Unusual rearrangement of (cyclopropylmethoxy)benzene and its derivatives in the presence of BF3 · Et2O

Article abstract of DOI:10.1134/S1070428011100071

(Cyclopropylmethoxy)benzene and its ortho- and para-bromo-substituted analogs in the presence of BF₃ · Et₂O undergo rearrangement with formation of (cyclobutyloxy)benzene and 2-ethyl-2,3-dihydro- 1- benzofuran. Pleiades Publishing, L

Full text of DOI:10.1134/S1070428011100071

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 10, pp. 1498–1500. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © D.O. Ptashko, M.D. Khanova, V.A. Dokichev, Yu.V. Tomilov, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47,
No. 10, pp. 1473–1474.
Unusual Rearrangement of (Cyclopropylmethoxy)benzene
and Its Derivatives in the Presence of BF3 ·Et2O
D. O. Ptashkoa, M. D. Khanovaa, V. A. Dokicheva, and Yu. V. Tomilovb
a Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences,
pr. Oktyabrya 71, Ufa, 450054 Bashkortostan, Russia
e-mail: dokichev@anrb.ru
b Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia
Received February 10, 2011
Abstract—(Cyclopropylmethoxy)benzene and its ortho- and para-bromo-substituted analogs in the presence
of BF3 ·Et2O undergo rearrangement with formation of (cyclobutyloxy)benzene and 2-ethyl-2,3-dihydro-1-
benzofuran.
DOI: 10.1134/S1070428011100071
Rearrangement of allyl phenyl ethers into allyl-
phenols (Claisen rearrangement) have become a model
of many pericyclic [3,3]-sigmatropic reactions [1–4].
The resulting o- and/or p-allylphenols are widely used
in organic synthesis as important intermediate products
[1]. Diverse and facile transformations of cyclo-
propanes into various structural fragments of organic
compounds have been considered in detail in numer-
ous reviews [5–9]. However, there no published data
on rearrangements of (cyclopropylmethoxy)benzene
derivatives under the Claisen reaction conditions; such
rearrangements could give rise to o-(but-2-en-1-yl)-
phenols as a result of cleavage of the three-membered
ring.
The reaction of (cyclopropylmethoxy)benzene (Ia)
with BF3·Et2O at a molar ratio of 2:1 in carbon tetra-
chloride at –10°C in 2 h afforded a mixture of (cyclo-
butyloxy)benzene (IIa) and 2-ethyl-2,3-dihydro-1-
benzofuran (III) in 40 and 8% yield, respectively
(Scheme 1). Bromo-substituted (cyclopropylmethoxy)-
benzenes Ib and Ic reacted with BF3·Et2O to produce
the corresponding bromo-substituted (cyclobutyloxy)-
benzenes IIb and IIc as the only products (yield
20 and 8%). In the presence of an equimolar amount of
the Lewis acid, the yields of (cyclobutyloxy)benzene
(IIa) and 2-ethyl-2,3-dihydro-1-benzofuran (III) de-
creased to 20 and 5%, respectively, whereas the yields
of cyclobutyl ethers IIb and IIc increased to 45 and
53%, respectively. In no case butenyl aryl ethers were
formed.
In the present work we examined transformations
of (cyclopropylmethoxy)benzene (Ia) and its 2- and
4-bromo derivatives Ib and Ic in the presence of
a Lewis acid, boron trifluoride–diethyl ether complex.
Prior to our study, isomerization of a cyclopropyl-
methyl fragment into cyclobutyl was reported only for
cyclopropylmethanols, cyclopropanols, and their esters
[5]. 2-Ethyl-2,3-dihydro-1-benzofuran (III) is likely to
be formed as a result of unusual migration of the
cyclopropylmethyl group to the ortho position of the
aromatic ring and subsequent intramolecular ring
Scheme 1.
BF3 ·Et2O
CCl4, –10°C
O
O
R2
R1
R2
R1
Ia–Ic
IIa–IIc
Scheme 2.
O
BF3 ·Et2O
CCl4, –10°C
+
Et
O
Et
III
OH
R1 = R2 = H (a); R1 = Br, R2 = H (b); R1 = H, R2 = Br (c).
IV
III
1498

UNUSUAL REARRANGEMENT OF (CYCLOPROPYLMETHOXY)BENZENE
1499
closure. This reaction path is confirmed by isomeriza-
tion of o-(cyclopropylmethyl)phenol (IV) into 2-ethyl-
2,3-dihydro-1-benzofuran (III) under analogous condi-
tions (yield 42%; Scheme 2).
From 0.3 g (2.0 mmol) of 2-(cyclopropylmethyl)-
phenol (IV) we obtained 0.125 g (42%) of 2-ethyl-2,3-
dihydro-1-benzofuran (III).
(Cyclobutyloxy)benzene (IIa). Light yellow
liquid. 1H NMR spectrum, δ, ppm: 1.72 t (1H, CH2, J =
8.1 Hz), 1.89 t (1H, CH2, J = 9.5 Hz), 2.21 m (2H,
CH2), 2.48 m (2H, CH2), 4.65 t.t (1H, CH, J = 6.8,
7.5 Hz), 6.86 d (2H, Harom, J = 8.6 Hz), 6.96 t (1H,
Harom, J = 8.6 Hz), 7.29 d (2H, Harom, J = 8.6 Hz).
13C NMR spectrum, δC, ppm: 13.4 (CH2), 30.8 (CH2),
71.5 (CH), 115.1 (Carom), 120.7 (Carom), 129.5 (Carom),
157.6 (Carom). Mass spectrum, m/z (Irel, %): 148.1 (34)
[M]+, 120.1 (100), 94 (54), 91.1 (34), 77 (15), 54.9
(39). Found: m/z 148.0864 [M]+. C10H12O. Calculated:
M 148.0888.
Thus (cyclopropylmethoxy)benzenes in the pres-
ence of BF3 ·Et2O undergo rearrangement which in-
volves isomerization of the cyclopropylmethyl frag-
ment into cyclobutane ring and/or its migration into
the ortho position of the phenyl ring and subsequent
intramolecular cyclization to 2-ethyl-2,3-dihydro-1-
benzofuran.
EXPERIMENTAL
1
The H and 13C NMR spectra were recorded on
a Bruker AM-300 spectrometer at 300.13 and
75.47 MHz, respectively, using CDCl3 as solvent and
tetramethylsilane as internal reference. The mass
spectra (electron impact, 70 eV) were obtained on
a Thermo Finnigan MAT 95 XP mass spectrometer
with direct sample admission into the ion source (ion
source temperature 250°C; batch inlet probe tempera-
ture 50–250°C, heating rate 10 deg/min). The reaction
mixtures were analyzed by GLC on a Shimadzu GL
2014 chromatograph equipped with a flame-ionization
detector and a 25000×0.2-mm DB-35MS capillary
column; carrier gas helium; the components were
quantitated by the internal standard technique (decane,
calibration factor 1.09).
2-Ethyl-2,3-dihydro-1-benzofuran (III). The
1H and 13C NMR spectra of III were identical to those
reported in [10].
From 0.45 g (2.0 mmol) of 2-bromo-1-(cyclopro-
pylmethoxy)benzene (Ib) we obtained 0.202 g (45%)
of 2-bromo-1-(cyclobutyloxy)benzene (IIb).
From 0.9 g (4.0 mmol) of compound Ib we ob-
tained 0.183 g (20%) of cyclobutyl ether IIb.
2-Bromo-1-(cyclobutyloxy)benzene (IIb). Light
1
yellow liquid. H NMR spectrum, δ, ppm: 1.73 t (1H,
CH2, J = 8.1 Hz), 1.90 t (1H, CH2, J = 9.5 Hz), 2.27 m
(2H, CH2), 2.49 m (2H, CH2), 4.70 t.t (1H, CH, J =
6.8, 7.5 Hz), 6.84 t (1H, Harom, J = 7.6 Hz), 6.90 d (1H,
H
arom, J = 8.3 Hz), 7.23 d.d (1H, Harom, J = 7.6,
Rearrangement of (cyclopropylmethoxy)ben-
zenes Ia–Ic (general procedure). Compound Ia–Ic,
2.0 or 4.0 mmol, was dissolved in 1.2 or 2.4 ml of
carbon tetrachloride, the solution was cooled to –10°C,
0.28 g (2.0 mmol) of boron fluoride–diethyl ether com-
plex was added under argon, and the mixture was
stirred for 2 h at –10°C. The mixture was then washed
with 6 ml of water at 0°C, treated with a 10% aqueous
solution of sodium hydrogen carbonate until neutral
reaction (pH 7), and extracted with methylene chloride
(3×5 ml). The extracts were combined, washed with
6 ml of water, and dried over MgSO4, and the solvent
was removed under reduced pressure.
8.3 Hz), 7.54 d (1H, Harom, J = 7.8 Hz). 13C NMR spec-
trum, δC, ppm: 13.1 (CH2), 30.4 (CH2), 72.3 (CH),
111.9 (CBr), 113.7 (Carom), 121.5 (Carom), 127.9 (Carom),
133.2 (Carom), 154.1 (Carom). Found: m/z 225.9988
[M – H]+. C10H11BrO. Calculated: M 227.10.
From 0.45 g (2.0 mmol) of 4-bromo-1-(cyclopro-
pylmethoxy)benzene (Ic) we obtained 0.237 g (53%)
of 4-bromo-1-(cyclobutyloxy)benzene (IIc).
From 0.9 g (4.0 mmol) of compound Ic we ob-
tained 0.072 g (8%) of IIc.
4-Bromo-1-(cyclobutyloxy)benzene (IIc). Light
1
yellow liquid. H NMR spectrum, δ, ppm: 1.67 t (1H,
From 0.3 g (2.0 mmol) of (cyclopropylmethoxy)-
benzene (Ia) we obtained 0.222 g of a dark yellow
liquid. Yield of (cyclobutyloxy)benzene (IIa) 20%,
yield of 2-ethyl-2,3-dihydro-1-benzofuran (III) 5%.
CH2, J = 8.1 Hz), 1.87 t (1H, CH2, J = 9.5 Hz), 2.16 m
(2H, CH2), 2.44 m (2H, CH2), 4.60 t.t (1H, CH, J =
6.8, 7.5 Hz), 6.75 d (2H, Harom, J = 8.9 Hz), 7.33 d
(2H, Harom, J = 8.9 Hz). 13C NMR spectrum, δC, ppm:
13.1 (CH2), 30.4 (CH2), 71.6 (CH), 112.7 (CBr), 116.7
(Carom), 132.3 (Carom), 158.0 (Carom). Found: m/z
225.9988 [M – H]+. C10H11BrO. Calculated: M 227.10.
From 0.6 g (4.0 mmol) of (cyclopropylmethoxy)-
benzene (Ia) we obtained 0.531 g of a dark yellow
liquid. Yield of IIa 40%, yield of III 8%.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 10 2011

PTASHKO et al.
6. Wong, H.-N.C., Hon, M.-Y., Tse, Ch.-W., Yip, Y.-Ch.,
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REFERENCES
Tanko, J., and Hudlicky, T., Chem. Rev., 1989, vol. 89,
p. 165.
1. The Claisen Rearrangement, Hiersemann, M. and Nubbe-
meyer, U., Eds., Weinheim: Wiley, 2007.
7. Reissig, H.-U. and Zimmer, R., Chem. Rev., 2003,
vol. 103, p. 1151.
2. Lutz, R.P., Chem. Rev., 1984, vol. 84, p. 205.
8. Baldwin, J.E., Chem. Rev., 2003, vol. 103, p. 1197.
3. Castro, A.M.M., Chem. Rev., 2004, vol. 104, p. 2939.
9. Rubin, M., Rubina, M., and Gevorgyan, V., Chem. Rev.,
4. Majumdar, K.C., Alam, S., and Chattopadhyay, B., Tetra-
hedron, 2008, vol. 64, p. 597.
2007, vol. 107, p. 3117.
5. Yanovskaya, L.A., Dombrovskii, V.A., and Khusid, A.Kh.,
Tsiklopropany s funktsional’nymi gruppami (Functional-
ized Cyclopropanes), Moscow: Nauka, 1980, p. 224.
10. Butov, S.A., Kozlikovskii, Ya.B., Chernyaev, B.V.,
and Koshchii, V.A., Zh. Org. Khim., 1990, vol. 26,
p. 2371.
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