Palladium nano-particles as a recyclable catalyst for C-O bond formation under solvent free conditions

Article abstract of DOI:10.1039/d0gc01194k

The development of green, economical and sustainable chemical processes is one of the primary challenges in organic chemistry. Herein, we report a recoverable heterogeneous palladium catalyst for the stereoselective synthesis of multi-functionalized allylic ethers under solvent free conditions. This method is based on in situ generated PdNP catalyzed O-allylation of alcohols with vinyl cyclic carbonates. Solvent-free conditions, easily recoverable catalysts, efficient recycling, and operational simplicity make the protocol green, economic and sustainable.

Full text of DOI:10.1039/d0gc01194k

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Palladium nano-particles as recyclable catalyst for C-O bond
formation under solvent free conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Ajmal Khan,* Junjie Zhang and Shahid Khan
DOI: 10.1039/x0xx00000x
The development of green, economical and sustainable
chemical processes is one of the primary challenges in organic
a) Previous work
O
chemistry. Herein, we report
a recoverable heterogenous
R1
O
O
PdNPs
palladium catalysis for the stereoselective synthesis of multi-
functionalized allylic ethers under solvent free conditions. This
method is based on an in situ generated PdNPs catalyzed O-
allylation of alcohols with vinyl cyclic carbonates. The solvent-
free conditions, easily recoverable catalyst, efficient recycling,
and operational simplicity renders the protocol green,
economic and sustainable.
HO
Ar
ArB(OH)2
R2
(Z/E) 4:1 typically
R1
R2
b) This work
O
HO
R1
O
O
PdNPs
R2OH
OR2
(Z/E) > 25:1
The development of green and economical organic processes
that can be performed under solvent-free, and at ambient
conditions in which the catalyst can be recovered by simple
filtration without the loss of catalytic activity is highly desirable
in organic synthesis. Efforts to develop economical and green
procedures, we focused to develop a practical method for the
preparation of allylic ethers since it can be used as starting
material in many organic reactions for the synthesis of valuable
building blocks.1 Generally, preparation of allyl aryl ethers relies
on Williamson-type ether synthesis that uses highly active allyl
halides and strongly basic alkoxide anions.2 Therefore,
considerable efforts have been committed to their synthesis by
employing transition metal-catalysed allylic substitution
reactions.3 However, most of these reported methods are carried
R1
Recyclable, Green, Regio- and Stereoselective Catalysis
Solvent and ligand-free conditions, Easily accessed materials
Scheme 1 Cross coupling reactions of vinyl cyclic carbonates catalyzed by palladium
nano-catalyst.
Recently, vinyl cyclic carbonates have found great importance in
organic synthesis serving as dipole precursors to generate -allyl
palladium intermediate after decarboxylation and hence provide
direct access to a variety of synthetically as well as medicinally
important compounds.4 Most recently, Kleij and co-workers have
reported a homogeneous palladium-catalyzed decarboxylative
etherification by the utilization of vinyl cyclic carbonates to form
allylic ethers.5a However, the use of bulky phosphine ligands and
nonrecyclable homogeneous Pd-catalyst create economic and
environmental problems to extending their scope. The fast-
growing field of palladium nano-catalysis could address these
difficulties, which has increasing interest in both organic
reactions and industry field, due to its stability, recyclability and
unique reactivity.6 In this account, we have proposed a “green’’, so-
called “ligandless”, palladium nano-catalyst, which has shown good
reactivity and recyclability without the addition of any stabilizer
or base under solvent free conditions. We planned to examine
the catalytic activity of PdNPs in CO bond formation reactions,
since very few reports are available for nanoparticle-catalysed
CO bond formation,7 and, to our knowledge, no report is
out by using
a homogeneous non-recyclable catalyst with
expensive ligands and toxic organic solvents. In the context of
low cost and environmental acceptability, the development of
cost-efficient, heterogeneous and recyclable catalysis under
solvent-free conditions is highly desirable and essential.
Department of Applied Chemistry, School of Science and Xi’an Key Laboratory of
Sustainable Energy Materials Chemistry, MOE Key Laboratory for Nonequilibrium
Synthesis and Modulation of Condensed Matter, Xi’an Jiao Tong University, Xi’an
710049, P. R. China. E-mail: ajmalkhan@xjtu.edu.cn
†
Electronic
Supplementary
Information
(ESI)
available:
See
existing for
PdNPs-catalysed CO bond formation, which
DOI: 10.1039/x0xx00000x
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involves no additional stabilizer, base and phosphine ligand With the optimized reaction conditions developed, the generality of
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under solvent free conditions.8 Our interest in green chemistry the substrate scope of the decarboxylative cross-coupling reaction
DOI: 10.1039/D0GC01194K
and efforts toward PdNPs catalysed sustainable organic reactions was investigated and the results are summarized in Table 2. A
(Scheme 1a),9 we herein report an external ligand-free and reusable variety of phenols reacted effectively to provide the desired allyl
aryl ethers in high yield with excellent (Z)-selectivity. Both electron-
donating and electron-withdrawing substituents on phenols could
react with 1a, affording the corresponding products in high yields
and stereoselectivities (Table 2, entry 2-13). As for the p-substituted
phenols, the products 3ab, 3ac, 3ad, 3ae and 3af were produced in
high yields (91%, 96%, 88%, 92%, and 93%) with excellent
stereoselectivities. In the cases of phenols with substituents on
ortho-position, allylic ethers 3ag, 3ah and 3ai were smoothly
produced in 84%, 89% and 82% yield respectively with excellent (Z)-
selectivity. Furthermore, the bi-substituted and tri-substituted
phenol furnished the desired product 3aj and 3ak in 90% and 72%
yields with excellent (Z)-selectivity. The lower yield obtained in case
of tri-substituted phenol could be possibly due to the steric
hindrance effect. Moreover, 1-naphthol and 2-naphthol gave 3al
and 3am in 94% and 92% yields, respectively with excellent
stereoselectivity. It should be mentioned that methanol, which
posed great challenge to be installed into allylic ethers due to their
low reactivity, were also suitable substrate in our optimized
condition (3an). Unfortunately, iso-propanol and tert-butanol could
not provide the desired products (3ao and 3ap) and in both cases,
the conversion was less than 5%.
PdNPs catalysed CO bond formation reaction for the synthesis of
allylic ethers with excellent yields and stereoselectivities under
solvent-free conditions at room temperature (Scheme 1b).
Table 1 Optimization of reaction conditionsa
O
HO
Pd2(dba)3 (2.5 mol %)
O
+
PhOH
O
solvent free, rt
Ph
OPh
2a
Ph
1a
3aa
Entry Deviation from optimal conditions
Yieldb[
%]
Z/Ec
1
2
3
4
5
6
7
8
Without palladium source
No deviation
PdCl2
Pd(OAc)2
Pd(acac)2
Solvents = THF, DMF, toluene
H2O used as solvent
Amount of Pd2(dba)3 = 1%, 0.5%
0
93
--
19:1
5:1
5:1
5:1
ND
﹤15
20
﹤10
﹤10
20
8:1
19:1
76, 54
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), rt, 15 hours. b The yields are
c
of isolated materials for the mixtures of the stereoisomers. Determined by 1H
NMR of crude reaction mixture. The (Z)-configuration was confirmed by the
comparison of 1H-NMR with that of the reported literature.5
Table 2 Alcohol substrate scopea
O
HO
Pd2(dba)3 (2.5 mol %)
O
+
ROH
O
solvent free, rt
Firstly, we use the reaction of Phenyl substituted vinyl cyclic
carbonate (1a) with phenol (2a) as model substrates to optimize
the reaction conditions (Table 1). Different palladium-precursors,
Pd2(dba)3, PdCl2, Pd(OAc)2 and Pd(acac)2 were used under external
ligand and solvent-free conditions at room temperature (Table 1,
entries 2-5). Among them, only Pd2(dba)3 provided the desired
trisubstituted allyl aryl ether (3aa) at an 93% isolated yield with
19:1 Z/E ratio. The use of other palladium-precursors resulted in
lower conversion (Table 1, entry 3-5). The solvent effect and
catalyst loading were next investigated. The reaction efficiency
was decreased dramatically in organic solvents like THF, DMF and
toluene (Table 1, entry 6). The reaction was also performed in pure
water, but only 20% of the desired product was obtained (Table 1,
entry, 7). When the catalyst amount decreased from 5 mol% to 2
mol% and 1 mol%, the isolated yield of the desired product 3aa
dropped from 93% to 76 % and 54% respectively (Table 1, entry
8). During the optimization studies we found that palladium was
essential for promoting the reaction; in the absence of palladium
source, the reaction did not proceed at all and only starting
materials were collected (Table 1, entry 1). The (Z)-configuration
Ph
OR
2
Ph
1a
3
Entry 2 (R)
3
Yieldb (%)
Z/Ec
19:1
25:1
25:1
19:1
15:1
10:1
25:1
25:1
25:1
25:1
1
2
3
4
5
6
7
8
9
2a (R = Ph)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
93
91
96
88
92
93
84
89
82
90
2b (R = 4-MeC6H4)
2c (R = 4-MeOC6H4)
2d (R = 4-BrC6H4)
2e (R = 4-NO2C6H4)
2f (R = 4-CNC6H4)
2g (R = 2- NO2C6H4)
2h (R = 2- PhC6H4)
2i (R = 2- CHOC6H4)
10
2j (R = 4-allyl-2-MeO- 3aj
C6H3)
11
2k (R = 2,3,4-trimethyl- 3ak
72
25:1
C6H2)
12
13
2l (R = 1-naphthyl)
2m (R = 2-naphthyl)
2n (R = Me)
3al
3am
3an
3ao
3ap
94
92
78
﹥5
﹥5
25:1
25:1
19:1
ND
14
15d
16d
2o (R = iPr)
2p (R = tBu)
ND
1
a
of 3aa was confirmed by the comparison of H-NMR with that of
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), Pd2(dba)3 (2.5 mol%), rt, 15
hours. b The yields are of isolated materials for the mixtures of the stereoisomers.
Determined by 1H NMR of crude reaction mixture. 1H-NMR-yield. The (Z)-
configuration of 3aa and 3ac was confirmed by the comparison of 1H-NMR with
that of the reported literature,5a those of the other products were assigned by
analogy.
reported in literature.5a The high (Z)-selectivity in these
transformations was found, because of the in situ formation of a
six-member palladacycle conformed by DFT calculations reported in
the previously related literature.5b-c
c
d
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mmol scale, producing 3aa at an 81.4% isolated yield (1.95 g). At
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DOI: 10.1039/D0GC01194K
the end of reaction on 10 mmol, the mixture was filtered and
As described in Table 3, various vinyl cyclic carbonates were tested
under the optimized conditions to further examine the reaction
scope. A variety of phenyl-substituted vinyl cyclic carbonates, with
different electronic and steric properties, were tolerated to provide
the desired allyl aryl ethers 3ba-3ka in high yields with excellent
stereoselectivities. Vinyl cyclic carbonates with bulky naphthyl
group also worked well to provide allylic ethers 3la and 3ma in high
yields with excellent (Z)-selectivity. Vinyl cyclic carbonate with
versatile furan moiety also reacted smoothly to furnish the desired
product 3na in high yield with high stereoselectivity. Furthermore,
the conditions were also suitable for the reaction of alkyl
substituted vinyl cyclic carbonates (1o-1p), providing the
corresponding products in good yields albeit with comparatively
low stereoselectivities.
washed to give a dark black solid, from which palladium nano-
particles were observed with size of 5-50 nm upon TEM analysis.10
In addition, we evaluated the recyclability of these free-formed
palladium nano-particles, replacing Pd2(dba)3, showing no
appreciable loss in the catalytic activity in a test of five cycles (Fig.
1b and Fig. S5) which clearly indicate that catalyst is free from any
sort of agglomeration. Notably, a small decrease in the product
yields in the subsequent catalytic cycles might be attributed the
minor loss of PdNPs catalyst during workup and recovery
procedures.
O
HO
Pd2(dba)3 (2.5 mol %)
O
(a)
+
PhOH
O
solvent free, rt, 15 h
2a
Ph
OPh
Ph
1a
3aa
1 mmol 3aa 0.21 g yield 87%
10 mmol 3aa 1.95 g yield 81%
Table 3 Vinyl cyclic carbonate substrate scopea
O
HO
Pd2(dba)3 (2.5 mol %)
O
O
solvent free, rt
R
OPh
O
PdNPs
(2.5 mol %)
PhOH
2a
HO
R
O
(b)
+
PhOH
O
1
3
solvent free, rt, 15 h
2a
Ph
OPh
Ph
1a
3aa
HO
HO
HO
OPh
OPh
OPh
cycle yeild of 3aa recovered PdNPs
Me
tBu
Cl
1
2
3
4
5
89%
85%
82%
78%
75%
85%
82%
84%
75%
76%
3ba: 90% yield, Z/E = >25/1
3ca: 92% yield, Z/E = >25/1
3da: 82% yield, Z/E = 12/1
HO
HO
HO
OPh
OPh
OPh
NO2
OMe
OMe
Fig. 1 Gram-scale synthesis (a) and recyclability of PdNPs (b).
3ea: 88% yield, Z/E = >25/1
3fa: 84% yield, Z/E = >19/1
3ga: 85% yield, Z/E = >25/1
HO
Furthermore, in the kinetic study for the coupling reaction, a hot
filtration test was used to examine the heterogeneity of Pd-catalyst
(Fig. S6).10 To understand whether the reaction is catalyzed by
heterogeneous or a leached catalyst, a hot filtration experiment
was performed for the preparation of 3aa. A set of four reactions
were separately carried out and catalyst was removed from the
reaction mixtures through centrifugation after 30 minutes for first
reaction (30%, NMR yield), after 60 minutes for second reaction
(56%, NMR yield), after 2 hours for third reaction (65%, NMR yield),
after 5 hours for fourth reaction (82%, NMR yield), and after 10
hours for fifth reaction (91%, NMR yield). The remaining reaction
mixtures were further allowed to react for next 13 h. However, no
improvements in the yields were observed in all five reactions. This
confirms that no leaching of PdNPs catalyst was observed during
the course of reactions.
HO
HO
OPh
OPh
OPh
Cl
MeO
OMe
F
F
Cl
3ha: 92% yield, Z/E = >19/1
3ia: 87% yield, Z/E = 19/1
3ja: 91% yield, Z/E = 5/1
HO
HO
HO
OPh
OPh
OPh
O
O
3ka: 90% yield, Z/E = >25/1
3la: 92% yield, Z/E = >25/1
3ma: 89% yield, Z/E = >25/1
HO
HO
HO
OPh
O
Ph
OPh
BnO
OPh
3na: 90% yield, Z/E = >25/1
3oa: 73% yield, Z/E = 7/1
3pa: 78% yield, Z/E = 8/1
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), Pd2(dba)3 (2.5 mol%), rt, 15
hours. b The yields are of isolated materials for the mixtures of the stereoisomers.
Determined by 1H NMR of crude reaction mixture. The (Z)-configuration of 3ba
c
Transmission electron microscopy (TEM) analysis was carried out to
determine the reason for the high activity of the palladium catalyst
in this decarboxylative cross coupling reaction. The Pd(0) precursor
serve as source of PdNPs due to the decomposition of Pd2(dba)3 in
the solid state to release (dba) and formation of Pd black: Pd2(dba)3
→ dba + [Pd].11 Analysis by TEM confirmed the presence of
palladium nano-particles with dimensions of 5-50 nm as shown in
was confirmed by the comparison of 1H-NMR with that of the reported
literature,5 those of the other products were assigned by analogy.
To demonstrate the application of the present protocol, gram scale
transformation was carried out (Fig. 1a). The compound 3aa can be
prepared from 1a and 2a (1.0 mmol) over Pd2(dba)3 (2.5 mol%) at
an 87% yield (0.21 g). This procedure can be directly enlarged to 10
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Figure 2.10 These results revealed that Pd2(dba)3 was reduced to
form palladium nano-particles and the PdNPs are likely stabilized by
dba.
1
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Fig. 2 TEM images of PdNPs for the coupling reaction of 1a with 2a under solvent
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In summary, we have developed an efficient, practical and
green method for the synthesis of allyl aryl ethers featuring tri-
substituted olefin scaffolds from phenols and vinyl cyclic
carbonates using inexpensive Pd2(dba)3 as the catalyst under
ligand and solvent free-conditions. The reaction need only 2.5
mol% of catalyst and the in situ formed PdNPs are found to be
active catalyst. The catalytic system shows good tolerance
towards both aryl and alkyl-substituted vinyl cyclic carbonates
and phenols, providing the corresponding allylic products in
high yields and stereoselectivities. This protocol can be easily
scaled-up and applied, as a demonstration, in the synthesis of
tri-substituted allyl aryl ether 3aa at a yield of “1.95g”.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
4
For selected reviews, see (a) W. Guo, J. E. Gómez, À.
Cristòfol, J. Xie and A. W. Kleij, Angew. Chem. Int. Ed., 2018,
57, 13735; (b) H. Zhang, H. B. Liu and J. M. Yue, Chem. Rev.,
2013, 114, 883; (c) A. W. Kleij and C. J. Whiteoak,
This work was supported by the “Fundamental Research Funds
for Central Universities" (No. 1191329135), Key Laboratory
Construction Program of Xi’an Municipal Bureau of Science
and Technology (No. 201805056ZD7CG40) and the starting
fund of Xi’an Jiao Tong University (No. 7121182002). We thank
the Instrumental Analysis Centre of Xi’an Jiao Tong University
for HRMS analysis.
ChemCatChem, 2015, 7, 51; (d) A. Khan and Y. J. Zhang,
Synlett, 2015, 26, 853; For selected original examples, see:
(e) A. Khan, S. Khan, I. Khan, C. Zhao, Y. Mao, Y. Chen and Y.
J. Zhang, J. Am. Chem. Soc., 2017, 139, 10733; (f) A. Cai, W.
Guo, L. Martínez-Rodríguez and A. W. Kleij, J. Am. Chem.
Soc., 2016, 138, 14194; (g) W. Guo, R. Kuniyil, J. E. Gómez, F.
Maseras and A. W. Kleij, J. Am. Chem. Soc., 2018, 140, 3981;
(h) W. Guo, L. Martínez-Rodríguez, R. Kuniyil, E. Martin, E. C.
Escudero-Adán, F. Maseras and A. W. Kleij, J. Am. Chem.
Soc., 2016, 138, 11970; (i) W. Guo, L. Martínez-Rodríguez, E.
Martin, E. C. Escudero-Adán and A. W. Kleij, Angew. Chem.
Notes and references
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