Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes

Article abstract of DOI:10.1021/acs.organomet.7b00668

Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene have been prepared and structurally characterized. Their possible role in gold(I)-catalyzed cycloisomerizations has been studied by experiment and by DFT calculations. Gold(I) acetylides are totally unproductive complexes in the absence of Br?nsted acids. Similarly, no cyclizations were observed by heating σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies provide a rationale for the much lower reactivity of digold species in reactions of enynes.

Full text of DOI:10.1021/acs.organomet.7b00668

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes
Sofia Ferrer† and Antonio M. Echavarren*,†,‡
†Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007
Tarragona, Spain
‡Departament de Química Analítica i Química Organ
̀
ica, Universitat Rovira i Virgili, C/Marcel·li Domingo s/n, 43007 Tarragona,
Spain
S
* Supporting Information
ABSTRACT: Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived
from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene
have been prepared and structurally characterized. Their possible role in
gold(I)-catalyzed cycloisomerizations has been studied by experiment and by
DFT calculations. Gold(I) acetylides are totally unproductive complexes in the
absence of Brønsted acids. Similarly, no cyclizations were observed by heating
σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies
provide a rationale for the much lower reactivity of digold species in reactions
of enynes.
which then react with each other in a dual-gold(I)-catalyzed
INTRODUCTION
■
process.31−33
Although digold(I) complexes have been known since the mid-
1970s,1 their relevance in homogeneous gold(I) catalysis has
only been recognized recently.2−4 They also display interesting
The first example of the reaction of alkynes via σ,π-activation
was proposed by Toste and Houk for the cycloisomerization of
1,5-allenynes catalyzed by [(Ph3PAu)3O]BF4 (Scheme 2).34
luminescence properties and are important building elements
for the design of supramolecular structures.5 Alkenyl,
and
6−9
aryl10−12 digold complexes with Au2C three-center−two-
Scheme 2. Proposed Role of gem-Diaurated Species in the
Gold(I)-Catalyzed Cycloisomerization of 1,5-Allenynes
electron bonds have been characterized.13
The vast majority of the gold(I)-catalyzed chemistry of
alkynes under homogeneous conditions can be understood by
the initial π coordination via ligand substitution to form π-
alkyne gold(I) complexes 1, which then react with carbo- or
heteronucleophiles (Scheme 1).14,15 Complexes of this type
Scheme 1. Formation of σ,π-Digold(I) Alkyne Complexes 3
from π-Gold(I) Alkyne Gold(I) Complexes 1
The experimental and computational data for the cyclization of
allenynes such as 4 to form 5 suggested that σ,π-alkyne
digold(I) intermediates 6 initiate a 5-endo-dig cyclization to
form gem-diaurated species 7 and 8, which give rise to 5 by
protodeauration. This and related mechanisms have been
recently reexamined by Fensterbank, Gandon, and Gimbert in
the context of a broader study on the ligand and anion effects in
the cycloisomerizations of allenynes.35
with nonterminal alkynes have been structurally character-
ized.16−18 Alternatively, deprotonation of the terminal alkyne
by the counterion X− can generate σ-alkynyl gold(I) complexes
2. Acetylides 2 react with 1 or with the initial catalyst [AuLL′]X
to form finally σ,π-digold(I) alkyne digold(I) complexes 3,19−21
which are stable species that have been structurally
characterized.22−29 The facile formation of complexes 3 is a
result of the stronger binding of gold(I) to acetylides 2 than to
free alkynes.30
Although similar species have been observed in the
cycloisomerization of 1,6-enynes by mass spectrometry under
Digold complexes 3 are excellent catalysts in reactions of
diynes in which one of the alkynes is a terminal one, by
allowing the simultaneous formation of a nucleophilic σ-alkynyl
gold(I) species and an electrophilic π-alkyne gold(I) species,
Special Issue: Organometallic Chemistry in Europe
Received: September 2, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00668
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Article
electrospray ionization, the real involvement of digold
complexes as intermediates in this reaction has been
questioned.36 Furthermore, a theoretical study on the gold-
(I)-catalyzed hydroamination of alkynes suggested that the
nucleophilic attack takes place on π-alkyne gold(I) complexes
1, rather than σ,π-digold(I) alkyne digold(I) complexes 3.37 On
the other hand, digold complexes of type 3 have been proposed
to be superior catalysts in several gold(I)-catalyzed trans-
formations.22
IPrAuCl (Figure 1). Further reaction of 13 with B or C led to
σ,π-digold(I) alkyne complexes 14a,b. Complexes 15−17 and
We have isolated σ,π-digold(I) alkyne digold(I) complexes 3
in the intermolecular [2 + 2 + 2] cycloaddition of alkynes with
oxoalkenes,24 as well as in the synthesis of cyclobutenes by
intermolecular [2 + 2] cycloaddition of alkynes with alkenes.38
However, these isolated complexes were shown to lay outside
of the main catalytic cycle. Actually, by a change in the
−
counterion of the cationic catalyst from SbF6 to the softer
anion BAr4F−, the formation of digold species could be
minimized,24 which had a positive effect on the overall yields
of the [2 + 2] cycloaddition and other related processes.39
The active role displayed by σ,π-digold(I) alkyne digold(I)
complexes in the gold(I)-catalyzed cycloisomerization of 1,5-
allenynes (Scheme 2)34 is in apparent contradiction with the
proposal that these species are actually “dead ends” in reactions
of cycloisomerizations of enynes and in the cyclobutene
synthesis.38 In order to shed light on the actual role played
by σ,π-digold(I) alkyne complexes (3) in catalytic trans-
formations of 1,n-enynes, we reinvestigated several gold(I)-
catalyzed cycloisomerization reactions by isolating the corre-
sponding gold(I) acetylides and digold complexes.
RESULTS AND DISCUSSION
■
Mechanistic studies of gold(I)-catalyzed cycloisomerizations of
1,n-enynes have been the benchmark for the understanding of
the fundamental reactivity of alkynes and alkenes,40−42 which
have recently been extended to intermolecular systems.39c The
gold(I)-catalyzed reaction of 1,6-enyne 9 at room temperature
leads cleanly to diene 10a by a single cleavage rearrangement
using complex A or B (Scheme 3).40 At longer reaction times,
or using complex C as catalyst, mixtures of 10a and 10b, the
products of double-bond isomerization, were obtained. We also
examined the reaction of 7-ethynyl-1,3,5-cycloheptatriene (11),
which reacts cleanly with A or B to give indene (12).43
Figure 1. Acetylide gold(I) and σ,π-digold(I) alkyne complexes 13−
19 (CHT = cycloheptatriene).
18a,b were similarly prepared from 7-ethynyl-1,3,5-cyclo-
heptatriene (11). We also synthesized gold acetylide complex
19 from 3-methoxy-7-methylocta-5,6-dien-1-yne and IPrAuCl,
although the corresponding digold complex could not be
obtained in pure form by reaction with B or C.
The structures of 13, 14a,b, 15−17, and 18a,b were
confirmed by X-ray diffraction. The structures of the two
representative complexes 15 and 16 are shown in Figure 2. For
the gold(I) acetylide complexes 13, 15, and 17, the C1−Au and
CC bond distances are 1.98−2.01 and 1.18−1.21 Å,
respectively. In the case of σ,π-digold(I) alkyne complexes
(14a,b, 16, and 18a,b) the C1−Au (1.98−2.03 Å) and CC
bond distances (1.21−1.23 Å) are similar. As expected, the
second π-coordinated gold atom is symmetrically bonded to the
alkyne: 2.25/2.20 Å for 14a, 2.21/2.23 Å for 14b, 2.22/2.25 Å
for 16, 2.20/2.20 Å for 18a, and 2.21/2.20 Å for 18b.
Gold(I) acetylide 13 was easily prepared by deprotonation of
1,6-enyne with n-BuLi at −50 °C followed by reaction with
Scheme 3. Cycloisomerizations of 1,6-Enyne 9 and 7-
Ethynyl-1,3,5-cycloheptatriene 11
Mononuclear species 13, 15, and 17 were heated under
refluxing conditions in CDCl3. Gold(I) acetylide 13 remained
unchanged, whereas 15 and 17 showed partial decomposition,
although indene (12) could not be detected after 2 h. These
results definitively confirm that gold(I) acetylides are not
intermediates in these cycloisomerizations.
Digold complexes 14a,b, 16, and 18a,b were heated from 25
to 130 °C in CDCl2CDCl2,44a and any reaction progress was
1
monitored by H, 31P, and 19F NMR. Complexes 14a and 18a
−
with SbF6 as the counterion remained unchanged up to 130
°C. Complexes 14b, 16, and 18b bearing the weakly
F‑
coordinating counterion BAr4 were stable up to 100 °C. At
B
DOI: 10.1021/acs.organomet.7b00668
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Article
F
heterolytically cleave the C−B bond of BAr4 , leading to the
aryl transfer from boron to gold.46
The presence of cyclization products 10a and 12 in the
thermolysis of 14b and 16 suggests two possible pathways. One
F−
is the aryl transfer from the BAr4 to σ-coordinated gold to
form LAuAr complexes together with alkynyl borates, which
undergo cyclization promoted by the second, π-coordinated
gold center. Hydrolysis of the C−B bonds could take place at
the enynes or the final products by reaction with water present
in the solvent. The other involves first a protodeauration of the
σ-coordinated gold with the water present in the solvent,
leading to the corresponding alkyne species and subsequent
F−
aryl transfer from the BAr4 to this gold to form LAuAr
complexes. The alkynes could then undergo cyclization
promoted by the second π-coordinated gold center. Similar
results were obtained when digold complexes 14b and 16 were
44b
heated in CDCl3 under refluxing conditions for 1 h.
We also studied the catalytic activity of the isolated gold(I)
acetylide and digold complexes in the cyclizations of substrates
9 and 11 (Tables 1 and 2). Gold(I) acetylide 13 was not
Table 1. Cycloisomerization of 1,6-Enyne 9 with Complexes
13a,b and 14a,b
[Au]
additive
(amt (mol %))
time
(h)
10a:10b yield
a
entry (amt (mol %))
(%)
1
2
3
4
5
6
7
8
13 (5)
24
8
Figure 2. ORTEP plots (50% thermal ellipsoids) for gold(I) acetylide
15 and σ,π-alkyne digold(I) Complex 16. The SbF6 anion of 16 is
omitted for clarity.
−
13 (5)
HSbF6·6H2O (5)
21:34
b
14a (2.5)
14a (2.5)
14a (2.5)
14a (5)
14b (3)
14b (5)
1
11:0
b
2
15:0
24
8
79:0
this temperature, a slow conversion of the digold species into
new gold complexes together with cyclization products were
observed. In the case of 14b at 130 °C, a mixture of the starting
complex and two new gold species were formed in a 1:1.2:1.5
ratio, together with cyclization product 10a, which was
obtained in 58% yield (1H NMR yield, using mesitylene as
internal standard). From the reaction mixture, we obtained a
crystalline compound, whose structure was determined by X-
ray diffraction analysis as the symmetrical gold(I) complex 20
(Figure 3).45
HSbF6·6H2O (5)
HSbF6·6H2O (5)
23:33
51:0
24
8
21:34
a
Yields determined by 1H NMR using mesitylene as internal standard.
An 84−87% of enyne 1 was recovered.
b
catalytically active (Table 1, entry 1), although addition of
HSbF6·6H2O, which cleaves the Au−C bond, generates a
catalytically active cationic gold(I) species (Table 1, entry
2).3 24 On the other hand, digold complexes 14a, were
b,
Table 2. Cycloisomerization of Cycloheptatriene 11 with
Complexes 17 and 18a,b
Figure 3. Complexes 20 and 21.
[Au]
(amt (mol %))
Aadditive
(amt (mol %))
time
(h)
12 yield
a
entry
(%)
1
2
3
4
5
6
7
17 (5)
24
8
17 (5)
HSbF6·6H2O (5)
61
40
46
50
37
48
In the case of 16, a 1:8 ratio between starting digold complex
and a new gold species was observed at 130 °C, together with
indene (12), which was formed in 40% NMR yield. The major
product was determined by X-ray diffraction to be complex 21,
arising by transmetalation from the tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate (BAr4F‑) counterion to the
gold cation. In fact, gold(I) is known to be able to
18a (2)
18a (3)
18a (5)
18b (2)
18b (5)
24
24
8
HSbF6·6H2O (5)
HSbF6·6H2O (5)
24
8
a
Yields determined by 1H NMR using mesitylene as internal standard.
C
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Article
moderately active in the cycloisomerization of 9. However,
even under the best conditions (Table 1, entry 5), the reaction
required 24 h to furnish 10a in 79% yield, while catalyst A or B
provided 10a in higher yields (93−98%) after just 1 h.
Scheme 4. Cycloisomerizations of Monogold (Ia−c) and
a b
,
Digold 1,6-Enyne (IVa−c) Complexes
Very similar results were obtained in the reaction of 11 to
give indene (12) (Table 2). Again, gold(I) acetylide 17 was
catalytically inactive (Table 2, entry 1), whereas in the presence
of a strong Brønsted acid (Table 2, entry 2) or using digold
complexes 18a,b (Table 2, entries 3−7), indene (12) was
obtained. Nevertheless, all of these reactions are slower than
those catalyzed by A or B (1.5 h, 25 °C, Scheme 3).
We also performed experiments by first mixing 1,6-enyne 9
with 3 mol % of gold acetylide 13 in CD2Cl2, followed by
addition of 3 mol % of catalyst B (Table 3). After 15 min, only
Table 3. Competition Studies between 1,6-Enyne 9 and Gold
Acetylide 13
a
entry
amt of B (mol %)
time
10a yield (%)
1
2
3
3
3
6
15 min
20 h
trace
b
45
15 min
95
a
b
Free energies in kcal/mol. The energy of TSIVc‑VIc was calculated by
a
Yields determined by 1H NMR using mesitylene as internal standard.
A 51% yield of enyne 9 was recovered.
freezing the following distance: d(C8−C55).48
b
incompetent in the cycloisomerization process. Gold acetylides
VIIa−c can only evolve by an ene reaction, via 1,5-hydrogen
transfer trough TSVII‑VIII, leading to VIIIa−c (Scheme 4).
Nevertheless, this transformation shows prohibitively high
energy barriers (ca. 38 kcal/mol).
Calculations also show higher energy barriers for the reaction
of digold complexes XIIa−c in comparison with the reaction of
monogold species IXa−c (Scheme 5). These reactions actually
proceed by cycloisomerization of the norcaradiene complexes
Xa−c, which are in tautomeric equilibrium with the cyclo-
heptatrienes.
The same reactivity pattern is reproduced in the reaction of
1,5-allenynes (Scheme 6).34,35 Thus, much higher barriers were
found in the first step of the reaction through digold complexes
XVIIIa−c in comparison to that initiated by the cyclization of
XVa−c to XVIa−c. However, in this case, the second step
actually makes the digold pathway more favorable, in
traces of product 10a together with digold complex 18a were
formed (Table 3, entry 1), suggesting a higher affinity of the
gold(I) for the gold acetylide species, in accordance with the
precedents.30,36 After 20 h (Table 3, entry 2) product 10a was
formed in 45% yield. At this stage, addition of an extra 3 mol %
of catalyst B led to a very fast conversion, affording 10a in 95%
yield (Table 3, entry 3). In the case of ethynylcycloheptatriene
(11), after addition of 3 mol % of gold acetylide 17 and 3 mol
% of catalyst B, 12 was obtained in 78% yield, along with digold
complex 18a, which suggests that the ligand exchange between
the catalyst B and 11 is faster than that between catalyst B and
gold acetylide 17.
Computational Results. DFT calculations (M06, 6-
31G(d) (C, H, P, N) and SDD (Au), CH2Cl2) were performed
in order to compare the energy barriers for the previous
gold(I)-catalyzed cycloisomerizations.47 In all cases, the
calculations were performed with three different ligands: L1 =
trimethylphosphine, L2 = triphenylphosphine, and L3 = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
Scheme 5. Cycloisomerizations of Monogold (IXa−c) and
a
Digold Cycloheptatriene (XIIa−c) Complexes
The gold(I)-catalyzed cycloisomerization of 1,6-enynes can
proceed by two pathways, leading to five- or six-membered
rings.14,40,41 The 5-exo-dig cyclization of Ia−c leads to
intermediates IIa−c, whereas IIIa−c are formed in a 6-endo-
dig process that is less favorable kinetically (Scheme 4). In most
cases slightly higher energy barriers were observed for the
cyclization with the more electron donating carbene ligand L3,
which reduces the electrophilicity of AuL+.
The cyclization in digold complexes IVa−c has a
considerably higher energy barrier (5-exo-dig, L1 22.3 kcal/
mol, L2 20.1 kcal/mol, L3 22.1 kcal/mol; 6-endo-dig, L1 23.3
kcal/mol, L2 21.5 kcal/mol, L3 33.4 kcal/mol), leading to gem-
diaurated species Va−c and VIa−c in thermodynamically
unfavorable reactions. These results are consistent with the
experimental data and clearly show that digold species are
a
Free energies in kcal/mol.
D
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Article
of enynes, being less efficient than cationic complexes bearing
nitriles as weakly coordinating ligands.
Scheme 6. Cycloisomerizations of Monogold (XVa−c) and
ab
,
Digold 1,5-Allenyne (XVIIIa−c) Complexes
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00668.
All procedures, characterization data for new compounds,
and full details on the theoretical calculations (PDF)
Cartesian coordinates for the calculated structures (XYZ)
Accession Codes
CCDC 1572064−1572073 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*aechavarren@iciq.es.
■
a
b
Free energies in kcal/mol. The energy of TSXVIIIc‑XIXc was calculated
by freezing the following distances: d(C1−C79) and d(C1−C80).49
ORCID
Antonio M. Echavarren: 0000-0001-6808-3007
Notes
agreement with a previous theoretical study employing PH3 as
ligand.34 Thus, the second step for the monogold pathway
(XVIa−c to XVIIa−c) has much higher barriers (28−29 kcal/
mol) in comparison to that of the digold pathway (XIXa−c to
XXa−c), which only requires from around 5 to 10 kcal/mol. In
this case aurophilic interactions are observed that could lower
the energy of the transition states. Therefore, since both XVa−
c and XVIIIa−c would be in equilibrium, the system will evolve
almost exclusively through digold species XIXa−c and XXa−c.
Products XXa−c possess strong aurophilic interactions and
thus displace the reaction toward the products.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the European Research Council (Advanced Grant
No. 321066), Agencia Estatal de Investigacion
■
́
(AEI)/FEDER,
UE (CTQ2016-75960-P), AEI-Severo Ochoa Excellence Acre-
ditation 2014−2018 (SEV-2013-0319), the AGAUR (2014
SGR 818), and CERCA Program/Generalitat de Catalunya for
financial support. We thank the ICIQ X-ray diffraction unit. We
also thank Yahui Wang for preliminary experiments on the
synthesis of 17 and 18 and M. Elena de Orbe for helpful
discussions.
CONCLUSIONS
■
We have prepared gold(I) acetylides and σ,π-digold(I) alkyne
digold(I) complexes from substrates that undergo isomer-
ization reactions under mild conditions in the presence of
gold(I) catalysts. Gold(I) acetylides are unproductive com-
plexes in cycloisomerization reactions, unless a strong Brønsted
acid is added to cleave the Au−C bond leading to a cationic
gold(I) complex. Well-characterized digold(I) complexes are
very robust and fail to cyclize on heating in solution up to 100−
130 °C. These results confirm that these digold(I) species are
not catalytic intermediates; rather, these complexes are “dead
ends” in catalytic reactions of enynes, in full agreement with
previous studies.36,38 Only in the case of cationic digold(I)
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F
DOI: 10.1021/acs.organomet.7b00668
Organometallics XXXX, XXX, XXX−XXX