Dual gold catalysis: σ,π-propyne acetylide and hydroxyl-bridged digold complexes as easy-to-prepare and easy-to-handle precatalysts

Article abstract of DOI:10.1002/chem.201203010

A series of dinuclear gold σ,π-propyne acetylide complexes were prepared and tested for their catalytic ability in dual gold catalysis that was based on the reaction of an electrophilic π-complex of gold with a gold acetylide. The air-stable and storable catalysts can be isolated as silver-free catalysts in their activated form. These dual catalysts allow a fast initiation phase for the dual catalytic cycles without the need for additional additives for acetylide formation. Because propyne serves as a throw-away ligand, no traces of the precatalyst are generated. Based on the fast initiation process, side products are minimized and reaction rates are higher for these catalysts. A series of test reactions were used to demonstrate the general applicability of these catalysts. Lower catalyst loadings, faster reaction rates, and better selectivity, combined with the practicability of these catalysts, make them ideal catalysts for dual gold catalysis. Traceless dual-activation catalysts: Air-stable π-coordinated propyne acetylide gold complexes proved to be powerful precatalysts for dual gold catalysis (see scheme). A fast catalyst transfer to the starting substrates initiates the catalytic cycle. Reaction yields and selectivity were improved without the need for basic additives or activation by silver salts. Copyright

Full text of DOI:10.1002/chem.201203010

FULL PAPER
DOI: 10.1002/chem.201203010
Dual Gold Catalysis: s,p-Propyne Acetylide and Hydroxyl-Bridged Digold
Complexes as Easy-to-Prepare and Easy-to-Handle Precatalysts
A. Stephen K. Hashmi,* Tobias Lauterbach, Pascal Nçsel, Mie Højer Vilhelmsen,
Matthias Rudolph, and Frank Rominger[a]
Abstract: A series of dinuclear gold
s,p-propyne acetylide complexes were
prepared and tested for their catalytic
ability in dual gold catalysis that was
based on the reaction of an electrophil-
ic p-complex of gold with a gold acety-
lide. The air-stable and storable cata-
lysts can be isolated as silver-free cata-
lysts in their activated form. These dual
catalysts allow a fast initiation phase
for the dual catalytic cycles without the
need for additional additives for acety-
lide formation. Because propyne serves
as a throw-away ligand, no traces of
the precatalyst are generated. Based on
the fast initiation process, side products
are minimized and reaction rates are
higher for these catalysts. A series of
test reactions were used to demon-
strate the general applicability of these
catalysts. Lower catalyst loadings,
faster reaction rates, and better selec-
tivity, combined with the practicability
of these catalysts, make them ideal cat-
alysts for dual gold catalysis.
Keywords: acetylides
·
diynes
·
dual gold catalysis · gold · traceless
precatalysts · vinylidenes
Introduction
The most common reactivity in the field of gold chemistry
is, without any doubt, the activation of multiple bonds to-
wards the attack of a nucleophile by the p-coordination of a
gold complex. Based on this reactivity, a manifold of differ-
ent reaction patterns have been developed throughout the
last decade.[1]
The use of gold acetylides as nucleophiles in gold-cata-
lyzed reactions has only rarely been mentioned in the litera-
ture to date. The first examples were the gold-catalyzed ver-
sion of the A3-coupling (aldehydes, alkynes, and amines
leading to propargylic amines) and related reactions that are
based on a Grignard-type reactivity of a gold acetylide.[2]
Recently, a new field of gold catalysis that is based on a
dual activation mode by the gold catalyst has been opened
(Scheme 1).[3] In these cases, the gold catalyst activates the
alkyne by the known p-activation mode and, in addition, a
second molecule of gold catalyst activates one alkyne by s-
coordination, which enables it to react as a nuleophile. This
s-activated alkyne can react at its a-carbon atom, leading to
enyne systems IIa/IIb.[4] A second option comprises an
attack at the b-carbon atom of the acetylide to deliver gold
vinylidenes I, which are interesting intermediates that open
Scheme 1. Reaction modes for dual gold catalysis.
up completely new reaction pathways.[5] We believe that the
concept of dual activation will lead to a completely new
field of gold chemistry, thus we started our efforts towards
establishing a generally applicable and easy-to-handle cata-
lyst system. So far, most of these reactions are conducted
without any additives or with basic additives to induce ace-
tylide formation. For these cases, the equilibrium depicted
in Scheme 2 plays a crucial role and therefore the amount of
activated catalyst (left side of the equilibrium) depends on
the acidity of the alkyne and the base that is applied. The
counter ion also influences the conjugated acid that is
formed from the acetylide. For many substrates, these fac-
tors might lead to a quantitative acetylide formation (with
respect to the gold catalyst) that would inhibit the catalytic
cycle. This is underlined by recent reports on the inactivity
[a] Prof. Dr. A. S. K. Hashmi, T. Lauterbach, Dipl.-Chem. P. Nçsel,
Dr. M. H. Vilhelmsen, Dr. M. Rudolph, Dr. F. Rominger+
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221-54-4205
E-mail: hashmi@hashmi.de
[+] Crystallographic investigation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203010.
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strate, which should allow high reproducibility and efficiency
(Scheme 2b). To circumvent a possible equilibrium of the
released alkyne after substrate activation (which should
matter at late reaction stages and/or for high catalyst load-
ings), we considered propyne precursors to be ideal because
gaseous propyne is released after the exchange process. The
preparation and catalytic abilities of these species are dis-
cussed within this contribution.
Results and Discussion
Preparation of the catalysts: To keep the synthesis of the
acetylide complexes as simple as possible, we used the com-
mercially available propynyl Grignard as a transmetalation
reagent, instead of the reported protocols that are based on
the use of condensed propyne in combination with a base.[8]
Starting from gold(I) chloride precursors with different li-
gands, a small series of propyne gold acetylides were availa-
ble in good to excellent efficiency (Table 1). The differences
Scheme 2. a) Activation with basic additives; b) activation with a trace-
less dual-activation catalyst.
of gold acetylides in cycloisomerization reactions of
enynes.[6]
During our investigations on the gold-catalyzed hydroary-
lation aromatization of terminal diynes,[5b] we provided ex-
perimental proof that the reformation of the s-activated
substrate, by catalyst transfer from an organogold intermedi-
ate onto a starting terminal alkyne, can close the catalytic
cycle (release of the product and activation of new starting
material). Furthermore, organogold compounds were used
as suitable additives to initiate a dual-catalysis cycle without
any basic additive. In the same report geminal-diaurated
compounds (prepared from the stoichiometric reaction of a
gold acetylide and one equivalent of activated catalyst)
could be used as instant dual-activation catalysts. To design
a generally applicable, instant catalyst system for dual gold
catalysis, we had the following requirements of the catalysts:
Table 1. Preparation of gold propyne acetylides by transmetalation.
Entry
Ligand L
Time
[h]
Compound
Yield
[%]
1
2
3
4
5
IPr
PPh3
BrettPhos
SPhos
L1
5
6
5
5
5
1a[a]
1b
1c
1d
1e
100
80
71
70
99
1) preparation must be easy;
2) handling and storage even under open flask conditions
should be possible;
3) a broad range of counter ions and ligands should be ap-
plicable;
[a] See the Supporting Information for the solid-state molecular struc-
ture.[9]
4) generation of byproducts derived from components of
the precatalyst should be minimized as much as possible;
5) the initiation of the dual activation cycle should be fast
to avoid competing reaction pathways that lead to unde-
sired byproducts (one example is the formation of an a-
naphtalene in the hydroarylating cyclization during the
initiation phase);[5b]
6) the ratio of acetylide and activated catalyst should be
well-defined after the initiation phase;
7) the catalyst should be preactivated to avoid the necessity
of in situ activation with silver salts;
in yields are based on the purification by crystallization.
This turned out to be slightly more efficient for carbene
complexes (Table 1, entries 1 and 5) but, nevertheless, yields
for phosphane complexes were also high (Table 1, entries 2–
4) and all of the products could be obtained as air-stable
crystalline solids.
As the next step, we focused on the preparation of a
series of s,p-acetylide complexes. First, we prepared a set of
compounds with the established IPr N-heterocyclic (NHC)
ligand[10] in combination with different weakly coordinating
counter ions. For preparing the complexes, two different
protocols were applied (see Table 2). The most straightfor-
ward process is the direct reaction of the gold acetylide with
IPrAuCl (method A). Simple addition of the corresponding
silver salt and filtration over Celite delivered the desired
complexes after evaporation of the solvents and washing
with diethyl ether. This process delivered high yields of pure
8) the catalyst should be thermally stable.
To conform to these requirements, we considered a cata-
lyst system that is based on dinuclear gold s,p-acetylide
complexes as possible precatalysts.[7]
The advantage of s-activation by ligand exchange, in con-
trast to activation by using a basic additive, is the well-de-
fined 1:1 ratio of activated catalyst and s-activated sub-
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ternative pathway.[7d] One explanation might be the function
of gold as a type of protecting group for the terminal
alkyne, which might suppress side reactions due to alkyne/
alkyne dimerizations.[4b] All the other derivatives were also
obtained as air-stable crystalline solids in excellent yields
(Table 3, entries 2–6).
Table 2. Preparation of s,p-propyne acetylide complexes with the IPr
ligand.
Next, we installed different ligands at the s,p-acetylide
complexes (Table 4). All of the complexes were prepared
with hexafluorophosphate as the counter ion under the
same conditions as for the IPr complexes. Unfortunately, the
Entry
Counter Ion
Method[a]
Time
AHCTUNGTERG[NNUN min]
Compound
Yield
[%]
À
Table 4. Preparation of hexafluorophosphate s,p-propyne acetylide com-
plexes with different ligands.
1
2
3
4
5
6
7
8
9
NTf2À
B
A
A
B
A
A
B
A
B
30
30
30
30
45
30
60
30
50
2a
2b
2c
2c
2d
2e
2e
2 f
2g
92
86
98
94
97
96
89
74
89
SbF6
À
PF6À
PF6 À
BF4
TsOÀ
TsOÀ
À
NNf2
TfOÀ
Entry
Ligand L
Method
Time
[min]
Compound
Yield
[%]
[a] Method A: simple mixing of gold acetylide/IPrAuCl and AgX; Meth-
od B: preactivation of IPrAuCl with AgX, then addition of the acetylide
AHCTUNGTRENNUNG
[a]
1
2
3
4
5
PPh3
BrettPhos
SPhos
L1
A
A
B
A
B
30
15
20
30
20
–
9
10
–
–
94
97
compounds for all of the applied counter ions (Table 2, en-
tries 2, 3, 5, 6, and 8). As an alternative (method B), we per-
formed the reaction in a sequential process. First, the corre-
sponding silver salt and the gold chloride complexes were
stirred for 30 min and then the gold acetylide was added to
the preactivated gold catalyst. This was practical for the
[a]
–
–
[a]
L1
–
[a] Decomposition.
A
reaction with PPh3–acetylide 1a and PPh3–AuCl led to a de-
composition of the starting material (Table 4, entry 1). The
lack of stability of this compound seems to be based on the
decreased steric hindrance of the phosphane ligand. By
switching to the bulkier phosphane ligands BrettPhos
(Table 4, entry 2) and SPhos (Table 4, entry 3), clean conver-
sions were observed and yields were nearly quantitative.
NAC–acetylide 1e (NAC=nitrogen acyclic carbene) led to
a decomposition of the starting material even if a sequential
addition of silver salt was used (Table 4, entries 4 and 5).
Once again, the increase in bulkiness of the NAC ligand
seems to be insufficiently stabilizing the resulting species.
(Table 2, entry 9). In addition, we compared the two proto-
cols and this showed no significant differences for the hexa-
fluorophosphate anion (Table 2, entries 3 and 4) or for silver
tosylate (Table 2, entries 6 and 7). Nonaflate as the anion
was also suitable, but the yield for the final complex was
slightly lower (Table 2, entry 8). To evaluate the effect of
other acetylenes on the preparation of these catalysts, we
synthesized six different phenylacetylene-derived s,p-acety-
lide complexes (Table 3). Different electronic properties and
counter ions were applied. Interestingly, the approach in-
volving preformed acetylides (instead of p-coordinated
alkyne complexes in the Widenhoefer protocol) delivered
air-stable complex 3 as a crystalline solid in excellent yield
(Table 3, entry 1), a compound that decomposed by the al-
Structures of the catalysts: During purification of the com-
plexes, most of the IPr-ligated compounds delivered single
crystals that were suitable for X-ray crystal-structure analy-
ses.[9] Two representative solid-state molecular structures of
2b and 3 are depicted in Figure 1. A summary of the key
data is depicted in Table 5 (for comparison, the structural
data of 10[7d] is also shown). Along the C2–Au2-axis, the
bond lengths C1–C2, as well as C1–Au2, are quite similar and
nearly no effects due to the counter ion or the substituent at
the alkyne are apparent. Interestingly, if at all, only small in-
fluences due to the counter ion/alkyne moiety are detected
for the distances between the p-bonded gold (Au1) and the
C1/C2/R1 atoms. Small variations in the values for the differ-
ent complexes can be found for the Au1–Au2 separation, as
Table 3. Preparation of s,p-phenylacetylene acetylide complexes with the
IPr ligand.
Entry
1
2
3
4
5
6
À
well as for the Au1-C1-Au2 angles. The exchange of SbF6
with the stronger coordinating TsOÀ leads to a shortening of
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A. S. K. Hashmi et al.
all of the prepared complexes showed equivalent
1H/13C NMR shifts for the two coordinating ligands, which
can be explained by a fast fluctuation of the gold fragments
even at low temperatures (À908C). To investigate the ther-
mal stability of the propyne catalysts, we measured a series
1
of H NMR spectra of compound 2a over a period of 2 h in
C2D2Cl4 at 1108C. The complex showed a remarkable stabil-
ity and no traces of decomposition could be monitored even
at this temperature.
Tests of the catalytic properties: As the first test reaction for
the catalytic abilities of the complexes, we investigated the
gold-catalyzed formation of b-phenyl naphthalenes (reaction
type I, Scheme 1) under the previously reported reaction
conditions (Scheme 3).[5b] In the first set of experiments, the
Scheme 3. Gold-catalyzed hydroarylating aromatization
propyne derived catalysts 2a–2g containing the IPr ligand
but different counter ions were compared. Figure 2 displays
the formation of the b-product (12b) at different reaction
times. A rather large effect due to the counter ions can be
monitored. By far the best results were obtained for
propyne catalyst 2c, which contains the hexafluoro-
phosphate ion. Complete conversions were ob-
tained within 1 hour, four times faster than under
our previously applied conditions (5 mol%
IPrAuNTf2, 10 mol% IPrAuPh as the additive).
Even higher reaction rates were obtained with cata-
Figure 1. Solid-state molecular structures of compounds 2b and 3.
Table 5. Comparison of the structural data of the s,p-digold species.
Compound
2b
2c
2e
2 f
3
10
(Ref.
G
ACHTUNGTNER[NUNG 7d])
lyst 2g, which contains the triflate counter ion;
however, complete conversion was not obtained.
Slightly lower reaction rates were detected for the
tetrafluoroborate anion, as well as for the nonaflate
anion, both of which perform in the same range
(complete conversions after 4.5 h). Although the
triflimide and the tosylate anion still delivered ac-
ceptable results (complete conversions after 6 to
7 h), only slow conversion rates were detected for
the hexafluoroantimonate anion. The pronounced
difference of the hexafluorophosphate and the hex-
afluoroantimonate anion seems unusual, but there
is precedence for this phenomenon in gold cataly-
sis.[11]
À
À
À
À
À
counter ion
SbF6
Me
PF6
Me
3.2
TsOÀ
Me
NNf2
Me
SbF6
SbF6
R1 =
R1 value [%] 2.9
3.6
6.8
4.5
¯
space group
distances [ꢂ]
C1–Au2
P21/n
P21/n
P1
P21/n
P21/c
P21/c
1.994(4) 1.993(4) 1.989(4) 1.997(9)
1.226(5) 1.231(5) 1.214(6) 1.203(11) 1.245(10)
1.476(6) 1.474(6) 1.479(6) 1.482(13) 1.433(11)
2.217(4) 2.220(4) 2.204(4) 2.212(9)
2.216(4) 2.192(4) 2.241(4) 2.230(9)
3.202(4) 3.207(4) 3.202(4) 3.184(9)
3.716(3) 3.690(3) 3.547(3) 3.677(8)
2.001(8)
1.997
1.214
1.450
2.193
2.234
3.247
3.624
C1–C2
C2–R1
C1–Au1
2.221(7)
2.194(7)
3.247(7)
3.745(6)
C2–Au1
Au1–R1
Au1–Au2
Angles [8]
Au1-C1-Au2
Au1-C2-R1
129.8
118.9
122.2
118.9
115.5
117.4
121.7
116.7
125.0
125.7
119.7
122.2
Table 6 presents the a/b ratios after complete
conversion for all the propyne dual catalysts and
the phenylacetylene derivatives. Even though only
one equivalent of the additive (acetylide), with re-
the Au1–Au2 separation by 0.17 ꢂ and the Au1-C1-Au2 angle
decreases from 129.88 to 115.58. The same parameters were
also affected by changing the electronic parameters of the
alkyne moiety. Analogously to the report of Widenhoefer,
spect to the catalyst, was available for s-activation of the
substrate, the b-selectivity for most of the counter ions was
excellent. This indicates a fast ligand exchange of the acety-
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their largest influence on selectivity and slow conversion
rates (which allow a collection of accurate data points, even
for the initiation phase). Figure 3 shows the formation of the
Figure 2. Yields of 12b at different reaction times for propyne catalysts
2a–2g (different counter ions)
Table 6. Yields and selectivities due to dependency of the counter ion
and the alkyne moiety of the dual catalysts.
Figure 3. Yield of 12a for different reaction times—dependency of the
alkyne unit.
Entry
Compound
Time
[h]
Yield
[%]
Ratio
12a/12b
a-phenylnaphtalene 12a for the different precatalysts at dif-
ferent reaction times. These results clearly demonstrate a
much faster initiation phase for the propyne catalyst 2b.
One reason might be the release of the volatile propyne
after acetylide exchange, which (in a traceless manner)
avoids the possibility of a back reaction. Furthermore, the
acidity seems to also play a role. Although the effect of the
acidity of the aromatic systems is inconsistent (compare re-
sults in Table 6), the faster ligand exchange for less acidic al-
kynes seems to occur for all of the propyne catalysts. This
seems reasonable as the pKa value for alkylacetylenes is ap-
proximately two units higher than for phenylacetylene deriv-
atives.[13]
1
2
3
4
5
2a
2b
3
6
48
48
48
1
5
8
8
4
36
98
92
98
98
77
69
61
92
85
84
97
97
66
4:96
11:89
25:75
33:67
2:98
2:98
4:96
3:97
7:93
20:80
14:86
4:96
4:96
4:96
4
2c
2c
5
6
2d
7
6[a]
7[a]
8[a]
9
10
11
12
13
14
5
5
7
8
2e
2 f
2g
5
2[b]
[a] Only 1 mol% catalyst loading; [b] only 87% conversion.
Complex 2c is the best catalyst and thus was used in the
subsequent tests, but here the kinetics were too fast for our
kinetic investigation and, in accordance with our model of a
short induction period leading to high selectivity, the selec-
tivity of the reactions conducted with 2c were so high that
we failed to detect the formation of 12a.
The faster initiation of the propyne catalyst leads to a
better selectivity for the b-product 12b at the same conver-
sion rates. This is demonstrated in Figure 4, which displays
the b/a ratios at different conversion rates for the three cat-
alysts. It is clear that even at the early stages of the reaction,
a very high selectivity for the b-product is obtained for the
propyne catalyst, whereas the phenylacetylene derivatives
predominantly deliver the a-product 12a at early stages of
the reaction and the b pathway becomes dominant only
after significant conversion.
lide, which circumvents the formation of a-phenylnaphta-
lene (12a; the side product that is formed if no activation of
the substrate by acetylide exists).[5b] It is also worth noting
that for catalyst 2c, not only the second fastest reaction rate
was observed, but also the highest b-selectivity, combined
with perfect yields even for low catalyst loadings (see
Table 6).
Furthermore, a strong effect of the alkyne moiety on the
precatalysts was visible. For all of the three tested counter
ions, poorer selectivity was obtained with phenylacetylene
derivatives. Only a minor effect on the selectivity was visible
for the different aromatic substituents, but a high dependen-
cy of the selectivity on the counter ion was observed. A re-
lated strong effect on the stability for diaurated aromatic
species was reported by Gagnꢃ and co-workers.[12]
Next, we evaluated the efficiency of the bulky phosphane
complexes 9 and 10 in comparison to the carbene ligand
(Figure 5). Unfortunately, these ligands turned out to be less
efficient and, as well as prolonged reaction times, unaccepta-
ble yields were obtained with both of the complexes. In ad-
For a comparison of the induction phase, propyne- and
phenylacetylene-derived catalysts with the hexafluoroantim-
onate counter ion were used. Complexes 2b, 3, and 4, which
contain different alkyne substituents, were tested due to
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A. S. K. Hashmi et al.
Figure 4. Dependence of selectivity on the alkyne unit of the dual cata-
lysts.
Figure 5. Yields of 12b at different reaction times for catalysts 2c, 9, 10,
and 13.
dition, we included a study of
hydroxyl-bridged carbene cata-
lyst 13, a species which was first
reported by Nolan et al.[14] This
complex can also be regarded
as a dual-activation catalyst, as
it provides NHC-Au+ and
NHC-Au-OH. The latter spe-
cies has been shown to directly
form gold acetylides with termi-
nal alkynes as well. However,
in this proACHTUNGTRENNUNGcess water is liberat-
ed, which could be problematic
for higher catalyst loadings. The
reaction rates for this class of
dual-activation catalysts is simi-
lar to the propyne-based system
with the same NHC ligand, but
the yields obtained with 13 are
slightly lower. Furthermore, no
complexes of type 13 with non-
NHC ligands have been report-
ed so far, and the preparation,
storage, and handling of 13
proved to be more difficult
than the traceless dual-activa-
Scheme 4. Set of test reactions for TDAC 2c.
tion catalysts (TDACs). The
only advantage of 13 is a slightly faster product formation.
After the catalytic testing for the hydroarylating reaction,
we wanted to know whether the traceless dual-activation
catalysts are generally applicable for this field of gold chem-
Scheme 1. With the use of 2.5 mol% of TDAC 2c, reaction
rates were faster and the isolated yield of the product could
be nicely improved.[5d] Next, we tested the mechanistically
related dibenzopentalene synthesis.[5c] As in the previous
case, a faster reaction was observed than under the original
conditions. Furthermore, better yields were obtained with
the TDAC (Scheme 4,b).
The intermolecular alkyne/alkyne cross dimerization of
18[4b] was also considered as a test reaction for our catalysts.
In contrast to the previously discussed reactions, this trans-
À
istry. We used the same counter ion (PF6 ) for our set of
test reactions. All other reaction parameters were in accord-
ance with the references, or were even milder. As a starting
point, we used TDAC 2c for the transformation of diyne 14
to the fulvene derivative 15 (Scheme 4,a), a transformation
which also follows the reactivity pattern I shown in
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formation is believed to proceed by reaction type II, shown
in Scheme 1. Our tests were concentrated on the dimeriza-
tion of phenylacetylene as a substrate, which delivered only
marginal yields under the original conditions. Without opti-
mization, our catalyst system allowed the reaction to be per-
formed under slightly milder conditions (Scheme 4,c).
Yields for this transformation could be significantly im-
proved but further optimization would be necessary for a
practical use of this transformation. Finally, we investigated
the alkyne/alkyne macrocyclization that was reported by the
Gagosz group (reaction type II, Scheme 1).[4a] The yield for
this transformation turned out to be slightly lower for the
TDAC but the selectivity of the reaction was much better
(almost pure 21a was obtained).
IPrAu+ moiety towards IPr–gold phenylacetylide exceeded
the binding affinity towards phenyl acetylide by a factor of
80. This change in binding affinity corresponds to an energy
difference of DDG=1.9 kcalmolÀ1. Furthermore, the stabili-
ty of the geminal-digold complex of phenylacetylene was
tested by exposure to triflic acid. Unlike the gold s-acety-
lide, which was rapidly protodeaurated even at low tempera-
tures, the geminal-digold compound showed no detectable
decomposition over a period of 12 h at room temperature
(see Scheme 6).
Alkynes versus gold acetylides: Computational study on the
relative affinity of [L-Au]+: The observation of the (desired)
very high affinity and stability of the [L-Au]+ to the aurated
alkynes in the preparation of the traceless dual-activation
catalysts immediately raised the question of the fate of [L-
Au]+ after the catalyst transfer step. This catalyst transfer
step in the diyne substrates chemospecifically generates a
gold acetylide at the terminal position of one of the alkynes.
Because the thermodynamic affinity of the second gold, the
[L-Au]+, which is needed to p-activate the second triple
bond (to form gold(I) vinylidene species), might be lower
than the affinity for the triple bond of the gold acetylide, we
conducted a computational study.
The relative stabilities of the two s,p-acetylide complexes
22 and 23 were examined by calculations and a clear prefer-
ence was found for the geminal digold species 22. The geom-
etry optimizations were carried out by using the B3LYP
hybrid functional along with the correlation-consistent
double-z basis (cc-pPDZ) for all atoms but gold.[15] For gold,
a relativistic effective core potential reported by the Stoll
group was used.[16] These studies revealed an enhanced sta-
bility of 7–10 kcalmolÀ1 for the geminal s,p-acetylide com-
plexes 22 compared to the corresponding non-geminal
digold complexes 23 (Scheme 5).
Scheme 6. Different stabilities against protodeauration for 24 and 25.[7d]
Along the same lines, Houk, Toste, and co-workers com-
putationally found that between a gold s-acetylide and an
allene, phosphinogold showed a strong preference towards
binding to the acetylide (DDG=22.6 kcalmolÀ1).[3] In con-
trast, there was no great difference in the affinity towards
an alkyne and an allene—the binding to the allene was fa-
vored slightly by 1.1 kcalmolÀ1 (Scheme 7).
Scheme 7. Similar preference of digold–acetylides in competition with al-
lenes.[3]
This suggests that a kinetic coordination of [L-Au]+ to
the non-aurated triple bond of the s-activated substrate or,
more probably, a dynamic equilibrium between 22 and 23
delivers the reactive dual-activated complex, leading to the
gold vinylidene intermediates.
Scheme 5. Calculating the relative energies of the isomers 22 and 23.
Conclusion
This energy difference between the two catalytic inter-
mediates is in accordance with the high thermostability that
was found experimentally for the dual catalyst described
herein and with the earlier experimental result of Widen-
hoefer and co-workers.[7d] From NMR experiments in
CD2Cl2, Widenhoefer found that the binding affinity of a
s,p-Acetylide complexes derived from propyne proved to be
generally applicable catalysts in the new field of dual gold
catalysis. Without optimization of the conditions, the cata-
lysts delivered improved results for the small series of test
reactions. Moreover, all of the complexes were air-stable
Chem. Eur. J. 2012, 00, 0 – 0
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A. S. K. Hashmi et al.
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cient amounts of cationic gold. We are convinced that this
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Acknowledgements
This work was supported by Umicore AG & Co. KG, the Deutsche For-
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Received: August 24, 2012
Revised: November 25, 2012
Published online: && &&, 0000
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running title
FULL PAPER
Gold
A. S. K. Hashmi,* T. Lauterbach,
P. Nçsel, M. H. Vilhelmsen,
M. Rudolph, F. Rominger . . &&&& — &&&&
Dual Gold Catalysis: s,p-Propyne
Acetylide and Hydroxyl-Bridged
Digold Complexes as Easy-to-Prepare
and Easy-to-Handle Precatalysts
Traceless dual-activation catalysts: Air-
stable p-coordinated propyne acetylide
gold complexes proved to be powerful
precatalysts for dual gold catalysis (see
scheme). A fast catalyst transfer to the
starting substrates initiates the cata-
lytic cycle. Reaction yields and selec-
tivity were improved without the need
for basic additives or activation by
silver salts.
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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