Synthesis, spectroscopy, photophysics and thermal behaviour of stilbene-based triarylamines with dehydroabietic acid methyl ester moieties

Article abstract of DOI:10.1039/b815711a

Novel stilbene-based triarylamines with dehydroabietic acid methyl ester moieties have been synthesised by palladium catalysed C-N coupling reactions. The presence of the bulky dehydroabietic acid group increases solubility, hinders crystallisation and pe

Full text of DOI:10.1039/b815711a

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Synthesis, spectroscopy, photophysics and thermal behaviour of
stilbene-based triarylamines with dehydroabietic acid methyl ester moietiesw
a
a
a
bc
´
Barbara Gigante,* M. Alexandra Esteves, N. Pires, Matthew L. Davies,
Peter Douglas,b Sofia M. Fonseca,c Hugh D. Burrows,*c Ricardo A. E. Castro,d
Joa
o Pinac and Joao Seixas de Meloc
˜
˜
Received (in Durham, UK) 9th September 2008, Accepted 14th January 2009
First published as an Advance Article on the web 5th February 2009
DOI: 10.1039/b815711a
Novel stilbene-based triarylamines with dehydroabietic acid methyl ester moieties have been
synthesised by palladium catalysed C–N coupling reactions. The presence of the bulky
dehydroabietic acid group increases solubility, hinders crystallisation and permits their
sublimation while preserving their electronic and optical characteristics. This makes them good
candidates for various molecular electronic applications. We report a detailed characterisation of
their electrochemical, spectroscopic and photophysical properties. In addition, the thermal
stability and behaviour is discussed with reference to the potential for preparation of stable
thin films.
amorphous). To avoid crystallisation, various derivatives
Introduction
(including polymers) with high glass transition temperatures
(Tg)5–7 have been developed. One extensively used material for
this purpose5 is N,N0-diphenyl-N,N0-bis(3-methylphenyl)-
[1,10-biphenyl]-4,40-diamine (TPD), with a Tg of 60 1C.8
However, TPD films prepared by sublimation tend to crystallise
over time, even at ambient temperature. To circumvent these
problems, many efforts have been devoted to the synthesis of
new HTL materials with better thermal and morphological
stability, and there is strong interest in the development of
amorphous molecular materials for these applications.1 We
have previously shown that triarylamines containing a bulky
aryl-substituent (a dehydroabietic acid methyl ester moiety)9,10
inhibit crystallisation and show good solubility characteristics.
In addition, we have also indicated the potential of dehydro-
abietic acid based bipolar arylamine–quinoxalines for device
applications.11 Here, we take one step further and study
the effect of stilbene substituents within a new series of
compounds 3–8 (Fig. 1). The triarylamine 9, without the
stilbene unit, was also studied for comparison with 8, in order
to understand its emission behaviour.
In addition to their potential as HTL materials,12 the
stilbene derivatives act as models for end-capped poly-
(p-phenylene vinylene)-based systems, which have been
studied in more detail elsewhere.13 They may also possess
interesting photonic properties, since arylamines with electron
accepting nitro groups are amongst the most important
organic systems in non-linear optics.14 Further, the enhanced
solubility of these compounds suggests that they may be of
interest as optical probes. Both the amorphous nature of these
compounds and their good solubility characteristics favour
many of these applications. We have therefore carried out
a detailed study of the spectroscopic and electrochemical
properties of these compounds, and have further characterised
the photophysical behaviour both in solution and in
rigid media. In addition, we have looked at the thermal
properties using thermomicroscopy and thermogravimetry.
To achieve organic materials offering high performance for
various molecular electronic applications, including organic
light-emitting diodes (OLEDs), photovoltaic systems and
non-linear optics, intensive research has been focused on the
development of new compounds with significant improve-
ments in non-crystallinity or amorphous film-forming ability,
thermal stability, electrochemical reversibility, emission colour
purity and lifetime. These have been accompanied by advances
in device architecture, leading to tremendous progress in the
design and synthetic strategies.1
For photovoltaic and LED applications, optimisation of
the charge carrier separation or balance using multilayer
structures is
a common method to improve efficiency.
Hole-transporting layers (HTL), which may also act as
electron-blocking layers, have been widely used for this.
Triarylamines offer particularly attractive properties as HTL,
such as high hole mobility and ease of sublimation, leading to
a wide range of practical applications,2 and are being explored
either as small molecules (as thin films formed by vacuum
sublimation or solution processing) or inserted, as
co-monomer units, in luminescent copolymers.3,4 To maximise
light output, HTL materials should not absorb in the visible
nor lead to significant light scattering (for which they must be
a INETI-DTIQ, Estrada do Pac¸o do Lumiar, 22, Lisboa, 1649-038,
Portugal. E-mail: barbara.gigante@ineti.pt;
Fax: +351 (0)217 168 100; Tel: +351 (0)210 924 750
b Chemistry Group, School of Engineering, Swansea University,
Singleton Park, SA2 8PP Swansea, UK
´
Departamento de Quımica, Universidade de Coimbra, Coimbra,
3004-535, Portugal. E-mail: burrows@ci.uc.pt;
c
Fax: +351 (0)239 827703; Tel: +351 (0)239 854482
´
CEF, Faculdade de Farmacia, Universidade de Coimbra, Coimbra,
3000-295, Portugal
d
w Electronic supplementary information (ESI) available: Experimental
data on hot stage microscopy (HSM) of compounds 3–9 and a
thermogravimetric curve up to 650 1C for compound 7 are given in
figures S1 and S2 of the ESI. See DOI: 10.1039/b815711a
ꢀc
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Fig. 1 Structures of new methyl dehydroabietic acid triarylamines 3–9.
The relevance of this to the film-forming properties of the
compounds and their potential applications will be discussed.
stilbene moiety, was synthesised by reaction of 2a with
1-bromo-4-nitrobenzene using the same palladium-catalysed
route.15
The structures of the new compounds were confirmed
1
by H NMR spectroscopy, mass spectrometry and elemental
Results and discussion
analysis. All the triarylamines are soluble in commonly used
organic solvents.
Synthesis
The synthetic pathway for the new stilbene-based triaryl-
amines with dehydroabietic acid methyl ester moieties (3–8)
is sketched out in Scheme 1. Different trans-4-bromostilbenes
(1a–d), prepared by a Horner–Wadsworth–Emmons procedure,12
were coupled to the diarylamines 2a or 2b using a suitable
palladium-catalysed route to aromatic carbon–nitrogen bond
formation (Pd(OAc)2/P(t-Bu)3/NaOt-Bu)15 to give the target
compounds 3–8. The starting stilbenes 1a–d, the diarylamine
intermediates 2a and 2b as well as the new stilbene–triaryl-
amines 3–8 were purified by either repeated recrystallisation or
by column chromatography. The triarylamine 9, without the
Electrochemical, spectroscopic and photophysical studies
To ensure that these new stilbene-based compounds (3–8)
could be useful for fabrication of OLEDs, the electrochemical,
spectroscopic and photophysical properties were studied in
various solvents and under different conditions.
Electrochemical properties
The redox behaviour of the new compounds was studied by
cyclic voltammetry in acetonitrile. All triarylamines show a
Scheme 1
ꢀc
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Table 1 Absorption and fluorescence data for triarylamines 3–9
Cpd
Solvent
Epa/Va
l
abs/nm
eabs ꢂ 104/Mꢃ1 cmꢃ1b
lem/nm
Ffe
tf f/ns
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
Cyclohexane
Acetonitrile
Isopropanol
371 (269)
367 (293)
367 (292)
375 (291)
369 (289)
371 (284)
367 (298)
365 (294)
366 (298)
371 (300)
367 (292)
370 (296)
379 (300)
373 (302)
377 (290)
432 (301)
428 (301)
430 (289)
388 (287)
398 (264)
397 (283)
3.64 (2.26)
2.81 (2.81)
2.93 (2.06)
3.77 (2.50)
3.70 (2.50)
3.52 (2.45)
3.90 (2.90)
4.05 (2.96)
3.79 (2.79)
3.90 (2.90)
4.02 (2.94)
3.56 (2.71)
4.82 (2.64)
4.15 (3.31)
4.24 (2.65)
3.29 (3.17)
2.96 (3.31)
2.83 (2.84)
1.90 (1.35)
2.14 (1.60)
1.43 (1.04)
404
479
453
418
517
472
400
473
438
411
493
471
410
445
436
483
N/Ed
N/Ed
495
N/Ed
N/Ed
0.34
0.68
0.67
0.43
0.27
0.42
0.39
0.56
0.56
0.35
0.49
0.54
0.50
0.24
0.29
0.57
N/A
N/A
0.50
N/A
N/A
1.60
3
0.77
1.96
1.29
1.58
1.26
2.38
1.05
4
5
6
7
8
0.69
0.74
c
0.52
0.84
c
9
a
b
c
d
Versus SCE, calibration is as discussed in the Experimental Section. Error in eabs around 10%. Not determined. No emission
(ff r 10ꢃ4). Error in Ff ꢁ 5%. Error in tf ꢁ 0.05 ns.
e
f
reversible first step upon oxidation. For triarylamines 3, 4 and
5 a second oxidation peak at more positive potential with no
cathodic counterpart in the reverse scan is observed indicating
that the oxidative process is associated with consecutive
chemical reactions of the initially formed radical cation. In
the case of 8 a cathodic redox process attributed to reaction
of the nitro substituent was noticed. In 7 a second quasi-
reversible oxidation process closely overlapped with the first
one, and can be attributed to the oxidation of the dimethyl-
amino group. The first oxidation potentials are presented in
Table 1. From the data, it can be seen that the increase of the
electron-releasing character of the substituent in the aryl
moiety shifts the first oxidation potential cathodically. This
effect, which has also been observed in diarylamines derived
from the dehydroabietic acid,16 is explained by the ability of
these substituents to inductively delocalise the charge in the
radical cation, thus improving its stability.
these compounds are summarised in Table 1. Typical absorp-
tion and emission spectra are shown in Fig. 2.
A more detailed study of the photophysical properties was
made in cyclohexane solution. The fluorescence quantum
yields were markedly higher than those of related dehydroabietic
Spectroscopic and photophysical studies
Absorption spectra of the seven triarylamines were recorded in
cyclohexane, acetonitrile and isopropanol, and showed a
lowest energy absorption band around 370 ꢁ 10 nm, which
was only slightly dependent on substituents and solvent. In
contrast, a strong solvent dependence was seen in the fluores-
cence spectra, with the most marked Stokes shifts occurring in
acetonitrile. In addition, significant substituent effects were
observed in both emission maxima and quantum yields, and in
the case of compound 8 and the derivative 9 without the
stilbene unit, no emission was observed in the polar solvents
acetonitrile and isopropanol. In the case of compounds
8 and 9, the absence of emission and the presence of the
electron-withdrawing NO2 group suggest that the lowest lying
singlet excited state is forbidden and n, p*, based, while both
the solvent and substituent effects suggest considerable charge
transfer character in the emitting level.12,17 Spectral data for
Fig. 2 Typical spectra: (A) absorption (left) and emission (right) of 5
in acetonitrile; (B) fluorescence spectrum of 3 in cyclohexane.
ꢀc
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acid based diarylamines in the same solvent.18 Although in
part this may be due to the presence of the strongly fluorescent
N-phenylaminostilbene group,12 the fact that compound 9,
lacking this moiety, also has a relatively high fluorescence yield
shows that this is not the only factor. The observation of
high luminescent yields is relevant to the potential of these
compounds either as probes or in light-emitting devices.
Fluorescence lifetimes were measured in this solvent, and
radiative lifetimes calculated using these and quantum yields.
They were also determined from the integral absorption and
emission band using the Strickler–Berg (SB) relationship
(eqn (1)):19,20
Table 3 Comparison of the low temperature fluorescence results in
methylcyclohexane glass with those in cyclohexane solution at room
temperature
Room temperature
Low temperature
lmax/nm
Ff
lmax/nm
Ff
Compound
3
4
5
6
7
8
9
404
418
400
411
410
483
495
0.34
0.43
0.39
0.35
0.50
0.57
0.50
408
418
403
413
416
484
510
0.67
0.75
0.72
0.95
0.80
0.68
0.93
ꢂ
ꢀ
ꢁ
ꢃ
Z
eðnaÞ
with room temperature measurements in Table 3. Slight red
shifts in emission maxima were observed, and were accompa-
nied by dramatic increases in fluorescence quantum yields,
which in some cases approached unity. We believe that this
is due to the increased rigidity in the glasses, which makes
non-radiative decay of the lowest stilbene–triarylamine excited
states more difficult. It is likely that the freedom of
rotation around the N-aromatic bonds is responsible for the
importance of the radiationless channel at 293 K (which has
been termed the loose bolt effect21), whereas at 77 K this
deactivation channel vanishes due to the rigidity imposed to
the system. Such effects of increased rigidity will also be
present in thin films used in electro-optic devices and are likely
to favour application of these materials in OLEDs.
ðtÞꢃ1 ¼ 2:88 ꢂ 10ꢃ9 n2½hnfꢃ3iꢄꢃ1
dn
ð1Þ
na
where n is the solvent refractive index; e is the molar absorp-
tion coefficient and the integration of this term is only
over the lowest energy absorption band; n is the frequency
of the transition in cmꢃ1. The term nfꢃ3 was determined from
the integral of the emission spectrum, in terms of corrected
intensities in units of relative numbers of quanta at each
frequency, divided by the integral of the emission spectrum
at nꢃ3 times this value. All data are summarised in Table 2.
The radiative lifetimes calculated from the Strickler–Berg
relationship are, in all cases, slightly lower than those obtained
from fluorescence quantum yields and lifetimes. The
long wavelength absorption band corresponds to various
overlapping electronic transitions, both from different
conformation and excited states, and a likely explanation for
the differences in experimental and calculated radiative life-
times is that the lowest energy (S0 - S1) transition is a weak
band buried in the more intense absorption around 370 nm,
such that the molar absorption coefficients used in the
Strickler–Berg calculation are too high and thus give an
underestimation of the radiative lifetime. This again suggests
the presence of a forbidden transition (likely to be of n, p*
origin) as the lowest lying transition in the case of compound
9, whereas with the other compounds a less intense, but still
allowed, transition (of p, p* origin in view of the tradiative(SB)
values in Table 2) is responsible for the S0 - S1 transition. As
we will see later, further clarification on the origin of the
nature of this lowest lying excited state can be drawn from
fluorescence anisotropy studies of the excitation spectra of
these compounds in a rigid medium.
Fluorescence anisotropy study
The anisotropy of the nitro-substituted triarylamine 9 was
studied in polar solvents of varying viscosity; it has already
been shown that its emission is quenched by polar solvents at
room temperature. The spectra of 9 were recorded at 77 K in a
mixture of methanol and ethanol (4 : 1 ratio) in order to
determine if the fluorescence is quenched at low temperature in
polar conditions. At 77 K quenching is not observed, and in
both cases the fluorescence quantum yield is high, with values
of 0.83 and 0.85 obtained for 9 and 8, respectively. This is most
likely to be due to the fact that at 77 K the compounds cannot
undergo molecular torsion, i.e. at 77 K emission is from a
Franck–Condon excited state to a planar ground state whereas
at room temperature deactivation is predominantly non-
radiative via a twisted excited state. A similar pattern has been
described for oligo and polythiophene derivatives.22,23
Fluorescence spectra were also measured in a methylcyclo-
hexane glass at liquid nitrogen temperature, and are compared
Compound 9 was dissolved in diisodecyl phthalate (DIDP),
which is a very viscous solvent.24 The anisotropy of 9 was
Table 2 Fluorescence quantum yields, lifetimes and radiative lifetimes in cyclohexane
Compound
Ffa
tf (ꢁ0.05)/ns
Radiative lifetimeb (ꢁ0.10)/ns
Calculated radiative lifetimec/ns
3
4
5
6
7
8
9
0.34
0.43
0.39
0.35
0.50
0.57
0.50
1.60
1.96
1.29
1.58
1.26
2.38
1.05
4.71
4.56
3.31
4.51
2.52
4.18
2.10
1.74
2.21
2.08
1.83
1.87
1.59
8.19
a
b
In cyclohexane solution with a-4-oligothiophene as reference. Calculated from quantum yield (Ff) and lifetime (texp) using trad = texp/Ff.
Calculated from the Strickler–Berg relationship.
c
ꢀc
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Table 4 Fluorescence anisotropy results for compound 9
Table 6 TGA results; thermal decomposition temperatures for all the
compounds
Sample
Solvent
Anisotropy value
Thermal decomposition
temperature (Td)/1C (ꢁ5 1C)
9a
9
9
9
9
9
Methylcyclohexane
DIDP
DIDP + 5 ml PEG
DIDP + 15 ml PEG
DIDP + 30 ml PEG
DIDP + 100 ml PEG
0.56
0.23
0.19
0.18
0.15
0.07
Compound
3
4
5
6
7
8
9
357
361
302
323
367
313
300
a
Carried out at 77 K, all other samples were at room temperature.
then monitored upon the addition of the polar and relatively
non-viscous polyethylene glycol 2000 (PEG).
point range arise due to the fact that the onset of melting
temperature is the value calculated from the extrapolation of
the intensity curves whereas the melting point range is the
observed value. The HSM shows that these compounds do not
crystallise readily; the compounds have also been kept in a
small sample for a time period greater than a year with no
crystallisation.
The observed value for methylcyclohexane is above the
theoretical maximum anisotropy (0.4), probably due to
problems with light scattering, and should be taken as showing
that there is no depolarisation during the fluorescence lifetime
of 9. The other results show, as expected, that frozen in a
‘glass’ or in a highly viscous solution 9 has a relatively high
anisotropy value. The anisotropy value then decreases with a
decrease in viscosity (Table 4). This is due to the fact that as
the viscosity decreases it becomes easier for it to undergo
molecular torsion (i.e. emission is from a twisted excited state
to a twisted ground state).
Thermogravimetric analysis (TGA) was also carried out on
all seven samples in order to determine the thermal properties
and in particular the degradation temperatures of the
compounds. A typical thermogram, for compound 7, is shown
as ESIw (Fig. SI 2). TGA showed that all compounds were
stable in nitrogen, with thermal degradation temperatures (Td)
equal to or greater than 300 1C (Table 6). It is important that
the thermal degradation temperatures are above 300 1C in
order to be well above operating temperatures of relevant
devices and also so that the compounds can be applied
using vacuum deposition techniques. Also a high thermal
degradation temperature means a wider temperature range
in which the compounds could be used, possibly leading to a
wider range of applications.
In summary both 9 and 8, which contain the strongly
electron-withdrawing –NO2 group, show unusual behaviour.
Emission changes significantly with solvent polarity; there is a
red shift with complete loss of structure of the emission band,
and a marked reduction in emission intensity as solvent
polarity is increased. This may be understood in terms of a
close lying twisted charge transfer state, the position of which
is solvent dependent. This is supported by the data from the
low temperature emission spectra as quenching does not occur
in a rigid glass at low temperatures due to the fact that the
compounds cannot undergo molecular torsion. This suggests
that room temperature deactivation in fluid solution is
predominantly non-radiative via a twisted CT excited state
to a twisted ground state, whereas emission in a rigid glass at
low temperatures or in a highly viscous environment is from a
planar excited state to a planar ground state.
During thermogravimetric analysis it was also found
that some of the compounds sublime. This was confirmed by
recording a UV/VIS spectrum of the sublimate and comparing it
to the known spectrum of that compound. Only two compounds
out of the seven (compounds 8 and 9) did not sublime.
Conclusions
The presence of a bulky dehydroabietic acid moiety in a series
of stilbene-based triarylamines has been shown to produce
thermally stable, soluble and vacuum sublimable materials
without adversely affecting their electrochemical, optical or
photophysical properties. These compounds can be used for
preparation of thin films which do not crystallise readily. In
addition, in some cases the presence of the bulky groups seems
to enhance their fluorescence emission properties. These
properties make the compounds attractive materials for
a variety of applications in molecular electronics. Their
potential applications as hole transport layers in LEDs are
being studied in detail and will be reported elsewhere.
Hot stage microscopy studies and thermogravimetric analysis
Hot stage microscopy (HSM) was carried out on all seven
samples as previously described.9 Table 5 shows the onset of
melting temperature and the melting point range of all the
compounds; this is also shown as ESIw (Fig. SI 1, which is the
graphical representation of these results). The differences
between the onset of melting temperature and the melting
Table 5 HSM results summary showing the onset temperature along
with the melting point range of all the compounds
Compound
Onset temperature/1C
Melting point range/1C
3
4
5
6
7
8
9
176.0
92.6
106.8
86.1
175.7
182.5
80.0
176–178
98–102
113–117
100–108
177–180
182–185
85–93
Experimental
Materials
Reagents were of the purest grade available, and generally
were used without further treatment. The stilbenes 1a–d, as
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well as the diarylamines 2a and 2b, were prepared as described
below. Tetrabutylammonium hexafluorophosphate (98%)
used for cyclic voltammetry (CV) was purified by recrystalli-
sation from ethanol and dried under vacuum prior to use.
Acetonitrile P.A. grade was distilled twice, first over CaH2 and
then over P2O5, under nitrogen. Other solvents were dried
according to standard methods.25
converter and multichannel analyser. Alternate measurements
(1000 counts per cycle), controlled by Decays software
(Biodinamica-Portugal), of the pulse profile at 337 or 356 nm
and the sample emission were performed until 1–2 ꢂ 104
counts at the maximum were reached.27 The fluorescence
decays were analysed using the modulating functions method
of Striker with automatic correction for the photomultiplier
‘‘wavelength shift.’’28 Thermogravimetric analysis was carried
out with a Rheometric Scientific Simultaneous Thermal
Analyser (STA). All samples were run in a nitrogen atmosphere
Equipment and methods
FTIR spectra were recorded on a Perkin-Elmer 1725 spectro-
meter. Fourier transform (FT) NMR spectra were run on
a General Electric QE-300 spectrometer with resonance
frequency of 300.65 MHz for 1H and 75.6 MHz for 13C, using
an appropriate solvent. The chemical shifts are reported in d
(ppm, TMS) and coupling constants in Hz. EI mass spectra
were determined on a Kratos MS 25RF instrument at 70 eV.
Microanalyses were performed on a Carlo Erba 1106R micro-
analyser. The electrochemical instrumentation for cyclic
voltammetry (CV) consisted of an EG&G Princeton Applied
Research Potentiostat Model 273A, connected to the data
acquisition software (EG&G PAR Electrochemical Analysis
Model 273 version 3.0). A three-electrode system was used,
with a platinum counter electrode, a platinum working
electrode and a potassium chloride saturated calomel reference
electrode (SCE). A 10ꢃ1 M solution of tetrabutylammonium
hexafluorophosphate in acetonitrile was used as the supporting
electrolyte. The solutions used in the cyclic voltammetry
studies contained 5 mM triarylamine in 0.1 M of supporting
electrolyte. Solutions were degassed and kept under nitrogen
throughout each experiment. Measurements were conducted
at 25 ꢁ 0.1 1C using a sweep rate of 50 mV sꢃ1. We used
ferrocene/ferrocinium (Fc/Fc+ = +0.41 V) to calibrate the
SCE. Thin layer chromatography (TLC) was performed on
silica gel 60 F254 plates with 0.2 mm layer thickness from
Macherey-Naguel, and the compounds visualised by illumin-
ation under UV light at 254 nm. Column chromatography was
carried out with Macherey-Naguel Si gel 60 (230–400 mesh).
Absorption spectral measurements were made on solutions
in 1 cm quartz cuvettes on a Shimadzu UV-2100 spectro-
photometer. Molar absorption coefficients were determined
from slopes of absorbance as a function of concentration
for a series of solutions for each compound at various
concentrations. Fluorescence spectra and quantum yields were
measured in 1 cm quartz cuvettes using 90 degree geometry on
a Jobin Yvon-Spex Fluorolog 3–22 instrument. Emission and
excitation spectra were corrected for variation in spectral
response of the light source (a 450 W xenon lamp), mono-
chromator and detector. Quantum yields were determined for
optically matched solutions from the ratio of integrated
corrected emission spectra.26 Tetrathiophene was used as
reference.22 Absorption and fluorescence spectra at 77 K were
run with a Perkin-Elmer Lambda 5 UV/VIS spectrophoto-
meter and a Jobin Yvon JY3D spectrofluorimeter, respectively.
Fluorescence decays were measured using a home-built
time-correlated single photon counting apparatus with an N2
filled IBH 5000 coaxial flashlamp as excitation source,
Jobin-Ivon monochromator, Philips XP2020Q photo-
multiplier and Canberra instruments time-to-amplitude
and were heated from 30–625 1C at a rate of 10 1C minꢃ1
.
Sample masses that were used were in the range of 0.5–1 mg;
all samples were run under identical conditions.
Synthesis
General procedure for the trans-4-bromostilbenes (1a–d)
trans-4-Bromostilbenes 1a–d used in this work were prepared
(Scheme 1) by a reaction previously described.12 A mixture of
bromobenzylbromide (4.00 mmol) and triethylphosphite
P(OEt)3 (4.80 mmol) was heated at 150 1C for 3 h under
argon. The solution was cooled and then 10 mL of anhydrous
DMF were added. The solution was placed in an ice bath for
15 min and then NaH (60% in mineral oil) (6 mmol) was
added. The mixture was stirred for 20 min and then the
appropriate benzaldehyde (4.00 mmol) was slowly added.
The mixture was stirred for 12 h before it was decanted into
ice water. Further purification was performed by recrystallisation.
With benzaldehyde (0.41 mL), trans-(4-bromostyryl)-
benzene 1a was afforded (0.88 g, 85%) as white crystals mp
132–135 1C (from CH2Cl2–MeOH) (lit.12 132–133 1C); dH
(300 MHz, CDCl3, Me4Si) 7.03 (1H, d, J 16.2), 7.11 (1H, d,
J 16.2), 7.27 (1H, t, J 6.0), 7.34–7.40 (4H, m), 7.47–7.53
(4H, m); dC (75 MHz, CDCl3, Me4Si) 121.3 (C-Br), 126.5
(2C); 127.4 (C-Olef), 127.9 (C), 127.9 (2C), 128.7 (2C), 129.4
(C-Olef), 131.7 (2C), 136.2 (C), 136.9 (C).
With 4-methoxybenzaldehyde (0.54 g), trans-4-(4-bromostyryl)-
methoxybenzene (1b) was afforded (0.92 g, 80%) as a white
solid mp 202–204 1C (from CH2Cl2–MeOH); (BrC15H13O
requires
C 62.30, H 4.53; found C 62.29, H 4.31);
nmax (KBr)/cmꢃ1 1604, 1513, 1254, 969; dH (300 MHz,
CDCl3, Me4Si) 3.83, (3H, s), 6.89 (1H, d, J 16.2), 6.90 (2H, d,
J 8.5), 7.04 (1H, d, J 16.2), 7.34 (2H, d, J 8.5), 7.42–7.47
(4H, m); dC (75 MHz, CDCl3, Me4Si) 55.3 (O-CH3) 114.1
(2C), 120.7 (C-Br), 125.2 (C-Olef), 127.7 (2C); 127.8 (2C),
128.9 (C-Olef), 129.7 (C9), 131.7 (2C), 136.6 (C), 159.5 (C);
m/z (EI) 290 (M+ 81Br, 97%), 289.0 (M+ 79Br, 100).
With 4-nitrobenzaldehyde (0.60 g), trans-4-(4-bromostyryl)-
nitrobenzene (1c) was obtained (0.96 g, 79%) as a white solid
mp 215–217 1C (from CH2Cl2–MeOH); (BrC14H10 NO2
requires C 55.29, H 3.31, N 4.61; found C 55.13, H 3.05,
N 4.86); nmax (KBr)/cmꢃ1 1592, 1506, 1340, 945; dH (300 MHz,
CDCl3, Me4Si) 7.12 (1H, d, J 16.2), 7.20 (1H, d, J 16.2), 7.41
(2H, d, J 9.0), 7.53 (2H, d, J 9.0), 7.63 (2H, d, J 9.0), 8.22
(2H, d, J 9.0); dC (75 MHz, CDCl3, Me4Si)122.2 (C-Br), 124.2
(2C), 126.9 (C-Olef and 3C); 128.4 (2C), 131.9 (C-Olef), 132.0
(2C), 135.1 (C), 143.7 (C), 146.9 (C); m/z (EI) 305 (M+ 81Br,
41%), 303 (M+ 79Br, 42), 178 (100).
ꢀc
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With 4-dimethylaminobenzaldehyde (0.60 g), trans-4-
(4-bromostyryl)dimethylaniline (1d) was afforded (0.80 g, 66%)
as a pale yellow solid mp 205–207 1C (from CH2Cl2–n-hexane),
(BrC16H16N requires C 63.56, H 5.34, N 4.63; found C 63.88,
H 5.08, N 4.54); nmax (KBr)/cmꢃ1 3448, 1605, 1351, 966; dH
(300 MHz, CDCl3, Me4Si) 2.99 (6H, s), 6.71 (2H, d, J 8.7),
6.83 (1H, d, J 16,2), 7.03 (1H, d, J 16,2), 7.26–7.45 (6H, m);
dC (75 MHz, CDCl3, Me4Si) 40.4 (N-(CH3)2), 112.4 (2 ꢂ
C-Ar), 120.1 (C-Br), 123.0 (C-Olef), 125.3 (C), 127.5 (2C);
127.7 (2C), 129.6 (C-Olef), 131.7 (2C), 137.2 (C), 150.3 (C);
m/z (EI) 303 (M+ 79Br, 96%), 301 (M+ 79Br, 100), 183 (44).
CH), 6.72 (1H, dd, J 8 and 2.1, CH), 6.83 (2H, m, 2 ꢂ CH), 7.02
(2H, m, 2 ꢂ CH). 7.09 (1H, d, J 8.2, CH); dC (75 MHz,
CDCl3, Me4Si) 16.4, 18.5, 21.6, 25.0, 30.1, 36.6 (2C), 38.1,
45.0, 47.6, 51.9, 55.5, 114.1, 114.6 (2C), 115.7, 121.5 (2C), 125.1,
136.0, 136.2, 141.4, 142.3, 154.8, 179.1; m/z (APCI) 392.7
[M + H]+.
General procedure for triarylamine synthesis. To a mixture of
diarylamine (0.4 mmol), trans-4-bromostilbenes (0.44 mmol)
and sodium tert-butoxide (0.48 mmol) in dry o-xylene (0.6 mL)
in a Schlenk tube under argon atmosphere were added
tri-tert-butylphosphine (0.04 mmol) and palladium acetate
(0.01 mmol). The mixture was stirred at 120 1C until the
complete consumption of amine (1–2 h) as judged by TLC.
The reaction mixture was then cooled to room temperature,
taken up in diethyl ether (5 mL) and washed with brine. The
organic phase was dried over anhydrous sodium sulfate,
filtered and concentrated under reduced pressure. The residue
obtained was purified by silica column chromatography using
mixtures of diethyl ether–petroleum ether (1.5 : 3.5 to 1 : 1) as
eluent.
Diarylamine synthesis
Methyl 13-(phenyl)aminodeisopropyldehydroabietate (2a).
Triphenylbismuth diacetate16 (0.85 g, 1.52 mmol) was added
in small portions with stirring to a mixture of methyl
13-aminodesisopropyldehydroabietate29 (0.40 g, 1.38 mmol)
and copper acetate (0.025 g, 0.138 mmol) in dry dichloro-
methane (15 mL). After 5 min the reaction mixture was poured
into a solution of 3 M hydrochloric acid, the organic phase
separated, washed with a 10% sodium bicarbonate solution
and brine and dried over anhydrous sodium sulfate. The
residue obtained after filtration and evaporation of the solvent
was purified by silica column chromatography using a mixture
of diethyl ether–petroleum ether (3.5 : 6.5) as eluent. The
product was isolated as a white foam (0.42 g, Z = 83%),
(C24H29NO2 requires C 79.30, H 8.04, N 3.85; found C 78.98,
H 8.24, N 3.72); nmax (KBr)/cmꢃ1 3382, 2928, 1723, 1598,
1498, 1248, 694; dH (300 MHz, CDCl3, Me4Si) 1.18
(3H, s, CH3), 1.27 (3H, s, CH3), 1.36–1.53 (2H, m, 2H,
2 ꢂ CH) 1.63–1.89 (5H, m, 2 ꢂ CH2 and CH), 2.23 (1H, dd,
J 12 and 2.1 Hz, CH), 2.27 (1H, brd, J 13, CH), 2.81–2.84
(2H, m, CH2), 3.66 (3H, s, CH3), 5.60 (1H, brs, NH), 6.75 (1H,
d, J 2.1, CH), 6.85–6.89 (2H, m, 2 ꢂ CH), 6.99–7.02 (2H, m,
2 ꢂ CH), 7.13 (1H, d, J 8.7, CH), 7.20–7.25 (2H, m, 2 ꢂ CH).
dC (75 MHz, CDCl3, Me4Si) 16.5, 18.6, 21.7, 25.2, 30.1, 36.7,
36.8, 38.1, 45.1, 47.7, 52.0, 116.3, 117.3, 118.2, 120.4, 125.2,
129.3, 136.1, 140.3, 142.9, 143.7, 179.2. MS (EI): m/z (%) 363
(M+, 39), 348 (33), 288 (55), 59 (100).
Methyl 13-[(trans-4-styryl-phenyl)-phenyl]aminodeisopropyl-
dehydroabietate (3). The general method was used with methyl
13-(phenyl)aminodeisopropyldehydroabietate 2a (0.15 g,
0.41 mmol) and trans-(4-bromostyryl)benzene 1a (0.12 g,
0.45 mmol) to afford 3 (0.15 g, 67%) as white crystals mp
175–176 1C (from diethyl ether–methanol); (C38H39NO2
requires C 84.25, H 7.26, N 2.59; found C 84.55, H 7.33,
N 2.52); nmax (KBr)/cmꢃ1 3448, 2933, 1715, 1592, 1494, 1250,
1174, 965, 752; dH (300 MHz, CDCl3, Me4Si) 1.23 (3H, s,
CH3), 1.28 (3H, s, CH3), 1.34–1.40 (1H, m, CH), 1.43–1.60
(1H, m, CH), 1.64–1.89 (5H, m, 2 ꢂ CH2 and CH), 2.23
(1H, dd, J 13 and 2.1, CH), 2.27 (1H, brd, J 13 Hz, CH),
2.75–2.80 (2H, m, CH2), 3.67 (3H, s, CH3), 6.77 (1H, d, J 2.4
Hz, CH), 6.86 (1H, dd, J 8.4 and 2.4, CH), 6.95–7.13 (9H, m,
9 ꢂ CH), 7.20–7.27 (2H, m, 2 ꢂ CH), 7.31–7.38 (4H, m,
4 ꢂ CH), 7.49 (2H, d, J 7.5, 2 ꢂ CH); dC (75 MHz, CDCl3,
Me4Si) 16.5, 18.6, 21.6, 25.1, 29.9, 36.7, 36.9, 38.0, 44.9, 47.6,
52.0, 122.4, 122.6, 123.2 (2C), 124.2 (2C), 124.7, 125.1, 126.3
(2C), 126.7, 127.2 (3C), 128.2, 128.6 (2C), 129.2 (2C), 131.0,
136.0, 137.7, 144.5, 144.8, 147.5, 147.6, 179.2; m/z (APCI)
541.7 [M + H]+.
Methyl
13-(4-methoxyphenyl)aminodeisopropyldehydro-
abietate (2b). To a solution of methyl 13-aminodesisopropylde-
hydroabietate29 (1 mmol, 0.29 g) and copper diacetate
(0.1 mmol, 18 mg) in dry dichloromethane (10 mL) was added,
Methyl 13-[(trans-4-styrylphenyl)-(4-methoxyphenyl)]amino-
deisopropyldehydroabietate (4). The general method was used
with methyl 13-(4-methoxyphenyl)-aminodeisopropylde-
hydroabietate 2b (0.16 g, 0.41 mmol) and trans-(4-bromostyryl)-
benzene 1a (0.12 g, 0.45 mmol) to afford 4 (0.16 g, 69%) as a
white powder mp 104–106 1C (from diethyl ether–methanol);
(C39H41NO3 requires C 81.93, H 7.23, N 2.45; found C 81.83,
H 7.24, N 2.44); nmax (KBr)/cmꢃ1 3448, 2926, 1723, 1594,
1505, 1243, 1178, 691; dH (300 MHz, CDCl3, Me4Si) 1.22
(3H, s, CH3), 1.28 (3H, s, CH3), 1.33–1.39 (1H, m, CH),
1.46–1.56 (1H, m, CH), 1.64–1.84 (5H, m, 2 ꢂ CH2 and
CH), 2.22 (1H, dd, J 13 and 1.8 Hz, CH), 2.26 (1H, brd,
J 13 Hz, CH), 2.74–2.79 (2H, m, CH2), 3.67 (3H, s, CH3), 3.81
(3H, s, CH3), 6.73 (1H, d, J 2.4, CH), 6.81–6.87 (3H, m,
3 ꢂ CH), 6.93–7.11 (7H, m, 7 ꢂ CH), 7.19–7.24 (1H, m, CH),
7.31–7.36 (4H, m, 4 ꢂ CH), 7.48 (2H, d, J 7.2, 2 ꢂ CH);
in
small
portions,
4-methoxyphenyllead
triacetate30
(1.1 mmol, 0.54 g). The mixture was stirred at room tempera-
ture, under N2, until the complete consumption of the amine
(TLC, 1 h). The reaction mixture was filtered through celite,
concentrated under reduced pressure and purified by silica
column chromatography (Et2O–petroleum ether, 1 : 1).The
product 2b (0.29 g, 74%) recrystallised as white needles mp
120–121 1C (from Et2O–petroleum ether); (C25H31NO3
requires C 79.30, H 8.04, N 3.85; found C 79.60, H 8.19,
N 3.58); nmax (KBr)/cmꢃ1 3398, 2933, 1723, 1608, 1510, 1238,
829; dH (300 MHz, CDCl3, Me4Si) 1.19 (3H, s, CH3), 1.26
(3H, s, CH3), 1.30–1.50 (2H, m, 2 ꢂ CH) 1.60–1.80 (5H, m,
2 ꢂ CH2 and CH), 2.21 (1H, dd, J 12 and 1.8, CH), 2.26
(1H, brd, J 12, CH), 2.78–2.84 (2H, m, CH2), 3.66 (3H, s, CH3),
3.78 (3H, s, CH3), 5.38 (1H, brs, NH), 6.59 (1H, d, J 2.1,
ꢀc
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dC (75 MHz, CDCl3, Me4Si) 16.5, 18.6, 21.6, 25.1, 30.0, 36.7,
36.9, 38.0, 44.9, 47.6, 52.0, 55.5, 114.7 (2C), 121.4, 121.8 (2C),
123.6, 125.0, 126.2 (2C), 126.3 (2C), 127.2 (2C), 127.3 (3C),
128.4, 128.6 (2C), 130.1, 135.9, 137.8, 140.5, 144.1, 144.8,
148.0, 156.1, 179.2; m/z (APCI) 572.0 [M + H]+.
(300 MHz, CDCl3, Me4Si) 1.22 (3H, s, CH3), 1.28 (3H, s, CH3),
1.35–1.40 (1H, m, CH), 1.42–1.56 (1H, m, CH), 1.64–1.85 (5H,
m, 2 ꢂ CH2 and CH), 2.23 (1H, d, J 13, CH), 2.27 (1H, brd, J
12, CH), 2.74–2.78 (2H, m, CH2), 2.98 (6H, s, 2 ꢂ CH3), 3.67
(3H, s, CH3), 6.71 (2H, d, J 8.4, 2 ꢂ CH) 6.76 (1H, d, J 2.4,
CH), 6.83–6.91 (3H, m, 3 ꢂ CH), 6.95–7.12 (6H, m, 6 ꢂ CH),
7.23 (2H, t, J 7.2, 2 ꢂ CH), 7.32–7.40 (4H, m, 4 ꢂ CH); dC
(75 MHz, CDCl3, Me4Si) 16.5, 18.6, 21.6, 25.1, 29.9, 36.7,
36.9, 38.0, 40.5, 44.9, 47.6, 52.0, 112.5 (2C), 122.1, 122.3 (2C),
123.8 (2C), 123.9, 124.0, 124.4, 125.0, 126.2, 126.7 (2C), 127.1,
127.3 (2C), 129.1 (2C), 132.3, 136.0, 144.5, 144.7, 146.6, 147.8,
149.9, 179.1; m/z (APCI) 585.2 [M + H]+.
Methyl 13-{[trans-4-(4-methoxystyryl)phenyl]-phenyl}amino-
deisopropyldehydroabietate (5). The general method was used
with methyl 13-(phenyl)aminodeisopropyldehydroabietate 2a
(0.12 g, 0.33 mmol) and trans-4-(4-bromostyryl)methoxy-
benzene 1b (0.11 g, 0.40 mmol) to afford 5 (0.12 g, 63%) as
white microcrystals mp 120–123 1C (from Et2O–petroleum
ether); (C39H41NO3 requires C 81.93, H 7.23, N 2.45; found C
81.70, H 7.40, N 2.47); nmax (KBr)/cmꢃ1 3448, 2930, 1720,
1511, 1493, 1250, 1174, 1037, 697; dH (300 MHz, CDCl3,
Me4Si) 1.22 (3H, s, CH3), 1.28 (3H, s, CH3), 1.34–1.40 (1H, m,
CH), 1.47–1.54 (1H, m, CH), 1.64–1.84 (5H, m, 2 ꢂ CH2
and CH), 2.22 (1H, dd, J = 12 and 2.1, CH), 2.27 (1H, brd,
J 13, CH), 2.74–2.80 (2H, m, CH2), 3.67 (3H, s, CH3), 3.82
(3H, s, CH3), 6.76 (1H, d, J 2.4), 6.83–6.95 (5H, m, 5 ꢂ CH),
6.96–7.05 (3H, m, 3 ꢂ CH), 7.06–7.14 (3H, t, J 8.7, 3 ꢂ CH),
7.2–7.27 (2H, m, 2 ꢂ CH), 7.34, (2H, d, J 8.7, 2 ꢂ CH), 7.42,
(2H, d, J 8.7, 2 ꢂ CH); dC (75 MHz, CDCl3, Me4Si) 16.5, 18.6,
21.6, 25.1, 29.9, 36.7, 36.9, 38.0, 44.9, 47.6, 52.0, 55.3, 114.1
(2C), 122.3, 122.5, 123.5 (2C), 124.1 (2C), 124.5, 125.1, 126.2,
126.4, 127.0 (2C), 127.4 (2C), 129.1 (2C), 130.5, 131.5,
136.0, 144.6, 144.7, 147.1, 147.7, 159.0, 179.1; m/z (APCI)
572.1 [M + H]+.
Methyl 13-{[trans-4-(4-nitrostyryl)phenyl]-phenyl}aminode-
isopropyldehydroabietate (8). The general method was used
with methyl 13-(phenyl)aminodeisopropyldehydroabietate 2a
(0.15 g, 0.41 mmol) and trans-4-(4-bromostyryl)-nitrobenzene
1d (0.14 g, 0.45 mmol) to afford 8 (0.18 g, 75%) as orange
plates mp 186–188 1C (from dichloromethane–n-hexane);
(C38H38N2O4: calculated C 77.79, H 6.53, N 4.77; found C
77.50, H 6.52, N 4.68); nmax (KBr)/cmꢃ1 3448, 2925, 1728,
1584, 1509, 1489, 1337, 1108, 698; dH (300 MHz, CDCl3,
Me4Si) 1.23 (3H, s, CH3), 1.28 (3H, s, CH3), 1.36–1.41 (1H, m,
CH), 1.45–1.58 (1H, m, CH), 1.65–1.85 (5H, m, 2 ꢂ CH2 and
CH), 2.23 (1H, dd, J 12 and 2.1 Hz, CH), 2.28 (1H, brd, J =
15, CH), 2.76–2.81 (2H, m, CH2), 3.68 (3H, s, CH3), 6.78
(1H, d, J 2.4, CH), 6.87 (1H, dd, J 9 and 2.4, CH), 7.01–7.18
(8H, m, 8 ꢂ CH), 7.23–7.30 (2H, m, 2 ꢂ CH), 7.38 (2H, d,
J 8.7, 2 ꢂ CH), 7.58 (2H, d, J 8.7, 2 ꢂ CH), 8.19 (2H, d, J 8.7,
2 ꢂ CH); dC (75 MHz, CDCl3, Me4Si) 16.5, 18.5, 21.6, 25.1,
29.9, 36.7, 37.0, 37.9, 44.9, 47.6, 52.0, 122.2 (2C), 122.8, 123.2,
123.8, 124.2 (2C), 124.8 (2C), 125.2, 125.3, 126.4 (2C), 127.9
(2C), 129.2, 129.3 (2C), 133.0, 136.2, 144.1, 144.4, 145.4, 146.1,
147.2, 148.7, 179.1; m/z (APCI) 587.1 [M + H]+.
Methyl 13-{[trans-4-(4-methoxystyryl)phenyl]-(4-methoxy-
phenyl)]}aminodeisopropyl-dehydroabietate (6). The general
method was used with methyl 13-(4-methoxyphenyl)amino-
deisopropyldehydroabietate 2b (0.11 g, 0.25 mmol) and trans-
4-(4-bromostyryl)-methoxybenzene 1b (0.08 g, 0.28 mmol) to
afford 6 (0.10 g, 77%) as a white solid mp 98–101 1C (from
Et2O–petroleum ether); (C40H43NO4: calculated C 79.84, H
7.20, N 2.33; found C 79.90, H 7.27, N 2.33); nmax (KBr)/cmꢃ1
3448, 2930, 1723, 1605, 1509, 1244, 1175, 1036, 830; dH
(300 MHz, CDCl3, Me4Si) 1.22 (3H, s, CH3), 1.27 (3H, s,
CH3), 1.33–1.39 (1H, m, CH), 1.47–1.54 (1H, m, CH),
1.66–1.83 (5H, m, 2 ꢂ CH2 and CH), 2.22 (1H, dd, J 12 and
2.1, CH), 2.27 (1H, brd, J 13, CH), 2.72–2.80 (2H, m, CH2),
3.67 (3H, s, CH3), 3.80 (3H, s, CH3), 3.82 (3H, s, CH3), 6.72
(1H, d, J 2.4, CH), 6.79–7.00 (11H, m, CH), 6.99–7–15 (2H, m,
CH), 7.31, (2H, d, J 8.7, CH), 7.42, (2H, d, J 8.7, CH); dC
(75 MHz, CDCl3, Me4Si) 16.5, 18.6, 21.6, 25.1, 29.9, 36.7,
36.9, 38.0, 44.9, 47.6, 51.9, 55.3, 55.5, 114.1 (2C), 114.7 (2C),
121.2, 122.0 (2C), 123.4, 125.0, 125.9, 126.3, 126.9 (2C), 127.2
(2C), 127.4 (2C), 130.6, 135.9, 140.6, 143.9, 144.9, 147.6, 156.0,
158.9, 170.1, 179.2; m/z (APCI) 602.4 [M + H]+.
Methyl 13-[(4-nitrophenyl)-phenyl]aminodeisopropyldehydro-
abietate (9). The general method was used with methyl
13-(phenyl)aminodeisopropyldehydroabietate 2a (0.15 g,
0.41 mmol) and 1-bromo-4-nitrobenzene (0.091 g, 0.45 mmol)
to afford 9 as a yellow foam (115 mg, 58%); (C30H32N2O4
requires C, 74.36, H 6.66, N 5.78; found C 74.32, H 6.65,
N 5.79); nmax (KBr)/cmꢃ1 3448, 2931, 1724, 1508, 1493, 1242,
696; dH (300 MHz, CDCl3, Me4Si) 1.24 (3H, s, CH3), 1.29
(3H, s, CH3), 1.31–1.39 (1H, m, CH), 1.44–1.56 (1H, m, CH),
1.63–1.83 (5H, m, CH2 and CH), 2.22 (1H, dd, J 12 and 2.1,
CH), 2.29 (1H, brd, J 12, CH), 2.80–2.88 (2H, m, CH2), 3.68
(3H, s, CH3), 6.83–6.94 (4H, m, CH), 67.16–7.26 (4H, m, CH),
7.33–7.38 (2H, t, CH), 8.00–8.03 (2H, d, J 9, CH); dC
(75 MHz, CDCl3, Me4Si) 16.5, 18.5, 21.5, 25.1, 29.8, 36.7,
37.2, 37.9, 45.0, 47.6, 52.0, 117.6 (2C), 124.0, 125.4 (2C), 125.6,
125.8 (3C), 126.5, 126.7, 129.8 (2C), 137.0, 139.7, 142.6, 145.6,
147.5, 153.7, 179.0; m/z (EI) 484 (M+, 100%), 485 (30),
386 (5).
Methyl 13-{[trans-4-(4-dimethylaminostyryl)phenyl]-phenyl}-
aminodeisopropyl-dehydroabietate (7). The general method was
used with methyl 13-(phenyl)amino-deisopropyldehydro-
abietate 2a (0.15 g, 0.41 mmol) and trans-4-(4-bromostyryl)-
dimethylaniline 1c (0.14 g, 0.45 mmol) to give 7 (0.14 g, 60%)
as a yellow powder mp 174–176 1C (from dichloromethane–
Acknowledgements
We thank POCI, FCT and FEDER for financial support
under contract POCI/QUI/58291/2004. SMF gratefully
acknowledges the award of a postdoctoral fellowship from
the FCT. We are also grateful to a referee for valuable
n-hexane); (C40H44N2O2 requires
C
82.15,
H
7.58,
N 4.79; found C 82.24, H 7.59, N 4.78); nmax (KBr)/cmꢃ1
3448, 2932, 1718, 1607, 1524, 1495, 1250, 1123, 696; dH
ꢀc
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884 | New J. Chem., 2009, 33, 877–885

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comments on the application of the Strickler–Berg procedure.
JP gratefully acknowledges a PhD grant (BD/18876/2004)
from the FCT.
12 J.-S. Yang, S.-Y. Chiou and K.-L. Liau, J. Am. Chem. Soc., 2002,
124, 2518; H.-C. Li, Y.-P. Lin, P.-T. Chou, Y.-M. Cheng and
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