Novel fluorinated amino-stilbenes and their solid-state photodimerization

Article abstract of DOI:10.1039/c0nj00264j

We synthesized a series of polyfluoro-amino-stilbenes in satisfactory yields by Wittig reaction of 4-piperazinyl-benzalhehydes with pentafluoro-benzylidene-triphenyl-phosphorane and we analyzed the photochemical behavior of these stilbenes in the solid st

Full text of DOI:10.1039/c0nj00264j

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Novel fluorinated amino-stilbenes and their solid-state photodimerizationw
Antonio Papagni,*a Paola Del Buttero,b Chiara Bertarelli,c Luciano Miozzo,ad
Massimo Moret,a Mary T. Prycee and Silvia Rizzatof
Received (in Montpellier, France) 7th April 2010, Accepted 28th June 2010
DOI: 10.1039/c0nj00264j
We synthesized a series of polyfluoro-amino-stilbenes in satisfactory yields by Wittig reaction of
4-piperazinyl-benzalhehydes with pentafluoro-benzylidene-triphenyl-phosphorane and we analyzed
the photochemical behavior of these stilbenes in the solid state; thanks to arene–perfluoroarene
p–p interactions, some of them have shown a good propensity to give the corresponding
cyclobutane photodimers in quantitative yields. The photocyclization is reversible both in
solution and in polymeric matrix, affording the corresponding stilbenes with different
cis–trans stereo-selectivity.
therefore these reactions are potentially very important for
Introduction
the synthesis of molecules with well-defined stereochemistry
The photochemical transformations of organic molecules in
(topochemical reaction). Arene–arene interactions play a
the solid state are known since the end of the 19th century.1
fundamental role in molecular recognition events, both in
Among these, the photochemical [2+2] reactions of olefins,
solution and in the solid state and their importance has been
affording the corresponding cyclobutanes, have been extensively
recognized in many areas, spanning from biology to materials
science.7 From this latter point of view, arene–perfluoroarene
investigated and today their mechanisms are well known. To
observe [2+2] photoreactions of olefins it is necessary to have
p–p interactions have gained interest as new supramolecular
double bonds parallelly arranged and with a distance between
synthons and have been exploited to promote stereoselective
them in the 3.5–4.2 A range.2 More recently, such reactions
topochemical reactions in the solid state. For instance single
have been exploited in materials science to produce new
crystals of 2,3,4,5,6-pentafluoro-stilbene have been irradiated to
photoresists3 or for the development of new solid-state optical
give the corresponding cyclobutane (a single diasteroisomer)
memories for high capacity storage.4 From the stereochemical
with high yield.8,9 In this case, the arene–perfluoroarene
point of view, in solution the photocyclization of olefins results
interactions control the alignment and the distance between
in a mixture of stereoisomeric cyclobutanes. This is further
the two reacting double bonds. Indeed, diffraction analysis
complicated by photochemical cis 2 trans isomerization
occurring in solution, which produces a mixture of reacting
pointed out that olefinic double bonds are parallel arranged
with an average distance of 3.704 A, falling in the 3.5–4.2 A
cis and trans stereoisomers, increasing the number of possible
range, therefore satisfying both the requisites to observe a
topochemical [2+2] photocyclization reaction.2
stereoisomers. As an example, the [2+2] photocyclization of
2-butene produces 4 diastereoisomeric cyclobutanes,5 but the
situation is even more complicated when asymmetric olefins
are reacting. A stereoselective photochemical [2+2] cyclo-
Results and discussion
addition can be performed in the solid state, where inter-
Few years ago, some of the authors prepared a series of
stilbenes bearing a pentafluoro-phenyl group; these systems
can work as ‘‘Push–Pull’’ systems10 for non-linear optics
applications (Scheme 1).
molecular interactions are responsible for the correct arrangement
of olefins.6 In this case the stereochemistry of the products is
determined by the crystal packing of the molecules and
a
`
Dipartimento di Scienza dei materiali, Universita degli Studi di
These molecules are structurally similar to the systems
studied by Coates et al., but the presence of dimethyl- or
diphenyl-amino groups makes the non-fluorinated aromatic
ring more electron-rich, and therefore it should enhance
arene–perfluoroarene interactions in the solid state, thus
facilitating the [2+2] photodimerization reaction. Starting
from this background, we planned both to study the photo-
reactivity of molecules 1 and 2 and a series of new 2,3,4,5-
pentafluoro-40-dialkylamino-stilbenes in order to investigate their
behavior in [2+2] photochemical reactions. In detail, we planned
the synthesis of new fluorinated amino-stilbenes, in order to test
the effect on the photoreactivity of bulkier substituents (such as
piperazine) and also to access more complex systems, such as
bis-stilbenes or photoreactive polymers. Such systems are the
starting point for new and more complex systems, such as
Milano Bicocca, via Cozzi 53, 20125 Milano, Italy.
E-mail: antonio.papagni@mater.unimib.it; Fax: +39 0264485400;
Tel: +39 0264485234
b
`
Dipartimento di Chimica Organica e Industriale, Universita degli
Studi di Milano, via Venezian 21, 20133 Milano, Italy
c Dipartimento di Chimica, Materiali e Ingegneria Chimica
‘‘G. Natta’’, Politecnico di Milano, P.zza L. da Vinci 32,
20133 Milano, Italy
d
´
ITODYS, Universite Paris Diderot-Paris 7, 15 rue Jean de Baıf,
¨
75205 Paris Cedex, France
e School of Chemical Sciences, Dublin City University, Dublin 9, Eire
f Dipartimento di Chimica Strutturale e Stereochimica Inorganica and
`
Facolta di Farmacia, Universita di Milano, via Venezian 21,
20133 Milano, Italy
`
w Electronic supplementary information (ESI) available: Copies of
1H, 13C 19F NMR spectra, copies of IR spectra of PMMA doped with
6b and 18b. CCDC 717576. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0nj00264j
ꢀc
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Table 1 Geometrical parameters for crystal of 1
A–B
B–C
y
0.01
97.51
69.41
3.726 A
0.5 A
1.30 A
0.01
108.91
87.81
3.693 A
1.22 A
0.13 A
y2
y3
d
D1
D2
The two crystallographically independent stacking inter-
actions along [001], described by the geometrical parameters
y1, y2, y3, D1 and D2 (Table. 1),12 are in accordance with the
highly efficient photodimerization, i.e. the relative arrangement
of adjacent reacting ethylenic moieties is quite favorable.
Indeed, photocyclization of 1 occurs under very mild irradiation
conditions and results in a quantitative yield for product 3
(Scheme 1). The stereochemistry of cyclobutane 3 on the basis
of 1H NMR data, the crystal structure of the parent molecule 1
and the topochemical rules for solid-state photoreactions
should correspond to the anti head-to-tail coupling of two
Scheme
1
General structure of 2,3,4,5-pentafluoro-40-amino-
stilbenes 1 and 2 (left) and their photodimerization in the solid state.
photo-polymerizable crystals and new cross-linkable polymers.
We prepared 2,3,4,5-pentafluoro-40-amino-stilbenes 1 and 2 by
Wittig reaction between a p-dialkyl-amino-benzaldehyde and
(2,3,4,5,6-pentafluoro-benzylidene)-triphenyl-phosphorane,
following the procedure reported in the literature.10 We
obtained polycrystalline powders of 1 and 2 by crystallization
from ethanol solution, but in the case of 1 we were able to
grow single crystals suitable for X-ray diffraction by slow
evaporation of ethanol solution. On the other side, in spite
of the large number of trials, all the attempts to grow single
crystals of 3 and of all the other compounds presented in this
paper, suitable for X-ray diffraction, were unsuccessful. The
crystal structure of 1, determined by means of single crystal
X-ray analysis, evidences nearly flat molecules (dihedral angle
between phenyl and pentafluoro-phenyl group 4.51) with a
trans arrangement of the phenyl and pentafluoro-phenyl
groups, in general referred as HT (Head-to-Tail) anti stereoisomer.
Molecules of 1 are stacked in columns parallel to [001] with a
head-to-tail arrangement dictated by perfluoroarene–arene
interactions (Fig. 1). According to the classification proposed
by Schmidt11 crystal packing of 1 is of the a-type with adjacent
molecular planes ca. 3.5 A apart. This feature in the crystal
structure of 1 is closely related to that of trans-pentafluoro-
stilbene8a for which the p-dimethyl-amino unit is missing.
Along the [001] columns there are two symmetry independent
stacking interactions with distances between centroids of
ethylenic moieties of 3.693 and 3.726 A (Table 1, Fig. 1).
2,3,4,5,6-pentafluoro-40-dimethylamino-stilbene
molecules.
1
In fact, the H NMR spectrum of cyclobutane shows only a
broad resonance for the hydrogen atoms, centered on 4.5 ppm;
this result proves that only one stereoisomer is present. On the
basis of the middle point method proposed by Ben-Efraim and
Green13 and according to the data reported by Coates et al. for
cyclobutanes obtained from the photodimerization of crystals
of pentafluoro-stilbene,8a the stereoisomer of 3 formed in
this reaction is that reported in Scheme 1, that is the HT
anti stereoisomer. The quantitative yield obtained for 1 is unusual
among photodimerization reactions, which normally are less
than 100% since the produced photodimers tend to isolate the
reactive molecules in the crystal matrix (for evenly spaced
olefin stacks the theoretical maximum yield is 86.5%).14
The different steric requirements for the parent and product
molecules often reduce the ability of the former to accommodate
product molecules as a stable solid solution quite early during
reaction. In general, the photoproduct can hardly be a
substitutional solute with significant concentration in the
parent crystal due to its high steric crowding. In 1, the
arrangement of CQC bonds makes crystals highly reactive
in photodimerization even with a very low power light source
(e.g. the light source of an optical microscope set at minimum
power). Within few minutes from the beginning of irradiation,
the crystals start to bend and crack under the mechanical
lattice strain promoted by photodimerization to the bulkier
cyclobutane derivative 3, eventually exploding. Observation
under the optical microscope revealed that the crystals crack
parallel to the {001} plane.
Compound 1 absorbs at 350 nm (the wavelength which
promotes the [2+2] cyclodimerization), while 3 is completely
transparent at this wavelength. When using a halogen lamp, at
the beginning the photodimerization of 1 occurs close to the
surface of the crystal, because the crystal is not completely
transparent at this wavelength and therefore the inner part of
the crystal cannot react. Later, during the photoreaction,
because the external layer formed by photocyclization is
transparent, the radiation can reach also the inner part of
Fig. 1 Crystal packing of 1 showing the distances between ethylenic
moieties of stacked molecules.
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the crystals, promoting the photoreaction. This feature
together with the favorable crystal packing is responsible for
the high conversion yield observed for this compound. Since
the photodimerization starts close to the surface, in order to
study its reactivity, we analyzed the surface topography of a
(010) crystal face under irradiation with an optical fiber lamp
by atomic force microscopy (AFM). During the early stages of
the photoreaction small holes on the terraces were observed
with concomitant appearance of spheroidal features, which
grew at the expenses of the surface material (Fig. 2a, b, g and h).
On continuing irradiation of the crystal, more dramatic
changes occurred, eventually leading to the formation of a
new crystalline phase on top of the reacting crystal, showing
no azimuthal orientation relative to the surface of the parent
crystal (Fig. 2c–e). Possible eroding effects due to the scanning
tip could be ruled out by observing that microcrystals of the
new phase grew even larger outside the scanned area (Fig. 2f).
The presence of the spheroids can be related to the enthalpy
released during the conversion of two CQC bonds to four
C–C bonds of a cyclobutane ring causing the formation of
product molecules in an initial amorphous phase (DH0 for gas
phase reaction of two molecules of ethylene to afford one
molecule of cyclobutane = ꢁ77.1 kJ molꢁ1 15) and showing a
high molecular mobility which allows post-crystallization in a
few minutes.
Scheme 2 Photostimulated retrocyclization of 3 in ethanol solution.
reversible; irradiation of an ethanol solution of 3 with
lexc = 254 nm results in a mixture of cis and trans isomers
of 1. These isomers arise from the two possible retrocyclization
pathways: the first (path a) yielding the starting trans-stilbene
1 and the second one (path b) affording the cis isomer
(Scheme 2). A useful feature emerging from this preliminary
study is that 1 and 3, having different absorption maxima,
exhibit both photodimerization and retrocyclization at
different wavelengths (l > 350 or l = 254 nm). Moreover,
compound 1 is also highly fluorescent, even in the solid state,
while compound 3 is not fluorescent.
While compound 1 underwent photodimerization in the
solid state affording quantitatively the corresponding cyclo-
butane derivative 3, we did not observe any photocyclization
for compound 2. We attribute the lack of reactivity in the solid
state to the steric hindrance of the phenyl groups, which keep
the stilbene cores separated, thus hindering the double bonds
from getting close enough to react.
Starting from these interesting results, we planned and
carried out the synthesis of new stilbene derivatives, starting
from p-fluoro-benzaldehyde, which was allowed to react in
DMSO with N-methyl- or N-BOC-piperazine in the presence
of K2CO3 at 100 1C, affording 4-piperazinyl-aldehydes 4a,b.
The [2+2]-photodimerization of olefins is, in principle, a
reversible reaction under photochemical conditions. Preliminary
studies have shown that our photodimerization reaction is
Fig. 2 1 - 3 Transformation observed with the AFM (scan size: a–b 20 mm; c–e 30 mm; f 45 mm. Elapsed time: 5 minutes between subsequent
images); g: zoom of frame b showing etch pits arising from the reaction of crystal surface of 1 to produce an amorphous phase of 3; h: cross-section
profile through the black line in g showing height and deepness of drop-like features and etch pits, respectively.
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compound was then transformed into the corresponding
phosphorane, by treatment with BuLi. The reaction of the
phosphorane with pentafluoro-benzaldehyde afforded the
desired compound 6a. Both benzyl alcohol 8 and triphenyl-
phosphonium bromide 9 have been never described so far in
the literature.
We investigated stilbenes 6a and 6b as starting materials to
access more functionalized substrates. In particular, we tested
the nucleofugacity of fluorine atoms of pentafluoro-phenyl
rings in nucleophilic aromatic substitution reactions with
piperazine or N-methyl-piperazine. As expected, under the
same conditions used in the preparation of compounds 4a–b,
we obtained the p-tetrafluoro substituted piperazine derivatives
10a and 10b in 63 and 29% yield, respectively. Both the
derivatives 7 and 10a react as nucleophiles with p-fluoro-
benzaldehyde affording the corresponding products of
N-arylation 11 and 12. The most ambitious target of this
strategy has been the synthesis of bis-stilbenes. Stilbene 12
was reacted with pentafluoro-benzylidene-phosphorane 5,
affording the desired, head-to-tail, bis-stilbene 13 (Scheme 4).
A similar approach was applied to prepare the analogous
head-to-head bis-stilbene. Stilbene 11 was reacted with the
phosphorane 5 under standard Wittig conditions but the
formyl-phenyl derivative was found totally unreactive.
In order to prepare this product, we explored also an alternative
route. N,N0-Bis(p-formyl-phenyl)-piperazine 14 was prepared as
described in the literature starting from p-fluoro-benzaldehyde
and piperazine in the presence of K2CO3 and then reduced to
the corresponding alcohol 15 by NaBH4. Alcohol 15 was then
transformed in one step into the corresponding phosphonium
salt 16 by reaction with HPPh3Br. Compound 16 was treated
with BuLi to prepare the corresponding phosphorane and then
Scheme 3 Synthesis of piperazinyl-based pentafluoro stilbenes.
We transformed the aldehydes 4, through Wittig reactions with
the pentafluoro-benzylidene-phosphorane 5, into the corres-
ponding trans pentafluoro-stilbenes 6a,b, collected, respectively,
in 60 and 71% yields. Treatment of 6b with trifluoro-acetic
acid afforded the corresponding piperazine derivative 7 in 90%
yield. The synthetic procedure is summarized in Scheme 3.
Compound 6a was prepared also following an alternative
procedure. p-(4-Methyl-piperazinyl)-benzyl alcohol (8) (prepared
by reduction of the corresponding aldehyde 4a with NaBH4)
was reacted with triphenyl-phosphonium bromide (HPPh3Br),
affording the corresponding phosphonium salt (9). This latter
Scheme 4 Reactions of piperazinyl-pentafluoro-stilbenes.
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reacted with pentafluoro-benzaldehyde, but it was not possible
to isolate the desired product. The outcome of this reaction
was an almost insoluble, fluorescent matter. The hypothesis of
uncontrolled photo-polymerization of the product, resulting
into an insoluble polymer, in principle should be ruled out,
because the reaction was carefully carried out in the dark and
the work up done in the presence of Erythrosine-B (a quencher
for triplet states, involved in the photodimerization) or with
Nafion-Hs, to protonate the piperazine nitrogen atoms and
hindering therefore the [2+2] reaction. The low solubility of
(see Experimental section for details) in order to determine
quickly the substrates with solid state reactivity comparable to
that of compound 1. We transformed compounds 6a, 6b and 7
into the corresponding cyclobutane derivatives 18a, 18b and
18c in quantitative yield. The 1H NMR analysis showed for all
the cyclobutanes a signal system for the hydrogen atoms of
cyclobutanic ring centered around 4.5 ppm, and, on the basis
of the middle point method proposed, we assigned the cyclo-
butanes 18a–c the same configuration as that of 3. The
stilbenes 10a, 10b, 11, 12, and 13 did not give any cyclization
product, even after a very long exposure. These findings clearly
suggest that the presence of bulky substituents attached to the
stilbene double bond hinders the reactivity of these derivatives,
probably because it increases the distance between double
bonds involved in the formation of the cyclobutane ring.
These results are in line with the observed photochemical
inertness found for compound 2. These data support the idea
that there is a limit to the dimension and nature of the
substituents present on the stilbene core, which are compatible
with the photocyclization, because bulky substituents can
destroy the arene–perfluoroarene interactions, responsible
for solid-state photocyclization. We fully characterized all
new cyclobutane derivatives by 1H and 19F NMR, mass
spectroscopy and melting point analysis. All cyclobutanes
show UV-Visible absorption bands below 300 nm and are
non-fluorescent.
the products hindered
a
full characterization; mass
spectroscopy, IR spectroscopy and elemental analysis are in
agreement with the presence of the desired product, but
NMR analysis was not possible, due to the low solubility.
We explored also alternative strategies to prepare this
bis-stilbene. In a first experiment, the bis-aldehyde 14 was reacted
with the phosphorane 5 under standard Wittig conditions, but
we obtained the same results observed with the phosphorane.
The alcohol 15 was transformed into the bis-phosphonate 17
by reaction with triethyl-phosphite, in the presence of
iodine, but compound 17 was found totally unreactive under
Wittig–Horner conditions (Scheme 5).
The reactions described so far underline the versatility of
pentafluoro-stilbenes towards further functionalization and
they widen the potential applications of these compounds.
Indeed all the reactions we performed can be exploited as
possible strategies for linking these molecules to polymeric
matrixes, in particular compound 7 could be used as a photo-
reactive unit in side-chain functionalized polymers. On the
other side, the preparations of bis-stilbenes showed a puzzling
situation: the head-to-tail dimer 13 was easily accessible and
we were able to carry out a complete analysis confirming its
structure, while it was not possible to isolate and fully
characterize the head-to-head dimer, because of the low
solubility of the products of the reaction. All the products
have been characterized in solution by 1H and 19F NMR,
UV-Visible absorption and fluorescence spectroscopy and
mass spectroscopy. All the stilbenes are highly fluorescent
and present fluorescence maxima between 430 and 450 nm.
All stilbene derivatives described so far (6a, 6b, 7, 10a, 10b,
11, 12, and 13) were exposed to a low power source lamp
We decided to investigate the retro-cyclization of these
cyclobutanes to their parent stilbenes, both by thermal
treatment and by UV irradiation (254 nm). We carried out
the thermal treatment by heating a toluene solution of 18b at
160 1C in a closed microwave reactor (power 180 W). After 1 h
of irradiation, the cyclobutane 18b was recovered quantitatively
with no traces of stilbene 6b detected, even by spectrofluorimetry,
which underlines the high thermal stability of 18b. As
anticipated, the photocyclization reaction is reversible in
solution by irradiation with UV radiation (254 nm). Thus,
an ethanol solution of 18b (3 mg in 10 mL of ethanol) has been
irradiated for 1 h and the crude reaction mixture analyzed by
1H-NMR. The analysis showed that the retro-photocyclization
occurs with a fairly good yield (6b: yield = 63%. 6c: yield = 57%)
but again gave a mixture of both trans and cis isomers with
10 : 1 ratio respectively (Scheme 6).
The prevalence of the trans isomer stimulated us to
investigate whether the retro-photocyclization could occur in
a polymeric matrix and whether the polymeric matrix can
influence the stereoselectivity of the retro-photocyclization,
since this possibility is of considerable importance in the
development of new optical memories. We preferred to blend
the cyclobutane derivative instead of the stilbene into the
polymer because in this way the retro-cyclization should
produce ‘couples’ of stilbene molecules able to give again the
[2+2] photoreaction, while a blend of the stilbene could
produce a random distribution of the isolated molecules,
hampering the photocyclization. For this investigation we
chose poly-methyl-methacrylate (PMMA), because of its high
optical quality. We irradiated casted films, prepared from a
CH2Cl2 solution of PMMA doped with 18b (5% weight) with
UV radiation (254 nm) produced by low-pressure mercury
vapor lamp (15 W). We monitored the progress of the reaction
Scheme 5 Attempts to synthesize head-to-head bis-stilbene.
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Scheme 6 Photocyclization and cycloreversion of photoreactive stilbenes.
by UV-Visible absorption (Fig. 3) and the results demonstrated
that a complete retro-photodimerization is obtained after 1 h
of irradiation as shown by the disappearance of the band at
260 nm (of the cyclobutane derivative, curve 1 in Fig. 3) and
the appearance of a new band at 340 nm (due to the stilbene
absorption, curve 2) as confirmed by comparing this absorption
band with that of a cast film of PMMA doped with 6b,
prepared in the same way. In addition, the film becomes
fluorescent under 365 nm light and this evidence indicates a
successful retro-photodimerization. These results confirm that
fluorinated stilbenes can be exploited as active materials for
new optical memories. We also tried to determine how many
cycles of photodimerization and retro-cyclization could be
performed. Thus, on the same sample, we attempted exposure
cycles at 365 nm (to transform the stilbene into the cyclobutane)
and at 254 nm (to open the cyclobutane). After 20 h of
exposure with lexc = 365 nm (4 W) we observed a reduction
of the absorption in the UV-Visible spectrum but not the
complete disappearance of absorption band centered at
330 nm (due to the stilbene absorption), therefore suggesting
that only a fraction of stilbene was converted into cyclobutane.
After that, we carried out irradiation with lexc = 254 nm, to open
the cyclobutane, but after 5 h we did not find any appreciable
variation in the absorption spectra (curve 3 in Fig. 3).
We attribute the origin of these results to preferential formation
of the cis isomer during the photolysis reaction, since the
TLC analysis of the stilbene extracted from the polymeric
matrix showed a higher amount of the cis isomer (50% vs. 10%
observed in alcoholic solution). Also the blue shift of the
stilbene absorption maximum in the UV-Visible absorption
confirmed the presence of a higher amount of cis isomer.
During the retro [2+2] both cis and trans isomers have
equal probability and a 1 : 1 mixture of two isomers is
expected. In solution the molecules can move around freely
and the cis isomer can be converted into the more stable
trans one and photostationary state is generally enriched with
trans-isomer. Within the polymeric matrix, the limited mobility
of the molecules hinders the cis–trans isomerization and
therefore this completely explains our findings. This result
can have a positive outcome, because the retro-cyclization of
cyclobutanes imbedded in polymeric matrixes can be viewed as
a mean of obtaining a mixture of olefins enriched in the
cis isomer, while usually the trans isomer is prevalent. Finally,
we carried out IR reflectance measurements (see ESIw for a
caption of the spectra) on films of PMMA doped with stilbene
6b and cyclobutane 18b (8% in weight for both of them).
Unfortunately the region of double bond bending (trans double
bond shows a strong band around 900 cmꢁ1) is partially
covered by the stretching and bending modes of the polymeric
Fig. 3 Absorption spectra of 18b in PMMA matrix. The different
spectra show the conversion of 18b into the corresponding stilbene,
compared with a spectrum of 6b in PMMA matrix.
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matrix and cannot be diagnostic for the analysis of the
photodimerization process. However, the PMMA is completely
transparent around 1600 cmꢁ1, a region where PMMA doped
by stilbene 6b and that doped with the cyclobutane 18b show
significant differences. In fact, the PMMA doped with stilbene
6b shows a peak at 1604 cmꢁ1, while the polymer doped with
18b shows a very weak peak at 1612 cmꢁ1; these signals arise
from the stretching vibrations of the CQC bonds of olefins
and aromatic rings. The absorption at 1604 cmꢁ1 is probably
due to stretching of CQC olefinic bond and aromatic double
bonds of stilbene, while the peak at 1612 cmꢁ1 detected for
PMMA doped with cyclobutane is only due to the stretching
of aromatic rings. In any case both these signals are clearly
distinguishable from those of the polymeric matrix and can be
used as possible readout region for an optical memory
functions of these compounds. Other parts of the IR spectrum
cannot be used to discriminate stilbene-doped PMMA from
cyclobutane-doped.
(254 nm and 365 nm); solid and solution (within quartz
cuvettes) samples were held at 10 cm from the bulbs.
Compounds 1, 2, 4a, b, 5, 14 were prepared as described in
the literature. nBuLi (1.6 M hexane solution) was purchased
from Merck Co. and titrated just before use. Tetrahydrofuran
(THF) was dried by refluxing in the presence of
Na/benzophenone. All manipulations were performed under
an inert gas (nitrogen). Flash chromatography was performed
with silica gel 230–400 mesh.
Single crystal X-ray data were measured at room temperature
(298 K) on a Bruker SMART CCD diffractometer using
Mo-Ka radiation, l = 0.71073 A. The structure was solved
by direct methods and refined by full-matrix least-squares
against Fo2 using SHELXL97 software.16 All non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms attached to carbon were placed in geometrically
idealized positions and refined using a riding model.w
AFM imaging was performed with a Nanoscope IIIa AFM
(VEECO) in contact mode with oxide sharpened Si3N4 tips of
nominal 0.06 N mꢁ1 force constant. Force applied to the
crystal surface by the tip was minimized to reduce influence of
scanning on steps. Scan rates were in the range 3–4 Hz. All
images are unfiltered. The photoreaction was triggered by an
optical fiber halogen lamp filtered with a white screen to reduce
the amount of photons reaching the sample surface in order to be
able to follow the surface reaction with the AFM.
Conclusions
In conclusion, we have synthesized a series of new polyfluoro-
amino-stilbenes and analyzed their photochemical behavior in
the solid state. A number of these compounds showed a good
propensity to give the corresponding cyclobutane photodimers
in quantitative yields. Arene–perfluoroarene p–p interactions
assure the correct position of the olefinic double bonds,
necessary to observe [2+2] cyclization. The photocyclization
is reversible both in solution and when the cyclobutane is
Syntheses
Synthesis of compounds 6a and 6b. 0.5 mmol of the aldehyde
(4a or 4b) were added at 0 1C, under an inert gas, to a yellow
THF solution (10 mL) of the phosphorane 5 (0.5 mmol
prepared as described in the literature). The mixture was
allowed to react at room temperature and the progress of
the reaction was monitored by TLC (eluent: diethyl ether/light
petroleum ether, 3 : 7). The reaction was carried out in the
dark. The reaction was complete after 15 h and then was
quenched by addition of few drops of water. The solvent was
distilled under reduced pressure, affording a bright yellow solid.
The solid was taken up with CH2Cl2 (20 mL), and the organic
layer was washed twice with distilled water. The organic phase
was dried with Na2SO4, and the solvent was evaporated under
reduced pressure. The crude reaction mixture was purified by dry
flash chromatography (eluent: light petroleum ether/ethyl acetate
3 : 7) affording the desired product.
dispersed in
a polymeric matrix. A different cis–trans
stereoselectivity is observed for the stilbenes obtained from
retro-cyclization in solution and in polymeric matrix with a
preference for the cis isomer in the polymeric matrix. Even
though the formation of cis isomer limits the number of
open–closure cycles, the results presented here confirm that
fluorinated stilbenes and/or cyclobutanes are potential systems
for at least non-rewritable optical data storage devices. The
stability of the cyclobutane under heating represents another
advantage for such an application. Moreover, the possibility
to obtain the cis isomer from the ring opening in a polymeric
matrix is an interesting result, since usually the photolysis of
cyclobutanes in solution affords solely the trans isomer. Thus
the incorporation of cyclobutanes in polymeric matrices can
be a useful method to drive the retro-cyclization in producing
the cis isomers, since this result is hard to obtain in solution,
where, for steric and stability reasons, the trans isomer is
always the preferred one.
Compound 6a. Yellow greenish solid. Reaction yield: 60%.
Mp 175 1C. IR (nujol): n (cmꢁ1) = 1603 (n, HCQCH trans),
1350.5 (n, Carom-N), 957 (out of plane deformation C-H trans).
UV-Vis (CH2Cl2): lmax = 355 nm. Fluorescence (CH2Cl2):
lmax = 455 nm. 1H NMR (300 MHz, CDCl3): d = 2.36
(s, 3H, CH3), 2.57 (t, 4H, CH2 of piperazine), 3.28 (t, 4H, CH2
of piperazine), 6.80 (d, 1H, C6F5-CHQCH-Ar), 6.91
(d, 2H, aromatic ring), 7.29 (d, 1H, C6F5-CHQCH-Ar), 7.43
(d, 2H, aromatic ring). 19F NMR (75 MHz, CDCl3):
d = ꢁ143.86 (m, 2F, Fortho), ꢁ158.56 (m, 1F, Fpara),
ꢁ163.89 (m, 2F, Fmeta). MS(EI): m/z = 368 [C19H17F5N2]+.
Elemental analysis for C19H17F5N2: calcd C 61.95, H 4.65,
N 7.61%; found C 61.89, H 4.35, N 7.57%.
Experimental section
Methods and materials
Melting points were determined with a melting point apparatus
1
in open tubes and are uncorrected. H NMR and 19F NMR
spectra were collected at rt, in CDCl3. Chemical shifts are
given as parts per million from tetramethylsilane and J values
are given in Hertz. 19F NMR spectra were collected using
CFCl3 as internal standard. Photochemical reactions were
carried out with a 5 W lamp, equipped with two bulbs
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Compound 6b. Yellow greenish solid. Reaction yield: 71%.
Mp 171 1C. IR (nujol): n (cmꢁ1) = 1696.5 (n, CQO), 1605
(n, HCQCH trans), 1370 (n, Carom-N), 957 (out of plane
deformation C-H trans). UV-Vis (CH2Cl2): lmax = 328 nm.
Fluorescence (CH2Cl2): lmax = 446 nm. 1H NMR (300 MHz,
CDCl3): d = 1.51 (s, 9H, C-CH3), 3.25 (t, 4H, CH2 of
piperazine), 3.64 (t, 4H, CH2 of piperazine), 6.85
(d, 1H, C6F5-CHQCH-Ar), 6.97 (d, 2H, aromatic ring), 7.32
(d, 1H, C6F5-CHQCH-Ar), 7.48 (d, 2H, aromatic ring).
19F NMR (75 MHz, CDCl3): d = ꢁ143.73 (d, 2F, Fortho),
ꢁ158.14 (t, 1F, Fpara), ꢁ163.69 (t, 2F, Fmeta). MS(EI): m/z = 454
[C23H23F5N2O2]+. Elemental analysis for C23H23F5N2O2:
calcd C 60.79, H 5.10, N 6.16%; found C 60.75, H 5.01, N 6.12%.
washed with water. The organic phase was dried with Na2SO4,
and the solvent was evaporated under reduced pressure. The
crude product was then purified by column chromatography
(eluent: CH2Cl2/MeOH 9 : 1), affording the desired product
(yield: 38%).
Compound 7. A solution of 0.5 mL of CF3COOH in
anhydrous CH2Cl2 (0.5 mL) was added to a solution of 6b
(0.3 mmol) in anhydrous CH2Cl2 (0.5 mL). The solution was
stirred for 5 h at room temperature in the dark, then was
quenched with an aqueous saturated solution of NaHCO3.
The aqueous layer was then extracted with CH2Cl2, the
organic phase dried with Na2SO4, and the solvent evaporated
under reduced pressure. The crude solid was purified by
column chromatography (eluent: light petroleum ether/ethyl
acetate 3 : 7) affording the desired product. Pale yellow solid.
Reaction yield 65%. Mp 168–169 1C. IR (nujol): n (cmꢁ1) = 3436
(n, N-H), 1603 (n, HCQCH trans), 1377 (n, Carom-N), 960
(out of plane deformation C-H trans). UV-Vis (CH2Cl2):
lmax = 319 nm. Fluorescence (CH2Cl2): lmax = 445 nm.
1H NMR (300 MHz, CDCl3): d = 2.81 (t, 4H, CH2 of
piperazine), 3.13 (t, 4H, CH2 of piperazine), 6.82
(d, 1H, C6F5-CHQCH-Ar), 6.94 (d, 2H, aromatic ring), 7.31
(d, 1H, C6F5-CHQCH-Ar), 7.51 (d, 2H, aromatic ring).
19F NMR (75 MHz, CDCl3): d = ꢁ143.62 (d, 2F, Fortho),
ꢁ157.04 (t, 1F, Fpara), ꢁ163.87 (m, 2F, Fmeta). Elemental
analysis for C18H15F5N2: calcd C 61.02, H 4.27, N 7.91%;
found C 60.75, H 4.15, N 7.85%.
Alternative synthesis for compound 6a. (a) Synthesis of
p-(4-methyl-piperazinyl)-benzyl-alcohol (compound 8): 1.554 g
of p-(4-methyl-piperazinyl)-benzaldehyde (4a) were dissolved
in 25 mL of EtOH and 336 mg of NaBH4 were added,
keeping the temperature in the range 18–20 1C, then the
solution was heated at 40 1C for 1 h. The solution was then
neutralized and extracted with CH2Cl2. The organic phase was
dried with Na2SO4, and the solvent was evaporated under
reduced pressure, affording the desired alcohol (pale yellow solid).
Yield: 73%. Mp 102–103 1C. IR (nujol): n (cmꢁ1) = 3186
broad (n, OH). 1H NMR (300 MHz, CDCl3): d = 2.19
(broad, 1H, OH), 2.35 (s, 3H, CH3), 2.57 (t, 4H, J = 5.1,
CH2 of piperazine), 3.18 (t, 4H, J = 5.1 Hz, CH2 of piperazine),
6.91 (d, 4H, J = 8.5, aromatic ring), 7.27 (d, 4H, J = 8.5,
aromatic ring). MS(EI): m/z = 206 [C12H18N2O]+. Elemental
analysis for C12H18N2O: calcd C 69.87, H 8.80, N 13.58%;
found C 69.01, H 8.50, N 13.0%. (b) Synthesis of p-(4-methyl-
piperazinyl)-benzyl-triphenyl-phosphonium bromide (compound 9):
711 mg of p-(4-methyl-piperazinyl)-benzyl alcohol (8) and
3.847 g of triphenyl-phosphonium bromide were suspended
in anhydrous CH2Cl2. The mixture was refluxed for 5 h, then
the solvent was removed under reduced pressure. The yellow
solid was dissolved in water and then extracted with CH2Cl2.
The organic phase was dried with Na2SO4, and the solvent was
evaporated under reduced pressure, affording the desired
product (white solid). Yield: 67%. IR (nujol): n (cmꢁ1) =
1342 broad (n, Carom-N), 1516 (n, CH2-P), 1436 (n, P-C6H5),
1608 (n, CQC). 1H NMR (300 MHz, CDCl3): d = 2.37
(s, 3H, CH3), 2.57 (m, 4H, CH2 of piperazine), 3.15
(m, 4H, CH2 of piperazine), 5.25 (d, 2H, J = 13.5, CH2-PPh3),
6.69 (d, 2H, J = 8.5, aromatic ring), 6.97 (d, 2H, J = 8.5,
aromatic ring), 7.64–7.78 (m, 15 H, -P(C6H5)3). 31P NMR
(300 MHz, CDCl3): d = 23.71 (s, -PPh3). Elemental analysis
for C30H32BrN2P: calcd C 67.80, H 6.07, N 5.27%; found C
67.50, H 6.10, N 5.10%. (c) Synthesis of 2,3,4,5,6-pentafluoro-
40-(4-methyl-piperazinyl)-stilbene (compound 6a): 287 mg of
p-(4-methyl-piperazinyl)-benzyl-triphenyl-phosphonium bromide
(9) were suspended in 4 mL of anhydrous THF; 0.58 mL of
n-BuLi (1.194 M) were added dropwise, keeping the temperature
at 0 1C, obtaining a dark red solution. The solution was stirred
for 30 min at rt, then 114 mg of pentafluoro-benzaldehyde
were added. The mixture was allowed to react for 3 h
in the dark, the reaction was quenched with few drops of
water and the solvent evaporated under reduced pressure.
The yellow crude solid was then taken up with CH2Cl2 and
Synthesis of compounds 10a and 10b. To a solution of
stilbene 6b (0.1 mmol) and the proper piperidine (0.15 mmol)
in anhydrous DMSO (13 mL) was added Na2CO3 (0.15 mmol).
The solution was heated at 90 1C for 33 h in the dark, then was
added water and the resulting solution was extracted with
ethyl acetate. The organic phase was dried with Na2SO4, and
the solvent was evaporated under reduced pressure. The crude
reaction mixture was purified by dry flash chromatography
(eluent: light petroleum ether/ethyl acetate 1 : 1 and then
methanol/ethyl acetate 1 : 1) affording the desired product.
Compound 10a. Yellow solid. Reaction yield: 63%.
Mp 165–166 1C. UV-Vis (CH2Cl2): lmax = 346 nm. Fluores-
cence (CH2Cl2): lmax = 438 nm. 1H NMR (300 MHz, CDCl3):
d = 1.52 (s, 9H, C-CH3), 2.31 (broad, 1H, N-H), 3.08 (s, 4H,
CH2 of methyl-piperazine), 3.24 (t, 4H, CH2 of BOC-piperazine),
3.32 (s, 4H, CH2 of methyl-piperazine), 3.63 (t, 4H, CH2 of
BOC-piperazine), 6.90 (d, 1H, C6F5-CHQCH-Ar), 6.94
(d, 2H, aromatic ring), 7.36 (d, 1H, C6F5-CHQCH-Ar), 7.48
(d, 2H, aromatic ring). Elemental analysis for C27H32F4N4O2:
calcd C 62.30, H 6.20, N 10.76%; found C 62.25, H 6.22,
N 10.68%.
Compound 10b. Yellow solid. Reaction yield: 29%. Mp
172–174 1C. UV-Vis (CH2Cl2): lmax = 347 nm. Fluorescence
(CH2Cl2): lmax = 436 nm. 1H NMR (300 MHz, CDCl3):
d = 1.52 (s, 9H, C-CH3), 2.50 (s, 3H, N-CH3), 2.74 (s, 4H, CH2
of methyl-piperazine), 3.24 (t, 4H, CH2 of BOC-piperazine),
3.44 (s, 4H, CH2 of methyl-piperazine), 3.63 (t, 4H, CH2
of BOC-piperazine), 6.86 (d, 1H, C6F5-CHQCH-Ar), 6.94
(d, 2H, aromatic ring), 7.36 (d, 1H, C6F5-CHQCH-Ar), 7.48
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(d, 2H, aromatic ring). Elemental analysis for C28H34F4N4O2:
calcd C 62.91, H 6.41, N 10.48%; found C 62.56, H 6.35,
N 10.44%.
Na2SO4, and the solvent was evaporated under reduced
pressure. The crude reaction mixture was purified by column
chromatography (eluent: CH2Cl2/ethyl acetate 95 : 5) affording
the desired product. Yellow solid. Reaction yield: 50%. UV-Vis
(CH2Cl2): lmax = 338 nm. Fluorescence (CH2Cl2): lmax = 458 nm.
1H NMR (300 MHz, CDCl3): d = 1.46 (s, 9H, C-CH3), 3.18
(t, 4H, J = 1.6 Hz, CH2 of N-BOC piperazine), 3.39 (m, 4H,
CH2 of piperazine), 3.42 (t, 4H, J = 1.6 Hz, CH2 of N-BOC
piperazine), 6.54 (m, 4H, CH2 of piperazine), 6.76–7.12
(m, 6H, olefinic and aromatic), 7.31–7.36 (m, 2H, olefinic),
7.44 (d, 2H, J = 2.9 Hz, aromatic), 7.47 (d, 2H, J = 3.0,
aromatic). 19F NMR (75 MHz, tetrachloroethane-D2):
d = ꢁ139.0 (m, 2F, tetrafluoro-phenyl ring), ꢁ143.73
(m, 2F, Fortho), ꢁ150.17 (m, 2F, tetrafluoro-phenyl ring),
ꢁ156.51 (m, 1F, Fpara), ꢁ161.04 (t, 2F, Fmeta). Elemental
analysis for C41H37F9N4O2: calcd C 62.43, H 4.73, N 7.10%;
found C 62.35, H 4.65, N 7.01%.
Compound 11. To a solution of stilbene 7 (0.17 mmol) and
the p-fluoro-benzaldehyde (0.53 mmol) in anhydrous DMSO
(3 mL) was added K2CO3 (0.2 mmol). The solution was heated
at 90 1C for 20 h in the dark, then was added water (10 mL)
and the resulting solution was extracted with ethyl acetate.
The organic phase was dried with Na2SO4, and the solvent was
evaporated under reduced pressure. The crude reaction mixture
was purified by dry flash chromatography (eluent: light petroleum
ether/ethyl acetate 2 : 8) affording the desired product. Yellow
solid. Reaction yield 23% Mp: decomposition starting at
200 1C. IR (nujol): n (cmꢁ1) = 1685 (n, CQO), 1600
(n, HCQCH trans), 1377 (n, Carom-N), 956 (out of plane
deformation C-H trans). UV-Vis (CH2Cl2): lmax = 339 nm.
Fluorescence (CH2Cl2): lmax = 444 nm. 1H NMR (300 MHz,
CDCl3): d = 3.47 (t, 4H, CH2 of piperazine), 3.63 (t, 4H, CH2
of piperazine), 6.86 (d, 1H, C6F5-CHQCH-Ar), 6.99
(d, 2H, aromatic ring of stilbene), 7.00 (d, 2H, aromatic ring
of benzaldehyde), 7.40 (d, 1H, C6F5-CHQCH-Ar), 7.51
(d, 2H, aromatic ring of stilbene), 7.82 (d, 2H, aromatic ring
of benzaldehyde), 9.84 (s, 1H, CHO). MS(EI): m/z = 458
[C25H19F5N2O]+. Elemental analysis for C25H19F5N2O: calcd
C 65.50, H 4.18, N 6.11%; found: C 65.32, H 4.11, N 6.05%.
Compound 15. 7.326 mmol of bis-aldehyde 14 were
suspended in 120 mL of EtOH and 14.3 mmol of NaBH4
were added, keeping the temperature in the range 18–20 1C,
then the stirred at rt for 3 h. The solution was then neutralized
with diluted HCl and extracted with CH2Cl2. The organic phase
was dried with Na2SO4, and the solvent was evaporated under
reduced pressure, affording the desired alcohol. Pale yellow
solid. Yield: 95%. Mp 197–198 1C. IR (nujol): n (cmꢁ1) =
1
3256 broad (n, OH). H NMR (300 MHz, CDCl3): d = 3.37
Compound 12. To a solution of stilbene 10a (0.088 mmol)
and the p-fluoro-benzaldehyde (0.123 mmol) in anhydrous
DMSO (15 mL) was added NaH (0.111 mmol). The solution
was heated at 90 1C for 20 h in the dark, then was added water
(25 mL) and the resulting solution was extracted with ethyl
acetate. The organic phase was dried with Na2SO4, and the
solvent was evaporated under reduced pressure. The crude
reaction mixture was purified by dry flash chromatography
(eluent: dichloromethane/ethyl acetate 96 : 4) affording the desired
product. Yellow solid. Reaction yield: 48%. Mp 183–184 1C.
UV-Vis (CH2Cl2): lmax = 336 nm. Fluorescence (CH2Cl2):
lmax = 427 nm. 1H NMR (300 MHz, CDCl3): d = 1.52
(s, 9H, C-CH3), 3.26 (t, 4H, CH2 of BOC-piperazine), 3.48
(t, 4H, CH2 of piperazine), 3.57 (t, 4H, CH2 of BOC-piperazine),
3.66 (t, 4H, CH2 of piperazine), 6.92 (d, 1H, C6F5-CHQCH-Ar),
7.01 (d, 2H, aromatic ring of stilbene), 7.10 (d, 2H, aromatic
ring), 7.38 (d, 1H, C6F5-CHQCH-Ar), 7.49 (d, 2H, aromatic
ring of stilbene), 7.82 (d, 2H, aromatic ring), 9.85 (s, 1H, CHO).
Elemental analysis for C34H36F4N4O3: calcd C 65.37, H 5.81,
N 8.97%; found C 65.25, H 5.75, N 8.85%.
(2, 8H, CH2 of piperazine), 4.41–4.94 (s, 4H, CH2-OH), 7.00
(d, 4H, J = 1.2, aromatic), 7.32 (d, 4H, J = 1.2, aromatic).
Elemental analysis for C18H22N2O2: calcd C 72.46, H 7.43,
N 9.39%; found C 72.28, H 7.35, N 9.35%.
Compound 16. 0.735 mmol of 15 with 3.011 mmol of
triphenyl-phosphonium bromide HPPh3Br were suspended
in anhydrous toluene. The mixture was heated at 75 1C for
7 h, then the solvent was removed under reduced pressure.
The pink solid was taken up with CH2Cl2 and then washed
with an aqueous saturated solution of NaHCO3. The organic
phase was dried with Na2SO4, and the solvent was evaporated
under reduced pressure, affording the desired product. White
solid. Yield: 68%. Mp 297–298 1C. IR (nujol): n (cmꢁ1) =
1352 broad (n, Carom-N), 1516 (n, CH2-P), 1436 (n, P-C6H5),
1604 (n, CQC). 1H NMR (300 MHz, CDCl3): d = 3.21
(s, H, CH2 of piperazine), 5.34 (d, 4H, J = 15.1, CH2-P), 6.45
(d, 4H, J = 8.6, aromatic), 7.00 (d, 4H, J = 8.6, aromatic),
7.61–7.80 (m, 30H, -P(C6H5)3). 31P NMR (300 MHz, CDCl3):
d = 23.94 (s, -PPh3). Elemental analysis for C53H48Br2N2P2:
calcd C 68.10, H 5.18, N 3.00%; found C 68.02, H 5.49, N 2.85%.
Compound 13. 0.04 mmol of the formyl-phenyl-stilbene 12
were added at 0 1C, under an inert gas, to a THF solution
(12 mL) of the phosphorane 5 (0.05 mmol prepared as
described in the literature). The mixture was allowed to react
at room temperature and the progress of the reaction was
monitored by TLC (eluent: diethyl ether/light petroleum ether,
3 : 7). The reaction was carried out in the dark. The reaction
was complete after 13 h and then was quenched by addition of
15 mL of water. The solvent was distilled under reduced
pressure, affording a bright yellow solid. The solid was taken
up with CH2Cl2 (10 mL), and the organic layer was washed
twice with distilled water. The organic phase was dried with
Compound 17. 0.69 mmol of 15 were suspended in 6.4 mL of
triethyl-phosphite at 0 1C and 1.4 mmol of iodine were added
portionwise. The reaction was stirred at rt for 15 h, then the
triethyl-phosphite was evaporated under reduced pressure. The
crude product was purified by column chromatography (eluent:
ethyl acetate/methanol 9 : 1, affording the desired product. Pale
1
yellow solid. Reaction yield 20%. H NMR (300 MHz, CDCl3):
d = 1.27 (t, 12H, CH3), 3.10 (d, 4H, -P-CH2-Ar), 3.35 (s, 8H, CH2
of piperazine), 4.04 (m, 8H, -CH2-CH3), 6.96 (d, 4H, aromatic),
7.24 (d, 4H, aromatic). MS(EI): m/z = 538 [C26H40N2O6P2]+,
ꢀc
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498, 401. Elemental analysis for C26H40N2O6P2: calcd C 57.98,
H 7.49, N 5.20%; found C 57.66, H 7.32, N 5.05%.
References
1 C. Liebermann and O. Bergami, Chem. Ber., 1889, 22, 782–786.
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Synthesis of compounds 3, 18a, 18b, and 18c. 40 mg of the
stilbene are put on a microscope slab and exposed to a 20 W
tungsten light bulb for 15 h. This solid state reaction is
monitored by TLC.
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5 N. J. Turro, Modern Molecular Photochemistry, University Science
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Compound 3. White solid. Quantitative yield. Mp 145–146 1C.
IR (nujol): n (cmꢁ1) = 1232 (n, C-N), 1011.5 (n, C-F). UV-Vis
(CH2Cl2): lmax = 400 nm. 1H NMR (300 MHz, CDCl3):
d = 2.90 (12H, s, CH3N), 4.73 (4H, s, CH of cyclobutane),
6.64 (d, 4H, J = 8.8, aromatic ring), 7.07 (d, 4H, J = 8.8,
aromatic ring). 19F NMR (75 MHz, CDCl3): d = ꢁ141.91
(m, 4F, Fortho), ꢁ157.26 (m, 2F, Fpara), ꢁ163.43 (m, 4F, Fmeta).
Elemental analysis for C32H24F10N2: calcd C 61.34, H 3.86,
N 4.47%; found C 61.15, H 3.90, N 4.59%.
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Compound 18a. White solid. Quantitative yield. Mp 173–174 1C.
1H NMR (300 MHz, CDCl3): d = 2.39 (6H, s, CH3N),
2.63 (m, 8H, CH2 of piperazine), 3.21 (t, 8H, J = 4.9, CH2
of piperazine), 4.79 (s, 4H, CH of cyclobutane), 6.81 (d, 4H,
J = 8.7, aromatic ring), 7.11 (d, 4H, J = 8.7, aromatic ring).
19F NMR (75 MHz, CDCl3): d = ꢁ141.88 (m, 2F, Fpara),
ꢁ156.71 (m, 4F, Fmeta), ꢁ163.16 (m, 4F, Fortho). Elemental
analysis for C38H34F10N4: calcd C 61.95, H 4.65, N 7.61%;
found C 61.74, H 4.48, N 7.54%.
Compound 18b. White solid. Quantitative yield. Mp 170–172 1C.
1H NMR (300 MHz, CDCl3): d = 1.50 (s, 18H, C-CH3),
3.11 (t, 8H, J = 5.0, CH2 of piperazine), 3.59 (m, 8H, CH2 of
piperazine), 4.81 (s, 4H, CH of cyclobutane), 6.89 (d, 4H,
J = 8.5, aromatic ring), 7.14 (d, 4H, J = 8.5, aromatic ring).
19F NMR (75 MHz, CDCl3): d = ꢁ142.58 (m, 2F, Fpara),
ꢁ157.78 (m, 4F, Fmeta), ꢁ163.47 (m, 4F, Fortho). Elemental
analysis for C46H46F10N4O4: calcd C 60.79, H 5.10, N 6.16%;
found C 60.65, H 5.21, N 6.12%.
7 (a) V. A. Kumar, N. S. Begum and K. Venkatesan, J. Chem. Soc.,
Perkin Trans., 1993, 463; (b) C. A. Hunter, Chem. Soc. Rev., 1994,
23, 101–109; (c) S. Bacchi, M. Benaglia, F. Cozzi, F. Demartin,
G. Filippini and A. Gavezzotti, Chem.–Eur. J., 2006, 12, 3538;
(d) F. Cozzi, S. Bacchi, G. Filippini, T. Pilati and A. Gavezzotti,
Chem.–Eur. J., 2007, 13, 7177.
8 (a) G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller,
E. B. Lobkovsky and R. H. Grubbs, J. Am. Chem. Soc., 1998,
120(15), 3641–3649; (b) K. Vishnumurthy, T. N. Guru Row and
K. Venkatesan, Photochem. Photobiol. Sci., 2002, 1, 427.
9 The crystals were irradiated for 24 h with a 450 W medium-
pressure mercury lamp. The stereochemistry was assigned on the
basis of the middle point method.13
.
Compound 18c. White solid. Quantitative yield. Mp 171–172 1C.
IR (nujol): n (cmꢁ1) = 3434 (n, N-H), 1377 (n, Carom-N).
1H NMR (300 MHz, CDCl3): d = 2.94 (broad, 8H, CH2 of
piperazine), 3.07 (broad, 8H, CH2 of piperazine), 4.77 (broad,
4H, CH of cyclobutane), 6.82 (d, 4H, J = 8.6, aromatic ring),
7.15 (d, 4H, J = 8.6, aromatic ring). 19F NMR (75 MHz, CDCl3):
d = ꢁ141.39 (t, 2F, Fpara), ꢁ156.14 (m, 4F, Fmeta), ꢁ163.29
(d, 4F, Fortho). Elemental analysis for C36H30F10N4: calcd C
61.02, H 4.27, N 7.91%; found C 60.85, H 4.15, N 7.85%.
10 A. Papagni, S. Maiorana, P. Del Buttero, D. Perdicchia, F.
Cariati, E. Cariati and W. Marcolli, Eur. J. Org. Chem., 2002,
1380–1384.
11 (a) M. D. Cohen, G. M. J. Schmidt and F. I. J. Sonntag,
J. Chem. Soc., 1964, 2000; (b) G. M. J. Schmidt, J. Chem. Soc.,
1964, 2014.
12 V. Ramamurthy and K. Venkatesan, Chem. Rev., 1987, 87,
433–481.
13 D. A. Ben Efraim and B. S. Green, Tetrahedron, 1974, 30,
2357.
14 K. D. M. Harris, J. M. Thomas and D. Williams, J. Chem. Soc.,
Faraday Trans., 1991, 87, 325.
15 D. R. Lide, CRC Handbook of Chemistry and Physics, Taylor and
Francis, Boca Raton, FL, 87th edn, 2007.
16 See: G. M. Sheldrick, SHELXL97-program for crystal structure
refinement, University of Goettingen, Germany, 1997.
Acknowledgements
We thank Alessio De Giuli and Vera Milan for their
experimental contribution.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 2612–2621 | 2621