Dioxastilbenophanes - Synthesis and charge transfer complexation studies

Article abstract of DOI:10.1016/j.tet.2004.01.008

Intramolecular McMurry coupling of dialdehydes derived from xylenyl dibromide and 4-hydroxy benzaldehyde afforded cis-stilbenophanes along with cyclophane diols. Stilbenophanes with a large cavity were also synthesized. Charge transfer complexations of th

Full text of DOI:10.1016/j.tet.2004.01.008

Tetrahedron 60 (2004) 2351–2360
Dioxastilbenophanes—synthesis and charge transfer
complexation studies
†
*
Perumal Rajakumar and Venghatraghavan Murali
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India
Received 25 July 2003; revised 23 December 2003; accepted 8 January 2004
This paper is dedicated to Professor S. Swaminathan (Emeritus Professor, Department of Organic Chemistry, University of Madras, India) on the occasion of
his 80th birthday
Abstract—Intramolecular McMurry coupling of dialdehydes derived from xylenyl dibromide and 4-hydroxy benzaldehyde afforded cis-
stilbenophanes along with cyclophane diols. Stilbenophanes with a large cavity were also synthesized. Charge transfer complexations of the
stilbenophanes with TCNE, TCNQ and PQT were studied. Some stilbenophanes form a relatively stronger complex with PQT rather than
with TCNE and TCNQ.
q 2004 Published by Elsevier Ltd.
1. Introduction
and then to study the donor–acceptor complexation proper-
ties of the resultant host molecules. Stilbenophanes 3a to 3c
and 8a to 8c are classified as electron rich cyclophanes due
to the presence of olefin and ether linkages. The electron
rich nature of such cyclophanes can be easily proved by
their complexation with electron deficient guest molecules
like tetracyanoethylene (TCNE), tetracyanoquinodimethane
(TCNQ) and 4,40-dimethylbipyridine hexaflurophosphate
(PQT). Hence we report herein the synthesis of stillbeno-
phanes 3a to 3c and 8a to 8c and their charge transfer
complexation studies with electron deficient acceptors.
Non-covalent interactions such as hydrogen bonding, p-
stacking, charge-transfer interaction and electrostatic inter-
action play a pivotal role in supramolecular host–guest
complexation.1 The ability of cyclophanes to include p-
electron donors within their cavities as a result of stabilizing
noncovalent interactions has led to the construction of a
number of mechanically interlocked compounds such as
catenanes and rotaxanes.2 A charge-transfer interaction in a
porphyrin–fullerene system was recently studied by
Diederich and co-workers.3 cis-and trans-stilbene have
been the subject of interest from the view-point of structure,
property and isomerization.4 The design and synthesis of
cyclophanes possessing rigidly defined cavities and shape-
persistent structures of molecular dimensions is of interest
to create molecular hosts in the areas of host–guest and
electron donor–acceptor complexation.5 The dimensions of
the cavity depend on the spacer group and its connectivity to
the arene units. Stilbenophanes are an interesting class of
cyclophanes and are synthesized by inter6 and intra-
molecular7 McMurry couplings. During our investigation
on the synthesis of various cyclophanes,8 an attention was
focused on introducing the 4,40-dioxa-cis-stilbene unit in the
cyclophane skeleton by intramolecular McMurry coupling
Keywords: McMurry coupling; Stilbenophane; Charge transfer
complexation.
*
Corresponding author. Tel.: þ91442351269x213; fax: þ91442353309;
e-mail address: perumalrajkumar@hotmail.com
Present address: School of Chemistry, Monash University, Clayton, Vict.
3800, Australia.
†
0040–4020/$ - see front matter q 2004 Published by Elsevier Ltd.
doi:10.1016/j.tet.2004.01.008

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P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
2. Result and discussion
Treatment of o-xylenyl dibromide (1a) with 2 equiv. of 4-
hydroxybenzaldehyde in DMF in the presence of K2CO3 at
70 8C for 48 h afforded the dialdehyde 2a in 80% yield.
Slow addition of 1 equiv. of the dialdehyde 2a to a solution
of 5 equiv. of TiCl4 and 10 equiv. of zinc in THF, followed
by refluxing for 24 h resulted in the formation of cis-
stilbenophanes 3a (20%) along with cyclophane diol 4a
(60%). It is interesting to note that increasing the
concentration of zero valent titanium solution increases
the yield of cis-stilbenophanes 3a, 3b and 3c. In fact, when
the reaction was carried out with 10 equiv. of TiCl4 and
20 equiv. of zinc for 1 equiv. of the dialdehyde 2a, 2b and
2c, stilbenophane 3a, 3b and 3c were obtained in 55, 60 and
53% yield. And the yield of the diols 4a, 4b and 4c
decreased. Stilbenophanes 3a, 3b, 3c and stilbenophane
diols 4a, 4b, 4c were characterized by spectral and
1
analytical data. From the H NMR, the diol 4a was found
to be a diastereomeric mixture in the ratio 7.5:2.5 (meso/
DL). Attempts to separate the diastereomeric mixture of the
diol 4a could not be achieved by crystallization and
chromatographic techniques. Further, in the present inves-
tigation significance was given to the synthesis of
cyclophanes rather than the diols. Similarly, other cyclo-
phane diols 4b and 4c were also observed as a diaster-
eomeric mixture in the ratio 7:3 and 6.5:3.5 (meso/DL)
(Scheme 1).
Semi empirical energy minimization calculations using the
MOPAC (PM3) method were carried out for cis and trans
isomers of stilbenophane 3c. The heat of formation of the cis
isomer of stilbenophane 3c is 29.5 kcal/mol (Fig. 1a) while
that of the trans isomer 62.1 kcal/mol (Fig. 1b). Hence the
cis isomer of 3c is more stable than the trans isomer by a
factor of 32.6 kcal/mol. Therefore, the formation of the cis
isomer is more favoured than the trans isomer. The results
are in good agreement with the experimental observations.
The formation of cis stilbenophane 3c was confirmed based
Figure 1. (a) Heat of Formation of cis isomer of stilbenophane 3c:
29.5 kcal/mol. (b) Heat of formation trans isomer of stilbenophane 3c:
62.1 kcal/mol.
8c, the dialdehydes 2a, 2b and 2c, were reduced with
NaBH4 in methanol to give the corresponding diols 5a to 5c,
which were then converted into corresponding dibromides
6a to 6c using PBr3 in CH2Cl2. The dibromides 6a to 6c
were treated with 2 equiv. of 4-hydroxy benzaldehyde in dry
DMF in the presence of K2CO3 to obtain the expanded
dialdehydes 7a to 7c. The expanded dialdehydes 7a to 7c
were subjected to intramolecular McMurry coupling by
slow addition (16 h) of the dialdehyde 7a to 7c to a mixture
of TiCl4 (10 equiv.) and Zn (20 equiv.) for 1 equiv. of
dialdehyde in THF, followed by reflux (24 h) to give the cis
stilbenophane 8a to 8c (Scheme 2).
1
on H NMR. XRD studies also confirmed the structures of
3a and 3c as cis.9
Synthesis of stilbenophanes with large cavities would be
more interesting so that a study of the cavity effect on
complexation behaviour with electron deficient acceptors
like TCNE, TCNQ and PQT could be carried out. With a
view to synthesize large cavity stilbenophanes 8a, 8b and
Scheme 1. (i) 4-Hydroxy benzaldehyde (2 equiv.), K2CO3, DMF, 70 8C, 48 h, (ii) TiCl4 (5 equiv.), Zn (10 equiv.) for dialdehyde (1 equiv.), THF, slow
addition 16 h, reflux 24 h or TiCl4 (10 equiv.), Zn (20 equiv.) for dialdehyde (1 equiv.), THF, slow addition 16 h, reflux 24 h.

P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
2353
Scheme 2. (i) NaBH4, MeOH, 12 h, (ii) PBr3, CH2Cl2, 12 h, (iii) 4-hydroxybenzaldehyde (2 equiv.), K2CO3, DMF, 70 8C, 2 days, (iv) TiCl4 (10 equiv.), Zn
(20 equiv.), THF, slow addition 16 h, reflux 24 h.
3. Complexation studies
transfer complexation of 3a with TCNE was observed at
473 nm (Fig. 2 CT behaviour of 3a with TCNE). Similarly
stilbenophanes 3a, 3b and 3c form charge transfer
complexes with TCNQ as indicated by absorption at 665.0
and 760.0 nm apart from 743.0 nm (Fig. 3 CT behaviour of
3a with TCNQ). Stilbenophanes 3a, 3b and 3c with PQT
shows charge transfer absorption maxima only at 394.0 nm
(Fig. 4 CT behaviour of 3a with PQT).
Stilbenophanes 3a, 3b and 3c show UV–Vis absorption
maxima at 238.8, 239.4 and 235.6 nm, respectively, in the
solvent medium CH3CN/CH2Cl2 (4:1). However, the
acceptor TCNE, TCNQ and PQT show absorption maxima
at 286.8, 274.0 and 263.0 nm, respectively, in the same
solvent mixture. Stilbenophanes 3a, 3b and 3c forms a
charge transfer complex with TCNE as evidenced by the
appearance of absorption maxima at 394.0, 414.6 and
473.0 nm, respectively. The equilibrium constant for charge
From the plot of the D0/A vs 1/A0 and by the Benesi–
Hildebrand equation10 the stability constant of the charge
transfer complex of stilbenophanes 3a to 3c with various
Table 1. Stability constant for the charge transfer complex of 3a to 3c with
acceptors TCNE, TCNQ and PQT
cis-Stilbenophane
TCNE
(473 nm)
TCNQ
(743 nm)
PQT (394 nm)
Table 2. Stability constant for the charge transfer complex of 8c with
acceptors TCNE, TCNQ and PQT
KACD
e AD
KACD
e AD
KACD
e AD
Cyclophane
TCNE (474 nm)
TCNQ (761 nm)
PQT (395.2 nm)
KACD
e AD
KACD
e AD
KACD
e AD
3a
3b
3c
20
30
1.3£104 130 4.5£103
1.7£104 300 3.3£103
200
100
1.0£103
5.0£103
100 2.5£104 100 1.0£103 1667 1.0£105
8c
20
5.0£104
142.8
1.0£104
2000
2.5£104

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P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
Figure 2. Charge transfer complexation behaviour of stilbenophane 3a with TCNE.
acceptors TCNE, TCNQ and PQT were determined
(Table 1).
cyclophanes 3c and 8c form a strong CT complex with
PQT due to electronic complementary matching between
donor and acceptor.
It is noteworthy that stilbenophane 3c forms a strong charge
transfer complex with PQT (KAc D, 1667 M21; e AD
1.0£105).
,
4. Experimental
The ability of stilbenophane 8c to form charge transfer
complex with electron deficient acceptors like TCNE,
TCNQ and PQT was then explored. Table 2 shows the Kc
values for the charge transfer complex of 8c with TCNE,
TCNQ and PQT.
All the melting points are uncorrected. The IR spectra were
1
recorded using Shimadzu FT-IR 8300 instrument. The H
and 13C NMR spectra of all compounds in CDCl3 were
recorded using Jeol GSX 400 (400 MHz) NMR spec-
trometer. The mass spectra were recorded using Jeol (EI,
70 eV and FAB-MS). The column chromatography was
performed using silica gel (100–200 mesh).
Comparing the stability constants values of 3a, 3b, 3c with
8c, it was found that the stability constant values are
relatively similar and hence the cavity size does not have
very high influence on the stability of charge transfer
complexes. Though Kc for the complex derived from 8c
with PQT is slightly on the higher side when compared with
the complex derived from 3c with PQT, the results are
comparable within experimental errors. Similarly the
stability constants values of 8a and 8b with TCNE, TCNQ
and PQT are comparable with that of 3a and 3b.
4.1. General procedure for O-alkylation (procedure A)
A mixture of p-hydroxybenzaldehyde (21 mmol), the
dibromide 1a/1b/1c (10 mmol) and potassium carbonate
(15 g) in anhydrous DMF (60 mL) was stirred under
nitrogen for 48 h at 60 8C. The reaction mixture was poured
into water (2 L) and stirred. The resulting precipitate was
filtered, washed with water (3£150 mL) and dissolved in
CH2Cl2 (350 mL). The organic layer was washed with
NaOH solution (5% w/v, 2£100 mL), dried (Na2SO4) and
evaporated to give a residue, which was chromatographed
(SiO2) using hexane/CHCl3 (1:2) to give the corresponding
dialdehyde 2a/2b/2c.
In conclusion, we have synthesized a new class of electro0n
rich cyclophanes (3a to 3c, and 8a to 8c) possessing a 4,4 -
dioxa-cis-stilbene unit and studied their CT complexation
ability with electron poor guest molecules like TCNE,
TCNQ and PQT. It was interesting to note that the

P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
2355
Figure 3. Charge transfer complexation behaviour of stilbenophane 3a with TCNQ.
4.1.1. Dialdehyde 2a. Following the general procedure A,
the dialdehyde 2a was obtained as a white solid from o-
xylenyl dibromide (1a).
4.1.3. Dialdehyde 2c. Following the general procedure A,
the dialdehyde 2c was obtained as a white solid from p-
xylenyl dibromide (1c).
Yield: 80%. Mp: 126 8C. IR: (KBr, cm21) 1687 (CvO). 1H
NMR (200 MHz, CDCl3, 25 8C): d¼5.18 (s, 4H, Ar-CH2O),
Yield: 86%. Mp: 110 8C. IR (KBr, cm21): 1684 (CvO). 1H
NMR (200 MHz, CDCl3, 25 8C): d¼5.13 (s, 4H, Ar-CH2O),
6.96 [d, 3JH,H¼7.2 Hz, 4H, Ar-H], 7.44 (m, 4H, Ar-H), 7.84
[d, 3JH,H¼7.2 Hz, 4H, Ar-H], 9.81 (s, 2H, CHO). 13C NMR
(75 MHz, CDCl3, 25 8C): d¼71.8, 115.1, 123.7, 126.3,
129.1, 132.4, 161.4, 190.9. Mass spectrum: m/z (EI, 70 eV)
346 (Mþ). Elemental analysis calcd for C22H18O4: C, 76.30;
H, 5.20; found, C, 76.11; H, 5.12.
3
6.94 [d, JH,H¼7.2 Hz, 4H, Ar-H], 7.32–7.45 (m, 4H, Ar-
3
H), 7.73 [d, JH,H¼7.2 Hz, 4H, Ar-H], 9.79 (s, 2H, CHO).
13C NMR (50 MHz, CDCl3, 25 8C): d¼71.3, 114.9, 125.7,
126.8, 128.3, 130.6, 134.7, 162.5, 190.9. Mass spectrum:
m/z (EI, 70 eV) 346 (Mþ). Elemental analysis calcd for
C22H18O4: C, 76.30; H, 5.20; found, C, 76.14; H, 5.08.
4.1.2. Dialdehyde 2b. Following the general procedure A,
the dialdehyde 2b was obtained as a white solid from m-
xylenyl dibromide (1b).
4.2. General procedure for intramolecular McMurry
coupling (procedure B)
A solution of zero valent titanium [Ti(0)] prepared from
TiCl4 (7.23 mmol, 5 equiv.) with zinc (14.5 mmol,
10 equiv.) in dry THF (300 mL) under a nitrogen atmos-
phere at 0 8C and was allowed to attain room temperature
after 0.5 h and then refluxed for 1 h. A solution of the
dialdehyde (1.45 mmol, 1 equiv.) in THF (100 mL) was
slowly added at reflux to the freshly prepared zero valent
titanium during a period of 16 h. After the addition was
over, the reaction mixture was refluxed for 24 h. The
reaction mixture was then quenched with saturated K2CO3
Yield: 86%. Mp: 163 8C. IR: (KBr, cm21) 1681 (CvO). 1H
NMR (200 MHz, CDCl3, 25 8C): d¼5.17 (s, 4H, Ar-CH2O),
3
7.07 [d, JH,H¼7.2 Hz, 4H, Ar-H], 7.43–7.45 (m, 3H, Ar-
3
H), 7.51 (s, 1H, Ar-H), 7.84 [d, JH,H¼7.2 Hz, 4H, Ar-H],
9.88 (s, 2H, CHO). 13C NMR (75 MHz, CDCl3, 25 8C):
d¼70.2, 115.3, 126.6, 127.5, 129.3, 130.4, 132.2, 136.8,
163.7, 190.9. Mass spectrum: m/z (EI, 70 eV) 346 (Mþ).
Elemental analysis calcd for C22H18O4: C, 76.30; H, 5.20;
found, C, 76.21; H, 5.11.

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P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
Figure 4. Charge transfer complexation behaviour of stilbenophane 3a with PQT.
solution. The precipitated inorganic material was removed
by filteration. The precipitate was thoroughly washed with
THF (100 mL) and the THF extract was evaporated to give a
residue which was then dissolved in CHCl3 (200 mL),
washed with water (2£200 mL), brine (100 mL) and dried
over Na2SO4. Crude product obtained after evaporation of
CHCl3, was purified by column chromatography. Elution
with hexane/CHCl3 (8:2) afforded the stilbenophanes and
further elution with hexane/CHCl3 (2:8) gave the stilbene
diols.
cedure B/C, the dialdehyde 2a was subjected to intramo-
lecular McMurry coupling to give the cyclophane 3a.
Yield: 20% (procedure B) and 55% (procedure C). White
solid, Mp: 168 8C. 1H NMR (400 MHz, CDCl3, 25 8C):
3
d¼4.82 (s, 4H, Ar-CH2O), 6.42 [d, JH,H¼8.8 Hz, 4H, Ar-
3
H], 6.49 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.02 (s, 2H,
3
stilbenic), 7.37 [dd, JH,H¼5.9, 2.4 Hz, 2H, Ar-H], 13C
NMR (100.3 MHz, CDCl3, 25 8C): d¼70.9, 120.1, 127.6,
129.6, 131.0, 134.5, 135.4, 135.8, 157.4. Mass spectrum:
m/z (EI, 70 eV) 314 (Mþ). Elemental analysis calcd for
C22H18O2: C, 84.08; H, 5.73; found, C, 83.96; H, 5.64.
4.3. Modified procedure for the preparation of
stilbenophanes (procedure C)
4.3.2. cis-Stilbenophane 3b. Following the general pro-
cedure B/C, the dialdehyde 2b was subjected to intramo-
lecular McMurry coupling to give the cyclophane 3b.
A solution of zero valent titanium [Ti(0)] prepared from
TiCl4 (14.5 mmol, 10 equiv.) with zinc (29 mmol,
10 equiv.) in dry THF (300 mL) under a nitrogen atmos-
phere at 0 8C and was allowed to attain room temperature
after 0.5 h and then refluxed for 1 h. A solution of
dialdehyde (1.45 mmol, 1 equiv.) in THF (100 mL) was
added slowly at reflux to the freshly prepared zero valent
titanium solution and the experiment was carried out and
worked up as mentioned in the procedure B to give the
stilbenophanes as the major product.
Yield: 20% (procedure B); and 60% (procedure C). White
solid, Mp: 140 8C. 1H NMR (400 MHz, CDCl3, 25 8C):
d¼4.96 (s, 4H, Ar-CH2O), 6.42 [d, 3JH,H¼8.8 Hz, 4H], 6.50
3
[d, JH,H¼8.3 Hz, 4H], 6.89 (s, 2H, stilbenic), 7.19–7.31
(m, 4H, Ar-H). 13C NMR (100.3 MHz, CDCl3, 25 8C):
d¼71.5, 117.3, 128.7, 128.9, 130.3, 130.4, 132.3, 134.2,
136.5, 155.2. Mass spectrum: m/z (EI, 70 eV) 314 (Mþ).
Elemental analysis calcd for C22H18O2: C, 84.08; H, 5.73;
found, C, 83.93; H, 5.59.
4.3.1. cis-Stilbenophane 3a. Following the general pro-

P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
2357
4.3.3. cis-Stilbenophane 3c. Following the general
procedure B/C, the dialdehyde 2c was subjected to
intramolecular McMurry coupling to give the cyclophane
3c.
obtained was filtered off. Evaporation of the solvent in
vacuo gave the diol 5a/5b/5c, which was purified by
recrystallization from CHCl3/MeOH (5:1).
4.4.1. Diol 5a. Following the general procedure D, the diol
5a was obtained from the dialdehyde 2a.
Yield: 20% (procedure B); and 53% (procedure C). White
solid, Mp: 140 8C. 1H NMR (400 MHz, CDCl3, 25 8C):
d¼5.09 (s, 4H, Ar-CH2O), 6.34 [d, 3JH,H¼6.8 Hz, 4H], 6.43
Ar-H). 13C NMR (100.3 MHz, CDCl3): d¼69.9, 116.8,
129.4, 129.5, 130.9, 133.2, 136.3, 154.6. Mass spectrum:
m/z (EI, 70 eV) 314 (Mþ). Elemental analysis calcd for
C22H18O2: C, 84.08; H, 5.73; found, C, 83.98; H, 5.65.
Yield: 90%. White solid, Mp: 196 8C. IR: (KBr, cm21) 3280
3
1
[d, JH,H¼6.4 Hz, 4H], 6.68 (s, 2H, stilbenic); 7.06 (s, 4H,
(b, OH). H NMR (90 MHz, DMSO-d6, 25 8C) d¼4.48 (s,
4H, Ar-CH2OH), 5.12 (s, 4H, Ar-CH2O), 6.35 (bs, 2H,
exchangeable with D2O), 7.02 (d, 3JH,H¼7.4 Hz, 4H, Ar-H),
3
7.22–7.49 (m, 4H, Ar-H); 7.53 (d, JH,H¼7.6 Hz, 4H, Ar-
H). Mass spectrum: m/z (EI, 70 eV) 350 (Mþ). Elemental
analysis calcd for C22H22O4: C, 75.43; H, 6.29; found, C,
75.22; H, 6.15.
4.3.4. Cyclophane diol 4a. Following the general procedure
B, the dialdehyde 2a was subjected to intramolecular
McMurry coupling to give the diol 4a.
4.4.2. Diol 5a. Following the general procedure D, the diol
5b was obtained from the dialdehyde 2b.
Yield: 60%. White solid, Mp: 201 8C. 1H NMR (400 MHz,
DMSO-d6, 25 8C): d¼4.85 (s, 4H, Ar-CH2O), 5.12 (s, 2H,
Ar-CH–OH), 6.12–6.36 (m, 6H, Ar-H), 6.86 [d,
Yield: 95%. White solid, Mp: 211 8C. IR: (KBr, cm21) 3300
1
(b, OH); H NMR: (90 MHz, DMSO-d6, 25 8C) d¼4.53 (s,
3
3JH,H¼7.8 Hz, 2H, Ar-H], 7.38 [dd, 2H, JH,H¼6.8,
4H, Ar-CH2OH), 5.14 (s, 4H, Ar-CH2O), 6.46 (bs, 2H,
exchangeable with D2O, OH); 7.07 [d, JH,H¼7.8 Hz, 4H,
3
2.6 Hz, Ar-H], 7.48 (s, 2H, exchanged with D2O, OH);
3
3
7.72 [dd, JH,H¼6.6, 2.7 Hz, 2H]. 13C NMR (100.3 MHz,
Ar-H], 7.24–7.47 (m, 4H, Ar-H); 7.53 [d, JH,H¼7.8 Hz,
DMSO-d6, 25 8C): d¼69.7, 74.1, 118.6, 119.4, 126.4, 127.2,
129.4, 135.5, 157.3. Mass spectrum: m/z (EI, 70 eV) 348
(Mþ). Elemental analysis calcd for C22H20O4: C, 75.86; H,
5.75; found, C, 75.67; H, 5.53.
4H, Ar-H]. Mass spectrum: m/z (EI, 70 eV) 350 (Mþ).
Elemental analysis calcd for C22H22O4: C, 75.43; H, 6.29;
found, C, 75.32; H, 6.08.
4.4.3. Diol 5c. Following the general procedure D, the diol
5c was obtained from the dialdehyde 2c.
4.3.5. Cyclophane diol 4b. Following the general procedure
B, the dialdehyde 2b was subjected to intramolecular
McMurry coupling to give the diol 4b.
Yield: 85%. White solid, Mp: 205 8C. IR: (KBr, cm21) 3320
1
(b, OH). H NMR (90 MHz, DMSO-d6, 25 8C) d¼4.48 (s,
Yield: 56%. White solid, Mp: 185 8C. 1H NMR (400 MHz,
DMSO-d6, 25 8C): d¼5.07 (s, 4H, Ar-CH2O), 5.41 (s, 2H,
Ar-CHOH), 6.26–6.58 (m, 8H, Ar-H), 6.86 (s, 2H,
exchanged with D2O, OH), 7.26–7.38 (m, 4H, Ar-H). 13C
NMR (100.3 MHz, DMSO-d6, 25 8C): d¼68.9, 80.01,
114.1, 116.9, 126.5, 127.9, 128.7, 135.0, 136.9, 155.2.
Mass spectrum: m/z (EI, 70 eV) 348 (Mþ). Elemental
analysis calcd for C22H20O4: C, 75.86; H, 5.75; found, C,
75.77; H, 5.65.
4H, Ar-CH2OH), 5.11 (s, 4H, Ar-CH2O), 6.38 (bs, 2H,
exchangeable with D2O, OH), 6.99 [d, JH,H¼7.7 Hz, 4H,
3
3
Ar-H], 7.43 (s, 4H, Ar-H), 7.52 [d, JH,H¼7.5 Hz, 4H, Ar-
H]. Mass spectrum: m/z (EI, 70 eV) 350 (Mþ). Elemental
analysis calcd for C22H22O4: C, 75.43; H, 6.29; found, C,
75.24; H, 6.18.
4.5. General procedure for the conversion of the diol into
the dibromide (procedure E)
4.3.6. Cyclophane diol 4c. Following the general procedure
B, the dialdehyde 2c was subjected to intramolecular
McMurry coupling to give the diol 4c.
To a stirred suspension of the diol 5a/5b/5c (6 mmol) in
CH2Cl2 (120 mL), PBr3 (3 mmol) was added and the
reaction mixture was stirred at 0 8C for 12 h. The reaction
mixture was poured into water (500 mL) and the organic
layer was rapidly washed with water (3£150 mL) followed
by brine (200 mL) and dried. The solvent was evaporated in
vacuo to give dibromide 6a/6b/6c, which was purified by
recrystallization from hexane/CH2Cl2 (1:5).
Yield: 62%. White solid, Mp: 212 8C. 1H NMR (400 MHz,
DMSO-d6, 25 8C): d¼5.09 (s, 4H, Ar-CH2O), 5.35 (s, 2H,
Ar-CHOH), 6.88–7.22 (m, 8H, Ar-H), 7.41 (s, 2H,
exchanged with D2O, OH), 7.68 (s, 4H, Ar-H). 13C NMR
(100.3 MHz, DMSO-d6, 25 8C): d¼65.9, 78.4, 125.2, 127.5,
129.6, 135.3, 141.1, 152.5. Mass spectrum: m/z (EI, 70 eV)
348 (Mþ). Elemental analysis calcd for C22H20O4: C, 75.86;
H, 5.75; found, C, 75.71; H, 5.63.
4.5.1. Dibromide 6a. Following the general procedure E,
the dibromide 6a was obtained from the diol 5a.
1
Yield: 75%. White solid, Mp: 134 8C. H NMR (90 MHz,
CDCl3, 25 8C): d¼4.51 (s, 4H, Ar-CH2Br), 5.04 (s, 4H, Ar-
4.4. General procedure for the reduction of dialdehyde
with NaBH4 (procedure D)
3
CH2O), 6.97 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.15–7.29 (m,
3
4H, Ar-H); 7.37 [d, 4H, JH,H¼8.8 Hz, Ar-H]. Mass
To a solution of the dialdehyde 2a/2b/2c (8 mmol) in
methanol (70 mL) was added NaBH4 (4 mmol) in portions
at 0 8C. The reaction mixture was stirred at rt for 6 h, after
which conc. HCl (10 drops) was added. The residue
spectrum: m/z (EI, 70 eV) 476 (Mþ).
4.5.2. Dibromide 6b. Following the general procedure E,
the dibromide 6a was obtained from the diol 5b.

2358
P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
1
Yield: 87%. White solid, Mp: 146 8C. H NMR (90 MHz,
CDCl3, 25 8C): d¼4.57 (s, 4H, Ar-CH2Br), 5.13 (s, 4H, Ar-
the dialdehyde 7a was subjected to intramolecular McMurry
coupling to give the cyclophane 8a.
3
CH2O), 7.00 [d, 4H, JH,H¼8.8 Hz, Ar-H], 7.24–7.32 (m,
3
4H, Ar-H), 7.39 [d, JH,H¼8.8 Hz, 4H, Ar-H]. Mass
Yield: 25%. White solid, Mp: 136 8C. 1H NMR (400 MHz,
CDCl3, 25 8C): d¼5.06 (s, 8H, Ar-CH2O), 6.72 [d,
3JH,H¼8.3 Hz, 8H, Ar-H], 6.81 (s, 2H, stilbenic), 7.38 [d,
8H, 3JH,H¼8.5 Hz, 8H, Ar-H], 7.85 (s, 4H, Ar-H). 13C NMR
(100.4 MHz, CDCl3, 25 8C): d¼69.3, 69.9, 114.9, 115.3,
121.4, 123.6, 126.8, 128.3, 129.7, 130.6, 131.4, 136.4,
157.2, 162.5. Mass spectrum: m/z (EI, 70 eV) 526 (Mþ).
Elemental analysis calcd for C36H30O4: C, 82.13; H, 5.70;
found, C, 82.08; H, 5.58.
spectrum: m/z (EI, 70 eV) 476 (Mþ).
4.5.3. Dibromide 6c. Following the general procedure E,
the dibromide 6c was obtained from the diol 5c.
1
Yield: 72%. White solid, Mp: 135 8C. H NMR (90 MHz,
CDCl3, 25 8C): d¼4.54 (s, 4H, Ar-CH2Br), 5.09 (s, 4H, Ar-
3
CH2O), 6.94 (d, 4H, JH,H¼8.4 Hz), 7.32 (s, 4H), 7.39 (d,
3
4H, JH,H¼8.8 Hz). Mass spectrum: m/z (EI, 70 eV) 476
(Mþ).
4.5.8. Cyclophane 8b. Following the general procedure C,
the dialdehyde 7b was subjected to intramolecular
McMurry coupling to give the cyclophane 8b.
4.5.4. Dialdehyde 7a. Following the general procedure A,
the dialdehyde 7a was obtained from dibromide 6a and p-
hydroxy benzaldehyde.
Yield: 30%. White solid, Mp: 145 8C. 1H NMR (400 MHz,
CDCl3, 25 8C): d¼5.05 (s, 8H, Ar-CH2O), 6.78 [d,
3JH,H¼8.3 Hz, 8H, Ar-H], 6.98 (s, 2H, stilbenic), 7.46 [d,
3JH,H¼8.3 Hz, 8H, Ar-H], 7.64–7.67 (m, 4H, Ar-H). 13C
NMR (100.3 MHz, CDCl3, 25 8C): d¼68.4, 70.2, 115.1,
115.5, 116.2, 119.3, 127.5, 128.1, 129.2, 129.7, 130.2,
131.7, 133.8, 156.7, 162.3. Mass spectrum: m/z (EI, 70 eV)
526 (Mþ). Elemental analysis calcd for C36H30O4: C, 82.13;
H, 5.70; found, C, 82.08; H, 5.61.
Yield: 70%. White solid, Mp: 156 8C. IR: (KBr, cm21) 1682
(CvO). 1H NMR (400 MHz, CDCl3, 25 8C): d¼5.07 (s, 4H,
3
Ar-CH2O); 5.09 (s, 4H, Ar-CH2O); 7.00 [d, JH,H¼8.3 Hz,
3
4H], 7.06 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.36 [d,
3JH,H¼8.8 Hz, 4H, Ar-H], 7.41 (s, 4H, Ar-H), 7.83 [d,
3JH,H¼8.3 Hz, 4H], 9.88 (s, 2H, CHO). 13C NMR
(100.3 MHz, CDCl3, 25 8C): d¼69.8, 70.0, 115.0, 115.1,
126.4, 127.1, 128.9, 129.3, 129.9, 131.9, 137.2, 158.8,
163.8, 190.8. Mass spectrum: m/z (EI, 70 eV) 558 (Mþ).
Elemental analysis calcd for C36H30O6: C, 77.42; H, 5.38;
found, C, 77.25; H, 5.16.
4.5.9. Cyclophane 8c. Following the general procedure C,
the dialdehyde 7c was subjected to intramolecular McMurry
coupling to give the cyclophane 8c.
4.5.5. Dialdehyde 7b. Following the general procedure A,
the dialdehyde 7b was obtained from dibromide 6b and p-
hydroxy benzaldehyde.
Yield: 35%. White solid, Mp: 144 8C. 1H NMR (400 MHz,
CDCl3, 25 8C): d¼5.09 (s, 8H, Ar-CH2O), 6.94 [d,
3JH,H¼8.8 Hz, 4H, Ar-H], 7.25 [d, J¼8.3 Hz, 4H, Ar-H],
3
7.31 (s, 2H, stilbenic), 7.38 [d, JH,H¼8.8 Hz, 4H, Ar-H],
Yield: 65%. White solid, Mp: 143 8C. IR: (KBr, cm21) 1686
(CvO). 1H NMR (400 MHz, CDCl3, 25 8C): d¼5.06 (s, 4H,
7.68 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.95 (s, 4H, Ar-H). 13C
3
NMR:(100.3 MHz, CDCl3, 25 8C) d¼68.3, 69.5, 115.2,
126.7, 127.2, 128.5, 120.8, 131.4, 132.3, 133.6, 144.6,
163.5, 167.3. Mass spectrum: m/z (EI, 70 eV) 526 (Mþ).
Elemental analysis calcd for C36H30O4: C, 82.13; H, 5.70;
found, C, 82.08; H, 5.59.
3
Ar-CH2O), 5.18 (s, 4H, Ar-CH2O), 6.99 [d, JH,H¼8.8 Hz,
3
4H, Ar-H], 7.05 [d, JH,H¼8.8 Hz, 4H, Ar-H], 7.25 (s, 1H,
3
Ar-H), 7.35 [d, JH,H¼8.8 Hz, 4H, Ar-H], 7.36–7.53 (m,
3
3H, Ar-H), 7.83 [d, JH,H¼8.3 Hz, 4H, Ar-H], 9.88 (s, 2H,
CHO). 13C NMR (100.3 MHz, CDCl3, 25 8C): d¼68.0,
69.9, 114.9, 115.1, 128.4, 128.5, 129.0, 129.3, 130.0, 131.9,
134.9, 158.7, 163.7, 190.8. Mass spectrum: m/z (EI, 70 eV)
558 (Mþ). Elemental analysis calcd for C36H30O6: C, 77.42;
H, 5.38; found, C, 77.31; H, 5.26.
4.6. Complexation studies
Charge transfer complexation studies were carried out by
preparing 1£1025 M solution of stilbenophane 3a to 3c, 8c
with gradual addition of acceptor (2 mg) in a 4:1 mixture of
CH3CN and CH2Cl2. (10 mL). Gradual addition of TCNE
with stilbenophanes 3a to 3c rapidly increases the intensity
of charge transfer bands at 394.0 and 414.6 nm. Hence the
equilibrium constant was measured at 473.0 nm only. The
equilibrium constant for the CT complex derived from the
3a, 3b and 3c with TCNQ was measured at 743 nm though
absorption bands were observed at 665 and 760 nm.
Similarly, the equilibrium constant for the charge transfer
complex derived from 3a, 3b and 3c with PQT was
measured at 394 nm. Absorbance is measured at a suitable
wavelength as mentioned above with change in the
concentration of the acceptor in term of 2 mg with
concentration of the stilbenophane receptor being kept
constant. Plot of D0/A (D0 is the concentration of
stilbenophane and A is the concentration of acceptor) vs
1/A0 (A0 is the absorbance of the complex at charge transfer
4.5.6. Dialdehyde 7c. Following the general procedure A,
the dialdehyde 7c was obtained from dibromide 6c and p-
hydroxy benzaldehyde.
Yield: 72%. White solid, Mp: 154 8C. IR: (KBr, cm21) 1687
(CvO). 1H NMR (400 MHz, CDCl3, 25 8C): d¼5.02 (s, 4H,
Ar-CH2O), 5.10 (s, 4H, Ar-CH2O), 6.92 [d, J¼8.8 Hz, 4H];
3
3
7.28 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.45 [d, JH,H¼8.8 Hz,
3
4H, Ar-H], 7.76 [d, JH,H¼8.3 Hz, 4H, Ar-H], 7.95 (s, 4H,
Ar-H), 9.81 (s, 2H, CHO). 13C NMR (100.3 MHz, CDCl3,
25 8C): d¼68.1, 69.7, 115.0, 127.6, 128.2, 128.8, 129.3,
130.9, 132.0, 132.4, 162.6, 166.8, 190.8. Mass spectrum:
m/z (EI, 70 eV) 558 (Mþ). Elemental analysis calcd for
C36H30O6: C, 77.42; H, 5.38; found, C, 77.27; H, 5.23.
4.5.7. Cyclophane 8a. Following the general procedure C,

P. Rajakumar, V. Murali / Tetrahedron 60 (2004) 2351–2360
2359
Figure 5. Plot of D0/A versus 1/A0 for determination of stability constant (e.g. stilbenophane 3c with PQT).
5265–5293. (f) Hubin, T. J.; Busch, D. L. Coord. Chem.
Rev. 2000, 200–202, 5–52. (g) Nielsen, M. B.; Jeppesen, J. O.;
Lau, J.; Lomholt, C.; Damgaard, D.; Jacobsen, J. P.; Becher, J.;
Stoddart, J. F. J. Org. Chem. 2001, 66, 3559–3563.
transition) gave a straight line indicating the stoichiometry
of the complex as 1:1 (Fig. 5 plot of D0/A vs 1/A0 for the
complex of 3c with PQT). By applying the Benesi–
Hildebrand equation, the reciprocal of the intercept on the
Y-axis provided the information about e AD (e of donor–
acceptor complex) and from the slope of the line, KcAD
(equilibrium constant of the donor acceptor complex) was
calculated. Similarly KAc D for the charge transfer complex of
8c with PQT was also determined. Thus by making use of
UV–Visible spectroscopy, the stability constant of charge
transfer complexes of stilbenophane 3a to 3c and 8c with
various acceptors TCNE, TCNQ and PQT were determined.
3. Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.;
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5. Tobe, Y.; Nagano, A.; Kawabata, K.; Sonoda, M.; Naemara,
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VM thanks CSIR, New Delhi for fellowship. The authors
thank DST, New Delhi for financial assistance and RSIC,
IIT Madras for spectral data.
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