Characterization of vinylgold intermediates: Gold-mediated cyclization of acetylenic amides

Article abstract of DOI:10.1002/anie.201106132

Hidden nuggets of gold: Mono- and divinylgold complexes (see scheme), key intermediates in the gold-mediated cyclization reaction of N-(propargyl) benzamides, are characterized by NMR and X-ray diffraction analyses. The monovinylgold intermediates undergo

Full text of DOI:10.1002/anie.201106132

Communications
DOI: 10.1002/anie.201106132
Reactive Intermediates
Characterization of Vinylgold Intermediates: Gold-Mediated
Cyclization of Acetylenic Amides**
Olga A. Egorova, Hyewon Seo, Yonghwi Kim, Dohyun Moon, Young Min Rhee, and
Kyo Han Ahn*
Gold complexes show unusual affinity toward the carbon–
carbon triple bond, thus activating it towards carbon, oxygen,
and nitrogen nucleophiles. Therefore, various gold-catalyzed
synthetic transformations involving carbon–carbon triple
bonds have been developed in recent years,[1] along with
characterization of some reaction intermediates involved. In
most cases, however, the gold-catalyzed reactions are inter-
preted with “suggested” mechanisms, as pointed out by
Hashmi in a recent review article on the gold intermediates
identified by direct observation and characterization.[2] So far,
a limited number of gold intermediates have been charac-
terized by X-ray crystallography,[3] including several vinyl-
gold(I) species obtained from alkynes[3a–f] or allenes,[3g,h] and
gem-digold(I) species from vinylboronic acids through an
exchange reaction.[3i,j]
The treatment of N-(propargyl)benzamide (1) with an
equimolar amount of AuCl3 in acetonitrile/water (1:1 v/v) at
room temperature for 1 hour produces the formyloxazole 2 as
a major product, along with an unknown compound that was
eventually characterized to be a dimerized product (see
below). Neither the exomethylene oxazoline 3 or methylox-
azole 4 is produced in any noticeable amounts, as determined
by in situ NMR analysis. When a gold(I) species, AuCl
(3 equiv), is used instead of AuCl3, 2 becomes the predom-
inant product (Scheme 1).
During our study on a gold(I)/(III)-mediated intramolec-
ular cyclization of an N-(propargyl)arylamides, we observed
that the major product was a 2-oxazole-5-carboxaldehyde, an
oxidation product of the vinylgold intermediate, rather than
the compound(s) expected from a proto-deauration process.[4]
Vinylgold intermediates involved in various gold-catalyzed
reactions are known to undergo proto-deauration in the
presence of a proton source. The unexpected result drew our
attention on the chemistry of vinylgold intermediates
involved and thus prompted us to investigate the gold-
mediated cyclization in detail with simple substrates, N-
(propargyl)benzamides. Described herein is identification of
the vinylgold(III) intermediates involved and their reaction
pathways, all of which expands our present understanding on
the vinylgold intermediates.
Scheme 1. Gold(I/III)-promoted cyclization reactions of N-(propargyl)-
benzamide.
Hashmi and co-workers reported that 4 can be obtained in
greater than 95% yield from the AuCl3-catalyzed cyclization
of 1 in organic media.[5a] They also noted that 3 could be
obtained by addition of water to the corresponding vinylgold
intermediate arising from 1 using a cationic gold(I) species
and triethylamine in tetrahydrofuran (THF).[3b]
We examined the product distribution in the above
reaction in aqueous media for the different gold species in
different amounts (see Table 1 in the Supporting Informa-
tion). The results can be summarized as follows: 1) the
formyloxazole 2 becomes the major product when more than
one equivalent of the gold species is used; 2) under catalytic
conditions, a hydration product becomes the major product,[6]
regardless of the catalyst used; 3) AuCl3 and HAuCl4 behave
similarly, whereas AuCl is required in excess to form 2;[7]
[*] Dr. O. A. Egorova,[+] H. Seo,[+] Y. Kim, Prof. Y. M. Rhee,
Prof. K. H. Ahn
Department of Chemistry
POSTECH (Pohang University of Science and Technology)
San 31, Hyoja-dong, Pohang, 790-784 (Republic of Korea)
E-mail: ahn@postech.ac.kr
Homepage: http://www.postech.ac.kr/lab/chem/mras
Dr. D. Moon
Small/Supramolecule Crystallography,
Pohang Accelerator Laboratory, POSTECH (Republic of Korea)
4) among the catalysts, only
a cationic gold species
([PPh3Au]OTf) produces the proto-deauration product 3
[+] Contributed equally to this paper.
together with 2.[5c]
[**] This work was supported by grants from the EPB Center (R11-2008-
052-01001). The X-ray diffraction studies were performed at the
Pohang Accelerator Laboratory (Beamline 6B1 MXI, 10A), which is
supported by Korean Ministry of Education, Science and Technology
(MOEST) and POSTECH.
The heterocyclic products should be produced through the
corresponding vinylgold intermediates. To characterize the
vinylgold intermediates, we carried out in situ NMR analysis
for an equimolar mixture of benzamide 1 (0.02–0.03m) and
AuCl3 in CD3CN at room temperature. Upon mixing the two
components, two complexes were formed rapidly in a ratio of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106132.
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1:2. By using [D]-1, we were able to confirm that the 5-exo-dig
attack is dominant over the 6-endo-dig attack, as reported in
the literature,[3b] and in such a case, the anti addition occurs
exclusively over the syn addition in the catalytic oxyauration
reaction.[5a] On the basis of these findings, we assigned the
minor component to be the (E)-vinylgold(III) species A,
which was later confirmed by crystal structure analysis
(Scheme 2). The vinylgold(III) A was characterized through
can be enriched to a single component by a repeated
precipitation process. In this way, a separate 1H NMR
spectrum for the major component, B, can be obtained. All
our efforts to obtain a crystal structure of B, however, were
unsuccessful because some of the crystals obtained were of
low crystallinity and very fragile. Fortunately, we were able to
obtain a reaction product from B, which provided us with
valuable information on its structure. When an isolated
sample of B was heated up to 508C in acetonitrile and slowly
cooled to room temperature, needle-like crystals were
obtained and identified to be the bis(oxazole) 6 by using
1H NMR spectroscopy and X-ray diffraction analyses.[8] We
were also able to identify the bis(oxazoline) 5, a precursor of
6, by X-ray diffraction analysis (Scheme 3).[8] These results
Scheme 2. AuCl3-promoted cyclization of benzamide 1. The structure
of A is shown with thermal ellipsoids at 50% probability.
NMR spectroscopy by its vinylic proton at d = 5.8 ppm
(triplet, J = 3.0 Hz) and methylene protons at d = 5.0 ppm
(doublet, J = 3.0 Hz). The carbon atoms attached to the
vinylic and methylene protons appeared at d = 104.0 and
49.6 ppm, respectively, in the 13C NMR spectrum. The other
complex, B, exhibited chemical shifts similar to those of A,
that is, showing only vinylic and methylene-type protons in
addition to the aromatic ring protons: 1H NMR: d = 6.4
(singlet, 1H), 5.1 ppm (singlet, 2H); 13C NMR: d = 103.3 and
49.4 ppm.
So far only a few vinylgold(I) intermediates have been
characterized by X-ray crystallography as mentioned
above,[3a–f] but no of vinylgold(III) species have been charac-
terized yet. Our efforts to obtain single crystals from the
reaction mixture resulted in pale yellow prisms, the structure
of whcih was resolved to be A by X-ray diffraction analysis.
The crystal structure shows that the AuIII atom is coordinated
to three chloride ligands in a typical square-planar geome-
try.[8] The Au(1)-C(1) bond length is 2.004(19) ꢀ and the
C(1)-C(2) length is 1.311(3) ꢀ. The five-membered ring is
Scheme 3. Identification of the dimerized products, bis(oxazoline) 5
and bis(oxazole) 6, formed from B. Structures of 5 and 6 in the solid
state are shown with thermal ellipsoids at 50% probability (the
counterions 2AuCl4ꢀ in the case of 5 are omitted for clarity).
suggest that B is a divinylgold(III) species, the structure of
which is assigned as shown in Scheme 3. This assignment is
additionally supported by a crystal structure resolved for a
dimeric vinylgold(III) species obtained from a similar reac-
tion starting with a derivative of benzamide 1 (see below).
The formation of the mono- and divinylgold species in the
gold-mediated cyclization reaction was also observed when
AuCl, instead of AuCl3, was used, as indicated by the same
NMR spectral patterns obtained for the crude reaction
mixture (see the Supporting Information).
ꢀ
virtually planar and the C C bond [C2-C3, 1.502(3) ꢀ] is
=
differentiated from that of the C C bond of methyloxazole 4
[1.3401(18)].[8] The gold atom has a formal negative charge;
hence, the oxazoline nitrogen atom should be protonated to
give a zwitterionic vinylgold species. The presence of the
protonated oxazoline moiety was confirmed by the peaks in a
1H NMR spectrum of a mixture containing A and B that
correspond to an acidic proton; the peaks appear downfield as
two broad singlets (d = 10.6 and 10.9 ppm), including the
peaks corresponding to B.
The oxidative dimerization of vinylgold species is a known
process, as observed in several gold-catalyzed cyclization
reactions.[9] The corresponding vinylgold intermediates such
as B, however, have remained elusive species.
The complex B is less soluble in acetonitrile than A and
thus tends to precipitate at lower temperature. Therefore, it
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Communications
To obtain additional information on the structure of B, we
C11 at 2.2743(9) ꢀ, the neighboring C9-C11 at 1.322(6) ꢀ,
and the vinylic C8-C11 at 1.499(6) ꢀ. C also exists as a stable
zwitterionic form in which the nitrogen atom is protonated.
Of particular note is that C is quite stable in aqueous media
and does not undergo proto-deauration readily. Additionally,
its solution in acetonitrile does not show any decomposition
even after two weeks at room temperature.
examined the same gold-promoted cyclization with N-(but-2-
ynyl)benzamide (7). According to the previous study by
Hashmi and co-workers,[3b] the cyclization should proceed
through the 6-endo-dig route for such an internal alkyne. An
in situ NMR study for an equimolar mixture of benzamide 7
and AuCl3 in CD3CN at room temperature indicated that two
organogold species, C and D, formed instantaneously in a
ratio of 2:1, similar to the case involving benzamide 1
In contrast to C, D was not stable and transformed into
the organic dimer 8 after standing for seven hours at room
temperature, as determined by NMR analysis. The intensity
of the peaks for the methylene proton of D at d = 4.44 ppm
decreased, and a new peak appeared at d = 4.32 ppm. In
1
(Scheme 4). Both H and 13C NMR analyses (see the Sup-
porting Information) suggested that C is a monovinylgold
species.
1
contrast to D, C remained unchanged during the H NMR
observation (see the Supporting Information). Colorless
single crystals of the dimeric compound 8 were obtained
from recrystallization of the final mixture containing C and 8
at ꢀ188C; the crystal structure of 8 thus confirmed that it is
the dimerized product that is produced from D.
Along with our efforts to obtain the crystal structures, we
carried out extensive MS analyses for the reaction intermedi-
ates, A/B and C/D. An ESI-MS analysis (in the positive mode)
for a 1:1 mixture of benzamide 1 and AuCl3 in CD3CN at
room temperature (immediately after mixing) showed inten-
sive peaks at m/z 548 and 550 (Figure 1), which were
attributed to the divinylgold species 9 and its 37Cl derivative,
respectively.
Figure 1. Divinylgold species 9 observed by ESI-MS.
In the negative ESI-MS mode, the cluster peaks observed
at m/z 461.0 and 463.0 were attributed to the deprotonated
form of A (C10H8AuCl3NO, exact mass 459.93) having differ-
ent isotopes (C10H7DAuCl3NO and C10H7DAu35Cl237ClNO,
respectively). Both data sets also supported the formation of
the mono- and divinylgold species. We found that deposition
of metal gold during the mass analysis contaminated the
instrument; hence, we carried out MALDI-TOF MS analysis
for the mixture of C and D. A freshly prepared sample showed
an intense peak at m/z 345, which corresponded to the
protonated form of 8; in this case, peaks that represent
vinylgold species were observed but as minor fragments.
It was difficult to obtain single crystals of less stable D
from a mixture containing C because C crystallizes out first
and, furthermore, D has low stability in organic solvents.
After many attempts, we were able to identify single crystals
of D whose structure was resolved by X-ray diffraction
analysis. D is a cis-dichlorobis(vinyloxazinyl)gold species, as
shown in Figure 2. The cocrystal of AuCl4ꢀ indicates that both
the oxazinyl nitrogen atoms are protonated to balance the
charge. To the best of our knowledge, this is the first
divinylgold intermediate characterized by X-ray diffraction
analysis. On the basis of this result, we have assigned the
Scheme 4. Identification of the vinylgold intermediate C and the dimer
product 8 in the gold-promoted cyclization of benzamide 7. The
structures of C and 8 in the solid state are shown with thermal
ellipsoids at 50% probability (the counterion AuCl2 in the case of 8
and solvent molecules are omitted for clarity).
ꢀ
Fortunately, we were able to obtain single crystals of C in
the presence of D, by diffusion of water vapor into a solution
of the reaction mixture in acetonitrile at room temperature.
The resolved crystal structure confirmed that C is the
vinylgold(III) species produced through the 6-endo-dig
attack. Hashmi and co-workers characterized a crystal
structure of the vinylgold(I) species formed through the 6-
endo-dig attack.[3b] The crystal structure of C shows that the
AuIIICl3 moiety is in a typical square-planar geometry
(Scheme 4), similar to that of A, with the bond lengths Au1-
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gold species such as A and B are generated, they are
chemically stable in aprotic media if there are no promoters
such as 1 or others for the proto-deauration.[11] However, in
aqueous media, the stable vinylgold species can follow new
reaction pathways such as the formation of the formyloxazole
2. Previously, we tentatively suggested a gold-mediated
hydration of vinylgold A as a possible mechanism for the
introduction the formyl group in 2. A high level of molecular
calculation has been carried out, which suggests that the
AuCl3-mediated hydration of vinylgold A is less probable
owing to its high activation energy. An alternative route may
involve a gold carbenoid intermediate E (Scheme 6), which
could be produced through an oxidation process (AuIII$AuI
+ Au0) to produce 2 in the presence of water. Gold carbenoids
have been assumed to be intermediates in various trans-
formations but they have rarely been characterized.[12] Addi-
tional studies are necessary to characterize the mechanism
suggested.
Figure 2. Structure of D in the solid state. Thermal ellipsoids shown at
50% probability. A counterion (AuCl4ꢀ) is omitted for clarity.
structure of B as the corresponding divinylgold(III) species
(Scheme 3). Before obtaining the crystal structure of D, we
suspected that B might be a dimerized species of A, a divinyl-
digold species in which two A species are bridged through two
chlorine ligands. Further experiments, however, indicate that
there is no exchange reaction between A and B or between C
and D; the results exclude the possibility of B as a divinyl
digold species.
Scheme 6. A gold carbenoid route for A to form formyloxazole 2 via E.
To examine whether B could be formed from A, we
followed NMR spectral changes for a mixture of benzamide 1
and AuCl3 in a ratio of 3:2 in CD3CN at room temperature.
We know that, under the stoichiometric and dilute (0.3m of
benzamide 1) reaction conditions, only A and B form
instantly in a ratio of 1:2, with no benzamide 1 left.[10]
Under the substoichiometric reaction conditions, we observed
A, B, and benzamide 1 in an approximate ratio of 1:4:3. This
result indicates that the divinylgold B is not produced by a
possible reaction between A (or its dissociated form) and
benzamide 1. If it were the latter case, we could not observe
the unreacted benzamide under the substoichiometric reac-
tion conditions. The distribution ratio also did not change
when the temperature of the mixture was raised to 408C. A
similar behavior was also observed in the case of benzamide 7.
The divinylgold intermediate seems to be formed by a 1:2
coordination between AuCl3 and the benzamide starting
material through the propargylic group, although the mech-
anism is not clear at present.
In summary, we have characterized key vinylgold(III)
intermediates in the gold-mediated cyclization reactions of N-
(propargyl)benzamides. Both mono- and divinylgold(III)
complexes are instantaneously produced in comparable
ratios upon the treatment of the N-(propargyl)benzamides
with an equimolar amount of AuCl3 in acetonitrile at room
temperature. The structures of the vinylgold intermediates
have been identified by NMR, MS, and X-ray diffraction
analyses. Two crystal structures of two different
monovinylgold(III) complexes and one crystal structure of a
divinylgold(III) complex have been resolved. We have also
identified that the monovinylgold intermediate from N-
(prop-2-ynyl)benzamide does not undergo the proto-deaura-
tion reaction even in the presence of water. However, it does
produce 2-phenyloxazole-5-carboaldehyde, an oxidation
product of the vinylgold intermediate, probably through a
gold(III) carbenoid intermediate. The proto-deauration of
the vinylgold intermediates takes place upon the addition of
the N-(propargyl)benzamides. The divinylgold intermediates
are not as stable as the corresponding monovinylgold species
and thus, on standing they produce the corresponding
dimerized organic compounds, which were identified by
NMR and X-ray diffraction analyses. These results expand
our present understanding of the vinylgold intermediates in
the gold-catalyzed addition reactions, the presumed proto-
deauration process, and new reaction pathways of the vinyl-
gold species.
Of particular note is that when benzamide [D]-1 was
treated with a catalytic amount (5 mol%) of a preformed
mixture of A and B, the vinylgold complexes undergo proto-
deauration to afford 4 via 3, as determined by 1H NMR
spectroscopy (Scheme 5). Therefore, it is apparent that the
substrate facilitates the proto-deauration step in this gold-
catalyzed cyclization reaction in aprotic media. Once vinyl-
Scheme 5. Substrate-assisted proto-deauration. Vinylgold-catalyzed
conversion of [D]-1 into [D]-4.
Received: August 30, 2011
Published online: October 5, 2011
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Communications
[7] AuCl disproportionates into AuIII and Au0 in water; hence, more
than an equivalent amount gave better results: a) R. Kissner, G.
Welti, G. Geier, J. Chem. Soc. Dalton Trans. 1997, 1773 – 1777.
[8] Crystal data for A (C10H9N1O1Cl3Au1): M = 462.50, monoclinic,
space group P21/c (No. 14); a = 9.7219(6), b = 8.4971(5), c =
14.9538(9) ꢀ, b = 97.364(3)8, V= 1225.11(13) ꢀ3; Z = 4, T=
Keywords: gold · reaction mechanisms · reactive intermediates ·
structure elucidation · X-ray diffraction
.
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100(2) K, m(l=0.71073 ꢀ) = 12.394 mmꢀ1
, ,
1calc = 2.508 gcmꢀ3
59124 reflections measured, 10447 unique (Rint = 0.0472), R1 =
0.0287, wR2 = 0.0542 (I > 2s(I)), R1 = 0.0434, wR2 = 0.0573 (all
data), GOF = 1.058. Crystal data for C (C11H11AuCl3N O3): M =
ꢀ
508.52, triclinic, space group P1 (No. 2), a = 7.8236(3), b =
9.2952(4), c = 10.9315(4) ꢀ, a = 75.575(2), b = 79.595(2), g =
84.014(3)8,
V= 755.77(5) ꢀ3,
Z = 2,
T= 100(2) K,
29010
m(l=0.71073 ꢀ) = 10.264 mmꢀ1
,
1calc = 2.235 gcmꢀ3
,
reflections measured, 7798 unique (Rint = 0.0728), R1 = 0.0465,
wR2 = 0.0666 for 5508 reflections (I > 2s(I)), R1 = 0.0837, wR2 =
0.0760 (all data), GOF = 0.997. Crystal data for
D
(C28H31Au2Cl6N5O2): M = 1076.21, orthorhombic, Pca21 (No.
29), a = 23.6666(14), b = 11.8359(8), c = 24.6210(13) ꢀ, V=
6896.7(7) ꢀ3,
Z = 8,
T= 100(2) K,
m(l=0.71073 ꢀ) =
8.998 mmꢀ1, 1calc = 2.073 gcmꢀ3, 64521 reflections measured,
13790 unique (Rint = 0.1264), R1 = 0.0638, wR2 = 0.1299 for
8285 reflections (I > 2s(I)), R1 = 0.1312, wR2 = 0.1541 (all
data), GOF = 1.026. Synthetic procedures and crystal data for
other organic compounds (4, 5, 6, and 8) are provided in the
Supporting Information. CCDC 801793 (A), 801792 (4), 806020
(6), 806021 (5), 813810 (C), 813809 (8), and 837674 (D) contain
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[10] See the Supporting Information for the NMR spectral data.
Under the stoichiometric but more concentrated conditions
(ꢁ0.054m of benzamide 1), a third species (ca. 14%, from the
NMR integration) appeared together with A and B; this
unknown compound also seems to be a vinylgold species, as
inferred from the characteristic vinyl proton at d = 5.76 ppm.
Under both the stoichiometric and dilute conditions at ꢀ208C, A
and B were formed in a ratio of 1:1; hence A is preferentially
formed at lower temperature over B (A/B = 1:2 at 258C).
[11] During preparation of this manuscript, Zhu and Yu also reported
that the proto-deauration of a vinylgold(I) species does not
proceed in protic solvent (MeOH or EtOH); they used triflic
acid to promote the protodeauration in a catalytic reaction. See
Ref. [3f].
[12] a) A. Correa, N. Marion, L. Fensterbank, M. Malacria, S. P.
Nolan, L. Cavallo, Angew. Chem. 2008, 120, 730 – 733; Angew.
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E. Tkatchouk, Y. Wang, W. A. Goddard III, F. D. Toste, Nat.
Chem. 2009, 1, 482 – 486; c) L. Ye, L. Cui, G. Zhang, L. Zhang, J.
Am. Chem. Soc. 2010, 132, 3258 – 3259; d) For a mini-review on
gold a-oxo carbenoids, see: J. Xiao, X. Li, Angew. Chem. 2011,
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11450 www.angewandte.org
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