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

• .12.126 Abstract Bis-protic N-heterocyclic carbene complexes of platinum and palladium (4) yield dimeric structures 6 when treated with sodium tert-butoxide in CH2Cl2. The use of a more polar solvent (THF) and a strong base (LiN(iPr)2) gave the lithium chloride adducts monobasic complex 7 or analogous

• 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

• for the NM nitrogen. The ∆∆ values for both nitrogens are relatively unchanged as the carbene character of the ligand is still intact. Interestingly, 6-Pd shows a similar small ∆∆ value for the NH (2.6) but a greater ∆∆ for the NM of 17.4 ppm. This is possibly a result of the differing ring strain in

On the mechanism of imine elimination from Fischer tungsten carbene complexes

• Philipp Veit,
• Christoph Förster and
• Katja Heinze

Beilstein J. Org. Chem. 2016, 12, 1322–1333, doi:10.3762/bjoc.12.125

• Philipp Veit Christoph Forster Katja Heinze Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University, Duesbergweg 10-14, 55128 Mainz, Germany 10.3762/bjoc.12.125 Abstract (Aminoferrocenyl)(ferrocenyl)carbene(pentacarbonyl)tungsten(0) (CO)5W=C(NHFc)Fc (W(CO)5(E-2)) is

• synthesized by nucleophilic substitution of the ethoxy group of (CO)5W=C(OEt)Fc (M(CO)5(1Et)) by ferrocenyl amide Fc-NH– (Fc = ferrocenyl). W(CO)5(E-2) thermally and photochemically eliminates bulky E-1,2-diferrocenylimine (E-3) via a formal 1,2-H shift from the N to the carbene C atom. Kinetic and

• dissociation, followed by an oxidative addition/pseudorotation/reductive elimination pathway with short-lived, elusive seven-coordinate hydrido tungsten(II) intermediates cis(N,H)-W(CO)4(H)(Z-15) and cis(C,H)-W(CO)4(H)(Z-15). Keywords: carbene complexes; ferrocene; imine; mechanism; tungsten; Introduction

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

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

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp² C–H and sp³ C–H bonds

• Nivesh Kumar,
• Santanu Ghosh,
• Subhajit Bhunia and
• Alakesh Bisai

Beilstein J. Org. Chem. 2016, 12, 1153–1169, doi:10.3762/bjoc.12.111

• -oxindoles while working on asymmetric synthesis of 3,3-disubstituted-2-oxindoles via a Pd-catalyzed (chiral N-heterocyclic carbene as ligands) intramolecular α-arylation of an amide [35][36][37]. For this 'intramolecular dehydrogenative coupling' (IDC) of Csp2-H and Csp3-H they used 2.2 equiv of CuCl2 and 5

Chiral cyclopentadienylruthenium sulfoxide catalysts for asymmetric redox bicycloisomerization

• Barry M. Trost,
• Michael C. Ryan and
• Meera Rao

Beilstein J. Org. Chem. 2016, 12, 1136–1152, doi:10.3762/bjoc.12.110

• ) fragment in a bidentate fashion and undergoes a redox isomerization reaction wherein the carbinol proton performs a 1,2-hydride shift. The resulting vinylruthenium intermediate can be seen as a resonance structure of a ruthenium carbene, which coordinates to a pendant alkene, performs a [2 + 2

• coordination. The pendant olefin prefers to approach the carbene anti to the aforementioned sulfoxide, resulting in the observed enantiomer of 53. In THF, the rate of epimerization is significantly slower than the [2 + 2] cycloaddition, which means that the enantiomeric ratios observed in the products are

The synthesis of functionalized bridged polycycles via C–H bond insertion

• Jiun-Le Shih,
• Po-An Chen and
• Jeremy A. May

Beilstein J. Org. Chem. 2016, 12, 985–999, doi:10.3762/bjoc.12.97

• systems rapidly and efficiently, though additional catalyst development is needed. Keywords: bridged rings; carbene; cascade reaction; C–H bond insertion; nitrene; Introduction Bridged polycyclic natural products are an inviting challenge to the synthetic chemist for their rich display of functional

• access to multiple targets from a single intermediate produced on large scale that may be stored until needed [17]. The C–H bond insertion has great potential as a method to access different polycyclic isomers (e.g., 1 or 3) through C–C or C–N bond formation from a carbene or nitrene, respectively

• have organized this review by the catalyst used, with free carbenes first, followed by Cu, Rh, Au, Pt, and then W-catalyzed reactions. Review Metal-free reactions While transition metal catalysis has seen widespread adoption for carbene and nitrene reactions, it is not necessary for a controlled

Enantioselective carbenoid insertion into C(sp³)–H bonds

• J. V. Santiago and
• A. H. L. Machado

Beilstein J. Org. Chem. 2016, 12, 882–902, doi:10.3762/bjoc.12.87

• enantioselective insertion of these organometallic species into these non-polarized bonds is a recent topic in the chemical literature, when compared to the first reports of carbenoid chemistry around the 1950s. Carbene is a molecule bearing a functional group with a divalent neutral carbon. This structural

• framework results in the presence of a nonbonding electron pair that may adopt two electronic configurations: singlet and triplet (Figure 1). A carbenoid is an organometallic complex where the carbene acts as a neutral ligand to a metal center. This ensures a greater stability of the carbene, allows the

• carbene carbon atom, the insertion reaction can be more or less selective. Very electrophilic carbenoid intermediates, for example, display little regio- and stereoselectivity, favoring the occurrence of side reactions. A less electrophilic carbenoid intermediate, on the other hand, has a lower reactivity

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

• Republic of China 10.3762/bjoc.12.85 Abstract A series of copper complexes (3–6) stabilized by 1,2,3-triazole-tethered N-heterocyclic carbene ligands have been prepared via simple reaction of imidazolium salts with copper powder in good yields. The structures of bi- and trinuclear copper complexes were

• 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

Recent advances in C(sp³)–H bond functionalization via metal–carbene insertions

• Bo Wang,
• Di Qiu,
• Yan Zhang and
• Jianbo Wang

Beilstein J. Org. Chem. 2016, 12, 796–804, doi:10.3762/bjoc.12.78

• –carbene; site-selectivity; Introduction Direct functionalization of inactivated C–H bonds, especially C(sp3)–H bonds, have attracted significant attentions in recent years. The C(sp3)–H bond activation strategies based on radical reactions and transition metal catalysis have been explored, alongside the

• simple aliphatic C(sp3)–H bonds. In this context, catalytic metal–carbene C(sp3)–H bond insertion represents an alternative and unique approach for this purpose. Metal–carbene insertion into a C(sp3)–H bond, well-recognized as one of the typical reactions of carbene species, have been studied extensively

• catalysts, the carbenic carbon of the metal–carbene species is positively charged in general, as shown by the resonance structure. Consequently, when the electron-deficient carbenic carbon approaches the C(sp3)–H bonds, the C–H bonds with high electron density will react preferentially [9]. However, the

Gold-catalyzed direct alkynylation of tryptophan in peptides using TIPS-EBX

• Gergely L. Tolnai,
• Jonathan P. Brand and
• Jerome Waser

Beilstein J. Org. Chem. 2016, 12, 745–749, doi:10.3762/bjoc.12.74

• . It has been achieved in the past for example by Francis and co-workers and Ball and co-workers using rhodium-catalyzed carbene-insertion reactions [21][22][23] or via direct C–H arylation [24][25][26][27][28][29]. If the installation of alkynes on peptides or proteins is desired, an indirect method

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

• borylation; iridium; N-heterocyclic carbene; Introduction Catalytic C–H borylation of arenes has become an essential tool in organic synthesis [1]. The eminent features of this methodology include 1) no directing group is needed, allowing the direct functionalization of simple arenes; 2) the

• 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

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

• popularly known as ‘Breslow intermediate’. This seminal discovery by Breslow paved the way for further developments in the area of carbene catalysis. Almost three decades later Bertrand and co-workers proved the existence of carbenes as catalytically active species in the benzoin reaction, with the

• nucleophile and then as a leaving group in cyanide-catalysed benzoin reactions [8]. Analogously, Breslow invoked the generation of a nucleophilic thiazolylidene species 1 via deprotonation of the thiazolium salt by base. The ylide 1 may also be represented as its resonance structure 1’ (carbene). Nucleophilic

• may be regarded as the first report of an NHC-catalysed benzoin reaction on a synthetically useful scale [9]. Much later, in 2005, Xu and Xia used N-alkyl-substituted imidazolium carbene 6 to efficiently promote benzoin reactions. Although a high catalyst loading (50 mol %) was required, the reactions

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

• in unsymmetrical alkynes can help to achieve high regioselectivity in the hydrophenoxylation. Keywords: cooperative catalysis; gold catalysis; hydrophenoxylation; N-heterocyclic carbene; vinyl ethers; Introduction During the last 30 years, N-heterocyclic carbenes (NHCs) have evolved from mere

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

• -heterocyclic carbene bearing precursor complexes M31, HovII or M32 with excess of 5,7-dichloro-8-hydroxyquinoline or 5,7-dibromo-8-hydroxyquinoline in the presence of excess Cs2CO3 as the base (see Scheme 1). The silver-free method [41] resulted in any case in the formation of at least two new products (as

• 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

• the initiators was investigated. For that purpose, 4 was mixed with 5 equiv of monomer 5 in CDCl3 and activated it with 5 equiv of etherical HCl in a NMR tube. After few minutes, the characteristic carbene peak for propagating alkylidenes at 18.1 ppm appeared (see Figure 3). To identify this carbene

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

• ; 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

• exhibit better activity, despite bearing its carbene functionality. Of course, these good results in basic research supposed and explosion of industrial efforts to design the right metal NHC-based catalyst for any kind of reaction. Anyway, neither a unique nor a few list of catalysts turned out to be

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

• with 4-chlorotoluene. Keywords: arylation; benzimidazolium salts; catalysis; N-heterocyclic carbene; PEPPSI complex; Suzuki–Miyaura cross-coupling reaction; Introduction The use of N-heterocyclic carbenes (NHCs) as ligands was started by Wanzlick [1] and Öfele [2] almost fifty years ago. There have

• been major advances in the design and synthesis of metal complexes containing N-heterocyclic carbene ligands in the last two decades, and they had a wide range of applications in different fields, particularly in homogeneous/heterogeneous catalysis [3][4][5][6][7][8] and bioorganometallic chemistry [9

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

• compounds even at 0 °C. It was also examined in more demanding systems such as conjugated dienes and polyenes. The catalyst is stable, storable and easy to purify. Keywords: chromane derivatives; metathesis catalyst; nitrogen heterocycles; olefin metathesis; Ru-carbene; Introduction Olefin metathesis is

• of organic chemistry since 1992, when Grubbs discovered the first well-defined ruthenium catalyst [2]. Nearly 400 ruthenium heterocyclic carbene-coordinated olefin metathesis catalysts were prepared until 2010 [3]. Since 2011, when Grubbs reported the synthesis of a Z-selective catalyst [4], several

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

• strongest electron-donating N-heterocyclic carbenes, resulted in the rate acceleration of the C–H arylation reaction of heterocycles over conventional ligands such as triphenylphosphine and a classical N-heterocyclic carbene. In situ observation and isolation of the 2-pyridylidene-gold(III) species, as well

• as a DFT study, indicated unusual stability of gold(III) species stabilized by strong electron donation from the 2-pyridylidene ligand. Thus, the gold(I)-to-gold(III) oxidation process is thought to be facilitated by the highly electron-donating 2-pyridylidene ligand. Keywords: carbene ligand; C–H

• numerous benefits such as high activity and stability of gold catalyst, thereby achieving otherwise-difficult oxidative transformations [37][38][39][40]. Recently, we have introduced highly electron-donating triaryl-2-pyridylidene (PyC: pyridine-based carbene) [82][83][84] as a new type of nonclassical N

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

• calculations and with an energy decomposition analysis. The investigated molecules include N-heterocyclic carbenes (NHCs), the cyclic alkyl(amino)carbene (cAAC), mesoionic carbenes and ylide-stabilized carbenes. The bonding analysis suggests that the carbene centre in cAAC and in diamidocarbene have the

• size of the backbone ring [39][40][41], variation of the α-heteroatoms [7], anti-Bredt NHCs [42][43], mesoionic NHCs [44][45][46][47], ylide stabilized carbenes [48][49][50][51] and other [52][53][54][55]. A remarkable variation was introduced with the cyclic alkyl(amino)carbene (cAAC) by Bertrand in

• 2005 [20][56][57]. The replacement of one amino substituent by a saturated alkyl group makes the carbene more nucleophilic and electrophilic at the same time [20][56][57]. Since then, cAACs have been used as a superior ligand for the stabilization of unstable chemical species, radical and main group

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

• microreactor, a new protocol for scalable aromatic trifluoromethylation was developed. From a mechanistic aspect, Vicic and co-workers explored the direct generation of CF3Cu from CF3CO2Cu. The use of (N-heterocyclic carbene)copper-trifluoroacetates prepared from trifluoroacetic acid (TFA) was investigated in

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

• , Portugal 10.3762/bjoc.11.278 Abstract The coordination versatility of a 1,3,5-triphenylbenzene-tris-mesoionic carbene ligand is illustrated by the preparation of complexes with three different metals: rhodium, iridium and nickel. The rhodium and iridium complexes contained the [MCl(COD)] fragments, while

• the nickel compound contained [NiCpCl]. The preparation of the tris-MIC (MIC = mesoionic carbene) complex with three [IrCl(CO)2] fragments, allowed the estimation of the Tolman electronic parameter (TEP) for the ligand, which was compared with the TEP value for a related 1,3,5-triphenylbenzene-tris

• and 3 were characterized by NMR and mass spectrometry. Both, the 1H and the 13C NMR spectra of the complexes were consistent with the expected threefold symmetry of the molecules, as exemplified by the appearance of one only signal for the carbene carbons, at 173.4 (1JRh–C = 41.5 Hz) and 172.1 ppm

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

• 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

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

• derivatives, whereas the use of triethyl orthoformate under microwave irradiation was most appropriate for the fast and efficient synthesis of imidazolinium salts. This strategy was applied to the synthesis of six common N-heterocyclic carbene precursors, namely, 1,3-dimesitylimidazolium chloride (IMes·HCl

• serves de facto as a carbene source for most catalytic and synthetic purposes. Alternative methods to generate NHCs without the intervention of a base, which might lead to unwanted side-reactions, include the facile cleavage of NHC·CO2 zwitterions (Scheme 1, path B) [31][32][33][34][35], the thermolysis

• scattered in the literature, often relegated to supporting information, and comparison of their respective merits has become more and more challenging. In this report, we aimed at collecting a series of efficient synthetic protocols for the preparation of eight common N-heterocyclic carbene precursors

Evidencing an inner-sphere mechanism for NHC-Au(I)-catalyzed carbene-transfer reactions from ethyl diazoacetate

• Manuel R. Fructos,
• Juan Urbano,
• M. Mar Díaz-Requejo and
• Pedro J. Pérez

Beilstein J. Org. Chem. 2015, 11, 2254–2260, doi:10.3762/bjoc.11.245

• Abstract Kinetic experiments based on the measurement of nitrogen evolution in the reaction of ethyl diazoacetate (N2CHCO2Et, EDA) and styrene or methanol catalyzed by the [IPrAu]+ core (IPr = 1,3-bis(diisopropylphenyl)imidazole-2-ylidene) have provided evidence that the transfer of the carbene group

• CHCO2Et to the substrate (styrene or methanol) takes place in the coordination sphere of Au(I) by means of an inner-sphere mechanism, in contrast to the generally accepted proposal of outer-sphere mechanisms for Au(I)-catalyzed reactions. Keywords: carbene transfer; inner sphere; gold catalysis; O–H

• . Most of the reported systems contain an unsaturated fragment that is activated upon coordination to the gold center, thus triggering further transformations, the formation of very reactive gold–carbene intermediates being proposed [4][5][6][7][8][9][10][11][12][13][14]. As a representative example, the