Influence of Electronic Effects on the Reactivity of Triazolylidene‐Boryl Radicals: Consequences for the use of N‐Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

A small library of triazolylidene‐boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen‐atom donors, as well as that of the corresponding N‐heterocyclic carbene (NHC)‐boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin‐trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene‐borane. The new NHC‐boranes were also used as co‐initiators for the Type I photopolymerization of acrylates. This allowed a structure–reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC‐boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.     

Radical trans‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes

Hydroboration of internal alkynes with N‐heterocyclic carbene boranes (NHC‐boranes) occurs to provide stable NHC (E)‐alkenylboranes upon thermolysis in the presence of di‐tert‐butyl peroxide. The E isomer results from an unusual trans‐hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical‐chain reaction involving addition of an NHC‐boryl radical to an alkyne to give a β‐NHC‐borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC‐borane provides the product and returns the starting NHC‐boryl radical. Experiments suggest that the observed trans‐selectivity results from kinetic control in the hydrogen‐transfer reaction.     

Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

N‐Heterocyclic Carbene–Phosphinidyne Transition Metal Complexes

The N‐heterocyclic carbene–phosphinidene adduct IPr?PSiMe₃ is introduced as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPr?P)ML_n] (ML_n=(η⁶‐p‐cymene)RuCl) and (η⁵‐C₅Me₅)RhCl). Their spectroscopic and structural characteristics, namely low‐field ³¹P NMR chemical shifts and short metal–phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPr?P ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi‐ and trimetallic RuAu, RhAu, Rh₂, and Rh₂Au complexes.     

Visible‐Light Emulsion Photopolymerization of Styrene

The photopolymerization of styrene in emulsion is achieved in a conventional double‐wall reactor equipped with a LED ribbon coiled around the external glass wall. Styrene mixed to acridine orange is added to the water phase containing sodium dodecyl sulfate, a water‐soluble N‐heterocyclic carbene–borane and disulfide, and irradiated. Highly stable latexes are obtained, with particles up to a diameter of 300 nm. The ability to reach such large particle sizes via a photochemical process in a dispersed medium is due to the use of visible light: the photons in the visible range are less scattered by larger objects and thus penetrate and initiate better the polymerizations. They are also greener and cheaper to produce via LEDs, and much safer than UVs. The method presented does not require any specific glassware; it works at lower temperature and delivers larger particles compared to thermal processes at similar solids contents and surfactant concentrations.     

N‐Heterocyclic Carbene Catalyzed Radical Coupling of Aldehydes with Redox‐Active Esters

N‐Heterocyclic carbene catalyzed radical reactions are challenging and underdeveloped. In a recent study, Ohmiya, Nagao and co‐workers found that aldehyde carbonyl carbon centers can be coupled with alkyl radicals under NHC catalysis. An elegant aspect of this study is the use of a redox‐active carboxylic ester that behaves as an single‐electron oxidant to convert the Breslow intermediate into a radical adduct and concurrently release an alkyl radical intermediate as a reaction partner.     

N‐Heterocyclic Carbene–Phosphinidene Complexes of the Coinage Metals

Coinage metal complexes of the N‐heterocyclic carbene–phosphinidene adduct IPr ? PPh (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were prepared by its reaction with CuCl, AgCl, and [(Me₂S)AuCl], which afforded the monometallic complexes [(IPr ? PPh)MCl] (M=Cu, Ag, Au). The reaction with two equivalents of the metal halides gave bimetallic [(IPr ? PPh)(MCl)₂] (M=Cu, Au); the corresponding disilver complex could not be isolated. [(IPr ? PPh)(CuOTf)₂] was prepared by reaction with copper(I) trifluoromethanesulfonate. Treatment of [(IPr ? PPh)(MCl)₂] (M=Cu, Au) with Na(BAr^F) or AgSbF₆ afforded the tetranuclear complexes [(IPr ? PPh)₂M₄Cl₂]X₂ (X=BAr^F or SbF₆), which contain unusual eight‐membered M₄Cl₂P₂ rings with short cuprophilic or aurophilic contacts along the chlorine‐bridged M???M axes. Complete chloride abstraction from [(IPr ? PPh)(AuCl)₂] was achieved with two equivalents of AgSbF₆ in the presence of tetrahydrothiophene (THT) to form [(IPr ? PPh){Au(THT)}₂][SbF₆]₂. The cationic tetra‐ and dinuclear complexes were used as catalysts for enyne cyclization and carbene transfer reactions.     

Metal‐Free Dehydrogenation of Amine‐Boranes by Tunable N‐Heterocyclic Iminoboranes

We report the synthesis of structurally tunable boron complexes supported by N‐heterocyclic imine ligands IPr=N?BR₂ (IPr=[(HCNDipp)₂C], Dipp=2,6‐iPr₂C₆H₃, R=Cl and/or Ph) that have the ability to abstract dihydrogen from amine‐boranes, and instigate their dehydrocoupling. In one instance, mild heating of the hydrogen addition product IPr=NH?B(Ph)HCl releases H₂ to regenerate the starting N‐heterocyclic iminoborane; accordingly IPr=N?B(Ph)Cl can be used as a metal‐free catalyst to promote the dehydrocoupling of MeNH₂ ? BH₃ to yield N‐methylaminoborane oligomers [MeNH‐BH₂]_x.     

Unexpected reactions associated with the xanthate‐mediated polymerization of N‐vinylpyrrolidone

The monomer N‐vinylpyrrolidone (NVP) undergoes side reactions in the presence of R group functional xanthates and impurities. The fate of the monomer NVP and a selection of six O‐ethyl xanthates during xanthate‐mediated polymerization were studied via NMR spectroscopy. A high number of by‐products were identified. Significant side reactions affecting NVP include the formation of an unsaturated dimer and hydration products in bulk or in solution in C₆D₆. In addition, the xanthate adjacent to a NVP unit was found to undergo elimination at moderate temperature (60–70 °C), resulting in unsaturated species and the formation of new xanthate species. The presence of the chlorinated compound α‐chlorophenyl acetic acid, ethyl ester, a precursor in the synthesis of the xanthate S‐(2‐ethyl phenylacetate) O‐ethyl xanthate, resulted in a dramatic increase in the rate of side reactions such as unsaturated dimer formation and a high ratio of unsaturated chain ends. The conditions for the occurrence of such side reactions are discussed in this article, with relevance to increasing the control over the polymerization kinetics, endgroup functionality, and control over the molar mass distribution. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6575–6593, 2008     

Proton‐Induced Generation of Remote N‐Heterocyclic Carbene–Ru Complexes

The proton‐induced Ru?C bond variation, which was previously found to be relevant in the water oxidation, has been investigated by using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)₂(1,5‐phen)]⁺ ([ 2 ]⁺), [Ru(bpy)₂(1,6‐phen)]⁺ ([ 3 ]⁺), and [Ru(bpy)₂(1,7‐phen)]⁺ ([ 4 ]⁺) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocols. Protonation of [ 4 ]⁺ triggered profound electronic structural change to form remote N‐heterocyclic carbene (rNHC), whereas protonation of [ 2 ]⁺ and [ 3 ]⁺ did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related ruthenium catalysts.     

N‐Heterocyclic Carbene Stabilized Boryl Radicals

The reaction of the 2‐(trimethylsilyl)imidazolium triflate 9 with diarylboron halides (4‐R‐C₆H₄)₂BX (R=H, X=Br; R=CH₃, X=Cl; R=CF₃, X=Cl) afforded the NHC‐stabilized borenium cations 10 a – c . Cyclic voltammetry revealed a linear correlation between the Hammett parameter σ _p of the para substituent and the half‐wave potential. Chemical reduction with decamethylcobaltocene, [(C₅Me₅)₂Co], furnished the corresponding radicals 11 a – c ; their characterization by EPR spectroscopy confirmed the paramagnetic character of 11 a – c , with large hyperfine coupling constants to the boron isotopes ¹¹B and ¹⁰B, while delocalization of the unpaired electron into the NHC is negligible. DFT calculations of the percentage of spin density distribution between the carbene (NHC) and the boryl fragments (BR₂) revealed for 11 a – c a spin density ratio (BR₂/NHC) of ca. 9:1, which underlines their distinct boryl radical character. The molecular structure of the most stable species 11 c was established by X‐ray diffraction analysis.     

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.     

From the N‐Heterocyclic Carbene‐Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N‐heterocyclic carbenes (NHCs) tested, only 1,3‐bis(tert‐butyl)imidazol‐2‐ylidene (NHC^tBu) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC‐catalyzed 1,4‐addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well‐defined α‐alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]₀/[ROH]₀ molar ratio, were ultimately obtained in N,N‐dimethylformamide at 25 °C. A hydroxyl‐terminated poly(ethylene oxide) (PEO‐OH) macro‐initiator was also employed to directly access PEO‐b‐PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC^tBu.     

Synthesis and Tunable Reactivity of N‐Heterocyclic Germylene

Modifying the β‐diketimine ligand LH 1 (LH=[ArN?C(Me)? CH?C(Me)? NHAr], Ar=2,6‐iPr₂C₆H₃) through replacement of the proton in 3‐position by a benzyl group (Bz) leads to the new ^BzLH ligand 2, which could be isolated in 77 % yield. According to ¹H NMR spectroscopy, 2 is a mixture of the bis(imino) form [(ArN?C(Me)]₂CH(Bz) 2a and its tautomer [ArN?C(Me)? C(Bz)?C(Me)NHAr] 2b. Nevertheless, lithiation of the mixture of 2a and 2b affords solely the N‐lithiated β‐diketiminate [ArN?C(Me)? C(Bz)?C(Me)? NLiAr], ^BzLLi 3. The latter reacts readily with GeCl_2?dioxane to form the chlorogermylene ^BzLGeCl 4, which serves as a precursor for a new zwitterionic germylene by dehydrochlorination with LiN(SiMe₃)₂. This reaction leads to the zwitterionic germylene ^BzL′Ge: 5 (^BzL′=ArNC(?CH₂)C(Bz)?C(Me)NAr) which could be isolated in 83 % yield. The benzyl group has a distinct influence on the reactivity of zwitterionic 5 in comparison to its benzyl‐free analogue, as shown by the reaction of 5 with phenylacetylene, which yields solely the 1,4‐addition product 6, that is, the alkynyl germylene ^BzLGeCCPh. Compounds 2–8 have been fully characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses, and single‐crystal X‐ray diffraction analyses.     

9‐Boraanthracene Derivatives Stabilized by N‐Heterocyclic Carbenes

2009: A boraanthracene odyssey : General routes to boraanthracene derivatives—long proposed, but never observed—are disclosed, along with the structural and photophysical properties of these compounds and their remarkable reactivity towards oxygen.

     

CC Coupling of Ketones with Methanol Catalyzed by a N‐Heterocyclic Carbene–Phosphine Iridium Complex

An N‐heterocyclic carbene–phosphine iridium complex system was found to be a very efficient catalyst for the methylation of ketone via a hydrogen transfer reaction. Mild conditions together with low catalyst loading (1 mol %) were used for a tandem process which involves the dehydrogenation of methanol, C?C bond formation with a ketone, and hydrogenation of the new generated double bond by iridium hydride to give the alkylated product. Using this iridium catalyst system, a number of branched ketones were synthesized with good to excellent conversions and yields.     

A Crystalline Diazadiborinine Radical Cation and Its Boron‐Centered Radical Reactivity

One‐electron oxidation of 1,4,2,5‐diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF₃)₃}₄ afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X‐ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π‐system of 2 b , as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p‐benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B?O bonds and the monocationic product 2b‐H containing a B?H bond, respectively, thus confirming the boron‐centered radical reactivity of 2 b .     

N‐Heterocyclic Carbene Boranes as Reactive Oxygen Species‐Responsive Materials: Application to the Two‐Photon Imaging of Hypochlorous Acid in Living Cells and Tissues

N‐Heterocyclic carbene (NHC) boranes undergo oxidative hydrolysis to give imidazolium salts with excellent kinetic selectivity for HOCl over other reactive oxygen species (ROS), including peroxides and peroxynitrite. Selectivity for HOCl results from the electrophilic oxidation mechanism of NHC boranes, which stands in contrast to the nucleophilic oxidation mechanism of arylboronic acids with ROS. The change in polarity that accompanies the conversion of NHC boranes to imidazolium salts can control the formation of emissive excimers, forming the basis for the design of the first fluorescence probe for ROS based on the oxidation of B?H bonds. Two‐photon microscope (TPM) ratiometric imaging of HOCl in living cells and tissues is demonstrated.     

Key Role of Intramolecular Metal Chelation and Hydrogen Bonding in the Cobalt‐Mediated Radical Polymerization of N‐Vinyl Amides

This work reveals the preponderance of an intramolecular metal chelation phenomenon in a controlled radical polymerization system involving the reversible trapping of the radical chains by a cobalt complex bis(acetylacetonato)cobalt(II). The cobalt‐mediated radical polymerization (CMRP) of a series of N‐vinyl amides was considered with the aim of studying the effect of the cobalt chelation by the amide moiety of the last monomer unit of the chain. The latter reinforces the cobalt? polymer bond in the order N‐vinylpyrrolidone<N‐vinyl caprolactam<N‐methyl‐N‐vinyl acetamide, and is responsible for the optimal control of the polymerizations observed for the last two monomers. Such a double linkage between the controlling agent and the polymer, through a covalent bond and a dative bond, is unique in the field of controlled radical polymerization and represents a powerful opportunity to fine tune the equilibrium between latent and free radicals. Possible hydrogen bond formation is also taken into account in the case of N‐vinyl acetamide and N‐vinyl formamide. These results are essential for understanding the factors influencing Co? C bond strength in general, and the CMRP mechanism in particular, but also for developing a powerful platform for the synthesis of new precision poly(N‐vinyl amide) materials, which are an important class of polymers that sustain numerous applications today.