Catalytic hydrogen-transfer reactions of benzylic and allylic
alcohols with palladium compounds in the presence of vinyl
acetate or under an ethylene atmosphere
1
Masahiko Hayashi,* Kanako Yamada and Shu-zo Nakayama
Department of Chemistry, Faculty of Science, Yamaguchi University, Yamaguchi 753-8512,
Japan. E-mail: hayashi@po.cc.yamaguchi-u.ac.jp
Received (in Cambridge, UK) 21st February 2000, Accepted 29th March 2000
Published on the Web 19th April 2000
An efficient and simple process for conversion of benzylic
and allylic alcohols into their corresponding carbonyl
compounds has been developed using a catalytic amount
of a palladium compound (Pd(OAc)2 or Pd/C) and an
alkene (vinyl acetate or ethylene).
ucts being obtained. For example, in the case of alcohol 1,
diphenylmethane was obtained in 16–24% yield, whereas under
an ethylene atmosphere the formation of diphenylmethane was
not observed at all. Primary allylic alcohols such as cinnamyl
alcohol, geraniol and nerol were less reactive, although isom-
erization was not observed under the reaction conditions. The
corresponding α,β-unsaturated aldehydes were obtained in 64,
36 and 21% yields, respectively. Steroidal allylic alcohol was
also oxidized. The reaction of 11 with Pd/C under an ethylene
atmosphere gave 19-nortestosterone (12) in 84% yield and
ketone 13 in 5% yield. In this catalytic system the allylic alcohol
was oxidized whilst the non-allylic hydroxy group was not. This
is in contrast with the products obtained via equimolar reac-
tions. For example, Czernecki and co-workers reported that
both of the hydroxy groups in 11 were oxidized by treatment
with a stoichiometric amount of Pd(OAc)2.7 The reaction of 11
with 9 equiv. of activated MnO2 (in CHCl3, 50 ЊC, 23 h) gave 12
in 68% yield.
The transformation of alcohols into aldehydes and ketones is
one of the most fundamental reactions in organic synthesis.1
For this purpose, some stoichiometric oxidizing agents such as
chromium and manganese salts have previously been used.
However, these metal salts are usually toxic and hazardous and
they often cause environmental problems. Therefore catalytic
processes using oxygen or aqueous H2O2 as oxidizing agents
with the aid of less toxic metal complexes are desirable from
an environmental viewpoint. Recently, Markó et al. reported an
efficient catalytic system consisting of CuCl–phenanthroline–
K2CO3–DBADH2
(1,2-bis(tert-butoxycarbonyl)hydrazine)2a
and TPAP (tetrapropylammonium perruthenate) and MS 4 Å2b
using oxygen or air as an oxidant. Noyori and co-workers
developed organic solvent- and halide-free oxidation of alco-
hols with aqueous H2O2 using Na2WO4 and a phase-transfer
catalyst (PTC) system.2c With regard to a Pd() catalyzed pro-
cess, Uemura et al. reported Pd(OAc)2-catalyzed oxidation of
The reactions of cyclohex-2-en-1-ol (14) and cyclohex-2-en-
1-one (16) with Pd(OAc)2 are shown in Tables 2 and 3. The
reaction of 14 with 5 mol% of Pd(OAc)2 in the absence of
ethylene in a sealing tube gave 24% phenol (15), 40% cyclo-
hexanone (17) and 36% cyclohexanol (18). On the other hand,
when the reaction was performed under an ethylene atmos-
phere, the yield of phenol was increased to 68% (cyclohexanone
17, 28%; cyclohexanol 18, 4%). The formation of cyclohex-2-
en-1-one (16) was not observed either case when the reactions
were completed.8 We then examined the reaction of cyclohex-
2-en-1-one (16) with Pd(OAc)2. The reaction of 16 was less
reactive than that of 14 and required a longer reaction time
(Table 3).
The reaction of 16 gave phenol (15) and cyclohexanone (17)
in almost equal yields (15, 53%; 17, 47%) in the absence of
ethylene, whereas under an ethylene atmosphere, the reactions
catalyzed by Pd(OAc)2 produced phenol in >99% yield (cyclo-
hexanone 17, <1%). A similar phenomenon was also observed
in the reaction when using Pd/C–ethylene. The palladium pre-
cipitate which was formed in the reaction using Pd(OAc)2 was
found to be reusable. The composition of this palladium
precipitate proved to be the same as palladium black, after
analysis by powder X-ray diffraction.
alcohols using molecular oxygen in the presence of MS 3 Å.2d
A
similar type of palladium-catalyzed oxidation of alcohols was
reported by Peterson and Larock.2e Hydrogen-transfer reac-
tions using transition metal catalysts have also been examined
intensively.3 During our study of C-glycoside synthesis using
-glucal,4 we found that the hydrogen-transfer reaction of
-glucal using a catalytic amount of a palladium compound
such as Pd(OAc)2 or Pd/C in the presence of vinyl acetate or
under an ethylene atmosphere, led to the catalytic and efficient
synthesis of 1,5-anhydrohex-1-en-3-uloses.5 This successful dis-
covery prompted us to examine the general utility of this hydro-
gen-transfer reaction for other alcohols, since this extremely
simple reaction could become a valuable method for the trans-
formation of alcohols into their corresponding carbonyl com-
pounds. Here we would like to report the hydrogen-transfer
reaction of some benzylic and allylic alcohols with a catalytic
amount of Pd(OAc)2 or Pd/C in the presence of vinyl acetate or
under an ethylene atmosphere.6
In summary, we have developed an efficient and catalytic
system which transforms benzylic and allylic alcohols into
their corresponding carbonyl compounds.† This extremely
simple process is not only economically viable but it is also
environmentally friendly. It is often the case that relatively
long reaction times are necessary for completion of the reac-
tion, therefore, the search for more reactive systems is now in
progress in our laboratory.
A variety of benzylic and allylic alcohols were converted into
the corresponding carbonyl compounds under the following
conditions; 20–50 weight% of 10% Pd/C, 3 equiv. of vinyl
acetate or an ethylene atmosphere in acetonitrile at 50–80 ЊC.
The results obtained are summarized in Table 1. In many cases,
the combination of Pd/C and ethylene exhibited the most effi-
cient conversion, although this proved to be slightly dependent
on the nature of the substrates. Secondary benzylic alcohols
such as benzhydrol, sec-phenethyl alcohol, and 2-hydroxy-2-
phenylacetophenone (benzoin) were converted into their
corresponding ketones in high yields (78–90% yield) under the
above conditions. It should be mentioned that in the absence of
ethylene, the secondary benzylic alcohols such as 1, 3 and 5
were dehydrogenated to the corresponding ketones 2, 4 and 6 in
lower yields, due to by-products such as dehydroxylation prod-
Experimental
General
1
All melting points are uncorrected. H and 13C NMR spectra
were recorded on a Bruker Avance 400S spectrometer (400 and
DOI: 10.1039/b001385o
J. Chem. Soc., Perkin Trans. 1, 2000, 1501–1503
This journal is © The Royal Society of Chemistry 2000
1501
Hayashi, Masahiko
