Asymmetric phthalocyanine synthesi by ROMP-capture-release

Article abstract of DOI:10.1021/ol900352f

Statistical condensation of norbornenyl-tagged phthalonitrile 3 (Pn A) and 4,5-di-4-methoxyphenoxyphthalonitrile 4 (Pn B) followed by ring-opening metathesis polymerization (ROMP) of Pcs AB₃ and B₄ produced asymmetric Pc-appended pol

Full text of DOI:10.1021/ol900352f

ORGANIC
LETTERS
2009
Vol. 11, No. 10
2061-2064
Asymmetric Phthalocyanine Synthesis
by ROMP-Capture-Release
Xiaochun Chen, Thaddeus R. Salmon, III, and Dominic V. McGrath*
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721
mcgrath@u.arizona.edu
Received February 18, 2009
ABSTRACT
Statistical condensation of norbornenyl-tagged phthalonitrile 3 (Pn A) and 4,5-di-4-methoxyphenoxyphthalonitrile 4 (Pn B) followed by ring-
opening metathesis polymerization (ROMP) of Pcs AB3 and B4 produced asymmetric Pc-appended polymers. Acidic cleavage of the resulting
polymers afforded 2,3,9,10,16,17-hexa-(4-methoxyphenoxy)-23-hydroxy Pc 9. A more soluble 2,3,9,10,16,17-hexa-4-pentylphenoxy-23-hydroxy
Pc 13 was synthesized by the same strategy and modified with sebacoyl chloride demonstrating that the unmasked hydroxyl site is reactive
as a nucleophile.
Phthalocyanines (Pcs) are 18 π-electron tetrapyrrolic mac-
rocycles that have generated interest since their first discov-
ery.1 A number of exceptional physical and chemical
properties arise from their characteristic π-electron delocal-
ization, such as high chemical and thermal stability, intense
absorption in infrared/near-IR, semiconductivity, and large
optical nonlinearities.2,3 To explore the utility of Pcs for their
applications in materials science, chemical modification of
the Pc ring has been extensively investigated. The attachment
of various substitutents to the periphery allows fine-tuning
of the Pc’s self-organization capabilities,4 as well as sup-
pression of aggregation,5 useful for applications in optical
limiting,6 photodynamic therapy (PDT),7 and thin-film pho-
tovoltaic devices.8
Of particular interest in a number of contexts are asym-
metrically substituted Pcs, which can possess improved
photodynamic properties9 and unique second-order nonlinear
optical effects.10 Syntheses of asymmetrically substituted Pcs
are relatively scarce. The simplest approach to the preparation
of asymmetrically substituted Pcs is the statistical crossover
Linstead macrocyclization11 of two different phthalonitrile
(Pn) precursors.12 However, this method inherently yields a
mixture of Pc products, and the isolation of the desired Pcs
from the mixture usually requires extensive chromatographic
purification. Methods to achieve direct synthesis of asym-
(1) Dandridge, A. G.; Drescher, H. A.; Thomas, J. (to Scottish Dyes
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Pacheco-Soares, C.; da Silva, N. S. Aust. J. Chem. 2008, 61, 741–754.
(8) (a) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005,
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N. B.; Thompson, N. J. Porphyrin Phthalocyanines 2000, 4, 460–464. (b)
Ng, D. K. P. C. R. Chim. 2003, 6, 903–910. (c) Kernag, C. A.; McGrath,
D. V. Chem. Commun. 2003, 1048–1049.
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(12) Schmid, G.; Sommerauer, M.; Geyer, M.; Hanack, M. Phthalo-
cyanines: Prop. Appl. 1996, 4, 1–18.
10.1021/ol900352f CCC: $40.75
Published on Web 04/23/2009
 2009 American Chemical Society

metric Pcs include the linking of two phthalonitrile units prior
to macrocyclization,13 ring expansion of a subphthalocya-
nine,14 and the preparation of “half-phthalocyanine” inter-
mediates,15 although none of these have provided a general
approach. Solid-phase techniques were first applied to
asymmetric Pc synthesis in 1982,16 but this method has also
not seen widespread application.17
Scheme 2. Crossover Condensation of 3 and 4
Prompted by the recent ROMP-capture-release synthesis
of asymmetric porphyrazines by Barrett and Hoffman,18 we
herein describe our development of the ROMP-capture-
release strategy to achieve asymmetric Pcs. Our methodology
employs solution phase crossover-Linstead cyclization of a
norbornenyl-tagged Pn with another Pn, followed by selective
capture of asymmetric norbornenyl-tagged Pc under ROMP
conditions and acidic cleavage of the target asymmetric Pc
from the ROMP polymer. Asymmetric Pcs are obtained
directly in relatively high purity, and require only minimal
additional purification.
To apply ROMP-capture-release during the synthesis of
asymmetric Pcs, our key precursor is norbornenyl-tagged Pn
3 bearing an acid-cleavable benzyl ether linkage (Scheme
1). Pn 3 was prepared by aromatic nucleophilic substitution
A/B ) 1:3, and the resultant crude product mixture was
examined by mass spectrometry (MALDI). The mixture
contained 5 (B4) as the most abundant product, followed by
6 (AB3) and bis-tagged A2B2, presumably as a mixture of
cis and trans isomers, as the least abundant product (Figure
1).
Scheme 1. Synthesis of Norbornenyl-Tagged Phthalonitrile 3
of commercially available 4-nitrophthalonitrile 2 and previ-
ously synthesized benzyl alcohol 118b in moderate yield.
A crossover-Linstead cyclization (A + B) was then carried
out between norbornenyl tagged Pn 3 (A) and Pn 419 (B)
(Scheme 2). To ensure the purity of cleaved products, an
appropriate ratio of 3 (A) and 4 (B) must be employed so
that the crossover condensation is biased toward the forma-
tion of only tag-free and monotagged Pcs 5 (B4) and 6 (AB3),
respectively. The initial stoichiometric ratio employed was
(13) (a) Kobayashi, N.; Kobayashi, Y.; Osa, T. J. Am. Chem. Soc. 1993,
115, 10994–10995. (b) Drew, D. M.; Leznoff, C. C. Synlett 1994, 623–
624.
Figure 1. MALDI mass spectrum of the crude mixture from the
reaction ratio of (left) A/B ) 1:3 and (right) A/B ) 1:6.
(14) (a) Kobayashi, N.; Kondo, R.; Nakajima, S.; Osa, T. J. Am. Chem.
Soc. 1990, 112, 9640–9641. (b) Kobayashi, N.; Ishizaki, T.; Ishii, K.;
Konami, H. J. Am. Chem. Soc. 1999, 121, 9096–9110.
(15) Nolan, K. J. M.; Hu, M.; Leznoff, C. C. Synlett 1997, 593–594.
(16) (a) Leznoff, C. C.; Hall, T. W. Tetrahedron Lett. 1982, 23, 3023–
3026. (b) Hall, T. W.; Greenberg, S.; McArthur, C. R.; Khouw, B.; Leznoff,
C. C. NouV. J. Chim. 1982, 653–658.
Based on this product composition, we concluded that a
lower A/B ratio was needed to prevent the formation of the
A2B2 product which, if present, would be incorporated into
the ROMP polymer. Thus, additional cyclization reactions
were carried out with A/B ratios that varied from 1:4 to 1:7.
As expected, MALDI mass spectra of the mixtures obtained
from these procedures indicated a decrease in the intensity
of the A2B2 peak as the A/B ratio decreased. An A/B ratio
of 1:6 was determined to be optimum as a mixture of
predominantly 5 (B4) and 6 (AB3) along with a negligible
amount of A2B2 was obtained under these conditions (Figure
1).
(17) (a) Hirth, A.; Sobbi, A. K.; Wo¨hrle, D. J. J. Porphyrins Phthalo-
cyanines 1997, 1, 275–279. (b) Leznoff, C. C.; Svirskaya, P. I.; Khouw,
B.; Cerny, R. L.; Seymour, P.; Lever, A. B. P. J. Org. Chem. 1991, 56,
82–90. (c) Wo¨hrle, D.; Krawczyk, G. Polym. Bull. 1986, 15, 193–200. (d)
Erdem, S. S.; Nesterova, I. V.; Soper, S. A.; Hammer, R. P. J. Org. Chem.
2008, 73, 5003–5007.
(18) (a) Fuchter, M. J.; Vesper, B. J.; Murphy, K. A.; Collins, H. A.;
Phillips, D.; Barrett, A. M.; Hoffman, B. J. Org. Chem. 2005, 70, 2793–
2802. (b) Fuchter, M. J.; Hoffman, B.; Barrett, A. J. Org. Chem. 2005, 70,
5086–5091.
(19) Wo¨hrle, D.; Eskes, M.; Shigehara, K.; Yamada, A. Synthesis 1993,
194–196.
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Org. Lett., Vol. 11, No. 10, 2009

Scheme 3. Preparation of Asymmetric Pc 9 by ROMP-Capture-Release
The crude mixture of 5 (AB3) and 6 (B4) was then filtered
through silica gel to remove excess magnesium butoxide
which has been reported to deactivate Grubbs’ catalyst in
the next step.18a The filtrate was concentrated to give a green
residue, which was subjected to typical ROMP conditions
in the presence of cross-linker 7 (Scheme 3).20 Polymeriza-
tion of this mixture led to the formation of insoluble green
polymer 8, which was then purified by Soxhlet extraction in
DCM for 24 h to remove symmetrical Pc 5. Finally, acidic
cleavage was performed using 10% trifluoroacetic acid in
CH2Cl2 to afford the final product, asymmetric Pc 9.
The low solubility of Pc 9 in common organic solvents
such as CH2Cl2, CHCl3, Et2O, THF, DMSO, benzene,
CH3CN, and pyridine precluded characterization by NMR.
However, we obtained mass spectroscopic (MALDI) data
identifying the expected molecular ion (Figure 2) and
Figure 3. (Left) UV-vis spectrum of asymmetric Pcs 9, 13, and
14 in CH2Cl2. (Right) GPC of Pcs 13 and 14 in CH2Cl2.
absorption spectrum exhibited a split Q-band at 701 (ε )
1.26 × 105 M-1 cm-1) and 666 nm, vibrational bands at 641
and 608 nm (Figure 2), and B-band at 342 nm, indicating
the existence of metal-free, nonaggregated Pc cores.21
A likely rationale for the low solubility of asymmetric Pc
9 stems from the relatively low solubilizing power of the
methoxy groups. Therefore, to increase solubility in organic
solvents to aid in further manipulation and feasible solution-
phase processing, we prepared a more soluble Pc using the
ROMP-capture-release strategy derivative by replacing the
methoxy groups with pentyl groups.
Similar to the synthesis of Pc 9, the preparation of soluble
asymmetric Pc 13 began with crossover condensation of
norbornenyl tagged phthalonitrile 3 and previously synthe-
sized phthalonitrile 12.22 The components of the resultant
Figure 2. MALDI mass spectrum of cleaved asymmetric Pc 9
showing an [M + H]+ peak at 1263.6238 (calcd 1263.3889).
(20) Barrett, A. G. M.; Hopkins, B. T.; Love, A. C.; Tedeschi, L. Org.
Lett. 2004, 6, 835–837.
UV-vis spectrophotometric data was consistent with the
chromophoric structure of 9 (Figure 3). The UV-vis
(21) Dodsworth, E. S.; Lever, A. B. P.; Seymour, P.; Leznoff, C. C. J.
Phys. Chem. 1985, 89, 5698–5705.
Org. Lett., Vol. 11, No. 10, 2009
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mixture were monitored by MS-MALDI and the ratio of 3
(A)/12 (B) )1:4 was chosen as the final stoichiometry
leading to a product mixture containing only AB3 and B4
Pcs. Subsequent ROMP-capture-release and acidic cleavage
provided the desired Pc 13 (Scheme 4). Not surprising, Pc
To demonstrate that the hydroxyl group on the unique
quadrant of the asymmetric Pcs can be further employed for
covalent chemical modification, Pc 13 was acylated using a
large excess of sebacoyl chloride to afford asymmetric Pc
14 (Scheme 4). The final methanolysis step not only
converted the reactive pendant acyl chloride to an ester, but
also helped to minimize the workup and purification. Pc 14
precipitated upon addition of MeOH to the reaction mixture,
while other soluble byproduct remained in solution. Filtration
followed by column chromatography gave Pc 14 which was
successfully characterized by 1H and 13C NMR, mass
spectrometry, combustion analysis, GPC, and UV-vis spec-
trophotometry (Figure 3).
Scheme 4. Synthesis of Asymmetric Pc 13 by
ROMP-Capture-Release and Transformation to Pc 14 by
Acylation
In summary, we have described the synthesis of asym-
metric Pcs via a ROMP-capture-release strategy. In addition
to the preparation of asymmetric Pcs 9 and 13 by this
method, we also demonstrated a straightforward modification
of Pc 13 with an excess of sebacoyl chloride followed by
methanolysis to give tethered Pc 14. This indicates that the
hydroxy group on the unique quadrant of the asymmetric
Pc is a useful nucleophile and therefore allows the fabrication
of potentially complex Pc-containing materials.
Acknowledgment. This work was supported by a grant
from NSF (CHE-0719437).
13 exhibited the anticipated enhanced solubility in most
organic solvents (e.g., CH2Cl2, CHCl3, THF, Et2O). Thus,
unlike Pc 9, we were able to characterize Pc 13 by NMR
and GPC (Figure 3) in addition to MS-MALDI and UV-vis.
Supporting Information Available: Experimental details
and characterization data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(22) Haas, M.; Liu, S. X.; Neels, A.; Decurtins, S. Eur. J. Org. Chem.
2006, 24, 5467–5478.
OL900352F
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Org. Lett., Vol. 11, No. 10, 2009