Hydrophenoxylation of alkynes by cooperative gold catalysis

Article abstract of DOI:10.1002/anie.201304182

Double impact: The title method gives aryl vinyl ethers in high yields, short reaction times, Z-stereospecificity, and good regioselectivities. Insights into the reaction mechanism highlight the role of [{Au(NHC)} ₂(μ-OH)][BF₄] (NHC=N-heterocyclic carbene) as both a Lewis acid, [Au(NHC)][BF₄], and a Bronsted base, [Au(NHC)(OH)], thereby generating a synergistic effect between the two gold moieties. Copyright

Full text of DOI:10.1002/anie.201304182

Angewandte
Chemie
DOI: 10.1002/anie.201304182
Cooperative Catalysis
Hydrophenoxylation of Alkynes by Cooperative Gold Catalysis**
Yoshihiro Oonishi, Adriꢀn Gꢁmez-Suꢀrez, Anthony R. Martin, and Steven P. Nolan*
Dedicated to Professor Carl D. Hoff on the occasion of his 65th birthday
Over the past decade, the concept of cooperative or dual
catalysis has emerged as an attractive and effective strategy to
access unique reactivity and selectivity in synthetic organic
chemistry.[1] This type of catalysis has been shown in dual
organo-catalyzed,[2] organo- and transition-metal-catalyzed,[3]
homobimetallic transition-metal-catalyzed[4] and heterobime-
tallic catalyzed[5] processes. Recently, it has also received
increased attention in gold chemistry owing to the synthesis
and isolation of dinuclear organogold species, such as gem-
diaurated or s,p-diaurated acetylide complexes.[6] These
species were first proposed and later identified as key
intermediates or catalyst reservoirs in gold-catalyzed reac-
tions.[7] We recently contributed to this area with the synthesis
of dinuclear gold hydroxide species [{Au(NHC)}2(m-OH)]-
[BF4] (1) (NHC = N-heterocyclic carbene), which can be
easily prepared from commercially available [Au(NHC)(X)]
(X = OH or Cl) complexes.[8] Complexes 1 have been shown
to be highly active catalysts for silver- and acid-free gold-
catalyzed transformations.[9] We have also recently reported
straightforward access to both gem-diaurated and s,p-diau-
rated acetylide species by reacting 1A (NHC = IPr= 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with aryl/vinyl
boronic acids and terminal alkynes, respectively.[10] In addi-
tion, complexes 1 exhibited particularly interesting catalytic
properties. For example, during our studies of the gold-
catalyzed nitrile hydration, the use of 1A afforded higher
conversions to the desired amide than the gold monomer
[Au(IPr)NTf2].[8b] We have previously postulated that [{Au-
(NHC)}2(m-OH)][BF4] (1) could be considered as a combina-
tion of [Au(NHC)][BF4] (2) and [Au(NHC)(OH)] (3).[8] We
believe that, under the appropriate reaction conditions, this
equilibrium could be displaced, thus liberating a Lewis acid 2
and a Brønsted base 3 that could produce a synergistic effect
leading to enhanced catalytic activity [Eq. (1)].
Most gold-catalyzed transformations take advantage of
À
the well-documented ability of gold to activate C C multiple
bonds, typically alkynes, towards nucleophilic attack.[11]
While the addition of primary and secondary alcohols to
alkynes is relatively well-known,[12] reports of the addition of
tertiary alcohols and phenols remain scarce. To our knowl-
edge, there is only one report dealing with the gold-catalyzed
hydrophenoxylation of alkynes.[13] In 2010, Sahoo and co-
workers described the reaction between internal alkynes 4
and phenols 5 (2 equiv) using AuCl3 (3–5 mol%) in the
presence of K2CO3 or Ag2CO3 (2 equiv) under very harsh
reaction conditions.[13] We envisioned that if 1 could act as
a bifunctional catalyst, 2 might react with 4 forming a p-gold–
alkyne complex I[14] and 3 might react with 5 forming a gold–
phenoxide complex II[15] (Scheme 1).
[*] Dr. Y. Oonishi, A. Gꢀmez-Suꢁrez, Dr. A. R. Martin,
Prof. Dr. S. P. Nolan
Scheme 1. Dual activation in the hydrophenoxylation of alkynes.
EaStCHEM School of Chemistry, University of St Andrews
St Andrews, KY16 9ST (UK)
E-mail: snolan@st-andrews.ac.uk
We began our studies by reacting diphenylacetylene 4a
and phenol 5a (1.1 equiv) in 1,4-dioxane at 808C using 1A
(0.5 mol%) as catalyst. We were pleased to observe after 1 h
an encouraging 15% conversion to the desired vinyl ether 6aa
by GC. With this result in hand, we proceeded to optimize the
reaction conditions (Table 1). Interestingly, the use of less
polar solvents allowed for better conversions (Table 1,
entries 1–4). In toluene, a 97% conversion was reached
within 1 h at 808C (Table 1, entry 5). The 1H NMR spectrum
of the isolated product (96%) confirmed the stereospecific
formation of Z-isomer 6aa.[13] Next, we screened various
Homepage: http://chemistry.st-and.ac.uk/staff/spn/group/SP_No-
lan/Home.html
[**] The ERC (Advanced Investigator Award-FUNCAT), EPSRC and
Syngenta are gratefully acknowledged for support. Umicore AG is
acknowledged for their generous gift of materials. The EPSRC
National Mass Spectrometry Service Centre (NMSSC) is gratefully
acknowledged for HRMS analyses. We thank Dr. David J. Nelson
and Dr. Alba Collado for helpful discussions. S.P.N. is a Royal
Society Wolfson Research Merit Award holder. Y.O. thanks the
Uehara Memorial Foundation for a Research Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304182.
Angew. Chem. Int. Ed. 2013, 52, 9767 –9771
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9767

.
Angewandte
Communications
Table 1: Optimization of the reaction conditions.[a]
desired aryl vinyl ethers 6ab and 6ac in high yields within 1 h
(Scheme 2). In contrast, phenols 5d–5g bearing electron-
withdrawing groups at the para-position needed longer
reaction times to reach completion. The corresponding
compounds 6ad–6ag were isolated in high yields (91–98%).
The reaction time increases as follows: MeO ꢀ Me ꢀ H ꢀ F <
Cl < CF3 < NO2. Interestingly, this trend is the opposite of
that observed by Sahoo and co-workers.[13] This observation
points to a different reaction mechanism operating in these
two systems. The use of sterically hindered substrates was
then tested. The reaction with ortho-substituted phenols (5j
and 5k) afforded the expected ethers (6aj and 6ak) in 99%
and 89% yields, respectively, although at a cost of longer
reaction times. Surprisingly, even the use of the highly
sterically hindered 2,6-dimethylphenol (5l) afforded the
desired product 6al in 67% yield.
Entry
NHC
Solvent
Conversion [%][b]
1
2
3
4
5
6
7
8
IPr
IPr
IPr
IPr
1,4-dioxane
CH3CN
DMF
1,2-dichloroethane
toluene
toluene
15
2
1
75
97 (96)
45
94
IPr
SIPr
IPrCl
IPent
toluene
toluene
86
[a] Conditions: 4a (0.50 mmol), 5a (0.55 mmol, 1.1 equiv), [Au]
(0.5 mol%), solvent (1 mL), 808C. [b] Conversions determined by GC.
Average of at least two runs. Yield of isolated product in parentheses.
Reactions between various alkynes 4 and phenol 5a
(Scheme 3) were investigated. Dialkyl-substituted 4b
afforded the desired product 6ba in 94% yield in a stereo-
specific manner. The hydrophenoxylation of unsymmetrical
digold hydroxide complexes. While the catalyst bearing SIPr
ligands performed poorly, IPrCl and IPent derivatives[9,16]
afforded good to excellent conversions (Table 1, entries 6–
8). Since [{Au(IPr)}2(m-OH)][BF4] afforded slightly better
conversions and can be easily synthesized from commercially
available complexes, 0.5 mol% of 1A in toluene at 808C was
selected as our optimized catalytic conditions. This new
methodology permits a turnover frequency (TOF) of nearly 4
orders of magnitude greater than the previous state of the art
(192 hÀ1 versus 0.05 hÀ1).[13] Having established the optimized
reaction conditions, the scope and limitations of the method
were explored.
We first examined the reaction between diphenylacety-
lene 4a and several phenol derivatives (Scheme 2). The
reaction of phenols 5b and 5c, bearing electron-donating
groups at the para-position, proceeded smoothly to afford the
Scheme 3. Hydrophenoxylation using various unsymmetrical alkynes.
Conditions: 4c–h (0.50 mmol), 5a or 5g (0.55 mmol, 1.1 equiv), 1A
(0.5 mol%), toluene (1 mL), 808C. Yields of isolated products are
given. Average of two runs. Ratio 6/6’ was determined by 1H NMR
spectroscopy. nd=not detected. [a] 1A (1 mol%), 1108C. [b] 4b
(0.55 mmol) and 5a (0.50 mmol).
diaryl-substituted alkynes 4c–4e provided the corresponding
products 6ca–6ea stereospecifically in high yields but mod-
erate regioselectivities. Better regioselectivities were
observed when one of the aryl moieties bears a methoxy
group at the para-position (1:0.28 versus 1:0.65). When 1-
phenylpropyne (4 f) was used, the regioselectivity improved
to 1:0.18. The reaction of 4 f with 4-nitrophenol (5g) instead
of 5a was also investigated. Vinyl ether 6 fg was obtained as
the major product. Interestingly, the regioselectivity in this
reaction is the opposite of that observed by Sahoo and co-
workers (6 fg/6 fg’ = 1:0.35 versus 0.61:1), thus pointing again
towards a different reaction mechanism in each method-
ology.[13] The reactions using alkynes 4g and 4h, having
a directing group such as pyridine, were then tested. Gratify-
ingly, the reaction proceeded with complete regioselectivity,
Scheme 2. Hydrophenoxylation using various phenols. Conditions: 4a
(0.50 mmol), 5a–l (0.55 mmol, 1.1 equiv), 1A (0.5 mol%), toluene
(1 mL), 808C. Yields of isolated products are given. Average of two
runs. [a] 1A (1 mol%), 1108C.
9768 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 9767 –9771

Angewandte
Chemie
although at lower rates, affording aryl vinyl ethers 6ga and
6ha in moderate and low yields of 62% and 34%, respec-
tively.
OH)][BF4] (1B)[16,19] a statistical distribution (1:1:2) was
observed between 1A, 1B, and the cross-over product
[{Au(IPr)}{Au(SIPr)}(m-OH)][BF4] (1C).[20] This result
strongly supports the existence of the above-mentioned
equilibrium.
To assess the recyclability of 1A, once the reaction
between alkyne 4a and phenol 5a was complete, iterative
additions of both substrates (0.5 and 0.55 mmol, respectively)
were conducted. As a result, 8.5 mmol of 4a were converted
over 36 h by using 2.5 mmol of 1A affording an impressive
turn over number (TON) of 3400! To relate this to the
previous state of the art, the procedure reported by Sahoo and
co-workers afforded a TON of 7 for 6aa (Scheme 4).[13]
Next, the use of 2A and 3A as catalysts was investigated
(Table 2). Since 2 is not a stable species, [Au(IPr)(CH3CN)]-
[BF4] (2A’) was employed. 2A’ and [Au(IPr)(OH)] 3A
performed poorly with 40% and < 1% conversion, respec-
tively (Table 2, entries 1 and 2). We have previously postu-
lated that 2A’ could lead to in situ generation of 1A in the
presence of water.[8b] Since the reaction was carried out using
technical-grade toluene, this could explain the moderate
conversions observed using 2A’ as catalyst. To confirm this,
the reaction was repeated in the presence of 3 drops of water,
where an improved conversion of 81% was observed,
whereas under anhydrous conditions poor conversions were
obtained (Table 2, entries 3–4). This series of experiments
unequivocally establishes that 2A or 3A alone cannot
efficiently catalyze the reaction. Nevertheless, the combina-
tion of both 2A’ (0.5 mol%) and 3A (0.5 mol%) showed high
conversion (87%; Table 2, entry 5). Next, p-gold–alkyne
complex I[14] and gold–phenoxide complex II[15] were synthe-
sized following the reported methodologies and tested in
catalysis. Gratifyingly, the combination of both I (0.5 mol%)
and II (0.5 mol%) afforded a 97% conversion (Table 2,
entry 6). These results are consistent with our proposal that
a digold hydroxide species 1 can act as cooperative bifunc-
tional catalyst to permit the efficient hydrophenoxylation of
alkynes.
Scheme 4. Iterative additions of 4a and 5a to the catalytic reaction.
Once the scope and limitations of the system were
established, the reaction mechanism was probed. A dual
activation mechanism was hypothesized for the hydrophe-
noxylation of internal alkynes as depicted in Scheme 5. In this
mechanistic scenario, [{Au(IPr)}2(m-OH)][BF4] (1A) is in
equilibrium with [Au(IPr)][BF4] (2A) and [Au(IPr)(OH)]
(3A) under the reaction conditions. [Au(IPr)][BF4] (2A)
activates alkyne 4 to form the p-gold–alkyne complex I,[14]
while [Au(IPr)(OH)] (3A) reacts with phenol 5 to provide the
gold–phenoxide complex II.[15] The latter then attacks from
the opposite side of I to give a gem-diaurated compound III[7]
or s-monoaurated compound IV[17] along with [Au(IPr)][BF4]
(2A). Similar intermediates have recently been proposed for
the addition of MeOH to alkynes.[18] Finally, protodeauration
of III or IV with phenol 5 or H2O takes place to afford vinyl
ether 6 (Scheme 5).
Table 2: Catalyst studies.[a]
Entry
Catalyst (mol%)
Conversion [%][b]
1
2
3
(2A’) [Au(IPr)(CH3CN)][BF4] (1)
(3A) [Au(IPr)(OH)] (1)
2A’ (1) + H2O
2A’ (1)
2A’ (0.5) + 3A (0.5)
I (0.5) + II (0.5)
40
<1
81
33
87
97
To support this mechanistic proposal, the possible equi-
librium between 1A, 2A, and 3A was examined. When
mixing [{Au(IPr)}2(m-OH)][BF4] (1A) and [{Au(SIPr)}2(m-
4[c]
5
6
[a] Conditions: 4a (0.50 mmol), 5a (0.55 mmol, 1.1 equiv), toluene
(1 mL), 808C. [b] Conversions determined by using GC. Average of two
runs. [c] Anhydrous conditions.
To further support our hypothesis of dual gold activation,
a number of stoichiometric reactions were carried out.[20]
Owing to solubility problems in [D8]toluene, experiments
were carried out in CD2Cl2.[20] Also, since the presence of
water can displace equilibria towards formation of 1A,
anhydrous conditions were used.[20] First we investigated if
1A could interact with either of the substrates.
After 16 h at room temperature no reaction was observed
between 1A and alkyne 4a.[20] However, complete conversion
into 1A and 4a was observed when mixing 3A and p complex
Scheme 5. Proposed reaction mechanism.
Angew. Chem. Int. Ed. 2013, 52, 9767 –9771
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9769

.
Angewandte
Communications
I, thus strongly suggesting the existence of an equilibrium
between these species [Eq. (2)].[20] When reacting 1A and
phenol 5a, approximately 50% conversion to a new species
could be observed.[20] In the presence of molecular sieves, only
In summary, we have developed a mild and straightfor-
ward methodology for the hydrophenoxylation of alkynes,
using digold–hydroxide complexes, to form various aryl vinyl
ethers in good to high yields with good regioselectivities. The
current data strongly support a dual activation pathway;
digold hydroxide 1A dissociates into a Lewis acid 2A and
a Brønsted base 3A and these species will independently
react with substrates 4 and 5 respectively. Owing to this
synergistic effect, the hydrophenoxylation of alkynes pro-
ceeds smoothly under relatively mild conditions. We postulate
that diaurated species III and V can be observed under
anhydrous conditions, but we are not able, at this point, to
determine their role in reactions conducted under catalytic
conditions. These findings provide new insights into the
chemistry of gold catalysis and open the door to the develop-
ment of new catalytic transformations. Further studies to
delineate the scope, limitations, and the detailed mechanism
of this and related reactions are in progress.
1
one species was observed. H and 13C NMR spectroscopy as
well as elemental analysis suggest the new species is [{Au-
(IPr)}2(m-OPh)][BF4] (V) [Eq. (3)].
The interactions between I, II, 4a, and 5a were also
examined. Only 8% conversion to 6aa was observed when
reacting phenol 5a and I for 16 h at room temperature
[Eq. (4)]. Moreover, no reaction was observed between
alkyne 4a and phenoxide II under identical reaction con-
ditions, whereas the reaction between p complex I and II
occurred within mixing time [Eq. (5)–(6)]. After 5 min,
complete consumption of the starting materials was observed
and two new species were detected by 1H NMR spectroscopy
in a 1:3 ratio.[20] The minor species was identified as digold
phenoxide V. We hypothesized that V may be in equilibrium
with I, II, and alkyne 4a. This equilibrium favors formation of
the diaurated species V, the same as the equilibrium in
Equation (2) favoring formation of 1A.[20] In addition, we
postulated that the major species could be the gem-diaurated
complex III, and what is observed is an equilibrium between
V, 4a, and III [Eq. (6)]. Therefore, when an excess of alkyne
4a was added to the reaction mixture, III and 4a are observed.
Unfortunately, attempts to isolate gem-diaurated species III
were not successful, because once the excess of 4a is removed
a mixture between digold–phenoxide V, 4a, and III was
obtained, thus further supporting the hypothesis that this step
is an equilibrium.[20,21] If the reaction mixture of p complex I
and gold–phenoxide II is quenched with H2O, complete
conversion to the desired vinyl ether 6aa and digold
hydroxide 1A can be observed [Eq. (7)].[20]
Received: May 15, 2013
Published online: July 29, 2013
Keywords: alkynes · cooperativity · gold ·
.
homogeneous catalysis · N-heterocyclic carbenes
[1] a) M. Shibasaki, M. Kanai, S. Matsunaga, N. Kumagai, Acc.
Chem. Res. 2009, 42, 1117 – 1127; b) J. J. Hirner, Y. Shi, S. A.
Blum, Acc. Chem. Res. 2011, 44, 603 – 613; c) D. C. Powers, T.
Ritter, Acc. Chem. Res. 2012, 45, 840 – 850.
[2] Selected publications: a) N. Z. Burns, M. R. Witten, E. N.
Jacobsen, J. Am. Chem. Soc. 2011, 133, 14578 – 14581; b) H.
Rahaman, ꢀ. Madarꢁsz, I. Pꢁpai, P. M. Pihko, Angew. Chem.
2011, 123, 6247 – 6251; Angew. Chem. Int. Ed. 2011, 50, 6123 –
6127; c) M. D. Pierce, R. C. Johnston, S. Mahapatra, H. Yang,
R. G. Carter, P. Ha-Yeon Cheong, J. Am. Chem. Soc. 2012, 134,
13624 – 13631.
[3] Selected publications: a) M. Rueping, A. P. Antonchick, C.
Brinkmann, Angew. Chem. 2007, 119, 7027 – 7030; Angew.
Chem. Int. Ed. 2007, 46, 6903 – 6906; b) D. A. DiRocco, T.
Rovis, J. Am. Chem. Soc. 2012, 134, 8094 – 8097.
[4] Selected publications: a) B. M. Trost, J. Jaratjaroonphong, V.
Reutrakul, J. Am. Chem. Soc. 2006, 128, 2778 – 2779; b) T. J. L.
Mustard, D. J. Mack, J. T. Njardarson, P. H.-Y. Cheong, J. Am.
Chem. Soc. 2013, 135, 1471 – 1475.
[5] Selected publications: a) M. J. Hostetler, R. G. Bergman, J. Am.
Chem. Soc. 1990, 112, 8621 – 8623; b) G. M. Sammis, H. Danjo,
E. N. Jacobsen, J. Am. Chem. Soc. 2004, 126, 9928 – 9929; c) Y.-F.
Wang, K. K. Toh, J.-Y. Lee, S. Chiba, Angew. Chem. 2011, 123,
6049 – 6053; Angew. Chem. Int. Ed. 2011, 50, 5927 – 5931.
[6] A. Gꢂmez-Suꢁrez, S. P. Nolan, Angew. Chem. 2012, 124, 8278 –
8281; Angew. Chem. Int. Ed. 2012, 51, 8156 – 8159.
[7] Selected publications: a) P. H.-Y. Cheong, P. Morganelli, M. R.
Luzung, K. N. Houk, F. D. Toste, J. Am. Chem. Soc. 2008, 130,
4517 – 4526; b) D. Weber, M. A. Tarselli, M. R. Gagnꢃ, Angew.
Chem. 2009, 121, 5843 – 5846; Angew. Chem. Int. Ed. 2009, 48,
5733 – 5736; c) G. Seidel, C. W. Lehmann, A. Fꢄrstner, Angew.
Chem. 2010, 122, 8644 – 8648; Angew. Chem. Int. Ed. 2010, 49,
8466 – 8470; d) T. J. Brown, D. Weber, M. R. Gagnꢃ, R. A.
Widenhoefer, J. Am. Chem. Soc. 2012, 134, 9134 – 9137;
e) A. S. K. Hashmi, I. Braun, P. Nçsel, J. Schꢅdlich, M. Wieteck,
M. Rudolph, F. Rominger, Angew. Chem. 2012, 124, 4532 – 4536;
Angew. Chem. Int. Ed. 2012, 51, 4456 – 4460; f) A. S. K. Hashmi,
I. Braun, M. Rudolph, F. Rominger, Organometallics 2012, 31,
9770 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 9767 –9771

Angewandte
Chemie
644 – 661; g) A. S. K. Hashmi, T. Lauterbach, P. Nçsel, M. H.
Vilhelmsen, M. Rudolph, F. Rominger, Chem. Eur. J. 2013, 19,
1058 – 1065; h) M. M. Hansmann, M. Rudolph, F. Rominger,
A. S. K. Hashmi, Angew. Chem. 2013, 125, 2653 – 2659; Angew.
Chem. Int. Ed. 2013, 52, 2593 – 2598.
[13] M. R. Kuram, M. Bhanuchandra, A. K. Sahoo, J. Org. Chem.
2010, 75, 2247 – 2258.
[14] a) J. A. Akana, K. X. Bhattacharyya, P. Mꢄller, J. P. Sadighi, J.
Am. Chem. Soc. 2007, 129, 7736 – 7737; b) S. Flꢄgge, A. Anoop,
R. Goddard, W. Thiel, A. Fꢄrstner, Chem. Eur. J. 2009, 15, 8558 –
8565; c) T. J. Brown, R. A. Widenhoefer, J. Organomet. Chem.
2011, 696, 1216 – 1220.
[15] a) J. D. Egbert, A. M. Z. Slawin, S. P. Nolan, Organometallics
2013, 32, 2271 – 2274; b) N. Ibrahim, M. H. j. Vilhelmsen, M.
Pernpointner, F. Rominger, A. S. K. Hashmi, Organometallics
2013, 32, 2576 – 2583.
[8] a) S. Gaillard, J. Bosson, R. S. Ramꢂn, P. Nun, A. M. Z. Slawin,
S. P. Nolan, Chem. Eur. J. 2010, 16, 13729 – 13740; b) R. S.
Ramꢂn, S. Gaillard, A. Poater, L. Cavallo, A. M. Z. Slawin, S. P.
Nolan, Chem. Eur. J. 2011, 17, 1238 – 1246.
[9] A. Gꢂmez-Suꢁrez, Y. Oonishi, S. Meiries, S. P. Nolan, Organo-
metallics 2013, 32, 1106 – 1111.
[10] A. Gꢂmez-Suꢁrez, S. Dupuy, A. M. Z. Slawin, S. P. Nolan,
Angew. Chem. 2013, 125, 972 – 976; Angew. Chem. Int. Ed.
2013, 52, 938 – 942.
[11] a) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180 – 3211; b) E.
Jimꢃnez-NfflÇez, A. M. Echavarren, Chem. Commun. 2007, 333 –
346; c) N. Marion, S. P. Nolan, Chem. Soc. Rev. 2008, 37, 1776 –
1782; d) H. Huang, Y. Zhou, H. Liu, Beilstein J. Org. Chem.
2011, 7, 897 – 936; e) S. P. Nolan, Acc. Chem. Res. 2011, 44, 91 –
100.
[12] Selected publications: a) Y. Fukuda, K. Utimoto, J. Org. Chem.
1991, 56, 3729 – 3731; b) J. H. Teles, S. Brode, M. Chabanas,
Angew. Chem. 1998, 110, 1475 – 1478; Angew. Chem. Int. Ed.
1998, 37, 1415 – 1418; c) R. Casado, M. a. Contel, M. Laguna, P.
Romero, S. Sanz, J. Am. Chem. Soc. 2003, 125, 11925 – 11935;
d) A. Corma, V. R. Ruiz, A. Leyva-Pꢃrez, M. J. Sabater, Adv.
Synth. Catal. 2010, 352, 1701 – 1710; e) R. S. Ramꢂn, C. Pottier,
A. Gꢂmez-Suꢁrez, S. P. Nolan, Adv. Synth. Catal. 2011, 353,
1575 – 1583.
[16] IPr: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; SIPr: 1,3-
bis(2,6-diisopropylphenyl)imidazolin-2-ylidene;
IPrCl:
4,5-
dichloro-1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene;
IPent: 1,3-bis(2,6-di(pentan-3-yl)phenyl)imidazol-2-ylidene.
[17] Selected publications: a) A. S. K. Hashmi, Gold Bull. 2009, 42,
275 – 279; b) L. Ye, Y. Wang, D. H. Aue, L. Zhang, J. Am. Chem.
Soc. 2012, 134, 31 – 34; c) A. S. K. Hashmi, M. Wieteck, I. Braun,
P. Nçsel, L. Jongbloed, M. Rudolph, F. Rominger, Adv. Synth.
Catal. 2012, 354, 555 – 562.
ˇ
ˇ
ˇ
[18] a) J. Roithovꢁ, S. Jankovꢁ, L. Jasíkovꢁ, J. Vꢁna, S. Hybel-
bauerovꢁ, Angew. Chem. 2012, 124, 8503 – 8507; Angew. Chem.
Int. Ed. 2012, 51, 8378 – 8382; b) A. Zhdanko, M. E. Maier,
Chem. Eur. J. 2013, 19, 3932 – 3942.
[19] A. Gꢂmez-Suꢁrez, R. S. Ramꢂn, A. M. Z. Slawin, S. P. Nolan,
Dalton Trans. 2012, 41, 5461 – 5463.
[20] See the Supporting Information for further experimental details.
À
[21] For another example of reversible C O bond formation see
Ref. [7d].
Angew. Chem. Int. Ed. 2013, 52, 9767 –9771
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9771