Gold(I) Complexes with Eight-Membered NHC Ligands: Synthesis, Structures and Catalytic Activity

Article abstract of DOI:10.1002/adsc.202000281

A series of expanded-ring NHC gold complexes of the formula (NaphtDHD?Ar)Au?X (NaphtDHD=4,5-dihydro-1H-naphtho[1,8-ef][1,3]diazocin-3(2H)-ylidene; Ar: Mes=2,4,6-trimethylphenyl, Dipp=2,6-diisopropylphenyl or Xyl=2,6-dimethylphenyl; X=Cl, NCCH₃, NTf₂) have been synthesized, including the first gold(I) triflimidate complex (5) stabilized by an eight-membered NHC ligand. The new organogold compounds have been characterized by mass spectrometry, IR spectroscopy, and ¹H and ¹³C NMR spectroscopy. The structural geometries of 3 b–c and 5 have been unequivocally established by crystallographic analysis revealing broad N-C-N angles (>121°) and high buried volume values (46–54%). The first catalytic studies were carried out on the cycloisomerization of 1,6-enynes, obtaining full conversions (0.5 mol% catalyst loading) and excellent endo/exo selectivity (up to 99:1), and on the gold-catalyzed phenol synthesis. Lastly, the (NaphtDHD-Dipp)Au⁺ NTf₂^? species was subjected to a kinetic experiment in the cyclization of a N-propargyl carboxamide to evaluate the efficiency of the pre-formed catalyst (5) and the in situ activated gold complex (3 b+AgNTf₂). (Figure presented.).

Full text of DOI:10.1002/adsc.202000281

Title: Gold(I) Complexes with Eight-Membered NHC Ligands:
Synthesis, Structures and Catalytic Activity
Authors: Alejandro Cervantes-Reyes, Frank Rominger, Matthias
Rudolph, and A. Stephen K. Hashmi
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000281
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.202000281
Gold(I) Complexes with Eight-Membered NHC Ligands:
Synthesis, Structures and Catalytic Activity
Alejandro Cervantes-Reyes,a Frank Rominger,a,+ Matthias Rudolph,a and
A. Stephen K. Hashmia,b*
a
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: (+49)-6221-54-4205
E-mail: hashmi@hashmi.de
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
b
+
Crystallographic investigation
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A series of expanded-ring NHC gold complexes of The first catalytic studies were carried out on the
the formula (NaphtDHD-Ar)Au-X (NaphtDHD = 4,5- cycloisomerization of 1,6-enynes, obtaining full conversions
dihydro-1H-naphtho[1,8-ef][1,3]diazocin-3(2H)-ylidene; Ar: (0.5 mol% catalyst loading) and excellent endo/exo
Mes = 2,4,6-trimethylphenyl, Dipp = 2,6-diisopropylphenyl selectivity (up to 99:1), and on the gold-catalyzed phenol
-
or Xyl = 2,6-dimethylphenyl; X = Cl, NCCH3, NTf2) have synthesis. Lastly, the (NaphtDHD-Dipp)Au+ NTf2 species
been synthesized, including the first gold(I) triflimidate was subjected to a kinetic experiment in the cyclization of a
complex (5) stabilized by an eight-membered NHC ligand. N-propargyl carboxamide to evaluate the efficiency of the
The new organogold compounds have been characterized by pre-formed catalyst (5) and the in situ activated gold
1
mass spectrometry, IR spectroscopy, and H and 13C NMR complex (3b + AgNTf2).
spectroscopy. The structural geometries of 3b–c and 5 have
been unequivocally established by crystallographic analysis
revealing broad N-C-N angles (>121°) and high buried
volume values (46–54%).
Keywords: Enynes; Expanded Ring NHC Ligands; Gold;
Heterocycles
complexes (where metal is different from gold) have
been reported.[12]
Introduction
During the preparation of this paper, Huynh and co-
workers reportedly synthesized some alkyl-chain
backbone eight-membered NHC gold(I)-bromide
complexes.[5d,13] Recently, we have developed the first
examples of sterically bulky nine- and ten-membered
NHC ligands and the synthesis of the corresponding
gold complexes. The catalytic activity of these species
was tested down to a 50 ppm catalyst loading and an
outstanding performance (TON = 3800) was observed
for the cyclization of an N-arylpropargylamide.[14] In
virtue of those findings we were interested in further
expanding the chemistry of expanded-ring NHC-Au(I)
complexes.
Herein, the synthesis, full characterization and
steric profiles of neutral and cationic eight-membered
NHC-Au(I) complexes is presented, along with a
preliminary catalytic survey of several synthetically
relevant cyclization reactions, including the
cycloisomerization of 1,6-enynes and the gold-
catalyzed oxazole synthesis.
N-heterocyclic carbenes (NHCs)[1] are efficient
stabilizing ligands for metals owing to their steric and
electronic features which can be modulated either from
the substituents at the N-atoms or from the alkyl/aryl
backbones.[2,3] In this context, expanded-ring
backbone NHCs turn out to increase the steric
environment around the first coordination sphere of
the metal, where the catalysis occurs.[4,5]
Since the pioneering work of Cavell et al., the
interest in expanded-ring NHC-Au(I) complexes has
surged due to their widespread applications,
particularly in homogeneous catalysis.[6–8] It has been
established, that this class of ligands can efficiently
stabilize π-acidic gold cations by increasing the ionic
character of the complex, leading to an increased
catalytic activity. Furthermore, the stereoelectronic
features of the NHC ligands can be tuned to obtain the
desired selectivity.[9,10]
Several examples of six- and seven-membered
NHC-Au(I) complexes have been reported, as well as
their structural and catalytic properties.[11] In addition,
a large family of eight-membered NHC metal
1
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
-
-
Results and Discussion
hexafluoroantimonate (SbF6 ) and triflimidate (NTf2 ),
as suitable moieties for the synthesis of cationic NHC-
Au(I) complexes.[15–17] To this end, the treatment of the
gold(I) chloride complexes with 1 equivalent of either
AgSbF6 or AgNTf2 was attempted (Scheme 1). The
reaction of 3a with AgSbF6 in acetonitrile at room
temperature afforded the cationic complex
[(NaphtDHD-Mes)AuNCCH3]SbF6 (4) in 64%
isolated yield as a colourless crystalline solid. By
analogy, complex 5 was obtained by the reaction of 3b
with 1 equivalent AgNTf2 in CH2Cl2 at room
temperature in 71% yield. Recrystallization of the
crude products from CH2Cl2/pentane afforded pure,
crystalline gold complexes.
Synthesis of NHC-Au(I) complexes
The first report of the 4,5-dihydro-1H-naphtho[1,8-
ef][1,3]diazocin-3(2H)-ylidene (NaphtDHD) NHC
ligand and the corresponding NHC-AgBr complexes
was made by Nechaev et. al. and the access to the
NHC-CuBr complexes was accomplished by ligand
transfer from the isostructural Ag(I) compounds.[12c]
As a direct synthetic pathway to the current NHC-
AuCl complexes, we considered the generation of the
free NHC in solution by deprotonation of a suitable
NHC-HX salt (i.e. X = BF4) as carbene precursor,
followed by direct complexation with a gold source
(i.e. (SMe2)AuCl). To this aim various bases and
reaction conditions were tested (see Supporting
Information).
Spectroscopic analysis
The presence of NCCH3 in the complex 4 was
indicated by the infrared spectra and the complexation
to gold was confirmed from the stretching frequency
compared to non-coordinated acetonitrile. FTIR-ATR
of neat 4 shows signals (νC≡N 2332, 2306 cm−1) which
are higher than those for NCCH3 (νC≡N 2254, 2293
cm−1) due to the change in the electronic configuration
of the acetonitrile upon complexation.[18] This is
expected since the electron donation from the lone pair
of the NCCH3 to the gold center removes weakly
antibonding electrons from the σ orbital at the CN
bond upon coordination, and therefore strengthening it
and increasing its stretching frequency (νC≡N).[17,19]
Initially, LiHMDS, MeONa and tBuOK in
combination with (NaphtDHD-Mes)HBF4 (2a)
afforded the desired (NaphtDHD-Mes)AuCl complex
(3a) in poor to moderated yields (17–52%). K2CO3 for
instance, was not basic enough for the deprotonation
1
according to H NMR analysis of the crude mixture.
By varying quantities of base and substrate, tBuOK
was established as the optimal deprotonating base at -
78°C (Scheme 1). Thus, the NHC precursors 2a–c
reacted with tBuOK in the presence of (SMe2)AuCl to
afford the neutral complexes 3a–c as colourless solids
in 43–78% yield. Their structures were all confirmed
by NMR, mass spectrometry and IR spectroscopy.
These new gold complexes are readily soluble in
CH2Cl2, CHCl3, acetone or THF but insoluble in
diethyl ether, hexane and pentane.
-
The presence of the triflimidate moiety (NTf2 ) in
complex 5 was confirmed by the νS=O (1398, 1195
cm−1) and νC-F (1131 cm−1) bands.[20] The 13C NMR
chemical shifts are summarized in Table 1. The
carbene carbon signal (CNHC) of the neutral complexes
(3a–c) range between 195–197 ppm, about Δ7 ppm
shifted downfield compared to their corresponding
cationic congeners 4 and 5 (188–190 ppm).
With the neutral NHC-Au(I) complexes in hand, we
aimed on the synthesis of highly electron-deficient
cationic complexes by anion metathesis using weakly
coordinating counter anions. We envisaged
Scheme 1. General formation of NHC-Au(I) species 3a–c and 4–5.
2
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
diffusion of pentane into a concentrated solution of the
complex in a CH2Cl2/CHCl3 mixture followed by slow
evaporation at room temperature overnight (Figure
3).[22] The N-CNHC-N angle in 5 (121.29(16)°) is
marginally bigger than that in complex 3b (121.2(4)°),
revealing a negligible change in the molecular
geometry of the NHC backbone after anion exchange
Table 1. 13C NMR shifts in neutral complexes (NHC)AuCl
(3) and cationic species 4–5.
CNHC
(ppm)
δ
Complex
CZ δ (ppm)
3a (NaphtDHD-Mes)AuCl
3b (NaphtDHD-Dipp)AuCl
3c (NaphtDHD-Xyl)AuCl
195.4
197.2
195.2
-
from Cl- (3b) to NTf2 (5). The CNHC–Au and Au–NTf2
bond lengths in complex 5 are 2.004(18) Å and
2.1069(17) Å, respectively.
2.8 (CCH3),
4 [(NaphtDHD-Mes)AuNCCH3]SbF6
5 [(NaphtDHD-Dipp)AuNTf2]
188.0
190.7
119.6 (CCN
)
In complex 4, the carbon signals of acetonitrile are
found at δ(13C) = 2.8 ppm (CCH3) and δ(13C) = 119.6
ppm (CCN), which appeared slightly downfield shifted
with respect to the non-coordinated acetonitrile [δ(13C)
= 1.9 ppm (CCH3), 116.9 ppm (CCN)], indicating a net
electron transfer from the coordinated NCCH3 to the
gold center.[19,21] The carbene carbon (CNHC) in 5 is
apparent from the signal at δ(13C) = 190.7 ppm,
significantly highfield shifted with respect to that of
the neutral species 3b [δ(13C) = 197 ppm] after halide
abstraction. The 19F{1H} NMR spectrum of complex 5
shows a singlet at δ(19F) = -74.5 ppm corresponding to
the CF3 group (see Supporting Information).
Figure 2. Ball-and-stick representation of cationic complex
[(NaphtDHD-Mes)Au(NCMe)]SbF6 (4, left) and a side-
view thought the N-CNHC-N plane (right). Hydrogen atoms
are omitted for clarity.
Solid state structures and steric profiling
X-ray diffraction studies on single crystals of 3b and
3c confirmed the structures of the NHC gold(I)
chloride complexes determined by NMR.[22] Suitable
single crystals were obtained by slow evaporation of
the solvent from a concentrated CH2Cl2/pentane
solution of the complex at room temperature. The
molecular structures are shown in Figure 1, and the
selected crystallographic data are listed in Table 2.
The CNHC-Au-Cl bond angle found in 3b (179.32(12)°)
is almost linear, while the structure of 3c showed a
stronger deviation from linearity (173.88(13)°).
Interestingly, both complexes exhibit broad N-CNHC-N
angles (around 121°), which match those of other
expanded-ring NHC-Au(I) complexes in the current
literature.[13] The CNHC–Au (2.000(4)–2.002(5) Å) and
Au−Cl (2.2955(1)–2.3158(12) Å) bond lengths lie in
the range of other comparable expanded-ring NHC-
Au(I) systems.[11,12,14]
Table 2. Selected structure and steric data of complexes 3b,
3c and 5.
CNHC-Au
CNHC-Au-X
N-CNHC-N
Torsion
α (°)24
25
Complex
Au-X (Å)
2.2955(11)
2.3158(12)
2.1069(17)
%Vbur
(Å)
(Å)
(°)
[(NaphtDHD-
Dipp)AuCl] (3b)
[(NaphtDHD-
2.000(4)
2.002(5)
2.004(18)
179.32(12)
173.88(13)
177.90(7)
121.2(4)
121.3(5)
30.9
22.2
49.4
54.0
46.4
51.6
Xyl)AuCl] (3c)
[(NaphtDHD-
121.29(16)
Dipp)AuNTf2](5)
Figure 1. Molecular structures of 3b (left) and 3c (right) in
the solid state (ORTEP representations). Ellipsoids are
drawn at 45% probability level and all hydrogen atoms are
omitted for clarity.
The torsion angle α(°) is a measurement of the
spatial twisting of the N-aryl substituents around the
CAr---N2—N5---CAr’ plane, with respect to the metal’s
coordination sphere.[24] The torsion angles found in 3b
(α = 30.9°) and 3c (α = 22.2°) reveal the influence of
the ortho-substituents in the N-aryls over the spatial
arrangement of the complex. The bulky ortho-
isopropyl substituents present in 3b promote the
twisting of the N-aryls resulting in an increased torsion
angle while, comparatively, the ortho-methyl
Gratifyingly, slow evaporation of a solution of 4 in
CH2Cl2 led to the formation of a single crystal suitable
for X-ray analysis (Figure 2). The presence of the
-
counterion (SbF6 ) and the coordination of acetonitrile
to the gold center were unequivocally established.[23]
A colourless single crystal from 5 was grown by slow
3
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
substituents found in 3c produce smaller impact on the
twisting of the molecule. In addition, the naphthalene
backbone exerts sharp influence on the ortho-
substituents of the N-aryls by pushing them down
towards the gold center. The solid state structure of 4
from a side-view (Figure 2, right) exhibits this effect.
The twisting of both N-aryls is also visible from this
privileged picture.
while top-right and bottom-left corners are more
occupied, a disproportionated steric distribution along
the X axis can be noticed. This is associated to the
molecular geometry of 3b, in which two ortho-
isopropyl substituents are relatively pointing down
towards the gold center whereas the other two are
pointing out of the sphere’s first coordination of the
complex. Analogously, the ortho-methyl substituents
in complex 3c leave some “free” or “uncovered”
region (depicted along the X axis).
-
The counteranion (NTf2 ) in complex 5 exert high
influence over two isopropyl groups of the N-aryls by
pushing them out of the first coordination sphere of the
gold center as evidenced by the increase in the torsion
angle (α = 49.4°) with respect to that of complex 3b (α
= 30.9°). This steric re-distribution can easily be
appreciated when comparing the steric maps of both
3b and 5, and particularly focusing on the top-right and
bottom-left corners. While the steric map of 3b reveals
those bulky corners (orange), the analogous ones in 5
seem to be less occupied (yellow). The
diminished %Vbur value of 5 (51.6%) with respect to
3b (%Vbur = 54.0%) proves numerically the influence
of the triflimidate moiety in the steric bulkiness of the
gold complex after halide abstraction.
Catalytic studies: Cycloisomerization of 1,6-
Enynes
Figure 3. ORTEP representation of [(NaphtDHD-
Dipp)AuNTf2 (5). Ellipsoids are drawn at 45% probability
level. Hydrogen atoms and one CHCl3 molecule were
omitted for clarity.
Gold complexes are excellent catalysts for the
cyclization of a variety of enynes.[29] This divergent
transformation has been examined in the presence of
phosphine-, phosphite- and NHC-based catalysts due
to the growing applicability in the synthesis of
complex molecules.[30–33]
The Topographic Steric Maps (TSM) and percent
buried volume (%Vbur) values were derived with help
of the user-friendly interface (SambVca 2.0)
developed by Cavallo et. al. using the information
from the CIF files of the X-ray diffraction analysis
(Table 2).[25–27] Orange-to-red zones correspond to a
more occupied (bulky) space while the green-to-blue
areas represent the less bulky zones in the first
coordination sphere of the gold center (Figure 4).
In the initial catalytic studies, we tested enyne 6a as
benchmark substrate under open-air conditions in non-
anhydrous CH2Cl2 by varying reaction times and
temperature. The results using complexes 3a–c as pre-
catalysts and the cationic species 4 and 5 are
summarized in Table 3. The cyclization of 6a
proceeded in >99% conversion within 15 minutes at
room temperature in the presence of the complexes
3a–c activated by AgSbF6 to give a mixture of
products 7–9 (Entries 1–3). Despite the unselective
transformation under the given conditions, it is worth
mentioning that a 1.0 mol% loading was able to
promote the conversion almost completely in only a
few minutes. Echavarren et. al. reported that the
phosphine donor gold complexes are efficient catalyst
at the 2.0–5.0 mol% loading in comparative reaction
times.[34]
Table 3. Exploration of cycloisomerization conditionsa
Figure 4. Steric maps of 3b, 3c and 5.[28]
Entry
[Au] cat
T [°C]
23
t [min]
15
Conversion [%]b
Ratio 7:8:9c
1/0.6/1
1
2
3
3a + AgSbF6
3b + AgSbF6
3c + AgSbF6
>99
>99
>99
The TSM of 3b confirms the above-mentioned
twisting of the N-aryls with respect to the gold center;
23
15
0.5/0.6/1
0.3/0.6/1
23
15
4
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
4
5
3a + AgSbF6
0
0
0
0
20
20
30
15
>99
>99
>99
>99
1/0.5/0.9
0.3/1/0.6
1/0.1/0.1
0.6/1/0.1
3b, in the presence of AgSbF6 as the activator (Table
5). Catalyst 3a afforded a minor quantity of the endo-
product 8 (3–10%, Entries 1 and 3) and predominantly
the exo-product 7 (88–94%) for both substrates,
accompanied by some traces of dienes 7b’ and 7c’ (2–
3%) which could be detected by 1H NMR analysis. The
bulkier complex 3b afforded no endo-product (8) and
a mixture of isomers 7 appeared after 45 minutes
(Entries 2 and 4). Interestingly, complex 3b yielded a
1:1 mixture of only the isomers 7b and 7b’ after 2
hours (see Supporting Information) with no traces of
product 8b. Similar product distribution was reported
by Fensterbank et. al. by using phosphine oxide-based
gold(I) complexes which afforded a mixture of
isomers [7b (88) : 7b’ (7)] upon 2 hours reaction, and
by Echavarren et. al. by using some phosphine-gold(I)
complexes.[35,36]
3b + AgSbF6
6d
7d
4
5
a) Reaction conditions: 6a (0.2 mmol), [Au] cat. (2.0 mol%),
CH2Cl2 (2.0 mL), 23°C. b) Determined by 1H NMR analysis.
c)
d)
Determined by GC-MS analysis. 0.5 mol% catalyst
loading was used.
The formation of product 9a can be explained by water
addition to the intermediate cation/carbene.[33] When
we settled the reaction at 0°C, by using complex 3a,
no significant variation of the result was observed
(Entry 4). In addition, the transformation of 3b at same
temperature remained unselective (Entry 5).
Importantly, the use of complexes 4 and 5,
considerably improved the selectivity, even under
“wet” conditions, affording 7a (Entry 6) and 8a (Entry
7) as the main products, respectively. Moreover, a
lower catalyst loading of the cationic species 4–5 (0.5
mol%) was enough to promote full conversion of the
substrate after 15–30 minutes. By setting up the test
reaction under anhydrous conditions (Table 4), only
products 7a and 8a were formed. In all cases 99%
conversion was achieved within 15 minutes. In the
presence of 3b a mixture of products was observed [7a
(67) : 8a (32), Entry 1]. The formation of the main
product 7a fits within the general exo-type mechanism
followed by a single cleavage of the gold-carbene
intermediate. No significant difference on the
regioselectivity was observed by employing the
cationic gold catalyst 4 [7a (64) : 8a (35), Entry 3]. In
contrast, the treatment of 6a with a catalytic 1:1
mixture of 3b and AgSbF6 produced almost
exclusively the endo-type single cleavage product 8a
(99%, Entry 2).
Table 5. Skeletal rearrangement of enynes 6b and 6ca
Entry
Enyne
6b
Catalyst
Products (Yield[%])b
7b (94) + 7b’ (3) + 8b (3)
7b (66) + 7b’ (34)
1
2
3
4
3a + AgSbF6
3b + AgSbF6
3a + AgSbF6
3b + AgSbF6
6b
6c
7c (88) + 7c’ (2) + 8c (10)
7c (90) + 7c’ (10)
6c
a) Reaction conditions: 6 (0.1 mmol), [Au] catalyst (2 mol%),
anhydrous CH2Cl2, 23°C, 45 min. b) Determined by 1H NMR
analysis.
Subsequently, the performance of complex 5 was
investigated at lower catalyst loadings (Table 6). 0.25
mol% loading afforded a combined yield of 92% after
1 hour (Entry 1). When the loadings were diminished
to 0.15–0.10 mol%, the yields proportionally
decreased (65–41%, Entries 2 and 3, respectively).
After 12 hours, 0.05 mol% of catalyst afforded 37%
yield (Entry 5), slightly higher than that after 1 hour
(29%, Entry 4). An 11% yield was observed after 12
hours using 0.01 mol% catalyst loading (TON = 1100,
Entry 7) while a 0.005 mol% loading provided 7%
yield after 24 hours (TON = 1400, Entry 9) which
proves that the gold complex remained catalytically
active in solution after several hours.
Table 4. Skeletal rearrangement of enyne 6aa
Entry
Catalyst
Conversion (%)b
Products (Yield[%])c
7a (67) + 8a (32)
8a (99)
1
2
3
4
3a + AgSbF6
>99
>99
>99
>99
3b + AgSbF6
4
5
7a (64) + 8a (35)
7a (28) + 8a (71)
a)
Reaction conditions: 6a (0.1 mmol), [Au] catalyst (2
Table 6. Efficiency of cationic complex 5 on the cyclization
of enyne 6aa
mol%), anhydrous CH2Cl2, 23°C, 15 min. b) Determined by
1H NMR analysis. c) Determined by GC-MS analysis.
To our surprise, the employment of cationic complex
5 led to a decrease in the selectivity of the reaction,
yielding a mixture of endo : exo products (71:28, Entry
4), despite a full conversion.
We then extended our study on other enyne systems.
Therefore, substrates 6b and 6c were treated under the
conditions mentioned below, using complexes 3a or
5
TOF
(h-1)
368
Entry
1
t [h]
1
7+8 (%)b
TON
368
(mol%)
0.250
92
5
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
2
3
4
5
6
7
8
9
0.150
0.100
0.050
0.050
0.025
0.010
0.005
0.005
1
65
41
29
37
16
11
4
433
410
433
410
580
62
presence of catalyst 5, while pre-catalyst 3b activated
with 1 equivalent AgNTf2 was found to be slightly
slower, but active enough to reach a full conversion
after 7.5 hours. For both catalytic systems, the reaction
followed first-order kinetics until 99% conversion
along 5 hours and 7.5 hours, respectively.
1
1
580
12
12
12
12
24
740
640
53
1100
800
92
67
7
1400
66
a) Reaction conditions: 6a (50 µmol), 5 (0.005–0.250 mol%),
anhydrous CH2Cl2 (0.05 M), 0°C. b) Determined by GC-MS
analysis using dodecane as the internal standard.
Next the activity of the complexes was evaluated in
the gold-catalyzed synthesis of phenols, by using
substituted furane 10 as the substrate and 5 mol%
catalyst at the 50 µmol scale.[37] As indicated in Table
7, the pre-catalysts 3a, 3b and 3c afforded moderated
to good yields (55–89%, Entries 1–3) whereas the
active gold catalysts 4 and 5 yielded 97% and 99%
respectively, under the same reaction conditions
(Entries 4 and 5).
Table 7. Gold-catalyzed phenol synthesisa
Figure 5. Cycloisomerization of N-propargyl carboxamide
12 to oxazoline 13 catalyzed by (NaphtDHD-Dipp)Au+
NTf2 species.
-
Conclusion
Entry
Catalyst
3a + AgNTf2
3b + AgNTf2
3c + AgNTf2
4
Yield (%)b
Efficient, and in some cases highly selective, catalysts
for the cycloisomerization of 1,6-enynes have been
synthesized using sterically demanding eight-
membered NHC ligands. This library of gold(I)
complexes has been fully characterized and the solid-
state structures have confirmed their structures as well
as their steric properties.
1
2
3
4
5
69
55
89
97
99
5
The X-ray diffraction analysis revealed broad N-CNHC
-
a) Reaction conditions: 10 (15.3 mg, 50 µmol), [Au] cat (5.0
mol%), AgNTf2 (5.0 mol%), Diphenylmethane (8.3 mg, 50
N angles (around 121°) and torsion angles between 12°
and 31° which are directly related to steric
environment around the gold center (%Vbur = 46–54%).
The catalytic performance of the complexes has been
b)
µmol; internal standard), CDCl3 (0.5 mL), 23°C, 2 h.
Determined by 1H NMR analysis.
evaluated at the
2
mol% loadings in the
cycloisomerization of 1,6-enynes obtaining full
conversions in all cases. The activity of cationic
complex 5 was tested down to 0.005 mol% loading
yielding 7% combined product after 24 hours.
Interestingly, the sterically demanding complex 3b
(%Vbur = 54.0) resulted less active than 3c (%Vbur
=
46.4) on this transformation affording only 55% yield.
Given that catalyst 5 afforded an excellent yield (99%),
the results from the in situ activated pre-catayst 3b
could be misleading, since the adventitious Ag+ (from
the NHC-AuCl activation) might intercept key
organogold intermediates and affect negatively the
catalysis.[38,39]
In the skeletal rearrangement of 6a the pre-catalyst 3b
afforded 99% conversion and excellent selectivity
(endo : exo = 99:1) after several minutes using 2 mol%
catalyst loading. In the presence of this sterically
demanding complex, the reaction proceeded via endo-
type single cleavage to give almost exclusively the
product 8a. The selectivity of the reaction is
considerably diminished using complex 3a and
cationic species 4 and 5 (endo = 32–71: exo = 28–67)
despite full conversions were observed under the
studied conditions.
Finally, we assessed the (NaphtDHD-Dipp)Au+
-
NTf2 species to ascertain the catalytic activity of either
the pre-formed catalyst or the in situ-activated gold
complex.[40] For direct comparison, 5 and the system
3b + AgNTf2 were employed in the cycloisomerization
of N-propargyl carboxamide 12 as the case reaction
(Figure 5). The conversion to form oxazoline 13
proceeded in 99% conversion after ca. 5 hours in the
In the latter, a comparative kinetic experiment on the
cyclization of
a
N-propargyl carboxamide
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000281
Advanced Synthesis & Catalysis
demonstrated no significant difference between the
performance of the active gold species (NaphtDHD-
134.43, 133.78, 132.42, 131.53, 131.48, 130.75, 130.00,
129.88, 125.94, 58.03, 21.10, 18.52, 18.30; IR (ATR, cm-1):
νmax = 2978, 2856, 1476, 1440, 1308, 1037, 877, 824, 686;
UV-vis (CH2Cl2) λ (log ε): 279 (3.82), 289 (3.84), 299
(3.68), 316 (2.79) nm; HR-MS (ESI): calc.
C31H32AuClN2Na [M+Na]+ = 687.1812, found: 687.1829.
(NaphtDHD-Dipp)AuCl (3b). Yield: 32 mg (43%); m.p:
>300 °C; 1H NMR (500 MHz, CDCl3) δ 7.89 (d, J = 8.2 Hz,
2H), 7.39 (t, J = 7.6 Hz, 2H), 7.28 (t, J = 7.8 Hz, 2H), 7.25
(d, J = 7.1 Hz, 2H), 7.17 (dd, J = 7.7, 1.2 Hz, 2H), 7.05 –
6.95 (m, 2H), 6.28 (d, J = 16.0 Hz, 2H), 4.56 (d, J = 16.0
Hz, 2H), 3.16 (dt, J = 13.6, 6.8 Hz, 2H), 2.24 (dt, J = 13.5,
6.7 Hz, 2H), 1.41 (d, J = 6.7 Hz, 6H), 1.40 (d, J = 6.8 Hz,
-
Dipp)Au+ NTf2 generated either by in situ activation
(3b + AgNTf2) or from the preformed gold catalyst 5.
With the results discussed above, this library of neutral
and cationic NHC-Au(I) complexes shows
a
tremendous potential to be employed as catalysts on
several other gold(I)-catalyzed transformations.
Experimental Section
Unless otherwise stated, the synthesis of gold complexes
was carried out using standard Schlenk techniques using dry
solvents dispensed from MB SPS 800 solvent system or
inside an argon-filled glove box when needed. Organogold
compounds were stored under an atmosphere of dry
nitrogen at low temperature (ca. -35°C) after syntheses to
avoid decomposition. THF was degassed by Freeze-Pump-
Thaw prior to use. NMR characterization data was collected
at 296 K at the department of chemistry of the University of
Heidelberg in Bruker Avance DRX 300 (300 MHz), Bruker
Avance III 400 (400 MHz), Bruker Avance III 500 (500
6H), 1.07 (d, J = 6.8 Hz, 6H), 0.56 (d, J = 6.7 Hz, 6H); 13
C
NMR (126 MHz, CDCl3) δ 197.17, 145.50, 144.99, 143.86,
135.12, 131.73, 131.68, 131.19, 130.95, 129.27, 125.88,
124.93, 124.87, 59.62, 29.30, 28.16, 25.37, 24.91, 24.80,
23.86. IR (ATR, cm-1): νmax = 2964, 2866, 1505, 1447, 1324,
1055, 823, 793; UV-vis (CH2Cl2) λ (log ε): 278 (3.90), 288
(3.92), 299 (3.94), 315 (2.91) nm; HR-MS (ESI): calc. C37-
H44AuClN2Na [M+Na]+ = 771.2751, found: 771.2762.
(NaphtDHD-Xyl)AuCl (3c). Yield: 37 mg (63%); m.p:
231 °C; 1H NMR (500 MHz, CDCl3) δ 7.90 (d, J = 8.2 Hz,
1H), 7.38 (t, J = 7.6 Hz, 1H), 7.14 (d, J = 7.0 Hz, 1H), 7.11
(d, J = 4.9 Hz, 2H), 6.92 (t, J = 4.5 Hz, 1H), 6.41 (d, J = 16.0
Hz, 1H), 2.42 (s, 3H), 1.46 (s, 3H); 13C NMR (126 MHz,
CDCl3) δ 195.23, 147.66, 135.25, 134.85, 134.21, 132.32,
131.60, 131.40, 130.88, 129.31, 128.41, 125.99, 57.90,
18.62, 18.41; IR (ATR, cm-1): νmax = 3043, 2953, 2852, 1509,
1470, 1305, 1170, 823, 771, 684. UV-vis (CH2Cl2) λ (log ε):
269 (4.98), 279 (4.90), 289 (4.90), 299 (4.77), 316 (3.72)
1
MHz) and Bruker Avance III 600 (600 MHz). H and 13C
spectra are given relative to TMS (0.00 ppm) or to the
residual protons in deuterated solvents (CD2Cl2: 5.32 ppm,
53.84 ppm; and CDCl3: 7.26 ppm, 77.16 ppm). Melting
points were determined in a BÜCHI automated B-545
apparatus and are uncorrected. High resolution mass spectra
(HRMS) were recorded on a Bruker Apex-Qe hybrid 9.4
FT‐ ICR spectrometer (ESI+, MALDI). IR spectra were
recorded on a Bruker Vector 22 Lumos FT‐ IR. UV-Vis
nm; HR-MS (ESI): calc. C31H32AuClN2Na [M+Na]+
659.1499, found: 659.1513.
=
were accomplished in
a
V-670 UV-VIS-NIR
[(NaptDHD-Mes)AuNCCH3]SbF6 (4). Inside the glove
box, a 4.5 mL ambar screw-cap Teflon coated vial provided
with stirring bar, was charged with gold(I)-chloride
complex 3a (50 mg, 0.075 mmol) and AgSbF6 (26 mg, 0.075
mmol, 1.0 equiv). The vial was taken out of the glove box
and anhydrous MeCN (1.0 mL) was added to the mixture
via syringe. The suspension was stirred at room temperature
for 4.5 hours and filtered through a short plug of Celite to
remove solid impurities and the solvent was removed in
vacuo. The oily residue was taken up in anhydrous CH2Cl2
(1 mL) and diethyl ether was added dropwise to solution
until a colorless precipitate appeared. The suspension kept
on the bench for 15 minutes in dark and solvent was
removed via syringe. The crude product was recrystallized
with CH2Cl2/pentane mixture and dried in vacuum to afford
44 mg (64%) of 4 as a colorless solid. m.p: 176 °C; 1H NMR
(600 MHz, CD2Cl2) δ 7.92 (d, J = 8.3 Hz, 2H), 7.38 (t, J =
7.6 Hz, 2H), 7.14 (d, J = 7.0 Hz, 2H), 6.98 (s, 2H), 6.76 (s,
2H), 6.43 (d, J = 16.1 Hz, 2H), 4.46 (d, J = 16.1 Hz, 2H),
2.35 (s, 6H), 2.22 (s, 6H), 2.03 (s, 3H), 1.35 (s, 6H); 13C
NMR (151 MHz, CD2Cl2) δ 188.06, 144.71, 138.91, 134.45,
134.37, 131.88, 131.85, 131.13, 130.12, 129.83, 126.11,
119.57, 58.20, 20.80, 18.20, 18.06, 2.84; IR (ATR, cm-1):
νmax = 2920, 2856, 2332, 2306, 1609, 1523, 1475, 1379,
1358, 1335, 1312, 1188, 1144, 1039, 1005, 947, 854, 825,
791, 766, 654; HR-MS (ESI): calc. C33H35AuN3 [M-SbF6]+
= 670.2497, found: 670.2505.
Spectrophotometer. NHC-HBF4 salts were prepared
according to literature.[12c]
General procedure for the synthesis of NHC-AuCl
complexes 3a-c
A 4.5 mL screw-cap vial provided with stirring bar, was
charged with NHC-HBF4 salt (0.10 mmol), DMS-AuCl
(0.05 mmol, 0.5 equiv.), t-BuOK (0.165 mmol, 1.65 equiv)
and 4A MS (30 mg) under open-air conditions at room
temperature. Vial was sealed with Teflon/silicone septa and
purged-back filled with nitrogen three times. After cooling
to -78°C in dry ice-acetone bath, dry degassed THF (3 mL)
was introduced to vial with a syringe and suspension was
stirred overnight, allowing to warm up slowly to room
temperature. Dark suspension was directly passed through a
pad of silica gel (1.5 x 3 cm), followed by washing the pad
with anhydrous CH2Cl2 (10 mL). The solvent was removed
in vacuo till a minimal volume followed by addition of n-
hexane (ca. 40 mL). The precipitated gold complex was
allowed to settle for 1 hour and the supernatant liquid was
decanted off, the solid residue was washed with n-hexane
and dried. Pure gold complexes are obtained as colorless
solids after recrystallization from CH2Cl2:pentane mixture
or by flash chromatography (ALOX, CH2Cl2/P.E, 4:1).
(NaptDHD-Mes)AuCl (3a). Yield: 52 mg (78%); m.p:
266–268 °C; 1H NMR (500 MHz, CDCl3) δ 7.88 (d, J = 8.1
Hz, 2H), 7.46 – 7.27 (m, 2H), 7.14 (d, J = 6.9 Hz, 2H), 6.90
(s, 2H), 6.71 (s, 2H), 6.37 (d, J = 16.0 Hz, 2H), 4.42 (d, J =
16.0 Hz, 2H), 2.37 (s, 6H), 2.19 (s, 6H), 1.40 (s, 6H); 13C
NMR (126 MHz, CDCl3) δ 195.35, 145.37, 137.84, 135.19,
[(NaptDHD-Dipp)Au]NTf2 (5). To a stirred solution of
AgNTf2 (39 mg, 0.10 mmol) in CH2Cl2 (2 mL) was added
7
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000281
Advanced Synthesis & Catalysis
gold(I)-chloride complex 3b (78 mg, 0.10 mmol) and
immediately a suspension was formed. The reaction mixture
was stirred in dark at room temperature for 8 hours and
filtered through a short plug of Celite. The solvent was
removed under a steam vent of nitrogen until an oily residue
was obtained. Precipitation of the product was effected by
crashing out the residue with the spatula over cold pentane.
The crude product was recrystallized with CH2Cl2/pentane
mixture and dried in vacuum to afford 70 mg (71%) of 5 as
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1
a colorless solid. m.p: 192°C; H NMR (500 MHz, CDCl3)
δ 7.97 (d, J = 7.8 Hz, 2H), 7.49 – 7.45 (m, 2H), 7.37 – 7.31
(m, 4H), 7.26 (dd, J = 7.1, 1.9 Hz, 2H), 7.08 (dd, J = 7.7, 1.3
Hz, 2H), 6.37 (d, J = 16.0 Hz, 2H), 4.68 (d, J = 16.0 Hz, 2H),
3.24 – 3.14 (m, 1H), 2.33 – 2.23 (m, 2H), 1.47 (d, J = 6.8
Hz, 6H), 1.46 (d, J = 6.8 Hz, 6H), 1.09 (d, J = 6.8 Hz, 6H),
0.62 (d, J = 6.7 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ
190.86, 145.34, 144.96, 143.54, 135.19, 132.01, 131.77,
131.27, 130.85, 129.57, 126.09, 125.27, 125.23, 60.04,
29.45, 28.33, 25.39, 25.12, 24.37, 24.08. IR (ATR, cm-1):
νmax = 2965, 2929, 2870, 1674, 1510, 1463, 1398, 1363,
1324, 1195, 1131, 1055, 951, 823, 803, 773, 653, 610. HR-
MS (ESI): calc. C39H44AuF6N3O4S2Na [M-Na]+ = 745.3438,
found: 745.3428.
General procedure for the catalyzed cycloisomerization
of 1,6-enynes
A screw-cap vial was charged with enyne (0.5 mmol), NHC-
Au(I) complex and corresponding silver halide. The vial
was sealed with a Teflon-coated septum and purged with
nitrogen once. Anhydrous CH2Cl2 (2 mL) was introduced to
mixture and reaction was stirred for the given time and
temperature (vide supra). Crude products were purified by
column chromatography (SiO2, P.E 100%).
Acknowledgements
A.C.R. is grateful for a Ph.D. fellowship from Consejo Nacional de
Ciencia y Tecnología (México).
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obtained defines the connectivity and confirms the
overall composition of 4, but bond lengths and angles
of the NHC complex are not reliably obtained from
this crystal structure and are therefore not included in
Table 2. The CIF file has been included in the
Supporting Information.
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Advanced Synthesis & Catalysis
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10
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10.1002/adsc.202000281
Advanced Synthesis & Catalysis
UPDATE
Gold(I) Complexes with Eight-Membered NHC
Ligands: Synthesis, Structures and Catalytic
Activity
Adv. Synth. Catal. Year, Volume, Page – Page
A. Cervantes-Reyes, F. Rominger, M. Rudolph,
A. Stephen K. Hashmi*
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