Two Amphoteric Silver Carbene Clusters

Article abstract of DOI:10.1002/anie.201803246

Two highly labile silver carbene cluster complexes are described, which are unique in that they mark the transition point at which the carbene center transmutes from a fairly common NHC-like nucleophilic behavior to an electrophilic character befitting reactive silver carbene intermediates of relevance in various catalytic transformations. This amphoteric character is the distinguishing attribute of a μ-bridged donor/donor carbene entity that connects two silver atoms of triangular or tetrahedral metallic core units.

Full text of DOI:10.1002/anie.201803246

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Accepted Article
Title: Two Amphoteric Silver Carbene Clusters
Authors: Alois Fürstner, Alexander G. Tskhovrebov, and Richard
Goddard
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803246
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10.1002/anie.201803246
Angewandte Chemie International Edition
Two Amphoteric Silver Carbene Clusters
Alexander G. Tskhovrebov, Richard Goddard, and Alois Fürstner*
Abstract: Two highly labile silver carbene cluster complexes are described which are unique in that
they mark the transition point at which the carbene center transmutes from a fairly common NHC-like
nucleophilic behavior to an electrophilic character befitting reactive silver carbene intermediates of
relevance in various catalytic transformations. This amphoteric character is the distinguishing
attribute of a -bridged donor/donor carbene entity that connects two silver atoms of the triangular
or tetrahedral metallic core unit of complexes 4 and 5, respectively.
Silver salts are amongst the best promoters of the Wolff rearrangement that allows diazoketones to
be transformed into ketenes and their downstream products.1,2 The intervention of electrophilic
silver carbene intermediates in this reaction is highly likely, even though a rigorous proof is still
missing, not least because of the presumably very low stability of such intermediates.1-3 Actually it
cannot be firmly excluded that silver carbenes decompose in statu nascendi, especially when carrying
electron withdrawing carbonyl substituents; in doing so, the carbene ligand might get released into
solution where the actual rearrangement could take place in the unbound state.4 A similar ambiguity
surrounds even more recent reports on silver carbene chemistry,5,6 including applications to the CH
activation of alkanes7,8,9 as well as activation of the CX bond of certain alkyl halides:10 once again,
the intervention of electrophilic silver carbenes is plausible and probable;11 yet, it has been prudently
mentioned in the literature that free carbenes afford similar products or product distributions.7a,10
The fact that silver catalysts allow otherwise poorly reactive olefins to be cyclopropanated is also
strong but again only indirect evidence for participation of silver carbene intermediates.12,13 Similar
arguments pertain to silver-catalyzed Doyle-Kirmse reactions14 and ring expansions of
*
Dr. A. G. Tskhovrebov, Dr. R. Goddard, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
Email: fuerstner@kofo.mpg.de
Supporting Information and the ORCHID identification number(s) for the author(s) of this article can be found
under: #############
1
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10.1002/anie.201803246
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cyclopropyldiazomethane derivatives.15 Even attempted asymmetric silver catalysis was not fully
conclusive: the very modest ee’s and/or low yields achieved in intramolecular CH insertion as well
as aziridination reactions upon silver catalyzed diazo decomposition have indeed been rationalized
by assuming that the chosen chiral silver complexes merely act as Lewis-acid catalysts rather than as
sources of true silver carbenes.16,17 Only recently has a highly enantioselective intramolecular
dearomatization reaction been reported which makes a strong case for the intervention of discrete
silver carbenes.18 Along the same lines, silver catalyzed (vinylogous) OH or NH insertion reactions
of diazo derivatives were originally proposed to be Lewis-acid catalyzed transformations,16a,19
whereas recent DFT calculations speak for true silver carbene intermediates.20,21,22 Collectively, these
examples illustrate that this entire research area remains somewhat opaque.
The problem is amplified by the paucity of secured experimental information about structure and
bonding of reactive silver carbenes. To the best of our knowledge only a single example of an
electrophilic silver carbene complex has been characterized by spectroscopic and crystallographic
means.23,24 Specifically, complex 1 features a dimesityl carbene unit (red) that, in itself, is highly
shielded by virtue of the twisting of the aryl rings enforced by the ortho-substituents; on top, the
strongly electron donating and massively overcrowded ancillary ligand (blue) safeguards the reactive
2
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carbene site to the maximum extent. This special design notwithstanding, 1 proved highly fragile; it
was the least stable carbene complex amongst the Group 11 triad (Ag < Cu < Au), a trend that had
already been noticed before for Fischer-type carbenes.25 The homolysis-prone AgC bond in
combination with marginal back-donation of electron density from the Ag atom into the carbene
center and missing stabilization of the carbene LUMO of largely p-character by the flanking arene
rings accounts for the sensitivity of 1.23 Unfortunately, no reactivity data whatsoever were reported
for this unique complex.
A much larger experimental data set is available for silver complexes carrying N-heterocyclic carbene
(NHC) or diamino-substituted cyclopropenylidene ligands, many of which are air- and moisture-stable
and thermally fairly robust.26,27,28,29 NHC’s (and relatives) are nucleophilic rather than electrophilic at
carbon; for their strong donor properties, steric bulk and inertia, they find widespread use as
ancillary ligands in organometallic chemistry;30 the NHC contained in 1 (blue) used to stabilize the
fragile complex illustrates this aspect. Silver-NHC complexes can also be used as versatile carbene
transfer agents.28,29,31 It is therefore not clear if the available structural information about silver-NHC
complexes has any particular relevance for a better understanding of the catalytic transformations
outlined above which likely involve highly electrophilic silver carbene intermediates. It is of note,
however, that silver-NHC complexes show quite diverse bonding motifs, encompassing monomeric,
bridging and staircase-like species as well as a few cluster architectures.27,32
As evident from this brief summary, the structural space populated by reactive silver carbenes is
largely uncharted. Apprehensive of the experimental challenges to be met when working with such
sensitive compounds, our initial foray was built on the lessons learnt in the related gold carbene
arena.33 With complex 2 comprising a substituted diphenyl carbene backbone, our group had isolated
and characterized the first reactive gold carbene:34,35 as this species cyclopropanates styrene
derivatives even at low temperature, its structure is relevant for a better understanding of gold
catalysis.36,37 Since 2 obviously reacts as an ordinary electrophilic carbene despite the remote
heteroelement substituents, it must not be mistaken for a vinylogous NHC-Au complex. Therefore,
we hoped that analogous silver carbenes derived from diphenyldiazo derivatives 3 might also be
sufficiently stable for full characterization yet reactive enough to be significant in the realm of
catalysis. For the same reason, ordinary silver salts as commonly used for preparative purposes were
3
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chosen for this study rather than silver sources endowed with specially designed ancillary ligand
frameworks. Moreover, different substituents X in substrates of type 3 should provide a handle for
tuning the character of the resulting silver carbene, at least to some extent.
Scheme 1. a) AgOTFA (excess), THF, 78°C, > 95% (NMR, 4+5, see Text); b) AgOTFA (2 equiv.), DME,
55°C  0°C, 57%; DME = 1,2-dimethoxyethane; TFA = trifluoroacetyl
Figure 1. Structure of the silver carbene cluster 4 in the solid state; only one of the two independent
molecules in the unit cell is shown; likewise, the loosely associated THF molecules in the unit cell are
omitted for clarity (for details, see the Supporting Information)
4
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Unfortunately though, silver carbenes generated from 3a (X = H) or 3b (X = OMe) defied attempted
isolation even at very low temperatures. Only complexes derived from 3c carrying the supposedly
more stabilizing NMe2 substituents were sufficiently stable for full characterization as long as kept
cold ( 20°C) under Ar in the dark (Scheme 1). The outcome of the reaction strongly depended on
the chosen conditions in that two distinct carbene clusters with different global architectures could
be isolated even though they comprise the very same -carbene entity.
Specifically, reaction of 3c with AgOTFA in THF at 78°C was fairly clean as judged by NMR. Yet, two
distinctly different complexes co-crystallized from the intense green solution, which could be easily
distinguished as one of them is dichroic yellow/green, whereas the other one is dichroic
purple/green. It is not clear at this point whether these distinct species equilibrate on the NMR
timescale in solution or whether their spectra are indistinguishable on a 500 MHz spectrometer; in
this context, it is of note that complex 4 basically represents one face of complex 5. The sensitive
yellow/green fraction (4) consists of a triangular silver core carrying a -bridging carbene entity
(Figure 1); the other coordination sites are occupied by three trifluoroacetate groups. The bridging
carbene center C1 is equidistant to Ag1 and Ag2 [av. 2.17(1) Å] but does not make any contact to the
distal Ag3 atom. The resulting four-membered framework is puckered; the trifluoroacetyl groups at
Ag1 and Ag3 are pseudo-axially oriented, whereas the third trifluoroacetate is di-equatorially
disposed. As a single resonance is observed by 19F NMR, this particular pose in the solid state can
only be a snapshot of a dynamic ligand environment in 4 that averages out on the NMR time scale.
The purple/green fraction, which had to be hand-picked from the conglomerate that crystallized
from THF, consists of a tetrahedral Ag-cluster. Gratifyingly, this particular complex can be prepared
much more selectively when the reaction of 3c with AgOTFA (2 equiv.) is carried out in DME rather
than THF at low temperature. The signal at C = 221.1 ppm most likely indicates the presence of a
bridging carbene unit;38 in any case, it is very different from the resonance of the terminal carbene in
1 (C = 359.3 ppm).23 Careful recrystallization at low temperature afforded single crystals suitable for
X-ray diffraction (Figure 2). In contrast to 4, complex 5 was found to consist of a tetrahedral Ag-core
unit with two bridging carbenes entities in the periphery and decorated with four (weakly) bridging
trifluoroacetyl ligands,39 variously bound. As in the case of 4, the pose of the trifluoroacetates in the
5
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solid state is only one of several interconverting conformers which render these anionic ligands
equivalent on the NMR time scale.
Figure 2. Top: Structure of the silver carbene cluster 5 (prepared in DME and crystallized from
hexane) in the solid state; bottom: core region showing the symmetry-related -carbene centers (C1,
C18) as well as the bridging trifluoroacetyl ligands, which interconvert in solution on the NMR time
scale (all ligands are truncated for the sake of clarity). Except for conformational details, the structure
of the complex crystallized from THF is akin (see the Supporting Information)
6
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As expected for a -bridging carbene, the CAg bonds in 5 are short [2.19(1)2.22(1) Å], yet distinctly
longer than those in the terminal silver carbene 1 [2.05(2) Å].23 By virtue of simultaneous ligation to
two AgI fragments, the carbene acceptor orbital in 5 (as well as 4) benefits from enhanced net back
donation of electron density compared to a terminal carbene contained in complex 1. A similar
situation had previously been encountered in the copper series, where -bridging dicopper carbene
complexes proved more stable than their terminal cousins.40,41,42,43 The fact that the carbene units in
4 and 5 entertain three-center four-electron bonding modes is vaguely reminiscent of the situation in
electrophilic dirhodium carbenes, despite the linearity of the RhRhC core.44,45 Moreover, Figures 1
and 2 suggest that the electron rich arene rings in 4 and 5 help stabilize these carbene species.34,35
Specifically, the C1C2/C1C10 bonds (1.41/1.43 Å) in 5 are slightly contracted relative to the
corresponding CCipso bonds in the terminal carbene 1 (1.45 Å), and the NMe2 groups are coplanar
with the arenes so as to maximize orbital overlap.
The presence of a three-center four-electron interaction in 4 and 5 is thought to indicate an electron-
deficient carbene unit, which in turn implies electrophilic behavior; yet, it is not intuitive to which
extent the mesomeric effect of the distal NMe2 substituents compensates this electronic attribute.
Neither 4 nor 5 would cyclopropanate styrene to any appreciable extent, perhaps because the olefin
substrate is incapable of disassembling the cluster to a potentially more reactive terminal carbene.
Since -bridged dicopper carbene complexes described in the literature had previously also failed in
cyclopropanation,40,41 they had been profiled by reaction with a diazo derivative, the nucleophilic N-
terminus of which will attack an electrophilic carbene center very efficiently.46 Even though azine
formation constitutes a highly sensitive probe, the reaction had not been overly effective in the
copper series.40 In stark contrast, treatment of the silver complexes 4 or 5 with 3c afforded azine 6 in
almost quantitative yield, indicating electrophilic behavior (Scheme 2).47 The same is manifest in the
formation of imine 8, which is thought to derive from NH insertion followed by oxidation of amine 7
by the concomitantly released AgI cation. Furthermore, complex 5 proved amenable to formal cross
coupling on treatment with PhMgBr at low temperature; this nucleophile was chosen because the
transmetalation of Grignard reagents with AgI salts is known to be facile.48 In contrast to the rapidly
growing number of cross coupling reactions involving transition metal carbene intermediates,49 the
putative primary adduct 10 cannot stabilize itself by -hydride elimination or protonation;50,51 the
7
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redox capability of silver, however, opened an alternative outlet which ultimately furnished the
stable cation 11 in good yield together with precipitated Ag0.
Scheme 2. a) 3c (X = NMe2), CH2Cl2, 78°C  RT, 96%; b) BnNH2, CH2Cl2, 78°C  RT, 65%; c)
PhMgBr, THF, 78°C  RT, then HCl, 75%; d) LAuNTf2, CH2Cl2, 50°C  0°C, 12a (63%, L = IPr), 12b
(84%, L = XPhos); Bn = benzyl; IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene; Tf =
trifluoromethanesulfonyl; XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl
An additional set of experiments showed that complex 5 is actually amphoteric in nature in that it
does not only exhibit modest electrophilic reactivity but can evidently act as a nucleophilic entity too
(Scheme 2). In analogy to silver-NHC complexes, which serve coordination chemistry well as carbene
transfer agents,31 5 reacted with LAuNTf2 at low temperatures to give the corresponding terminal
gold carbene complexes 12a (L = IPr) and 12b (L = XPhos). The structures of 12a (Figure 3) and 12b
(Figure S6) in the solid state resemble those of the other known  but still exceedingly rare 
donor/donor gold carbenes.33,34,35 The characteristic quinoid distortion in 12a with short C1C2
(1.433 Å) and C5N1 (1.359 Å) bonds shows that the electron rich carbene backbone rather than the
8
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metal center largely accounts for the metastability of this complex. In contrast, the actual C1Au1
carbene unit itself seems to lack significant -back donation:52 in this particular case it has the exact
same length [2.023(6) Å] as the bond between the gold atom and the IPr ligand, which is appreciated
by the organometallic community for its poor -acceptor qualities.28,30
Figure 3. Structure of the cationic gold carbene complex 12a (L = IPr) in the solid state; the
disordered [NTf2] counterion is not shown for clarity
Although a systematic investigation into how para-substituents X with different electronic properties
impact on structure and reactivity of silver carbene species has not worked out as originally planned
for stability reasons, the example with X = NMe2, which was obtained in two different cluster
formats, led to several important insights. The carbene unit bridges two silver centers and shows
electrophilc as well as nucleophilic behavior; this peculiar amphotheric nature marks the transition
point at which silver carbenes transmute from an NHC-like behavior to the electrophilic reactivity
mode characteristic for intermediates invoked in a variety of catalytic transformations. In this context
reference is made to a very recent study on a gold carbene solely characterized by spectroscopic
means, which also seems to exhibit amphotheric character due to interaction of the carbene ligand
with a cluster surface.53,54 In any case, the newly isolated cluster species speak for an extraordinary
propensity of (electrophilic) silver carbenes to assemble to larger aggregates unless they carry
massively overcrowded and/or strongly donating ancillary ligands; this tendency is likely relevant for
mechanistic discussions and attempted optimizations of silver-catalyzed reactions of diazo
9
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derivatives (or other relevant carbene sources), which are usually carried out with bare unligated
silver salts. We suspect that the two specimens presented herein provide nothing but a glimpse of a
potentially large structural diversity, which future investigations into this challenging field promise to
uncover.
Acknowledgements
Generous financial support by the MPG is gratefully acknowledged. We thank the analytical
departments of our Institute for expert assistance, especially Mrs. S. Tobegen, Dr. C. Farès and Mr. N.
Nöthling for assistance with NMR analysis and X-ray crystallography.
10
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1
W. Kirmse, Eur. J. Org. Chem. 2002, 2193-2256.
T. Ye, M. A. McKervey, Chem. Rev. 1994, 94, 1091-1160.
2
3
a) S. G. Sudrik, T. Maddanimath, N. K. Chaki, S. P. Chavan, S. P. Chavan, H. R. Sonawane, K.
Vijayamohanan, Org. Lett. 2003, 5, 2355-2358; for an alternative proposal invoking carbenes
bound via the O-atom to silver nanoclusters, see: b) S. G. Sudrik, N. K. Chaki, V. B. Chavan, S. P.
Chavan, S. P. Chavan, H. R. Sonawane, K. Vijayamohanan, Chem. Eur. J. 2006, 12, 859-864.
Thermally, photochemically or sonochemically generated metal-free carbenes are known to
rearrange analogously, see ref. [1].
4
5
6
Review: Z. Li, C. He, Eur. J. Org. Chem. 2006, 4313-4322.
C. J. Lovely, in Silver in Organic Chemistry (Ed.: M. Harmata), Wiley, Hoboken, 2010, pp. 229-
257.
7
8
a) H. V. R. Dias, R. G. Browning, S. A. Richey, C. J. Lovely, Organometallics 2004, 23, 1200-1202;
b) C. J. Lovely, J. A. Flores, X. Meng, H. V. R. Dias, Synlett 2009, 129-132.
a) A. Caballero, E. Despagnet-Ayoub, M. Mar Díaz-Requejo, A. Díaz-Rodriguez, M. E. Gonzáles-
Núñez, R. Mello, B. K. Muñoz, W.-S. Ojo, G. Asensio, M. Etienne, P. J. Pérez, Science 2011, 332,
835-838; b) J. Urbano, T. R. Belderraín, M. C. Nicasio, S. Trofimenko, M. M. Díaz-Requejo, P. J.
Pérez, Organometallics 2005, 24, 1528-1532.
9
J. A. Flores, N. Komine, K. Pal, B. Pinter, M. Pink, C.-H. Chen, K. G. Caulton, D. J. Mindiola, ACS
Catal. 2012, 2, 2066-2078.
10
11
H. V. R. Dias, R. G. Browning, S. A. Polach, H. V. K. Diyabalanage, C. J. Lovely, J. Am. Chem. Soc.
2003, 125, 9270-9271.
For computational evidence, see: A. A. C. Braga, A. Caballero, J. Urbano, M. M. Diaz-Requejo, P.
J. Pérez, F. Maseras, ChemCatChem 2011, 2, 1646-1652.
12
13
a) J. L. Thompson, H. M. L. Davies, J. Am. Chem. Soc. 2007, 129, 6090-6091; b) L. Chen, M. O.
Bovee, B. E. Lemma, K. S. M. Keithley, S. L. Pilson, M. G. Coleman, J. Mack, Angew. Chem. Int.
Ed. 2015, 54, 11084-11087; Angew. Chem. 2015, 127, 11236-11239.
For cyclopropenation and Buchner reactions, see: a) J. F. Briones, H. M. L. Davies, Org. Lett.
2011, 13, 3984-3987; b) C. J. Lovely, R. G. Browning, V. Badarinarayana, H. V. R. Dias,
11
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Tetrahedron Lett. 2005, 46, 2453-2455; c) N. Komine, J. A. Flores, K. Pal, K. G. Caulton, D. J.
Mindiola, Organometallics 2013, 32, 3185-3191.
14
In this case, the silver atom is almost certainly no longer associated once the ylide has been
formed by attack of the sulfide onto the (silver-bound ?) carbene, see: P. W. Davies, S. J.-C.
Albrecht, G. Assanelli, Org. Biomol. Chem. 2009, 7, 1276-1279.
15
16
H. Xu, W. Zhang, D. Shu, J. B. Werness, W. Tang, Angew. Chem. Int. Ed. 2008, 47, 8933-8936;
Angew. Chem. 2008, 120, 9065-9068.
a) S. Bachmann, D. Fielenbach, K. A. Jorgensen, Org. Biomol. Chem. 2004, 2, 3044-3049; b) K.
Juhl, R. G. Hazell, K. A. Jorgensen, J. Chem. Soc. Perkin Trans. 1 1999, 2293-2297; see also: c) K.
Burgess, H.-J. Lim, A. M. Porte, G. A. Sulikowski, Angew. Chem. Int. Ed. Engl. 1996, 35, 220-222;
Angew. Chem. 1996, 108, 192-194.
17
For a Lewis acid/Lewis base adduct between a silver scorpionate complex and diazo malonate,
see: H. V. R. Dias, S. A. Polach, Inorg. Chem. 2000, 39, 4676-4677.
18
19
H. Nakayama, S. Harada, M. Kono, T. Nemoto, J. Am. Chem. Soc. 2017, 139, 10188-10191.
a) Y. Yue, Y. Wang, W. Hu, Tetrahedron Lett. 2007, 48, 3975-3977; b) G.-F. Zha, J.-B. Han, X.-Q.
Hu, H.-L. Qin, W.-Y. Fang, C.-P. Zhang, Chem. Commun. 2016, 52, 7458-7461.
a) J. H. Hansen, H. M. L. Davies, Chem. Sci. 2011, 2, 457-461; see also: b) G. Xu, K. Liu, Z. Dai, J.
Sun, Org. Biomol. Chem. 2017, 15, 2345-2348.
20
21
Computational studies suggest that silver carbenes account for [3+2] cycloadditions of nitriles
with diazo esters, see: J. A. Flores, K. Pal, M. E. Carroll, M. Pink, J. A. Karty, D. J. Mindiola, K. G.
Caulton, Organometallics 2014, 33, 1544-1552.
22
23
24
For analogous silver catalyzed fluorination of vinyl diazoacetates, see: C. Qin, H. M. L. Davies,
Org. Lett. 2013, 15, 6152-6154.
M. W. Hussong, W. T. Hoffmeister, F. Rominger, B. F. Straub, Angew. Chem. Int. Ed. 2015, 54,
10331-10335; Angew. Chem. 2015, 127, 10472-10476.
For a recent review on late-transition metal carbene complexes, see: R. Peloso, E. Carmona,
Coord. Chem. Rev. 2018, 355, 116-132.
25
26
L. Batiste, P. Chen. J. Am. Chem. Soc. 2014, 136, 9296-9307.
The first example was reported by: A. J. Arduengo, H. V. R. Dias, J. C. Calabrese, F. Davidson,
Organometallics 1993, 12, 3405-3409.
12
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27
J. C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978-4008.
S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612-3676.
28
29
30
31
32
D. Holschumacher, C. G. Hrib, P. G. Jones, M. Tamm, Chem. Commun. 2007, 3661-3663.
M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature, 2014, 510, 485-496.
S.-T. Liu, K. Rajender Reddy, Chem. Soc. Rev. 1999, 28, 315-322.
For representative NHC-containing silver clusters, see: a) J. C. Garrison, R. S. Simons, W. G.
Kofron, C. A. Tessier, W. J. Youngs, Chem. Commun. 2001, 1780-1781; b) J. C. Garrison, R. S.
Simons, C. A. Tessier, W. J. Youngs, J. Organomet. Chem. 2003, 673, 1-4; c) V. J. Catalano, M. A.
Malwitz, Inorg. Chem. 2003, 42, 5483-5485.
33
34
R. J. Harris, R. A. Widenhoefer, Chem. Soc. Rev. 2016, 45, 4533-4551.
G. Seidel, A. Fürstner, Angew. Chem. Int. Ed. 2014, 53, 4807-4811; Angew. Chem. 2014, 126,
4907-4911.
35
a) C. Werlé, R. Goddard, A. Fürstner, Angew. Chem. Int. Ed. 2015, 54, 15452-15456; Angew.
Chem. 2015, 127, 15672-15676; b) W. Debrouwer, A. Fürstner, Chem.. Eur. J. 2017, 23, 4271-
4275; c) G. Seidel, R. Mynott, A. Fürstner, Angew. Chem. Int. Ed. 2009, 48, 2510-2513; Angew.
Chem. 2009, 121, 2548-2551.
36
37
38
a) D. J. Gorin, F. D. Toste, Nature 2007, 446, 395-403; c) A. S. K. Hashmi, Angew. Chem. Int. Ed.
2010, 49, 5232-5241; Angew. Chem. 2010, 122, 5360-5369.
a) A. Fürstner, P. W. Davies, Angew. Chem. Int. Ed. 2007, 46, 3410-3449; Angew. Chem. 2007,
119, 3478-3519; b) A. Fürstner, Acc. Chem. Res. 2014, 47, 925-938.
R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, 4th ed., Wiley,
Hoboken, 2005.
39
40
41
42
The AgO distances are of different lengths but seem all to be binding.
X. Dai, T. H. Warren, J. Am. Chem. Soc. 2004, 126, 10085-10094.
P. Hofmann, I. V. Shishkov, F. Rominger, Inorg. Chem. 2008, 47, 11755-11762.
a) B. F. Straub, P. Hofmann, Angew. Chem. Int. Ed. 2001, 40, 1288-1290; Angew. Chem. 2001,
113, 1328-1330; b) I. V. Shishkov, F. Rominger, P. Hofmann, Organometallics 2009, 28, 1049-
1059.
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For related heterobimetallic complexes with a -bridging diarylcarbene unit, see: G. Seidel, B.
Gabor, R. Goddard, B. Heggen, W. Thiel, A. Fürstner, Angew. Chem. Int. Ed. 2014, 53, 879-882;
Angew. Chem. 2014, 126, 898-901.
44
45
J. F. Berry, Dalton Trans. 2012, 41, 700-713.
a) C. Werlé, R. Goddard, P. Philipps, C. Farès, A. Fürstner, J. Am. Chem. Soc. 2016, 138, 3797-
3805; b) C. Werlé, R. Goddard, P. Philipps, C. Farès, A. Fürstner, Angew. Chem. Int. Ed. 2016,
55, 10760-10765; Angew. Chem. 2016, 128, 10918-10923; c) D. J. Tindall, C. Werlé, R. Goddard,
P. Philipps, C. Farès, A. Fürstner, J. Am. Chem. Soc. 2018, 140, 1884-1893.
46
47
For azine formation from discrete metal carbene species and Ph2CN2, see: a) H. Werner, P.
Schwab, E. Bleuel, N. Mahr, B. Windmüller, J. Wolf, Chem. Eur. J. 2000, 6, 4461-4470.
A control experiment described in the Supporting Information showed that addition of 3c to
catalytic amounts of AgOTFA affords the same outcome, suggesting that the cluster complex 5
is a competent intermediate.
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G. van Koten, S. L. James, J. T. B. H. Jastrzebski, in Comprehensive Organometallic Chemistry II,
Vol. 3 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson), Elsevier, Oxford, 1995, pp. 57-133.
a) Y. Xia, D. Qiu, J. Wang, Chem. Rev. 2017, 117, 13810-13889; b) J. Barluenga, C. Valdés,
Angew. Chem. Int. Ed. 2011, 50, 7486-7500; Angew. Chem. 2011, 123, 7626-7640.
Mechanistic details are currently unknown; the depicted intermediates 9 and 10 are therefore
nothing but formal representations of a conceivable ionic pathway; they could very well be
oligomeric in nature. Moreover, many organosilver complexes are homolysis-prone and the
observed reaction might proceed via radical intermediates. Similar arguments pertain to more
conventional silver catalyzed cross coupling reactions of Grignard reagents with e. g. alkyl or
silyl halides; for leading references, see the following and literature cited therein: a) M.
Tamura, J. K. Kochi, J. Am. Chem. Soc. 1971, 93, 1483-1485; b) H. Someya, H. Ohmiya, H.
Yorimitsu, K. Oshima, Org. Lett. 2008, 10, 969-971; c) K. Murakami, K. Hirano, H. Yorimitsu, K.
Oshima, Angew. Chem. Int. Ed. 2008, 47, 5833-5835; Angew. Chem. 2008, 120, 5917-5919.
For the reaction of a tertiary organopalladium intermediate similar to putative 10 with formate
as external reducing agent to release triarylmethane products, see: Y. Xia, F. Hu, Z. Liu, P. Qu,
R. Ge, C. Ma, Y. Zhang, J. Wang, Org. Lett. 2013, 15, 1784-1787.
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While dp back donation from Au into the „empty“ carbene orbital is low, electron push in
the -framework is non-negligible, see: R. G. Carden, N. Lam, R. A. Widenhoefer, Chem. Eur. J.
2017, 23, 17992-18001.
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54
J. Oliver-Meseguer, M. Boronat, A. Vidal-Moya, P. Concepción, M. Á. Rivero-Crespo, A. Leyva-
Pérez, A. Corma, J. Am. Chem. Soc. 2018, 140, 3215-3218.
Gold carbenes seem to transmute from electrophilic to nucleophilic behavior by increasing the
oxidation state, see: A. Pujol, M. Lafage, F. Rekhroukh, N. Saffon-Merceron, A. Amgoune, D.
Bourissou, N. Nebra, M. Fustier-Boutignon, N. Mézailles, Angew. Chem. Int. Ed. 2017, 56,
12264-12267; Angew. Chem. 2017, 129, 12432-12435.
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