5664
J . Org. Chem. 1997, 62, 5664-5665
Ta ble 1. Ca ta lytic Activities of Ar ylbor on Com p ou n d s in
th e Op p en a u er Oxid a tion of 2a
Bis(p en ta flu or op h en yl)bor in ic Acid a s a
High ly Effective Op p en a u er Oxid a tion
Ca ta lyst for Allylic a n d Ben zylic Alcoh ols
Kazuaki Ishihara,† Hideki Kurihara, and
Hisashi Yamamoto*
Graduate School of Engineering, Nagoya University,
Research Center for Advanced Waste and Emission
Management (ResCWE), Nagoya University, and
CREST, J apan Science and Technology Corporation (J ST),
Chikusa, Nagoya 464-01, J apan
catalyst
(mol %)
conditions
(°C, h)
yield
(%)
entry
1
2
3
4
5
6
7
C6F5B(OH)2 (2)
1 (1)
80, 3
40, 3b
80, 3
40, 3b
80, 3
40, 3b
80, 3
0
92
>99
48
>99
58
47
Received J une 2, 1997
1 (2)
B(C6F5)3 (1)
B(C6F5)3 (2)
(3,5-(CF3)2C6H3)2BOH (1)
(3,4,5-F3C6H2)2BOH (2)
Oppenauer (OPP) oxidation is one of the most useful
methods for transforming secondary alcohols into ke-
tones.1,2 Functional groups such as carbon-carbon double
and triple bonds, aldehydes, amino groups, halogens, or
sulfur atom-containing groups are not affected by this
reaction, which is a great advantage over many oxygen-
transferring oxidation processes. In general, however,
it is difficult to oxidize primary alcohols to the corre-
sponding aldehydes by the OPP method. Although a
variety of basic organo- and inorganometallic reagents
have been reported as OPP catalysts, there are only a
few examples of the oxidation of primary allylic alco-
hols.3,4 For the selective oxidation of allylic alcohols in
the presence of saturated alcohols, activated MnO2 is still
one of the most useful reagents, despite the large amount
required.5 We report here that bis(pentafluorophenyl)-
borinic acid (1) is a suitable OPP catalyst for primary
and secondary allylic and benzylic alcohols.6
a
Unless otherwise noted, the oxidation of 2 (1 equiv, 0.25 M)
was carried out in benzene in the presence of pivalaldehyde and
a catalyst. The oxidation of 2 (1 equiv, 0.5 M) in toluene.
b
Sch em e 1
a hydride acceptor in toluene or benzene. Representative
results are summarized in Table 1. As expected, the
catalytic activity of borinic acid 1 was much higher than
those of other diarylborinic acids (entries 2 and 3 versus
entries 6 and 7). In contrast, (pentafluorophenyl)boronic
acid was inert (entry 1). Their catalytic activities were
in proportion to their Lewis acidities. Surprisingly,
tris(pentafluorophenyl)boron,9 a commercially available
Lewis acid, was also active as a catalyst for the present
oxidations (entries 4 and 5). These results can be
understood by assuming that borinic acid 1, generated
from tris(pentafluorophenyl)boron in situ, acted as the
actual catalyst (Scheme 1). In fact, we determined by
19F NMR analyses that tris(pentafluorophenyl)boron
gradually decomposed to 1 and pentafluorobenzene, and
finally to (pentafluorophenyl)boronic acid, under these
reaction conditions. In general, triarylborons and di-
arylborinic acids with electron-withdrawing substituents
at their aryl groups are relatively stable in acidic aqueous
solutions, but are unstable in neutral and basic aqueous
solutions, to give arylboronic acids and arenes.
The decomposition of borinic acid 1 in OPP oxidation
may be caused by a nucleophilic attack on the boron atom
of 1 by water, alcohols, or aldehydes. To prevent the
decomposition of 1, several additives were examined in
the oxidation of 2 under milder conditions (1 mol % of 1,
ambient temperature). The results are summarized in
Table 2. Fortunately, the addition of magnesium sulfate
efficiently prevented the inactivation of 1 and obviously
promoted the oxidation; 2 was completely oxidized to the
Borinic acid 1 was prepared from the known chlorobo-
rane (C6F5)2BCl by hydrolysis with aqueous 2 N HCl.7
Compound 1 is a white, microcrystalline solid which can
be readily handled in air and is soluble in many organic
solvents. Furthermore, 1 is a stronger Lewis acid than
(pentafluorophenyl)boronic acid, but is weaker than
tris(pentafluorophenyl)boron.6
We first examined several arylboron compounds with
electron-withdrawing substituents at their aryl groups
as catalysts for the OPP oxidation of (S)-perillyl alcohol
(2). Catalysis was carried out using 1-2 mol % of the
catalysts in the presence of 6 equiv of pivalaldehyde8 as
† ResCWE, Nagoya University.
(1) Oppenauer, R. V. Rec. Trav. Chim. Pays-Bas 1937, 56, 137.
(2) Reviews, see: (a) Djerassi, C. Org. React. 1951, 6, 207. (b) de
Graauw, C. F.; Peters, J . A.; van Bekkum, H.; Huskens, J . Synthesis
1994, 1007.
(3) For Cp2ZrH2 (2 mol %, 130 °C), see: Ishii, Y.; Nakano, T.; Inada,
A.; Kishigami, Y.; Sakurai, K.; Ogawa, M. J . Org. Chem. 1986, 51,
240.
(4) For Zr(Ot-Bu)4 (20 mol %, 20 °C), see: Krohn, K.; Knauer, B.;
Ku¨pke, J .; Seebach, D.; Beck, A. K.; Hayakawa, M. Synthesis 1996,
1341.
(5) Fatiadi, A. J . Organic Syntheses by Oxidation with Metal
Compounds; Mijs, W. J ., de J onge, C. R. H. I., Eds.; Plenum Press:
New York, 1986; p 119.
(6) For the selective dehydration of anti aldols to R,â-enones
catalyzed by 1, see: Ishihara, K.; Kurihara, H.; Yamamoto, H. Synlett
1997, 597.
(7) (a) Chambers, R. D.; Chivers, T. J . Chem. Soc. 1964, 4782. (b)
Chambers, R. D.; Chivers, T. J . Chem. Soc. 1965, 3933.
(8) Pivalaldehyde was a more suitable oxidant than other aliphatic
aldehydes and ketones. Although benzaldehyde was also a good
oxidant, it was not easy to remove excess benzaldehyde and benzyl
alcohol produced after the oxidation
(9) (a) Ishihara, K.; Hanaki, N.; Yamamoto, H. Synlett 1993, 577.
(b) Ishihara, K.; Funahashi, M.; Hanaki, N.; Miyata, M.; Yamamoto,
H. Synlett 1994, 963. (c) Ishihara, K.; Hanaki, N.; Funahashi, M.;
Miyata, M.; Yamamoto, H. Bull. Chem. Soc. J pn. 1995, 68, 1721. (d)
Ishihara, K.; Hanaki, N.; Yamamoto, H. Synlett 1996, 721. (e) Parks,
D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118, 9440.
S0022-3263(97)00959-6 CCC: $14.00 © 1997 American Chemical Society