N-Heterocyclic Carbene Boranes Research Articles

A Second-Generation Chameleon N-Heterocyclic Carbene–Borane Coinitiator for the Visible-Light Oxygen-Resistant Photopolymerization of Both Organic and Water-Compatible Resins

N-Heterocyclic carbene–boranes (NHC-boranes, NHCBs) were recently described as efficient coinitiators for the visible light photopolymerization of hydroxyethyl methacrylate (HEMA) in the presence of water. In this work, a new, more water-soluble, NHC-borane—2,4-dimethyl-1,2,4,5-tetrazol-3-ylidene borane—has been synthesized, and its efficiency in three-component systems (dye (Acridine Orange), disulfide, and NHC-borane coinitiators) for the polymerization of methacrylate resins under visible light under air has been studied. In fluid resins this new photoinitiating system (PIS) gives better results than the one previously studied. More importantly, this system is competitive with the widely used type II system—camphorquinone/4-(dimethylamino)benzonitrile (DMABN)—for the polymerization of methacrylate resins but better for the photopolymerization of poly(2-hydroxyethyl methacrylate) leading to hydrogels, where the former PIS fails. The excellent ability of the NHC-boranes, and especially the new one, to be...

Endoplasmic Reticulum-Targeted Ratiometric N-Heterocyclic Carbene Borane Probe for Two-Photon Microscopic Imaging of Hypochlorous Acid.

The naphthoimidazolium borane 4 is shown to be a selective probe for HOCl over other reactive oxygen species. Unlike other boronate-reactive oxygen species (ROS) fluorogenic probes that are oxidized by HOCl through a nucleophilic borono-Dakin oxidation mechanism, probe 4 is distinguished by its electrophilic oxidation mechanism involving B-H bond cleavage. Two-photon microscopy experiments in living cells and tissues with the probe 4 demonstrate the monitoring of endogenous HOCl generation and changes in HOCl concentrations generated in the endoplasmic reticulum during oxidative stress situations.

Reactions of N-Heterocyclic Carbene Boranes with 5-Diazo-2,2-dimethyl-1,3-dioxane-4,6-dione: Synthesis of Mono- and Bis-hydrazonyl NHC-Boranes.

N-Heterocyclic carbene boranes (NHC-boranes) react with 5-diazo-2,2-dimethyl-1,3-dioxane-4,6-dione at 40 °C in dichloromethane to provide NHC-boryl hydrazone derivatives of 2,2-dimethyl-1,3-dioxane-4,6-dione. These hydrazones disproportionate to bis-hydrazones on treatment with diiodine in dichloromethane at room temperature. The mono- and bis-hydrazones are yellow solids that are stable to chromatography and storage.

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.

Juyoung Yoon

Angewandte Chemie International EditionVolume 57, Issue 32 p. 10010-10010 Author ProfileFree Access Juyoung Yoon First published: 19 April 2018 https://doi.org/10.1002/anie.201803425AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract “… If I were not a scientist, I would be a medical doctor. My favorite food is pasta …” Find out more about Juyoung Yoon in his Author Profile. Juyoung Yoon The author presented on this page has recently published his 10th article in Angewandte Chemie in the last 10 years: “N-Heterocyclic Carbene Boranes as Reactive Oxygen Species-Responsive Materials: Application to the Two-Photon Imaging of Hypochlorous Acid in Living Cells and Tissues”: Y. L. Pak, S. J. Park, D. Wu, B. Cheon, H. M. Kim, J. Bouffard, J. Yoon, Angew. Chem. Int. Ed. 2018, 57, 1567; Angew. Chem. 2018, 130, 1583. The work of J. Yoon has been featured on the inside cover of Angewandte Chemie: “Thin-Film Formation of Imidazolium-Based Conjugated Polydiacetylenes and Their Application for Sensing Anionic Surfactants”: X. Chen et al., Angew. Chem. Int. Ed. 2010, 49, 1422; Angew. Chem. 2010, 122, 1464. Date of birth: June 23, 1964 Position: Professor, Ewha Womans University E-mail: jyoon@ewha.ac.kr Homepage: http://home.ewha.ac.kr/∼jyoon/ ORCID: 0000-0002-1728-3970 Education: 1987 Undergraduate degree, Seoul National University 1994 PhD supervised by Anthony W. Czarnik, The Ohio State University 1994–1996 Postdoctoral work with Donald J. Cram, University of California, Los Angeles 1996–1998 Postdoctoral work with Kim D. Janda, The Scripps Research Institute, La Jolla Awards: 2012 Knowledge Creation Grand Prize, Korean Ministry of Science, ICT and Future Planning; 2016 Korean Chemical Society Award; 2016 Ewha Academic Award, Ewha Womans University Research: Fluorescent probes; activatable photosensitizers; organic functional materials; theranostics Hobbies: Watching movies, travelling If I were not a scientist, I would be a medical doctor. My favorite food is pasta. My top three film (series) of all time are Masquerade, The Sound of Music, Harry Potter. My favorite piece of music is Fifty Days of Summer (made by MARUKO on Soundcloud). My favorite motto is to do your best and then relax. If I won the lottery, I would buy a summer house for my family. When I'm frustrated, I talk to my wife. The most important thing I learned from my parents is that parents have to make great sacrifices for their children. If I could have dinner with three famous scientists from history, they would be Albert Einstein, John Dalton, and Marie Skłodowska Curie. The biggest challenge that scientists face is finding a cure for cancer and Alzheimer's disease. I chose chemistry as a career because I thought I could use it to make anything that I wanted. The best advice I have ever been given is to take care of my health. Guaranteed to make me smile is an acceptance mail from an editor. My 5 top papers: 1“Unique Sandwich Stacking of Pyrene-Adenine-Pyrene for Selective and Ratiometric Fluorescent Sensing of ATP at Physiological pH”: Z. Xu, N. J. Singh, J. Lim, J. Pan, H. N. Kim, S. Park, K. S. Kim, J. Yoon, J. Am. Chem. Soc. 2009, 131, 15528. (Selective recognition for ATP could be obrained through an intelligent molecular recognition process.) 2“Zn2+–Triggered Amide Tautomerization Produces a Highly Zn2+–Selective, Cell–Permeable, and Ratiometric Fluorescent Sensor”: Z. Xu, K.-H. Baek, H. N. Kim, J. Cui, X. Qian, D. R. Spring, I. Shin, J. Yoon, J. Am. Chem. Soc. 2010, 132, 601. (Explains ligand transformation toward different metal ions and is one of the most highly cited papers from our group.) 3“Development of Imidazoline-2-Thiones Based Two-Photon Fluorescence Probes for Imaging Hypochlorite Generation in a Co-Culture System”: Q. Xu, C. H. Heo, G. Kim, H. W. Lee, H. M. Kim, J. Yoon, Angew. Chem. Int. Ed. 2015, 54, 4890; Angew. Chem. 2015, 127, 4972. (An old organic reaction is still very valuable.) 4“A Fluorescent Sensor for Dual-Channel Discrimination between Phosgene and a Nerve-Gas Mimic”: X. Zhou, Y. Zeng, C. Liyan, X. Wu, J. Yoon, Angew. Chem. Int. Ed. 2016, 55, 4729; Angew. Chem. 2016, 128, 4807. (A single probe can display different fluorescent and colorimetric changes for phosgene and nerve gas mimic through the formation of different products.) 5“Nanostructured Phthalocyanine Assemblies with Protein-Driven Switchable Photoactivities for Biophotonic Imaging and Therapy”: X. Li, C-y. Kim, S. Lee, D. Lee, H.-M. Chung, G. Kim, S.-H. Heo, C. Kim, K. S. Hong, J. Yoon, J. Am. Chem. Soc. 2017, 139, 10880. (We expect that these results can enable us to start a new scientific journey.) Volume57, Issue32August 6, 2018Pages 10010-10010 ReferencesRelatedInformation

Synergistic Photoredox Catalysis and Organocatalysis for Inverse Hydroboration of Imines.

The first catalytic inverse hydroboration of imines with N-heterocyclic carbene (NHC) boranes has been realized by means of cooperative organocatalysis and photocatalysis. This catalytic combination provides a promising platform for promoting NHC-boryl radical chemistry under sustainable and radical-initiator-free conditions. The highly important functional-group compatibility and possible application in late-stage hydroborations represent an important step forward to an enhanced α-amino organoboron library.

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.

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.

Chiral organoboranes built by bacteria

Although there are natural products that contain boron, the organisms that make these compounds do so by using small molecules that react spontaneously with boric acid in the environment. No enzymes are involved. What’s more, natural organisms that create carbon-boron bonds, or organoboranes, are unknown. But that didn’t stop Frances H. Arnold, S. B. Jennifer Kan, Xiongyi Huang, and coworkers at Caltech, who reasoned that with a little genetic tinkering they could create organoborane-making bacteria. The new approach could embellish current methods to make organoboranes, which are important synthetic building blocks for organic chemists. The researchers took Escherichia coli cells that were engineered with wild-type cytochrome c from the geothermal bacterium Rhodothermus marinus and incubated them with an N-heterocyclic carbene borane and a diazoester, finding they could create a chiral organoborane (reaction shown) in modest yield and with good enantioselectivity (Nature 2017, DOI: 10.1038/nature24996). ...

N-Heterocyclic Carbene Boranes are Hydrogen Donors in Masamune-Bergman Reactions of Benzo[3,4]cyclodec-3-ene-1,5-diynes.

Thermal reactions of benzo[3,4]cyclodec-3-ene-1,5-diyne with N-heterocyclic carbene boranes (NHC-boranes) provided mixtures of 9-borylated 1,2,3,4-tetrahydroanthracenes along with 1,2,3,4-tetrahydroanthracene. These products indicate that NHC-boranes serve as hydrogen donors to a p-benzyne intermediate formed by the Masamune-Bergman reaction. Experimental results support a radical mechanism in nonpolar solvents, but suggest that ionic mechanisms compete in the production of 1,2,3,4-tetrahydroanthracene when the reaction is performed in a polar solvent.

Synthesis of N-heterocyclic carbene boranes via silver N-heterocyclic carbene complexes

The reaction of N-heterocyclic carbene (NHC) complexes of silver with BH3 or BEt3 was investigated. On treatment of IiPr·AgCl (1-AgCl) (IiPr=1,3-diisopropylimidazol-2-ylidene) with two equivalents of NaBH4-free BH3·THF (purified BH3·THF) in THF at room temperature for 2h, IiPr·BH3 complex (1-BH3) was obtained in excellent yield (96%). In the reaction of various silver NHC complexes with purified BH3·THF, the corresponding NHC·BH3 complexes were obtained in moderate to good yields. The reaction of 1-AgCl with two equivalents of BEt3, instead of BH3, afforded IiPr·BEt3 complex (1-BEt3) in 86% yield. This procedure enabled the preparation of BEt3-adducts of imidazol- and benzimidazol-2-ylidenes.

N-Heterocyclic Carbene Boryl Iodides Catalyze Insertion Reactions of N-Heterocyclic Carbene Boranes and Diazoesters.

Boron-hydrogen bond insertion reactions of N-heterocyclic carbene (NHC) boranes and diazoesters can be catalyzed by NHC-boryl iodides and produce stable α-NHC-boryl esters. The conditions of the reaction resemble the previous rhodium-catalyzed transformations (only the catalyst is different); however, the mechanisms of the two reactions are probably very different. The new boryl iodide catalyzed method is adept at producing α-substituted-α-NHC-boryl esters, and this has led to a family of NHC-boryl esters with amino acid and amino-acid-like side chains.

Catalytic Electrophilic C−H Borylation Using NHC⋅Boranes and Iodine Forms C2‐, not C3‐, Borylated Indoles

Activation of N-heterocyclic carbene boranes (NHC⋅BH3 ) by I2 enables the metal-free catalytic C-H borylation of heteroarenes with formation of H2 as the by-product in a process that uses only bench stable precursors. The borylation of indoles using NHC⋅BH3 /I2 produces C2-borylated indoles exclusively in contrast to other catalytic electrophilic C-H borylation methods. Mechanistic studies indicate that this is due to C3 to C2 boron migration facilitated by the absence of exogenous Brønsted bases. Thus this C-H borylation methodology proceeds under sufficiently Brønsted acidic conditions to enable the thermodynamic C2-borylated indole isomer to be formed instead of the C3 borylated-isomer. This demonstrates that electrophilic C-H borylation can be used to access a wider range of borylated regioisomers than reported to date.

Radical Silyldifluoromethylation of Electron-Deficient Alkenes.

A reaction of bromo- and iododifluoromethyl-substituted silanes with electron-deficient alkenes in the presence of an N-heterocyclic carbene borane complex is described. The reaction is performed under irradiation with light-emitting diodes and proceeds via a radical chain mechanism. The resulting products, the functionalized silicon reagents, can undergo chemoselective transformations involving either the silyldifluoromethyl fragment or the functional group.

Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-diynes and N-Heterocyclic Carbene-Boranes.

Borylative radical cyclization of benzo[3,4]cyclodec-3-ene-1,5-diynes to provide 5-borylated 6,7,8,9-tetrahydrobenzo[a]azulenes has been developed. The experimental results suggest that the reaction proceeds by a radical chain mechanism, in which di-tert-butyl hyponitrite (TBHN) works as a good radical initiator to form boryl radicals from N-heterocyclic carbene-boranes (NHC-boranes). The present reaction is a rare model that illustrates addition of boryl radicals to alkynes.

Synthesis of Boriranes by Double Hydroboration Reactions of N-Heterocyclic Carbene Boranes and Dimethyl Acetylenedicarboxylate.

Reaction of bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene borane with dimethyl acetylenedicarboxylate gives 80% yield of a stable borirane (boracyclopropane) formed by formal double hydroboration along with 5% of the (E)-alkenylborane. DFT calculations suggest a mechanism where divergence to the two products occurs after a common initial stage of hydride transfer from the NHC-borane to the acetylenedicarboxylate.

An efficient FeCl3-mediated approach for reduction of ketones through N-heterocyclic carbene boranes

An efficient FeCl3-mediated approach for reduction of ketones by NHC-BH3 has been developed. A series of ketones were smoothly converted to the corresponding alcohols in good to excellent yields through NHC-BH3 reductive process.

Diastereoselective Radical Hydrogen Transfer Reactions using N-Heterocyclic Carbene Boranes.

Reported herein are the first diastereoselective and Lewis acid-mediated radical reactions of N-heterocyclic carbene (NHC) boranes. We applied these reactions to the synthesis of four propionate diastereoisomers combining an aldol reaction, followed by a stereoselective radical-based reduction in which the NHC borane serves as the hydrogen donor, thus obviating the use of tin-based reagents. The 2,3-syn isomer is obtained by combining an NHC-borane and a Lewis acid (MgBr2·OEt2), while using a reverse polarity strategy provides the 2,3-anti isomer.

Radical and Thermal Reactions of N-Heterocyclic Carbene Boranes with Diazo Compounds

Reactions of NHC-boranes with several diazo compounds and one diazonium salt give new classes of N-substituted NHC-boranes. Reactions with diazo compounds and azo initiators such as AIBN provide N-(cyanoalkyl)-N-borylhydrazones. However, the reactant diazo 2,2-dimethyl-1,3-dioxane-4,6-dione adds directly to the NHC-borane to give an N-boryl hydrazone product arising formally from 1,1-hydroboration. A similar thermal reaction of an NHC-borane with a diazonium salt gives an intermediate that upon neutralization undergoes air oxidation to provide the first NHC-boryl azoxy compound.

ChemInform Abstract: Asymmetric Reduction of tert‐Butanesulfinyl Ketimines by N‐Heterocyclic Carbene Boranes.

N-heterocyclic carbene borane (NHC-borane) based on a triazole core is demonstrated for the first time to be efficient for reduction of a variety of tert-butanesulfinyl ketimines. Up to 95% yield and up to >99% diastereomeric excess were achieved. NHC-borane exhibited excellent activities that are more efficient than or comparable to commonly used reductive reagents such as NaBH4, NaBH3CN, l-selectride, Ru catalyst, or BH3–THF.