4H-pyran-4-ylidenes: Strong proaromatic donors for organic nonlinear optical chromophores

Article abstract of DOI:10.1021/jo901142f

(Chemical Equation Presented) Merocyanines where a polyenic spacer separates a 4H-pyran-4-ylidene moiety and different strong organic acceptors have been synthesized. According to NMR studies and X-ray diffraction data, these compounds have weakly alterna

Full text of DOI:10.1021/jo901142f

pubs.acs.org/joc
4H-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear
Optical Chromophores
†
Raquel Andreu, Laura Carrasquer, Santiago Franco, Javier Garın,* Jesus Orduna,
†
†
,†
†
ꢀ
Natalia Martınez de Baroja, Raquel Alicante, Belen Villacampa, and Magali Allain
†
‡
‡
§
ꢀ
†
‡
Departamento de Quımica Organica and Departamento de Fısica de la Materia Condensada, ICMA,
´
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, and Laboratoire de Chimie,
ꢀ
´
§
ꢀ ꢀ
Ingenierie Moleculaire et Materiaux d’Angers (UMR CNRS 6200), Universite d’Angers, 2,
ꢀ
Boulevard Lavoisier, 49045-Angers CEDEX, France
jgarin@unizar.es
Received May 29, 2009
Merocyanines where a polyenic spacer separates a 4H-pyran-4-ylidene moiety and different strong
organic acceptors have been synthesized. According to NMR studies and X-ray diffraction data,
these compounds have weakly alternated structures and remarkably zwitterionic ground states, with
a partial aromatic character that is compared to those of other pyran derivatives. The proaromaticity
of the 4H-pyran-4-ylidene donor lies behind the cyanine-like behavior and low (positive or negative)
second-order optical nonlinearities of the shorter derivatives. On the other hand, lengthening the
π-spacer gives rise to rapidly increasing μβ1907 values up to 17,400 ꢀ 10-48 esu.
Introduction
It was soon discovered that chromophores incorporating
aromatic subunits displayed lower β values than the corre-
sponding polyenes of the same conjugation length, since
their ground states are usually dominated by neutral, aro-
matic forms that lose resonance energy on charge separa-
tion.4 To overcome this drawback, Marder proposed a
strategy consisting in the use of fragments that, conversely,
gain aromaticity on charge transfer. This strategy has since
then been applied to proaromatic acceptors,5 spacers,6 and
donors, the most widely used proaromatic electron donors
being pyrid-4-ylidenes7 and 1,3-dithiol-2-ylidenes.8 On the
other hand, the donor ability of the isoelectronic pyran-
4-ylidene moiety has received much less attention in the
search for new NLO chromophores. Thus, from a theoretical
point of view, Marks suggested that heavily substituted
4-quinopyrans could display very high hyperpolarizabilities
due to their twisted intramolecular charge transfer nature
Conjugated donor-acceptor (D-π-A) compounds con-
stitute the blueprint in the search for new materials endowed
with nonlinear optical (NLO) properties and are being
widely investigated because of their technological and funda-
mental interest.1 These compounds display first molecular
hyperpolarizabilities (β), related to intramolecular charge
transfer (ICT) excitations, which are dependent on the
nature of the donor, acceptor, and spacer moieties.2 One of
the successful strategies in the design of these NLO chromo-
phores involves tuning the ground-state polarization of the
chromophore, which in valence bond terminology depends
on the relative contributions of the neutral and zwitterionic
limiting forms.3
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DOI: 10.1021/jo901142f
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Published on Web 08/05/2009
J. Org. Chem. 2009, 74, 6647–6657 6647
2009 American Chemical Society

Andreu et al.
JOCArticle
and, therefore, zwitterionic character.9 On the experimental
side, the only studies aimed at taking advantage
of the potential proaromaticity of 4-pyranylidenes in the
field of compounds with second-order NLO responses were
reported by Caro, who prepared and studied push-pull
compounds in which such donors are conjugated to
Fischer-type carbenes10a,b and organoiron(II) fragments.10c
In this paper we describe the synthesis and characteriza-
tion of new D-π-A compounds (1-5) where a 4H-pyran-4-
ylidene donor unit is linked to different strong organic
acceptors and report the first study on the second-order
NLO properties of these purely organic pyranylidene-based
merocyanines. Experimental and theoretical evidence sup-
porting the proaromaticity of the pyran-4-ylidene moiety are
presented, and its partly aromatic character is compared to
that of other pyran/pyrylium derivatives.
SCHEME 1
Results and Discussion
Synthesis . The shortest derivatives (1a,11 2a,11 3a,12 4a,13
and 5a12) with n=0 are known compounds and were prepared
as previously reported. For the synthesis of the longer mole-
cules, 4H-pyranylidene aldehydes 7b,c were chosen as pre-
cursors. Atthis stage, the replacement of the methyl groups on
the pyran ring by tert-butyl seemed convenient to avoid
possible side-reactions in the (vinylogous) formylation step.
Thus, aldehyde 7b was prepared as previously described,14
and the previously unreported aldehyde 7c was synthesized
using a similar procedure, namely, the reaction of salt 615 with
3-dimethylaminoacrolein/Ac2O,16 followed by hydrolysis
(overall yield, 39% from 6) (Scheme 1).
Knoevenagel reactions of 7b,c with acceptors 8-12, where
12 is the so-called “tricyanofuran” or TCF acceptor, affo-
rded the vinylogous push-pull compounds 1b,c-5b,c, re-
spectively (Scheme 2). On passing, it should be noted that
there are very few pyran-4-ylidene derivatives linked to an
acceptor through a polyenic spacer, and most of them feature
weak acceptors, such as carbonyl groups.17a-d (Moreover,
some cyanines having a formal pyrylium cation linked to a
pyran-4-ylidene moiety through a conjugated spacer with an
odd number of carbon atoms (the so-called pyrylocyanines)
have also been described.)17e-h
The synthesis of molecules with more extended π-systems
required another approach because, under a variety of condi-
tions, we were not able to prepare the higher vinylogue of
aldehyde 7c. Therefore, we decided to take advantage of the
well-known electrophilicity of enamido substituents linked to
strong acceptor groups and of the nucleophilic character of the
exocyclic carbon atom of 4-methylpyrylium derivatives (as their
corresponding anhydrobases). To that end, acceptor vinylo-
gues 80-120 were prepared by the reaction of acceptors 8-12,
respectively, with glutaconaldehyde dianilide hydrochloride
(6) Andreu, R.; Blesa, M. J.; Carrasquer, L.; Garın, J.; Orduna, J.;
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6648 J. Org. Chem. Vol. 74, No. 17, 2009

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SCHEME 2
derivative 1d turned out to be unstable and could not be isolated)
(Scheme 3).
SCHEME 3
1H NMR Studies . Concerning the new aldehyde 7c, 3JHH
coupling constants reveal that it has an all-trans geometry,
with an s-trans conformation for the formyl group (see
Experimental Section). This contrasts to the s-cis conforma-
tion, which minimizes steric crowding in the corresponding
doubly vinylogous ketones.17b
Concerning compounds 1-5, 3JHH coupling constants ana-
lysis along the spacer shows that CHdCH and CH-CH bonds
have (E)-configuration and s-trans conformation, respectively.
On the other hand, the isoxazolone-exocyclic CdC bond in
compounds 2 has a (Z)-geometry, as demonstrated by the
downfield shift of the H atom located five bonds away from
the carbonyl oxygen atom.8e,20 Similarly, the HCdC(CN) bond
in compounds 3 (see below) shows the expected (Z)-geometry.
Concerning compounds 4, NOE and NOESY experiments at
different temperatures on model compound 4b did not allow to
establish the configuration of the HCdC(CN) bond, but pre-
vious studies on related compounds show a (Z)-geometry for
that fragment, configuration that is also assumed for 4.21
1H NMR spectra also afford valuable information con-
cerning the contribution of the neutral (N) and zwitterionic
(ZW) limiting forms to the description of the ground state of
these compounds, that is, about the proaromatic character of
the 4H-pyran-4-ylidene moiety (Figure 1).
In fact, previous NMR experiments22 (and dipole moment
measurements)23 onsimple 4-methylene-4H-pyranderivatives
7c
(Scheme 3). Compounds 80,18 90,18,19 and 120 have been
described, but only the latter has been adequately characterized.
New compounds 100 and 110 were also prepared by the same
general method. The reaction of these extended acceptors with
pyrylium salt 6 in the presence of triethylamine afforded the
desired merocyanines 2d-5d (unfortunately, the thiobarbituric
(20) Maquestiau, A.; Van Haverbeke, Y.; Muller, R. N.; Lo Vecchio, G.;
Grassi, G. Tetrahedron Lett. 1973, 14, 4249–4251.
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(b) Akkurt, M.; Payze, Z.; Kepez, M. Cryst. Res. Technol. 2000, 35, 349–353.
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(c) Alıas, S.; Andreu, R.; Cerdan, M. A.; Franco, S.; Garın, J.; Orduna, J.;
Romero, P.; Villacampa, B. Tetrahedron Lett. 2007, 48, 6539–6542.
(22) (a) Belsky, I.; Dodiuk, H.; Shvo, Y. J. Org. Chem. 1974, 39, 989–995.
(b) Belsky, I.; Dodiuk, H.; Shvo, Y. J. Org. Chem. 1977, 42, 2734–2741.
(c) Balaban, A. T.; Wray, V.; Furmanova, N. G.; Minkin, V. I.; Minkina, L.
S.; Czernysch, Yu. E.; Borodkin, G. S. Liebigs Ann. Chem. 1985, 1587–1595.
(23) Wortmann, R.; Poga, C.; Twieg, R. J.; Geletneky, C.; Moylan, C. R.;
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D. M. J. Chem. Phys. 1996, 105, 10637–10647.
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J. Org. Chem. Vol. 74, No. 17, 2009 6649

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FIGURE 1. Limiting forms of compounds 1-5.
bearing acceptor groups on the exocyclic carbon atom have
provided some evidence for the contribution of the zwitter-
ionic form, mainly based on the study of rotational barriers
around the exocyclic C4dCexo bond.
1
FIGURE 2. Molecular structures of 2b (left) and 3c (right).
In a similar way, 2D H EXSY experiments on 4b (at
30 °C, in CDCl3, see Supporting Information) show a slow
exchange process between the otherwise inequivalent H-3
and H-5 atoms of the pyran ring. This fact can be accounted
for by the slow rotation around the partially single C4dCexo
bond, thus pointing to a certain zwitterionic character of
these derivatives. Moreover, H-3 and H-5 of the pyranyl-
idene ring are known to undergo downfield shifts as the
pyrylium character of the ring increases.10c,24 For the herein
reported compounds, the chemical shifts of the pyranylidene
ring protons increase (2d being the only exception) on short-
ening the polyenic spacer (δ: d<c<b), thus suggesting that
the contribution of the aromatic form (ZW) is higher for the
shorter derivatives.
3JHH coupling constants along the polyenic spacer also
give a clear indication of the ground-state polarization of
compounds 1-5, given the known correlation between 3JHH
values and C-C bond lengths.25 The fact that compounds b
show high 3JHH values for the vinylic protons of the dCH-
CHd spacer (e.g., 1b: 14.2 Hz; 2b: 13.6 Hz; 3b: 13.3 Hz; 4b:
13.0 Hz) also supports the important contribution of the ZW
form to the description of these compounds. Moreover, the
longer derivatives show very close 3JHH values for every pair
of vicinal protons along the polyenic chain. Thus, every 3JHH
along the spacer equals 13.0 Hz for 1c, whereas for 2c-d to
5c-d most of these coupling constants lie in the range of
12.5-13.5 Hz. This means that the ΔJ values (defined as the
difference between the averaged 3JHH values of the formally
double and single bonds along the polymethine chain)26 are
very small (e1 Hz). These data indicate increased
(decreased) bond order for the formally single (double)
bonds and weakly alternated structures, closer to cyanines
(ΔJ ≈ 0 Hz) than to all-trans polyenes (ΔJ ≈ 6 Hz).27
Moreover, the chemical shifts of the polymethine protons
show an oscillatory behavior, reflecting the alternation in the
electron densities of the carbon atoms to which they are
bonded, which is characteristic of strongly polarized mer-
ocyanines.10b,25,27a,28 Taken together, these evidence clearly
show the proaromaticity of the 4H-pyran-4-ylidene moiety.
X-ray Diffraction Studies and the Proaromatic Character
of 4H-Pyran-4-ylidene Derivatives . Single crystals of 2b and
3c were grown from C2D6CO solutions (Figure 2). X-ray
analysis of 2b confirms the (Z)-geometry of the isoxazolone-
exocyclic CdC bond, as is usually found in 3-phenyl-4-
polyenylideneisoxazol-5-ones.5b,7a,8e Concerning 3c, it can
be seen that the geometry of the CdC double bond formed in
the Knoevenagel reaction is (Z) and that the PhC-C(CN)
has a s-trans conformation, which are typical features of this
moiety.8d,12 Moreover, the π-system of 3c is essentially
planar, except for a slight deviation of the tricyanopropene
fragment and for the phenyl ring (the angle between the best-
fit planes of the pyran and the phenyl rings equals 68.1°).
The strong delocalization suggested by ΔJ values along
the spacer is confirmed by the very similar C-C bond lengths
of the polymethine chain. Thus, bond length alternation
(BLA) of the polyenic spacer, defined as the difference
between the average carbon-carbon single and double bond
lengths,3,29 is very close to 0 A both for 2b and 3c, reinforcing
the view that these compounds are, at least in the solid state,
very close to the cyanine limit. This bond length equalization
is accompanied by an alternation of the C-C-C angles
along the spacer, which are greater (smaller) for the carbon
atoms with low (high) electron density, as is also typical of
(mero)cyanines.30
From a structural point of view, strongly polarized 4H-
pyran-4-ylidene derivatives with a noticeable zwitterionic
character10b,31 should display structural features intermediate
between those of aromatic and quinoid derivatives, therefore
showing a shortening of the C-O bonds, a lengthening of the
pyran exocyclic bond (C4-Cexo), and a decreased degree of
C-C bond length alternation, which can be measured
through the δr parameter, defined as (a - b + c - d)/2
(Figure 3).6,32
This is indeed the case, as shown by the parameters
collected in Table 1, where we have also included for the
sake of comparison the corresponding data for two extreme
models: the 2,4,6-triphenylpyrylium cation (13),33 expected
to show “full” aromatic character, and 2,20,6,60-tetraphenyl-
(29) Marder, S. R.; Perry, J. W.; Tiemann, B. G.; Gorman, C. B.;
Gilmour, S.; Biddle, S. L.; Bourhill, G. J. Am. Chem. Soc. 1993, 115, 2524–
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C.; Saillard, J. Y.; Kahlal, S.; Ouahab, L.; Golhen, S. Eur. J. Org. Chem.
2000, 577–581. (b) Caro, B.; Le Poul, P.; Robin-Le Guen, F.; Saillard, J. Y.;
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7519–7530.
(25) Radeglia, R.; Dahne, S. J. Mol. Struct. 1970, 5, 399–411.
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B. G.; Mansour, K. Science 1993, 261, 186–189.
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FIGURE 4. Aromatic and quinoid pyran derivatives.
FIGURE 3. Bond lengths involved in the δr parameter.
TABLE 1. Structural Parameters (A) of 2b, 3c, 13, and 14
compd
C-O
C4-Cexo
δr
2b
3c
1.360
1.361
1.354
1.389
1.393
1.391
1.466
1.385
0.087
0.084
0.043
0.107
13a
14b
a Reference 33. b Reference 34.
bipyranylidene (14),34 which can be safely considered as a
quinoid derivative (Figure 4). As expected, δr for 14 is much
larger than for 13, but the latter still shows a substantial δr
value, in good agreement with the low aromaticity of pyryl-
ium cations.35
Moreover, the geometries of 2b and 3c are similar to those
reported for other push-pull pyran-4-ylidene derivatives,
such as 4-dicyanomethylene-2,6-dimethyl-4H-pyran,36 3a,12
and 4-methylenepyran Fischer-type carbene complexes,10b,31
where available structural data show that these compounds
have intermediate features between those of cation 13 and
compound 14 (with the following average values: C-O 1.36-
1.37 A; C4-Cexo 1.39-1.40 A; δr 0.08-0.09 A).
These experimental geometries, together with those of
pyrylium cations, allow a qualitative estimation of the
proaromatic character of the herein reported compounds
and of other 4-methylenepyran derivatives. It is known that
geometry-based indices of aromaticity37 suffer from some
limitations, but their easy availability and reasonable success
on treating the aromaticity of closely related compounds
make them adequate enough for qualitative comparisons.
Thus, we have calculated the Bird index (I6)38 for a wide
variety of pyran derivatives using experimental (X-ray
diffraction) geometries. Concerning the extreme models,
pyrylium cations33,39 show I6 values of ca. 50 (for benzene
I6=100), whereas that of compound 1434 is much lower (I6=
25.4). Compounds 2b and 3c show intermediate values (39.2
and 38.5, respectively), nearly identical to those calculated
FIGURE 5. Pyrylium cations 4-substituted with donor groups.
for the previously mentioned push-pull pyran-4-ylidene
derivatives (I6=38-40).
Thus, whereas the geometries of all these compounds are
closer to the quinoid than to the aromatic type as judged from
bond distance parameters (Table 1), their I6 values indicate a
noticeable contribution of the zwitterionic limiting form. This
intermediate quinoid/aromatic character may be due to the
fact that the energetic cost of the charge separation needed to
adopt a more aromatic structure (ZW, Figure 1) is hardly
compensated for by the low aromaticity of the pyrylium form.
In fact, these compounds can be considered either as pyran-
4-ylidene derivatives bearing acceptor groups (N, Figure 1) or
as pyrylium salts substituted with strong (negatively charged)
donor groups (ZW, Figure 1), a situation reminiscent of that
foundfor cations 15 (Figure 5),40 where thestabilization of the
positive charge by the electron donor group (D) leads to an
increase in the quinoid contribution and, therefore, to less
aromatic structures (e.g, for 15b40b (R=Ph): C-O 1.363 A;
C4-Cexo 1.438 A; δr 0.068 A; I6=42.7).
These examples suggest that the relatively high alternation
found in the pyran ring of push-pull pyran-4-ylidenes
should not be taken as evidence against the proaromatic
character of the pyranylidene moiety.
Calculated Structures . We have also carried out theoretical
calculations at the B3P86/6-31G* level on the model com-
pounds 10-50, where the alkyl groups on the pyran ring of
compounds 1-5 have been replaced by H atoms for the sake of
simplicity (the validity of such replacement has been confirmed
by comparison of the DFT calculated geometries of compound
3a and its simplified model 30a, see Supporting Information).
(34) Chasseau, D.; Gaultier, J.; Hauw, C.; Fugnitto, R.; Gianis, V.;
Strzelecka, H. Acta Crystallogr., Sect. B 1982, 38, 1629–1631.
(35) (a) Hepworth, J. D.; Gabbutt, C. D.; Heron, B. M. In Comprehensive
Heterocyclic Chemistry II; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.;
Pergamon Press: Oxford, 1996; Vol. 5, pp 301-350. (b) Balaban, A. T.; Oniciu, D.
C.; Katritzky, A. R. Chem. Rev. 2004, 104, 2777–2812. (c) Balaban, A. T. In
Aromaticity in Heterocyclic Compounds; Krygowski, T. M., Cyranski, M. K.,
Eds.; Springer: Berlin, 2009; pp 203-246.
(36) Fu, X.-S.; Zhou, M.-J.; Sun, J.-W.; Xiao, D.-D. Acta Crystallogr.,
Sect. E 2006, 62, o4917–o4918.
(37) (a) Krygowski, T. M.; Cyranski, M. K. Chem. Rev. 2001, 101, 1385–
1419. (b) Krygowski, T. M.; Cyranski, M. K. Phys. Chem. Chem. Phys. 2004,
6, 249–255.
(38) (a) Bird, C. W. Tetrahedron 1985, 41, 1409–1414. (b) Bird, C. W.
Tetrahedron 1986, 42, 89–92.
(39) (a) Tamamura, T.; Yamane, T.; Yasuoka, N.; Kasai, N. Bull. Chem.
Soc. Jpn. 1974, 47, 832–837. (b) Turowska-Tyrk, I.; Krygowski, T. M.;
Milart, P.; Butt, G.; Topsom, R. D. J. Mol. Struct. 1991, 245, 289–299. (c)
Gdaniec, M.; Turowska-Tyrk, I.; Krygowski, T. M. J. Chem. Soc., Perkin
Trans. 2 1989, 613–616. (d) Krygowski, T. M.; Anulewicz, R.; Milart, P.;
Zimmermann, T. J. Prakt. Chem. 1994, 336, 649–653. (e) Krygowski, T. M.;
Cyranski, M. Tetrahedron 1996, 52, 10255–10264.
(40) (a) Sammes, M. P.; Harlow, R. L.; Simonsen, S. H. J. Chem. Soc.,
Perkin Trans. 2 1981, 303–309. (b) Turowska-Tyrk, I.; Krygowski, T. M.;
Milart, P. J. Mol. Struct. 1991, 263, 235–245.
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Andreu et al.
JOCArticle
TABLE 2. B3P86/6-31G* Calculated Values for Compounds 10a-d
oxidation potentials of compounds 1c, 3b, and 5a (all of them
with the same number of carbon atoms in the conjugation
path) are compared, it becomes clear that the electron-with-
drawing ability of the corresponding acceptor groups de-
creases in the sequence 5>3>1.
compd
δr (A)
C4-Cexo (A)
BLAa (A)
I6
chargeb (e)
10a
10b
10c
10d
0.090
0.097
0.100
0.101
1.404
1.385
1.380
1.379
47.1
42.7
41.1
40.6
+0.39
+0.34
+0.29
+0.27
0.022
0.034
0.037
UV-vis Spectroscopy . The UV-vis absorption data of
compounds 1-5 are collected in Table 3, inspection of which
reveals some common trends. For a given acceptor, λmax
values increase with the conjugation length, showing large
vinylene shifts of ca. 100 nm (on the energy scale these shifts
decrease on lengthening the spacer; thus, for the series 5a, 5b,
5c, 5d, the observed shifts are of 0.48, 0.26, and 0.20 eV,
respectively). This is indicative of a cyanine-like (rather than a
polyene-like) behavior, thus suggesting an important zwitter-
ionic character for these derivatives, in agreement with theo-
reticalcalculations(Table2). The observedbathocromicshifts
and the increased log ε values on lengthening the spacer are
also well reproduced by TDDFT calculations (see below).
When different acceptors are compared, λmax approxi-
mately decrease in the following order: 5>3>4 ≈ 1 ≈ 2. This
order, although based on a crude comparison obviating the
very different structures of the acceptor groups, suggests a
parallel decrease in the corresponding acceptor strengths, in
agreement with the results of cyclic voltammetry. A more
detailed comparison can be made between the structurally
similar derivatives 3 and 4. The replacement of the phenyl
group in compounds 3 by an amino group in 4 gives rise to a
strong hypsochromic shift. Those substituents are linked to
an odd-numbered carbon atom of the polymethine chain and
the lower λmax values are therefore expected on the basis of
Dewar’s rules.41 In fact, the amino group not only causes a
strong destabilization of the LUMOs of compounds 4, when
compared to 3 (Table 3), but also a slight and unexpected
stabilization of the corresponding HOMOs. Both factors
result in increased HOMO-LUMO gaps and higher excita-
tion energies (Table 4) for compounds 4.
The solvatochromism of compounds 1-5 is small. With a
few exceptions, compounds 3 and 5 show small, positive
solvatochromic effects, whereas the reverse holds for com-
pounds 4. For compounds 1 and 2, the shorter derivatives
show a small, negative solvatochromism, which becomes
positive for the longer ones (2c,d). This behavior reinforces
the previously mentioned cyanine-like character of 1-5,
which decreases on lengthening the spacer, in good agree-
ment with the calculated increase in BLA values on passing
from the shorter to the longer derivatives (Table 2).
Nonlinear Optical Properties . The second-order NLO
properties of compounds 1-5 were measured by electric
field-induced second harmonic generation (EFISH) at 1907
nm in dichloromethane (except where noted), and the static
(zero-frequency) μβ0 values were calculated by the two-level
model,42 using the lowest energy absorption band of each
compound (for the sake of comparison, a common bench-
mark of organic NLO chromophores, Disperse Red 1, shows
a μβ0 value of ca. 400 ꢀ 10-48 esu).43
aAlong the polyenic spacer. b In the pyranylidene moiety.
In the few cases where experimental geometries are avail-
able (2b, 3a,12 3c) we have found that calculations on the
corresponding model compounds overestimate both BLA
along the spacer (by ca. 0.03 A) and δr values while under-
estimating C-O bond distances, giving rise to I6 values
higher (by ca. 3-5 units) than those of the experimentally
determined structures. While this precludes a direct compar-
ison between the calculated and solid-state stuctures, the
DFT geometries should still give information about the main
structural features of the model compounds. They are ex-
emplified by the values shown in Table 2 for the series 10a-d
(the same trends are observed for the remaining series).
It can be seen that the pyran rings are calculated to display
high δr values and relatively short C4-Cexo bonds, but the
charges in the donor moiety are also high, pointing to a
noticeable zwitterionic (proaromatic) character that de-
creases on lengthening the polyenic spacer, reflecting the
higher energy required to separate charges over longer
distances. This trend is in good agreement with the decrease
in I6 values and the increase in BLA on passing from 10a to
10d. The progressive shortening of the C4-Cexo bond also
supports this reasoning and confirms the previously ob-
served good correlation between that bond length and the
aromatic character of pyrylium ions.39d
Electrochemistry . The electrochemical properties of com-
pounds 1-5 were studied by cyclic voltammetry (Table 3).
Most of these merocyanines show two irreversible waves,
corresponding to the oxidation of the donor and the reduc-
tion of the acceptor moieties, respectively. Lengthening the π
spacer gives rise to a shift of Eox values toward less anodic
potentials and to a shift of Ered values toward less cathodic
potentials. The easier oxidation and reduction processes on
chain lengthening point to a decrease in the interaction
between the donor and acceptor end groups and, therefore,
to a smaller contribution of the zwitterionic form to the
ground state of the longer derivatives, in line with the
calculated Mulliken charges (Table 2). Moreover, the ob-
served trends in Eox (Ered) are also confirmed by theoretical
calculations, which show that EHOMO (ELUMO) values in-
crease (decrease) with the length of the spacer. For any given
acceptor, the decrease in Eox is much more noticeable than
that in |Ered|, in agreement with the trends shown by the
energy levels of the frontier orbitals (Table 3). Eox values also
allow a broad comparison of the electron-withdrawing
abilities of the different acceptor groups. Thus, values in
Table 3 show that thiobarbiturate is a stronger acceptor than
isoxazolone, in line with the lower Mulliken charges sup-
ported by the donor moiety in compounds 2 (2a, +0.32; 2b,
+0.30; 2c, +0.26; 2d, +0.25; cf. Table 2), and that the
acceptor strength slightly decreases on passing from com-
pounds 3 to 4. Broader comparisons between structurally
disparate acceptors must take into account the length of the
spacer separating the D and A moieties, since this parameter
has a marked influence on Eox values. For instance, when the
(41) (a) Griffiths, J. Colour and Constitution of Organic Molecules; Aca-
demic Press: London, 1976. (b) Fabian, J.; Hartmann, H. Light Absorption of
Organic Colorants; Springer: Berlin, 1980.
(42) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195–
242.
(43) Singer, K. D.; Sohn, J. E.; King, L. A.; Gordon, H. M.; Katz, H. E.;
Dirk, C. W. J. Opt. Soc. Am. B 1989, 6, 1339–1350.
6652 J. Org. Chem. Vol. 74, No. 17, 2009

Andreu et al.
JOCArticle
TABLE 3. Electrochemical Data,a EHOMO and ELUMO Valuesb (eV), and UV-vis Data
compd
Eox
Ered
EHOMO
ELUMO
λmax (CH2Cl2) (log ε)
λmax (DMSO)
1a
1b
1c
1de
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
+1.56
+1.19
+0.83
-1.40
-1.11c
-0.94d
-6.73
-6.55
-6.25
-6.03
-6.58
-6.23
-5.99
-5.81
-6.95
-6.52
-6.26
-6.06
-7.02
-6.60
-6.32
-6.10
-6.67
-6.43
-6.21
-6.03
-3.36
-3.50
-3.58
-3.64
-3.04
-3.26
-3.36
-3.44
-3.62
-3.74
-3.79
-3.82
-3.32
-3.61
-3.69
-3.74
-3.81
-3.87
-3.90
-3.92
392 (sh), 409 (4.79)
501 (4.81), 530 (5.18)
584 (4.93), 628 (5.25)
407
502, 530
585, 627
+1.50
+1.10
+0.77
+0.57
+1.64
+1.20
+0.88
+0.64
+1.67
+1.14
+0.87
+0.59
+1.32
+1.00
+0.68
+0.51
-1.50
-1.15f
-1.04
397 (4.35)
396
498 (4.72), 530 (4.68)
579 (4.82), 623 (4.76)
610 (sh), 654 (4.86), 715 (4.68)
455 (4.42)
535 (4.72), 568 (4.89), 601 (sh)
629 (4.89), 678 (5.11)
719 (4.92), 787 (5.07)
382 (4.26)
506 (4.62), 532 (4.64)
584 (4.72), 625 (4.71)
643 (4.71), 719 (4.56)
513 (4.60)
592 (4.74), 639 (4.97)
675 (4.86), 740 (5.03)
688 (sh), 753 (4.92), 841 (4.94)
502, 529
583, 626
663, 725
460
536 (sh), 570, 600 (sh)
629 (sh), 678
718 (sh), 782
358
-0.93
-1.11, -1.70
-0.89g
-0.76
-0.71
-1.35
-1.12
485
-1.04
552, 603 (sh)
558, 713 (sh)
517
595, 637
680, 739
763, 847
-0.86
-1.10
-0.83h
-0.77
-0.69
a In volts, 10-3 M in CH2Cl2 vs Ag/AgCl (KCl 3 M), glassy carbon working electrode, Pt counter electrode, 20 °C, 0.1 M Bu4NPF6, 100 mV s-1 scan
rate. Ferrocene internal reference E1/2=+0.43 V. b Calculated for model compounds 10-50 in the gas phase for the geometries shown in Schemes 2 and 3,
at the B3P86/6-31G*//B3P86/6-31G* level. cEred1/2 (ΔEp=0.12 V). dEred1/2 (ΔEp=0.10 V). e Not isolated. fEred1/2 (ΔEp=0.10 V). gEred1/2 (ΔEp=0.11 V).
hEred1/2 (ΔEp = 0.08 V).
TABLE 4. Experimentala and Calculatedb,c NLO Properties
experimental
μβf
TD-DFTd
Δμge(z) (D)
CPHFe
f
g
μβ0
compd
μβ0
μg (D)
μe (D)
Δμge (D)
E (eV)
f
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4c
4d
5ah
5ai
5bh
5bi
5ch
5ci
5dh
5di
-50
-90
600
-40
-60
300
6.83
9.18
11.36
13.31
7.99
14.29
3.52
9.35
18.39
2.89
6.27
21.13
12.65
2.20
5.16
9.46
9.52
2.39
5.58
7.29
2.37
5.59
9.34
13.88
12.90
6.58
2.65
12.88
13.80
11.79
10.37
7.11
2.62
1.30
5.70
-21.13
-12.63
-2.04
+5.05
-9.08
-7.41
-2.31
+4.67
+2.27
+0.56
+4.20
+8.06
-11.35
-11.40
-6.17
+2.09
-11.86
-13.01
-11.26
-9.96
-6.99
-2.60
-0.36
+5.47
3.43
3.13
2.85
2.60
3.48
3.13
2.89
2.64
3.15
2.96
2.68
2.44
3.45
3.03
2.79
2.56
2.90
2.92
2.60
2.71
2.44
2.51
2.28
2.31
0.3900
0.9923
1.6607
2.1456
0.1204
0.4950
1.2031
1.8227
0.3717
1.1168
1.6086
2.0492
0.2460
0.7356
1.4039
2.0463
0.5395
0.7621
0.8545
1.2921
1.4440
1.9578
2.0312
2.4691
-131
-157
255
1421
-63
-21
312
1096
-82
-54
787
2820
-109
-103
540
0
100
740
3500
60
210
1550
7500
25
190
705
2650
140
0
64
380
1320
43
120
670
2000
21
120
360
980
90
9.31
10.80
12.23
11.71
14.38
16.42
18.37
11.36
13.98
16.06
17.97
14.23
15.57
15.83
18.01
17.84
20.27
19.59
22.37
8.51
17.17
15.61
15.12
20.95
26.85
7.99
6.56
10.14
20.13
5.55
5.28
5.76
2198
-119
-355
306
630
4500
310
1520
3110
8.54
336
10.92
17.67
19.27
27.89
1824
2404
4745
6341
17400
a In CH2Cl2 (in DMSO for 4a), at 1907 nm. b On B3P86/6-31G* geometries. c On model compounds 10-50 for the geometries shown in Schemes 2 and 3
d B3P86/6-31G* level. eCPHF/6-31G* level. f In 10-48 esu. Experimental accuracy: ( 10%, except for 3a, 5a ((15%), and 1a, 1b, 2b, 4a ((20%). g In 10-
48 esu. hCalculated value for the conformer with TCF-exocyclic s-cis C-C bond. iCalculated value for the conformer with TCF-exocyclic s-trans C-C
bond.
Inspection of Table 4 reveals that μβ0 values increase with
the conjugation length and that compounds 5 and 3, bearing
the strongest acceptors, display the highest second-order
NLO responses. CPHF-calculated μβ0 values are in reason-
ably good agreement with the experimental ones. Thus, the
small hyperpolarizabilities shown by the shorter derivatives
(series a and b) are qualitatively predicted, although in most
cases the calculated β values are negative, indicating an
overestimation of the zwitterionic character of such com-
pounds. On the other hand, positive μβ0 values are correctly
predicted for the longer derivatives (series c and d). Even
though DFT calculated hyperpolarizabilities are usually
overestimated,44 we have carried out TDDFT calculations,
which readily afford the parameters involved in the two-level
model and provide a better understanding of the factors
responsible for the NLO response. In this approach β0 µ
Δμge f/E3, where Δμge stands for the difference between the
(44) Suponitsky, K. Yu.; Tafur, S.; Masunov, A. E. J. Chem. Phys. 2008,
129, 44109-1–44109-11.
J. Org. Chem. Vol. 74, No. 17, 2009 6653

Andreu et al.
JOCArticle
excited- and ground-state dipole moments (μe and μg,
respectively), f is the oscillator strength, and E is the first
excitation energy. Within each series of compounds (1-5) f
increases and E decreases on lengthening the spacer, thus
explaining the increase in μβ0 values on passing from series a
to d. On the other hand, the variation of Δμge is more
complex, and this is partly due to the widely different relative
orientations of the μe and μg vectors for these compounds. To
get a clearer picture of this factor, we have included in Table 4
the Δμge(z) parameter, defined as the projection of Δμge
onto the ground state dipole moment direction, arbitrarily
named z. Inspection of Δμge(z) values reveals that, except for
compounds 3, they are predicted to pass from strongly
negative to positive values on increasing the chain length.
Since the experimentally determined μβ0 values of most
compounds herein reported are positive, this parameter
indicates that TDDFT also overestimates the zwitterionic
character of these derivatives, although it correctly predicts
the negative μβ0 values observed for 1a-b.
Comparison of the NLO properties of compounds 3 and 4
provides convincing evidence about an important aspect to
which only a few experimental45 and theoretical46 efforts
have been devoted, namely, the influence of the substituents
of either the spacer or the acceptor on the NLO properties of
merocyanines. Thus, and concerning a structure perturba-
tion of the acceptor, it can be seen that μβ values of
compounds 3 are higher than those of 4, as a consequence
of the replacement of a phenyl substituent by a strong
electron-donating amino group at an odd-numbered carbon
atom of the polymethine chain. Inspection of Table 4 reveals
that this modification results both in higher excitation
energies and lower oscillator strengths and, therefore, has a
detrimental effect on the NLO response of compounds 4.
Compounds 5 may be present as mixtures of conformers
(s-cis/s-trans around the TCF-exocyclic C-C bond) under
the conditions of the EFISH experiment, since calculations
show that their energies are very close. The calculated
parameters of each conformer are gathered in Table 4, but
it can be seen that their μβ0 values are very similar, except for
5c,d. This fact lends support to previous theoretical results
suggesting that, in many cases, the conformational depen-
dence of β is small, thus making it possible to consider only
one conformation for computational studies.47
b > c > d) for every series of compounds (1-5). Comparison
of the different series reveals that compounds 3 and 5 are
the most thermally stable, with Td values in the range
304-184 °C for 3b-d, and 290-205 °C for 5b-d. On the
other hand, isoxazolone-containing compounds 2 have the
lowest thermal stability (Td range for 2b-d: 240-152 °C), in
agreement with previous observations on NLO chromo-
phores containing such acceptor.48
Finally, it seems pertinent to compare the NLO properties
of the herein reported compounds to those of related deri-
vatives. Concerning the acceptor moiety, the only directly
comparable compounds are the pyranylidene-derived
Fischer carbene complexes reported by Caro.10a,b They show
high, rapidly increasing μβ1907 values, which are similar to
those of compounds 3, but lower than those of compounds 5.
The shortest Fischer carbene derivative shows a negative β
value, thus pointing to a strong zwitterionic character which
decreases for the longer derivatives, a trend also shown by
the present compounds. Therefore, series 1-5 and the pre-
viously reported organometallic derivatives show a qualita-
tively similar behavior, the electron-withdrawing ability of
the C(OMe)W(CO)5 fragment being surpassed by that of the
TCF acceptor. Concerning the donor ability of the 4H-
pyran-4-ylidene moiety, a straightforward comparison with
another proaromatic donor (1,3-dithiol-2-ylidene) is possi-
ble thanks to the use of a common acceptor. To that end,
compounds 30 and 16, featuring a common acceptor, have
been chosen as models, since the linear and nonlinear optical
properties of the dimethyl analogues of 16 have been studied
both theoretically and experimentally.8d
A comparison of the ground-state Mulliken charges on the
donor fragment in compounds 30 (30a, +0.38; 30b, +0.33;
30c, +0.29; 30d, +0.27) and 16 (16a, +0.23; 16b, +0.18; 16c,
+0.14; 16d, +0.12) reveals that compounds 30 are more
polarized (have more zwitterionic ground states) than their
dithiole counterparts, although their μg values are very
similar, (cf. Table 4 and ref 8d) probably because of the
strong negative charge supported by the oxygen atom of the
pyran ring. This effect must also lie behind the lower μe (and
hence, Δμge) values of compounds 30, compared to those of
16. Even though oscillator strengths are larger for the pyran
derivatives, the dithiole compounds have lower excitation
energies and higher Δμge values and therefore, it is unsurpris-
ing that they show higher μβ0 values8d than their pyran
analogues. These trends also hold when other dithiole/pyran
families are compared, although the higher NLO-efficiency of
the former tends to level-off for the longer derivatives.8e,21c
Besides, the lower Δμge values, the higher oscillator strengths
and the solvatochromism of pyranylidenes 1-5, together
The thermal stabilities of NLO chromophores 1-5 were
studied by thermogravimetric analysis (TGA) (and differ-
ential scanning calorimetry (DSC) in some cases), their
decomposition temperatures (Td) being estimated as the
temperature that is the intercept of the leading edge of the
weight loss by the baseline of the TGA scans. TGA shows
that the shortest derivatives studied (b) have high Td values
(Td (°C): 1b, 279; 2b, 240; 3b, 304; 4b, 246; 5b, 290) and that
the thermal stability decreases on lengthening the spacer (Td:
(45) (a) Luo, J.; Huang, S.; Cheng, Y.-J.; Kim, T.-D.; Shi, Z.; Zhou,
X.-H.; Jen, A. K.-Y. Org. Lett. 2007, 9, 4471–4474. (b) Cheng, Y.-J.; Luo, J.;
Huang, S.; Zhou, X.; Shi, Z.; Kim, T.-D.; Bale, D. H.; Takahashi, S.; Yick,
A.; Polishak, B. M.; Jang, S.-H.; Dalton, L. R.; Reid, P. J.; Steier, W. H.; Jen,
A. K.-Y. Chem. Mater. 2008, 20, 5047–5054.
(46) (a) Schmidt, K.; Barlow, S.; Leclercq, A.; Zojer, E.; Jang, S.-H.;
ꢀ
Marder, S. R.; Jen, A. K.-Y.; Bredas, J.-L. J. Mater. Chem. 2007, 17, 2944–
2949. (b) Chafin, A. P.; Lindsay, G. A. J. Phys. Chem. C 2008, 112, 7829–
7835.
(47) Suponitsky, K. Yu.; Masunov, A. E.; Antipin, M. Yu. Mendeleev
Commun. 2008, 18, 265–267.
(48) (a) Zhang, C.; Ren, A. S.; Wang, F.; Zhu, J.; Dalton, L. R.;
Woodford, J. N.; Wang, C. H. Chem. Mater. 1999, 11, 1966–1968. (b) Gong,
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6654 J. Org. Chem. Vol. 74, No. 17, 2009

Andreu et al.
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with the small (positive or negative) μβ0 values of the shorter
derivatives (a,b) clearly support the important zwitterionic
character of these compounds, which are near the cyanine
limit and, therefore, lie in region C of Marder’s plot.3 On
increasing the chain length, BLA (Table 2) and μβ0 values
(Table 4) also increase, the longer derivatives (c,d) falling in
region B of such plot.
and finally saturated aqueous NaHCO3 (5ꢀ75 mL). The
organic layer was dried (MgSO4) and evaporated. To the crude
iminium salt was added methanolic KOH (5%, 37.5 mL), and
the mixture was heated at 60 °C for 30 min. Then, a second
portion of methanolic KOH (37.5 mL) was added to complete
the hydrolysis, and the reaction was stirred for another 30 min.
The mixture was cooled to room temperature, and CH2Cl2
(100 mL) and water (100 mL) were added. The organic layer
was separated and washed with water (2ꢀ100 mL), dried
(MgSO4), and evaporated. The crude product was purified by
flash chromatography (silica gel) using hexane/AcOEt (9:1) to
afford a yellow to brown oil that precipitated (205 mg, 39%) on
standing at 4 °C. Mp 101-103 °C. IR (Nujol, cm-1): 1575
(CdC), 1643 (CdO). 1H NMR (CDCl3, 400 MHz): δ 9.50 (d, J=
8.2 Hz, 1H), 7.49 (dd, J=14.4 Hz, J0=12.3 Hz, 1H), 6.24 (d, J=
1.6 Hz, 1H), 6.00 (dd, J=14.4 Hz, J0=8.2 Hz, 1H), 5.80 (d, J=
1.6 Hz, 1H), 5.65 (d, J=12.3 Hz, 1H), 1.25 (s, 9H), 1.21 (s, 9H).
13C NMR (CDCl3, 75 MHz,): δ 193.0, 166.7, 166.5, 148.2, 141.5,
124.6, 109.6, 105.4, 99.0, 36.0, 35.7, 27.8, 27.7. HRMS (ESIþ):
calcd for C17H25O2 (M þ H)þ 261.1849; found 261.1863; calcd
for C17H24NaO2 (M þ Na)þ 283.1669; found 283.1682. Anal.
Calcd for C17H24O2: C 78.42, H 9.29. Found: C 78.77, H 9.06.
Compounds 1b,c: General Procedure . 1,3-Diethylthiobarbi-
turic acid (8) (72 mg, 0.36 mmol) was added to a solution of 0.36
mmol of the corresponding aldehyde (7b-c) in absolute ethanol
(2 mL). The solution was refluxed under argon atmosphere for
40-50 min (TLC monitoring), then cooled. The resulting solid
was filtered off, washed with cold ethanol, and dried. For 1b
evaporation of the filtrate and chromatography (silica gel) using
CH2Cl2 as eluent gave a second fraction.
5-(2-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)ethylidene)-1,3-di-
ethyl-2-thioxodihydropyrimidine-4,6-dione (1b) . Yield: maroon
solid (66 mg; 44%). Mp 234-236 °C. IR (Nujol, cm-1) 1643
(CdO), 1240 (CdS). 1H NMR (CD2Cl2, 400 MHz): δ 8.56 (d,
J=14.2 Hz, 1H), 7.61 (d, J=14.2 Hz, 1H), 7.04 (d, J=1.7 Hz,
1H), 6.55 (d, J=1.7 Hz, 1H), 4.53 (q, J=6.9 Hz, 2H), 4.52 (q, J=
6.9 Hz, 2H), 1.35 (s, 9H), 1.31 (s, 9H), 1.25 (t, J=6.9 Hz, 3H),
1.24 (t, J=6.9 Hz, 3H). 13C NMR (CDCl3, 75 MHz): δ 178.5,
173.5, 172.9, 162.3, 156.7, 150.2, 113.2, 110.0, 104.6, 102.8, 43.4,
42.9, 36.9, 36.6, 28.0, 27.9, 12.5. MS (EIþ): m/z 416 (Mþ 3 , 80),
383 (100), 328 (50). Anal. Calcd for C23H32N2O3S: C 66.31, H
7.74, N 6.72. Found: C 66.59, H 7.61, N 6.83.
(E)-5-(4-(2,6-di-tert-butyl-4H-pyran-4-ylidene)but-2-enyl-
idene)-1,3-diethyl-2-thioxodihydropyrimidine-4,6-dione (1c). Yield:
bright blue-violet solid (104 mg; 66%). Mp 215-217 °C. IR
(Nujol, cm-1) 1654 (CdO), 1193 (CdS). 1H NMR (C2D6CO, 300
MHz): δ 8.15 (t, J=13.0 Hz, 1H), 8.02 (d, J=13.0 Hz, 1H), 7.83
(t, J=13.0 Hz, 1H), 7.15 (d, J=1.8 Hz, 1H), 6.51 (d, J=1.8 Hz,
1H), 6.14 (d, J=13.0 Hz, 1H), 4.50 (q, J=6.9 Hz, 2H), 4.49 (q,
J=6.9 Hz, 2H), 1.36 (s, 9H), 1.32 (s, 9H), 1.22 (t, J=6.9 Hz, 3H),
1.20 (t, J=6.9 Hz, 3H). 13C NMR (C2D6CO, 75 MHz): δ 173.1,
172.8, 162.8, 161.7, 158.8, 157.8, 152.1, 125.4, 117.2, 110.0, 104.3,
44.5, 43.9, 38.4, 38.1, 29.1, 29.0, 13.8, 13.7. MS (EIþ): m/z 442
(Mþ 3 , 100), 409 (30), 57 (55). Anal. Calcd for C25H34N2O3S: C
67.84, H 7.74, N 6.33. Found: C 68.10, H 7.65, N 6.52.
Further comparisons between the donor ability of 4H-
pyran-4-ylidenes and other proaromatic six-membered do-
nors can also be established, although they are somewhat
restricted by the lack of appropriate reference compounds or
by the different experimental setups used in the measurement
of the corresponding second-order NLO properties. Thus,
N-substituted 4-pyridylidenes linked to the TCF acceptor
(12) through a polyenic chain display zwitterionic ground
states and strong negative solvatochromism,7c pointing to a
superior donor ability of the nitrogen heterocycle when
compared to its oxygen analogue. Although a reliable com-
parison between the NLO properties of these pyridylidene
derivatives and those of 5 is hampered by the fact that the
former were measured by hyper-Rayleigh scattering (HRS),
the high negative value (μβ0=-500 ꢀ 10-48 esu)3 determined
by EFISH in CH2Cl2 for a 4-pyridylidene analogue of 1b
reinforces the previous assumption. Concerning other clo-
sely related donors, such as chalcogenopyranylidenes, the
very few reports to date show that replacement of the oxygen
atom of the pyranylidene ring by sulfur generally leads to an
enhancement of the second-order NLO responses, although
the reversed trend is observed for chromene derivatives.10a,b
In summary, polyenic merocyanines featuring a 4H-pyr-
an-4-ylidene donor fragment have been prepared in one or
two steps from 4-methylpyrylium salts. Experimental (X-ray
diffraction, NMR, and UV-vis) data and theoretical calcu-
lations demonstrate the proaromaticity of the 4H-pyran-
4-ylidene moiety in these push-pull compounds. Its partial
aromatic character has been estimated and compared, for the
first time, to that of pyrylium cations and simpler 4-methyl-
enepyrans. The short merocyanines, with an important aro-
matic (zwitterionic) contribution are close to, or even sur-
pass, the cyanine limit, whereas lengthening the spacer gives
rise to more alternated structures with high second-order
nonlinearities. The NLO properties of these all-organic,
thermally stable compounds compare favorably to those of
related organometallic pyranylidenes.
Experimental Section
Compounds 6,15 7b,14 10,49 12,50 80,18 90,18,19 and 120 were
7c
prepared as previously described.
(E)-4-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)-but-2-enal (7c).
To a solution of 2,6-di-tert-butyl-4-methylpyrylium triflate (6)
(706 mg, 2 mmol) in acetic anhydride (10 mL) was added
3-dimethylaminoacrolein (90%) (0.6 mL, 6 mmol) under argon.
The mixture was stirred at room temperature for 2 h. CH2Cl2
(100 mL) was added, and the resulting organic layer was washed
with diluted H3PO4 (1/50 v/v; 2 ꢀ 100 mL), water (2 ꢀ 100 mL),
Compounds 2b,c: General Procedure . 3-Phenyl-5-isoxazolone
(9) (77 mg, 0.48 mmol) was added to a solution of 0.48 mmol of
the corresponding aldehyde (7b-c) in absolute ethanol (3 mL).
The solution was refluxed under argon atmosphere for 40-50
min (TLC monitoring), then cooled. The resulting solid was
filtered off, washed with cold ethanol and cold pentane, and
dried. For 2c evaporation of the filtrate and chromatography
(silica gel) using hexane/AcOEt (7:3) as eluent gave a second
fraction.
(49) (a) Elgemeie, G. E. H.; Elfahham, H. A.; Elgamal, S.; Elnagdi, M. H.
Heterocycles 1985, 23, 1999–2003. (b) Abdelrazek, F. M.; Michael, F. A. J.
Heterocycl. Chem. 2006, 43, 7–10.
(50) (a) Melikian, G.; Rouessac, F. P.; Alexandre, C. Synth. Commun.
1995, 25, 3045–3051. (b) Ermer, S.; Lovejoy, S. M.; Bedworth, P. V.; Leung,
D. S.; Warren, H. B.; Epstein, J. A.; Girton, D. G.; Dries, L. S.; Taylor, R. E.;
Barto, R. R., Jr.; Eades, W.; Van Eck, T. E.; Moss, A. S.; Anderson, W. W.
Adv. Funct. Mater. 2002, 12, 605–610.
(Z)-4-(2-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)ethylidene)-3-
phenyl-5-isoxazolone (2b) . Yield: darkblue microcrystalline solid
(74 mg; 41%). Mp 222-224 °C. IR (Nujol, cm-1) 1709 (CdO),
1654 (CdC), 1545 (CdN). 1H NMR (CDCl3, 400 MHz): δ 7.70
J. Org. Chem. Vol. 74, No. 17, 2009 6655

Andreu et al.
JOCArticle
(d, J = 13.6 Hz, 1H), 7.64-7.61 (m, 2H), 7.54-7.51 (m, 3H),
7.26 (d, J=13.6 Hz, 1H), 6.51 (d, J=1.8 Hz, 1H), 6.37 (d, J=
1.8 Hz, 1H), 1.29 (s, 9H), 1.28 (s, 9H). 13C NMR (CDCl3, 75
MHz): δ 172.4, 171.3, 162.8, 152.3, 142.7, 130.1, 129.4, 128.9,
128.6, 128.3, 110.0, 108.7, 104.3, 101.3, 101.0, 36.5, 36.4, 27.9,
27.6. MS (EIþ): m/z 377 (Mþ 3 , 100), 221 (30). Anal. Calcd
for C24H27NO3: C 76.36, H 7.21, N 3.71. Found: C 76.51, H
7.00, N 3.98.
(Z)-4-[(E)-4-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)but-2-enyl-
idene]-3-phenyl-5-isoxazolone (2c) . Yield: dark green solid (147
mg; 76%). Mp 123-125 °C. IR (Nujol, cm-1) 1708 (CdO), 1654
(CdC), 1551 (CdN). 1H NMR (CDCl3, 300 MHz): δ 7.70-7.40
(m, 7H), 7.33 (d, J=12.6 Hz, 1H), 6.40 (d, J=1.8 Hz, 1H), 6.02 (d,
J=1.8Hz, 1H), 5.90 (d, J=12.8Hz, 1H), 1.27 (s, 9H), 1.26 (s, 9H).
13C NMR (CDCl3, 100 MHz): δ 169.5, 169.3, 162.7, 150.2, 149.7,
146.1, 130.1, 129.2, 128.9, 128.4, 121.5, 114.2, 107.3, 107.1, 100.8,
36.4, 36.2, 31.0, 27.9. MS(EIþ): m/z 403 (Mþ•, 100), 57(75). Anal.
Calcdfor C26H29NO3: C 77.39, H 7.24, N 3.47. Found: C 77.61, H
7.04, N 3.69.
Compounds 3b,c: General Procedure . To a solution of the
corresponding aldehyde (7b-c) (0.37 mmol) in 2 mL of Ac2O,
1,1,3-tricyano-2-phenylpropene (10) (71 mg, 0.37 mmol) was
added. The mixture was heated under argon at 75 °C for 1 h
(TLC monitoring) and then allowed to cool to room tempera-
ture. The resulting solid was isolated by filtration, washed with
cold pentane, and dried. In the case of 3b, CH2Cl2 (50 mL) was
added to the filtrate, and then, washed with NaOH 10% (3 ꢀ
50 mL) and water (2 ꢀ 50 mL). The organic layer was dried over
MgSO4 and evaporated, giving a second fraction, after purifica-
tion by column chromatography (silica gel, eluent: hexane/
AcOEt (7:3)).
(Mþ 3 , 100). Anal. Calcd for C21H24N4O: C 72.39, H 6.94, N
16.08. Found: C 72.14, H 7.11, N 16.33.
(3Z,5E)-2-Amino-7-(2,6-di-tert-butyl-4H-pyran-4-ylidene)he-
pta-1,3,5-triene-1,1,3-tricarbonitrile (4c) . To a solution of alde-
hyde 7c (54 mg, 0.21 mmol) and 2-amino-1,1,3-tricyanopropene
(11) (27 mg, 0.21 mmol) in absolute ethanol (4 mL) were added
pyridine (0.1 mL) and acetic acid (0.05 mL). The mixture was
refluxed under argon for 2 h 20 min. (TLC monitoring). The
solvent was evaporated and the crude product was purified by
chromatography (silica gel) using hexane/AcOEt (8:2), then
hexane/AcOEt (7:3), and finally hexane/AcOEt (6:4). Yield: dark
purple solid (34 mg; 44%). Mp 158-162 °C. IR (Nujol, cm-1
)
2209 (CtN). 1H NMR (CDCl3, 400 MHz): δ 8.02 (d, J=12.2 Hz,
1H), 7.49 (t, J=13.0 Hz, 1H), 6.50 (t, J=12.6 Hz, 1H), 6.49 (d, J=
1.7 Hz, 1H), 6.05 (d, J=1.7 Hz, 1H), 5.80 (d, J=13.0 Hz, 1H), 5.77
(br s, 2H), 1.31 (s, 9H), 1.27 (s, 9H). 13C NMR (CDCl3, 100
MHz): δ 169.9, 169.8, 164.2, 154.2, 148.9, 147.0, 120.4, 116.5,
116.1, 115.9, 113.3, 107.4, 101.1, 90.1, 48.3, 36.5, 36.1, 27.9, 27.8.
HRMS (ESIþ): calcd for C23H27N4O (M þ H)þ 375.2179; found
375.2169; calcd for C23H26N4NaO (M þ Na)þ 397.1999; found
397.1986. Anal. Calcd for C23H26N4O: C 73.77, H 7.00, N 14.96.
Found: C 73.89, H 6.82, N 15.19.
Compounds 5b,c: General Procedure . To a solution of the
corresponding aldehyde (7b-c) (1.46 mmol) and TCF (12) (290
mg, 1.46 mmol) in absolute ethanol (3 mL) was added piperidine
(three drops). The mixture was refluxed under argon for 2-2.5 h
(TLC monitoring) and then cooled to room temperature. The
solvent was distilled, and the residue was purified by column
chromatography (silica gel) using hexane/AcOEt (65:35) as
eluent.
(E)-2-[3-Cyano-4-(3-(2,6-di-tert-butyl-4H-pyran-4-ylidene)prop-
1-enyl)-5,5-dimethylfuran-2(5H)-ylidene]malononitrile(5b) . Yield:
dark blue-gray solid (89 mg; 15%). Mp 297-298 °C. IR (Nujol,
cm-1) 2217 (CtN). 1H NMR (C2D6CO, 300 MHz): δ 8.58 (t, J=
13.2 Hz, 1H), 6.86 (br s, 1H), 6.55 (d, J=1.5 Hz, 1H), 6.18 (d, J=
14.1 Hz, 1H), 6.11 (d, J=12.9 Hz, 1H), 1.66 (s, 6H), 1.34 (s, 9H),
1.32 (s, 9H). 13C NMR (CDCl3, 75 MHz) δ 177.4, 172.6, 172.0,
171.8, 150.5, 145.7, 114.0, 113.6, 113.1, 108.3, 107.8, 102.3, 95.7,
53.3, 36.7, 36.5, 27.8, 26.4. MS (MALDIþ): m/z 415 (Mþ•). Anal.
Calcd for C26H29N3O2: C 75.15, H 7.03, N 10.11. Found: C 75.01,
H 7.21, N 10.36.
2-[3-Cyano-4-((1E,3E)-5-(2,6-di-tert-butyl-4H-pyran-4-ylidene)-
penta-1,3-dienyl)-5,5-dimethylfuran-2(5H)-ylidene]malononitrile
(5c) . Yield: dark blue solid (129 mg; 20%). Mp 231-232 °C. IR
(Nujol, cm-1) 2216 (CtN). 1H NMR (CDCl3, 400 MHz): δ 7.38
(t, J=12.8 Hz, 2H), 6.46 (d, J=1.4 Hz, 1H), 6.29 (t, J=12.6 Hz,
1H), 6.02-5.99 (m, 2H), 5.77 (d, J=12.8 Hz, 1H), 2.17 (s, 6H),
1.31 (s, 9H), 1.26 (s, 9H). 13C NMR (CDCl3, 100 MHz): δ 177.0,
172.0, 170.0, 169.6, 150.2, 146.2, 145.5, 126.0, 114.5, 113.5,
113.1, 112.9, 111.8, 107.5, 101.1, 96.0, 36.5, 36.2, 29.7, 28.0,
27.9, 26.6. MS (MALDIþ): m/z 441 (Mþ•), 464 (M þ Na)þ, 480
(M þ K)þ. Anal. Calcd for C28H31N3O2: C 76.16, H 7.08, N
9.52. Found: C 75.93, H 7.37, N 9.71.
Compounds 100 and 110: General Procedure . A mixture of
N-((2E,4E)-5-(phenylamino)penta-2,4-dienylidene)aniline hy-
drochloride (1 g, 3.52 mmol), the corresponding acceptor (10,
11) (3.52 mmol), and anhydrous sodium acetate (289 mg,
3.52 mmol) in acetic anhydride (5 mL) was refluxed under argon
for 5-10 min. After cooling, the resulting solid was isolated
by filtration and washed with water, methanol, and finally
ether.
(Z)-5-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)-2-phenylpenta-
1,3-diene-1,1,3-tricarbonitrile (3b) . Yield: violet microcrystalline
solid (64 mg; 42%). Mp 243-245 °C. IR (Nujol, cm-1) 2204
1
(CtN), 1643 (CdC), 1548 (CdC, Ar). H NMR (CDCl3, 400
MHz): δ 8.25 (d, J=13.3 Hz, 1H), 7.56-7.40 (m, 5H), 6.56 (d, J=
1.5 Hz, 1H), 6.36 (d, J=1.5 Hz, 1H), 6.22 (d, J=13.3Hz, 1H), 1.31
(s, 9H), 1.28 (s, 9H). 13C NMR (CDCl3, 100 MHz): δ 173.7, 172.9,
167.2, 153.4, 149.4, 135.7, 131.4, 129.3, 128.9, 116.4, 115.7, 115.5,
111.2, 108.8, 102.7, 96.4, 70.8, 36.8, 36.7, 29.7, 28.1, 27.8. MS
(EIþ): m/z 409 (Mþ 3 , 100). Anal. Calcd for C27H27N3O: C 79.19,
H 6.65, N 10.26. Found: C 79.28, H 6.47, N 10.08.
(3Z,5E)-7-(2,6-Di-tert-butyl-4H-pyran-4-ylidene)-2-phenyl-
hepta-1,3,5-triene-1,1,3-tricarbonitrile (3c) . Yield: dark blue solid
(80 mg; 50%). Mp 234-236 °C. IR (Nujol, cm-1) 2203 (CtN),
1
1648 (CdC), 1552 (CdC, Ar). H NMR (CDCl3, 400 MHz): δ
7.58-7.49 (m, 3H), 7.41-7.34 (m, 4H), 6.68 (t, J=12.7 Hz, 1H),
6.49 (d, J=1.5 Hz, 1H), 6.13 (d, J=1.5 Hz, 1H), 5.88 (d, J=13.0
Hz, 1H), 1.30 (s, 9H), 1.28 (s, 9H). 13C NMR (CDCl3, 75 MHz): δ
171.2, 170.9, 166.6, 155.9, 149.5, 148.5, 135.2, 131.5, 129.6, 129.0,
122.1, 115.9, 115.5, 114.9, 114.4, 108.1, 101.7, 98.7, 36.6, 36.4,
28.0, 27.8. MS (EIþ): m/z 435 (Mþ 3, 100), 57 (50). Anal. Calcd for
C29H29N3O: C 79.97, H 6.71, N 9.65. Found: C 80.22, H 6.56, N
9.90.
(Z)-2-Amino-5-(2,6-di-tert-butyl-4H-pyran-4-ylidene)penta-
1,3-diene-1,1,3-tricarbonitrile (4b) . To a stirred solution of the
aldehyde 7b (200 mg, 0.85 mmol) and 2-amino-1,1,3-tricyano-
propene (11) (113 mg, 0.85 mmol) in absolute ethanol (3 mL) was
added piperidine (two drops). The mixture was refluxed under
argon for 1 h 15 min (TLC monitoring). After cooling, the
resulting solid was isolated by filtration, washed with water and
cold pentane, and dried. Yield: bright purple solid (127 mg; 43%).
Mp 213-215 °C. IR (Nujol, cm-1) 2209 (CtN). 1H NMR
(CDCl3, 400 MHz): δ 8.78 (d, J=13.0 Hz, 1H), 6.76 (d, J=1.8
Hz, 1H), 6.28 (d, J=1.8 Hz, 1H), 6.02 (d, J=13.0 Hz, 1H), 5.70 (br
s, 2H) 1.33 (s, 9H), 1.32 (s, 9H). 13C NMR (CDCl3, 75 MHz):
δ 172.6, 172.1, 163.9, 152.5, 148.4, 117.6, 116.4, 108.9, 107.7,
102.3, 87.5, 58.4, 54.8, 46.9, 36.7, 36.5, 27.8. MS (EIþ): m/z 348
N-Phenyl-N-((1E,3E,5Z)-6,8,8,-tricyano-7-phenylocta-1,3,5,-
7-tetraenyl)acetamide (100) . Yield: bright maroon solid (789 mg;
58%). Mp 217-220 °C. IR (Nujol, cm-1) 2212 (CtN).
1H NMR (CDCl3, 400 MHz): δ 8.12 (d, J = 13.7 Hz, 1H),
7.62-7.50 (m, 6H), 7.40-7.37 (m, 2H), 7.20-7.14 (m, 3H), 6.98
(dd, J=14.1 Hz, J0=11.6 Hz, 1H), 6.58 (dd, J=14.1 Hz, J0=12.0
Hz, 1H), 5.28 (dd, J=13.7 Hz, J0 =11.6 Hz, 1H), 1.96 (s, 3H).
6656 J. Org. Chem. Vol. 74, No. 17, 2009

Andreu et al.
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13C NMR (CDCl3, 100 MHz): δ 169.3, 166.4, 157.6, 151.9,
141.6, 137.9, 133.5, 132.5, 130.7, 130.0, 129.6, 129.3, 128.1,
125.1, 114.0, 113.6, 112.8, 112.3, 105.2, 80.5, 23.3. MS (ESIþ):
m/z 391 (M þ H)þ, 413 (M þ Na)þ, 429 (M þ K)þ. Anal. Calcd
for C25H18N4O: C 76.91, H 4.65, N 14.35. Found: C 77.23, H
4.40, N 14.52.
N-((1E,3E,5Z)-7-Amino-6,8,8,-tricyanoocta-1,3,5,7-tetraenyl)-
N-phenylacetamide (110) . Yield: red-maroon solid (602 mg;
52%). Mp 224-226 °C. IR (Nujol, cm-1) 3339, 3220 (NH),
2215, 2198 (CtN). 1H NMR (DMSO-d6, 300 MHz): δ 8.75 (s,
2H), 8.12 (d, J=13.6 Hz, 1H), 7.66-7.53 (m, 4H), 7.37-7.30 (m,
3H), 6.42 (dd, J=14.1 Hz, J0=11.8 Hz, 1H), 5.33 (dd, J=13.5
Hz, J0 =11.5 Hz, 1H), 1.93 (s, 3H). 13C NMR (DMSO-d6, 75
MHz): δ 168.9, 164.7, 154.8, 150.3, 140.6, 138.1, 130.1, 129.1,
128.4, 123.0, 115.6, 115.1, 114.1, 111.7, 98.3, 48.5, 23.0. MS
(EIþ): m/z 329 (Mþ•, 15), 287 (100), 195 (50), 93 (100), 77 (70).
Anal. Calcd for C19H15N5O: C 69.29, H 4.59, N 21.26. Found: C
69.50, H 4.45, N 21.55.
2-(3-Cyano-4-((1E,3E,5E)-7-(2,6-di-tert-butyl-4H-pyran-4-
ylidene)hepta-1,3,5-trienyl)-5,5-dimethylfuran-2(5H)-ylidene)malo-
nonitrile (5d) . A solution of 2,6-di-tert-butyl-4-methylpyrylium
triflate (6) (270 mg, 0.76 mmol), acceptor 120, (150 mg,
0.38 mmol), triethylamine (0.21 mL), and catalytic pyridine in
ethanol (12 mL) was refluxed under argon atmosphere for
30 min (TLC monitoring). After cooling, the resulting solid
was isolated by filtration, washed with cold ethanol, and dried.
A second fraction is recovered from the filtrate. Ether (50 mL)
was added, the organic layer was washed with HCl 10% (2 ꢀ
50 mL) and water (2 ꢀ 50 mL) and dried (MgSO4), and the
solvent was evaporated. The crude product was purified by
column chromatography on silica gel (eluent, hexane/AcOEt (3:1)).
Yield: green-brown solid (134 mg, 76%). Mp 211-213 °C.
IR (Nujol, cm-1) 2211 (CtN). H NMR (CDCl3, 400 MHz):
δ 7.65 (dd, J=13.7 Hz, J0 =11.8 Hz, 1H), 7.17-7.04 (m, 2H),
6.35 (dd, J=13.2 Hz, J0=12.1 Hz, 1H), 6.29 (t, J=12.8 Hz, 1H),
6.27 (d, J=1.5 Hz, 1H), 6.13 (d, J=14.8 Hz, 1H), 5.88 (d, J=1.5
Hz, 1H), 5.69 (d, J=12.7 Hz, 1H), 1.63 (s, 6H), 1.27 (s, 9H), 1.23
(s, 9H). 13C NMR (CDCl3, 75 MHz): δ 176.6, 172.0, 167.9,
167.7, 151.4, 149.4, 142.3, 141.8, 127.3, 126.2, 113.9, 113.4,
113.2, 112.5, 112.5, 112.2, 106.6, 100.3, 96.2, 36.2, 35.9, 27.9,
1
27.8, 26.6. MS (EIþ): m/z 467 (Mþ , 25), 78 (80), 63 (100). Anal.
Calcd for C30H33N3O2: C 77.06, H 7.11, N 8.99. Found: C 77.23,
H 7.01, N 9.22.
3
Acknowledgment. We thank Prof. Larry Falvello and
Dr. Pilar Romero (University of Zaragoza) for helpful
discussions. Financial support from MICINN-FEDER
(CTQ2008-02942 and MAT2008-06522C02-02) and Gobier-
ꢀ
no de Aragon-Fondo Social Europeo (E39) is gratefully
acknowledged. A predoctoral grant (Departamento de
ꢀ
Educacion, Gobierno de Navarra) to N.M.deB. is also
acknowledged.
Supporting Information Available: General experimental
methods, analytical data for 80 and 90, synthesis and analytical
data for 2d-4d, NMR and UV-vis spectra of new compounds,
NLO measurements, DSC curves of 3b-d, X-ray crystallo-
graphic data in CIF format and diagram of the crystal structures
of 2b and 3c, computed energies and Cartesian coordinates of
optimized geometries, and contour plots of frontier molecular
orbitals of compounds 30. This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 17, 2009 6657