Magnesium-promoted reductive silylation and acylation of tropone and tropone acetal

Article abstract of DOI:10.1246/bcsj.20130156

Magnesium-promoted reductive coupling of chlorotrialkylsilanes and tropone or tropone acetal in DMF, followed by oxidation with 2,3-dichloro-5,6-dicyano-1, 4-benzoquinone in benzene gives 2-trialkylsilyltropones as the main product, and the reductive coup

Full text of DOI:10.1246/bcsj.20130156

© 2013 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 86, No. 10, 1183-1189 (2013) 1183
Magnesium-Promoted Reductive Silylation and Acylation
of Tropone and Tropone Acetal
Hirofumi Maekawa,* Keisuke Nagakubo, Midori Nishi, Makoto Nishimura, and Daisuke Nagahara
Department of Materials Science and Technology, Nagaoka University of Technology,
1603-1 Kamitomioka-cho, Nagaoka, Niigata 940-2188
Received June 4, 2013; E-mail: maekawa@vos.nagaokaut.ac.jp
Magnesium-promoted reductive coupling of chlorotrialkylsilanes and tropone or tropone acetal in DMF, followed
by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in benzene gives 2-trialkylsilyltropones as the main
product, and the reductive coupling of tropone with acid anhydrides under similar reaction conditions yields benzene
derivatives after ring contraction.
Tropone, tropolone, and azulene, a series of compounds
that contain cycloheptatriene rings, are called non-benzenoid
aromatic compounds and have attracted interest owing to their
special reactivity, aromaticity, color, etc.1 Tropone has a single
seven-membered ring, and its structure is quite simple;2-12
however, direct and selective introduction of a substituent,
especially electrophilic substitution in the tropone ring remains
difficult because of the electron-deficient nature of the cyclo-
heptatriene ring.4,13-18 On the other hand, it is well known that
Grignard reagents attack the 2-position of tropone to give
2-alkyl- or 2-phenyltropones after subsequent oxidation.19
Therefore, the general synthetic route to substituted tropones
is ring expansion of six-membered rings bearing a functional
group.20-23
In this study, the magnesium-promoted reduction of tropone
in the presence of chlorotrialkylsilane, followed by oxidation
of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) resulted
in the selective formation of 2-trialkylsilyltropones in only
two steps. Furthermore, the dimethyl acetal of tropone was also
transformed into 2-trialkylsilyltropones under similar reaction
conditions (Scheme 1).
the optimized conditions, excess quantities of magnesium and
chlorotrialkylsilane were required for effective disilylation.
Furthermore, as shown in Table 1, compound 2 was the
predominant product in most cases, although a small quantity
of isomers was detected by gas chromatography and identified
with mass spectrometry. Chlorotrialkylsilanes bearing bulky
alkyl groups decreased the yield of the coupling product, and in
particular, tert-butyl and phenyl groups completely prevented
this coupling reaction.
Then, the transformation of 2a into the corresponding
substituted tropone 3a using various oxidizing agents was
investigated, and DDQ in benzene was found to be the best
oxidant (Table 2). Compounds 2b and 2c were also oxidized
by DDQ to give the corresponding tropones in good yields
(Scheme 2).
The dimethyl acetal of tropone (7,7-dimethoxycyclohepta-
1,3,5-triene, 4) is known to be highly reactive with nucleo-
philes, such as Grignard and organolithium reagents.5,24-27
Reductive coupling of the acetal with chlorotrialkylsilanes
was also performed under reaction conditions that were similar
to those used for tropone. The results are shown in Table 3.
The yield of the tropone precursors was improved for the
trimethylsilyl and isopropyldimethylsilyl groups. In addition,
surprisingly, dimethylphenylsilylation partially occurred de-
spite the presence of the bulky moiety at the 2-position.
Results and Discussion
Reaction conditions for the reductive coupling between
tropone and chlorotrimethylsilane were investigated, and under
OSiMe2R
O
SiMe2R
Mg / RMe2SiCl
DMF
DDQ
O
Benzene
SiMe2R
2
1
OMe
MeO
OMe
SiMe2R
DDQ
Benzene
Mg / RMe2SiCl
DMF
3
5
4
Scheme 1.
Published on the web August 2, 2013; doi:10.1246/bcsj.20130156

1184 Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013)
Table 1. Magnesium-Promoted Silylation of Troponea)
Reductive Silylation and Acylation of Tropone and Tropone Acetal
Table 3. Reductive Coupling of 4 with Silylating Agenta)
OSiMe2R
SiMe2R
OMe
O
MeO
OMe
Mg / RMe2SiCl
SiMe2R
Mg / RMe2SiCl
DMF
DMF
2
1
5
4
Entry
R
GC yield/%
Entry
R
Yield/%
1
2
3
4
5
Me
Et
iso-Pr
tert-Bu
Ph
2a, 48
2b, 46
2c, 27
0b)
1
2
3
4
5
Me
Et
iso-Pr
tert-Bu
Ph
5a, 59
5b, 42
5c, 43
®
5e, 27
b)
0b)
a) Reaction conditions: Tropone (5 mmol), Mg (8 mol equiv),
RMe2SiCl (12 mol equiv), DMF (30 mL), r.t., 12 h, under N2
atmosphere. b) Complex mixture.
a) Reaction conditions: Tropone acetal (5 mmol), Mg (8
mol equiv), RMe2SiCl (12 mol equiv), DMF (30 mL), 2-3 °C,
2 h, under N2 atmosphere. b) Complex mixture.
Table 2. Oxidation of 2a under Various Reaction Conditions
Table 4. Synthesis of 2-Silyltropones
O
O
OMe
OTMS
TMS
SiMe2R
SiMe2R
DDQ
TMS
Oxidation
Benzene
3a
3
2a
5
Entrya)
R
Yield/%
Entry
Oxidant
Solvent
Yield/%
1a)
2
3
4
DDQ
DDQ
Ph3CBF4
C7H7BF4
Benzene
CH2Cl2
CH2Cl2
CH2Cl2
70
18
0
1b)
2
3
4
Me
Et
iso-Pr
Ph
3a, 55
3b, 59
3c, 76
3e, 49
0
a) Silyl enol ether (3 mmol), DDQ (1.2 mol equiv), benzene
(30 mL), r.t., 1 h, under N2 atmosphere.
a) 5 (3 mmol), DDQ (1.2 mol equiv), benzene (30 mL), r.t., 1 h,
under N2 atmosphere. b) 5 (5 mmol), DDQ (1.2 mol equiv),
benzene (50 mL), r.t., 1 h, under N2 atmosphere.
OSiMe2R
O
SiMe2R
SiMe2R
OMe
MeO
OMe
SiMe3
DDQ
Pt-
Pt+
- e
Benzene
3
MeOH, Et4NOTs, NaOMe
2
4
5a
R = Et, 3b
Yield 56%
7.5 F/mol, Quantitative yield
R = iso-Pr, 3c Yield 56%
Scheme 2.
Scheme 3.
Compounds 5 were also oxidized with DDQ to give the
corresponding tropones in moderate to good yields, as shown
in Table 4. The oxidant is also important for this oxidation,
because anodic oxidation of 5a in methanol in an undivided
cell gave tropone acetal 4 as a single product owing to elimi-
nation of the trimethylsilyl cation instead of a proton in the
course of the oxidation, while the elimination of the silyl group
at the allylic position was not the main reaction in the oxida-
tion with DDQ (Schemes 2 and 3).
Next, reductive coupling of 2-methoxytropone and 2-
phenyltropone with chlorotrimethylsilane was performed under
the same reaction conditions, but no coupling products were
formed. Meanwhile, the reaction of 2-methyltropone gave a
mixture of regioisomeric silylated compounds in low yield.
From the perspective of new carbon-carbon formation, the
acylation of tropone with an acid anhydride was investigated
under similar reaction conditions to afford ring contraction
compounds as the main products with a small amount of simply
reduced compounds. The results are summarized in Table 5.
For the coupling reactions of tropone with less reactive acid
anhydride, three equivalent of chlorotrimethylsilane was used to
avoid the formation of silylated compounds (Entries 2 and 3).
The use of acetyl chloride, or propionyl chloride in this
acylation in place of the acid anhydride failed to give a com-
plex mixture. On the other hand, acylation of tropone acetal 4
with acetic anhydride gave the corresponding acylated product
8 without ring contraction as the main product; however, the
yield of 8 was not satisfactory, as shown in Scheme 4.

H. Maekawa et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1185
Table 5. Magnesium-Promoted Acylation of Troponea)
is ineffective and results in undesirable ring contraction in the
case of acylation of tropone.
O
Mg / (RCO)2O / TMSCl
Experimental
1H NMR and 13C NMR were measured on a JEOL JNM AL-
400 (400 MHz), or a JEOL JNM EX-400 (400 MHz) spectrom-
eter. Proton chemical shifts are expressed in parts per million
(ppm) downfield from internal standard (tetramethylsilane,
0.00 ppm), and coupling constants are reported in hertz (Hz).
Carbon chemical shifts are referenced to the carbon signal
of the solvent at 77.00 ppm (CDCl3). Infrared (IR) spectra
were recorded on a JASCO 470Plus FTIR spectrometer. Mass
spectra and high-resolution mass spectra were recorded on a
JEOL JMS-600H spectrometer. Elemental analyses were mea-
sured on Yanaco CHN coder MT-6. Cyclic voltammograms
were measured by ALS model 600. Cyclic voltammetry was
carried out with a three-electrode system using a platinum
working electrode, a platinum counter electrode, and Ag/AgCl
DMF
1
O
OCOR
R
+
O
O
R
6
7
TMSCl
/equiv
Yield 6
/%
Yield 7
/%
Entry
R
ortho:para
1
2
3
Me
Et
n-Pr
5
3
3
6a, 49
6b, 45
6c, 48
3:2
3:1
3:1
0
7b, 20
7c, 11
¹1
as a reference electrode with sweeping rate of 200 mV s in
1% tetrabutylammonium perchlorate/N,N-dimethylformamide
(DMF).
a) Reaction conditions: Tropone (2.55 mmol), Mg (3
mol equiv), acid anhydride (15 mol equiv), DMF (30 mL),
16 h, under N2 atmosphere.
DMF was distilled under reduced pressure after drying over
calcium hydride. For thin-layer chromatography (TLC) analy-
sis, Merck precoated plates (silica gel 60 F254, Art 5715,
0.25 mm) were used. Tropone (1)5,9,10,27 and 7,7-dimethoxy-
cyclohepta-1,3,5-triene (4)27,31 were synthesized according to
the reported procedures.
O
OMe
MeO
OMe
Mg
TMSCl / (CH3CO)2O
DMF
Typical Procedure for Coupling of Tropone with Chloro-
trialkylsilane. Non-pretreated magnesium turnings (0.97 g,
40.0 mmol) for Grignard reagent in dry DMF (25 mL) were
placed in a 100-mL four-necked flask at room temperature
under nitrogen atmosphere and then chlorotrimethylsilane (7.67
mL, 60.0 mmol) was added in one portion. After activation
of the magnesium for 1 h, the mixture was cooled in an ice bath
and tropone (0.53 g, 5.0 mmol) in dry DMF (5 mL) was added
dropwise. Stirring was continued at room temperature for
12 h. Then the reaction mixture was extracted with ether. The
combined organic layers were washed with brine, dried over
anhydrous magnesium sulfate, and the solvent was evaporated
under reduced pressure. The residue was purified by column
chromatography to give 2a.
4
8
Yield 24%
Scheme 4.
To clarify the reaction mechanism, the redox potential of the
starting materials was evaluated by cyclic voltammetry. Acetyl
chloride, acetic anhydride, and chlorotrimethylsilane showed
no significant reduction peaks, while tropone (1) and tropone
acetal 4 showed reduction peaks at ¹1.59 and ¹1.64 V vs. Ag/
AgCl, respectively. Therefore, silylation and acylation may be
initiated by a single electron transfer from magnesium to 1 or 4.
Considering our previous results which suggested that the
more stable anion species would be formed finally after the
second electron transfer,28-30 no cycloheptatrienide derivative
should be generated. A plausible reaction mechanism for the
silylation and acylation of 1 is proposed in Scheme 5. The
carbonyl group derived from the acylating agent stabilizes the
anion 10 and 11 at the ¡-position of the carbonyl group, which
would be the driving force for migration to the benzene ring,
while the silyl group may not stabilize the anion. On the other
hand, in the silylation and acylation of 4, the methoxy group of
13 and 14 is a good eliminating group and thus, the reaction
may proceed without ring contraction (Scheme 6). The reason
for the regioselective coupling is not clear and a reactive
electrophile such as chlorotrimethylsilane may coordinate the
oxygen atom of tropone or the acetal of tropone and attack
the seven-membered ring quickly to give the precursor of 2-
trialkylsilyltropone, predominantly.
1-Trimethylsiloxy-7-trimethylsilylcyclohepta-1,3,5-triene
(2a).
1H NMR (400 MHz, CDCl3): ¤ 0.09 (9H, s), 0.20
(9H, s), 1.65 (1H, d, J = 8.2 Hz), 5.09 (1H, t, J = 8.2 Hz), 5.38
(1H, d, J = 7.2 Hz), 6.02-6.08 (3H, m). 13C NMR (100 MHz,
CDCl3): ¤ ¹1.88, 0.00, 37.99, 104.62, 120.74, 125.12, 127.10,
127.90, 148.21. IR (neat): 3012, 2956, 1610, 1593, 1254, 1186,
1139, 842, 755, 712 cm¹1. MS: m/z 252 (M+). HRMS: Calcd
for C13H24OSi2: 252.1366. Found: 252.1346.
1-Ethyldimethylsiloxy-7-ethyldimethylsilylcyclohepta-
1,3,5-triene (2b). 1H NMR (400 MHz, CDCl3): ¤ 0.06 (3H, s),
0.08 (3H, s), 0.19 (6H, s), 0.61 (2H, q, J = 8.1 Hz), 0.67 (2H, q,
J = 8.1 Hz), 0.94 (3H, t, J = 8.1 Hz), 0.96 (3H, t, J = 8.1 Hz),
1.76 (1H, d, J = 8.5 Hz), 5.09 (1H, t, J = 8.5 Hz), 5.36 (1H, d,
J = 7.0 Hz), 5.98-6.04 (3H, m). 13C NMR (100 MHz, CDCl3):
¤ ¹3.99, ¹3.90, 6.53, 6.61, 7.40, 8.25, 37.36, 104.60, 121.06,
125.23, 127.03, 128.01, 148.44. IR (neat): 3012, 2955, 2876,
1254, 1184, 1138, 837, 789, 699 cm¹1. MS: m/z 280 (M+).
HRMS: Calcd for C15H28OSi2: 280.1679. Found: 280.1659.
In summary, magnesium-promoted silylation of tropone and
tropone acetal yields 2-silyltropones in only two steps, while
acylation of tropone and tropone acetal under similar conditions

1186 Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013)
Reductive Silylation and Acylation of Tropone and Tropone Acetal
Silylation of Tropone
OSiMe2R
SiMe2R
O
O
O
SiMe2R
RMe2SiCl
RMe2SiCl
Mg (+ e)
Mg (+ e)
1
7
8
2
Acylation of Tropone
Formation of ortho Isomer
O
O
O
O
COR
COR
(RCO)2O
Mg (+ e)
Mg (+ e)
1
7
9
OCOR
O
O
(RCO)2O
COR
6 (ortho)
COR
COR
10
Formation of para Isomer
O
O
O
O
O
(RCO)2O
Mg (+ e)
Mg (+ e)
7
1
COR
COR
OCOR
O
(RCO)2O
COR
COR
11
COR
6 (para)
Scheme 5.
Silylation of Tropone Acetal
OMe
OMe
OMe
SiMe2R
- MeO-
MeO
MeO
MeO
OMe
Mg (+ e)
SiMe2R
RMe2SiCl
Mg (+ e)
4
5
12
Acylation of Tropone Acetal
OMe
13
OMe
OMe
MeO
MeO
MeO
OMe
Mg (+ e)
COR
COR
- MeO-
(RCO)2O
Mg (+ e)
4
12
14
8
Scheme 6.

H. Maekawa et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1187
1-Isopropyldimethylsiloxy-7-isopropyldimethylsilylcyclo-
hepta-1,3,5-triene (2c). 1H NMR (400 MHz, CDCl3): ¤ 0.04
(3H, s), 0.07 (3H, s), 0.17 (3H, s), 0.18 (3H, s), 0.97-1.03
(14H, m), 1.90 (1H, d, J = 8.6 Hz), 5.11 (1H, t, J = 8.6 Hz),
5.36 (1H, d, J = 7.2 Hz), 5.96-6.00 (3H, m). 13C NMR (100
MHz, CDCl3): ¤ ¹5.24, ¹3.98, ¹3.71, 13.08, 14.41, 16.67
(©2), 17.72, 17.78, 36.51, 104.68, 121.41, 125.31, 126.78,
127.98, 148.31. IR (neat): 3012, 2955, 2865, 1254, 1183, 1140,
884, 832, 780 cm¹1. MS: m/z 308 (M+). HRMS: Calcd for
C17H32OSi2: 308.1992. Found: 308.1953.
reagent in dry DMF (25 mL) were placed in a 100-mL four-
necked flask at room temperature under nitrogen atmosphere
and then chlorotrimethylsilane (7.67 mL, 60.0 mmol) was added
in one portion. After activation of the magnesium for 1 h, the
mixture was cooled in an ice bath and 7,7-dimethoxycyclo-
hepta-1,3,5-triene (0.76 g, 5.0 mmol) in dry DMF (5 mL) was
added dropwise. Stirring was continued at 2-3 °C for 2 h. Then
the reaction mixture was poured into a saturated sodium
hydrogen carbonate solution and was extracted with ether. The
combined organic layers were washed with brine, dried over
anhydrous magnesium sulfate, and the solvent was evaporated
under reduced pressure. The residue was purified by column
chromatography to give 5a.
1-Methoxy-7-trimethylsilylcyclohepta-1,3,5-triene (5a).
1H NMR (400 MHz, CDCl3): ¤ 0.07 (9H, s), 1.95 (1H, dd,
J = 8.1 Hz, 0.7 Hz), 3.57 (3H, s), 5.16 (1H, dt, J = 8.1 Hz,
2.1 Hz), 5.22 (1H, d, J = 6.6 Hz), 5.97-6.03 (2H, m), 6.08-6.14
(1H, m). 13C NMR (100 MHz, CDCl3): ¤ ¹1.92, 37.06, 55.66,
95.81, 121.58, 124.57, 126.94, 128.04, 154.63. IR (neat): 3012,
2954, 2898, 2831, 1613, 1572, 1247, 1200, 1167, 1138, 1034,
1013, 841, 715 cm¹1. MS: m/z 194 (M+). HRMS: Calcd for
C11H18OSi: 194.1127. Found: 194.1117.
7-Ethyldimethylsilyl-1-methoxycyclohepta-1,3,5-triene
(5b). 1H NMR (400 MHz, CDCl3): ¤ 0.05 (6H, s), 0.59 (2H, q,
J = 7.9 Hz), 0.93 (3H, t, J = 7.9 Hz), 2.01 (1H, dd, J = 8.9 Hz,
1.0 Hz), 3.57 (3H, s), 5.16 (1H, t, J = 8.9 Hz), 5.21 (1H, d, J =
6.9 Hz), 5.96-6.02 (2H, m), 6.07-6.15 (1H, m). 13C NMR (100
MHz, CDCl3): ¤ ¹4.02, ¹3.91, 6.56, 7.38, 36.08, 55.74, 95.93,
121.74, 124.71, 126.99, 128.20, 154.68. IR (neat): 3012, 2953,
2909, 2874, 2832, 1613, 1247, 1200, 1167, 1139, 1013, 836,
819, 714, 699 cm¹1. MS: m/z 208 (M+). HRMS: Calcd for
C12H20OSi: 208.1284. Found: 208.1267.
7-Isopropyldimethylsilyl-1-methoxycyclohepta-1,3,5-tri-
ene (5c). 1H NMR (400 MHz, CDCl3): ¤ 0.03 (3H, s), 0.04
(3H, s), 0.82-1.00 (7H, m), 2.06 (1H, d, J = 9.0 Hz), 3.57 (3H,
s), 5.22 (1H, d, J = 6.6 Hz), 5.71 (1H, t, J = 9.0 Hz), 5.96-6.04
(2H, m), 6.10-6.17 (1H, m). 13C NMR (100 MHz, CDCl3):
¤ ¹5.46, ¹5.23, 13.14, 17.69, 17.77, 34.79, 55.72, 96.01,
121.79, 124.75, 126.91, 128.26, 154.42. IR (neat): 3012, 2954,
2864, 2833, 1613, 1464, 1247, 1200, 1167, 1140, 1013, 1001,
883, 808, 715, 690 cm¹1. MS: m/z 222 (M+). HRMS: Calcd for
C13H22OSi: 222.1440. Found: 222.1483.
Typical Procedure for DDQ Oxidation of 2 and 5. To a
solution of 2a (3.0 mmol) in dry benzene (30 mL) was added
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (0.83 g, 3.6
mmol) at room temperature under nitrogen atmosphere, and
the reaction mixture was stirred for 1 h. Then a 5% sodium
hydroxide solution (100 mL) was added to the reaction mixture
and the products were extracted with ether. The combined
organic layers were washed with sodium hydrogen carbonate
and brine, dried over anhydrous magnesium sulfate, and the
solvent was evaporated under reduced pressure. The residue
was purified by column chromatography to give 3a.
2-Trimethylsilylcyclohepta-2,4,6-trien-1-one
(3a).5
1H NMR (400 MHz, CDCl3): ¤ 0.15 (9H, s), 6.73-6.94 (4H,
m), 7.24 (1H, d, J = 7.8 Hz). 13C NMR (100 MHz, CDCl3):
¤ ¹0.08, 135.09, 135.73, 135.94, 140.83, 142.81, 158.58,
¹1
193.00. IR (neat): 3026, 2953, 1627, 1577, 1246, 841 cm
MS: m/z 178 (M+).
.
2-Ethyldimethylsilylcyclohepta-2,4,6-trien-1-one (3b).
1H NMR (400 MHz, CDCl3): ¤ 0.22 (6H, s), 0.78 (2H, q,
J = 7.7 Hz), 0.93 (3H, t, J = 7.7 Hz), 6.82 (1H, d, J = 11.6 Hz),
6.85-6.92 (1H, m), 6.93-6.99 (1H, m), 6.97-7.03 (1H, m), 7.30
(1H, dd, J = 7.8 Hz, 1.2 Hz). 13C NMR (100 MHz, CDCl3): ¤
¹3.35, 6.82, 7.52, 134.00, 134.62, 134.70, 139.80, 141.96,
156.81, 192.11. IR (neat): 3026, 2952, 2907, 2873, 1627, 1580,
1513, 1458, 1410, 1383, 1273, 1245, 1210, 1010, 981, 837,
813, 785, 697, 646, 617 cm¹1. MS: m/z 192 (M+). HRMS:
Calcd for C11H16OSi: 192.0971. Found: 192.0992.
2-Isopropyldimethylsilylcyclohepta-2,4,6-trien-1-one
(3c). 1H NMR (400 MHz, CDCl3): ¤ 0.19 (6H, s), 0.94 (6H, d,
J = 7.5 Hz), 1.28 (1H, sept, J = 7.5 Hz), 6.82 (1H, d, J =
11.9 Hz), 6.85-9.92 (1H, m), 6.93-6.99 (1H, m), 6.97-7.03
(1H, m), 7.29 (1H, dd, J = 7.8 Hz, 1.2 Hz). 13C NMR (100
MHz, CDCl3): ¤ ¹5.11, 12.63, 17.72, 133.95, 134.54, 134.63,
139.64, 142.18, 156.40, 192.12. IR (neat): 3027, 2952, 2890,
2862, 1628, 1579, 1513, 1459, 1273, 1245, 1210, 1005, 976,
882, 834, 809, 784, 768, 686, 648 cm¹1. MS: m/z 206 (M+).
HRMS: Calcd for C12H18OSi: 206.1127. Found: 206.1121.
2-Dimethylphenylsilylcyclohepta-2,4,6-trien-1-one (3e).
1H NMR (400 MHz, CDCl3): ¤ 0.57 (6H, s), 6.82-6.91 (3H,
m), 6.97-7.04 (1H, m), 7.15-7.20 (1H, m), 7.35-7.41 (3H, m),
7.59-7.63 (2H, m). 13C NMR (100 MHz, CDCl3): ¤ ¹2.74,
127.79, 129.13, 134.32, 134.39, 134.66, 134.96, 137.71,
140.30, 143.18, 156.32, 191.86. IR (neat): 3068, 3047, 3024,
2955, 2899, 1626, 1574, 1514, 1459, 1427, 1275, 1246, 1212,
1113, 977, 814, 787, 739, 702, 658 cm¹1. MS: m/z 240 (M+).
HRMS: Calcd for C15H16OSi: 240.0971. Found: 240.0986.
Typical Procedure for Coupling of 7,7-Dimethoxycyclo-
1-Methoxy-7-dimethylphenylsilylcyclohepta-1,3,5-triene
(5e). 1H NMR (400 MHz, CDCl3): ¤ 0.38 (3H, s), 0.42 (3H,
s), 2.21 (1H, d, J = 8.9 Hz), 3.53 (3H, s), 5.16 (1H, dt, J =
8.9 Hz, 1.6 Hz), 5.27 (1H, d, J = 6.6 Hz), 6.01-6.06 (2H, m),
6.13-6.19 (1H, m), 7.32-7.39 (3H, m), 7.54-7.59 (2H, m).
13C NMR (100 MHz, CDCl3): ¤ ¹3.11, ¹3.07, 36.31, 55.70,
96.39, 121.02, 124.77, 127.39, 127.50, 128.24, 128.85, 133.93,
138.37, 153.97. IR (neat): 3068, 3048, 3011, 2956, 2903, 2832,
1613, 1427, 1248, 1200, 1168, 1140, 1113, 1034, 1013, 813,
701 cm¹1. MS: m/z 256 (M+). HRMS: Calcd for C16H20OSi:
256.1284. Found: 256.1312.
Anodic Oxidation of 5a.
Into a 50-mL beaker-type
undivided cell equipped with platinum plates (4 cm2) as the
cathode and the anode, were introduced 30 mL of freshly dis-
tilled methanol as a solvent, tetraethylammonium p-toluene-
sulfonate (2 g, 6.6 mmol) and sodium methoxide (2 g, 37 mmol)
as supporting electrolytes, and 5a (0.58 g, 3 mmol) as the
hepta-1,3,5-triene (4) with Chlorotrialkylsilane.
Non-
pretreated magnesium turnings (0.97 g, 40.0 mmol) for Grignard

1188 Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013)
Reductive Silylation and Acylation of Tropone and Tropone Acetal
¹1
starting substrate. The electrolysis was carried out under the
constant current conditions (current density: 50 mA cm¹2) until
7.5 F mol of electricity passed through the system at ¹5-
2940, 1760, 1713, 1508, 1459, 1355, 1206, 1167, 1146 cm .
MS: m/z 220 (M+). Anal. Calcd for C13H16O3: C, 70.89; H,
7.32%. Found: C, 70.60; H, 7.51%.
¹1
5 °C. After the electrolysis, the reaction mixture was poured
into a saturated sodium bicarbonate solution, and organic mate-
rial was extracted with ether. The combined ethereal solution
was washed with a saturated sodium chloride solution and
dried over anhydrous magnesium sulfate. After evaporation of
the solvent, the product 4 was obtained in quantitative yield.
The product was characterized by comparison of its spectro-
scopic and chromatographic behaviors with those of authentic
samples.26
Typical Procedure for Coupling Reaction of Tropone
with Acid Anhydride. A mixture of acetic anhydride (3.9 g,
38.25 mmol), chlorotrimethylsilane (1.38 g, 12.75 mmol), and
non-pretreated magnesium turnings (0.18 g, 7.65 mmol) for
Grignard reagent in dry DMF (25 mL) was placed in 50-mL
four-necked flask at 5 °C under nitrogen atmosphere and
tropone (0.27 g, 2.55 mmol) in dry DMF (5 mL) was added
dropwise over 15 min. Stirring was continued at room tem-
perature for 16 h. Then the reaction mixture was poured into
a mixture of ether and saturated sodium hydrogen carbo-
nate solution. Further stirring was continued for 30 min. The
products were extracted with ether. The combined organic
layers were washed with brine, dried over anhydrous magne-
sium sulfate, and the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography
to give 6a.
[2-(2-Oxopropyl)]phenyl Acetate (6a) ortho-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 2.11 (s, 3H), 2.29 (s, 3H),
3.60 (s, 2H), 7.10-7.34 (m, 4H). 13C NMR (100 MHz, CDCl3):
¤ 20.77, 28.94, 46.21, 122.70, 126.40, 126.96, 128.58, 131.45,
149.17, 169.15, 205.53. IR (neat): 3065, 3032, 2934, 1764,
1713, 1490, 1370, 1210, 1173, 1102 cm¹1. MS: m/z 192 (M+).
Anal. Calcd for C11H12O3: C, 68.74; H, 6.29%. Found: C,
68.60; H, 6.41%.
[4-(2-Oxopropyl)]phenyl Acetate (6a) para-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 2.16 (s, 3H), 2.29 (s, 3H),
3.69 (s, 2H), 7.05-7.22 (m, 4H). 13C NMR (100 MHz, CDCl3):
¤ 21.03, 29.29, 50.10, 121.77, 130.33, 131.67, 149.68, 169.39,
206.04. IR (neat): 3039, 3003, 2935, 1760, 1714, 1673, 1508,
1371, 1200, 1165, 1018, 912 cm¹1. MS: m/z 192 (M+). Anal.
Calcd for C11H12O3: C, 68.74; H, 6.29%. Found: C, 68.93; H,
6.52%.
[2-(2-Oxopentyl)]phenyl Butanoate (6c) ortho-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 0.87 (t, 3H, J = 7.3 Hz), 1.04 (t,
3H, J = 7.6 Hz), 1.57 (sext, 2H, J = 7.3 Hz), 1.78 (sext, 2H,
J = 7.6 Hz), 2.36 (t, 2H, J = 7.3 Hz), 2.53 (t, 2H, J = 7.6 Hz),
3.58 (s, 2H), 7.08-7.33 (m, 4H). 13C NMR (100 MHz, CDCl3):
¤ 13.58, 13.65, 17.07, 18.33, 35.96, 43.50, 45.27, 122.62,
126.15, 127.00, 128.37, 131.44, 149.20, 171.75, 207.48. IR
(neat): 3064, 3033, 2965, 2876, 1761, 1710, 1490, 1457, 1364,
1220, 1143, 1106, 753 cm¹1. MS: m/z 248 (M+). Anal. Calcd
for C15H20O3: C, 72.55; H, 8.12%. Found: C, 72.78; H, 8.00%.
[4-(2-Oxopentyl)]phenyl Butanoate (6c) para-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 0.87 (t, 3H, J = 7.3 Hz), 1.04
(t, 3H, J = 7.6 Hz), 1.58 (sext, 2H, J = 7.3 Hz), 1.78 (sext, 2H,
J = 7.6 Hz), 2.36 (t, 2H, J = 7.3 Hz), 2.53 (t, 2H, J = 7.6 Hz),
3.66 (s, 2H), 7.03-7.21 (m, 4H). 13C NMR (100 MHz, CDCl3):
¤ 13.57, 13.62, 17.05, 18.30, 36.16, 43.93, 49.29, 121.73,
130.30, 131.42, 149.67, 172.09, 208.20. IR (neat): 3062, 3016,
2964, 2935, 2876, 1758, 1714, 1507, 1414, 1303, 1203, 1146,
752 cm¹1. MS: m/z 248 (M+). Anal. Calcd for C15H20O3: C,
72.55; H, 8.12%. Found: C, 72.53; H, 7.85%.
1-Propionyloxycyclohepta-1,3,5-triene (7b).
1H NMR
(400 MHz, CDCl3): ¤ 1.20 (t, 3H, J = 7.6 Hz), 2.45 (q, 2H,
J = 7.6 Hz), 2.56 (d, 2H, J = 6.8 Hz), 5.43 (dt, 1H, J = 6.8 Hz,
J = 9.4 Hz), 5.93 (d, 1H, J = 6.3 Hz), 6.22 (dd, 1H, J = 5.5 Hz,
J = 9.4 Hz), 6.38 (dd, 1H, J = 6.3 Hz, J = 11.2 Hz), 6.53 (dd,
1H, J = 5.5 Hz, J = 11.2 Hz). 13C NMR (100 MHz, CDCl3):
¤ 8.89, 27.44, 31.91, 112.88, 120.59, 126.81, 127.29, 129.48,
139.55, 172.76. IR (neat): 3021, 2982, 2943, 2883, 1753, 1643,
1462, 1423, 1355, 1334, 1144, 1068, 711 cm¹1. MS: m/z 164
(M+). Anal. Calcd for C10H12O2: C, 73.15; H, 7.37%. Found:
C, 73.16; H, 7.32%.
1-Butyryloxycyclohepta-1,3,5-triene (7c). 1H NMR (400
MHz, CDCl3): ¤ 0.80 (t, 3H, J = 7.4 Hz), 1.51 (sext, 2H,
J = 7.4 Hz), 2.19 (t, 2H, J = 7.4 Hz), 2.36 (d, 2H, J = 6.8
Hz), 5.23 (dt, 1H, J = 6.8 Hz, J = 9.3 Hz), 5.73 (d, 1H, J = 6.3
Hz), 6.02 (dd, 1H, J = 5.6 Hz, J = 9.3 Hz), 6.18 (dd, 1H, J =
6.3 Hz, J = 11.2 Hz), 6.33 (dd, 1H, J = 5.6 Hz, J = 11.2 Hz).
13C NMR (100 MHz, CDCl3): ¤ 13.56, 18.29, 31.96, 35.93,
112.90, 120.57, 126.82, 127.29, 129.47, 139.57, 171.91. IR
(neat): 3021, 2967, 2935, 2876, 1753, 1643, 1423, 1376, 1335,
1235, 1122, 1070, 846, 710 cm¹1. MS: m/z 178 (M+). Anal.
Calcd for C11H14O2: C, 74.13; H, 7.92%. Found: C, 74.38; H,
7.89%.
Coupling Reaction of 7,7-Dimethoxycyclohepta-1,3,5-tri-
ene (4) with Acetic Anhydride. Non-pretreated magnesium
turnings (0.36 g, 15.0 mmol) for Grignard reagent in dry DMF
(55 mL) were placed in 100-mL four-necked flask at room
temperature under nitrogen atmosphere and acetic anhydride
(3.06 g, 30.0 mmol) and chlorotrimethylsilane (1.92 mL, 15.0
mmol) was added in one portion. After activation of the mag-
nesium for 1 h, the mixture was cooled in an ice bath and
7,7-dimethoxycyclohepta-1,3,5-triene (0.76 g, 5.0 mmol) in dry
DMF (5 mL) was added dropwise. Stirring was continued at
room temperature for 2 h. Then the reaction mixture was
poured into a 1 M sodium hydroxide solution (300 mL) and the
products were extracted with ether. The combined organic
[2-(2-Oxobutyl)]phenyl Propionate (6b) ortho-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 1.01 (t, 3H, J = 7.3 Hz), 1.26
(t, 3H, J = 7.6 Hz), 2.40 (q, 2H, J = 7.3 Hz), 2.58 (q, 2H, J =
7.6 Hz), 3.59 (s, 2H), 7.08-7.32 (m, 4H). 13C NMR (100 MHz,
CDCl3): ¤ 7.69, 9.02, 27.51, 34.81, 45.00, 122.65, 126.19,
127.14, 128.40, 131.45, 149.22, 172.55, 208.01. IR (neat):
3064, 3030, 2979, 2941, 1761, 1713, 1457, 1353, 1220, 1142,
755 cm¹1. MS: m/z 220 (M+). Anal. Calcd for C13H16O3: C,
70.89; H, 7.32%. Found: C, 70.79; H, 7.13%.
[4-(2-Oxobutyl)]phenyl Propionate (6b) para-Isomer.
1H NMR (400 MHz, CDCl3): ¤ 1.03 (t, 3H, J = 7.3 Hz), 1.26
(t, 3H, J = 7.6 Hz), 2.48 (q, 2H, J = 7.3 Hz), 2.58 (q, 2H, J =
7.6 Hz), 3.67 (s, 2H), 7.04-7.22 (m, 4H). 13C NMR (100 MHz,
CDCl3): ¤ 7.66, 8.97, 27.67, 35.24, 48.92, 121.70, 130.27,
131.75, 149.68, 172.97, 208.91. IR (neat): 3067, 3038, 2980,

H. Maekawa et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 10 (2013) 1189
59, 795.
12 T. Rosenau, P. Schmid, P. Kosma, Tetrahedron 2005, 61,
3483.
13 J. H. Rigby, C. H. Senanayake, S. Rege, J. Org. Chem.
1988, 53, 4596.
layers were washed with brine, dried over anhydrous magne-
sium sulfate, and the solvent was evaporated under reduced
pressure. The residue was purified by column chromatography
to give 8.
7-Acetyl-1-methoxycyclohepta-1,3,5-triene (8).5 1H NMR
(400 MHz, CDCl3): ¤ 2.11 (3H, s), 3.54 (1H, d, J = 8.9 Hz),
3.65 (3H, s), 5.22 (1H, t, J = 8.9 Hz), 5.57 (1H, d, J = 6.8 Hz),
6.27 (1H, dd, J = 5.6 Hz, 10.9 Hz), 6.33 (1H, dd, J = 5.6 Hz,
8.9 Hz), 6.39 (1H, dd, J = 6.8 Hz, 10.9 Hz). 13C NMR (100
MHz, CDCl3): ¤ 28.22, 56.52, 56.73, 97.93, 117.96, 124.23,
127.78, 128.56, 150.51, 203.42.
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This work was supported in part by JSPS KAKENHI Grant
Number 22605003 and The Uchida Energy Science Promotion
Foundation.
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