Full text of DOI:10.3762/bjoc.7.115

Triazole–Au(I) complex as chemoselective catalyst in
promoting propargyl ester rearrangements
Dawei Wang, Yanwei Zhang, Rong Cai and Xiaodong Shi*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1014–1020.
Department of Chemistry, West Virginia University, Morgantown, WV
26506, USA
doi:10.3762/bjoc.7.115
Received: 23 April 2011
Accepted: 06 July 2011
Published: 25 July 2011
Email:
Xiaodong Shi* - Xiaodong.Shi@mail.wvu.edu
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
Keywords:
allene; chemoselectivity; gold catalysis; ligand effect; organometallic
Guest Editor: F. D. Toste
© 2011 Wang et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Triazole–Au (TA–Au) catalysts were employed in several transformations involving propargyl ester rearrangement. Good chemose-
lectivity was observed, which allowed the effective activation of the alkyne without affecting the reactivity of the allene ester inter-
mediates. These results led to the investigation of the preparation of allene ester intermediates with TA–Au catalysts under anhy-
drous conditions. As expected, the desired 3,3-rearrangement products were obtained in excellent yields (generally >90% yields
with 1% loading). Besides the typical ester migrating groups, carbonates and carbamates were also found to be suitable for this
transformation, which provided a highly efficient, practical method for the preparation of substituted allenes.
Introduction
The past decade has seen rapid growth in the use of homoge- lysts involved in alkyne and alkene activation are of the type
neous gold catalysis for conducting powerful organic transfor- [L–Au]+, with the open coordination site on the opposite side of
mations [1-9]. Like many other transition metal complexes, the the ligand (L) for substrate binding [5,6]. The recent success in
reactivity of gold catalysts greatly depends on the nature of the obtaining the complexes of the alkyne-coordinated [L–Au]+,
ligands coordinating with the metal cations [10-15]. Of the two reported by Toste and coworkers, greatly supported this mecha-
typical oxidation states, Au(I) and Au(III), more studies have nistic model [16]. Generally, the PR3 compounds can be applied
been focused on the former cation due to the easier preparation as the ligand in Au(I) catalysis. The recent development of
of the catalyst and better pre-catalyst stability. It is currently N-heterocyclic carbene (NHC) derivatives has significantly
accepted by the research community that Au(I) complexes expanded the choice of ligands by improving the catalyst
adopt one of two coordination sites with 180° linear geometry stability through metal-ligand backbonding [17-20]. To access
(Scheme 1) (although some exceptions exist). The actual cata- the active catalyst [L–Au]+, stable precursors L–Au–X or
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Beilstein J. Org. Chem. 2011, 7, 1014–1020.
Scheme 1: The counter ligands, an important factor in Au(I) catalysis.
[L–Au–X]+·A− were typically used. While the ligands (L) are the gold activated allene. Therefore, selective activation of the
certainly considered critical in gold catalysis, more and more alkyne over the allene was considered as a significant challenge
attention have been paid to the evaluation of whether the choice in gold catalysis.
of counter ligand “X” can be used to adjust the overall catalyst
reactivity.
Results and Discussions
Recently, our group reported the synthesis and characterization
The propargyl ester rearrangement was considered as one of the of the 1,2,3-triazole [36-40] coordinated gold(I) complexes. As
most important reaction modes in the Au(I) promoted transfor- revealed by the X-ray crystal structures (Scheme 3), both
mation [21-31]. Recent experimental and computational mecha- neutral and anionic triazoles can coordinate with Au(I) cation,
nistic studies revealed the 3,3-rearrangement to form the allene forming stable TA–Au complexes [41].
ester intermediate [32,33] as the key step in this transformation
(Scheme 2a) [34]. Both experimental and theoretical investi- The preparation of these complexes was very straightforward.
gations confirmed the reversibility between allene and Simply treating the NH-triazoles with PPh3AuCl in methanol
propargyl ester due to effective activation of both functional under basic conditions (K2CO3, 1 equiv) at room temperature
groups by the Au(I) catalysts. As a result, it was extremely chal- gave the neutral TA–Au-1 in >90% yield. The “cationic” com-
lenging to obtain the allene intermediates with good yields. plex TA–Au-2 was prepared either from the addition of HOTf
Many strategies have been developed to make the Au(I)-acti- to TA–Au-1 or by the reaction between PPh3Au–OTf (prepared
vated allene esters react with other proper substrates, forming from PPh3–Au–Cl and AgOTf) and benzotriazole. Both
interesting new products in a cascade fashion. The indene syn- complexes were stable and could be further purified by recrys-
thesis (Scheme 2b), reported by Nolan and coworkers, is one tallization to ensure no extra Ag+ or acid in the catalysts. The
good example highlighting the importance of the cascade crystal structures revealed nearly identical Au–P bond length
process [35].
for both the anionic and neutral triazole coordinated Au(I)
complexes. The longer Au–N bond in TA–Au-2 implies that the
As shown in Scheme 2b, with the [IPr–Au]+ catalyst, only trace neutral triazole dissociates more easily to release the coordina-
amount of the allene 2a was obtained. The major product tion site for substrate activation. This new class of compounds
derived from the Friedel–Crafts addition of the aromatic ring to offers improved thermal stability and substrate stability in the
Scheme 2: The challenge of the synthesis of allenes through gold activated alkynes.
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Beilstein J. Org. Chem. 2011, 7, 1014–1020.
cationic TA–Au catalyst, on the other hand, produced the kinet-
ically favored (E)-isomer. Notably, treating the allene ester 2a
with NIS gave the (E)-isomer as the dominant product. These
results imply that the allene iodination should favor the forma-
tion of the (E)-isomer (Scheme 4b). The typical [L–Au]+ cata-
lyst not only promoted the propargyl ester 3,3-rearrangement,
but also influenced the allene reactivity, probably through gold
catalyzed allene activation. The fact that TA–Au gave the domi-
nant (E)-isomers strongly suggests that these complexes may be
applied as the chemoselective catalyst in alkyne activation over
allene. The reactions of propargyl ester 1a with TA–Au cata-
lysts were then investigated as shown in Figure 1.
As expected, with the cationic catalyst TA–Au-2 or TA–Au-3,
the allene ester 2a was formed in excellent yields (1% loading,
91% yield). It is important to note here, that indene 3a was not
observed even after 48 h reaction time, thus indicating excel-
lent chemoselectivity of the triazole coordinated gold
complexes. Various propargyl esters were synthesized to test
the reaction substrate scope. The results are summarized in
Table 1.
As shown in Table 1, the transformation proceeded smoothly
with substrates having both an aromatic group on the propargyl
side and an aliphatic group on the alkyne side (entries 1–6). The
desired allene products were formed in excellent yields, with
1% catalyst loading. The electronic density on the aromatic ring
Scheme 3: X-ray crystal structures of the two different types of 1,2,3-
triazole–Au complexes.
gold(I) promoted hydroamination and Hashmi phenol synthesis did not have a strong impact on the transformation: Both elec-
[42], which makes them interesting novel catalysts in the field tron donating and electron-withdrawing groups were suitable
of gold catalysis. One particular new development of the for the reaction. Again, no indene by-products were observed in
TA–Au catalysis that attracted our attention was the synthesis any of the tested cases, even with the electron-enriched p-OMe
of α-iodoenone from propargyl esters (Scheme 4a) [43].
substituted alkyne 1d. These results highlighted the excellent
chemoselective nature of the TA–Au catalyst.
As indicated in Scheme 4a, the typical [L–Au]+ catalyst
promoted the sequential rearrangement and iodination, giving The terminal alkyne 1i did not give any product when treated
the thermally, dynamically stable (Z)-isomer [44-46]. The with TA–Au catalyst, even after an extended reaction time
Scheme 4: Synthesis of α-iodoenone compounds from propargyl esters.
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Beilstein J. Org. Chem. 2011, 7, 1014–1020.
Figure 1: Chemoselective activation of alkyne over allene by the TA–Au catalysts.
Table 1: The reaction substrate scope.a
Entry
1
Substrate
Product
Yield
1a
1b
2a
2b
91%
90%
2
3
1c
1d
1e
2c
2d
2e
87%
89%
89%
4
5
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Beilstein J. Org. Chem. 2011, 7, 1014–1020.
Table 1: The reaction substrate scope.a (continued)
6
1f
2f
85%
Substrates that did not form the desired allenesb
aGeneral reaction conditions: 1 (0.25 mmol) and TA–Au-2 (1.0 mol %) in dry DCM (2.5 mL), the reactions were monitored by TLC (2–10 h), rt.
bTA–Au-1, TA–Au-2 and TA–Au-3 did not catalyze the reaction under the standard conditions.
(24 h). This was probably caused by the preferred 1,2- suggests that the propargyl ester rearrangement to form allene is
rearrangement with the formation of a vinyl–Au intermediate. highly substrate dependent. This could either be due to the
The aliphatic propargyl esters (1g, 1h) also did not give any similar reactivity of the alkyne and the allene (giving an equilib-
desired allene products (enones from hydrations were produced rium state favoring the alkyne over the allene) or it be could be
after a long reaction time, 24–48 h; the crude NMR of the reac- due to a preferred alternative migration path (2,3-migration
tion mixtures did not show any allene products). This may be versus 3,3-migration). In any case, the TA–Au catalyst clearly
caused by the high reactivity of the corresponding aliphatic displayed the interesting chemoselectivity, if the reaction could
allenes under the reaction conditions (activated by TA–Au) and occur. To study the feasibility of this migration, we then investi-
the overall better stability of the propargyl ester compared to the gated migrating groups other than esters. The results are
aliphatic substituted allenes (equilibrium favored the starting summarized in Table 2.
material). The reaction of cyclopropyl substituted propargyl
ester 1j with the TA–Au catalyst gave a complex reaction mix- As indicated in Table 2, carbonates (entries 1–5) and carbamate
ture, which suggests possible ring opening and sequential (entry 6) were also suitable for this transformation. Compared
cyclization as reported previously [47]. Overall, this study to the allene-acetates, the allene-carbonates and allene-carba-
Table 2: Different migrating groups.a
Entry
Substrate
Product
Yield
92%
1
4a
4b
4c
5a
5b
5c
2
3
91%
88%
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Beilstein J. Org. Chem. 2011, 7, 1014–1020.
Table 2: Different migrating groups.a (continued)
4
4d
4e
4f
5d
5e
5f
92%
89%
85%
5
6
aGeneral reaction condition: 4 (0.25 mmol) and TA–Au-2 (1.0 mol %) in dry DCM (2.5 mL), the reactions were monitored by TLC (2–10 h), rt.
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