Carbene Catalysts

Moore, Jennifer L.; Rovis, Tomislav

doi:10.1007/978-3-642-02815-1_18

Cited by 47 publications

(19 citation statements)

References 175 publications

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“…In the course of developing enantioselective, NHC-catalyzed reactions of catalytically generated reactive species 25, 26, 27 we identified an intriguing annulation reaction of ynals and stable enols. Key to this approach is the catalytic generation of the α,β -unsaturated acyl azolium ( I , Scheme 2) by an internal redox reaction of ynals.…”

Section: Introductionmentioning

confidence: 99%

Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates–Claisen Rearrangement

2012

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A combination of a chiral N-heterocyclic carbene catalyst and α,β-unsaturated aldehyde leads to a catalytically generated α,β-unsaturated acyl azolium, which participates in a highly enantioselective annulation to give dihydropyranone products. This full account of our investigations into the scope and mechanism of this reaction reveals the critical role of both the type and substitution pattern of the chiral triazolium precatalyst in inducing and controlling the stereochemistry. In an effort to explain why stable enols such as naphthol, kojic acid, and dicarbonyl are uniquely efficient, we have postulated that this annulation occurs via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal prior to a sigmatropic rearrangement. Detailed kinetic investigations of the catalytic annulation are consistent with this mechanistic postulate.

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Section: Introductionmentioning

confidence: 99%

Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates–Claisen Rearrangement

2012

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“…In particular, their use as synthetic precursors of N-heterocyclic carbenes (NHCs), 1,2 ionic liquids for various synthetic reactions 3 and electrochemical procedures, 4 redox reagents or mediators, 5 and building blocks for organic materials 6 is noteworthy. The electrochemical behavior of azolium salts is thus of particular interest because it affects each of these areas, and continued tabulation of redox properties of new azolium species is necessary to expand beyond this scope.…”

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confidence: 99%

Electrochemical Characterization of Azolium Salts

Ogawa

Boydston

2014

Chemistry Letters

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Redox properties of a series of azolium salts, including benzothiazolium, thiazolinium, thiazolium, triazolium, imidazolium, and imidazolinium salts, have been systematically investigated. The series includes a broad range of N-arylthiazolium salts that collectively demonstrate the ability to finetune the reduction potential of the thiazolium ring via electronic modification of the N-aryl moiety. Additionally, a novel class of N-arylthiazolinium salts has been synthesized and characterized.In contrast to what has been observed for imidazolium and imidazolinium counterparts, saturation of the thiazolium backbone to give thiazolinium salts results in a more facile electrochemical reduction.The azolium cation ( Figure 1) is a versatile centerpiece for multiple applications in organic chemistry. In particular, their use as synthetic precursors of N-heterocyclic carbenes (NHCs), 1,2 ionic liquids for various synthetic reactions 3 and electrochemical procedures, 4 redox reagents or mediators, 5 and building blocks for organic materials 6 is noteworthy. The electrochemical behavior of azolium salts is thus of particular interest because it affects each of these areas, and continued tabulation of redox properties of new azolium species is necessary to expand beyond this scope. For example, we recently observed undesirable thiolate oxidation by thiazoliumbased NHC precursors, which may be avoided in future studies through judicious selection of azolium precatalysts based on their reduction potentials. 5 Unfortunately, a few studies have been reported on the electrochemical behavior of azolium salts, with specific noteworthy examples by Pichon, Clyburne, Coughlin, and Sun. 7 To the best of our knowledge, C2-unsubstituted thiazolium salts, which are commonly used precursors of thiazolylidene-based NHCs, were not investigated. Moreover, N-arylthiazolinium salts have not been reported in the literature, although their N-alkyl counterparts have received limited attention.2f,8 Herein, we describe a systematic comparison of reduction potentials of various classes of azolium cations, including a series of electronically varied thiazolium species, and report N-arylthiazolinium salts for the first time.To address the paucity of reported thiazolinium compounds, we prepared N-arylthiazolinium iodides 6a and 6b as depicted in Scheme 1. Previously reported N-alkylthiazolinium salts, which were used as chiral ionic liquids and Brønsted acid catalysts, were prepared by alkylation of thiazoline precursors.2f,8 However, it was not obvious to us that this approach could be used to access the desired N-aryl analogues. Our initial attempts at synthesizing N-arylthiazolinium moieties via cyclization of 2-aminoethanethiols 8 with (EtO) 3 CH and Brønsted acids were unsuccessful, 9 as were double alkylation approaches via 9, which are similar to those reported by Grubbs for the synthesis of imidazolinium salts.10 A successful route was finally realized via iodine-catalyzed cyclization of 8 with (EtO) 3 CH. This reaction is thought to be facilit...

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“…1 In the area of Lewis base-promoted reactions, 2 N -heterocyclic carbene (NHC) catalysis is a rapidly growing field that employs electron lone pair-bearing heterocycles to facilitate a wide breadth of chemical transformations. 3 Carbene catalysis is highly versatile and allows access to acyl anions, 4 homoenolates, 5 enolates, 6 Cannizarro-type reductions, 3 c and oxidations. 7 The majority of NHC catalysis currently involves the 1,2-addition of the carbene to a carbonyl-containing species which then proceeds to produce an acyl anion or homoenolate equivalent.…”

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confidence: 99%

N-heterocyclic carbene-catalyzed rearrangements of vinyl sulfones

2011

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N-heterocyclic carbenes catalyze the rearrangement of 1,1-bis(arylsulfonyl)ethylene to the corresponding trans-1,2-bis(phenylsulfonyl) under mild conditions. Tandem rearrangement/cycloadditions have been developed to capitalize on this new process and generate highly substituted isoxazolines and additional heterocyclic compounds. Preliminary mechanistic studies support a new conjugate addition/Umpolung process involving the ejection and subsequent unusual re-addition of a sulfinate ion.

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Carbene Catalysts

Cited by 47 publications

References 175 publications

Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates–Claisen Rearrangement

Chiral N-Heterocyclic Carbene-Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates–Claisen Rearrangement

Electrochemical Characterization of Azolium Salts

N-heterocyclic carbene-catalyzed rearrangements of vinyl sulfones

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