An efficient Pd–NHC catalyst system in situ generated from Na₂PdCl₄ and PEG-functionalized imidazolium salts for Mizoroki–Heck reactions in water

• Nan Sun,
• Meng Chen,
• Liqun Jin,
• Wei Zhao,
• Baoxiang Hu,
• Zhenlu Shen and
• Xinquan Hu

Beilstein J. Org. Chem. 2017, 13, 1735–1744, doi:10.3762/bjoc.13.168

• functionalization of the N atom of the NHC ring allows for the possible incorporation of water soluble moieties, thus providing more opportunities for water soluble catalyst design [37][38][39]. Since the pioneering report of a sulfonate-functionalized NHC ligand by Shaughnessy [40], a number of water-soluble NHC

• activity in Mizoroki–Heck reactions with DMF as solvent [66]. With this regards, we herein report the development of a new poly(ethylene glycol, PEG) and pyridine bi-functionalized imidazolium salt L1 (Figure 1), which was employed as a water soluble NHC ligand precursor for an in situ generated Pd–NHC

• –L3 from commercially available MeO-PEG1900-OH, imidazole, and various arylmethyl bromides (2-bromomethylpyridine for L1, benzyl bromide for L2 and 1-bromomethylnaphthalene for L3). It was shown that these imidazolium salts L1–L3 could be utilized as water soluble NHC ligand precursors in combination

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

• -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

• of cis- and trans-CO Rh-NHC complexes bearing an additional P donor reveals a smaller 1JRhC coupling constant for the trans-CO ligand and a larger coupling constant for the cis-CO ligand (relative to the NHC ligand) [57]. This trend is also observed for the 1JRhC coupling constants of carbonyl

• for complexes with the unsaturated NHC ligand III, which favors the respective NHC/COapic intermediate by 14.9 kJ/mol. Assuming similar low activation barriers for the CO association and the dissociation, release of the CO ligand from the trigonal plane in the intermediate 3a NHC/COapic leads to 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

• 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

• -NHC ligand [33]. To study the differences between both systems we synthesized palladium [34] and platinum [35] “hybrid complexes” with ligands combining both structural elements the pyrimidine as well as the NHC fragment. We also used density functional theory (DFT) calculations to investigate the

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

• 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

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

• 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

• complexes 4 upon deprotonation with K2CO3. Complex 4 contains only one NHC ligand at the copper, as shown by elemental analysis and ESIMS. Attempts to coordinate Cu(II) to the NHC ligand failed. However, the terpyridyl (terpy) ligand is a promising candidate to support Cu(II) ions. Therefore, the terpy

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

• Inspired by the catalytic activity of Cu(I) species supported by NHC ligand in Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction under mild conditions, copper complexes 2–6 were investigated in the CuAAC reaction of azide and phenylacetylene. Firstly, we compared the catalytic activity of different

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

• 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

• 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

• bond [7][8], which favored the synthesis of new NHCs and to their use as ligands in transition metal complexes. The latter complexes were usually obtained by an easy replacement of a phosphine by the new NHC ligand, displaying a very high stability under many catalytic conditions. Furthermore, NHCs

• reveals a quadrant highly sterically hindered (37.2) due to the rotation of the aromatic ring on the NHC which facilitates the allocation of such a NHC ligand in the dinuclear complexes. The other quadrant where this aromatic ring participates displays a value of 29.2, whereas the other two are innocuous

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; 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

• yet powerful procedure gives access to a modified complex with multitude of possible applications [43][44][45][46][47]. We have found that the introduction of an ammonium chloride tag into the NHC ligand results in catalysts with interesting properties such as solubility in neat water as well as

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

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

• 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

• obtained for the analogous Rh and Ir complexes with the tris-NHC ligand A (E1/2 = 0.67 mV, for both complexes) [25], the observed lower E1/2 values for complexes 2 and 3 are consistent with a stronger electron-donating character of the tris-MIC ligand B. Further the observation of only one redox wave for

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

• outcome of the ACA reaction with α,β,δ,γ-unsaturated ketones [18]. Using a Grignard reagent as the nucleophile, it appeared that catalytic systems based on phosphoramidite ligands favored the formation of the 1,6-adduct. However, the use of catalytic systems based on an hydroxyalkyl NHC ligand (Cu(OTf)2

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

• 2010, the group of Markó designed a very bulky, yet flexible NHC ligand by replacing the methyl groups of IDip with phenyl rings [71]. This new highly hindered carbene that we shall designate as IDip* (but it is also trivially named IPr*) was readily obtained by deprotonation of 1,3-bis(2,6-bis

Beyond catalyst deactivation: cross-metathesis involving olefins containing N-heteroaromatics

• Kevin Lafaye,
• Cyril Bosset,
• Lionel Nicolas,
• Amandine Guérinot and
• Janine Cossy

Beilstein J. Org. Chem. 2015, 11, 2223–2241, doi:10.3762/bjoc.11.241

• complex 8 that was isolated and characterized using X-ray diffraction (Scheme 3, reaction 2). In both cases, free PCy3 was observed by NMR confirming amine-induced phosphine displacement. The decomposition of GI was hypothesized to go through a bimolecular coupling from 7. On the contrary, the bulky NHC

• ligand present in 8 could prevent this side reaction. However, in the presence of diethyl diallylmalonate and n-butylamine, GII decomposed readily probably due to an increased instability of the less hindered methylidene 9 compared to benzylidene 8 (Scheme 3, reaction 3). Fogg et al. completed this study

Computational study of productive and non-productive cycles in fluoroalkene metathesis

• Markéta Rybáčková,
• Jan Hošek,
• Ondřej Šimůnek,
• Viola Kolaříková and
• Jaroslav Kvíčala

Beilstein J. Org. Chem. 2015, 11, 2150–2157, doi:10.3762/bjoc.11.232

• ruthenium forming 3,3-difluororuthenacyclobutane intermediate a2I. The computations started from an alkene weakly coordinated to the NHC ligand without any coordination to ruthenium (structures 1a and 2a), and continued with the first mechanistic step, the coordination of alkene to ruthenium with partial

Ru complexes of Hoveyda–Grubbs type immobilized on lamellar zeolites: activity in olefin metathesis reactions

• Hynek Balcar,
• Naděžda Žilková,
• Martin Kubů,
• Michal Mazur,
• Zdeněk Bastl and
• Jiří Čejka

Beilstein J. Org. Chem. 2015, 11, 2087–2096, doi:10.3762/bjoc.11.225

• ][17][18][19][20]. Although the character of this interaction is not completely clear, they are firm enough to ensure low Ru leaching and catalyst reusability. Recently, we reported Hoveyda–Grubbs type catalysts bearing quaternary ammonium tag on NHC ligand (HGIIN+X, where X = Cl−, I−, PF6−, or BF4

• with sterically enlarged NHC ligand supported on SBA-15 exhibited high stability and was effective in flow reactions [22]. According to our knowledge, zeolites have not been considered as perspective supports for the immobilization of Ru metathesis catalysts due to small diameters of their pores (<1 nm

Olefin metathesis in air

• Lorenzo Piola,
• Fady Nahra and
• Steven P. Nolan

Beilstein J. Org. Chem. 2015, 11, 2038–2056, doi:10.3762/bjoc.11.221

• groups of Nolan (14) [40][41], Grubbs (15) [42][43][44][45] and Hermann [46][47][48] reported on the synthesis of this family of complexes. The combination of a labile phosphine group with a non-labile NHC ligand provided a significant improvement in terms of reactivity and stability. The bulky NHC

Hexacoordinate Ru-based olefin metathesis catalysts with pH-responsive N-heterocyclic carbene (NHC) and N-donor ligands for ROMP reactions in non-aqueous, aqueous and emulsion conditions

• Shawna L. Balof,
• K. Owen Nix,
• Matthew S. Olliff,
• Sarah E. Roessler,
• Arpita Saha,
• Kevin B. Müller,
• Ulrich Behrens,
• Edward J. Valente and
• Hans-Jörg Schanz

Beilstein J. Org. Chem. 2015, 11, 1960–1972, doi:10.3762/bjoc.11.212

• employed for emulsion ROMP bearing an NHC ligand. This may a consequence of the low accessibility of these catalysts and one of the reasons for the relatively low observed activities knowing that the NHC ligand dramatically increases the propagation rates of the metathesis reaction [57]. The higher

• complex 13 [46], the only other (DMAP)2Cl2Ru–alkylidene complex bearing an NHC ligand for which a crystal structure was solved, all metal ligand bond distances are very similar (within 2 pm) with the exception of one Ru–N distance to the DMAP ligand trans to the NHC ligand (Table 1). In complex 12 this

Stereochemistry of ring-opening/cross metathesis reactions of exo- and endo-7-oxabicyclo[2.2.1]hept-5-ene-2-carbonitriles with allyl alcohol and allyl acetate

• Piotr Wałejko,
• Michał Dąbrowski,
• Lech Szczepaniak,
• Jacek W. Morzycki and
• Stanisław Witkowski

Beilstein J. Org. Chem. 2015, 11, 1893–1901, doi:10.3762/bjoc.11.204

• -tolyl substituted NHC ligand in [Ru]5 exerts a smaller steric effect than the -PCy3 residue in [Ru]1–4. Furthermore, the bulky phosphine ligand (PCy3) expands away from the transition metal center (coordination sphere), while the N,N’-o-tolyl substituent attached to the central imidazole ring penetrates

Nitro-Grela-type complexes containing iodides – robust and selective catalysts for olefin metathesis under challenging conditions

• Andrzej Tracz,
• Mateusz Matczak,
• Katarzyna Urbaniak and
• Krzysztof Skowerski

Beilstein J. Org. Chem. 2015, 11, 1823–1832, doi:10.3762/bjoc.11.198

• with iodide can – at least for some substrates – provide similar catalyst stabilization as the introduction of a bulky NHC ligand. In the case of the most sterically crowded nG-SIPr-I2, initiation was delayed, but a very fast reaction propagation was observed. This catalyst was the most stable and

Synthesis and structures of ruthenium–NHC complexes and their catalysis in hydrogen transfer reaction

• Chao Chen,
• Chunxin Lu,
• Qing Zheng,
• Shengliang Ni,
• Min Zhang and
• Wanzhi Chen

Beilstein J. Org. Chem. 2015, 11, 1786–1795, doi:10.3762/bjoc.11.194

• ligands and two acetonitrile ligands in an octahedral geometry. One NHC ligand, one acetonitrile ligand and one carbon atom of the other NHC ligand occupy the equatorial plane in which two carbon atoms of two NHC ligands are mutually trans-arranged. The remaining acetonitrile ligand and one nitrogen atom

• of the NHC ligand lie on the axial positions. The angles (N–Ru–N) of adjacent nitrogen atoms and Ru(II) ion are in the range of 83.9 to 94.0°. The Ru–C distance (2.066 Å) is consistent with the reported values in known Ru–NHC complexes [17][18][19][20][21][22][23][24][25][26][27][28][29]. The Ru

• –Npyrimidine distance (2.081 Å) is slightly longer than Ru–Nacetonitrile (2.033 Å). The cationic structure of 2 is shown in Figure 2. The central Ru(II) ion is surrounded by one pyrimidine-functionalized NHC ligand and four acetonitrile ligands also in a typical octahedral geometry. The Ru ion lies on a

A comprehensive study of olefin metathesis catalyzed by Ru-based catalysts

• Albert Poater and
• Luigi Cavallo

Beilstein J. Org. Chem. 2015, 11, 1767–1780, doi:10.3762/bjoc.11.192

• bottom attack and how to favor the cis one [8][32][33][34], with an open debate still for the(NHC)Ru-based catalysts [35][36][37][38]. The postulated binding of the substrate can be preferentially trans to the NHC ligand (bottom path in Scheme 2) or cis to this ligand with the simultaneous shift of an

• hand, complex 19 is an example of an asymmetric catalyst, suggesting that the NHC ligand is the source of asymmetry [52]. In this paper we contribute in the understanding of the side- and bottom-bound coordination intermediates and the stability of the corresponding transition states as well as the

• will center our interests. All studied systems are displayed in Scheme 4. System 7 has been thoroughly studied because of its simplicity, playing especially with the NHC ligand, replacing one or both mesityl groups by CH3, CF3, t-Bu, H, F, and combinations between them to also observe the effects of an

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

• , H2ITapMe2 = 1,3-bis(2’,6’-dimethyl-4’-trimethylammoniumphenyl)-4,5-dihydroimidazol-2-ylidene, OTf− = CF3SO3−) based on a dicationic N-heterocyclic carbene (NHC) ligand was prepared. The reactivity was tested in ring opening metathesis polymerization (ROMP) under biphasic conditions using a nonpolar organic

• gave the dicationic ruthenium alkylidene Ru-2 in 90% isolated yield (Scheme 1). This is to the best of our knowledge the first example of a ruthenium alkylidene bearing an NHC ligand with permanent dicationic charge. Crystals of Ru-2 suitable for single-crystal X-ray analysis were obtained from DMF