Hidden Brønsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates

Dang, Tuan Thanh; Boeck, Florian; Hintermann, Lukas

doi:10.1021/jo201631x

Cited by 277 publications

(204 citation statements)

References 66 publications

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“…Alternatively, triflic acid might be generated in situ from the metal triflate. [23][24][25][26][27] We feel our work confirms Agosta and Wolff's long-standing hypothesis that alkene 6 is the key intermediate in the acid-promoted formation of the norcamphor 7. Our unified mechanistic proposal is shown in Scheme 3.…”

supporting

confidence: 81%

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Jiang

Pan

Douglas

2015

Tetrahedron Letters

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a b s t r a c tThe cyclization of an alkene-bearing cyclopentanone to a [2.2.1]-norcamphor ring system is described. The reaction is catalyzed by a combination of rhodium and Brønsted acid. Control experiments indicate that both are needed for acceptable yield. Control experiments with bulky base additives show that rhodium promotes alkene isomerization, likely the first step of this cascade reaction, and that rhodium alone does not promote cyclization. Cyclization is promoted by Brønsted acid in a Prins-type cyclization and carbocation rearrangement process. Trace Brønsted acid present in commercial samples of Rh(cod) 2 OTf is likely responsible for the observed reaction. Indeed, the norcamphor product can be obtained simply with strong acid, presumably initiated by acid-promoted alkene isomerization. Since our initial motivation for this work was the development of rhodium catalysts for the activation of C-C bonds adjacent to ketones, this communication serves to identify other, perhaps less obvious, pathways for the reactions of unsaturated ketone compounds by the action of rhodium catalysts.Ó 2015 Elsevier Ltd. All rights reserved.Carbon-carbon sigma (C-C) bond activation is a growing area of research for organic synthesis. 1 Our group has developed alkene carboacylation reactions based on the insertion of rhodium adjacent to unstrained ketones. 2,3 The general mechanistic strategy is to couple the activation of the C-C bond between a carbonyl carbon and the a-carbon with the insertion of an alkene. 4,5 In our prior work, we placed a nitrogen five atoms away from the carbonyl carbon to provide a chelate for C-C activation and prevent decarbonylation prior to an alkene engaging the activated bond in a productive capturing event. 6,7 Others have developed related alkene carboacylation reactions (sometimes termed 'cut-andsew') without the judiciously placed heteroatom within the substrate, but this work is largely limited to strained ring ketones for starting materials, most often cyclobutanones. [8][9][10][11][12] Our goal at the outset of this work was to investigate similar chemistry of cyclopentanones, which have substantially less ring strain than the corresponding cyclobutanones. Herein, we report an unexpected cyclization encountered in our study. We show that a combination of alkene isomerization and a likely acid-mediated Prins-type cyclization and semi-pinacol rearrangement can account for the formation of this unexpected product. Our purpose in communicating this work is to demonstrate the need for an appropriate control experiment to rule out classic reaction pathways when developing C-C bond activation approaches to alkene carboacylation reactions.We designed cyclopentanone 1 with the notion that rhodium might reversibly insert adjacent to the carbonyl. The tethered alkene could then coordinate to the metal center to promote carboacylation potentially generating the bridged compound 2 via alkene carboacylation (Scheme 1). This mechanistic thinking is in line with the documented cyclobutanone carboac...

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confidence: 81%

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Jiang

Pan

Douglas

2015

Tetrahedron Letters

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“…21 Since no significant decrease in catalytic activity was observed, the Bi(III)-species is assumed to be the active catalyst. 22 In all cases the amidoalkylated product was obtained as a mixture of regioisomers (15:1 ortho/para-vs. ortho/ortho-substitution) With the optimized conditions established, the scope of reaction was examined (Scheme 1).…”

Section: Three-component Reaction With Formaldehydementioning

confidence: 99%

Bi(OTf)₃-Catalyzed Multicomponent α-Amidoalkylation Reactions

Schneider

Manolikakes

2015

J. Org. Chem.

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“…18 For this purpose, we chose benzhydryl bromide (5) as a test substrate (Scheme 3). 19a Heterolytic cleavage of its C-Br bond and a subsequent Ritter-like solvolysis reaction of the intermediate carbocation 6 in wet acetonitrile yields amide 7.…”

Section: Figurementioning

confidence: 99%

Multidentate Halogen-Bond Donors as Lewis Acidic Activators or Catalysts in Halide Abstraction Reactions

Jungbauer

Schindler

Kniep

et al. 2013

Synlett

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Although halogen bonds share many similarities with hydrogen bonds, they have so far found virtually no application in organic synthesis. This account summarizes our efforts to use multidentate halogen-bond donors (halogen-based Lewis acids) in formal halide abstraction reactions. Following a first proof-ofprinciple study, we recently reported the first halogen-bond-based organocatalytic carbon-carbon bond-forming reaction.Hydrogen bonds are ubiquitous in Nature. As arguably the most important type of non-covalent interactions, they are crucial for various processes of life, for example, the specific binding of substrates in enzyme pockets. 1 Mimicking Nature, hydrogen-bond donors such as thiourea derivatives have, in the last 15 years, been established as potent non-covalent organocatalysts. 2 Although it has been known for a long time 3 that certain halogen substituents form similar non-covalent interactions with Lewis bases (LB; Figure 1, a), these so-called 'halogen bonds' 4 have so far found very little use in synthetic organic chemistry. Figure 1Halogen bonding: a) general definition (X = F, Cl, Br, I; LB = Lewis base); b) charge-transfer interaction; c) electrostatic potential (red = negative potential, blue = positive potential) of CF 3 I (violet = iodine, grey = carbon, cyan = fluorine) illustrating the σ-hole (in blue) on the iodine substituent Traditionally, halogen bonds have predominantly been described as n→σ* charge-transfer interactions (Figure 1, b), 5 while in recent years, the electrostatic contribution to the overall interaction energy has been more in focus. 6 The latter is often visualized by the 'σ-hole', a region of positive electrostatic potential on the halogen substituent ( Figure 1, c). 6a Strong halogen-bond donors (halogenbased Lewis acids) R-X are obtained for X = I > Br > Cl, and very electron-withdrawing (or highly polarizable) 7a R substituents. In contrast to hydrogen bonds, halogen bonds invariably feature a high directionality, with an R-X···LB angle of approximately 180°. 4,7b,c In the last two decades, halogen bonds have mostly found applications in the solid state, especially for crystal engineering. 8 With few exceptions, 9 studies in solution have only started to appear in the last few years. 10 In addition to fundamental investigations, 11 further examples include trans-membrane anion transport, 12 catenanes and rotaxanes, 13 and anion receptors. 14 In the field of organocatalysis, there are only two reports in which the involvement of halogen bonds has been postulated (Scheme 1). 15 In both cases, it was shown that typical halogen-bond donors (a perfluoroiodoalkane or iodine trichloride) acted as catalysts. The exact mode of activation was not elucidated, however, and it cannot be ruled out that traces of acid are responsible for part of the observed activity. Similar Stefan M. Huber (3 rd from left) studied chemistry at the FriedrichAlexander University of Erlangen-Nuremberg, Germany. He obtained his PhD in 2007 in the group of Prof. Robert Weiss, working on...

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Hidden Brønsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates

Cited by 277 publications

References 66 publications

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Bi(OTf)₃-Catalyzed Multicomponent α-Amidoalkylation Reactions

Multidentate Halogen-Bond Donors as Lewis Acidic Activators or Catalysts in Halide Abstraction Reactions

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Hidden Brønsted Acid Catalysis: Pathways of Accidental or Deliberate Generation of Triflic Acid from Metal Triflates

Cited by 277 publications

References 66 publications

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Cyclization of an alkene-bearing cyclopentanone: The role of rhodium and Brønsted acid

Bi(OTf)3-Catalyzed Multicomponent α-Amidoalkylation Reactions

Multidentate Halogen-Bond Donors as Lewis Acidic Activators or Catalysts in Halide Abstraction Reactions

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Bi(OTf)₃-Catalyzed Multicomponent α-Amidoalkylation Reactions