Water-Soluble Gold-N-Heterocyclic Carbene Complexes for the Catalytic Homogeneous Acid- and Silver-Free Hydration of Hydrophilic Alkynes

Article abstract of DOI:10.1002/adsc.201500729

Water-soluble gold(III/I) N-heterocylic carbene complexes behave as efficient catalysts for the hydration of terminal alkynes in neat water. The transformation proceeds in the absence of Bronsted acids or halide scavengers and is suitable for sensitive substrates. Kinetic profiles and DFT studies provide a clear picture of intermediates present during catalysis.

Full text of DOI:10.1002/adsc.201500729

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DOI: 10.1002/adsc.201500729
Water-Soluble Gold–N-Heterocyclic Carbene Complexes for the
Catalytic Homogeneous Acid- and Silver-Free Hydration of
Hydrophilic Alkynes
Houssein Ibrahim,a,b,c Pierre de FrØmont,c Pierre Braunstein,c Vincent ThØry,a,b
Lionel Nauton,a,b Federico Cisnetti,a,b,* and Arnaud Gautiera,b,*
a
UniversitØ Clermont Auvergne, UniversitØ Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000
Clermont-Ferrand, France
E-mail: federico.cisnetti@univ-bpclermont.fr or arnaud.gautier@univ-bpclermont.fr
CNRS, UMR 6296, ICCF, F-63178 Aubi›re, France
UniversitØ de Strasbourg, Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), 4 rue Blaise
b
c
Pascal, CS 90032, F-67081 Strasbourg, France
Received: August 6, 2015; Revised: September 21, 2015; Published online: December 9, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500729.
Abstract: Water-soluble gold(III/I) N-heterocylic
carbene complexes behave as efficient catalysts for
the hydration of terminal alkynes in neat water.
The transformation proceeds in the absence of
Brønsted acids or halide scavengers and is suitable
for sensitive substrates. Kinetic profiles and DFT
studies provide a clear picture of intermediates
present during catalysis.
As for numerous other catalytic reactions in homo-
geneous conditions,[9] the use of N-heterocyclic car-
benes as phosphine surrogates has allowed for the
synthesis of catalysts with improved activity and ro-
bustness (Scheme 1). Herrmann disclosed the first
gold(I)-NHC hydration catalyst in 2003.[10] In 2009,
Nolan reported the use of other classical lipophilic
Keywords: alkynes; gold; N-heterocyclic carbenes
(NHCs); water-soluble catalysts
The hydration of alkynes is a classical process in car-
bochemistry, since Kutcherovꢀs discovery (1881, Hg
catalyst).[1] Nowadays, this reaction is regarded as
a safe, atom-economical and environmentally friendly
route to carbonyl compounds. Transition metal com-
plexes (Ru, Rh, Pd, Pt, Cu or Ag) have been used as
catalysts.[2] In the 1970s, gold entered this field[3] with
a report of alkyne hydration mediated by HAuCl4.[4]
Fukuda and Utimoto found that NaAuCl4 catalyzes
the addition of water or methanol to non-activated al-
kynes to give ketones or acetals, and the Meyer–
Schuster rearrangement.[5] Rare examples of gold(III)
organometallic complexes {e.g., [Au(C6F5)2(m-Cl)]2}
have also been reported.[6] However, cationic Au(I)
systems are usually preferred and highly efficient.
These are generated from [AuX(PPh3)3] or
[AuMe(PPh3)3] in the presence of Brønsted or Lewis
acids.[7] The coordinated phosphine may also bring
about water-solubilizing properties.[8]
Scheme 1. Selected examples of gold(I)-NHC complexes
used for alkyne hydration.
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NHC ligands.[11] These catalysts are effective in water/ phases.[22] We report herein the use of water soluble
organic solvent mixtures. Impressively, very low cata- gold(I)- and gold(III)-NHC complexes containing
lyst loadings could be used (ꢀ100 ppm) for the hydra- a triazolylammoniomethyl substituent. A similar strat-
tion of terminal or internal alkynes. Silver(I) salts are egy has already allowed the preparation of hydrophil-
frequently used to activate the gold pre-catalysts and, ic and oxidation-resistant CuAAC catalysts[23] or iridi-
by halide abstraction, may lead to a silver/gold bimet- um complexes for NMR signal amplification by rever-
allic process.[12] In acidic aqueous media, the gold pre- sible exchange (SABRE).[24] Importantly, our synthet-
cursors may evolve into a catalytically active dinu- ic route does not involve any silver(I) intermediate
clear m-hydroxido species,[13] which was shown to cata- (commonly used for the carbene transfer step), thus
lyze alkyne hydration without any added silver(I) co- ruling out any possible silver contamination issue.
catalyst. Consequently, reaction optimization requires Water-soluble gold-NHC complexes were prepared as
an adequate choice of metal source and metal:acid depicted in Scheme 2.
ratio.[14] Other gold(I)-NHC systems with lipophilic li-
Starting from the easily prepared diazido precursor
gands were also described with the very bulky IPr** 1,[25] the introduction of trimethylammonium groups
ligand,[15] using supramolecular encapsulation,[16] or was accomplished by a click CuAAC reaction on the
micelles.[17] Extended ring[18] or abnormal[19] carbenes bis-azido diamine, yielding 2 in excellent yield. Subse-
were also used. In contrast, gold(III)-NHC complexes quent orthoformate cyclization led to the isolation of
have been much less investigated: Nolan reported hy- the ammonium salt 3 in very good yield. Its reaction
dration reactions with NHC-AuBr3, albeit with with Au(I) – generated by reaction of HAuCl4 with
a small acceleration as compared to other Au(III) tdg (thiodiglycol) in the presence of NaOH as a base,
salts.[20]
afforded cleanly the functionalized NHC gold(I) com-
Joóꢀs and Silbestriꢀs groups reported the prepara- plex 4a.[26] Facile oxidation by iodobenzene dichlor-
tion of water-soluble NHC-Au(I) complexes bearing ide[27] yielded the corresponding Au(III)-NHC com-
sulfonated groups either on the N-alkyl or N-aryl sub- plex 4b.
stituents of imidazol-2-ylidene ligands (Scheme 1). In-
This allowed an evaluation of the capability of
terestingly, comparative studies between complexes Au(I)/Au(III) complexes 4a and 4b to behave as
bearing sulfonated ligands and unfunctionalized hy- alkyne hydration catalysts in additive-free homogene-
drophobic ligands showed the superiority of the ous aqueous solution (Table 1) with hex-5-yn-1-ol as
former.[21] However, except for the hydration of prop- water-miscible substrate.
argyl alcohol, the scope of sulfonated catalysts was
only investigated with water-insoluble alkynes, which temperature (Table 1, entries 1, 2) after 8 h and full
required the use of water-methanol mixtures. conversion after one day. An increase of the reaction
Both complexes afford a good conversion at room
Ammonium tagging is also a strategy frequently rate was observed when the temperature was raised
used to solubilize organometallic catalysts in aqueous to 508C (entries 3 and 4) and completion was reached
Scheme 2. Preparation of water-soluble Au(I)/Au(III) complexes (tdg=thiodiglycol).
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1
Table 1. Hydration of hex-5-yn-1-ol: H NMR conversion as
a function of time. [alkyne]=0.8 mol. LÀ1
Entry
Catalyst
T [8C]
t=6 h
t=8 h
t=24 h
1
2
3
4
4a
4b
4a
4b
22
22
50
50
ND[a]
ND
100%
100%
90%
80%
100%
100%
100%
100%
ND
ND
[a]
ND=not determined.
after a maximum of 6 h. The addition of methanol
(H2O/MeOH, v/v 1:1) had no influence on the reac-
tion rate and the expected ketone was obtained with-
out visible contamination by ketal or enol ether prod-
uct. The pH variation was monitored during the reac-
tion (Figure 1).
Figure 2. 1H NMR kinetic plots of the hydration of hex-5-
yn-1-ol catalyzed by 4a, 4b.
The kinetic profile of the reactions catalyzed by 4a
1
and 4b at 508C was probed using H NMR in water at
[alkyne]=0.8 mol·LÀ1 and a catalyst loading of 0.5
mol% (Figure 2).
In the presence of Au(I) or Au(III) NHC com-
plexes, the reactions reach completion after 3 and 5 h,
respectively, but their kinetic profiles differ strongly.
The reaction with the Au(I) catalyst 4a follows a clas-
sical first order rate law (see below), whereas the
product formation with Au(III) 4b is described by
Figure 1. Hydration of hex-5-yn-1-ol at room temperature:
pH variation.
Upon addition of the catalysts to the aqueous solu- a sigmoid curve. Very interestingly, addition of one
tion of hex-5-yn-1-ol, the pH of the medium immedi- equivalent of hex-5-yn-1-ol at ca. 95% completion
ately drops to 2.7, a value consistent with a release of gives a kinetic profile close to that of 4a (Figure 3).
1 equiv. of H+ per Au.[28] Interestingly, a slow increase This similarity strongly suggests that 4b converts to 4a
of the pH value is observed for the gold(I) catalyst during the catalytic cycle. The reduction of 4b into
4a, reaching 5 after 4 h. Regarding the gold(III) cata- 4a[31] is possibly a slow process responsible for the ob-
lyst 4b the solution becomes more acidic (pH 2.2) served induction period.
after 30 min. Its acidity remains constant throughout
To ascertain that Au(III) is reduced in situ to
the course of the reaction. This pH variation, which is Au(I), we tried to fit the experimental data with an
impeded by buffering the medium,[29] has to be taken apparent first order rate equation for Au(I) 4a and
into account when considering the reaction mecha- for Au(III) 4b, after the addition of the second equiva-
nism (see below). A drop of mercury does not inhibit lent of alkyne.
the reaction, suggesting that it does not proceed
through decomposition of molecular precursors to
colloidal gold.[30]
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Figure 5. Determination of the order in Au(I) catalyst 4a.
Table 2. Apparent rate constant as a function of the Au(I)
catalyst concentration
Figure 3. 1H NMR survey of the hydration of hex-5-yn-1-ol
profile reaction catalyzed by 4b: further addition of 1 equiv.
substrate at 95% conversion.
[4a] (mol·LÀ1)
0.012
0.008
0.004
0.002
Loading (mol%)
1.5
0.062
1.0
0.045
0.5
0.019
0.25
0.0081
k’ (minÀ1)
The corresponding apparent alkyne hydration rate
constants k’ exhibits very close values of 0.018 and
0.017 minÀ1, respectively (Figure 4).
Furthermore, we determined for 4a the dependency
Therefore, the determination of the rate orders for
of the apparent rate constant k’ on gold concentra- 4a both in substrate and catalyst allows us to conclude
tion. The amount of catalyst was varied from 0.25 to that the NHC-Au(I) alkyne hydration follows the rate
1.5 mol%. The plot of ln([catalyst]) vs. ln(k’) displays equation:
a linear dependency (Figure 5 and Table 2) with
a slope of 1.1 (R2 =0.989). We concluded that the re-
action is also first order in catalyst.
Then, we focused on the determination of the fate
of the Au(III) catalyst 4b (Scheme 3). Indeed, litera-
ture precedents for reductive elimination from
gold(III) have highlighted that Glaser-type products
are formed with terminal alkynes under anhydrous
conditions[32] while fluoro ketones are formed during
the hydration of alkynes in the presence of Select-
fluorꢂ, a re-oxidant of gold(I).[33] Moreover, aryl
chlorides are also formed under UV irradiation of
aryl gold(III) complexes.[34]
To discriminate between reductive elimination and
alkyne homocoupling, we selected a model reaction
to monitor easily the formation of associated products
by chromatographic techniques. The hydration of phe-
nylacetylene in water/methanol in the presence of 10
mol% of catalyst was performed and the results ana-
Figure 4. First order linear plots at 0.5 mol% [Au] loading.
Scheme 3. Possible Au(III) reduction pathways.
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lyzed by HPLC and GC-mass spectrometry. Besides
To shed more light on the various reaction steps,
the formation of acetophenone, we were able to ob- we performed DFT calculations with the Gaussian
serve chloroacetophenone (Scheme 4).
G09 software[37] on a simplified model: hydration of
propyne by the cationic 2,5-dimethylimidazolin-2-yli-
dene dichloro-gold(III) fragment surrounded by 11
water molecules as explicit solvent located randomly
in the vicinity of the alkyne.[38] The calculations were
performed at DFT level using 6-31G** basis set for
all atoms except gold for which SDD basis was used.
We positioned and characterized each transition state
(TS) using a scan procedure leading to a high energy
structure which was furthermore freely optimized
(Supporting Information). Starting from each TS, IRC
was performed to return to the expected products and
reactants on which thermodynamic corrections were
applied. For the hydration of complex 5 leading to
the enol 6a, an exothermic reaction of 39.1 kcal·molÀ1
is calculated. Interestingly, this TS is only
Scheme 4. Outcome of the reaction of phenylacetylene with
water in the presence of 4b.
These observations prompted us to propose the fol- 1.0 kcal·molÀ1 higher in energy than the reagent: hy-
lowing unified mechanism implying vinylgold inter- dration is very fast. We found that the conversion of
mediates[35] for the hydration of alkynes by both 6a to 6b is also an exothermic process (17.0 kcal·
NHC-Au(I) and NHC-Au(III) catalysts, depicted in molÀ1) with a relatively low TS energy of 6.1 kcal·
Scheme 5.
molÀ1. Finally, the release of the chloro ketone 7 from
The Au(III) precatalyst would form first a p-com- 6b is an exothermic step by 30.2 kcal·molÀ1 with the
plex 5 with the alkyne by displacement of the chloride highest TS positioned at 21.8 kcal·molÀ1 (Figure 6).
ligand trans to the strongly s-donating NHC ligand. Thus, our calculations reveal that neither the hydra-
This step would be followed by the addition of water tion nor the protonation represent rate-determining
with concomitant release of a proton to produce a s- steps for the overall transformation. It is the reductive
bonded enol 6a. Upon tautomerization of 6a, the re- elimination step that is responsible for the induction
sulting ketone complex 6b could release the a-chloro period.
ketone 7 by reductive elimination and generate a cata-
The scope of the catalysts 4a and 4b was prelimi-
lytically active NHC-Au(I)-Cl species.[36] The gold(I)- narily studied with a series of water-soluble terminal
NHC catalyst would lead to ketone formation by alkynes.[39] An experiment with an internal alkyne
a similar sequence (via complex 8 and aurated ketone (but-2-yn-1,4-diol) revealed no conversion at 24 h,
9) in which the last step consists in the protodeaura- even at 808C.
tion of the s-bonded complex, leading to the ketone
10.
Examination of Table 3 shows that both gold(I) and
gold(III) complexes behave similarly, as expected.
Scheme 5. Proposed alkyne hydration pathway for gold(I)- and gold(III)-NHC complexes.
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tronegative group and steric hindrance may decrease
the reaction rate. Interestingly, our conditions allow
the formation of acyloins (entries 2, 4 and 5) rather
than their corresponding 1,4-dioxane dimers, as previ-
ously reported with catalysts employed under acidic
methanolic conditions.[7a] Regarding 2,2-dimethylpro-
pyn-2-ol (entry 5), the conversion is low at 508C and
affords a mixture of hydration and Meyer–Schuster
products at 808C. The same type of secondary prod-
uct, albeit in much smaller proportions, was also ob-
served for but-3-yn-2-ol (entry 4). In contrast, the in-
troduction of the less hindered and less electronega-
tive acetamide group (entry 6) restores milder reac-
tion conditions (508C). This example may be viewed
as a minimalistic model of propargyl amides in the C-
terminal position of peptides.[40] Also (entry 7), our
gold catalysts allow for the conversion of propargyl a-
Figure 6. Energetic profile for the formation of chloro d-mannopyranoside[41] into a ketopropyl mannoside[42]
ketone promoted by the gold(III) complex 4b. NHC=N,N’-
under mild conditions, albeit with longer reaction
time. Remarkably, a carbohydrate-derived compound
amenable to further modification on the carbonyl
moiety was obtained without the need for any protec-
tion/deprotection sequence. This latter example illus-
trates the advantages of avoiding acidic cocatalysts
which might lead to the formation of undesired glyco-
side products.
dimethylimidazolin-2-ylidene. Energies given in kcal·molÀ1.
Table 3. Hydration of water-soluble terminal alkynes at 0.9
mol·LÀ1 with 0.5 mol% catalyst loading after 20 h of reac-
tion.
In summary, we have reported herein two gold(I)
and gold(III) complexes derived from the same hy-
drophilic ligand and demonstrated that they behave
as precatalysts in alkyne hydration in neat water, and
without any additive. The gold(III) compound is re-
duced to gold(I) with concomitant formation of
a chloro ketone, and catalysis proceeds with the same
gold(I)-based cycle regardless of the oxidation state
of the pre-catalyst. As preliminary experiments to
evaluate the scope and limitations of the systems, the
hydration reaction was performed successfully with
various terminal alkynes, indicating that the presence
of pending O-, or N- function was well tolerated. Fur-
ther studies towards the modification of more com-
plex substrates are in progress.
Experimental Section
General Procedures
[a]
After 18 h.
Total completion after 40 h.
Isolated yield: 95%
[b]
A stock solution of Au(I) or Au(III) pre-catalyst was pre-
pared by dissolving the appropriate complex (4a: 24.3 mg,
4b: 26.1 mg) (see synthetic details in the Supporting Infor-
mation) in 5 mL H2O and 0.5 mL D2O. The alkyne
(0.5 mmol) was weighed out (or transferred with a microsyr-
inge) in a vial. 0.55 mL of the pre-catalyst solution was
added to the alkyne, the solution homogenized and heated
for the appropriate time before it was analyzed by H NMR
(Supporting Information).
In the case of propargyl a-d-mannopyranoside, the reac-
[c]
Propargylic, homopropargylic alcohols and pent-4-yn-
2-ol afford the expected ketones in 40–60% yield at
508C after 20 h (results not shown). However, a slight
increase of the temperature to 608C or 808C allows
complete conversions (entries 2, 3, 4). By comparison
1
with hex-5-yn-1-ol (entry 1), the presence of an elec- tion was performed on a 1.5 mmol scale, with 0.5 mol% 4a
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Water-Soluble Gold–N-Heterocyclic Carbene Complexes
in 1.5 mL H2O. After 40 h at 608C, the reaction mixture was
introduced on the top of a Waters Sep-Pak cartridge. Elu-
tion with water afforded 335 mg of the product (95% yield).
ESI-HR-MS: m/z=259.0787, calcd. for C9H16O7Na [M+
Na]+: 259.0794. See the Supporting Information for product
1H- and 13C NMR spectra.
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