Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols

Article abstract of DOI:10.1021/ja031936l

In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 °C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields. Copyright

Full text of DOI:10.1021/ja031936l

Published on Web 05/07/2004
Homogeneous Palladium Catalyst Suppressing Pd Black Formation in Air
Oxidation of Alcohols
Tetsuo Iwasawa, Makoto Tokunaga, Yasushi Obora, and Yasushi Tsuji*
Catalysis Research Center and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, CREST,
Japan Science and Technology Corporation (JST), Sapporo 001-0021, Japan
Received December 24, 2003; E-mail: tsuji@cat.hokudai.ac.jp
A homogeneous transition metal-catalyzed reaction is one of the
most versatile methods for synthesizing useful and necessary
molecules. However, there is a persistent problem that metal
aggregation and precipitation cause catalyst decomposition and a
considerable loss of catalytic activity.1 In particular, Pd catalysts
are known to aggregate easily and form Pd black, although they
realize a wide variety of useful reactions in organic synthesis.2 To
overcome this intrinsic problem of homogeneous Pd catalysis, we
explored a new class of Pd catalyst with adopting aerobic oxidation
of alcohols as a probe reaction.
Recently, aerobic oxidation of alcohols catalyzed by a homo-
geneous transition metal complex has received considerable atten-
tion as an environmentally benign reaction.3 Among them, Pd
catalyst shows a good catalytic activity4,5 and asymmetric oxidation
also has been realized.6 However, if reaction conditions were not
Figure 1. Novel pyridine ligands having 2,3,4,5-tetraphenylphenyl moiety.
fixed properly in these reactions, the Pd black formed very
easily.3a,4a,d,e,5 There are two keys to successful catalytic reaction
without the Pd black formation. One is sufficient oxygen partial
pressure (typically 1 atm molecular oxygen,4a-e,g,5c,6 rather than
air4f,5). The other is low substrate to catalyst molar ratio (S/C:
typically 204a,c-f,6a,d,e or less4f,6b).7
Herein we report a new catalyst system that suppresses the Pd
black formation even under 1 atm air and with a high S/C (>1000).
None of the homogeneous palladium catalysts reported so far4-7
would survive under these reaction conditions. We designed and
prepared pyridine derivatives having a 2,3,4,5-tetraphenylphenyl8
Figure 2. ORTEP drawing of Pd(OAc)2(1)2 with thermal ellipsoids at 50%
probability levels. Hydrogen atoms have been omitted for clarity.
substituent and its higher dendritic unit at the 3-position (1 and 2).
The palladium complexes having 1 and 2 as a ligand, namely,
Actually, Pd(OAc)2(Py)2 and Pd(OAc)2(1)2 showed comparable
Pd(OAc)2(1)2 and Pd(OAc)2(2)2, successfully suppress the Pd black
catalytic activity at S/C ) 20 in the aerobic oxidation of alcohols.
formation and achieved the highest TON ) 1480 in homogeneous
The Pd(OAc)2(ligand)2 was employed as a catalyst in the air
palladium-catalyzed air oxidation of alcohols.9,10
oxidation in toluene at 80 °C (Table 1). The molar ratio of the
Dendrimers having a 2,3,4,5-tetraphenylphenyl moiety were
alcohol to Pd(OAc)2(ligand)2 was fixed to 1000 (S/C ) 1000). As
synthesized by Mu¨llen et al. and utilized as polyphenylene
an additive, NaOAc4b,5a (0.1 equiv based on the alcohol) was
nanomaterials.8 We are interested in their spatially spread rigid
employed.
structure. A series of novel pyridine derivatives bearing the 2,3,4,5-
tetraphenylphenyl substituent, 1-6, were synthesized (Figure 1).
Due to its limited solubility of 3, a methylated analogue (2) was
prepared. These pyridine ligands 1-5 as well as pyridine (Py),
Pd(OAc)2 did not show catalytic activity in the oxidation of
1-phenylethanol (run 1). On the other hand, Pd(OAc)2(Py)2
catalyzed the oxidation and afforded acetophenone in 23% yield
in 2 h. However, at this moment, the Pd catalyst decomposed
3-phenylpyridine (3-PhPy), 3,5-diphenylpyridine (3,5-diPhPy), and
2,2′-bipyridine (Bipy) readily reacted with Pd(OAc)2 in toluene and
afforded the Pd(OAc)2(ligand)2 complexes quantitatively. The
molecular structure of Pd(OAc)2(1)2 determined by crystallographic
analysis is shown in Figure 2.11 It is evident that the 2,3,4,5-
tetraphenylphenyl substituent at the 3-position of the pyridine ring
spatially spreads out and covers the wide area over the long-range
from the Pd center. On the other hand, steric congestion around
the Pd coordination sphere is essentially the same as that of corre-
sponding pyridine complex Pd(OAc)2(Py)2,12 implying that the large
substituent at the 3-position would not obstruct the metal center.
completely into Pd black13 and the oxidation stopped (run 2).
Pd(OAc)2(3-PhPy)2 and Pd(OAc)2(3,5-diPhPy)2 also resulted in
complete Pd black formation within 6 h and afforded acetophenone
in 34 and 32% yields, respectively (run 3 and 4). The reaction with
Pd(OAc)2(Bipy)2 was very sluggish, although no Pd black formed
(run 5). In contrast, Pd(OAc)2(1)2 afforded acetophenone in 87%
yield without the Pd black formation (run 6) and the product was
isolated in 97% yield after a prolonged reaction time (130 h). The
higher dendritic analogue Pd(OAc)2(2)2 is a more efficient catalyst
(run 7), realizing the highest TON ) 1480 with S/C ) 2000 (run
8). Pd(OAc)2(4)2 also catalyzed the reaction without the Pd black
9
6554
J. AM. CHEM. SOC. 2004, 126, 6554-6555
10.1021/ja031936l CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Effect of Pyridine Ligands on Palladium-Catalyzed Air
The marked effects of 1 and 2 over pyridine (Py) as a ligand at
a high S/C ratio were also observed in the oxidation of other
alcohols under air (Table 2). Pd(OAc)2(1)2 and Pd(OAc)2(2)2
maintained the catalytic activity to provide the products in high
yields, while Pd(OAc)2(Py)2 afforded the Pd black within 1-4 h.
A similar oxidation of allylic and primary aliphatic alcohols with
Pd(OAc)2(1)2 and Pd(OAc)2(Py)2 resulted in low yields (5-30%).
Further studies on prevention of catalyst deactivation are now in
progress.
Oxidation of Alcoholsa
time
(h)
Pd black
c
run
alcohol
ligandb
none
% yield
formationd
1
1-phenylethanole
7
24
2
trace
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
+
+
+
+
-
-
-
trace
23
23
34
32
3
2
Py
24
6
6
3
4
5
3-PhPy
3,5-diPhPy
Bipy
24
72
24
72
24
72
96
24
72
2
5
3
3
96
96
96
Supporting Information Available: Experimental procedures and
spectroscopic data for 1-6 (PDF) and crystallographic data for
Pd(OAc)2(1)2 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
6
6
7
1
2
57
87
78
>99 (95)
74
44
63
21
23
8f
9
2
4
References
(1) (a) van Leeuwen, P. W. N. M. Appl. Catal., A 2001, 212, 61-81. (b)
Parshall, G. D.; Ittel, S. D. Homogeneous Catalysis; John Wiley and
Sons: New York, 1992; p 168.
(2) (a) Tromp, M.; Sietsma, J. R. A.; van Bokhoven, J. A.; van Strijdonck,
G. P. F.; van Haaren, R. J.; van der Eerden, A. M. J.; van Leeuwen, P.
W. N. M.; Koningsberger, D. C. Chem. Commun. 2003, 128-129. (b)
Tsuji, J. Palladium Reagents and Catalysts; Wiley-VCH: New York,
1995. (c) Heck, R. F. Palladium Reagents in Organic Syntheses; Academic
Press: New York, 1985.
10
11h
12
13
14
15
16
2-octanolg
Py
Py
3-PhPy
3,5-diPhPy
1
2
4
21
15
69
79 (75)
67
(3) (a) Sheldon, R. A.; Arends, I. W. C. E.; ten Brink, G.-J.; Dijksman, A.
Acc. Chem. Res. 2002, 35, 774-781. (b) Sheldon, R. A.; Arends, I. W.
C. E.; Dijksman, A. Catal. Today 2000, 57, 157-166. (c) Marko´, I. E.;
Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch, C. J. Science 1996,
274, 2044-2046.
(4) (a) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org. Chem. 1999,
64, 6750-6755. (b) Blackburn, T. F.; Schwartz, J. J. Chem. Soc., Chem.
Commun. 1977, 157-158. (c) Larock, R. C.; Peterson, K. P. J. Org. Chem.
1998, 63, 3185-3189. (d) Steinhoff, B. A.; Stahl, S. S. Org. Lett. 2002,
4, 4179-4181. (e) Schultz, M. J.; Park, C. C.; Sigman, M. S. Chem.
Commun. 2002, 24, 3034-3035. (f) Hallman, K.; Moberg, C. AdV. Synth.
Catal. 2001, 343, 260-263. (g) Kaneda, K.; Fujii, M.; Morioka, K. J.
Org. Chem. 1996, 61, 4502-4503.
(5) (a) ten-Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Science 2000,
287, 1636-1639. (b) ten-Brink, G.-J.; Arends, I. W. C. E.; Hoogenraad,
M.; Verspui, G.; Sheldon, R. A. AdV. Synth. Catal. 2003, 345, 1341-
1352. (c) Jensen, D. R.; Schultz, M. J.; Mueller, J. A.; Sigman, M. S.
Angew. Chem., Int. Ed. 2003, 42, 3810-3813. (d) Kakiuchi, N.; Maeda,
Y.; Nishimura, T.; Uemura, S. J. Org. Chem. 2001, 66, 6620-6625. (e)
Kakiuchi, N.; Nishimura, T.; Inoue, M.; Uemura, S. Bull. Chem. Soc.
Jpn. 2001, 74, 165-172.
(6) (a) Mandal, S. K.; Jensen, D. R.; Pugsley, J. S.; Sigman, M. S. J. Org.
Chem. 2003, 68, 4600-4603. (b) Bagdanoff, J. T.; Ferreira, E. M.; Stoltz,
B. M. Org. Lett. 2003, 5, 835-837. (c) Jensen, D. R.; Sigman, M. S.
Org. Lett. 2003, 5, 63-65. (d) Mueller, J. A.; Jensen, D. R.; Sigman, M.
S. J. Am. Chem. Soc. 2002, 124, 8202-8203. (e) Ferreira, E. M.; Stoltz,
B. M. J. Am. Chem. Soc. 2001, 123, 7725-7726.
a At 80 °C with S/C ) 1000 in toluene under air (balloon). b Ligand of
the Pd(OAc)2(ligand)2. c Determined by GC. Isolated yields in parentheses.
d +: Complete Pd black formation. -: No Pd black formation. e 1-
Phenylethanol (2 mmol), Pd(OAc)2(ligand)2 (0.002 mmol), NaOAc (0.2
mmol), and toluene (1 mL). f S/C ) 2000, Pd(OAc)2(2)2 (0.001 mmol).
g 2-Octanol (2.5 mmol), Pd(OAc)(ligand)2 (0.0025 mmol), NaOAc (0.25
mmol), and toluene (0.8 mL). h With additional pyridine (0.02 mmol).
Table 2. Palladium-Catalyzed Air Oxidation of Various Alcoholsa
time
Pd black
c
run
alcohol
benzyl alcohol
S/C ligandb (h) % yield formation
1
2
3
4
5
6
7
8
9
1000
1000
1250
2
1
Py
2
1
Py
2
1
Py
1
Py
1
Py
1
Py
1
Py
1
Py
48
48
2
72
72
8
96
96
4
96
4
44
3
45
1
96
2
74
3
78d
74d
23d
72
52
27
63
65
18
89
32
81
32
74d
24d
72
24
65
17
-
-
+
-
-
+
-
-
+
-
+
-
+
-
+
-
+
-
+
2-heptanol
3-octanole
10 3,3-dimethyl-2-butanol
11
12 1-(4-methylphenyl)ethanol
13
14 4-isopropylbenzyl alcohol
15
16 2-hexanol
17
1000
750
(7) Very recently, Sigman et al. successfully carried out the oxidation with
S/C ) 1000 (under 1 atm molecular oxygen) and S/C ) 200 (under 1
atm air) by using N-heterocyclic carbene a Pd complex/3 Å molecular
sieves/HOAc catalyst system.5c
720
(8) (a) Watson, M. D.; Fechtenkotter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267-1300. (b) Berresheim, A. J.; Muller, M.; Mu¨llen, K. Chem. ReV.
1999, 99, 1747-1785 and references therein.
1000
(9) Under 30 bar5a air and 50 bar5b O2/N2 (8/92), TONs of 400 and 1000
were realized, respectively, with water-soluble Pd catalyst.
18 trans-2-methylcyclohexanol 1300
19
(10) With a heterogeneous Pd catalyst, Kaneda et al. reported very high TON
(236 000) in oxidation of 1-phenylethanol at 160 °C under molecular
oxygen. Mori, K.; Yamaguchi, K.; Hara, T.; Mizugaki, T.; Ebitani, K.;
Kaneda, K. J. Am. Chem. Soc. 2002, 124, 11572-11573.
(11) Crystal data of Pd(OAc)2(1)2: monoclinic, space group C2/c, colorless,
a ) 36.15(9) Å, b ) 10.09(5) Å, c ) 20.68(6) Å, â ) 98.94(4)°, V )
7451.5(47) Å3, Z ) 4, T ) -160 °C, dcalcd ) 1.257 g cm-3, µ(Mo, KR)
) 3.05 cm-1, R1 ) 0.067, wR2 ) 0.093, GOF ) 1.373.
a At 80 °C with Pd(OAc)2(ligand)2 (0.002 mmol) and NaOAc (0.1 equiv
to the alcohol) in toluene (1 mL) under air (balloon). b Ligand of the
Pd(OAc)2(ligand)2. c Determined by GC. d Corresponding aldehyde. e Tolu-
ene (0.8 mL).
formation (run 9). However, the use of Pd(OAc)2(5)2 and a mixture
of Pd(OAc)2 and 6 (6/Pd ) 2) as the catalyst resulted in trace yields
due to the Pd black formation. Thus, the spatially spread moiety at
the 3-position, not at the 2-position, effectively suppresses the Pd
black formation and maintains the catalytic activity for a long time.14
Similar suppression was also observed in oxidation of 2-octanol
(runs 10-16). Pd(OAc)2(Py)2 afforded Pd black within 2 h (run
10). It is noteworthy that even if an excess of pyridine (8 equiv)
with respect to Pd(OAc)2(Py)2 was added (N/Pd ) 10) in this
system, Pd black formed within 5 h (run 11).
(12) As for the molecular structure of Pd(OAc)2(Py)2, see: Kravtsova, S. V.;
Romm, I. P.; Stash, A. I.; Belsky, V. K. Acta Crystallogr., Sect. C 1996,
52, 2201-2204.
(13) Pd black formation is affected considerably by the S/C ratio. Maintaining
the same concentration of Pd(OAc)2(Py)2 as that in run 2 in Table 1 led
to observation of complete Pd black formation within 2 h (S/C ) 1000,
run 2 in Table 1), 3 h (S/C ) 700), 5 h (S/C ) 400), and 7 h (S/C )
100), but no Pd black formation was observed with S/C ) 50.
(14) Such a “long-range steric effect” was also observed in a rhodium-catalyzed
hydrosilylation of ketones with a bowl-shaped phoshine.15
(15) Niyomura, O.; Tokunaga, M.; Obora, Y.; Iwasawa, T.; Tsuji, Y. Angew.
Chem., Int. Ed. 2003, 42, 1287-1289.
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