Silver-free activation of ligated gold(I) chlorides: The use of [Me3NB12Cl11]- as a weakly coordinating anion in homogeneous gold catalysis

Article abstract of DOI:10.1002/chem.201404487

Phosphane and N-heterocyclic carbene ligated gold(I) chlorides can be effectively activated by Na[Me₃NB₁₂Cl₁₁] (1) under silver-free conditions. This activation method with a weakly coordinating closo-dodecaborate anion was shown to be suitable for a large variety of reactions known to be catalyzed by homogeneous gold species, ranging from carbocyclizations to heterocyclizations. Additionally, the capability of 1 in a previously unknown conversion of 5-silyloxy-1,6-allenynes was demonstrated.

Full text of DOI:10.1002/chem.201404487

DOI: 10.1002/chem.201404487
Full Paper
&
Homogeneous Catalysis
Silver-Free Activation of Ligated Gold(I) Chlorides: The Use of
[Me3NB12Cl11]ꢀ as a Weakly Coordinating Anion in Homogeneous
Gold Catalysis
Michael Wegener,[a] Florian Huber,[a] Christoph Bolli,[b] Carsten Jenne,[b] and
Stefan F. Kirsch*[a]
Abstract: Phosphane and N-heterocyclic carbene ligated
gold(I) chlorides can be effectively activated by
Na[Me3NB12Cl11] (1) under silver-free conditions. This activa-
tion method with a weakly coordinating closo-dodecaborate
anion was shown to be suitable for a large variety of reac-
tions known to be catalyzed by homogeneous gold species,
ranging from carbocyclizations to heterocyclizations. Addi-
tionally, the capability of 1 in a previously unknown conver-
sion of 5-silyloxy-1,6-allenynes was demonstrated.
Introduction
by Teles et al., it is known that treatment of methyl gold com-
plexes with a Brønsted acid generates catalytically active cat-
ionic species with release of methane.[6] Following a similar
concept, Nolan and co-workers developed an N-heterocyclic
carbene (NHC) gold(I) hydroxide precatalyst that can also be
activated under acidic conditions.[7] In addition, there has been
evidence for the catalytic activity of (Ph3P)AuCl in the presence
of TfOH.[8] Non-silver-containing Lewis acids have successfully
been used as activating reagents for gold(I) sulfonate or ace-
tate precatalysts,[9] and in particular the combination of gold(I)
chloride complexes with Cu(OTf)2 appears to give an effective
catalyst system.[10] The generation of active catalysts from
gold(I) chloride complexes and alkali metal salts of borates
Homogeneous gold catalysis as a means for activating multiple
bonds to effect complex transformations of small molecules is
of ever-growing importance in organic synthesis.[1] As a result,
recent reports on previously unsuspected complications that
arise from the commonly used activation of gold(I) chloride
complexes by silver salts are of major significance.[2] It was
shown that, in many cases with silver activation, the success of
gold-catalyzed reactions strongly depends on subtle changes
in the reaction conditions, particularly with regard to handling
of the gold precatalyst and the silver salt. Other, more obvious
downsides of silver salts as activating reagents (e.g., light sen-
sitivity, solubility) have spurred the desire for alternative, silver-
free methods for quite some time now,[3] and in light of these
recent revelations, interest is rapidly increasing.
ꢀ
such as B(C6F5)4 (extensively used by Bertrand and co-workers
to activate gold(I) NHC complexes) is also well-documented.[11]
Recently, Echavarren and co-workers demonstrated that, in
a series of gold(I) complexes with different counterions,
Gagosz and co-workers introduced the widely used gold(I)
bis(trifluoromethanesulfonyl)imidates as stable, highly reactive
catalysts that can be isolated free of silver.[4] Stable cationic ni-
trile complexes, which were applied by Echavarren and co-
workers, among others, had a similar impact.[5] On the other
hand, a variety of concepts for silver-free in situ generation of
active gold(I) catalysts from stable gold(I) precatalysts have
also been introduced. For instance, since the pioneering work
those
with
the
bulky
and
poorly
coordinating
ꢀ
ꢀ
BArF (=B[3,5-(CF3)2C6H3]4 ) ion are particularly efficient in
4
substantially raising the yields of intermolecular reactions.[12]
However, studies on the general applicability of the BArF
ꢀ
4
counterion through the experimentally quite simple in situ ac-
tivation of gold(I) chlorides are somewhat scarce.[13]
Due to their broad availability, gold(I) chloride precatalysts
remain the prominent gold source for all types of homogene-
ous gold catalysis. Therefore, a general method for their activa-
tion, apart from the classical silver activation, would be of
great significance. We now report the application of the
sodium salt of an anionic boron cluster to activate a range of
widely used gold(I) chloride precatalysts. The active gold spe-
cies with the weakly coordinating counterion are formed in
situ under experimentally simple, silver-free conditions. The ef-
ficiency of this new method is demonstrated by a broad spec-
trum of known reactions and an unprecedented cascade reac-
tion of 1,6-allenynes.
[a] M. Wegener, F. Huber, Prof. S. F. Kirsch
Organic Chemistry, Bergische Universitꢀt Wuppertal
Gaussstrasse 20, 42119 Wuppertal (Germany)
Fax: (+49)202-4392648
E-mail: sfkirsch@uni-wuppertal.de
[b] C. Bolli, Prof. C. Jenne
Inorganic Chemistry, Bergische Universitꢀt Wuppertal
Gaussstrass 20, 42119 Wuppertal (Germany)
1
Supporting information (experimental procedures, analytical data, H and
13C NMR spectra, and crystal structure data) for this article is available on
the WWW under http://dx.doi.org/10.1002/chem.201404487.
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Most of the salts generated catalytically active species, albeit
with varying reactivity. The boron hydride cluster (Table 1,
entry 3) and its perfluorinated analogue (Table 1, entry 4) ef-
fected hardly any or no conversion after 24 h. Of the remaining
dianionic clusters, the chlorinated cluster proved to be the
most effective, giving complete conversion after 3 h and
a yield of 66% (Table 1, entry 5). The reactivity decreased sig-
nificantly when the brominated and iodinated clusters were
used (Table 1, entries 6 and 7). Employing different alkali metal
salts of the chlorinated cluster (Table 1, entries 8 to 10) also did
not bring any improvement either, most likely due to solubility
issues.
Results and Discussion
Inspired by the ongoing investigation of halogenated icosahe-
dral borates as weakly coordinating anions for access to reac-
tive cations,[14] we envisaged the application of such borates as
a means to generate catalytically active, cationic gold(I) com-
plexes from the corresponding gold(I) chlorides. These boron
clusters are very robust, only weakly basic, and can be pre-
pared and used as their alkali metal salts. During our studies
(see below), Na[Me3NB12Cl11] (1) emerged as the reagent of
choice. The singly charged anion has been prepared as its
sodium salt by a simple two-step procedure: The reaction of
[H3NB12H11]ꢀ with an excess of SbCl5 as the chlorinating agent
gives the perchlorinated [H3NB12Cl11]ꢀ anion. In the second
step, this anion is methylated by methyl iodide to yield
[Me3NB12Cl11]ꢀ.[14i]
In contrast, 1, which is the sodium salt of a monoanionic
cluster, gave the desired product 3 in 92% yield after only 1 h,
which matches our best results so far using silver-salt activa-
tion.[15] This silver-free activation mode seemed to be of gener-
al value, since it proved to be equally effective when applied
to 1,6-enyne 4, which gave the expected product 5 in excellent
yield [Eq. (1)].[17]
Carbocyclization reactions of enynes
Our initial studies focused on the pinacol-terminated cycliza-
tion of 1,5-enyne 2 to generate bicyclic aldehyde 3 (Table 1).[15]
(Ph3P)AuCl shows no catalytic reactivity in this reaction, unless
it is activated with a suitable silver salt (Table 1, entries 1 and
2), in which case the removal of silver salts by filtration
through Celite before addition of the catalyst is essential, since
they cause decomposition of the starting material.[15a] We then
replaced the silver salt with a variety of closo-dodecaborates
[B12X12]2ꢀ. In initial screening, different alkali metal salts of the
clusters and (Ph3P)AuCl were directly added to the substrate
solution to trigger the conversion of enyne 2 to aldehyde 3.
In the context of these silver-sensitive reactions 2!3 and
4!5, we were thus able to adjust the reaction conditions in
a way that would be impossible with the in situ use of silver
salts. Preactivation of the catalyst, notably in the absence of
the substrate, and subsequent filtration to remove the silver
are no longer necessary; instead, gold(I) catalyst and 1 are
both simply added directly to the substrate solution.
The commercially available Na[BArF ] is also a potent addi-
4
Table 1. Cascade reaction of 2 in the presence of dodecaborates.
tive for the in situ activation of (Ph3P)AuCl: 3-silyloxy-1,5-enyne
2 was converted to aldehyde 3 in 90% yield (Table 1, entry 12),
which is comparable to the yields we obtained using AgSbF6
or cluster 1. However, under otherwise identical conditions,
the conversion of 3-silyloxy-1,6-enyne 4 with Na[BArF ] instead
4
of cluster 1 gave a significantly reduced yield of 51% of 5. Fur-
ther experiments with both additives confirmed our impres-
sion that 1 is the more reliable and robust reagent for a variety
of gold-catalyzed conversions (see below).
Entry
MX
t [h]
3 [%][a]
2 [%][a]
1[b]
2[b]
3[c]
–
24
<1
24
24
3
24
24
48
48
24
1
0
93
11
0
66
50
28
43
37
0
>95
0
71
100
–
31
39
37
48
100
–
AgSbF6
Na2[B12H12]
Na2[B12F12]
Na2[B12Cl12]
Na2[B12Br12]
Na2[B12I12]
Li2[B12Cl12]
K2[B12Cl12]
Cs2[B12Cl12]
1
4[c]
Encouraged by our results with cluster 1 in the cascade reac-
tions of silyloxy-enynes 2 and 4, we next tested how this acti-
vation method applies to other enyne systems that were previ-
ously converted by gold catalysis. As was generally the case
with all previously described reactions herein, unless stated
otherwise, no further attempt was made to optimize the reac-
tion conditions reported in the respective literature; we simply
changed the original activation method by adding 1 instead of
the silver salt, without filtration or any other intermediary pu-
rification step.
5[c]
6[c]
7[c]
8[c]
9[c]
10[c]
11[d]
12[c]
92
90
Na[BArF ]
24
–
4
[a] Yield of isolated product after column chromatography. (Ph3P)AuCl
and MX were directly added to a solution of substrate unless stated oth-
erwise. [b] Entry 1: gold catalyst (5 mol%) was directly added to the solu-
tion. Entry 2: gold catalyst (10 mol%) was preactivated by silver catalyst
(5 mol%) and filtered through Celite; see ref. [15a,b]. [c] 10 mol%
(Ph3P)AuCl, 5 mol% MX. For the synthesis of the borates, see ref. [16].
[d] 5 mol% (Ph3P)AuCl, 5 mol% 1.
We first chose the cycloisomerization of 1,6-enyne 6 to test
the broad applicability of our activation method,[6d] and found
that the combination of (Ph3P)AuCl and cluster 1 is indeed
highly reactive in the formation of diene 7 (Scheme 1). An
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reaction in the absence of a silver-salt catalyst.[20] Nevertheless,
the desired furan 16 could indeed be obtained in 77% yield.
An equally good yield was observed for the acetylenic Schmidt
reaction of azide 17 to give pyrrole 18 with (dppm)Au2Cl2
(dppm=1,1-bis(diphenylphosphino)methane) as precatalyst
[Eq. (4)].[21]
Scheme 1. Carbocyclization reactions of 1,6-enyne 6.
equally high yield was obtained for the methoxycyclization of
6 to 8 with (JohnPhos)AuCl (instead of the originally used
(Ph3P)AuMe/protic acid system) in methanol,[6d] although the
reaction rate was considerably lower. The same catalyst was
also effective in the cyclization of 1,5-enynes 9 and 10 to the
two different bicyclohexanone isomers 11 and 12, respectively
(Scheme 2), whereby the high-yield two-step conversion of
acetate 10 is particularly noteworthy.[18] The combination of
1 with the originally reported (Ph3P)AuCl precatalyst gave
slightly inferior yields and lower reaction rates in those cases.
(Ph3P)AuCl and 1 again proved to be a very effective combi-
nation in the cyclization of propargylic amide 19, which rapidly
gave oxazoline 20 in high yield [Eq. (5)].[22] The same combina-
tion, however, fell short compared to the highly reactive
(Ph3P)AuNTf2 catalyst in the cyclization of propargylic tert-butyl
carbonate 21 to dioxolanone 22 [Eq. (6)], in which case we
had to use slightly elevated temperatures to force conver-
sion.[4c]
Scheme 2. Cycloisomerization reactions of 1,5-enynes 9 and 10.
Changing the solvent from toluene to chloroform was nec-
essary to perform the cyclization of homopropargylic hydroxyl-
amine 23 to pyrrolidinone 24 [Eq. (7)], which we obtained in
a moderate yield close to the previously reported result.[23] The
use of molecular sieves described in the original procedure
was not necessary when we conducted the reaction. For the
formation of dihydrobenzofuran 26 from allyl ether 25, which
likely proceeds via Claisen rearrangement and subsequent cyc-
lization, we applied slightly higher temperatures to obtain the
product in 51% yield [Eq. (8)].[24] This pretty poor result sup-
Heterocyclization reactions
We were also able to successfully perform a variety of hetero-
cyclization reactions with cluster-activated gold(I) catalysts. For
example, alkynone 13 was converted to furanone 14 in the
presence of (Ph3P)AuCl and cluster 1 [Eq. (2)].[19] Compared to
the activation of the catalyst with various silver salts, this is by
far the best result we could obtain with gold(I) catalysts at
room temperature for this reaction, which runs more efficiently
with platinum(II) catalysts at elevated temperatures.
In the case of applying the same conditions to propargyl
vinyl ether 15 to effect the propargyl Claisen rearrangement
and heterocyclization cascade to give furan 16 [Eq. (3)], our ex-
pectations were low, since we were never able to perform this
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ports the original assumption that an assisting role can be at-
tributed to the silver salt in this reaction.[2a]
Cyclization cascade of 5-silyloxy-1,6-allenynes
Apart from the above known reactions, we also found applica-
tion for this new activation method in a previously undisclosed
gold(I)-catalyzed reaction of 1,6-allenynes. Although a variety
of allenynes have been shown to engage in cyclization reac-
tions similar to corresponding enyne systems in the presence
of transition-metal catalysts, their reactivity has not been inves-
tigated nearly as thoroughly as that of their enyne counter-
parts.[1k,m,28] That fact, along with our work on cyclization reac-
tions of silyloxy-enynes, fueled our interest in the reactivity of
allenynes bearing a silyloxy group.
Other gold(I)-catalyzed reactions
Other gold(I)-catalyzed transformations in which we applied
1 include the hydration of alkyne 27, in which case we could
successfully activate the NHC–gold precatalyst in aqueous
medium and obtain ketone 28 in high yield [Eq. (9)];[25] howev-
er, we were forced to increase catalyst loading to 5 mol% com-
pared to the reported procedure to reach full conversion. The
combination of (JohnPhos)AuCl and 1 worked well in the
tandem hydroamination/hydrogenation of phenylacetylene
(30) with N-methylaniline (29) in the presence of diethyl 1,4-di-
hydro-2,6-dimethyl-3,5-pyridinedicarboxylate (Hantzsch ester),
as shown in [Eq. (10)].[26] We were able to isolate tertiary amine
31 in 72% yield, although conversion of amine 29 was not yet
complete after 48 h.
In various instances, 1,6-enynes in the presence of base or
gold catalysts have been shown to engage in a cascade reac-
tion consisting of a heterocyclization step followed by Claisen
rearrangement to give complex and often polycyclic cyclohep-
tenones.[17,29] We now report that 5-silyloxy-1,6-allenynes un-
dergo a similar gold(I)-catalyzed transformation to (bi)cyclic,
polyunsaturated products.
As shown in Table 2, we initially found that 1,6-allenyne 34a
in the presence of (Ph3P)AuCl and AgSbF6 gives both bicyclic
ketone 35a and the rearrangement product 36 (Table 2,
entry 1). Conducting the same experiment solely with AgSbF6
gave exclusively ketone 36 (Table 2, entry 2), that is, the silver
salt seems to be responsible for the formation of this unde-
sired byproduct. Indeed, preactivation and filtration of the cat-
alyst resulted in the sole formation of ketone 35a in 73% yield
(Table 2, entry 4). Other silver salts such as AgOTf and AgNTf2
proved to be less efficient than AgSbF6. In an effort to dis-
pense with silver salts altogether, we also tried to convert alle-
nyne 34a in the presence of (Ph3P)AuCl and cluster 1, in which
case we were able to match the best yield of 73% obtained
with silver-salt activation. Under experimentally simple condi-
tions, the gold(I) catalyst and 1 are both added to the sub-
Also in case of the [2+2] cycloaddition of phenylacetylene
(30) with a-methylstyrene (32) [Eq. (11)], (XPhos)AuCl activated
in situ by 1 proved to be reactive, although the yield was
lower than those obtained with the isolated [(XPhos)Au-
(NCMe)]SbF6 catalyst.[5e] Recently, Echavarren and co-workers
reported that cyclobutene 33 is formed in 95% yield when the
isolated [(tBuXPhos)Au(MeCN)]BArF4 catalyst is employed.[12,27]
Table 2. Optimization of catalyst activation for the conversion of 34a.
Several reactions discussed above can be efficiently per-
Entry
Catalyst (mol%)
t [h]
35a [%][a]
36 [%][a]
formed with Na[BArF ] instead of cluster 1 under otherwise
4
1[b]
2
(Ph3P)AuCl (5)/AgSbF6 (5)
AgSbF6 (5)
(Ph3P)AuCl (5)
(Ph3P)AuCl (10)/AgSbF6 (5)
(Ph3P)AuCl (10)/AgOTf (5)
(Ph3P)AuCl (10)/AgNTf2 (5)
(Ph3P)AuCl (5)/Na[BArF ] (5)
3
3
24
1
22
6
2
42[c]
0
32[c]
64
0[d]
0
0
0
0
0
0[d]
identical conditions. The in situ activation of gold(I) complexes
with Na[BArF ] gave, for example, good results for the forma-
4
3
0[d]
73
60
65
59
73
0[d]
tion of 7 (82%), 12 (94%), 20 (82%), 31 (74%), and 33 (41%),
whereby the yields are in the range of those obtained with
cluster 1. In other cases, however, we found markedly reduced
4[e]
5[e,f]
6[e]
7[b]
8[b]
9
yields when employing Na[BArF ]: 8 (42%), 11 (69%), 14 (44%),
4
4
(Ph3P)AuCl (5)/1 (5)
1 (5)
2
24
16 (43%), 18 (36%), 26 (no product formation), and 28 (39%).
We conclude that for a couple of reactions Na[BArF ] and clus-
4
[a] Yield of isolated product after column chromatography. [b] (Ph3P)AuCl
and activating reagent were directly added to a solution of substrate.
[c] Obtained as mixture; individual yields determined by NMR spectrosco-
py. [d] No reaction. [e] (Ph3P)AuCl and silver salt were stirred separately
and filtered through Celite before addition to substrate. [f] Reaction was
performed at rt.
ter 1 are equally effective for the in situ activation of gold
complexes, but cluster 1 is easily prepared and shows, at least
in our hands, superior performance in several reactions in
which the anionic boron cluster appears to be the more effi-
cient counterion.
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strate solution. In comparison, the use of Na[BArF ] under the
clic double bond of the product. Furthermore, substrate 34 f
featuring a five-membered ring gave the corresponding bicy-
clic product 35 f in good yield (Table 3, entry 6).
4
same conditions also led to complete conversion of the start-
ing material, but gave a markedly inferior yield.
After further optimization of the reaction conditions (nota-
bly, the use of wet CH2Cl2 instead of iPrOH to effect hydrolysis
of the silyl ether resulted in slightly improved yields), we next
investigated the scope of the reaction. Entries 1–5 in Table 3
show that substrates with a variety of substituents on the ter-
minal allene carbon atom, such as methyl, ethyl, and phenyl
groups, were successfully converted in mostly good to high
yields, with only the cyclohexylidene derivative 35e being ob-
tained in a moderate yield of 50% (Table 3, entry 5). In the
case of unsymmetrically substituted allenes, comparison of the
diastereomeric ratios of products and substrates indicate that
the stereoinformation of the allene is transferred to the exocy-
Whereas all of the substrates with two substituents at the
terminal allene carbon atom resulted exclusively in the forma-
tion of products with one exocyclic double bond, substrate
34g, which bears no further substituent on the allene moiety,
gave a,b-unsaturated ketone 35g with two endocyclic double
bonds in very good yield (Table 3, entry 7), which suggests
that, depending on the substitution pattern of the allene,
slightly different mechanisms may be at work and lead to the
more highly substituted double bond in each case. Allenyne
34h gave the analogous product 35h in moderate yield
(Table 3, entry 8). Interestingly, allenyne 34i, which has just
one methyl substituent on the terminal allene carbon atom,
gave both products 35i and 35i’, which could be isolated sep-
arately [Eq. (12)].
Table 3. Cascade reaction of 5-silyloxy-1,6-allenynes 34.
Entry
Substrate
Product
Yield
[%][a]
1
2
3
4
34a: R1 =R2 =Me
35a
35b
35c
35d
82
Although terminal alkynes readily reacted under the present
conditions, internal alkynes proved to be much more difficult
to engage in an analogous reaction. The conversion of phenyl-
ated alkyne 34j to ketone 35j, however, shows that it is gener-
ally feasible (Table 3, entry 9). We found that in this particular
instance the use of iPrOH in dry CH2Cl2 improved the conver-
sion, which was still not complete after 3 d at room tempera-
ture, and resulted in a comparatively low yield of 46%. Fur-
thermore, the only product isolated after the reaction of meth-
ylated alkyne 34k was the unexpected allene 36, which is ob-
viously formed by carbocyclization followed by a hydride shift
[Eq. (13)], a reaction that has already been reported for similar
substrates.[30] In addition to cyclic substrates, acyclic allenynes
34l and 34m were also successfully converted to cyclopente-
nones 35l and 35m in moderate and high yields, respectively
(Table 3, entries 10 and 11).
34b[b]: R1 =Me, R2 =Et
34c: R1 =R2 =Et
68[c]
85
34d[d]: R1 =Me, R2 =Ph
85[e]
5
6
34e:
34 f:
35e:
35 f:
50
73
7
34g:
34h:
34j:
35g:
35h:
35j:
84
8
54
9[f]
46[g]
On comparing the in situ activation of the gold(I) catalyst
with cluster 1 to preactivation with AgSbF6, we found that our
silver-free method was generally equal or even superior to pre-
activation with a silver salt. For instance, applying the latter
method to allenyne 34 f afforded ketone 35 f in only 34%
yield, as opposed to 73% with cluster 1. Even more notewor-
thy is the fact that internal alkyne 34j gave only traces of the
product 35j after 4 d at room temperature when AgSbF6 was
used. This indicates that the catalyst formed during preactiva-
10
11
34l: R=C6H13
34m: R=Ph
35l
35m
83
61
[a] Yield of isolated product after column chromatography; conditions:
(Ph3P)AuCl (5 mol%), 1 (5 mol%), wet CH2Cl2 (0.2m), 08C. [b] d.r.=1.2:1.
[c] d.r.=1.2:1. [d] d.r.=1.7:1. [e] d.r.=1.5:1. [f] With iPrOH (1.1 equiv), rt.
[g] 68% Yield based on recovered starting materials.
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tion in the absence of the substrate is less reactive compared
to activation with cluster 1 in the presence of the substrate. By
implication, if a substrate of this nature had been chosen as
a starting point to investigate the reactivity of this allenyne
system, the use of common silver salts would have provided
a false negative result.
pot procedure with N-iodosuccinimide (NIS). Consistent with
the results of the deuteration experiments, we found that con-
verting allenyne 34g in dry CH2Cl2 in the presence of NIS and
under otherwise standard reaction conditions led to the forma-
tion of the somewhat unusual allylic iodide 39, which we were
able to isolate in 52% yield (Scheme 5). Acyclic substrates 34l
and 34m behaved in analogous fashion, and iodides 40 and
41 were obtained in higher yields of 73 and 74%, respectively.
To gain a better understanding of the mechanistic details
that influence the outcome of the reaction regarding the exo-
or endocyclic position of the double bond in the cyclization
products, we conducted deuteration experiments with alle-
nynes 34a and 34m (Scheme 3). Carrying out the reaction
with substrate 34m in the presence of deuterium oxide led ex-
clusively to the incorporation of deuterium at the methyl
group of ketone 38. In contrast, conducting the same experi-
ment with substrate 34a afforded product 37 deuterated at
the expected a position to the ketone.
Scheme 5. Synthesis of allylic iodides.
Characterization of cationic gold(I) species
To better understand the interaction of cluster 1 with gold(I)
catalysts, we then isolated the activated cationic gold(I) com-
plex. The chlorido-bridged digold complex 42 was formed on
stirring equimolar amounts of (Ph3P)AuCl and 1 in CH2Cl2
(Figure 1). An X-ray diffraction study[31] on complex 42 revealed
an A-frame structure of the cation with a short Au···Au dis-
Scheme 3. Deuteration experiments with allenynes 34a and 34m.
A possible mechanism that is in line with these results is of-
fered in Scheme 4. After activation of alkyne A by the gold cat-
alyst towards heterocyclization, the resulting species B under-
goes [3,3]-sigmatropic rearrangement to complex C, which
then collapses with release of the gold catalyst to give silyl
enol ether D. Subsequent hydrolysis of the silyl enol ether pro-
ceeds by protonation at the a or g position leading to product
E or F, respectively, depending on the steric demand of the
substituents R1 and R2; g protonation is favored only when
R1 =R2 =H.
To further expand the scope of the reaction, we attempted
to convert the intermediately formed silylenol ether in a one-
Figure 1. Formation of digold complex 42 (top). Part of the crystal structure
of 42 (bottom); ellipsoids are drawn at 50% probability, and hydrogen
atoms are omitted for clarity.
Scheme 4. Probable mechanism for the cascade reaction of A.
Chem. Eur. J. 2015, 21, 1328 – 1336
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1333
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Full Paper
tance of 310.08(4) pm due to an aurophilic interaction.[31] This
result, which is in accordance with previous reports on analo-
gous dinuclear complexes generated from phosphine gold(I)
halides and silver salts of different weakly coordinating
anions,[2b,33] suggests that chloride abstraction from the preca-
talyst is not complete in the absence of a substrate or coordi-
nating solvent. However, in our standard reaction setup, in
which the precatalyst is activated in the presence of the sub-
strate, we assume that chlorido-bridged complex 42 is not
formed.
131.6, 123.7, 123.6, 49.1, 48.0, 40.7, 38.3, 36.0, 28.7, 26.6, 21.5,
~
20.6 ppm; IR (ATR): n=2922, 2853, 1708, 1626, 1444, 1372, 1341,
1312, 1273, 1251, 1232, 1188, 1156, 1118, 1065, 1026, 967, 944, 878,
848, 746, 638, 586, 538, 514, 492, 450, 410 cmꢀ1; LRMS (EI): m/z (%):
204 (100) [M+], 189 (14) [M+ꢀCH3], 171 (10), 161 (59), 147 (36),
133 (49), 119 (57), 105 (52), 91 (75), 77 (28), 67 (13), 53 (12); HRMS
(ESI): m/z calcd for C14H20ONa+: 227.1404; found: 227.1406
[M+Na+].
Acknowledgements
Applying the isolated digold complex 42 directly to allenyne
34a effected the conversion to ketone 35a almost as readily
as in situ activation [Eq. (14)], which implies that this particular
substrate is able to dissociate the chlorido-bridged complex to
release the catalytically active species.
Support of this research by DFG through grant KI 1289/1-3 is
gratefully acknowledged. We thank RockwoodLithium for the
kind donation of chemicals.
Keywords: borates
· cyclization · gold · homogeneous
catalysis · weakly coordinating anions
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Conclusion
We have introduced a general and simple method for the acti-
vation of all types of ligated gold(I) chloride precatalysts LAuCl
in which L is a phosphane or an NHC. Under silver-free condi-
tions, the action of Na[Me3NB12Cl11] (1) produces gold species
with the weakly coordinating closo-dodecaborate anion
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catalysts for a range of literature-known conversions including
carbocyclizations and heterocyclizations. Furthermore, we used
this activation method to make the conversion of 5-silyloxy-
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a novel reaction that proved sensitive to silver additives. We
feel that the activation of gold(I) chloride precatalysts with 1 is
a general alternative to the classical usage of silver salts that
will become of great value to future developments.
Experimental Section
General procedure for the AuI-catalyzed cyclization cascade
of allenynes
8-Propan-2-ylidene-3,4,4a,5,7,8-hexahydro-1H-benzo[7]annulen-
6(2H)-one (35a): (Ph3P)AuCl (2.4 mg, 4.88 mmol, 5 mol%) and
Na[Me3NB12Cl11] (2.9 mg, 4.88 mmol, 5 mol%) were added to a solu-
tion of allenyne 34a (27.0 mg, 97.7 mmol, 1 equiv) in 0.49 mL of
wet CH2Cl2 at 08C. After stirring at that temperature for 2 h, the
crude mixture was mounted on silica and purified by flash chroma-
tography (petroleum ether (PE)/EtOAc 98/2) to afford ketone 35a
(16.4 mg, 80.3 mmol, 82%) as a pale yellow oil. TLC: Rf =0.33 (PE/
1
EtOAc 90/10, [UV] [KMnO4]); H NMR (400 MHz, CDCl3): d=6.10 (s,
1H), 3.29 (d, J=17.9 Hz, 1H), 3.19 (d, J=17.9 Hz, 1H), 2.67 (dd, J=
12.2, 9.2 Hz, 1H), 2.57 (dd, J=12.3, 3.8 Hz, 1H), 2.41–2.31 (m, 1H),
2.30–2.23 (m, 1H), 2.03 (td, J=13.1, 4.0 Hz, 1H), 1.85–1.72 (m, 3H),
1.76 (s, 3H), 1.70 (s, 3H), 1.45–1.35 (m, 1H), 1.35–1.26 (m, 1H),
1.26–1.17 ppm (m, 1H); 13C NMR (151 MHz, CDCl3): d=211.8, 142.5,
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Received: July 21, 2014
Published online on November 13, 2014
Chem. Eur. J. 2015, 21, 1328 – 1336
www.chemeurj.org
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