In Situ Generated Zinc(II) Catalyst for Incorporation of CO2 into 2-Oxazolidinones with Propargylic Amines at Atmospheric Pressure

Article abstract of DOI:10.1002/cssc.201601469

Incorporation of CO₂ into heterocyclic compounds (i.e., 2-oxazolidinones) under mild conditions, especially at atmospheric pressure still remains challenging. The mononuclear Zn^II complex ZnCl₂(TBD)₂, where TBD=

Full text of DOI:10.1002/cssc.201601469

Accepted Article
Title: Zinc(II) Catalyst In Situ Formed for Incorporation of CO2 into 2-
Oxazolidinones with Propargylic Amines at Atmospheric Pressure
Authors: Xi Liu, Mei-Yan Wang, Si-Yuan Wang, Qi Wang, and Liang-
Nian He
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Zinc(II) Catalyst In Situ Formed for Incorporation of CO into 2-
2
Oxazolidinones with Propargylic Amines at Atmospheric
Pressure
[a]
[a]
[a]
[a]
*[a,b]
Xi Liu, Mei-Yan Wang, Si-Yuan Wang, Qi Wang, Liang-Nian He
Abstract: Incorporation of CO
2
into heterocycle compounds i.e. 2-
sustainable feedstock by constructing 2-oxazolidinone motif has
[
3]
oxazolidinones under mild conditions, especially at atmospheric
pressure still remains challenging. The mononuclear zinc(II) complex
received considerable interest,
economy reaction through coupling easily accessible starting
materials e.g. propargylic amines with renewable C synthon
CO is particularly appealing. Besides, the 2-oxazolidinone
among which the atom
viz. ZnCl
a robust catalyst for the carboxylative cyclization of propargylic
amines with CO to exclusively afford various 2-oxazolidinones in
excellent yields. Notably, the zinc(II) catalytic species is readily
generated in situ from zinc dichloride and 1,5,7-
triazabicyclo[4.4.0]dec-5-ene (TBD) without pre-preparation and
further isolation. Such CO fixation protocol could proceed
smoothly under atmospheric pressure at mild temperature in an
2
(TBD)
2
in this study was for the first time demonstrated as
1
2
2
framework bearing an exocyclic double bond can be further
functionalized, for instance, through Sonogashira cross-coupling
reaction with aryl halides, as demonstrated by Nevado et al.
[
4]
recently.
To date, several catalytic systems for the
have
a
2
carboxylative cyclization of propargylic amines with CO
2
been developed, including transition metal catalysts based on
13
[5]
[4,6]
[7]
[8]
[9]
atom economy and environmentally benign manner. C NMR and
control experiments were performed to explore possible interaction
between zinc(II) and carbon-carbon triple bond of propargylic amine.
The dual catalytic role of zinc catalyst on enhancing O-nucleophilicity
of the carbamate anion intermediate and activating carbon-carbon
triple bond is proposed based on mechanistic investigations.
Ru, Pd,
Ag, Au, Cu, and organocatalysts such as
[
10]
[11]
superbases,
protic ionic liquids,
and N-heterocyclic
[
12]
carbene. In addition, with stoichiometric amounts of tBuIO, 2-
oxazolidinone bearing iodomethyl group can be obtained from
[
13]
allyl or propargyl amine and CO
Particularly, such carboxylative cyclization could also proceed
under supercritical CO conditions in the absence of any
2
under mild conditions.
2
[14]
catalyst. Despite of great achievements, examples of efficient
processes based on readily available metal catalysts with
Introduction
utilization of CO
2
under atmospheric pressure are limited.
Therefore, development of alternative methodologies with cheap,
low-toxic metal catalysts for the synthesis of 2-oxazolidinones
With the continuous growth of CO
atmosphere, great efforts have been made to establish efficient
strategies for the consumption and utilization of CO . In this
context, chemical fixation of CO into value-added chemicals
represents a promising field in view of sustainable development
and green synthesis. Though CO as a highly attractive C
resource has been successfully incorporated into various
organic compounds, efficient CO transformation still remains
great challenging especially at atmospheric pressure originating
2
concentration in the
using CO
From the mechanistic perspective, the carboxylative
cyclization is supposed to proceed through carbamate
2
under mild conditions is still highly desirable.
2
2
a
intermediate and subsequent intramolecular nucleophilic
addition of carbamate anion on carbon-carbon triple bond is
2
1
[11]
considered to be the rate-determining step. Thus, activation of
2
carbon-carbon triple bond is the key to render the carboxylative
cyclization to proceed smoothly under mild conditions. In this
aspect, coinage metals i.e. Ag, Au, Cu have been employed
originating from efficient activation of carbon-carbon triple bond
due to the π-coordination with the alkyne, thus exhibiting
superior properties. In particular, Yamada et al. have
demonstrated that silver acetate is an efficient catalyst for
[
1]
from the thermodynamic stability and kinetic inertness of CO
2
.
2-Oxazolidinones are a class of heterocyclic compounds
which have broad applications as chemical intermediates, chiral
auxiliaries as well as antibacterial pharmaceuticals in synthetic
[2]
and medicinal chemistry.
2
Recently, upgrading CO as a
incorporating CO
propargylic amines as substrates under ambient conditions.
Interestingly, this dual-component catalytic system being
composed of AgNO and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) can fix CO directly from air by propargylic amines to
deliver corresponding 2-oxazolidinones.
2
into 2-oxazolidinones by adopting various
[
7a]
[
a]
b]
X.Liu, Dr. M.-Y.Wang, S.-Y. Wang, Q. Wang, Prof. Dr. L.-N. He
State Key Laboratory and Institute of Elemento-Organic Chemistry
Nankai University
Tianjin, 300071, P R China
E-mail: heln@nankai.edu.cn
3
2
[7c]
On the other hand,
[
Prof. Dr. L.-N. He
zinc appears to be ponderable since it has been well
demonstrated to possess the capacity of activating carbon-
Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University
Tianjin, 300071, P R China
[
15]
carbon triple bond.
The coordinative flexibility combined with
E-mail: heln@nankai.edu.cn
soft Lewis acidity makes zinc(II) significant in alkynes
Supporting information for this article is given via a link at the end of
the document.
transformation. Carreira et al. have documented that zinc salts,
typically Zn(OTf)
2
in conjunction with tertiary amine lead to the
[15a,d,e]
generation of the zinc acetylide species,
being also
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ChemSusChem
10.1002/cssc.201601469
FULL PAPER
[
16]
confirmed by in situ IR spectroscopy study.
Remarkably, the
Table 1. Catalyst screening for the carboxylative cyclization of propargylic
[
a]
zinc-based catalysts are also receiving continuous attention in
CO transformations given its superiority of tunable Lewis acidity,
high natural abundance and low environmental impact. In this
context, we envisioned that elaborately-designed zinc catalyst
with potential activation of carbon-carbon triple bond could be an
alternative to noble metal catalysts in promoting the coupling
2
amine 1a with CO .
2
[17]
reaction of propargylic amines with CO
As expected, the well-defined monodentate zinc(II) complex
ZnCl (TBD) , being in situ generated from ZnCl and TBD, was
proven to be a robust catalyst for the carboxylative cyclization of
propargylic amines with CO to perform under mild conditions
especially at atmospheric pressure, as shown in Scheme 1.
2
.
2
2
2
2
[
b]
[b]
Entry
Catalyst
Additive
Conversion
Yield
1
2
3
4
5
6
7
8
9
10
–
–
0
0
Zn(OAc)
2
–
53
13
39
19
2
33
3
TBD
–
2 2
Scheme 1. In situ formed ZnCl (TBD) as a robust catalyst for the
carboxylative cyclization of propargylic amines with CO
2
Zn(OAc)2
PPh3
Bipy
TMEDA
Pyridine
DMAP
31
3
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
Zn(OAc)
2
2
2
2
2
2
2
2
2
Results and Discussion
2
34
23
48
91
99
99
19
18
43
82
94
97
The carboxylative cyclization of N-benzylprop-2-yn-1-amine (1a)
with CO was selected as the model reaction as summarized in
Table 1. When no catalyst was employed, the starting material
was recovered quantitatively (entry 1). Zn(OAc) was initially
tested in consideration of the potential hydrogen-bonding
2
2
3
Et N
[
18]
DABCO
DBU
TMG
TBD
TBD
TBD
TBD
TBD
TBD
–
interaction between acetate anion and 1a. However, only 33%
yield of 3-benzyl-5-methyleneoxazolidin-2-one (2a) was obtained,
11
12
13
implying that single Zn(OAc)
the applied reaction conditions (entry 2). Then a series of
additives were investigated in conjunction with Zn(OAc) to
accelerate the cyclization reaction. Monophosphine ligand e.g.
PPh was confirmed to be ineffective (entry vs. 2).
Unexpectedly, 2,2’-bipyridine (Bipy), tetramethylethylenediamine
TMEDA), pyridine and 4-dimethylaminopyridine (DMAP) had an
2
was not effective enough under
2
[c]
[c]
[c]
99, 93
99, 92
98, 99
53
[
19]
[
c]
3
4
14
ZnCl
Zn(OTf)
Zn(ClO
Zn(BF
Zn(NO
ZnCl (TBD)
2
99, 99
15
16
17
18
2
57
(
adverse effect (entries 5–8 vs. 2), probably being attributed to
their insignificant interaction towards carbon-carbon triple bond
as a result of strong coordination of such nitrogen ligands with
4
)
2
59
52
4
)
2
53
45
Zn(OAc)
with steric hindrance gave a better result for enhancing the
catalytic activity of Zn(OAc) than trimethylamine (Et N) (entry 10
2
. Gratefully, 1,4-diazabicyclo[2.2.2]octane (DABCO)
3
)
2
73
50
[
d]
[e]
[e]
2
3
19
2
2
99, 99
95, 96
vs. 9). Those results inspired us to examine sterically hindered
guanidine and amidine compounds (entries 11–13). As expected,
[a] Reaction conditions: 1a (145.2 mg, 1 mmol), catalyst (0.1 mmol, 10 mol%
relative to 1a), additive (0.2 mmol, 20 mol% relative to 1a), CO balloon, 60
NMR with 1,3,5-
2
2a was obtained in a quantitative yield by cooperating TBD with
o
1
C, 12 h, 0.5 mL CH
3
CN; [b] Determined by
H
Zn(OAc) even though TBD itself was almost inactive (entry 13
2
trimethoxybenzene as internal standard. [c] Catalyst (0.05 mmol, 5 mol%
vs. 3). 1,1,3,3-Tetramethylguanidine (TMG) exhibited the similar
catalytic behavior to that of TBD (entry 12 vs. 13). Noticeably,
relative to 1a), TBD (13.9 mg, 0.1 mmol, 10 mol% relative to 1a); [d]
ZnCl
2 2
(TBD) (20.7 mg, 0.05 mmol, 5 mol% relative to 1a) was prepared
according to Ref.15a prior to use; [e] Under solvent-free conditions.
ZnCl
2 2
displayed a comparable activity to Zn(OAc) but other zinc
salts showed moderate activities (entries 14–18 vs 13).
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Halving the Zn(OAc)
whereas ZnCl still remained high catalytic efficiency (entry 14
vs. 13). Therefore, the combination of ZnCl and TBD with best
catalytic performance was selected for further investigation.
The mononuclear zinc(II) complex ZnCl (TBD) can be readily
generated from ZnCl and TBD and stabilized by intramolecular
N–H···Cl hydrogen bonding, X-ray crystal structure of which has
2
caused a slightly reduction in 2a yield,
2
2
Table 2. ZnCl
with atmospheric pressure of CO .
2
2
(TBD)
2
-catalyzed cyclization of various propargylic amines
[
a]
2
2
2
[
20]
been reported.
situ generated could be an active species for this kind of
cyclization reaction. To validate our hypothesis, ZnCl (TBD)
was prepared according to the published method (See
2 2
We speculated that the ZnCl (TBD) that in
[
b]
Entry
1
Substrate
Product
Yield
96
2
2
[20a]
Supproting Information for details ).
complex gave a comparable result in comparison with the two-
component ZnCl /TBD system under otherwise identical
conditions (entry 19 vs. 14), presumably suggesting that the
ZnCl (TBD) could be the active catalytic species. Notably, the
reaction could also proceed under solvent-free conditions (entry
9).
Having established the ZnCl
Indeed, the zinc(II)
2
2
3
94
93
2
2
1
2
2
(TBD) -catalyzed carboxylative
cyclization protocol, we then examined the reactivity of various
propargylic amines to further explore the utility of this protocol.
N-aryl substituted terminal propargylic amines were smoothly
converted into the corresponding 2-oxazolidinones in excellent
yields under optimized conditions (entries 1–5, Table 2). Notably,
the catalytic system worked well with the substrates bearing
electron-donating or electron-withdrawing group (1d and 1e).
Terminal propargylic amines with n-butyl and cyclohexyl
substituent at N-position were also practicable, delivering the
desired products in good yields (entries 6 and 7). Unfortunately,
N-phenylprop-2-yn-1-amine (1h) displayed a moderate reactivity
4
5
6
7
95
93
91
90
(
entry 8). This is understandable because the weak N-
nucleophilicity may hinder the nucleophilic attack at CO to
2
further generate the carbamate intermediate. The introduction of
a phenyl group at terminal position caused a drastic decrease in
the 2-oxazolidinone yield (entry 9), probably indicating that the
internal propargylic amine is unreactive with atmospheric
2
pressure of CO in the present catalytic system.
Control experiments were carried out to gain insight into the
reaction mechanism for identifying the possible intermediate as
depicted in Scheme 2. Reaction of propargylic amine 1a with
2
equimolar TBD in DMSO under atmospheric pressure of CO at
8
9
52
8
+
room temperature resulted in the formation of [TBDH]
carbamate 3a in a quantitative yield (Step 1). Similar processes
have been well demonstrated by Jessop group and Weiss group.
[
21]
They have developed binary system DBU/alkyl alcohol or
DBU/alkyl amine as absorbents for CO capture, generating the
amidinium carbonate or amidinium carbamate, kind of
2
a
reversible ionic liquid. Remarkably, analogous structure of 3a
has been proposed by Costa et al. in the guanidine-promoted
[
a] All the reactions were carried out with propargylic amine (1 mmol) and
ZnCl (TBD) (20.7 mg, 0.05 mmol, 5 mol% relative to the substrate 1)
under atmospheric pressure of CO2 at 60 C within 12 h. [b] Determined
by H-NMR with 1,3,5-trimethoxybenzene as internal standard.
2
2
o
[10b]
cyclization of propargylic amines with CO
2
.
This species is
1
supposed to contain two intramolecular hydrogen bridges for
carbamate anion stabilization. To verify the hypothesis, the
structure of 3a was identified by NMR analyses (See Supporting
Information for details). Then 3a was further converted into 2-
yield), suggesting the possible decomposition of 3a can be ruled
out. A reasonable interpretation may be that the hydrogen-
bonding interaction between [TBDH] and carbamate anion
+
oxazolidinone 2a in the presence of catalytic amount of ZnCl
elevated temperature in just 68% yield (Step 2). In addition, step
in the presence of CO also gave the similar result (69%
2
at
partially hinders subsequent nucleophilic addition of the
carbamate anion on carbon-carbon triple bond as illustrated in
Scheme 3 (mode A). Therefore, we proposed the bidentate
2
2
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[
24]
ligation with alkyne and carbamate anion to zinc center as mode
B. The presence of the bulky cation [(TBD)ZnCl] may be
and terminal alkynes.
Similarly, Carreira et al. has reported
+
ZnI /DMAP-catalyzed intramolecular cyclization of propargylic N-
2
favorable for stabilizing the carbamate anion; Whereas, the
Lewis acid-carbamate interaction could be diminished due to
steric hindrance of such bulky cation. As a result, the zinc(II)
hydroxylamines. They also speculated that zinc (II) participates
in the complexation and activation of carbon-carbon triple bond.
The nature of amine and counterion effect might affect the
[25]
may play
a
critical role in facilitating the intramolecular
equilibrium between free zinc (II) and the coordinated zinc(II).
nucleophilic addition towards carbon-carbon triple bond by
enhancing the O-nucleophilicity of carbamate anion relative to
Thus, it is reasonable to suppose that zinc(II) may serve as an
activator of alkyne to make the carbon-carbon triple bond more
vulnerable to the nucleophile such as carbamate anion.
[
22]
3a.
A plausible mechanism for ZnCl
atmospheric pressure of CO with propargylic amine 1a is
illustrated in Scheme 4. The reaction of ZnCl and TBD initially
furnishes ZnCl (TBD) , which then coordinates with the carbon-
carbon triple bond of 1a to afford the intermediate I with release
2 2
(TBD) -promoted fixation of
2
2
2
2
of TBD. TBD is able to activate CO
2
by reversibly forming the
[
26]
zwitterionic adduct.
The TBD-CO adduct further reacts with
2
the zinc-coordinated propargylic amine I (the activated substrate)
to afford the carbamate II by inserting CO into the N–H bond.
2
Subsequently, intramolecular nucleophilic cyclization occurs to
deliver the alkenylzinc(II) species III. Finally, the desired 2-
oxazolidinone 2a is obtained by proto-demetallation of the
+
intermediate III with assistance of [TBDH] , concomitant with the
2 2
regeneration of ZnCl (TBD) .
Scheme 2. Control experiments. Reaction conditions: Step 1: 1a (145.2 mg, 1
o
mmol), TBD (139.2 mg, 1 mmol), CO
: 3a (330.4 mg, 1 mmol), ZnCl (6.8 mg, 0.05 mmol), 60 C, 12 h, 0.5 mL
DMSO, in the presence or absence of CO balloon.
2
balloon, 20 C, 6 h, 0.5 mL DMSO; Step
C3
(a)
C1
C4
o
C2
2
2
2
(
b)
c)
(
1
3
Scheme 3. Two patterns of the ring-closure step promoted by TBD and the
zinc complex, respectively
Figure 1. C NMR study. (a) 1a (29.0 mg, 0.2 mmol) in 0.5 mL d
ZnCl
-DMSO. (c) ZnCl
6
-DMSO. (b)
2
(13.6 mg, 0.1 mmol) was added into 1a (29.0 mg, 0.2 mmol) in 0.5 mL
d
6
2
(27.2 mg, 0.2 mmol) was added into 1a (29.0 mg, 0.2
mmol) in 0.5 mL [D
ambient conditions.
6
]DMSO. All the NMR experiments were performed under
1
3
C NMR analyses were further applied to investigate the
1
3
interaction between zinc(II) with propargylic amine 1a. In the
C
NMR spectrum of the mixture of half equivalent of ZnCl with 1a,
2
the signals assigned to C1 and C2 of carbon-carbon triple bond
shifted from δ = 73.78 to 74.38 ppm, and 82.82 to 82.21 ppm,
respectively (Figure 1, (a) vs (b)). Increasing ZnCl amount to
2
one equivalent relative to 1a resulted in further shift of signals
corresponded to C1 and C2 ((b) vs (c)). However, the signals of
C3 and C4 had no significant change upon the addition of ZnCl
The above observation probably indicates that alkyne has an
interaction with ZnCl rather than the amine coordinates with
2
.
2
zinc center. Notably, the coordination of the carbon-carbon triple
bond with zinc (II) has been described in the nucleophilic
addition reaction of terminal alkynes to C=O and C=N
[
15a,d,e]
[23]
bond,
intramolecular hydroamination of alkynes,
redox
cross-dehydrogenative coupling reaction of propargylic amines
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ChemSusChem
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organic phases were collected, which were further purified by column
chromatography on silica gel (petroleum ether/ethyl acetate as an eluent)
1
to afford the pure product 3a as a white solid. Characterization of 3a:
NMR (400 MHz, CDCl
H), 4.47 (s, 2H), 4.24 (dd, J = 5.2, 2.3 Hz, 1H), 4.02 (t, J = 2.4 Hz, 1H)
H
3
): δ 7.39–7.27 (m, 5H), 4.75 (dd, J = 5.6, 2.7 Hz,
1
13
ppm. C NMR (101.6 MHz, CDCl
3
): δ 155.78, 149.02, 135.05, 129.11,
1
1
7
28.39, 128.31, 86.95, 47.96, 47.33 ppm. IR (neat, KBr): 1783, 1680,
427, 1282, 1237, 1083, 1060, 969, 877, 835, 754, 702 cm . GC-MS (EI,
0 eV) m/z (%) = 189.15 (14), 105.15 (2), 92.10 (16), 91.10 (100), 89.10
-
1
(3), 77.10 (2), 65.10 (14).
Acknowledgements
This work was financially supported by National Key Research
and Development Project (2016YFA0602900), the National
Natural Science Foundation of China, the Natural Science
Foundation
of
Tianjin
Municipality
(16JCZDJC39900),
2 2
Scheme 4. Mechanistic proposal for in situ formation of ZnCl (TBD) complex
and subsequent catalytic cycle.
Specialized Research Fund for the Doctoral Program of Higher
Education (project 20130031110013), MOE Innovation Team
(
IRT13022) of China and the “111” Project of Ministry of
Education of China (project No. B06005).
Conclusions
In summary, an elaborate protocol has been demonstrated for
efficient chemical fixation of atmospheric pressure of CO to
produce 2-oxazolidinones. The active zinc(II) complex i.e.
ZnCl (TBD) , being in situ generated from ZnCl and TBD,
serves as a robust catalyst for the carboxylative cyclization of
propargylic amines with CO under mild conditions. Control
2
Keywords: Carbon dioxide chemistry • carboxylative cyclization
• dual activation • synthetic methods • zinc catalysis
2
2
2
2
[1]
2
Recent reviews on CO transformation, see: a) G. Fiorani, W.-S. Guo, A.
13
W. Kleij, Green Chem. 2015, 17, 1375–1389; b) B. Yu, L.-N. He,
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1
3
in C NMR spectra, indicating that zinc(II) may act as an
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carbamate anion towards carbon-carbon triple bond thus
rendering the cyclization reaction to proceed smoothly under
mild conditions. The application of non-noble, earth-abundant
and nontoxic metal zinc features this method a sustainable
1
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building block.
2
as a
[
3]
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2 2 2
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Entry for the Table of Contents
FULL PAPER
The zinc(II) complex viz. ZnCl
being in situ generated from ZnCl
2
(TBD)
2
,
Xi Liu, Mei-Yan Wang, Si-Yuan Wang,
Qi Wang, Liang-Nian He*
2
and TBD, serves as a robust catalyst
for incorporating CO
oxazolidinones with
2
into 2-
propargylic
Page No. – Page No.
Zinc(II) Catalyst In Situ Formed for
Incorporation of CO into 2-
2
Oxazolidinones with Propargylic
Amines at Atmospheric Pressure
amines. The readily formed zinc
catalyst is presumably capable of both
activating carbon-carbon triple bond
and enhancing O-nucleophilicity of the
carbamate intermediate. Such dual
activation
effects
render
the
carboxylative cyclization to proceed
smoothly under atmospheric pressure.
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