Efficient palladium-catalyzed C-O hydrogenolysis of benzylic alcohols and aromatic ketones with polymethylhydrosiloxane

Article abstract of DOI:10.1002/adsc.201200832

A simple method has been developed for the reductive deoxygenation of aromatic ketones and benzylic alcohols in the presence of polymethylhydrosiloxane (PMHS). The reductive deoxygenation of aromatic ketones and benzylic alcohols, including secondary alcohols, to the corresponding methylene hydrocarbons has been achieved in good to excellent yields using palladium chloride (PdCl₂) as catalyst and PMHS as hydride source. Such deoxygenations were successfully with aryl alkyl ketones and diaryl ketones, as exemplified by the reductive deoxygenation of acetophenone and benzopheneone, respectively. The corresponding benzylic alcohols and secondary alcohol analogues could also be converted into their respective methylene hydrocarbons by the PdCl₂/PMHS system.

Full text of DOI:10.1002/adsc.201200832

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DOI: 10.1002/adsc.201200832
À
Efficient Palladium-Catalyzed C O Hydrogenolysis of Benzylic
Alcohols and Aromatic Ketones with Polymethylhydrosiloxane
Hu Wang,a Li Li,a Xing-Feng Bai,a Jun-Yan Shang,a Ke-Fang Yang,a,*
and Li-Wen Xua,b,*
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal
University, Hangzhou, Peopleꢀs Republic of China
Fax : (+86)-571-2886-5135; phone: (+86)-571-2886-7756; e-mail: yangkefang@gmail.com or liwenxu@hznu.edu.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
b
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
E-mail: licpxulw@yahoo.com
Received: September 14, 2012; Revised: October 27, 2012; Published online: February 1, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200832.
Abstract: A simple method has been developed for
the reductive deoxygenation of aromatic ketones
and benzylic alcohols in the presence of polyme-
thylhydrosiloxane (PMHS). The reductive deoxyge-
nation of aromatic ketones and benzylic alcohols,
including secondary alcohols, to the corresponding
methylene hydrocarbons has been achieved in good
to excellent yields using palladium chloride (PdCl2)
as catalyst and PMHS as hydride source. Such de-
oxygenations were successfully with aryl alkyl ke-
tones and diaryl ketones, as exemplified by the re-
ductive deoxygenation of acetophenone and benzo-
pheneone, respectively. The corresponding benzylic
alcohols and secondary alcohol analogues could
also be converted into their respective methylene
hydrocarbons by the PdCl2/PMHS system.
the past decades, PMHS has been used in the hydrosi-
lylation or reduction of carbonyl compounds,[3]
imines,[4] olefins,[5] or alkynes[6] that require activation
with a transition metal complex, fluoride ion or
a Lewis acid.[3–7] We have also previously reported
that the combinational use of polymethylhydrosilox-
ane and malononitrile as reductive system could allow
for the bismuth-catalyzed regioselective formation of
substituted ketones from enones under mild condi-
tions.[8]
Although the palladium-catalyzed selective 1,4-re-
duction of a,b-unsaturated ketones in the presence of
PMHS is known,[9] the 1,2-reduction of ketones is not
an easy task for palladium catalysis. Thus we are in-
terested in the development of a highly efficient palla-
dium catalyst encapsulated in a polysiloxane gel to
reduce ketones to alcohols or alkanes completely in
the presence of PMHS. In this context, Maleczka
et al.[10] has reported very recently an important and
Keywords: deoxygenation; hydrogenolysis; palladi-
um; reduction; polymethylhydrosiloxane (PMHS);
silanes
effective catalytic system with PdACTHNUTRGENUG(N OAc)2 (5 mol%)
and PMHS (2.5 equiv.) in conjunction with aqueous
KF (4 equiv.) and a catalytic amount of aromatic chlo-
ride (10 mol%) that promoted the highly chemoselec-
tive deoxygenation of aromatic ketones. The ability to
Polymethylhydrosiloxane (PHMS) is an important re- achieve such a reductive deoxygenation is highly at-
ducing agent for eco-friendly reductive processes on tractive, since it allows the rapid, efficient, and selec-
organic functional groups because it is a safe, easy-to- tive construction of alkanes in excellent yields. Al-
handle, cheap and environmentally benign reagent. It though the preparative advantages of fluoride activa-
is a by-product of the organosilicon industry and an tion in terms of reaction rate, generality, and stereose-
inexpensive hydrosilane that can be employed as the lectivity in the synthetic applications of hydrosilane
stoichiometric reducing agent in organic synthesis.[1] have been known for many years, the cost and corro-
In the context of PMHS, the development of cleaner, siveness of fluoride sources as well as their incompati-
more chemoselective, and safer synthetic methods bility with common protective groups limit the wide-
and technologies for the reduction of functional spread application of this method. In view of the
groups to meet ever stricter environmental regula- power of palladium catalysis and the importance of
tions remains an active area in organic chemistry.[2] In the deoxygenation reaction of aromatic ketones,[11] we
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341

Hu Wang et al.
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solvents, of which methanol proved to be the most ef-
fective. It was thus clear that the methanol was influ-
encing the reactivity of the palladium-catalyzed deox-
ygenation of benzylic alcohol. Interestingly, employ-
ing 1 mol% of PdCl2 also led to a reductive deoxyge-
nation in high yield. The observation that 5 mol% of
PdCl2 resulted in complete conversion and excellent
yield indicates that the reaction conditions could be
suitable for a broad substrate scope.
Under our optimized conditions with 5 mol% of
PdCl2 and 3 equiv. of PMHS, the generality of the re-
ductive deoxygenation reaction was investigated using
a number of substituted benzylic alcohols. As shown
in Table 1, the reduction reaction of various aromatic
alcohols using the same conditions as in the case of
benzylic alcohol (1a) affords the desired alkanes in
excellent yields. For example, the reduction of o-tolyl-
methanol (1b), m-tolylmethanol (1c), or p-tolylmetha-
nol (1d), gives o-xylene (3b), m-xylene (3c), or p-
xylene (3d) in excellent yields, respectively (entries 1–
Scheme 1. Comparison of conventional palladium-based de-
oxygenation approaches with the PdCl2/PMHS/MeOH
system used in this work
À
sought to develop a new strategy to achieve the C O 3). The more electron-rich substrates (1e–1h) afford
hydrogenolysis of benzylic alcohols and aromatic ke- the desired products in almost quantitative yields (en-
tones through fluoride-free palladium catalysis tries 4–7). As summarized in Table 1, benzylic alco-
(Scheme 1). With the success of the reductive deoxy- hols substituted at the phenyl ring with MeO, Me,
genation described above, we could investigate the phenyl, hydroxy moieties were all converted into the
feasibility and potential of the preparation of various corresponding alkanes completely. Unfortunately,
aromatic alkanes via palladium-catalyzed reductive nitro and halide substituents at the phenyl ring show
deoxygenation.
limited selectivity and led to mixtures under the same
In our initial investigation of the palladium-cata- reaction conditions because the nitro (NO2) or halide
lyzed oxidative esterification of benzylic alcohol in (Cl) group could be reduced partially to amine (NH2)
the presence of PMHS,[12] we noted a significant dif- or hydrogen (H), respectively (Scheme 3). It is note-
ference in the reaction results in accord with the reac- worthy that for 4-(hydroxymethyl)phenol (1i), p-
tion conditions. When PMHS was used an additive in cresol was also isolated in 99% yield (entry 8). We
the PdACHTUNGTRENNUNG(OAc)2-catalyzed transformation of benzylic al- have also investigated the reductive deoxygenation of
cohol (1a), the aerobic oxidation reaction occurred secondary alcohols, such as 1-phenylethanol and ana-
smoothly to afford aldehyde and ester. However, logues. The results indicated that secondary aryl alco-
treatment of benzylic alcohol with a large amount of hols could also be successfully transformed into the
PMHS (3 equiv.) in the presence of PdCl2 afforded corresponding products in good to excellent yields
toluene by deoxygenation of benzylic alcohol (entries 9–19). Generally, aromatic alcohols bearing
(Scheme 2). Subsequently, we examined a variety of electron-rich substituents proceeded more efficiently
than those of bearing electron-donating substituents.
Although some functional groups, such as nitro and
halide moieties, exhibited remarkable and negative
electronic effects, we were pleased to find that the
present palladium-catalyzed reductive deoxygenation
proved to be a highly efficient procedure of the prep-
aration of alkanes from aromatic alcohols.
As can be seen from Table 1, the scope of this Pd-
catalyzed reductive deoxygenation of benzylic alco-
hols with PMHS appears to be very broad, and the re-
action is satisfactory even with versatile secondary al-
cohols. Since the ability of PMHS to reduce ketones
to alcohols has been recognized in the past,[13] we hy-
pothesized this reductive system would be expected
to achieve the reductive deoxygenation of aromatic
ketones. Then the palladium-catalyzed reductive de-
oxygenation of aromatic ketones was also tested with
Scheme 2. Transformation of benzyl alcohol catalyzed by
palladium in the presence of PMHS.
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Efficient Palladium-Catalyzed C O Hydrogenolysis of Benzylic Alcohols and Aromatic Ketones
Table 1. Reductive deoxygenation of alcohols by palladium
Table 1. (Continued)
in the presence of PMHS.[a]
PdCl2 as catalyst and PMHS as reducing agent. As
shown in Scheme 4, the reaction conditions were
found to be very effective in the reaction of aromatic
ketones, and the corresponding alkanes were obtained
in good to excellent yields. Notably, the selectivities
of all these substrates shown in Scheme 4 were excel-
lent, as no alcohol intermediate was detected. The ex-
periments showed that the reactivities of ketones
were the same as those of the corresponding alcohols.
Therefore, the excellent chemoselectivity and conver-
sion of present process make it an attractive and prac-
tical alternative to the use of aryl chlorides as addi-
tives and large amounts of KF as activator of PMHS.
Similarly to alcohol substrates, the reductive deoxy-
genation of related aromatic ketones containing the
nitro or halide substituent resulted in low selectivity
under the same reaction conditions. In addition, it is
[a]
Reaction conditions: benzylic alcohols (1 mmol), PMHS
(3 equiv.), PdCl2 (5 mol%), in MeOH (2 mL), at 408C.
The isolated yields given in this table are the average
values of three runs.
found that the reactivity of unconjugated aromatic ke- important information for the mechanistic process of
tones with an isolated carbonyl was not good, for ex- palladium-catalyzed reductive deoxygenation, espe-
ample, the reduction of 2-indanone (4n, Scheme 5) cially the by-product of methyl ether supported that
led to low yield of alkane and poor chemoselectivity. the possible nucleophilic addition of methanol had oc-
However, these unexpected products provided some curred in the transformation step of the key inter-
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Hu Wang et al.
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Scheme 3.
Scheme 4. Reductive deoxygenation of aromatic ketones by
palladium in the presence of PMHS.
Scheme 6.
Scheme 5.
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Adv. Synth. Catal. 2013, 355, 341 – 347

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Efficient Palladium-Catalyzed C O Hydrogenolysis of Benzylic Alcohols and Aromatic Ketones
Scheme 8.
hydride intermediate (I). If toluene was used as sol-
vent instead of methanol, a trace of ketone was de-
tected as major by-product (see the Supporting Infor-
mation), which suggests that the step of oxidative ad-
dition was possible. In the key step, the HCl generat-
ed from PdCl2 and PMHS in MeOH facilitates the
Scheme 7. Possible explanation for the palladium-catalyzed
reductive deoxygenation of aromatic alcohols and ketones.
mediate. In addition, even though we did not examine formation of alkane through a possible chloride inter-
all aromatic ketones containing reducible functional mediate or corresponding halogenated palladium in-
groups, such as amide, cyano, ester, carboxylic acid, termediate (II). Scheme 8 shows that the reduction of
sulfoxide, alkeneyl, or alkynyl moieties, we evaluated benzyl chloride was easy in the presence of the PdCl2/
the reactivity of these functional groups in this reduc- PMHS system and indicted that the reductive deoxy-
tion. Except for alkenyl and alkynyl moieties, amide, genation of alcohols or ketone via a chloride inter-
cyano, ester, carboxylic acid, and amine (for example, mediate was resonable. In addition, to elucidate the
1-phenylethanamine) were stable and tolerated under hydrogen source of the alkane in the reductive deoxy-
the reaction conditions. Also sulfoxide and epoxide genation, a deuterium incorporation experiment was
groups were reduced partially to sulfanes (see the carried out. When deuterium-labelled methanol
Supporting Information).
(CD3OD) was used as a solvent in this reaction,
It should be noted that benzylic ethers were suc- a 52% deuterium incorporation was detected (see the
cessfully converted to the corresponding alkanes via Supporting Information), which showed that the hy-
À
C O cleavage with complete conversion (Scheme 6). drogen of the alkane could come from methanol by
We also found that tertiary alcohols were converted hydride transfer between intermediate I and solvent.
to the corresponding alkanes easily under the same However, it is hard to draw a final conclusion to ex-
reaction conditions (Scheme 6), and the reduction was clude the presence of a carbocation intermediate[16] in
not limited to benzylic alcohols and ketones, allylic or the last step because the methyl ether products, for
propargylic alcohols were suitable substrates in this example, 4-(1-methoxyethyl)benzenamine or 1-
reaction (see the Supporting Information).
Although the mechanistic details of the palladium- spectively, in the deoxygenated reaction of 1-(4-nitro-
catalyzed deoxygenation of ketones or alcohols are phenyl)ethanol and 1-(2-chlorophenyl)ethanone
not yet clear, we envisaged the pathway of palladium- (Scheme 3 and Scheme 5).
catalyzed reductive deoxygenation to proceed in two In summary, we have demonstrated a new applica-
chloro-2-(1-methoxyethyl)benzene was detected, re-
steps on the basis of experimental data. In the first tion of palladium as reductive deoxygenation catalyst
step, we assumed that the palladium-catalyzed hydro- for the reduction of aromatic ketones and benzylic al-
silyation of aromatic ketones or oxidative addition of cohols in the presence of PMHS. The reductive deox-
Pd(0) to benzylic alcohols has occurred, respectively. ygenation of aromatic ketones and benzylic alcohols,
Then hydrogenolysis of the benzylic alcohol with the including secondary alcohols to the corresponding
aid of the PMHS/PdCl2 system took place to yield the methylene hydrocarbons has been achieved in good
corresponding chloride or halogenated palladium in- to excellent yields by PdCl2 catalyst and PMHS as hy-
termediate that was easily reduced to the hydrocar- dride source. Such deoxygenations were successfully
bons in the presence of a hydrogen source.[14]
A
achieved with aryl ketones and diaryl ketones, as ex-
postulated catalytic cycle for representative alcohols emplified by the reductive deoxygenation of aceto-
is shown in Scheme 7. Initially, the PdCl2 would be re- phenone and benzopheneone, respectively. The corre-
duced to metallic Pd(0)[14,15] with the aid of PMHS, sponding benzylic alcohols and secondary alcohol an-
which was supported by the evolution of H2. The by- alogues could also be converted into their respective
À
product of halogenated PMHS containing an Si Cl methylene hydrocarbons by PdCl2/PMHS system. Al-
moiety could readily react with methanol (solvent) to though the mechanistic details of the palladium-cata-
provide traces of HCl. Then, the benzylic alcohol lyzed deoxygenation of ketones or alcohols are not
would add oxidatively to Pd(0) to generate palladium yet clear, the method is of importance and utility in
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All reaction flasks and solvent were used directly. The hy-
drogenated silicone [viscosity (258C), mm2/MS: 10–50; hy-
droxyl content: 1.55–1.60; density (258C), gcmÀ3: 0.995–
1.015] was purchased from Cheng xing County Chemical
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1
formed over silica (200–300 mesh). H NMR and 13C NMR
spectra were recorded at 400 and 100 MHz, respectively, on
an Advance (Bruker) 400 MHz nuclear magnetic resonance
spectrometer, and were referenced to the internal solvent
signals. Thin layer chromatography was performed using
silica gel; GF254 TLC plates and visualized with ultraviolet
light. The products of the reductive deoxygenation reaction
were known and confirmed by GC-MS, and the usual spec-
tral methods (1H NMR).
General Procedure for Reductive Deoxygenation of
Aromatic Ketones or Benzylic Alcohols
Under a nitrogen atmosphere, the benzylic alcohol or aro-
matic ketone (1 mmol) was added to a dry tube containing
PdCl2 (5 mol% or 10 mol%), PMHS (0.2 mL: 3 equiv. for al-
cohols or 0.34 mL: 5 equiv. for ketones, calculated on the
À
hydrogen content of Si H) and methanol (2 mL). And then
the mixture was stirred at 408C for 12–24 h. After the com-
pletion of the reaction, the product was dissolve in n-
hexane, the reaction course was monitored by GC-MS or
TLC. The combined organic layers were dried (Na2SO4),
concentrated under vacuum, and purified by column chro-
matography on silica gel to gain the pure product. All the
products are confirmed by GC-MS, NMR, and IR, and rep-
resentative characterization data for products 3 are listed in
the Supporting Information.
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Acknowledgements
This project was supported by the National Natural Science
Foundation of China (No. 21173064 and 51203037) as well as
Zhejiang Provincial Natural Science Foundation of China
(Q12B020037), and the Program for Excellent Young Teach-
ers in Hangzhou Normal University (HNUEYT, JTAS 2011-
01-014) is appreciated. XLW is greatly indebted to Prof.
Chun-Gu Xia, Lanzhou Institute of Chemical Physics, Chi-
nese Academy of Sciences, for his help.
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