethoxyphenyl nitrile N‐oxide to afford the respective nitrosoethylenes, only the six‐membered DAC and NHC adducts rearranged to the corresponding nitrones at elevated temperature. Copyright © 2012 John Wiley & Sons, Ltd.

Alkyne and Reversible Nitrile Activation: N,N′-Diamidocarbene-Facilitated Synthesis of Cyclopropenes, Cyclopropenones, and Azirines

We report the synthesis of a variety of diamidocyclopropenes by combining an isolable and readily accessible N,N'-diamidocarbene (DAC) with a range of alkynes (nine examples, 68-97% yield). Subsequent hydrolysis of selected cyclopropenes afforded the corresponding cyclopropenones or α,β-unsaturated acids, depending on the reaction conditions. In addition, the combination of a DAC with alkyl or aryl nitriles was found to form 2H-azirines in a reversible manner (four examples, K(eq) = 4-72 M(-1) at 30 °C in toluene).

Retracted Article: Homonuclear bond activation using a stable N,N′-diamidocarbene

The activation of molecules possessing homonuclear bonds (i.e., X–X, where X = Br, O, S, or C) using a stable N,N′-diamidocarbene (DAC) is described. Exposing bromine to the DAC at 25 °C afforded a substituted tetrahydropyrimidinium salt in good yield (70%). In contrast, treatment of the DAC with benzoyl peroxide or various disulfides afforded the corresponding diamidoketal or diamidothioketal products, respectively, under mild conditions (25 °C) and in good yield (60–80%). Mechanistic and kinetic studies of the latter reactions were consistent with a concerted process involving the nucleophilic attack of the DAC on the aforementioned substrates. While the diamidoketal decomposed to the corresponding urea (91% yield) at 25 °C, the diamidothioketals produced a thiolate which induced rearrangement to the corresponding ring-opened thioesters (78–90% yield) at elevated temperatures (60 °C). The DAC also inserted into the C(O)–C(O) bond of cyclic and acyclic diones as well as the C(O)–CR bond of a cyclopropenone under mild conditions (25–60 °C); the corresponding products were subsequently isolated in good yield (75–90%).

A Seven-MemberedN,N′-Diamidocarbene

Condensation of N,N′-dimesitylformamidine with phthaloyl chloride afforded 1·HCl, which, upon treatment with base, afforded 2,4-dimesitylbenzo[e][1,3]diazepin-1,5-dione-2-ylidene (1), a seven-membered N,N′-diamidocarbene (DAC), in high yield (85%). The free DAC was used to synthesize four new, late transition metal complexes: [Rh(cod)(1)Cl] (2a) (cod = 1,5-cyclooctadiene), [Ir(cod)(1)Cl] (2b), [Rh(CO)2(1)Cl] (3a), and [1−AuCl] (5). The Tolman electronic parameter (TEP) of 1 was calculated to be 2047 cm−1 from the IR spectrum of 3a. This TEP value is approximately 10 cm−1 lower than known DACs and 5 cm−1 lower than known imidazol-2-ylidenes, indicating that DAC 1 is a relatively strong electron donor. Additionally, electrochemical analyses of 2a and 2b corroborated the IR data obtained on 3a and revealed E1/2 values that were shifted cathodically by ca. 0.16 V when compared to analogous complexes supported by N-heterocyclic carbenes. The gold complex 5 was found to catalyze the hydration of phenylacetylene...