Rearrangement of a Transient Gold Vinylidene into Gold Carbenes

Article abstract of DOI:10.1002/chem.201700326

The gold acetylide complex 20 endowed with a biaryl backbone provides opportunities for a study on the formation and fate of gold vinylidenes. Although the formyl group in 20 is not sufficiently electrophilic to get attacked by the acetylide in proximity, its activation with TBSOTf (TBSOTf=tert-butyldimethylsilyl trifluoromethanesulfonate) at low temperature triggered instantaneous formation of a gold vinylidene (21). This metastable species evolved into the cationic gold carbene complex 22 bearing a phenanthrene unit and a hydroxyl group at the aurated center; the recorded data suggest that this product might be better viewed as an acylgold species protonated by triflic acid. The use of [Me₃O?BF₄] as the activating agent led to formation of the analogous Fischer-type carbene 24, whereas replacement of gold by the [CpRu(PPh₃)₂]⁺ fragment allowed the ruthenium vinylidene 27 to be isolated, which closely resembles the proposed gold intermediate 21. The starting gold complex 20, the derived products 22 and 24, as well as vinylidene 27 were characterized by X-ray diffraction.

Full text of DOI:10.1002/chem.201700326

A Journal of
Accepted Article
Title: Rearrangement of a Transient Gold Vinylidene into Gold
Carbenes
Authors: Alois Fürstner and Wouter Debrouwer
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10.1002/chem.201700326
Chemistry - A European Journal
Rearrangement of a Transient Gold Vinylidene into Gold Carbenes
Wouter Debrouwer and Alois Fürstner*
Abstract: The gold acetylide complex 20 endowed with a biaryl backbone provides
opportunities for a study on the formation and fate of gold vinylidenes. While the formyl
group in 20 per se is not sufficiently electrophilic to get attacked by the acetylide in proximity,
its activation with TBSOTf at low temperature triggered instantaneous formation of a gold
vinylidene (21). This metastable species evolved into the cationic gold carbene complex 22
bearing a phenanthrene unit and a hydroxyl group at the aurated center; the recorded data
suggest that this product might be better viewed as an acylgold species protonated by triflic
acid. The use of [Me3OBF4] as the activating agent also led to formation of the analogous
Fischer-type carbene 24, whereas replacement of gold by the [CpRu(PPh3)2]+ fragment
allowed the ruthenium vinylidene 27 to be isolated, which closely resembles the proposed
gold intermediate 21. The starting gold complex 20, the derived products 22 and 24, as well
as vinylidene 27 were characterized by X-ray diffraction.
During the course of comprehensive studies on the synthesis of phenanthrenes and related
polycyclic arenes by -acid catalyzed cycloisomerization,1,2,3 we noticed that haloalkynes 1 (X
= Br, I) rearrange on treatment with catalytic amounts of AuI to give product 3 in which the
halide has migrated by one C-atom (Scheme 1). Intervention of a gold vinylidene of type 2
formed by 1,2-halide shift provided the best explanation for this unusual outcome,1,4
although such intermediates had never been invoked before. This mechanistic proposal was
confirmed by related observations subsequently made in other laboratories5 and by a first
set of DFT calculations.6,7
*
Dr. W. Debrouwer, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
E-mail: fuerstner@kofo.mpg.de
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Scheme 1. Gold vinylidenes: prior art and project outline; counterions are not shown for
clarity; L = neutral two-electron donor ligand; LA = Lewis acid.
This research area gained momentum when the groups of Zhang8 and Hashmi9
independently suggested that gold acetylides such as 5 and 9 react with tethered gold-
alkyne -complexes to form the gold vinylidenes 6 and 10, respectively (Scheme 1). In its
essence, such “dual / activation”10,11 represents an innovative extension of the arguably
most general entry into metal vinylidenes by reaction of a transition metal acetylide with
various kinds of electrophiles, including H+, R+ or X+.4 If this approach pertains to the gold
series, it is reasonable to expect that electrophilic partners other than gold-alkyne -
complexes12 can be used for the same purpose. In fact, Harris and Widenhoefer cleverly
2
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used this strategy to generate the gold vinylidene 14, which is the first and so far only
species of its kind to be fully characterized by NMR.13 Complex 14 was derived from 12 upon
in situ release of a tethered silylium cation (13); its vinylidene carbon atom resonates at a
surprisingly high field ( (90°C, CD2Cl2) = 206 ppm), likely as a consequence of the stabilizing
effect exerted by the two silyl groups onto the polarized vinylidene unit. Such stabilization is
arguably necessary to give a gold vinylidene a discrete lifetime: one can safely extrapolate
from the growing body of secured information on structure and bonding in gold
carbenes14,15,16 that gold vinylidenes in general must be exceptionally reactive species unless
they are tempered by an appropriate ligand framework and/or substitution pattern.
Specifically, a vinylcation resonance extreme makes a substantial contribution to their
ground state structure;17 this qualitative picture is well in line with the available
experimental and computational data.10,18,19
We conjectured that a ligand architecture as shown in A might also lend itself to an
investigation into gold vinylidenes (Scheme 1). The aromatic backbone was thought to be a
robust and well accessible platform that ensures proximity of the nucleophilic and
electrophilic sites; if necessary, the reactivity of the carbonyl can be tuned by activation with
an appropriate Lewis-acid as shown in B, whereas substituents on the arene rings might
ultimately help to stabilize the resulting complex C. In a first foray, we prepared the parent
compound of this series by a sequence of palladium catalyzed cross coupling reactions
exploiting the differential in the reactivity of the C-I and C-Br bond of 16 (Scheme 2).2c
Treatment of 18 with the gold hydroxide complex 1920 afforded the corresponding gold
acetylide 20 in essentially quantitative yield.21 Even though this reaction was carried out in
alcoholic medium at 60°C, spontaneous cyclization with formation of a gold vinylidene did
not occur. The aldehyde per se is obviously not sufficiently electrophilic to succumb to
spontaneous attack by the acetylide unit of 20, despite the fact that the two sites supposed
to react with each other are fairly close in space  at least in the solid state (C5C18 3.02 Å)
 and the biaryl is only slightly twisted (C6-C11-C12-C17 58.5°) (Figure 1).
3
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Scheme 2. a) HCCSiMe3, [(Ph3P)2PdCl2] (1 mol%), CuI (3 mol%), Et3N, 84%; b) 2-formyl-
phenylboronic acid, Pd2(dba)3 (5 mol%), RuPhos (10 mol%), K3PO4, toluene, reflux, 82%; c)
19, MeOH/EtOH (1:1), 60°C, quant.; d) TBSOTf, CH2Cl2, 78°C, 75%; e) CD2Cl2, RT, 23a (X = H),
quant. (NMR); f) CD2Cl2, MeOH, 23b (X
= COOMe), quant. (NMR); dba =
bisbenzylideneacetone, TBS = tert-butyldimethylsilyl; L = 1,3-bis(2,6-diisopropyl)imidazole-2-
ylidene.
Figure 1. Structure of the gold acetylide complex 20 in the solid state; anisotropic
displacement parameters are shown at the 50% probability level. Selected bond lengths (Å)
and angles (°): C5C4 1.207(6), C4Au1 1.983(5), Au1C1 2.012(5), C5-C4-Au1 173.8(4).
4
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Upon addition of TBSOTf at 78°C, however, the almost colorless solution of 20 in
CH2Cl2 instantaneously turned orange-red; 13C NMR spectra recorded at this temperature
showed the presence of an intermediate with a distinctive low-field resonance at C = 293
ppm, which proved metastable and evolved within minutes into a new compound with a
characteristic resonance at C = 283.5 ppm; concurrently, the intense orange-red color faded
away, giving rise to a yellow solution. The constitution of the resulting complex 22 was
unambiguously established by NMR and X-ray diffraction (Figure 2).
Figure 2. Structure of the gold carbene complex 22 in the solid state; anisoptropic
displacement parameters are shown at the 50% probability level; the thin red line indicates
the O1(H)O2 hydrogen bonding array. Selected bond lengths (Å): Au1C1 2.010(9),
Au1C16 2.011(9), C1C2 1.467(9), C2C3 1.337(8), C1O1 1.274(9).
Complex 22 turned out to be a cationic Fischer-type gold carbene bearing a
phenanthrene entity and a hydroxyl group as the substituents flanking the carbene center,
even though the proton of the OH group has not been localized on the Fourier map. The
gold “carbene” Au1C1 bond is as long (2.010(9) Å) as the gold-NHC distance Au1C16
(2.011(9) Å), in line with a very low bond order in either case.22 In contrast, the C1O1
distance (1.274(9) Å) is remarkably short, thus indicating substantial carbonyl character. 22
5
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10.1002/chem.201700326
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can therefore rightfully be viewed as an acylgold species protonated by triflic acid. This
interpretation is reinforced by the tight ion pairing of the cationic entity with the escorting
triflate anion, which manifests itself in a very short O1(H)O2 distance (2.551 Å). In line
with the notion that 22 shows substantial acyl metal character, this compound was found to
decarbonylate to give 23a (X = H) when kept in solution,23 whereas treatment with MeOH at
low temperature afforded the corresponding ester 23b (X = COOMe). To the best of our
knowledge, 22 is the first isolated gold complex of its kind.
We propose that 22 is formed upon attack of the alkynylgold group of 20 onto the
activated formyl substituent in vicinity, which gives rise to a transient gold vinylidene species
21 that likely manifests itself in the C = 293 ppm signal observed at the outset of the
reaction. Whether the subsequent conversion into 22 proceeds in a metathesis fashion via a
discrete oxete intermediate D or in a stepwise manner passing through a stabilized benzylic
cation such as E cannot be decided at this point, although some preliminary data speak for
an ionic pathway (see below). In any case, aromatization of the backbone certainly drives
this formal alkyne/carbonyl metathesis reaction.24
Scheme 3. a) Me3OBF4, CH2Cl2, 78°C  RT, 55%; b) TBAF, MeOH/THF, 0°C  RT, 95%; c)
TBSOTf, CH2Cl2, 78°C  RT, 82%; d) [CpRu(PPh3)2Cl], NH4PF6, MeOH, reflux, 88% (R = H,
Me); Cp = cyclopentadienyl; TBAF = tetra-n-butylammonium fluoride
Although the significant carbonyl character of the emerging [LAuC(=O)R] unit in 22
explains why the silyl group derived from TBSOTf used to activate the aldehyde is not
6
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retained in the resulting gold complex but was cleaved off in situ or during product isolation
by adventitious water, we explored whether a more “sticky” activating agent stays on. To
this end, compound 20 was treated with [Me3OBF4]; while no spontaneous reaction was
observed at 78°C, gentle warming of the mixture led to the formation of the corresponding
methoxycarbene 24 (Scheme 3). Its structure in the solid state also features the
characteristic pattern of a distinctively long Au1C1 but a notably short Au1O1 bond and is
hence similar to that of 22 (Figure 3).
Another control experiment proved that the gold atom attached to the alkyne unit
plays a crucial role in determining the course of the reaction. Specifically, the terminal alkyne
25 derived from 18 was exposed to TBSOTf in CH2Cl2 at low temperature. In line with our
expectation, no phenanthrene system was formed in this case; rather, the known seven-
membered enone 26 could be isolated in good yield.25
Figure 3. Structure of the gold carbene complex 24 in the solid state; anisoptropic
displacement parameters are shown at the 50% probability level; solute CH2Cl2 and the
disorder of the [BF4]- counterion are not shown for clarity. Selected bond lengths (Å):
Au1C1 2.018(2), Au1C17 2.017(2), C1C2 1.457(2), C1O1 1.296(2), O1C16 1.466(2),
C2C3 1.374(2).
7
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Figure 4. Structure of the ruthenium vinylidene complex 27b (R = Me) in the solid state;
anisoptropic displacement parameters are shown at the 60% probability level, the PF6
counterion and cocrystallized water are not shown for clarity. Selected bond lengths (Å) and
angles (°): Ru1C1 1.844(1), C1C2 1.322(2), C2C3 1.529(2), Ru1-C1-C2 174.0(1), C1-C2-C3-
O1 124.3(2)
Finally, the proposed intervention of a metal vinylidene of type B was corroborated
by an experiment in which the terminal alkyne 25 was treated with [CpRu(PPh3)2Cl] and
NH4PF6; ruthenium vinylidenes in general are well behaved and many members of this series
had been characterized in the past.4,26 In fact, substrate 25 readily furnished the expected
ruthenium vinylidene 27, in which the new C-C-bond has already formed by attack of the
aldehyde onto the (transient) metal acetylide but the subsequent rearrangement resulting in
aromatization and concomitant Fischer carbene formation has not yet taken place (Scheme
3). As the reaction was carried out in MeOH at elevated temperature, product 27a (R = H)
was admixed with the corresponding methyl ether 27b (R = Me) formed by solvolysis. This
latter compound could be isolated in pure form by recrystallization; its structure in the solid
state is shown in Figure 4. The ready –OH for –OMe exchange indicates facile ionization at
the benzylic position adjacent to the vinylidene -C-atom; this reactivity profile, in turn,
suggests that an ionic scenario might also govern the conversion of the gold vinylidene 21
into the final carbene complex 22. The fact that the –OMe substituent on the puckered
8
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backbone of 27b points away from the electrophilic vinylidene C1-atom also renders oxete
formation unlikely. Yet, we emphasize that this mechanistic aspect needs further scrutiny.
In any case, the preliminary results outlined above on the intramolecular reaction of a gold
acetylide with an activated aldehyde partner provide evidence for the transient formation of
gold vinylidenes as discrete yet highly reactive species; the spectroscopic signature of one
representative was recorded at low temperature. The particular ligand design used in this
study, however, allowed this particular type of intermediate to escape by an unprecedented
rearrangement to a gold carbene. Attempts to refine the approach with the hope of being
able to fully characterize such an exceedingly rare gold vinylidene complex of immediate
relevance for catalysis are in progress.
Acknowledgements
Generous financial support by Fund for Scientific Research Flanders (FWO) (fellowship to W.
D.) and the Max-Planck-Gesellschaft is gratefully acknowledged. We thank Mr. J. Rust and
Dr. R. Goddard for solving the X-ray structures, the Service Departments of our Institute for
excellent support and Umicore AG & Co KG, Hanau, for a generous gift of noble metal salts.
9
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