Expanded-ring N-heterocyclic carbenes efficiently stabilize gold(I) cations, leading to high activity in π-acid-catalyzed cyclizations

Article abstract of DOI:10.1002/chem.201303760

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X ^-=BF₄^-, NTf₂^-, OTf ^-) were generated in solution. According to their ¹³C NMR spectra, the ionic character of the complexes increases in the order X ^-=Cl^-2^--4^-. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er-NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au⁺] cations. A comparative study has been carried out of the catalytic activities of five-, six-, and seven-membered carbene complexes [(NHC)AuX], [(Ph₃P)AuX], [(Me₂S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF₄ (1,3-bis(2,6-diisopropylphenyl) -3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol% catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations. What a difference a ring makes: Expanded-ring N-heterocyclic carbenes (er-NHC) surpass their five-membered ring counterparts, as well as Ph₃P and Me₂S, in the stabilization of cationic gold(I) species due to increased steric protection and electron-donating properties. A monoligand cationic [(THD-Dipp)Au]BF ₄ (Dipp=2,6-diisopropylphenyl) complex bearing a seven-membered ring carbene and bulky Dipp substituents exhibits unprecedentedly high catalytic activity in indole and benzofuran synthesis (see scheme).

Full text of DOI:10.1002/chem.201303760

DOI: 10.1002/chem.201303760
Full Paper
&
Intramolecular Hydroamination
Expanded-Ring N-Heterocyclic Carbenes Efficiently Stabilize
Gold(I) Cations, Leading to High Activity in p-Acid-Catalyzed
Cyclizations
[
a]
[a]
[a]
[a]
Oleg S. Morozov, Andrey V. Lunchev, Alexander A. Bush, Aleksandr A. Tukov,
[a]
[b]
[c]
[c]
Andrey F. Asachenko, Victor N. Khrustalev, Sergey S. Zalesskiy, Valentine P. Ananikov,
and Mikhail S. Nechaev*
[a, d]
Abstract: A series of six- and seven-membered expanded-
ring N-heterocyclic carbene (er-NHC) gold(I) complexes has
been synthesized using different synthetic approaches. Com-
membered carbene complexes [(NHC)AuX], [(Ph P)AuX],
3
[(Me S)AuX], and inorganic compounds of gold in model re-
2
actions of indole and benzofuran synthesis. It was found
that increased ionic character of the complexes was correlat-
ed with increased catalytic activity in the cyclization reac-
tions. As a result, we developed an unprecedentedly active
ꢀ
plexes with weakly coordinating anions [(er-NHC)AuX] (X =
ꢀ
ꢀ
ꢀ
BF4 , NTf , OTf ) were generated in solution. According to
2
13
their C NMR spectra, the ionic character of the complexes
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
increases in the order X =Cl <NTf <OTf <BF . Addi-
monoligand cationic [(THD-Dipp)Au]BF (1,3-bis(2,6-diisopro-
2
4
4
tional factors for stabilization of the cationic complexes are
expansion of the NHC ring and the attachment of bulky sub-
stituents at the nitrogen atoms. These er-NHCs are bulkier li-
gands and stronger electron donors than conventional NHCs
as well as phosphines and sulfides and provide more stabili-
pylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetra-
fluoroborate) catalyst bearing seven-membered-ring carbene
and bulky Dipp substituents. Quantitative yields of cyclized
products were attained in several minutes at room tempera-
ture at 1 mol% catalyst loadings. The experimental observa-
tions were rationalized and fully supported by DFT calcula-
tions.
+
zation of [(L)Au ] cations. A comparative study has been car-
ried out of the catalytic activities of five-, six-, and seven-
[
1]
Introduction
catalysis. Both inorganic gold salts and organic gold coordi-
nation complexes have been extensively used in various organ-
[2]
Over the past decade, a burgeoning area of transition metal-
mediated transformations has centered on the use of gold in
ic transformations. Efficient activation of multiple carbon-
+
carbon bonds is due to the unique p-acidic properties of LAu
[
3]
species. Recently, it was found that gold can form stable co-
valent as well as cationic complexes with N-heterocyclic car-
[
a] O. S. Morozov, A. V. Lunchev, A. A. Bush, A. A. Tukov, A. F. Asachenko,
M. S. Nechaev
A. V. Topchiev Institute of Petrochemical Synthesis
Russian Academy of Sciences
[4]
bene (NHC) ligands. The strong s-donating properties of N-
heterocyclic carbenes make them effective stabilizing ligands
in organometallic chemistry as well as important ligands in cat-
Leninsky Prosp., 29, Moscow, 119991 (Russian Federation)
E-mail: tips@ips.ac.ru
alytic applications. To date, research has largely focused on
[4b,5]
five-membered-ring carbene gold complexes.
Examples of
[
b] V. N. Khrustalev
“
expanded-ring”, six- and seven-membered-ring carbene (er-
A. N. Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
Vavilov Str., 28, Moscow, 119991 (Russian Federation)
[6]
NHC) complexes have also been reported. Utilization of er-
NHCs in the stabilization of gold cations seems promising due
to their enhanced donor and steric properties compared to
[
c] S. S. Zalesskiy, V. P. Ananikov
Zelinsky Institute of Organic Chemistry
Russian Academy of Sciences
[6b]
five-membered-ring NHCs.
However, their properties have
been little studied. In this work, we report synthetic ap-
proaches to er-NHC gold(I) covalent and cationic complexes,
and studies of their catalytic activities in heterocyclization reac-
tions. The factors governing covalent versus cationic nature
and the catalytic activity of the complexes have been delineat-
ed. The experimental observations have been rationalized by
DFT calculations.
Leninsky Prospect 47, Moscow, 119991 (Russian Federation)
[
d] M. S. Nechaev
Department of Chemistry
M. V. Lomonosov Moscow State University
Leninskie Gory 1/3, Moscow, 119991 (Russian Federation)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303760 and contains the preparation of
1
13
starting materials and H and C NMR spectra for all compounds. Main
geometrical parameters and crystallographic data for [(THD-Mes)AuCl] and
[(THD-Dipp)AuCl] are also included.
Chem. Eur. J. 2014, 20, 1 – 10
1
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The second approach involves interaction of a free carbene
with a gold(I) complex (Method B). Free carbenes can be ob-
tained from the corresponding amidinium tetrafluoroborates
by deprotonation with sodium bis(trimethylsilyl)amide and
Results and Discussion
Synthesis of the complexes
[10]
To date, three basic approaches have been devised for the syn-
thesis of five-membered N-heterocyclic carbene gold(I) com-
plexes: i) interaction of amidinium salts with gold(III) species
subsequent recrystallization from diethyl ether. Addition of
chloro(dimethylsulfide) gold(I) to a solution of a free carbene
gives the corresponding gold(I) complex in good yield after
simple work-up (Table 1, Method B). For less sterically bulky
[(THP-Mes)AuCl] and [(THD-Mes)AuCl] complexes, however, the
[
7]
(
Scheme 1, Method A), ii) interaction of free or in situ generat-
[8]
ed carbenes with chloro(dimethylsulfide) gold(I) (Method B),
[
8]
[9]
and iii) transmetalation of silver(I) or copper(I) NHC com-
plexes (Methods C and D). We implemented all three ap-
proaches in the synthesis of 1,3-bis(aryl)-3,4,5,6-tetrahydropyri-
midin-2-ylidene (THP-Ar) and 1,3-bis(aryl)-3,4,5,6-tetrahydrodi-
azepin-2-ylidene (THD-Ar) er-NHC gold(I) complexes.
formation of bis-carbene complexes [(er-NHC) Au]Cl as side
2
products was observed. Recently, Nolan reported a similar
complication for analogous gold(I) complexes of five-mem-
[8]
bered-ring carbenes. Nevertheless, we have been able to iso-
late the target mono-carbene complexes by column chroma-
tography with no significant loss
in yields.
An alternative approach can
be used to solve the problem of
selectivity in the synthesis of
these complexes (Method C). Re-
action of equimolar amounts of
[
(Me S)AuCl] and [(er-NHC)AgBr]
2
affords only mono-carbene com-
plexes [(er-NHC)AuCl]. For less
bulky Mes-substituted deriva-
tives, this method gives higher
yields than interaction of a gold
precursor with a free carbene
(
Method B). For more bulky
Dipp-substituted derivatives,
however, yields are only around
3
0% (Table 1).
Scheme 1. Synthetic approaches to [(er-NHC)AuCl].
Recently, we reported on er-
I
NHC migration from Ag to give
[
7]
I
[11]
Recently, Zhu et al. described a direct approach for the syn-
thesis of NHC gold(I) complexes from imidazolium salts and
commercially available gold(III) salts in the presence of 3-chlor-
opyridine and K CO (Method A). Under similar conditions, we
Cu complexes. We have since found that [(er-NHC)CuBr] can
be transmetalated using [(Me S)AuCl] to give the correspond-
2
ing mono-carbene gold complexes [(er-NHC)AuCl] (Method D).
Analogously to Methods A and C, less bulky Mes-substituted
derivatives were obtained in good yields, whereas more bulky
Dipp-substituted derivatives were obtained in very low yields
or not at all (Table 1). Mes-substituted complexes [(er-
NHC)CuBr] were obtained by a modified procedure, that is, in-
teraction of [(er-NHC)AgBr] with two equivalents of CuBr in re-
fluxing dichloromethane under aerobic conditions.
2
3
obtained complexes bearing mesityl (Mes)-substituted car-
benes [(THP-Mes)AuCl] and [(THD-Mes)AuCl] in good yields
(Table 1, Method A). For more bulky Dipp-substituted carbenes,
however, this method gives little or none of the desired prod-
uct. Interestingly, if amidinium tetrafluoroborates are used in-
stead of the corresponding chlorides, the formation of gold
complexes is not observed.
All of the obtained er-NHC gold(I) complexes were charac-
1
13
[6b]
terized by H and C NMR.
Crystals suitable for X-ray crystal-
lography were obtained for
Table 1. Comparison of synthetic approaches to [(er-NHC)AuCl] complexes.
[
(THD-Mes)AuCl] and [(THD-Dip-
[
a]
Method
[(THP-Mes)AuCl] [%]
[(THP-Dipp)AuCl] [%]
[(THD-Mes)AuCl] [%]
[(THD-Dipp)AuCl] [%]
p)AuCl] by crystallization from
a mixture of CH Cl and hexane
2
2
A
B
C
D
68
67
78
60
0
75
32
0
72
71
83
41
14
82
30
5
at low temperature (<08C) (crys-
tallographic data are presented
in the Supporting Information).
Our findings indicated that
a more general approach for the
synthesis of [(er-NHC)AuCl] com-
[
a] General conditions for A): KAuCl
at 1108C for 30 h; B): free er-NHC, (Me
1.0 equiv), CHCl , at r.t. for 4–20 h; D): [(er-NHC)CuBr], (Me
4
, amidinium salt (1.07 equivs), K
2
CO
3
(5 equivs), 3-chloropyridine (40 equivs)
S)AuCl
2
S)AuCl (1.0 equiv), THF, at r.t. for 1–6 h; C): [(er-NHC)AgCl], (Me
2
(
3
2
2 2
S)AuCl (1.0 equiv), CH Cl , at r.t. for 6–30 h.
&
&
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plexes is a straightforward reaction of equimolar amounts of
[
(Me S)AuCl] and free er-NHCs (Method B). Transmetalation of
2
[(er-NHC)AgBr] complexes is the best choice for the prepara-
tion of complexes bearing less bulky Mes-substituted er-NHCs
(Method C).
Three complexes with weakly coordinating anions [(THD-
ꢀ
ꢀ
ꢀ
Dipp)AuX] (X=OTf , BF4 , NTf ) were prepared in good yields
2
by salt metathesis with AgX. It was previously supposed that
ꢀ
the use of NTf2 is preferable to that of the other anions due
Scheme 2. Cyclization reactions mediated by gold catalysts.
ꢀ
to the instability of cationic complexes of gold with BF and
4
ꢀ
[12]
OTf . In the case of [(THD-Dipp)AuX], all three complexes
are stable upon storage. Unfortunately, we were unable to
obtain single crystals of these complexes suitable for X-ray
crystal structure analyses. However, we were able to determine
Initial catalytic tests were carried out at room temperature in
ethanol with catalyst loadings of 2 mol%. The progress of the
reactions was monitored by thin-layer chromatography (n-
hexane/ethyl acetate, 5:1). Upon completion of the reaction or
after 48 h, the reaction mixture was quenched by passage
through silica gel to remove the catalytic species and analyzed
13
the ionic character of the complexes in solution by C NMR
analysis.
Protic acids are arranged in the following order of acid
strengths in chlorinated hydrocarbons: HNTf @TfOH>HBF @
1
by H NMR spectroscopy.
2
4
[
13]
[14]
HCl.
However, Redel et al.
reported that anions can be
It is well established that the catalytic activity of gold species
[3]
ranked according to their binding energies to the gold cation
is due to their p-acidic nature. To increase the acidity, gold
complexes are converted into cationic forms with appropriate
activators bearing weakly coordinating anions.
ꢀ
ꢀ
ꢀ
in the following order: BF4 <OTf !Cl . It is still unclear as to
ꢀ
where NTf2 should be placed in this series. Arduengo et al.
13
showed that the C NMR chemical shifts of the ylidene car-
bons in NHC complexes are between the values for amidinium
Catalytic tests were performed in the presence of an equi-
molar amount of AgOTf as an activator. The results are sum-
marized in Table 2.
[
15]
salts and free carbenes.
Herrmann et al. found that the
C NMR chemical shifts of ylidene carbon atoms in NHC-metal
complexes are shifted upfield with increasing Lewis acidity of
13
[
16]
13
the metal fragment.
Table 2. C NMR chemical shifts of the carbene carbon atom of gold
complexes.
We found that the highest upfield shift of the ylidene
carbon signal was seen for [(THD-Dipp)AuBF ] (d=179.8 ppm;
4
Complex
X
Table 3), and that the lowest shift was seen for [(THD-Dip-
p)AuCl] (d=201.8 ppm). In carbene gold (pseudo)halide sys-
tems, an increase in the s-donor ability of the (pseudo)halide
causes a decrease in the Lewis acidity of the metal, resulting in
Cl
NTf2
OTf
BF4
[
(SIPr)Au]X
196.1
193.0
201.3
–
–
184.5
181.4
189.3
–
–
[(THP-Dipp)Au]X
[(THD-Dipp)Au]X
194.3
179.8
13
[17]
an increased C chemical shift of the ylidene carbon. Thus,
it can be argued that the chemical shift of the ylidene carbon
atom indicates the degree of cationic character of the
The catalytic activities of the studied complexes were found
to vary over a wide range. We obtained virtually no yield after
48 h in the case of non-activated phosphane or sulfide com-
plexes. However, activated er-NHC supported catalysts led to
complete conversion within a few minutes. In the case of com-
plexes bearing Ph P, Me S, IMes, and SIMes ligands, violet colo-
[(NHC)AuX] complex.
Consequently, for [(THD-Dipp)AuX] the degree of dissocia-
ꢀ
tion giving gold cations increases in the order: Cl<NTf
<
2
ꢀ
ꢀ
OTf <BF4
.
3
2
ration of the reaction mixtures was observed, indicating the
formation of colloidal gold. This led to deactivation of the cata-
lysts and significant decreases in yields.
Catalytic studies
To reveal the influence of expansion of the ring in er-NHCs on
the catalytic activity in cyclization reactions, we performed
comparative catalytic studies. The [(er-NHC)AuCl] systems were
compared with five-membered-ring carbene complexes, as
well as conventional KAuCl , (Ph P)AuCl, and (Me S)AuCl cata-
According to the data from the catalytic tests, several trends
in activity of the studied complexes can be discerned: i) Activa-
tion with AgOTf drastically increases (by one or two orders of
magnitude) the catalytic activity in virtually all cases. This is
consistent with the cationic nature of the catalytically active
species. ii) When activated, gold(III) surpasses simple phosphine
and sulfide complexes, but is inferior to Dipp-substituted NHC-
supported gold(I) complexes. iii) NHC-supported complexes are
more active than their SMe and PPh counterparts. iv) Expan-
4
3
2
lysts. Catalytic tests were performed on three model reactions:
heterocyclizations of 2-(phenylethynyl)aniline (1a), N-methyl-2-
(
phenylethynyl)aniline (1b), and 2-(phenylethynyl)phenol (1c)
to give the corresponding indoles (2a,b) and benzofuran (2c)
Scheme 2). The reaction conditions were similar to those re-
2
3
(
sion of the ring in NHCs leads to increased catalytic activity.
This factor plays a dual role. Firstly, expansion of the ring leads
to an increase in NHC donor strength, which favors thermody-
[
18]
ported by Arcadi for the cyclization of 2-(phenylethynyl)ani-
line in ethanol in the presence of sodium tetrachloroaurate.
Chem. Eur. J. 2014, 20, 1 – 10
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ing indole in less than 10 min at 303 K (see the Sup-
porting Information and Figure 1). Therefore, sub-
strate 1b was chosen for further kinetic studies.
According to the NMR spectra, the conversion of
Table 3. Comparison of catalytic activities of gold compounds (total conversion times
are given in minutes; if total conversion was not reached, NMR yields are given in pa-
rentheses).
[
a]
[b]
No activator
Activated by AgOTf
1
b into 2b catalyzed by [(THD-Dipp)AuOTf] proceed-
Complex
X=NH ([%]) NMe ([%]) O ([%]) NH ([%]) NMe ([%]) O ([%])
ed cleanly. No signals corresponding to side products
were detected. Complete conversion of 1b was ach-
ieved in around 8 min. The reaction mixture did not
change color and remained homogeneous; no visible
precipitate was formed, revealing high stability of the
catalytically active gold cationic species under the re-
action conditions.
(
(
(
(
(
(
[
[
[
[
Ph
Me
IMes)AuCl
3
P)AuCl
– (<3)
– (<3)
– (45)
– (67)
– (<3)
2460
– (10)
– (12)
1170
1020
– (60)
250
960
100
60
45
2880
1440
– (30)
1446
1860
– (50)
– (60)
– (87)
2400
2580
420
– (53)
1400
250
134
660
50
137
18
137
15
– (80)
– (47)
25
20
60
10
12
11
10
– (<3)
– (30)
350
15
400
8
18
9
8
6
2
S)AuCl
[
c]
[c]
IPr)AuCl
SIMes)AuCl
[
c]
[c]
SIPr)AuCl
(THP-Mes)AuCl]
– (70)
(THP-Dipp)AuCl] 2460
(THD-Mes)AuCl] 1060
(THD-Dipp)AuCl] 960
1200
To examine the counterion effect, we monitored
the catalytic reactions of three complexes [(THD-Dip-
8
200
KAuCl
4
60
66
60
ꢀ
ꢀ
ꢀ
p)AuX] bearing X=OTf , BF4 , and NTf2 weakly co-
ordinating anions. Kinetic curves for 1b ! 2b con-
version are presented in Figure 1 (for details, see the
Supporting Information).
1
[
a] Maximum reaction time 48 h; conversion based on H NMR spectroscopy. [b] Maxi-
1
mum reaction time 24 h; conversion based on H NMR spectroscopy. [c] Abbrevia-
tions: N,N’-bis(2,6-diisopropylphenyl) imidazol-2-ylidene (IPr), N,N’-bis(2,4,6-trimethyl-
phenyl) imidazol-2-ylidene (IMes), N,N’-bis(2,6-diisopropylphenyl) 4,5-dihydroimidazol-
2-ylidene (SIPr), N,N’-bis(2,4,6-trimethylphenyl) 4,5-dihydroimidazol-2-ylidene (SIMes).
It was revealed that the counterion has a strong
effect on the catalytic activity of [(THD-Dipp)AuX]
complexes. [(THD-Dipp)AuBF ] gave a higher reaction
4
rate than [(THD-Dipp)AuOTf]. Complete conversion
namic stabilization of the gold cation. Secondly, expansion of
was achieved in around 6.5 min. Changing the counterion to
[
19]
ꢀ
the ring leads to an increase in steric stabilization. v) Increase
in the steric bulk of the NHC ligand on going from Mes to
Dipp leads to a marked increase in catalytic activity. vi) The
effect of increased steric bulk is more pronounced than the
effect of increased donor properties of the ligands.
NTf2 resulted in a dramatic loss of activity. Only 60% conver-
sion was observed after 30 min.
Thus, we found that all factors leading to thermodynamic
and kinetic (steric) stabilization of gold cations led to increased
catalytic activity of the gold(I) complexes. [(THD-Dipp)AuCl]
was found to be the most active precatalyst in our study. The
formation of a catalytically active gold cation is favored by
i) addition of weakly coordinating triflate anion, ii) electronic as
well as steric stabilization by seven-membered-ring NHCs, and
iii) steric stabilization by bulky Dipp substituents (complex
[
(THD-Dipp)AuCl] has the largest calculated %Vbur value of
[
6]
any gold(I) NHC complex reported to date).
The synthesis of indoles 2a,b and benzofuran 2c was suc-
cessfully implemented on a multi-gram scale. Reactions were
carried out in 96% EtOH in the presence of 1 mol% of [(THD-
Dipp)AuCl]/AgOTf under aerobic conditions at room tempera-
ture. In all cases, virtually quantitative yields were obtained in
less than 1 h. On this basis, [(THD-Dipp)AuCl]/AgOTf consti-
tutes the most active gold-based catalytic system hitherto re-
ported for the intramolecular hydroamination and hydrophe-
noxylation of alkynes under mild, aerobic conditions.
Figure 1. Kinetic curves for cyclization of 1b mediated by [(THD-Dipp)AuX]
ꢀ
ꢀ
ꢀ
(X=BF , OTf , NTf ).
4
2
Kinetic studies of catalytic reactions were performed by
As noted above, the degree of dissociation of the [(THD-Dip-
1
ꢀ
ꢀ
H NMR spectroscopy. Spectra were recorded from solutions in
p)AuX] complexes increases in the order: Cl<NTf2 <OTf <
ꢀ
CDCl3 containing a catalyst loading of 1 mol% [(THD-Dip-
p)AuOTf]. At 303 K, no trace of product 2a from 1a was de-
tected. Increasing the temperature to 333 K led to a minor in-
crease in reaction rate (ca. 5% conversion was achieved in
BF4 . This sequence perfectly fits the observed increase in cata-
lytic activity of the corresponding complexes. These findings
support the assumption that cationic gold complexes are true
catalytic species in indole and benzofuran syntheses promoted
by [(L)AuX].
3
0 min). On the other hand, 1c was completely converted into
the corresponding benzofuran in less than 1 min at 303 K, thus
making NMR monitoring of the reaction difficult. Methylated
substrate 1b was completely converted into the correspond-
&
&
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DFT considerations
To rationalize our experimental findings, we performed a DFT
study. We calculated potential energy surfaces for the sup-
posed catalytic cycle of heterocyclization of 1b catalyzed by
+
LAu . All calculations were conducted in the gas phase. When
modeling the catalytic cycle, the counter-ion was omitted as it
was not possible to account for the counter-ion and/or solvent
effects at the chosen level of theory within reasonable compu-
tational time.
Geometry optimization was carried out using the PBE gener-
[
20]
alized gradient functional. Triple-x valence basis sets includ-
ing TZ2P polarization functions [5,1,1,1,1/5,1,1,1,1/5,1,1,1] for
Au, [3,1,1/3,1,1/1,1] for S, P, N, and C, and [3,1,1/1] for H were
used. The innermost electrons of Au, S, P, N, and C atoms were
Figure 2. Schematic representation of potential energy surface for catalytic
heterocyclization of 1b (E in kcalmol ).
0
ꢀ1
[
21]
treated using ECP-SBKJC effective core potentials. Total ener-
0
gies E, and zero-point vibrational energies ZPE, E =E + ZPVE,
were calculated for all stationary points either as minima (i=0)
or first-order transition states (i=1). All calculations were per-
Table 4. Calculated energy parameters of potential energy surfaces of
0
ꢀ1
0
0
[
22]
catalytic heterocyclization of 1b (E in kcalmol , E =E (TS)ꢀE (PC)).
formed using the PRIRODA program.
a
[
23]
Based on experimental and theoretical data, we propose
the following reaction mechanism (Scheme 3). The catalytically
Ligand
Cat+1b PC
TS
SAd
PAd
Cat+2b
E
a
Me
2
S
P
0.0
0.0
0.0
0.0
ꢀ20.1 ꢀ12.1 ꢀ30.3 ꢀ57.1 ꢀ36.4
8.0
9.5
10.2
10.2
9.8
Ph
IPr
3
ꢀ13.6 ꢀ4.1
ꢀ13.0 ꢀ2.8
ꢀ12.7 ꢀ2.5
ꢀ11.3 ꢀ1.5
ꢀ10.8 ꢀ0.9
ꢀ19.9 ꢀ49.5 ꢀ36.4
ꢀ18.5 ꢀ48.2 ꢀ36.4
ꢀ18.0 ꢀ47.2 ꢀ36.4
ꢀ15.1 ꢀ45.6 ꢀ36.4
ꢀ14.4 ꢀ45.4 ꢀ36.4
SIPr
THP-Dipp 0.0
THD-Dipp 0.0
9.9
molecule was added where necessary to keep relative energies
comparable.
We found that intramolecular nucleophilic addition (cycliza-
tion step) has a very low activation barrier. Indeed, for the
ꢀ
1
studied ligands, transition states TS are only 8.0–10.2 kcalmol
above the PCs. The values of activation energies E lie in
a
ꢀ
1
a narrow range of 2.2 kcalmol . Such small differences in acti-
vation energies cannot account for the observed differences in
the activities of the catalysts. It can be supposed that the
nature of the ligand does not have a significant influence on
the catalytic activity once a catalytically active cationic species
is formed.
Thus, we supposed that initiation of the catalytic cycle is
likely to be the rate-determining step. We calculated the disso-
ciation energies of the [(NHC)AuOTf] complexes. In order to
take solvent effects into account, at least roughly, we opti-
mized the complexes with three molecules of solvent, thus ap-
proximating the first solvent sphere. We used methanol in-
stead of ethanol to save computational time. The calculated
dissociation energies are presented in Table 5.
Scheme 3. Proposed mechanism for the heterocyclization of 1b.
active gold cation is solvated by one molecule of methanol
(
Cat). A molecule of substrate 1b substitutes methanol to give
a p-complex PC. Then, via transition state TS, a s-adduct SAd
bearing the metal moiety at the 3-position of the indole ring is
formed. Migration of a proton from the nitrogen to the 3-posi-
tion leads to the formation of a p-adduct PAd. Finally, indole
The absolute values of the dissociation energies are mean-
ingless. Relative energies should be considered for comparison
of the ligand properties. Calculated dissociation energies de-
crease in the order Me S>Ph P>SIMes>IPrꢁTHP-Mesꢁ
2
b is replaced by a methanol molecule to regenerate Cat.
Calculated energy parameters of the potential energy sur-
face are presented in Figure 2 and Table 4. For SIPr, we found
that proton transfer from SAd to PAd could proceed with vir-
tually no barrier. Thus, we suggest the same for the other com-
plexes. The sum of catalyst and substrate energies was set as
zero. The energy of an isolated methanol and/or substrate
2
3
SIPrꢁTHD-MesꢁTHP-DippꢁTHD-Dipp. A decrease in dissocia-
tion energy leads to an increase in the concentration of cata-
lytically active cationic species in solution. This explains the
much higher activity of carbenes over phosphines and sulfides.
Chem. Eur. J. 2014, 20, 1 – 10
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5
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calculations fully support the experimental data on the catalyt-
ic activities of [(L)AuX] complexes.
0
Table 5. Calculated E energies for the dissociation of [(L)AuOTf]*3MeOH
ꢀ
1
[kcalmol ].
Conclusion
A series of expanded-ring NHC gold(I) complexes has been
synthesized. The most general method for the synthesis of
[
[
(er-NHC)AuCl] is interaction of
a
free carbene with
Ligand
Me
Ph
E
diss
Ligand
78.5 SIMe
70.2 THP-Me 68.4 THP-Mes 61.6 SIPr
E
diss
Ligand
E
diss
Ligand
E
diss
62.0
61.5
(Me S)AuCl].
2
2
S
P
70.2 SIMes
64.1 IPr
Cationic complexes with weakly coordinating anions [(er-
NHC)AuX] have been prepared by salt metathesis with silver
3
THD-Me 68.3 THD-Mes 60.4 THP-Dipp 60.0
THD-Dipp 59.8
ꢀ
ꢀ
ꢀ
salts AgX (X=OTf , BF4 , and NTf ). The cationic character of
2
ꢀ
ꢀ
the [(NHC)AuX] complexes increases in the order Cl <NTf
<
2
ꢀ
ꢀ
OTf <BF4 . Cationic species are additionally stabilized by ex-
pansion of the ring in NHCs and the attachment of bulky sub-
stituents. The er-NHCs exhibit superior steric and donor prop-
erties compared to their five-membered-ring counterparts. No-
tably, steric factors dominate over the donor properties of the
Furthermore, computational data show that complexes bear-
ing carbenes with the same ring size but bulkier substituents
(
Dipp vs Mes) should be more active, which is also in agree-
ment with the experimental results. The sequence of decreas-
ing dissociation energies is in qualitative agreement with the
sequence of activities of the complexes (Table 2) observed ex-
perimentally.
+
ligands in the stabilization of [(L)Au ] species.
The increase in the ionic character of the complexes leads to
increased catalytic activity in cyclization reactions. Utilization of
a seven-membered-ring carbene bearing bulky Dipp groups in
In order to investigate the influence of steric factors, we con-
sidered carbenes with methyl substituents, for which experi-
ments on catalytic activity were not performed. It should be
noted that, despite a huge difference in electronic properties,
ꢀ
combination with weakly coordinating anions such as X=OTf
ꢀ
or BF4 in [(THD-Dipp)AuX] results in extremely active catalytic
systems for the synthesis of indoles and benzofurans under un-
precedentedly mild conditions. Quantitative yields of products
were attained in a few minutes at room temperature and at
low catalyst loadings. er-NHC gold(I) complexes surpass con-
ventional gold(I) complexes bearing five-membered-ring NHCs
the dissociation energies for complexes of Ph P and SIMe are
3
equal. Introduction of bulky ligands in carbenes leads to a sig-
nificant decrease in their dissociation energies. For SIMe!
ꢀ
1
SIMes, E decreases by 6.1 kcalmol , and for THD-Me!THD-
diss
ꢀ
1
as well as PPh and SMe ligands in terms of activity. Further
Dipp it decreases by 8.5 kcalmol . Such energy differences
correspond to increases in the concentrations of the catalyti-
cally active gold cations by two to three orders of magnitude
at room temperature. Thus, the introduction of bulky ligands
significantly contributes to stabilization of the cationic species.
One may also note that bulky ligands destabilize the covalent
form due to steric repulsion between the ligand and the sol-
vated anion.
3
2
studies on the catalytic activity of er-NHC gold(I) complexes in
amination reactions are under way in our laboratories.
Experimental Section
General information
The computational data show that the primary stabilization
factors are steric in origin. Carbenes are much more “bulky”
than phosphines due to their structure (in carbene complexes,
substituents are directed towards the metal atom). This results
in much higher catalytic activity of the carbene complexes.
Within carbenes, steric properties depend on cycle size and
the size of the substituents. The difference between five- and
six-membered cyclic diaminocarbenes is sharp, while the six-
and seven-membered-ring analogues are similar from both ex-
Unless otherwise stated, all manipulations were carried out using
standard Schlenk techniques under an argon atmosphere. Di-
chloromethane was distilled over CaH2 in an argon atmosphere
prior to use. Diethyl ether and THF were distilled over sodium-ben-
zophenone. Unsaturated amidinium precursors were prepared by
[24]
the method published by Hintermann.
Six- and seven-mem-
bered-ring amidinium salts were prepared by our previously pub-
[11]
lished method. Six- and seven-membered-ring NHC silver and
copper complexes were synthesized according to a slightly modi-
[10]
fied literature method. Other chemicals and solvents were ob-
[
19]
perimental and theoretical points of view.
tained from commercial sources and used without further purifica-
tion. NMR spectra were acquired on Bruker Avance II 600 and
Bruker DRX-500 spectrometers equipped with 5 mm inverse probe
heads with Z-gradient coils. All spectra were measured at 303 K,
unless indicated otherwise. All deuterated solvents were stored
over molecular sieves (4 ꢁ). Chemical shifts are reported in parts
To summarize the results of the calculations, the nature of
+
the ligand in LAu complexes has only a minor influence on
the catalytic activity in heterocyclization reactions. Catalytic ac-
tivity is mostly determined by the concentration of the cationic
form of the complex. Stabilization of the cation and destabili-
zation of the covalent form by strongly donating bulky ligands
such as er-NHCs leads to a higher concentration of the cationic
complex. The effect of bulky substituents outweighs that of
the donor properties of the ligands in [(L)AuX]. The results of
1
13
per million relative to TMS ( H) or residual solvent signals ( C).
Coupling constants J are given in hertz as positive values regard-
less of their real individual signs. Signal multiplicities are indicated
as s (singlet), d (doublet), or m (multiplet), and br denotes broad-
ened signals.
&
&
Chem. Eur. J. 2014, 20, 1 – 10
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General procedure for the synthesis of [(er-NHC)AuCl]
[(THD-Mes)AuCl]: Method A: yield 408 mg (72%); Method B: yield
of free carbene THD-Mes 495 mg (74%), yield of [(THD-Mes)AuCl]
Method A from azolium salt: A mixture of potassium chloroaurate
dihydrate (414 mg, 1 mmol), the azolium salt (1.07 equiv), and po-
tassium carbonate (691 mg, 5 mmol) was placed in a Schlenk flask
equipped with a magnetic stirring bar. 3-Chloropyridine (3.80 mL,
596 mg (71% based on gold source); Method C: yield 800 mg
1
(83%); Method D: yield 116 mg (41%). H NMR (600 MHz, CDCl ):
3
d=6.89 (s, 4H), 3.87 (t, 4H, J=5.7 Hz), 2.32 (s, 12H), 2.24 (s, 6H),
13
2.13 ppm (q, 4H, J=5.8 Hz); C NMR (151 MHz, CDCl ): d=200.8,
3
4
0 mmol) was then added. The reaction mixture was stirred at
144.2, 137.9, 134.1, 129.9, 53.2, 25.0, 21.1, 18.6 ppm.
1
10 8C for 30 h. The flask was then allowed to cool to room tem-
[
(THP-Dipp)AuCl]: Method A: formation of the product was not
observed after 5 days; Method B: yield of free carbene THP-Dipp
05 mg (50%), yield of [(THP-Dipp)AuCl] 478 mg (75% based on
perature, dichloromethane (10 mL) was added to dissolve the
product, and the mixture was passed through a thin pad of silica
gel (eluent CH Cl ). All volatiles were removed in a rotary evapora-
tor; the residual solid was redissolved in the minimum volume of
dichloromethane and the solution was added to hexane (ca.
4
2
2
gold source); Method C: yield 346 mg (32%); Method D: formation
of the product was not observed after 7 days. H NMR (600 MHz,
CDCl ): d=7.33 (t, 2H, J=7.7 Hz), 7.17 (d, 4H, J=7.7 Hz), 3.47 (t,
1
3
5
0 mL). The precipitate was collected by filtration, washed with
4
5
H, J=5.8 Hz), 2.98 (dt, 4H, J=13.7, 6.9 Hz), 2.34 (quin, 2H, J=
.6 Hz), 1.36 (d, 12H, J=6.8 Hz), 1.27 ppm (d, 12H, J=6.8 Hz);
hexane, and dried to afford [(er-NHC)AuCl].
1
3
C NMR (151 MHz, CDCl ): d=200.8, 145.3, 141.6, 129.5, 124.6,
Method B from free er-NHC: A 1m solution of sodium hexame-
thyldisilazide in THF (2.0 mL) was added to a solution of amidinium
tetrafluoroborate (2.0 mmol) in dry THF (50 mL) at ꢀ308C and the
mixture was allowed to warm to room temperature (ca. 30 min).
After evaporation of the solvent under reduced pressure, the resi-
due was taken up in dry diethyl ether (60 mL) and the obtained
suspension was filtered through Celite. The filtrate was concentrat-
ed under vacuum until free carbene began to precipitate and was
then cooled at ꢀ208C for 12 h. The precipitate was collected by fil-
tration and dried under vacuum. The obtained free carbene was re-
dissolved in dry THF (50 mL). One equivalent of chloro(dimethyl
sulfide) gold(I) was added to the solution and the mixture was
stirred in the dark for 1–6 h (TLC control). Subsequent manipula-
tions were carried out in air. The solution obtained was filtered
through Celite and the filtrate was concentrated to dryness under
vacuum. The residue was redissolved in the minimum volume (3–
3
4
7.5, 28.7, 24.7, 24.5, 20.2 ppm.
[(THD-Dipp)AuCl]: Method A: yield 91 mg (14%); Method B: yield
of free carbene THD-Dipp 502 mg (60%), yield of [(THD-Dipp)AuCl]
641 mg (82% based on gold source); Method C: yield 332 mg
(30%); Method D: yield 16 mg (5%). H NMR (600 MHz, CDCl
7.30 (t, J=7.7 Hz, 2H), 7.15 (d, J=7.7 Hz, 4H), 3.93–4.00 (m, 4H),
3.16 (dt, J=13.7, 6.9 Hz, 4H), 2.27–2.34 (m, 4H), 1.38 (d, J=6.8 Hz,
12H), 1.30 ppm (d, J=6.8 Hz, 12H); C NMR (151 MHz, CDCl ): d=
3
1
): d=
3
13
201.3, 144.8, 144.0, 129.1, 124.7, 54.7, 28.9, 25.0, 24.6, 24.5 ppm.
General procedure for catalytic cyclization
The gold complex (0.013 mmol, 2 mol%) was added to a solution
of 2-(phenylethynyl)aniline [N-methyl-2-(phenylethynyl)aniline or 2-
(phenylethynyl)phenol] (0.65 mmol) in EtOH (6 mL). The mixture
was stirred at room temperature under ambient atmosphere. The
progress of the reaction was monitored by TLC (hexane/EtOAc,
5:1); after its completion (or after 48 h), the solvent was evaporat-
ed and the residue was passed through a thin pad of silica gel
(hexane/EtOAc, 8:1) and analyzed by NMR spectrometry.
6
mL) of dichloromethane and then hexane (35 mL) was added,
which resulted in the immediate precipitation of a bright white
solid. The solid was collected by filtration, washed with hexane,
and dried under vacuum. The product could also be purified from
the admixture of biscarbene compound by column chromatogra-
phy (eluent CH Cl ).
2
2
Method C from [(er-NHC)AgCl]: A mixture of [(er-NHC)AgCl]
1.7 mmol) and chloro(dimethyl sulfide) gold(I) (500 mg, 1.7 mmol)
General procedure for catalytic cyclization with addition of
AgOTf
(
was taken up in CHCl (40 mL) under ambient atmosphere. After
3
AgOTf (3.3 mg, 0.013 mmol) (or 0.039 mmol in the case of KAuCl4)
was added to a solution of the gold complex (0.013 mmol) in EtOH
stirring for 4–20 h (TLC control; strong heating of TLC plates makes
it easy to identify compounds containing gold by the characteristic
purple color of colloidal gold), the mixture was filtered through
Celite and concentrated. The residue was redissolved in the mini-
mum volume of CH Cl (2–5 mL) and hexane (20 mL) was added.
(
1 mL). After stirring for 15 min, 2-(phenylethynyl)aniline [N-methyl-
-(phenylethynyl)aniline or 2-(phenylethynyl)phenol] (0.65 mmol) in
EtOH (5 mL) was added. After completion of the reaction (or after
4 h), the solvent was evaporated and the residue was passed
2
2
2
2
The precipitate was collected by filtration to give the product as
a white microcrystalline solid.
through a thin pad of silica gel (hexane/EtOAc, 3:1) and analyzed
by NMR spectrometry.
Method D from [(er-NHC)CuBr]:
A
mixture of [(NHC)CuBr]
[18]
2
-Phenylindole (2a): [(THD-Dipp)AuCl] (163 mg, 0.25 mmol) and
(
0.5 mmol) and chloro(dimethyl sulfide) gold(I) (147 mg, 0.5 mmol)
AgOTf (64 mg, 0.25 mmol) were added to a solution of 2-(phenyl-
ethynyl)aniline (4.83 g, 25 mmol) in EtOH (100 mL). The mixture
was stirred under ambient atmosphere for 45 h. The solvent was
then removed under reduced pressure and the product was puri-
fied by flash chromatography (hexane/EtOAc, 8:1). Yield: 4.64 g
96%). H NMR (600 MHz, CDCl ): d=8.33 (brs, 1H), 7.72–7.65 (m,
H), 7.47 (m, 2H), 7.42 (m, 1H), 7.36 (m, 1H), 7.17 (m, 1H), 7.13 (m,
H), 6.87 ppm (m, 1H); C NMR (151 MHz, CDCl ): d=137.9, 136.9,
32.4, 129.3, 129.1, 127.7, 125.2, 122.4, 120.7, 120.3, 110.9,
was taken up in dichloromethane (15 mL). The mixture was stirred
for 6–30 h (TLC control; CH Cl ), then passed through silica gel cov-
2
2
ered by a thin pad of Celite. The obtained solution was concentrat-
ed to a volume of about 5 mL and added to hexane (50 mL). The
precipitate was collected by filtration and dried. (Graphical NMR
spectra are presented in the Supporting Information)
1
(
3
3
1
1
1
13
[
(THP-Mes)AuCl]: Method A: yield 376 mg (68%); Method B: yield
of free carbene THP-Mes 506 mg (79%), yield of [(THP-Mes)AuCl]
85 mg (67% based on gold source); Method C: yield 733 mg
3
00.0 ppm.
5
1
[18]
(
78%); Method D: yield 166 mg (60%). H NMR (600 MHz, CDCl ):
1-Methyl-2-phenylindole (2b):
[(THD-Dipp)AuCl] (163 mg,
3
d=6.92 (s, 4H), 3.43 (q, 4H, J=5.8 Hz), 2.33 (q, 2H, J=5.6 Hz),
0.25 mmol) and AgOTf (64 mg, 0.25 mmol) were added to a solu-
tion of N-methyl-2-(phenylethynyl)aniline (5.18 g, 25 mmol) in EtOH
(100 mL). The mixture was stirred under ambient atmosphere for
13
2
.27 ppm (brs, 18H); C NMR (151 MHz, CDCl ): d=192.3, 142.0,
3
1
38.2, 134.4, 129.8, 45.4, 21.1, 20.8, 17.9 ppm.
Chem. Eur. J. 2014, 20, 1 – 10
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3
0 min. The solvent was then removed under reduced pressure
progress of the reaction was monitored by TLC (hexane/EtOAc,
5:1); after its completion (or after 48 h), the solvent was evaporat-
ed and the residue was passed through a thin pad of silica gel
(hexane/EtOAc, 8:1) and analyzed by NMR spectrometry.
and the product was purified by flash chromatography (hexane/
EtOAc, 9:1). Yield: 5.03 g (97%). H NMR (600 MHz, CDCl ): d=
7
1
1
1
1
3
.73–7.64 (m, 3H), 7.49 (m, 2H), 7.41 (m, 2H), 7.34 (m, 1H), 7.18 (m,
13
H), 6.86 (m, 1H), 3.42 ppm (s, 3H); C NMR (151 MHz, CDCl ): d=
3
41.7, 138.5, 133.0, 132.6, 129.5, 128.6, 128.0, 121.8, 120.6, 120.0,
09.7, 101.8, 31.2 ppm.
General procedure for catalytic cyclization with addition of
AgOTf
[18]
2
-Phenyl-1-benzofuran
(2c):
[(THD-Dipp)AuCl]
(163 mg,
0
.25 mmol) and AgOTf (64 mg, 0.25 mmol) were added to a solu-
tion of 2-(phenylethynyl)phenol (4.86 g, 25 mmol) in EtOH
100 mL). The mixture was stirred under ambient atmosphere for
5 min. The solvent was then removed under reduced pressure
and the product was purified by flash chromatography (hexane/
AgOTf (3.3 mg, 0.013 mmol (or 0.039 mmol in the case of KAuCl4)
was added to a solution of the gold complex (0.013 mmol) in EtOH
(
1
(1 mL). After stirring for 15 min, 2-(phenylethynyl)aniline [N-methyl-
2
-(phenylethynyl)aniline or 2-(phenylethynyl)phenol] (0.65 mmol) in
EtOH (5 mL) was added. After completion of the reaction (or after
24 h), the solvent was evaporated and the residue was passed
through a thin pad of silica gel (hexane/EtOAc, 3:1) and analyzed
by NMR spectrometry.
1
EtOAc, 8:1). Yield: 4.71 g (97%). H NMR (600 MHz, CDCl ): d=7.88
3
(
dd, J=7.9 Hz, 1.4 Hz, 2H), 7.59 (m, 1H), 7.53 (d, J=7.6 Hz, 1H),
13
7
.47 (m, 2H), 7.37 (m, 1H), 7.28 (m, 2H), 7.04 ppm (s, 1H); C NMR
(
151 MHz, CDCl ): d=155.9, 154.9, 130.5, 129.2, 128.8, 128.5, 124.9,
3
1
24.2, 122.9, 120.9, 111.2, 101.3 ppm.
Experimental protocol for kinetic measurements
General procedure for anion exchange
A solution of substrate 1a–c (0.241 mmol) in CDCl (0.6 mL) was
3
quickly added to an NMR tube containing the corresponding gold
complex (0.00241 mmol). The tube was transferred to the thermo-
stabilized (303 K) magnet of an NMR spectrometer. 41 points were
acquired for each kinetic run. The total experiment time was set to
[
(
(NHC)AuCl] (0.3 mmol) was added to a suspension of AgX
0.3 mmol) in dichloromethane under ambient atmosphere. The
mixture was stirred for 15 min in the dark and then filtered
through Celite. The filtrate was concentrated to a volume of about
1
added. The solution was allowed to stand for 30 min. The precipi-
tate formed was collected by filtration and dried under vacuum.
(
3
1
2 min; points were acquired at intervals of 20 s for the first
0 min and at intervals of 2 min over the next 20 min. Kinetic
mL and then diethyl ether (15 mL) and hexane (5 mL) were
curves for the cyclization of 1b to 2b in the presence of [(THD-
Dipp)Au]BF , [(THD-Dipp)Au]OTf, and [(THD-Dipp)Au]NTf catalysts
4
2
Full NMR spectra are presented in the Supporting Information).
are presented in the Supporting Information (Figures S2–S4).
1
[
(SIPr)AuOTf]: H NMR (600 MHz, CDCl ): d=7.42 (t, J=7.52 Hz,
3
2
H), 7.23 (d, J=7.57 Hz, 4H), 4.12 (brs, 4H), 2.93–3.02 (m, 4H), 1.36
13
(
(
d, J=6.47 Hz, 12H), 1.31 ppm (d, J=6.33 Hz, 12H); C NMR
151 MHz, CDCl ): d=184.5, 146.5, 133.5, 130.3, 124.7, 53.5, 29.0,
X-ray structure determination
3
Data were collected on a Bruker SMART APEX-II CCD diffractometer
2
4.9, 24.2 ppm.
(
l (MoK )-radiation, graphite monochromator, f and w scan mode)
1
a
[(THP-Dipp)AuOTf]: H NMR (600 MHz, CDCl ): d=7.35 (t, J=
3
[25]
and corrected for absorption using the SADABS program (for de-
tails, see the Supporting Information, Table S1).
7
.7 Hz, 2H), 7.19 (d, J=7.7 Hz, 4H), 3.53 (t, J=5.2 Hz, 4H), 2.91
(
ddd, J=13.3, 6.6, 6.5 Hz, 4H), 2.31–2.41 (m, 2H), 1.34 (d, J=6.7 Hz,
13
1
1
2H), 1.27 ppm (d, J=6.7 Hz, 12H); C NMR (151 MHz, CDCl ): d=
88.4, 145.2, 141.4, 129.8, 124.8, 47.4, 28.8, 24.8, 24.3, 20.0 ppm.
3
1
Acknowledgements
[(THD-Dipp)AuOTf]: Yield 204 mg (89%) of white solid. H NMR
(
4
6
600 MHz, CDCl ): d=7.34 (t, J=7.8 Hz, 2H), 7.18 (d, J=7.7 Hz, 4H),
3
DFT calculations were performed on an MVS-100 K supercom-
puter at the Joint Supercomputer Center of the Russian Acade-
my of Sciences. NMR measurements were carried out in the
Laboratory of Magnetic Tomography and Spectroscopy, Faculty
of Fundamental Medicine of Moscow State University.
.00–4.05 (m, 4H), 3.07–3.11 (m, 4H), 2.31–2.35 (m, 4H), 1.36 (d, J=
13
.8 Hz, 12H), 1.30 ppm (d, J=6.9 Hz, 12H); C NMR (151 MHz,
CDCl ): d=189.3, 144.7, 143.8, 129.4, 125.0, 54.5, 28.9, 24.7, 24.6,
2
3
4.4 ppm.
1
[
(THD-Dipp)AuBF ]: Yield 141 mg (67%) of white solid. H NMR
4
(
600 MHz, CDCl ): d=7.32 (t, J=7.7 Hz, 2H), 7.17 (d, J=7.7 Hz, 4H),
3
4
6
1
.04 (brs, 4H), 3.05–3.13 (m, 4H), 2.33 (brs, 4H), 1.29 ppm (d, J=
1
3
Keywords: alkynes
hydroamination · N-heterocyclic carbenes
· density functional theory · gold ·
.7 Hz, 24H); C NMR (151 MHz, CDCl ): d=179.8, 145.0, 143.8,
3
29.6, 125.0, 54.7, 28.9, 24.8, 24.6, 24.5 ppm.
1
[
(THD-Dipp)AuNTf ]: Yield 258 mg (96%) of white solid. H NMR
2
(
4
600 MHz, CDCl ): d=7.32 (t, J=7.7 Hz, 2H), 7.17 (d, J=7.8 Hz, 4H),
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.00–4.07 (m, 4H), 3.07–3.14 (m, 4H), 2.32–2.38 (m, 4H), 1.37 (d,
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J=6.8 Hz, 12H), 1.30 ppm (d, J=6.9 Hz, 12H); C NMR (151 MHz,
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Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 10
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
&
Intramolecular Hydroamination
O. S. Morozov, A. V. Lunchev, A. A. Bush,
A. A. Tukov, A. F. Asachenko,
V. N. Khrustalev, S. S. Zalesskiy,
V. P. Ananikov, M. S. Nechaev*
&& – &&
Expanded-Ring N-Heterocyclic
Carbenes Efficiently Stabilize Gold(I)
Cations, Leading to High Activity in p-
Acid-Catalyzed Cyclizations
What a difference a ring makes: Ex-
panded-ring N-heterocyclic carbenes
(er-NHC) surpass their five-membered
erties. A monoligand cationic [(THD-Dip-
p)Au]BF (Dipp=2,6-diisopropylphenyl)
4
complex bearing a seven-membered
ring carbene and bulky Dipp substitu-
ents exhibits unprecedentedly high cat-
alytic activity in indole and benzofuran
synthesis (see scheme).
ring counterparts, as well as Ph P and
3
Me S, in the stabilization of cationic
2
gold(I) species due to increased steric
protection and electron-donating prop-
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
10
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!