Investigation of N‐Heterocyclic Carbene‐Supported Group 12 Triflates as Pre‐catalysts for Hydrosilylation/Borylation

Roy, Matthew M. D.; Ferguson, Michael J.; McDonald, Robert; Rivard, Eric

doi:10.1002/chem.201603704

Cited by 25 publications

(7 citation statements)

References 71 publications

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“…Very recently, we noted that the organocatalytic hydroborylation of ketonic substrates can be accomplished with hindered NHOs such as IPrCH 2 and its backbone methylated counterpart Me IPrCH 2 (Scheme ). , As a benchmark reaction, benzophenone underwent catalytic borylation with HBpin (Bpin = B(OCMe 2 ) 2 ) in the presence of 5 mol % of IPrCH 2 in THF to give quantitative conversion to the hydroborylation product Ph 2 HCO(Bpin) after 18 h at room temperature; when IPr was used as a catalyst under similar conditions, only trace amounts of Ph 2 HCO(Bpin) formed . A range of functionalized substrates, including the sterically hindered aldehyde MesC(O)H (Mes = 2,4,6-Me 3 C 6 H 2 ) and alkyl-functionalized ketones (such as cyclohexanone) could also be hydroborylated under mild conditions.…”

Section: N-heterocyclic Olefins In Organocatalysismentioning

confidence: 99%

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry

2017

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N-Heterocyclic olefins (NHOs) have gone from the topic of a few scattered (but important) reports in the early 1990s to very recently being a ligand/reagent of choice in the far-reaching research fields of organocatalysis, olefin and heterocycle polymerization, and low oxidation state main group element chemistry. NHOs are formally derived by appending an alkylidene (CR) unit onto an N-heterocyclic carbene (NHC), and their pronounced ylidic character leads to high nucleophilicity and soft Lewis basic character at the ligating carbon atom. These olefinic donors can also be structurally derived from imidazole, triazole, and thiazole-based heterocyclic carbenes and, as a result, have highly tunable electronic and steric properties. In this Account, we will focus on various synthetic routes to imidazole-2-ylidene derived NHOs (sometimes referred to as deoxy-Breslow intermediates) followed by a discussion of the electron-donor ability of this structurally tunable ligand group. It should be mentioned that NHOs have a close structural analogy with Breslow-type intermediates, N-heterocyclic ketene aminals, and β-azolium ylides; while these latter species play important roles in advancing synthetic organic chemistry, discussion in this Account will be confined mostly to imidazole-2-ylidene derived NHOs. In addition, we will cover selected examples from the literature where NHOs and their anionic counterparts, N-heterocyclic vinylenes, are used to access reactive main group species not attainable using traditional ligands. Added motivation for these studies comes from the emerging number of low coordinate main group element based compounds that display reactivity once reserved for precious metal complexes (such as H-H and C-H bond activation). Moreover, NHOs are versatile precursors to new mixed element (P/C and N/C), and potentially bidentate, ligand constructs of great potential in catalysis, where various metal oxidation states and coordination environments need to be stabilized during a catalytic cycle. The most active area of recent growth for NHOs is their use as nucleophiles to promote efficient organocatalytic transformations, including transesterification, carbonyl reduction, and the conversion of CO into value added products. Polyesters have also been generated through the NHO-promoted ring-opening polymerization of lactones, and the highly tunable nature of NHO organocatalysts allows for the rapid screening and enhancement of catalytic performance. Therefore, the growing utility of NHOs in the realm of organic and polymer chemistry can be viewed as evidence of the widespread impact of N-heterocyclic olefins on the chemical community. It is hoped that through this Account others will join this flourishing research domain and that the rapid recent growth of NHO chemistry is sustained for the foreseeable future.

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Section: N-heterocyclic Olefins In Organocatalysismentioning

confidence: 99%

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry

2017

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“…Here, we introduce a new class of EDL: 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbene (NHC) ligands, with hydrogen (H), methyl (Me), or chloride (Cl) substituents (Scheme ), that address many of the issues with dithiocarbamate and thiolate EDLs. We targeted NHCs as EDLs because, much like dithiocarbamates and thiolates, NHCs interact through both s- and p-type orbitals located on the carbene carbon, − which has the potential to bind to either cadmium − or selenium − or both on the QD surface, and because there is literature precedent for NHCs as ligands for metal clusters − and metal nanoparticles. − The imidazolylidene NHCs in particular are partially aromatic; thus, modification of substituents at the 4,5-positions affects their π acidity and should affect the degree of exciton delocalization enabled by the molecule . We find that all three NHC derivatives studied in this work delocalize the QD exciton; as expected for an EDL, the magnitude of the apparent increase in excitonic radius (Δ R ) per bound NHC ligand depends on the substituent.…”

Section: Introductionmentioning

confidence: 99%

N-Heterocyclic Carbenes as Reversible Exciton-Delocalizing Ligands for Photoluminescent Quantum Dots

2020

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Delocalization of excitons within semiconductor quantum dots (QDs) into states at the interface of the inorganic core and organic ligand shell by so-called “exciton-delocalizing ligands (EDLs)” is a promising strategy to enhance coupling of QD excitons with proximate molecules, ions, or other QDs. EDLs thereby enable enhanced rates of charge carrier extraction from, and transport among, QDs and dynamic colorimetric sensing. The application of reported EDLswhich bind to the QDs through thiolates or dithiocarbamatesis however limited by the irreversibility of their binding and their low oxidation potentials, which lead to a high yield of photoluminescence-quenching hole trapping on the EDL. This article describes a new class of EDLs for QDs, 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbenes (NHCs), where the 4,5-substituents are Me, H, or Cl. Postsynthetic ligand exchange of native oleate capping ligands for NHCs results in a bathochromic shift of the optical band gap of CdSe QDs (R = 1.17 nm) of up to 111 meV while the colloidal stability of the QDs is maintained. This shift is reversible for the MeNHC-capped and HNHC-capped QDs upon protonation of the NHC. The magnitude of exciton delocalization induced by the NHC (after scaling for surface coverage) increases with the increasing acidity of its π system, which depends on the substituent in the 4,5-positions of the imidazolylidene. The NHC-capped QDs maintain photoluminescence quantum yields of up to 4.2 ± 1.8% for shifts of the optical band gap as large as 106 meV.

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“…Hydroboration reduction of carbonyl compounds is a vital transformation in organic synthesis as the resultant borates provide an efficient approach to alcohols via hydrolysis. , In comparison to the traditional stoichiometric reduction by using light metal hydrides, catalyzed hydroboration of aldehydes and ketones has been realized by means of a vast number of catalysts ranging from transition metals, ,,− main group elements, − and lanthanide complexes. , In most cases, however, these compounds/complexes are either expensive or difficult to prepare. As noted earlier, studies on the hydroboration reactions driven by simple main group metal catalysts are rather limited.…”

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confidence: 99%

n-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones

Zhu

et al. 2018

J. Org. Chem.

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Highly efficient and selective hydroboration of aldehydes and ketones with HBpin is achieved by using the simple and convenient n-BuLi as a catalyst. The reaction proceeds rapidly with low catalyst loading (0.1-0.5 mol %) under mild conditions. Key features include the high catalytic efficiency, exceptional functional group compatibility, ample substrate scope, and high selectivity for aldehydes over ketones. Computational studies were carried out to provide a mechanistic insight into the n-BuLi catalyzed hydroboration of aldehydes/ketones with HBpin.

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Investigation of N‐Heterocyclic Carbene‐Supported Group 12 Triflates as Pre‐catalysts for Hydrosilylation/Borylation

Cited by 25 publications

References 71 publications

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry

Pushing Chemical Boundaries with N-Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element Chemistry

N-Heterocyclic Carbenes as Reversible Exciton-Delocalizing Ligands for Photoluminescent Quantum Dots

n-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones

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