Synthesis of 4,16-dimethoxy-1,2-dimethyl[2.4]metacyclophan-1-ene and 8,17-dimethoxy-1.2-dimethyl-10-thia[2.3.4](1,3,5)cyclophan-1-ene

Article abstract of DOI:10.3184/030823410X12795373456980

McMurry cyclisation of 1,4-bis(3-acetyl-4-methoxyphenyl)butane afforded flexible 4,16-dimethoxy-1,2-dimethyl[2.4] metacyclophan-1-ene, which was converted to the corresponding triple bridged 8,17-dimethoxy-1,2-dimethyl-10- thia[2.3.4](1,3,5)cyclophan-1-en

Full text of DOI:10.3184/030823410X12795373456980

JOURNAL OF CHEMICAL RESEARCH 2010
AUGUST, 445–448
RESEARCH PAPER 445
Synthesis of 4,16-dimethoxy-1,2-dimethyl[2.4]metacyclophan-1-ene and
8,17-dimethoxy-1.2-dimethyl-10-thia[2.3.4](1,3,5)cyclophan-1-ene
Tomoe Shimizu, Rika Kato, Shinpei Miyamoto and Takehiko Yamato*
Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga 840-8502, Japan
McMurry cyclisation of 1,4-bis(3-acetyl-4-methoxyphenyl)butane afforded flexible 4,16-dimethoxy-1,2-dimethyl[2.4]
metacyclophan-1-ene, which was converted to the corresponding triple bridged 8,17-dimethoxy-1,2-dimethyl-
10-thia[2.3.4](1,3,5)cyclophan-1-ene. The conformational studies of these cyclophan-1-enes in solution are also
described.
Keywords: cyclophanes, [2.4]metacyclophan-1-ene, McMurry reaction, conformation, strained molecules, triple bridged
cyclophane
Although the parent [2.2]metacyclophane (MCP = metacyclo-
phane) was first reported as early as in 1899 by Pellegrin,1 the
synthesis of syn-[2.2]MCP was not realised until 85 year later.
Mitchell et al.2 have successfully prepared syn-[2.2]MCP at
low temperature by using (arene)chromiumcarbonyl complex-
ation to control the stereochemistry. Later, Itô and coworkers3
have also isolated and characterised syn-[2.2]MCP without
complexation. However, syn-[2.2]MCP isomerises readily to
the anti-isomer above 0°C. On the other hand, Boekelheide4
and Staab5 succeeded in synthesising intra-annularly sub-
stituted syn-[2.2]MCPs, respectively. However, reports on syn-
thesis and reactions of syn-[2.n]MCP have not been published
so far. On the other hand, Boekelheide et al. reported the
synthesis of triple bridged [23](1,3,5)cyclophanes as a key
compound to synthesise the superphane.6,7 Bodwell et al. also
reported the synthesis of [2.2.n](1,3,5)cyclophane-1,9-dienes
to afford 1,n-dioxa[n](2,7)pyrenophanes.8 These cyclophanes
adopt rigid syn-conformation with overlaying aromatic rings.
Thus there is substantial interest to synthesise the flexible [2.n]
MCP-1-enes and conversion to triple bridged [2.3.n](1,3,5)-
thiacyclophane-1-enes or [2.2.n](1,3,5)cyclophane-1,9-dienes,
which can adopt the syn-conformation and the flexibility
arising from the ring inversion can be completely restricted.
Recently, we have reported the preparation of 1,2-dimethyl
[2.3]MCP-1-enes9–11 by using the reductive coupling of
carbonyl compounds by low-valent titanium, the McMurry
reaction12–15 as a key step. While in [2.3]MCP-1-enes the aro-
matic rings preferentially appear to adopt the syn-arrangement,
its hgher homologue, i.e. [2.n]MCP-1-enes, can be expected to
adopt the mobile anti- or syn-conformation. We now report on
the synthesis of [2.4]MCP-1-ene using the low-valent titanium
induced McMurry reaction and conversion to syn-10-thia
[2.3.4](1,3,5)cyclophan-1-ene. The conformational studies of
these cyclophane-1-enes in solution are also described.
Results and discussion
1,4-Bis(4-methoxyphenyl)butane 1 has been prepared accord-
ing our previous papers.9,10 Thus the cross coupling reaction16,17
of 4-methoxyphenylmagnesium bromide with 1,4-dibromobu-
tane has been carried out in the presence of cuprous bromide as
a catalyst in a mixture of hexamethylphosphoric triamide
(HMPA) and tetrahydrofuran at reflux temperature to give
the desired 1,4-bis(4-methoxyphenyl)butane 1 in 80% yield.
AlCl3–MeNO2-catalysed acetylation of compound 1 with
acetic anhydride or acetyl chloride at 20°C led to regioselec-
tive acylation at the meta positions of the 1,4-diphenylbutane
affordingthedesired1,4-bis(3-acetyl-4-methoxyphenyl)butane
2 in 71% yield. 1,4-Bis(3-acetyl-4-methoxyphenyl)butane
2 was subjected to reductive coupling by the McMurry reac-
tion following the improved Grützmacher’s procedure13
(Scheme 1). Thus, the reductive coupling reaction of 2 carried
out using TiCl4-Zn in refluxing THF under the high dilution
conditions afforded the desired compound 4,16-dimethoxy-
1,2-dimethyl of [2.4]MCP-1-ene 3 in 23% yield along with an
intractable mixture of products. This result was different result
from that of the similar McMurry cyclisation of 1,4-bis(5-
acetyl-2-methoxyphenyl)butane in the absence of pyridine,
which afforded the corresponding [4.1]MCP by the TiCl4 or
acids induced pinacol rearrangements.11 Surprisingly, when
the present cyclisation reaction was carried out in the presence
of pyridine, the yield of 3 increased to 69%.
Fig. 1 Possible conformations of [2.n]MCP-enes and conversion to the triple bridged cyclophanes.
* Correspondent: E-mail: yamatot@cc.saga-u.ac.jp

446 JOURNAL OF CHEMICAL RESEARCH 2010
Scheme 1
The structure of 3 was elucidated based on their elemental
analysis and spectral data. Especially, the mass spectral data
for 3 (M+ = 322) strongly supports the cyclic structure. [2.n]
MCP-1-enes adopt either a “stair-case” anti conformation
or a syn conformation with overlaying aromatic rings18,19
(Fig. 1). Depending on the size of the bridges20 and on the pres-
ence of intraannular substituents,18,19 the interconversion
between the syn and anti conformers occur by ring flipping. 1H
NMR spectrum of 3 showed the doublet of the intra-annular
proton Hi at δ = 6.90 (J = 2.4 Hz) apart from at δ = 6.41 and
6.59 ppm of the other two protons at the aromatic rings. The
conformation of 3 was readily apparent from its 1H NMR spec-
trum. Thus, the internal aromatic proton of anti-conformation
should show an upfield shift due to the ring current of the
1
opposite benzene ring.19,21,22 The H NMR spectrum of the
[2.4]MCP-1-ene 3 prepared here shows that its structure
corresponds exclusively to the syn-conformer. In addition, the
protons of the butane bridge give rise to two multiplets centred
at δ = 1.55 and 2.36 ppm, respectively, providing a fast syn–syn
interconversion of the two syn conformations of 3 by ring
flipping which would exchange HA and HB of each CH2 group.
However, as the temperature of the solution in CDCl3/CS2 (1:3)
is decreased, a single peak of the benzyl protons splits into two
multiplets at δ 2.2 and 2.5 ppm below 0°C (Fig. 2). The energy
barrier to the conformational ring flipping estimated from the
coalescence
temperature
(T = 0°C)
is
13.0 kcal
c
mol−1. Interestingly, similar findings were also observed in the
Fig. 2 Dynamic 1H NMR spectrum of 3 at 300 MHz (CDCl3/CS2;
corresponding anti-6,14-dimethoxy-1,2-dimethyl[2.4]MCP-1-
1:3).
ene 4 (T = -30°C, ΔG≠ = 10.7 kcal mol−1)11,23 in spite of being
c
expected to be similar flexible structure attributable to the
same cyclophane ring size. These observations suggest that the
introduction of a double bond of the ethylene bridge as well as
the substituents such as methyl groups and methoxy groups
might control the syn- and anti-conformation of the present
[2.4]MCP-1-ene 3.
(bromomethyl)-4,16-dimethoxy-1,2-dimethyl[2.4] MCP-1-
ene 8 has been prepared in 64% yield by reduction of 5 with
sodium borohydride in ethanol reflux followed by bromination
of bis(hydroxymethyl) derivative 7 with PBr3 in dioxane at
room temperature for 2 h. The cyclisation of 8 has been carried
out under the conditions of high dilution and in ethanolic Na2S/
Al2O324 to afford the corresponding 8,17-dimethoxy-
1,2-dimethyl-10-thia[2.3.4](1,3,5)cyclophan-1-ene 9 in 28%
yield.
The structure of 9 was elucidated based on its elemental
analysis and spectral data. The mass spectral data for 9 (M+ =
380) strongly supports the cyclic structure. The 300 MHz
1H NMR spectrum of 9 in CDCl3 showed a doublet of the two
protons of the aromatic rings at δ 6.66 and 7.18 ppm (J =
2.1 Hz). These observations strongly suggest that its structure
corresponds exclusively to the syn-conformation. The
The formylation of 3 with dichloromethyl methyl ether
in the presence of TiCl4 afforded the desired 5,15-diformyl
[2.4]MCP-1-ene 5 as a colourless prisms in 66% yield. Several
attempted reductive coupling reaction of 5 carried out using
TiCl4–Zn in the presence of pyridine in refluxing THF under
the high dilution conditions failed. No formation of the desired
8,16-dimethoxy-1,2-dimethyl[2.2.4](1,3,5)cyclophan-
1,9-diene 6 was observed under the conditions used. Only an
intractable mixture of products was obtained.
Therefore,wehaveattemptedtoprepare10-thia[2.3.4](1,3,5)
cyclophan-1-ene 9 as shown in Scheme 3. Thus, 5,15-bis

JOURNAL OF CHEMICAL RESEARCH 2010 447
Scheme 2
Scheme 3
intra-annular proton Hi was observed at the slightly lower
field (δ 7.18 ppm) than that of the corresponding syn-4,16-
dimethoxy-1,2-dimethyl[2.4]MCP-1-ene 3 (δ 6.99 ppm) due
to being in a deshielding region of the bridged double bond.
The protons of the tetra-methylene bridge gave rise to a com-
plicated signal pattern as expected for a rigid [2.4.3](1,3,5)
cyclophan-1-ene 9. The protons of the benzylic CH2 group
were observed as two multiplets centred at δ 3.46 and 4.13
ppm J = 13.2 Hz which were further split by coupling with
the protons of the central CH2 group. This central CH2 group
was also observed as two multiuplets centred at δ 1.56
and 2.50 ppm. The peak pattern ascribed to eight chemically
distinct protons of the butano bridge proved the absence of a
syn–syn interconversion which would exchange HA and HB of
each CH2 group. These findings suggest the rigid structure of
[2.3.4](1,3,5)cyclophan-1-ene 9 at this temperature. In fact,
the signals of the benzyl protons of 9 do not coalescence below
150°C in CDBr3, and the energy barrier of flipping is above
25 kcal mol−1. This result suggests that the introduction of
one extra CH2SCH2 bridge into the flexible [2.4]MCP-1-ene 3
can completely inhibit the flexibility arising from the ring
inversion.
reaction of 1,4-bis(3-acetyl-4-methoxy-phenyl)butane 2.
The conversion of 3 to the corresponding triple bridged 8,17-
dimethoxy-1,2-dimethyl-10-thia[2.3.4](1,3,5)cyclophan-1-
ene 9, which adopts rigid syn-conformation. Further studies on
the chemical properties of 8,17-dimethoxy-1,2-dimethyl-10-
thia[2.3.4](1,3,5)cyclophan-1-ene 9 and conversion to the
corresponding [2.2.4](1,3,5)cyclophan-1,9-diene 6 are now in
progress.
Experimental
All melting points are uncorrected. 1H 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-HX110A ultrahigh performance mass spectrometer at 75 eV
using a direct-inlet system. Elemental analyses were performed by
Yanaco MT-5.
Preparation of 1,4-bis(4-methoxyphenyl)butane 1 has been previously
described.9
1,4-Bis(3-acetyl-4-methoxyphenyl)butane (2): To a solution of 1,3-
bis(4-methoxyphenyl)butane 1 (3.84 g, 15 mmol) and acetyl chloride
(3.15 mL, 45 mmol) in methylene dichloride (60 mL) was added a
solution of aluminum chloride (8.91 g, 67.5 mmol) in nitromethane
(15 mL) at 0°C. After the reaction mixture had been stirred at room
temperature for 3 h, it was poured into ice-water (100 mL). The
organic layer was extracted with CH2Cl2 (50 mL × 2). The extract was
Conclusions
We have demonstrated a convenient preparation of flexible syn-
4,16-dimethoxy-1,2-dimethyl[2.4]MCP-1-ene 3 by McMurry

448 JOURNAL OF CHEMICAL RESEARCH 2010
washed with water (50 mL × 2), dried (Na2SO4), and concentrated.
The residue was chromatographed over silica gel (Wako
C-300, 300 g) with CHCl3 as eluent to give crude 2b as a colourless
solid. Recrystallisation from hexane:benzene (1:1) gave 1,4-bis
(3-acetyl-4-methoxyphenyl)butane (2) (4.42 g, 71%) as colourless
prisms [from hexane:benzene (1:1)]; m.p. 74–76°C; νmax(KBr)/cm−1
1669 (C=O); δH (CDCl3) 1.60 (4H, m, CH2), 2.57 (4H, m, CH2), 2.60
(6H, s, Me), 3.88 (6H, s, OMe), 6.87 (2H, d, J = 8.3, ArH), 7.25 (2H,
dd, J = 8.3, 2.4, ArH), and 7.52 (2H, d, J = 2.4, ArH); m/z 354 (M+)
(Found: C, 74.26; H, 7.18. C22H26O4 (354.5) requires C, 74.55;
H, 7.39%).
McMurry coupling reaction of 2: The McMurry reagent was pre-
pared from TiCl4 [23.8 g (13.8 mL), 125 mmol] and 18 g (275 mmol)
of Zn powder in 500 mL of dry THF, under nitrogen. A solution of
2 (3.06 g, 9 mmol) and pyridine (22.5 mL, 200 mmol) in dry THF
(250 mL) was added within 60 h from two Hershberg funnels to the
black mixture of the McMurry reagent by using a high-dilution tech-
nique25–28 with continuous refluxing and stirring. The reaction mixture
was refluxed for additional 8 h, cooled to room temperature, and
hydrated with aqueous 10% K2CO3 (200 mL) at 0°C. The reaction
mixture was extracted with CH2Cl2 (200 mL × 3). The combined
extracts were washed with water, dried with Na2SO4 and concentrated.
The residue was chromatographed over silica gel (Wako C-300,
300 g) with benzene as eluents to give crude 3 as a colourless solid.
Recrystallisation from hexane gave syn-4,16-dimethoxy-1,2-dimethyl
[2.4]metacyclophan-1-ene (syn-3) (2.0 g, 69%) as colourless prisms
(from hexane); m.p. 160–161°C; νmax(KBr)/cm-1 2923, 1493, 1442,
1252, 1226, 1032, 805 and 797; δH (CDCl3) (27°C) δ: 1.55 (4H, broad
s, CH2), 2.14 (6H, s, Me), 2.36 (4H, broad s, CH2), 3.63 (6H, s, OMe),
6.41 (2 H, d, J = 8.3, ArH), 6.59 (2H, dd, J = 2.4, 8.3, ArH), and 6.90
(2 H, d, J = 2.4, ArH). (–40°C, CDCl3/CS2 1:3) δ: 1.1–1.4 (2H, broad
s, CH2), 1.72–1.92 (2H, broad s, CH2), 2.17 (6H, s, Me), 2.1–2.3
(2H, broad s, CH2), 2.4–2.6 (2H, broad s, CH2), 3.67 (6H, s, OMe),
6.41 (2H, d, J = 8.3, ArH), 6.64 (2 H, dd, J = 2.4, 8.3, ArH), and 6.93
(2H, d, J = 2.4, ArH); m/z 322 (M+) (Found: C, 82.07; H, 8.15.
C22H26O2 (322.45) requires C, 81.95; H, 8.13%).
5,15-Diformyl-4,16-dimethoxy-1,2-dimethyl[2.4]metacyclophan-
1-ene (5): TiCl4 (0.21 mL, 27.3 mmol) in CH2Cl2 (2 mL) at 0 °C
was added to a solution of 3 (100 mg, 0.31 mmol) and Cl2CHOCH3
(0.366 mL, 4.1 mmol) in CH2Cl2 (8 mL). After the reaction mixture
was stirred at room temperature for 1 h, it was poured into a large
amount of ice-water (5 mL) and extracted with CH2Cl2 (10 mL × 2).
The combined extracts were washed with water, dried with Na2SO4
and concentrated. The residue was chromatographed over silica gel
(Wako C–300, 200 g) with benzene as eluent to give 5 as a colourless
solid. Recrystallisation from hexane gave 77 mg (66%) of 5 as colour-
less prisms, m.p. 130–131°C; νmax (KBr)/ cm−1 1683 (C=O); δH(CDCl3)
1.61 (4H, broad s, CH2), 2.24 (6H, s, Me), 2.47 (4H, broad s, CH2),
3.86 (6H, s, OMe), 7.20 (2H, d, J = 2.2 Hz, ArH), 7.27 (2H, d, J = 2.2
Hz, ArH) and 9.86 (2H, s, CHO); m/z 378 (M+) (Found C, 75.92; H,
6.93. C24H26O4 (378.47) requires C, 76.17; H, 6.92%).
Preparation of 5,15-bis(hydroxymethyl)-4,16-dimethoxy-1,2-
dimethyl[2.4]metacyclo-phan-1-ene (7): Sodium borohydride
(250 mg, 6.61 mmol) was added gradually to a solution of 5 (250 mg,
0.66 mmol) in methanol (15 mL) under gently refluxing. After the
reaction mixture was stirred under reflux for 24 h, it was poured into
a large amount of ice-water (10 mL) and extracted with CH2Cl2
(10 mL × 2). The combined extracts were washed with water, dried
with Na2SO4 and concentrated to afford 7 (240 mg) as a colourless
solid. Recrystallisation from a mixture of hexane:CH2Cl2 (1:5) gave
166 mg (66%) of 7 as colourless prisms, m.p. 139–140°C; νmax (KBr)/
cm−1 3335 (OH); δH(CDCl3) 1.56 (4H, broad s, CH2), 2.21 (6H, s, Me),
2.36 (4H, broad s, CH2), 2.62 (2H, s, OH), 3.76 (6H, s, OMe), 4.45
(4H, s, CH2), 6.65 (2H, d, J = 2.2 Hz, ArH) and 6.77 (2H, d, J = 2.2
Hz, ArH); m/z 382 (M+) (Found C, 75.32; H, 7.87. C24H30O4 (382.5)
requires C, 75.36; H, 7.91%).
1.47 (4H, broad s, CH2), 2.21 (6H, s, Me), 2.38 (4H, broad s, CH2),
3.36 (6H, s, OMe), 4.38 (4H, s, CH2), 6.71 (2H, d, J = 2.1 Hz, ArH)
and 6.86 (2H, d, J = 2.1 Hz, ArH); m/z 506, 508, 510 (M+) (Found C,
56.57; H, 5.67. C24H28O2Br2 (508.3) requires C, 56.71; H, 5.55%).
8,17-Dimethoxy-1,2-dimethyl-10-thia[2.3.4](1,3,5)cyclophan-
1-ene (9): Lumpy reagent (Na2S/Al2O3) (58.2 mg, 0.394 mmol) was
added to a solution of 8 (50 mg, 0.0985 mmol) in ethanol (10 mL) at
room temperature. After the reaction mixture was stirred at room tem-
perature for 24 h, it was treated with celite by filtration. The filtrate
was concentrated to afford a yellow solid. The residue was chromato-
graphed over silica gel (Wako C–300, 100 g) with benzene as eluent
to give 9 (42mg) as a colourless solid. Recrystallisation from a
mixture of hexane:AcOEt (5:1) gave 11 mg (28%) of 9 as colourless
prisms, m.p. 162–163°C; δH(CDCl3) 1.56 (4H, m, CH2), 2.23 (6H,
s, Me), 2.50 (4H, m, CH2), 3.46 (2H, d, J = 13.2 Hz, CH2), 3.81 (6H,
s, OMe), 4.13 (2H, d, J = 13.2 Hz, CH2), 6.66 (2H, d, J = 2.1 Hz, ArH)
and 7.18 (2H, d, J = 2.1 Hz, ArH); m/z 380 (M+) (Found C, 75.32;
H,7.57. C24H28O2S (380.55) requires C, 75.75; H, 7.42%).
The estimation of the activation energy of the ring flipping
The rate constant (kc) of the observed conformational interconversion
at the coalescence (Tc) can be calculated by using Eqn (1). The free
energy of activation (ΔGc≠) at coalescence can then be estimated by
using Eyring equation [Eqn (2)].29,30
kc = πΔυ / 21/2
ΔGc≠ = 2.303RTc (10.32 + logTc – logkc)
(1)
(2)
Received 12 May 2010; accepted 22 June 2010
Paper 1000120 doi: 10.3184/030823410X12795373456980
Published online: 30 August 2010
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5,15-Bis(bromomethyl)-4,16-dimethoxy-1,2-dimethyl[2.4]
metacyclo-phan-1-ene (8): A solution of PBr3 (0.1 mL, 1.06 mmol)
in dioxane (2 mL) was added to a solution of 7 (50 mg, 0.131 mmol)
in 1,4-dioxane (2 mL) at 0°C. After the reaction mixture was stirred at
room temperature for 12 h, it was poured into aqueous 10% Na2S2O8.
The mixture was stirred at room temperature for 15 min and extracted
with CH2Cl2 (10 mL × 2). The combined extracts were washed with
water, dried with Na2SO4 and concentrated to afford 8 as a yellow
solid. Recrystallisation from a mixture of hexane:CH2Cl2 (5:1) gave
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