Preparation of Triazole Gold(III) Complex as an Effective Catalyst for the Synthesis of E-α-Haloenones

Article abstract of DOI:10.1002/adsc.201600243

The pyridyltriazole ligand was prepared and applied as a new ligand system for the coordination towards gold(III) cations. The resulting complex showed excellent stability and effective catalytic reactivity for the Meyer–Schuster rearrangement of propargyl alcohols and sequential allene halogenation, giving α-haloenones with excellent yields and good E/Z selectivity. More importantly, by using this new gold(III) catalytic system, the challenging α-chloroenone substrates were synthesized in good yields for the first time. This work also permits the avoidance of preparing of acetate derivatives as required in the case of Au(I) catalysts and validates that the triazole gold(III) complex is an effective new catalyst for alkyne activation. (Figure presented.).

Full text of DOI:10.1002/adsc.201600243

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DOI: 10.1002/adsc.201600243
Preparation of Triazole Gold(III) Complex as an Effective
Catalyst for the Synthesis of E-a-Haloenones
Yongchun Yang,a Wenkang Hu,a Xiaohan Ye,b Dawei Wang,a,* and Xiaodong Shib,*
a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, Peopleꢀs Republic of China
Fax : (+86)-510-8591-7763; e-mail: wangdw@jiangnan.edu.cn
Department of Chemistry, University of South Florida, 4202 E. Fowler Ave, Tampa, Florida 33620, USA
b
Fax : (+1)-813-974-3203; e-mail: xmshi@usf.edu
Received: March 2, 2016; Revised: April 30, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600243.
Abstract: The pyridyltriazole ligand was prepared
and applied as a new ligand system for the coordi-
nation towards gold(III) cations. The resulting com-
plex showed excellent stability and effective catalyt-
ic reactivity for the Meyer–Schuster rearrangement
of propargyl alcohols and sequential allene halogen-
ation, giving a-haloenones with excellent yields and
good E/Z selectivity. More importantly, by using
this new gold(III) catalytic system, the challenging
a-chloroenone substrates were synthesized in good
yields for the first time. This work also permits the
avoidance of preparing of acetate derivatives as re-
quired in the case of Au(I) catalysts and validates
that the triazole gold(III) complex is an effective
new catalyst for alkyne activation.
are excellent ligands for the Au(I) cation, are not
viable in Au(III) catalysis due to the rapid redox
chemistry. Electron-rich NR3 ligands, such as
TMEDA, often behave in a similar way. Consequent-
ly, only neutral or electron-deficient compounds, such
as pyridine derivatives,[6] have been used as ligands
for Au(III) cations. However, these complexes usually
suffer from decomposition and low catalytic activity.[7]
Recently, we reported that 1,2,3-triazole (TA) li-
gands could greatly enhance the stability of Au(I)
with good catalytic activity.[8] Based on the previous
results, we hypothesized that triazole derivatives can
also be used in the application of Au(III) complexes
(Scheme 1). Herein, we report pyridyltriazole
gold(III) complex [TA-Py-Au(III)] as a new stable
gold(III) catalyst. Furthermore, this catalyst can be
successfully applied in promoting propargyl alcohol
activation and sequential allene halogenation to pro-
duce a-haloenones in good yields. Notably, the intro-
duction of the triazole core may enhance the reactivi-
ty and stability of gold(III) complexes, offering new
potential reactivity and applications in alkyne activa-
Keywords: gold; haloenones; propargyl alcohols;
rearrangement; triazoles
Gold catalysis has attracted significant attention tion.
during the past few decades and is still considered as
It has been reported that gold(I) complexes can
a synthetically appealing approach towards carbon- catalyze various rearrangements of propargyl esters
carbon multiple bond activation. This is further high- and alcohols.[9] A well-known example is the Meyer–
lighted by its superior reactivity, excellent selectivity Schuster rearrangement, including the dehydrogena-
and high functional group tolerance.[1] To date, chem- tive Meyer–Schuster rearrangement developed by
ists have designed many gold(I) complexes and there- Hashmi.[10] Another interesting example involved the
by achieved various attractive synthetic transforma-
tions.[2] However, gold(III) catalysis is much less de-
veloped due in part to the lack of effective and non-
redox active ligand systems.[3]
Typical gold(III) complexes usually possess
a square planar geometry with four coordination sites,
which should be easily tuned through the design of
different ligands in a modular approach.[4] However,
the stability of gold(III) complexes can be a concern.[5]
Scheme 1. Au(III) complexes with N-ligands and 1,2,3-tri-
Generally, the electron-rich PR3 compounds, which
ACHTUNGTRENaNGNU zole complexes.
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Figure 1. Synthesis of TA-Py-Au(III).
terization data supported our hypothesis that 1,2,3-tri-
azoles could serve as a good ligand for Au(III) coor-
dination.
Scheme 2. Gold-catalyzed rearrangements of propargyl
esters and alcohols.
After heating the solution of TA-Py-Au(III) in tol-
synthesis of Z-a-iodoenones from propargyl esters uene at 1008C for 1 hour, no obvious decomposition
and NIS under Au(I) catalytic conditions, which was of gold was observed. Furthermore, the TGA experi-
first
reported
by
Zhang
and
co-workers ments indicated that the thermal stability of this TA-
(Scheme 2A).[11] Many other groups have also used Py-Au(III) complex was significantly improved in
NXS in the context of homogeneous gold catalysis.[12] comparison with the pyridine acid Au(III) complex
We have developed triazole-gold(I) complexes (TA- PicAuCl2. In order to evaluate the reactivity of this
Au) as effective catalysts for the preparation of more new complex, TA-Py-Au(III) was treated with prop-
challenging kinetic E-haloenones through the chemo- argyl alcohol and NIS. As shown in Scheme 2B, di-
selective activation of alkynes over allenes.[13] Shep- iodide ketone was formed instead of the desired vinyl
pard et al. developed the gold(I)-catalyzed regioselec- iodide when the Ph3PAuNTf2 was used.[12] To obtain
tive conversion of propargylic alcohols into previously the desired vinyl iodide product using a gold(I) cata-
unreported a,a-diiodo-ß-hydroxy ketones with good lyst, other additives [such as MoO2(acac)2] are usually
yields (Scheme 2B).[14] Another interesting example needed to promote the 3,3-rearrangement of proparg-
reported by Zhang and co-workers is the dual activa- yl alcohols.[13] However, based on the reaction mecha-
tion of propargyl alcohol using the combination of nism, it is possible to achieve this transformation with
Ph3PAuNTf2 and MoO2(acac)2 with Ph3PO as an addi- gold(III) catalyst alone if a formal dehydration occurs
tive, which could suppress the formation of the unde- (Scheme 3).
sired enone/enal, giving the Z-isomer of a-haloenones
To test this hypothesis, a series of gold catalysts
(Scheme 2C).[15] In this work, an efficient reaction cas- were used to react with propargyl alcohol 1a. The re-
cade directly from propargyl alcohol (instead of ester) sults are summarized in Table 1.
was achieved, thus greatly enhancing synthetic effi-
ciency.
A challenge in the execution of this strategy was
the competition between propargyl alcohol hydration
We were interested in exploring whether 1,2,3-tri- and iodination. As shown in entry 3, the undesired
ACHTUNGTRENNUNG
1
(Figure 1) was confirmed by H NMR, 13C NMR and
elemental analysis. Attempts to obtain X-ray crystal-
lographic analysis were unsuccessful, probably due to
Scheme 3. Proposed vinyl halide synthesis from propargyl al-
the chloride anion exchange. Nevertheless, the charac- cohol with gold(III) catalyst alone.
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Table 1. Screening of the reaction conditions.[a]
Entry
Catalyst (2%)
Solvent
Conv. (E/Z)[b]
2a [%][c]
3a [%]
[d]
1
2
3
4
5
6
7
8
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
TA-Py-Au(III)
KAuCl4
toluene
acetone
MeOH
THF
CH2Cl2
MeCN
H2O
MeCN:H2O (5:1)
MeCN:H2O (5:1)
MeCN:H2O (5:1)
MeCN:H2O (5:1)
none
none
16
<5
15
65
<5
>95 (>20:1)
21
mess
–
–
–
–
–
–
16
–
10[c]
54
–
5
8
–
91
15
–
–
5
12
13
14
AuCl3
PicAuCl2
<5
–
>95 (>20:1)
86
[a]
Reaction conditions: 1a (0.5 mmol), NIS (0.75 mmol), [Au] (2 mol%), solvent (3 mL), 6–12 h, room temperature.
Determined by H NMR.
Isolated yields.
“–” means no product was detected.
[b]
[c]
[d]
1
enone 3a was the only product when MeOH was tion conditions. Both results strongly supported the
used. No reaction was observed with toluene, acetone proposed formation of allene as the key intermediate
or THF as the solvent. With DCM as solvent, low en route to the formation of the desired vinyl iodide.
conversion was observed. The conversion was in-
In conclusion, we have developed a new type of
creased in CH3CN. The best result was obtained with pyridyltriazole gold(III) complex TA-Py-Au(III) and
a mixed solvent system (CH3CN:H2O=5:1), in which demonstrated its use as an effective catalyst in the
the hydration product 3a was completely subdued.
Meyer–Schuster rearrangement of propargyl alcohols
With the optimal conditions in hand, the substrate and sequential allene halogenation. Compared with
scope of the gold(III)-catalyzed rearrangement of the previously developed method based on propargyl
propargyl alcohols was assessed (Table 2).
acetate substrates, this method is more efficient.
Considering the good stability of this TA-Py-
Au(III) complex, we believed that this catalyst could
also be used to promote the synthesis of a-chloro-
ACHTUNGTRENNUNGenones through Meyer–Schuster rearrangement of
propargyl alcohols under the above conditions. How-
ever, the reaction did not work with NCS as the chlor-
ine source. On changing the “Cl+” source to TCICA
(trichloroisocyanuric acid),[15] the desired a-chloro-
ACHTUNGTRENNUNGenone products were formed with good yields. The re-
sults are summarized in Table 3. It should be noted
that a-chloroenones could not be achieved in the ab-
sence of the pyridyltriazole gold(III) catalyst.
To explore the reaction mechanism, an intramolec-
ular competition experiment was conducted between
electron-withdrawing and electron-donating group-
modified arenes (Figure 2). The experimental data re-
vealed that the substrates with an electron-donating
group reacted faster than those with an electron-with-
drawing group, which was consistent with the pro-
posed propargyl alcohol dehydration reaction path-
way. Additionally, the vinyl iodide product was not
observed when treating the enone with the same reac- Figure 2. Brief mechanistic investigation.
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Table 2. Substrate scope of propargyl esters.[a,b,c]
Table 3. Substrate scope of propargyl esters with trichloro-
AHCTUNGTRENNUNG
isocyanuric acid (TCICA).[a,b,c]
[a]
Reaction conditions: 1 (0.5 mmol), TCICA (1.5 equiv.),
TA-Py-Au(III) (2 mol%), CH3CN/H2O (5:1, 3 mL), 6 h,
room temperature.
[b]
[c]
Isolated yields. The yields are based on the mixture of
isomers.
1
The E/Z ratio was determined by H NMR.
ethanolic solution of KAuCl4·2H2O (1.0 mmol, 5 mL) was
added. The resulting mixture was stirred overnight at room
temperature, then the cloudy solution was filtered through
a short pad of celite. The clear solution was concentrated
under vacuum. The yellow solid was dissolved with CH2Cl2,
and then added dropwise to a flask containing petroleum
ether. The precipitated solid was washed several times with
diethyl ether, dried and filtered to afford the complex;
yield: 89%.
[a]
Reaction conditions: propargyl alcohol (0.5 mmol), NIS
(0.75 mmol), TA-Py-Au(III) (2 mol%), MeCN:H2O (5:1,
3 mL), 6–12 h, room temperature.
Isolated yields. The yields are based on the mixture of
General Procedure for Synthesis of a-Haloenones
[b]
At room temperature, propargyl alcohol (0.5 mmol) was
added to a stirring solution of NIS (0.75 mmol) and Au(III)
catalyst (2 mol%) in MeCN/H2O (3 mL, 5:1). After the re-
action was completed (6–12 h) according to TLC, the sol-
vent was removed under reduced pressure and the residue
was purified by flash chromatography on silica gel (ethyl
acetate/hexane=1:20, v/v) to give a-haloenones as yellow or
light yellow oils.
isomers.
[c]
1
The E/Z ratio was determined by H NMR.
Moreover, it revealed that triazole gold(III) is anoth-
er effective catalyst in propargyl alcohol activation.
Mechanistic investigations and studies on further ap-
plications of this catalyst to other reactions are cur-
rently on going in our group.
Acknowledgements
We gratefully acknowledge financial support of this work by
NSF (CHE-1362057), NSFC (21401080), NSF of Jiangsu
(BK20130125) and MOE & SAFEA for the 111 Project
(B13025).
Experimental Section
Synthesis of Pyridyltriazole Gold(III) Complex
To a solution of ligand (196 mg, 1.0 mmol, 1 equiv.) in
EtOH (5 mL) was added KOH (56 mg, 1.0 mmol, 1 equiv.).
The mixture was stirred for 15 min at room temperature. An
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ÞÞ
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COMMUNICATIONS
Preparation of Triazole Gold(III) Complex as an Effective
7
Catalyst for the Synthesis of E-a-Haloenones
Adv. Synth. Catal. 2016, 358, 1 – 7
Yongchun Yang, Wenkang Hu, Xiaohan Ye, Dawei Wang,*
Xiaodong Shi*
Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!