Synthesis of phthalonitrile derivatives by photoinduced reactions. New unsymmetrical substituted zinc phthalocyanines

Article abstract of DOI:10.1142/S1088424615500844

The synthesis of new phthalonitrile derivatives by photoinduced reaction is described. Novel unsymmetrically substituted Zn(II) phthalocyanine bearing an aryl alcohol group (ArZnPc) was synthesized by the ring expansion reaction of boron(III) subphthalocyanine chloride with an appropriated phthalonitrile. The spectroscopic and photodynamic properties of these ArZnPc were studied.

Full text of DOI:10.1142/S1088424615500844

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–7
DOI: 10.1142/S1088424615500844
Published at http://www.worldscinet.com/jpp/
Synthesis of phthalonitrile derivatives by photoinduced
reactions. New unsymmetrical substituted zinc
phthalocyanines
Tomas C.Tempesti and María T. Baumgartner*
INFIQC (CONICET), Dpto. de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de
Córdoba, Ciudad Universitaria s/n, Córdoba 5000, Argentina
Received 30 April 2015
Accepted 16 July 2015
ABSTRACT: The synthesis of new phthalonitrile derivatives by photoinduced reaction is described.
Novel unsymmetrically substituted Zn(II) phthalocyanine bearing an aryl alcohol group (ArZnPc) was
synthesized by the ring expansion reaction of boron(III) subphthalocyanine chloride with an appropriated
phthalonitrile. The spectroscopic and photodynamic properties of these ArZnPc were studied.
KEYWORDS: phthalonitrile, phthalocyanine, photodynamic properties, hydroxyaryls, S 1.
RN
as increased solubility and reactivity, and may facilitate
enhanced applications [6]. Different strategies have been
INTRODUCTION
Phthalocyanines (Pc) are used as conventional dyes and
employed for the preparation of A B structures. Statistical
3
pigments. A number of other applications such as efficient
condensation is the most widely used. This non­selective
photosensitizers in photodynamic therapy (PDT) have
method is based on the reaction of two differently
also been found [1]. These photosensitizers exhibit a high
substituted phthalonitriles and it affords a mixture of
absorption coefficient in the visible region of the spectrum,
products which complicate the purification process [7].
mainly in the phototherapeutic window (600–800 nm) [2].
However, the most efficient selective approach involves
The PDT utilizes visible light to active a photosensitizer
ring expansion reaction of subphthalocyanine [8]. Good
which can react with molecules by the electron or hydrogen
yields have been obtained when the subphthalocyanine [9]
transfer, leading to the production of radicals (type I reac­
derivative is treated with a phthalonitrile in the presence
tion), or it can transfer its energy to oxygen, generating the
highly reactive singlet molecular oxygen, O ( Dg) (type II
of a strong base, such as 1,8­diazabicyclo[5.4.0]undec­7­
1
2
ene (DBU), and a metal salt.
reaction). Both pathways can cause cell damages [3].
On the other hand mono and multi hydroxy­substituted
However, the photosensitizing ability of the phthalocyani­
Pc have been reported and shown to be promising
nes can be affected by its aggregation tendency due to large
p conjugated systems. In particular bulky substituents (such
as tert­butyl groups) decrease macrocycle aggregation [4].
photosensitizers for PDT (in vitro, the 2­hydroxy ZnPc
was the most active) [10]. The synthesis of substituted
phthalocyanines is related to the preparation of the new
Many of these new applications require the modifica­
phthalonitriles. Much effort has been geared towards
tion of the phthalocyanine macrocycle. The symmetrically
different synthetic strategies in order to increase the
substituted phthalocyanines have been largely investi­
range of phthalonitriles.
gated; only minor attention has been devoted to the
preparation of asymmetrical phthalocyanines (A B type)
The radical nucleophilic substitution can be considered
an alternative route for the formation of carbon–carbon
bonds [11]. This type of reactions finds increasing
application in the synthesis of complex organic molecules
3
[5]. The presence of a different group is particularly
interesting because it may provide additional features such
[12], particularly, since such reactions are generally
*
Correspondence to: María T. Baumgartner, email: tere@fcq.
carried out under mild conditions and the substrates
tolerate many functional groups. It is well­known that in
unc.edu.ar, tel: +54 351­5353867, fax: +54 351­4333030
Copyright © 2015 World Scientific Publishing Company

2
T. C. TEMPESTI & M. T. BAUMGARTNER
hν
General synthesis procedure of phthalonitrile
derivatives
ArX + Nu-
ArX•-
ArX•- + Nu•
(1)
(2)
(3)
(4)
Ar + X-
•
The reactions were carried out in a 50 mL three­neck
round bottomed flask equipped with a nitrogen inlet and
a magnetic stirrer. To 10 mL of dry and degassed DMSO
under nitrogen were added potassium tert­butoxide
Ar• + Nu-
ArNu•- + ArX
ArX + Nu-
ArNu•-
ArNu + ArX•-
(KOBu­t) and then the corresponding phenol. After 5 min
ArNu + X-
4
­iodophthalonitrile was added and the reaction mixture
was irradiated for 180 min. The reaction was quenched
with an excess of ammonium nitrate and water (30 mL).
The mixture was extracted three times with methylene
chloride (20 mL); the organic extract was washed twice
Scheme 1 SRN 1 reaction
a S 1 reaction (Scheme 1), a nucleophile is combined
RN
with water, dried magnesium sulfate (MgSO ). The
4
with an aryl radical to provide the corresponding coupling
product. Traditionally, aryl halide substrates give the
aryl radical in the photoelectron transfer reaction from
the nucleophile (Equations 1 and 2). The radicals thus
formed can react with the nucleophile to give the radical
anion of the substitution product (Equation 3).
On the other hand, a special regiochemistry has been
determined in the reactions of radicals with anions
able to show ambident behavior [13]. The anion of the
iodide ions in the aqueous solution were determined
potentiometrically. All products are unknown and they
were isolated by column or radial chromatography
1
[
CH Cl :CH OH (95:5)] and characterized by H NMR
2
13
2
3
and C NNR and mass spectrometry.
4
-(4-Phthalonitryl)-2,6-di-tert-butylphenol (4). mp
1
2
(
7
14–216°C. H NMR (400 MHz, CDCl ): d , ppm 1.50
s, 18H), 5.52 (s, 1H, OH), 7.37 (s, 2H), 7.81 (d, 1H),
.86 (dd, 1H), 7.94 (d, 1H). C NMR (400 MHz, CDCl ):
3
H
13
3
2
­naphthol have been reported to react with aryl radicals
dC, ppm 42.69, 114.29, 114.53, 114.89, 117.12, 125.16,
134.06, 134.44, 142.00, 142.95, 150.15, 150.84. CG­MS:
to give C ­substitution at their naphthyl moiety [14].
1
In this paper we propose a selective arylation approach,
based on the radical nucleophilic substitution mechanism,
for the generation of 4­(hydroxyaryl)phthalonitriles from
commercially available reactants. Then, we used these
to synthetize a novel unsymmetrical phthalocyanines
bearing either one or two hydroxy groups, as well as bulky
tert­butyl groups. The spectroscopic and photodynamic
characteristics of Pcs synthetized were also studied.
m/z (%) 55 (11); 57 (32); 137 (11); 289 (21); 317 (100);
­1
3
18 (21); 332 (31). IR (AgBr disc): n, cm 2871–2956
(C­H), 2231 (C≡N), 3619 (O–H).
4
-(1,3-Dihydroxi-4-phenyl)phthalonitrile(6). mp139–
1
1
6
1
1
1
44°C. H NMR (400 MHz, CDCl ): d , ppm 6.58 (t, 1H),
3
H
.62 (dd, 1H), 6.78 (dd, 1H), 7.22–7.32 (m, 2H), 7.72 (d,
13
H). C NMR (400 MHz, CDCl ): d , ppm 108.1, 108.8,
3
C
12.4, 113.5, 114.9, 115.4, 117.6, 121.6, 131.3, 135.4,
54.7, 157.9, 161.6. CG­MS: m/z (%) 63 (17); 64 (11); 65
(
57); 179 (18); 208 (22); 235 (18); 236 (100); 237 (13). IR
­1
EXPERIMENTAL
(AgBr disc): n, cm 2231 (C≡N), 3334–3415 (O–H).
All starting materials were purchased from Sigma­
Aldrich. They were used without further purification.
Synthesis procedure of SubPc
1
DMSO was stored under molecular sieves (4 Å). H
1
,2­Dicyanobenzene (110 mg, 0.86 mmol), boron
1
3
NMR and C NMR spectra were recorded on 400 MHz a
trichloridein heptane 1 M (250 ml), and 1­chlorona­
phthalene (4 mL) were taken in a microwave tube. The
contents were irradiated in a commercial microwave
oven at 100 W and 200°C for 10 min. After cooling,
the reaction mixture was poured onto a hexane solvent.
The residue was purified by column chromatography
nuclear magnetic resonance spectrometer with CDCl as
3
solvent. Gas chromatographic analyses were performed
on a cromatograph with a flame­ionization detector and
using a HP­5 capillary column (30 m × 0.32 mm × 0.25 mm
film thickness). The GS/MS analyses were carried out
on a Shimadzu GC­MS QP 5050 spectrometer, using a
Vf­5ms 30 m × 0.25 mm × 0.25 mm column. Irradiation
was conducted in a reactor equipped with two 400­W
lamps emitting maximally at 350 nm (Philips Model
HPT, air and water refrigerated). Potentiometric titration
of halide ions was performed in a pH meter using an
(
hexane/CH Cl , (98:2)) to give SubPc (103 mg, 85%),
2 2
1
characterized by UV and H NMR.
General procedure of A B-phthalocyanine formation
3
A solution of phthalonitrile derivate (0.19 mmol) and
DBU (7 mL, 0.07 mmol) in 3 mL of DMSO/1­chloronaph­
thalene (5:1) was added drop­wise to a suspension
of SubPc (43 mg, 0.10 mmol) and zinc(II) acetate
dihydrate (22 mg, 0.10 mmol) in 1 mL of DMSO/1­
chloronaphthalene (5:1). The mixture was placed in a
microwavereactor(100W,140°C,2h).Thenitwascooled
+
Ag/Ag electrode. Melting points were not corrected.
Column chromatography was performed on silica gel
(
70–270 mesh ASTM). IR spectra were obtained by
a Nicolet­510 FT­IR spectrometer. UV spectra were
performed on an UV­1800 Shimadzu spectrophotometer.
Microwave monomode CEM­Discovery reactor.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 2–7

SYNTHESIS OF PHTHALONITRILE DERIVATIVES BY PHOTOINDUCED REACTIONS
3
to room temperature and precipitated with 50 mL of water.
The solid was separated by centrifugation and washed
with hexane. Pc1, yield 40%. ESI­MS: m/z 781.2390
CN
CN
CN
CN
CN
CN
(i)
(ii)
+
+
[
M + H] (781.2376 calcd. for C H N OZn + H ).
4
6
36
8
(iii)
+
Pc2, yield 37%. ESI­MS: m/z 685.4389 [M + H]
(
NO2
NH2
I
+
685.1073 calcd. for C H N O Zn + H ).
3
8
20
8
2
2
1
Spectroscopic studies
Scheme 2 Synthesis of 4­iodophthalonitrile (i) Fe, methanol,
HCl, D, 2h. (ii) NaNO , HCl, <5°C, 1.5 h. (iii) KI, H O, 0.5 h.
Absorption and fluorescence spectra were recorded
2
2
at 25.0 ± 0.5°C using 1 cm path length quartz cells.
The fluorescence quantum yield (F ) of Pc1 y Pc2
were calculated by comparison of the area below the
corrected emission spectrum in DMF with that of Zn(II)
F
C of the anion, 4­(4­phthalonitryl)­2,6­di­tert­butylphenol
4
[19] 4 (36%) (Equation 5, Table 1, Exp. 1).
phthalocyanine (ZnPc) as a reference (F = 0.28) [15].
F
OH
CN
Absorbance of sample and reference were matched at
the excitation wavelength (640 nm) and the areas of the
emission spectra were integrated in the range 650–800 nm.
hν
1 +
HO
CN
(5)
KOBu-t
3
4
Steady state photolysis
The by­products observed correspond to decomposi­
tion of phthalonitrile. No reaction was observed in the
absence of light irradiation (Table 1, Exp. 2). In order
to confirm the electron transfer mechanism, inhibition
reactions were performed by adding amounts of
2,2,6,6­tetramethyl­1­piperidinyloxy (TEMPO) or para­
dinitrobenzene (p­DNB), commonly used as inhibitors in
the mechanistic study (Scheme 1) [11]. Inhibition of the
synthesis of 4 was observed in both reactions (Table 1,
Exps. 3 and 4). These effects provide good evidence of
the S 1 mechanism in the C ­arylation.
Solutions of 9,10­dimethylanthracene (DMA, 35 mM,
absorbance 0.7) and photosensitizer (Q­band, absorbance
0
.2) in different media were irradiated in 1 cm path
length quartz cells (2 mL) with monochromatic light
at lirr = 670 nm, from a LED NES110NR lamp (Red
(
625 nm) Lustrous­green Technology of Lightings).
The kinetics of DMA photooxidation were studied by
following the decrease of the absorbance (A) at lmax
=
3
78 nm. The observed rate constants (k ) were obtained
obs
by a linear least­squares fit of the semilogarithmic plot
RN
4
of Ln A /A vs. time. Photooxidation of DMA was used
0
Subsequently, solvent and the number of equivalents of
3 or base were tested to identify optimal conditions. The
formation of 4 was favored by increasing the equivalents of
1
to determine O ( D ) production by the photosensitizer
2
g
[
[
16]. ZnPc (F = 0.56) was used as a reference in DMF
D
17]. Measurements of the sample and reference under
3
and decreasing the excess of base (Table 1, Exps. 5 and 6).
By changing the solvent from DMSO to NH3
the same conditions afforded F for phthalocyanines by
direct comparison of the slopes in the linear region of
D
accompanied by a decrease in the concentration of
the reactants, 4 was the main product formed (65%)
accompanied by decrease of decomposition of ArH
the plots. All the experiment were performed at 25.0 ±
0
.5°C. The Pc and DMA controls do not change along
the photolysis experiment (see Supplementary material).
The pooled standard deviation of the kinetic data, using
different prepared samples, was less than 10%.
(
Table 1, Exp. 7). The small change in the yield does not
justify the use of this solvent.
Thus, the best C­substitution yield was obtained using
3
equivalents of 3, defect of base, DMSO and 180 min of
irradiation (Table 1, Exp. 8).
RESULTS AND DISCUSSION
Resorcinol (5). The best conditions developed during
the study with anion of 3 were applied to resorcinol (5)
reactions. The reaction of 5, base and 1 (Equation 6) gave
the substitution product 6 in good yield (Table 1, Exp. 9).
Synthesis of 4-substituted phthalonitriles
The required starting material for the radical nucleo­
philicsubstitutionreactionis4­iodophthalonitrile(1)which
was prepared in two sequential steps from 4­nitrophthalo­
nitrile (2) (Scheme 2) [18] with good yield (71%).
OH
OH
CN
hν
1
+
HO
CN
(6)
KOBu-t
OH
2,6-Di-tert-butylphenol (3). The photoinitiated reaction
5
6
of 1 with anion of 2,6­di­tert-butylphenol (3) obtained by
deprotonation with potassium tert­butoxide (KBuO­t) in
DMSO, (substrate:nucleophile:base = 1:3:4) afforded 72%
yield of iodide ion release (indicator of the total yield of the
reaction), and the product corresponding to substitution at
When the crude product was purified by direct
crystallization the yield increased to 77% (Table 1,
Exp. 10). In this reaction, we observed that the yield of
product decreased with excess base.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 3–7

4
T. C. TEMPESTI & M. T. BAUMGARTNER
a
Table 1. Photoinitiated reactions of 4­iodophthalonitrile 1 with nucleophiles in DMSO
3
3
3
­
b
c
Exp.
1
1 M × 10
Nucleophile M × 10
Base M × 10
X , %
Products (% yields)
32
32
31
32
30
31
9.4
50
79
79
79
3, 95
128
128
122
122
204
323
51
72
<5
35
58
81
94
70
69
89
88
4, 36
—
d
2
3, 95
e
3
3, 91
4, 13
4, —
4, 34
4, 53
4, 65
f
4
3, 93
5
6
3, 149
3, 303
3, 50
h
7
g
8
9
3, 148
5, 234
5, 252
5, 218
144
234
240
245
4, 49
g
6, 48
i
1
0
1
6, 77
i
1
6, 27
a
Photoinitiated reactions (unless indicated) carried out under nitrogen. Reaction time = 180 min.
b
c
Potentiometrically determined on the basis of the ArX concentration. Determined by GLC and
d
the internal standard method on the basis of the ArX concentration. Reaction carried out in the
dark. p­dinitrobenzene (40 mmol.%). TEMPO (41 mmol.%). Isolated yield by chromatography
column. Solvent = NH (l). T = ­33°C. Purified by direct crystallization.
e
f
g
h
i
3
Synthesis of phthalocyanine derivatives
Spectroscopic studies
The 4­(hydroxyaryl)phthalonitrile derivatives were
The absorption spectra of the Pc1 and Pc2 were
studied in different solvents (Fig. 1). These show the
typical Soret and Q­bands characteristic of zinc(II)
phthalocyanine derivatives, in the visible region at ca.
600–750 nm (Q­band) attributed to the p–p* transition
[15]. A sharp absorption bands were obtained indicating
that there was no aggregation of these phthalocyanines
[20]. Organic solvents are known to reduce aggregation
whereas aqueous solvents result in highly aggregated
complexes and the peripheral substituents increase the
distance between the planar macrocycle rings thereby
making solvation easier.
By varying the solvent polarity, a small effect is
observed on the location of Q­bands. The main Q­band
for the unsubstituted ZnPc shows similar values to
those observed for Pc1 and Pc2 in the different solvents
used [15].
employed to synthesize the A B­phthalocyanine by ring
3
expansion of SubPc (Scheme 3). SubPc was obtained
from an appropriate phthalonitrile fused with boron
trichloride in 1­chloronaphthlene by conventional heating
9] or microware irradiation [19]. In our procedure, the
SubPc was prepared using microware irradiation (100 W,
[
2
00°C, 10 min) with 85% yield.
The reaction of SubPc and phthalonitriles 4 and 6 were
performed in the presence of DBU and Zn(II) acetate
dehydrate in DMSO/1­chloronaphthalene using microwave
irradiation. Undersuchconditions, onlyonephthalocyanine
was obtained facilitating the purification process.
After heating the mixture of SubPc and 4 for 2 h, Pc1
afforded with a 40% yield (Scheme 3). The SubPc was
also ring expanded with 6 to afford Pc2 (37%).
In all organic solvents, Pc1 and Pc2 are soluble (~2 ×
­6
Cl
1
0
M). The spectrum of Pc2 has similar intensity
N
N
CN
CN
in all these. However, the intensity of Pc1 spectrum
decreased in methanol as a consequence of the increase
in the lipophilic (two tert­butyl groups) character of
macrocycle.
Both Pcs were very poorly solubilized in water, as
shown by the broadening and low signal intensity.
Figure 2 shows the Pc2 spectrum in water at different
pHs. It can be seen that solubility increases with
increasing pH, i.e. with the deprotonation of the hydroxyl
group.
N
N
N
N
N
N
B
N
N
Zn
N
R
N
N
Zn(II)
DMSO/DBU
MW
N
OH
R
SubPc
Pc1
Pc2
R =
The spectroscopic properties of Pc1 and Pc2 were
compared with that of ZnPc in DMF, see Table 2. The
Q­bands of Pc1 and Pc2 present a ~2 nm bathochromic
HO
OH
Scheme 3 Synthesis of phthalocyanine
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 4–7

SYNTHESIS OF PHTHALONITRILE DERIVATIVES BY PHOTOINDUCED REACTIONS
5
­6
Fig. 1. Absorption spectra of (a) Pc1 and (b) Pc2 in different solvents. Concentrations ~2 × 10 M
Table 2. Spectroscopic data, fluorescence quantum yield (FF),
kinetic parameters (k ) for the photooxidation reaction of DMA
obs
1
and quantum yield of O ( D ) production (F ) of Pcs in DMF
2
g
D
Pc
labs, nm lems, nm FF
kobs
FD
a
ZnPc
668
670
671
676
678
680
0.28
0.56
­
4
Pc1
0.23 (1.86 ± 0.1)10 0.30 ± 0.02
­4
Pc2
0.29 (3.83 ± 0.1)10 0.62 ± 0.02
a
ZnPc = zinc phthalocyanine (R = H). Value from Ref. 23.
Photodynamic activity in DMF solution
2
1
Taking into account that DMA quenches O ( D )
g
exclusively by chemical reaction [23], and that the
typical first­order kinetic plots of the DMA absorption at
378 nm with time describing the progress of the reaction.
Consequently, this substrate, DMA, can be used as a
method to evaluate the ability of the photosensitizers to
Fig. 2. Absorption spectra of Pc2 in water at different pH.
Concentrations ~2 × 10 M
­
6
2
1
shift with respect to ZnPc (668 nm) [21] due to the effect
of the hydroxyaryl groups.
produce O ( D ) in solution [8, 24].
g
1
The quantum yield of O ( D ) production (F ) was
2
g
D
The steady­state fluorescence emission spectra of Pc1
and Pc2 were performed in DMF (Fig. 3b). The spectra
show two bands in the red spectral region, which are
characteristic for similar zinc(II) phthalocyanines [2]. A
small Stokes shift (≈8 nm) was observed, indicating that
the spectroscopic energy is nearly identical to the relaxed
energy of the singlet state.
As expected from the absorption data, the emission
maxima are bathochromically shifted with respect to
that of ZnPc. By comparison with ZnPc as a Ref. [22],
calculated by comparing the slope for the Pc1 and Pc2
(Fig. 4) with the corresponding slope obtained for the
reference ZnPc.
The results for F (Table 2) follow the order Pc2 >
D
ZnPc > Pc1. Very close values of F were obtained for
D
these phthalocyanines indicating that they are higher
1
efficient photosensitizers to produce O ( D ) in DMF
2
g
(Table 2).
In summary, we describe in this work a different and
direct protocol to obtain new phthalonitriles (biaryl type)
using photoinduced nucleophilic substitution. We have
developed a simple and versatile route for the synthesis
of novel hydroxyl aryl derivates in good yields using
commercial and stable starting materials.
the values of fluorescence quantum yields (F ) were
F
calculated in DMF. The results of F are summarized in
F
Table 2. They are appropriate values for quantification
of phthalocyanine by fluorescence emission techniques.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 5–7

6
T. C. TEMPESTI & M. T. BAUMGARTNER
Fig. 3. (a) Absorption spectra of ZnPc (...), Pc1 (­­­) and Pc2 (—) in DMF. (b) Normalized fluorescence emission of Pc1 (­­) and
­6
Pc2 (—) in DMF, lexc = 640 nm. Concentrations ~2 × 10 M
Acknowledgements
This work was supported by Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET) of
Argentina, SECYT UNC and FONCyT.
Supporting information
1
13
General methods and materials. H NMR and
C
NMR spectra, and MS of compounds 4, 6, Pc1, Pc2 are
given in the supplementary material. This material is
available free of charge via the Internet at http://www.
worldscinet.com/jpp/jpp.shtml.
Fig. 4. First­order plots for the photooxidation of DMA
photosensitized by Pc1 (•), and ZnPc () in DMF. Values
represent mean±standard deviation of three separate
experiments. The insert plots the electronic absorption spectra
of DMA at different times
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