AgOAc-mediated rearrangement of gem-dibromospiropentanes in trifluoroacetic acid

Article abstract of DOI:10.1016/j.tetlet.2009.01.116

AgOAc-mediated intramolecular skeleton rearrangement reaction of gem-dibromospiropentanes produced the corresponding naphthalene and indene derivatives in moderate to good yields under mild conditions.

Full text of DOI:10.1016/j.tetlet.2009.01.116

Tetrahedron Letters 50 (2009) 1636–1638
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Tetrahedron Letters
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AgOAc-mediated rearrangement of gem-dibromospiropentanes
in trifluoroacetic acid
*
Lei Wu, Min Shi
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
AgOAc-mediated intramolecular skeleton rearrangement reaction of gem-dibromospiropentanes pro-
duced the corresponding naphthalene and indene derivatives in moderate to good yields under mild
conditions.
Received 20 November 2008
Revised 20 January 2009
Accepted 21 January 2009
Available online 27 January 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Recently, the chemical transformations and skeleton rearrange-
ments of highly strained oligospirocyclopropanes have attracted
much attention from both synthetic and mechanistic viewpoints.1
Up till now, the chemical transformation of oligospirocyclopro-
panes can formally be divided into two major sections: (i) reactions
with retention of the triangulane skeleton and (ii) reactions accom-
panied by ring opening or ring enlargement of one or more rings.2,3
In this Letter, we wish to report a novel example of AgOAc-medi-
ated skeleton rearrangement of easily available gem-dibromospir-
opentanes in trifluoroacetic acid under mild conditions, affording
two cyclopropane rings and one cyclopropane ring opened prod-
ucts, naphthalene, and indene derivatives, in moderate to good to-
tal yields.4
Initial examinations using 2,2-diphenyl-1,1-dibromospiropen-
tane 1a5 as the substrate in the presence of AgOAc were aimed
at determining the best conditions for this intramolecular skeleton
rearrangement reaction and their results are summarized in Table
1. We found that using AgOAc as a mediator, two cyclopropane
rings and one cyclopropane ring opened products, naphthalene
derivative 2a and indene derivative 3a, were formed in 83% total
yield with the ratio of 1.28:1 at room temperature (20 °C) (Table
1, entry 1). Examination of the solvent effects revealed that
CF3COOH was the solvent of choice because in dichloromethane,
no reaction occurred and in trifluoromethanesulfonic acid CF3SO3H
(HOTf), complex product mixtures were formed (Table 1, entries 1–
3). Raising the reaction temperature to 72 °C, 2a was obtained as
the major product along with only trace of 3a (Table 1, entry 4).
Lowering the temperature to À10 °C, the total yield of 2a and 3a
could be improved to >99% with the ratio of 1:1.13 (Table 1, entry
5). Using Lewis acids such as Sc(OTf)3, Yb(OTf)3, Zr(OTf)4, BF3ÁEt2O,
or In(OTf)3 as the additive (0.1 equiv), no improvement on the total
yield of 2a and 3a could be realized whether under reflux, or at
room temperature or at À10 °C (Table 1, entries 6–12). We
assumed that Lewis acid as the additive might have no effect on
the reaction outcomes, and the acidity of the reaction system
would play a significant role in this reaction. To prove this, we used
the Brønsted acid HOTf as the additive to examine the reaction out-
come and found that in higher concentration of HOTf or at higher
reaction temperature, 2a was obtained as the major product,
whereas in lower concentration of HOTf or at lower reaction tem-
perature, 2a and 3a were usually obtained as the product mixtures
in good total yields with different ratios (Table 1, entries 13–17).
However, no distinct improvement could be realized if compared
with that of the reaction carried out in the absence of HOTf as
shown in entry 5 of Table 1. Using strong Brønsted acids C8F17SO3H
and Tf2NH, similar results were observed (Table 1, entries 18 and
19). Therefore, the best reaction conditions are to carry out the
reaction in CF3COOH (2.0 mL) at À10 °C using AgOAc (0.30 mmol,
1.5 equiv) as a mediator. Under these optimal conditions, the reac-
tion could complete within 3 h.
With these optimal reaction conditions in hand, we next ex-
plored the scope and limitations of this AgOAc-mediated skeleton
rearrangement reaction using various gem-dibromospiropentanes
1b–k as the substrates. The results of these examinations are sum-
marized in Table 2. As for the spiropentanes 1b and 1c in which
both the two aromatic rings had an electron-withdrawing group
such as F and Cl atom, the major products were the two cyclopro-
pane rings opened products 2 (Table 2, entries 1 and 2). Moreover,
when one of the aromatic rings in spiropentane 1 bore an electron-
donating group, such as CH3 and CH3O group, one cyclopropane
ring opened products 3 were obtained as the major products in
good yields (Table 2, entries 3–5). As for spiropentanes 1g–k in
which R2 is an alkyl group, such as CH3 or CH3CH2 group, the cor-
responding naphthalene derivatives 2g–k were mainly obtained in
moderate to good yields whether aromatic R1 group bearing an
electron-donating or electron-withdrawing group (Table 2, entries
6–10).
On the basis of the above results, a plausible mechanism of
AgOAc-mediated rearrangement reaction is tentatively outlined
in Scheme 1.6 The initial process is more likely to be the generation
of spiropentyl cationic intermediate A by the elimination of a
* Corresponding author. Tel.: +86 21 54925137; fax: +86 21 64166128.
E-mail address: Mshi@mail.sioc.ac.cn (M. Shi).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.01.116

L. Wu, M. Shi / Tetrahedron Letters 50 (2009) 1636–1638
1637
Table 1
Br
Br
Optimization of the reaction conditionsa
R1
+
AgOAc
R2
1
C6H5
C6H5
Br
Br
Br
Br
AgOAc (1.5 equiv), additive
Br
AgBr
+
R2
R1
R2
C6H5
solvent, temp
Br
C6H5
1a
R1
2a
ring opening
R1
Br
F-C
3a
R2
3
B
A
Entry
Additive (mol %)
Solvent
Temperature
Yieldb (%)
ring opening
Br
2a
45
3a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
—
—
—
—
CF3COOH
DCM
HOTf
rt
rt
rt
38
No reaction
Complex
52
47
49
56
54
49
53
50
50
46
33
49
61
44
46
R2
R1
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3CO2H
CF3CO2H
Reflux
À10 °C
Reflux
rt
rt
rt
Trace
53
Trace
32
35
42
36
36
50
Trace
Trace
39
Trace
56
—
Sc(OTf)3(10)
Yb(OTf)3(10)
Zr(OTf)4(10)
Sc(OTf)3(10)
BF3ÁEt2O(10)
In(OTf)3(10)
Yb(OTf)3(10)
HOTf (100)
HOTf (50)
HOTf (20)
HOTf (100)
HOTf (50)
C8F17SO3H (100)
Tf2NH (100)
C
F-C
rt
rt
R2
R2
Br
Br
tautomerization
À10 °C
R1
R1
rt
rt
rt
D
2
À10 °C
À10 °C
À10 °C
À10 °C
Scheme 1. A plausible reaction mechanism.
44
30
58
intermediates B and C. In intermediate B, the aromatic ring conju-
gates to the positively charged allylic moiety, but in intermediate
C, it is not in such a case. When both the aromatic rings have an
electron-withdrawing group, intermediate B is not stable and can
easily undergo the ring opening process of another cyclopropane
to give intermediate C. Namely, the formation of intermediate C
is favored, and the product 2 is the major one. When R2 is an ali-
phatic group and R1 has an electron-donating group, the only one
electron-rich aromatic ring is not enough to stabilize the positively
charged intermediate B, which can still undergo a similar process
as mentioned above. However, when both the aromatic rings have
an electron-donating group, there exist two electron-rich aromatic
rings to stabilize the cationic intermediate B. The reaction can
mainly proceed directly through intramolecular Friedel–Crafts
reaction via intermediate B to afford the corresponding indene
derivative 3 as the major product.
In conclusion, we have disclosed an interesting AgOAc-medi-
ated skeleton rearrangement reaction of various gem-dibromo-
spiropentanes in CF3COOH to afford the corresponding
naphthalene and indene derivatives in moderate to good total
yields under mild conditions. Such novel synthetic approach pro-
vides an easy access to the synthesis of the interesting naphtha-
lene and indene derivatives, which might be useful in organic
synthesis.8 Efforts are underway to further elucidate the reaction
mechanism and to understand the scope and limitations of this
process.
a
All reactions were carried out using 1a (0.20 mmol) and AgOAc (0.30 mmol,
1.5 equiv) in solvent (2.0 mL) otherwise specified.
b
Isolated yields.
Table 2
AgOAc-mediated skeleton rearrangement of gem-dibromospiropentanes 1 under the
optimal reaction conditionsa
R2
R2
Br
Br
Br
AgOAc (1.5 equiv)
R1
R1
+
Br
R1
CF3COOH, -10 o
C
R2
1
2
3
Entry
R1
R2
No.
Yieldb of 2 (%)
Yieldb of 3 (%)
1
2
3
4
5
6
7
8
9
p-F
p-FC6H4
p-ClC6H4
p-CH3OC6H4
p-CH3C6H4
p-CH3OC6H4
CH3
CH3CH2
CH3
CH3
1b
1c
1d
1e
1f
1g
1h
1i
2b, 81
2c, 84
Trace
3b, 18
3c, 10
3d, 63
3e, 91
3f, 60
Trace
Trace
Trace
Trace
Trace
p-Cl
p-CH3O
p-CH3
H
H
H
p-CH3
p-Cl
m-Br
Trace
2f, 12
2g, 71
2h, 72
2i, 61
2j, 95
2k, 50c
1j
1k
10
CH3
a
All reactions were carried out using 1 (0.20 mmol) and AgOAc (0.30 mmol,
1.5 equiv) in CF3COOH (2.0 mL) otherwise specified.
b
Isolated yields.
c
The mixture of o-and p-isomers (1:1) determined by 1H NMR spectroscopic
Acknowledgments
data.
Financial support from the Shanghai Municipal Committee of
Science and Technology (06XD14005, 08dj1400100-2), National
Basic Research Program of China (973)-2009CB825300, and the
National Natural Science Foundation of China (20472096,
20672127, 20872162, and 20732008) is greatly acknowledged.
bromo anion upon the treatment of 1 with AgOAc. The following
process might be one of the cyclopropane ring opening reaction
to produce the allylic cation B, which can directly undergo intra-
molecular Friedel–Crafts reaction to furnish the corresponding
product 3.7 In the mean time, intermediate B can undergo the ring
opening process of another cyclopropane to produce the allylic cat-
ion C, which is followed by intramolecular Friedel–Crafts reaction
to afford intermediate D. The tautomerization of intermediate D
produces the corresponding product 2. The distribution of products
2 and 3 is mainly dependent on the stability of the two cationic
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.01.116.

1638
L. Wu, M. Shi / Tetrahedron Letters 50 (2009) 1636–1638
7. (a) Xu, G.-C.; Liu, L.-P.; Lu, J.-M.; Shi, M. J. Am. Chem. Soc. 2005, 127, 14552–
References and notes
14553; (b) Xu, G.-C.; Ma, M.; Liu, L.-P.; Shi, M. Synlett 2005, 1869–1872; (c)
Zhang, Y.-P.; Lu, J.-M.; Xu, G.-C.; Shi, M. J. Org. Chem. 2007, 72, 509–516; (d) Lu,
J.-M.; Shi, M. Org. Lett. 2007, 9, 1805–1808.
1. For recent review, see: de Meijere, A.; Kozhushkov, S. I. Chem. Rev. 2000, 100, 93–
142.
8. Typical reaction procedure: gem-Dibromospiropentanes
1 (0.20 mmol) and
2. Selected recent articles about reactions with retention of the triangulane
skeleton: (a) Wiberg, K. B.; Snoonian, J. R. J. Org. Chem. 1998, 63, 1402–1407; (b)
Wade, P. A.; Kondracki, P. A.; Carroll, P. J. J. Am. Chem. Soc. 1991, 113, 8807–8811;
(c) Zorn, C.; Anichini, B.; Goti, A.; Brandi, A.; Kozhushkov, S. I.; de Meijere, A.;
Citti, L. J. Org. Chem. 1999, 64, 7846–7855; (d) Zorn, C.; Goti, A.; Brandi, A.;
Johnsen, K.; Noltemeyer, M.; Kozhushkov, S. I.; de Meijere, A. J. Org. Chem. 1999,
64, 755–763. and references cited therein.
3. Selected recent articles about reactions accompanied by ring opening or ring
enlargement of one or more rings: (a) Bissember, A. C.; Phillis, A. T.; Banwell, M.
G.; Willis, A. C. Org. Lett. 2007, 9, 5421–5424; (b) Matsuda, T.; Tsuboi, T.;
Murakami, M. J. Am. Chem. Soc. 2007, 129, 12596–12597.
4. For silver catalyzed/mediated reactions of gem-dihalocyclopropanes, see: (a)
Murphy, J. A.; Scott, K. A.; Sinclair, R. S.; Martin, C. G.; Kennedy, A. R. J. Chem. Soc.,
Perkin Trans. 1 2000, 2395; (b) Trost, B. M.; Oslob, J. D. J. Am. Chem. Soc. 1999,
121, 3057; (c) Banwell, M. G.; Harrey, J. E.; Jolliffe, K. A. J. Chem. Soc., Perkin Trans.
1 2001, 2002; (d) Lee, J.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1995, 117, 9919; (e)
Banwell, M. G.; Edwards, A.; Harrey, J.; Hockless, D.; Willis, A. J. Chem. Soc., Perkin
Trans. 1 2000, 2175; (f) Banwell, M. G.; Harrey, J. E.; Hockless, D. C. R. J. Org.
Chem. 2000, 65, 4241.
AgOAc (0.30 mmol, 1.5 equiv) were added into a Schlenk tube, and the solvent
of CF3COOH (2.0 mL, prechilled to À10 °C) was added into the mixture of 1 and
AgOAc. The reaction mixture was stirred at À10 °C for 2–3 h (monitored by TLC).
After the starting materials (gem-dibromospiropentanes 1) were consumed, the
solvent was removed under reduced pressure, and the residue was subjected to
a flash column chromatography to give the desired product 2 or 3.Compound
2a: A yellow solid, mp: 87–89 °C. 1H NMR (CDCl3, 300 MHz, TMS) d 2.63 (s, 3H,
CH3), 7.24–7.34 (m, 4H, Ar), 7.42–7.55 (m, 4H, Ar), 7.74–7.79 (m, 2H, Ar). 13C
NMR (CDCl3, 75 MHz, TMS) d 24.5, 125.4, 125.8, 126.0, 126.6, 127.1, 127.6,
128.2, 128.3, 130.0, 132.27, 132.29, 135.4, 140.4, 140.5. IR (CH2Cl2) m 3055, 2952,
2922, 2853, 1739, 1440, 1029, 880, 748, 699, 605 cmÀ1. MS (%) m/e 296 (M+, 37),
215 (55), 202 (58), 189 (12), 108 (32), 85 (40), 71 (60), 57 (100), 43 (94). HRMS
(EI) calcd for C17H13Br: 296.0201, found: 296.0202.Compound 3a: A colorless
solid, mp: 114–119 °C. 1H NMR (CDCl3, 300 MHz, TMS) d 1.62–1.66 (m, 2H, CH2),
1.76–1.80 (m, 2H, CH2), 7.02–7.05 (m, 1H, Ar), 7.18–7.27 (m, 2H, Ar), 7.38–7.43
(m, 2H, Ar), 7.47–7.52 (m, 2H, Ar), 7.56–7.60 (m, 2H, Ar). 13C NMR (CDCl3,
75 MHz, TMS) d 16.0, 29.7, 35.2, 117.7, 120.1, 124.7, 125.7, 127.8, 128.4, 128.5,
129.1, 134.2, 139.0, 142.3, 146.5. IR (CH2Cl2)
m 3053, 3006, 2925, 1603, 1457,
1345, 965, 769, 746, 699, 661, 619, 574 cmÀ1. MS (%) m/e 296 (M+, 13), 217 (38),
202 (34), 189 (8), 108 (14), 95 (12), 71 (7), 58 (25), 43 (100). HRMS (EI) calcd for
C17H13Br: 296.0201, found: 296.0214.
5. For the preparation of gem-dihalocyclopropanes, see: Matsuda, T.; Tsuboi, T.;
Murakami, M. J. Am. Chem. Soc. 2007, 129, 12596–12597.
6. (a) Applequist, D. E.; Johnston, M. R.; Fisher, F. J. Am. Chem. Soc. 1970, 92, 4614–
4617; (b) Applequist, D. E.; Nickel, G. W. J. Org. Chem. 1979, 44, 321–323.