Cerium(IV) ammonium nitrate-mediated oxidation of mono-aryl-substituted methylenecyclobutanes: A convenient method for the synthesis of spirocyclobutyl-1,2-dioxethanes

Article abstract of DOI:10.1080/00397911.2010.505705

Cerium(IV) ammonium nitrate (CAN)-mediated oxidation of methylenecyclobutanes was investigated. Compared with the similar reactions of methylenecyclopropanes, the reaction products were quite different. Instead of the desired ring-enlarged product cyclope

Full text of DOI:10.1080/00397911.2010.505705

Synthetic Communications1, 41: 2530–2538, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.505705
CERIUM(IV) AMMONIUM NITRATE–MEDIATED
OXIDATION OF MONO-ARYL-SUBSTITUTED
METHYLENECYCLOBUTANES: A CONVENIENT
METHOD FOR THE SYNTHESIS OF
SPIROCYCLOBUTYL-1,2-DIOXETHANES
Lei Yu, Lingfeng Ren, Rong Yi, and Rong Guo
School of Chemistry and Chemical Engineering, Yangzhou University,
Yangzhou, China
GRAPHICAL ABSTRACT
Abstract Cerium(IV) ammonium nitrate (CAN)-mediated oxidation of methylenecyclo-
butanes was investigated. Compared with the similar reactions of methylenecyclopro-
panes, the reaction products were quite different. Instead of the desired ring-enlarged
product cyclopentanones, a cyclobutyl ring-intact product, spirocyclobutyl-1,2-dioxethane,
was obtained. The structures of these spiroheterocycle-containing compounds are interest-
ing, and the reactions are potentially valuable in both organic synthesis and mechanism
research.
Keywords CAN; 1,2-dioxetane; methylenecyclobutane; oxidation; spiro compound
During the past decade, highly activated small organic molecules have attracted
much attention, and their novel reactions have been widely investigated. These
include methycyclopropanes (MCPs),[1] vinylidenecyclopropanes (VCPs),[2] allenes,[3]
cyclopropyl allenes.[4] Because of their high intramolecular ring strain or cumulated
carbon–carbon double bonds, these compounds have high reactivity and therefore
can undergo a series of interesting reactions under mild conditions with high selec-
tivity, providing convenient methods for the construction of many useful organic
skeletons. Hence, they are all useful building blocks in organic synthesis. Containing
a four-membered carbon ring and an exocyclic double bond, the molecular structure
of methylenecyclobutanes (MCBs) are very similar to that of MCPs.[5] Therefore,
Received May 24, 2010.
Address correspondence to Lei Yu, School of Chemistry and Chemical Engineering, Yangzhou
University, Yangzhou 225 002, China. E-mail: yulei@yzu.edu.cn
2530

SPIROCYCLOBUTYL-1,2-DIOXETHANES
2531
Scheme 1. CAN-promoted reactions of MCPs with 1,3-dicarbonyl compounds.
MCBs may undergo a series of reactions like MCPs. Recently, the novel reactions of
MCBs have also been reported in the literature.[6]
Cerium(IV) ammonium nitrate (CAN) is a good single-electron-transfer (SET)
reagent in organic synthesis. In the field of MCPs, CAN-mediated reactions have
been widely investigated, providing novel entry to many useful organic com-
pounds.[7] CAN-mediated oxidative additions of MCPs with 1,3-dicarbonyl com-
pounds provide a convenient method for the synthesis of spirocyclopropyl
dihydrofurans. In this reaction, CAN first reacts with 1,3-dicarbonyl anions through
the SET mechanism to generate the corresponding intermediate carbon-free radicals,
which then react with MCPs (Scheme 1).[7c,7d] In 2006, Nair and coworkers reported
that the direct reaction of CAN with MCPs in the presence of oxygen led to cyclo-
butanones (Scheme 2),[7b] which are also useful building blocks in both organic syn-
thesis and drug design.[8] Encouraged by these works, we became interested in the
exploration of CAN-mediated reactions. Herein, we report our recent interesting
findings in the CAN-mediated oxidation of MCBs.
We initially chosen MCB 1a as the model starting compound and examined its
CAN-mediated oxidations under different conditions. When 1a and CAN were stir-
red in methanol or ethanol under air, no desired ring-enlarged product 2a was
observed and a series of unidentified complexes were obtained (Table 1, entries 1
and 2). Interestingly, when the reaction was carried out in tetrahydrofuran (THF),
a spiroheterocycle-containing product, 3a, was obtained in 15% yield (Table 1, entry
3). Further screening demonstrated that the yield of 3a could be enhanced in
CH3CN-THF (4:1) mixed solvent at room temperature (Table 1, entry 7). The struc-
ture of compound 3a was confirmed by analytical spectra as well as the literature.[9]
1
In the H NMR spectrum, the disrupt alkyl peaks (chemical shifts: 1.35–2.61 ppm)
Scheme 2. CAN-promoted SET reactions of MCPs.

2532
L. YU ET AL.
Table 1. CAN-mediated oxidation of MCBs under different conditionsa
Entry
Solvent
MeOH
EtOH
Temp.
Time (h)b
Yield of 3a (%)c
1
2
3
4
5
6
7
8
9
rt
rt
6
10
5
0d
0d
THF
rt
15
31
CH3CN–THF (1:1)
CH3CN–THF (2:1)
CH3CN–THF (3:1)
CH3CN–THF (4:1)
CH3CN–THF (4:1)
CH3CN–THF (4:1)
rt
4
rt
3
38
46
rt
3
rt
3
51
30
Traced
40 ꢁC
60 ꢁC
1
1
aReaction conditions: 1a (0.3 mmol) and CAN (0.6 mmol) were stirred in 1 mL of solvent under different
conditions.
bThe reaction was monitored by TLC (eluent: petroleum ether).
cIsolated yields.
dUnidentified complexes were observed.
indicate the asymmetric cyclobutyl structure with different substituents on its two
sides.[1d] The single peak at 6.36 ppm indicated the proton of the carbon adjacent
to the O-O group, as the literature reported.[9a] In infrared (IR) spectrum, the
absorption at 1074 cmꢀ1 indicated the O-O structure. In the mass spectrum, the
fragments at 71 (16) and 185 (95) are cyclobutanone [M þ Hþ] and 4-bromo-
benzaldehydes [M þ Hþ], respectively. Since literature contains reports of thermo-
lysis of 1,2-dioxethanes generating aldehydes or ketones,[9b] the existence of these
two fragments in the mass spectrum confirmed the spirocyclobutyl-1,2-dioxethane
structural unit.
Containing
a
spirocyclobutyl ring and
a dioxetane structure unit,
spirocyclobutyl-1,2-dioxethanes 3 may have potential value in organic synthesis.[6,10]
Therefore, we next examined the application scope of this reaction. A series of MCBs
1 were employed to synthesize the corresponding spirocyclobutyl-1,2-dioxethane 3.
The experimental results are listed in Table 2. When monosubstituted MCBs were
employed, the corresponding spirocyclobutyl-1,2-dioxethanes 3 could be smoothly
obtained (Table 2, entries 1–7). The product yields are especially good when MCBs
with alkyl groups on their aryl ring were employed (Table 2, entries 6 and 7). How-
ever, only a series of unidentified complexes were obtained when disubstituted MCBs

SPIROCYCLOBUTYL-1,2-DIOXETHANES
2533
Table 2. Synthesis of spirocyclobutyl-1,2-dioxethanes 3a
Entry
R1, R2
Yield of 3 (%)b
1
2
3
4
5
6
7
8
9
p–BrC6H4, H (1a)
C6H5, H (1b)
51 (3a)
60 (3b)
56 (3c)
50 (3d)
64 (3e)
72 (3f)
86 (3g)
Tracec
Tracec
o–BrC6H4, H (1c)
p–ClC6H4, H (1d)
m–ClC6H4, H (1e)
p-MeC6H4, H (1)
p-ButC6H4, H (1g)
C6H5, C6H5 (1h)
C6H5, Me (1i)
aReaction conditions: 1 (0.3 mmol), CAN (0.6 mmol), CH3CN (0.8 mL), and THF
(0.2 mL), and the reactions were carried out in air at room temperature.
bIsolated yields.
cUnidentified complexes were observed.
were employed (Table 2, entries 8 and 9). The products spirocyclobutyl-1,2-
dioxethanes are not very stable and should be kept in a refrigerator. When they
are stored for a long time, traces of decomposition products, cyclobutanone or alde-
1
hydes, might be observed in H NMR spectra.
Based on these experimental results as well as literature reports, we supposed
the probable mechanisms. The initial CAN-mediated SET reaction of MCBs 1 furn-
ished the intermediate radical cation 4.[7b] It is reasonable that 4 could be oxidized to
5 by trapping molecular oxygen in air. After capturing a proton from THF,[11] the
peroxide free radical 5 was transformed to 6, which then led to the final product 3
(Scheme 3).[12]
Scheme 3. Probable mechanisms.

2534
L. YU ET AL.
Scheme 4. CAN-promoted oxidation of other olefins.
We then tried to expand the application scope of this interesting reaction by
employing some other kinds of olefins. The experimental results showed that the
cyclobutyl ring in the substrate is necessary. When styrene 7, methylenecyclopentane
8, or methylenecyclohexane 9 was employed, no desired product was obtained
(Scheme 4).
In conclusion, the reaction products of MCBs’ CAN-mediated oxidations are
quite different from those of MCPs. Instead of the ring-enlarged cyclopentanones,
cyclobutyl ring-intact products spirocyclobutyl-1,2-dioxethanes were obtained. The
structures of these spiroheterocycle-containing compounds are interesting and
potentially valuable in organic synthesis. This CAN-mediated SET reaction might
provide an oxidation of olefins with air and therefore have some successive appli-
cable value in future.
EXPERIMENTAL
1H NMR spectra were recorded on a Bruker Avance (600-MHz) spectrometer
in CDCl3 with tetramethylsilane (TMS) as the internal standard. 13C NMR spectra
were recorded on a Bruker Avance (150-MHz) spectrometer in CDCl3. IR spectra
were recorded on a Shimadzu IR-408 spectrometer. Electron-impact mass spec-
trometry (EIMS) studies were run on a HP 5989B mass spectrometer. Elemental
analysis was recorded on a Carlo Erba 1106 elemental analysis instrument.
Typical Procedure for the Preparation of 3
A solution of MCBs 1 (0.3 mmol) in CH3CN (0.8 mL) and THF (0.2 mL) was
added to a Schlenk tube containing 0.6 mmol of CAN. The mixture was stirred in air,
and the reaction was monitored by thin-layer chromatography (TLC; eluent: pet-
roleum ether). When the reaction terminated, 5 mL of water were added, and the
mixture was extracted by ether (5 mL) three times. The combined organic layer
was dried with anhydrous Na2SO4, and then the solvent was evaporated under

SPIROCYCLOBUTYL-1,2-DIOXETHANES
2535
vacuum. The residue was purified by preparative TLC (eluent: petroleum ether–
EtOAc 16:1) and gave the corresponding product 3.
Data
Compound 3a. A colorless oil. IR (film): 2961, 1641, 1302, 1281, 1251, 1108,
1074, 1012, 981, 840 cmꢀ1; 1H NMR (600 MHz, CDCl3, TMS): d 1.35–1.40 (m, 1H),
1.90–1.94 (m, 1H), 2.32–2.38 (m, 1H), 2.44–2.50 (m, 2H), 2.59–2.61 (m, 1H), 6.36 (s,
1H), 7.31–7.59 (m, 4H) ppm; 13C NMR (150 MHz, CDCl3, TMS): d 14.0, 28.7, 29.7,
81.7, 87.8, 124.0, 128.5, 132.2, 132.3 ppm; MS (EI) m=z (%): 254 (1) [Mþ], 185 (95)
(p-BrC6H4CHO þ Hþ), 183 (100), 71 (16) (C4H6O þ Hþ). Anal. calcd. for
C11H11BrO2: C, 51.79; H, 4.35. Found: C, 52.12; H, 4.11.
Compound 3b. A colorless oil. IR (film): 2962, 1640, 1498, 1455, 1281, 1091,
1
978, 849, 751, 702 cmꢀ1; H NMR (600 MHz, CDCl3, TMS): d 1.28–1.30 (m, 1H),
1.85–1.89 (m, 1H), 2.34–2.36 (m, 1H), 2.43–2.53 (m, 2H), 2.63–2.65 (m, 1H), 6.42
(s, 1H), 7.44 (s, 5H) ppm; 13C NMR (150 MHz, CDCl3, TMS): d 13.9, 28.5, 29.9,
82.4, 88.1, 126.9, 129.0, 129.7, 133.2 ppm; MS (EI) m=z (%): 176 (2) [Mþ], 105
(100) (C6H5CHO þ Hþ), 71 (7) (C4H6O þ Hþ). Anal. calcd for C11H12O2: C,
74.98; H, 6.86. Found: C, 74.70; H, 6.63.
Compound 3c. A colorless oil. IR (film): 2962, 1644, 1471, 1429, 1300, 1282,
1
1029, 979, 845, 753 cmꢀ1; H NMR (600 MHz, CDCl3, TMS): d 1.36–1.43 (m, 1H),
1.91–1.94 (m, 1H), 2.40–2.49 (m, 2H), 2.58–2.69 (m, 2H), 6.92 (s, 1H), 7.29–7.68 (m,
4H) ppm; 13C NMR (150 MHz, CDCl3, TMS): d 14.5, 28.5, 30.2, 80.7, 88.6, 124.3,
128.1, 128.2, 131.1, 133.1, 133.7 ppm; MS (EI) m=z (%): 255 (1) [Mþ þ 1], 185 (95)
(p-BrC6H4CHO þ Hþ), 183 (100), 71 (16) (C4H6O þ Hþ). Anal. calcd. for
C11H11BrO2: C, 51.79; H, 4.35. Found: C, 51.60; H, 4.67.
Compound 3d. A colorless oil. IR (film): 2962, 1704, 1643, 1597, 1493, 1302,
1
1281, 1093, 1015, 983, 942, 838 cmꢀ1; H NMR (600 MHz, CDCl3, TMS): d 1.35–
1.40 (m, 1H), 1.90–1.93 (m, 1H), 2.34–2.38 (m, 1H), 2.44–2.51 (m, 2H), 2.59–2.62
(m, 1H), 6.38 (s, 1H), 7.37–7.43 (m, 4H) ppm; 13C NMR (150 MHz, CDCl3,
TMS): d 13.9, 28.7, 29.7, 81.7, 87.8, 128.2, 129.3, 129.5, 130.9 ppm; MS (EI) m=z
(%): 210 (2) [Mþ], 141 (32) (p-ClC6H4CHO þ Hþ), 139 (100), 71 (9) (C4H6O þ Hþ).
Anal. calcd. for C11H11ClO2: C, 62.72; H, 5.26. Found: C, 63.01; H, 5.22.
Compound 3e. A colorless oil. IR (film): 2962, 1643, 1576, 1477, 1433, 1299,
1282, 1087, 986, 846, 784, 753 cmꢀ1; 1H NMR (600 MHz, CDCl3, TMS): d 1.37–1.40
(m, 1H), 1.91–1.94 (m, 1H), 2.33–2.39 (m, 1H), 2.43–2.53 (m, 2H), 2.60–2.64 (m,
1H), 6.37 (s, 1H), 7.31–7.43 (m, 4H) ppm; 13C NMR (150 MHz, CDCl3, TMS): d
14.0, 28.7, 29.8, 81.5, 87.8, 125.1, 127.0, 130.0, 130.3, 135.1, 135.3 ppm; MS (EI)
m=z (%): 210 (3) [Mþ], 141 (31) (m-ClC6H4CHO þ Hþ), 139 (100), 71 (29)
(C4H6O þ Hþ). Anal. calcd. for C11H11ClO2: C, 62.72; H, 5.26. Found: C, 62.55;
H, 5.58.
Compound 3f. A colorless oil. IR (film): 2960, 1641, 1515, 1424, 1299, 1281,
1
1252, 1087, 975, 851, 758 cmꢀ1; H NMR (600 MHz, CDCl3, TMS): d 1.28–1.33
(m, 1H), 1.85–1.88 (m, 1H), 2.32–2.45 (m, 2H), 2.40 (s, 3H), 2.49–2.53 (m, 1H),

2536
L. YU ET AL.
2.61–2.65 (m, 1H), 6.39 (s, 1H), 7.23–7.33 (m, 4H) ppm; 13C NMR (150 MHz,
CDCl3, TMS): d 13.9, 21.3, 28.5, 29.9, 82.5, 88.2, 126.8, 129.6, 130.1, 139.7 ppm;
MS (EI) m=z (%): 190 (1) [Mþ], 120 (8) (p-MeC6H4CHO), 119 (100), 71 (2)
(C4H6O þ Hþ). Anal. calcd. for C12H14O2: C, 75.76; H, 7.42. Found: C, 75.51;
H, 7.08.
Compound 3g. A colorless oil. IR (film): 2965, 2907, 2871, 1642, 1515, 1463,
1423, 1366, 1281, 1252, 1126, 1109, 979, 851, 750 cmꢀ1; 1H NMR (600 MHz, CDCl3,
TMS): d 1.19–1.25 (m, 1H), 1.25 (s, 9H), 1.76–1.80 (m, 1H), 2.25–2.37 (m, 2H),
2.43–2.56 (m, 2H), 6.34 (s, 1H), 7.27–7.36 (m, 4H) ppm; 13C NMR (150 MHz,
CDCl3, TMS): d 13.9, 28.5, 29.9, 31.2, 34.8, 82.4, 88.3, 125.8, 126.7, 130.0,
152.8 ppm. MS (EI) m=z (%): 162 (25) (p-ButC6H4CHO), 147 (100), 71 (2)
(C4H6O þ Hþ). Anal. calcd. for C15H20O2: C, 77.55; H, 8.68. Found: C, 77.22;
H, 8.54.
ACKNOWLEDGMENTS
This work was supported by the Natural Scientific Foundation of Jiangsu
Province (No. BK2010321), University Natural Scientific Foundation of Jiangsu
Province (No. 09KJB150014), the 45th Postdoctoral Foundation of China (No.
20090451249), and the National Natural Scientific Foundation of China (Nos.
20633010 and 20773106).
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