Atomically precise palladium nanocluster catalyzed tandem oxidation processes of alcohols and phosphorous ylides: Facile access to α,β-unsaturated esters

Article abstract of DOI:10.1016/j.tetlet.2020.152385

The mesoporous silica nanosphere supported palladium nanocluster was synthesized through a simple impregnation method and well characterized. The as prepared catalyst shows excellent catalytic performance in tandem oxidation process (TOP) of alcohol and phosphorous ylide, which provides an efficient access to α,β-unsaturated esters with high yields utilizes 1 atmosphere of molecular oxygen as sole oxidant. This work represents the first example of atomically precise palladium nanocluster catalyzed tandem oxidation process of alcohol and phosphorous ylide and is expected to open new horizons for nanoclusters in the tandem reactions.

Full text of DOI:10.1016/j.tetlet.2020.152385

Tetrahedron Letters 61 (2020) 152385
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Atomically precise palladium nanocluster catalyzed tandem oxidation
processes of alcohols and phosphorous ylides: Facile access
to
a
,b-unsaturated esters
⇑
Taiping Gao , Jialei Du, Wenyan Liu, Mingxia Ren
School of Chemistry and Materials Science, Ludong University, Yantai 264025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2020
Revised 18 August 2020
Accepted 19 August 2020
Available online 29 August 2020
The mesoporous silica nanosphere supported palladium nanocluster was synthesized through a simple
impregnation method and well characterized. The as prepared catalyst shows excellent catalytic perfor-
mance in tandem oxidation process (TOP) of alcohol and phosphorous ylide, which provides an efficient
access to a,b-unsaturated esters with high yields utilizes 1 atmosphere of molecular oxygen as sole oxi-
dant. This work represents the first example of atomically precise palladium nanocluster catalyzed tan-
dem oxidation process of alcohol and phosphorous ylide and is expected to open new horizons for
nanoclusters in the tandem reactions.
Keywords:
Atomically precise palladium nanocluster
Heterogeneous catalysis
Ó 2020 Elsevier Ltd. All rights reserved.
Tandem oxidation-wittig reaction
Mesoporous silica nanoparticles
a,b-Unsaturated esters
Introduction
purification of intermediates. The tandem oxidation processes ini-
tiated by alcohol oxidation to the corresponding more reactive
Atomically precise nanoclusters (APNCs) [1], as a new family of
nanomaterials, have emerged as one of the most important
branches of nanoscience and nanotechnology during the past dec-
ade and attracted significant attentions due to its applications in
catalysis [2], biology [3] and sensing [4]. Owing to its high sur-
face-to-volume ratio and the unique geometric structure and elec-
tronic structure, nanocluster show distinct catalytic activity in
selective oxidation [5], reduction [6] and carbon–carbon coupling
aldehydes, which can be easily trapped in situ by many nucle-
ophilic agent leading to a variety of synthetically useful com-
pounds such as esters, imines, amides and heterocycles [10].
The tandem oxidation-Wittig reaction, when the nucleophilie is
stable phosphorous ylide, is an important reaction in organic syn-
thesis because it provides a facile method to
which is a very important 1,4-addition synthon in many organic
reactions. Conventionally, various oxidants such as MnO [11],
a,b-unsaturated ester,
2
[
7] reactions. Nanocluster with determined crystal structure and
Dess-Martin agent [12], IBX [13], PCC [14] were studied for the
one-pot tandem oxidation-Wittig reaction. However, these proto-
cols require the use of stoichiometric or even large excess amounts
of oxidant, which generated many unpredictable by-products and
much effort is needed to remove the oxidant (Scheme 1(a)). More-
over, some of this oxidant themselves are highly toxic and hard to
handle. To avoid the use of hazardous oxidants, a great deal of
effort has been devoted to design of new catalysts with green oxi-
dants (Scheme 1(b)). For example, Williams and co-works [15]
reported the tetra-n-propylammonium perruthenate (TPAP) cat-
alyzed tandem oxidation-process of alcohols with stable ylides to
compositions make it possible to reveal the relationship between
catalytic performance and their atomic level structure, which
enable it have huge advantages over the uncertain surface struc-
ture of traditional metal nanoparticles [8]. Therefore, the develop-
ment of new chemical processes promoted by atomically precise
nanoclusters is a high desirable, yet challenging task.
One-pot tandem reactions [9], in which two or more transfor-
mation processes are combined into one synthetic operation, are
great importance for efficient and practical organic synthesis and
for green and sustainable chemistry, which avoid the isolation
and purification of the intermediate. Thus allowing a reduction in
time, labor and yield losses associated with the isolation and
0
0
give the
a
,b-unsaturated esters using O
2
as oxidant and N,N,N ,N -
) as co-oxidant.
Tetramethylethylenediamine dioxide (TMEDAO
2
Park and co-works [16] reported porous aluminum oxyhydroxide
embedded ruthenium nanoparticles catalyzed alcohol oxidation-
Wittig reaction in toluene with O as oxygen source. However, this
2
⇑
Corresponding author.
E-mail address: gaotp@ldu.edu.cn (T. Gao).
https://doi.org/10.1016/j.tetlet.2020.152385
040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
0

2
T. Gao et al. / Tetrahedron Letters 61 (2020) 152385
Scheme 1. Tandem oxidation-wittig reaction of alcohol and phosphorous ylide.
system suffers from high temperature and the poor recyclability of
the catalysts. The Au/Pd bimetallic nanoparticles were also
explored by Kobayashi and co-works to promote the tandem pro-
cess of alcohol oxidation and Horner–Wadsworth–Emmons reac-
tion [17]. Recently, using CuI/TEMPO as co-catalyst and air as the
oxidant, Yu and co-works reported an efficient method to synthesis
with positive model in Fig. 1(B) shows mass peak at m/z = 1511.9
+
(assigned to molecular ion peak of [Pd
ical molecular weight: 1511.9). There are two fragment peaks at
1249.9 and 986.6, assigned to [Pd Cl(PPh (PPh ] and [Pd
ClPPh (PPh ] respectively, after lost one or two PPh from the
Pd Cl clusters, caused by the high energy of MALDI-MS method
3 3 3 2 2
Cl(PPh ) (PPh ) ] , theoret-
3
3
)
2
2
)
2
3
-
3
2
)
2
3
3
of
a
,b-unsaturated esters [18]. Despite this advances, the novel cat-
(The fragment is inevitable in MALDI-MS analysis because of the
hard ionization method (high laser intensity)). No other peaks
are found in the wide range of MALDI-MS spectrum indicating
alyst with excellent catalytic activity, easy recyclability and green
oxidant are still highly demanding. So we developed and herein
report the first mesoporous silica nanoparticles (MSNs) supported
1
31
3
the purity of the as-obtained Pd Cl clusters. The H and P NMR
+
-
atomically precise palladium nanocluster [Pd
3
3
Cl(PPh )
3 2
(PPh )
2
] Cl
spectra of Pd Cl demonstrated that there is phenyl and P in the
3
(
abbreviated Pd Cl) catalyzed tandem oxidation-Wittig reaction of
3
nanocluster, which is well consistent with the result of MALDI-
MS analysis. The uniform spherical morphology with average par-
ticle size ~100 nm of MSNs can be easily observed from the TEM
alcohol and phosphorous ylide with 1 atmosphere O
as oxidant (Scheme 1(c)).
2 2
(O balloon)
image (Fig. 2(A) and (B)). The as-prepared Pd
onto MSNs by impregnation of MSNs powder in a DCM solution of
cluster. As expected, the Pd Cl nanoclusters are absorbed into the
pore of the MSNs. After the Pd Cl loading, the structure and mor-
3
Cl cluster was loaded
Results and discussions
3
3
phology of MSNs are well maintained (Fig. 2(C) and (D)). To con-
form the Pd nanoclusters after loaded on the MSNs are well
dispersed, the energy dispersive X-ray spectroscopy (EDX) map-
ping analysis of Si, O, and Pd was conducted and shown in Fig. 3.
The results demonstrate that the Pd element is distributed
The Pd
3
Cl nanocluster was synthesized following the previously
Cl nanocluster shows
reported method [19]. The as-prepared Pd
distinct peak at 418 nm and a weak absorption band at 485 nm
in UV–vis absorption spectra (Fig. 1(A)), which is consistent with
3
3
the literature. MALDI-MS analysis of the as-prepared Pd Cl cluster
3
Fig. 1. (A) UV–vis spectrum and (B) positive mode MALDI mass spectrum of Pd Cl nanoclusters. The peak marked with an asterisk (*) in (B) is a fragment caused by the MALDI
method.

T. Gao et al. / Tetrahedron Letters 61 (2020) 152385
3
3
Fig. 2. (A) and (B) TEM image of MSNs. (C) and (D) TEM image of Pd Cl/MSNs catalyst.
3
Fig. 3. TEM image of Pd Cl/MSNs (a), EDX mapping analysis of Si (b), O (c) and Pd (d).
homogenously throughout the whole MSNs. In addition, ICP-MS
analysis shows that the Pd content of the Pd Cl/MSNs catalyst is
.1 wt%.
The free Pd
and the results are shown in Fig. 4A. A strong absorption at
of PhAP bond, which is the characteristic absorption peak of triph-
enylphosphine group. After loading on the MSNs, the absorption
peak was still observed (Fig. 4A, red line). The peaks at
3
1
ꢀ1
ꢀ1
3
Cl, MSNs and Pd
3
Cl/MSNs were analyzed by FT-IR
2940 cm
bending vibration of CAH bond, respectively. To gain insight into
the oxidation state of palladium nanocluster, the free Pd Cl and
and 693 cm
assigned to stretching vibration and
ꢀ1
1
439 cm was observed in Pd
3
Cl owing to the stretching vibration
3
3 3 3 3
Fig. 4. FT-IR spectra of free Pd Cl, MSNs and Pd Cl/MSNs (A), XPS spectra in Pd 3d of free Pd Cl and Pd Cl/MSNs (B).

4
T. Gao et al. / Tetrahedron Letters 61 (2020) 152385
Table 1
Palladium nanocluster catalyzed one-pot tandem oxidation-wittig reaction of benzyl
a
alcohol and phosphorous ylide .
Entry
1
Solvent
Additive
Yieldb (%)
Toluene
Toluene
Toluene
Toluene
Toluene
–
–
–
–
[c]
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Na
NEt
2
CO
3
46
19
52
26
33
48
trace
71
89
27.
n.r.
n.r.
n.r.
3
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
3
CH
3
CN
3
3
3
3
3
3
3
3
3
3
DMSO
Dioxane
THF
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
0[
1
2
3
4
5
d]
[c]
[
c,e]
c,f]
c,g]
c,h]
3
Fig. 5. Recyclability of the Pd Cl/MSNs.
[
[
[
between zero and two. The distinctive oxidation state of Pd in pal-
ladium nanocluster may play an important role in the tandem
reactions. After immobilized on the MSNs, the binding energy of
Pd 3d has no obvious change (Fig. 4B blue line).
Then, the tandem oxidation-Wittig reaction of benzyl alcohol 1a
and phosphorous yilde 2a was chosen as a model reaction to test
[
a]
Reaction conditions: benzyl alcohol (0.2 mmol), ylide (0.24 mmol), additive
[b]
[c]
(
0.24 mmol), solvent (1 ml), catalyst (20 mg) at rt.
Isolated yield. The reaction
[
d]
[e]
was carried out at 50 °C.
was carried out under air atmosphere.
atmosphere.
carried out without catalyst.
The reaction was carried out at 30 °C.
The reaction
[f]
The reaction was carried out at N
2
[
g]
The reaction was catalyzed with bare MSNs. [h] The reaction was
the catalytic activity of Pd
parameters (Table 1). Unfortunately, no reaction occurred in the
solvent of toluene under the O balloon, even the reaction temper-
ature was increased to 50 °C (Table 1, entries 1 and 2). To our
delighted, when 1.2 equiv. of Na CO was added, the desired
3
Cl/MSNs and optimize the reaction
2
Table 2
Substrate scope of the tandem oxidation-Wittig reaction.a,b
2
3
one-pot tandem oxidation-Wittig reaction took place smoothly
with moderate yield (Table 1, entry 3). Further investigation
showed that, compared with Na
promising additive (Table 1, entries 4 and 5). Screening of solvents
revealed that toluene was the best one among CH CN, DMSO, Diox-
2 3 3 2 3
CO and NEt , Cs CO was the
3
ane and THF, although the yields are still unsatisfied (Table 1,
entries 6–9). Gratifyingly, significant improvement of yield was
obtained when the reaction was carried out at 50 °C (Table 1,
entries 10 and 11). The results of control and blank experiment
clearly suggest that palladium nanocluster is the true catalytic spe-
2
cies and O is the oxidant (Table 1, entries 12–15).
With the reaction conditions optimized, the generality of the
tandem oxidation-Wittig reaction was then tested and the results
are summarized in Table 2. Generally, the reaction proceeded
smoothly to give the corresponding a,b-unsaturated esters in good
to excellent E/Z selectivity and yields (68–96%). It was found that
the substituents electronic effect has slightly impact in the yield
of the tandem reaction. Benzyl alcohol with electron-withdraw
2
substituents (4-NO , 4-Br, 4-Cl, 4-F) and electron-donating sub-
stituents (4-Me, 4-OMe, 3-OMe) all reacted efficiently and give
the desired products with good yields. In addition, the more hin-
dered ortho-substitute alcohols such as 2-bromobenzyl alcohol
and 2-methyl benzyl alcohol could also be tolerated in this tandem
a
2 3
Reaction conditions: alcohol (0.2 mmol), ylide (0.24 mmol), Cs CO (0.24 mmol),
oxidation-Wittig reaction, which generated the corresponding
unsaturated esters with 82% and 71% yield, respectively (Table 2,
g and 3i). The semi-stabilized phosphorous ylide was also exam-
a,b-
b
toluene (1 ml), catalyst (20 mg) at 50 °C. Isolated yield. Values in [ ] represent E:Z
isomer ratios determined by 1H NMR.
3
ined under the optimized reaction conditions and the desired a,b-
unsaturated ketone was obtained in good yield (Table 2, 3 l).
3
Pd Cl/MSNs were analyzed by X-ray photoelectron spectra (XPS).
3
The recyclability of the as-prepared Pd Cl/MSNs was then
As shown in Fig. 4B, there are two observable peaks centred at
tested. When the reaction completed, the catalyst was collected
simply by centrifugation, washed with water and ethanol three
times, and dried at 50 °C. A new catalytic cycle was run under
the same conditions using the recycled catalyst. To our surprise,
the recycled catalyst showed almost as the same activity as the
3
3
36.4 and 341.7 eV, which are assigned to Pd 3d5/2 and Pd
d3/2, respectively. In comparison of the standard binding energy
of Pd(0) 3d5/2 (~335.9 eV) and Pd(Ⅱ) 3d5/2 (~337.0 eV), we can
conclude that the oxidation state of Pd in Pd Cl nanocluster is
3

T. Gao et al. / Tetrahedron Letters 61 (2020) 152385
5
fresh catalyst after 5 cycles (Fig. 5). So we can conclude that Pd
MSNs is a good recyclable catalyst.
3
Cl/
f) Y. Lu, W. Chen, Chem. Soc. Rev. 41 (2012) 3594–3623;
g) Y. Negishi, W. Kurashige, Y. Niihori, K. Nobusada, Phys. Chem. Chem. Phys.
5 (2013) 18736–18751.
1
[
2] a) Y. Du, H. Sheng, D. Astruc, M. Zhu, Chem. Rev. (2019);
b) G. Li, R. Jin, Acc. Chem. Res. 46 (2013) 1749–1758;
Conclusions
c) H. Qian, M. Zhu, Z. Wu, R. Jin, Acc. Chem. Res. 45 (2012) 1470–1479;
d) S. Yamazoe, K. Koyasu, T. Tsukuda, Acc. Chem. Res. 47 (2014) 816–824;
e) Y. Zhu, R. Jin, Y. Sun, Catalysts 1 (2011) 3–17;
f) Y. Zhu, H. Qian, R. Jin, J. Mater. Chem. 21 (2011) 6793.
3] X.-D. Zhang, J. Chen, Z. Luo, D. Wu, X. Shen, S.-S. Song, Y.-M. Sun, P.-X. Liu, J.
Zhao, S. Huo, S. Fan, F. Fan, X.-J. Liang, J. Xie, Adv. Healthcare Mater. 3 (2014)
133–141.
In summary, we have disclosed a novel, efficient and recyclable
catalyst for the one-pot tandem oxidation-Wittig reaction of alco-
hol and phosphorous ylide. With this protocol, a wide range of a,b-
unsaturated esters were produced in good to excellent yields. This
also represents the first tandem reaction catalyzed by atomically
precise nanocluster. This work is expected to open up a new fron-
tier for atomically precise nanocluster in tandem reactions.
[
[4] A. Mathew, P.R. Sajanlal, T. Pradeep, Angew. Chem. Int. Ed. 51 (2012) 9596–
600.
5] a) W. Li, C. Liu, H. Abroshan, Q. Ge, X. Yang, H. Xu, G. Li, J. Phys. Chem. C 120
2016) 10261–10267;
b) S. Xie, H. Tsunoyama, W. Kurashige, Y. Negishi, T. Tsukuda, ACS Catal. 2
2012) 1519–1523.
6] a) Y. Zhu, Z. Wu, C. Gayathri, H. Qian, R.R. Gil, R. Jin, J. Catal. 271 (2010) 155–
60;
b) H. Chen, C. Liu, M. Wang, C. Zhang, G. Li, F. Wang, Chin. J. Catal. 37 (2016)
787–1793.
9
[
[
[
(
(
Declaration of Competing Interest
1
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
1
7] a) H. Abroshan, G. Li, J. Lin, H.J. Kim, R. Jin, J. Catal. 337 (2016) 72–79;
b) G. Li, H. Abroshan, C. Liu, S. Zhuo, Z. Li, Y. Xie, H.J. Kim, N.L. Rosi, R. Jin, ACS
Nano 10 (2016) 7998–8005.
[
[
8] G. Li, R. Jin, J. Am. Chem. Soc. 136 (2014) 11347–11354.
9] a) X. Luo, Y. Xu, G. Xiao, W. Liu, C. Qian, G. Deng, J. Song, Y. Liang, C. Yang, Org.
Lett. 20 (2018) 2997–3000;
Acknowledgement
We are grateful to the Shandong Province Natural Science Foun-
dation (ZR2018PB003).
b) L.F. Tietze, Chem. Rev. 96 (1996) 115–136.
[
10] a) A.M. Faisca Phillips, A.J.L. Pombeiro, M.N. Kopylovich, ChemCatChem 9
2017) 217–246;
(
b) V. Jeena, R.S. Robinson, RSC Adv. 4 (2014) 40720–40739;
Appendix A. Supplementary data
c) H. Miyamura, S. Kobayashi, Acc. Chem. Res. 47 (2014) 1054–1066.
11] R.J.K. Taylor, M. Reid, J. Foot, S.A. Raw, Acc. Chem. Res. 38 (2005) 851–869.
12] D.H. Anthony, G.M. Barrett, M. Ohkubo, J. Org. Chem. 62 (1997) 9376–9378.
13] A. Maiti, J.S. Yadav, Synth. Commun. 31 (2001) 1499–1506.
[
[
[
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152385.
[14] A.R. Bressette, L.C. Glover Iv, Synlett (2004) 738–740.
[15] C.D.G. Read, P.W. Moore, C.M. Williams, Green Chem. 17 (2015) 4537–4540.
[16] E.Y. Lee, Y. Kim, J.S. Lee, J. Park, Eur. J. Org. Chem. 2009 (2009) 2943–2946.
[17] H. Miyamura, A. Suzuki, T. Yasukawa, S. Kobayashi, Adv. Synth. Catal. 357
References
(
2015) 3815–3819.
[
1] a) L. Cademartiri, V. Kitaev, Nanoscale 3 (2011) 3435–3446;
b) I. Chakraborty, T. Pradeep, Chem. Rev. 117 (2017) 8208–8271;
c) R. Jin, Nanoscale 2 (2010) 343–362;
d) R. Jin, C. Zeng, M. Zhou, Y. Chen, Chem. Rev. 116 (2016) 10346–10413;
e) B.H. Kim, M.J. Hackett, J. Park, T. Hyeon, Chem. Mater. 26 (2013) 59–71;
[18] C. Ren, Z. Shi, W. Ding, Z. Liu, H. Jin, X. Yu, S. Wang, Tetrahedron Lett. 59 (2018)
14–17.
[19] F. Fu, J. Xiang, H. Cheng, L. Cheng, H. Chong, S. Wang, P. Li, S. Wei, M. Zhu, Y. Li,
ACS Catal. 7 (2017) 1860–1867.