Acyl anion free N-heterocyclic carbene organocatalysis

Ryan, Sarah J.; Candish, Lisa; Lupton, David W.

doi:10.1039/c3cs35522e

Cited by 761 publications

(121 citation statements)

References 62 publications

(45 reference statements)

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“…The corresponding heavier Group 14 analogues (diamidotetrylenes; (R 2 N) 2 E, E = Si, Ge, Sn, Pb) have a much longer history. [4] It is, perhaps, surprising then that the corresponding P-substituted species (diphosphatetrylenes; (R 2 P) 2 E) [5][6][7] and their carbene homologues (PHCs) [8] are rather less well established.…”

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confidence: 99%

“…Since the discovery of the first stable example in 1991, Nheterocyclic carbenes (NHCs) and their acyclic analogues have become indispensable, both as ligands for catalytically active transition-metal complexes and as organocatalysts themselves; [1,2] recent reports even suggest that NHC complexes of transition metals may exhibit anti-tumor activity. [3] The versatility of NHCs and related compounds is due to their unusually strong s-donor (and correspondingly weak pacceptor) properties and their remarkable stabilities.…”

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Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

et al. 2014

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N-Heterocyclic carbenes and their heavier homologues are, in part, stabilized by delocalization of the N lone pairs into the vacant p-orbital at carbon (or a heavier Group 14 element center). These interactions are usually absent in the corresponding P-substituted species, owing to the large barrier to planarization of phosphorus. However, judicious selection of the substituents at phosphorus has enabled the synthesis of a diphosphagermylene, [(Dipp) 2 P] 2 Ge, in which one of the P centers is planar (Dipp = 2,6-diisopropylphenyl). The planar nature of this P center and the correspondingly short PÀGe distance suggest a significant degree of PÀGe multiple bond character that is due to delocalization of the phosphorus lone pair into the vacant p-orbital at germanium. DFT calculations support this proposition and NBO and AIM analyses are consistent with a Ge À P bond order greater than unity.

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Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

et al. 2014

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“…NHCs can also react with acyl halides/anhydrides/esters 24,25 , electron-deficient olefins 26,27 and ketenes 28,29 , forming either highly reactive acylating agents or Baylis-Hillman-type intermediates through a nucleophilic addition reaction (Fig. 1b,c) 30 . These intermediates can undergo a series of stereoselective nucleophilic and electrophilic reactions to produce functionalized carbonyl compounds.…”

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Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates

2014

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N-heterocyclic carbenes are a class of persistent carbenes stabilized by adjacent heteroatoms that are part of a heterocycle. They play a central role in multiple enzymatic biosynthetic reactions that involve thiamine diphosphate. Inspired by this biocatalysis machinery, N-heterocyclic carbenes have emerged as one of the most versatile classes of organocatalysts for organic reactions. However, the asymmetric synthesis of carbon-carbon bonds through a non-covalent interaction mechanism has not been previously established for chiral carbenes. Here, we report an N-heterocylic carbene-catalysed, highly enantioselective process that uses weak hydrogen bonds to relay asymmetric bias. We find that catalytic amounts of hexafluoroisopropanol are the critical proton shuttle that facilitates hydrogen transfer to provide high-reaction rates and high enantioselectivity. We demonstrate that a successful asymmetric reaction of this type can be accomplished through a rational design that balances the pKa values of the substrate, the carbene precursor and the product.

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“…According to the hypothesis of Breslow, the heart of the mechanism is the nucleophilic attack of NHC to the carbonyl function of an aldehyde yielding a zwitterionic structure, which is converted (by an intramolecular proton transfer) into an acylanion equivalent (the Breslow intermediate). The coupling of the Breslow intermediate with a second aldehyde molecule yields the product (α-hydroxyketone, benzoin) and regenerates the NHC [15][16][17][18].…”

Section: Umpolung and The Peculiar Chemistry Of Nhcsmentioning

confidence: 99%

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

2016

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In the last twenty years, N-heterocyclic carbenes (NHCs) have acquired considerable popularity as ligands for transition metals, organocatalysts and in metal-free polymer synthesis. NHCs are generally derived from azolium based salts NHCH+X− by deprotonation or reduction (chemical or electrochemical) of NHCH+. The extensive knowledge of the physicochemical properties of NHCH+/NHC system could help to select the conditions (scaffold of NHC, nature of the counter-ion X−, solvent, etc.) to enhance the catalytic power of NHC in a synthesis. The electrochemical behavior of NHCH+/NHC system, in the absence and in the presence of solvent, was extensively discussed. The cathodic reduction of NHCH+ to NHC and the anodic oxidation of NHC, and the related effect of the scaffold, solvent, and electrodic material were emphasized. The electrochemical investigations allow acquiring further knowledge as regards the stability of NHC, the acidic and nucleophilic properties of NHCH+/NHC system, the reactivity of NHC versus carbon dioxide and the effect of the hydrogen bond on the catalytic efficiency of NHC. The question of the spontaneous or induced formation of NHC from particular ionic liquids was reconsidered via voltammetric analysis. The results suggested by the classical and the electrochemical methodologies were compared and discussed.

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Acyl anion free N-heterocyclic carbene organocatalysis

Cited by 761 publications

References 62 publications

Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

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