Design and Synthesis of Alanine Triazole Ligands and Application in Promotion of Hydration, Allene Synthesis and Borrowing Hydrogen Reactions

Article abstract of DOI:10.1002/adsc.201600141

A new type of alanine triazole (ATA) ligand was found to be an efficient partner for the stabilization of gold(III), thus rendering improved stability and high catalytic activities in allene synthesis and hydration reactions through the 3,3′-arrangement of propargyl esters and propargyl alcohols. In addition to Au(I), ATA-gold(III) was also proven to be an effective catalyst in promoting the formation of allenes with high yields. Furthermore, alanine triazole ruthenium has exhibited excellent potential as a means to catalyze borrowing hydrogen of alcohols and amines, ketones and alcohols.

Full text of DOI:10.1002/adsc.201600141

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DOI: 10.1002/adsc.201600141
Design and Synthesis of Alanine Triazole Ligands and
Application in Promotion of Hydration, Allene Synthesis and
Borrowing Hydrogen Reactions
Yongchun Yang,a Anni Qin,a Keyan Zhao,a 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
E-mail: wangdw@jiangnan.edu.cn
Department of Chemistry, University of South Florida, 4202 E Fowler Ave, Tampa, Florida, USA
b
E-mail: xmshi@usf.edu
Received: February 2, 2016; Revised: March 7, 2016; Published online: April 24, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600141.
Abstract: A new type of alanine triazole (ATA)
ligand was found to be an efficient partner for the
stabilization of gold(III), thus rendering improved
stability and high catalytic activities in allene syn-
thesis and hydration reactions through the 3,3’-ar-
rangement of propargyl esters and propargyl alco-
hols. In addition to Au(I), ATA-gold(III) was also
proven to be an effective catalyst in promoting the
formation of allenes with high yields. Furthermore,
alanine triazole ruthenium has exhibited excellent
potential as a means to catalyze borrowing hydro-
gen of alcohols and amines, ketones and alcohols.
It is widely accepted that there are two main states
of gold catalyst: gold(I) and gold(III). Among the
two typical oxidation states of gold catalyst, gold(I)
complexes are considered as having two coordination
sites with linear geometry, while those of gold(III) are
accepted as having a square planar geometry with
four coordination sites, which should be conveniently
fine-tuned by ligand changes (Scheme 1).[6] For
gold(I), excellent examples are the development of
PR3 derivatives[7] and N-heterocyclic carbene (NHC)
ligands,[8] which greatly enhanced the catalyst stability
and enriched the catalytic applications of gold cata-
lysts through metal-ligand back-bonding. Meanwhile,
gold(III) complexes have usually been stabilized by
bidentate and tridentate ligands.[9–11] Typical examples
are PicAuCl2 and its derivatives,[10] which exhibit
high stability in gold(III)-catalyzed transformations
(Scheme 1).
Keywords: alanine; allenes; gold; rearrangement;
triazoles
The use of ligands represents one of the most power-
ful and versatile methods to build organic molecular
and functional groups, and has attracted a great deal
of attention from the chemical community.[1] The de-
velopment of new ligands promotes lots of new reac-
tions and methodologies and has played a great role
in modern synthetic chemistry. Notably, the role of li-
gands in gold chemistry is more prominent and signif-
icant because of the easy decomposition of gold cata-
lysts.[2] The reactivity and stability of gold complexes
largely depends on ligands coordinating with gold cat-
ions, stabilizing the catalytically active species and not
on accelerating the catalysis.[3] Therefore, the develop-
ment of new skeleton ligands is an effective and im-
portant method to solve the stability problems of gold
catalysts, especially for gold(III) catalysts.[4] However,
the design the synthesis of effective ligands to stabi-
lize gold(III) is rarely reported and remains a chal-
lenging scientific issue.[5]
Scheme 1. Alanine triazole ligands.
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Natural amino acid derivatives are universal and
common organic compounds, which are very cheap
and conveniently obtained and achieved huge applica-
tions in hot research areas, like the catalytic area, life
science, pharmaceutical chemistry, biochemistry, agri-
culture and so on. Now, we have a proposal about
this: if the triazole ligand[12,13] was introduced into
amino acid derivatives, the designed new ligands
could improve the reactivity and stability of gold(III)
through three kinds of functional groups or coordina-
tion sites: ester group, amino group and triazole part
(Scheme 1B). Based on the easy decomposition of
gold(III) with the strong coordination ability of tria-
zole, the alanine triazole ligand may be a good choice
to stabilize gold(III) serving as a tridentate or a biden-
tate ligand with three possible coordination sites.
Herein, we have synthesized the alanine triazole li-
gands (ATA) and explored their coordination func-
tion with gold(III). It was found that ATA-Au(III)Cl
Figure 1. Thermogravimetric curves of ATA-Au(III).
exhibited improved stability and good catalytic activi- that ATA-Au(III)Cl exhibited much greater stability
ties in allene synthesis and hydration reactions than the simple gold salt.
through the 3,3’-arrangement of propargyl esters and
Next, the classical substrate propargyl ester (2a)
alcohols. Besides, alanine triazole ruthenium has been was selected to test the catalytic activity of ATA-
proved to be excellent as a means to promote borrow- gold(III). The 3,3’-arrangement of the propargyl
ing hydrogen of alcohols and amines, ketones and al- ester[14,15] was carried out under KAu(III)Cl4/1d or
cohols.
KAu(III)Cl4/1e conditions and the results are sum-
Initially, the alanine triazole ligand (1e) was pre- marized in Table 1. According to the literature, the
pared conveniently in totally 74% yield for four steps simple Au(I) promotes allene synthesis from proparg-
through alkylation, hydration and condensation reac- yl esters through a 3,3’-rearrangement. Nolan report-
tions (Scheme 2). It was found that ATA was easily ed that [(IPr)AuOH](I) and [(IPr)Au(pyr)][BF4](I)
coordinated with gold(III), and a yellow solid ATA- could catalyze this transformation.[15b] We found that
gold(III)Cl was obtained with 83% yield, but it failed TA-Au(I) could promote allene synthesis with high
to cultivate crystals even though many solvents were yields and this was verified by Hashmi.[14c,15c] A con-
tried. In order to test the thermal stability of this cern is that Au(III) can realize allene synthesis from
ATA-gold(III) complex, a TG experiment was carried propargyl esters? Here, it was found that the simple
on and the thermal curves are shown in Figure 1. The Au(III) salts could not catalyze the formation of al-
stability experiment indicated that the ATA-Au(III)Cl lenes, even when several common solvents were tried
complex is stable up to 1708C and the results showed (entries 1–6, Table 1). Interestingly, a much higher
yield of allene was achieved when the ligand 1d or 1e
was added in order to adjust the gold(III) catalyst
under the same conditions (entries 5–7, Table 1). It
was found that allenes could not be achieved in the
absence of ligands with this methodology (entries 5
and 6, Table 1). The best result was achieved in DCM
with 1e as a ligand (entry 9, Table 1), while a slightly
lower yield was achieved with 1d as a ligand. Notably,
the simple gold salt (AuCl3) could not catalyze the re-
action, but this was consistent with the published re-
sults (entry 11, Table 1).[15a] The reaction solution
began to gradually become dark in 0.5 h for allene
synthesis from propargyl ester with AuCl3 as a catalyst
(entry 10, Table 1), meaning that the gold salt began
to partly decompose even after half an hour.[12] This is
further evidence to explain the importance of the
ligand in the stabilization of gold(III).
Encouraged by the promising results, we further ex-
Scheme 2. The synthesis of alanine triazole ligand.
amined the reaction scope of gold-catalyzed 3,3-rear-
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Design and Synthesis of Alanine Triazole Ligands and Application
Table 1. Screening of the reaction conditions.[a]
Table 2. Substrate scope of propargyl esters.[a,b]
Entry Au
Solvent Conv.
[%][b]
3a
4a
[%][c] [%][c]
1
2
3
4
5
6
7
8
KAuCl4
KAuCl4
KAuCl4
KAuCl4
THF
23
<5
<5
<5
<5
<5
51
<5
<5
12
<5
8
6
13
8
CH3CN 14
MeOH 20
acetone <5
toluene 28
toluene 66
toluene 57
DCM
DCM
DCM
DCE
DCM
DCM
KAuCl4
KAuCl4/1e
KAuCl4/1d
KAuCl4/1d
KAuCl4/1e
AuCl3
43
73
81
86
>95
<5
<15
<15
>95
[a]
Conditions:
KAu(III)Cl4
(2 mol%),
ligand
1e
9
8
(2.2 mol%), 2 (1 mmol), dry DCM (5 mL), 6–12 h, room
temperature.
Isolated yields based on 2.
10
11
12
13
<5[d] <5
[e]
15a
AuCl3
-
-
-
-
[b]
AuPPh3Cl(I)
AuPPh3Cl(I)/
AgBF4
IPrAuCl(I)/
AgBF4
8[f]
<5
<5
<5
Table 3. Substrate scope of propargyl alcohols.[a,b]
14
DCM
DCM
>95
<5
<5[g]
<5
15
IPrAuCl(I)
[a]
Conditions: 2a (1 mmol), [Au(III)] (2 mol%), ligand 1d
or 1e (2.2 mol%), dry solvent (5 mL), 3-12 h, room tem-
perature.
[b]
[c]
[d]
Determined by 1H NMR.
Isolated yields based on 2a.
The reaction solution began to gradually become dark
for 0.5 h.
[e]
Similar reference reaction: another alkynyl acetate 1-(2-
acetyl-phenyl)hept-2-yn-1-yl acetate was used as a sub-
strate, ref.[15a]
[f]
51% of indene instead, ref.[14k]
92% of indene instead, ref.[14k]
[g]
rangement of propargyl esters (Table 2). Generally,
various propargyl esters could be smoothly turned
into the corresponding allene products with moderate
to good yields. When the substrate containing a termi-
nal alkyne was used to test this reaction, the reaction
did not occur.
The Meyer–Schuster rearrangement of propargyl
alcohols[16] was conducted with KAuCl4/1e as a catalyst
under wet MeOH conditions. We were pleased to find
[a]
Conditions: 5 (1 mmol), KAu(III)Cl4 (2 mol%), ligand 1e
(2.2 mol%), MeOH/H2O (20:1), 8 h, 508C.
Isolated yields based on 5.
[b]
that the enones were obtained with overall moderate 1e was an effective catalyst for the hydration of al-
to high yields, and the substrate experiments are kynes. A series of ketones was obtained with good
listed in Table 3. Nearly all the propargyl alcohols re- yields, as the data in Table 4 show. All three reactions
acted smoothly to give the enones with only 2.2% of (allene synthesis, 3,3’-rearrangement of propargyl
1e added in the catalytic system. These experiments esters and hydration reaction) indicated that the ala-
revealed that alanine triazole ligands (ATA) greatly nine triazole ligand (ATA) is an excellent partner in
improved the stability of Au(III), and therefore en- the stabilization of gold(III).
hanced the catalytic activity of these catalysts.
The concept of borrowing hydrogen reaction also
Compared to the Meyer–Schuster arrangement of known as hydrogen autotransfer (BH/HA) has at-
propargyl alcohols, the hydration of alkynes was tracted considerable attention over the past ten years,
much more difficult. Here, we found that the KAuCl4/ mainly because it offers an alternative to the alkyla-
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Table 4. Gold-catalyzed hydration of alkynes.[a,b]
Table 5. Screening of the reaction conditions.[a,b]
Entry
Ligand
Solvent
Base
3a [%]
1
2
3
4
5
6
7
8
none
1d
1e
1d
1d
1d
1d
1d
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
t-BuONa
t-BuONa
t-BuONa
Na2CO3
KOH
t-BuONa
t-BuONa
t-BuONa
35
88
63
41
76
21
45
<5
[a]
Conditions: KAu(III)Cl4 (2 mol%), ligand 1e (2.2 mol%),
7 (1.0 mmol), HOAc/H2O, 12 h, room temperature.
Isolated yields based on 7.
DMF
[a]
[b]
Conditions: 9a (1 mmol), 10a (1.1 mmol), [RuCp*Cl2]n
(2 mol%), ligand (4 mol%), base (1.2 mmol), dry solvent
(5 mL), 48 h, reflux.
[b]
Isolated yields based on 9a.
tion of amines with a more greener and atom-efficient
process, in which water is the only by-product.[17] The
process usually involves the dehydrogenation of alco-
hols and reduction of imines to obtain N-alkylated or
C-alkylated products. However, an often overlooked
fact is that transfer hydrogenation is an important
competitive reaction. Here, we found that the alanine
triazole ligand was effective as a means to promote
the iridium-catalyzed N-alkylation reactions of aryla-
mines and various alcohols.
Table 6. Ru-catalyzed N-alkylation of anilines.[a,b]
In order to show the excellent catalytic activity of
these ATA ligands, we explored their application in
borrowing hydrogen reactions. According to our pre-
vious efforts,[18] the classical aniline and benzyl alco-
hol were selected as the model substrates to test the
N-alkylation reaction. All screening experiments for
the reaction conditions are shown in Table 5. They re-
vealed that the ATA ligand (1d) greatly enhanced this
transformation (entries 1 and 2, Table 5), which is
consistent with the reported results.[19]
Subsequently, we examined the N-alkylation reac-
tions of other arylamines with benzyl alcohols
through the use of the catalyst [Ru]/1d. The experi-
ments showed that different N-alkylated anilines were
achieved with moderate to high yields (Table 6). Gen-
erally, the reaction had a better substituent tolerance
scope. The substituents with different electronic prop-
erties on the aryl rings of aromatic amines significant-
ly affected the reaction yields. Most often, those aro-
matic amines possessing electron-donating groups
gave the corresponding products in higher yields as
compared to those with electron-poor ones.
[a]
Conditions: 9 (1 mmol), 10 (1.1 mmol), [RuCp*Cl2]n
(2 mol%), 1d (4 mol%), t-BuONa (1.2 mmol), dry tolu-
ene (5 mL), 48 h, reflux.
Isolated yields based on 9.
[b]
The borrowing hydrogen reaction of ketones with
alcohols is a very important method to build function- reaction. In some cases, the competition of the two
al ketones. However, an often overlooked fact is that transformations greatly reduces the application of
transfer hydrogenation is an unavoidable competitive useful functional ketones (Scheme 3). When the
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Design and Synthesis of Alanine Triazole Ligands and Application
In summary, the new type of alanine triazole
(ATA) ligand has been developed with good yield.
The ATA-gold(III)Cl revealed improved stability and
good catalytic activities in hydration reactions and
allene synthesis through the rearrangement of prop-
argyl esters and alcohols. Additionally, the alanine tri-
azole Ru catalyst has proved to be excellent as
a means to promote borrowing hydrogen of alcohols
and amines, ketones and alcohols. Further investiga-
tions on the extension of this kind of ligand to other
transformations are currently undergoing in our
Scheme 3. The competition of transfer hydrogenation and
alkylation reactions.
upper catalytic system was used for this transforma- group.
tion, it was found that only the C-alkylated product
was obtained.
Next, we further employed the above methods to Experimental Section
the borrowing reaction of alcohols with ketones. The
results were summarized in Table 7. All the substrates The Synthesis of Alanine Triazole Ligand
with electron-neutral, electron-rich, and electron-defi-
cient functional groups at the arene moiety were well
tolerated, giving 13a–13i with generally good yields.
The furan group in the substrate could also be incor-
porated into the product 13e with a high yield.
To the solution of 1c (1.772 g, 10 mmol) in dry DCM was
added methyl 2-aminopropanoate (2.094 g, 15 mmol), 4-di-
methylaminopyridine (DMAP, 1.222 g, 10 mmol), EDCI
(3.834 g, 20 mmol), and NEt3 (2.530 g, 25 mmol) under N2,
The reaction mixture was stirred overnight at room temper-
ature and monitored by TLC, and then the solvent was re-
moved under reduced pressure and purification of the crude
product by column chromatography on silica-gel (petroleum
ether/ethyl acetate=2:1) afforded compound 1d as a white
solid; yield: 88%.
Table 7. Ru-catalyzed C-alkylation of ketones.[a,b]
To a mixed solution of THF/MeOH(1:1) (20 mL) and
LiOH (1M, 20 mL, in H2O) was successively added the solu-
tion of 1d (20 mmol) in THF/MeOH (1:1) (20 mL) at room
temperature. After one hour, the reaction mixture was acidi-
fied to pH 2 by adding an aqueous solution of 1N HCl, the
acid product (1e) was washed with EtOH, then evaporated
to dryness to give 1e; yield: 94%.
Typical Procedure for Synthesis of Allenes
To a 20-mL Schlenk tube were added KAuCl4 (2 mol%),
ligand 1e (2.2 mol%), dry CH2Cl2 (1.0 mL), after the mix-
ture was stirred for 30 min, 2a (1 mmol) in CH2Cl2 (4.0 mL)
was added to the reaction mixture at room temperature.
After stirring for 3–4 h and monitored by TLC, the solvent
was removed under reduced pressure and purification of the
crude product by column chromatography on silica-gel (pe-
troleum ether/ethyl acetate=80:1) afforded the title com-
pound 3a as a colorless oil; yield: 81%.
Acknowledgements
We gratefully acknowledge financial support of this work by
NSF (YCHE-1362057), NSFC (21401080), NSFC of Jiangsu
(BK20130125) and MOE & SAFEA for the 111 Project
(B13025).
[a]
Conditions: 9 (1.2 mmol), 12 (1.0 mmol), [RuCp*Cl2]n
(2 mol%), 1d (4 mol%), t-BuONa (1.5 mmol), dry tolu-
ene (5 mL), 48 h, reflux.
Isolated yields based on 9.
The ligand 1d was not added.
[b]
[c]
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