Mechanistic investigations of a stable, highly active, extremely sterically shielded molecular gold catalyst

Article abstract of DOI:10.1002/cctc.201200944

An N-heterocyclic carbene gold complex IPr**AuNTf₂ has been synthesized, spectroscopically investigated, structurally characterized, and used as a highly active and stable catalyst in the Hashmi phenol synthesis (IPr=1,3-di-p-tolylimidazol-2-ylidene with four di-tert-butylbenzhydryl ortho substituents, Tf=trifluoromethansulfonyl). A side reaction comprises an irreversible arene oxide ring opening with subsequent 1,2 methyl shift. The advantage of the steric demand of the ancillary ligand was explained by higher equilibrium concentrations of the cationic gold species, the circumvention of inactive dinuclear intermediates, and the inhibition of catalyst decomposition pathways. The methanol addition-hydration of alkynes featured a turnover-limiting proton transfer step of an alkenylgold catalyst resting state, indicated by a large primary kinetic isotope effect and an alkyne competition experiment.

Full text of DOI:10.1002/cctc.201200944

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DOI: 10.1002/cctc.201200944
Mechanistic Investigations of a Stable, Highly Active,
Extremely Sterically Shielded Molecular Gold Catalyst
[
a]
Simone G. Weber, David Zahner, Frank Rominger, and Bernd F. Straub*
th
Dedicated to Prof. Dr. A. Stephen K. Hashmi on the occasion of his 50 birthday
An N-heterocyclic carbene gold complex IPr**AuNTf has been
explained by higher equilibrium concentrations of the cationic
gold species, the circumvention of inactive dinuclear inter-
mediates, and the inhibition of catalyst decomposition path-
ways. The methanol addition–hydration of alkynes featured
a turnover-limiting proton transfer step of an alkenylgold cata-
lyst resting state, indicated by a large primary kinetic isotope
effect and an alkyne competition experiment.
2
synthesized, spectroscopically investigated, structurally charac-
terized, and used as a highly active and stable catalyst in the
Hashmi phenol synthesis (IPr**=1,3-di-p-tolylimidazol-2-yli-
dene with four di-tert-butylbenzhydryl ortho substituents, Tf=
trifluoromethansulfonyl). A side reaction comprises an irreversi-
ble arene oxide ring opening with subsequent 1,2 methyl shift.
The advantage of the steric demand of the ancillary ligand was
Introduction
Results and Discussion
Owing to its inertness and noble character, gold “was consid-
The gold chloride complex IPr**AuCl (1) was activated with
[
1]
ered to be catalytically dead.” Only a modest research activity
silver bis(trifluoromethanesulfonyl)imide (AgNTf ). We isolated
2
[2]
in the field of gold catalysis was observed until 1998. In that
year, Teles and co-workers reported the catalytic activity of
a cationic gold complex in the addition of methanol
and completely characterized the resulting complex
IPr**AuNTf (2) (Scheme 1 and Figure 1).
2
[
3]
across alkyne triple bonds. This publication initiated
a “gold rush”, accompanied by an exponential in-
[
4]
crease of publications in this field. The reduction of
gold complexes to colloidal gold limits the maximum
turnover number. However, phosphine and N-hetero-
cyclic carbene ligands stabilize cationic 12-valence
electron gold complexes, thereby suppressing cata-
[
4c,k,5]
lyst decomposition.
The unraveling of mechanis-
tic pathways is of particular interest: Several inter- Scheme 1. Synthesis of IPr**AuNTf (2).
2
mediates of gold-catalyzed reaction were character-
ized, for example, arene oxides in the Hashmi phenol
[
6]
synthesis.
Aurophilic attraction favors dinuclear gold species. The for-
mation of dinuclear gold intermediates is prevented by the
steric bulk of the IPr** ligand. In complex 2, even the medium-
sized NTf2 ligand faces repulsive interactions of its sulfonyl
groups and the arene substituents in IPr**, thereby featuring
a minimum distance of sulfonyl O and arene H of 241 pm.
The catalytic activity of complex 2 was tested in the rear-
rangement reaction of 2-methyl-5-[(prop-2-yn-1-yloxy)methyl]-
furan to 1,3-dihydro-5-methyl-4-isobenzofuranol by using the
same reaction conditions as described by Hashmi et al. with
the exception of the internal standard (Hashmi group, n-tetra-
We aimed at isolating intermediates of catalytic cycles by
using an ancillary N-heterocyclic carbene ligand with a record
steric shielding, attempting to slow down elementary steps at
[
7]
the gold center. Unexpectedly, we found a very high catalytic
activity of our IPr**Au system in the Hashmi phenol synthesis,
and the hydration of alkynes (IPr**=1,3-bis{2,6-bis[bis(4-tert-
butylphenyl)methyl]-4-methylphenyl}imidazol-2-ylidene).
[
a] S. G. Weber, D. Zahner, Dr. F. Rominger, Prof. Dr. B. F. Straub
Organisch-Chemisches Institut
[8]
decane; this work, dodecane) (Scheme 2).
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: straub@oci.uni-heidelberg.de
Homepage: http://www.uni-heidelberg.de/fakultaeten/chemgeo/oci/ak-
straub
Catalysts, catalyst concentrations, reaction conditions, overall
phenol yields, conversions, turnover numbers (TONs) and turn-
over frequencies (TOFs) are summarized in Figure 2 and
Table 1.
2
-Methyl-5-[(prop-2-yn-1-yloxy)methyl]furan is a challenging
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200944.
substrate for gold catalysts because of the often mediocre
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Figure 1. Space-filling model of the single-crystal X-ray structure of
Figure 2. Catalyst complexes in the Hashmi phenol synthesis. Catalysts A
and B are from Ref. [10], C from Ref. [11], D from Ref. [12], and E, F, and G
from Ref. [8].
IPr**AuNTf
2
(2). Selected bond angles (8): N5ÀC1ÀN2 102.1(6), C1ÀAuÀN1
[9]
179.3(3); selected bond lengths (pm): AuÀC1 196.1(7), AuÀN1 212.9(8).
anistic perspective, on the other hand, its detection offers a val-
uable opportunity for mechanistic investigations. Thus, we in-
troduced a methyl substituent to the propargyloxymethyl
group of the substrate (Scheme 3). The methyl marker helps to
decide whether the methyl group migrates or whether the re-
arrangement towards the minor isomer is based on an epoxide
walk over the arene system.
Scheme 2. Catalyst 2 in the Hashmi phenol synthesis.
From this reaction the major isomer was isolated in pure
form. The minor isomer was enriched to 33% by column chro-
matography. HSQC and HMBC NMR spectroscopy revealed that
the methyl groups were on the same side in the minor isomer.
This structure assignment proved a Wagner–Meerwein-type
TONs and phenol yields. The highest TON of 5500 was ob-
tained with complex B from the Hashmi group, but without
[
10]
À1
full conversion (55% yield). A record TOF of 450 min was
reached by complex G from the same group,
[
8]
again without full conversion (54% yield). In
comparison, the product of the Hashmi phenol
synthesis could be reached in good yields and
with high TONs and TOFs by the IPr**Au system.
Interestingly, amounts of 12 to 13% of the
isomer 1,3-dihydro-6-methyl-5-isobenzofuran-5-
ol were also found. If traces of protic acids were
the catalyst for the rearrangement of benzene
oxide, then no isomeric phenol product would
be observed, as it was the case in other gold-
catalyzed phenol syntheses with the same sub-
strate. Traces of the isomer, however, had been
Table 1. Parameters for the gold-catalyzed Hashmi phenol syntheses at 258C in the literature
(entries 1–10), and in this work (11–14).
[i]
[j]
Catalyst Solvent [mol%] Substrate
t
Conversion
Yield [%]
GC or Isolated
NMR
TON TOF
concentration
À1
À1
[
molL
]
[h] [%]
[min
]
[
[
a]
a]
[10]
[10]
[10]
[11]
[12]
[12]
[12]
[8]
[c]
[c]
[c]
[d]
[c]
1
2
3
4
5
6
7
8
9
A
B
B
C
D
D
D
E
F
G
2
2
2
CD
CD
CD
2
2
2
Cl
Cl
Cl
2
2
2
0.1
0.2
0.2
0.2
0.08
0.4
0.4
0.4
0.4
0.4
0.4
0.4
10 100
24 100
72 69
14 100
97
90
55
–
38
30
–
–
–
94
–
–
970
1800
5500
94
1.6
[c]
[c]
0.05
0.01
1
1.3
1.3
0.1
6.3
[a]
b]
[
CH NO
3
2
[
a]
a]
a]
[c]
[c]
[c]
CDCl
CDCl
3
3
0.1
1
1
1
1
–
–
–
380
[
[
0.05
0.25
0.02
600 10.0
348 5.8
3050 50.8
670 2.7
540 450.0
900 60.0
1200 80.0
1780 29.7
[h]
CD
CH
2
Cl
Cl
2
82 87
[a]
a]
[e]
[e]
[e]
[f]
[e]
2
2
85
61
67
54
–
–
–
[
[8]
[e]
[e]
previously reported in the PtCl -catalyzed
2
CH Cl 0.1
4.13 96
0.02 99
0.25 100
0.25 100
1 100
2
2
[
13]
[a]
[8]
phenol synthesis. Its formation was apparent
10
11
CH
2
Cl
2
0.1
[
[
a]
a]
[h,k]
[h,k]
[h,k]
[h,k]
1
CH Cl 0.1
quant. 90
quant. 90
quant. 89
2
2
2
2
2
2
by the H NMR singlet signals at 6.98 and
[f]
1
1
2
3
CH
CH
Cl
Cl
0.075 0.4
0.05
6.64 ppm. Small traces of this isomeric phenol
[a]
[f]
0.4
0.4
[
a]
[f]
can be seen in figures of NMR spectra in the
Supporting Information of these publications, so
that this gold-catalyzed rearrangement appears
14
2
2
CH Cl 0.02
168 90
–
50
2500
0.2
2
[a] In situ activation of LAuCl by AgX; [b] No silver salt was added; [c] determined by NMR, by in-
tegration and comparison to the internal standard 1,3,5-tri-tert-butylbenzene; [d] monitored by
TLC; [e] determined by GC using n-tetradecane as internal standard; [f] determined by GC using
dodecane as internal standard; [g] determined by NMR; [h] average of two catalytic experi-
ments; [i] n(product)/n(catalyst) [j] TON/t; [k] ratio of isomers 7:1 (Scheme 2).
[
10–12,14]
to be a general side reaction.
On the one
hand, formation of the minor isomer must be
considered as a synthetic nuisance. From a mech-
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The hydration of alkynes to methylketones underlines the
catalytic activity of the IPr**Au system for further alkyne con-
[3,8,17]
version reactions (Scheme 5).
Reaction conditions, sub-
strate conversions, and ketone yields for the hydration of al-
kynes by catalyst 2 are summarized in Table 2. Catalyst 2 gave
good results and a wide range of alkynes was converted to
a ketone.
Scheme 3. Reaction of a 1-propargyloxyethyl-substituted furan.
methyl migration (Scheme 4). An epoxide walk was ruled out;
the CÀO bond cleavage of the oxirane moiety appeared to be
irreversible. In accordance with these results, a gold catalysis
with an isopropyl group migration to a chinoid product has
[
15]
been reported in the literature.
Scheme 5. Hydration of alkynes to ketones.
Table 2. Hydration of terminal alkynes RCꢀCH with D
gold complex 2.
2
O catalyzed by
[
b]
[b]
[b]
R
1+AgNTf
mol%]
2
T
t
Conversion
[%]
Yield
[%]
[
[8C] [h]
[
[
c]
e]
[d]
1
2
3
4
5
6
7
8
9
0
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
1
1
0.1
0.1
0.1
–
–
1
0.1
1
0.1
0.1
1
0.1
0.1
1
0.1
0.1
1
50
80
RT
80
80
80
80
80
80
80
80
80
50
80
80
50
80
80
50
80
80
80
80
18.5
1.5
25.5
168
1
3
22
28
1.5
70
0.25 >99
2.5
30
0.25 >99
1.5
30
0.25 >99
1.5
30
0.5
28
0.25 >99
15 >99
>99
>99
>99
>99
>99
>99
7
>99
>99
>99
99
[
d]
82
92
81
93
89
–
[f]
[f]
[d]
d]
[
[
[
[
[
g]
h]
g]
i]
[d]
[d]
[d]
67
94
80
88
88
88
94
93
95
97
98
96
82
78
92
89
[
[
d]
d]
p-CH
p-CH
3
O-Ph
O-Ph
1
3
1
1
C
C
C
C
C
C
C
C
C
3
3
3
4
4
4
6
6
6
H
H
H
H
H
H
H
H
H
7
7
7
9
9
9
1
1
1
2
3
4
>99
>99
[
6a,13]
+
15
>99
>99
Scheme 4. Hashmi phenol synthesis
with an IPr**Au catalyst system,
1
1
1
1
2
2
2
2
6
7
8
9
0
1
2
3
and proposed mechanism for the formation of a minor phenol isomer.
13
13
13
>99
>99
>99
>99
tBu
tBu
Thus, complex 2 combines high scores in three important
benchmarks of catalysis: activity (TOF), stability (TON), and
product yield. Additionally, pathways to isomeric phenol prod-
ucts become competitive. Four contributions of the high steric
bulk are probably responsible for the experimental observa-
tions. 1) Steric repulsion of the IPr** substituents and the
0.1
1
1-cyclohexenyl
1-cyclohexenyl 0.1
[
a] Reaction conditions: alkyne (0.2 mmol), D
2 4
O (0.05 mL), methanol-d
(
0.5 mL); [b] determined by NMR spectroscopy, 1,4-dioxane as internal
standard; [c] reaction performed in CD Cl in a sealed NMR tube; [d] iso-
2
2
À
anionic ligand NTf2 leads to a destabilization of complex 2,
lated yield; [e] reaction conditions: phenylacetylene (0.293 mmol), di-
chloromethane/water 2:1 (0.05 mL), CF COOH (10 mol%); [f] determined
by GC; [g] addition of HNTf (10 mol%); [h] addition of HNTf (1 mol%);
i] 10 mol% AgNTf
3
and thus to higher concentrations of the active cationic cata-
+
2
2
lyst species LAu . 2) The steric shielding of the reactive gold
[
2
.
center hampers catalyst decomposition pathways, for example,
redox reactions with concomitant metal precipitation, or ancil-
lary ligand protonation and dissociation. 3) The steric bulk ren-
ders dinuclear catalyst resting states impossible. Then, the un-
derlying attractive aurophilic interaction in diaurated catalyst
Besides these terminal alkynes, also 1-phenyl-1-propyne was
tested with 1 mol% catalyst in methanol at 808C. Full conver-
sion could be achieved after one hour and with an isolated
yield of 63%. Based on the different reaction times with a cata-
lyst loading of 0.1 mol%, the possibility of a chemoselective re-
action was checked. A 1:1 mixture of 1-octyne and phenylace-
tylene was reacted with the catalyst (0.1 mol%) in methanol at
808C. Interestingly, the conversion of 1-octyne was considera-
bly slowed down. In the case of this 1:1 substrate mixture, full
conversion of 1-octyne was achieved after 37 h. After this time,
[16]
resting states cannot raise the overall activation energy.
) Electronically disfavored but sterically less demanding path-
4
ways towards phenol isomers become competitive (Scheme 3).
The cationic gold complex was apparently the source of Lewis
[
6a]
acidity for the benzene oxide rearrangement
because
a proton-induced rearrangement would consistently lead to
the same isomer ratio for all gold catalysts.
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only 67% of the phenylacetylene had been converted. If
0.2 mol% catalyst were used, the conversion of 1-octyne was
completed after 7 h, along with 49% of phenylacetylene.
To explore the influence of the acid concentration, different
amounts of HNTf (1 mol% and 10 mol%) were added. Without
2
additional acid, completion of the reaction of phenylacetylene
to acetophenone under 0.1 mol% catalyst loading took seven
days (Table 2, entry 4). With addition of 10 mol% HNTf , the re-
2
action was finished after 1 h (Table 2, entry 5). Even the addi-
tion of 1 mol% HNTf strongly influenced the reaction rate, re-
2
sulting in a reaction time of only 3 h (Table 2, entry 6). It is
known in the literature that also HNTf alone catalyzes the re-
Scheme 6. Proposed mechanism for the addition of methanol to alkyne
substrate.
2
[
18]
action of phenylacetylene to acetophenone,
so we deter-
mined the catalytic activity of 10 mol% HNTf in the absence
2
of gold complexes. Only 7% of phenylacetylene were convert-
ed to acetophenone after 22 h (Table 2, entry 7), which thereby
proved the pivotal role of the gold catalyst 2.
Two other organic–synthetic transformations were efficiently
catalyzed by complex 2. In a modified protocol of Lavoie
[22]
et al., the NH bond of aniline was added across the C–C
triple bond of phenylacetylene (Scheme 7). The catalyst load-
ing was decreased from 5 mol% to 0.4 mol%. Instead of
10 mol% AgBF in acetonitrile, we employed 0.4 mol% AgNTf
Silver salts such as AgSbF also catalyze the hydration of ter-
6
[
19]
minal alkynes. However, the complete hydration of phenyla-
cetylene substrate to acetophenone catalyzed by 10 mol%
4
2
AgNTf required 28 h (Table 2, entry 8) compared to 1.5 h with
in THF. The obtained activity was similar to that of other cata-
2
[23]
1
mol% of gold complex 2 (Table 2, entry 2). These experi-
lysts described in the literature.
Catalyst 2 was also active in the three-component coupling
ments demonstrate that only a very minor portion of the sub-
strate is converted by the catalytic properties of the acid or
silver additives.
[17i,24]
reaction of aldehyde, amine and alkyne (Scheme 8).
Again, the steric hindrance did not hamper the catalytic ac-
The alkyne hydration proceed 3 to 5 times faster with H O
tivity of IPr**AuNTf compared to the performance of other
2
2
[17i,24b,d,g,h]
and CH OH than with D O and CD OD. This result is equivalent
systems.
3
2
3
to a primary kinetic isotope effect (KIE) typical for a rate-deter-
mining proton transfer step. Thus, the protonolysis of the
gold-carbon bond is the turnover-limiting step of this intermo-
lecular reaction. Recently, the group of Gagnꢁ and Widenhoe-
fer reported a KIE of 5.3 for the gold-catalyzed intramolecular
hydroalkoxylation of 2,2-diphenyl-4,5-hexadien-1-ol to a 2-vi-
[
20]
nyltetrahydrofuran derivative.
Methanol is usually the solvent of choice for the hydration
[
17a,d–f]
of alkynes with different catalysts.
The higher nucleophi-
licity of methanol compared to that of water at cationic gold
alkyne complexes has been reported in the literature: Leyva
and Corma observed a faster transformation of terminal al-
kynes to ketones in the presence of methanol, proposing
Scheme 7. Amination of phenylacetylene.
[17e]
a mechanism in which methanol attacks the triple bond.
,4-Dioxane at 1208C instead of methanol has been used as
solvent by Nolan et al., thereby enforcing water as the
1
[
17f]
nucleophile.
Combining the results of the competition experiment and
the large primary KIE, we propose a mechanism with fast
methanol addition to the alkyne ligand (Scheme 6). Because of
the extreme steric bulk of the IPr** ligand, we presume an anti
Scheme 8. Three-component coupling reaction of benzaldehyde, piperidine
and phenylacetylene.
[
21]
attack of the nucleophile. The catalyst resting state is a 2-
methoxyvinyl gold complex, which rationalizes the outcome of
the competition experiment: The 2-phenylvinyl gold species of
the phenylacetylene/acetophenone cycle accumulates, thereby
inhibiting the turnover of 1-octyne (see below).
Conclusions
The extreme steric shielding of the IPr**AuNTf2 catalytic
system resulted in high catalytic activity, high catalyst stability,
and high phenol yield in the Hashmi phenol synthesis. The for-
mation of two phenol isomers indicated that cationic gold spe-
cies and not protons induced the rearrangement of a benzene
The proposed turnover-limiting protonation of the vinyl
complex also correlates with the higher reaction rate of al-
kynes with more electron-donating substituents
Table 2).
R (see
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oxide intermediate to the phenol products. In a side reaction,
a Wagner–Meerwein-type methyl shift was involved. A large
primary kinetic isotope effect and alkyne competition experi-
ments indicated a turnover-limiting protonation of an alkenyl-
gold catalyst resting state in the methanol addition–hydration
of alkynes.
at RT. The suspension was filtered over a pad of Celite and the sol-
vent removed in vacuo to obtain 2 as a colorless solid (113.0 mg,
1
6
3
8
5
1.4 mmol, 92% yield). M. p.:> 3308C. H NMR (300.190 MHz,
3
3
00.0 K, CDCl ): d=7.26 (d, JHH =8.3 Hz, 8H, H ), 7.11 (d, JHH
.3 Hz, 16H, H ), 6.87 (s, 4H, H ), 6.73 (d, J =8.3 Hz, 8H, HPh),
.38 (s, 2H, NCH=CHN), 5.21 (s, 4H, CHPh ), 2.26 (s, 6H, CH ), 1.28
s, 36H, C(CH ) ), 1.22 ppm (s, 36H, C(CH ) ).
=
3
Ph
3
Ph
Ph
HH
2
3
13
1
(
C{ H} NMR
3
3
3 3
(
75.483 MHz, 300.0 K, CDCl ): d=167.4 (N CAu), 149.2 (2 q-C ),
3
2
Ph
1
41.4 (q-C ), 140.2 (q-C ), 139.9 (q-C ), 139.3 (q-C ), 133.2 (q-C ),
Ph Ph Ph Ph Ph
Experimental Section
129.8 (C ), 129.4 (C ), 128.9 (C ), 125.3 (C ), 124.9 (C ), 123.3
Ph Ph Ph Ph Ph
(
(
3
NCH=CHN), 50.7 (CHPh ), 34.4 (q-C(CH ) ), 34.3 (q-C(CH ) ), 31.3
2 3 3 3 3
General
19
C(CH ) ), 31.3 (C(CH ) ), 21.8 ppm (CH ). F NMR (282.762 MHz,
3 3 3 3 3
+
00.0 K, CDCl ): d=À75.83 ppm (s, CF ). MS (FAB ) m/z (%): 1557.2
Chemicals and solvents used in this work were supplied by the In-
stitute of Organic Chemistry at the Ruprecht-Karls University Hei-
delberg or bought directly from Acros Organics, Sigma Aldrich,
Strem, and TCI. Reactions involving air-sensitive reagents were per-
formed in an atmosphere of nitrogen or argon by using standard
Schlenk techniques or an MBraun LABmaster 130 glovebox operat-
3
3
+ +
(
(
40) [MÀC F NO S ] , 1838.1(4) [M] . IR (KBr): n˜ =3418 (br), 2963
2
6
4 2
w), 2868 (w), 1608 (w), 1511 (m), 1464 (m), 1404 (m), 1363 (m),
1
8
344 (w), 1255 (m), 1237 (m), 1197 (s), 1136 (m), 1061 (w), 1019 (w),
41 (m), 653 (w), 612 (m), 574 cm (m); elemental analysis (%) cal-
À1
culated for C103H120AuF N O S : C 67.26, H 6.58, N 2.28; found: C
6
3
4 2
[25]
67.43, H 6.62, N 2.31. Literature: This synthesis is according to a pro-
ed with nitrogen. Absolute solvents were taken from an MBraun
MB SCS-800 solvent purification system containing appropriate
drying agents. Deuterated solvents for NMR spectroscopy were
supplied by Deutero GmbH and Euriso-top. All reported yields are
[29]
tocol of Cazin and Nolan et al.
1
General procedure for the Hashmi phenol synthesis
isolated yields. H NMR spectra were recorded at RT using hard-
ware by Bruker. The following instrument was employed: Bruker
The furan (0.2 mmol) and dodecane as an internal standard (25 mL)
were dissolved in dichloromethane (0.5 mL). A solution of the gold
Avance 300 (300 MHz). Chemical shifts d are reported in ppm rela-
1
tive to TMS and were determined by reference to the residual H
catalyst and AgNTf were added and the conversion determined
2
[26]
solvent peaks (dichloromethane, d=5.32 ppm),
coupling con-
by gas chromatography. After full conversion, the reaction mixture
was filtered over a pad of Celite and diluted with dichloromethane
stants J in Hz. The following abbreviations were used for describ-
ing the signals’ multiplicities: s; singlet; d, doublet; m, multiplet.
(25 mL). The solvent was removed in vacuo and the residue puri-
1
3
1
C{ H} NMR spectra were recorded at RT with a Bruker Avance 300
75 MHz) spectrometer. The spectra were calibrated with respect to
fied by column chromatography on silica gel.
(
[27]
the solvent (dichloromethane at 54.00 ppm). The assignment of
1
3
1
signals in the C{ H} NMR spectra was achieved by interpretation
of 2D NMR spectra (HMBC, HSQC). Mass spectra were recorded by
the Mass Spectrometry Service Facility of the Chemical Department
at Ruprecht Karls University Heidelberg. The following machines
were employed: Bruker ICR Apex-Qe, Finnigan MAT LCQ und JEOL
JMS-700. Apart from the method of ionization and the peak of the
molecular ion, the basis peak and characteristic fragmentation
peaks with their relative intensities are reported. Elemental analy-
ses were performed by the Laboratory of Microanalysis in the De-
partment of Chemistry at the Ruprecht-Karls-University Heidelberg.
IR spectra were recorded on an IR spectrometer Vector22 FTIR by
Bruker. Measurements were performed with KBr pellets. The follow-
ing abbreviations were used to describe both the intensity and
profile of the signals: w (weak), m (medium), s (strong), br (broad).
Determination of melting points was performed in open capillaries
by using a Gallenkamp hot-stage microscope. X-ray analyses were
performed at the Department of Organic Chemistry at Ruprecht-
Karls-University Heidelberg by Dr. Rominger and co-workers using
the following hardware: Bruker Smart Apex diffractometer, Bruker
Smart Apex II Quazar diffractometer and Bruker Smart 1000 diffrac-
tometer. For analysis and graphic representation, the programs
Hydration of alkynes
The alkyne (0.2 mmol) was dissolved in dichloromethane/methanol
(0.45 mL). Water (50 mL) and the solution of the catalyst (1 mol% or
0
5
.1 mol%) were added and the reaction mixture was heated to
08C or 808C. The conversion was determined by NMR/GC. In the
case of phenylacetylene and p-methoxyphenylacetylene, the reac-
tion mixture was filtered over Celite and diluted with diethylether
(25 mL). The solvent was removed in vacuo and the residue puri-
fied by column chromatography on silica gel. In the other cases
the yield was determined by NMR by comparison to 1,4-dioxane.
Hydration of phenylacetylene at RT
Phenylacetylene (32.3 mL, 0.293 mmol) and dodecane (25 mL) were
dissolved in dichloromethane/water 2:1 (0.5 mL). Trifluoroacetic
acid (2.3 mL, 0.03 mmol) and the solution of the catalyst (1 mol%)
were added and the reaction mixture was stirred at RT. The conver-
sion was determined by GC. After full conversion the reaction mix-
ture was filtered over Celite and diluted with diethylether (25 mL).
The solvent was removed in vacuo and the residue purified by
column chromatography on silica gel.
[9a]
[9b]
ORTEP and POV-Ray were used. 2-Methyl-5-[(prop-2-yn-1-ylox-
[8]
[28]
y)methyl]furan and silver bistriflylimide
cording to the literature.
were synthesized ac-
Procedure for the hydroamination of phenylacetylene
In an NMR tube, phenylacetylene (50 mL, 455 mmol) and aniline
Synthesis and characterization of IPr**AuNTf (2)
2
(
46 mL, 505 mmol) were dissolved in dry [D ]THF (0.5 mL). The cata-
8
1
,3-Bis{2,6-bis[bis(4-tert-butylphenyl)methyl]-4-methylphenyl}-1H-
lyst (0.4 mol%) was added and the reaction mixture was heated to
708C for three days. The mixture was filtered over a pad of Celite
and diluted with CH Cl . The solvent was removed in vacuo and
imidazol-2-ylidengold(I) chloride (1) (105.7 mg, 66.3 mmol) was dis-
solved in dichloromethane (10 mL), and AgNTf2 (27.0 mg,
2
2
6
9.6 mmol) was added. The reaction mixture was stirred for 20 min
the residue purified by column chromatography on silica gel.
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5
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ÞÞ
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CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Procedure for the three-component condensation
A. M. Echavarren, Angew. Chem. 2001, 113, 4890–4893; Angew. Chem.
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In an NMR tube, complex 2 (4.6 mg, 2.5 mol, 1 mol%) was dis-
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solved in dry toluene-d (0.5 mL). Under an argon atmosphere phe-
8
nylacetylene (44.0 mL, 400 mmol), benzaldehyde (25.3 mL, 250 mmol),
and piperidine (27.2 mL, 275 mmol) were added and the reaction
mixture was heated to reflux for one day. The mixture was filtered
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over a pad of Celite and diluted with Et O. The solvent was re-
2
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Keywords: alkynes · arenes · carbene ligands · gold · steric
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Published online on && &&, 0000
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FULL PAPERS
Deeply buried but very vivid: An un-
precedented combination of high activi-
ty, stability, selectivity, and Lewis acidity
as well as mechanistic insight is provid-
ed by the gold bistriflimide complex of
an IPr** N-heterocyclic carbene ancillary
ligand with its record buried volume.
S. G. Weber, D. Zahner, F. Rominger,
B. F. Straub*
&& – &&
Mechanistic Investigations of a Stable,
Highly Active, Extremely Sterically
Shielded Molecular Gold Catalyst
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 7
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These are not the final page numbers!