Influence of a very bulky N- Heterocyclic Carbene in Gold-Mediated Catalysis

Article abstract of DOI:10.1021/om200705y

The syntheses of the free carbene IPr* (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) and related gold complexes [Au(IPr*)Cl] (C1) and [Au(IPr*)(NTf₂)] (C2) were achieved in high yields. The % V_Bur of IPr* for both gold complexes was measured, revealing IPr* as one of the bulkiest NHCs on gold complexes reported to date. In addition, the catalytic activity of C1 and C2 in several reactions, typically catalyzed by Au^I complexes, was investigated. Examples include the tandem alkoxylation/lactonization of γ-hydroxy-α,β-acetylenic esters, the [3,3]-rearrangement of propargylic acetates leading to the formation of conjugated enones and substituted indenes, and the rearrangement of allylic acetates. These studies revealed a strong solvent effect on the catalytic activity with 1,2-dichloroethane as the solvent of choice. The screening of C1 and C2 demonstrated only slightly diminished activities in comparison to [Au(NHC)(L)] complexes bearing bulky ligands such as IPr and SIPr.

Full text of DOI:10.1021/om200705y

ARTICLE
pubs.acs.org/Organometallics
Influence of a Very Bulky N-Heterocyclic Carbene in Gold-Mediated
Catalysis
Adriꢀan Gꢀomez-Suꢀarez, Rubꢀen S. Ramꢀon, Olivier Songis, Alexandra M. Z. Slawin, Catherine S. J. Cazin,* and
Steven P. Nolan*
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, KY16 9ST, U.K.
S Supporting Information
b
ABSTRACT: The syntheses of the free carbene IPr* (IPr* =
1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-
ylidene) and related gold complexes [Au(IPr*)Cl] (C1) and
[Au(IPr*)(NTf2)] (C2) were achieved in high yields. The %
VBur of IPr* for both gold complexes was measured, revealing
IPr* as one of the bulkiest NHCs on gold complexes reported to
date. In addition, the catalytic activity of C1 and C2 in several reactions, typically catalyzed by AuI complexes, was investigated.
Examples include the tandem alkoxylation/lactonization of γ-hydroxy-α,β-acetylenic esters, the [3,3]-rearrangement of propargylic
acetates leading to the formation of conjugated enones and substituted indenes, and the rearrangement of allylic acetates. These
studies revealed a strong solvent effect on the catalytic activity with 1,2-dichloroethane as the solvent of choice. The screening of C1
and C2 demonstrated only slightly diminished activities in comparison to [Au(NHC)(L)] complexes bearing bulky ligands such as
IPr and SIPr.
’ INTRODUCTION
have been performed to evaluate its role in metal-catalyzed
organic reactions.
In the past decade, the popularity of gold complexes has grown
exponentially due to their very broad catalytic activity,1 e.g., in
nitrile2 and alkyne3 hydration, hydroamination,4 carboxylation5
and decarboxylation6 reactions, skeletal rearrangements,7 and
many others.8 Initially, Au-catalyzed organic reactions were carried
out using simple AuI/AuIII salts such as AuCl, AuCl3, or NaAuCl4.9
Recently, more elaborate organogold chloride complexes bearing
monodentate ancillary ligands have been developed and used to
this end. The catalytically active Au species are usually generated
in the presence of a halogen abstractor, typically silver salts,
generating a cationic AuI complex.10 Initially, gold catalysts bearing
phosphine ligands such as [Au(PPh3)Cl] were ubiquitous in litera-
ture reports.11 During the past few years, the use of N-hetero-
cyclic carbenes (NHCs) as ancillary ligands in gold complexes
has, however, gained increased attention, as they provide
unique properties such as unprecedented σ-donation and
significant steric bulk.8 Further advantages offered by NHC
ligands, in practical terms, are their ease of synthesis and modifica-
tion, providing straightforward access to a wide number of
[Au(NHC)Cl] complexes.12
Figure 1. IPr* ligand reported by Markꢀo et al.
Intrigued by the steric and electronic properties of IPr*, we
decided to study its effect on the catalytic activity of AuI complexes.
Herein, we report the synthesis of [Au(IPr*)Cl] (C1) and
[Au(IPr*)(NTf2)] (C2) complexes and a study of their catalytic
activity in several reactions known to be mediated by bulky NHC-
gold complexes.
’ RESULTS AND DISCUSSION
Recently, Markꢀo et al. reported the synthesis of IPr* (1) (IPr* =
1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-
ylidene)13 inspired by the NHCs with “flexible sterics” described
by Glorius14 and Bertrand.15 This new NHC was found to be an
Synthesis and Characterization. [Au(NHC)Cl] complexes
are generally synthesized using three different routes: (a) transfer
of the NHC ligand from a silver complex,12a (b) the recently
disclosed method involving transmetalation with copper-NHC
complexes,17 or (c) direct reaction of a free carbene with a gold(I)
precursor (Scheme 1).12a Since Markꢀo reported the failure of the
silver complex 2 as a transmetalation agent but did not specify on
16
extraordinarily sterically bulky ligand, with a %VBur of 53.6
calculated for [Ag(IPr*)Cl] (2), and also exhibits good electron-
donating capability (Figure 1).13
To the best of our knowledge, the IPr* ligand has been employed
only in the synthesis of [Ag(IPr*)Cl], [Rh(IPr*)(CO)2Cl], and
[Rh(acac)(IPr*)(CO)] complexes.13 Moreover, no catalytic studies
Received: July 29, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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|

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ARTICLE
what metal the reaction was tested,13 the transmetalation be-
tween [Ag(IPr*)Cl] and [Au(DMS)Cl] (DMS = dimethylsulfide)
was carried out in order to synthesize C1. Unfortunately, no
product was generated (route A, Scheme 1). As the transmetala-
tion with silver was also invalid for gold, [Cu(IPr*)Cl] (3) was
tested as a NHC transfer agent. Copper complex 3 was easily
obtained as a white crystalline material, following (with slight
modifications) the literature protocol for the synthesis of [Cu-
(NHC)Cl] complexes,18 in 98% yield. Crystals suitable for
single-crystal diffraction studies were obtained. An ORTEP
representation of complex 3 is presented in Figure 2. Unfortu-
nately, the transmetalation using 3 also proved ineffective (route
B, Scheme 1). This was, as in the case of silver, presumably
because of the steric bulk of IPr*.
(route C, Scheme 1). Noteworthy, our optimized standard
procedure for the synthesis of the free NHC involves the use
of the corresponding BF4 salt; therefore IPr* HCl (4)13 was first
3
3
reacted with aqueous HBF4, affording IPr* HBF4 (5) in 87%
yield (route D, Scheme 1). A significant upfield shift of the
imidazolium proton can be observed by 1H NMR spectroscopy,
shifting from 12.95 to 10.13 ppm. Crystals suitable for single-
crystal diffraction studies were obtained by slow diffusion, at
À20 °C, of pentane in a saturated dichloromethane solution of 5
(Figure 3).
Figure 3. ORTEP representation for IPr* HBF4 showing 50% thermal
3
ellipsoid probability. H atoms, except the imidazolium proton, were
omitted for clarity purposes. Selected bond distances (Å) and angles
(deg): C1ÀN2 1.337(4), C1ÀN2 1.337(4), N2ÀC1ÀN2 108.1(4).
Figure 2. ORTEP representation for [Cu(IPr*)Cl] showing 50%
thermal ellipsoid probability. H atoms were omitted for clarity purposes.
Selected bond distances (Å) and angles (deg): Cu1ÀC1 1.867(3),
Cu1ÀCl1 2.0944(9), C1ÀN2 1.351(4), C1ÀN5 1.364(4), C1ÀCu1ÀCl1
176.21(9), N2ÀC1ÀN5 103.8(2).
IPr* HBF4 (5) was then deprotonated in the presence of a
3
slight excess of NaH and a catalytic amount of KOtBu to afford
carbene 1 as a white powder in 62% yield (route D, Scheme 1).
After recrystallization of the free NHC, 1 was reacted with
[Au(DMS)Cl] leading to the desired complex C1 in 82% yield
(route C, Scheme 1).
The synthesis of [Au(IPr*)Cl] (C1) was next attempted using
the reaction between the free carbene 1 and [Au(DMS)Cl]
Scheme 1. Synthetic Approaches to [Au(IPr*)Cl]
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ARTICLE
Crystals suitable for single-crystal diffraction studies were
obtained by slow diffusion, at À20 °C, of pentane in a saturated
dichloromethane solution of C1 (Figure 4). As expected, C1 is a
linear complex with a C1ÀAu1ÀCl1 angle of 178.3(2)° and a
Au1ÀC1 bond length of 1.987(7) Å. The Au1ÀC1 bond length
is longer than the corresponding bond in Au complexes bearing
IPr (1.942(3) Å) or SIPr (1.979(3) Å) ligands, but within the
normal range for [Au(NHC)Cl] complexes.12a
of silver additives.23 The air-stable [Au(NHC)(NTf2)] com-
plexes, reported by Gagosz et al.,23d represent a very attractive
and practical alternative to the use of protocols involving silver
cocatalysts. The synthesis of [Au(IPr*)(NTf2)] (C2) was at-
tempted following the established procedure.23d Gratifyingly, the
desired complex was isolated in 93% yield as a white micro-
crystalline solid (Scheme 2). A saturated solution of C2 in
toluene at 40 °C was prepared. Upon cooling to room tempera-
ture (rt), diffraction-quality crystals were obtained (Figure 6). As
expected C2 is a linear complex with a C1ÀAu1ÀCl1 angle of
178.2(3)° and a Au1ÀC1 bond length of 1.986(7) Å. The Au1ÀC1
bond length is longer than the corresponding bond in Au complexes
bearing IPr (1.969(2) Å) or IMes (1.976(3) Å) ligands, but within
the normal range for [Au(NHC)(NTf2)] complexes.23d As ex-
pected the %VBur of IPr* for C2 (44.8) is smaller than for C1
(50.1). This can be explained by the steric bulk of the ligand trans
to the carbene, more significant for NTf2 than for Cl.
Scheme 2. Synthesis of [Au(IPr*)(NTf2)]
Figure 4. ORTEP representation for [Au(IPr*)Cl] showing 50%
thermal ellipsoid probability. H atoms were omitted for clarity purposes.
Selected bond distances (Å) and angles (deg): Au1ÀC1 1.987(7),
Au1ÀCl1 2.274(2), C1ÀN2 1.332(9), C1ÀN5 1.347(9), C1ÀAu1ÀCl1
178.3(2), N2ÀC1ÀN5 106.3(6).
Computational studies using SambVca19 reveal that the %VBur
of IPr* for [Au(IPr*)Cl] C1 (50.4) and [Cu(IPr*)Cl] 3 (50.1)
are smaller than that calculated for [Ag(IPr*)Cl] 2 (53.5).13,20
The variation of %VBur of IPr* with different metals can be
explained by the flexibility of the ligand, allowing it to adapt its
shape to the metal center coordination environment. The results
also explain the different %VBur for [Au(IPr*)Cl] calculated by
us, in comparison to values provided by Glorius in his recent
review on NHCs,21 which were calculated using the closely
related [Ag(IPr*)Cl] complex. The honorific title of “bulkiest
NHC” based on %VBur still, to the best of our knowledge, belongs
to the CAAC ligand reported by Bertrand et al., with a %VBur
of 51.2 (Figure 5).22 Although extremely useful, it is recommended
to compare %VBur for NHCs bound to the same metal center and
calculate the values using the same parameters to prevent incon-
sistencies. The %VBur values are obtained from solid-state crystal
structures; therefore they might not be completely representative
of the actual coordination environment a metal center experi-
ences in solution. The %VBur values should therefore be used to
represent a trend rather than an absolute number and in any case
should be interpreted with caution.
Figure 6. ORTEP representation for [Au(IPr*)(NTf2)] showing 50%
thermal ellipsoid probability. H atoms were omitted for clarity purposes.
Selected bond distances (Å) and angles (deg): Au1ÀC1 1.986(7),
Au1ÀN1 2.086(6), C1ÀN2 1.344(9), C1ÀN5 1.357(9), C1ÀAu1ÀN1
178.2(3), N2ÀC1ÀN5 105.9(6).
Catalytic Studies. Next, the catalytic activities of C1 and C2
were evaluated in several important organic transformations
catalyzed by gold, such as the tandem alkoxylation/lactonization
of γ-hydroxy-α,β-acetylenic esters,24 the [3,3]-rearrangement of
propargylic acetates leading to the formation of conjugated
enones25 and substituted indenes,7a,26 and the rearrangement
of allylic acetates.7d The Au complexes used for the catalytic
studies are presented in Scheme 3. To avoid the use of silver salts
in the reaction mixture, complex C2 was preferably used instead
of complex C1 whenever possible.
Figure 5. Bulkiest NHC on Au reported to date with a %VBur of 51.2.
Since silver salts are generally hygroscopic and light sensitive
and have unpredictable catalytic activity, some protocols have
been reported for Au-catalyzed organic reactions in the absence
Gold-Catalyzed Formation of Furanones. We have recently
reported the base-free gold-catalyzed formation of several
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ARTICLE
Scheme 3. Au Complexes Used in the Catalytic Studies
4-alkoxy-2(5H)-furanones from a variety of propargylic alcohols
using a simple and straightforward procedure consisting in the
use of C3 (2 mol %) in MeOH at rt to afford the desired furanones
in good yields (48À98%) within 2 h.24 Initial catalytic studies
revealed that C3, C8, and C9 afforded compound 7 in good to
excellent conversions (entries 2, 6, and 7; Table 1). For the reaction
catalyzed by C5 and C6 no conversion could be determined due
to the complexity of the mixture obtained (entries 4 and 5; Table
5). Unfortunately, C2 showed poor catalytic activity for this
transformation, allowing only 18% conversion of the desired
furanone 7 (entry 1; Table 1). Interestingly, 1H NMR analysis of
the reaction mixture revealed a second unidentified intermediate
species along with the expected (E)-ethyl 4-hydroxy-3-methoxy-
4-phenylbut-2-enoate (6a),27 suggesting a different reaction
pathway when the reaction takes place in the presence of catalyst
C2. This is presently being further explored.
shorter reaction times, allowing better comparison between catal-
ysts (Table 2). In comparison to our previous results using [Au-
(NHC)Cl] complexes activated by AgSbF6,25b [Au(NHC)(NTf2)]
complexes furnished diminished yields of 9 (e.g., from 98% with
[Au(ItBu)Cl]/AgSbF6 to 89% with [Au(ItBu)(NTf2)]). Initial
optimization studies indicated that sterically demanding ligands
afforded 9 in better yields (entries 2, 3, and 8; Table 2).
Unfortunately, rearrangement of propargylic acetate 8 in the
presence of C2 furnished disappointing conversions (entry 1;
Table 2). A possible solvent effect was considered as the source of
the problem, due to the ability of THF to coordinate to the gold
center.29 The reaction was therefore tested in benzene and 1,2-
dichloroethane (DCE). In both cases a remarkable increase in
the yield of 9 was observed (entries 4 and 6; Table 3). Note-
worthy, reactions in benzene furnished a significant increase of
allene 11 (entry 4; Table 3). The selective formation of allenes
and inhibition of subsequent gold-catalyzed transformations was
already reported by our group to be strongly dependent on the
labile gold ligand. The allene 11 was selectivily formed in gold-
catalyzed rearrangement of 9 applying several [Au(NHC)(L)]-
[BF4] complexes, with L = nitrogen-based ligand.28 A similar
effect induced by benzene acting as a labile ligand to stabilize the
cationic gold center in the reaction mixture can rationalize the
analogous observations in the current case.
Satisfyingly, C2 performed better than C3 (entries 5 and 6;
Table 3) for reactions carried out in DCE. These results strongly
support the hypothesis of coordinating solvents dampening the
catalytic activity of C2, due to the effective steric protection of the
gold center by both the NHC and the coordinating solvent,
resulting in no coordination of substrate to the catalyst (see
entries 2, 4, and 6; Table 3). The use of coordinating solvents was
therefore avoided in the following reactions.
The [3,3]-rearrangement/intramolecular hydroarylation of
propargylic acetate 8 into indene 10 has also been reported by
Nolan et al.7a,26 In these studies, the conversion of the standard
substrate 8 into 10 was accomplished in good yields after 5 min in
CH2Cl2 using [Au(IPr)Cl)]/AgBF4 (2 mol %) as the catalytic
system. Initial studies revealed that complex C2 was unsuitable
for this transformation, as the hydroarylation of 11 does not
occur under these reaction conditions. We had already observed
formation of 10 under microwave heating in the presence of C2
(see entries 5 and 6; Table 3). Encouraged by these results, the
transformation of 8 into 10 was achieved using microwave
irradiation in anhydrous DCE in order to avoid competing enone
formation. The catalyst screening under these conditions re-
vealed the superiority of complexes bearing bulky NHCs (entries
1, 2, 3, and 8; Table 4). A reasonable yield was afforded in the
presence of complex C9; however good to excellent yields were
achieved when the reaction was catalyzed by C2, C3, and C4
(entries 1, 2, and 3; Table 4).
Table 1. Catalyst Screening for the Synthesis of Furanone 7a
entry
catalyst
[Au(IPr*)(NTf2)]
7 (%)b
1
C2
C3
C4
C5
C6
C8
C9
18
67
45
nd
nd
85
83
2
[Au(IPr)(NTf2)]
[Au(SIPr)(NTf2)]
[Au(IMes)(NTf2)]
[Au(SIMes)(NTf2)]
[Au(IAd)(NTf2)]
[Au(ItBu)(NTf2)]
3
4c
5c
6
7
a Reaction conditions: propargylic alcohol 6 (245 μmol), [Au] (2 mol %),
MeOH (2 mL). b Conversions determined by 1H NMR as an average of
2 runs. c nd = not determined. Along with the desired product various
side products were observed.
Gold-Catalyzed Isomerization of Propargylic Acetates
into Enones and Indenes. The activity of gold(I) complexes
C1 and C2 in the rearrangement of propargylic acetates was also
studied. The rearrangement of propargylic acetate 8 can afford
four different compounds depending on reaction conditions: (a)
α,β-unsaturated ketone 9 under aqueous conditions,25 (b) sub-
stituted indenes 10 (kinetic product)7a or 10b (thermodynamic
product) under anhydrous conditions,26 and (c) depending on
the gold complex, the reaction could be stopped at allene 11
(Scheme 4).28
Zhang25a and Nolan25b have independently reported the iso-
merization of 8 into 9 under mild conditions. Taking into account
our reported catalytic protocol,25b the reaction procedure was
slightly modified to study the activity of [Au(NHC)(NTf2)] at
Finally, the rearrangement of allylic acetates was tested in
order to study the potential of the new catalysts in reactions
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Scheme 4. Rearrangement of Propargylic Acetates into Conjugated Enones or Substituted Indenes
Table 2. Catalyst screening of the Isomerization of Propargylic Acetate 8 into α,β-Unsaturated Ketone 9a
entry
catalyst
9 (%)b
11 (%)
1
2
3c
4
5
6
7
8
[Au(IPr*)(NTf2)]
[Au(IPr)(NTf2)]
[Au(SIPr)(NTf2)]
[Au(IMes)(NTf2)]
[Au(SIMes)(NTf2)]
[Au(ICy)(NTf2)]
[Au(IAd)(NTf2)]
[Au(ItBu)(NTf2)]
C2
C3
C4
C5
C6
C7
C8
C9
8
79
83
38
61
40
51
89
18
17
13
33
24
32
28
7
a Reaction conditions: propargylic acetate 8 (217 μmol), [Au] (2 mol %), THF (2 mL), H2O (0.2 mL). b Yields determined by 1H NMR as an average of
at least 2 runs. c Average of 4 runs: 2 with 70% and 2 with 96% yield of 9.
Table 3. Solvent Influence on the Catalyst Activitya
entry
solvent
catalyst
9 (%)b
11 (%)
1
THF
THF
C6H6
C6H6
DCE
DCE
[Au(IPr)(NTf2)]
[Au(IPr*)(NTf2)]
[Au(IPr)(NTf2)]
[Au(IPr*)(NTf2)]
[Au(IPr)(NTf2)]
[Au(IPr*)(NTf2)]
C3
C2
C3
C2
C3
C2
79
8
17
18
32
67
0
2
3
48
23
65
78
4
5c
6c
0
a Reaction conditions: propargylic acetate 8 (217 μmol), [Au] (2 mol %), solvent (2 mL), H2O (0.2 mL). b Yields determined by 1H NMR as an average
of at least 2 runs. c Indene 10 was found in 31% yield in entry 5 and 22% yield in entry 6.
beyond alkyne activation. This classical skeletal rearrangement
has been described to be catalyzed by several transition metals,30
including gold.7d In the latter case, complete conversion of 12
into 13 after 12 min at 80 °C in DCE using microwave heating
and [Au(IPr)Cl] (3 mol %)/AgBF4 (2 mol %) as catalyst had
been described by Nolan et al.7d Initial catalytic studies revealed
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Table 4. Catalyst Screening for the Synthesis of Substituted
Indenes 10a
typically catalyzed by AuI complexes was investigated, revealing a
strong solvent effect. DCE has been shown to be the solvent of
choice, allowing for good activity of [Au(IPr*)(L)] complexes.
The screening of C1 and C2 demonstrated a catalytic activity
similar to [Au(NHC)(L)] complexes bearing bulky NHCs such
as IPr (C3/C10) and SIPr (C4/C11). Further studies aimed at
extending and exploring the catalytic effect of the bulky IPr* in
gold and other metal-mediated transformations are ongoing in
our laboratories.
entry
catalyst
10 (%)b
11 (%)
1
2
3
4
5
6
7
8
[Au(IPr*)(NTf2)]
[Au(IPr)(NTf2)]
[Au(SIPr)(NTf2)]
[Au(IMes)(NTf2)]
[Au(SIMes)(NTf2)]
[Au(ICy)(NTf2)]
[Au(IAd)(NTf2)]
[Au(ItBu)(NTf2)]
C2
C3
C4
C5
C6
C7
C8
C9
72
85
79
3
11
0
’ EXPERIMENTAL SECTION
0
General Considerations. Unless otherwise stated, all solvents and
reagents were used as purchased and all reactions were performed under
air. Dry solvents were obtained from a solvent purification system. NMR
spectra were recorded on 400 and 300 MHz spectrometers at room
temperature in CDCl3. Chemical shifts are given in parts per million
(ppm) with respect to TMS. Reactions under microwave irradiation
were performed in a single-mode microwave apparatus, producing
controlled irradiation at 2450 MHz. Reaction times refer to hold times
at the indicated temperature and not total irradiation times with constant
cooling via propelled air flow at a set power of 200 W. Elemental analysis
was carried out by the analytical services of London Metropolitan
University. Compounds 4,13 6,24 8,7a and 127d were synthesized according
to literature procedures.
Synthesis of Free IPr* (1). As the free NHC is the foundation of
this study, a detailed synthetic protocol is provided here. In a glovebox
(under Ar atmosphere), NaH (8.48 mmol, 203 mg, 1.50 equiv) was
added to a suspension of 5 (5.65 mmol, 5.65 g, 1.00 equiv) in anhydrous
THF (150 mL). A tip of a spatula of KOtBu was added. The reaction
mixture was stirred overnight at rt. The mixture was filtered over a pad of
Celite and was concentrated in vacuo. The resulting solid was then
dissolved in the minimum amount of THF, and the product was
precipitated by the addition of hexane. It was then collected by filtration
to afford 1 (3.50 mmol, 3.19 g, 62%) as a white powder. 1H NMR (400
MHz; C6H6): δ 7.36 (m, 8H, CHAr), 7.10À7.08 (m, 8H, CHAr), 7.03À
6.92 (m, 28H, CHAr), 6.03 (s, 4H, CHPh2), 5.78 (s, 2H, CH4,5), 1.85 (s,
6H, CH3). 13C NMR (101 MHz; C6H6): δ 220.02 (Ccarb), 145.0 (CAr),
143.9 (CAr), 142.0 (CAr), 138.4 (CAr), 138.3 (CAr), 130.2 (CHAr), 130.0
(CHAr), 129.9 (CHAr), 128.6 (CHAr), 128.5 (CHAr), 126.5 (CHAr),
126.5 (CHAr), 122.6 (CH4,5), 51.5 (CHPh2), 21.4 (CH3). Anal. Calcd
for C69H56N2 (913.20): C, 90.75; H, 6.18; N, 3.07. Found: C, 90.65; H,
6.04; N, 3.14.
10
28
50
35
23
8
5
17
45
a Reaction conditions: propargylic acetate 8 (217 μmol), [Au] (2 mol
%), DCE (2 mL). b Yields determined by 1H NMR as an average of at
least 2 runs. The formation of enone 9 (E and Z) due to the presence of
water was observed.
Table 5. Catalyst Screening for the Rearrangement of Allylic
Acetate 12a
entry
catalyst
[Au(IPr*)Cl]
12 (%)b
1
2
3
4
5
C1
79
84
55
68
76
[Au(IPr)Cl]
[Au(SIPr)Cl]
[Au(IAd)Cl]
[Au(ItBu)Cl]
C10
C11
C15
C16
a Reaction conditions: allylic acetate 12 (284 μmol), [Au] (3 mol %),
AgOTf (2 mol %), DCE (5 mL). b Conversions determined by GC as an
average of 2 runs.
Synthesis of [Cu(IPr*)Cl] (3). In a microwave vial, inside a
glovebox (under Ar atmosphere), 4 (263 μmol, 250 mg, 1.00 equiv)
and Cu2O (175 μmol, 25.0 mg, 0.66 equiv) were dissolved in anhydrous
toluene (1 mL). The vial was then removed from the glovebox and
heated in a microwave reactor at 150 °C for 15 min, affording a white
crystalline solid in solution. The crude product was dissolved in CH2Cl2
(10 mL) and filtered through Celite to remove the excess Cu2O. The pad
of Celite was washed with CH2Cl2 (3 Â 10 mL), and solvent evaporated
in vacuo, affording complex 3 as a white solid (258 μmol, 261 mg, 98%).
1H NMR (300 MHz; CD2Cl2): δ 7.16À7.21 (m, 24H, CHAr), 7.02À
7.05 (m, 8H, CHAr), 6.92 (s, 4H, CHAr), 6.88À6.91 (m, 8H, CHAr), 5.87
(s, 2H, CH4,5), 5.21 (s, 4H, CHPh2), 2.25 (s, 6H, CH3). 13C NMR (75
MHz, CDCl3): δ 180.7 (Ccarb), 143.3 (CAr), 143.0 (CAr), 141.4 (CAr),
140.6 (CAr), 134.6 (CAr), 130.5 (CHAr), 130.0 (CHAr), 129.7 (CHAr),
129.0 (CHAr), 128.8 (CHAr), 127.1 (CHAr), 127.0 (CHAr), 123.7
(CH4,5), 51.6 (CHPh2), 21.9 (CH3). Anal. Calcd for C69H56ClCuN2
(1012.20): C, 81.88; H, 5.58; N, 2.77. Found: C, 81.73; H, 5.44; N, 2.65.
that no rearrangement was promoted by the [Au(NHC)(NTf2)]
complexes. In this context, C1, along with AgOTf, was chosen as
catalytic system for the initial screening instead of using C2.
Moreover, the reaction time was reduced to 6 min to allow a
better comparison between catalysts. As shown in Table 5, C10
exhibited the best catalytic performance, achieving 84% conver-
sion (entry 2; Table 5). Unfortunately, C11 and C15, which
allowed almost full conversion under the initial conditions, were
not very active at shorter reaction times (entries 3 and 4; Table 5).
We were pleased to see that C1 promoted the reaction in excellent
conversions (79%), showing similar catalytic activity to C10, the
best catalyst in our previous report (entries 1, 2, and 5; Table 5).7d
’ CONCLUSION
The syntheses of carbene 1 and complexes [Au(IPr*)Cl] (C1)
and [Au(IPr*)(NTf2)] (C2) have been performed in high yields.
The %VBur of IPr* for C1 and C2 was evaluated, revealing it as
the second bulkiest NHC on [Au(NHC)Cl] complexes reported
to date. The catalytic activity of C1 and C2 in several reactions
Synthesis of IPr* HBF4 (5). To a stirred suspension of 4 (3.95
3
mmol, 3.75 g, 1.00 equiv) in water (250 mL) was added aqueous HBF4
(5.90 mmol, 0.58 mL, 1.50 equiv). The mixture was stirred for 3 h at rt. It
was then extracted with CH2Cl2 (3 Â 50 mL), and the combined organic
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Organometallics
ARTICLE
layers were dried over anhydrous MgSO4 and concentrated in vacuo. The
resulting solid was dissolved in a minimum amount of CH2Cl2, precipitated
by the addition of Et2O, and collected by filtration to afford 5 (3.38
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1
mmol, 3.39 g, 86%) as an off-white powder. H NMR (300 MHz;
CDCl3): δ 10.13 (t, 4J = 1.5 Hz, 1H, H2), 7.28À7.13 (m, 24H, CHAr),
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140.3 (CH2) 130.9 (CHAr), 129.8 (CHAr), 129.3 (CHAr), 128.9 (CHAr),
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white solid. 1H NMR (300 MHz; CDCl3): δ 7.19À7.13 (m, 24H, CHAr),
7.10À7.07 (m, 8H, CHAr), 6.89À6.84 (m, 12H, CHAr), 5.81 (s, 2H,
CH4,5), 5.26 (s, 4H, CHPh2), 2.23 (s, 6H, CH3). 13C NMR (75 MHz;
CDCl3): δ 175.2 (Ccarb), 143.0 (CAr), 142.3 (CAr), 140.9 (CAr), 140.2
(CAr), 133.8 (CAr), 130.3 (CHAr), 129.7 (CHAr), 129.4 (CHAr), 128.5
(CHAr), 128.4 (CHAr), 126.7 (CHAr), 126.7 (CHAr), 123.2 (CH4,5), 51.2
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72.39; H, 4.93; N, 2.45. Found: C, 72.49; H, 4.84; N, 2.36.
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6.83À6.80 (m, 12H, CHAr), 5.49 (s, 2H, CH4,5), 5.27 (s, 4H, CHPh2),
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129.9 (CHAr), 129.3 (CHAr), 128.7 (CHAr), 128.5 (CHAr), 127.02
(CHAr), 126.9 (CHAr), 123.9 (CH4,5), 120.7 (CAr), 117.5 (CAr), 51.7
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Found: C, 61.50; H, 3.89; N, 2.87.
’ ASSOCIATED CONTENT
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Supporting Information. This material is available free
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: snolan@st-andrews.ac.uk. Fax: +44(0)1334 463808.
’ ACKNOWLEDGMENT
The ERC (Advanced Investigator Award-FUNCAT) and the
EPSRC are gratefully acknowledged for support of this work. S.P.N.
is a Royal Society Wolfson Research Merit Award holder.
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