Tandem molecular rearrangement in the alkylation of phenol with camphene

Article abstract of DOI:10.1134/s1070428008010077

The alkylation of phenol with camphene in the presence of boron trifluoride in glacial acetic acid was accompanied by tandem molecular rearrangement with formation of a mixture of ortho- and para-substituted phenols having 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yl and 5,5,6-trimethylbicyclo[2.2.1] hept-exo-2-yl substituents. The same products were obtained by rearrangement of 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yloxybenzene under analogous conditions. Similar reactions performed in the presence of aluminum phenoxide as catalyst resulted in predominant formation of the corresponding ortho-substituted phenols.

Full text of DOI:10.1134/s1070428008010077

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 1, pp. 62–66. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © I.Yu. Chukicheva, L.V. Spirikhin, A.V. Kuchin, 2008, published in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 1, pp. 69–73.
Dedicated to Full Member of the Russian Academy of Sciences
G.A. Tolstikov on his 75th anniversary
Tandem Molecular Rearrangement in the Alkylation of Phenol
with Camphene
I. Yu. Chukichevaa, L. V. Spirikhinb, and A. V. Kuchina
a Institute of Chemistry, Komi Research Center, Ural Division, Russian Academy of Sciences,
ul. Pervomaiskaya 48, Syktyvkar, 167982 Russia
e-mail: chukicheva-iy@chemi.komisc.ru
b Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences,
pr. Oktyabrya 69, Ufa, 450054 Bashkortostan, Russia
e-mail: chemorg@anrb.ru
Received July 12, 2007
Abstract—The alkylation of phenol with camphene in the presence of boron trifluoride in glacial acetic acid
was accompanied by tandem molecular rearrangement with formation of a mixture of ortho- and para-sub-
stituted phenols having 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yl and 5,5,6-trimethylbicyclo[2.2.1]hept-exo-2-
yl substituents. The same products were obtained by rearrangement of 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-
yloxybenzene under analogous conditions. Similar reactions performed in the presence of aluminum phenoxide
as catalyst resulted in predominant formation of the corresponding ortho-substituted phenols.
DOI: 10.1134/S1070428008010077
A large number of naturally occurring compounds,
so-called terpenophenols, are products of mixed bio-
genesis of phenols and terpenoids [1, 2]. These com-
pounds and structurally related phenols having bulky
alkyl substituents have found application as interme-
diate products in the synthesis of fragrant substances
[3, 4] and antioxidants [5]. Large-scale manufacture of
alkylphenols is usually based on the alkylation of
phenols with olefins in the presence of acid catalysts,
which leads to complex mixtures of isomeric products
[6]. Some homogeneous aluminum-based catalysts
showed high ortho-selectivity in the alkylation of
phenols; among these, the most active is aluminum
phenoxide [7–9]. The use in such reactions of terpene
derivatives that are characterized by very strong ability
to undergo various skeletal rearrangements [10] adds
much specificity to the alkylation process and high-
lights terpenophenols from other alkylphenols [11].
Scheme 1.
9
Me
15
8
Me
OH
OH
Me
Me
16
Me
7
14
13
Me
10
Me11
Catalyst
+
6
+
Me
CH2
1
2
12
4
5
OH
3
I
II
IV
VI
OH
Me
Me
Me
Me
Me
Me
Catalyst
O
+
Me
OH
Me
Me
III
V
VII
62

TANDEM MOLECULAR REARRANGEMENT
63
Alkylation of phenol (I) with camphene (II) and rearrangement of 1,7,7-trimethyl-exo-2-phenoxybicyclo[2.2.1]heptane (III)
Initial reactants
(molar ratio)
Ratio of ortho/para-
substituted phenols
Catalyst (10 wt %)
Temperature, °C Reaction time, h Conversion, %
I, II (1:1)
III
BF3 in AcOHa
BF3 in AcOHa
(PhO)3Al
080
080
120
160
3
3
6
6
85
70
80
90
01:1
02:1
25:1
20:1
III
I, II, (1:1)
(PhO)3Al
a
A 35% solution of BF3 in acetic acid.
The composition of alkylation products of phenol with
camphene is very complex due to various types of
isomerism [12–16]. Reactions of phenol with terpenes
below 100°C resulted mainly in the formation of phen-
oxy-substituted terpenoids [12, 13, 17, 18].
ing to our previous data [19], the ratio of ortho-sub-
stituted phenols IV and V in the alkylation of phenol
(I) with camphene was 10:1.
Our results indirectly indicate that the C-alkylation
process in the presence of aluminum phenoxide in-
volves preliminary etherification followed by rear-
rangement. In the first step, the aluminum atom in
Al(OPh)3 coordinates an additional phenol molecule
with localization of proton in the vicinity of oxygen
atoms, leading to the formation of ether III. When the
reaction is carried out at low temperature, it may be
stopped at the O-alkylation step with selective forma-
tion of ether III. Raising the temperature promotes
rearrangement of III into C-substituted phenol.
In the present work we examined direct alkylation
of phenol (I) with camphene (II) and rearrangement of
phenoxybornane III catalyzed by boron trifluoride in
glacial acetic acid and aluminum phenoxide (see
table). The alkylation of phenol with camphene, as
well as rearrangement of ether III, in the presence of
BF3 in AcOH gave a mixture of ortho- and para-sub-
stituted phenols IV–VII with different structures of the
terpene substituent (Scheme 1). The product structure
was unambiguously determined on the basis of their
1H and 13C NMR and IR spectra. The ratio of ortho-
substituted phenols IV and V was 1:4, and the ratio of
para-substituted phenols VI and VII was 1:2. The
overall ortho/para ratio in the rearrangement of ether
III was 2:1 (see table). In keeping with published data
[13], this ratio suggests intermolecular mechanism of
the rearrangement, i.e., alkylation of the aromatic ring
is preceded by dissociation of the ether bond with
formation of trimethylbicyclo[2.2.1]heptyl cation. The
same mechanism is likely to be responsible for the
formation of isomer mixture IV–VII in the reaction of
camphene with phenol in the presence of BF3; this
means that alkyl phenyl ethers are not necessarily
formed as precursors of alkylphenols. Thus the alkyla-
tion of phenol (I) with camphene (II) and rearrange-
ment of ether III in acid medium give rise to the same
set of products, and C-alkylation occurs under the
same conditions as does the rearrangement of III.
The first step in the rearrangement of ether III in
the presence of Al(OPh)3 is coordination of the ether
oxygen atom to aluminum. Next follows intramolecular
tandem rearrangement of the ether into alkylphenol (in
a way similar to Claisen rearrangement) and Wagner–
Meerwein rearrangement of the terpene fragment.
Thus aluminum phenoxide as catalyst promotes
selective alkylation of phenol (I) and rearrangement of
ether III with predominant formation of ortho isomer
IV with exo orientation of the benzene ring (80%).
The observed regio- and stereoselectivity in the
alkylation of phenol (I) with camphene (II) may be
rationalized in terms of intramolecular mechanism of
the process with participation of aluminum coordina-
tion sphere (Scheme 2). Initially, addition of phenol to
aluminum phenoxide gives HAl(OC6H5)4 [20, 21].
Protonation of camphene with HAl(OC6H5)4 generates
cation VIII. The latter reacts with the catalyst, yielding
intermediate cyclic complex IX. Spatial proximity of
the electrophilic C1 center to the ortho-carbon atom in
the aromatic ring favors intramolecular ortho-alkyla-
tion which is accompanied by Wagner–Meerwein rear-
rangement [22]. A small amount of phenol V may be
formed via alkylation of I with the corresponding
cation generated by the action of HAl(OC6H5)4 or as
a result of secondary isomerization processes.
However, different results were obtained in the
reaction of phenol with camphene and in the rearrange-
ment of ether III in the presence of aluminum phen-
oxide. The first process required a higher temperature,
and the ortho/para isomer ratio was 25:1 against 20:1
in the rearrangement of III. Furthermore, no phenols V
and VII were formed in the second reaction. Accord-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 1 2008

CHUKICHEVA et al.
64
Scheme 2.
O
Me
Al(OPh)3
Me
Me
OH
H+
Me
HAl(OPh)4
PhOH
1
6
2
Me
CH2
1
Me
–HAl(OPh)4
6
Me
Me
Me
Me
IX
II
VIII
IV
Our experimental data on the rearrangements of
Alkylation of phenol (I) with camphene (II) in
the presence of aluminum phenoxide. Phenol, 5.6 g
(59.5 mmol), was heated to 160°C, 0.06 g (2.2 mmol)
of aluminum turnings was added in small portions, the
resulting solution was cooled to 40°C, and 7.2 g
(52.8 mmol) of camphene was added. The mixture was
heated at 160–170°C until complete consumption of
camphene (GLC), cooled, diluted with diethyl ether,
treated with dilute hydrochloric acid to decompose the
catalyst, and washed with water until neutral reaction.
The organic phase was dried over anhydrous sodium
sulfate and evaporated under reduced pressure.
According to the GLC data, the residue contained
93 wt % of orto-substituted phenols. The product mix-
ture was separated by column chromatography on
aluminum oxide using hexane–diethyl ether as eluent
(gradient elution) to isolate 80% of IV, 3% of III, 4%
of V, and 8% of VI.
ether III in the presence of BF3 in acetic acid and in
the presence of aluminum phenoxide led us to presume
that in the first case the process follows intermolecular
mechanism, and in the second, intramolecular.
Unlike isotope mass spectrometry, quantitative
2H NMR spectroscopy provides a unique possibility for
studying selective intramolecular isotope distribution
which is very important for analysis of mechanisms of
chemical reactions and paths of formation of organic
2
compounds [23]. The results of H NMR study on the
molecular rearrangements in the alkylation of phenol
with camphene will be reported later.
EXPERIMENTAL
The IR spectra were recorded on a Specord M-80
spectrometer from samples prepared as KBr pellets or
1
thin films (neat). The H and 13C NMR spectra were
2-(1,7,7-Trimethylbicyclo[2.2.1]hept-exo-2-yl)-
phenol (IV). Colorless crystals, mp 100°C (from hot
hexane). IR spectrum (KBr), ν, cm–1: 3560, 3476 (OH);
1152 (C–O); 1604, 1588, 1458 (C=C); 754, 828 (δCH).
1H NMR spectrum, δ, ppm: 0.85 s (3H, C10H3), 0.90 s
(3H, C8H3), 0.96 s (3H, C9H3), 1.36–1.45 m (1H,
6-Hα), 1.47–1.55 d.d.d (1H, 6-Hβ, J = 6.4, 9.0 Hz),
1.63–1.72 m (2H, 3-H), 1.74–1.97 m (2H, 5-H), 2.23–
2.32 m (1H, 4-H), 3.18 t (1H, 2-H, J = 8.9 Hz), 4.80 s
(1H, OH), 6.78–6.81 d.d (1H, 13-H, J = 1.3, 6.6 Hz),
6.92–6.98 t.d (1H, 15-H, J = 1.2, 6.4 Hz), 7.08–
7.14 t.d (1H, 14-H, J = 1.5, 6.0 Hz), 7.37 br.d (1H,
18-H, J = 7.0 Hz). 13C NMR spectrum, δC, ppm: 12.38
(C10), 20.34 (C9), 21.49 (C8), 27.57 (C5), 33.95 (C3),
39.99 (C6), 45.44 (C2), 45.65 (C4), 48.04 (C7), 49.80
(C1), 114.99 (C16), 120.24 (C14), 126.47 (C15), 128.13
(C13), 129.54 (C11), 154.62 (C12). Found, %: C 84.09;
H 9.31. C16H22O. Calculated, %: C 83.43; H 9.63.
measured on a Bruker AM instrument at 300.13 and
75.03 MHz, respectively, using chloroform-d as sol-
vent and reference (δ 7.26 ppm, δC 77.0 ppm). Signals
were assigned using JMOD, COSY, NOESY, and
HETCOR 13C–1H techniques.
The initial compounds and products were analyzed
by GLC on a Kristall-2000 chromatograph (30-m×0.3-
mm capillary column, stationary phase PEG 20; oven
temperature programming from 100 to 240°C at a rate
of 6 deg/min; carrier gas argon). Thin-layer chroma-
tography was performed on Silufol UV-254 plates
using diethyl ether–hexane (3:1) as eluent; spots were
detected by treatment with a solution of vanillin, fol-
lowed by heating to 110°C, or with a saturated solution
of KMnO4 at room temperature. Preparative column
chromatography was performed on silica gel L (100–
250 μm) or aluminum oxide (neutral, Brockmann ac-
tivity grade I). The melting points were determined on
a Kofler hot stage.
1,7,7-Trimethyl-exo-2-phenoxybicyclo[2.2.1]-
heptane (III). Oily liquid, nD20 = 1.5255. IR spectrum
(neat), ν, cm–1: 1604, 1592, 1458 (C=C); 754, 692
According to the GLC data, initial camphene (II,
racemate) contained 5% of tricyclene; ether III: nD20
=
(δC–H); 1248 (C–O–C); 1084 (O–C). H NMR spec-
1
1.5247, bp 190°C (0.98 atm).
trum, δ, ppm: 1.32 s (3H, C10H3), 1.19 s and 1.41 s (3H
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 1 2008

TANDEM MOLECULAR REARRANGEMENT
65
each, C8H3, C9H3), 1.46–1.61 m (2H, 5-H, 6-H), 1.87–
1.94 m (1H, 6-H), 2.06 m (2H, 5-H, 4-H), 2.12–2.2 m
(2H, 3-H), 4.32 q (1H, 2-H, J = 3.7, 3.3 Hz), 7.13–
7.20 m (3H, Harom), 7.50–7.55 m (2H, Harom). 13C NMR
spectrum, δC, ppm: 11.93 (C10), 20.26 (C9), 20.45 (C8),
27.50 (C5), 34.32 (C6), 39.61 (C3), 45.38 (C4), 47.09
(C7), 49.23 (C1), 84.39 (C2), 115.46 (C13, C15), 120.08
(C14), 129.38 (C12, C16), 158.00 (C11).
Alkylation of phenol (I) with camphene (II) in
the presence of boron trifluoride in glacial acetic
acid. A 35% solution of BF3 in glacial acetic acid
(10 wt % with respect to the initial phenol) was slowly
added at room temperature to a mixture of 5.0 g
(53.1 mmol) of phenol (I) and 7.23 g (53.1 mmol) of
camphene (II). The mixture was heated to 80°C,
stirred for 3 h at that temperature, cooled, diluted with
diethyl ether, and washed first with a solution of
NaHCO3 and then with a solution of NaCl until neutral
reaction. The organic phase was dried over anhydrous
Na2SO4, the solvent was removed under reduced pres-
sure, and the residue was distilled in a vacuum, a frac-
tion boiling at 135°C (3 mm) being collected. The
ortho and para isomers were separated by column
chromatography on silica gel L (100–160 μm) using
benzene–hexane as eluent. A mixture of para-substi-
tuted phenols V and VII was thus isolated. The NMR
spectra contained signals typical of the corresponding
terpene fragments. The data for phenol VII coincided
with those reported in [24].
4-(1,7,7-Trimethylbicyclo[2.2.1]hept-exo-2-yl)-
phenol (VI). Colorless crystals, mp 150°C (from hot
heptane). IR spectrum (KBr), ν, cm–1: 3276 (OH);
1
1620, 1604 (C=C); 824 (δC–H); 1244 (C–O). H NMR
spectrum, δ, ppm: 0.75 s (3H, C10H3), 0.81 s (3H,
C8H3), 0.84 s (3H, C9H3), 1.25–1.39 m (2H, 6-H),
1.59–1.69 m (2H, 3-H), 1.74–1.87 m (2H, 5-H),
2.25 d.d.d.d (1H, 4-H, J = 3.5, 4.0, 4.5 Hz), 2.85 t (1H,
2-H, J = 9.0 Hz), 4.75 s (1H, OH), 6.74–6.77 d.t (2H,
12-H, 16-H, J = 3.0, 2.0, 6.8 Hz), 7.14 s and 7.16 s (1H
each, 13-H, 15-H). 13C NMR spectrum, δC, ppm: 14.66
(C10), 20.11 (C9), 21.44 (C8), 27.64 (C5), 33.68 (C3),
40.53 (C6), 45.68 (C2), 47.84 (C7), 49.46 (C1), 51.73
(C4), 114.39 (C13, C15), 130.36 (C12, C16), 135.59 (C11),
153.15 (C14). Found, %: C 83.60; H 9.10. C16H22O.
Calculated, %: C 83.43; H 9.63.
4-(5,5,6-Trimethylbicyclo[2.2.1]hept-exo-2-yl)-
1
phenol (VII). H NMR spectrum, δ, ppm: 0.98 s (3H,
C8H3), 1.02 d (3H, C10H3, J = 2.6 Hz), 1.14 s (3H,
C9H3), 1.50–1.56 m (3H, 3-H, 6-H, 7-H), 1.81–1.94 m
(2H, 7-H, 1-H), 1.97 m (1H, 4-H), 2.28–2.32 m (1H,
6-H), 2.80 t (1H, 5-H, J = 7.4 Hz), 6.66 s (1H, OH),
6.96–6.97 m (2H, 12-H, 16-H), 7.18 s and 7.20 s (1H
each, 13-H, 15-H). 13C NMR spectrum, δC, ppm: 16.38
(C10), 24.88 (C8), 27.74 (C9), 32.68 (C6), 33.57 (C7),
39.66 (C2), 40.67 (C5), 48.83 (C3), 49.89 (C1), 50.87
(C4), 115.29 (C16), 120.59 (C14), 125.97 (C15), 126.52
(C13), 133.40 (C11), 153.35 (C12).
2-(5,5,6-Trimethylbicyclo[2.2.1]hept-exo-2-yl)-
phenol (V). Colorless crystals, mp 70°C (from hot
hexane). IR spectrum (KBr), ν, cm–1: 3345 (OH);
1
1609, 1591 (C=C); 750 (δC–H); 1208 (C–O). H NMR
spectrum, δ, ppm: 1.02 s (3H, C8H3), 1.04 d (3H,
C10H3, J = 2.6 Hz), 1.17 s (3H, C9H3), 1.37–1.45 m
(3H, 1-H, 6-H, 7-H), 1.76–1.84 m (2H, 7-H, 4-H),
2.02 s (1H, 3-H), 2.26–2.34 m (1H, 6-H), 2,90 t (1H,
5-H, J = 7.4 Hz), 4.83 s (1H, OH), 6.79 d (1H, 13-H,
J = 6.5 Hz), 6.91 t (1H, 14-H, J = 3.7 Hz), 7.16 t (1H,
15-H, J = 6.0 Hz), 7.29 d (1H, 16-H, J = 8.3 Hz).
13C NMR spectrum, δC, ppm: 16.23 (C8), 24.77 (C10),
27.67 (C9), 32.52 (C6), 33.47 (C7), 39.63 (C2), 40.61
(C5), 48.82 (C1), 49.82 (C4), 50.75 (C3), 115.13 (C13),
120.44 (C14), 125.79 (C15), 126.48 (C16), 133.02 (C11),
153.31 (C12). Found, %: C 83.80; H 9.35. C16H22O.
Calculated, %: C 83.43; H 9.63.
The rearrangement of ether III in the presence of
BF3 in acetic acid was performed in a similar way.
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