A facile synthetic route to benzimidazolium salts bearing bulky aromatic N-substituents

• Gabriele Grieco,
• Olivier Blacque and
• Heinz Berke

Beilstein J. Org. Chem. 2015, 11, 1656–1666, doi:10.3762/bjoc.11.182

• : benzimidazolium salts; bulky ligands; cyclization; ligand design; N-heterocyclic carbenes (NHC); ring formation; Introduction Imidazole-based N-heterocyclic carbenes (NHCs) are stable systems serving as ancillary ligands mainly to construct organometallic complexes. These NHCs are sterically and electronically

• used as electroluminescent materials (OLEDs) [16][17][18][19]. After the discovery of NHCs in 1968 by Wanzlick and Öfele, who isolated stable diamino-substituted carbenes, around 20 years later Arduengo further stabilized these potential ligand groups by embedding them into imidazole rings and

• increased in addition the steric congestion [20]. It is important to recognize that synthetically the stable forms for handling of imidazole carbenes are imidazolium salts. In imidazolium-based NHC chemistry two synthetic aspects are important: 1) there are several fundamental synthetic routes available

Grubbs–Hoveyda type catalysts bearing a dicationic N-heterocyclic carbene for biphasic olefin metathesis reactions in ionic liquids

• Maximilian Koy,
• Hagen J. Altmann,
• Benjamin Autenrieth,
• Wolfgang Frey and
• Michael R. Buchmeiser

Beilstein J. Org. Chem. 2015, 11, 1632–1638, doi:10.3762/bjoc.11.178

• can also be prepared with the aid of N-heterocyclic carbenes (NHCs) that bear pendant ionic groups (Figure 1) [10][16][17][18][19]. We addressed that issue by preparing a novel ionic Ru-NHC-alkylidene using NHCs with ionic groups. Here we report our results. Results and Discussion Catalyst synthesis

Latent ruthenium–indenylidene catalysts bearing a N-heterocyclic carbene and a bidentate picolinate ligand

• Thibault E. Schmid,
• Florian Modicom,
• Adrien Dumas,
• Etienne Borré,
• Loic Toupet,
• Olivier Baslé and
• Marc Mauduit

Beilstein J. Org. Chem. 2015, 11, 1541–1546, doi:10.3762/bjoc.11.169

• of cocatalyst. While first examples of latent catalysts were based on phosphine-containing ruthenium complexes bearing a Schiff base ligand (O–N) [9] replacement of the phosphine ligand by sterically demanding and strongly σ-donor N-heterocyclic carbenes (NHC) afforded catalysts with improved

Selected synthetic strategies to cyclophanes

• Sambasivarao Kotha,
• Mukesh E. Shirbhate and
• Gopalkrushna T. Waghule

Beilstein J. Org. Chem. 2015, 11, 1274–1331, doi:10.3762/bjoc.11.142

• to chiral concave N-heterocyclic carbenes. Molecular encapsulation was achieved by using double RCM to generate insulated oligoynes 204. Here, the masked hexayne plays an important role to lock the flanking chains [158]. The synthesis of planer chiral cyclophanes is a difficult task owing to the

Highly selective palladium–benzothiazole carbene-catalyzed allylation of active methylene compounds under neutral conditions

• Antonio Monopoli,
• Pietro Cotugno,
• Carlo G. Zambonin,
• Francesco Ciminale and
• Angelo Nacci

Beilstein J. Org. Chem. 2015, 11, 994–999, doi:10.3762/bjoc.11.111

• activity is still mandatory in these kind of reactions. Among various ligands, N-heterocyclic carbenes (NHCs) have gained greater importance in organometallic chemistry. Unlike phosphanes, NHCs are not toxic and insensitive to air, heat and moisture. Moreover, the introduction of substituents onto the

• favourably also with the analogous and more widely used NHC carbenes deriving from imidazole. Complexes and ligands employed. Pd-catalyzed α-allylation of active methylene compounds. Optimization of reaction conditions.a α-allylation of 1,3-dicarbonyl compoundsa. Supporting Information Supporting

Cross-dehydrogenative coupling for the intermolecular C–O bond formation

• Igor B. Krylov,
• Vera A. Vil’ and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2015, 11, 92–146, doi:10.3762/bjoc.11.13

• are used as the solvents or are taken in a large excess relative to the CH-reagent. 2.3 Reactions catalyzed by N-heterocyclic carbenes N-heterocyclic nucleophilic carbenes 105 have found use for the oxidative esterification. Scheme 23 shows, in a simplified way, the proposed mechanism of this type of

• quinone 116 afforded esters 117. This method was also applied to the synthesis of esters 117a–c from amino alcohols (Scheme 25) [115][116]. Different conditions were proposed for the cross-dehydrogenative C–O coupling of aldehydes with alcohols and phenols catalyzed by N-heterocyclic carbenes (118, 129

• -heterocyclic carbenes was used for the kinetic separation of enantiomers of alcohols [125][126]. 2.4 Reactions using halogen-containing oxidative systems This section considers cross-dehydrogenative C–O coupling reactions with aldehydes and primary alcohols as C-reagents in which halogens and their compounds

One-pot functionalisation of N-substituted tetrahydroisoquinolines by photooxidation and tunable organometallic trapping of iminium intermediates

• Joshua P. Barham,
• Matthew P. John and
• John A. Murphy

Beilstein J. Org. Chem. 2014, 10, 2981–2988, doi:10.3762/bjoc.10.316

• BrCCl3 and its byproduct CHCl3 [22] were responsible for poor organometallic reaction profiles. Generation of radical intermediates or carbenes upon reacting Grignard reagents or magnesium salts with BrCCl3 or CHCl3 are evidenced in the literature [38][39]. Initially, we took advantage of the

Organic chemistry on surfaces: Direct cyclopropanation by dihalocarbene addition to vinyl terminated self-assembled monolayers (SAMs)

• Malgorzata Adamkiewicz,
• David O’Hagan and
• Georg Hähner

Beilstein J. Org. Chem. 2014, 10, 2897–2902, doi:10.3762/bjoc.10.307

• dibromo- (:CBr2), dichloro- (:CCl2) and difluoro- (:CF2) carbenes [21][22][23]. The resultant SAMs were analysed by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, ellipsometry, and atomic force microscopy. Results and Discussion After exposure to carbenes the vinyl-terminated SAMs were

• 1s, consistent with a CF2 group present on the surface [27]. It was anticipated that gem-dibromo-, gem-dichloro- and gem-difluorocyclopropane-terminated SAMs will be formed, following the usual transformations of these carbenes with double bonds. To add further support to this expectation, model

• reactions demonstrate that these organic chemical transformations, which have been relatively widely used in solution reactions of olefins, can be extended to surface reactions of SAMs. This opens up prospects too of modifying surfaces in this manner with carbenes carrying more elaborate functional groups

A novel 4-aminoantipyrine-Pd(II) complex catalyzes Suzuki–Miyaura cross-coupling reactions of aryl halides

• Claudia A. Contreras-Celedón,
• Darío Mendoza-Rayo,
• José A. Rincón-Medina and
• Luis Chacón-García

Beilstein J. Org. Chem. 2014, 10, 2821–2826, doi:10.3762/bjoc.10.299

• develop a stable and efficient Pd catalyst for these reactions. Recently, palladium complexes containing imidazole-imines [12], binary nanoclusters [13], N-heterocyclic carbenes (NHCs) [14], nanoparticles [15], palladacycles [16], and Schiff bases [17] have been developed as highly effective phosphine

Isoxazolium N-ylides and 1-oxa-5-azahexa-1,3,5-trienes on the way from isoxazoles to 2H-1,3-oxazines

• Alexander F. Khlebnikov,
• Mikhail S. Novikov,
• Yelizaveta G. Gorbunova,
• Ekaterina E. Galenko,
• Kirill I. Mikhailov,
• Viktoriia V. Pakalnis and
• Margarita S. Avdontceva

Beilstein J. Org. Chem. 2014, 10, 1896–1905, doi:10.3762/bjoc.10.197

• obtained in 14% yield when heated under reflux in CH2Cl2 and with the use of dirhodium tetraoctanoate instead of Rh2(OAc)4 as a catalyst. This unsatisfactory result prompted us to test a carbene instead of a Rh(II) carbenoid, since it has been found [23] that carbenes can be successfully generated by

Comparing kinetic profiles between bifunctional and binary type of Zn(salen)-based catalysts for organic carbonate formation

• Carmen Martín and
• Arjan W. Kleij

Beilstein J. Org. Chem. 2014, 10, 1817–1825, doi:10.3762/bjoc.10.191

• carbenes [31], polyphenolic compounds [32], quaternary ammonium salts [33] or metal-containing complexes [34][35][36][37]. Also, a detailed theoretical analysis combined with experimental evidences using a Zn(salphen) complex has been reported [36]. Beside these computational activities, experimental

Stereocontrolled synthesis of 5-azaspiro[2.3]hexane derivatives as conformationally “frozen” analogues of L-glutamic acid

• Beatrice Bechi,
• David Amantini,
• Cristina Tintori,
• Maurizio Botta and
• Romano di Fabio

Beilstein J. Org. Chem. 2014, 10, 1114–1120, doi:10.3762/bjoc.10.110

• rotation around the C3–C4 bond present in the azetidine derivative Ia by incorporating an appropriate spiro moiety. The cyclopropyl moiety was introduced by a diastereoselective rhodium catalyzed cyclopropanation reaction. Keywords: Amino acids; carbenes; cyclopropanation; rhodium; spiro compounds

Site-selective covalent functionalization at interior carbon atoms and on the rim of circumtrindene, a C₃₆H₁₂ open geodesic polyarene

• Hee Yeon Cho,
• Ronald B. M. Ansems and
• Lawrence T. Scott

Beilstein J. Org. Chem. 2014, 10, 956–968, doi:10.3762/bjoc.10.94

• geodesic polyarenes at their interior carbon atoms (i.e., carbon atoms that are shared by three rings) [4][38]. The curved PAHs corannulene (1) and diindenochrysene (10), for example, show fullerene-type reactivity towards carbenes and nucleophiles, respectively (Scheme 2). Dichlorocarbene adds to

Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles

• Zong Guan,
• Jan C. Namyslo,
• Martin H. H. Drafz,
• Martin Nieger and
• Andreas Schmidt

Beilstein J. Org. Chem. 2014, 10, 832–840, doi:10.3762/bjoc.10.79

• . Virtasen aukio 1), FIN-00014 University of Helsinki, Finland 10.3762/bjoc.10.79 Abstract Deprotonation of indazolium salts at low temperatures gives N-heterocyclic carbenes of indazoles (indazol-3-ylidenes) which can be trapped as rhodium complexes (X-ray analysis). In the absence of Rh, the indazol-3

• review articles dealing with syntheses [1][2][3][4], synthetic potentials [4], and biological activities [4][5] of this ring system. In view of the rapid development of the class of N-heterocyclic carbenes (NHC) [6][7][8][9][10][11][12] it is not unexpected, that attention was also directed towards the

• NHCs of indazole which have been generated and applied in heterocyclic synthesis (vide infra) as well as in complex chemistry [13]. Undoubtedly the N-heterocyclic carbenes of imidazole, imidazoline and the triazoles play the most important roles as ligands in metal-organic chemistry [14] or as

Practical synthesis of aryl-2-methyl-3-butyn-2-ols from aryl bromides via conventional and decarboxylative copper-free Sonogashira coupling reactions

• Andrea Caporale,
• Stefano Tartaggia,
• Andrea Castellin and
• Ottorino De Lucchi

Beilstein J. Org. Chem. 2014, 10, 384–393, doi:10.3762/bjoc.10.36

• in piperidine as the solvent [39][78] or by using aminophosphines [18] as well as phenanthryl imidazolium carbenes as the catalyst ligands [79]. A more practical methodology has been reported by Shirakawa in which a Pd(OAc)2/PPh3 catalyst system in DMSO and in the presence of K3PO4 as the base was

Carbenoid-mediated nucleophilic “hydrolysis” of 2-(dichloromethylidene)-1,1,3,3-tetramethylindane with DMSO participation, affording access to one-sidedly overcrowded ketone and bromoalkene descendants^§

• Rudolf Knorr,
• Thomas Menke,
• Johannes Freudenreich and
• Claudio Pires

Beilstein J. Org. Chem. 2014, 10, 307–315, doi:10.3762/bjoc.10.28

• possible SNV reaction of 12 with DMSO and/or dimsyl-K (11), as proposed in Scheme 3 and later on as follows. In a rapid oxygen-transfer reaction that is known [31][32] for saturated carbenes or carbenoids, 12 will attack the solvent DMSO to generate the K,O-carbenoid 15 which decays in an E1cb-like

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

• Alexander O. Terent'ev,
• Dmitry A. Borisov,
• Vera A. Vil’ and
• Valery M. Dembitsky

Beilstein J. Org. Chem. 2014, 10, 34–114, doi:10.3762/bjoc.10.6

Modulating NHC catalysis with fluorine

• Yannick P. Rey and
• Ryan Gilmour

Beilstein J. Org. Chem. 2013, 9, 2812–2820, doi:10.3762/bjoc.9.316

• derivatives are presented and the conformations are discussed. Upon deprotonation, the fluorinated triazolium salts generate catalytically active N-heterocyclic carbenes, which can then participate in the enantioselective Steglich rearrangement of oxazolyl carbonates to C-carboxyazlactones (e.r. up to 87.0

• converted to the respective N-heterocyclic carbenes (NHCs) by simple deprotonation. Given the importance of NHCs in modern organic synthesis [23][24][25][26][27][28] it was envisaged that these systems would be intriguing candidates for investigation. Moreover, structural information gleaned from the

• (e.r. 62.5:37.5), although the reactions did not display the same sensitivity to changes in base (Table 2, entries 11 and 12). Conclusion In conclusion, the ability of fluorine to modulate the catalytic performance of N-heterocyclic carbenes in the Steglich rearrangement of oxazolyl carbonates has been

Advancements in the mechanistic understanding of the copper-catalyzed azide–alkyne cycloaddition

• Regina Berg and
• Bernd F. Straub

Beilstein J. Org. Chem. 2013, 9, 2715–2750, doi:10.3762/bjoc.9.308

• N-heterocyclic carbenes, azolium cations cannot compete for free coordination sites at the copper(I) centres so that the formation of copper acetylide complexes is greatly facilitated. This “built-in” base, the irreversible deprotonation of the alkyne under the reaction conditions of this protocol

Gold(I)-catalyzed enantioselective cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2013, 9, 2250–2264, doi:10.3762/bjoc.9.264

• could be formally categorized as a [2 + 1] cycloaddition. In particular, they showed that it is possible to trap the intermediate gold–carbenes resulting from a 1,2-acyloxy migration in propargyl esters such as 1, with external alkenes (e.g. vinylarenes), to give cyclopropane products [41]. The racemic

• -workers reported another example of a highly enantioselective gold-catalyzed cyclopropanation reaction by using diazo compounds as a source of gold carbenes. In particular, the authors demonstrated that the chiral bisgold complex Au6, derived from the spiroketal bisphosphine ligand, was a very efficient

• -complexes (B, B’ and B’’), planar σ-allylic cations C, zwitterionic carbenes C’ or η1-bent allenes C’’ (Figure 2) [48]. In some cases, experimental and theoretical evidence supports the participation of one of those structures, but in many other cases the precise nature of these species remains unknown. The

Gold(I)-catalyzed 6-endo hydroxycyclization of 7-substituted-1,6-enynes

• Ana M. Sanjuán,
• Alberto Martínez,
• Patricia García-García,
• Manuel A. Fernández-Rodríguez and
• Roberto Sanz

Beilstein J. Org. Chem. 2013, 9, 2242–2249, doi:10.3762/bjoc.9.263

• and co-workers, as substrates in the identification of new reactivities catalyzed by gold and other transition metal complexes [5][6][7][8][9][10][11][12][13]. Cyclopropyl metal carbenes II are usually formed by exo-dig processes from enynes I bearing a terminal alkyne, which in the absence of

• external nucleophiles undergo skeletal rearrangements to afford products such as III (single cleavage) [14]. However, reactions of II with alcohols or water give the corresponding products of alkoxy(hydroxy)cyclization IV [15][16][17] (Scheme 1). The less common 6-endo cyclization via metal carbenes V was

Synthesis, characterization and luminescence studies of gold(I)–NHC amide complexes

• Adrián Gómez-Suárez,
• David J. Nelson,
• David G. Thompson,
• David B. Cordes,
• Duncan Graham,
• Alexandra M. Z. Slawin and
• Steven P. Nolan

Beilstein J. Org. Chem. 2013, 9, 2216–2223, doi:10.3762/bjoc.9.260

• catalysis; N-heterocyclic carbenes; synthesis; Introduction The synthesis of organogold complexes has recently attracted wide attention due to their considerable range of applications, in areas such as materials and medicinal chemistry, as well as their potential role as reaction intermediates or new

Synthesis of enantiomerically pure N-(2,3-dihydroxypropyl)arylamides via oxidative esterification

• Akula Raghunadh,
• Satish S More,
• T. Krishna Chaitanya,
• Yadla Sateesh Kumar,
• Suresh Babu Meruva,
• L. Vaikunta Rao and
• U. K. Syam Kumar

Beilstein J. Org. Chem. 2013, 9, 2129–2136, doi:10.3762/bjoc.9.250

• purity was observed. Keywords: N-(2,3-dihydroxypropyl)arylamide; nitrogen heterocyclic carbenes (NHC); oxidative esterification of aromatic aldehyde; Introduction Chiral structures with three carbons are an integral part of many biologically active compounds including alkaloids, pharmaceuticals and

• carbenes, such as 9a to 9k, with different bases (triethylamine, DBU, and DABCO) and with or without an additive to optimize the reaction conditions (Table 1). When the NHC was used in 0.25 equivalents in THF or THF/butanol (10:1) the major product isolated in the reaction was benzoin, and the desired

Gold-catalyzed regioselective oxidation of propargylic carboxylates: a reliable access to α-carboxy-α,β-unsaturated ketones/aldehydes

• Kegong Ji,
• Jonathan Nelson and
• Liming Zhang

Beilstein J. Org. Chem. 2013, 9, 1925–1930, doi:10.3762/bjoc.9.227

• carboxylate; Introduction We reported in 2010 [1] that α-oxo gold carbenes could be conveniently generated as reactive intermediates in gold-catalyzed intermolecular oxidation of alkynes. By using pyridine N-oxides [1] and later 8-substituted quinoline N-oxides [2] as the external oxidants, this approach

• permits a safe and efficient access to α-oxo gold carbenes without resorting to the dediazotization strategy [3][4][5] using hazardous and potentially explosive diazo substrates (Scheme 1). Since then an array of versatile synthetic methods has been developed based on the general approach by us [2][6][7

• migration might be an even less meaningful event in the reaction; b) it is known that the gold carbenes of type B can be readily oxidized by Ph2S=O [33], which, however, is an inefficient oxidant for generating α-oxo gold carbenes of type A via alkyne oxidation [34][35]; when the N-oxide 3 is replaced by

[3 + 2]-Cycloadditions of nitrile ylides after photoactivation of vinyl azides under flow conditions

• Stephan Cludius-Brandt,
• Lukas Kupracz and
• Andreas Kirschning

Beilstein J. Org. Chem. 2013, 9, 1745–1750, doi:10.3762/bjoc.9.201

• cycloaddition reactions. They are commonly formed through three routes which are a) the addition of electrophilic carbenes to nitriles, b) the dehydrochlorination of imidoyl chlorides, and c) the photochemical ring opening of strained 2H-azirines 2 [2][3][4][5]. The latter route can be initiated by the