Lewis acid catalyzed rearrangement of vinylcyclopropenes for the construction of naphthalene and indene skeletons

Article abstract of DOI:10.1021/ol0626746

The choice of Lewis acid catalyst can result in dramatic differences in the chemoselectivity of the rearrangement reactions of vinylcyclopropenes. When BF₃·OEt₂ was used as the catalyst, naphthalenes were formed. However, when Cu(OTf

Full text of DOI:10.1021/ol0626746

ORGANIC
LETTERS
2007
Vol. 9, No. 1
117-120
Lewis Acid Catalyzed Rearrangement of
Vinylcyclopropenes for the Construction
of Naphthalene and Indene Skeletons
Li-Xiong Shao,† Yun-Peng Zhang,† Ming-Hui Qi,† and Min Shi*,†,‡
School of Chemistry and Molecular Engineering, East China UniVersity of Science and
Technology, 130 Meilong Road, Shanghai, China 200237, and State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, 354 Fenglin Road, Shanghai, China 200032
mshi@mail.sioc.ac.cn
Received November 2, 2006
ABSTRACT
The choice of Lewis acid catalyst can result in dramatic differences in the chemoselectivity of the rearrangement reactions of vinylcyclopropenes.
When BF3 OEt2 was used as the catalyst, naphthalenes were formed. However, when Cu(OTf)2 was used as the catalyst, indenes were obtained.
‚
Thermal and photochemical skeleton rearrangements of
strained small rings with multiple bonds and functional
groups have attracted much attention from both synthetic
and mechanistic viewpoints. However, only a few examples
of such thermal1 and photochemical2 skeletal conversions
of vinylidenecyclopropanes 1 have been reported. Previously,
we observed that naphthalene, 6aH-benzo[c]fluorine, and
indene derivatives can be obtained by Lewis acid catalyzed
rearrangement of arylvinylidenecyclopropanes 1 depending
on the substitution pattern on the cyclopropyl ring and the
electronic nature of aryl groups.3 In this paper, we wish to
report the Lewis acid catalyzed rearrangement of vinylcy-
clopropenes 2, which are derived from the corresponding
vinylidenecyclopropanes 1 under basic conditions, to afford
naphthalene and indene skeletons in good to high yields
under mild conditions.
As shown in Table 1, vinylcyclopropenes 2 are readily
accessible from simple isomerization of the corresponding
vinylidenecyclopropanes 1 under basic conditions. For all
(2) (a) Akasaka, T.; Misawa, Y.; Ando, W. Tetrahedron Lett. 1990, 31,
1173. (b) Mizuno, K.; Sugita, H.; Isagawa, K.; Goto, M.; Otsuji, Y.
Tetrahedron Lett. 1993, 34, 5737. (c) Mizuno, K.; Nire, K.; Sugita, H.;
Otsuji, Y. Tetrahedron Lett. 1993, 34, 6563. (d) Mizuno, K.; Sugita, H.;
Hirai, T.; Maeda, H. Chem. Lett. 2000, 1144. (e) Mizuno, K.; Nire, K.;
Sugita, H.; Maeda, H. Tetrahedron Lett. 2001, 42, 2689. (f) Mizuno, K.;
Maeda, H.; Sugita, H.; Nishioka, S.; Hirai, T.; Sugimoto, A. Org. Lett.
2001, 3, 581. (g) Mizuno, K.; Sugita, H.; Hirai, T.; Maeda, H.; Otsuji, Y.;
Yasuda, M.; Hashiguchi, M.; Shima, K. Tetrahedron Lett. 2001, 42, 3363.
(h) Maeda, H.; Zhen, L.; Hirai, T.; Mizuno, K. ITE Lett. Batteries, New
Technol. Med. 2002, 3, 485.
(3) (a) Shi, M.; Lu, J.-M. J. Org. Chem. 2006, 71, 1920. (b) Shi, M.;
Lu, J.-M. Synlett 2005, 2352. (c) Shi, M.; Ma, M.; Shao, L.-X. Tetrahedron
Lett. 2005, 46, 7609. (d) Shi, M.; Lu, J.-M.; Xu, G.-C. Tetrahedron Lett.
2005, 46, 4745. (e) Xu, G.-C.; Ma, M.; Liu, L.-P.; Shi, M. Synlett 2005,
1869. (f) Xu, G.-C.; Liu, L.-P.; Lu, J.-M.; Shi, M. J. Am. Chem. Soc. 2005,
127, 14552.
† East China University of Science and Technology.
‡ Shanghai Institute of Organic Chemistry.
(1) (a) Paulson, D. R.; Crandall, J. K.; Bunnell, C. A. J. Org. Chem.
1970, 35, 3708. (b) Jones, M., Jr.; Hendrick, M. E.; Hardie, J. A. J. Org.
Chem. 1971, 36, 3061. (c) Patrick, T. B.; Haynie, E. C.; Probst, W. J. J.
Org. Chem. 1972, 37, 1553. (d) Sadler, I. H.; Stewart, J. A. G. J. Chem.
Soc., Perkin Trans. 2 1973, 278. (e) Sugita, H.; Mizuno, K.; Saito, T.;
Isagawa, K.; Otsuji, Y. Tetrahedron Lett. 1992, 33, 2539. (f) Mizuno, K.;
Sugita, H.; Kamada, T.; Otsuji, Y. Chem. Lett. 1994, 449. (g) Maeda, H.;
Hirai, T.; Sugimoto, A.; Mizuno, K. J. Org. Chem. 2003, 68, 7700.
10.1021/ol0626746 CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/07/2006

were aimed at determining the optimal conditions for
rearrangement to 3a. The results are summarized in the
Supporting Information. It was found that BF3‚OEt2 was the
best catalyst and that 1,2-dichloroethane (DCE) was the
solvent of choice, and 3a was produced in 60% yield at 40 °C
(Scheme 1) (see the Supporting Information for details). The
Table 1. Isomerization of Vinylidenecyclopropanes 1 to
Vinylcyclopropenes 2
Scheme 1. Optimal Conditions for the Formation of
Naphthalene 3a
generality of this transformation was examined, and we found
that for all of the employed substrates (2) the corresponding
rearranged product 3 could be obtained in good yield (Table
2).
A plausible mechanism for the BF3‚OEt2-catalyzed rear-
rangement of vinylcyclopropenes 2 to naphthalenes 3 is
outlined in Scheme 1. Initially, reaction of BF3‚OEt2 with
trace amounts of water in the system generates the Brφnsted
acid catalyst A,5 which is similar to the Brφnsted acid TfOH.
The reaction of catalyst A with 2 produces cationic inter-
mediate B (the corresponding counteranion in all of the
intermediates has been omitted for convenience), which will
rearrange to either ring-opened cationic intermediate C or
resonance-stabilized intermediate C’ via σ-bond b cleavage.6
Subsequent intramolecular Friedel-Crafts reaction of C’ with
the aromatic group R1 gives the cyclized intermediate D,
which affords intermediate E via deprotonation. Finally,
aromatization of intermediate E furnishes the corresponding
naphthalene 3 (Scheme 2).
a All reactions were carried out using 1 (0.2 mmol), NaOH (20.0 equiv),
and Bu4NHSO4 (2.0 equiv) in toluene (5.0 mL) at 60 °C for 5 h. b Isolated
yields.
vinylidenecyclopropanes 1 examined, the isomerization reac-
tion proceeded smoothly to give vinylcyclopropenes 2 in
good to high yields within 5 h under mild conditions (Table
1). Compound 2b was unambiguously determined by an
X-ray diffraction (Figure 1).4
Interestingly, we found that vinylcyclopropenes 2 can
rearrange to the corresponding indene derivatives 4 in the
(4) The crystal data of 2b have been deposited in the CCDC with number
602862. Empirical formula: C25H20F2. Formula weight: 358.41. Crystal
color, habit: colorless, prismatic. Crystal system: monoclinic. Lattice
type: primitive. Lattice parameters: a ) 12.5896(12) Å, b ) 9.1294(8) Å,
c ) 17.6833(16) Å, R ) 90°, â ) 102.192(2)°, γ ) 90°, V ) 1986.6(3)
Å3. Space group: P2(1)/n. Z ) 4; Dc ) 1.198 g/cm3; F000 ) 752.
Diffractometer: Rigaku AFC7R. Residuals: R. Rw: 0.0431, 0.1098.
(5) For related papers, which report that trace amounts of water are
required to trigger the formation of cation B with BF3 (BF3 + H2O + Me2-
CH ) CH2 f Me3CH+BF3OH-), see: (a) Evans, A. G.; Holden, D.; Plesch,
P.; Polanyi, M.; Skinner, H. A.; Weinberger, M. A. Nature 1946, 157, 102.
(b) Evans, A. G.; Meadows, G. W.; Polanyi, M.; Skinner, H. A.; Weinberger,
M. A. Nature 1946, 158, 94. (c) Evans, A. G.; Polanyi, M. J. Chem. Soc.
1947, 252. (d) Prakash, G. K. S.; Mathew, T.; Hoole, D.; Esteves, P. M.;
Wang, Q.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 2004, 126, 15770.
The rearrangement of 2a in anhydrous DCE was also studied by bubbling
BF3 gas into the reaction solution by means of a Schlenk tube, under an
argon atmosphere. The reaction proceeded slowly under these conditions.
However, when the solution was exposed to the ambient atmosphere for
several minutes, the reaction proceeded smoothly to give product 3a in
good yield.
Figure 1. ORTEP drawing of compound 2b.
Initial examinations using vinylcyclopropene 2a (R1 ) R2
) C6H5; R3 ) R4 ) Me) as the substrate in the presence of
various Brφnsted acids, such as trifluoromethanesulfonic acid
(TfOH), CF3CO2H, CH3CO2H, and the Lewis acid BF3‚OEt2,
(6) Reaction of electrophilic reagents with strained bonds has been
extensively studied. See: (a) Bishop, K. C. Chem. ReV. 1976, 76, 461. (b)
Leftin, J. H.; Gil-Av, G. Tetrahedron Lett. 1972, 13, 3368. (c) Padwa, A.;
Rieker, W. F.; Rosenthal, R. J. J. Am. Chem. Soc. 1985, 107, 1710. (d)
Padwa, A.; Blacklock, T. J.; Loza, R. J. Am. Chem. Soc. 1981, 103, 2404.
118
Org. Lett., Vol. 9, No. 1, 2007

solvent of choice, and 4a was produced in 98% yield at 50
°C (Scheme 3) (see the Supporting Information for details).
Table 2. BF3‚OEt2-Catalyzed Rearrangement of 2 for the
Formation of Naphthalenes 3
Scheme 3. Optimal Conditions for the Formation of Indene
4a
It should be emphasized here that, using Brφnsted acid CF3-
CO2H as a catalyst, both indene and naphthalene products
can be obtained as a mixture. Compound 4a was unambigu-
ously determined by an X-ray diffraction (Figure 2).7 The
a All reactions were carried out using 2 (0.2 mmol) in the presence of
BF3‚OEt2 (10 mol %) and DCE (5.0 mL) at 40 °C for 5 h. b Isolated yields.
Figure 2. ORTEP drawing of compound 4a.
presence of the Lewis acid Cu(OTf)2 in DCE at 50 °C. The
optimization of the reaction conditions for this process is
summarized in the Supporting Information in the presence
of various Lewis acids, such as Sc(OTf)3, Sn(OTf)2, Eu-
(OTf)3, and Zr(OTf)4. It was found that Cu(OTf)2 was the
best catalyst and that 1,2-dichloroethane (DCE) was the
generality of this Cu(OTf)2-catalyzed rearrangement reaction
is satisfactory, and the reaction can tolerate various substit-
uents, whether they are electron-rich or -poor aryl groups
(R1 and R2) or methyl groups or hydrogen atoms (R3 and
R4). All reactions proceeded smoothly to give the corre-
sponding indene derivative 4 in good to high yields under
the optimal conditions (Table 3).
A plausible mechanism for the formation of indenes 4 is
illustrated in Scheme 4. The coordination of 2 to the Lewis
acid initially gives zwitterionic intermediate F, which results
in the formation of cyclopropyl ring-opened zwitterionic
intermediate G via σ-bond a-cleavage. Intermediate G also
can be drawn as its delocalized representations as intermedi-
ate H or intermediate H’.6,8 Subsequent intramolecular
Friedel-Crafts reaction of intermediate H’ with aromatic
group R1 affords intermediate I. Elimination of Lewis acid
and deprotonation from intermediate I furnish 4.
Scheme 2. Plausible Mechanism for the Formation of
Naphthalenes 3
(7) The crystal data of 4a have been deposited in the CCDC with number
615026. Empirical formula: C25H22. Formula weight: 322.43. Crystal color,
habit: colorless, prismatic. Crystal system: Triclinic. Lattice type: primitive.
Lattice parameters: a ) 8.1168(19) Å, b ) 10.068(2) Å, c ) 11.493(16)
Å, R ) 93.001(4)°, â ) 98.563(4)°, γ ) 98.080(4)°, V ) 917.0(4) Å3.
Space group: P1h. Z ) 2; Dc ) 1.168 g/cm3; F000 ) 344. Diffractometer:
Rigaku AFC7R. Residuals: R. Rw: 0.0655, 0.1722.
(8) For the mechanism of the 1,3-proton shift, see: Carey, F. A.;
Sundberg, R. J. AdVanced Organic Chemistry, 3rd ed.; Plenum Press: New
York, 1990; p 609.
Org. Lett., Vol. 9, No. 1, 2007
119

Scheme 4. Plausible Mechanism for the Formation of Indenes
Table 3. Cu(OTf)2-Catalyzed Rearrangement of
Vinylcyclopropenes 2 for the Formation of Indenes 4
4
In conclusion, we have discovered efficient and mild Lewis
acid catalyzed rearrangement reactions of vinylcyclopropenes
2. In these reactions, the choice of Lewis acid catalyst is
important and naphthalenes 3 or indenes 4 can be obtained
in good to high yields. Stronger Lewis acid catalysts (or
Brφnsted acid in this case) and somewhat weak Lewis acid
catalysts will result in the different σ-bond cleavage of 2,
which directly affects the subsequent Friedel-Crafts reaction
and determines the product structure. Efforts are underway in
the laboratory to further elucidate the mechanistic details of
these reactions and to understand their scope and limitations.
a All reactions were carried out using 2 (0.2 mmol) in the presence of
Cu(OTf)2 (10 mol %) and DCE (5.0 mL) at 50 °C for 5 h. b Isolated yields.
Acknowledgment. Financial support from the Shanghai
Municipal Committee of Science and Technology (04JC14083,
06XD14005), the National Natural Science Foundation of
China (203900502, 20472096, and 20672127), and the
Cheung Kong Scholar Program is greatly appreciated.
At the present stage, we believe that the coordination of
sterically bulky LA to the cyclopropene moiety of 2
exclusively produces intermediate F due to the steric
repulsion of LA and the aromatic R2 group. However, the
coordination of a proton to the cyclopropene moiety of 2
gives a more stabilized intermediate B. The different
coordination place in the cyclopropene moiety of 2 causes
the different reaction products.
Supporting Information Available: Spectroscopic data
of all the new compounds, detailed experimental procedures,
and X-ray data for compounds 2b and 4a. This material is
available free of charge via the Internet at http://pubs.acs.org.
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