Triazole acetyl gold(III) catalyzed Meyer-Schuster rearrangement of propargyl alcohols

Article abstract of DOI:10.1016/j.tetlet.2016.04.043

A new type triazole acetyl gold(III) was prepared and found to be an effective catalyst in Meyer-Schuster rearrangement of propargyl alcohols. The reactions proceeded well under much milder conditions to afford enones bearing a wide range of functional groups, thereby opening a new avenue for gold(III) catalysis. In addition, TriaAuCl₂ catalyst was also effective on promotion of a-haloenones synthesis.

Full text of DOI:10.1016/j.tetlet.2016.04.043

Tetrahedron Letters 57 (2016) 2280–2282
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Triazole acetyl gold(III) catalyzed Meyer–Schuster rearrangement
of propargyl alcohols
b,
Yongchun Yang a, Yanan Shen a, Xiaoli Wang a, Yao Zhang a, Dawei Wang a, , Xiaodong Shi
⇑
⇑
a The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu
Province, China
b 4202 E Fowler Ave, Department of Chemistry, University of South Florida, Tampa, FL, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type triazole acetyl gold(III) was prepared and found to be an effective catalyst in Meyer–Schuster
rearrangement of propargyl alcohols. The reactions proceeded well under much milder conditions to
afford enones bearing a wide range of functional groups, thereby opening a new avenue for gold(III)
catalysis. In addition, TriaAuCl2 catalyst was also effective on promotion of a-haloenones synthesis.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 28 January 2016
Revised 11 April 2016
Accepted 13 April 2016
Available online 13 April 2016
Keywords:
Triazole
Gold
Meyer–Schuster rearrangement
Propargyl alcohols
Haloenones
Introduction
1A
1B
HO
O
OH
Gold catalysis has been widely applied in organic transforma-
tions, and methods for their efficient, selective functionalization
to construct more complex molecules during the past several
decades.1,2 Gold(I) catalysts are usually two coordinated with
180° linear geometry, while gold(III) catalysts have a planar coor-
dination geometry, and hence the spatial environment around
the gold center can be more easily fine-tuned through ligand
design studies. However, gold(I) catalysis has achieved much more
development and concern than gold(III) catalysis, which should be
attributed to the heat stability of gold catalyst.3 The typical gold
(III) catalyst is the PicAuCl2 and their derivatives, which enriched
and evidenced the development of gold(III) catalysis (Scheme 1A).
After some pioneering works on PicAuCl2,4 Hashmi reported
PicAuCl2 catalyzed phenol synthesis, which revealed that gold(III)
precatalysts represent a typical step in catalyst-tuning by ligand
design, while they found Au(I) showed low selectivity.5 Toste
et al. described a PicAuCl2 catalyzed synthesis of azepines via inter-
molecular [4+3]-annulation reaction.6 In 2013, Waser et al devel-
oped the selective synthesis of 2- and 3-alkynylated furans based
on a domino cyclization/alkynylation process with PicAuCl2 as
catalyst.7 Chan group showed that PicAuCl2 catalyzed selective
O
O
N
N
Cl Au
Cl
N
Cl Au
Cl
N
N
O
O
O
O
N
Au O
Cl
Cl Au
Cl
O
Cl
PicAuCl2
TriaAuCl2
Scheme 1. PicAuCl2 and TriaAuCl2.
1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to
afford cis-cyclopenten-2-yl d-diketones.8
Our research in developing new triazole ligands to balance the
stability and reactivity of gold catalysts has led to the recent
discovery of triazole gold(I) complexes (TA-Au),9,10 which was
achieved several applications on hydroamination, Hashmi phenol
synthesis, and 3,3-rearrangements with TA-Au(I) as a catalyst.
We have a concern about triazole ligands: could triazole adjust
the stability and reactivity of gold catalysts gold(III)? Herein, we
synthesized the TriaAu(III)Cl2 complex, which showed excellent
catalytic activity in Meyer–Schuster rearrangement of propargyl
alcohols for enone synthesis under mild conditions, which avoid
the preparation of corresponding acetate derivatives and have
large substrate exploration (Scheme 1B).
The designed (triazol-1-yl)acetic acid (1a) was synthesized
from benzotriazole and methyl 2-bromoacetate via two steps with
⇑
Corresponding authors. Tel./fax: +86 510 85917763.
E-mail addresses: wangdw@jiangnan.edu.cn (D. Wang), xmshi@usf.edu (X. Shi).
http://dx.doi.org/10.1016/j.tetlet.2016.04.043
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

Y. Yang et al. / Tetrahedron Letters 57 (2016) 2280–2282
2281
86% yield,11 following dealt with potassium tetrachloroaurate
Table 2
Substrate expansion of propargyl alcoholsa,b
under base conditions and the obtained TriaAu(III)Cl2 complex
(1b) was obtained in 81% yield.12 Next, we explored the catalytic
activity of this new catalyst TriaAu(III)Cl2. Compared to the 3,30-
rearrangement of propargyl esters,13 Meyer–Schuster rearrange-
ment of propargyl alcohols is quite difficult. Propargyl alcohol
(2a) was selected as a model substrate for reaction screening. It
is very disappointing that the hydration product was found only
with low yields (Table 1, entry 1). Based on our previous results
on Au(I),14 we firstly examined the effect of solvents on the reactiv-
ity using TriaAuCl2 as the catalyst. As shown in Table 1, the
reaction was strongly solvent-dependent. Excellent reactivity was
achieved in MeOH. It should be noted that the Meyer–Schuster
rearrangement of propargyl alcohols could not take place in the
absence of gold catalyst (Scheme 2).
Having established the optimal conditions, the substrate scope
of TriaAuCl2 catalyzed Meyer–Schuster rearrangement of propar-
gyl alcohols was examined. The reactions were carried out in
MeOH (10% H2O) at 60 °C. Generally, all the propargyl alcohols
were converted completely into the corresponding enones. High
yields were achieved regardless of the electronic properties and
steric hindrance of substituent groups (Table 2). The propargyl
alcohols with the MeO group (3m) gave the highest yield, while
a slightly lower yield was obtained when the nitro substituent
was in the substrate (3l).
OH
R3
O
1b
) (1 mol%)
TriaAu(III)Cl2
(
R1
R1
R2
R3
MeOH/H2O, 60 oC
R2
2
3
O
O
O
O
Bu
Bu
Ph
Bu
F
3c
3a
3b
3d
: 90%
: 89%
E/Z > 20:1
: 87%
: 84%
E/Z > 20:1
E/Z = 19:1
E/Z > 20:1
O
O
O
Ph
O
Bu
Bu
Ph
Ph
OMe
Cl
3e
3f
3g
3h
: 78%
E/Z > 20:1
: 80%
E/Z = 16:1
: 91%
: 88%
E/Z > 20:1
E/Z > 20:1
O
O
O
O
Bu
Bu
Bu
Bu
NO2
Cl
Cl
3i
3j
3k
: 89%
E/Z > 20:1
3l
: 76%
E/Z = 13:1
: 85%
E/Z > 20:1
: 87%
E/Z = 19:1
O
O
O
O
Bu
Bu
O2N
Bu
Bu
MeO
In addition, we attempted the experiment with alkyl-substi-
tuted propargyl alcohol as a substrate. As illustrated in Scheme 3,
the alkyl-substituted enone was achieved in only E isomer 81%
yield.
Moreover, TriaAu(III)Cl2 was also an effective catalyst on
a-haloenones synthesis,15 although a slightly lower yield was
achieved (Table 3). It was found that the electronic and steric
natures of aromatic ring of propargyl esters had no obvious effect
on the results. For example, the propargyl esters bearing
electron-donating (like Me, MeO) or electron-withdrawing groups
3m
3n
3o
3p
: 93%
E/Z > 20:1
: 82%
E/Z > 20:1
: 83%
E/Z > 20:1
: 86%
E/Z > 20:1
a
Conditions: 2 (0.5 mmol), TriaAu(III)Cl2 (1b) (1 mol %), MeOH (10% H2O), 60 °C,
Isolated yields based on 2.
6 h.
b
O
OH
TriaAu(III)Cl2 (1b) (1 mol%)
Bu
Solvent
Bu
3q
: 81%
2q
E/ Z > 20:1
Table 1
Screening of reaction conditionsa
Scheme 3. Substrate experiment with alkyl-substituted propargyl alcohol.
O
OH
TriaAu(III)Cl2 (1b) (1 mol%)
Ph
Bu
Ph
Solvent
Bu
Table 3
3a
2a
Substrate expansion of propargyl estersa,b
Entry
Solvent
T
Convn.
3a [%] (E/Z)b
R1
O
OAc
1b
) (1 mol%)
TriaAu(III)Cl2
(
1
2
3
4
5
6
7
8
9
1,4-Dioxane
Acetone
Toluene
CH2Cl2
H2O
MeOH
MeOH
MeOH
MeOH
rt
rt
rt
rt
rt
rt
40
60
60
23
<5
<5
<5
<5
26
75
>95
<5
15
<5
<5
<5
<5
20
62
R2
R1
NIS, CH2Cl2, rt
I
R2
4
5
Cl
O
O
O
MeO
O
89 (>20:1)
Bu
Bu
Bu
Bu
<5c
I
I
I
5b
I
5c, 70%
E/Z = 7:1
5d, 77%
5a
, 80%
, 72%
E/Z = 2.5:1
a
Conditions: 2a (0.5 mmol), TriaAu(III)Cl2(1b) (1 mol %), wet solvent (10% H2O),
E/Z > 10:1
E/Z = 6:1
6 h.
b
Isolated yields based on 2a.
No[Au] was used.
c
a
Conditions: 4 (0.5 mmol), NIS (0.75 mmol), TriaAu(III)Cl2 (1b) (1 mol %), CH2Cl2
(3 mL), 12 h, rt.
b
Isolated yields based on 4.
KAuCl4
N
N
N
N
O
KOH, MeOH
81% yield
at para-, meta-, or ortho-positions were well tolerated, and afforded
the corresponding products in moderate yields. Notably, this trans-
formation didn’t work for the terminal alkyne since the 1,2-rear-
ramgement was usually preferred for terminal alkyne propargyl
acetate.
N
N
O
Au O
Cl
HO
Cl
1b
1a
TriaAuCl2
Scheme 2. The synthesis of TriaAu(III)Cl2 complex.

2282
Y. Yang et al. / Tetrahedron Letters 57 (2016) 2280–2282
10668; (e) Wang, Y.; Zheng, Z.; Zhang, L. J. Am. Chem. Soc. 2015, 137, 5316; (f)
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In summary, we developed a new type TriaAu(III)Cl2 complex,
which showed good catalytic activity in Meyer–Schuster rearrange-
ment of propargyl alcohols for enone synthesis under mild condi-
tions, which avoid the preparation of corresponding acetate
derivatives and have large substrate exploration. In addition,
TriaAuCl2 complex was also effective on promotion of a-haloenones
synthesis. Further applications of TriaAu(III)Cl2 catalyst to other
reactions for the construction of carbon-heteroatom bonds are
currently in progress in our laboratory.
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Acknowledgements
We gratefully acknowledge financial support of this work by
the National Natural Science Foundation of United States
(CHE-1362057), the National Natural Science Foundation of China
(21401080, 51503086), the Natural Science Foundation of Jiangsu
Province of China (BK20130125), China Postdoctoral Science
Foundation (2014M550262, 2015T80495) and Programme of
Introducing Talents of Discipline to Universities (111 Project
B13025).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.04.
043.
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