Synthesis of swallowtail-substituted multiporphyrin rods

Article abstract of DOI:10.1021/jo049348t

The availability of multiporphyrin arrays with defined architectures and good solubility in organic solvents is essential for a wide variety of physical studies. Herein the synthesis of linear multiporphyrin arrays (triads, tetrad, pentad) bearing solubil

Full text of DOI:10.1021/jo049348t

SCHEME 1
Syn th esis of Sw a llow ta il-Su bstitu ted
Mu ltip or p h yr in Rod s
Patchanita Thamyongkit and J onathan S. Lindsey*
Department of Chemistry, North Carolina State University,
Raleigh, North Carolina 27695-8204
jlindsey@ncsu.edu
Received April 17, 2004
Abstr a ct: The availability of multiporphyrin arrays with
defined architectures and good solubility in organic solvents
is essential for a wide variety of physical studies. Herein
the synthesis of linear multiporphyrin arrays (triads, tetrad,
pentad) bearing solubilizing 7-tridecyl (swallowtail) groups
is presented. The rodlike arrays are composed of zinc
porphyrins at the termini and 1, 2, or 3 free base porphyrins
at the core. The free base porphyrins in the tetrad and
pentad are joined to each other via p-phenylene linkers
whereas the zinc porphyrins in each array are attached to
the core free base porphyrins via 1,4-diphenylethyne linkers.
The arrays are designed for studies of interporphyrin
electronic communication.
zinc porphyrins in a triad motif owing to the star-shaped
architecture.
A wide variety of molecular constructs containing
multiple porphyrins have been prepared to serve as light-
harvesting arrays, optoelectronic gates, charge-separa-
tion units, and charge-storage reservoirs.1-6 The design
of molecular architectures with prescribed electronic
properties (energy transfer, electronic switching, charge
separation/storage) requires a firm understanding of the
mechanisms of electronic communication between inter-
acting constituents. We previously prepared a series of
multiporphyrin arrays for studies of ground-state elec-
tronic communication. The method for examining ground-
state electronic communication involved hole-hopping in
partially oxidized arrays. The arrays examined include
(1) dimers of zinc porphyrins,7,8 (2) linear or right-angle
triads with zinc porphyrins at the termini of the linear
array and a free base porphyrin at the core,9 and (3) star-
shaped pentads with four zinc porphyrins at the periph-
ery and a free base porphyrin at the core.7 Despite the
larger size of the pentads, hole-hopping occurred between
A chief problem encountered upon examination of the
linear triads was the limited solubility of the compounds.
Indeed, adequate solubility for electrochemical studies
could only be achieved in mixtures of chlorinated solvents
(e.g., CH2Cl2 and CHCl3).9 The previous arrays incorpo-
rated porphyrins bearing mesityl groups at all nonlinking
meso positions (Tr ia d -1, Chart 1). We recently showed
that 7-tridecyl (“swallowtail”) groups impart solubility to
meso-substituted porphyrin building blocks without af-
fecting the physical properties of the macrocycles.10
Herein we present the synthesis of a series of rodlike
arrays composed of multiple swallowtail-substituted por-
phyrins, including four triads, one tetrad, and one pentad.
The triads enable in-depth studies of electronic com-
munication while the tetrad and pentad enable examina-
tion of whether hole-hopping persists over longer dis-
tances than that in the triad arrays.
The four triads (Tr ia d -2-5) are shown in Chart 1.
Tr ia d -2 bears swallowtail substituents at nonlinking
meso positions of the terminal porphyrins and mesityl
substituents at nonlinking meso positions of the central
free base porphyrin. Tr ia d -3 is a zinc-chelated derivative
of Tr ia d -2. Tr ia d -4 bears swallowtail groups at the
terminal porphyrins while the free base porphyrin bears
pentafluorophenyl groups to tune the energy levels of the
porphyrin molecular orbitals. Tr ia d -5 bears swallowtail
and mesityl groups in the reversed pattern of that in
Tr ia d -2. The tetrad or pentad each incorporates zinc
porphyrins at the termini and two or three free base
porphyrins as the intervening unit (Chart 2). The linker
between the free base porphyrins is a p-phenylene unit,
(1) Harvey, P. D. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vol.
18, pp 63-250.
(2) (a) Kim, D.; Osuka, A. J . Phys. Chem. A 2003, 107, 8791-8816.
(b) Aratani, N.; Osuka, A. Macromol. Rapid Commun. 2001, 22, 725-
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(3) Holten, D.; Bocian, D. F.; Lindsey, J . S. Acc. Chem. Res. 2002,
35, 57-69.
(4) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem.
Rev. 2001, 101, 2751-2796.
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40-48.
(6) Wasielewski, M. R. Chem. Rev. 1992, 92, 435-461.
(7) Seth, J .; Palaniappan, V.; J ohnson, T. E.; Prathapan, S.; Lindsey,
J . S.; Bocian, D. F. J . Am. Chem. Soc. 1994, 116, 10578-10592.
(8) (a) Strachan, J .-P.; Gentemann, S.; Seth, J .; Kalsbeck, W. A.;
Lindsey, J . S.; Holten, D.; Bocian, D. F. J . Am. Chem. Soc. 1997, 119,
11191-11201. (b) Yang, S. I.; Lammi, R. K.; Seth, J .; Riggs, J . A.; Arai,
T.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J . S. J . Phys. Chem. B
1998, 102, 9426-9436.
(10) Thamyongkit, P.; Speckbacher, M.; Diers, J . R.; Kee, H. L.;
Kirmaier, C.; Holten, D.; Bocian, D. F.; Lindsey, J . S. J . Org. Chem.
2004, 69, 3700-3710.
(9) Seth, J .; Palaniappan, V.; Wagner, R. W.; J ohnson, T. E.; Lindsey,
J . S.; Bocian, D. F. J . Am. Chem. Soc. 1996, 118, 11194-11207.
10.1021/jo049348t CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/28/2004
5796
J . Org. Chem. 2004, 69, 5796-5799

CHART 1
CHART 2
CHART 3
which affords enhanced electronic coupling between the
porphyrins compared with that of the diphenylethyne
unit.3 Each array is pseudocentrosymmetric along the
long axis (ignoring torsional motions and NH tautomer-
ism) and the two terminal porphyrins are identical with
one another.
The general approach for synthesis of the triads
entailed a Sonogashira reaction with a difunctional core
porphyrin and 2 equiv of the monofunctional terminal
porphyrin. Porphyrin building blocks Zn -1,10 Zn -2,10 Zn -
3,11 4,12 and Zn -412 are known compounds (Chart 3).
Condensation of dipyrromethane 513 and 4-(2-(trimeth-
J . Org. Chem, Vol. 69, No. 17, 2004 5797

SCHEME 2
SCHEME 4
SCHEME 3
ylsilyl)ethynyl)benzaldehyde with catalysis by TFA14
followed by oxidation with DDQ afforded porphyrin 6 in
16% yield. Subsequent treatment with TBAF in THF
gave ethynyl porphyrin 7, a free base precursor for the
synthesis of Tr ia d -4, in 75% yield (Scheme 1). Acylation
of dipyrromethane 810 with pyridyl benzothioate 915 led
to 1-acyldipyrromethane 10. Reduction of 10 and self-
condensation15 of the resulting carbinol in CH2Cl2 con-
taining Yb(OTf)3 followed by oxidation with DDQ gave
16
porphyrin 11, required for the synthesis of Tr ia d -5, in
18% yield (Scheme 2). The condensation of a dipyr-
romethane-carbinol can be achieved with a mild Lewis
15
(11) Wagner, R. W.; J ohnson, T. E.; Li, F.; Lindsey, J . S. J . Org.
Chem. 1995, 60, 5266-5273.
(12) Li, J .; Ambroise, A.; Yang, S. I.; Diers, J . R.; Seth, J .; Wack, C.
R.; Bocian, D. F.; Holten, D.; Lindsey, J . S. J . Am. Chem. Soc. 1999,
121, 8927-8940.
(13) Laha, J . K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J . S. Org. Proc. Res. Dev. 2003, 7, 799-812.
(14) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
(15) Rao, P. D.; Littler, B. J .; Geier, G. R., III; Lindsey, J . S. J . Org.
Chem. 2000, 65, 1084-1092.
acid such as Yb(OTf)3 in CH2Cl2 or by trifluoroacetic
acid in CH3CN,16 though the former conditions generally
are superior in terms of diminished propensity to aci-
dolysis of the dipyrromethane and greater ease of puri-
fication of the resulting porphyrin.
(16) Geier, G. R., III; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
5798 J . Org. Chem., Vol. 69, No. 17, 2004

SCHEME 5
16
The synthesis of the triads is summarized in Scheme
3. The Sonogashira reaction was performed under copper-
free conditions in toluene/TEA (5:1) at 35 °C in the
presence of Pd2(dba)3 and P(o-tol)3.17 In each case, the
desired triad was isolated by a straightforward combina-
tion of adsorption chromatography and size exclusion
chromatography. The yields ranged from 37% to 87%.
Tr ia d -2, Tr ia d -3, and Tr ia d -4, which bear swallowtail
groups at the porphyrin termini, exhibit satisfactory
solubility (>5 mg/mL) in common organic solvents (e.g.,
toluene, CH2Cl2, CHCl3, THF) for routine handling,
characterization, and physical studies. By contrast, Tr iad-
5, which has swallowtail groups at the central porphyrin,
was less soluble in the same solvents. In general, the
swallowtail-substituted porphyrin arrays are more soluble
than the corresponding mesityl-substituted porphyrin
arrays (e.g., Tr ia d -1).
The synthetic approach toward the target tetrad and
pentad similarly entailed Sonogashira coupling of a core
unit of p-phenylene-linked free base porphyrins and the
terminal swallowtail-substituted zinc porphyrin. The
reaction of 4-iodobenzaldehyde with excess pyrrole con-
taining InCl313 afforded dipyrromethane 12 in 66% yield
(Scheme 4). Diacylation of 12 was carried out with 3,5-
di-tert-butylbenzoyl chloride (13)18 under catalysis by
SbCl5.19 Treatment of the mixture with dibutyltin dichlo-
ride10 and TEA gave the corresponding diacyldipyr-
romethane-tin complex 14 in 39% yield. Reduction of 14
with NaBH4 in THF/methanol for 4 h gave the dipyr-
romethane-dicarbinol. Reaction of the latter with 1,4-
bis(dipyrromethyl)benzene (15)20,21 in CH2Cl2 containing
Yb(OTf)3 followed by oxidation with DDQ afforded the
difunctional dimer 16 in 8% yield. Although the overall
yield of 16 was low, the starting materials are readily
available in sizable quantities.
The coupling reactions to form the tetrad and pentad
were performed under the standard conditions for Pd-
mediated reactions of iodo and ethynyl porphyrins. The
reaction of dimer 16 and porphyrin Zn -2 was complicated
by the incomplete dissolution of the dimer (16) at the
start of the reaction. Tetr a d -1 was obtained in 19% yield
(Scheme 4). The low yield is attributed to the low
solubility of 16 and the formation of trimeric byproducts
(as shown by analytical SEC). On the other hand, the
Pd-mediated coupling of triad 1722 and porphyrin Zn -
210 afforded P en ta d -1 in 71% yield (Scheme 5). As with
the triads, the tetrad and pentad were purified by
straightforward combination of adsorption and size ex-
clusion chromatography. Both Tetr a d -1 and P en ta d -1
exhibit satisfactory solubility (>5 mg/mL) in a variety of
common organic solvents. Studies of electronic com-
munication in the various rodlike arrays can now be
undertaken and will be reported elsewhere.
Ack n ow led gm en t. This research was supported by
a grant from the Division of Chemical Sciences, Office
of Basic Energy Sciences, Office of Energy Research,
U.S. Department of Energy.
Su p p or tin g In for m a tion Ava ila ble: Full Experimental
Section including characterization data (NMR spectra, mass
spectra) for all new compounds, as well as analytical SEC
traces for the arrays. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
J O049348T
(18) (a) Korshunov, M. A.; Bodnaryuk, F. N.; Fershtut, E. V. Zh.
Org. Khim. 1967, 3, 140-143. (b) Alberti, A.; Benaglia, M. J . Orga-
nomet. Chem. 1992, 434, 151-158.
(20) Lee, C.-H.; Lindsey, J . S. Tetrahedron 1994, 50, 11427-11440.
(21) Bennis, V.; Gallagher, J . F. Acta Crystallogr. 1998, C54, 130-
132.
(19) Tamaru, S.-I.; Yu, L.; Youngblood, W. J .; Muthukumaran, K.;
Taniguchi, M.; Lindsey, J . S. J . Org. Chem. 2004, 69, 765-777.
(22) Yu, L.; Lindsey, J . S. Tetrahedron 2001, 57, 9285-9298.
J . Org. Chem, Vol. 69, No. 17, 2004 5799