4502
J . Org. Chem. 1996, 61, 4502-4503
Ta ble 1. Oxid a tion of Cin n a m yl Alcoh ol w ith Va r iou s
P d Ca ta lysts in th e P r esen ce of Molecu la r Oxygen a
High ly Selective Oxid a tion of Allylic
Alcoh ols to r,â-Un sa tu r a ted Ald eh yd es
Usin g P d Clu ster Ca ta lysts in th e P r esen ce
of Molecu la r Oxygen
yield of
conversion cinnamaldehyde
run
catalyst
(%)
(%)
TOFb
1
2
3
4
Pd4Phen2(CO)(OAc)4
Pd4Phen2(CO)(OAc)4
Pd4Phen4(CO)(OAc)4
Pd4(CO)4(OAc)‚
2AcOH [PCA]
Pd(OAc)2
100
18
1
quantitative
14
1
30
4
0
Kiyotomi Kaneda,* Masanori Fujii, and
Kengo Morioka
c
42
38
13
Department of Chemical Engineering,
Faculty of Engineering Science, Osaka University,
Toyonaka, Osaka 560, J apan
5
6
7
30
39
15
28
29
11
8
9
3
Pd/C
PdCl2
Received March 19, 1996
a
Pd catalyst: 0.05 mmol, cinnamyl alcohol: 1.5 mmol, solvent:
b
Much attention has been paid to metal cluster com-
plexes having metal-metal bonds because unprecedented
chemical reactions might occur on multimetallic centers
of the cluster catalysts, and also because they are model
compounds for heterogeneous metal catalysts.1 Pal-
ladium complexes have unique catalytic activities and
thus have been widely used in many organic syntheses.2
However, little is known about selective organic trans-
formations assisted by Pd cluster complexes.3 We wish
to report selective dehydrogenation of allylic alcohols to
R,â-unsaturated aldehydes catalyzed by Pd4 clusters in
the presence of molecular oxygen (eq 1). To the best of
our knowledge, this is the first example of selective
oxidations with molecular oxygen using metal cluster
complex catalysts.
benzene, 5 mL, 50 °C, 24 h, O2 atmosphere. TOF ) alde-
hyde[mol]/Pd[mol]. c N2 atmosphere.
the filtrate was subjected to column chromatography on
silica (hexane:ethyl acetate (10:1)) to yield the product
of cinnamaldehyde (0.56 g, 93%).
Oxidations of cinnamyl alcohol using various Pd com-
pounds in a benzene solvent with oxygen yielded the
results shown in Table 1. Pd4Phen2(CO)(OAc)4 complexes
had the highest catalytic activity for the dehydrogenative
oxidation (a quantitative yield of cinnamaldehyde, run
1).6 The use of Pd4Phen4(CO)(OAc)4, PCA, and Pd(OAc)2
resulted in low yields of cinnamaldehyde. Pd/C and
PdCl2 were poor catalysts for the selective oxidation.
Other metal carbonyl clusters such as Rh6(CO)16, Ru3-
(CO)12, Fe3(CO)12, and Os3(CO)12 showed low catalytic
activities for the oxidation of allylic alcohols under the
present reaction conditions. In order to examine the
additive effect of amines and pyridine derivatives, cin-
namyl alcohol was oxidized by oxygen using mixtures of
PCA and basic additives at 50 °C. Triethylamine and
N,N,N′,N′-tetramethylpropanediamine additives were
also effective. Both pyridine and bipyridine gave low
yields of cinnamaldehyde.
Oxidations of various allylic alcohols with the Pd4-
Phen2(CO)(OAc)4 cluster in benzene under an oxygen
atmosphere yielded the results in Table 2. Many primary
allylic alcohols were oxidized to yield the corresponding
R,â-unsaturated aldehydes in high yields together with
small amounts of hydrogenation products. Generally, the
Pd4 cluster catalyst oxidized allylic alcohols possessing
disubstituted olefinic moieties at faster rates than trisub-
stituted allylic ones. When compared with other catalyst
systems using molecular oxygen,7 this Pd4 cluster catalyst
has the highest selectivity for dehydrogenation of allylic
alcohols to R,â-unsaturated aldehydes. In oxidation to
two E/ Z iosmers (nerol and geraniol), we observed
retention of the stereochemistry in the product R,â-
unsaturated aldehydes (runs 13 and 14). Oxidation of
nerol to neral was slower than that of geraniol. This
might be explained by the steric congestion in active Pd
intermediate species.8 A (E,E)-dienol of sorbic alcohol
was also oxidized stereoselectively to give (E,E)-2,4-
hexadienal in high yield (eq 2). The Pd4Phen2(CO)(OAc)4
Pd4 clusters were prepared by known procedures.4
Treatment of an acetic acid solution of Pd(OAc)2 with CO
gave Pd4(CO)4(OAc)‚2AcOH (PCA). Reaction of PCA
(0.04 mmol) with 1,10-phenanthroline (Phen, 0.08 mmol)
under a nitrogen atmosphere at room temperature for
0.5 h afforded a quantitative yield of Pd4Phen2(CO)-
(OAc)4. [Anal. Calcd for Pd4C33H28N4O9: C, 37.73; H,
2.67; N, 5.34. Found: C, 37.41; H, 2.98; N, 5.25.] Pd4-
Phen4(CO)(OAc)4 was also obtained by a similar treat-
ment of PCA with 4 equiv of Phen. A typical procedure
for the oxidation reaction is as follows. Benzene (15 mL)
and cinnamyl alcohol (0.61 g, 4.5 mmol) was first added
to a reaction vessel containing Pd4Phen2(CO)(OAc)4 (0.04
mmol). The mixture was then stirred at 50 °C under an
oxygen atomsphere.5 After 24 h, the reaction mixture
was cooled, Pd clusters were removed by filtration, and
(1) (a) Gates, B. C. Chem. Rev. 1995, 95, 511. (b) Lewis, L. N. Chem.
Rev. 1993, 93, 2693. (c) Knozinger, H. Cluster Models for Surface and
Bulk Phenomena; Plenum: New York, 1992. (d) Shriver, D. F.; Kaesz,
H. D.; Adams, R. D. The Chemistry of Metal Cluster Compounds; VCH
Publishers: New York, 1990.
(2) (a) Wilkinson, S. G.; Stone, F. G. A.; Abel, E. W., Eds. Compre-
hensive Organometallic Chemistry II; Pergamon Press Ltd.: New York,
1995; Vols. 7-9. (b) Tsuji, J . Palladium Reagents and Catalysts, J ohn
Wiley, New York, 1995. (c) Heck, R. F. Palladium Reactions in Organic
Syntheses; Academic Press: London, 1985.
(3) (a) Moiseev, I. I.; Stromnova, T. A.; Vargaftig, M. N. J . Mol. Catal.
1994, 86, 71. (b) Vargaftik, M. N.; Zagorodnikov, V. P.; Storarov, I.
P.; Moiseev, I. I. J . Mol. Catal. 1989, 53, 315. (c) Stolyarov, I. P.;
Vargaftik, M. N.; Nefedov, O. M.; Moiseev, I. I. Kinet. Katal. 1982, 23,
376. (d) Blackburn, T. F.; Schwartz, J . J . Chem. Soc., Chem. Commun.
1977, 157.
(5) When scale up above the 15 mL reaction volume is carried out,
the reaction vessel should be placed behind an appropriate shield.
(6) Moiseeve et al. have reported the catalysis of Pd4 complexes for
acetoxylation of olefins and toluene and for hydrogenation of olefins;
the Pd4Phen2(CO)(OAc)4 complex had the highest catalytic activity for
the above reactions.3c
(7) (a) Bi-Pt/Alumina: Mallat, T.; Bodnar, Z.; Hug, P.; Baiker, A.
J . Catal. 1995, 153, 131. (b) Rh6(CO)16
: Kaneda, K.; Miyoshi, T.;
(4) (a) Moiseev, I. I. J . Organomet. Chem. 1995, 488, 183. (b)
Moiseev, I. I.; Stromnova, T. A.; Vargaftig, M. N.; Mazo, G. J . J . Chem.
Soc., Chem. Commun. 1978, 27.
Imanaka, T. J . Mol. Catal. 1991, 64, L7. (c) MnO2: Xiao-yi, X.;
Prestwitch, G. D. Synth. Commun. 1990, 20, 3125. (d) RuO2: Mat-
sumoto, M.; Watanabe, N. J . Org. Chem. 1984, 49, 3435.
S0022-3263(96)00530-0 CCC: $12.00 © 1996 American Chemical Society