Cobalt-Catalyzed Oxygenation/Dearomatization of Furans

Article abstract of DOI:10.1021/acs.joc.8b01183

The dearomatization of aromatic compounds using cobalt(II) acetylacetonate with triplet oxygen and triethylsilane converts furans, benzofurans, pyrroles, and thiophenes to a variety of products, including lactones, silyl peroxides, and ketones.

Full text of DOI:10.1021/acs.joc.8b01183

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Article
Cobalt-Catalyzed Oxygenation/Dearomatization of Furans
Jonathan P. Oswald, and Keith A. Woerpel
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01183 • Publication Date (Web): 16 May 2018
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The Journal of Organic Chemistry
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Cobalt-Catalyzed Oxygenation/Dearomatization of Furans
Jonathan P. Oswald and K. A. Woerpel*
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Department of Chemistry, New York University, New York, New York 10003, United States
Supporting Information
ABSTRACT: The dearomatization of aromatic compounds using cobalt(II) acetylacetonate with triplet oxygen and triethylsilane
converts furans, benzofurans, pyrroles, and thiophenes to a variety of products, including lactones, silyl peroxides, and ketones.
considering that it involved not only dearomatization, but also
loss of a substituent (the acetyl group).
INTRODUCTION
The cobalt-catalyzed oxygenation of alkenes1 is a syntheti-
cally useful reaction for the production of a number of oxidized
products, including alcohols,2,3 peroxides,2–6 epoxides,6 and ke-
tones.7 The conditions are mild, requiring a bis-(1,3-diketo-
nato)cobalt(II) catalyst, such as Co(acac)2, Co(thd)2,7,8 or
Co(modp)2 (Figure 1),8–10 in the presence of a silane and triplet
oxygen, usually at room temperature. The reaction is chemose-
lective for highly substituted electron-rich carbon–carbon dou-
ble bonds over less substituted ones.10 Because the reaction gen-
erates a carbon-centered radical intermediate, functionalization
typically occurs at the more substituted carbon atom. Alkenes
with aromatic groups are also generally oxidized readily.8,10,11
In this Article, we report that the cobalt catalyst is reactive
enough that it can functionalize the π-system of some aromatic
compounds, including furans, pyrroles, and thiophenes, result-
ing in dearomatization. These reactions lead to the formation of
products such as lactones, silyl peroxides, and ketones.
Scheme 1. Co-Catalyzed Oxygenation of 2-Acetylfuran
The yield of the dearomatized product 2 could be improved
by adjusting the reaction conditions (Table 1). Because lactone
2 (Scheme 1) decomposed in the presence of Co(acac)2 over
time, the catalyst loading was decreased. Furthermore, t-
BuOOH was added to decrease the induction period of the re-
action,11,16 which limited the exposure of the product to the re-
action conditions. The addition of acetylacetone also allowed
for decreased catalyst loading in addition to reduced reaction
times (Table 1, entry 5), likely because it served as a supporting
ligand to limit catalyst decomposition. Excessive amounts of
acetylacetone did not have a positive impact on the yield, how-
ever (Table 1, entry 4). The addition of more equivalents of
Et3SiH also increased the yield, suggesting that an unproductive
pathway consumed the silane, although addition of more than
15 equivalents did not increase the yield significantly (Table 1,
entry 10 and 11). Increasing the reaction temperature to 40 °C
allowed the catalyst loading to be decreased to 2 mol %, and
reactions were complete in four hours with a 62% yield, as de-
1
termined by H NMR spectroscopy (Table 1, entry 12). Other
Figure 1. Structure of Co(II) catalysts
cobalt(II) catalysts were screened (Table 1, entry 7 and 8) using
the optimized conditions, but they proved to be less effective
than Co(acac)2.
RESULTS AND DISCUSSION
During our investigation into the synthesis of endoperoxides
using Co(acac)2, Et3SiH, and oxygen gas,9–11 we observed that
furan-substituted alkenes gave products that suggested that the
aromatic ring had reacted. Although phenyl groups are gener-
ally unreactive under these conditions,8,10,11 furyl groups, con-
sidering that they are not as strongly stabilized by aromatic-
ity,12,13 could be reactive, as they are in other oxidation trans-
formations.14,15 Control experiments supported this hypothesis:
when 2-acetylfuran (1) was treated with the standard oxidation
conditions, lactone 2 was formed as the sole identifiable product
(Scheme 1). The formation of this product was not anticipated,
An important clue as to how the acetyl group was removed
was provided by experiments with a 2-furoic acid ester. When
2-methylfuroate (3) was subjected to the reaction conditions
(Scheme 2), the lactone 2 was formed as the major isolable
product, indicating that a carbomethoxy group can also be re-
moved upon oxidation. In addition, however, two products were
isolated that retained the carbomethoxy group. These products
were the two diastereomers of a bis(silylperoxy) ester (4 and 5).
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Table 1. Optimization of Co-Catalyzed Oxgenation of Fu-
rans
Scheme 4. Proposed Mechanism for the Co-Catalyzed Ox-
ygenation of Furans
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The cobalt-catalyzed oxygenation was general for several
substituted furans (Table 2). When the furan was substituted at
the 2-position with a carbonyl group, lactones were observed as
the principal product. More sterically hindered furans 8a–8c,
which were synthesized from furan 6 as outlined in Scheme 5,
exhibited decreased yields and typically required longer reac-
tion times (Table 2, entries 4-7), likely because of steric hin-
drance during the initial addition of the cobalt-hydride spe-
cies.11 Whereas furan-derived ketones and esters both gave the
product where the side-chain had been removed, the corre-
sponding carboxylic acid was unreactive (entry 3). The peroxi-
dation of disubstituted furans was diastereoselective, forming
the resulting lactones as single diastereomers (entries 4-7). The
relative configuration of the disubstituted lactones was found to
be trans, which was determined by converting silyl peroxide 9
to the known alcohol (9a, Scheme 6) and comparing the 1H and
13C NMR chemical shifts to reported values.21 Isolating only
one diastereomer22 of product was not expected because the co-
balt-catalyzed oxygenation of alkenes is typically not diastere-
oselective.23–25 Selectivity has only been observed in systems
where the diastereotopic faces of the alkenes are sterically quite
distinct.26–29
a Yield by 1H NMR spectroscopy with mesitylene as internal stand-
ard.
Scheme 2. Co-Catalyzed Oxygenation of 2-Methyl Furoate
The isolation of both lactone products and bis(silylperoxy)
esters suggested that the bis(peroxide), or a structurally similar
intermediate, could be a precursor to the lactone products. Upon
standing in a deuterated solvent, a purified sample of bis(si-
lylperoxy) ester 4 decomposed into the lactone (Scheme 3).
This observation suggests that an intermediate resembling the
bis(peroxide) is the first-formed product in these reactions.
Scheme 5. Disubstituted Furan Synthesis
Scheme 3. Bis(peroxide) Decomposition
Table 2. Substituted Furan Screen
A mechanism can be proposed that is consistent with these
observations (Scheme 4). First, addition of a cobalt-hydride11 to
the C4–C5 double bond of furan 1 would form a stabilized, cap-
todative radical (I).17 Capture of this radical by O2 at C4, fol-
lowed by silyl peroxide formation, precedes a second peroxida-
tion via a captodative radical at C2, which would form Co(III)–
alkylperoxo complex II. This Co(III)–alkylperoxo complex re-
sembles the intermediate in the conversion of bis(peroxides) 4
and 5 to lactone 2 (Scheme 3). Conversion of Co(III)–
alkylperoxo complex II to lactone 2 likely first involves cy-
clization of II to dioxetane III, followed by a [2 + 2] cyclorever-
sion of dioxetane III, resulting in cleavage of the carbonyl
group and formation of lactone 2. Related cleavage reactions
have been observed for other α-acylperoxides.18–20 The Co(III)–
alkylperoxo complex II is a more plausible intermediate than
the corresponding silyl peroxide because although bis(perox-
ides) 4 and 5 are converted to lactone 2, it occurs more slowly
(Scheme 3).
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Scheme 6. Formation of Alcohol 9a
The Journal of Organic Chemistry
An experiment with dihydropyran 18 provided additional ev-
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idence supporting this ring-opening mechanistic hypothesis
(Scheme 9, eq 2). Treatment of pyran 18 to the reaction condi-
tions also produced a ring-opened product (19, Scheme 9, eq 4),
likely through a similar pathway. The product, a primary per-
oxide, is difficult to synthesize by traditional cobalt-catalyzed
peroxidation of alkenes, which forms secondary and tertiary
peroxides.4
Considering that the yields determined by NMR spectros-
copy (Table 1) for 2-acetylfuran (1) are about 40% higher than
the isolated yield of this compound (Table 2, entry 1), the prod-
uct must be unstable. In general, α-peroxy carbonyl compounds
can be difficult to synthesize and isolate in high yield.30–32 The
products were found to be sensitive to chromatography, which
was determined by converting the silyl peroxide to the more
stable silyl ether (10, Scheme 7) using triphenylphosphine,33
which resulted in an increased yield compared to the silyl per-
oxide (Table 2, entry 1). The products were also found to de-
compose during the course of the reaction. In the dearomatiza-
tion reaction, the peroxide moiety is sensitive to the reaction
conditions used, particularly to the presence of the metal cata-
lyst.11 Evidence for this sensitivity was found when furan 8c
was subjected to peroxidation conditions, resulting in formation
of lactone 9 in addition to carboxylic acid 11 (Scheme 8). Silyl
ester 12 could have been an intermediate, but it likely hydro-
lyzed to the carboxylic acid 11.34 Based on the yields of carbox-
ylic acid 11 (Scheme 8), most of the starting material was con-
verted to lactone 9 through the mechanism shown in Scheme 4,
but during the course of the reaction, lactone 9 decomposed.
Scheme 9. Co-Catalyzed Oxygenation of 2-Methylfuran
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Scheme 7. Silyl Ether Formation
The position of the carbonyl group on the furan influences
reactivity. When the carbonyl group was placed at the 3-posi-
tion, as seen in ethyl 3-furoate (20), lactone 21 and bis(si-
lylperoxide) 22 were isolated (Scheme 10). A possible explana-
tion for the formation of lactone 21 first involves formation of
cobalt-alkyl species VI (Scheme 10). Decomposition of the per-
oxide at C2 of cobalt-alkyl species VI would generate the lac-
tone (Scheme 10), a process observed for furans substituted at
this position with a hydroperoxyl group.36–38 Finally, another
peroxide formation at C4 would occur to give lactone 21
(Scheme 10). Bis(silylperoxide) 22 (Scheme 10) could be
formed through a ring-opening mechanism analogous to the one
shown in Scheme 9.
Scheme 8. Formation of Carboxylic Acid 11a
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a Yields by H NMR spectroscopy with mesitylene as internal
Scheme 10. Co-Catalyzed Oxygenation of Ethyl 3-Furoate
standard.
The presence of a carbonyl group was not essential to observe
peroxidation of furans, although different types of products
were formed. Peroxidation of 2-methylfuran (13) formed only
one product,35 bis(silylperoxide) 14 (Scheme 9, eq 1), although
the yield was low due to instability of bis(silylperoxide) 14 to
chromatography. This product can be rationalized as involving
initial peroxidation of 13 to form IV, followed by fragmentation
to generate cobalt-alkyl species V (Scheme 9, eq 2). This com-
plex would lead to the primary silyl peroxide, and then a second
peroxidation would form bis(silylperoxide) 14 (Scheme 9, eq
2). 2-Benzylfuran (15, Scheme 9, eq 3) was also subjected to
peroxidation conditions and instead of isolating a ring-opened
product as seen with 2-methylfuran (13, Scheme 9, eq 1), dia-
stereomeric bis(silylperoxides) 16 and 17 were isolated
(Scheme 9, eq 3). In this case, the electron-withdrawing effect
of the phenyl group seems to discourage the ring-opening mech-
anism depicted in Scheme 9, eq 2.
Other furan-containing aromatic compounds undergo
dearomatization during cobalt-catalyzed oxygenation. Both
2,3-benzofuran (23) and 1,3-diphenyl isobenzofuran (25) pro-
duced oxidized products upon exposure to the reaction condi-
tions optimized for 2-acetylfuran (Scheme 11). 2,3-Benzofuran
(23) produced the corresponding silyl peroxide (24, Scheme 11,
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Scheme 13. Co-Catalyzed Oxygenation of Indoles
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eq 5), which likely involves a stabilized benzylic radical inter-
mediate.10 The reaction of 1,3-diphenylisobenzofuran (25) pro-
duced dione 26 (Scheme 11, eq 6), a transformation that may
follow a similar mechanism to the one proposed for reactions of
25 with peroxyl radicals.39
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Scheme 11. Co-Catalyzed Oxygenation of Benzofurans
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The cobalt-catalyzed oxygenation is not restricted to het-
eroaromatic compounds. Phenanthrene (34) was oxidized under
these reaction conditions to form dione 35 (Scheme 14, eq 10).
Anthracene (36) and naphthalene (37) were unreactive to co-
balt-catalyzed oxygenation (Scheme 14, eq 11).2,41
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Scheme 14. Co-Catalyzed Oxygenation of Arenes
This cobalt-catalyzed dearomatization is not limited to fu-
rans. Pyrroles and thiophenes showed similar reactivity. Cobalt-
catalyzed oxygenation of protected pyrrole 27 generated lactam
28 and only one isolable diastereomer of bis(silylperoxide) 29
(Scheme 12, eq 7), which is structurally analogous to bis(si-
lylperoxide) 4 and 5 generated from 2-methylfuroate (Scheme
2). Just as in the case of the furan, this bis(silylperoxide) was
shown to be a competent intermediate for the formation of the
lactam (Scheme 12, eq 8). Similarly, 2-acetylthiophene (30)
generated thiolactone 31, although this reaction only proceeded
to 1% conversion after 23 h (Scheme 12, eq 9). The yields and
rates of reaction of the five-membered ring heteroaromatic
compounds correlate with the degree of aromaticity that these
three heteroarenes exhibit. Furans, the least aromatic of the
three, converted most rapidly, while thiophenes showed the
lowest conversions because of their greater aromaticity.13
CONCLUSION
In summary, we have demonstrated that cobalt-catalyzed ox-
ygenation of aromatic compounds can occur. Under optimal
conditions, furans, pyrroles, and thiophenes were dearomatized,
yielding the corresponding lactones, lactams, and thiolactones.
A mechanism has been suggested involving the formation of a
bis(peroxy) intermediate, followed by cleavage of the carbonyl
group. More structurally complex aromatic compounds, such as
benzofuran and indole derivatives, were also oxidized. Cobalt-
catalyzed oxygenation of indole allows for the incorporation of
acetylacetonate, generating a highly conjugated indole deriva-
tive. Even arenes could be oxidized, indicating this reaction is
not restricted to heteroarenes.
Scheme 12. Co-Catalyzed Oxygenation of Pyrroles and
Thiophenes
EXPERIMENTAL SECTION
General Information. All 1H and 13C NMR spectra were rec-
orded at ambient temperature using Brunker AVIII-400 (400 and
100 MHz, respectively), AV-500 (500 and 125 MHz, respectively)
and AVIII-600 with TCI cryoprobe (600 and 150 MHz, respec-
tively) spectrometers. The data were reported as follows: chemical
shifts are reported in ppm from internal tetramethylsilane or from
residual solvent peaks (1H NMR: CDCl3 δ 7.26; C6D6 δ 7.16;
DMSO δ 2.50; and 13C NMR: CDCl3 δ 77.23; C6D6 δ 128.4; DMSO
δ 39.5) on the δ scale, multiplicity (s = singlet, br = broad, d = dou-
blet, t = triplet, q = quartet, quint = quintet, m = multiplet), coupling
constants (Hz), and integration. Multiplicity of carbon peaks was
determined using HSQC or DEPT experiments. Infrared (IR) spec-
tra were recorded using a Thermo Nicolet AVATAR Fourier Trans-
form IR spectrometer using attenuated total reflectance (ATR).
High-resolution mass spectra (HRMS) were recorded using an Ag-
ilent 6224 Accurate-Mass time-of-flight spectrometer with atmos-
pheric pressure chemical ionization (APCI) or electrospray ioniza-
tion (ESI) ionization sources. Melting points were reported uncor-
rected. Analytical thin layer chromatography was performed on
Silicycle silica gel 60 Å F254 plates. Liquid chromatography was
performed using forced flow (flash chromatography) of the indi-
cated solvent system on Silicycle silica gel (SiO2) 60 (230-400
mesh). THF and Et2O were dried and degassed using a solvent pu-
rification system before use. All reactions were run under a nitro-
Indole (32) also reacted when subjected to cobalt-catalyzed
oxygenation, although the reaction was slow. A trace amount of
conjugated indolinone 33 (Scheme 13) was isolated, but indole
32 went mostly unreacted after 18 h. Indolinone 33 could be
formed by incorporation of acetylacetonate into a 3H-indol-3-
one.40
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The Journal of Organic Chemistry
gen atmosphere in glassware that had been flame-dried under vac-
3.8, 1H), 2.28 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 153.5 (C),
153.0 (C), 144.9 (C), 130.2 (q, J = 31.5, C), 127.1 (CH), 125.5 (q,
J = 3.0, CH), 124.3 (q, J = 270.3, C), 109.1 (CH), 106.5 (CH), 69.6
(CH), 13.7 (CH3); IR (ATR) 3332, 1620, 1322, 1162, 1120, 1066,
1016, 779 cm -1; HRMS (ESI) m/z calcd for C13H10F3O (M + H –
H2O)+ 239.0678, found 239.0681. Anal. Calcd for C13H11F3O2: C,
60.94; H, 4.33. Found: C, 61.03; H, 4.41.
Ketone 8a. To a solution of alcohol 7a (0.316 g, 2.50 mmol) in
EtOAc (4.5 mL) was added 2-iodoxybenzoic acid (2.10 g, 7.51
mmol). The resulting suspension was heated to reflux for 16 h. The
reaction mixture was cooled to room temperature and filtered
through a glass frit. The filter cake was washed with EtOAc (3 × 5
mL) and the filtrate was concentrated in vacuo. Purification by
flash chromatography (hexanes:EtOAc = 90:10) afforded ketone
8a (0.227 g, 73%) as a yellow oil. The spectroscopic data are con-
sistent with the data reported:47 1H NMR (500 MHz, CDCl3) δ 7.10
(d, J = 3.4, 1H), 6.16 (d, J = 3.4, 1H), 2.43 (s, 3H), 2.40 (s, 3H);
13C NMR (125 MHz, CDCl3) δ 186.3, 158.1, 151.8, 119.6, 109.1,
25.9, 14.3; IR (ATR) 1669, 1515, 1372, 1209, 1027, 667 cm-1;
HRMS (APCI) m/z calcd for C7H9O2 (M + H)+ 125.0597, found
125.0596.
uum unless otherwise stated. All reactions that were ran under an
oxygen atmosphere used glassware that was not flame dried. The
synthesis and characterization of compounds were reported previ-
ously. Unless otherwise noted, all reagents were commercially
available. Compounds 3,42 15,43 and 27,44 were prepared by known
methods. Co(acac)2,45 Co(thd)2,6 and Co(modp)2,8 were synthe-
sized by known methods.
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Optimization of Co-Catalyzed Oxygenation of Furans. Opti-
mization reactions were performed as follows: A flask containing
DCE (1,2-dichloroethane, 5 mL) was sparged with oxygen for 15
min. To a separate flask was added CoL2 followed by 2-acetylfuran
(0.050 g, 0.227 mmol). The oxygenated DCE (1.1 mL) was added
to the reaction flask and the reaction flask was sparged with oxygen
for 5 min. A solution of tert-butyl hydroperoxide in CH2Cl2 (1.0 M,
0.011 mL, 0.011 mmol) was added, followed by acetylacetone. Tri-
ethylsilane was added and the mixture stirred at the corresponding
temperature under a balloon of oxygen for the time described in
Table 1. The reaction mixture was filtered through a 3-cm plug of
silica, eluted with CH2Cl2 (50 mL), and concentrated in vacuo.
CDCl3 (0.70 mL) was added and the crude reaction mixture was
transferred to an NMR tube followed by mesitylene (0.020 mL,
0.144 mmol, internal standard). The yield of lactone 2 was deter-
mined based on 1H NMR spectroscopic analysis of the area of the
internal standard (δ 6.77) and the area of the methine group (δ 4.80)
of lactone 2.
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Ketone 8b. To a solution of alcohol 7b (0.301 g, 1.38 mmol) in
EtOAc (2.5 mL) was added 2-iodoxybenzoic acid (1.15 g, 4.14
mmol). The resulting suspension was heated to reflux for 16 h. The
reaction mixture was cooled to room temperature and filtered
through a glass frit. The filter cake was washed with EtOAc (3 × 5
mL) and the filtrate was concentrated in vacuo. Purification by
flash chromatography (hexanes:EtOAc = 90:10) afforded ketone
Synthesis of Disubstituted Furan Substrates
Alcohol 7a. To a solution of furan 6 (0.271 mL, 2.73 mmol) in
THF (4.1 mL) was added a solution of methylmagnesium chloride
in Et2O (3.0 M, 1.0 mL, 3.0 mmol). The resulting reaction mixture
was stirred for 30 min. Saturated aqueous NH4Cl (4 mL) was added
and the mixture was extracted with diethyl ether (3 × 4 mL). The
combined organic layers were dried over Na2SO4, and concentrated
in vacuo. Alcohol 7a was isolated as a yellow oil (0.316 g, 92%)
and used without further purification. The spectroscopic data are
consistent with the data reported:46 1H NMR (500 MHz, CDCl3) δ
6.07 (d, J = 3.0, 1H), 5.87 (d, J = 2.9, 1H), 4.78 (q, J = 6.5, 1H),
2.54 (s, 1H), 2.26 (s, 3H), 1.48 (d, J = 6.5, 3H); 13C NMR (125
MHz, CDCl3) δ 156.0, 151.6, 106.0, 105.9, 63.6, 21.3, 13.6; IR
(ATR) 3345, 2979, 1563, 1220, 1073, 1015, 782 cm-1.
Alcohol 7b. To a solution of magnesium turnings (0.078 g, 3.21
mmol) in THF (2.4 mL) was added 4-bromoanisole (0.201 mL,
1.60 mmol). The resulting solution was stirred for 1 h, then furan 6
(0.160 mL, 1.60 mmol) was added. The resulting solution was
stirred for 1 h. Saturated aqueous NH4Cl (3 mL) was added and the
mixture was extracted with Et2O (3 × 3 mL). The combined organic
layers were dried over Na2SO4 and concentrated in vacuo. Alcohol
7b was isolated as a yellow oil (0.302 g, 86%) and used without
further purification: 1H NMR (500 MHz, CDCl3) δ 7.37 (d, J = 8.5,
2H), 6.91 (d, J = 8.5, 2H), 5.95 (d, J = 3.0, 1H), 5.89 (d, J = 2.9,
1H), 5.73 (d, J = 4.2, 1H), 3.82 (s, 3H), 2.32 (d, J = 4.3, 1H), 2.29
(s, 3H); 13C NMR (125 MHz, CDCl3) δ 159.6 (C), 154.6 (C), 152.5
(C), 133.5 (C), 128.2 (CH), 114.0 (CH), 108.5 (CH), 106.3 (CH),
70.0 (CH), 55.5 (CH3), 13.8 (CH3); IR (ATR) 3409, 1611, 1511,
1172, 1030, 778 cm -1; HRMS (APCI) m/z calcd for C13H13O2 (M
+ H – H2O)+ 201.0910, found 201.0909.
1
8b (0.115 g, 39%) as a yellow oil: H NMR (500 MHz, CDCl3) δ
7.97 (d, J = 8.9, 2H), 7.10 (d, J = 3.4, 1H), 6.96 (d, J = 8.9, 2H),
6.20 (d, J = 3.5, 1H), 3.87 (s, 3H), 2.44 (s, 3H); 13C NMR (125
MHz, CDCl3) δ 181.1 (C), 163.2 (C), 158.2 (C), 151.4 (C), 131.6
(CH), 130.4 (C), 122.0 (CH), 113.8 (CH), 109.0 (CH), 55.6 (CH3),
14.3 (CH3); IR (ATR) 1631, 1597, 1502, 1254, 1161, 1025, 883,
762 cm-1; HRMS (APCI) m/z calcd for C13H13O3 (M + H)+
217.0859, found 217.0857.
Ketone 8c. To a solution of alcohol 7c (0.245 g, 0.960 mmol) in
EtOAc (1.7 mL) was added 2-iodoxybenzoic acid (0.621 g, 2.87
mmol). The resulting suspension was heated to reflux for 16 h. The
reaction mixture was cooled to room temperature and filtered
through a glass frit. The filter cake was washed with EtOAc (3 × 5
mL) and the filtrate was concentrated in vacuo. Purification by
flash chromatography (hexanes:EtOAc = 95:5) afforded ketone 8c
(0.124 g, 51%) as a white solid: mp 110−114 °C; 1H NMR (600
MHz, CDCl3) δ 8.02 (d, J = 7.8, 2H), 7.75 (d, J = 7.8, 2H), 7.15 (d,
J = 2.8, 1H), 6.26 (d, J = 2.6, 1H), 2.49 (s, 3H); 13C NMR (150
MHz, CDCl3) δ 181.0 (C), 159.7 (C), 150.8 (C), 140.9 (C), 133.8
(q, J = 31.5, C), 129.6 (CH), 125.6 (q, J = 3.0, CH) 123.9 (q, J =
271.7, C), 123.7 (CH), 109.7 (CH), 14.4 (CH3); IR (ATR) 1637,
1514, 1329, 1163, 1067, 767 cm-1; HRMS (APCI) m/z calcd for
C13H10F3O2 (M + H)+ 255.0627, found 255.0631. Anal. Calcd for
C13H9F3O2: C, 61.42; H, 3.57. Found: C, 61.15; H, 3.53.
Representative Procedure for the Co-Catalyzed Oxygena-
tion of Aromatic Compounds (Lactone 2). A flask containing
DCE (1,2-dichloroethane, 10 mL) was sparged with oxygen for 15
min. To a separate flask was added Co(acac)2 (0.013 g, 0.050
mmol) followed by furan 1 (0.110 g, 1.00 mmol). The oxygenated
DCE (4.5 mL) was added to the reaction flask and the reaction ves-
sel was sparged with oxygen for 5 min. A solution of tert-butyl hy-
droperoxide in CH2Cl2 (1.0 M, 0.050 mL, 0.050 mmol) was added
followed by acetylacetone (0.103 mL, 1.00 mmol). Triethylsilane
(2.39 mL, 15.0 mmol) was added and the reaction mixture was
stirred at 40 °C under a balloon of oxygen for 2 h. The reaction
mixture was filtered through a 3-cm plug of silica, eluted with
CH2Cl2 (50 mL), and concentrated in vacuo. Purification by flash
chromatography (pentane:Et2O = 80:20) afforded lactone 2 (0.050
g, 23%) as a colorless oil: 1H NMR (500 MHz, CDCl3) δ 4.81 (t, J
Alcohol 7c. To a solution of magnesium turnings (0.076 g, 3.11
mmol) in THF (2.3 mL) was added 4-bromobenzotrifluoride (0.220
mL, 1.56 mmol). The resulting solution was stirred for 1 h, then 6
(0.155 mL, 1.56 mmol) was added. The resulting solution was
stirred for 1 h. Saturated aqueous NH4Cl (3 mL) was added and the
mixture was extracted with Et2O (3 × 3 mL). The combined organic
layers were dried over Na2SO4 and concentrated in vacuo. Purifi-
cation by flash chromatography (hexanes:EtOAc = 90:10) afforded
1
alcohol 7c (0.317 g, 80%) as a yellow oil: H NMR (600 MHz,
CDCl3) δ 7.63 (d, J = 8.3, 2H), 7.57 (d, J = 8.1, 2H), 5.97 (d, J =
3.1, 1H), 5.91 (d, J = 3.1, 1H), 5.82 (d, J = 3.9, 1H), 2.61 (d, J =
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The Journal of Organic Chemistry
Page 6 of 10
= 5.5, 1H), 4.57 (d, J = 10.9, 1H), 4.34 (dd, J = 10.9, 4.7, 1H), 2.71
raphy (twice, pentane:Et2O = 90:10) afforded lactone 9 as a color-
(dd, J = 18.5, 6.5, 1H), 2.62 (d, J = 18.5, 1H), 0.96 (t, J = 8.0, 9H),
0.67 (q, J = 8.0, 6H); 13C NMR (125 MHz, CDCl3) δ 175.3 (C),
79.9 (CH), 71.7 (CH2), 33.1 (CH2), 6.7 (CH3), 3.8 (CH2); IR (ATR)
2957, 2878, 1785, 1165, 1005, 793, 741 cm-1; HRMS (APCI) m/z
calcd for C10H21O4Si (M + H)+ 233.1204, found 233.1203.
less oil (0.004 g, 4%). The spectroscopic data match the reported
values above for lactone 9.
1
2
3
4
5
6
7
8
Lactone 9 prepared from furan 6. Lactone 9 was prepared us-
ing the representative procedure for the Co-catalyzed oxygenation
of aromatic compounds using furan 6 (0.100 mL, 1.00 mmol),
Co(acac)2 (0.051 g, 0.200 mmol), tert-butyl hydroperoxide (0.050
mL, 0.050 mmol), acetylacetone (0.103 mL, 1.00 mmol), and tri-
ethylsilane (2.39 mL, 15.0 mmol) in DCE (4.5 mL). The reaction
time was 4 h. Purification by flash chromatography (pentane:Et2O
= 95:5 → pentane:Et2O = 90:10) afforded lactone 9 as a colorless
oil (0.020 g, 8%). The spectroscopic data match the reported values
above for lactone 9.
Lactone 2 and bis(silylperoxy) esters 4 and 5. Lactone 2 and
bis(silylperoxy) esters 4 and 5 were prepared using the representa-
tive procedure for the Co-catalyzed oxygenation of aromatic com-
pounds using methyl 2-furoate (3, 0.085 mL, 0.793 mmol),
Co(acac)2 (0.010 g, 0.040 mmol), tert-butyl hydroperoxide (0.040
mL, 0.040 mmol), acetylacetone (0.081 mL, 0.793 mmol), and tri-
ethylsilane (1.89 mL, 11.9 mmol) in DCE (3.6 mL). The reaction
time was 2.5 h. Purification by flash chromatography (hex-
ane:EtOAc = 90:10) afforded lactone 2 as a colorless oil (0.028 g,
15%) and diastereomeric silyl peroxides 4 and 5 separately as col-
orless oils (0.015 g, 4%; 0.017 g, 5%, respectively). The spectro-
scopic data are consistent with the data reported above for lactone
2. Bis(silylperoxy) ester 4: 1H NMR (600 MHz, CDCl3) δ 4.70 (m,
1H), 4.36 (d, J = 10.4, 1H), 4.05 (dd, J = 10.4, 4.3, 1H), 3.80 (s,
3H), 2.42 (dd, J = 15.8, 2.6, 1H), 2.36 (dd, J = 15.7, 6.4, 1H), 0.95
(m, 18H), 0.66 (m, 12H); 13C NMR (150 MHz, CDCl3) δ 167.6 (C),
110.6 (C), 83.5 (CH), 72.2 (CH2), 52.8 (CH3), 38.9 (CH2), 6.84
(CH3), 6.76 (CH3), 3.87 (CH2), 3.82 (CH2); IR (ATR) 2954, 1772,
1751, 1127, 1005, 800, 730 cm-1; HRMS (ESI) m/z calcd for
C18H38NaO7Si2 (M + Na)+ 445.2048, found 445.2050. Bis(si-
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Alcohol 9a. To a solution of lactone 9 (0.038 g, 0.154 mmol) in
EtOAc (0.70 mL) was added triphenylphosphine (0.061 g, 0.231
mmol). The reaction mixture was stirred at room temperature for 5
h, then it was concentrated in vacuo. Purification by flash column
chromatography (hexane:EtOAc = 90:10) afforded the silyl ether
1
product as a colorless oil (0.020 g, 57%): H NMR (600 MHz,
C6D6) δ 4.02 (m, 1H), 3.48 (q, J = 5.4, 1H), 2.24 (dd, J = 17.2, 6.6,
1H), 2.10 (dd, J = 17.2, 5.3, 1H), 0.89 (d, J = 6.6, 3H), 0.82 (t, J =
8.0, 9H), 0.37 (q, J = 8.0, 6H); 13C NMR (150 MHz, C6D6) δ 173.5
(C), 83.3 (CH), 74.0 (CH), 38.5 (CH2), 18.5 (CH3), 7.1 (CH2), 5.2
(CH3); IR ATR 2956, 2877, 1786, 1168, 1077, 1010, 747 cm-1;
HRMS (ESI) m/z calcd for C11H22NaO3Si (M + Na)+ 253.1230,
found 253.1231.
To a solution of the above silyl ether (0.020 g, 0.089 mmol) in
Et2O (0.89 mL) was added a solution of tetrabutylammonium fluo-
ride in THF (1.0 M, 0.13 mL, 0.13 mmol). The resulting reaction
mixture was stirred for 1.5 h. H2O (1 mL) was added and the mix-
ture was extracted with Et2O (3 × 1 mL). The combined organic
layers were dried over Na2SO4 and concentrated under a stream of
nitrogen. Purification by flash chromatography (hexane:EtOAc =
50:50) afforded alcohol 9a as a colorless oil (0.001 g, 10%). The
spectroscopic data are consistent with the data reported:21 1H NMR
(600 MHz, CDCl3) δ 4.50 (m, 1H), 4.27 (m, 1H), 2.87 (dd, J = 17.9,
6.5, 1H), 2.53 (dd, J = 17.9, 3.8, 1H), 2.00 (d, J = 4.2, 1H), 1.39 (d,
J = 6.6, 3H); 13C NMR (150 MHz, CDCl3) δ 174.7, 83.9, 73.3, 37.6,
18.8.
1
lylperoxy) ester 5: H NMR (600 MHz, CDCl3) δ 4.71 (m, 1H),
4.32 (dd, J = 10.2, 6.0, 1H), 4.15 (dd, J = 10.2, 3.6, 1H), 3.82 (s,
3H), 2.38 (d, J = 4.8, 2H), 0.99 (t, J = 8.4, 18H), 0.70 (m, 12H); 13
C
NMR (150 MHz, CDCl3) 168.1 (C), 109.9 (CH), 82.7 (CH), 72.6
(CH2), 52.8 (CH3), 39.1 (CH2), 6.85 (CH3), 6.74 (CH3), 3.8 (CH2);
IR (ATR) 2954, 1767, 1240, 1106, 807, 740 cm-1; HRMS (ESI) m/z
calcd for C18H38NaO7Si2 (M + Na)+ 445.2048, found 445.2047.
Lactone 9 prepared from furan 8a. Lactone 9 was prepared
using the representative procedure for the Co-catalyzed oxygena-
tion of aromatic compounds using ketone 8a (0.100 g, 0.806
mmol), Co(acac)2 (0.010 g, 0.040 mmol), tert-butyl hydroperoxide
(0.040 mL, 0.040 mmol), acetylacetone (0.083 mL, 0.806 mmol),
and triethylsilane (1.93 mL, 12.1 mmol) in DCE (3.7 mL). The re-
action time was 4 h. Purification by flash chromatography (hex-
ane:EtOAc = 90:10) afforded lactone 9 as a colorless oil (0.038 g,
19%): 1H NMR (600 MHz, C6D6) δ 4.55 (q, J = 6.9, 1H), 3.85 (d,
J = 6.9, 1H), 2.31 (dd, J = 18.4, 2.2, 1H), 2.09 (dd, J = 18.4, 6.9,
1H), 0.94 (t, J = 8.0, 9H), 0.75 (d, J = 6.9, 3H), 0.60 (q, J = 7.9,
6H); 13C NMR (150 MHz, CDCl3) δ 172.7 (C), 84.8 (CH), 78.4
(CH), 31.9 (CH2), 18.5 (CH3), 6.5 (CH3), 3.6 (CH2); IR (ATR)
1789, 1617, 1453, 1329, 1166, 743 cm-1; HRMS (APCI) m/z calcd
for C11H22NaO4Si (M + Na)+ 269.1180, found 269.1184.
Lactone 9 prepared from furan 8b. Lactone 9 was prepared
using the representative procedure for the Co-catalyzed oxygena-
tion of aromatic compounds using ketone 8b (0.100 g, 0.463
mmol), Co(acac)2 (0.006 g, 0.023 mmol), tert-butyl hydroperoxide
(0.023 mL, 0.023 mmol), acetylacetone (0.047 mL, 0.463 mmol),
and triethylsilane (1.11 mL, 6.94 mmol) in DCE (2.1 mL). The re-
action time was 3 h. Purification by repeated flash chromatography
(twice, pentane:Et2O = 90:10) afforded lactone 9 as a colorless oil
(0.008 g, 7%). The spectroscopic data matched the reported values
above for lactone 9.
Silyl ether 10. A flask containing DCE (1,2-dichloroethane, 10
mL) was sparged with oxygen for 15 min. To a separate flask was
added Co(acac)2 (0.013 g, 0.05 mmol) followed by 2-acetylfuran
(0.110 g, 1.00 mmol). The oxygenated DCE (4.5 mL) was added to
the reaction flask and the reaction vessel was sparged with oxygen
for 5 min. A solution of tert-butyl hydroperoxide (0.050 mL, 0.050
mmol) in CH2Cl2 was added followed by acetylacetone (0.103 mL,
1.00 mmol). Triethylsilane (2.39 mL, 15.0 mmol) was added and
the reaction mixture was stirred at 40 °C under a balloon of oxygen
for 2 h. Triphenylphosphine (0.262 g, 1.00 mmol) was added di-
rectly to the reaction mixture, which was stirred at room tempera-
ture for 3 h. The reaction mixture was filtered through a 3-cm plug
of silica, eluted with CH2Cl2 (50 mL), and concentrated in vacuo.
Purification by flash chromatography (pentane:Et2O = 80:20) af-
1
forded silyl ether 10 (0.072 g, 33%) as a colorless oil: H NMR
(500 MHz, CDCl3) δ 4.58 (m, 1H), 4.35 (dd, J = 9.7, 4.7, 1H), 4.14
(dd, J = 9.7, 1.8, 1H), 2.67 (dd, J = 17.5, 6.2, 1H), 2.41 (dd, J =
17.5, 2.4, 1H), 0.92 (t, J = 8.1, 9H), 0.59 (q, J = 8.0, 6H); 13C NMR
(125 MHz, CDCl3) δ 175.9 (C), 76.3 (CH), 67.9 (CH2), 38.3 (CH2),
6.7 (CH3), 4.7 (CH2); IR (ATR) 2955, 2877, 1780, 1162, 1092, 999,
727 cm-1; HRMS (APCI) m/z calcd for C10H21O3Si (M + H)+
217.1254, found 217.1256. Anal. Calcd for C10H20O3Si: C, 55.52;
H, 9.32. Found: C, 55.78; H, 9.21.
Lactone 9 and Carboxylic acid 11 from furan 8c. Lactone 9
was prepared using the using the representative procedure for the
Co-catalyzed oxygenation of aromatic compounds using ketone 8c
(0.055 g, 0.220 mmol), Co(acac)2 (0.003 g, 0.011 mmol), tert-butyl
hydroperoxide (0.011 mL, 0.011 mmol), acetylacetone (0.022 mL,
Lactone 9 prepared from furan 8c. Lactone 9 was prepared
using the representative procedure for the Co-catalyzed oxygena-
tion of aromatic compounds using ketone 8c (0.100 g, 0.393
mmol), Co(acac)2 (0.005 g, 0.020 mmol), tert-butyl hydroperoxide
(0.020 mL, 0.050 mmol), acetylacetone (0.040 mL, 0.393 mmol),
and triethylsilane (0.942 mL, 15.0 mmol) in DCE (1.8 mL). The
reaction time was 2.25 h. Purification by repeated flash chromatog-
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The Journal of Organic Chemistry
0.220 mmol), and triethylsilane (0.530 mL, 3.30 mmol) in DCE
2877, 1726, 1168, 1006, 863, 730 cm-1; HRMS (APCI) m/z calcd
for C11H24KO4Si (M + K)+ 287.1075, found 287.1067.
(1.0 mL). The reaction time was 2.5 h. CDCl3 (0.70 mL) was added
and the crude reaction mixture was transferred to an NMR tube fol-
lowed by mesitylene (0.031 mL, 0.220 mmol, internal standard).
The yield of lactone 9 and carboxylic acid 11 was determined based
on 1H NMR spectroscopic analysis of the area of the internal stand-
ard (δ 6.77) and the area of the methine group of lactone 9 (δ 4.81)
and carboxylic acid 11 (δ 8.16). Purification by flash chromatog-
raphy (hexane:EtOAc = 75:25 → hexane:EtOAc = 50:50) afforded
an analytical sample of carboxylic acid 11 (0.004 g, 10%) as a white
solid. The spectroscopic data are consistent with the data re-
ported:48 1H NMR (600 MHz, DMSO) δ 8.14 (d, J = 8.0, 2H), 7.88
(d, J = 8.1, 2H); 13C NMR (150 MHz, DMSO) δ 166.2, 134.9, 132.3
(q, J = 31.6), 130.1, 125.6, 124.7 (q, J = 272.2); IR (ATR) 3312,
1762 cm-1.
Silyl peroxide 14. Silyl peroxide 14 was prepared using the rep-
resentative procedure for the Co-catalyzed oxygenation of aromatic
compounds using 2-methylfuran (13, 0.080 mL, 0.914 mmol),
Co(acac)2 (0.012 g, 0.046 mmol), tert-butyl hydroperoxide (0.046
mL, 0.046 mmol), acetylacetone (0.093 mL, 0.914 mmol), and tri-
ethylsilane (2.19 mL, 13.7 mmol) in DCE (4.2 mL). The reaction
time was 2 h. Purification by flash chromatography (hexane:EtOAc
= 95:5) afforded silyl peroxide 14 as a colorless oil (0.015 g, 4%):
1H NMR (600 MHz, CDCl3) δ 4.44 (m, 1H), 4.34 (dd, J = 12.1, 3.8,
1H), 4.27 (dd, J = 12.1, 5.7, 1H), 4.20 (dd, J = 12.1, 5.6, 1H), 4.08
(dd, J = 12.1, 4.9, 1H), 2.08 (s, 3H), 0.98 (m, 18H), 0.70 (m, 12H);
13C NMR (150 MHz, CDCl3) δ 171.0 (C), 80.8 (CH), 74.4 (CH2),
62.2 (CH2), 21.0 (CH3), 6.85 (CH3), 6.84 (CH3), 3.81 (CH2), 3.79
(CH2); IR (ATR) 2958, 2878, 1747, 1236, 849, 740 cm-1; HRMS
(APCI) m/z calcd for C17H39O6Si2 (M + H)+ 395.2279, found
395.2277.
1
2
3
4
5
6
7
8
Lactone 21 and Silyl peroxide 22. Lactone 21 and silyl perox-
ide 22 were prepared using the representative procedure for the Co-
catalyzed oxygenation of aromatic compounds using ethyl 3-fu-
roate (20, 0.096 mL, 0.714 mmol), Co(acac)2 (0.009 g, 0.036
mmol), tert-butyl hydroperoxide (0.036 mL, 0.036 mmol), acety-
lacetone (0.073 mL, 0.714 mmol), and triethylsilane (1.71 mL, 10.7
mmol) in DCE (3.2 mL). The reaction time was 2.5 h. Purification
by flash chromatography (hexane:EtOAc = 95:5) afforded lactone
21 as a colorless oil (0.037 g, 17%) and silyl peroxide 22 as a col-
orless oil (0.044 g, 14%). Lactone 21: 1H NMR (600 MHz, CDCl3)
δ 4.58 (d, J = 10.9, 1H), 4.53 (d, J = 10.9, 1H), 4.27 (q, J = 7.2,
2H), 3.04 (d, J = 18.5, 1H), 2.90 (d, J = 18.6, 1H), 1.31 (t, J = 7.2,
3H), 0.96 (t, J = 8.0, 9H), 0.68 (q, J = 7.9, 6H); 13C NMR (150
MHz, CDCl3) δ 173.4 (C), 168.4 (C), 87.9 (C), 71.7 (CH2), 62.5
(CH2), 36.3 (CH2), 14.2 (CH3), 6.7 (CH3), 3.7 (CH2); IR (ATR)
2957, 1794, 1743, 1310, 1030, 803, 742 cm-1; HRMS (APCI) m/z
calcd for C13H25O6Si (M + H)+ 305.1415, found 305.1417. Silyl
peroxide 22: 1H NMR (600 MHz, CDCl3) δ 8.08 (s, 1H), 4.81 (d,
J = 12.0, 1H), 4.62 (d, J = 12.1, 1H), 4.32 (d, J = 12.0, 1H), 4.22
(m, 2H), 4.17 (d, J = 12.0, 1H), 1.29 (t, J = 7.2, 3H), 0.95 (t, J =
8.1, 18H), 0.66 (m, 12H); 13C NMR (150 MHz, CDCl3) δ 168.6
(C), 160.7 (CH), 85.6 (C), 74.0 (CH2), 61.6 (CH2), 60.6 (CH2), 14.2
(CH3), 6.77 (CH3), 6.73 (CH3), 3.79 (CH2), 3.73 (CH2); IR (ATR)
2956, 2878, 1735, 1172, 1019, 847, 733 cm-1; HRMS (APCI) m/z
calcd for C19H44NO8Si2 (M + NH4)+ 470.2600, found 470.2600.
Silyl peroxide 24. Silyl peroxide 24 was prepared using the rep-
resentative procedure for the Co-catalyzed oxygenation of aromatic
compounds using benzofuran 23 (0.090 mL, 0.847 mmol),
Co(acac)2 (0.011 g, 0.042 mmol), tert-butyl hydroperoxide (0.042
mL, 0.042 mmol), and triethylsilane (0.676 mL, 4.23 mmol) in
DCE (3.8 mL). The reaction time was 6 h. Purification by flash
chromatography (pentane → pentane:Et2O = 95:5) afforded silyl
peroxide 24 (0.099 g, 44%) as a colorless oil: 1H NMR (600 MHz,
CDCl3) δ 7.43 (d, J = 7.2, 1H), 7.30 (t, J = 7.8, 1H), 6.93 (t, J = 7.2,
1H), 6.89 (d, J = 7.8, 1H), 5.61 (dd, J = 6.0, 1.8, 1H), 4.78 (dd, J =
11.4, 2.0, 1H), 4.47 (dd, J = 11.1, 6.0, 1H), 0.99, (t, J = 7.8, 9H),
0.70 (q, J = 7.8, 6H); 13C NMR (150 MHz, CDCl3) δ 161.8 (C),
131.6 (CH), 127.3 (CH), 123.5 (C), 120.7 (CH), 110.7 (CH), 84.9
(CH), 75.4 (CH2), 6.9 (CH3), 3.9 (CH2); IR (ATR) 2954, 2877,
1612, 1600, 1479, 1246, 1015, 963, 832 cm-1; HRMS (APCI) m/z
calcd for C14H21O2Si (M + H – H2O)+ 249.1305, found 249.1306.
Dione 26. Dione 26 was prepared using the representative pro-
cedure for the Co-catalyzed oxygenation of aromatic compounds
using isobenzofuran 25 (0.050 g, 0.185 mmol), Co(acac)2 (0.002 g,
0.009 mmol), tert-butyl hydroperoxide (0.009 mL, 0.009 mmol),
and triethylsilane (0.440 mL, 2.78 mmol) in DCE (1.7 mL). The
reaction time was 18 h. Following the workup as described above,
dione 26 isolated as a white, off-yellow solid (0.041 g, 78%). Char-
acterization was performed without further purification. The spec-
troscopic data are consistent with the data reported:49 mp 130–133
°C, which is lower than reported literature value (138–139 °C)49
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Bis(silylperoxide) 16 and 17. Bis(silylperoxides) 16 and 17
were prepared using the representative procedure for the Co-cata-
lyzed oxygenation of aromatic compounds using 2-benzylfuran
(15, 0.055 g, 0.348 mmol), Co(acac)2 (0.005 g, 0.017 mmol), tert-
butyl hydroperoxide (0.017 mL, 0.017 mmol), acetylacetone (0.036
mL, 0.348 mmol), and triethylsilane (0.833 mL, 5.22 mmol) in
DCE (1.6 mL). The reaction time was 2.5 h. Purification by flash
chromatography (hexane → hexane:EtOAc = 98:2) afforded bis(si-
lylperoxides) 16 and 17 as an inseparable 59:41 mixture of diastere-
1
omers: H NMR (600 MHz, CDCl3) δ 7.32–7.21 (m, 8.5H), 4.75
(m, 0.6H), 4.43 (m, 1H), 4.17 (d, J = 9.8, 0.7H), 4.13 (dd, J = 9.8,
4.4, 0.7H), 4.09 (dd, J = 9.6, 6.4, 1H), 4.00 (dd, J = 9.7, 4.9, 1H),
3.25 (d, J = 10.6, 0.7H), 3.23 (d, J = 10.5, 1H), 3.15 (d, J = 13.9,
1H), 2.27 (dd, J = 15.0, 7.2, 0.7H), 2.08 (dd, J = 15.0, 8.1, 1H),
1.96 (m, 1.7H), 1.05–0.93 (m, 36.9H), 0.80–0.62 (m, 24.6H); 13C
NMR (150 MHz, CDCl3) δ 136.9 (C), 136.6 (C), 130.74 (CH),
130.71 (CH), 128.2 (CH), 128.1 (CH), 126.69 (CH), 126.66 (CH),
114.6 (C), 114.0 (C), 85.3 (CH), 83.7 (CH), 72.7 (CH2), 71.9 (CH2),
42.2 (CH2), 40.9 (CH2), 37.5 (CH2), 37.2 (CH2), 6.93 (CH3), 6.91
(CH3), 6.90 (CH3), 6.84 (CH3), 4.04 (CH2), 3.99 (CH2), 3.89 (CH2),
3.80 (CH2); IR (ATR) 2954, 2877, 1456, 1239, 1006, 803, 740, 701
cm-1; HRMS (APCI) m/z calcd for C23H46NO5Si2 (M + NH4)+
472.2909, found 472.2904.
1
due to residual impurities; H NMR (600 MHz, CDCl3) δ 7.72 (d,
Silyl peroxide 19. Silyl peroxide 19 was prepared using the rep-
resentative procedure for the Co-catalyzed oxygenation of aromatic
compounds using pyran 18 (0.054 mL, 0.594 mmol), Co(acac)2
(0.008 g, 0.030 mmol), tert-butyl hydroperoxide (0.030 mL, 0.030
mmol), acetylacetone (0.061 mL, 0.594 mmol), and triethylsilane
(1.42 mL, 8.92 mmol) in DCE (2.7 mL). The reaction time was 2
h. Purification by flash chromatography (hexane:EtOAc = 95:5) af-
forded silyl peroxide 19 as a colorless oil (0.038 g, 26%): 1H NMR
(600 MHz, CDCl3) δ 8.05 (s, 1H), 4.18 (t, J = 6.3, 2H), 4.00 (t, J =
6.1, 2H), 1.73 (m, 4H), 0.99 (t, J = 8.0, 9H), 0.69 (q, J = 7.9, 6H);
13C NMR (150 MHz, CDCl3) δ 161.3 (CH), 76.3 (CH2), 63.9 (CH2),
25.5 (CH2), 24.6 (CH2), 6.9 (CH3), 3.9 (CH2); IR (ATR) 2955,
J = 1.2, 2H), 7.70 (d, J = 1.1, 2H), 7.63 (d, J = 1.1, 4H), 7.53 (m,
2H), 7.39 (m, 4H); 13C NMR (150 MHz, CDCl3) δ 196.8 (C), 140.2
(C), 137.4 (C), 133.2 (CH), 130.6 (CH), 130.0 (CH), 129.9 (CH),
128.5 (CH); IR (ATR) 1659, 1596, 1278, 937, 704 cm-1; HRMS
(ESI) m/z calcd for C20H14NaO2 (M + Na)+ 309.0886, found
309.0898.
Lactam 28 and Bis(silylperoxy) ketone 29. Lactam 28 and
bis(silyl peroxy) ketone 29 were prepared using the representative
procedure for the Co-catalyzed oxygenation of aromatic com-
pounds using pyrrole 27 (0.100 g, 0.380 mmol), Co(acac)2 (0.020
g, 0.076 mmol), tert-butyl hydroperoxide (0.019 mL, 0.019 mmol),
acetylacetone (0.039 mL, 0.380 mmol), and triethylsilane (0.909
mL, 5.70 mmol) in DCE (1.7 mL). The reaction time was 2.5 h.
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Purification by flash chromatography (pentane:Et2O = 95:5 → pen-
tane:Et2O = 90:10) afforded lactam 28 as a colorless oil (0.024 g,
16%) and silyl peroxide 29 as a colorless oil (0.004 g, 2%). Lactam
28: 1H NMR (600 MHz, CDCl3) δ 7.93 (d, J = 8.4, 2H), 7.33 (d, J
= 8.4, 2H), 4.64 (t, J = 5.4, 1H), 4.24 (d, J = 12.0, 1H), 4.00 (dd, J
= 12.0, 5.4, 1H), 2.72 (dd, J = 18.4, 6.7, 1H), 2.53 (d, J = 18.3, 1H),
2.44 (s, 3H), 0.92 (t, J = 7.8, 9H), 0.61 (q, J = 7.8, 6H); 13C NMR
(150 MHz, CDCl3) δ 170.9 (C), 145.4 (C), 135.4 (C), 129.8 (CH),
128.3 (CH), 76.0 (CH), 51.6 (CH2), 37.3 (CH2), 21.9 (CH3), 6.8
(CH3), 3.8 (CH2); IR (ATR) 2956, 2877, 1742, 1362, 1169, 1089,
663 cm-1; HRMS (ESI) m/z calcd for C17H27NNaO5SSi (M + Na)+
408.1271, found 408.1268. Anal. Calcd for C17H27NO5SSi: C,
52.99; H, 7.06. Found: C, 52.69; H, 7.06. Bis(silylperoxy) ketone
29: 1H NMR (600 MHz, CDCl3) δ 7.80 (d, J = 8.4, 2H), 7.29 (d, J
= 8.3, 2H), 4.87 (m, 1H), 3.78 (dd, J = 9.6, 6.6, 1H), 3.48 (dd, J =
9.6, 3.6, 1H), 2.60 (dd, J = 14.4, 7.6, 1H), 2.42 (s, 3H), 2.40 (s, 3H),
2.28 (dd, J = 14.2, 5.7, 1H), 0.99 (t, J = 7.8, 9H), 0.94 (t, J = 7.8,
9H), 0.73 (q, J = 7.8, 6H), 0.69 (q, J = 7.8, 6H); 13C NMR (150
MHz, CDCl3) δ 201.6 (C), 143.7 (C), 136.0 (C), 129.5 (CH), 128.4
(CH), 103.3 (C), 82.2 (CH), 53.4 (CH2), 41.0 (CH2), 27.4 (CH3),
21.8 (CH3), 6.84 (CH3), 6.83 (CH3), 4.0 (CH2), 3.8 (CH2); IR
(ATR) 2956, 2877, 1737, 1349, 1011, 784, 676 cm-1; HRMS
(APCI) m/z calcd for C25H49N2O7SSi2 (M + NH4)+ 577.2794, found
577.2799.
lower than reported literature value (207–208 °C)50 due to impuri-
ties which could not be removed by chromatography; H NMR
1
1
2
3
4
5
6
7
8
(500 MHz, CDCl3) δ 8.20 (d, J = 8.0, 2H), 8.03 (d, J = 8.0, 2H),
7.73 (t, J = 7.5, 2H), 7.48 (t, J = 8.0, 2H); 13C NMR (125 MHz,
CDCl3) δ 180.6 (C), 136.2 (CH), 136.1 (C), 131.3 (C), 130.7 (CH),
129.8 (CH), 124.2 (CH); IR (ATR) 1679, 1596, 1452, 1286, 924,
766 cm-1; HRMS (APCI) m/z calcd for C14H9O2 (M + H)+
209.0597, found 209.0597.
ASSOCIATED CONTENT
Supporting Information
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Spectral data and stereochemical proof of alcohol 9a. The Support-
ing Information is available free of charge on the ACS Publications
website.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kwoerpel@nyu.edu
Notes
The authors declare no competing financial interest.
Thiolactone 31. Thiolactone 31 was prepared using the repre-
sentative procedure for the Co-catalyzed oxygenation of aromatic
compounds using thiophene 30 (0.085 mL, 0.793 mmol), Co(acac)2
(0.041 g, 0.159 mmol), tert-butyl hydroperoxide (0.039 mL, 0.039
mmol), acetylacetone (0.081 mL, 0.793 mmol), and triethylsilane
(1.89 mL, 11.9 mmol) in DCE (3.6 mL). The reaction time was 23
h. Purification by flash chromatography (hexane:EtOAc = 90:10)
afforded thiolactone 31 as a colorless oil (0.001 g, 1%). 1H NMR
(600 MHz, CDCl3) δ 4.85 (m, 1H), 3.78 (dd, J = 12.2, 2.2, 1H),
3.59 (dd, J = 12.3, 4.6, 1H), 2.77 (dd, J = 17.4, 5.4, 1H), 2.72 (dd,
ACKNOWLEDGMENT
This research was supported by the National Institute of General
Medical
Sciences
of
the
National
Institutes
of
Health (1R01GM118730). NMR spectra acquired with the
TCI cryoprobe and Bruker Advance 400 were supported by the Na-
tional Institutes of Health (OD016343) and the National Sci-
ence Foundation (CHE-01162222), respectively. We thank Dr.
Chin Lin for his assistance with NMR spectroscopy and mass spec-
trometry.
J = 17.4, 3.0, 1H), 1.00 (t, J = 7.8, 9H), 0.71 (q, J = 7.8, 6H); 13
C
REFERENCES
NMR (150 MHz, CDCl3) δ 205.3 (C), 81.2 (CH), 45.3 (CH2), 36.3
(CH2), 6.9 (CH3), 3.9 (CH2); IR (ATR) 2955, 1711, 1008, 792, 742
cm-1; HRMS (APCI) m/z calcd for C10H19O2SSi (M + H – H2O)+
231.0870, found 231.0870.
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Dione 33. Dione 33 was prepared using the representative pro-
cedure for the Co-catalyzed oxygenation of aromatic compounds
using indole 32 (0.100 g, 0.854 mmol), Co(acac)2 (0.044 g, 0.171
mmol), tert-butyl hydroperoxide (0.043 mL, 0.043 mmol), and tri-
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1
33 as a red solid (0.003 g, 2%): mp 205−208 °C; H NMR (600
MHz, CDCl3) δ 10.19 (br, 1H), 7.65 (d, J = 9.0, 1H), 7.52 (t, J =
9.6, 1H), 7.03 (t, J = 9.0, 1H), 6.95 (d, J = 9.6, 1H), 2.58 (s, 3H),
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188.8 (C), 153.1 (C), 140.4 (C), 137.9 (CH), 125.9 (CH), 122.8
(CH), 119.6 (C), 117.0 (C), 112.4 (CH), 32.5 (CH3), 29.2 (CH3); IR
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are consistent with the data reported:50 mp 192–194 °C, which is
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