Fluorine-enabled cationic gold catalysis: Functionalized hydration of alkynes

Article abstract of DOI:10.1002/anie.201003593

Fluorine makes it possible: A cationic fluorine-gold species generated by fluorination of a low-valence gold complex mediates the functionalized hydration of alkynes. A triple bond is used to install a carbonyl group and two new bonds on the a-carbon atom, all occurring in one pot, under mild conditions and in very good.

Full text of DOI:10.1002/anie.201003593

Angewandte
Chemie
DOI: 10.1002/anie.201003593
Oxidative Gold Catalysis
Fluorine-Enabled Cationic Gold Catalysis: Functionalized Hydration
of Alkynes**
Weibo Wang, Jacek Jasinski, Gerald B. Hammond,* and Bo Xu*
Dedicated to Professor Donald J. Burton
Transition-metal catalysis plays a major role in chemical
synthesis.[1] Many transition metals need to be in cationic form
to be catalytically active; a case in point is gold.[2] Cationic
gold species have been regarded as the most powerful
catalysts for electrophilic activation of alkynes toward a
variety of nucleophiles.[2,3] Simply put, a nucleophilic attack
on a [AuL]+-activated alkyne proceeds through a p complex
to give a trans-alkenyl gold complex intermediate capable of
reacting with an electrophile (E+), usually a proton, to yield
the final product through protodeauration (Scheme 1). A
active cationic gold complex (Scheme 1). We sought an
alternate mode of catalysis arising from a fluorine-enabled
cationic metal species, generated by fluorination of a low-
valence metal complex (e.g. [ClAuIL]) with an ammonium
+
À
[N F] type fluorination reagent containing a poorly coordi-
nating counterion (Scheme 2). Such species (intermediate A)
is expected to have a Lewis acidity that is stronger than for the
Scheme 1. Traditional methods for the generation of [LAuI]+ and its
catalytic cycle.
Scheme 2. Fluorination-enabled cationic gold complex and its catalytic
cycle.
common cationic gold precursor, such as [Ph3PAuCl], is not
catalytically active by itself. It is typically treated with a silver
salt of a non-coordinating anion (e.g. AgBF4) to generate the
corresponding metal catalyst prior to fluorination. This is due
to the higher oxidation state of the metal and the presence of
a non-coordinating counter anion (e.g. BF4 in the case of
[*] W. Wang, Prof. Dr. G. B. Hammond, Prof. Dr. B. Xu
Department of Chemistry, University of Louisville
Louisville, KY 40292 (USA)
À
Selectfluor) (Scheme 2). Our premise is that this cationic
gold–fluorine species A would be able to catalyze not only
well-known reactions of [AuIL]+ (e.g. hydration or cyclization
of alkynes) (Scheme 2, inner cycle), with similar or even
higher efficiency, but that it could also mediate additional
transformations not traditionally associated with [AuIL]+.
This is illustrated in Scheme 2 (outer circle). For example, the
fluoro–gold vinyl intermediate C may undergo reductive
elimination to give a vinyl fluoride, or transmetalation with an
Fax: (+1)502-852-5998
E-mail: gb.hammond@louisville.edu
bo.xu@louisville.edu
Dr. J. Jasinski
Materials Characterization
Conn Center for Renewable Energy Research
University of Louisville, Louisville, KY 40292 (USA)
[**] We are grateful to the National Science Foundation for financial
support (CHE-0809683). We acknowledge the support provided by
the CREAM Mass Spectrometry Facility (University of Louisville)
funded by NSF/EPSCoR grant no. EPS-0447479.
organometallic reagent R3M (e.g. M = B, Si, Sn, etc.). The
[4]
À
À
À
À
weak Au F bond and the strong B F, Si F, and Sn F bonds
would drive the transmetalation. After reductive elimination,
the low-valence gold(I) complex is re-fluorinated to resume
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003593.
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7247

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its catalytic cycle. Overall, this approach should deliver a
diversity of fluorinated and non-fluorinated building blocks
or targets.
complex C to furnish the ketone product. The cyclization of
alkynoic acid is also an example of transformations catalyzed
by gold (Table 2). In the cyclization of an alkynoic acid,
[PPh3AuCl] or Selectfluor alone did not enable the cycliza-
tion, but, when combined in catalytic amounts, the resulting
lactone was obtained in high yields. These results support the
synergism that exists between the gold catalyst and Select-
fluor.
It is well known that alkynes do not undergo hydration
using solely AuCl or [ClAuPPh3]. Zhang and others have
developed an oxidative coupling chemistry involving a AuI/
AuIII catalytic cycle.[5] In Zhangꢀs proposed mechanism for the
gold-catalyzed coupling reaction of arylboronic acids with
propargyl esters,[5b] cationic AuI initiates the activation of
alkyne in a propargyl ester, and Selectfluor is used to oxidize
the resulting vinyl AuI intermediate to a AuIII species. In this
Table 2: Cyclization of alkynyl acid.
report, Zhang and co-workers also showed that cationic
I
À
P Au complexes were poor catalysts, which would seem to
hint that AuI is oxidized to cationic AuIII prior to the addition
of nucleophiles to alkynes. In their most recent communica-
tion on the gold-catalyzed carboheterofunctionalization of
alkenes,[5a] Zhang and co-workers have indeed revised their
earlier mechanistic proposal and proposed the initial oxida-
tion of AuI to AuIII with Selectfluor. Of late, fluorination
reagents have become the oxidant of choice in transition-
Entry
Catalyst (equiv)
Yield
1
2
3
[ClAuPPh3] (5%)
Selectfluor (100%)
[ClAuPPh3] (5%)/Selectfluor (10%)
no reaction
no reaction
88%
À
metal-mediated C H activation, fluorination, oxidation, and
coupling reactions, as demonstrated by Ritter,[6] Sanford,[7]
Gouverneur,[8] Michael,[9] Yu,[10] and Liu[11] among others. In
most of those reports, the fluorination reagent (e.g. Select-
fluor)[12] acts as a powerful oxidant that increases the
oxidation state of the transition-metal intermediate within
the catalytic cycle. We propose that fluorine enables new
modes of cationic gold catalysis, and, depending on the
application, the fluorinated reagent can be even used as an
activator in catalytic amounts.
A major shortcoming of a transition-metal-catalyzed
hydration is that it only adds the elements of H2O to an
alkyne (Scheme 3a).[16] We believe that the fluorine-enabled
The hydration of alkynes is a straightforward method for
preparing carbonyl compounds.[13,14] We used a gold-catalyzed
synthesis of g-keto esters through regioselective hydration of
3-alkynoate[15] to test our cationic fluorine–gold complex
(Table 1). Traditional gold(I) catalysts, like AuCl or
Table 1: Hydration of internal alkyne.
Scheme 3. Concept of functionalized hydration.
cationic gold catalysis could do more than simple hydration.
We propose that the vinyl–metal complex hydration inter-
mediate (obtained by the reaction of fluorine-generated
cationic metal species A with an alkyne) can react further
during the hydration process to give an a,a-disubstituted
ketone (Scheme 3b) in a one-pot reaction. We coined this
process “functionalized hydration”. We speculated that our
fluorine–gold intermediate could react with organoboronic
acid (reagent X) and an electrophilic fluorine source such
as Selectfluor (reagent Y) to give a-aryl-a-fluoroketone
(Scheme 3b). A proof of principle is the synthesis of
functionalized a-fluoroketones—well-known targets and
important synthetic intermediates.[17] Tertiary a-fluorinated
ketones have received much attention recently because
compounds having an a-fluorocarbonyl moiety are biologi-
cally active, they are effective mimics of a-hydroxy ketones,
useful probes in various biological processes, and also enzyme
inhibitors.[17a,c] They also provide configurationally stable
Entry
Catalyst (equiv)
Yield
1
2
3
4
5
AuCl (2%)
[ClAuPPh3] (2%)
[ClAuPPh3] (2%)/AgBF4 (2%)
NaAuCl4 (2%)
[ClAuPPh3] (2%)/Selectfluor (4%)
no reaction
no reaction
45%
81%
85%
[PPh3AuCl], do not catalyze this hydration (Table 1, entries 1
and 2), whereas [PPh3AuCl]/AgBF4 gave moderate yield of
the ketoester. As expected, gold(III) led to a better yield
(Table 1, entry 4) but our gold(I)/Selectfluor system (both
components in catalytic amounts) produced the best results
(Table 1, entry 5). The mechanism of this hydration is
illustrated in Scheme 2 (inner cycle); water, acting as
nucleophile (Nu = H2O), attacks the activated alkyne (inter-
mediate B), followed by protodeauration of the vinyl–gold
substituents for molecules containing
a tertiary chiral
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carbon atom next to a carbonyl group, an important structural
motif in medicinal chemistry.[17c] This one-pot tandem addi-
tion/oxidative coupling/fluorination sequence using readily
available starting materials (alkyne, water, organoboronic,
acid and Selectfluor) would have clear advantages over
literature methods, all of which require multiple synthetic
steps.[16]
12). With electron-rich 4-methoxy- or 3,4-dimethoxyphenyl-
boronic acids, we observed by-products arising from fluori-
nation or oxidation of the electron-rich aromatic rings by
Selectfluor.[12] Steric and electronic effects determine the
regioselectivity.[13,14] For internal alkynes possessing a nucle-
ophilic site nearby (e.g. the ester group in 1a), this site may
influence the regioselectivity through neighboring-group
participation.[13–15]
Our proposed mechanism, shown in Scheme 4, resembles
the reaction of boronic acid with alkynes.[18] Initially, water
attacks the gold-activated alkyne to form a vinyl–gold
We examined the reaction of 1a with phenyl boronic acid
(Table 3). Various metal catalysts were screened. Without
Table 3: Functionalized hydration: screening of conditions.[a]
Entry
Catalyst
Selectfluor
(equiv)
Yields [%] (3a/3a’)
1
2
3
4
5
6
AuCl
none
none
0.1
1.5
2.2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
no reaction
no reaction
no reaction
50 (5.1 :1)
57 (4.7:1)
88 (6.7:1)
83 (5.7:1)
no reaction
75 (6.5:1)
78 (5.0:1)
65 (4.7:1)
41 (5.7:1)
29 (5.6:1)
complex mixture
complex mixture
complex mixture
[PPh3AuCl]
[PPh3AuCl]
[PPh3AuCl]
[PPh3AuCl]
[PPh3AuCl]
[PPh3AuCl]
[PPh3AuCl]
[(o-Tol)3PAuCl]
[(p-CF3-C6H4)3PAuCl]
AuCl
AuCl3
AuCl3/AgOTf (15%)
CuBr
AgOTf
PdCl2
7[b]
8[c]
9
10
11
12
13
14
15
16
Scheme 4. Proposed mechanism for the functionalized hydration.
complex C,[19] which reacts with a metal reagent RM (e.g.,
PhB(OH)2) through a transmetalation process, to give
À
[a] 5% catalyst loading, RT, 18 h. Bz=benzoyl. [b] 2% catalyst loading,
RT, 24 h. [c] NFSi was used instead of Selectfluor; Selectfluor: 1-
chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoro-
borate); NFSi: N-fluorobenzenesulfonimide.
intermediate D. We believe that the strong B F bond and
[4]
À
the weak Au F bond are the driving forces behind this
transmetalation. Reductive elimination of D gives E. The
reaction does not stop at this stage; instead E can be
fluorinated by Selectfluor to give the functionalized ketone
3.[20] Because we had never isolated 5, it is also possible that
intermediate D reacts with Selectfluor first to give F, and,
additives, AuCl or [Ph3PAuCl] could not catalyze the reaction
of 1 with phenyl boronic acid (Table 3, entries 1 and 2), but in
combination with Selectfluor (1.5 equiv), we obtained a 50%
yield of 3 (Table 3, entry 4). Increasing the amount of
Selectfluor further increases the yield (Table 3, entry 5).
Reducing the loading of [Ph3PAuCl] from 5% to 2% had
no deleterious effect (Table 3, entry 7). Replacing Selectfluor
with NFSi was not effective (Table 3, entry 8). We tested
other gold(I) catalysts, but they were less efficient (Table 3,
entries 9–11). We also tested gold(III) catalysts (AuCl3 and
AuCl3/AgOTf, Table 3, entries 12 and 13), but they gave much
lower yields. Other transition metals like copper, silver, and
palladium could not catalyze this transformation under
similar conditions (Table 3, entries 14–16). On exploring the
scope of this novel functionalized hydration, we found that
functionalized and unfunctionalized internal alkynes reacted
with aryl boronic acid, giving very good yields of a-substituted
ketones, with moderate regioselectivity (Table 4, entries 1–
following reductive elimination, gives the final product 3. The
transmetalation of Au Cl complex with boronic acid has
I
À
been demonstrated by Hashmi and co-workers in a recent
paper.[21] The transmetalation of Au F with boronic acid has
À
also been proposed by Zhang and co-workers in their gold-
catalyzed cross-coupling reaction of propargylic acetate.[5b] To
probe the proposed mechanism in Scheme 4 we conducted
two control experiments (Scheme 5). According to Scheme 4,
fluoroketones 4 could be formed in the absence of a coupling
partner R3M, in which case, the vinyl–metal complex C
undergoes reductive elimination or fluorodemetalation[15] to
give fluoroketone 4. This was the case experimentally,
although so far only moderate yields have been obtained
(Scheme 5a). The reaction of terminal alkyne 1i with 2a gives
the oxidative coupling product 6.[22] The formation of 6 hints
towards a tandem mechanism that involves fluorination of the
gold complex, transmetalation with boronic acid or terminal
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Communications
Table 4: Scope of the functionalized hydration.[a]
Entry
1
1
2
Products 3
Yield (ratio)
1a PhB(OH)2 (2a)
3a, 88% (6.7:1)
2
3
4
5
6
1b 2a
1c 2a
1d 2a
1e 2a
1 f 2a
3b, 88% (4.6:1)
3c, 63%
3d, 79%
3e, 85% (2.2:1)
3 f, 47% (2.1:1)
7
8
1g 2a
1h 2a
3g, 83%( 5:1)
3h, 70% (5.0:1)
9
1a p-F-C6H4B(OH)2 (2b)
3i, 88% (5.3:1)
10
11
12
1a m-Cl-C6H4B(OH)2 (2c)
1a p-Me-C6H4B(OH)2 (2d)
1a p-Ph-C6H4B(OH)2 (2e)
3j, 71% (3.4:1)
3k, 88% (6.7:1)
3l, 90% (5.3:1)
[a] 1 (0.4 mmol), 2 (0.8 mmol), Selectfluor (1.0 mmol), [ClAuPPh3] (5%), 3 mL CH3CN/water (20:1), RT, 18–24 h.
alkyne, and reductive elimination, as shown in Scheme 4. It is
interesting to note that gold catalysts undergo transmetala-
tion and reductive elimination readily, although they are not
able to undergo oxidative insertion—as Pd does in coupling
reactions.
The key step of this mechanism is the generation of
cationic gold species A by fluorination (Scheme 4). Our
preliminary experiments show that Selectfluor did react with
gold(I) complex at room temperature. For example, when
CH3CN was used as solvent, AuCl readily reacted with
Selectfluor at room temperature; the net result being the
transfer of fluorine from Selectfluor to gold (Scheme 4). This
process could be monitored by 19F NMR spectroscopy. After
addition of Selectfluor to AuCl in CD3CN, a new peak (br.s,
d = À184 ppm) appeared and its intensity increased with time
Scheme 5. Synthesis of fluoroketones and the oxidative coupling of
alkynes.
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(see the Supporting Information for spectral analyses). When
we added PhB(OH)2 to the system, this peak disappeared
immediately and biphenyl was detected in the reaction
mixture (see the Supporting Information). We believe that
the new peak represents a reactive metal–fluoride species,
which we tentatively assigned as [AuIIIClF(L)]+ (L =
CH3CN). The formation of biphenyl can be rationalized by
a transmetalation with phenylboronic acid, followed by
reductive elimination to give biphenyl (see the Supporting
Information). The high reactivity rules out the possibility that
it is a simple fluoride anion. [Ph3PAuCl] can also be oxidized
under the same conditions but at a lower rate; however, in this
case the phosphine ligand Ph3P was also oxidized (by the
high-valence gold or Selectfluor) after prolonged times.
=
Ph3P O was detected in the reaction mixture (confirmed by
31P NMR and ESI-MS analysis).[5b,c]
Although the oxidation of AuI to AuIII by Selectfluor has
been postulated as a reasonable process, it has never been
confirmed experimentally in the literature. As the AuI to AuIII
oxidation was a key step in our mechanism, we studied this
process using X-ray photoelectron spectroscopy (XPS).[23]
XPS has been used in the determination of the chemical
states of supported gold catalysts.[24] The binding energy (BE)
of the Au 4f7/2 electron in each gold oxidation state is usually
large enough to be differentiated (for [ClAuIPPh3], Au 4f7/2
=
85.7 eV, for Na[AuIIICl4], Au 4f7/2 = 87.6 eV).[23] We used this
technique to investigate the valence change of gold in the
reaction. First, we tested two gold standards ([ClAuIPPh3] and
Na[AuIIICl4], Figure 1); the Au 4f7/2 photoelectron peak is
located at a BE value of 85.7 and 87.5 eV, respectively, which
is quite consistent with the literature.[23] We investigated our
samples A–C using the same conditions as for the standards.
Sample A appears as a mixture of two gold oxidation states
(Figure 1). We then tested the chloride-stabilized samples
(chloride reacts with the gold(III) complex to give a more
stable chloroaurate); they gave similar spectra but with a
higher percentage of gold(III) species (see Table S2 in the
Supporting Information). Because the BE difference between
Au 4f peak of two gold states is large (ca. 3.0 eV), we assigned
them to be Au0 and AuIII species, respectively. The spectra of
these Au 4f peaks are very similar to a literature report,[24] and
have been ascribed to be a mixture of Au0 and AuIII (see
Supporting Information for more details). Thus, our XPS
measurements confirm the existence of gold(III) (BE of Au
4f7/2 = 87.6 eV) in the reaction mixture of gold(I) catalyst and
Selectfluor (Figure 1). The existence of gold(0) can be
explained by disproportionation of unreacted AuI complexes
or decomposition/reduction of AuIII species in high vacuum
during XPS measurements. Because the AuIII species is the
major component in all tests, the existence of AuIII species
cannot be attributed to a disproportionation of unreacted AuI
complexes alone.
Figure 1. XPS curve fitting of the Au 4f photoelectron peaks.
mismatched with the cations formed by late transition
metals, especially gold.[25]
In summary, a potentially new role for fluorine in cationic
gold catalysis is proposed, an example of which is the
hydration of alkynes to give a-substituted a-fluoroketones,
in one pot and under mild conditions. The ready availability of
alkynes and organoboronic acids, and the current interest in
a-fluoroketones make this reaction quite attractive. Studies
probing the broader implications of cationic metal species
enabled by fluorine are underway in our laboratory.
Experimental Section
General procedure for preparation of 3. Selectfluor (354 mg,
1.0 mmol, 2.5 equiv) was added into a solution of alkyne 1a (92 mg,
0.4 mmol), [Ph3PAuCl] (9.8 mg, 0.02 mmol, 5% equiv), and phenyl-
boronic acid (98 mg, 0.8 mmol, 2 equiv) in 3 mL MeCN/H2O (20:1).
The reaction was stirred at room temperature for 18 h. The reaction
mixture was quenched with saturated NH4Cl solution, the resulting
aqueous mixture was extracted with diethyl ether (3 ꢁ 15 mL), and
then the combined organic layers were dried over Na2SO4. The
solvent was removed under reduced pressure to give the crude
product, which was purified by flash silica gel chromatography (30%
dichloromethane in hexane/60% dichloromethane in hexane) to give
We also conducted other investigations on the potential
valence change of gold in the reaction, using high-resolution
ESI mass spectrometry (see the Supporting Information). We
were able to detect various cationic AuIII species, but
[AuIII(L)ClF]+ itself was not detected; this may be because
metal–fluorine bonds tend to be labile and reactive. The
fluoride ion, according to hard/soft acid–base theory, is
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Communications
3a as a mixture of two regioisomers (3a/3a’ = 6.7:1); 118 mg (88%).
2009, 131, 9651; c) N. D. Ball, M. S. Sanford, J. Am. Chem. Soc.
2009, 131, 3796.
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Chem. Soc. 2009, 131, 9488.
[10] X. Wang, T.-S. Mei, J.-Q. Yu, J. Am. Chem. Soc. 2009, 131, 7520.
[11] a) T. Wu, G. Yin, G. Liu, J. Am. Chem. Soc. 2009, 131, 16354;
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132, 2856.
IR (neat): 2958, 1724, 1450, 1275, 1116, 711 cmÀ1; 1H NMR (500 mhz,
CDCl3), major isomer: d = 0.87 (t, J = 6.5 Hz, 3H), 1.29–1.31 (m, 4H),
2.03–2.18 (m, 1H), 2.22–2.38 (m, H), 3.08–3.1 (m, 2H), 4.55 (t, J =
6 Hz, 2H), 7.31–7.53 (m, 8H), 7.82 ppm (d, J = 7.5 Hz, 2H); 13C NMR
(125 mhz, CDCl3), major isomer: d = 14.0, 22.8, 22.5, 36.6, 37.8 (d, J =
22 Hz), 59.4, 103.5 (d, J = 186 Hz), 124.5 (d, J = 10.5 Hz), 128.4, 128.8,
129.7, 133.1, 137.6(d, J=22 Hz), 166.4, 208.5 ppm (d, J = 30 Hz);
19F NMR (470 mhz, CDCl3): major isomer: d = À170.04 ppm (t, J =
25.4 Hz); GC/MS (EI) m/z: 221, 201, 177, 135, 77; anal. calcd. for
C21H23FO3: C 73.66, H 6.77; found: C 73.42, H 6.75.
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Received: June 12, 2010
Revised: July 13, 2010
Published online: August 27, 2010
Keywords: alkynes · fluorination · fluoroketones · gold
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