Oxidation of anisoles to p-benzoquinone monoketals catalyzed by a ruthenium complex of 1,4,7-trimethyl-1,4,7-triazacyclononane with tert-butyl hydroperoxide

Article abstract of DOI:10.1139/v05-060

A protocol based on [Ru^III(Me₃tacn)(CF ₃CO₂)₂(H₂O)]CF₃CO ₂ (1, Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) as catalyst and tert-butyl hydroperoxide (TBHP) as oxidant was developed for oxidation of anisoles to p-benzoquinone monoketals. This reaction can be formally considered as regioselective aromatic C-H oxidation. With 2-methoxyanisole as substrate, 3,4-dimethoxy-4-tert-butoxy-2,5-cyclohexadienone can be obtained in up to 82% yield based on 84% substrate conversion.

Full text of DOI:10.1139/v05-060

521
Oxidation of anisoles to p-benzoquinone
monoketals catalyzed by a ruthenium complex of
1,4,7-trimethyl-1,4,7-triazacyclononane with tert-
butyl hydroperoxide1
Wai-Hung Cheung, Wing-Ping Yip, Wing-Yiu Yu, and Chi-Ming Che
Abstract: A protocol based on [RuIII(Me3tacn)(CF3CO2)2(H2O)]CF3CO2 (1, Me3tacn = 1,4,7-trimethyl-1,4,7-
triazacyclononane) as catalyst and tert-butyl hydroperoxide (TBHP) as oxidant was developed for oxidation of anisoles
to p-benzoquinone monoketals. This reaction can be formally considered as regioselective aromatic C-H oxidation.
With 2-methoxyanisole as substrate, 3,4-dimethoxy-4-tert-butoxy-2,5-cyclohexadienone can be obtained in up to 82%
yield based on 84% substrate conversion.
Key words: oxidation, quinones, tert-butyl hydroperoxide, ruthenium catalyst.
Résumé : Un protocole basé sur l’utilisation du [RuIII(Me3tacn)(CF3CO2)2(H2O)]CF3CO2 (1, Me3tacn = 1,4,7-trimethyl-
1,4,7-triazacyclononane) comme catalyseur et de l’hydroperoxyde de tert-butyle comme oxydant a été élaboré pour
l’oxydation des anisoles menant à la formation de monocétals de p-benzoquinone. On peut considérer formellement
cette réaction comme une oxydation C-H aromatique régiosélective. Avec le 2-méthoxyanisole comme substrat, on ob-
tient la 3,4-diméthoxy-4-tert-butoxycyclohexa-2,5-diénone avec un rendement pouvant atteindre 82 %, en se basant sur
un taux de conversion du substrat de 84 %.
Mots clés : oxydation, quinones, hydroperoxyde de tert-butyle, catalyseur de ruthénium.
[Traduit par la Rédaction] Cheung et al. 526
Introduction
alkyl hydroperoxides are known to give poor product selec-
tivity because of coupling of the phenoxy radicals (8). Re-
cently, selective oxidation of phenols by TBHP has been
achieved by Murahashi et al. (9) using Ru(PPh3)3Cl2 as cata-
lyst, and 4-(tert-butyldioxy)-4-alkyl-2,5-cyclohexadienones
were produced in good yields.
It is well documented that ruthenium complexes are versa-
tile catalysts for oxidative functionalization of C=C and C—H
bonds (10). Extensive studies have shown that oxo-ruthen-
ium complexes of chelating tertiary amines are powerful ox-
idants for alkenes, alkynes, and alkanes (11). We previously
reported the preparation and X-ray structure characterization
(12c) of [RuIII(Me3tacn)(CF3CO2)2(H2O)]CF3CO2 (1, Fig. 1),
which is a robust catalyst (12) for homogeneous and hetero-
geneous oxidation of alcohols and alkenes using TBHP as
the terminal oxidant; aldehydes and (or) ketones and
epoxides can be obtained in excellent yields with product
turnovers >6000. As part of our continuous effort to develop
p-Benzoquinones constitute a key structural element in
many biologically interesting natural products such as neo-
lignans and anthraquinone antibiotics (1, 2). Stoichiometric
oxidation of substituted phenols by oxidants such as ammo-
nium cerium(IV) nitrate (3) and thallium(III) nitrate (4) rep-
resents a common approach for quinone synthesis. Similarly,
quinone monoketal, which is a valuable class of synthetic in-
termediates, can be readily obtained through oxidation of
para-methoxy phenols by either electrochemical (5) or
chemical oxidation using hypervalent iodine reagents (6). In
view of growing environmental concern, there is an upsurge
of interest in developing environmentally friendly and opera-
tionally safe catalytic protocols for hydrocarbon oxidations
using more tractable oxidants such as molecular oxygen,
aqueous hydrogen peroxide, and tert-butyl hydroperoxide
(TBHP) (7). However, metal-catalyzed phenol oxidations by
Received 9 December 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 13 May 2005.
W.-H. Cheung,2 W.-P. Yip, W.-Y. Yu,3 and C.-M. Che.4 Department of Chemistry and Open Laboratory of Chemical Biology of
the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong
Kong.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Present address: Department of Surgery, 9/F, Laboratory Block, 21 Sassoon Road, Faculty of Medicine Building, The University of
Hong Kong.
3Corresponding author (e-mail: wyyu@hku.hk).
4Corresponding author (e-mail: cmche@hku.hk).
Can. J. Chem. 83: 521–526 (2005)
doi: 10.1139/V05-060
© 2005 NRC Canada

522
Can. J. Chem. Vol. 83, 2005
Table 1. Ruthenium-catalyzed TBHP oxidation of anisoles.
b
Yield (%)
b
Yield (%)
a
Conversion (%) Product
a
Conversion (%) Product
Entry
Substrate
Entry
Substrate
OMe
O
MeO
OtBu
OMe
OMe
OMe
1
48
7
76
MeO
OMe
(3g)
46
100
MeO
OMe
O
(2g)
(2a)
(3a)
O
MeO
MeO
OtBu
OMe
OMe
OMe
tBuO
OMe
2
3
4
53
57
41
8
84
32
19
12
99
(2b)
(2h)
(3b)
(3b)
OtBu
O
O
OtBu
Me
tBuO
OMe
Me
100
(3c)
(3h)
(2c)
O
O
O
MeO OtBu
OMe
OMe
NHAc
NHAc
9
32
56
29
95
100
45
Me
Me
(2i)
(3i)
(2d)
(3d)
O
O
O
O
OMe
OMe
NHAc
20
15
5
6
15
89
10
11
Me
Me
AcHN
Me
Me
(3e)
O
O
(2e)
(2j)
(3i)
OMe
Br
OMe
OMe
COOH
OtBu
Br
MeO
Me
Me
Me
(3f)
(2f)
(2k)
(3k)
O
Note: Reaction conditions (protocol I): 1:anisole:TBHP = 1:100:230, CH2Cl2 (5 mL), room temperature for 14 h.
aSubstrate conversions were determined by GLC analysis.
bIsolated yields.
Fig. 1. Molecular structure of [RuIII(Me3tacn)(CF3CO2)2(H2O)]-
CF3CO2.
Results and discussion
At the outset, treatment of 2-methoxyanisole (2a, 1 mmol)
with TBHP (2.3 mmol) in the presence of [RuIII(Me3tacn)-
(CF3CO2)2(H2O)]CF3CO2 (1, 1 mol%) in CH2Cl2 at ambient
temperature for 14 h produced 3,4-dimethoxy-4-tert-butoxy-
2,5-cyclohexadienone (3a) in 48% isolated yield based on
46% anisole consumption (Table 1, entry1). Other oxidants
such as m-chloroperoxybenzoic acid, hydrogen peroxide –
urea adduct, Oxone®, chloramine-T, TEMPO, and 2,6-
dichloropyridine N-oxide have been tested for their ability to
oxidize 2a. Under our reaction conditions (i.e., 1:2a:oxidant =
1:100:230 in CH2Cl2), formation of 3a was not detected, and
the starting anisole (>90%) was recovered. According to
Murahashi et al. (9), Ru(PPh3)3Cl2 and RuCl3·H2O were re-
ported to be effective catalysts for selective phenol oxidation
by TBHP to give 4-(tert-butyldioxy)-4-alkyl-2,5-cyclohex-
anedienone. In this work, when Ru(PPh3)3Cl2 and RuCl3·H2O
were employed as catalysts for the oxidation of 2a using
THBP as oxidant, no effective consumption of the starting
anisole was observed without product 3a being detected.
N
Me
III
N
N
Me
Me
CF3COO
Ru
CF3CO2
OH2
CF3CO2
structurally characterized ruthenium complexes for organic
catalysis, we are attracted to a recent work by Hirobe and co-
workers (13), which states that [RuII(Por)(CO)] (Por =
porphyrinato) can effect catalytic oxidation of anisoles by
2,6-dichloropyridine N-oxide to give p-benzoquinones as the
major product. The transformation was proposed to occur
via oxidation of an aromatic C—H bond as a principal step
(13). In this work we report that complex 1 can catalyze
anisole to p-benzoquinone monoketals with up to 90% yield
using TBHP as the oxidant. Results obtained under various
reaction conditions (e.g., solvent systems, temperature) and
with use of various substrates will be discussed.
The catalytic oxidation of other anisole derivatives has
been examined by employing the following reaction condi-
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Cheung et al.
523
Table 2. Effect of various experimental parameters on the catalytic oxidation of 2-methoxyanisole.
tBuO
OMe
OMe
OMe
III
[Ru (Me tacn)(CF CO ) (H O)]CF CO
OMe
3
3
2 2
2
3
2
TBHP
O
Entry
Loading (%)
TBHP (equiv.)
Temp (°C)
Time (h)
Solvent
Yielda (%)
Conversionb (%)
1
2
3
4
5
6
7
8
9
2.3
2.3
2.3
2.3
2.3
7.0
2.3
2.3
7.0
1.0
1.0
1.0
1.0
1.0
1.0
5.0
0.5
1.0
1.0
1.0
1.0
24
24
24
24
24
24
24
24
80
24
80
80
14
14
14
14
14
14
14
14
14
14
14
4
CH2Cl2
48
68
N/A
N/A
50
60
49
78
86
46
9
n-hexane
t-BuOH
MeOH
1,2-DCEc
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
<1
<1
45
49
57
28
62
45
62
82
10
11
12d
7.0
14.0
3.5/3.5
66
93
84
MeOH/t-BuOH/1,2-DCE
aIsolated yield.
bThe percentage of conversion was determined by GLC analysis.
c1,2-DCE = 1,2-dichloroethane.
dA dissolved mixture of complex 1 (1 mol%) in MeOH:t-BuOH with TBHP (3.5 mmol) was added slowly by a syringe pump (0.5 mL min–1) to a solu-
tion of 2a (1 mmol) with TBHP (3.5 mmol) in 1,2-DCE at 80 °C for 2 h. The reaction mixture was further stirred for 2 h, followed by a general work-up
procedure.
tions (protocol I): 1 (1 mol%), TBHP (2.3 equiv.), and
CH2Cl2 at room temperature. Unsubstituted anisole 2b re-
acted with TBHP to afford quinone monoketal 3b in 53%
yield based on 84% conversion (Table 1, entry 2). Likewise,
ortho- and meta-substituted methyl anisoles (2c and 2d)
were converted to their corresponding ketals in 57% and
41% yields, respectively (entries 3 and 4) under the Ru-
catalyzed conditions. Yet the analogous reaction of 3,5-
dimethylanisole gave quinone 3e in 15% yield despite com-
plete substrate conversion (entry 5). Analysis of the crude
diates for polycycle constructions. In this work, we have
studied the reactions of 2- and 3-methoxyanilides (2i and 2j)
with TBHP by employing protocol I, and the product ortho-
aminoquinone 3i was formed in merely 12%–20% yield (Ta-
ble 1, entries 9–10). Oxidation of 2-bromoanisole also met
with limited success, and the product 3k was isolated in only
15% yield based on 29% substrate conversion (entry 11).
Having found a protocol for anisole oxidation by TBHP,
we examined the effect of various experimental parameters
on the efficiency of the transformation. Using 2a as sub-
strate, the 1-catalyzed anisole oxidation was found to be sen-
sitive to the solvent used. For instance, with hexane as solvent
and under the reaction conditions 1:2a:TBHP = 1:100:230 at
room temperature for 14 h, a higher yield (68%) of 3a was
obtained, albeit with 9% conversion (Table 2, entry 2). Poor
substrate conversion could be explained by the insolubility
of the catalyst in hexane. Our previous results showed that 1
dissolves in alcoholic solvents such as MeOH and t-BuOH,
leading to the formation of an alkoxyruthenium complex
(12c). In this work, the ruthenium catalyst was found to be
inactive when MeOH or t-BuOH was used as the solvent,
and negligible substrate conversion was observed (entries 3
and 4). Compared to the case when CH2Cl2 was the solvent,
other halogenated solvents, such as 1,2-dichloroethane, gave
similar results (ca. 50% yield; 45% conversion) under identi-
cal experimental conditions (entry 5). Increasing the TBHP
amount to 7.0 equiv. produced moderate improvement in
product yields (60%) and substrate conversion (49%) for the
1-catalyzed oxidation of 2a (entry 6).
1
reaction mixture by H NMR failed to identify any defined
products other than 3e. Interestingly, when 2,4-
dimethylanisole (2f) was employed as substrate, the 1-cata-
lyzed TBHP reaction furnished benzoic acid 3f as the only
identifiable product in 89% yield (entry 6), presumably via
benzylic C-H oxidation. Based on these findings, the 1-cata-
lyzed oxidation of anisoles to quinone monoketal could be
regarded as a formal regioselective oxidation of the aromatic
C—H bond para to the methoxy group.
Adopting protocol I, oxidaton of other functionalized
anisoles was also explored. For example, the oxidation of
1,3,5-trimethoxybenzene was found to give dimethoxy-
quinone 3g in 76% yield, without any ketal product (Table 1,
entry 7). Notably, effective oxidation of 1,4-dimethoxyben-
zene, which does not bear any available aromatic C—H
bonds, was observed under the “1 + TBHP” conditions, and
a mixture of monoketals 3b (32%) and 3h (19%) was ob-
tained (entry 8).
Recently, Nicolaou et al. (14) reported a facile synthesis
of aminoquinones by oxidation of anilides with hypervalent
iodine reagents (Dess–Martin periodinane, DMP); amino-
quinones have been shown to be valuable synthetic interme-
At higher catalyst loading (5.0 mol%), little improvement
in the substrate conversion (57%) and product yield (49%)
was observed for the 1-catalyzed TBHP oxidation of 2a (en-
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Can. J. Chem. Vol. 83, 2005
Table 3. Catalytic oxidation of selected anisoles using protocol II.
Entry
Substrate
Product
Yield (%)a
Conversion (%)b
1
2
3
4
5
2a
2k
2i
2c
2d
3a
3k
3i
3c
3d
84
49
39
49
33
82
54
31
75
76
Note: Reaction conditions: a mixture of complex 1 (1.0 mol%) and TBHP (3.5 mmol) dissolved with MeOH-t-
BuOH (1:1 by volume) was added slowly through a syringe pump (0.5 mL min–1) over 2 h at 80 °C to a solution of
the substrate (1 mmol) with TBHP (3.5 mmol). The reaction mixture was stirred for a further 2 h and quenched with
sodium thiosulphate (100 mg).
aIsolated yield based on conversion.
bConversions were determined by GLC analysis.
try 7, compare results in entry 1). However, reducing the
catalyst loading to 0.5 mol% led to a higher product yield of
78% for the catalytic oxidation in spite of a 28% substrate
conversion (entry 8). This finding suggests that higher cata-
lyst loading may favor TBHP decomposition; however, the
possibility of catalyst deactivation under the reaction condi-
tions cannot be excluded.
Interestingly, when the catalytic oxidation was conducted
at 80 °C (1:2a:TBHP = 1:100:700), the product ketal was
obtained in 86% yield based on 62% anisole consumption
(entry 9). At room temperature, the identical transformation
resulted in only 45% substrate conversion and 66% yield for
the formation of 3a. Up to 93% yield of 3a was attained
when 14 equiv. TBHP was employed, though the substrate
conversion remained at 62% (entry 11).
complexes (15, 16). To gain insight about the plausible reac-
tive intermediates responsible for the 1-catalyzed anisole
oxidation by TBHP, we performed a stoichiometric reaction
of
a structurally defined oxo-ruthenium complex —
[RuVI(Me3tacn)(CF3CO2)O2](ClO4) — with 2-methoxyanisole
(2a). When 2a (10 mmol) was treated with [RuVI(Me3tacn)-
(CF3CO2)O2](ClO4) (0.5 mmol) in degassed aqueous tert-
butanol [t-BuOH:H2O = 6:3 (by volume)] at room temperature
for 16 h under an inert atmosphere, 2-methoxy-p-benzoquinone
(3l) was isolated in 81% yield. The dioxoruthenium(VI)
complex was reduced to [RuIII2(Me3tacn)(CF3CO2)2(O)](ClO4)
in 76% yield. This finding appears to be consistent with the
results reported by Hirobe and co-workers (13) that reactive
oxo-ruthenium complex is an active intermediate for anisole
oxidation.
Based on the above studies, halogenated solvents are the
solvents of choice for the 1-catalyzed anisole oxidation, and
an elevated temperature seems to produce high product yield
and substrate conversion. While the amount of the TBHP
has a moderate effect on the catalytic oxidation, low catalyst
loading was found to favor high product yield. Taking these
results into consideration, a modified catalytic procedure
(protocol II) was developed (see experimental section for de-
tails). Complex 1 was first dissolved in an MeOH:t-BuOH
mixture (1:1 by volume) and TBHP (3.5 equiv.). The ho-
mogenous mixture was added slowly through a syringe
pump (0.5 mL min–1) to a 1,2-dichloroethane solution con-
taining 2a (1 mmol) and TBHP (3.5 equiv.) at 80 °C. After
4 h of reaction, monoketal 3a was obtained in 84% yield
with 82% substrate conversion (entry 12). By employing
protocol II and 2-bromoanisole (2k) as substrate, the Ru-
catalyzed oxidation afforded 3k in 49% yield based on 54%
conversion (Table 3, entry 2). This result is a significant im-
provement when compared with the results using protocol I
(yield 29%, conversion 15%; see Table 1, entry 11). As
noted earlier, oxidation of 2-methoxyanilide 2i using proto-
col I afforded quinone 3i in 12% yield based on 32% con-
version (Table 1, entry 9). However, when protocol II was
adopted for the same reaction, a better yield of 39% for the
formation of 3i was observed, albeit with 31% conversion
(Table 3, entry 3). Using protocol II, oxidation of methyl
anisoles 2c and 2d by TBHP produced the corresponding
ketals in 49% (3c) and 33% (3d) yields (Table 3, entries 4
and 5). These findings are comparable to those obtained by
employing protocol I (Table 1, entries 3 and 4).
Experimental section
Materials
All the solvents were distilled and dried before used. All
the common laboratory reagents were used as received.
[RuIII(Me3tacn)(CF3CO2)2(H2O)]CF3CO2 (1) was synthesized
using a literature procedure (12c). tert-Butyl hydroperoxide
was obtained commercially as a 70% aqueous solution, and
the oxidant was pretreated according to a procedure de-
scribed by Sharpless and Verhoeven (17) using 1,2-dichloro-
ethane. The TBHP oxidant concentration was standardized
1
by H NMR (5.38 mol L–1 in 1,2-dichloroethane) prior to
use (17).
General procedure for the catalytic oxidation of
anisoles
Protocol I
A mixture of anisole (1 mmol), TBHP in 1,2-dichloro-
ethane (0.65 mL, 2.3 equiv.), and 1 (6.3 mg, 1.0 mol%) was
stirred in CH2Cl2 (5 mL) at room temperature for 14 h. The
reaction was quenched by the addition of sodium thiosul-
phate (100 mg), and the mixture was analyzed by GLC to
determine the substrate consumption. To isolate the reaction
products, the solvent was removed by rotary evaporation,
and the residue was purified by flash column chromatogra-
phy using 1% ethyl acetate in n-hexane as eluant.
Protocol II
Reaction of peroxides with [RuIII(N4)(H2O)2]3+ (N4 = macro-
cyclic amine ligand) is known to generate oxo-ruthenium
A mixture of complex 1 (10.6 mg, 1.6 mol%, MeOH-t-
BuOH, 1:1 by volume, 2 mL) and TBHP (3.5 equiv.) was
© 2005 NRC Canada

Cheung et al.
525
added dropwise (0.5 mL min–1) to a solution of 1,2-dichloro-
ethane (2 mL) containing anisole (1 mmol) and TBHP
(3.5 equiv.) at 80 °C over 2 h. The reaction mixture was
stirred for an additional 2 h. The reaction was quenched by
adding sodium thiosulphate (100 mg), and the substrate con-
sumption was determined by GLC analysis. After removal
of solvent by rotary evaporation, the residue was purified by
flash column chromatography using 0.5%–1% ethyl acetate
in n-hexane as eluant.
3-Bromo-4-tert-butoxy-4-methoxy-2,5-cyclohexadienone
(3k)
TLC: Rf = 0.6 (30% ethyl acetate in n-hexane as develop-
ing solvent). IR (neat KBr, υ cm–1): 1674. 1H NMR
(400 MHz, CDCl3) δH: 7.20 (d, J = 10.3 Hz, 1H), 6.82 (d,
J = 1.9 Hz, 1H), 6.40 (dd, J = 10.3, 1.9 Hz, 1H), 3.35 (s,
3H), 1.27 (s, 9H). 13C NMR (100 MHz, CDCl3) δc: 182.83,
143.50, 143.24, 134.98, 129.86, 96.34, 82.00, 52.07, 26.38.
EI (%): 203 ([M – O-t-Bu], 47), 201 ([M – O-t-Bu], 100).
HRMS (EI) calcd. for [M – O-t-Bu]+: 200.9550; found:
200.9551 (C7H6BrO2).
3,4-Dimethoxy-4-tert-butoxy-2,5-cyclohexadienone (3a)
TLC: Rf = 0.5 (30% ethyl acetate in n-hexane). IR (neat
KBr, υ cm–1): 1670, 1634, 1607. 1H NMR (400 MHz,
CDCl3) δH: 6.92 (d, J = 10.3 Hz, 1H), 6.25 (dd, J = 10.3,
1.7 Hz, 1H), 5.59 (d, J = 1.7 Hz, 1H), 3.80 (s, 3H), 3.42 (s,
3H), 1.24 (s, 9H). 13C NMR (100 MHz, CDCl3) δc: 187.89,
169.89, 141.68, 131.41, 104.99, 98.22, 83.14, 57.71, 53.86,
26.72. EI (%): 153 ([M – O-t-Bu], 100). HRMS (EI) calcd.
for [M – O-t-Bu]+: 153.1552; found: 153.1554 (C8H9O3).
Acknowledgments
This work is supported by the Areas of Excellence
Scheme (AoE/P–10/01) established under the University
Grants Committee (HKSAR, China), The University of
Hong Kong (University Development Fund), and the Hong
Kong Research Grants Council (HKU7384/02P).
4-tert-Butoxy-4-methoxy-2,5-cyclohexadienone (3b)
TLC: Rf = 0.6 (20% ethyl acetate in n-hexane as develop-
ing solvent). IR (neat KBr, υ cm–1): 1644. 1H NMR
(400 MHz, CDCl3) δH: 6.94 (d, J = 10.3 Hz, 2H), 6.26 (d,
J = 10.3 Hz, 2H), 3.45 (s, 3H), 1.25 (s, 9H). 13C NMR
(100 MHz, CDCl3) δc: 185.13, 141.39, 129.59, 95.47, 81.11,
50.75, 26.29. EI (%): 123 ([M – O-t-Bu], 100). HRMS (EI)
calcd. for [M – O-t-Bu]+: 123.0447; found: 123.0446
(C7H7O2).
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3-Methyl-4-tert-butoxy-4-methoxy-2,5-cyclohexadienone
(3c)
TLC: Rf = 0.6 (30% ethyl acetate in n-hexane as develop-
ing solvent). IR (neat KBr, υ cm–1): 1678. 1H NMR
(400 MHz, CDCl3) δH: 7.14 (d, J = 10.4 Hz, 1H), 6.32 (d,
J = 10.4 Hz, 1H), 6.16 (m, 1H), 3.26 (s, 3H), 2.00 (s, 3H),
1.25 (s, 9H). 13C NMR (100 MHz, CDCl3) δc: 185.48,
153.85, 143.57, 130.69, 129.52, 98.02, 81.74, 52.04, 26.72,
17.56. EI (%): 137 ([M – O-t-Bu], 100). HRMS (EI) calcd.
for [M – O-t-Bu]+: 137.0598; found 137.0603 (C8H9O2).
2-Methyl-4-tert-butoxy-4-methoxy-2,5-cyclohexadienone
(3d)
TLC: Rf = 0.6 (30% ethyl acetate in n-hexane as develop-
ing solvent). IR (neat KBr, υ cm–1): 1651. 1H NMR
(400 MHz, CDCl3) δH: 6.94 (dd, J = 10.3 Hz, 1H), 6.67 (m,
1H), 6.24 (d, J = 10.3 Hz, 1H), 3.44 (s, 3H), 1.93 (s, 3H),
1.25 (s, 9H). 13C NMR (100 MHz, CDCl3) δc: 186.20,
141.91, 137.28, 136.64, 129.86, 96.43, 81.35, 51.12, 26.74,
16.00. EI (%): 137 ([M – O-t-Bu], 100). HRMS (EI) calcd.
for [M – O-t-Bu]+: 137.0598; found: 137.0603 (C8H9O2).
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4,4-Di-tert-butoxy-2,5-cyclohexadienone (3h)
TLC: Rf = 0.7 (20% ethyl acetate in n-hexane as develop-
ing solvent), colorless crystal. IR (neat KBr, υ cm–1): 1644.
1H NMR (400 MHz, CDCl3) δH: 7.02 (d, J = 10.4 Hz, 2H),
6.25 (d, J = 10.4 Hz, 2H), 1.26 (s, 18H). 13C NMR
(100 MHz, CDCl3) δc: 185.81, 140.33, 129.67, 99.05, 81.77,
26.72. EI (%): 123 ([M – O-t-Bu], 100). HRMS (EI) calcd.
for [M – O-t-Bu]+: 123.0447; found: 123.0448 (C7H7O2).
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© 2005 NRC Canada