Cyclization of gold acetylides: Synthesis of vinyl sulfonates via gold vinylidene complexes

Article abstract of DOI:10.1002/anie.201310280

Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β-carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold-catalyzed reactions, but for the new substrates, only one gold atom is needed for substrate activation. After formation of a gold vinylidene complex, which forms a tight contact ion pair with the sulfonate leaving group, recombination of the two parts delivers vinyl sulfonates, which are valuable targets that can serve as precursors for cross-coupling reactions, for example. Gold vinylidene intermediates are generated by the cyclization of gold acetylides that carry a sulfonate leaving group. This result demonstrates for the first time that the formation of these species is not restricted to a dual activation mode. The cyclization products obtained herein contain a vinyl sulfonate moiety, which makes them useful building blocks for cross-coupling reactions.

Full text of DOI:10.1002/anie.201310280

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DOI: 10.1002/anie.201310280
Gold Catalysis
Hot Paper
Cyclization of Gold Acetylides: Synthesis of Vinyl Sulfonates via Gold
Vinylidene Complexes**
Janina Bucher, Thomas Wurm, Kumara Swamy Nalivela, Matthias Rudolph, Frank Rominger,
and A. Stephen K. Hashmi*
Abstract: Differently substituted terminal alkynes that bear
sulfonate leaving groups at an appropriate distance were
converted in the presence of a propynyl gold(I) precatalyst.
After initial formation of a gold acetylide, a cyclization takes
place at the b-carbon atom of this species. Mechanistic studies
support a mechanism that is related to that of dual gold-
catalyzed reactions, but for the new substrates, only one gold
atom is needed for substrate activation. After formation of
a gold vinylidene complex, which forms a tight contact ion pair
with the sulfonate leaving group, recombination of the two
parts delivers vinyl sulfonates, which are valuable targets that
can serve as precursors for cross-coupling reactions, for
example.
Scheme 1. Dual activation pathway leading to vinylidenes (top) and
strategy for the synthesis of vinylidenes based on acetylides and
a leaving group (bottom).
H
omogeneous gold catalysis is dominated by the electro-
philic p-activation of a multiple bond as the initial step. Based
on this principle, numerous fruitful reports contributed to an
enormous boom in gold catalysis.[1]
chemistry, which gives access to interesting polycyclic struc-
tures (Scheme 1).
Significantly less examples exist for the nucleophilic
activation of alkynes. Thus far, this reactivity was mostly
studied with gold acetylides, which reacted in a Grignard-like
fashion at their a-carbon atom.[2] Recently, Zhangꢀs and our
group independently discovered a new reactivity pattern,
which is based on a dual activation principle.[3] This approach
uses diyne systems, such as Ia, in which one of the alkyne
moieties is electrophilically activated by p-coordination,
while the second alkyne is activated by s-coordination,
which leads to an enhanced nucleophilicity. Depending on
the backbone, this synergy allows the formation of vinylidene
or carbene intermediates that enable a rich follow-up
Inspired by these findings, we wondered whether the p-
activated alkyne could be replaced with suitable other
electrophiles. This should enable a completely new access to
gold vinylidene complexes. As a leaving group replaces the p-
activated alkyne in this approach, we assumed that no cationic
gold source should be necessary for the generation of
vinylidenes of type IIb. Related transformations are known
for X = I or X = OSO2Ar and are either initiated by the
formation of ate complexes of boron[4] or zinc[5] or by the
formation of lithium acetylides.[6] Because of the high
reactivity of the applied precursors, these reactions have to
be performed in dry solvents, and nucleophilic addition of the
starting metal acetylides onto the intermediates can lead to
undesired side reactions. Herein, we report the first cycliza-
tion of gold acetylides into gold vinylidene intermediates that
does not rely on the principle of dual activation. Owing to the
properties of the gold catalyst, the reactions can be performed
in air, and they are insensitive to moisture.
[*] M. Sc. J. Bucher, M. Sc. T. Wurm, Dr. K. S. Nalivela, Dr. M. Rudolph,
Dr. F. Rominger,[+] Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
Inspired by the work of Harada and co-workers, who
could cyclize a variety of hexynyl tosylates, such as 1a, via the
corresponding lithium acetylides,[6a] we used 1a as a test
system for a possible gold-catalyzed process. As mentioned
earlier, only the s-activation should be addressed by the gold
catalyst; therefore, we used gold propyne acetylides as
precursors that do not form cationic gold complexes. After
ligand exchange with the starting material, a s-activated
substrate should be formed along with propyne as a volatile
and labile throw-away ligand. Indeed, heating 1a in benzene
at reflux in the presence of [IPrAu(propynyl)] 3a (5 mol%;
IPr= 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) effi-
ciently delivered vinyl tosylate 2a (Scheme 2).
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University (KAU)
Jeddah 21589 (Saudi Arabia)
[+] Crystallographic investigation.
[**] J.B. is grateful for a Chemiefonds scholarship of the Fonds der
Chemischen Industrie, K.S.N. for a postdoctoral fellowship of the
Alexander von Humboldt foundation. We thank Umicore AG & Co.
KG for the generous donation of gold salts.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310280.
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perform this reaction with halides as the leaving group were
unsuccessful.
Next, we tested differently substituted alkynyl sulfonates
(Table 2). With ether derivatives instead of the alkyl tethers,
longer reaction times were observed, and the yields were only
moderate (entries 1 and 2). As the presence of the ether
linkage breaks the symmetry of the cyclopentylidene ring, the
products were obtained as mixtures of diastereoisomers,
favoring the E isomers (assigned by 1H,1H NOESY NMR
spectroscopy). The corresponding substrates with a sulfona-
mide group in the chain were also investigated. Whereas no
reactivity was observed with the tosyl-substituted substrate 1i
(entry 3), switching to the brosylate (OBs = para-bromoben-
zenesulfonate) leaving group delivered the desired product 2j
in 83% yield as a 4:1 mixture of diastereoisomers (entry 4).
Based on the higher reactivity that was observed for the
brosylate substrates, we performed further studies with this
leaving group. As a next step, we investigated substrates with
an additional substituent at the propargylic position. A
phenethyl substituent not only increased the reaction rate
and yield, but also led to the exclusive formation of the
sterically less hindered E isomer (entry 5). It is noteworthy
that the competing CH insertion reaction was not observed.
A completely different picture was obtained with a directly
attached phenyl group. Most probably because of the highly
activated propargylic/benzylic ether position, decomposition
of the starting material was observed under the reaction
conditions (entry 6). The branched alkyl substrate 1m, which
does not bear additional heteroatoms, was also investigated.
The cyclized product 2m was obtained in good yield, but
again as a mixture of diastereoisomers (entry 7). Next, we
evaluated whether this transformation would be possible for
systems with aromatic moieties as part of the tether.
Unfortunately, substrate 1n, which bears a tether with six
carbon atoms, was not converted into the analogous annu-
lated cyclopentylidene, and only incomplete conversion was
observed, along with an unselective transformation (entry 8).
Whereas the expected reactivity was restored by increasing
the chain length, problems still occurred with unfunctional-
ized substrate 1o (entry 9). Aside from incomplete conver-
sion, the final product turned out to be fairly unstable.
Fortunately, the introduction of an oxygen atom into the
phenolic substrates 1p–1s delivered the stable products 2p–
2s, which could be isolated in good yields (entries 10–13).
Crystal structure analysis of compound 2p confirmed the
constitution of the target compound as well as the E geometry
of the exocyclic double bond.[7,10]
Scheme 2. Transformation of tosylate 1a in the presence of
[IPrAu(propynyl)] (3a; 5 mol%). Ts=tosyl=4-toluenesulfonyl.
An optimization of the reaction conditions[7] revealed that
gold propyne acetylide 3a was superior to the corresponding
methyl or phenyl gold complexes as well as to [IPrAuOH].[8]
Next, we performed a ligand screening; unsaturated NHC(15)
ligand 3b[9] turned out to be the best candidate (NHC = N-
heterocyclic carbene; NHC(15) bears a cyclopentadecyl sub-
stituent on one of the nitrogen atoms). Different solvents
were screened, but the yields dropped significantly in other
reaction media. A control experiment without any catalyst
showed no conversion; the addition of catalytic amounts of p-
TsOH also led to no reaction. A radical pathway can be
excluded as no drop in reactivity was observed when 2,6-
bis(1,1-dimethylethyl)-4-methylphenol (BHT) was added as
a radical inhibitor.[7]
With the optimized conditions in hand (5 mol% 3b,
benzene, reflux), we set out to investigate the scope of this
transformation. First, we tested different sulfonate moieties.
As shown in Table 1, sulfonates with aromatic substituents
Table 1: Gold-catalyzed cyclization of hexynyl sulfonates.[a]
Entry
R
Starting material
Product
Yield [%]
1
2
3
4
5
6
p-MeC6H4
p-BrC6H4
p-NO2C6H4
p-MeOC6H4
mesityl
1a
1b
1c
1d
1e
1 f
2a
2b
2c
2d
2e
2 f
92
88
53
84
93
72
Me
[a] All reactions were performed in benzene (150 mm) at reflux with 3b
Finally, we tested different chain lengths for the open-
chain substrates as well (entries 14–17). The formation of six-
membered rings was possible, but longer reaction times were
required than for the formation of the corresponding cyclo-
pentylidene rings. For both leaving groups, allenes 4t and 4u
were isolated as side products in small amounts (entries 14
and 15). Derivative 1v did not undergo cyclization; instead,
allene 4v, along with the HOBs addition product 5v,[11] was
obtained (entry 16). The structure of 5v could be confirmed
by X-ray crystal structure analysis.[7,10] A similar result was
obtained for butynyl derivative 1w (entry 17). In this case, the
analogous HOTs addition product 5w was isolated together
(5 mol%). All yields refer to isolated products.
delivered the corresponding products in good to excellent
yields, only a p-NO2 substituent resulted in a slightly lower
yield (entry 3). Mesylate (OMs = methanesulfonate) as an
aliphatic substituent was also suitable, and the corresponding
product 2 f could be obtained in 72% yield (entry 6).
For product 2b, single crystals that were suitable for X-ray
crystallography[7,10] could be obtained; therefore, the struc-
tural assignment of a cyclopentylidene ring with an exocyclic
double bond could be confirmed. Unfortunately, attempts to
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Table 2: Gold-catalyzed cyclization of substituted 1,n-yne sulfonates.[a]
Entry Starting material
Product
t
Yield
[%]
Entry Starting material
Product
t
Yield
[%]
1
6 days 21
12
48 h
77
2
3
4
39 h
15 h
39 h
53
–
13
14
15 h
87 h
87 h
78
49
12
2i not formed
83
5
6
15 h
15 h
65
–
15
63 h
63 h
54
12
decomposition, 2l not
formed
7
8
9
15 h
64 h
64 h
79
–
16
21 h
21 h
15 h
22
32
33
incomplete conversion
into 2n, unselective
25[b]
17
10
11
15 h
15 h
82
85
15 h
16
[a] All reactions were performed in benzene (150 mm) at reflux with 3b (5 mol%). All yields refer to isolated products. [b] 16% of the starting material could
be re-isolated.
with diene 6w, which was most probably formed by elimi-
nation of the tosylate group of compound 5w.
Our next experiments were aimed at investigating the
reaction mechanism. First, we isolated acetylide 7b by
reacting substrate 1b with [IPrAuOH] (1 equiv) at room
temperature (see the Supporting Information for the solid-
state molecular structure).[8a,10] Heating of 7b to 808C indeed
resulted in the formation of product 2b, which was confirmed
by GC/MS and thin-layer chromatography (TLC). Unfortu-
nately, no aurated intermediate could be isolated, and the
stoichiometric reaction turned out to be less selective than the
catalytic process (Scheme 3).
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scrambling of the 18O label in the product (see the Supporting
Information, Scheme S1).
Based on these findings, our proposed mechanism for the
cyclization is depicted in Scheme 6. Some of the key
elementary steps parallel those from Zhangꢀs and our work
on the dual activation mode.[3a,b] The reaction cascade is
initiated by catalyst transfer from the gold propyne acetylide
onto the starting material. The resulting acetylide 7 then
cyclizes through substitution of the sulfonate leaving group by
its b-carbon atom, which leads to gold vinylidene intermedi-
ate IIb. The so-formed contact ion pair of the gold vinylidene
complex and the sulfonate counterion
Scheme 3. Stoichiometric reaction of gold acetylide 7b.
then recombines, forming the vinyl gold
species III. The catalytic cycle is com-
pleted either by another catalyst transfer
onto the next substrate molecule (which
places a deuterium at the vinylic position
when a deuterated alkyne is used as the
starting material) or, as water is not
Scheme 4. Labeling experiments.
excluded, by protodeauration to form
product 2 and a gold hydroxide complex,
Next, we synthesized substrate [D]-1e with a deuterium
label at the terminal alkyne position (Scheme 4, condi-
tions A). After conversion under the optimized conditions,
50% of the deuterium had been transferred to the vinylic
position of the product. This indicates that after the cycliza-
tion, a vinyl gold complex is formed, which then transfers the
catalyst onto the next substrate molecule with simultaneous
release of the product.[3b] A control experiment with non-
deuterated substrate 1e in the presence of D2O (5 equiv) also
led to a deuterium incorporation of 50% at the vinylic
position (Scheme 4, conditions B). This uptake of deuterium
can be rationalized by exchange processes that take place at
the terminal alkyne position prior to cyclization. It is also
possible that protodeauration of the vinyl gold intermediate
by traces of water competes with the catalyst transfer.
A cross-over experiment was conducted to investigate
whether the recombination of the sulfonate anion with the
gold vinylidene species is an inter- or an intramolecular
process. We therefore synthesized substrate [D,18O]-
1a with deuterium and 18O labels.
Scheme 5. Cross-over experiment.
The complete conversion of a 1:1 mixture of
labeled compound [D,18O]-1a and non-labeled 1a
under the optimized conditions was confirmed by
TLC and GC/MS analysis. The absence of cross-over
products was affirmed by mass spectrometry
(EI(+)/MS; Scheme 5). This indicates that the
sulfonate counterion and the respective gold vinyl-
idene complex must be in close contact during the
reaction. This could either be due to the formation
of a tight ion pair in the hydrocarbon solvent or arise
À
from the fact that when the C O bond of the
starting material is broken, the sulfonate group
directly rebinds to the forming vinylidene complex
in an intramolecular fashion through one of its
sulfonyl oxygen atoms. However, the latter possi-
bility could be ruled out by the transformation of
Scheme 6. Proposed reaction mechanism. OTs is used as a representative for the
18O-labeled substrate [18O]-1a, which resulted in the sulfonate leaving groups.
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which then initiates another cycle by reformation of acety-
lide 7. The formation of allenes in a side reaction remains
unclear. Attempts to obtain the allenes by elimination from
the HOBs addition product 4v were unsuccessful,[7] which
excludes a trivial explanation. Further investigations con-
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building blocks for cross-coupling reactions was quickly
demonstrated by a Suzuki coupling of vinyl tosylate 2a with
phenylboronic acid, which delivered the desired coupling
product in an acceptable yield of 61% without any optimi-
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In summary, we could demonstrate that the formation of
gold vinylidene intermediates is also possible by the cycliza-
tion of gold acetylides alone and therefore not strictly
dependent on a dual activation mode. The cyclization
presented herein works well for a range of differently
substituted sulfonates and leads to interesting products with
a vinyl sulfonate moiety, which allows for further function-
alization by cross-coupling reactions. Further investigations
concerning the mechanism as well as the formation of the
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Received: November 27, 2013
Published online: March 5, 2014
Keywords: acetylides · alkynes · cyclization · gold · vinylidenes
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