2,3-Dichloro-5,6-dicyano-1,4-benzoquinone ((DDQ)/tert-butyl nitrite/oxygen: A versatile catalytic oxidation system

Article abstract of DOI:10.1002/adsc.201100429

A new catalytic oxidation system using catalytic amounts of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tert-butyl nitrite with molecular oxygen serving as the environmentally benign, terminal oxidant has been developed. This aerobic catalytic system was established for the selective oxidation of non-sterically hindered benzylic alcohols and electron-rich benzyl methyl ethers, and successfully extended to an application in the oxidative deprotection of PMB ethers to the alcohols in high selectivity. Copyright

Full text of DOI:10.1002/adsc.201100429

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DOI: 10.1002/adsc.201100429
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tert-Butyl
Nitrite/Oxygen: AVersatile Catalytic Oxidation System
Zhenlu Shen,a Jialiang Dai,a Jie Xiong,a Xijun He,a Weimin Mo,a,* Baoxiang Hu,a
Nan Sun,a and Xinquan Hua,b,*
a
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, Peopleꢀs
Republic of China
Fax : (+86)-571-8832-0103; phone: (+86)-571-8832-0103; e-mail: mowm@zjut.edu.cn or xinquan@zjut.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
Received: May 25, 2011; Published online: November 7, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100429.
Abstract: A new catalytic oxidation system using cat- tron-rich benzyl methyl ethers, and successfully ex-
alytic amounts of 2,3-dichloro-5,6-dicyano-1,4-benzo- tended to an application in the oxidative deprotec-
quinone (DDQ) and tert-butyl nitrite with molecular tion of PMB ethers to the alcohols in high selectivity.
oxygen serving as the environmentally benign, termi-
nal oxidant has been developed. This aerobic catalyt- Keywords: alcohols; deprotection; 2,3-dichloro-5,6-
ic system was established for the selective oxidation dicyano-1,4-benzoquinone
of non-sterically hindered benzylic alcohols and elec- oxygen
(DDQ);
oxidation;
Introduction
Previously, we had successfully developed an effi-
cient aerobic oxidation of alcohols with a transition
Oxidation reactions play an important role in organic metal-free system, TEMPO/Br2/NaNO2.[3] The key
chemistry research and the organic chemical industry, point of this aerobic oxidation is that NaNO2 acted as
and many oxidation methodologies and reagents have the equivalent of NO/NO2, which can efficiently acti-
been developed to date.[1] Although numerous stoi- vate molecular oxygen. With this concept, some aero-
chiometric inorganic oxidants have been traditionally bic alcohol oxidation systems have been developed
employed in oxidation reactions, there are considera- and also extended to other reactions.[4] Our later stud-
ble drawbacks such as their high cost, toxicity, and se- ies in this area were narrowly limited in the TEMPO-
rious environmental effluents. To meet the require- catalyzed selective oxidation of alcohols.[5] We then
ment for sustainable and environmentally benign pro- wished to expand this transition metal-free system to
cesses, the use of molecular oxygen as the terminal other aerobic oxidations. Thus, some organic com-
oxidant in oxidation reactions has been received great pounds with redox ability came to our sight, for ex-
attraction in recent years due to its remarkable ad- ample,
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
vantages, including great abundance, lower cost, and (DDQ). It is well known that DDQ is a powerful oxi-
generating water as the only by-product. However, dant for a number of organic transformations.[6] As
the conflict between the triplet ground state of molec- the stoichiometric oxidant, it has been successfully ap-
ular oxygen and the singlet ground states of common plied in alcohol oxidation,[7] protecting group remov-
organic compounds results in a difficulty for the appli- al,[8] aromatization,[9] benzylic acetoxylation,[10] oxida-
cation of molecular oxygen as the direct oxidant. This tive coupling,[11] oxidative cyclization,[12] biaryl con-
is also the reason that molecular oxygen possesses a struction,[13] and many other reactions.
high oxidative potential but exhibits unusual stability. However, the use of stoichiometric DDQ may lead
Therefore, many efforts have been focusing on the ac- to purification difficulties because of the concomitant
tivation and the application of molecular oxygen by-product
2,3-dichloro-5,6-dicyano-hydroquinone
under mild condition.[2]
(DDHQ) and also resulted in cost concern. To avoid
both these disadvantages, a combination of a catalytic
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amount of DDQ and a stoichiometric amount of a Results and Discussion
less-expensive co-oxidant is an interesting alternative.
Chandrasekhar and co-workers reported that 4-meth- With our familiar TEMPO/TBN/O2 system in the al-
oxy- and 3,4-dimethoxybenzyl ethers could be depro- cohol oxidation, we explored the possibility that the
tected with a catalytic amount of DDQ by oxidative DDQ/TBN/O2 system can be applied to the oxidation
recycling of the DDHQ with FeCl3.[14] Mn
(OAc)3 has of alcohols. After detailed exploration of the reaction
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
also been developed as a mild oxidant for the regen- conditions with benzyl alcohol as the substrate, we
eration of DDQ from HDDQ, and the combination concluded that 5 mol% of DDQ and 5 mol% TBN in
of Mn
successfully applied to the deprotection of p-meth- dation.[20] With the optimized reaction conditions, the
oxybenzyl ethers.[15] Very recently, Floreancig and co- results of DDQ/TBN-catalyzed aerobic oxidation of
ACHTUNGTRENNUNG(OAc)3 (3 equiv.) and DDQ (0.1 equiv.) was 1,2-dichloroethane were suitable for the aerobic oxi-
ACHTUNGTRENNUNG
worker described that a number of oxidation reac- selected benzylic alcohols and their heteroaromatic
tions including oxidative cyclization, deprotection and analogues are summarized in Table 1. As can be seen
aromatization, can be effected by using catalytic from Table 1, benzylic alcohols 1a–1c, 1e–1f were
DDQ (0.15–0.2 equiv.) with a large excess of MnO2 fully converted into their corresponding benzalde-
(up to 6 equiv.) serving as terminal oxidant.[16] While hydes in high selectivity (entries 1–3, 5 and 6). How-
this manuscript was being prepared, the chemoselec- ever, sterically hindered 2-methylbenzyl alcohol (1d)
tive oxidation of alcohols employing catalytic quanti- could not achieve a high conversion even upon elon-
ties of DDQ as the oxidant and MnACTHNUTRGENUG(N OAc)3 as the co- gating the reaction time (entry 4). For heteroaromatic
oxidant was described by Helquist et al.[17] There is no analogues, 2-furfurylmethanol (1g) and 2-thiophene-
doubt that using of a great amount of these inorganic methanol (1h), this catalytic oxidation system can also
oxidants did cut down the cost but did not change the afford clean aldehyde products (entries 7 and 8). 4-
waste problem which resulted in the serious purifica- Methylthiobenzyl alcohol (1i), which possesses two
tion problem and the environmental effluent. Thus, it easily oxidizable functional groups, could be selective-
would be ideal if molecular oxygen can serve as the ly converted into 4-methylthiobenzaldehyde (2i) with-
terminal oxidant in DDQ-mediated reactions. Howev- out any observable sulfur oxidation (entry 9). For
er, molecular oxygen cannot directly oxidize DDHQ some secondary benzylic alcohols, a-phenylethanols
to DDQ under mild conditions.[18] In other words, a (1j–1m) could also be smoothly oxidized into their
redox compound should be employed as the bridge corresponding acetophenones (2j–2m) in high selec-
between molecular oxygen and the cycling DDHQ/ tivity (entries 10–13). When 1,2-diphenylethane-1,2-
DDQ. With our previous TEMPO-catalyzed aerobic diol (1o) was used as the substrate, it could be con-
À
oxidation, we have clearly understood that NO is an verted to benzoin in 95% isolated yield and no C C
effective reagent to activate molecular oxygen and bond cleavage was observed (entry 14). Surprisingly,
tert-butyl nitrite (TBN) can served as an NO equiva- 3-pyridinemethanol, which was a representative sub-
lent.[5b,c] Thus we wished to establish a new protocol strate in our previous TEMPO-catalyzed aerobic oxi-
with the DDQ/TBN/O2 system for the oxidation as dation systems, could not be oxidized in this catalytic
described in Scheme 1. Up to now, very limited exam- oxidation system.[21] When an aliphatic alcohol, such
ples of the DDQ-catalyzed aerobic oxidative dehy- as 1-octanol, was used as the substrate, no oxidation
drogenation of dihydroarenes with an NO equivalent product was observed after a reaction time of 24 h.[21]
were reported.[19]
Although we have successfully developed this
DDQ/TBN catalytic system and applied it in the
Scheme 1. Proposed DDQ-mediated aerobic oxidation reactions.
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2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tert-Butyl Nitrite/Oxygen
Table 1. DDQ/TBN-catalyzed aerobic oxidation of benzylic alcohols.[a]
Entry
1
Substrate
Product
t [h]
Conversion [%]
99
Selectectivity [%][b]
1a
1b
1c
2a
2b
2c
5
>99
2
3
3
3
100
100
>99
98
4
5
1d
1e
2d
2e
4
6
47
98
100
>99
6[c]
7
1f
1g
1h
1i
2f
2g
2h
2i
6
3
3
2
100
100
100
100
98
>99
>99
>99
8
9
10[c]
11
1j
2j
12
6
98
>99
>99
97
1k
1l
2k
2l
99
12[c]
13
12
6
98
1m
2m
100
>99
14
1n
1o
2n
2o
8
100
100
>99
15[c]
15
(95)
[a]
All reactions were carried out with 10 mmol of substrate in 10 mL of ClCH2CH2Cl, DDQ (5 mol%), TBN (5 mol%), O2
(0.2 MPa), 808C. The conversion and selectivity were determined by GC with area normalization.
Value in parentheses is isolated yield.
[b]
[c]
DDQ (10 mol%), TBN (10 mol%).
aerobic oxidations of the benzylic alcohols, it is obvi- ty that TEMPO cannot compete with as described
ous that this catalytic system could not compare with before. Because the DDQ-catalyzed aerobic oxidation
our previously developed TEMPO/TBN system, showed a significantly different oxidative ability be-
either in terms of substrate scope or catalyst loading. tween benzylic alcohols and aliphatic alcohols, we
However, DDQ possesses a broad oxidative capabili- reasoned that this catalyst combination can be applied
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Table 2. DDQ/TBN-catalyzed aerobic oxidation of benzyl methyl ethers.[a]
Entry
1
Substrate
Product
t [h]
Conversion [%]
99
Selectivity [%]
98
3a
3b
3c
3d
3e
3f
3g
2a
2b
2c
2d
2e
2f
4a
3
2
3
4
5
6
7
1
100
100
90
>99
>99
98
1.5
2
1.5
3.5
3.5
100
88
>99
98
32
95
8
3h
3i
4b
4c
3.5
4
11
82
9[b]
100
>99
[a]
Reaction conditions: substrate (8 mmol), EGDE (10 mL), O2 (0.2 MPa), 1408C. The conversion and selectivity were de-
termined by GC with area normalization.
DDQ (5 mol%), TBN (5 mol%), 1208C.
[b]
to the oxidation of benzyl ethers. With our primarily
Indeed, the transformation of benzyl methyl ethers
optimized reaction condition,[20] the DDQ/TBN/O2 to benzaldehydes by DDQ/TBN/O2 greatly inspired
system was applied in the oxidation of benzyl methyl us to explore the potential of DDQ-catalyzed aerobic
ethers to the corresponding benzaldehydes in ethyl- oxidative deprotection. Considering that the oxidation
ene glycol diethyl ether (EGDE), and the results are of 3,4-dimethoxybenzyl methyl ether (3i) could be car-
listed in Table 2. From Table 2, we can see that those ried out under a lower temperature with less catalyst,
electron-donating and non-sterically hindered benzyl- we would like to choose para-methoxybenzyl methyl
ic methyl ethers can obtain high conversions with ether (3e) as the substrate to further optimize the oxi-
high selectivities (entries 2, 3 and 5), while the elec- dation condition because PMB (para-methoxybenzyl
tron-donating but sterically hindered 3d and weakly group) is the more commonly used alcohol protecting
electron-deficient but non-sterically hindered 3f af- group in the type of benzyl series.[22] After carefully
forded reasonable conversions in high selectivities optimizing the reaction conditions, it was found that
(entries 4 and 6). The weakly electron-deficient and 5 mol% of DDQ, 5 mol% of TBN in EGDE under
sterically hindered 3g and the strong electron-defi- 0.2 MPa of oxygen at 1208C could complete the oxi-
cient 3h only achieved low conversions (entries 7 and dation of para-methoxybenzyl methyl ether in 4 h
8). On the contrary, the oxidation of the strong elec- (Table 3).
tron-donating substrate, 3,4-dimethoxybenzyl methyl
To explore the substrate scope, we applied this
ether (3i) was completed at under 1208C even when newly developed method to the oxidative deprotec-
reducing DDQ and TBN to 5 mol% (entry 9).
tion of a variety of PMB ethers, and the results are
summarized in Table 4. PMB ethers of primary and
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2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tert-Butyl Nitrite/Oxygen
Table 3. Optimization of aerobic oxidation of p-methoxy-
Conclusions
benzyl methyl ether.[a]
In conclusion, we have developed a new catalytic oxi-
dation system using catalytic amounts of DDQ and
TBN with molecular oxygen serving as the environ-
mentally benign terminal oxidant. The catalytic oxida-
tion system was established from the aerobic oxida-
tion of non-sterically hindered benzylic alcohols and
applied in the oxidative deprotection of PMB ethers.
Under the optimal reaction conditions, a variety of
PMB ethers can be deprotected in high conversions
to their corresponding alcohols with high selectivities.
This aerobic oxidative deprotection can tolerate a
broad range of other protecting groups under the re-
action conditions.
Entry DDQ
TBN
T
t
Conversion Selectivity
[mol%] [mol%] [oC] [h] [%]
[%]
1
2
3
4
5
6
7
8
9
10
5
5
5
5
3
3
5
5
10
5
5
5
5
3
5
3
5
140 1.5 100
>99
>99
>99
>99
>99
>99
98
140
120
120
120
120
120
120
100
2
2
3
4
4
4
4
4
100
58
92
100
43
74
72
56
>99
>99
Experimental Section
[a]
Reaction conditions: para-methoxybenzyl methyl ether
(8 mmol), EGDE (10 mL), O2 (0.2 MPa), temperature
(oil bath). The conversion and selectivity were deter-
mined by GC with area normalization.
General Remarks
Gas chromatography (GC) analyses were conducted on an
Agilent GC6890N system with a flame ionization detector
(FID) and an FFAP capillary column. Conversions and se-
1
lectivities were determined by area normalization. H NMR
and 13C NMR spectra were carried out on a Bruker Avance
III (500 MHz) spectrometer. CDCl3 and DMSO-d6 were
used as the solvent with tetramethylsilane (TMS) as the in-
ternal standard. GC-MS was performed on Finnigan Trace
GC Ultra-Finnigan Trace DSQ instrument. Low- and high-
resolution mass spectra were recorded in the ESI mode on
an Agilent 6210 LC/TOF mass spectrometer. All solvents
and chemicals were used as received from commercial sup-
pliers, unless otherwise noted. 1,2-Diphenylethane-1,2-diol
was reduced from benzoin with NaBH4. Benzyl methyl
ethers (3a–3i) were synthesized from the corresponding
benzyl alcohols and dimethyl sulfate. PMB ethers (5a–5f,
5k–5o) were obtained from corresponding alcohols and 4-
methoxybenzyl chloride. Bifunctional PMB ethers (5g–5j)
were synthesized as follows: (i) one hydroxy group of 1,6-
hexanediol was protected as PMB ether with 4-methoxyben-
zyl chloride to give 6-(4-methoxybenzyloxy)hexan-1-ol; (ii)
the hydroxy group of 6-(4-methoxybenzyloxy)hexan-1-ol
was protected by other alcohol protecting groups (Me, Ac,
Bz, MOM) to obtain 5g–5j.
secondary alcohols 5a–5f underwent a facile and com-
plete deprotection of the PMB group to furnish the
corresponding alcohols 6a–6f in high selectivities (en-
tries 1–6). To test the tolerance of this newly devel-
oped aerobic oxidative deprotection with DDQ/TBN
catalytic system, one hydroxy group of 1,6-hexanediol
was protected with PMB and the other one was con-
verted to other protecting groups [Me (5g), Ac (5h),
Bz (5i), MOM (5j)], and these different bifunctional
ethers were also submitted to the oxidative deprotec-
tion reactions (entries 7–10). As can be seen, not only
the stable methyl ether (5g), but also the base-sensi-
tive groups Ac (5h) and Bz (5i), proceeded through
an efficient PMB removal to afford 6g, 6h and 6i (en-
tries 7–9). The isolated yield of 6i was 93%. Substrate
5j with an acid-sensitive MOM group could also be
converted to the deprotection product 6j in 98% se-
lectivity (entry 10). It should be noted that prolonging
of oxidative deprotection of 5j would result in a drop
of the reaction selectivity, and we observed some re-
moval of MOM group in a by-product which was
Typical Procedure for the Oxidation of Alcohols
(Table 1, entry 15)
probably caused by the weak acidity of the reaction To a teflon-lined 316L stainless steel autoclave (300 mL)
was added 2.14 g of 1,2-diphenylethane-1,2-diol (1o,
10 mmol), 227 mg of DDQ (1 mmol, 10 mol%), 58 mL of
TBN (51 mg, 0.5 mmol, 5 mol%), and 10 mL of 1,2-dichloro-
ethane. Then the autoclave was closed and charged oxygen
to 0.2 MPa. The autoclave was put into an oil bath that was
preheated to 808C. The autoclave was taken out of the heat-
ing bath after a reaction time of 15 h, cooled to room tem-
perature, and carefully depressurized. The sample from the
reaction mixture was diluted with CH2Cl2 and the conver-
sion and selectivity were detected by GC without any purifi-
cation. The GC result showed that the reaction was com-
system. Similarly, substrate 5l, bearing two acid-sensi-
tive isopropylidene groups as well as a glycosidic ring,
can achieve a satisfactory result by increasing the cat-
alyst loading and cutting down the reaction time
(entry 12). Further study showed that the oxidative
deprotection can endure the Boc group (entry 13). 8-
Hydroxy-2-chloro-5,6,7,8-tetrahydroquinoline (6o), an
intermediate for synthesis of chiral pyridine-phosphite
ligands,[23] could also be obtained from its PMB ether
5o in 90% yield (entry 15).
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Table 4. Oxidative deprotection of PMB ethers.[a]
Entry
1
Substrate
(CH2)7OPMB
Product
(CH2)7OH
t [h]
Conversion [%]
100
Selectivity [%][b]
CH
U
5a
5b
CH
G
6a
6b
2
>99
2
2
100
>99
3
4
5c
6c
1.5
1.5
100
100
>99
>99
5d
6d
5
6
5e
6e
2
100
>99
5f
6f
1.5
100
>99
7
8
9
10
11
MeO
AcO(CH2)6OPMB
BzO(CH2)6OPMB
MOMO(CH2)6OPMB
PMBO(CH2)6OPMB
G
5g
5h
5i
5j
5k
MeO
AcO(CH2)6OH
BzO(CH2)6OH
MOMO(CH2)6OH
HO(CH2)6OH
A
6g
6h
6i
6j
6k
2
100
100
100
100
100
>99
>99
>99 (93)
98
G
E
1.5
1.5
1.5
2
R
N
G
>99
12[c]
5l
6l
0.5
100
(84)
13
14
5m
5n
6m
6n
2
100
100
>99 (93)
>99
1.5
15
5o
6o
1
100
97 (90)
[a]
Reaction conditions: para-methoxybenzyl methyl ether (8 mmol), EGDE (10 mL), O2 (0.2 MPa), temperature (oil bath).
The conversion and selectivity were determined by GC with area normalization.
Values in parentheses are isolated yields.
[b]
[c]
DDQ (12 mol%), TBN (12 mol%).
pleted. Then the mixture was concentrated under reduced
pressure, and the residue was purified by chromatography
of 5i (8 mmol)), 90.8 mg of DDQ (0.4 mmol, 5 mol%),
46 mL of TBN (41 mg, 0.4 mmol, 5 mol%), 10 mL of EGDE,
and 0.2MPa of oxygen were heated to 1208C for 1.5 h.
After the vessel had been cooled and carefully depressur-
ized, the conversion and selectivity were detected by GC.
The reaction mixture was directly purified by chromatogra-
phy on silica gel, and 6-hydroxyhexyl benzoate (6i) was ob-
on silica gel to afford benzoin (2o) as a white solid; yield:
1
2.02 g (95%). H NMR (CDCl3): d=5.95 (s, 1H), 7.26–7.35
(m, 5H), 7.38–7.41 (m, 2H), 7.50–7.53 (m, 1H), 7.91–7.92
(2H).
1
Typical Procedure for the Deprotection of PMB
Ethers (Table 4, entry 9)
tained as a pale yellow liquid; yield: 1.65 g (93%). H NMR
(CDCl3): d=1.44–1.48 (m, 4H), 1.59–1.62 (m, 2H), 1.77–
1.80 (m, 3H), 3.65 (t, J=6.5 Hz, 2H), 4.32 (t, J=6.5 Hz,
2H), 7.45 (t, J=7.5 Hz, 2H), 7.55 (t, J=7.5 Hz, 1H), 8.04
(d, J=7.0 Hz, 2H).
Caution! Potentially a peroxide could be formed in the re-
action when EGDE was used as the solvent in the O2/NOx
system!
A similar operating procedure was used for the deprotec-
tion of 6-(benzyloxy)hexyl benzoate (5i). A mixture of 2.5 g
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2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tert-Butyl Nitrite/Oxygen
[9] Some examples of aromatization, see: a) T. Ohmura,
Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (NSFC 20772111, 20876149, 21146001)
and the Zhejiang Provincial Natural Science Foundation of
China (Y4090517).
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3038
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Adv. Synth. Catal. 2011, 353, 3031 – 3038