Synthesis and properties of fluorous benzoquinones and their application in deprotection of silyl ethers

Article abstract of DOI:10.1039/c4ob00783b

1,4-Benzoquinone derivatives bearing trifluoromethyl, perfluorobutyl and perfluorohexyl groups were prepared and employed in the deprotection of silyl ethers. The fluorous character of these compounds was examined by measuring the partition coefficient between the fluorous and organic solvents. The benzoquinone derivatives showed significant fluorous character, indicating that they can be recovered from the reaction mixtures using a fluorous/organic biphasic system. The oxidising ability of the fluorous benzoquinones was estimated by cyclic voltammetry, and these compounds were found to be strong oxidisers. The fluorous benzoquinones were utilised in the oxidative desilylation of silyl ethers to afford the deprotected alcohols in high yield. In addition, the reduced fluorous benzoquinones were recovered from the reaction mixtures in good yields using a fluorous/organic biphasic system. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00783b

Organic &
Biomolecular Chemistry
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Synthesis and properties of fluorous
benzoquinones and their application in
deprotection of silyl ethers†
Cite this: Org. Biomol. Chem., 2014,
12, 5442
Hiroshi Matsubara,*a Takahiko Maegawa,a Yasuaki Kita,a Takato Yokojia and
Akihiro Nomotob
1,4-Benzoquinone derivatives bearing trifluoromethyl, perfluorobutyl and perfluorohexyl groups were
prepared and employed in the deprotection of silyl ethers. The fluorous character of these compounds
was examined by measuring the partition coefficient between the fluorous and organic solvents. The
benzoquinone derivatives showed significant fluorous character, indicating that they can be recovered
from the reaction mixtures using a fluorous/organic biphasic system. The oxidising ability of the fluorous
benzoquinones was estimated by cyclic voltammetry, and these compounds were found to be strong oxi-
disers. The fluorous benzoquinones were utilised in the oxidative desilylation of silyl ethers to afford the
deprotected alcohols in high yield. In addition, the reduced fluorous benzoquinones were recovered from
the reaction mixtures in good yields using a fluorous/organic biphasic system.
Received 16th April 2014,
Accepted 23rd May 2014
DOI: 10.1039/c4ob00783b
www.rsc.org/obc
chromatography. Perfluoroalkyl groups reveal strong electron-
Introduction
withdrawing effects in nature and many studies5 in fluorous
The fluorous biphasic system (FBS)1 has been developed in chemistry utilise this property.
organic chemistry as an environmentally benign recycling
It is well known that 1,4-benzoquinone is easily reduced to
process. This concept is based on the physical phenomena hydroquinone, and 1,4-benzoquinones bearing electron-with-
that highly fluorinated compounds (fluorous materials) are drawing groups, for example 2,3-dichloro-5,6-dicyano-1,4-benzo-
immiscible in organic solvents and form another phase (fluo- quinone (DDQ, 1) and 2,3,5,6-tetrachloro-1,4-benzoquinone
rous phase). They are then easily separable from organic com- (chloranil, 2), have been used in many organic reactions6,7 as
pounds by extraction using perfluorinated solvents such as oxidising or dehydrogenating agents. However, DDQ is toxic,8
FC-72 (perfluorohexanes) or by liquid/solid extraction using while the oxidising ability of chloranil is generally modest. In
fluorous silica gel.2 Work in our laboratories3 in this field has addition, a stoichiometric amount of the oxidiser is usually
been directed towards the design, synthesis and application of required for the reactions. These drawbacks of benzoquinone
novel environmentally benign fluorous reaction media,4 as derivatives do not fit with the principles of green chemistry
well as effective fluorous reagents and catalysts. To this end, and prompted us to develop reusable benzoquinone deriva-
we previously reported fluorinated versions of diethyl ether3a–c tives. Herein, we report the preparation of 1,4-benzoquinones
and DMF3d that function as easily recyclable reaction media. bearing perfluoroalkyl chains 3. The fluorous character and
In addition, we prepared chiral and achiral lithium diisopropyl- electronic properties of benzoquinones 3a–c were examined
amide3e derivatives bearing a perfluoroalkyl chain, which pro- and it was found that the benzoquinones would work as
duced lithium enolates in excellent chemical and optical strongly oxidising fluorous reagents; benzoquinones 3 were
yields comparable with the parent non-fluorous lithium then applied to catalytic desilylation reaction of silyl ethers.9
amides. After the reaction, the fluorous lithium amides were
recovered as fluorous amines by liquid–liquid extraction or
aDepartment of Chemistry, Graduate School of Science, Osaka Prefecture University,
Sakai, Osaka 599-8531, Japan. E-mail: matsu@c.s.osakafu-u.ac.jp
bDepartment of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture
University, Sakai, Osaka 599-8531, Japan
†Electronic supplementary information (ESI) available: CV of compounds 3a
and 3c, catalytic desilylation using compounds 1 and 2, experimental details
and spectroscopic data. See DOI: 10.1039/c4ob00783b
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Partition coefficient of fluorous benzoquinone
Results and discussion
The approximate partition coefficients of fluorous benzo-
quinones 3a–c were measured at 25 °C according to the pro-
cedure reported by the Curran group.18 The results are
summarised in Fig. 1. Most of the trifluoromethyl analogue 3a
is distributed in the organic solvents, whereas perfluorobutyl
and perfluorohexyl analogues 3b and 3c are preferentially dis-
tributed in the FC-72 (perfluorohexanes) phase. Chloroform
was found to be the best solvent to recover 3b from the reac-
tion mixture using the biphasic system, while all solvents
(except THF) can be used to recover 3c efficiently (Fig. 1).
Preparation of fluorous benzoquinone
Fluorous benzoquinones 3a–c were synthesised according to
the procedure outlined in Scheme 1.10 Thus, 2,5-diiodo-1,4-
dimethoxybenzene (5) was prepared from 1,4-dimethoxy-
benzene (4), iodine and iodic acid11,12 in 76% yield. The di-
iodide was then converted to perfluoroalkyl compounds using
copper catalysis. Treatment of diiodide 5 with sodium trifluor-
oacetate and copper iodide afforded 1,4-dimethoxy-2,5-bis(tri-
fluoromethyl)benzene (6a)10 in 51% yield, while reacting 5
with perfluorobutyl iodide in the presence of copper and 2,2′-
bipyridine13 afforded 1,4-dimethoxy-2,5-bis(perfluorobutyl)-
benzene (6b) in 82% yield. Deprotection of the dimethoxy
groups using sodium ethanethiolate14 and boron tribromide10
afforded hydroquinone derivative 7.15 Hydroquinone 7 was oxi-
dised to benzoquinone 3 using chromium oxide10 (Rf = CF3) or
silver oxide16 (Rf = C4F9) in 77% and 73% yield, respectively.
Fluorous benzoquinones 3a and 3b were yellow columns and
brown needles with melting point of 110 °C (sublimation) and
107–109 °C, respectively.
Chlorine-substituted fluorous benzoquinone 3c was also
synthesised as shown in Scheme 2. Perfluorohexyl iodide was
treated with diiodobenzene 5 in the presence of copper and
2,2′-bipyridine to afford 2,5-bis(perfluorohexyl)-1,4-dimethoxy-
benzene (8) in 95% yield. Compound 8 was chlorinated using
sulphuryl chloride and disulphur dichloride in the presence of
aluminium chloride.17 Demethylation and oxidation pro-
ceeded simultaneously during the chlorination, affording
fluorous benzoquinone 3c in 86% yield. Compound 3c was
orange columns with a melting point of 78.5–80.0 °C.
Cyclic voltammograms of fluorous benzoquinones
To assess the oxidising ability of the prepared fluorous benzo-
quinones, their cyclic voltammograms were measured in aceto-
nitrile solution. The voltammogram of fluorous benzoquinone
3b is illustrated in Fig. 2, and the voltammograms of com-
pounds 3a and 3c are displayed in ESI.†
Inspection of the voltammogram of fluorous benzo-
quinones revealed two reversible redox steps (Fig. 2). The first
and second reduction peaks were found at −0.38 V and
−1.33 V (vs. Ag/Ag+), respectively, while the first and second
oxidation peaks were observed at −0.29 V and −1.13 V, respect-
ively. The first reduction peaks (E1p.c) of the fluorous benzoqui-
nones were chosen to estimate their oxidising ability (Fig. 3).
Fig. 1 Partition coefficient of fluorous benzoquinones (yellow: organic
phase; purple: fluorous phase).
Scheme 1 Synthetic pathway of fluorous benzoquinones 3a and 3b.
Scheme 2 Synthetic pathway of fluorous benzoquinone 3c.
Fig. 2 Cyclic voltammogram of fluorous benzoquinone 3b.
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Scheme 4 Aromatisation of tetralin.
Fig. 3 The first reduction peak of benzoquinone derivatives.
E1p.c of xyloquinone, chloranil and DDQ were also measured
and included for comparison.
As shown in Fig. 3, fluorous benzoquinones 3a and 3b
showed a similar oxidising ability to chloranil and much larger
than that of xyloquinone. In addition, 3c was a stronger oxidiz-
ing agent than 3a, 3b and chloranil. The order of oxidising
ability of the benzoquinones is: xyloquinone ≪ chloranil ∼ 3a
< 3b < 3c ≪ DDQ, indicating that fluorous benzoquinones
3a–c would work as strong oxidising agents. To test fluorous
benzoquinones 3a–c as alternative oxidising agents to DDQ,
the fluorous compounds were tested in oxidation reactions.
Scheme 5 Deprotection of TBDMS ether of dodecanol.
recovered in 92% yield as the corresponding hydroquinone
7b by fluorous/organic biphasic workup using FC-72 and
chloroform.
The scope and limitation of this desilylation reaction were
then investigated. Deprotection of various silyl ethers using
catalytic amounts of fluorous benzoquinones 3b and 3c in
acetonitrile–water (9 : 1) were carried out; the results are shown
in Tables 1 and 2, respectively. For reference, results for chlora-
nil and DDQ are summarized in Tables S1 and S2 in ESI,†
respectively.
As shown in Table 1, tert-butyldimethylsilyl (TBDMS)-pro-
tected primary and secondary alcohols were cleaved using
fluorous benzoquinone 3b in high yield at room temperature,
while TBDMS-protected tertiary alcohol and phenol were
stable under these conditions (entries 1–6). The triisopropyl-
silyl (TIPS) group could not be deprotected at room tempera-
ture; however, it was deprotected when it was at 50 °C (entry
7). In comparison, tert-butyldiphenylsilyl (TBDPS) and p-meth-
oxybenzyl (PMB) groups were stable at ambient and elevated
temperatures (entries 8 and 9). It is worth noting that selective
deprotection of the TBDMS group in the presence of TIPS and
PMB groups was achieved using 3b (entries 10–12). After the
reactions, 3b was recovered as hydroquinone 7b in high yields
using the fluorous/organic biphasic workup. Hydroquinone 7b
was converted to 3b using Ag2O (75% yield) and reused in the
second reaction.
Application of fluorous benzoquinone in organic synthesis
Cinnamyl alcohol (9) was successfully converted to cinnam-
aldehyde (10)19 using 1.5 equivalents of fluorous benzoqui-
none 3b (Scheme 3). This result shows that the product yield
and the reaction time using 3b are comparable to those
obtained using chloranil and DDQ. After fluorous/organic
biphasic workup using FC-72 and chloroform, 3b was recov-
ered in 98% yield from the fluorous phase as the corres-
ponding hydroquinone derivative.
DDQ is known to work as a dehydrogenating reagent.6a
Thus, aromatisation of tetralin (11)20 was attempted using
fluorous benzoquinone 3b. However, the reaction did not
proceed as expected and the starting material was recovered
quantitatively (Scheme 4).
Next, desilylation of silyl ethers using catalytic amounts of
fluorous benzoquinone 3b was investigated. In the presence of
3b, the desilylation9 of tert-butyldimethylsilyl ether of dodeca-
nol (12) proceeded smoothly to afford the corresponding
alcohol 13 in 96% yield, which was vastly superior to the yield
achieved when DDQ was used (Scheme 5). In addition, 3b was
Results revealed that 3c was more efficient in cleaving the
silyl protecting groups compared to 3b (Table 2). For example,
the TIPS group was cleaved using 3c at room temperature
(entry 3). The TBDMS group of phenol was also deprotected
under the same conditions (entry 2). TBDMS-protected tertiary
alcohol and TBDPS-protected primary alcohol were not cleaved
using 3c at room temperature, but were cleaved at 50 °C
(entries 1 and 4) while PMB was still stable under elevated
temperatures (entry 5). In addition, compound 3c could be
Scheme 3 Oxidation of cinnamyl alcohol.
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Table 1 Deprotection of silyl ethers using fluorous benzoquinone 3ba
Entry
Substrateb
Product
Temp.
Yieldc (recovery of 3b)d
1
2
12
13
rt
rt
96% (88%)
90% (83%)
3
—
rt
50 °C
nr
nr
4
5
rt
rt
92% (90%)
78% (86%)
6
—
rt
50 °C
nr
nr
7
8
9
13
—
—
rt
50 °C
nr
97% (61%)
rt
50 °C
nr
nr
rt
50 °C
nr
nr
10
11
12
rt
rt
rt
76%
73% (79%)
71% (88%)
a Reaction conditions: 3b (10 mol%), CH3CN–H2O
= 9 : 1, 6
h. b TBDMS: tert-butyldimethylsilyl; TIPS: triisopropylsilyl; TBDPS: tert-
butyldiphenylsilyl; PMB: p-methoxybenzyl. c Isolated yield. d 3b was recovered as hydroquinone 7b.
ether (12) proceeded smoothly to afford decanol (13) in 79%
Table 2 Deprotection of silyl ethers using fluorous benzoquinone 3c
yield. However, recovery of 3a using fluorous/organic biphasic
workup was unsuccessful because of the less fluorous charac-
ter of 3a.
Entry Substrate
1
Product
Temp. Yield (recovery of 3c)a
r.t
nr
50 °C
96%
2
21
r.t
90%
Conclusions
3
4
22
23
13
13
r.t
r.t
86% (73%)
nr
The results obtained suggest that fluorous benzoquinones 3b
and 3c are strong oxidising agents, which can be used in
organic synthesis. The oxidising ability of 3b and 3c lies
between that of chloranil and DDQ, and selective deprotection
of silyl groups was achieved using the synthesised fluorous
benzoquinones. The fluorous benzoquinones 3b and 3c were
subsequently recovered as their corresponding hydroquinones
using a fluorous/organic biphasic workup. In addition, the
hydroquinones were converted to fluorous benzoquinones 3b
and 3c with Ag2O and reused successfully.
50 °C
r.t
50 °C
84%
nr
nr
5
24
—
a Catalyst 3c was recovered as a hydroquinone.
recovered using the fluorous/organic biphasic workup as the
corresponding hydroquinone and was successfully converted
to 3c using the same procedure as that for hydroquinone 7b.
As can be seen in Tables S1 and S2 in ESI,† chloranil did
not cleave the TBDMS group at room temperature, while DDQ
deprotected TIPS, PMB and TBDMS even at room temperature.
This suggests that fluorous benzoquinones 3b and 3c are
superior to chloranil and DDQ in achieving selective deprotec-
tion of silyl groups. It should be noted that using fluorous
benzoquinone 3a under the same reaction conditions as those
for 3b, deprotection of the TBDMS group of dodecyl TBDMS
Experimental
1,4-Bis(perfluorobutyl)-2,5-dimethoxybenzene (6b)
A mixture of 5 (1.01 g, 2.59 mmol), perfluorobutyl iodide
(3.50 g, 7.70 mmol), copper powder (976 mg, 15.4 mmol), 2,2′-
bipyridine (80 mg, 0.51 mmol) and DMSO (5 mL) was heated
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at 120 °C under N2 for 2 days. After cooling to room tempera- 28%), 525 (36), 375 (68), 347 (100). HRMS (EI) (found: [M+]
ture, water was added. The mixture was filtered through a pad 543.9761. C14H2F18O2+ requires 543.9767).
of Celite and the pad was washed with diethyl ether. The
aqueous layer was separated and extracted with diethyl ether. 1,4-Bis(perfluorohexyl)-2,5-dimethoxybenzene (8)
The combined organic layer was washed with brine, dried over
sodium sulphate and concentrated under reduced pressure.
The residue was purified by column chromatography on silica
A mixture of 5 (1.95 g, 5.00 mmol), perfluorohexyl iodide
(6.69 g, 15.0 mmol), copper powder (1.91 g, 30.0 mmol) and
2,2′-bipyridine (156 mg, 1.00 mmol) in DMSO (25 mL) was
heated to 110 °C under N2 for 2 days. After cooling to room
gel (eluent: chloroform) to yield 6b (1.37 g, 92%) as lemon-
yellow crystals. Mp 63.0–64.0 °C; νmax/cm−1 (KBr) 2976, 2952,
temperature, water was added. The mixture was then filtered
2868, 2843, 1520, 1406, 1352, 1248, 1131, 836, 741; 1H NMR
through a pad of Celite and the pad was washed with diethyl
(500 MHz, CDCl3) δH 3.86 (6H, s, OCH3), 7.14 (2H, s, ArH); 13C
ether. The aqueous layer was separated and extracted with
diethyl ether. The combined organic layer was washed with
NMR (126 MHz, CDCl3) δC 56.8, 113.8, 110.0–118.0, 121.5,
152.0; LRMS (EI) 574(M+, 100%), 405(64), 188(93). HRMS (EI)
brine, dried over sodium sulphate, and concentrated under
(found: [M]+ 574.0232. C16H8F18O2+ requires 574.0237).
reduced pressure. The residue was purified by column
chromatography on silica gel (eluent: chloroform) to yield 8
2,5-Bis(perfluorobutyl)-1,4-hydroquinone (7b)
(3.55 g, 91%) as white crystals. Mp 66.0–67.5 °C; νmax/cm−1
A mixture of ethanethiol (2.61 g, 42.1 mmol), sodium hydride,
(KBr) 2948, 2869, 1514, 1469, 1398, 1273, 1202, 1174, 1122,
60% oil dispersion (1.69 g, 42.1 mmol) and DMF (25 mL) was
1047, 744, 719; 1H NMR (500 MHz, CDCl3) δH 3.86 (6H, s,
stirred at room temperature under N2 for 1 h. Then, 6b (4.03 g,
OCH3), 7.14 (2H, s, ArH); 13C NMR (126 MHz, CDCl3) δC 56.8,
7.02 mmol) dissolved in DMF (70 mL) was added to the reac-
114.0, 108.4–118.4, 121.5–121.9, 152.1; LRMS (EI) 774 (M+,
tion mixture and heated to 70 °C for one day. After cooling to
100%), 755 (17), 505 (39), 188 (59); HRMS (EI) (found: [M+]
room temperature, 2 M HCl was added until the pH was
774.0113, C20H8F26O2+ requires 774.0109).
neutral. The aqueous layer was then separated and extracted
with diethyl ether. The combined organic layer was washed
with brine, dried over sodium sulphate and concentrated
under reduced pressure to afford a crude solid (4.55 g). A
mixture of the crude product (4.55 g) and dichloromethane
2,5-Bis(perfluorohexyl)-3,6-dichloro-1,4-benzoquinone (3c)
A mixture of aluminium trichloride (255 mg, 2.00 mmol) and
sulphuryl chloride (60 mL) was heated under reflux. Then, 8
(3.10 g, 4.00 mmol) dissolved in sulphuryl chloride (20 mL)
(120 mL) was stirred under N2 and cooled to −15 °C using an
and disulphur dichloride (1.2 mL, 15.0 mmol) was added
ice-salt bath. Boron tribromide (7.0 mL, 74 mmol) was added
dropwise. After stirring for 24 h, the mixture was quenched
with water. The aqueous layer was separated and extracted
dropwise and the mixture was then warmed slowly to room
temperature. After stirring for
2 days, the mixture was
with diethyl ether. The combined organic layers were washed
with brine, dried over sodium sulphate and concentrated
under reduced pressure. The residue was purified by column
chromatography on silica gel (eluent: chloroform) to yield 3c
(2.65 g, 86%) as orange crystals. Mp 78.5–80.0 °C; νmax/cm−1
(KBr) 1694, 1586, 1235, 1219, 1174, 1145, 1126, 1095, 743, 725;
13C NMR (126 MHz, CDCl3) δC 108.0–119.0, 131.5, 149.8, 171.2;
LRMS (EI) 814 (1.4%), 812 (M+, 1.7), 795 (6), 793 (9), 774 (11),
565 (16), 517 (82), 515 (100), 286 (22), 261 (45), 259 (46), 246
(33), 233 (37), 231 (38); HRMS (EI) (found: [M]+ 811.8865,
C1835Cl2F26O2+ requires 811.8860).
quenched by adding water dropwise at 0 °C. The aqueous layer
was separated and extracted with diethyl ether. The combined
organic layer was washed with brine, dried over sodium sul-
phate and concentrated under reduced pressure. The residue
was purified by column chromatography on silica gel (eluent:
hexane–diethyl ether = 3/1) to yield 7b (3.21 g, 84%) as light
brown crystals. Mp 129–131 °C; νmax/cm−1 (KBr) 3467, 1427,
1
1353, 1236, 1204, 1131, 796, 741; H NMR (500 MHz, CDCl3)
δH 7.23 (2H, s, ArH), 9.19 (2H, s, OH); 13C NMR (126 MHz,
CDCl3) δC 107.9–119.8, 118.6, 120.3, 150.0; LRMS (EI) 546 (M+,
61%), 377 (100), 208 (76); HRMS (EI) (found: [M+] 545.9927,
C14H4F18O2+ requires 545.9924).
Deprotection of silyl ether using benzoquinone 3b; typical
procedure
2,5-Bis(perfluorobutyl)-1,4-benzoquinone (3b)
A mixture of 7b (3.21 g, 5.88 mmol), silver oxide (5.45 g, A mixture of silyl ether (1.00 mmol), 3b (54 mg, 0.10 mmol)
23.5 mmol) and sodium sulphate (426 mg, 3.00 mmol) in and acetonitrile (4.5 mL) in water (0.5 mL) was stirred for 6 h
diethyl ether (50 mL) was heated under reflux for 24 h. After at room temperature. Diethyl ether was added to the reaction
cooling to room temperature, the insoluble salt precipitate was mixture and the aqueous layer was separated off. The organic
filtered off. The filtrate was then concentrated under reduced layer was dried over sodium sulphate and evaporated under
pressure and the residue was purified by column chromato- reduced pressure. The residue was diluted with chloroform,
graphy on silica gel (eluent: hexane–toluene = 3/1) to yield 3b and perfluorohexane (FC-72) was then added to the solution.
(3.17 g, 73%) as brown crystals. Mp 107–109 °C; νmax/cm−1 The FC-72 layer was separated, and the chloroform layer was
(KBr) 1674, 1363, 1261, 1236, 1205, 1138, 1082, 940, 747, 498; extracted with FC-72 three times. The combined FC-72 layer
1H NMR (500 MHz, CDCl3) δH 7.22 (s); 13C NMR (100 MHz, was evaporated under reduced pressure to yield 2,5-bis(per-
CDCl3) δC 110.0–118.7, 135.2, 139.2, 179.3; LRMS (EI) 544 (M+, fluorobutyl)-1,4-hydroquinone (7b), while the chloroform layer
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was evaporated under reduced pressure to afford the unpro-
tected alcohol.
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and M. D. McLaws, in Encyclopedia of reagents for organic
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Acknowledgements
7 For some reviews of chloranil, see: (a) D. R. Buckle, in Encyclo-
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D. Crich, P. L. Fuchs and G. A. Molander, Wiley, Chichester,
2nd edn, 2009, vol. 3, p. 2188; (b) M. Balci, M. Celik and
M. S. Gultekin, Sci. Synth., 2006, 28, 31.
This work was financially supported by a Grant-in-Aid for
Scientific Research (C) (no. 24550213) from the Japan Society
for the Promotion of Science (JSPS). HM thanks Prof. Ilhyong
Ryu for helpful discussion.
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15 Demethylation of 6a using only BBr3 resulted in a complex
mixture, while the reaction of 6b under the same condition
took 5 days to complete, and afforded hydroquinone 7b in
lower yield (75%).
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