N-Heterocyclic Carbene-Catalyzed (4 + 2) Cycloaddition/Decarboxylation of Silyl Dienol Ethers with α,β-Unsaturated Acid Fluorides

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

doi:10.1021/ja111067j

Cited by 221 publications

(62 citation statements)

References 66 publications

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“…99 ) have been employed as precursors for unsaturated acylammonium salts 100 , whereas acylazoliums are typically prepared from unsaturated aldehydes 97 but require oxidation of the enol homoenolate to the acylazolium ( 103→102 ). Recently, the use of ynals, 36 α-bromo aldehydes 98 , 37 α,β-unsaturated acyl fluorides, 38 and p -nitrophenol esters 39 as starting materials is enabling direct generation of α,β-unsaturated acylazolium intermediates.…”

Section: Complimentarity To αβ-Unsaturated Iminium and Acylazoliumentioning

confidence: 99%

Asymmetric Organocatalysis: The Emerging Utility of α,β‐Unsaturated Acylammonium Salts

Vellalath

Romo

2016

Angew Chem Int Ed

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Although acylammonium salts are well-studied, chiral α,β--unsaturated acylammonium salts have received much less attention. While these intermediates are convenient synthons readily available from several commodity unsaturated acids and acid chlorides and possess three reactive sites, their application in organic synthesis has been limited likely due to a lack of appropriate chiral Lewis bases for their generation. In recent years, the utility of chiral, unsaturated acylammonium salts has expanded considerably demonstrating the unique reactivity of this intermediate leading to the development of a diverse array of catalytic, asymmetric transformations including organocascade processes. This Minireview highlights the recent and growing interest in these intermediates which might spark further research into their untapped potential for asymmetric organocascade catalysis. A cursory comparison is made to related unsaturated iminium and acylazolium intermediates.

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Section: Complimentarity To αβ-Unsaturated Iminium and Acylazoliumentioning

confidence: 99%

Asymmetric Organocatalysis: The Emerging Utility of α,β‐Unsaturated Acylammonium Salts

Vellalath

Romo

2016

Angew Chem Int Ed

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“…1a, for example, Benzoin condensation 21 and the Stetter reaction 22,23 ). 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 .…”

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confidence: 99%

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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“…In this case one postulated the formation of 2-acylimidazolium ion intermediates of type 24 (Scheme 15), the NHC acting as a nuleophilic catalyst. The formation of 2-acylimidazolium ion intermediates has been postulated also in the NHC-catalyzed (4+2)-cycloaddition/decarboxylation of silyl dienol ethers with α,β-unsaturated acyl fluorides [125,126] and for several other NHC-catalyzed reactions involving carboxylic moieties [127][128][129]. …”

Section: N-heterocyclic Carbene-catalyzed Transesterificationsmentioning

confidence: 96%

Organocatalysis: Fundamentals and Comparisons to Metal and Enzyme Catalysis

et al. 2016

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Catalysis fulfills the promise that high-yielding chemical transformations will require little energy and produce no toxic waste. This message is carried by the study of the evolution of molecular catalysis of some of the most important reactions in organic chemistry. After reviewing the conceptual underpinnings of catalysis, we discuss the applications of different catalysts according to the mechanism of the reactions that they catalyze, including acyl group transfers, nucleophilic additions and substitutions, and C-C bond forming reactions that employ umpolung by nucleophilic additions to C=O and C=C double bonds. We highlight the utility of a broad range of organocatalysts other than compounds based on proline, the cinchona alkaloids and binaphthyls, which have been abundantly reviewed elsewhere. The focus is on organocatalysts, although a few examples employing metal complexes and enzymes are also included due to their significance. Classical Brønsted acids have evolved into electrophilic hands, the fingers of which are hydrogen donors (like enzymes) or other electrophilic moieties. Classical Lewis base catalysts have evolved into tridimensional, chiral nucleophiles that are N-(e.g., tertiary amines), P-(e.g., tertiary phosphines) and C-nucleophiles (e.g., N-heterocyclic carbenes). Many efficient organocatalysts bear electrophilic and nucleophilic moieties that interact simultaneously or not with both the electrophilic and nucleophilic reactants. A detailed understanding of the reaction mechanisms permits the design of better catalysts. Their construction represents a molecular science in itself, suggesting that sooner or later chemists will not only imitate Nature but be able to catalyze a much wider range of reactions with high chemo-, regio-, stereo-and enantioselectivity. Man-made organocatalysts are much smaller, cheaper and more stable than enzymes.

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N-Heterocyclic Carbene-Catalyzed (4 + 2) Cycloaddition/Decarboxylation of Silyl Dienol Ethers with α,β-Unsaturated Acid Fluorides

Cited by 221 publications

References 66 publications

Asymmetric Organocatalysis: The Emerging Utility of α,β‐Unsaturated Acylammonium Salts

Asymmetric Organocatalysis: The Emerging Utility of α,β‐Unsaturated Acylammonium Salts

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

Organocatalysis: Fundamentals and Comparisons to Metal and Enzyme Catalysis

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