Squaraines based on 2-arylpyrroles

Article abstract of DOI:10.1016/j.tet.2004.07.023

Various derivatives of 2-phenylpyrrole were condensed with squaric acid to give the corresponding squaraines. The products are drawn with the anti geometry rather than the syn geometry generally shown in the past: the arguments for this formulation are gi

Full text of DOI:10.1016/j.tet.2004.07.023

Tetrahedron 60 (2004) 8913–8918
Squaraines based on 2-arylpyrroles
*
Raymond Bonnett, Majid Motevalli and Jason Siu
Department of Chemistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
Received 15 April 2004; revised 14 June 2004; accepted 8 July 2004
Available online 23 August 2004
Abstract—Various derivatives of 2-phenylpyrrole were condensed with squaric acid to give the corresponding squaraines. The products are
drawn with the anti geometry rather than the syn geometry generally shown in the past: the arguments for this formulation are given,
including the analogy with 2,5-bis(4-ethyl-3,5-dimethylpyrrol-2-yl)-1,4-benzoquinone, for which an X-ray structure is presented. Solutions
of the new bis(5-arylpyrrol-2-yl)squaraines have intense, sharp absorption bands shifted to the red. Condensation of squaric acid with
arylpyrroles possessing fused ring systems, and condensation with 2-styrylpyrrole, gave chromophores with high values for lmax and 3max
Certain of these chromophores appear to be suitable for further structural elaboration to give materials having potential in optoelectronic and
.
photodynamic applications.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
properties of bis(4-acetyl-3,5-dimethylpyrrol-2-yl)-
squaraine have been reported.12 Recently there have been
several reports on the non-linear optical properties,13,14
electrical conductivities,15–18 and lithium ion sequestra-
tion19 of various polymeric (N-alkylpyrrol-2-yl)squaraines.
Copolymers comprised of bis(pyrrol-2-yl)squaraine units
based on N-substituted and N-unsubstituted pyrroles have
also been prepared.20
The squaraines are a series of organic dyestuffs character-
ised by intense absorption in the red region of the spectrum.
They have attracted considerable commercial interest
because of their optoelectronic properties, finding appli-
cations in such areas as solar cells, xerographic sensitisers,
near-IR dyes and optical recording.1–4 There has also been
some limited activity in applications as sensitisers in
photodynamic therapy.5–9 In both of these areas, two
current interests are (i) to shift the absorption maximum to
lower energies, and (ii) to alter the solution properties, since
many of the squaraines have poor solubility in polar and in
non-polar solvents. Here we address the first of these.
Our present aim has been to investigate the preparation and
electronic absorption spectra of 2,4-bis(5-arylpyrrol-2-
yl)squaraines 1. Such molecules might be advantageous
here since they would be expected to have red-shifted
absorption maxima (with respect to the parent skeleton)
because of the aryl conjugation; and the aryl ring would
provide sites for substituents aimed at modulating solution
and spectroscopic properties.
The bis-pyrrolylsquaraines are highly p-delocalised sys-
tems which may be regarded as possessing 2pe aromaticity,
as shown in Scheme 1. Thus the carbonyl stretching
Although the bis(pyrrol-2-yl)squaraines were amongst the
first of the squaraine dyes to be reported,10,11 they have
subsequently been relatively little studied, and the 5-aryl-
pyrrol-2-yl derivatives are unknown. The photophysical
Keywords: Oxocarbon acids and derivatives; Electronic spectra; Pyrroles,
Aryl; Red-absorbing dyes.
* Corresponding author. Tel.: C44-020-7882-5024; fax: C44-020-7882-
7794; e-mail: r.bonnett@qmul.ac.uk
Scheme 1. Delocalisation in the bis(pyrrol-2-yl)squaraine system.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.07.023

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R. Bonnett et al. / Tetrahedron 60 (2004) 8913–8918
frequency in the IR spectrum appears at about 1600 cmK1
consistent with considerable single bond character.
,
original papers and reviews have followed suit:3,12,24
reference 25, where an anti structure is given without
specific comment, appears to be the only exception. The
matter has been given some attention for squaraines derived
from N-substituted pyrroles. Thus the squaraine obtained
from 2,4-dimethyl-1-octadecylpyrrole behaves as a single
substance, and has been thought most likely to prefer the
anti geometry 4 for steric reasons.13 Polymeric systems
derived from N-alkylpyrroles have also sometimes been
represented with the anti stereochemistry.19,26
For the various applications already mentioned, the
squaraines have some distinct advantages, especially the
sharp intense absorption in the red region. Thus the
squaraine 2 has lmax 560 nm (3 200,000 MK1 cmK1) in
chloroform (Fig. 1) which is little affected by modest
changes in the acidity of the medium (small hypsochromic
shift in CHCl3–HCl). Secondly, such compounds can be
synthesised, usually in good yield, in one step from squaric
acid and an a-free pyrrole. Thus in the original work of
Treibs and Jacob10,11 compound 2 was obtained in 90%
isolated yield from kryptopyrrole as shown in Scheme 2.
Such a one step preparation is a clear benefit with respect to
cost and environmental considerations when a potentially
commercial interest is in view, although the starting
materials still have to be obtained. Thirdly, although most
squaraines that have been described have symmetrical
structures, it is possible to carry out the synthesis in two
independent stages, and so attach two different p-excessive
units, thus increasing the range of structural variation.4,21–23
We believe that for the squaraines derived from N-unsubsti-
tuted pyrroles the anti stereochemistry shown in Scheme 1
will be preferred because it maximises intramolecular
hydrogen bonding. Such intramolecular hydrogen bonding
involving pyrrolic imino hydrogen has been encountered
elsewhere, for example in bilirubin (where it is also thought
to be associated with low solubility in polar solvents).27
Attempts to grow crystals of the new bis(2-arylpyrrol-2-
yl)squaraines (below) suitable for X-ray analysis have not
so far been successful, and we have found no closely related
X-ray structure in the literature. However, we have carried
out a crystal analysis of a structurally related compound, the
2,5-bis(pyrrol-2-yl)-1,4-benzoquinone 5,28 which is found
to have the intramolecularly hydrogen bonded structure
shown in Figure 2. Moreover, the observed bond lengths
accord with contributions from canonicals such as 6, again
analogous to the situation shown in Scheme 1.
Figure 1. The electronic absorption spectrum of 2,4-bis(4-ethyl-3,5-
dimethylpyrrol-2-yl)cyclobutenediylium-1,3-diolate [2,4-bis(4-ethyl-3,5-
dimethylpyrrol-2-yl)squaraine] 2 in chloroform.
Scheme 2. Synthesis of 2,4-bis(4-ethyl-3,5-dimethylpyrrol-2-yl)squaraine
2. (i) n-BuOH/PhHZ1:1, reflux, 3 h.
2. Results and discussion
2.1. Structure
Figure 2. Molecular structure of 2,5-bis(4-ethyl-3,5-dimethylpyrrol-2-yl)-
1,4-benzoquinone [2,5-bis(4-ethyl-3,5-dimethylpyrrol-2-yl)cyclohexa-
diene-1,4-dione] 5 (crystallographic numbering shown).
˚
˚
Thus, in 5 the C–O bond lengths are 1.242 A (1.222 A in
1,4-benzoquinone)29 indicating increased single bond
character, confirmed by the IR stretching frequency at
1602 cmK1. These and other bond lengths selected for their
relevance to this point are shown in Figure 3. Particularly
It is necessary at the outset to address the question of the
detailed structure of 2,4-bis(pyrrol-2-yl)squaraine [2,4-
bis(pyrrol-2-yl)cyclobutenediylium-1,3-diolate]. When
Treibs and Jacob first described 2, the structure was drawn
as shown at 3, and the majority of subsequent authors of

R. Bonnett et al. / Tetrahedron 60 (2004) 8913–8918
8915
significant are the lengthening in the six membered ring of
the C1–C2 and C2–C3 bonds, and the shortening of the
C3–C4 bond (chemical numbering) with respect to the
corresponding values in 1,4-benzoquinone.
Scheme 3. Condensation of squaric acid with 2-phenylpyrrole and
analogues to give the corresponding squaraines. [(i)Zn-BuOH–benzene,
6, 4 h, molecular sieves].
˚
Figure 3. Selected bond lengths (A) of compound 5 compared with
corresponding bond lengths in 1,4-benzoquinone (in parenthesis).29,30
(Chemical numbering shown).
broadening by radical impurities. The electronic spectra of
compounds 11–14 are compared graphically in Figure 4,
and refer to chloroform solution: treatment with traces of
acid and base (cf. Fig. 1) caused little or no discernible
change (data not shown).
Thus in this paper we show the bispyrrol-2-ylsquaraines
with the anti geometry (as in Scheme 1) throughout. This is
not to imply that the alternative geometry may not be
accessible in solution by overcoming the barrier to restricted
rotation. Although 1H NMR work has not led to its detection
with the new compounds described here, such equilibration
has been observed in this way with other systems (e.g. with
dialkylaminohydroxyphenyl squaraines).25 Recent studies
in non-protic solvents on semisquaraines with o-diethyl-
aminophenyl substitution have been interpreted in terms of
intramolecular hydrogen bonding which is disrupted in
protic solvents.31
It is apparent that the introduction of the 2-phenyl
substituent into the pyrrole moiety causes a marked
bathochromic shift (lmax 564 nm for 2, 621 nm for 11).
The peak is further shifted to lower energy by the p-methoxy
group in 13 (lmax 643 nm, cf. delocalisation pathway in 15)
but is essentially unaffected by the m-methoxy group in 12
(lmax 624 nm, oxygen lone pair not delocalisable onto the
squaraine system), except that the molecular absorbance
decreases. The absorption bands are particularly sharp, with
little absorption in the remaining part of the visible region,
and the solutions are strikingly coloured (purple-blue
shades) the brilliance of which we suppose to be associated
with the narrowness of this absorption band.
2.2. 2,4-Bis(5-arylpyrrol-2-yl)squaraines
Our approach has thus been to push the absorption
maximum of the bis-pyrrolylsquaraine system towards the
red by introducing a-aryl substituents and by constraining
such substituents more or less to the plane by non-
conjugative ring formation. Such an approach has been
used recently in modifying the chromophores of difluoro-
boron(III) pyrromethene derivatives used as fluorescent
labels for DNA sequencing to increase the range of
absorption and fluorescence.32,33
2-Phenylpyrrole 7 and its derivatives 8–10 were prepared by
coupling N-Boc-2-bromopyrrole34 with the appropriate
arylboronic acid using the Suzuki reaction.33 Condensation
of these 2-arylpyrroles 7–10 with squaric acid in butanol–
benzene in the presence of molecular sieve gave the
corresponding squaraines 11–14 (Scheme 3) as crystalline
or amorphous greenish solids. The squaraines were
characterised by elemental analysis and/or HRMS and by
spectroscopic methods.
Some minor problems were encountered: the crystals tended
to retain water (observed also by others11): and in some
cases it was not possible to obtain satisfactory H NMR
1
Figure 4. The electronic absorption spectra of squaraines 11–14 in
chloroform.
spectra, which is attributed to low solubility and/or to line

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R. Bonnett et al. / Tetrahedron 60 (2004) 8913–8918
bis[5-(E)-styrylpyrrol-2-yl]squaraine has been reported to
have lmax 695 nm, showing the pronounced auxochromic
effect of additional methoxy groups.3
However, the spectrum of the a-(1-naphthyl) derivative 14
showed quite different behaviour: the lmax value had
decreased (613 nm) and the peak had become broader and
less intense (Fig. 4). This is attributed to a steric effect which
causes the larger naphthyl substituent to rotate out-of-plane,
leading to reduced conjugation.
3-Methyl-4H-indano[1,2-b]pyrrole 2032 was condensed
with squaric acid to give the corresponding squaraine 17
as fine lustrous greenish needles in 42% yield. This
substance was especially difficult to study because of its
poor solubility: it was sparingly soluble in pyridine and in
trifluoroacetic acid, which proved to be satisfactory solvents
for several of the squaraines studied here. Squaraine 17 gave
a satisfactory analysis, but it was not sufficiently soluble
The example of the a-(1-naphthyl) derivative 14 pointed to
the need to facilitate conjugation by introducing features
which enhance coplanarity of the benzenoid and squaraine
chromophores. We sought to do this in two ways: (i) by
using styryl rather than phenyl substitution, as in 16; and (ii)
by introducing an additional saturated ring to force the
benzenoid substituent to lie more or less in the plane of the
bispyrrolylsquaraine nucleus, as in 17 and 18.
1
(DTFA–CDCl3; pyridine-d5) to give a H NMR spectrum.
The electronic spectrum (chloroform) is shown in Figure 5
(lmax 654 nm, 3 347,000 MK1 cmK1). In an analogous
manner, 3-methyl-4,5-dihydro-1H-benz[1,2-g]indole 2132
condensed with squaric acid to give the squaraine 18 as fine
green needles in 80% yield. It gave satisfactory elemental
analysis and high resolution molecular ion measurements.
Although in this case the substance did dissolve in pyridine–
chloroform to give a bright green solution, a satisfactory 1H
NMR spectrum was not obtained. The electronic spectrum
showed a sharp band at 660 nm with a high 3 value
(403,500 MK1 cmK1).
Thus with respect to the 2-phenylpyrrole squaraine
derivative 11, both structural variations, i.e. the extended
conjugation in 16, and the constraints imposed in 17 and 18,
have led to significant shifts of the absorption band into the
red region, with the additional advantage of marked
hyperchromic effects for 17 and 18.
2-(E)-Styrylpyrrole 1935 was condensed with squaric acid as
before to give 2,4-bis[5-(E)-styrylpyrrol-2-yl]cyclobutene-
diylium-1,3-diolate 16 as a dark green solid in 54% yield.
The electronic spectrum in chloroform (Fig. 5) showed lmax
673 nm (3 146,000 MK1 cmK1). Although the molecular
absorbance is lower than might have been expected, the lmax
of the absorption band is shifted well into the red. Since this
work was completed, a tetramethoxy derivative of 2,4-
3. Conclusions
(i) It is argued that the preferred geometry of the
N-unsubstituted bis(pyrrol-2-yl)squaraine system is
anti because this geometry allows greater intra-
molecular hydrogen bonding than the syn structure.
This conclusion is supported by an X-ray crystal
structure of a closely analogous compound, 2,5-bis(4-
ethyl-3,5-dimethylpyrrol-2-yl)-1,4-benzoquinone 5.
(ii) Squaraines derived from 2-arylpyrroles are prepared
for the first time. They possess sharp strong absorption
bands shifted to the red region of the spectrum when
compared with the known alkylated analogues.
Especially is this so for the molecules in which the
aryl ring is constrained to be more or less in the plane
of the pyrrole ring, as in 18 (lmax 660 nm, 3
403,500 MK1 cmK1).
(iii) The chromophores represented by 13, 16, 17 and 18
may have potential for optoelectronic and photo-
therapeutical applications. Solubility in common
solvents remains a problem with the present com-
pounds, but the presence within these structures of
benzenoid sites which can carry groups designed to
modulate physical properties (particularly solubility
Figure 5. The electronic absorption spectra of squaraine compounds 11 and
16–18 in chloroform. The base line for 17 and 18 has been displaced for
clarity.

R. Bonnett et al. / Tetrahedron 60 (2004) 8913–8918
8917
and photophysical parameters) offers a pathway for
future development.
4.2.3. 2,4-Bis[5-(4-methoxyphenyl)pyrrol-2-yl]cyclo-
butenediylium-1,3-diolate (13). Lustrous greenish crystals
(63%), mp 230 8C; dH (400 MHz; d5-pyridine–CDCl3;
Me4Si) 7.99 (4H, d, JZ8.8 Hz, AA0BB0 benzenoid C20
and C60 H), 7.95 (2H, d, JZ3.8 Hz, 3-H of pyrrole), 7.02
(2H, d, 0JZ3.8 Hz, 4-H of pyrrole), 6.94 (4H, d, JZ8.8 Hz,
AA0BB benzenoid C30 and C50 H), 3.70 (6H, s, OMe). lmax
(CHCl3)/nm 643 (3 248,000 MK1 cmK1); nmax (KBr)/cmK1
3393, 1603, 1583, 1560, 941, 916, 829, 797, 783; m/z (FAB)
425 (35, MCH), 424 (35, M), 336 (55); HRMS MCH,
C26H21N2O4 calcd 425.1501, found 425.1516. Anal. calcd
for C26H20N2O4$0.1H2O requires C, 73.26; H, 4.78; N,
6.57. Found C, 73.19; H, 4.83; N, 6.34.
4. Experimental
4.1. General
Electronic spectra were measured on a Perkin–Elmer 552
spectrometer; 1H NMR spectra were measured with
tetramethylsilane as internal standard and were recorded
either with a Bru¨ker AM-250, a JEOL EX-270, a Bru¨ker
AMX-400 or a Bru¨ker AMX-600 (ULIRS service) instru-
ment. The coupling constants (J values) are given in Hz;
peak assignments for 12–14 by COSY and NOESY
spectroscopy. Mass spectra were recorded on a Micromass
ZAB-2SE (ULIRS service): relative abundances and assign-
ments are given in parentheses. Unless otherwise stated
ionisation was by fast atomic bombardment (FAB): the
matrix was m-nitrobenzyl alcohol (NOBA). Mps were
measured on a hot-stage apparatus, and are uncorrected.
Reactions were monitored using TLC on Merck Kieselgel
60 silica gel plastic sheets. Column chromatography was
carried out on Merck Kieselgel 60 silica gel (0.040–
0.063 mm). The 2-aryl pyrroles 7–10 were prepared by
coupling N-t-butyloxycarbonyl-2-bromopyrrole with the
appropriate arylboronic acid as described in the literature.33
4.2.4. 2,4-Bis[5-(1-naphthyl)pyrrol-2-yl]cyclobutene-
diylium-1,3-diolate (14). Greenish solid (78%), mpO
300 8C; RfZ0.36 (EtOAc/CHCl3Z3:7); dH (400 M0Hz;
DTFA–CDCl3; Me4Si) 8.20 (2H, m, naphthyl C-1 H),
8.05, 7.95 (each 2H, m, naphthyl C–H), 7.82 (2H, m, 3-H
of pyrrole), 7.70 (2H, bs, naphthyl C–H), 7.61 (6H, m,
naphthyl C-30, 60 and 70 H), 7.15 (2H, m, 4-H of pyrrole).
lmax (CHCl3)/nm 613 (3 149,000 MK1 cmK1); nmax (KBr)/
cmK1 3425, 3051, 1624, 1593, 1558, 1516, 945, 768 and
664; m/z (FAB) 487 (13, MCNa), 465 (90, MCH), 464
(100, M), 443 (25), 349 (33), 336 (57), 329 (87), 307 (48),
289 (40), 264 (28); HRMS M, C32H20N2O2 calcd 464.1525,
found 464.1502. Anal. calcd for C32H20N2O2, C, 82.74; H,
4.34; N, 6.03. Found C, 81.91; H, 4.29; N, 5.75.
4.2. General procedure for the synthesis of squaraine
derivatives
4.2.5.
2,4-Bis[(E)-5-styrylpyrrol-2-yl]cyclobutene-
diylium-1,3-diolate (16). Greenish solid (54%), mpO
300 8C; RfZ0.40 (EtOAc/CHCl3Z3:7). This substance
did not give a satisfactory H NMR spectrum in CDCl3/
DTFA or in pyridine-d5. lmax (CHCl3)/nm 673 (3
146,000 MK1 cmK1); nmax (KBr)/cmK1 3421, 1593, 1551,
1501, 941, 847, 799, 745, 687 and 640; m/z (FAB) 417 (78,
MCH), 416 (82, M), 338 (45); HRMS M, C28H20N2O2
calcd 416.1525, found 416.1505.
4.2.1. 2,4-Bis[5-phenylpyrrol-2-yl]cyclobutenediylium-
1,3-diolate (11). 2-Phenylpyrrole 7 (35 mg, 0.24 mmol),
and squaric acid (11.5 mg, 0.10 mmol) were dissolved in
1
˚
butanol/benzene (1:1) (10 mL) and refluxed for 4 h with 4 A
molecular sieves (ca. 50 mg). The reaction mixture was
cooled to room temperature; ethanol (4 mL) was added and
the mixture was decanted from the molecular sieves and
kept in the freezer overnight. The crystalline product was
filtered off, washed with ethanol (20 mL), and dried to give
the title squaraine (12.9 mg, 35%) as lustrous greenish
needles, mpO300 8C; RfZ0.40 (EtOAc:CHCl3Z3:7). This
substance did not give satisfactory NMR signals in DMSO-
d6, DTFA–CDCl3, and pyridine-d5, due to poor solubility.
lmax (CHCl3)/nm 621 (3 237,000 MK1 cmK1); nmax (KBr)/
cmK1 3400, 1618, 1583, 1560, 945, 918, 758; m/z (FAB)
365 (100, MCH), 364 (50, M), 363 (35), 199 (75); HRMS
MCH, C24H17N2O2 calcd 365.1290, found 365.1285. Anal.
calcd for C24H16N2O2$1.8H2O, C, 72.64; H, 4.98; N, 7.06.
Found C, 72.53; H, 4.34; N, 6.82.
4.2.6. 2,4-Bis[5-(4-methyl)indano-4H-[1,2-b]pyrrol-2-
yl]cyclobutenediylium-1,3-diolate (17). Lustrous greenish
1
fine needles (42%), mpO300 8C. An H NMR spectrum
could not be obtained (low solubility in CDCl3/DTFA and
pyridine-d5). lmax (CHCl3)/nm 654 (3 347,000 MK1 cmK1);
nmax (KBr)/cmK1 3450, 1612, 1535, 1447, 1408, 1045, 945,
814, 795, 768 and 714; m/z (FAB); HRMS M, C28H20N2O2
calcd 416.1525, found 416.1545. Anal. calcd for
C28H20N2O2, C, 80.75; H, 4.84; N, 6.73. Found C, 80.92;
H, 4.90; N, 6.49.
4.2.2. 2,4-Bis[5-(3-methoxy)phenylpyrrol-2-yl]cyclo-
butenediylium-1,3-diolate (12). Lustrous greenish needles
(58%), mpO300 8C; RfZ0.35 (EtOAc/CHCl3 Z3:7); dH
(400 MHz; CDCl3; Me4Si) 7.73 (2H, bd, pyrrole 3-H), 7.46
(4H, m, benzenoid C50 and C60), 7.38 (2H, bs, benzenoid
C20), 7.10 (2H, m, benzenoid C40), 7.06 (2H, d, JZ4.8 Hz,
4-H of pyrrole), 4.00 (6H, s, OMe). lmax (CHCl3)/nm 625 (3
177,000 MK1 cmK1); nmax (KBr)/cmK1 3393, 1635, 1603,
1558, 1508, 934, 822, 800; m/z (FAB) 425 (100, MCH),
346 (35); HRMS MCH, C26H21N2O4 calcd 425.1501,
found 425.1516. Anal. calcd for C26H20N2O4$0.5H2O, C,
72.04; H, 4.88; N, 6.46. Found C, 72.07; H, 4.78; N, 6.21.
4.2.7. 2,4-Bis[5-(3-methyl-4,5-dihydro-1H)benz[g]-
indolyl-2-yl]cyclobutenediylium-1,3-diolate (18). Fine
greenish crystals (80%), mp 280 8C; RfZ0.72 (EtOAc/
1
CHCl3Z3:7). H NMR spectra could not be obtained (low
solubility in CDCl3/DTFA and pyridine-d5). lmax (CHCl3)/
nm 660 (3 403,500 MK1 cmK1); nmax (KBr)/cmK1 3400,
1612, 1528, 1472, 1420, 1369, 1302, 1283, 1180, 1113,
1088, 1061, 972, 955, 885 and 764; m/z (FAB) 445 (33, MC
H), 444 (50, M), 329 (40); HRMS M, C30H24N2O2 calcd
444.1838, found 444.1846. Anal. calcd for C30H24N2O2, C,
81.05; H, 5.44; N, 6.30. Found C, 81.25; H, 5.44; N, 6.27.

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R. Bonnett et al. / Tetrahedron 60 (2004) 8913–8918
11. Treibs, A.; Jacob, K. Liebig’s Ann Chem. 1966, 699, 153–167.
4.3. Model compound
12. Patrick, B.; George, M. V.; Kamat, P. V.; Das, S.; Thomas,
K. J. J. Chem. Soc., Faraday Trans. 1992, 88, 671–676.
13. Lynch, D. E.; Geisler, U.; Peterson, I. R.; Floersheimer, M.;
Terbrack, R.; Chi, L. F.; Fuchs, H.; Calos, N. J.; Wood, B.;
Kennard, C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans.
1997, 2, 827–832.
4.3.1. 2,5-Bis[3,5-dimethyl-4-ethylpyrrol-2-yl]cyclohexa-
diene-1,4-dione (5). 3-Ethyl-2,4-dimethylpyrrole (30 mg,
0.24 mmol) in ethanol (3 ml) and 1,4-benzoquinone
(36.3 mg, 0.34 mmol.) in chloroform/ethanol mixture (1:1)
(3 ml) were mixed together and stirred at room temperature
for 20 min. The solution became purple in colour, and then
very dark. The reaction was diluted with ethanol (6 ml) and
kept in the freezer overnight. The greenish purple solid was
filtered off and washed with ethanol (10 ml) to give 12 mg
(28% yield) of the title quinone as a dark purple solid with a
greenish iridescence, mp 245–250 8C (lit28 mp 250 8C);
RfZ0.33 (light petroleum/CHCl3 Z1:4); dH (400 MHz;
CDCl3; Me4Si) 10.90 (2H, bs, N–H), 6.60 (2H, s, quinone
C–H), 2.40 (8H, quartet, JZ7.4 Hz, CH2CH3), 2.30 (12H,
bs, C-3, C-5 CH3), 1.10 (6H, t, JZ7.4 Hz, CH2CH3);
(CHCl3)/nm 569 (3 30,000 MK1 cmK1); nmax (KBr)/cmK1
3346, 1602, 1560, 1535, 991, 962, 864, 791; m/z (FAB) 353
(32), 352 (86), 351 (100, MCH), 350 (79, M) and 349 (42);
HRMS calcd for MCH, C22H27N2O2 351.2073, found
351.2080. It proved possible to obtain crystals of 5 suitable
for X-ray analysis by slow evaporation of a chloroform
solution at room temperature.30
14. Lynch, D. E.; Peterson, I. R.; Floersheimer, M.; Essing, D.;
Chi, L. F.; Fuchs, H.; Calos, N. J.; Wood, B.; Kennard,
C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans.
1998, 2, 779–784.
15. Ajayaghosh, A.; Chenthamarakshan, C. R.; George, M. V.
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Acknowledgements
We thank the Medical Research Council (MRC) and Scotia
Pharmaceuticals Plc., Stirling for an industrial collaborative
research award (to J.S.), Dr. H. Toms and Mr. P. Haycock
for 600 and 400 MHz NMR spectroscopic studies (ULIRS
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