Solid supported Ru(0) nanoparticles: An efficient ligand-free heterogeneous catalyst for aerobic oxidation of benzylic and allylic alcohol to carbonyl

Article abstract of DOI:10.1016/j.tetlet.2013.03.106

Polymer immobilized stable, spherical ruthenium nanoparticles were prepared, characterized, and acted as a heterogeneous catalyst for the selective benzylic and allylic alcohol oxidation into the corresponding carbonyls using molecular oxygen. The solid supported Ru(0) (SS-Ru) as a heterogeneous catalyst exhibits good reusability and easy separation from the reaction mixture by filtration.

Full text of DOI:10.1016/j.tetlet.2013.03.106

Tetrahedron Letters 54 (2013) 2924–2928
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solid supported Ru(0) nanoparticles: an efficient ligand-free heterogeneous
catalyst for aerobic oxidation of benzylic and allylic alcohol to carbonyl
⇑
Pralay Das , Nidhi Aggarwal, Nitul Ranjan Guha
Natural Plant Products Division, CSIR, Institute of Himalayan Bioresource Technology, Palampur 176061, HP, India.
a r t i c l e i n f o
a b s t r a c t
Article history:
Polymer immobilized stable, spherical ruthenium nanoparticles were prepared, characterized, and acted
as a heterogeneous catalyst for the selective benzylic and allylic alcohol oxidation into the corresponding
carbonyls using molecular oxygen. The solid supported Ru(0) (SS-Ru) as a heterogeneous catalyst exhibits
good reusability and easy separation from the reaction mixture by filtration.
Received 15 November 2012
Revised 19 March 2013
Accepted 22 March 2013
Available online 3 April 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Heterogenous catalyst
Ruthenium nanoparticles
Aerobic alcohol oxidation
Ligand free
Solid supported Ru(0) (SS-Ru)
Oxidation of primary and secondary alcohol to carbonyl repre-
sents an important class of compounds or intermediates presence
in several pharmaceuticals, agrochemicals, perfumes, and flavoring
agents.1–3 Successful examples for the aerobic oxidation of alcohols
include both homogeneous as well as heterogeneous Ru4 catalyst.
Most of the ruthenium catalyzed oxidation reactions have been
explored either in perruthenates5 or ruthenium complex preferen-
tially with salen6 and phosphate ligands.7 Further, Ru immobilized
on different solid supports include Ru-hydroxyapatite (HAP),8
Herein, we report a facile process for in situ generation of Ru(0)
nanoparticles and their simultaneous deposition over solid matrix
to produce SS-Ru and their applicability in aerobic oxidation of
benzylic and allylic alcohols to their corresponding carbonyls, with
excellent recyclability of catalyst up to seven cycles.
SS-Ru was prepared following the same strategy which has
been applied for SS-Pd preparation. Amberlite IRA 900 chloride
form resin was partially exchanged with -BH4 ion and then washed
with water and acetone properly. Further it was treated with cat-
alytic quantity of RuCl3ÁH2O in DMF solvent for 1 h at 100 °C. The
[RuCl2(p-cymene)]2/activated carbon,9 Al2O3,10 carbon,11 zeolite12
,
and MgO13 have been designed and developed for aerobic oxida-
tion reactions. Polymer incarcerated Ru catalyst in a flow sys-
tem,14,15 and zeolite-confined nano-RuO2 have been explored
16
extensively but the major drawbacks of these system were the pre-
treatment of catalyst with costly silver salts, use of N-methylmor-
pholine (NMO),17 TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl)
which act as a co-catalyst, use of oxidants either an amine N-oxide
or an iodosobenzene derivative18 and longer reaction time re-
quired for the synthesis of nano-catalyst, respectively. Solid sup-
ported RuII and RuII-Dower catalyzed reaction were also reported,
but the use of harsh oxidant, NaIO4 made it less convenient.19
In recent reports, our group described the synthesis of solid
supported palladium and rhodium (SS-Pd and Rh) nano/micropar-
ticles as heterogeneous catalyst and their application in different
areas of organic synthesis.20
Figure 1. UV–vis absorption spectrophotometric study of in situ conversion of
Ru(III) salts to Ru(0). Reagents and conditions: (a) RuCl3. H2O, DMF at initial stage;
(b) after 1–2 min at rt RuCl3ÁH2O + sodium borohydride exchange resins
(Ru(III) + Ru(0)); (c) after heated at 100 °C for 1 h (SS-Ru(0)).
⇑
Corresponding author. Fax: +91 1894 230433.
E-mail addresses: pdas@ihbt.res.in, pdas_nbu@yahoo.com (P. Das).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.106

P. Das et al. / Tetrahedron Letters 54 (2013) 2924–2928
2925
Figure 3. EDX analysis of SS-Ru(0) for elemental detection of Ru.
Table 1
Optimization studies for benzylalcohol oxidation
CHO
OH
Catalyst, Solvent
Base, 90 °C
OMe
OMe
O2, 4h
1a
2a
Entry
Catalystb
Base/solvent
Yielda (%)
1
2
3
4
5
6
7
8
SS-Ru
SS-Ru
SS-Ru
SS-Ru
SS-Ru
SS-Ruc
SS-Rud
RuCl3
SS-Ru
SS-Ru
SS-Ru
Toulene
20
60
70
58
90
51
90
65
Cs2CO3/Toulene
K2CO3/Toulene
Pyridine/Toulene
Et3N/Toulene
Et3N/Toulene
Et3N/Toulene
Et3N/Toulene
Et3N/Toulene
Et3N/Toulene
Et3N/Toulene
9e
10f
11g
89
70
no reaction
aIsolated yield. Reaction condition: substrate (1 equiv), catalyst (0.05b, 0.03c and
0.06d mmol), solvent (3 mL), base (1.5 equiv), was heated for 3 h at 90 °C; 5 h at
f
80 °C (entry 9); e9 h at 70 °C (entry 10); air at 90 °C (entry 11).g
Figure 2. A gradual appearance of transmission electron micrograph (TEM) and
corresponding histogram of SS-Ru to illustrate particle size distributions.
white solid surface of resin soon turned gray after complete
impregnation of Ru (Supplementary data). After complete immobi-
lization of Ru over solid surface, the resin was washed with water,
then with acetone and dried under reduced pressure. The conver-
sion of Ru(III) to Ru(0), was monitored by UV–vis spectroscopic
studies (Fig. 1).
apparently affect the efficiency of the oxidation in terms of yield
and chemo-selectivity (Table 2, entries 1–20). Alcohols bearing
electron donating substituents such as 2-ethoxy, 4-methyl, 4-
methoxy, and 2,3,4-trimethoxy readily converted into the corre-
sponding aldehydes 2b–e in considerably good yields. Halogen
substituted benzyl alcohols afforded the corresponding aldehydes
2f–h in satisfactory yields and no dehalogenation product was ob-
served. Generally electron deficient functional group attached to
the aromatic ring causes electron deficiency on –CH2OH group
and thereby retards the rate of the oxidation process. In our study,
we also observed that nitro substituted benzyl alcohols 1i–j gave
lesser yields than the electron rich analogues (Table 2, entries 8
and 9). Similarly, secondary benzyl alcohols 1k–q containing both
the electron rich and deficient substituents showed comparable re-
sults as primary benzylic derivatives leading to desired ketones
2k–q22. Interestingly, SS-Ru was also found to be applicable to
the oxidation of heterocyclic alcohol 1r–s to respective aldehydes
2r–s in high yields. Allyl alcohols 1t–v were also found to be active
in our standard reaction conditions and underwent selective oxida-
tion to give excellent yields of cinnamylaldehydes 2t–v. Oxidation
reaction of geraniol 1w under the optimized condition even after
12 h ended with poor yield (20%) of the corresponding oxidized
product 2w and major unreacted geraniol was detected (Table 2).
Disappearance of the peak at 425 nm showed the conversion of
Ru(III) to Ru(0)21 (Fig. 1). The change of oxidation state, nanoparti-
cles formation, and metal detection were determined by UV–vis,
TEM (Fig. 2), EDX (Fig. 3), and XRD (Supplementary data) analysis.
In order to optimize the reaction conditions several bases, sol-
vents, and catalytic conditions were investigated. SS-Ru
(0.05 mmol Ru), triethyl amine in toluene at 90 °C was found suit-
able to give the highest yield of 2a in 90% (Table 1). With the de-
crease of temperature the rate of reaction as well as product
yield were decreased assertively. The same reaction was also per-
formed in open air condition in place of oxygen but no oxidized
product was observed. The generality of this methodology was
investigated using optimized reaction conditions with different
primary and secondary benzylic as well as allylic alcohols
(Table 2).
The reaction was also quite tolerant concerning the electronic
nature of the functional groups on the aromatic ring and did not

2926
P. Das et al. / Tetrahedron Letters 54 (2013) 2924–2928
Table 2
SS-Ru catalyzed aerobic oxidation of alcohols to carbonyls
Entry
1
Reactant
Product
Time
3
Yielda (%)
90
CHO
OH
1b
2b
OEt
OEt
CHO
OH
2
3
3
3
80
85
1c
2c
Me
Me
CHO
OH
1d
2d
MeO
MeO
MeO
CHO
OH
2e
4
4
84
OMe
1e
MeO
OMe
OMe
OMe
CHO
OH
5
6
7
3
3
3
92
86
85
2f
1f
Cl
Br
Cl
Br
CHO
OH
2g
1g
CHO
OH
2h
2i
1h
Br
Br
CHO
OH
1i
8
3
87
NO2
NO2
CHO
OH
2j
9
3
4
72
96
1j
O2N
O2N
OH
O
10
1k
2k
OH
O
11
12
4
4
89
87
2l
1l
OH
O
2m
2n
1m
1n
MeO
MeO
MeO
MeO
OH
O
13
14
15
16
4
4
4
4
85
90
87
85
OMe
OMe
OH
O
O
2o
1o
Br
Cl
Br
Cl
OH
2p
1p
OH
O
2q
1q
17
18
3
3
94
88
2r
CH2OH
CHO
CHO
1r
S
S
S
S
CH2OH
2s
1s

P. Das et al. / Tetrahedron Letters 54 (2013) 2924–2928
2927
Table 2 (continued)
Entry
Reactant
Product
CHO
2t
Time
4
Yielda (%)
CH2OH
19
20
94
90
1t
CH2OH
CHO
4
1u
2u
MeO
MeO
CH2OH
CHO
21
22
8
70
20
1v
2v
NO2
NO2
OH
CHO
12
2w
1w
Reaction condition: substrate (1 equiv), solvent (3 mL), catalyst (0.05 mol), Et3N (1.5 equiv) were heated using O2 at 90 °C for appropriate time.
a
Isolated yield after column chromatography.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.03.
106.
References and notes
1. Uozumi, Y.; Yamada, Y. M. A. Chem. Rec. 2009, 9, 51.
2. Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds;
Academic Press: New York, 1981.
3. Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer:
Berlin, 1984.
4. (a) Ten Brink, G. J.; Arends, I. W. C. E.; Sheldon, R. A. Science 2000, 287, 1636; (b)
Matsumoto, T.; Ueno, M.; Kobayashi, S. Chem. Asian J. 2008, 3, 239; (c) Liu, J.;
Wang, F.; Sun, K.; Xu, X. Adv. Synth. Catal. 2007, 349, 2439.
5. Hasan, M.; Musawir, M.; Davey, P. N.; Kozhevnikov, I. V. J. Mol. Catal. A: Chem.
2002, 180, 77.
6. (a) Miyata, A.; Muratami, M.; Irie, R.; Katsuki, T. Tetrahedron Lett. 2001, 42,
7067; (b) Miyata, A.; Furukawa, M.; Irie, R.; Katsuki, T. Tetrahedron Lett. 2002,
43, 3481.
Figure 4. Recovery and reuse of SS-Rua,b a3-methoxybenzyl alcohol (1 equiv), SS-
,
Ru (0.05 mol), Et3N (1.5 equiv). bConversion was determined on the basis of GC–MS
analysis.
7. (a) Meijer, R. H.; Ligthart, G. B. W. L.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof,
L. A.; Mills, A. M.; Kooijman, H.; Spek, A. L. Tetrahedron 2004, 60, 1065; (b)
Dijksman, A.; Arends, I. W. C. E.; Sheldon, R. A. Chem. Commun. 1999, 1591; (c)
Dijksman, A.; Martino-Gonzalez, A.; Mairata, A.; Payeras, I.; Arends, I. W. C. E.;
Sheldon, R. A. J. Am. Chem. Soc. 2001, 123, 6823; (d) Elzinga, A. J. M.; Li, Y.-X.;
Arends, I. W. C. E.; Sheldon, R. A. Tetrahedron Asymmetry 2002, 13, 879; (e)
Takahashi, M.; Oshima, K.; Matsubara, S. Tetrahedron Lett. 2003, 44, 9201.
8. Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc.
2000, 122, 7144.
9. Choi, E.; Lee, C.; Na, Y.; Chang, S. Org. Lett. 2002, 4, 2369.
10. Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 41, 4538.
11. Mori, S.; Takubo, M.; Makida, K.; Yanase, T.; Aoyagi, S.; Maegawa, T.; Monguchi,
Y.; Sajiki, H. Chem. Commun. 2009, 5159.
12. Zhan, B. Z.; White, M. A.; Sham, T. K.; Pinock, J. A.; Doucet, R. J.; Rao, K. V. R.;
Robertsin, K. N.; Camerson, T. S. J. Am. Chem. Soc. 2003, 125, 125.
13. Kantam, M. L.; Pal, U.; Sreedhar, B.; Bhargava, S.; Iwasawa, Y.; Tada, M.;
Choudary, B. M. Adv. Synth. Catal. 2008, 350, 1225.
14. (a) Akiyama, R.; Kobayashi, S. J. Am. Chem. Soc. 2003, 125, 3412; (b) Okamoto,
K.; Akiyama, R.; Kobayashi, S. J. Org. Chem. 2004, 69, 2871; (c) Kobayashi, S.;
Miyamura, H.; Akiyama, R.; Ishida, T. J. Am. Chem. Soc. 2005, 127, 9251.
15. Matsumoto, T.; Ueno, M.; Kobayashi, J.; Miyamura, H.; Mori, Y.; Kobayashi, S.
Adv. Synth. Catal. 2007, 349, 531.
Recovery of the catalyst was done by filtration and washing
with water, acetone and then dried under reduced pressure. The
catalyst so obtained could be reused without any activation. More-
over, the reused SS-Ru catalyst exhibit similar efficiency for oxida-
tion reaction, which was confirmed by the example of oxidation of
3-methoxybenzyl alcohol (Fig. 4). Upto seven cycles, minor loss of
product yields 2–5% were observed.
Also the reaction mixture showed no further conversion on
removing the catalyst from the system, indicating that metal is
not leaching from supports.
In summary, we have developed active and environmentally
sustainable immobilized ruthenium nanoparticles (SS-Ru) as het-
erogeneous catalyst. This catalyst has been utilized successfully
in an aerobic oxidation of benzylic and allylic alcohols to corre-
sponding carbonyls with high efficiency, selectivity, and reusabil-
ity. Use of milder base, oxygen as an inexpensive primary
oxidant and water as the only by product, highlights the environ-
mentally benign green chemical aspect of the present process.
16. Zhan, B. Z.; Mary Anne White, M. A.; Tsun-Kong Sham, T. K.; James, A.; Pincock,
J. A.; Doucet, R. J.; Ramana Rao, K. V.; Robertson, K. N.; Cameron, T. S. J. Am.
Chem. Soc. 2003, 125, 2195.
17. Kobayashi, S.; Miyamura, H.; Akiyama, R.; Ishida, T. J. Am. Chem. Soc. 2005, 127,
9251.
18. Muller, P.; Godoy, J. Tetrahedron Lett. 1981, 22, 2361.
19. Hu, Z.; Hongxia Du, H.; Chi-Fai Leung, C. F.; Liang, H.; Lau, T. C. Ind. Eng. Chem.
Res. 2011, 50, 12288.
Acknowledgments
20. (a) Das, P.; Sharma, D.; Shil, A. K.; Kumari, A. Tetrahedron Lett. 2011, 52, 1176;
(b) Bandana, C.; Aggarwal, N.; Das, P. Tetrahedron Lett. 2011, 52, 4954; (c) Shil,
A. K.; Sharma, D.; Guha, N. R.; Das, P. Tetrahedron Lett. 2012, 53, 4858–4861; (d)
Bandna, C.; Guha, N. R.; Shil, A. K.; Sharma, D.; Das, P. Tetrahedron Lett. 2012, 53,
5318; (e) Guha, N. R.; Reddy, C. B.; Aggarwal, N.; Sharma, D.; Shil, A. K.; Bandna,
C.; Das, P. Adv. Synth. Catal. 2012, 354, 2911; (f) Sharma, D.; Kumar, S.; Shil, A.
K.; Guha, N. R.; Bandna, C.; Das, P. Tetrahedron Lett. 2012, 53, 7044.
21. Nandanwar, S. U.; Chakraborty, M.; Mukhopadhyay, S.; Shenoy, K. T. Cryst. Res.
Technol. 2011, 46, 393.
Authors are grateful to Dr. P.S. Ahuja, Director IHBT and the
Department of Biotechnology for providing necessary facilities
during the course of the work. We gratefully acknowledge financial
assistance from the Department of Science & Technology (Nano
Mission), New Delhi (Grant No. SR/NM/NS-95/2009). N.A. (SRF)
and N.R.G. (SRF) thank the CSIR and the UGC, New Delhi for award-
ing fellowships.

2928
P. Das et al. / Tetrahedron Letters 54 (2013) 2924–2928
22. Typical experimental procedure for SS-Ru catalyzed oxidation of alcohols.
mixture of 2-phenyl ethanol 1k (100 mg, 0.818 mmol) SS-Ru (914 mg,
0.05 mmol), Et3 N (171 l, 1.22 mmol) and 3 ml of toluene was purged with
A
organic layer was evaporated under reduced pressure and the crude residue
was purified by silica gel (mesh 60–120) column chromatography (hexane/
EtOH::95:5) afforded 2k as colorless liquid (94 mg, 96%). 1H NMR (300 MHz,
CDCl3) d 2.36 (s, 3H), 7.31–7.37 (m, 3H), 7.74–7.77 (m, 2H); 13C NMR (75 MHz,
CDCl3) d 25.49, 127.64 (2C), 128.25 (2C), 132.04, 136.06, 197.11.
l
molecular oxygen and stirred at 90 °C for 4 h. The progress of the reaction was
monitored by TLC. After completion of the reaction, the reaction was cooled,
diluted with ethyl acetate, and filtered through cotton bed. The combined