Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

• Michael Nonnenmacher,
• Dominik M. Buck and
• Doris Kunz

Beilstein J. Org. Chem. 2016, 12, 1884–1896, doi:10.3762/bjoc.12.178

• (current address of corresponding author) 10.3762/bjoc.12.178 Abstract Rh(CO)2Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C–H coupling constant of the respective imidazolium salts and the N–C–N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor

• highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)2Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene. Keywords: carbonyl complexes

• -parameter [25][26][27][28], the 13C NMR chemical shift of special Pd(NHC)2 complexes [29][30], and electrochemical properties [31][32][33] (see [34][35] for reviews). In all these cases, only the overall donor-abilities of the NHC ligand are obtained. In the case of the Tolman parameter, not only the

Dinuclear thiazolylidene copper complex as highly active catalyst for azid–alkyne cycloadditions

• Anne L. Schöffler,
• Ata Makarem,
• Frank Rominger and
• Bernd F. Straub

Beilstein J. Org. Chem. 2016, 12, 1566–1572, doi:10.3762/bjoc.12.151

• Anne L. Schoffler Ata Makarem Frank Rominger Bernd F. Straub Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany 10.3762/bjoc.12.151 Abstract A dinuclear N-heterocyclic carbene (NHC) copper complex efficiently catalyzes azide–alkyne

• less time-consuming and more cost-efficient synthesis. Commercially available, inexpensive 4,5-dimethylthiazole is used as azole starting material instead of 4-aryl-1,2,4-triazoles. The precursor 1a for the NHC ancillary ligand is synthesized via a double SN2-reaction of two equivalents of thiazole

• characterized in our group [40]. The similarity of NMR-spectroscopic data of the 1,2,4-triazol-5-ylidene and the 1,3-thiazol-2-ylidene dicopper complex indicate that the complexes of both NHC ligand types consist of a bis-NHC ligand, two copper(I) ions and a labile acetate ligand that bridges the metal centers

Methylpalladium complexes with pyrimidine-functionalized N-heterocyclic carbene ligands

• Dirk Meyer and
• Thomas Strassner

Beilstein J. Org. Chem. 2016, 12, 1557–1565, doi:10.3762/bjoc.12.150

• Dirk Meyer Thomas Strassner Physikalische Organische Chemie, TU Dresden, Bergstraße 66, 01062 Dresden, Germany 10.3762/bjoc.12.150 Abstract A series of methylpalladium(II) complexes with pyrimidine-NHC ligands carrying different aryl- and alkyl substituents R ([((pym)^(NHC-R))PdII(CH3)X] with X

• complexes were fully characterized by standard methods and in three cases also by a solid state structure. Keywords: alkane activation; alkyl complex; NHC; palladium; solid state structure; Introduction Palladium complexes have been shown to be versatile homogeneous catalysts in a variety of reactions [1

• byproduct of the methanol synthesis. Sen reported the activity of palladium(II) catalysts for the oxidation of methane [21] and several groups contributed to the progress in the field which is summarized in recent reviews [22][23]. The system based on N-heterocyclic carbene (NHC) ligands developed in our

Reactivity studies of pincer bis-protic N-heterocyclic carbene complexes of platinum and palladium under basic conditions

• David C. Marelius,
• Curtis E. Moore,
• Arnold L. Rheingold and
• Douglas B. Grotjahn

Beilstein J. Org. Chem. 2016, 12, 1334–1339, doi:10.3762/bjoc.12.126

• dibasic complex 8. Keywords: NHC; 15N NMR spectroscopy; palladium; platinum; protic N-heterocyclic carbene; Introduction N-Heterocyclic carbenes (NHCs) have been extensively researched for a number of purposes since 1991 when Arduengo first isolated free NHCs [1][2][3]. NHCs as ligands have been known

• even longer. In 1968, Wanzlick and Öfele separately synthesized mercury(II) and chromium(0) imidazol-2-ylidene complexes [3]. Nearly 50 years of NHC ligand research have demonstrated the importance of the electronic and steric effects that can be modified by altering the alkyl or aryl groups on each

• , whereas for 4-PdCl, ∆ref = −13.1. This difference is likely due to a variety of factors related to the electronics of the nonprotic substituent of the PNHC and/or the ring size of the chelates, which is beyond the scope of this paper. For a given metal center and mono- or bis-NHC framework, we want to

Artificial Diels–Alderase based on the transmembrane protein FhuA

• Hassan Osseili,
• Daniel F. Sauer,
• Klaus Beckerle,
• Marcus Arlt,
• Tomoki Himiyama,
• Tino Polen,
• Akira Onoda,
• Ulrich Schwaneberg,
• Takashi Hayashi and
• Jun Okuda

Beilstein J. Org. Chem. 2016, 12, 1314–1321, doi:10.3762/bjoc.12.124

• copper(II) complexes were covalently linked to an engineered variant of the transmembrane protein Ferric hydroxamate uptake protein component A (FhuA ΔCVFtev). Copper(I) was incorporated using an N-heterocyclic carbene (NHC) ligand equipped with a maleimide group on the side arm at the imidazole nitrogen

• hydroxamate uptake protein component A (FhuA) as host for defined Cu(I) NHC or Cu(II) terpyridyl complexes with a maleimide moiety. By covalently bonding these copper complexes to the protein artificial Diels–Alderases based on a membrane protein have been obtained. Results and Discussion Synthesis of the

• contains a cysteine residue at position 545 for conjugation [31], an NHC ligand containing a maleimide function was prepared (Scheme 1). The imidazolium salt 3 was synthesized by nucleophilic substitution of mesityl imidazol 1 with maleimide derivative 2. These salts were used to generate the Cu(I) NHC

Conjugate addition–enantioselective protonation reactions

• James P. Phelan and
• Jonathan A. Ellman

Beilstein J. Org. Chem. 2016, 12, 1203–1228, doi:10.3762/bjoc.12.116

• without significantly affecting the reaction (93–97% yield, 92:8 to 96:4 er). The authors also explored further elaboration of the products to access enantioenriched cysteine analogues. The Glorius lab has made use of N-heterocyclic carbene (NHC) catalysts for intermolecular Stetter reactions between

• aldehydes and α,β-unsaturated esters (Scheme 8) [25][26]. Catalyzed by triazolium NHC-catalyst 35, electron-poor and neutral aromatic aldehydes reacted with methyl 2-acetamidoacrylate to access α-amino esters 34a with excellent enantioselectivity [25]. As expected, less electrophilic 4-methoxybenzaldehyde

• that NHC’s possessing a 2,6-dimethoxyphenyl moiety led to both superior reactivity and selectivity, with catalyst 36 being optimal. Catalyst 35 provided the product in less than 5% yield. It was proposed that a more electron-rich, and thereby more nucleophilic, NHC was needed because the α-carbon

Bi- and trinuclear copper(I) complexes of 1,2,3-triazole-tethered NHC ligands: synthesis, structure, and catalytic properties

• Shaojin Gu,
• Jiehao Du,
• Jingjing Huang,
• Huan Xia,
• Ling Yang,
• Weilin Xu and
• Chunxin Lu

Beilstein J. Org. Chem. 2016, 12, 863–873, doi:10.3762/bjoc.12.85

• imidazolium backbone and N substituents. The copper–NHC complexes tested are highly active for the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction in an air atmosphere at room temperature in a CH3CN solution. Complex 4 is the most efficient catalyst among these polynuclear complexes in an air

• atmosphere at room temperature. Keywords: copper; CuAAC reaction; : N-heterocylic carbene; 1,2,3-triazole; Introduction N-Heterocyclic carbene (NHC) have interesting electronic and structural properties. This resulted in their use as versatile ligands in organometallic chemistry and homogeneous catalysis

• , reports concerning their preparation and use of 1,4-disubstituted-1,2,3-triazoles bearing NHC ligands are rare [22][23]. Elsevier et al. [23] reported several of palladium(II) complexes containing a heterobidentate N-heterocyclic carbene-triazolyl ligand. These palladium(II) complexes are active

Iridium/N-heterocyclic carbene-catalyzed C–H borylation of arenes by diisopropylaminoborane

• Mamoru Tobisu,
• Takuya Igarashi and
• Naoto Chatani

Beilstein J. Org. Chem. 2016, 12, 654–661, doi:10.3762/bjoc.12.65

• derivatives by treatment with protecting groups in a one-pot reaction sequence. The reactivity of 1g has previously been well-exploited in catalytic borylation of aryl halides [22][23][24][25][26][27]. Herein, we report the C–H borylation of arenes using 1g catalyzed by an Ir/N-heterocyclic carbene (NHC

• C–H borylation [2], the reaction failed to give 2-B under these conditions (Table 1, entry 1). Several mono- and diphosphine ligands were found to be active for the formation of 2-B, but the best yield was only 21% (Table 1, entries 2–6). Our success in C–H borylation using NHC ligands [8][10] led

• us to investigate a series of NHC ligands for this process. Among the NHC ligands examined, 1,3-dicyclohexylimidazol-2-ylidene (ICy) [28][29][30][31][32][33] was found to be most effective, giving 2-B in 33% yield with a 2-/3-borylation ratio of 88:12 (Table 1, entry 9). It should be noted that [Ir

Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions

• Rajeev S. Menon,
• Akkattu T. Biju and
• Vijay Nair

Beilstein J. Org. Chem. 2016, 12, 444–461, doi:10.3762/bjoc.12.47

• the earliest known carbon–carbon bond-forming reactions catalysed by NHCs. The rapid growth of NHC catalysis in general has resulted in the development of a variety of benzoin and benzoin-type reactions. An overview of such NHC-catalysed benzoin reactions is presented. Keywords: acyloin reaction

• -heterocyclic carbenes (NHCs) are the two main classes of catalysts that are known to mediate benzoin reactions. This review focuses on the recent advancements made in the area of NHC-catalysed benzoin reactions. Historically, the first benzoin reaction was reported by Wöhler and Liebig in 1832. They discovered

• . Breslow postulated in 1958 a mechanistic rationale for the thiazolium salt-catalysed benzoin reaction [3]. He depicted the catalytically active species as a thiazolium zwitterion (the resonance structure of an NHC) and proposed that the reaction proceeds via an enaminol intermediate. The latter is now

Scope and limitations of the dual-gold-catalysed hydrophenoxylation of alkynes

• Adrián Gómez-Suárez,
• Yoshihiro Oonishi,
• Anthony R. Martin and
• Steven P. Nolan

Beilstein J. Org. Chem. 2016, 12, 172–178, doi:10.3762/bjoc.12.19

• metal and main-group chemistry [10][11][12][13]. We have been interested in exploring the use of NHC ligands in transition metal complexes for the development of highly active and well-defined catalysts. One of our main interests during the last decade has been to study the use of gold–NHC species as

• powerful catalysts for the construction of valuable organic molecules via the construction of C–C or C–O bonds [14][15][16][17][18][19][20][21][22][23]. In this regard, we have recently reported the use of [{Au(NHC)}2(µ-OH)][BF4] species as dual-activation catalysts for the hydrophenoxylation of alkynes

Simple activation by acid of latent Ru-NHC-based metathesis initiators bearing 8-quinolinolate co-ligands

• Julia Wappel,
• Roland C. Fischer,
• Luigi Cavallo,
• Christian Slugovc and
• Albert Poater

Beilstein J. Org. Chem. 2016, 12, 154–165, doi:10.3762/bjoc.12.17

• ppm in 3) when the N-heterocyclic carbene ligand is situated trans to the N-atom of the quinolinolate. The corresponding proton of the second quinolinolate ligand (with the N-atom situated cis to the NHC) is high-field shifted and resonates at 5.48 (in 1b), 5.32 (in 2b) and 5.9 (in 4). The OC-6-14

• similar to each other. For example, the Ru–NHC bond is in both complexes 2.05 ± 0.01 Å and the Ru–benzylidene bonds measure 1.89 ± 0.01 Å. Of interest are the different bond lengths of the two quinolinolate ligands. While the first quinolinolate in the OC-6-14 derivative 2a (N trans to the NHC and O trans

• of 2a, leading to species 2aIH+ and 2aIIH+, is unfavored, as well as protonation of the O atom of 2b cis to the NHC ligand, leading to 2bIH+. The only O atom presenting favorable protonation energy is the trans one to the Ru–alkylidene bond of 2b, leading to 2bIIH+. The accuracy of the absolute

Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes

• Albert Poater

Beilstein J. Org. Chem. 2016, 12, 117–124, doi:10.3762/bjoc.12.13

• Albert Poater Institut de Química Computacional i Catàlisi, Departament de Química, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Spain 10.3762/bjoc.12.13 Abstract Bearing the versatility of N-heterocyclic carbene (NHC) ligands, here density functional theory (DFT) calculations

• unravel the capacity of coordination of a deprotonated NHC ligand (pNHC) to generate a doubly C2,N3-bridged dinuclear complex. Here, in particular the discussion is based on the combination of the deprotonated 1-arylimidazol (aryl = mesityl (Mes)) with [M(cod)(μ-Cl)] (M = Ir, Rh) generated two geometrical

• ; iridium; isomerization; N-heterocyclic carbene; rhodium; Introduction In the framework of organometallic chemistry, N-heterocyclic carbenes (NHC) centre a well stablished class of relatively new ligands since in 1991 Arduengo and collaborators isolated the first stable NHC of the imidazole type with

N-Methylphthalimide-substituted benzimidazolium salts and PEPPSI Pd–NHC complexes: synthesis, characterization and catalytic activity in carbon–carbon bond-forming reactions

• Senem Akkoç,
• Yetkin Gök,
• İlhan Özer İlhan and
• Veysel Kayser

Beilstein J. Org. Chem. 2016, 12, 81–88, doi:10.3762/bjoc.12.9

• , 44280 Malatya, Turkey 10.3762/bjoc.12.9 Abstract A series of novel benzimidazolium salts (1–4) and their pyridine enhanced precatalyst preparation stabilization and initiation (PEPPSI) themed palladium N-heterocyclic carbene complexes [PdCl2(NHC)(Py)] (5–8), where NHC = 1-(N-methylphthalimide)-3

• ][10][11]. This is because NHC complexes are easily obtained by deprotonating imidazolium or benzimidazolium salts and most are relatively stable in air and moisture. They are weak π-acceptors and strong σ-donors and can form strong M–C bonds with transition metal ions compared to trivalent phosphine

• ligands [12][13]. As catalysts, palladium N-heterocyclic carbene (Pd–NHC) complexes display remarkable activities in coupling reactions [5][14][15][16][17]. Among various Pd–NHC complexes such as [Pd(NHC)(dmba)Cl] (dmba = N,N-dimethylbenzylamine) and [Pd(NHC)(Im)Cl2] (Im = imidazole) [18][19], [PEPPSI Pd

Effective immobilisation of a metathesis catalyst bearing an ammonium-tagged NHC ligand on various solid supports

• Krzysztof Skowerski,
• Jacek Białecki,
• Stefan J. Czarnocki,
• Karolina Żukowska and
• Karol Grela

Beilstein J. Org. Chem. 2016, 12, 5–15, doi:10.3762/bjoc.12.2

• ruthenium [19][20][21][22][23][24]. In other contributions originating from the same group, heterogeneous catalysts covalently connected to a monolithic support via NHC ligands were presented [25][26]. These initiators were suitable for continuous metathesis processes and provided products with low residual

• ruthenium; however, they were less active than complex 1. A very similar idea was explored by Grubbs et al. who obtained catalysts 2 and 3 covalently bonded to silica gel through the NHC ligand [27][28][29]. This work revealed that, for complexes supported on silica gel, the heterogenization via the NHC

• quaternary ammonium group attached to the NHC ligand (Figure 2) [43][44][45][46][47]. These, now commercially available [48], complexes were synthesised by on-site quarternisation of catalysts containing a tertiary amine functional group with the use of either methyl chloride or methyl iodide. This simple

New metathesis catalyst bearing chromanyl moieties at the N-heterocyclic carbene ligand

• Agnieszka Hryniewicka,
• Szymon Suchodolski,
• Agnieszka Wojtkielewicz,
• Jacek W. Morzycki and
• Stanisław Witkowski

Beilstein J. Org. Chem. 2015, 11, 2795–2804, doi:10.3762/bjoc.11.300

• catalyst bearing a modified N-heterocyclic carbene ligands is reported. The new catalyst contains an NHC ligand symmetrically substituted with chromanyl moieties. The complex was tested in model CM and RCM reactions. It showed very high activity in CM reactions with electron-deficient α,β-unsaturated

• reaction. Changes in the NHC ligand are also very important because this part of the catalyst participates all along the metathetic process. As a result, the catalyst may gain new properties, increased activity or stereoselectivity. Following this idea, we decided to synthesise a new catalyst bearing two

• may confer new properties of the NHC ligand. Results and Discussion Synthesis of the carbene precursor The synthesis of an imidazolinium salt as a carbene precursor was started from 2,2,5,7,8-pentamethylchromane (10), which was prepared by the reaction between 2,3,5-trimethylphenol and 3-methylbut-2

Pyridylidene ligand facilitates gold-catalyzed oxidative C–H arylation of heterocycles

• Kazuhiro Hata,
• Hideto Ito,
• Yasutomo Segawa and
• Kenichiro Itami

Beilstein J. Org. Chem. 2015, 11, 2737–2746, doi:10.3762/bjoc.11.295

• at 65 °C in the presence of AuCl(PPh3) (5 mol %), iodosobenzoic acid (IBA: 1 equiv) and (+)-10-camphorsulfonic acid (CSA: 1 equiv), the corresponding C–H arylation product 3aa was obtained in only 10% yield (Table 1, entry 1). Although the application of IPr, a conventional NHC ligand, to the

• shows that the four gold bonds are in a planar surface, and the pyridylidene face and the added two chlorine atoms are in vertical positions. The ligand arrangement is quite similar to a series of reported NHC–AuCl3 complexes [114]. With the authentic AuCl3(PyC) in hand, we next carried out the direct

Direct estimate of the internal π-donation to the carbene centre within N-heterocyclic carbenes and related molecules

• Diego M. Andrada,
• Nicole Holzmann,
• Thomas Hamadi and
• Gernot Frenking

Beilstein J. Org. Chem. 2015, 11, 2727–2736, doi:10.3762/bjoc.11.294

• been developed to quantify the π-acceptor ability of carbenes [61][62]. Thus, NMR methods have been reported that allow the measurement of the π-acidity of NHCs [63]. Bertrand et al. and Ganter et al. have proposed the use of 31P and 77Se NMR chemical shift of the NHC-phenylphosphinidene and NHC

• quest of a direct estimate of the NHC π-acceptor properties and its connection with the π-stabilization exerted by the adjacent α-heteroatoms to the carbene carbon atom, herein we report on the use of the EDA-NOCV (energy decomposition analysis with natural orbitals for chemical valence) method to

• )HC=CHN(Me), associated energies ∆E in kcal/mol. The color-code of the charge flow is red→blue. Plot of the ΔEπ values against NBO pπ occupation for the NHC family 1–15. Schematic view of the calculated carbenes 1–15. Schematic view of the major orbital interactions between a carbon atom in the 3P

Carbon–carbon bond cleavage for Cu-mediated aromatic trifluoromethylations and pentafluoroethylations

• Tsuyuka Sugiishi,
• Hideki Amii,
• Kohsuke Aikawa and
• Koichi Mikami

Beilstein J. Org. Chem. 2015, 11, 2661–2670, doi:10.3762/bjoc.11.286

• the decarboxylative trifluoromethylation of aryl halides [37] (Scheme 4). Not only iodobenzene but also 4-bromotoluene was trifluoromethylated by the [(NHC)Cu(TFA)] complex. The perfluoroalkylation reactions mentioned above require a stoichiometric amount of copper reagent, whereas it was found that

• . Decarboxylative pentafluoroethylation and its application. Trifluoromethyation with trifluoroacetate in a flow system. Trifluoromethylation of 4-bromotoluene by [(NHC)Cu(TFA)]. Trifluoromethylation of aryl iodides with small amounts of Cu and Ag2O. aThe yield was determined by GC analysis. bThe yield was

Recent advances in copper-catalyzed asymmetric coupling reactions

• Fengtao Zhou and
• Qian Cai

Beilstein J. Org. Chem. 2015, 11, 2600–2615, doi:10.3762/bjoc.11.280

• the copper/NHC-catalyzed asymmetric allylic substitution of allyl phosphates with arylboronates. Furthermore, they applied the method to the construction of quaternary carbon stereocenters with good enantioselectivity (up to 90% ee) with disubstituted allyl phosphates. The enantioselectivity was later

• improved to 92% ee with a new chiral catalyst (Scheme 25) [65]. In 2012, Hoveyda and Jung reported a copper/NHC-catalyzed asymmetric allylic substitution of allyl phosphates with allenylboronates [66], leading to chiral allenes bearing a tertiary or quaternary carbon stereogenic center in high yields and

• with excellent enantioselectivity (Scheme 26). The copper/NHC catalyst system was also applied to the allylic substitution of allyl phosphates with commercially available or easily accessible vinylboron reagents, leading to chiral alkenes bearing a quaternary carbon stereocenter. The utility of this

Rhodium, iridium and nickel complexes with a 1,3,5-triphenylbenzene tris-MIC ligand. Study of the electronic properties and catalytic activities

• Carmen Mejuto,
• Beatriz Royo,
• Gregorio Guisado-Barrios and
• Eduardo Peris

Beilstein J. Org. Chem. 2015, 11, 2584–2590, doi:10.3762/bjoc.11.278

• -NHC ligand. The electronic properties of the tris-MIC ligand were studied by cyclic voltammetry measurements. In all cases, the tris-MIC ligand showed a stronger electron-donating character than the corresponding NHC-based ligands. The catalytic activity of the tri-rhodium complex was tested in the

• its ability to form multimetallic catalysts whose catalytic performances can be compared with analogous monometallic NHC complexes [23][24][25]. On several occasions their activity has proven higher than the activities shown by their monometallic counterparts [23][26]. In the last few years we became

• higher nanolocal concentration of metal sites in the multimetallic catalyst [31]. In this context, we obtained the 1,3,5-triphenylbenzene-based C3-symmetrical tris-NHC ligand A (Scheme 1), which was coordinated to rhodium and iridium [25]. The catalytic activity of the trirhodium complex was tested in

Comparison of the catalytic activity for the Suzuki–Miyaura reaction of (η⁵-Cp)Pd(IPr)Cl with (η³-cinnamyl)Pd(IPr)(Cl) and (η³-1-t-Bu-indenyl)Pd(IPr)(Cl)

• Patrick R. Melvin,
• Nilay Hazari,
• Hannah M. C. Lant,
• Ian L. Peczak and
• Hemali P. Shah

Beilstein J. Org. Chem. 2015, 11, 2476–2486, doi:10.3762/bjoc.11.269

• ; homogeneous catalysis; NHC ligands; palladium; Suzuki–Miyaura reaction; Introduction The Suzuki–Miyaura reaction is a powerful synthetic method for forming C–C bonds between aryl halides or pseudo halides and organoborane containing species [1][2][3][4][5]. The most active catalysts are generally based on Pd

• and feature strongly electron-donating and sterically bulky phosphine or N-heterocyclic carbene (NHC) ancillary ligands [6][7]. In particular, precatalysts of the type (η3-allyl)Pd(NHC)(Cl) have shown excellent activity for the Suzuki–Miyaura reaction, with systems incorporating an η3-cinnamyl moiety

• be closely related to cyclopentadienyl (Cp) ligands [17]. Nevertheless, to the best of our knowledge there are only two reports describing the catalytic activity of complexes of the type (η5-Cp)Pd(NHC)(Cl) for the Suzuki–Miyaura coupling, as well as related cross-coupling reactions [18][19]. These

N-Heterocyclic carbenes

• Steven P. Nolan

Beilstein J. Org. Chem. 2015, 11, 2474–2475, doi:10.3762/bjoc.11.268

• Steven P. Nolan Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia 10.3762/bjoc.11.268 The N-heterocyclic carbenes (NHC) now hold a preferred-ligand status in organic and organometallic chemistry. Their role in catalysis continues to grow. When the

• through detailed mechanistic work, which permits the design of ever-better performing catalytic systems. This design/mechanistic study/redesign cycle will ensure the continued evolution of the field. The area of NHC-based research is a worldwide effort, as exemplified by contributions in this NHC-focused

Copper-catalyzed stereoselective conjugate addition of alkylboranes to alkynoates

• Takamichi Wakamatsu,
• Kazunori Nagao,
• Hirohisa Ohmiya and
• Masaya Sawamura

Beilstein J. Org. Chem. 2015, 11, 2444–2450, doi:10.3762/bjoc.11.265

• as PPh3 and PCy3 or the DPPE bisphopshine was also effective in promotion of the reaction, but resulted in a reduced stereoselectivity (Table 1, entries 2–4). No reaction occurred with N-heterocyclic carbenes (NHC) such as IMes or IPr (Table 1, entries 5 and 6). The reaction with (IMes)CuCl or (IPr

Copper-catalyzed asymmetric conjugate addition of organometallic reagents to extended Michael acceptors

• Thibault E. Schmid,
• Sammy Drissi-Amraoui,
• Christophe Crévisy,
• Olivier Baslé and
• Marc Mauduit

Beilstein J. Org. Chem. 2015, 11, 2418–2434, doi:10.3762/bjoc.11.263

• catalytic applications [22][23]. Amongst the myriad of available NHC ligands, chiral unsymmetrical NHC ligands appeared as particularly potent in asymmetric catalysis, and were investigated in copper-catalyzed conjugate additions [24]. Recently, a multicomponent synthesis enabled the facile access to a wide

• variety of unsymmetrical NHC precursors [25]. With this new methodology in hand, Mauduit and co-workers synthesized several bidentate chiral NHC precursors, using amino acids and amino alcohols as starting materials, and tested them in copper-catalyzed ACA [26]. Leucine-based L5 displayed the best

• performance in terms of enantioselectivity, and was used in combination with Cu(OTf)2 in the 1,6-ACA of cyclic dienones of the type 30 (Scheme 6). NHC ligands also enabled a total regioselectivity and ees ranging from 58 to 91%. Given the efficiency of (R)-Binap L4 in Cu-catalyzed 1,4-ACA on α,β-unsaturated

Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors

• Morgan Hans,
• Jan Lorkowski,
• Albert Demonceau and
• Lionel Delaude

Beilstein J. Org. Chem. 2015, 11, 2318–2325, doi:10.3762/bjoc.11.252

• just a single example, NHC ligands played a crucial role in the development of highly efficient ruthenium initiators for olefin metathesis and related reactions [18][19][20][21]. Lately, these divalent carbon species have also emerged as powerful nucleophilic organocatalysts for polymer chemistry [22

• of labile imidazolidine adducts (Scheme 1, path C) [36][37][38], or the recourse to Ag(I)–NHC complexes as NHC delivery agents (Scheme 1, path D) [39][40]. In many cases, however, these NHC surrogates are obtained from azolium intermediates. Hence, imidazolium and imidazolinium salts are the most

• common NHC precursors and their synthesis from acyclic starting materials is of utmost practical importance [41]. One of the most atom-economical and straightforward path to elaborate symmetrical imidazolium salts involves the combination of glyoxal, which provides the C4–C5 heterocyclic backbone of the