Lewis acid-catalyzed rearrangement of multi-substituted arylvinylidenecyclopropanes

Article abstract of DOI:10.1021/ja054988e

An interesting rearrangement of arylvinylidenecyclopropanes having three substituents at the 1- and 2-positions of the corresponding cyclopropane catalyzed by Lewis acids to give 6aH-benzo[c]fluorine derivatives via a double intramolecular Friedel-Crafts reaction or to give an indene derivative via an intramolecular Friedel-Crafts reaction is described. Copyright

Full text of DOI:10.1021/ja054988e

Published on Web 10/01/2005
Lewis Acid-Catalyzed Rearrangement of Multi-Substituted
Arylvinylidenecyclopropanes
Guang-Cai Xu,† Le-Ping Liu,‡ Jian-Mei Lu,‡ and Min Shi*,†,‡
School of Chemistry and Pharmaceutics, East China UniVersity of Science and Technology, 130 Meilong Lu,
Shanghai 200237 China, and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032 China
Received July 25, 2005; E-mail: Mshi@pub.sioc.ac.cn
Table 2. Lewis Acid Sn(OTf)2-Catalyzed Rearrangement of a
Variety of Arylvinylidenecyclopropanes 1 in DCE at 80 °C
Thermal and photochemical skeletal conversions of vinylidenecy-
clopropanes 1 have attracted much attention from mechanistic, theo-
retical, spectroscopic, and synthetic viewpoints.1,2 Recently, we have
been investigating the Lewis acid-catalyzed ring-opening reactions
of methylenecyclopropanes (MCPs)3 and vinylidenecyclopropanes
1.4,5 Thus far, we have found that aryl-substituted derivatives of 1
undergo interesting rearrangements in the presence of Lewis or
Brønsted acids to give the corresponding naphthalene derivatives
in good to high yields.4 We now wish to report the Lewis acid-
catalyzed rearrangement of arylvinylidenecyclopropanes 1 having
three substituents at the 1- and 2-positions of the corresponding
cyclopropane to give the corresponding 6aH-benzo[c]fluorine or
phenyl-1H-indene derivatives 2 in good to high yields under mild
conditions.
An initial examination was carried out using 1a (Z/E ) 1/2
mixture determined by NOESY NMR spectroscopic analysis; see
Supporting Information) as substrate in the presence of a variety
of Lewis acids. We found that an interesting rearrangement took
place to give 6aH-benzo[c]fluorine derivative 2a stereoselectively
with syn-configuration upon heating. Using Sn(OTf)2 (10 mol %)
as the catalyst in 1,2-dichloroethane (DCE), no reaction occurred
at room temperature, and the reaction was sluggish at 40 °C. How-
ever, it proceeded smoothly under reflux (80 °C) to give 2a in 74%
yield after 6 h (Table 1, entries 2-4). Using other Lewis acids,
Table 1. Rearrangement of Diphenylvinylidenecyclopropane 1a to
(6a,7,11b)-7,11b-Dihydro-7,11b-dimethyl-5-phenyl-6aH-benzo[c]-
fluorine 2a in the Presence of a Variety of Lewis Acids (0.1 equiv)
entry
solvent
catalyst
temp (
°C)
time (h)
yielda 2a (%)
1
2
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
toluene
none
80
25
40
80
80
40
80
80
80
80
66
80
78
69
120
120
20b
6b
c
c
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Zr(OTf)4
Zr(OTf)4
Cu(OTf)2
BF3Et2O
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
Sn(OTf)2
3
21
74
31
30
30
21
65
c
35
c
c
56
4
5
8b
6
2b
7
2b
8
5b,d
9
6b
10
11
12
13
14
CH3CN
6
THF
6b
DMF
12
12
6b
a Isolated yield.
EtOH
hexane
the presence of lanthanide Lewis acids Yb(OTf)3 and Sc(OTf)3,
no reaction occurred. Solvent effects have been examined with Sn-
(OTf)2 (10 mol %) at 80 °C. DCE is the solvent of choice (Table
1, entries 9-14). Therefore, these optimized reaction conditions
are to carry out the reaction in DCE at 80 °C using Sn(OTf)2 (10
mol %) as a catalyst.
Next, we carried out the reaction of a variety of arylvinylidene-
cyclopropane derivatives 1 (Z/E isomeric mixtures) in the presence
of Sn(OTf)2 under these optimized conditions. The results are sum-
a Isolated yield. b Until all of the starting material 1a was consumed.
c No reaction occurred. d At room temperature, 2a was obtained in 52% in
DCE with 10 mol % of BF3OEt2.
such as Zr(OTf)4, Cu(OTf)2, and BF3‚OEt2, under identical condi-
tions, 2a was obtained in lower yields (Table 1, entries 5-8). In
† East China University of Science and Technology.
‡ Shanghai Institute of Organic Chemistry.
9
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J. AM. CHEM. SOC. 2005, 127, 14552-14553
10.1021/ja054988e CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. A Plausible Mechanism for the Rearrangement of
Arylvinylidenecyclopropanes 1 and 2 in the Presence of Lewis Acid
Scheme 2. Rearrangement of Diphenylvinylidenecyclopropane 1i in
the Presence of Sn(OTf)2 in DCE at 40 °C and a Plausible Mechanism
shift along with the release of Lewis acid produces the correspond-
ing indene derivative 2i. Its structure was determined by spectro-
scopic data and X-ray diffraction analysis (Supporting Information).
Further details regarding this reaction will be reported in due course.
In conclusion, we have identified an efficient Lewis acid-
catalyzed rearrangement of arylvinylidenecyclopropanes 1 having
three substituents at the 1,2-positions of the cyclopropane to provide
easy access to 6aH-benzo[c]fluorine derivatives via a double
intramolecular Friedel-Crafts reaction or a 1-methyl-3-phenyl-1H-
indene derivative via an intramolecular Friedel-Crafts reaction
under mild reaction conditions in good to excellent yields. Efforts
are in progress to elucidate further mechanistic details of these
reactions and to understand their scopes and limitations.
Acknowledgment. We thank the State Key Project of Basic
Research (Project 973) (No. G2000048007), Shanghai Municipal
Committee of Science and Technology, and the National Natural
Science Foundation of China (20472096, 203900502, and 20272069)
for financial support.
Supporting Information Available: 13C and 1H NMR spectro-
scopic and analytic data for compounds 1 and 2, X-ray crystal data of
2a and 2i. This material is available free of charge via the Internet at
http://pubs.acs.org.
marized in Table 2. As can be seen from Table 2, the corresponding
rearranged products 2, 6aH-benzo[c]fluorine derivatives, were ob-
tained in good to high yields within 5 h with syn-configuration
(Table 2, entries 1-7).
1
The product structures were determined by H and 13C NMR
spectroscopic data and HRMS or microanalyses (Supporting
Information). The structure of 2a was further confirmed by X-ray
diffraction analysis (Supporting Information).
A plausible mechanism for the observed rearrangements of 1 in
the presence of Lewis acids is outlined in Scheme 1. The coordi-
nation of 1 to the Lewis acid5 initially gives zwitterionic intermedi-
ate A-1, a vinyl group stabilized cyclopropyl cationic intermediate,6
which results in the formation of cyclopropane ring-opened zwit-
terionic intermediate B-1 or the resonance-stabilized zwitterionic
intermediate C-1 by the aromatic R3 group. Subsequently, intramo-
lecular Friedel-Crafts reaction with either the aromatic R1 or R2
group produces the cyclized zwitterionic intermediate D-1, which
affords the zwitterionic intermediate E-1 via an allylic rearrange-
ment. This is followed by another sterically demanding intramole-
cular Friedel-Crafts reaction with the aromatic R3 group to produce
the cyclized zwitterionic intermediate F-1 with syn-configuration.
Deprotonation of F-1 affords the corresponding intermediate G-1,
and the addition of the corresponding released proton produces
zwitterionic intermediate H-1. The 1,3-proton shift along with the
release of Lewis acid produces the corresponding thermodynami-
cally favored 6aH-benzo[c]fluorine derivatives 2.7 This appears to
be the driving force in these reactions to move the reaction forward
and to allow for the formation of 6aH-benzo[c]fluorine derivatives
2 (Scheme 1).
Interestingly, with diphenylvinylidenecyclopropane 1i, which has
two phenyl groups at the C-1 position of the cyclopropane, as the
substrate under the similar conditions, we found that 2-(2,2-di-
phenylvinyl)-1-methyl-3-phenyl-1H-indene 2i was formed in 95%
yield (Scheme 2). A plausible mechanism is shown in Scheme 2.
Similar to the previous examples, the corresponding cyclopropane
ring-opened zwitterionic intermediate B-2 or the resonance-sta-
bilized zwitterionic intermediate C-2 is formed from the initial
zwitterionic intermediate A-2. Intramolecular Friedel-Crafts reac-
tion with the phenyl group at the C-1 position produces zwitterionic
intermediate D-2, which affords the corresponding zwitterionic in-
termediate E-1 via an allylic rearrangement. Subsequent 1,3-proton
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