Gold(I) Carbenoids: On-Demand Access to Gold(I) Carbenes in Solution

Article abstract of DOI:10.1002/anie.201611705

Chloromethylgold(I) complexes of phosphine, phosphite, and N-heterocyclic carbene ligands are easily synthesized by reaction of trimethylsilyldiazomethane with the corresponding gold chloride precursors. Activation of these gold(I) carbenoids with a variety of chloride scavengers promotes reactivity typical of metallocarbenes in solution, namely homocoupling to ethylene, olefin cyclopropanation, and Buchner ring expansion of benzene.

Full text of DOI:10.1002/anie.201611705

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201611705
German Edition: DOI: 10.1002/ange.201611705
Carbenoids
Gold(I) Carbenoids: On-Demand Access to Gold(I) Carbenes in
Solution
Juan M. Sarria Toro, Cristina Garcꢀa-Morales, Mihai Raducan, Ekaterina S. Smirnova, and
Antonio M. Echavarren*
In memory of Josꢁ Barluenga
Abstract: Chloromethylgold(I) complexes of phosphine,
phosphite, and N-heterocyclic carbene ligands are easily
synthesized by reaction of trimethylsilyldiazomethane with
the corresponding gold chloride precursors. Activation of these
gold(I) carbenoids with a variety of chloride scavengers
promotes reactivity typical of metallocarbenes in solution,
namely homocoupling to ethylene, olefin cyclopropanation,
and Buchner ring expansion of benzene.
low-temperature in situ formation of Mg(CH2Cl)Cl.[12d] We
have developed a more convenient approach that utilizes the
methanol promoted decomposition of trimethylsilyldiazome-
thane[13,14] at room temperature (Scheme 1). Treatment of
C
arbene complexes of transition metals are reactive inter-
mediates[1] frequently invoked in a wide variety of C C bond-
forming processes that range from industrial scale olefin
metathesis[2] to natural product synthesis.[3] Despite their
central role in gold catalysis,[4] few non-heteroatom-stabilized
gold(I) carbene complexes have been structurally character-
ized.[5] The reduced steric shielding provided by commonly
used ligands and the intrinsically high electrophilicity[6]
exhibited by simple gold(I) carbenes preclude their isolation
in condensed phase. Such species are however of high interest
as demonstrated by gas-phase studies by Chen,[7] Schwarz,[8]
and others.[9] The isolation of this type of compounds, or their
functional equivalents, is therefore of great importance for
the fundamental understanding of the reactivity of electro-
philic gold carbenes. Transition-metal complexes of the type
[M(CHXR)], formally defined as carbenoids,[10] show similar
reactivity to their carbene counterparts[11] and offer an
attractive alternative to otherwise non-isolable species. To
date, however, very little is known about gold(I) carbenoids
and their reactivity.[12] Herein we report the synthesis of easily
accessible gold carbenoids bearing bulky ligands as gold
carbene equivalents in solution.
À
Scheme 1. Synthesis of chloromethylgold(I) complexes 1a–d and struc-
tures of complexes a) 1a, b) 1b, c) and 1c.[38] ORTEP plots with
ellipsoids set at 50% probability; hydrogen atoms and solvent
molecules are omitted for clarity.
Previous synthesis of chloromethylgold(I) complexes used
either toxic and potentially explosive diazomethane[12a] or
phosphine, phosphite, and NHC gold chloride complexes with
trimethylsilyldiazomethane in benzene solution in presence
of methanol afforded gold carbenoids 1a–d within minutes.
Complexes 1a–c could be easily purified by column chroma-
tography and have been fully characterized. Remarkably,
these gold(I) carbenoids can be stored indefinitely when
protected from air, and only decompose slowly when left
under ambient conditions.
[*] Dr. J. M. Sarria Toro, C. Garcꢀa-Morales, M. Raducan, E. S. Smirnova,
Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institute of Science and Technology
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
Prof. A. M. Echavarren
Departament de Quꢀmica Analꢀtica i Quꢀmica Orgꢂnica
Universitat Rovira i Virgili
À
Measured Au C distances (Table 1) are close to 2.088-
(9) ꢀ observed for the previously reported [(PPh3)AuCH2Cl]
complex (2).[12d] On the other hand, C Cl distances (1.828–
À
C/Marcel·li Domingo s/n, 43007 Tarragona (Spain)
1.830 ꢀ) are markedly longer than found in 2 (1.68(1) ꢀ), and
in fact are the longest among all 79 published crystal
structures containing a [MCH2Cl] motif.[15] The longest
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201611705.
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Table 1: Selected bond distances [ꢃ] and angles [8] from solid-state
molecular structures and chemical shifts[a] [d, ppm] for the new
carbenoid complexes.
Complex
Au1 C2
Au1-C2-Cl3
d 1H[a]
d
13C[a]
À
À
C2 Cl3
1a
1b
1c
1d
2.058(9)
2.088(5)
2.060(2)
–
1.828(1)
1.829(5)
1.830(2)
–
110.3(5)
111.1(3)
110.4(1)
–
2.96
2.94
3.34
3.63
53.6
53.9
47.2
49.1
[a] Chemical shifts corresponding to the chloromethyl moiety measured
in CD2Cl2 solution at 238C.
Figure 1. 1H NMR spectra of intermediates 7a–d obtained by reaction
of 1a with different TMSX reagents in [D8]toluene solution.
À
previously reported C Cl distance in a (chloromethyl)metal
fragment was 1.826(2) ꢀ in a RhIII complex.[16] On the other
hand, the Au1-C2-Cl3 angles are closer to the ideal tetrahe-
dron as opposed to the majority of other structures in which
the M-C-Cl angle is about 1168.[17] Chemical shifts for the
chloromethyl moiety are found between 2.94 and 3.63 ppm in
1H NMR and 47.2 and 53.9 ppm in 13C NMR spectra.
The reaction of 1a with an excess of AgOTf (Scheme 3)
could be conveniently monitored by 1H NMR spectroscopy to
obtain accurate kinetic concentration profiles (Supporting
Information, Figure S15) of carbenoid 1a, triflate substituted
species 7a, and the final gold salt 6a. Even though ethylene
When solutions of complexes 1a–d were treated with
chloride scavengers possessing weakly coordinating counter-
ions, for example, TMSX and AgX (X = Tf2NÀ, TfOÀ,
MeSO3À, CF3CO2 ) as well as AgSbF6, formation of ethylene
À
(3) and simple gold salts 6 was observed (Scheme 2). Analysis
Scheme 3. Reaction of 1a with excess AgOTf used for the determina-
tion of the corresponding activation parameters by 1H NMR spectros-
copy.
was detected by 1H NMR, its signal cannot be accurately
integrated owing to its rapid equilibration with the gas phase.
For this reason, ethylene was quantified by means of pressure
measurements in a closed system (Supporting Information,
Figures S3 and S4).
The consumption of carbenoid 1a follows a first-order
decay as expected from the pseudo first-order conditions
employed. Surprisingly, the subsequent decay of intermediate
7a into complex 6a and ethylene also follows a first-order
kinetic regime contrary to what would be expected for
a dimerization process and the reported second order
homocoupling of well-characterized methylidene complexes
of Ta[21] and Re.[22] A final yield of 60% ethylene could be
quantified independently in the closed system. More impor-
tantly, at room temperature the pressure increase, that is, the
formation of ethylene, also follows a first-order regime in
good agreement with the decay of 7a observed by NMR.
Variable-temperature measurements allowed the determina-
tion of the activation parameters for both processes. The
initial substitution of chloride for triflate (1a to 7a) exhibits
values of DH° = 12.9 Æ 0.5 kcalmolÀ1 and DS° = À19.2 Æ
2 calKÀ1 molÀ1, whereas the conversion of 7a to 3 and
ethylene shows values of DH° = 15.0 Æ 0.7 kcalmolÀ1 and
DS° = À21.5 Æ 2 calKÀ1 molÀ1.
Scheme 2. General reactivity of gold carbenoids after activation by
chloride scavengers.
of the volatile products by GC-MS confirmed the identity of
ethylene and excluded the formation of any additional
organic compounds (Supporting Information, Figure S5).
1
The H NMR spectrum obtained upon chloride abstraction
on the partially monodeuterated carbenoid 1a-d1 (Supporting
Information, Figure S1) exhibits signals with characteristic
deuterium couplings arising from the formation of the mono-
and bi-deuterated ethylenes 3-d1[18] and 3-d2[19] alongside 3, as
expected from the bimolecular ethylene formation. Further-
more, intermediates of the type 7, in which the chloride has
been replaced by the counterion of the scavenger, could be
detected in all cases except when using AgSbF6 (Figure 1).
The chemical shift for the methylidene resonance of 7a–d
strongly depends on the counterion used and ranged from
3.45 to 5.01 ppm. Despite the apparently counterintuitive
trend (chemical shift does not follow the acidity of the
counterion), a similar behavior has been observed on the
methyl compounds MeX.[20]
Activation of 1a in THF suppressed the formation of
ethylene and yielded instead a gold-stabilized oxonium ylide 8
(Figure 2). Trapping is irreversible and no ethylene is formed
even upon heating of the sample. Despite the lack of
2
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with [(JohnPhos)AuCl] provided chloro(phenyl)methylgold
carbenoid 9 (Figure 3). In comparison with its chloromethyl
À
analogues, complex 9 exhibits a longer C Cl bond at 1.838-
(1) ꢀ and a more acute Au-C-Cl angle of 106.7(5) degrees.[25]
Activation of carbenoid complex 9 with TMSOTf in CD2Cl2
solution showed complete consumption of 9 and formation of
homocoupling products cis- and trans-stilbene 10 together
with their cyclopropanation product 1,2,3-triphenylcyclopro-
pane 11 among other not identified products (Scheme 4). The
observed reactivity closely resembles that previously reported
for the transition-metal-catalyzed decomposition of phenyl-
diazomethane[26] and the cyclopropanation of stilbenes by
gold carbenes generated by retro-Buchner reaction (decar-
benation).[27]
Figure 2. Molecular structures of complexes a) 8 and b) 9.[38] ORTEP
plots with ellipsoids set at 50% probability; solvent molecules and
hydrogen atoms are omitted for clarity. Selected bond distances [ꢃ]
and angles [8] for 8: Au1–C2 2.065(2), C2–O3 1.517(2), C4–O3
1.479(2), C5–O3 1.486(2), Au1–P5 2.3069(5); C2-O3-C4 117.8(1), C2-
O3-C5 115.4 (1), C4-O3-C5 108.8(1); and 9: Au1–C2 2.056(1), C2–Cl3
1.838(1), Au1–P4 2.287(2); Au1-C2-Cl3 106.7(5).
reactivity of 8, its formation hinted at the possibility of an
intermolecular trapping of the methylene moiety with a suit-
able substrate. When the activation of the new carbenoid
complexes was carried out in the presence of olefins,
methylene transfer to form cyclopropanes 4 alongside ethyl-
ene was observed (Scheme 2). This process closely resembles
the gold-catalyzed cyclopropanation of olefins with diazo
compounds.[23] Thus, complexes 1a and 1c could methylenate
cyclohexene and norbornene, yielding norcarene 4a and exo-
tricyclo[3.2.1.02,4]octane 4b in moderate to good yields when
activated with either a silver salt or a TMS reagent (Table 2,
entries 1a,b–4a,b). Phosphite complex 1d could only generate
traces of cyclopropanes with cyclohexene (Table 2, entrie-
s 5a,b and 6a,b). These results highlight the aptitude of good
s-donor and poor p-acceptor ligands (phosphines and N-
heterocyclic carbenes) to enhance the stabilization of carbene
fragments by gold.[4] Cycloheptatriene 5 was formed when the
activation of carbenoids 1a and 1c was performed in
benzene[23a] (Table 2, entries 1c–4c). In this case, initial
methylenation to form norcaradiene is followed by electro-
cyclic opening to yield 5.[24] Carbenoid 1d yielded only traces
of 5 in parallel with its low reactivity towards cyclopropana-
tion (Table 2, entries 5c,6c).
Figure 3. Calculated reaction profiles for the cyclopropanation of
norbornene and formation of ethylene on the model system. DFT
calculations were performed at B3LYP-D3/6-31G(d,p)+SDD(f,g) on
Au. Toluene was represented with the PCM. Free energies are in
kcalmolÀ1. Optimized structure of the proposed intermediates 15 and
17 are shown. Non-CH2 H atoms as well as front facing TfOÀ in 17 are
omitted for clarity. Selected bond distances [ꢃ] and angles [8] for 15:
Au–CH2 1.906, Au–O 2.281,[30] Au–P 2.403; H2C-Au-O 144.2, H2C-Au-P
139.7, O-Au-P 76.1; and 17: Au–CH2 2.079, Au–Au 2.983, CH2–CH2
2.461, Au–O 2.222, Au–P 2.406; H2C-Au-O 100.5, H2C-Au-P 98.4, O-Au-
P 87.9.
The formation of gold carbenoids is not limited to the
insertion of diazomethane. Reaction of phenyldiazomethane
Table 2: Yields[a] for methylene transfer from complexes 1a,c,d to
olefins[b] to form cyclopropanes 4a,b and to benzene[c] to form cyclo-
heptatriene 5 (see Scheme 2).
Entry
Complex
Chloride scavenger
a)
b)
c)
4a
4b
5
1
2
3
4
5
6
1a
1a
1c
1c
1d
1d
TMSNTf2
AgNTf2
TMSNTf2
AgNTf2
TMSNTf2
AgNTf2
51
60
67
45
0
97
74
75
51
0
12
7
28
12
2
4
0
1
1
[a] Yield determined from H NMR spectroscopy (average of two runs).
[b] Reactions performed with 0.017 mmol of carbenoid complexes and
20 equiv of olefin in CD2Cl2 solution. [c] Same scale as cyclopropanation
reactions using [D6]benzene as solvent and substrate.
Scheme 4. Activation of complex 9 with TMSOTf. Yields determined by
GC-FID.
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The possible reaction mechanisms leading to cyclopro-
panes and the kinetically unusual formation of ethylene were
examined on a model system by means of DFT calculations
(Figure 3). Initially, a purely dissociative mechanism analo-
gous to the proposed formation of carbenes from carbenoids
of Pd[28] and Ni[29] was considered. Complete dissociation of
the triflate anion from the activated carbenoid 12 leads to the
formation of a gold methylidene 13 from which cyclopropa-
nation of norbornene can occur without an apparent barrier
(Supporting Information, Figure S31). This scenario is how-
ever unlikely as the energy required to split a neutral
molecule into two charged species is prohibitively high
(70.9 kcalmolÀ1). Nevertheless, a species corresponding to
cationic [(JohnPhos)AuCH2]+ could be experimentally
detected upon ESI-MS of 1a, suggesting that in sufficiently
energetic conditions gold(I) carbenes can be accessed from
stable carbenoids.
Cyclopropanation can alternatively occur via a three-
centered transition state TS-I, in analogy to the Simmons–
Smith reaction,[31] leading to [LAuOTf] (14) and cyclopro-
pane 4b (Figure 3). This pathway is energetically accessible
and can be considered competent for the formation of
cyclopropanes. Alternative mechanisms involving reductive
elimination from metallacyclobutane[32] structures were also
considered but found to be unlikely (Supporting Information,
Figure S33). No path for the formation of ethylene from two
carbenoids 12 could be located. It should be noted that, to the
best of our knowledge, no homocoupling of a Simmons–Smith
carbenoid has been reported to date. Ethylene most probably
arises from the coupling of two bridging methylene units in
a dimeric structure as originally proposed for Re[22] and Sc[33]
methylidenes and found experimentally in well-characterized
Co[34] and Rh[35] complexes (Scheme 5).
oxidative addition.[37] The AuI center in this intermediate can
perform an intramolecular SN2 type attack on the CH2OTf
moiety bound to the AuIII center over TS-III, yielding 17 in
a second oxidative addition step. Since the formation of 15
(RDS according to our calculations) and the ethylene
extrusion are both unimolecular reactions, the experimentally
observed first order decay of 7a can be rationalized in terms
of our proposed mechanism. The difference in activation
energies for cyclopropanation and ethylene formation pro-
cesses accounts for the observed reactivity pattern, while
cyclopropanations can be performed at very low temper-
atures, ethylene generation only reaches completion after
more than one hour at room temperature.
In summary, we have developed a simple method for the
preparation of well-defined gold(I) carbenoids [LAuCH2Cl]
that, upon activation with a chloride scavenger, exhibit the
reactivity expected from gold carbenes in solution, that is,
homocoupling, olefin cyclopropanation, and Buchner reac-
tion. We expect these complexes to become a useful tool for
the examination and mechanistic understanding of processes
involving gold carbenes in solution, particularly elusive
methylidenes, which have been so far primarily studied in
the gas phase.
Acknowledgements
We acknowledge funding from MINECO (CTQ2013-42106-P,
Severo Ochoa Excellence Accreditation 2014–2018, SEV-
2013-0319, and FPI fellowship to C.G.M.), the European
Research Council (Advanced Grant No. 321066), the
AGAUR (2014 SGR 818), Swiss National Science Foundation
(Early Postdoc. Mobility fellowship to J.M.S.T.), and CERCA
Programme/ Generalitat de Catalunya. We thank Prof. Pedro
J. Pꢁrez and Dr. Manuel R. Fructos at CIQSO (University of
Huelva) for their help in the quantification of gaseous
ethylene. We also thank the reviewers of this manuscript for
their valuable suggestions, Dr. Dirk Spiegel for additional
work, and the help of the ICIQ X-ray and NMR research
support units.
To reach the analogous dimeric structure 17 (Figure 3),
from which ethylene can be generated with a low energy
barrier over transition state TS-IV, we propose the involve-
ment of the neutral gold carbene 15. This species can be
formed by migration of the triflate anion from the carbon
atom to the gold center via TS-II. Intermediate 15 exhibits
a distorted trigonal planar geometry around gold center and
À
a short Au CH2 bond length (1.906 vs. 2.087 ꢀ in 12). Triflate
À
remains coordinated to gold as evidenced by the Au O
distance (2.281 vs. 2.101 ꢀ in 14). Three-gold(I) complexes
bearing two neutral ligands and one halogen are known and
have been structurally characterized.[36] The analogous spe-
cies 15b could be located using the complete JohnPhos ligand
and despite the increased steric demand imposed by the full
ligand 15b lies only 28.8 kcalmolÀ1 above the parent carbe-
Conflict of interest
The authors declare no conflict of interest.
Keywords: Buchner reaction · carbenes · carbenoids ·
cyclopropanation · gold
noid 7a (Supporting Information, Figure S35). Carbene 15
I
À
can react with one molecule of carbenoid 12 to yield the Au
AuIII dimer 16 without an apparent barrier by a formal
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Manuscript received: December 1, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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Angewandte
Communications
Chemie
Communications
Carbenoids
J. M. Sarria Toro, C. Garcꢁa-Morales,
M. Raducan, E. S. Smirnova,
A. M. Echavarren*
&&& — &&&
Gold(I) Carbenoids: On-Demand Access
to Gold(I) Carbenes in Solution
Gold in the pocket: Activation of well-
defined gold(I) carbenoids LAuCH2Cl
with chloride scavengers promotes reac-
tivity typical of metallocarbenes in solu-
tion, namely homocoupling to form eth-
ylene, olefin cycloproanation, and Buch-
ner ring expansion of benzene.
6
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!