Poly(ethylene glycol) (400) as superior solvent medium against ionic liquids for catalytic hydrogenations with PtO2

Article abstract of DOI:10.1021/jo052604x

Adams' catalyst in poly(ethylene glycol) (PEG) (400) has been found to be a superior solvent over the ionic liquids by severalfold in promoting the hydrogenation of various functional groups. Both the catalyst and PEG were recycled efficiently over 10 runs without loss of activity, and substrate cross contamination was not observed despite a change in the substrate four times.

Full text of DOI:10.1021/jo052604x

SCHEME 1
Poly(ethylene Glycol) (400) as Superior Solvent
Medium against Ionic Liquids for Catalytic
Hydrogenations with PtO2
Srivari Chandrasekhar,* Samala Jaya Prakash, and
Chennamaneni Lohitha Rao
Organic DiVision-1, Indian Institute of Chemical Technology,
into the limelight. These can be recycled more vigorously
compared to conventional solvents, thus minimizing the solvent
waste. Keeping in mind the challenges posed by traditional
solvents and also the limitation of nonconventional solvents
(ionic liquids, scCO2 etc.), a program was initiated by our group5
and later by others6 where the biologically compatible poly-
(ethylene glycol) (PEG)7 was demonstrated to be a recyclable
solvent for various transformations. In these studies, the catalysts
used (viz., palladium and osmium) were also efficiently recycled
for 5-10 runs without loss of activity. Several other groups8
have utilized ionic liquids as solvent, which are relatively
expensive9 compared to PEG for recycling similar metal
catalysts as well as solvent. The scCO210 and fluorous11 solvents
were also popularized for various transformations with claims
of eco-compatibility and recyclability. An ambitious exercise
has been taken up to develop recyclable solvent medium for
Adams’ catalyst12 (PtO2), one of the most expensive9 metal
catalysts that richly deserves recyclability. The study taken up
by us unambiguously proved that PEG is a superior solvent over
the ionic liquids by severalfold in promoting the hydrogenation
of various functional groups (Schemes 1 and 2) using the
catalyst. The details are presented in this Note.
Hyderabad 500007, India
sriVaric@iict.res.in
ReceiVed December 20, 2005
Adams’ catalyst in poly(ethylene glycol) (PEG) (400) has
been found to be a superior solvent over the ionic liquids
by severalfold in promoting the hydrogenation of various
functional groups. Both the catalyst and PEG were recycled
efficiently over 10 runs without loss of activity, and substrate
cross contamination was not observed despite a change in
the substrate four times.
In the first instance, cinnamyl alcohol 1a (entry 1, Table 1)
was subjected to PtO2-catalyzed hydrogenation at atmospheric
pressure with the aid of a balloon to observe clean formation
of 3-phenyl-n-propanol 1b in greater than 95% yield in less
Catalysis and atom efficiency go hand in hand for efficient
organic synthesis.1 Recovery of materials used in the reaction
and purification difficulties limit the yield and utility of a
particular organic reaction in a multistep synthesis. Enormous
efforts continue in this direction, and already several protocols
are being practiced as efficient alternatives to the “not so
efficient” methods of older times. The major/bulk waste created
in most reactions is the solvent medium, and new nonvolatile
ionic liquids (IL),2 scCO2,3 and fluorous4 solvents have come
(5) (a) Chandrasekhar, S.; Narsihmulu, Ch.; Sultana, S. S.; Reddy, N.
R. Org. Lett. 2002, 4, 4399. (b) Chandrasekhar, S.; Narsihmulu, Ch.; Sultana,
S. S.; Reddy, N. R. Chem. Commun. 2003, 1716.
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70, 5347. (b) Li, J.-H.; Liu, W.-J.; Xie, Y.-X. J. Org. Chem. 2005, 70,
5409. (c) Huaxing, Z.; Yuhong, Z.; Leifang, L.; Hailiang, X.; Yanguang,
W. Synthesis 2005, 13, 2129. (d) Zhang, Z.-H.; Yin, L.; Wang, Y.-M.; Liu,
J.-Y.; Li, Y. Green Chem. 2004, 6, 563.
(1) (a) Trost, B. M. Science 1991, 254, 1471. (b) Trost, B. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 259. (c) Perkas, N.; Wang, Y.; Koltypin,
Y.; Gedanken, A. Chandrasekaran, S. Chem. Commun. 2001, 988. (d)
Sheldon, R. A.; Sawyer, D. T.; Martell, A. E. In Industrial EnVironmental
Chemistry; Plenum: New York 1992; pp 99-119. (e) Sheldon, R. A. Pure
Appl. Chem. 2000, 72, 1233.
(2) (a) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 63, 351. (b)
Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (c) Earle, M. J.;
Katdare, S. P.; Seddon, K. R. Org. Lett. 2004, 6, 707. (d) Welton, T. Chem.
ReV. 1999, 99, 2071. (e) Wasserscheid, P.; Keim, W. Angew. Chem., Int.
Ed. 2000, 39, 3772. (f) Sheldon, R. A. Chem. Commun. 2001, 2399.
(3) (a) Baiker, K. Chem. ReV. 1999, 99, 453. (b) Jessop, P. G.; Ikariya,
T.; Noyori, R. Chem. ReV. 1999, 99, 475. (c) Darr, J. A.; Poliakoff, M.
Chem. ReV. 1999, 99, 495. (d) Messiano, A. J.; Beckman, E. J.; Russell, A.
J. Chem. ReV. 1999, 99, 623. (e) Behles, J. A.; Desimone, J. M. Pure Appl.
Chem. 2001, 73, 1281. (f) Heldeberant, D. J.; Jessop, P. G. J. Am. Chem.
Soc. 2003, 125, 5600.
(4) (a) Studer, A.; Hadida, S.; Ferritto, R.; Kim, S. Y.; Jeger, P.; Wipf,
P.; Curran, D. P. Science 1997, 275, 823. (b) Horvath, I. T.; Rabai, J. Science
1994, 266, 72. (c) Foster, D. F.; Adams, D. J.; Gudmunsen, D.; Stuart, A.
M.; Hope, E. G.; Cole-Hamilton, D. J. Chem. Commun. 2002, 722. (d)
Perperi, E.; Huang, Y. L.; Angeli, P.; Manos, G.; Mathison, C. R.; Cole-
Hamilton, D. J.; Adams, D. J.; Hope, E. G. J. Chem. Soc., Dalton Trans.
2004, 2062.
(7) Harris, J. M.; Zalipsky, S. Poly(ethylene Glycol): Chemistry and
Biological Applications; American Chemical Society: Washington, DC,
1997.
(8) (a) Baan, Z.; Finta, Z.; Keglevich, G.; Hermecz, I. Tetrahedron Lett.
2005, 46, 6203. (b) Mo, J.; Xu, L.; Xiao, J. J. Am. Chem. Soc. 2005, 127,
751. (c) Xie, X.; Chen, B.; Lu, J.; Han, J.; She, X.; Pan, X. Tetrahedron
Lett. 2004, 45, 6235. (d) Chhikara, B. S.; Chandra, R.; Tandon, V.
Tetrahedron Lett. 2004, 45, 7585. (e) Ranu, B. C.; Banerjee, S. J. Org.
Chem. 2005, 70, 4517. (f) Yao, Q. Org. Lett. 2002, 4, 2197. (g) Teo, Y.-
C.; Goh, E.-L.; Loh, T.-P. Tetrahedron Lett. 2005, 46, 4573. (h) Yadav, J.
S.; Reddy, B. V. S.; Baishya, G.; Harshavardhan, S. J.; Chary, C. J.; Gupta,
M. K. Tetrahedron Lett. 2005, 46, 3569.
(9) Ionic liquids cost 60-400 USD/50 g, compared to PEG (400), which
costs 23 USD/500 g. Platinum(IV) oxide costs 60 USD/1 g (Aldrich 2005-
2006).
(10) (a) Komoto, I.; Kobayashi, S. Org. Lett. 2002, 4, 1115. (b)
Nishiyama, Y.; Kaneda, M.; Saito, R.; Mori, T.; Wada, T.; Inoue, Y. J.
Am. Chem. Soc. 2004, 126, 6568. (c) Selva, M.; Tundo, P.; Perosa, A.
Tetrahedron Lett. 2002, 43, 1217.
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Yasuda, S.; Curran, D. P. Org. Lett. 2003, 5, 1167. (b) Crich, D.; Zou, Y.
Org. Lett. 2004, 6, 775.
(12) Adams, R.; Voorhees, V.; Shriner, R. L. Organic Syntheses; Wiley
& Sons: New York, 1941; Collect. Vol. 1, p 463.
10.1021/jo052604x CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/09/2006
2196
J. Org. Chem. 2006, 71, 2196-2199

SCHEME 2
TABLE 1. Catalytic Hydrogenation in PEG under PtO2
FIGURE 1. Plot of consumed volume of hydrogen versus pulse No.
namely the carvone 9a (entry 1, Table 2) and the sugar
derivative 11a (entry 3, Table 2), having a hydrogenation-
sensitive benzyl ether were effectively saturated in greater than
90% yields in PEG. The benzyl ether cleavage was not observed
even in traces. These two substrates when subjected to PtO2-
catalyzed hydrogenation in ionic liquid (EMIMBF4) yielded
products in poor conversion and yields (entries 2 and 3, Table
2). To understand more closely the chemisorption of Adams’
catalyst in PEG over ionic liquids, quantitative adsorption of
hydrogen on PtO2 in both solvents was taken up.
The pulse chemisorption of H2 was employed to know the
amount of H2 chemisorbed on PtO2 (catalyst) in the presence
of solvent (PEG/IL). The pretreatment of catalyst was performed
by passing N2 at ambient temperature for 1 h. Then the hydrogen
was chemisorbed on pretreated catalyst at room temperature in
pulses through a six-port valve. In a typical experiment, 10 mg
of the catalyst in 1 mL of PEG/IL was taken in a 5-mL round-
bottom flask. The outlet of the reactor was connected to a
microthermal conductivity detector (TCD) equipped with GC-
17A (M/S Shimadzu Instruments, Japan) through a blank
column. After the pretreatment of the catalyst, pulses of H2
(purity 99.99%) were injected at room temperature through a
500-µL loop connected to the six-port valve until no further
change in the intensity of the outlet H2 (monitored by a data
station with CLASS 10 GC software) was observed. The
cumulative H2 consumption was recorded against five pulses,
and the data recorded (Figure 1) clearly demonstrated that the
H2 adsorption into PtO2 in PEG is severalfold superior to ionic
liquid. This is direct evidence for the efficient reduction of
functional groups in PEG over the ionic liquids.
a Time in hours. b All the products were characterized by 1H, 13C NMR,
IR, and mass spectra.
TABLE 2. Comparison of Catalytic Hydrogenation in PEG and IL
Using PtO2
This advantage is further exploited with various substrates.
For instance, in entry 2 (Table 1), the selective reduction of
electron-deficient olefin was rapid while that of the N-Bn group
was stable. However, partial O-Bn cleavage was observed in
the 4-nitro-1-benzyloxy aryl system 5a (entry 5, Table 1). The
reduction of the nitro group was also very efficient in the case
of 4-NO2-benzyl ether 7a (entry 7, Table 1).
Furanose derivative 8a (entry 8, Table 1) not only allowed
reduction of both azido and olefin groups but also facilitated
intramolecular cyclization to furnish the tricyclic compound 8b
in 95% yield. The recyclability of both PEG as solvent and PtO2
as catalyst was amply demonstrated in Table 3 for 10 runs, and
substrate cross contamination was not observed despite a change
in the substrate four times (entries 1, 4, 7, and 9 in Table 3).
a Time in hours. b All the products were characterized by 1H, 13C NMR,
IR, and mass spectra.
than 3 h. Similarly, ethyl-4-methoxycinnamate 10a (entry 2,
Table 2) was subjected to hydrogenation with 5 mol % PtO2 in
PEG (400 Da) for 2 h to isolate the saturated ester 10b in greater
than 93% yield and greater than 95% purity just by ethereal
workup (vide infra). The same substrate (entry 2, Table 2) when
subjected to hydrogenation in ionic liquid (EMIMBF4) was
resistant even after 24 h (15% conversion). Two other substrates,
J. Org. Chem, Vol. 71, No. 5, 2006 2197

and 743 cm-1; ESI (MS): 308 [M + H]+, 330 [M + 23]+; HRMS
(ESI) Calcd for C20H22NO2: 308.1650 [M + H]+; Found: 308.1648
[M + H]+.
TABLE 3. Recyclability of Both PEG and PtO2 as a Solvent and
Catalyst
Ethyl-3-(3-phenyl-2-oxiranyl)-propanoate (3b): 1H NMR (200
MHz, CDCl3) δ 7.36-7.19 (m, 5H), 4.15 (q, J ) 7.2 Hz, 2H),
3.61 (d, J ) 2.4 Hz, 1H), 3.01-2.93 (m, 1H), 2.50 (t, J ) 7.2 Hz,
2H), 2.21-2.03 (m, 1H), 2.00-1.84 (m, 1H), 1.28 (t, J ) 7.2 Hz,
3H); 13C NMR (75 MHz, CDCl3) δ 172.6, 137.3, 128.4, 128.0,
125.5, 61.6, 60.5, 58.6, 30.3, 27.5, 14.1; IR (KBr): 2981, 1732,
1179, and 699 cm-1; ESI (MS): 221 [M + H]+, 243 [M + 23]+;
HRMS (ESI) Calcd for C13H17O3: 221.1177 [M + H]+; Found:
221.1187 [M + H]+.
2-Methyl-2-propyl-1,3-cyclohexanedione (4b): 1H NMR (300
MHz, CDCl3) δ 2.78-2.54 (m, 4H), 2.11-1.96 (m, 1H), 1.92-
1.70 (m, 3H), 1.25-1.08 (m, 5H), 0.88 (t, J ) 7.55 Hz, 3H); 13C
NMR (75 MHz, CDCl3) δ 209.8, 65.3, 39.4, 37.4, 18.1, 17.5, 17.2,
13.8; IR (KBr): 2963, 1695, 1618, and 1130 cm-1; ESI (MS): 169
[M + H]+.
4-Benzyloxyaniline (5b): 1H NMR (300 MHz, CDCl3) δ 7.42-
7.28 (m, 5H), 6.78 (d, J ) 9.0 Hz, 2H), 6.61 (d, J ) 9.0 Hz, 2H),
4.98 (s, 2H), 1.90-1.46 (bs, 2NH); 13C NMR (50 MHz, CDCl3) δ
151.8, 140.1, 137.4, 128.4, 127.7, 127.4, 116.2, 115.9, 70.6; IR
(KBr): 3416, 2220, 1618, and 618 cm-1; ESI (MS): 200 [M +
H]+; HRMS (ESI) Calcd for C13H14NO: 200.1075 [M + H]+;
Found: 200.1078 [M + H]+.
1-(1-Benzyloxybutyl)-benzene (6b): 1H NMR (300 MHz,
CDCl3) δ 7.35-7.20 (m, 10H), 4.42 (d, J ) 12.0 Hz, 1H), 4.25
(dd, J ) 5.2, 7.5 Hz, 1H), 4.20 (d, J ) 12.0 Hz, 1H), 1.89-1.77
(m, 1H), 1.66-1.54 (m, 1H), 1.52-1.40 (m, 1H), 1.34-1.23 (m,
1H), 0.89 (t, J ) 7.5 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 142.6,
138.6, 128.2, 128.1, 127.6, 127.4, 127.3, 126.6, 81.1, 70.2, 40.4,
19.0, 13.8; IR (KBr): 2958, 1454, 1085, and 736 cm-1; ESI (MS):
241 [M + H]+; HRMS (ESI) Calcd for C17H24NO: 258.1857[M
+ NH4]+; Found: 258.1862 [M + NH4]+.
a Time in hours. b All the products were characterized by 1H, 13C NMR,
IR, and mass spectra.
In conclusion, we have demonstrated that the very expensive
PtO2 can be recycled several times in PEG (400) without
considerable loss of activity. Also, dissolution of H2 in PEG
and ionic liquid was studied using a gas chromatography
technique to prove the fact that the H2 dissolution plays an
advantageous role in favor of PEG over ionic liquid. It is
needless to mention of course that ionic liquid will be superior
in catalysis of acid-catalyzed reactions where PEG may not be
stable.
1
4-Benzyloxymethylaniline (7b): H NMR (300 MHz, CDCl3)
δ 7.33-7.24 (m, 5H), 7.11 (d, J ) 8.1 Hz, 2H), 6.60 (d, J ) 8.1
Hz, 2H), 4.47 (s, 2H), 4.40 (s, 2H), 1.45-1.34 (bs, 2-NH); 13C
NMR (75 MHz, CDCl3) δ 146.0, 138.5, 129.5, 128.3, 128.1, 127.8,
127.5, 115.0, 72.0, 71.6; IR (KBr): 3458, 1619, 1359, 1067, and
742 cm-1; ESI (MS): 214 [M + H]+; HRMS (ESI) Calcd for
C14H16NO: 214.1231 [M + H]+; Found: 214.1232 [M + H]+.
2,2-Dimethylperhydro-[1,3]-dioxolo-[4′,5′:4,5]-furo-[3,2-b]-py-
1
ridin-7-one (8b): H NMR (300 MHz, CDCl3) δ 7.88 (s, 1-NH),
5.87 (d, J ) 3.7 Hz, 1H), 4.55-4.51 (bs, 1H), 4.48 (d, J ) 3.7 Hz,
1H), 3.81 (d, J ) 3.7 Hz, 1H), 2.57-2.44 (m, 1H), 2.30-2.15 (m,
2H), 1.96-1.83 (m, 1H), 1.49 (s, 3H), 1.30 (s, 3H); 13C NMR (75
MHz, CDCl3) δ 171.7, 112.0, 104.9, 85.6, 71.8, 60.5, 26.7, 26.2,
25.6, 22.4; IR (KBr): 3179, 2904, 1669, 1227, 1072, and 1010
cm-1; ESI (MS): 214 [M + H]+, 236 [M + 23]+; HRMS (ESI)
Calcd for C10H16NO4: 214.1079 [M + H]+; Found: 214.1078 [M
+ H]+.
Experimental Section
General. The chemicals PtO2, ionic liquid (EMIMBF4), and poly-
(ethylene glycol) (400) were purchased from commercial suppliers
and used without further purification. Diethyl ether was distilled
over sodium wire. IR spectra were recorded on Perkin-Elmer 683
1
spectrometer. H and 13C NMR spectra were recorded in CDCl3
5-Isopropyl-2-methyl-1-cyclohexanone (9b): 1H NMR (300
MHz, CDCl3) δ 2.43-2.27 (m, 2H), 2.16-1.99 (m, 1H), 1.93-
1.84 (m, 1H), 1.76-1.28 (m, 5H), 1.02 (d, J ) 6.0 Hz, 3H), 0.96-
0.91 (m, 6H); 13C NMR (50 MHz, CDCl3) δ 213.5, 46.4, 45.2,
44.7, 34.9, 32.6, 28.7, 19.4, 19.2, 14.2; IR (KBr): 2962, 1712, and
1617 cm-1; ESI (MS): 155 [M + H]+, 177 [M + 23]+; HRMS
(ESI) Calcd for C10H19O: 155.1435 [M + H]+; Found: 155.1432
[M + H]+.
solution on a Varian Gemini 200, Brucker Avance 300. Chemical
shifts were reported in ppm with respect to internal TMS. Coupling
constants (J) are quoted in Hz. Mass spectra were obtained on an
Agilent Technologies LC/MSD Trap SL.
Spectroscopic (IR, 1H, and 13C NMR) data for the new
compounds are presented in order of their entries in Tables 1and
2.
Ethyl-3-(4-methoxyphenyl)-propanoate (10b): 1H NMR (300
MHz, CDCl3) δ 7.07 (d, J ) 8.9 Hz, 2H), 6.76 (d, J ) 8.9 Hz,
2H), 4.10 (q, J ) 7.4 Hz, 2H), 3.77 (s, 3H), 2.86 (t, J ) 7.4 Hz,
2H), 2.55 (t, J ) 7.4 Hz, 2H), 1.24 (t, J ) 7.4 Hz, 3H); 13C NMR
(50 MHz, CDCl3) δ 172.8, 157.9, 132.5, 129.1, 113.7, 60.2, 55.1,
36.1, 30.0, 14.1; IR (KBr): 2982, 1732, 1613, 1513, 1247, 1178,
and 1037 cm-1; ESI (MS): 209 (M+ + H), 231 (M+ + 23); HRMS
(ESI) Calcd for C12H17O3: 209.1177 [M + H]+; Found: 209.1181
[M + H]+.
3-Phenyl-1-propanol (1b). Data are in accordance with the data
of a commercially available sample.
Ethyl-3-(1-benzyl-1H-3-indolyl)-propanoate (2b): 1H NMR
(200 MHz, CDCl3) δ 7.61 (d, J ) 7.55 Hz, 1H), 7.33-7.07 (m,
8H), 6.94 (s, 1H), 5.27 (s, 2H), 4.12 (q, J ) 7.55 Hz, 2H), 3.10 (t,
J ) 7.55 Hz, 2H), 2.70 (t, J ) 7.55 Hz, 2H), 1.21 (t, J ) 7.55 Hz,
3H); 13C NMR (75 MHz, CDCl3): δ 173.4, 137.7, 136.7, 128.7,
127.9, 127.5, 126.8, 125.6, 121.8, 119.0, 118.9, 114.2, 109.7, 60.3,
49.9, 35.1, 20.6, 14.3; IR (KBr): 2980, 1729, 1467, 1177, 912,
2198 J. Org. Chem., Vol. 71, No. 5, 2006

6-Benzyloxy-2,2-dimethyl-5-pentylperhydrofuro-[2,3-d][1,3]-
dioxole (11b): 1H NMR (300 MHz, CDCl3) δ 7.34-7.25 (m, 5H),
5.83 (d, J ) 3.8 Hz, 1H), 4.68 (d, J ) 12.0 Hz, 1H), 4.53 (d, J )
4.5 Hz, 1H), 4.45 (d, J ) 12.0 Hz, 1H), 4.07-4.01 (m, 1H), 3.69
(d, J ) 3.0 Hz, 1H), 1.76-1.57 (m, 2H), 1.45 (s, 3H), 1.33-1.25
(m, 9H), 0.88 (t, J ) 6.7 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ
137.5, 128.3, 127.8, 127.7, 111.1, 104.6, 82.1, 81.7, 80.4, 71.6,
31.9, 27.8, 26.6, 26.2, 25.7, 22.5, 14.0; IR (KBr): 2929, 1456, 1077,
and 1022 cm-1; ESI (MS): 321 (M+ + H); HRMS (ESI) Calcd
for C19H29O4: 321.2065 [M + H]+; Found: 321.2073 [M + H]+.
Acknowledgment. S.J.P. and C.L.R. thank CSIR, New
Delhi, for financial support. We acknowledge Dr. K. S. Rama
Rao and Mr. V. Siva Kumar for their help in gas chromato-
graphic studies. IICT Communication No. 050917.
Supporting Information Available: General experimental
procedure and copies of 1H and 13C NMR spectra for all new
compounds 2b, 3b, 4b, 5b, 6b, 7b, 8b, 9b, 10b, and 11b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JO052604X
J. Org. Chem, Vol. 71, No. 5, 2006 2199