Synthesis of polyconjugated carbazolyl-oxazolones by a tandem hydrozirconation-Erlenmeyer reaction. Study of their hyperpolarizability values

Article abstract of DOI:10.1016/S0040-4039(02)00806-7

New push-pull systems with carbazole as donor moiety and oxazolone as electron withdrawing group have been synthesized by an AgClO₄-catalyzed hydrozirconation and ulterior Erlenmeyer reaction. Studies carried out with the semiempirical quantum method PM3 (MOPAC93) pointed a low dependence of the hyperpolarizability value on the number of double bonds at the conjugated bridge by β(0)∝n^0.8. Introduction of rigidity in the polyene chain or additional donor groups in the carbazole moiety pointed to exertion of a large effect on hyperpolarizability values that could avoid unnecessary efforts in costly path elongation synthesis.

Full text of DOI:10.1016/S0040-4039(02)00806-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4333–4337
Synthesis of polyconjugated carbazolyl–oxazolones by a tandem
hydrozirconation–Erlenmeyer reaction. Study of their
hyperpolarizability values
Jose´ Luis D´ıaz,a Bele´n Villacampa,b Francisco Lo´pez-Calahorraa and Dolores Velascoa,*
aDepartament de Qu´ımica Orga`nica, Facultad de Qu´ımica, Universidad de Barcelona, Mart´ı i Franque`s, 1-11,
E-08028 Barcelona, Spain
bDepartamento de F´ısica de la Materia Condensada, Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna, 12,
E-50009 Zaragoza, Spain
Received 25 January 2002; accepted 23 April 2002
Abstract—New push–pull systems with carbazole as donor moiety and oxazolone as electron withdrawing group have been
synthesized by an AgClO4-catalyzed hydrozirconation and ulterior Erlenmeyer reaction. Studies carried out with the semiempirical
quantum method PM3 (MOPAC93) pointed a low dependence of the hyperpolarizability value on the number of double bonds
at the conjugated bridge by i(0)8n0.8. Introduction of rigidity in the polyene chain or additional donor groups in the carbazole
moiety pointed to exertion of a large effect on hyperpolarizability values that could avoid unnecessary efforts in costly path
elongation synthesis. © 2002 Elsevier Science Ltd. All rights reserved.
Carbazole derived compounds have technological inter-
est due to their photoconducting1 and second-order
non-linear optical2 properties, being suitable candidates
for photorefractive3,4 and electroluminescence5,6 appli-
cations. In a previous paper,7 the synthesis and study of
non-linear optical properties of push–pull 2,3,7,9-poly-
substituted-carbazole chromophores has been reported.
Conformational and solvent effects7 were studied by
semiempirical quantum calculations. The incidence of
these aspects and others like the aromatization effect
has been widely reported.8 However, one of the main
factors that modifies the NLO response is the length of
the conjugated linkage between donor and withdrawing
moieties.9–16 So, the elongation of the conjugated path
gated the effect of placing additional donor groups into
the carbazole unit to enhance the non-linear optical
response. Here we present the synthesis of new carba-
zole–oxazolone (Cz-OXA) chromophores (Fig. 1) and
their experimentally and theoretically determined
hyperpolarizabilities. These new systems have the car-
bazole donor moiety and the oxazolone ring linked
through a polyethylene bridge of n double bonds.
The synthesis of the Cz-[n]-OXA compounds was per-
formed starting from 9-methyl-9H-3-carbazolecarbalde-
hyde 1, prepared from commercially available
carbazole, following the scheme of Fig. 2. Among the
different options for the homologation of polyethylene
systems from conjugated aldehydes,17–20 the hydrozir-
conation method of Maeta and Suzuki has been consid-
ered.21 Hydrozirconation is a one-step Grignard-type
is
a well established strategy to enhance NLO
properties.
Following our studies on non-linear optical (NLO) and
electronic properties of polysubstitued-carbazoles,7 we
have examined carbazole–oxazolone compounds as new
NLO-chromophores. We were interested in analyzing
the NLO-behaviour of carbazole–oxazolone com-
pounds as well as the influence of the length and the
nature of the conjugated bridge in the push–pull system
on hyperpolarizability values. We have also investi-
* Corresponding author. Tel.: +34 93 4021252; fax: +34 93 3397878;
e-mail: dvelasco@qo.ub.es
Figure 1. Cz-[n]-OXA systems.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00806-7

4334
J. L. D´ıaz et al. / Tetrahedron Letters 43 (2002) 4333–4337
Figure 2. Synthetic scheme for 2, 4 and 6. Reagents and conditions: CH3Cl, 60°C, 12 h; (b) DMF, POCl3, 90°C; (c)
2-(4-nitrophenylcarboxamido)acetic acid, Ac2O, NaOAc, D; (d) 1-ethoxyethyne, Cp2ZrHCl, AgClO4; (e) HCl (3N).
Hyperpolarizability
values
were
experimentally
reaction for two- or four-carbon homologation of alde-
hydes by an AgClO4-catalyzed addition of zirconocene
chloride (Schwarz’ reactive) to an alkoxyalkyne.22 After
acidic hydrolysis23,24 the desired product with the elon-
gated alkenyl chain was obtained (3, 65%; 5, 60%).
Product 3, 3-(9-methyl-9H-3-carbazolyl)-2-propenal,25
was yielded as an E/Z 95:5 mixture. Pure E isomer 3a
was obtained by recrystallization in CHCl3 in a 40%
yield. After Erlenmeyer reaction26–28 with 2-(4-nitro-
obtained for all the synthesized systems. vi values
calculated by PM3-MOPAC9332,33 and experimental
EFISH values for compounds 2 (n=0), 4 (n=1) and 6
(n=2) are registered in Table 1. PM3 method was
chosen because of the good accordance with experimen-
tal data observed for other polysubstituted-carbazole
systems.7 Optimized geometries for compounds 2, 4 and
6 resulted in an all trans disposition of the polyethylene
chain. When n increases, a slight deviation from planar-
ity is observed in the polyethylene bridge. The represen-
tation of vi(0) values in front of n (number of double
bonds) shows that no substantial variation is expected
above n=9 (Fig. 3), being the estimated dependence of
vi(0) on n by vi(0)8n0.8. In spite of the good concor-
dance found between the vi(0) experimental value and
that calculated by PM3 for n=0, the theoretical vi(0)
values show poor accordance with experimental EFISH
values when n increases (n=1, n=2). The experimen-
tally found dependence of vi(0) values on n was
vi(0)8n0.5, sensible lower than the expected one. The
two level model34 has been largely used in push–pull
systems in which charge transfer occurs along the
molecular dipole axis. So, the hyperpolarizability tensor
b has only a significant component parallel to that axis
(one-dimensional model). Although the above men-
tioned difference for experimental and theoretical vi(0)
values are significant for those molecules with more
phenylcarboxamido)acetic acid, system
4
4-[3-(9-
methyl)-9H-3-carbazolyl)-2-propenylidene]-2-(4-nitro-
phenyl)-4,5-dihydro-1,3-oxazol-5-one,29 was recovered
(85%) as a diastereomeric mixture Z,2E,/E,2E=6:4 (1H
NMR). Pure Z,2E isomer (4a) was obtained after
recrystallization in CHCl3 (35%). Synthesis of 4-[5-
(9-methyl)-9H-3-carbazolyl)-2,4-pentadienylidene]-2-(4-
nitrophenyl)-4,5-dihydro-1,3-oxazol-5-one 6 was carried
out by a two-carbon homologation reaction of com-
pound 3a to achieve first 5-(9-methyl)-9H-3-carba-
zolyl)-2,4-pentadienal30
5
in 60% yield with
a
diastereomeric excess of 78% (2E,4E/2Z,4E=89:11)
before recrystallization. Pure 2E,4E system (5a) was
obtained after recrystallization in CHCl3. Erlenmeyer
reaction yields compound
6
(60%).31 Pure 4-
[(Z,2E,4E)]-4,5-dihydro-1,3-oxazol-5-one isomer (6a)
was isolated with a 44% yield after recrystallization in
CHCl3/AcOEt.

J. L. D´ıaz et al. / Tetrahedron Letters 43 (2002) 4333–4337
4335
Table 1. Theoretical values for Cz-[n]-OXA systems and EFISH experimental values at 1.38 mm in CHCl3 for systems with
n=0 (2), 1 (4a), 2 (6a)
n
DHf0 (kcal/mol)
v (10−18 esu)
i(0) (10−30 esu) vi(0) (10−48 esu) uICT (nm)
viEFISH (10−48
esu)
vi(0)EFISH (10−48 esu)
0
1
2
3
4
5
6
7
8
9
36.5
48.3
62.5
76.4
90.3
104.3
118.2
132.2
146.1
160.0
6.2
7.0
7.2
7.4
7.5
7.6
7.6
7.7
7.7
7.7
33.5
59.0
81.9
102.5
119.9
134.2
145.7
154.9
162.3
168.2
208
413
590
758
900
1020
1107
1193
1250
1295
479
501
526
430950
400950
570960
200923
260930
365940
Figure 3. Theoretical dependence of vi(0) values on n by PM3 method.
than one double bond between the donor and the
acceptor group and not for the first example of the
series (n=0), it could suggest the presence of significant
b components along other directions. Regarding the
nature of the chromophores, where the carbonyl of the
oxazolone moiety and the nitrophenyl groups act as
acceptors in different directions, we may consider the
oxazolone chromophore to be a two-dimensional (2D)
chromophore. Some articles have been recently
published35,36 pointing to the non one-dimensional
character of the optical non-linearity of several carba-
zole derivatives. However, semiempirical calculations
(MOPAC93/PM3) carried out for all the polyene com-
pounds of Table 1 have shown a main diagonal compo-
nent bxxx along the dipole axis, clearly higher than the
other off-diagonal components (Table 2). Moreover,
the poor accordance between experimental and theoret-
ical values increases as n increases, in opposite sense of
the relative meaning of the calculated off-diagonal
components.
A plausible explanation for the deviation between
experimental and theoretical vi(0) values could be the
loss of planarity that long polyconjugated systems
Table 2. Tensor components of the hyperpolarizability (i(0)/10−30 esu) calculated by PM3 method for the systems repre-
sented in Fig. 1
n
ixxx
ixyy
ixzz
iyxx
iyyy
iyzz
izxx
izyy
izzz
1
2
3
4
5
6
7
8
87
130
169
202
228
249
266
279
12
10
8
6
5
4
3
3
0
0
0
0
0
0
0
0
28
30
29
25
21
17
13
10
4
2
1
0
0
0
0
1
0
0
0
0
0
0
0
0
2
2
2
2
2
2
2
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0

4336
J. L. D´ıaz et al. / Tetrahedron Letters 43 (2002) 4333–4337
exhibit in solution. When n increases, a progressive
coiling occurs in the polyconjugated chain and elec-
determined from EFISH measurements at 1.9 mm in
CHCl3 being vi=550 50×10−48 esu and vi(0)=330
40×10−48 esu. Comparing the NLO-properties obtained
applying the two structural modifications, it can be
pointed that the effect of the addition of two conju-
gated double bonds to the polyene chain (n=2, Fig. 1)
is equivalent to the introduction of two alkoxy donor
groups into the carbazole unit. That points to be a
synthetic alternative to avoid unnecessary, difficult and
low yielded synthesis of high polyconjugated systems.
tronic delocalization on
p
orbitals decreases.
Intramolecular charge transfer is not as effective as
expected for a planar conformation and hyperpolariz-
ability rises only in a soft way. To investigate whether
the loss of planarity could be related to the lower
correlation found (vi(0)8n0,5) a theoretical study with
more rigid systems was undertaken. Table 3 shows the
theoretical vi(0) values referred to rigid polyconju-
gated Cz-OXA systems, where a cyclohexenyl unit has
been introduced to restrict conformational freedom
(Fig. 4). Both vi(0) theoretical values of non-rigid
Cz-[n]-OXA systems (Fig. 1, Table 1) and rigid-Cz-[n%]-
OXA systems (Fig. 4, Table 3) are very close for short
conjugated chains, but when the length of the conju-
gated bridge increases higher theoretical hyperpolariz-
ability values are predicted for rigid systems.
Although the semiempirical calculations (MOPAC/
PM3) carried out for all the compounds have shown a
main diagonal component along the dipole axis, there
are too some non-negligible components. HRS mea-
surements could be interesting in order to obtain fur-
ther information about these components. Synthesis of
carbazole–oxazolone push–pull systems substituted
with different additional donor groups and rigid poly-
conjugated chain are also currently being undertaken in
our group.
The effect of two additional donor groups, alkoxy
groups, placed in the donor moiety of the push–pull
Cz-[n]-OXA system has also been considered. System 7,
4-(2-methoxy-9-methyl-7-undecyloxy-9H-carbazol-3-yl-
methylene)-2-(4-nitrophenyl)-4,5-dihydro-1,3-oxazol-5-
one (Fig. 5), has also been synthesized following the
same reaction scheme reported for compound 2 in Fig.
2, from 2,7-dimethoxycarbazole as starting material.37
Hyperpolarizability values of compound 7 have been
Acknowledgements
Financial support from Comisio´n Interministerial de
Ciencia
y
Tecnolog´ıa (Project PB 96-1491) is
acknowledged.
Table 3. Theoretical values for systems represented in Fig.
4 by PM3 method
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1
8
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CH, J=9 Hz, 2H), 8.14 (dd, CH, J=7 Hz, J=1 Hz, 1H),
7.79 (dd, CH, J=8.5 Hz, J=1.5 Hz, 1H), 7.74 (dd, CH,
J=15.5 Hz, J=11.5 Hz, 1H), 7.52 (m, CH, J=8.0 Hz,
J=7.0 Hz, J=1 Hz, 1H), 7.43 (dd, CH, J=15.5 Hz, J=1
Hz, 1H), 7.42 (d, CH, J=8 Hz, J=1 Hz, 1H), 7.41 (d,
CH, J=8.5 Hz, 1H), 7.34 (dd, CH, J=11.5 Hz, J=1 Hz,
1H), 7.31 (m, CH, J=7 Hz, J=1 Hz, 1H), 3.89 (s,
N-CH3, 3H) ppm. 13C NMR (75 MHz, CDCl3): l=
168.90, 164.84, 149.61, 148.23, 142.33, 141.34, 132.32,
128.37, 128.28, 127.57, 126.49, 124.10, 121.70, 120.66,
120.08, 109.17, 109.13, 29.36 ppm. MS-FAB(+)-NBA:
423 (M, 100%). Anal. (calcd for C25H17N3O4, found): %C
(70.91, 70.20), %H (4.05, 4.14), %N (9.93, 9.85). (Calcd
for C25H17N3O4·CHCl3 [recrystallization in CHCl3],
found): %C (57.53, 58.80), %H (3.34, 3.50), %N (7.74,
7.82), %Cl (19.59, 18.32).
18. Contreras, B.; Duhamel, L.; Ple´, G. Synth. Commun.
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22. General procedure for hydrozirconation: 14.40 mmol of
1-ethoxyethyne (40% hexane) was added under a N2
atmosphere to a solution of 14.40 mmol of zirconocene
(Cp2ZrHCl) in anhydrous CH2Cl2 (25 mL) cooled to 0°C.
After stirring at 0°C for 15 min, a solution of 9.6 mmol
of formilcarbazole in anhydrous CH2Cl2 (25 mL) was
added at room temperature and then 0.48 mmol of
AgClO4. After 30 min at room temperature, Et2O (200
mL) was added. The mixture was poured into an aqueous
saturated NaHCO3 solution, filtered through Celite and
the organic layer was separated. The aqueous layer was
washed with Et2O and the organic extracts were mixed. A
solution of 200 ml HCl (3N) was added and the mixture
was stirred under a N2 atmosphere for 2 h. The organic
layer was then separated and washed with a NaHCO3
solution until pH 7 was reached. The organic layer was
dried with Na2SO4 and filtered. Evaporation gave a
residue which was purified by flash chromatography
(CH2Cl2).
1
30. System 5: TLC (SiO2, CH2Cl2, UV): Rf=0.23. H NMR
(200 MHz, CDCl3): l=9.62 (dd, CHO, J=8 Hz, J=0.8
Hz, 1H), 8.23 (d, CH, J=1.8 Hz, J=1.4 Hz, 1H), 8.12
(dd, CH, J=7.6 Hz, J=1 Hz, 1H), 7.67 (dd, CH, J=8.4
Hz, J=1.4 Hz, 1H), 7.57–7.00 (m, CH, 7H), 6.27 (dd,
CH, J=14.8 Hz, J=8 Hz, 1H), 3.87 (s, N-CH3, 3H)
ppm. Anal. (calcd for C18H15NO, found): %C (82.73,
82.80), %H (5.79, 5.81), %N (5.36, 5.27).
31. System 6: TLC (SiO2, CH2Cl2, UV): Rf=0.57. Mp: 263–
266°C. 1H NMR (500 MHz, CDCl3): l=8.36 (d, CH,
J=1.5 Hz, 1H), 8.34 (d, CH, J=9 Hz, 2H), 8.19 (d, CH,
J=9 Hz, 2H), 8.10 (dd, CH, J=7.5 Hz, J=1 Hz, 1H),
7.65 (dd, CH, J=8.5 Hz, J=1.5 Hz, 1H), 7.61 (dd, CH,
J=14 Hz, J=1 Hz, 1H), 7.50 (m, CH, J=8 Hz, J=7 Hz,
J=1 Hz, 1H), 7.40 (dd, CH, J=8 Hz, J=1 Hz, 1H), 7.36
(dd, CH, J=14 Hz, J=12 Hz, 1H), 7.28 (m, CH, J=7
Hz, J=7.5 Hz, 1H), 7.22–7.09 (m, CH, 4H), 3.86 (s,
N-CH3, 3H) ppm. MS-MALDI-TOF: m/z=449.2 (M).
UV [u(nm), (m), CHCl3]: 526 (33300), 366 (11170). Anal.
(calcd for C27H19N3O4·CHCl3, found): %C (59.12, 61.13),
%H (3.54, 3.73), %N (7.39, 7.70), %Cl (18.70, 18.43).
32. Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. Stewart,
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25. System 3: TLC (SiO2, CH2Cl2, UV): Rf=0.28. Mp:
1
126.3–128.0°C. H NMR (200 MHz, CDCl3): l=9.72 (d,
CHO, J=8 Hz, 1H), 8.30 (dd, CH, J=1.8 Hz, J=1.4 Hz,
1H), 8.13 (dd, CH, J=8 Hz, J=1.2 Hz, 1H), 7.73 (dd,
CH, J=8.4 Hz, J=1.4 Hz, 1H), 7.69 (d, CH, J=15.8 Hz,
1H), 7.55 (m, CH, J=1.2 Hz, J=8.4 Hz, J=7.6 Hz, 1H),
7.44 (d, CH, J=8.4 Hz, J=1.8 Hz, 1H), 7.43 (d, CH,
J=8.4 Hz, J=1.2 Hz, 1H), 7.31 (m, CH, J=7.6 Hz, J=8
Hz, J=1.2 Hz, 1H), 6.80 (dd, CH, J=8 Hz, J=15.8 Hz,
1H), 3.90 (s, N-CH3, 3H) ppm. 13C NMR (75 MHz,
CDCl3): l=193.67, 154.54, 142.6, 141.41, 126.51, 126.08,
125.79, 125.01, 123.23, 122.46, 121.73, 120.43, 119.91,
108.99, 108.94, 29.34 ppm. Anal. (calcd for C16H13NO,
found): %C (81.68, 81.70), %H (5.57, 5.71), %N (5.95,
5.96).
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Ac2O were added under a N2 atmosphere to a mixture of
0.42 mmol of formilcarbazole, 0.42 mmol of 4-nitroben-
zoilglycine and 0.42 mmol of anhydrous NaAcO. The
mixture was stirred first at 60°C for 30 min, and then at
110°C for 1 h. H2O was added dropwise and the mixture
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