1,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes

Article abstract of DOI:10.1021/jacs.8b11501

Metal-catalyzed dimerization reactions of terminal acetylenes are well known in the literature. However, only a few examples of the dimerization of halogen-substituted acetylenes are described. The products of the latter metal-catalyzed dimerization are the branched head-to-tail enynes. The formation of the corresponding linear head-to-head enynes has not been reported yet. Herein, we demonstrate by means of quantum chemical methods and experiments that the head-to-head dimerization of chloroarylacetylenes can be achieved via mono gold catalysis. Under the optimized conditions, a clean and complete conversion of the starting materials is observed and the dimeric products are obtained up to 75% NMR yield. A mechanistic investigation of the dimerization reaction reveals that the branched head-to-tail vinyl cation is energetically more stable than the corresponding linear head-to-head cation. However, the latter can rearrange by an unusual 1,3-chlorine shift, resulting in the highly stereoselective formation of the trans product, which corresponds to the gold complex of the head-to-head E-enyne. The activation barrier for this rearrangement is extremely low (ca. 2 kcal/mol). As the mono gold-catalyzed dimerization can be conducted in a preparative scale, this simple synthesis of trans-1,2-dichloroenynes makes the gold(I)-catalyzed head-to-head dimerization of chloroarylacetylenes an attractive method en route to more complex conjugated enyne systems and their congeners.

Full text of DOI:10.1021/jacs.8b11501

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Article
1,3-Chlorine Shift to a Vinyl Cation: A Combined
Experimental and Theoretical Investigation of the E-
Selective Gold(I)-Catalyzed Dimerization of Chloroacetylenes
Mathis Kreuzahler, Alyssa Daniels, Christoph Wölper, and Gebhard Haberhauer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11501 • Publication Date (Web): 27 Dec 2018
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1
,3-Chlorine Shift to a Vinyl Cation: A Combined Experimental and Theoretical
Investigation of the E-Selective Gold(I)-Catalyzed Dimerization of
Chloroacetylenes
†
†
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Mathis Kreuzahler , Alyssa Daniels , Christoph Wölper and Gebhard Haberhauer*
†
Institut für Organische Chemie, Universität Duisburg-Essen, Universitätsstr. 7, D-45117
Essen, Germany
Email address: gebhard.haberhauer@uni-due.de
ABSTRACT:
Metal-catalyzed dimerization reactions of terminal acetylenes are well known in literature.
However, only a few examples for the dimerization of halogene-substituted acetylenes are
described. The products of the latter metal-catalyzed dimerization are the branched head-to-
tail enynes. The formation of the corresponding linear head-to-head enynes has not been
reported yet. Herein, we demonstrate by means of quantum chemical methods and
experiments that the head-to-head dimerization of chloroarylacetylenes can be achieved via
mono gold catalysis. Under the optimized conditions, a clean and complete conversion of the
starting materials is observed and the dimeric products are obtained up to 75% NMR yield. A
mechanistic investigation of the dimerization reaction reveals that the branched head-to-tail
vinyl cation is energetically more stable than the corresponding linear head-to-head cation.
However, the latter can rearrange by an unusual 1,3-chlorine shift resulting in the highly
stereoselective formation of the trans product which corresponds to the gold complex of the
head-to-head E-enyne. The activation barrier for this rearrangement is extremely low (ca. 2
kcal/mol). As the mono gold-catalyzed dimerization can be conducted in a preparative scale,
this simple synthesis of trans-1,2-dichloroenynes makes the gold(I)-catalyzed head-to-head
dimerization of chloroarylacetylenes an attractive method en route to more complex
conjugated enyne systems and their congeners.
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Introduction
In the 1970s Bergman et al. could already demonstrate that two conjugated alkyne units can
react with each other (Scheme 1a).1-3 In this context, the resulting diradical intermediate
proved to be highly reactive and could be trapped by hydrogen donors to yield the
corresponding aromatic compounds. The activation barrier for the Bergman reaction, the
thermal cyclization of (Z)-hex-3-en-1,5-diyne (1) was determined both experimentally and
theoretically and amounts to approximately 29 kcal/mol.4,5
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Recently, dos Passos Gomes and Alabugin could impressively show by means of quantum
chemicals calculations that the activation barrier for the Bergman cyclization can be
tremendously reduced using gold(I)6-17 as a catalyst.18 At the CCSD(T) level of theory they
calculated an activation barrier of 14.6 kcal/mol (Scheme 1a), which corresponds to a
difference of 15 kcal/mol compared to the radical pendant.4,5 Similar values were obtained in
an independent work.19
a)

E (exp) = 29.2 ± 0.5
a
E (CCSD(T)) = 29.5
a
1
2
E (CCSD(T)) = 14.6
a
[
Au] +
[Au]
3
4
b)

E (exp) = 29.7 ± 0.4
a
E (CCSD(T)) = 28.1
a
5
6
or
E (CCSD(T)) = 12.6
a
[
Au]+
[Au]
[Au]
7
8
9
c)
Cl

E (exp) = 22.4 ± 0.1
E (UB3LYP) = 22.2
a
2
Cl
a
Cl
10a
11a
Scheme 1. a) Thermal and mono gold(I)-catalyzed Bergman cyclization. b) Thermal and
gold(I)-catalyzed transannular ring closure of 1,6-cyclodecadiyne (5). c) Thermal dimerization
of the chloroarylacetylene 10a. The activation energies (E ) are given in kcal/mol.
a
A similar drastic reduction of the activation barrier for the reaction of two alkyne units can be
found for the transannular cyclization of non-conjugated cyclic diynes (Scheme 1b). The
thermal transannular ring closure of 1,6-cyclodecadiyne (5) occurs at temperatures above
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00 °C and proceeds via the bicyclic 1,3-butadiene-1,4-diyl intermediate 6.20-23 Both
1
experimentally and theoretically determined values for the activation barrier are in the range
of 28-30 kcal/mol.8,10 Trapping of the diradical intermediate 6 by hydrogen donors leads to
the corresponding hydrocarbons. Only recently, we were able to show that here again the
gold(I)-catalyzed version causes a tremendous acceleration compared to the radical pendant
(Scheme 1b).24 Depending on the system and the method used either the corresponding
cyclopropenylmethyl cation 8 or the butadienyl cation 9 represents the stable intermediate.24
The activation barrier for these intermediates corresponds to 12.6 kcal/mol on the CCSD(T)
level of theory. In this context we were also able to show that trapping of these cations with
aromatic nucleophiles leads to the corresponding hexahydronaphthalene systems.24
In contrast to the intramolecular reaction of two alkyne units (see Schemes 1a and 1b), the
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intermolecular reaction is clearly impeded except for the thermal dimerization of alkynes with
electron-withdrawing substituents, which are directly bound to the triple bond.25,23 Here
fluorine and alkoxy groups should be named as electron-withdrawing substituents. In
particular, chloroarylacetylenes were studied intensively (Scheme 1c).26-30 For the
chloroacetylene 10a the experimental and theoretical activation barrier amounts to 22
kcal/mol.29 By using electron-rich aromatic systems, the activation barrier can be reduced to
less than 19 kcal/mol.30 Taking all above mentioned results into account, following question
arises: Can the dimerization of chloroarylacetylenes also be accelerated in analogy to the
Bergman cyclization and the transannular ring closure by using gold(I) as a catalyst?
Exploiting the preparative dimerization of chloroarylacetylenes would be of great benefit as
the thermal dimerization can yet only be utilized for methoxyacetylene: The thermal
dimerization of chloroarylacetelynes via an intermediary formed diradical leads to a variety of
non-separable products.28,30
A glance at the literature shows that haloacetylenes have been employed multiple times in
gold(I)-catalyzed31-32,17 reactions. Hashmi et al. accomplished the head-to-tail coupling of
iodoalkynes via dual gold catalysis33-44 which delivered the gem-enynes 14 (Scheme 2a).45
Besides the iodoalkynes 12 also one example for each bromo- and chloroacetylene was
presented. In either case they isolated the corresponding head-to-tail coupling products 15
and 16, respectively.45 This is analogous to the gold(I)-catalyzed dimerization of terminal
alkynes 17 (Scheme 2b).46-49 The formation of the enynes 18 proceeds via a dual ,
activation.33-44 Both for the alkynes 17 and iodoalkynes 12 the key step in the catalytic cycle
is the formation of a gold acetylide50-52,39 ( activation) starting from an alkyne and a gold
complex. In a following step, the cationic gold complex activates the triple bond of a further
alkyne unit ( complex), which is subsequently attacked by the -activated gold acetylide.45
Recently, two intermolecular alkyne-alkyne couplings via mono gold(I) catalysis leading to
either naphthalenes or azulenes were reported.53-54
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a)
X
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[
Au]+
2
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X = I, Br, Cl
X = I, Br, Cl
12 13 10
14 15 16
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R'
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20
d)
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[Au]+
I
1
2a
21a
Scheme 2. a) Gold(I)-catalyzed dimerization of haloarylacetylenes 10, 12 and 13. b) Gold(I)-
catalyzed dimerization of terminal alkynes 17. c) Gold(I)-catalyzed [2+2] cycloaddition of
chloroarylacetylenes 10 and alkenes 19. d) Gold(I)-catalyzed synthesis of the iodofulvene
2
1a via cyclization of the mono-iodinated diyne 12a.
Zhang et al. used chloroacetylenes for the intermolecular [2+2] cycloaddition between
55
alkynes 10 and non-activated alkenes 19 (Scheme 2c). The first step is the activation of the
chloroacetylene by the cationic gold complex, which then reacts with an alkene via a
cyclopropylmethyl cation to the corresponding cyclobutyl cation. The last step delivers the
desired cyclobutene derivates 20 via desauration.55
An intramolecular dimerization of two alkyne units, whereby one unit is substituted with
iodine, is shown in Scheme 2d.56 This reaction also proceeds via a dual gold catalysis
mechanism which involves the formation of a gold acetylide by direct metal-halogen
exchange of the iodoacetylene 12a with a gold complex. It could be shown that this
cyclization only works with ,-dinuclear propyne–gold acetylides (DAC, dual-activation
39
catalysts ) or one cationic gold complex combined with an organogold compound (e.g.
IPrAuPh) as additive. No reaction occurred without an appropriate organogold compound as
additive.56
To the best of our knowledge, and in contrast to the above-mentioned head-to-tail
dimerization of haloacetylenes and terminal acetylenes, there is no report about the gold-
catalyzed head-to-head dimerization of haloacetylenes. The head-to-head dimerization is
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only known for terminal acetylenes like compound 17 which is dimerized in the presence of
transition metal complexes (e.g. rhodium).57-58 The cyclometalation of two dichloroacetylenes
at a metal center (e.g. Co, Rh, Ir) form the corresponding tetrachlorometallole.59-61 Herein, we
describe our investigation of the gold(I)-catalyzed head-to-head dimerization of
chloroarylacetylenes. By means of quantum chemical calculations we are able to show that
the head-to-head dimerization is not favored with regard to the activation barrier, but can
lead to a highly stable gold(I)-enyne complex via an unusual 1,3-chlorine shift. Further
experimental studies support our thesis and therefore we present the synthesis of trans-1,2-
dichloroenynes, a simple structure which, however, has only been supposed as a byproduct
during the investigation of the thermal oligomerization of chlorophenylacetylenes.26-27
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Results and Discussion
a) Quantum Chemical Model Studies on Gold(I)-Catalyzed Dimerization of Chloro-
phenylacetylene
As model system for the gold(I)-catalyzed dimerization of chloroarylacetylenes we have
chosen chlorophenylacetylene (10a; Scheme 3). This compound is not only the simplest
representative of this class, but it is also well investigated regarding its thermal dimerization
by means of experiments as well as quantum chemical calculations.28-30 This allows us a
direct comparison of both reaction mechanisms (radical vs. Au(I)-catalyzed ionic). For the
quantum chemical studies we assumed the simplest reaction mechanism, namely the mono
gold(I)-catalyzed dimerization. As reference system for the comparison of the energetic
values, the isolated molecules chlorophenylactelyene (10a) and the corresponding 
complex 22a were defined. As ligand for the cationic gold complex the neutral phosphine
+
+
PMe was chosen ([Au] = Au -PMe ).
3
3
For the reaction of the alkyne units 10a and 22a we considered two realistic ways (Scheme
): In the former case, the 1’,1 connection leads to the head-to-head coupling product 24a
3
while the 1’,2 connection delivers the head-to-tail coupling product 26a. In both cases the
positive charge located on carbon atom C2’ is stabilized by a phenyl group. As a result,
cations 24a and 26a should be the energetically most stable intermediates. A connection via
2’,1 or 2’,2 can be ruled out as the positive charge would be located at carbon atom C1’
being stabilized only by a chlorine atom. During our investigation of the radical dimerization
via the intermediary formed diradicals, the aromatic units showed a stronger stabilizing effect
on the radical centers than alkoxy groups or halogen atoms (see 11a in Scheme 1c).29 In
case of the radical dimerization the carbon atom C2’ is sp-hybridized so the aromatic unit is
aligned perpendicular to the butadiene unit.29 As a consequence we expect a similar
geometry for cationic intermediates 24a and 26a.
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head-to-head coupling
',1 dimerization
2'
1
'
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Cl
Cl
Cl 1'
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'
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Au]
[Au]
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head-to-tail coupling
',2 dimerization
1
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2
1
[Au]
[
Au]
25a
Scheme 3. The mono gold(I)-catalyzed dimerization of chlorophenylacetylene 10a can lead
to the head-to-head cationic product 24a (1‘,1 dimerization) or to the head-to-tail product 26a
+
+
(
1‘,2 dimerization). [Au] = Au -PMe .
3
The stationary points of the Au(I)-catalyzed dimerization were optimized using the PBE062-63
method. As basis set 6-311++G(d,p)64-67 has been used for C, H, Cl and P. For Au the def2-
TZVP+ECP68-69 basis set has been employed. This level of theory proved to give the most
reliable results for similar Au(I)-catalyzed reactions.18,70 Frequency calculations were carried
out at each of the structures to verify the nature of the stationary point. It turned out that all
transition states have exactly one imaginary frequency, whereas all other structures have
none. Furthermore, single point calculations by means of MP271 as well as the density
functionals PBE0-D372-73, B3LYP74-76, B3LYP-D372-73 and M06-2X77 were performed on the
PBE0 optimized structures. Here again, as basis set 6-311++G(d,p) has been used for C, H,
Cl and P. For Au the def2-TZVP+ECP basis set has been employed. To determine the
solvent effect on the reactivity, the single point calculations on the PBE0 optimized structures
were also performed using dichloromethane as solvent.
Table 1. Free energies (G in kcal/mol) and energies (E in kcal/mol) of 22a-32a relative to
the starting materials (10a and 22a) calculated by means of different methods. As basis sets
+
+
6
-311++G(d,p)/def2-TZVP+ECP have been used. [Au] = Au -PMe .
3
Ga
Ea
Eb
Ec
Ed
Ee
Ef
Eg
2
7a
8a
31.0
19.7
12.4
24.6
23.0
11.4
15.7
10.4
2
-42.1
-56.5
-61.3
-52.5
-48.3
-55.9
-57.3
-63.8
2
3a
21.3
-3.4
9.2
2.7
15.2
-11.8
-9.8
12.6
-12.0
-9.2
2.6
5.9
-1.7
24a
-17.5
-16.6
-56.3
-22.2
-21.2
-61.3
-19.4
-16.5
-55.6
-12.0
-10.0
-56.5
-14.0
-14.0
-63.6
3
1a
2a
-1.3
3
-41.2
-52.4
-47.7
2
9a
21.5
9.1
1.5
15.2
11.3
-0.5
6.5
1.3
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0a
-7.4
-22.0
-28.3
-16.4
-14.6
-24.3
-16.9
-25.1
25a
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-4.7
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-4.3
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-4.8
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-7.3
26a
-19.2
-25.4
-14.1
-12.1
-21.4
-14.0
-22.0
a
b
c
d
e
P
B
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.
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-
D
3
/
/
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B
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(
d
i
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o
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o
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e
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a
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a
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s
o
l
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e
n
t
)
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/
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B
E
0
.
B
3
L
Y
P
/
/
P
B
E
0
.
f
g
B3LYP-D3//PBE0. M06-2X//PBE0. MP2//PBE0.
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The calculated data are summarized in Table 1 and in Figure 1. A closer look at the possible
reaction pathways reveals two routes for the gold(I)-catalyzed head-to-head coupling (1’,1
dimerization) of chlorophenylacetylene 10a. The reaction can not only proceed via transition
state 23a, but also via transition state 27a (Figure 1). The transition states 23a and 27a differ
in their relative orientation of the gold complex at the carbon atom C2. The same applies for
the head-to-tail coupling (1’,2 dimerization) for which we could locate the stereoisomeric
transition states 29a and 25a (Figure 1).
At first we would like to discuss the results calculated with PBE0/6-311++G(d,p)/def2-
TZVP+ECP. To consider the loss of entropy for a dimerization reaction, the stationary points
for the free energy (G in kcal/mol) are depicted in Figure 1. An analogue figure for the
relative energy (E in kcal/mol) can be found in the supporting information (see Figure S1).
The addition of one molecule 10a to the cationic gold complex leads to  complex 22a
accompanied by a gain in free energy of 28.5 kcal/mol. A comparison of the free energy
values for the transition states 23a and 27a (head-to-head coupling) and transition states 25a
and 29a (head-to-tail coupling) reveals that the former always exhibit higher energy values.
Therefore the 1’,2 dimerization should be favored towards the 1’,1 dimerization, if the free
energies of these four transition states are considered. However, an entirely different picture
emerges by looking at the formed intermediates and their secondary products (Figure 1). The
1’,2 dimerization leads to vinyl cations 26a and 30a which are slightly more stable (G = -4.7
kcal/mol for 26a and -7.4 kcal/mol for 30a) than the starting materials 10a and 22a. The
intermediate 24a, which is formed via the head-to-head coupling is even less stable than the
intermediates 26a and 30a. However, cation 24a can undergo rearrangement via a 1,3-
chlorine shift to form the highly stable enyne complex 32a. The free energy of the transition
state for this rearrangement is only 2.1 kcal/mol higher than that of the cationic intermediate
24a. In case of the 1’1 dimerization via transition state 27a no vinyl cation could be located
on the energy hypersurface. Here only enyne complex 28a, which differs from 32a by the
relative orientation of the gold center, was found as a minimum.
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[
Au]
Cl
Ph
Cl
Ph
Ph
Cl 10a
Cl 10a
[Au]
4
0
0
0
0
Cl
Ph
Cl
Cl
Ph
Ph
27a
Ph
Ph
[Au]
Cl
Ph
Cl
[
Au] 23a
Cl 29a
Ph
2
[
Au]
2
5a
0
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2
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8
9
0
1
2
3
4
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1
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8
9
0
1
2
3
4
5
6
7
8
9
0
3
1a
[
Au]
Cl
[
Au]
Ph
Ph
Ph
Cl 22a
Cl 10a
Cl
[Au]
Ph
Ph
Ph
Cl
Ph
2
[Au]
Cl
Ph
Cl
Cl
24a
30a
26a
40
Ph
Cl
Ph
[
Au]
Cl
Ph
Cl
[
Au]
60
Cl
Ph
32a
28a
Figure 1. Free energy (G) profile for the gold(I)-catalyzed dimerization of two
chlorophenylacetylenes 10a calculated by means of PBE0/6-311++G(d,p)/def2-TZVP+ECP.
+
+
[
Au] = Au -PMe .
3
Evaluating the data calculated via the PBE0 method following predictions can be made: The
cationic intermediates of the 1’,2 dimerization 26a and 30a should be preferably formed and
could be trapped in presence of an appropriate nucleophile. Without a nucleophile in the
reaction mixture these pathways should be considered as reversible. Consequently, the 1’,1
dimerization (head-to-head coupling) proceeding via transition state 23a to the highly stable
product 32a should become the dominant reaction as the reverse reaction to the starting
materials can almost be excluded due to the low energy of the enyne complex 32a (G = -
41.2 kcal/mol). Typically, triple bonds are consumed in gold(I)-catalyzed reactions.
Therefore, the predicted reaction path, in which one triple bond reappears in the product by
elimination of the gold complex, is very rare and only a few exceptions were reported.78-79
Comparing the calculated PBE0 energies to the data obtained by other methods it becomes
obvious that the tendencies for the four reaction pathways (Figure S1) are the same.
However, the energy values strongly vary on the method used (Table 1). The usage of
PBE0-D3, B3LYP-D3, M06-2X and MP2, methods which describe dispersion interactions
more accurate for larger atomic distances, leads to an overall decrease of both transition
states and intermediates compared to PBE0. The additional stabilization amounts up to ca. 9
kcal/mol. On the contrary, employing B3LYP as the density functional or taking solvent
effects into account, the energy of both transition states and intermediates increases. Also
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here, changes can amount up to ca. 9 kcal/mol. In all cases the activation barrier for the
gold(I)-catalyzed dimerization is significantly lower than the values for the radical pendant.29
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Molecular structures of the transition state 23a (left), the intermediate 24a (middle)
and the complex 32a (right) calculated using PBE0/6-311++G(d,p)/def2-TZVP+ECP.
In the following, the stationary points of the 1’,1 dimerization (dominant route, highlighted in
black in Figure 1) will be contemplated. For the transition state 23a both alkyne units are
almost perpendicular to each other (Figure 2). The distance between C1 and C1’ in 23a
amounts to 2.30 Å and is therefore distinctly larger than that calculated for the thermal
dimerization (1.99 Å).29 For the cationic intermediate 24a both phenyl groups are aligned
perpendicular to the butadiene unit (Figure 2). The carbon atom C2’ is almost sp-hybridized,
the C1’-C2’-C(phenyl) angle amounts to 172° accordingly (for numbering of atoms see Figure
3). As expected, the highest positive charge (natural charge = +0.41; see Figure 3a) is
located on this carbon atom (C2’). An additional stabilization of the positive charge occurs
through the adjacent phenyl group, as well as through the interaction with the n orbitals of
p
the chlorine atoms Cl3 and Cl3’. This can easily be explained by looking at the LUMO of 24a
(Figure 3a).
After the 1,3-chlorine shift,  complex 32a adopts an E configuration for the double bond
(Figure 2). This is due to the fact, that the shift of the chlorine atom Cl3 from C1 to C2’ takes
place on the opposite site of the C1’-Cl3’ bond. The energy of the transition state for the 1,3-
chlorine shift (31a) is only slightly higher than the energy of vinyl cationic intermediate 24a
(2.1 kcal/mol for PBE0/6-311++G(d,p)/def2-TZVP+ECP). The natural charges of transition
state 31a reveal that the positive charge of the cationic center C2’ (formerly +0.41 for 24a) is
partially being transferred onto the chlorine atom Cl3 (formerly +0.07 for 24a) as both atoms
adopt a natural charge of about +0.2 (Figure 3b). Furthermore, the LUMO of 31a closely
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3
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resembles the LUMO of 24a, only the interaction between the vacant p orbital and the np
orbital on Cl3 is more distinct (Figure 3b).
a)
+0.136
' Cl
3
+
0.411
2'
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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6
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8
9
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2
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8
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1
2
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4
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7
8
9
0
Ph
-0.222
1
Ph
1
'
-
0.276
-0.034
2
Au
+
Cl
+0.066
3
0.221
Me P
3
24a
b)
+0.146
' Cl
3
+
0.198
2'
Ph
-0.147
Ph
1'
2
1
-
0.251
-0.089
3
Cl
Au
+0.213
+0.227
Me P
3
31a
Figure 3. Natural charges and LUMOs of the cation 24a (a) and transition state 31a (b)
calculated using PBE0/6-311++G(d,p)/def2-TZVP+ECP.
b) Experimental Studies on the Gold(I)-Catalyzed Dimerization of Chloroarylacetylenes
To verify our calculations, we have chosen the alkoxy compound 10b as testing system for
the investigation of a possible gold(I)-catalyzed dimerization reaction to the head-to-head
product 33b (Scheme 4). In order to gather experimental details, the gold(I)-catalyzed
1
conversion of 10b was investigated in an NMR tube and monitored by H NMR spectroscopy.
This allows us to directly determine the conversion of 10b and the yield of possible
dimerization products and by-products (selectivity). Therefore, an internal standard was
added. By using the methoxy group attached to the aromatic system two benefits derive. On
1
the one hand, the yield can easily be determined by the characteristic H NMR signal for the
methyl group (at ca. 4 ppm). On the other hand, the formation of a cationic intermediate is
facilitated by the electron donating effect of the methoxy group; thus, we expect that the
reaction occurs at ambient temperature. Additionally, higher concentrations of the starting
material 10b were employed. As a result, the dimerization should be favored with respect to
all reactions having rates only depending on the concentration of 10b in first order. The
hydration of chloroacetylenes should be mentioned here as a possible side reaction.80 At
higher concentrations this side reaction can be neglected even in non-dried solvents. The
gold(I)-catalyzed reaction of 10b was carried out in dichloroethane and deuterated
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dichloromethane as well as in deuterated benzene. Both chlorinated solvents are inert and
should react, if any, very slowly with an intermediary formed vinyl cation, whereas benzene
24
rapidly reacts with a vinyl cation to the corresponding trapping product. The experiment was
carried out at temperatures between 0 and 80 °C. To obtain the best results (e.g. yields), we
screened several cationic gold complexes with different ligands and counterions under
various reaction conditions (see Table 2).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cl
catalyst, conditions
O
2
O
Cl
O
Cl
10b
33b
Scheme 4. Gold(I)-catalyzed dimerization of chloroacteylene 10b under different conditions.
a
Table 2. Optimization of the reaction conditions for the head-to-head dimerization of 10b.
Entry
catalyst
conditions
yield [%]
1
2
3
4
5
6
7
8
9
[IPrAu(NCMe)]SbF (5 mol%)81
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
DCE, 40 °C, 1 h, 1 M
54b
52b
63b
32b
36b
24b
6
IPrAuCl (5 mol%), AgSbF (10 mol%)82
6
IPrAuCl (5 mol%), AgNTf (10 mol%)
2
IPrAuCl (5 mol%), NaBArF (10 mol%)
24
*
83
[IPr Au(NCMe)]SbF (5 mol%)
6
84
(Ph) PAuCl (5 mol%), AgSbF (10 mol%)
3
6
85
0b,d
9b,d
(OMe) PAuCl (5 mol%), AgSbF (10 mol%)
3
6
Dichloro(2-picolinato)gold(III) (5 mol%)86
87
IPrAuNTf (5 mol%)
DCM-d , rt, 1 h, 1 M
68
71
2
2
10
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , rt, 1 h, 1 M
6
2
11
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , rt, 1 h, 0.015 M
33c
65c
6
2
1
2
3
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , rt, 2 h, 0.25 M
6
2
1
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , rt, 1 h, 0.5 M
62
58
65
61
75
31
66
33
66
69
6
2
14
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , rt, 1 h, 2 M
6
2
15
16
17
[IPrAu(NCMe)]SbF (5 mol%)
DCM-d , 0 °C, 1 h, 1 M
6
2
[IPrAu(NCMe)]SbF (5 mol%)
CDCl , rt, 1 h, 1 M
6
3
[JohnPhosAu(NCMe)]SbF (5 mol%)88
DCM-d , rt, 1 h, 1 M
6
2
18
[JohnPhosAu(NCMe)]SbF (5 mol%)
C D , 80 °C, 16 h, 0.25 M
6
6
6
89
19
20
21
JohnPhosAuNTf (5 mol%)
DCM-d , rt, 1 h, 1 M
2
2
XPhosAu(NCMe)SbF (5 mol%)90
DCM-d , rt, 1 h, 1 M
6
2
91
BrettPhosAuNTf (5 mol%)
DCM-d , rt, 1 h, 1 M
2
2
22
tBuXPhosAuNTf (5 mol%)89
DCM-d , rt, 1 h, 1 M
2
2
a
1
Yield determined by H NMR using hexamethylbenzene as internal standard. b Here
c
d
cylohexane and cyclooctane were used as internal standards. No complete conversion.
At first, experiments with different gold(I) complexes were run at a concentration of one molar
(entry 1-7 in Table 2) at 40 °C. Here, DCE (dichloroethane) was used as the reaction solvent.
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1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
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5
5
5
5
5
5
5
6
AgSbF , AgNTf and NaBArF were employed as halide scavengers for the non-activated
6
2
24
gold(I) complex IPrAuCl. When using N-heterocyclic carbene (NHC) ligand complexes (entry
-5), we were able to identify a dimerization product (for a detailed characterization see
1
below) in a yield around 32-63%. We could not identify any byproduct, as all further reaction
products give a yield of lower than 5% (see Figure 4). Phosphorus-based ligands like a
phospine complex (entry 6) delivers the dimer in a significant lower yield, whereas a
phosphite ligand (entry 7) leads to no dimerization product, despite of an almost complete
conversion of 10b. Also the usage of the gold(III) complex dichloro(2-
pyridinecarboxylato)gold results in a massive drop of the yield (entry 8).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
By changing the solvent from dichloroethane (DCE) to dichloromethane (DCM) the yield
could be increased, if the nitrile-stabilized gold(I) complex [IPrAu(NCMe)]SbF is used (cf.
6
entry 1 and 10). At this point it is worth mentioning that this gold(I)-catalyzed dimerization
already occurred at ambient temperature with full conversion of 10b within one hour (entry
10). The variation of the concentration of 10b has a major impact on the reaction: At lower
concentrations (entry 11) the hydration of 10b becomes the dominant reaction, whereas a
higher concentration (two molar) also results in a decrease in yield (entry 14). The best yields
were obtained within a concentration range from 0.25 to 1.0 molar. Lowering the reaction
temperature to 0 °C leads to a slight decrease in the yield (entry 15). Changing the solvent
from DCM to CHCl has no increasing effect on the yield (entry 16). To our delight we could
3
further improve the yield for the dimerization product to 75% by using the electron-rich nitrile-
-
stabilized JohnPhos ligand with SbF as counteranion (entry 17). Changing the counteranion
6
-
to NTf resulted in a slightly lower yield (entry 19). We then tested bulkier phosphine ligands
2
such as XPhosAu(NCMe)SbF (entry 20), BrettPhosAuNTf (entry 21) and tBuXPhosAuNTf
2
6
2
(
entry 22) which proved to be not superior compared to JohnPhosAu(NCMe)SbF (entry 17).
6
1
The H NMR spectra for the conversion of 10b under these optimized reaction conditions is
1
depicted in Figure 4. The H NMR spectra show a clean conversion of 10b leading to only
one dimerization product with an overall yield of 75% (for integrals see Figure S6 in the
Supporting Information). No further dimerization product with more than 5% yield could be
1
identified. By means of the H NMR signals for both methyl groups at  = 3.8 ppm it becomes
obvious that the main product must be an asymmetric dimer. This would be in accordance
with the calculated proposed structure (see above).
In order to examine if the cationic intermediates 24a, 26a and 30a (Figure 1) can be trapped
in the presence of an appropriate nucleophile, the experiment was also performed with
1
[
JohnPhosAu(NCMe)]SbF in deuterated benzene (entry 18, for H NMR spectra see Figure
6
S7 in the Supporting Information). Apart from many by-products, the asymmetric dimerization
product could be determined with a yield of 31%. This suggests that the cationic
intermediates 26a and 30a are now trapped by benzene. However, we were neither able to
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identify nor able to isolate any benzene trapping products. The cationic intermediate 24a will
#
probably undergo no reaction with benzene since the energy for the 1,3-chlorine shift (G =
2.1 kcal/mol, see Table 1), which leads to the stable enyne complex 32a (see Figure 1), is
extremely low according to our calculations.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CH3
H
Cl
H
hexamethylbenzene
10b
H CO
3
DCM-d2
H
H
H'
H'
OCH3
CH3
Cl
H C
3
H
hexamethylbenzene
Cl
H
33b
H CO
3
DCM-d2
H' H
H'
H
1
Figure 4. H NMR spectra of 10b (1 M) and hexamethylbenzene as internal standard in
dichloromethane-d2 at 600 MHz before (top) and after (bottom) addition of 5 mol%
[JohnPhosAu(NCMe)]SbF6.
For an unambiguous identification of the dimerization product, the experiment was repeated
under the optimized conditions (5 mol% [JohnPhosAu(NCMe)]SbF in dry DCM) on a
6
preparative scale. The preparative reaction delivered the dimerization product in 51% yield.
The reason for the difference between the yield for the isolated compound (51%) and the
yield determined by NMR (75%) is due to the separation by flash column chromatography.
The latter was difficult because of the similar retention of the main product and small
amounts of by-products. By analyzing the 13C NMR spectra (see Supporting Information) of
the isolated compound, we could confirm the formation of an asymmetric dimerization
product, which has one triple bond and one double bond. Nevertheless, a structural
elucidation of the dimerization product was still not possible. For this reason, we reduced the
dimerization product with H /Pd and could isolate the non-separable hydrogenation products
2
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
4b and 35b in an overall yield of 46% (Scheme 5a). The structure of both hydrogenation
products could be confirmed by 2D NMR (see Figure S11). Product 34b can be explained by
simple hydrogenation of the triple bond of 33b. Compound 35b represents the fully
hydrogenated product of 33b where not only both double bond and triple bond were reduced,
but also both chlorine atoms were removed (Scheme 5a). So far, the head-to-head
dimerization product 36b, which would deliver hydrogenation products like 37b and 38b
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Scheme 5b), could not clearly be ruled out. As we could not detect 38b in the isolated
product mixture, the head-to-tail coupling product 36b can now be entirely excluded as the
isolated dimer.
a)
Cl
Cl
O
O
Pd/C, H2
O
O
AcOEt, 48 h
O
Cl
O
Cl
33b
34b (28%)
35b (18%)
b)
O
O
O
Pd/C, H2
AcOEt, 48 h
O
Cl
O
Cl
Cl
Cl
36b
37b
38b
Scheme 5. a) Structural elucidation of the head-to-head dimerization product 33b by
reduction to 34b and 35b. b) For the possible head-to-tail dimerization product, the
compounds 37b and 38b should be expected.
Whether 33b or 34b is the trans or cis alkene could not be determined based on the obtained
data. Fortunately, we were able to grow crystals which were examined by X-ray diffraction.
The molecular structure of 33b in the solid state is shown in Figure 5 and confirms the head-
to-head dimerization product as the trans isomer. These results are in agreement with our
predications made by quantum chemical calculations.
Figure 5. Molecular structure of the head-to-head dimerization product 33b in solid state.
Two independent molecules are found in the unit cell; only one of them is displayed. The
minor component of the disordered part was omitted for clarity (complete asymmetric unit
see Supporting Information). Displacement ellipsoids are drawn at 50% probability level and
hydrogen atoms as spheres of arbitrary radii.
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After the structural elucidation of 33b, we evaluated the scope of the head-to-head
dimerization under the optimized reaction conditions. Therefore, other chloroarylacetylenes
10 were converted into the corresponding head-to-head dimers 33 (Scheme 6). Besides the
alkoxy compounds 10b and 10c, also simpler chloroarylacetylenes like 10a, 10d and 10f
could be dimerized and isolated under these conditions. Changing the substitution pattern
from para (10d) to the ortho position (10f) of the aromatic system had no negative effect on
the reaction yield. Chloroarylacetylenes bearing a larger aromatic backbone like a biphenyl
unit (10c), led to a significant decrease of the reaction yield for 33c. With a naphthylic
backbone, the corresponding dimerization product 33h could only be isolated in 3% yield due
to an unselective reaction of 10h. As might be expected, no reaction occurred when an
aromatic system bearing an electron withdrawing group like CN in para position (10g) was
used, even if the sample was heated up to 50 °C. To our surprise, by substituting chlorine
with bromine, the bromoacetylene 12a could be dimerized to the dimerization product 39a,
though with a significant lower yield compared to the corresponding chlorinated dimer 33a. In
case of 12b, no selective reaction was observed.
1
1
1
1
1
1
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1
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0
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0
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0
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8
9
0
X
[
JohnPhosAu(NCMe)]SbF6
Ar
2 Ar
X
Ar
X
X = Cl, Br
X = Cl, Br
10 12
33 39
Cl
Cl
Cl
Cl
O
O
O
O
Cl
Cl
33a (50%)
33b (51%)
33c
(57%)
Cl
Cl
Cl
Cl
33d (42%)
33e (31%)
Cl
Br
Cl
Br
33f (52%)
39a (36%)
Limitation
NC
Cl
Cl
O
Br
Cl
33h (3%; not selective)
10g (no reaction)
12b (not selective)
Schema 6. Substrate scope for the gold(I)-catalyzed head-to-head dimerization of
haloarylacetylenes 10 and 12.
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We have also tried cross coupling reactions using two different haloarylacetylenes.
Therefore, we mixed one reactive compound (10b) with one non-reactive compound (10g) as
well as two reactive species (10b and 10d). However, no coupling products were obtained in
any of these cases.
Based on the above discussed results we suggest the mechanism for the mono gold(I)-
catalyzed head-to-head dimerization of chloroarylacetylenes shown in Scheme 7a. The first
step is the coordination of the cationic gold complex to the triple bond of chloroarylacetylene
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
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0
1
2
3
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5
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8
9
0
10 to form the cationic  complex 22, which causes the electrophilicity of the coordinated
triple bond. The rate determining step is the following nucleophilic attack of another
chloroarylacetylene molecule 10 on the  complex 22 resulting in the formation of the vinyl
cation 24 as intermediate. The latter rapidly undergoes rearrangement via a 1,3-chlorine shift
due to the low activation barrier to form the stable cationic enyne complex 32 which
corresponds to its cationic vinyl resonance structure. The enyne complex is formed out of the
latter by an elimination process of the gold catalyst. The last step of the catalytic cycle is the
removal of the cationic gold(I) complex (desauration) which releases the trans-1,2-
chloroenyne 33.
a)
Cl
Ar
Cl
b)
Cl
Ar
Cl
Cl
Cl
Ar
Ar
[Au]+
Ar
10
[Au]+
Ar
10
33
33
+
"Cl+"
"
Cl "
Ar
Cl
[Au]+
Cl
Au]+
Cl
[
Cl
Ar
[Au]
[Au]
Ar
Cl
[
Au]
Ar
Ar
[Au]
Ar
39
22
32
Ar
Au]+
[
4
2
Ar
Ar
4
0
Cl
Cl
10
10
Ar
Ar
Ar
[Au]
Cl
Cl
+
Ar
Cl
[Au]+
[
Au]
41
2
4
Scheme 7. Possible reaction mechanisms for the head-to-head dimerization of
chloroarylacetylenes 10 via mono (a) and dual (b) gold catalysis.
IPrAuPh
O
Cl
O
AuIPr
C D , 80 °C, 24 h
6
6
10b
43
Scheme 8. Attempted synthesis of organogold compound 43 monitored by 1H NMR
spectroscopy.
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41
An alternative reaction mechanism can be described via a dual gold catalysis (Scheme 7b).
However, this mechanism can be excluded for the following reasons: If the head-to-head
dimerization proceeds via a dual gold catalysis, the gold acetylide 39 must be formed by a
metal-halogen exchange. Except for iodine56,45, no analogous proof for chlorine can be found
in the literature to the best of our knowledge.92-93 In the case of iodoacetylenes it could be
shown that no free cationic iodine source is involved; the formation of the gold acetylide
proceeds via a stepwise mechanism.56 Therefore, the usage of organogold compounds as
additives (e.g. IPrAuPh) or DAC39 are necessary. To verify if a gold acetylide is involved in
the dimerization of the chloroarylacetylenes 10, our model compound 10b was heated with
IPrAuPh under the same conditions (Scheme 8), which delivered the gold acetylide from the
iodoacetylene 12a (for the structure of 12a see Scheme 2d).56 However, no conversion of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
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9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
0b into the corresponding gold acetylide 43 was observed (Figure S8).
Another issue corroborating the exclusion of a dual gold catalysis mechanism is the reaction
behavior of the vinylgold species 4294 (Scheme 7b). The latter should deliver the dimeric
product 33 after a gold-chlorine exchange. That means, in presence of a proton source this
gold-chlorine exchange would compete with a protodesauration35,95,19. To verify if the
vinylgold species 42 is present in solution, the dimerization was conducted in the protic
1
solvent CD OH. By means of H NMR spectroscopy (see Figure S9) neither the dimerization
3
product 33 nor a corresponding protonated dimerization product was observed. Here, the
80
hydration of the chloroarylacetylene 10b became the dominant reaction.
Conclusion
In conclusion, a gold(I)-catalyzed head-to-head dimerization of chloroarylacetylenes to trans-
1
,2-dichloroenynes is presented. The dimerization reaction proceeds via a completely
different mechanism compared to that which has been described so far for other
haloacetylenes. According to quantum chemical calculations, the head-to-head reaction
pathway competes with the head-to-tail dimerization. However, an unusual 1,3-chlorine shift
to the intermediary formed vinyl cation leads to the exclusive formation of E-enynes (head-to-
head coupling product). The latter are formed by an elimination process of the gold complex
resulting in the reappearance of one triple bond in the product. This is untypical as triple
bonds are normally consumed in gold(I)-catalyzed reactions. Mechanistically, this
investigation opens a new approach on the reactivity of haloacetylenes under gold(I)
catalysis. Whereas so far only metal-halogene exchange has been taken into account for
these systems, the halogene shift leads to a variety of new possible reaction pathways for
intermediary formed vinyl cations. This shift will also be considered for other reactions
involving haloacetylenes. Furthermore, this gold(I)-catalyzed head-to-head dimerization of
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6
chloroarylacetylenes is an attractive method en route to more complex conjugated enyne
systems and their congeners as the halogen pattern allows for potential further
transformations.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
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0
Computational Details
96
All calculations were performed by using the program package Gaussian 16 . The
geometrical parameters of all stationary points were optimized by means of PBE062-63. As
basis set 6-311++G(d,p)64-67 has been used for C, H, Cl and P. For Au the def2-
TZVP+ECP68-69 basis set has been employed. For all stationary points no symmetry
restriction was applied. Frequency calculations were carried out at each of the structures to
verify the nature of the stationary point. It turned out that all transition states have exactly one
imaginary frequency, whereas all other structures have none. Furthermore, the energies of
the stationary points were calculated using the density functionals PBE0-D372-73, B3LYP74-76
,
B3LYP-D372-73 and M06-2X77 as well as MP2 . Here again, as basis set 6-311++G(d,p) has
been used for C, H, Cl and P. For Au the def2-TZVP+ECP basis set has been employed. To
determine the solvent effect, the single point calculations on the PBE0 optimized structures
were also performed using dichloromethane as solvent.
71
Supporting Information
1
Molecular structures, synthesis of the new compounds, H NMR experiments, cartesian
coordinates and absolute energies for all calculated compounds, H NMR and 13C NMR
1
spectra, crystal structure data of 33b and complete reference 92.
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft (DFG; HA 2973/13-1).
The authors would like to thank Prof. Dr. Oliver J. Schmitz, Dr. Sven Meckelmann and Dr.
Lidia Montero for measuring the molecular mass.
Dedication
Dedicated to Professor Professor Julius Rebek, Jr. on the occasion of his 75th birthday.
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Table of Contents graphic:
[
Au]
Ar
Ar
Cl
Cl
Cl
Ar
Ar
[Au]
Cl
Cl
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Ar
[
Au]
Cl
Ar
2 Ar
Cl
Ar
Cl
Ar
Cl
[
Au]
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