Ipso-Acylation of 5, 13-di-tert-butyl-8, 16-dimethyl[2.2]metacyclophane with acid anhydrides: Through-space electronic interaction among the two benzene rings

Article abstract of DOI:10.3184/030823409X447727

Acylation of 5, 13-di-tert-butyl-8, 16-dimethyl[2.2]metacyclophane with acid anhydrides led to mono-ipso-acylation at the tert-butyl group to give 5-acyl-13-tert-butyl-8, 16-dimethyl[2.2]metacyclophanes, from which the second electrophilic substitition wi

Full text of DOI:10.3184/030823409X447727

298 RESEARCH PAPER
MAY, 298–301
JOURNAL OF CHEMICAL RESEARCH 2009
ipso-Acylation of 5,13-di-tert-butyl-8,16-dimethyl[2.2]metacyclophane
with acid anhydrides: through-space electronic interaction among the
two benzene rings
Tomoe Shimizu, Arjun Paudel and Takehiko Yamato*
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga 840-8502, Japan
Acylation of 5,13-di-tert-butyl-8,16-dimethyl[2.2]metacyclophane with acid anhydrides led to mono-ipso-acylation
at the tert-butyl group to give 5-acyl-13-tert-butyl-8,16-dimethyl[2.2]metacyclophanes, from which the second
electrophilic substitition with acid anhydrides can be strongly suppressed because of deactivation of the second
aromatic ring by acyl group introduced by the through-space electronic interaction.
Keywords: cyclophanes, [2.2]metacyclophanes, ipso-acylation, Clemmensen reduction
For many years various research groups have been attracted
by the chemistry and spectral properties of the [2.2]MCP
the acylation of 5,13-di-tert-butyl-8,16-dimethyl[2.2]MCP 1
with various acid anhydrides. Further Clemmensen reduction
of the acylation products to prepare 8,16-dimethyl[2.2]benzo-
napthaleno- and benzoanthracenoMCPs by Friedel–Crafts
intramolecular cyclisation was also described.
([2.2]MCP
=
[2.2]metacyclophane)
skeleton.1-3 Its
conformation, which was elucidated by X-ray measurements,4
is apparently frozen into a chair-like non-planar form.
The two halves of the molecule form a stepped system. The
benzene rings are not planar, but have a boat conformation,
with the result that the molecule avoids the steric interaction
of the central carbon atoms C-8 and C-16 and of the attached
hydrogen atoms. The C(8)–C(16) distance is 2.689 Å.
The increased strain in the molecule 8,16-dimethyl[2.2]MCP
as compared with that in the parent hydrocarbon can be seen,
in particular, in the distance between C-1 and C-2 (1.573 Å).5
Previously, we reported that6–8 nitration of 5,13-di-tert-
butyl-8,16-dimethyl[2.2]MCP 1 with fuming HNO3 afforded
13-tert-butyl-5-nitro-8,16-dimethyl[2.2]MCP 2 along with the
transannular reaction product, 2,7-di-tert-butyl-4,9-dinitro-
trans-10b,10c-dimethyl-10b,10c-dihydropyrene 3.
Results and discussion
Whenacetylationof5,13-di-tert-butyl-8,16-dimethyl[2.2]MCP
(1)19 with acetic anhydride in the presence of TiCl4 as a
catalyst was carried out at 0°C for 2 h, 5-acetyl-13-tert-butyl-
8,16-dimethyl[2.2]MCP (5a) and 2,7-di-tert-butyl-trans-
10b,10-dimethyl-10b,10c-dihydropyrene (7)20 were obtained
in 83% and 17% yield, respectively. Interestingly, acetylation
of 1 with acetic anhydride in the presence of AlCl3–MeNO2
as a catalyst was carried out at 0°C for 2 h led to the two-fold
ipso-acetylation to give 5,13-diacetyl-8,16-dimethyl[2.2]MCP
(6a) in 90% yield along with the monoacylation product 5a in
10% yield.
TiCl4 catalysed acylation of 1 with benzoic anhydride
carried out at 0°C for 2 h afforded 5-benzoyl-13-tert-butyl-
8,16-dimethyl[2.2]MCP (5b) in 95% yield along with a
small amount of 7. A similar reaction was carried out in the
presence of AlCl3–MeNO2 that led to ipso-acylation at just
one tert-butyl group to give 5b in quantitative yield. However,
attempted further acylation of 1 with benzoic anhydride
failed. In spite of increasing the amount of benzoic anhydride
and AlCl3–MeNO2 or increasing the reaction temperature
to 50°C and prolonging the reaction time, no formation of
two-fold ipso-acylation product 6b was observed. Only the
mono-ipso-acylation product 5b was obtained in good yields.
Although the replacement of a tert-butyl group by a nitro
groupinelectrophilicaromaticsubstitutionshasfrequentlybeen
described,9–16 generally the yields are modest because of the
accompanying side reactions.17 Only in activated compounds
are better yields obtained. However, the mechanistic aspects
for ipso-attack in electrophilic aromatic substitutions having
more than two aromatic rings are still not clear in spite
of the possibility of through space electronic interactions
among the other benzene rings.18 Thus there is substantial
interest in investigating the acylation of the internally
substituted [2.2]MCPs, which might afford single mono- and
di-acylated products. We report here on the through-space
electronic interaction among the two benzene rings during
Me
HNO3
CH2Cl2
Me
1
Me
NO2
Me
+
Me
O2N
Me
NO2
2
3
Scheme 1
* Correspondent. E-mail: yamatot@cc.saga-u.ac.jp

JOURNAL OF CHEMICAL RESEARCH 2009 299
(RCO)2O (4)
1
Lewis acid
CH2Cl2
0°C for 2h
O
Me
Me
R
Me
+
+
Me
O
O
Me
Me
R
R
6a; R= CH3
5 a; R= CH3
b; R= C6H5
b; R= C6H5
c; R= (CH2)2COOH
c; R= (CH2)2COOH
7
d; R= C6H4(o-COOH)
d; R= C6H4(o-COOH)
Scheme 2
Similar treatment of 1 with 3.0 equiv. of succinic anhydride or
phthalic anhydride in the presence of AlCl3–MeNO2 under the
same conditions afforded the corresponding mono-acylation
product 5c and 5d in 90 and 95% yields, respectively. Thus,
the number of ipso-acylation of 1 was strongly affected by the
acid anhydrides and the reaction conditions used.
Similarly, 5-tert-butyl-1,2,3-trimethylbenzene (8)21 with
excess succinic anhydride in the presence of AlCl3-MeNO2
at room temperature only gave a quantitative recovery of the
starting compound. Raising the reaction temperature to 50°C
and prolonging the reaction time resulted only the recovery of
the starting compound. No formation of the ipso-acylation at
the tert-butyl group was observed. In contrast with 8, acylation
of [2.2]MCP 1 with excess succinic anhydride in the presence
of AlCl3–MeNO2 led to ipso-acylation only at one of the
tert-butyl groups to give 5c in good yield. This result seems
to indicate that the metacyclophane structure in 1 plays an
important role in the present ipso-acylation reaction. The ipso-
acylation of 1 is attributed to the highly activated character
of the aryl ring and the increased stabilisation of s-complex
intermediate A arising from the through-space electronic.
Recently, Cacace et al. reported22 that the intramolecular
proton shift, namely, ring-to-ring proton migration in
(b-phenylethyl)arenium ions from the higher cationic
alkylation rate of 1,2-diphenylethane than that of toluene
in the gas-phase. Thus in the present system, s-complex
intermediate A would be stabilised by a through-space
electronic interaction through intraannular 8,16-positions
The present acylation behaviour of [2.2]MCP 1 can be
explained by the stability of the cationic intermediates, which
could arise from the through-space electronic interaction with
the benzene ring located on the opposite side. Thus, a first s-
complex intermediate (A) would be stabilised by the through-
space electronic intraannular interaction through 8,16-positions
with the opposing benzene ring, thus accelerating the reaction.
However, the second electrophilic substitition with acyl
group can be strongly suppressed in the intermediate (B)
because of deactivation of the second aromatic ring by acyl
group like nitration of 8,16-dimethyl[2.2]MCP, which only
afforded mono-nitration product even in the drastic nitration
conditions.8 This effect seems to be increased for benzoyl,
3-(carboxyl)propionyl and (2-carboxyl)benzoyl group in
comparison with that of acetyl group.
O
+
Me
Me
O
O
O
Me
Me
O
Me
Me
Me
(4c)
Me
R
+
Me
Me
O
R
AlCl3-MeNO2
CH2Cl2
room temp. for 2 h
R
CO(CH2)2COOH
9
B
A
8
Fig. 1 The through-space electronic interaction of s-complex
intermediates
Scheme 3
Table 1 Lewis acid catalysed acylation of 5,13-di-tert-butyl-8,16-dimethyl[2.2]MCP (1) with acid anhydrides (4)
Run
Reagent (4)
Lewis acida
4/1 (mol mol-1)
Product/%b,c
1
2
3
4
5
6
7
8
9
Acetic anhydride (4a)
Acetic anhydride (4a)
Benzoic anhydride (4b)
Benzoic anhydride (4b)
Succinic anhydride (4b)
Succinic anhydride (4b)
Succinic anhydride (4b)
Phthalic anhydride (4c)
Phthalic anhydride (4c)
A
B
A
B
A
B
B
A
B
3.0
3.0
3.0
3.0
1.5
1.5
3.0
1.5
3.0
5a (83) [70]d
5a (10) [ 5]
5b (95) [90]d
5b (100) [95]
5c (0)
6a (0)
6a (90) [85]
6b (0)
6b (0)
6c (0)
5c (85) [73]
5c (90) [80]
5d (0)
6c (0)
6c (0)
6d (0)
5d (95) [89]
6d (0)
aA: TiCl4, Catalyst/reagent (4) = 7.0 (mol/mol); B: AlCl3–MeNO2, Catalyst/reagent (4) = 3.0 (mol/mol). bYields were determined by
G.L.C. analyses. Isolated yields are shown in square brackets. d2,7-Di-tert-butyl-trans-10b,10-dimethyl-10b,10c-dihydropyrene (7)
c
was also obtained in 17 and 3% yields, respectively.

300 JOURNAL OF CHEMICAL RESEARCH 2009
Me
Zn(Hg), HCl
5c
toluene-H2O
Me
reflux for 24 h
HOOC
10c
(60%)
Zn(Hg), HCl
5d
toluene-H2O
reflux for 24 h
Me
Me
+
O
Me
Me
O
HOOC
(38%)
11d
10d (17%)
Scheme 4
Materials
with the opposing benzene ring, therefore accelerating the
reaction like the formylation of tert-butyl[n.2]MCPs.23,24
However, only one tert-butyl group is ipso-acylated because
of deactivation of the second aromatic ring by the acyl group
introduced (intermediate B).
The preparations of 5,13-di-tert-butyl-8,16-dimethyl[2.2]metacyclo-
phane 119, and 5-tert-butyl-1,2,3-trimethylbenzene 821 have been
previously described.
Titanium tetrachloride catalysed acylation of 5,13-di-tert-butyl-8,16-
dimethyl [2.2]metacyclophane (1); typical procedure
Clemmensen reduction of 5c with Zn–Hg afforded the
desired 10c in 60% yield. In contrast, in the case of 5d the
desired product 10d was obtained only in 17% yield along with
5-tert-butyl-13-(3-oxo-1,3-dihydroisobenzofuran-1-yl)-8,16-
dimethyl[2.2]metacyclophane 11d was obtained in 38% yield.
The structure of 11d was assigned on the basis of elemental
A solution of TiCl4 (1.2 ml, 10.92 mmol) in CH2Cl2 (1 mL) at 0°C
was added to a solution of 5,13-di-tert-butyl-8,16-dimethyl[2.2]meta-
cyclophane (1) (181 mg, 0.52 mmol) and acetic anhydride (0.16 mL,
1.56 mmol) in CH2Cl2 (4 mL). After the reaction mixture was stirred
at 0°C for 2 h, it was poured into ice-water (10 mL). The organic
layer was extracted with CH2Cl2 (10 mL ¥ 2). The extract was washed
with water (5 mL), dried (Na2SO4), and concentrated. The residue
was column chromatographed over silica gel with hexane, hexane:
benzene 1:1, and benzene as eluent to give 30 mg (17%) of 7 and 144
mg (70%) of 5a, respectively.
1
analyses and spectral data. The H NMR spectrum of 11d
shows two kinds of methyl protons, each as a singlet and
the methyl protons shifted strongly up-field at d –0.26
and 0.35 ppm in comparison with those of 10d (d 0.54 and
0.58 ppm). In contrast, the cyclophane aromatic protons of 11d
are observed as four sets of doublet (J = 1.8 Hz) at much lower
fields (d 7.00, 7.02, 7.24 and 7.45 ppm) than those of 10d at
d 6.85 and 7.08 ppm as a singlet. The methine proton was
also observed at d 7.32 ppm as a singlet. The above data show
that the structure of 11d is the 8,16-dimethyl[2.2.MCP having
the isobenzofuran group at the 13-position in which benzene
ring cause one of the methyl protons to the upper field shift at
d –0.26 ppm due to the ring current effect.
5-Acetyl-13-tert-butyl-8,16-dimethyl[2.2]metacyclophane
(5a):
Colourless prisms (hexane), m.p. 157–161°C; nmax/cm-1 (KBr) 1665
(C=O); dH (CDCl3) 0.50 (3H, s, Me), 0.63 (3H, s, Me), 1.30 (9H, s,
tBu), 2.55 (3H, s, Me), 2.73–3.04 (8H, m, CH2), 7.13 (2H, s, ArH) and
7.73 (2H, s, ArH); m/z 334 (M+) (Found: C, 86.65; H, 8.98. C24H30O
(334.51) requires C, 86.18; H, 9.04%).
2,7-Di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene
(7): Deep green prisms (hexane), m.p. 203–204°C (lit.20 m.p. 203–
204°C).
Compound 5b was obtained by the acylation of 1 with benzoic
anhydride in the same manner described above. The yields are
compiled in Table 1.
We conclude that the ipso-acylation reactions of 1 lead
to the first-reported direct introduction of one acyl group.
The selective ipso-acylation of 1 is attributed to the highly
activated character of the aryl ring and the increased
stabilisation of s-complex intermediate.Also we have deduced
that a first s-complex intermediate, (b-phenylethyl)arenium
ion is stabilised by the through-space electronic interaction
with the other benzene ring in acylation like the electrophilic
aromatic substitution of MCPs. Further studies on ipso-
acylation and Friedel–Crafts intramolecular cyclisation of
10c and 10d to prepare 8,16-dimethyl[2.2]benzonapthaleno-
and benzoanthracenoMCPs are currently in progress in our
laboratory.
5-Benzoyl-13-tert-butyl-8,16-dimethyl[2.2]metacyclophane (5b):
Colourless prisms (hexane), m.p. 179–182°C; nmax/cm-1 (KBr) 1648
(C=O); dH (CDCl3) 0.58 (3H, s, Me), 0.67 (3H, s, Me), 1.30 (9H, s,
tBu), 2.74–3.03 (8H, m, CH2), 7.14 (2H, s, ArH), 7.45–7.78 (5H, m,
ArH) and 7.65 (2H, s, ArH); m/z 396 (M+) (Found: C, 87.74; H, 8.22.
C29H32O (396.58) requires C, 87.83; H, 8.13%).
Acylation of 1 with acid anhydrides in the presence of AlCl3-MeNO2;
typical procedure
To a solution of 1 (1.0 g, 2.87 mmol) and succinic anhydride (432 mg,
4.31 mmol) in CH2Cl2 (17 mL) was added a solution of aluminum
chloride (1.73 g, 12.9 mmol) in nitromethane (3 mL) at 0°C.
After the reaction mixture was stirred at room temperature for 2 h,
it was poured into a large amount of water. The organic layer was
extracted with diethyl ether (20 mL ¥ 3). The extract was washed
with 10% hydrochloric acid (10 mL ¥ 2) and water (10 mL ¥ 2), dried
with Na2SO4, and evaporated in vacuo. The residue was recrystallised
from benzene to afford 13-tert-butyl-5-(3-carboxylpropionyl)-8,16-
dimethyl[2.2]metacyclophane (5c) (821 mg, 73%) as colourless
prisms, m.p. 176–178°C; nmax/cm-1 (KBr) 1712, 1676 (C=O);
Experiment
1
All melting points are uncorrected. H NMR spectra were recorded at
300 MHz on a Nippon Denshi JEOL FT-300 NMR spectrometer in
deuteriochloroform with Me4Si as an internal reference. IR spectra were
measured as KBr pellets on a Nippon Denshi JIR-AQ2OM spectrometer.
Mass spectra were obtained on a Nippon Denshi JMS-HX110Aultrahigh
performance mass spectrometer at 75 eV using a direct-inlet system.
Elemental analyses were performed by Yanaco MT-5.
d
H (CDCl3) 0.50 (3H, s, Me), 0.63 (3H, s, Me), 1.30 (9 H, s, tBu),
2.69–2.86 (6H, m, CH2), 2.90–3.07 (4H, m, CH2), 3.27–3.33 (2H, m,
CH2), 7.13 (2H, s, ArH) and 7.69 (2H, s, ArH); m/z 392 (M+) (Found:
C, 79.89; H, 8.13. C26H32O3 (392.56) requires C, 79.56; H, 8.22%).

JOURNAL OF CHEMICAL RESEARCH 2009 301
Acylation of 1 with acetic anhydride carried out as described above
afforded 5,13-diacetyl-8,16-dimethyl[2.2]metacyclophane 6a in 85%
yield as colourless prisms (hexane), m.p. 284–285°C; nmax/cm-1
(KBr) 1666 (C=O); dH (CDCl3) 0.59 (6H, s, Me), 2.58 (6H, s, Me),
2.79–3.10 (8H, m, CH2), 7.76 (4H, s, ArH); m/z 320 (M+) (Found: C,
82.56; H, 7.56. C22H24O2 (320.44) requires C, 82.46; H, 7.55%).
Acylation of 1 with phthalic anhydride carried out as described
above afforded 13-tert-butyl-5-[(2-carboxyl)benzoyl]-8,16-dimethyl
[2.2]metacyclophane 5d in 89% yield as colourless prisms, m.p.
257°C; nmax/cm-1 (KBr) 1690, 1649 (C=O); dH (CDCl3) 0.49 (3H,
s, Me), 0.57 (3H, s, Me), 1.29 (9H, s, tBu), 2.65–2.78 (4H, m, CH2),
2.83–2.93 (4H, m, CH2), 7.10 (2H, s, ArH), 7.27–7.30 (1H, m, ArH),
7.46 (2H, s, ArH), 7.50–7.57 (1H, m, ArH), 7.60–7.68 (1H, m, ArH)
and 8.06–8.10 (1 H, m, ArH); m/z 440 (M+) (Found: C, 81.67; H,
7.26. C30H32O3 (440.57) requires C, 81.78; H, 7.32%).
Compound 10d was obtained as prisms [hexane–benzene (1:2)];
m.p. 215°C; nmax/cm-1 (KBr) 1695 (C=O); dH (CDCl3) 0.54 (3H, s,
Me), 0.58 (3H, s, Me), 1.27 (9H, s, tBu), 2.69–2.88 (8H, m, CH2),
4.33 (2H, s, CH2), 6.85 (2H, s, ArH), 7.08 (2H, s, ArH), 7.22 (1H, d,
J = 7.3 Hz, ArH), 7.31 (1H, t, J = 7.3 Hz, ArH), 7.46 (1H, t, J = 7.3 Hz,
ArH) and 8.05 (1H, d, J = 7.3 Hz, ArH); m/z 426 (M+) (Found: C,
84.33; H, 8.05. C30H34O2 (426.6) requires C, 84.47; H, 8.03%).
Compound 11d was obtained as prisms [hexane–benzene (1:2)];
m.p. 235–237°C; nmax/cm-1 1775 (C=O); dH (CDCl3) –0.26 (3H, s,
Me), 0.35 (3H, s, Me), 1.23 (9H, s, tBu), 2.59–2.85 (8H, m, CH2),
7.00 (1H, d, J = 1.8 Hz, ArH), 7.02 (1H, d, J = 1.8 Hz, ArH), 7.24
(1H, d, J = 1.8 Hz, ArH), 7.32 (1H, s, CH), 7.35 (1H, t, J = 7.9 Hz,
ArH), 7.45 (1H, d, J = 1.8 Hz, ArH), 7.56 (1H, d, J = 7.9 Hz, ArH),
7.74 (1H, t, J = 7.9 Hz, ArH) and 8.41 (1H, d, J = 7.9 Hz, ArH);
m/z 424 (M+) (Found: C, 84.63; H, 7.75. C30H32O2 (424.59) requires
C, 84.87; H, 7.6%).
Reduction of 5c with Zn-Hg
Received 14 January 2009; accepted 6 March 2009
Paper 09/0391 doi: 10.3184/030823409X447727
Published online: 20 May 2009
To a solution of HgCl2 (206 mg, 0.76 mmol) in conc. HCl (0.1 mL)
and water (3.44 mL) was added zinc powder (2.06 g, 31.5 mmol) and
a mixture was stirred for 5 min. at room temperature. A suspension
was decantated to leave the residue to which conc. HCl (3.1 mL),
water (1.3 mL) was added. To the reaction mixture was added a
solution of 5c (500 mg, 1.28 mmol) in toluene (1.7 mL) and refluxed
for 6 h. After the fresh conc. HCl (2 mL) was added three times
every 6 h, the reaction mixture was cooled to room temperature.
The organic layer was extracted with ether (10 mL ¥ 3). The extract
was washed with water (10 mL ¥ 2), dried with Na2SO4, and
evaporated in vacuo. The residue was recrystallised from hexane–
benzene (1:2) to afford 10c (290 mg, 60%) as colourless prisms, m.p.
150–156°C; nmax/cm-1 (KBr) 1700 (C=O); dH (CDCl3) 0.56 (3H, s,
Me), 0.59 (3H, s, Me), 1.29 (9H, s, tBu), 1.90–1.99 (2H, m, CH2),
2.35–2.41 (2H, m, CH2), 2.52–2.58 (2H, m, CH2), 2.74–2.93 (8H, m,
CH2), 6.92 (2H, s, ArH) and 7.11 (2H, s, ArH); m/z 378 (M+) (Found:
C, 82.22; H, 9.05. C26H34O2 (378.56) requires C, 82.49; H, 9.05%).
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Reduction of 5d with Zn-Hg
Zinc powder (1.84 g, 28.2 mmol) was added to a solution of HgCl2
(184 mg, 0.68 mmol) in conc. HCl (0.1 mL) and water (3.1 mL)
and the mixture was stirred for 5 min. at room temperature.
The suspension was decantated to leave the residue to which conc.
HCl (2.8 mL), water (1.2 mL) was added. A solution of 5d (500 mg,
1.14 mmol) in toluene (1.5 mL) was added to the reaction mixture
and refluxed for 6 h. After the fresh conc. HCl (2 mL) was added three
times every 6 h, the reaction mixture was cooled to room temperature.
The organic layer was extracted with ether (10 cm3 ¥ 3). The
extract was washed with water (10 mL ¥ 2), dried with Na2SO4, and
evaporated in vacuo. The residue was recrystallised from hexane–
benzene (1:2) to afford 10d (83 mg, 17%) as colourless prisms.
Chromatography on silica gel (Wako, C-300; 100 g) eluting with
hexane–benzene (1:3) afforded 11d (178 mg, 38%) as colourless
solid.