Rational syntheses of cyclic hexameric porphyrin arrays for studies of self-assembling light-harvesting systems

Article abstract of DOI:10.1021/jo010742q

Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner. The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like array with a diameter of ～35 A. One array is comprised of

Full text of DOI:10.1021/jo010742q

7402
J . Org. Chem. 2001, 66, 7402-7419
Ra tion a l Syn th eses of Cyclic Hexa m er ic P or p h yr in Ar r a ys for
Stu d ies of Self-Assem blin g Ligh t-Ha r vestin g System s
Lianhe Yu and J onathan S. Lindsey*
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
jlindsey@ncsu.edu
Received J uly 25, 2001
Two new cyclic hexameric arrays of porphyrins have been prepared in a rational, convergent manner.
The porphyrins in each cyclic hexamer are joined by diphenylethyne linkers affording a wheel-like
array with a diameter of ∼35 Å. One array is comprised of five zinc (Zn) porphyrins and one free
base (Fb) porphyrin (cyclo-Zn 5F bU) while the other is comprised of an alternating sequence of
two Zn porphyrins and one Fb porphyrin (cyclo-Zn 2F bZn 2F bU). The prior synthesis employed a
one-flask template-directed process and afforded alternating Zn and Fb porphyrins or all Zn
porphyrins. More diverse metalation patterns are attractive for manipulating the flow of excited-
state energy in the arrays. The rational synthesis of each array employed three Pd-mediated coupling
reactions with four tetraarylporphyrin building blocks bearing diethynyl, diiodo, bromo/iodo, or
iodo/ethynyl groups. The final ring closure yielding the cyclic hexamer was achieved by reaction of
a porphyrin pentamer + porphyrin monomer or the joining of two porphyrin trimers. In the presence
of a tripyridyl template, the yields of the 5 + 1 and 3 + 3 reactions ranged from 10 to 13%. The 5
+ 1 reaction in the absence of the template proceeded in 3.5% yield, thereby establishing the
structure-directed contribution to cyclic hexamer formation. The 3 + 3 route relied on successive
ethyne + iodo/bromo coupling reactions. One template-directed route to cyclo-Zn 2F bZn 2F bU
employed a magnesium porphyrin, affording cyclo-Zn 2F bZn 2MgU from which magnesium was
selectively removed. The arrays exhibit absorption spectra that are nearly the sum of the spectra
of the component parts, indicating weak electronic coupling. Fluorescence spectroscopy showed
that the quantum yield of energy transfer in toluene at room temperature from the Zn porphyrins
to the Fb porphyrin(s) was 60% in cyclo-Zn 5F bU and 90% in cyclo-Zn 2F bZn 2F bU. Two dipyridyl-
substituted porphyrins, a Zn tetraarylporphyrin and a Fb oxaporphyrin, have been synthesized
for use as guests in the cyclic hexamers, affording self-assembled arrays for light-harvesting studies.
In tr od u ction
example of this hybrid approach, we prepared a shape-
persistent cyclic hexameric array of covalently linked
porphyrins with a cavity diameter of ∼35 Å.6 This work
was motivated by the finding that pigments in the
photosynthetic antenna complexes are arranged in cyclic
architectures3 and inspired by the extensive work of
Sanders on the chemistry of cyclic multiporphyrin
arrays.7-32 The 35 Å cavity size proved ideal for binding
The ability to design and construct molecular archi-
tectures in which the flow of excited-state energy can be
directed in a controlled manner, as occurs in the photo-
synthetic light-harvesting complexes, is expected to have
substantial impact in materials chemistry.1 The photo-
synthetic light-harvesting complexes consist of up to
hundreds of porphyrinic pigments organized in elaborate
three-dimensional structures.2,3 An attractive approach
for meeting the daunting synthetic challenge of organiz-
ing a large number of pigments is to design small- or
medium-sized multichromophoric arrays which self-as-
semble into light-harvesting architectures.4,5 As one
(6) 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.
(7) Some of the cyclic multiporphyrin arrays prepared by Sanders
bind porphyrin guests. See refs 10, 17, 21, 23, 25, and 28.
(8) Hunter, C. A.; Meah, M. N.; Sanders, J . K. M. J . Chem. Soc.,
Chem. Commun. 1988, 692-694.
(9) Hunter, C. A.; Meah, M. N.; Sanders, J . K. M. J . Chem. Soc.,
Chem. Commun. 1988, 694-696.
(1) Holten, D.; Bocian, D. F.; Lindsey, J . S. Acc. Chem. Res., in press.
(2) (a) Larkum, A. W. D.; Barrett, J . Adv. Bot. Res. 1983, 10, 1-219.
(b) Photosynthetic Light-Harvesting Systems; Scheer, H., Siegried, S.,
Eds.; W. de Gruyter: Berlin, 1988. (c) Mauzerall, D. C.; Greenbaum,
N. L. Biochim. Biophys. Acta 1989, 974, 119-140.
(3) (a) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawethornth-
waite-Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J .; Isaacs, N. W. Nature
1995, 374, 517-521. (b) Karrasch, S.; Bullough, P. A.; Ghosh, R. EMBO
J . 1995, 14, 631-638. (c) Pullerits, T.; Sundstro¨m, V. Acc. Chem. Res.
1996, 29, 381-389.
(4) Haycock, R. A.; Yartsev, A.; Michelsen, U.; Sundstro¨m, V.;
Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39, 3616-3619.
(5) (a) Tamiaki, H.; Miyatake, T.; Tanikaga, R.; Holzwarth, A. R.;
Schaffner, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 772-774. (b)
Miyatake, T.; Tamiaki, H.; Holzwarth, A. R.; Schaffner, K. Photochem.
Photobiol. 1999, 69, 448-456. (c) Miyatake, T.; Tamiaki, H.; Holzwarth,
A. R.; Schaffner, K. Helv. Chim. Acta 1999, 82, 797-811.
(10) Anderson, H. L.; Hunter, C. A.; Sanders, J . K. M. J . Chem. Soc.,
Chem. Commun. 1989, 226-227.
(11) Anderson, H. L.; Sanders, J . K. M. J . Chem. Soc., Chem.
Commun. 1989, 1714-1715.
(12) Anderson, H. L.; Sanders, J . K. M. Angew. Chem., Int. Ed. Engl.
1990, 29, 1400-1403.
(13) Mackay, L. G.; Anderson, H. L.; Sanders, J . K. M. J . Chem.
Soc., Chem. Commun. 1992, 43-44.
(14) Anderson, S.; Anderson, H. L.; Sanders, J . K. M. Angew. Chem.,
Int. Ed. Engl. 1992, 31, 907-910.
(15) Anderson, H. L.; Sanders, J . K. M. J . Chem. Soc. Chem.
Commun. 1992, 946-947.
(16) Walter, C. J .; Anderson H. L.; Sanders J . K. M. J . Chem. Soc.,
Chem. Commun. 1993, 458-460.
(17) Anderson, S.; Anderson, H. L.; Sanders, J . K. M. Acc. Chem.
Res. 1993, 26, 469-475.
10.1021/jo010742q CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/09/2001

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7403
Ch a r t 1. Self-Assem bled Ar r a ys P r ep a r ed P r eviou slya
a
In both structures, a mesityl group is present at each non-linking meso position (not displayed on coordinated metalloporphyrins for
clarity).

7404 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
Sch em e 1
tripyridyl template 1 or trans-dipyridyl substituted por-
phyrin 2 with high affinity (Chart 1). The binding of the
dipyridyl-substituted free base (Fb) porphyrin 2 in the
cyclic host comprised of six zinc (Zn) porphyrins (cyclo-
Zn 6U) yielded a self-assembled light-harvesting array in
a wheel and spoke architecture.33
The synthesis of the cyclic hexameric porphyrin array
was achieved in a one-flask template-directed reaction
of two porphyrin building blocks (Scheme 1).6 This
method relied on the Pd-mediated coupling of a p/ p-
substituted ethynylphenyl Zn porphyrin (Zn -3) and a
m/ m-substituted iodophenyl Fb porphyrin (4) in the
presence of a tripyridyl template (1). The template was
readily removed on workup but could be reinserted into
the purified cyclic hexamer. Because the template-
directed synthesis with the tripyridyl template requires
coordination of the three p/ p-substituted Zn porphyrins,
the only cyclic hexamers that were accessible incorpo-
rated alternating Zn porphyrins and Fb porphyrins
(cyclo-Zn 3F b3U) or all Zn porphyrins (cyclo-Zn 6U).
Other desirable patterns of metalloporphyrins, such as
those present in the new cyclic hexamers cyclo-Zn 5F bU
and cyclo-Zn 2F bZn 2F bU (Chart 2), were not available
using this one-flask synthesis.
The motivation for preparing cyclic hexamers with
various metalation patterns is to study and ultimately
control the flow of excited-state energy in the self-
assembled light-harvesting arrays. In the arrays, energy
transfer proceeds very rapidly between covalently linked
porphyrins via a through-bond process.1 Binding of the
guest via pyridyl ligation to a Zn porphyrin causes a red-
shift (∼10 nm) in the absorption spectrum, in effect
lowering the excited-state energy of the pyridyl-coordi-
nated Zn porphyrin. For example, in cyclo-Zn 6U-1,
photoexcitation of the Zn porphyrins is followed by energy
transfer to the pyridyl-coordinated Zn porphyrins.33 In
cyclo-Zn 6U-2, energy transfer can occur via a through-
space process from any porphyrin in the wheel to the Fb
porphyrin guest. However, the more rapid through-bond
(18) (a) Vidal-Ferran, A.; Mu¨ller, C. M.; Sanders, J . K. M. J . Chem.
Soc., Chem. Commun. 1994, 2657-2658. (b) Vidal-Ferran, A.; Mu¨ller,
C. M.; Sanders, J . K. M. Chem. Commun. 1996, 1849.
(19) Anderson, H. L.; Bashall, A.; Henrick, K.; McPartlin, M.;
Sanders, J . K. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 429-431.
(20) Walter, C. J .; Sanders, J . K. M. Angew. Chem., Int. Ed. Engl.
1995, 34, 217-219.
(21) Anderson, S.; Anderson, H. L.; Bashall, A.; McPartlin, M.;
Sanders, J . K. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1096-
1099.
(22) Anderson, H. L.; Sanders, J . K. M. J . Chem. Soc., Perkin Trans.
1 1995, 2223-2229.
(23) Anderson, H. L.; Anderson, S.; Sanders, J . K. M. J . Chem. Soc.,
Perkin Trans. 1 1995, 2231-2245.
(24) Anderson, S.; Anderson, H. L.; Sanders, J . K. M. J . Chem. Soc.,
Perkin Trans. 1 1995, 2247-2254.
energy-transfer process among porphyrins in the wheel
of the array results in localization of energy on the
pyridyl-coordinated Zn porphyrins. Energy transfer from
the latter to the guest Fb porphyrin apparently is slow,
occurring in a yield commensurate with a through-space
process.33 The flow of energy in cyclo-Zn 6U-2 is il-
lustrated in Figure 1. The sluggish transfer from the
pyridyl-coordinated Zn porphyrins to the guest Fb por-
phyrin limits the performance of the self-assembled
complex as a light-harvesting system.
In an array containing five Zn porphyrins and one Fb
porphyrin (cyclo-Zn 5F bU), pyridyl coordination of two
of the Zn porphyrins creates a cascade wherein the
excited-state energy levels decrease in the following
order: Zn porphyrin > pyridyl-coordinated Zn porphyrin
(25) Anderson, S.; Anderson, H. L.; Sanders, J . K. M. J . Chem. Soc.,
Perkin Trans. 1 1995, 2255-2267.
(26) Mackay, L. G.; Anderson, H. L.; Sanders, J . K. M. J . Chem.
Soc., Perkin Trans. 1 1995, 2269-2273.
(27) Anderson, H. L.; Walter, C. J .; Vidal-Ferran, A.; Hay, R. A.;
Lowden, P. A.; Sanders, J . K. M. J . Chem. Soc., Perkin Trans. 1 1995,
2275-2279.
(28) McCallien, D. W. J .; Sanders, J . K. M. J . Am. Chem. Soc. 1995,
117, 6611-6612.
(29) Marvaud, V.; Vidal-Ferran, A.; Webb, S. J .; Sanders, J . K. M.
J . Chem. Soc., Dalton Trans. 1997, 985-990.
(30) Wylie, R. S.; Levy, E. G.; Sanders, J . K. M. Chem. Commun.
1997, 1611-1612.
(31) Vidal-Ferran, A.; Clyde-Watson, Z.; Bampos, N.; Sanders, J . K.
M. J . Org. Chem. 1997, 62, 240-241.
(32) Vidal-Ferran, A.; Bampos, N.; Sanders, J . K. M. Inorg. Chem.
1997, 36, 6117-6126.
(33) Ambroise, A.; Li, J .; Yu, L.; Lindsey, J . S. Org. Lett. 2000, 2,
2563-2566.

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7405
Ch a r t 2
> Fb porphyrin. With a guest having an excited-state of
lower energy than that of the Fb porphyrin (e.g., phtha-
locyanine, bacteriochlorin, heteroatom-substituted Fb
porphyrin), excited-state energy should rapidly reach the
Fb porphyrin via through-bond transfer among porphy-
rins in the wheel and then transfer to the guest via a
through-space process. Three paths of energy transfer for
cyclo-Zn 5F bU-gu est are displayed in Figure 1. Paths 1
and 2 proceed downhill in energy in a stepwise manner
and are unremarkable. Path 3 proceeds along the wheel
from Zn porphyrin f pyridyl-coordinated Zn porphyrin
f Fb porphyrin; the transfer from the nonadjacent
pyridyl-coordinated Zn porphyrin to the Fb porphyrin
occurs via the intermediacy of a Zn porphyrin. We have
shown that through-bond energy transfer can occur
efficiently among distant (i.e., nonadjacent) sites in
covalently linked arrays via a superexchange process.1,34
In cyclo-Zn 2F bZn 2F bU, two Fb porphyrins can serve as
funnels for energy transfer to a suitable guest (Figure
1). Two paths for energy transfer in the wheel lead to
the Fb porphyrins, from which through-space transfer to
the guest can occur. The objective of the work described
in this paper has been to gain efficient access to arrays
such as cyclo-Zn 5FbU and cyclo-Zn 2FbZn 2FbU, thereby
enabling studies of the energy-transfer dynamics and
light-harvesting properties.
To gain access to cyclic hexamers containing diverse
patterns of metalloporphyrins requires a stepwise syn-
thesis of one or more linear arrays followed by cyclization
to give the desired cyclic structure. Gossauer recently re-
ported the synthesis of very large shape-persistent hex-
americ arrays of porphyrins (46 Å diameter), including
an array comprised of five Zn porphyrins and one Fb por-
phyrin.35,36 The synthesis employed the iterative divergent/
convergent construction of linear hexamers, which were
(35) Mongin, O.; Schuwey, A.; Vallot, M.-A.; Gossauer, A. Tetrahe-
dron Lett. 1999, 40, 8347-8350.
(36) Rucareanu, S.; Mongin, O.; Schuwey, A.; Hoyler, N.; Gossauer,
A.; Amrein, W.; Hediger, H.-U. J . Org. Chem. 2001, 66, 4973-4988.
(34) Lammi, R. K.; Ambroise, A.; Balasubramanian, T.; Wagner, R.
W.; Bocian, D. F.; Holten, D.; Lindsey, J . S. J . Am. Chem. Soc. 2000,
122, 7579-7591.

7406 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
F igu r e 1. Paths for the flow of excited-state energy in self-
assembled light-harvesting arrays. The color and height of the
bars in the histograms on the left indicate the relative excited-
state energy level of the various components. Code: Zn
porphyrin (blue), pyridyl-coordinated Zn porphyrin (green), Fb
porphyrin (orange), guest chromophore (red). (Top) In cyclo-
Zn 6U-2, energy flows from the Zn porphyrin to the pyridyl-
coordinated Zn porphyrin to the guest Fb porphyrin. (Middle)
In cyclo-Zn 5F bU-guest, energy flows from the Zn porphyrins
via several paths to the Fb porphyrin, which can funnel energy
to the guest. The dashed line in path 3 indicates the super-
exchange process from a pyridyl-coordinated Zn porphyrin to
a Fb porphyrin via the intermediacy of a Zn porphyrin.
(Bottom) In cyclo-Zn 2F bZn 2F bU-guest, energy flows to the
Fb porphyrins from which energy is funneled to the guest.
F igu r e 2. Possible roles of the template in different routes
to the cyclic hexamer cyclo-Zn 5F bU. (A) A 5 + 1 route with
triligation of the pentamer Zn 5-p/p-CCH prior to reaction with
the Fb-m/ m-diiodo-porphyrin. (B) A 5 + 1 route with only di-
ligation possible of the pentamer Zn 2F bZn 2-m /m -CCH; trili-
gation occurs upon reaction with the Zn-p/ p-diiodo-porphyrin.
(C) A 3 + 3 route with triligation possible upon reaction of
the two trimers, Zn 3-m /m -CCH and Zn F bZn -p/p-Br . The
legend employed here also applies to Schemes 7-10.
cyclized to the corresponding cyclic hexamers. Although
templating was not observed during the cyclization pro-
cess of the linear hexamers, the cyclic hexamers exhibited
very high affinity for a number of polydentate ligands.
In this paper, we first describe a refined synthesis of
the tripyridyl template (1). We then consider several
rational, stepwise strategies with differing degrees of
convergence for the preparation of cyclic hexamers.
Several such strategies are implemented, affording the
two new cyclic hexamers cyclo-Zn 5F bU and cyclo-
Zn 2F bZn 2F bU. The recent availability of porphyrin
building blocks bearing up to four different meso sub-
stituents enables both the synthesis of the porphyrin
building blocks and the stepwise synthesis of the cyclic
arrays to be carried out in a rational manner (i.e., without
any statistical reactions at any step). A trans-dipyridyl
substituted oxaporphyrin has been prepared as one
example of a low-energy chromophore for binding inside
the host. The studies of energy transfer in the complexes
formed upon binding various guests in the cyclic hexam-
ers will be presented elsewhere.
porphyrins. The distances are such that template 1 binds
to the p/ p-substituted porphyrins rather than the m/ m-
substituted porphyrins. Therefore, for template-directed
synthesis with 1, the three p/ p-substituted porphyrins
must be employed in the metalated state. Three ap-
proaches for the template-directed formation of cyclo-
Zn 5F bU are shown in Figure 2. (A) 5 + 1 route: All three
p/ p-substituted Zn porphyrins are located in the linear
pentamer Zn 5-p/p-CCH. The sixth porphyrin is a m/ m-
substituted Fb porphyrin. While the template cannot
facilitate intermolecular reaction yielding the linear Zn5-
Fb hexamer, binding of the template to the linear Zn5Fb
hexamer can facilitate intramolecular ring closure. (B) 5
+ 1 route: Only two p/ p-substituted Zn-porphyrins are
located in the linear pentamer Zn 2F bZn 2-m /m -CCH. The
sixth porphyrin is a p/ p-substituted Zn porphyrin. For
the template to assist formation of the linear hexamer,
the template must bis-ligate to Zn 2F bZn 2-m /m -CCH and
mono-ligate with the p/ p-substituted Zn porphyrin mono-
mer. The template can also facilitate intramolecular ring
closure in the last step. In both routes A and B, the
templated ring closure is effectively the same.37 (C) 3 +
3 route: Two p/ p-substituted Zn porphyrins are located
in one trimeric building block (Zn F bZn -p/p-Br ) while
Resu lts a n d Discu ssion
Syn th etic Str a tegy. The cyclic hexamers have 3-fold
rather than 6-fold symmetry due to the presence of three
p/ p-substituted porphyrins and three m/ m-substituted

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7407
proach to achieve a template-directed synthesis is to
construct a linear pentamer (Zn 2F bZn 2-m /m -CCH) for
reaction with a Mg porphyrin. In this case, the tripyridyl
template can bind to two p/ p-substituted Zn porphyrins
in Zn 2F bZn 2-m /m -CCH and the incoming p/ p-substi-
tuted Mg porphyrin. The resulting Zn2FbZn2Mg cyclic
hexamer can be selectively demetalated to give the
desired hexamer cyclo-Zn 2F bZn 2F bU. Mg porphyrins
(but not Zn porphyrins) readily demetalate upon exposure
to silica gel.38 (C) An alternate route is to introduce
magnesium into Zn 2F bZn 2-p/p-CCH. The resulting Zn2-
MgZn2-pentamer incorporates three p/ p-substituted met-
alloporphyrins (two Zn porphyrins and one Mg porphyrin)
for binding with the template in the cyclization step.
Demetalation of the Mg-porphyrin again affords cyclo-
Zn 2F bZn 2F bU. We chose routes A and B for preparing
cyclo-Zn 2F bZn 2F bU.
Tr ip yr id yl Tem p la te. For large-scale synthesis, we
sought a more efficient preparation of template 1 than
that employed previously.6 Key starting materials for the
construction of the tripyridyl template are 4-ethynylpy-
ridine (6) and 1,3,5-tris(4-iodophenyl)benzene (7). In our
hands, the synthesis of ethynylpyridine intermediate 5
in diethylamine following a reported method39 gave a 50%
yield in contrast to the reported 84%. To overcome the
poor solubility of the starting material 4-bromopyridine
hydrochloride in diethylamine, we performed the reaction
in the mixed solvent system of THF/diethylamine (2:5),
obtaining a 92% yield. Treatment of 5 in a suspension of
NaOH/toluene gave 6 in 64% yield (Scheme 2). Com-
pound 6 darkens on storage at room temperature over 2
days and should be used immediately.
Sch em e 2
F igu r e 3. Possible roles of the template in different routes
to the cyclic hexamer cyclo-Zn 2F bZn 2F bU. (A) A structure-
directed 5 + 1 route wherein triligation is not possible due to
the metalation states of the pentamer Zn 2F bZn 2-p/p-CCH and
the Fb-m/ m-diiodo-porphyrin. (B) A 5 + 1 route with triliga-
tion upon reaction of the pentamer Zn 2F bZn 2-m /m -CCH and
the monomer Mg-p/ p-diiodo-porphyrin. (C) A 5 + 1 route with
triligation of the pentamer Zn 2MgZn 2-p/p-CCH prior to
reaction with the Fb-m/ m-diiodo-porphyrin. The legend em-
ployed here also applies to Schemes 7-10.
the third p/ p-substituted Zn porphyrin is located in the
other trimeric building block (Zn 3-m /m -CCH). For the
template to assist formation of the linear hexamer, the
template must bis-ligate and mono-ligate to the respec-
tive trimers. As in cases A and B, the template can
facilitate intramolecular ring closure of the linear hex-
amer. We chose routes A and C to synthesize the target
molecule cyclo-Zn 5F bU.
Examples of 5 + 1 routes for the rational synthesis of
cyclo-Zn 2F bZn 2F b U are outlined in Figure 3. (A) All
three p/ p-substituted porphyrins are located in the linear
pentamer Zn 2F bZn 2-p/p-CCH. However, only two are Zn
porphyrins and one is a Fb porphyrin. In this case
triligation with the tripyridyl template is not possible due
to the absence of the third binding site. Thus, this route
constitutes a structure-directed synthesis. (B) One ap-
Preparation of 1,3,5-tris(4-iodophenyl)benzene (7) was
first carried out by condensation of 4-iodoacetophenone
in alcoholic hydrogen chloride, affording 17% yield after
30 days of reaction.40 The same acid-catalyzed reaction
using trimethyl orthoformate in chloroform instead of
(38) Lindsey, J . S.; Woodford, J . N. Inorg. Chem. 1995, 34, 1063-
1069.
(39) Ciana, L. D.; Haim, A. J . Heterocycl. Chem. 1984, 21, 607-
(37) The linear hexamers formed via the two routes differ only in
whether the iodo or ethynyl group is attached to the m/ m- or p/ p-
substituted porphyrin or vice versa.
608.
(40) Lyle, R. E.; DeWitt, E. J .; Nichols, N. M.; Cleland, W. J . Am.
Chem. Soc. 1953, 75, 5959-5961.

7408 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
Sch em e 3
Sch em e 4
yield (Scheme 4). Cleavage of the TMS groups by using
tetra-n-butylammonium fluoride (TBAF) afforded 12 in
84% yield.
Porphyrin building blocks bearing iodophenyl groups
and/or ethynylphenyl groups in a trans architecture are
essential for the preparation of the cyclic hexameric
arrays. Porphyrins bearing one iodophenyl group and one
ethynylphenyl group have been prepared using mixed
aldehyde condensations43 affording a mixture of three
porphyrins which were separated by column chromatog-
raphy. A rational synthesis is now available that makes
use of a diacyldipyrromethane and a dipyrromethane to
construct the desired trans-AB2C-porphyrin.44,45 The
monoacylation of dipyrromethanes is readily achieved
with S-2-pyridyl benzothioates. Thioester 13a was pre-
pared in 81% yield by coupling S-2-pyridyl 3-iodoben-
zothioate46 and trimethylsilylacetylene using Pd2(dba)3
in THF/triethylamine (3:1) (eq 1). Thioesters 13b and 13c
were prepared as described in the literature.44
alcohol gave a yield of 37% after 15 h. The byproduct 8
was also isolated in 12% yield. The Pd-mediated coupling
of 4-ethynylpyridine (6) and 7 in THF/diethylamine (1:1)
at 45 °C afforded the tripyridyl template 1 in 83% yield
(Scheme 3), compared with the previous 71% yield.6
P or p h yr in Bu ild in g Block s. Trans-A2B2-porphyrins
are readily formed by reaction of an aldehyde with a
sterically hindered dipyrromethane.41 We chose 5-mesi-
tyldipyrromethane (9) as a convenient starting material
for the synthesis of the various porphyrins. The mesityl
group imparts good solubility and 5-mesityldipyrrometh-
ane is especially resistant to acidolytic scrambling in
porphyrin syntheses.41 The mesityl substituted trans-
A2B2-porphyrins Zn -3 and 4 have been prepared previ-
ously.6 A slightly refined synthesis of 3 is provided in the
Supporting Information. Condensation of 5-mesityldipyr-
romethane (9) with 3-[2-(trimethylsilyl)ethynyl] benzal-
dehyde (10)43 followed by oxidation with DDQ afforded
m/ m-TMS-protected ethynylphenyl porphyrin 11 in 58%
(41) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
(42) Littler, B. J .; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J . S. J . Org. Chem. 1999, 64, 1391-1396.
(43) Ravikanth, M.; Strachan, J .-P.; Li, F.; Lindsey, J . S. Tetrahe-
dron 1998, 54, 7721-7734.

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7409
Sch em e 5
15a -d ica r bin ol and 5-mesityldipyrromethane (9) under
nonscrambling conditions (30 mM TFA in CH3CN at
room temperature) followed by oxidation with DDQ gave
the single porphyrin product 16a . Metalation with zinc
acetate gave Zn -16a . The analogous diacyldipyrrometh-
anes 15b,c were treated in the same manner, affording
porphyrins 16b,c without scrambling. Sizable quantities
(0.5-0.7 g) of pure porphyrins were prepared with only
minimal chromatography by this rational route.
The Mg porphyrin monomer (Mg-17) was prepared
with the room temperature, heterogeneous magnesium-
insertion procedure.38 Treatment of Fb porphyrin 1741
with N,N-diisopropylethylamine (DIEA) and MgI2 in
CHCl3 gave Mg-17 in 89% yield after chromatography
on alumina (eq 2).
P or p h yr in Gu ests. The guest in the cyclic hexamers
cyclo-Zn 5F bU and cyclo-Zn 2F bZn 2F bU must be able
to serve as the energy-transfer acceptor from Zn or Fb
porphyrin energy donors. Free base oxaporphyrins (i.e.,
an N3O-porphyrin) have slightly red-shifted absorption
bands compared with the normal (i.e., N4-) porphyrins.47
Thus, a dipyridyl-substituted N3O-porphyrin was attrac-
tive for use as a guest that would function as an energy-
transfer acceptor in the self-assembled cyclic hexameric
arrays. We previously developed methods for preparing
(44) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(45) Retrosynthetic analysis shows several different routes for
constructing a given trans-AB2C-porphyrin: (1) An A-dipyrromethane
can be diacylated to give the BAB-dipyrromethane, which is then
reacted with a C-dipyrromethane. (2) A B-dipyrromethane is mono-
acylated to give an AB-dipyrromethane, followed by a second mono-
acylation to give an ABC-dipyrromethane. Reaction of the latter with
a B-dipyrromethane gives the trans-AB2C-porphyrin. While the former
route is expedient and generally superior, conditions have not been
identified for reducing the mesitoyl dipyrromethane to the correspond-
The reaction of 5-mesityldipyrromethane (9) with Et-
MgBr followed by a pyridyl benzothioate (13a -c) gave
the corresponding monoacyldipyrromethane 14a -c in
good yield (Scheme 5). The second acyl group was
introduced by reaction of a monoacyldipyrromethane
(14a -c) with an iodobenzoyl chloride, affording the
corresponding diacyldipyrromethane 15a -c. Treatment
of a diacyldipyrromethane with NaBH4 gives the corre-
sponding dipyrromethanedicarbinol.44 Condensation of
ing carbinol. Accordingly, the latter route wherein
B is mesityl
constitutes the appropriate fallback approach for preparing the desired
trans-AB2C-porphyrins.
(46) Rao, P. D.; Littler, B. J .; Geier, G. R., III; Lindsey, J . S. J . Org.
Chem. 2000, 65, 1084-1092.
(47) Cho, W.-S.; Kim, H.-J .; Littler, B. J .; Miller, M. A.; Lee, C.-H.;
Lindsey, J . S. J . Org. Chem. 1999, 64, 7890-7901.

7410 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
Sch em e 6
As is typical of oxaporphyrins, 20 and 21 are signifi-
cantly more basic than normal N4-porphyrins. Indeed, the
absorption spectrum of 20 or 21 in neat CH2Cl2 showed
a long-wavelength shoulder on the Soret band (∼440 nm)
due to the formation of the protonated N3O-porphyrin.
Addition of triethylamine caused disappearance of this
1
shoulder. The H NMR spectrum of the N3O-porphyrin
20 or 21 recorded in CDCl3 showed no NH peak even
when the CDCl3 was treated overnight with K2CO3.
1
However, the H NMR spectrum of each N3O-porphyrin
revealed the nonsymmetric structure of the macrocycle,
with clear resolution of all of the resonances arising from
the â-protons on the furan and pyrrole rings.
For comparison purposes, we also prepared a dipyridyl-
substituted Zn porphyrin (Zn -2) as shown in eq 3.
Syn th esis of cyclo-Zn 5F bU via th e 5 + 1 Rou te. We
initially pursued the 5 + 1 route outlined in Figure 2A.
The synthesis employs four porphyrin building blocks,
three Pd-mediated coupling reactions, and two deprotec-
tion steps. The coupling reactions were performed using
refined conditions suitable for joining porphyrin building
blocks but at slightly higher concentration:48 ∼3.6-5 mM
[ethyne] and [iodo], ∼15 mol % Pd2(dba)3 based on
[ethyne], and ∼120 mol % P(o-tol)3 based on [ethyne], in
toluene/triethylamine (5:1) at 35 °C under argon. Each
purified product was characterized by TLC, analytical
SEC, LD-MS, UV-vis spectroscopy, fluorescence spec-
heteroatom-substituted porphyrin building blocks.47 The
synthesis began with 5-(4-iodophenyl)furylpyrromethane
which was acylated under Friedel-Crafts conditions,
affording the diacyl derivative 18.47 Reduction of 18 with
NaBH4 in THF/methanol (3:1) gave the furylpyrrometh-
anedicarbinol, which was condensed with 5-(4-iodo-
phenyl)dipyrromethane (19) under nonscrambling con-
densation conditions (30 mM TFA in CH3CN at room
temperature)44 followed by oxidation with DDQ. The yield
of the desired N3O-porphyrin 20 from the reaction of 18
and 19 was 14%. The Pd-mediated coupling of 20 and
4-ethynylpyridine (6) was performed under conditions
similar to those for the preparation of multiporphyrin
arrays.48 The desired dipyridyl N3O-porphyrin 21 was
obtained in 36% yield after purification by column
chromatography on basic alumina (Scheme 6).
1
troscopy, and H NMR spectroscopy.
The synthesis began by coupling p/ p-diethynyl Fb
porphyrin 3 with 2 mol equiv of m/ m-iodo/TMS-ethynyl
Zn porphyrin Zn -16a (Scheme 7). The distribution of
products observed by analytical SEC was typical, consist-
ing of desired trimer, mono-coupled byproduct (dimer),
and high molecular weight material (HMWM). Purifica-
tion by one silica column, one preparative SEC column,
and one silica column afforded trimer Zn F bZn -m /m -
CCTMS in 47% yield. Deprotection of the TMS-ethynyl
groups using TBAF49 gave the diethynyl trimer Zn F bZn -
m /m -CCH in 90% yield. A similar sequence of reactions
was employed to prepare the pentamer. The coupling of
Zn F bZn -m /m -CCH and 2 mol equiv of the p/ p-iodo/
TMS-ethynyl Zn -16b gave the TMS-protected pen-
(48) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7411
Sch em e 7
Chromatography on one silica column (CHCl3) removed
non-porphyrin species and some of the HMWM; four
repetitive preparative SEC columns (THF) removed most
of the HMWM. A final silica column afforded the pure
cyclo-Zn 5F bU in 13% yield.
Bin d in g of th e Lin ea r P en ta m er Zn 5-p/p-CCH a n d
th e Tr ip yr id yl Tem p la te (1). The cyclic hexamers such
as cyclo-Zn 6U have high affinity for template 1, ef-
fectively giving quantitative association at concentrations
typically employed for static electronic spectroscopy (i.e.,
10-6 to 10-7 M).33 We investigated whether the less-
structured linear pentamer Zn 5-p/p-CCH exhibited simi-
lar affinity for 1 (Scheme 8). Upon addition of 1 mol equiv
Sch em e 8
of the tripyridyl template 1 to a solution of Zn 5-p/p-CCH
in toluene (3.0 × 10-7 M each), the Soret band red-shifted
from 423 to 432 nm, indicating the binding of the
template with Zn 5-p/p-CCH (Figure 4A). Illumination of
tamer Zn 2F bZn 2-p/p-CCTMS in 41% yield after silica
chromatography and SEC. Cleavage of the TMS groups
gave the ethynyl substituted pentamer Zn 2F bZn 2-p/p-
CCH in 85% yield. Metalation of Zn 2F b Zn 2-p /p -CCH
with Zn(OAc)2‚2H2O afforded Zn 5-p/p-CCH in 95% yield.
A survey reaction of Zn 5-p /p -CCH and m/ m-diiodo
Fb porphyrin 4 was performed in the presence of a stoi-
chiometric amount of the template (Scheme 7). The
reaction required 2.5 h for complete consumption of the
starting porphyrins. The analytical SEC trace of the
crude reaction mixture showed a sharp peak assigned as
the desired hexamer (see Supporting Information).50 LD-
MS analysis of the same reaction mixture showed the
molecule ion peak at m/z ) 4645.4 (calcd avg mass 4642.3
for C312H230N24Zn5) as the only significant peak in the
region m/z ) 3000-10000. The preparative synthesis of
cyclo-Zn 5F bU was carried out in the same manner.
(49) Wagner, R. W.; J ohnson, T. E.; Lindsey, J . S. J . Am. Chem.
Soc. 1996, 118, 11166-11180.
(50) Analytical SEC generally is a very effective tool for monitoring
array-forming reactions.49 However, in this reaction
a significant
amount of precipitation occurred. The analytical SEC traces of aliquots
from the crude reaction mixture can be misleading because typically
only the soluble species are sampled. Such precipitation was not
observed in our prior work.6 The analytical SEC trace of the soluble
fraction from this reaction, and that of the purified product, are shown
in the Supporting Information.
F igu r e 4. Template binding effect of Zn 5-p/p-CCH with
tripyridyl template 1 in toluene at room temperature (3.0 ×
10-7 M each). (A) UV-vis absorption spectra before and after
adding template 1. (B) Fluorescence emission spectra before
and after adding template 1.

7412 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
Zn 5-p/p-CCH at 550 nm in toluene resulted in emission
typical of a Zn porphyrin, with λem ) 598 and 642 nm.
However, illumination at 550 nm of the solution contain-
ing a 1:1 ratio of Zn 5-p/p-CCH and template 1 resulted
in a red-shifted emission (λem ) 612 and 662 nm)
characteristic of pyridyl-coordinated Zn porphyrins, with
no emission observed from the uncoordinated Zn porphy-
rin (Figure 4B). Given that fluorescence from the fully
uncoordinated array (Zn 5-p/p-CCH) at a level of 5%
would be readily detected, the association constant for
formation of Zn 5-p/p-CCH-1 must be >109 M-1. These
results suggest that the linear pentamer and the tem-
plate must be quantitatively associated under the condi-
tions of the Pd-coupling reactions, which are performed
at a concentration 4 orders of magnitude higher than in
this binding experiment.
Syn th esis of cyclo-Zn 5F bU via th e 3 + 3 Rou te. We
recently developed methodology for successive Sonogash-
ira coupling reactions of iodo and bromo groups on
porphyrin building blocks.51 This method enabled inves-
tigation of a 3 + 3 route for preparing cyclo-Zn 5F b U
(Figure 2C). This route requires the preparation of two
trimer precursors, a diethynyl trimer Zn 3-m /m -CCH and
a dibromo trimer Zn F bZn -p/p-Br . The latter was pre-
pared by the selective reaction of 2 mol equiv of bromo/
iodo porphyrin Zn -16c with diethynyl porphyrin 12 at
room temperature (Scheme 9). SEC analysis of an aliquot
pure trimer Zn F bZn -p/p-Br in 35% yield. This approach
affords good selectivity for the iodo + ethyne reaction in
the presence of a bromo group.
The 3 + 3 reaction of Zn F bZn -p/p-Br and Zn 3-m /m -
CCH (Scheme 9) was carried out using the conditions
for the bromo + ethyne coupling reaction, which are
essentially identical to those of the iodo + ethyne reaction
but with reaction at 80 °C.51 Purification of this mixture
required an SEC column (THF) to remove the undis-
solved Pd species and some of the HMWM, three pre-
parative SEC columns to remove the remaining HMWM
which chromatographed closely with the desired cyclic
hexamer, and a final silica column chromatography.50
Some insoluble materials remained on top of the first
short SEC column, which upon dissolution in pyridine
and analysis by SEC were found to consist mostly of
HMWM. The desired product cyclo-Zn 5F bU was ob-
tained in 13.6% yield. The precipitate in this reaction
appears to be associated with the template.52
Syn th esis of cyclo-Zn 2F bZn 2F bU. All approaches
envisaged in Figure 3 for the synthesis of cyclo-
Zn 2F bZn 2F bU construct the array in a 5 + 1 route. We
initially investigated the approach shown in Figure 3B,
which employs four porphyrin building blocks, three Pd-
coupling reactions, and three functional group deprotec-
tion steps (including magnesium demetalation). Coupling
of m/ m-diethynyl porphyrin 12 and 2 mol equiv of p/ p-
iodo/TMS-ethynyl Zn porphyrin Zn -16b afforded the
desired trimer (Zn F bZn -p/p-CCTMS) and byproduct
(dimer) in 36% and 3.7% yield, respectively, after puri-
fication by chromatography (silica, SEC, silica). Cleavage
of the TMS groups (TBAF) afforded the diethynyl trimer
Zn F bZn -p/p-CCH in 80% yield (Scheme 10). Coupling
of the latter with 2 mol equiv of m/ m-iodo/TMS-ethynyl
Zn -16a afforded the TMS-protected porphyrin pentamer
Zn 2F bZn 2-m /m -CCTMS in 51% yield after chromatog-
raphy (silica, SEC, silica). Cleavage of the TMS groups
with TBAF afforded Zn 2F bZn 2-m /m -CCH in 85% yield.
The coupling reaction of Zn 2F bZn 2-m /m -CCH and
p/ p-diiodo Mg porphyrin (Mg-17) in the presence of
template 1 was complete within 1.5 h. The analytical SEC
trace of the crude reaction mixture exhibited a sharp
peak assigned as the hexamer cyclo-Zn 2F bZn 2MgU.50
LD-MS analysis also showed an intense molecule ion
Sch em e 9
peak at m/z ) 4602.9 (calcd avg mass 4601.2 for C312H230
-
MgN24Zn4). Demetalation of magnesium was achieved by
treatment of the crude reaction mixture with silica gel
in CHCl3. Chromatography on silica removed Pd species
and some of the HMWM, repetitive preparative SEC
removed most of the HMWM, and a final silica chroma-
tography followed by precipitation from CHCl3/methanol
afforded cyclo-Zn 2F bZn 2F bU in 10% yield.
We also investigated the coupling reaction of Zn 2FbZn 2-
p /p-CCH and m/ m-diiodo Fb porphyrin (4) in the
absence of the template (route A, Figure 3). A survey
reaction required 3 h for the complete consumption of
the starting porphyrins (see Supporting Information for
the SEC trace of the crude reaction mixture). LD-MS
analysis of this crude reaction mixture exhibited an
removed after 2.5 h showed the presence of 7% HMWM,
58% trimer, 7% dimer, and 28% monomers (uncorrected
data). No additional catalyst was added for this reaction.
Chromatographic workup (silica, SEC, silica) afforded the
(52) The reaction in the absence of the template resulted in complete
consumption of the starting porphyrin trimers after 2 h (analytical
SEC). No precipitation was noticed in this reaction. The integrated
intensity (uncorrected) of the desired hexamer only accounted for 4.4%
of the total, and LD-MS analysis of this crude mixture showed a weak
peak at m/z ) 4630.1 (calcd 4642.2 for C312H230N24Zn5). This material
was not isolated.
(51) Loewe, R. S.; Lammi, R. K.; Diers, J . R.; Kirmaier, C.; Bocian,
D. F.; Holten, D.; Lindsey, J . S. Manuscript in preparation.

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7413
Sch em e 10
of the component Zn and Fb porphyrins. The close
similarity of the spectra of the component parts and that
of the array are typical of a weakly coupled set of
chromophores.1 Illumination of cyclo-Zn 5F bU at 550 nm,
where the Zn porphyrins absorb over 90% of the light,
results in emission from both the Zn and Fb porphyrins
(integrated Φf ) 0.085). The emission from the Zn
porphyrins is decreased to 40% of that of a benchmark
Zn porphyrin, commensurate with a quantum yield of
60% for energy transfer reaching the Fb porphyrin. For
cyclo-Zn 2F bZn 2F bU, illumination at 550 nm, where the
Zn porphyrins absorb 85% of the light, results in emission
predominantly from the Fb porphyrins (integrated Φf )
0.098). The Zn porphyrin emission is decreased to 10%
of that of a benchmark Zn porphyrin, commensurate with
a quantum yield of 90% for energy transfer reaching the
Fb porphyrin (see fluorescence spectra in the Supporting
Information). Thus, for both arrays the dominant excited
singlet-state process is energy transfer from the Zn to
the Fb porphyrins. The spectroscopic characterization of
the self-assembled arrays derived from cyclo-Zn 5F bU
and cyclo-Zn 2F bZn 2F bU with various guests will be
presented elsewhere.
Con sid er a tion of Syn th etic Str a tegies. The syn-
thesis of multiporphyrin light-harvesting arrays is a
relatively new area of materials chemistry. A diversity
of synthetic methods is required for this field to flourish.
The arrays produced by Gossauer have 6-fold symmetry
with six p/ p-substituted porphyrins joined via a 1,3-bis-
(phenylethynyl)phenylene linker.36 The synthesis re-
ported by Gossauer employs an iterative divergent/
convergent strategy. This approach begins with one
porphyrin building block (prepared via earlier statistical
methods), employs four Pd-coupling reactions (one site
per coupling reaction), and requires six functional group
transformations as the array grows in size. Two advan-
tages of this strategy are as follows: (1) The metalation
state of each porphyrin can be controlled. (2) The linear
hexamers bearing iodo and ethynyl functional groups
enable cyclization to be performed in dilute solution. Such
large relatively rigid molecules, which have rarely been
accessible, can be used in fundamental studies of mac-
rocyclization. The cyclizations (untemplated) were per-
formed at 0.25 mM and afforded up to 0.39-1.8 mg of
cyclic hexamer in yields of 8-31%.
The arrays described herein have 3-fold symmetry with
alternating p/ p- and m/ m-substituted porphyrins. Each
synthetic route is performed in a stepwise manner,
employs four porphyrin monomers, and requires three
Pd coupling reactions (two sites per coupling reaction)
and one to three functional group deprotection steps.
Reaction at two coupling sites with each Pd-coupling step
affords rapid synthesis but restricts the range of patterns
of distinct porphyrins that can be produced. The final
ring-forming step also entails reaction at two sites (the
first an intermolecular reaction, the second an intramo-
lecular reaction) for which the conditions have yet to be
optimized. The cyclizations (templated) were performed
at 1.7-1.8 mM of each reactant and afforded 7.8-9.8 mg
of cyclic hexamer in yields of 10-13.6%. Two advantages
of this approach are as follows: (1) The desired functional
groups are introduced at the earliest stage in the
synthesis, during the construction of the porphyrin
monomers, for which powerful methodology is now avail-
able. (2) A high degree of convergence minimizes syn-
thetic manipulations of the growing arrays.
intense molecule ion peak at m/z ) 4583.8. The cyclo-
Zn 2F bZn 2F bU was isolated by chromatography (silica;
three SEC columns) in 3.5% yield with 95% purity as
judged by analytical SEC. This reaction without the
template establishes the structure-directed contribution
to cyclic hexamer formation. Although the yield was low,
the appropriate comparison is the one-flask synthesis
without a template, which afforded no isolated cyclic
hexamer (a trace was detected by LD-MS). The one-flask
synthesis with a template afforded cyclo-Zn 3F b3U in
5.3% yield.6 Thus, the rational syntheses can be carried
out to obtain cyclic hexamers with diverse metalation
patterns for which a template-directed synthesis is not
possible, obtaining yields comparable to those in the one-
flask synthesis with a template. A detailed comparison
of the synthesis and purification methods employed in
the one-flask and stepwise syntheses is shown in a Table
in the Supporting Information.
Sp ectr oscop ic Ch a r a cter iza tion . The absorption
and fluorescence spectra of the cyclic arrays were col-
lected in toluene at room temperature. The absorp-
tion spectrum of each array (cyclo-Zn 5F bU, cyclo-
Zn 2F bZn 2F bU) exhibits a slightly broadened Soret band
while the visible bands are essentially identical to those

7414 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
grade). Analytical SEC49 (λdet ) 420 nm) was performed to
monitor the progress of the coupling reactions and assess the
purity of the porphyrin oligomers and cyclic hexameric arrays.
Flu or escen ce Spectr oscopy. Fluorescence quantum yields
were determined in toluene at room temperature as described
previously.6 Tetraphenylporphyrin (TPP, Φf ) 0.11),54 zinc
tetraphenylporphyrin (ZnTPP, Φf ) 0.033),54 and cyclo-Zn 6U
(Φf ) 0.039)6 were used as quantitative standards.
Con clu sion s. For studies of the flow of excited-
state energy in self-assembled light-harvesting sys-
tems, two new cyclic hexamers (cyclo-Zn 5F bU, cyclo-
Zn 2F bZn 2F bU) were prepared. Several convergent ap-
proaches for constructing the macrocyclic arrays were
examined. The major findings from this work are as
follows: (1) The yields in the ring-forming reaction in the
template-directed 5 + 1 route were 13% (cyclo-Zn 5F bU,
Figure 2A) and 10% (cyclo-Zn 2F bZn 2F bU, Figure 3B),
while that in the template-directed 3 + 3 route was 13.6%
(cyclo-Zn 5F bU, Figure 2C). The yield in the structure-
directed (i.e., no template) 5 + 1 route was 3.5% (cyclo-
Zn 2F bZn 2F bU, Figure 3A). For comparison, the overall
yield of the template-directed one-flask synthesis was
5.3% (cyclo-Zn 3F b3U) while the structure-directed one-
flask synthesis gave only a trace of product. (2) The
template-directed one-flask route is superior for those
cyclic hexamers with a suitable pattern of metalation
states (e.g., cyclo-Zn 3F b3U and cyclo-Zn 6U), affording
a simple and expedient synthesis. On the other hand, the
stepwise synthesis is applicable to those cyclic hexamers
with metalation patterns incompatible with a one-flask
synthesis (e.g., cyclo-Zn 5FbU and cyclo-Zn 2FbZn 2FbU).
The stepwise syntheses can be implemented with or
without a template whereas the one-flask synthesis can
be performed only with a template. (3) Both the 5 + 1
route and the 3 + 3 route afford convergent syntheses.
The 3 + 3 route employs the reaction of a diethynyl
trimer and a dibromo trimer; the latter is available via
successive iodo + ethyne and bromo + ethyne coupling
reactions. (4) The linear pentamer exhibits high affinity
for the tripyridyl template with association constant >
109 M-1. (5) The rational synthesis of the cyclic hexamer
employs trans-AB2C-porphyrins and trans-A2B2-porphy-
rins bearing ethyne, iodo, and/or bromo functional groups.
The availability of ample quantities of the requisite
porphyrin building blocks enabled the cyclic hexamers
to be obtained in quantities of ∼10 mg, which is sufficient
for spectroscopic studies.
1,3,5-Tr is{4-[2-(4-p yr id yl)eth yn yl]p h en yl}ben zen e (1).
Following a standard method,55 samples of 6 (0.84 g, 8.1 mmol),
7 (1.37 g, 2.0 mmol), Pd(PPh3)2Cl2 (428 mg, 0.60 mmol), and
CuI (8 mg, 0.04 mmol) were weighed into a 100 mL Schlenk
flask. The flask was pump-purged with argon three times.
THF/diethylamine (60 mL, 1:1) was added, and the mixture
was stirred at 45 °C under argon. TLC analysis [silica, CHCl3/
ethyl acetate (1:1)] showed the product as the second bright
fluorescent spot upon UV illumination. Additional 4-ethy-
nylpyridine (90 mg, 0.87 mmol), Pd(PPh3)2Cl2 (60 mg, 0.093
mmol), and CuI (3.0 mg, 0.015 mmol) were added after a total
of 3 h and stirring was continued for another 3 h. The reaction
mixture was cooled in a freezer overnight. The precipitate was
filtered, and the filtrate was concentrated and purified by
chromatography [silica, CH2Cl2/methanol (96.5:3.5)]. The sec-
ond band (bright fluorescence) was collected, which afforded
a slightly yellow solid (1.01 g, 83%): mp > 260 °C; 1H NMR δ
8.63 (d, J ) 5.7 Hz, 6H), 7.84 (s, 3H), 7.74 (d, J ) 8.1 Hz, 6H),
7.69 (d, J ) 8.1 Hz, 6H), 7.42 (d, J ) 5.7 Hz, 6H); FAB-MS
obsd 610.2306, calcd 610.2283 (C45H27N3). Anal. Calcd: C,
88.64; H, 4.46; N, 6.89. Found: C, 87.86; H, 4.53; N, 6.87.
Zn (II)-5,15-Dim esityl-10,20-bis{4-[2-(4-p yr id yl)eth yn yl]-
p h en yl}p or p h yr in (Zn -2). A solution of 233 (15.0 mg, 16.6
µmol) in CHCl3 (5 mL) was treated with a solution of Zn(OAc)2‚
2H2O (18.3 mg, 83.3 µmol) in methanol (0.5 mL) at room
temperature for 4 h. The standard workup and chromatogra-
phy [silica, CHCl3/methanol (95:5)] afforded a purple solid (15.2
mg, 94%): 1H NMR δ 8.70-8.66 (m, 8H), 8.12 (d, J ) 8.1 Hz,
4H), 7.66 (d, J ) 7.8 Hz, 4H), 7.25 (m, 8H), 6.58 (s, br, 4H),
2.60 (s, 6H), 1.78 (s, 12H); LD-MS obsd 960.1, FAB-MS obsd
963.3130, calcd 963.3154 (C64H46N6Zn); λabs (log ꢀ) 426 (5.42),
551 (4.27), 593 (3.74) nm; λem (λex ) 550 nm) 600, 646 nm (Φf
) 0.046).
2-Meth yl-4-(4-p yr id yl)-3-bu tyn -2-ol (5). Following a stan-
dard method,55,56 4-bromopyridine hydrochloride (5.01 g, 25.7
mmol), Pd(PPh3)2Cl2 (180 mg, 0.26 mmol), and CuI (29 mg,
0.15 mmol) were weighed into a 100 mL Schlenk flask which
was then pump-purged with argon three times. 2-Methyl-3-
butyn-2-ol (2.60 g, 31.0 mmol) and diethylamine/THF (60 mL,
5:2) were added under argon. After stirring for 4.5 h at room
temperature, the reaction was judged to be complete by TLC
analysis [silica, CHCl3/ethyl acetate (1:1)]. The solvent was
evaporated to dryness. The crude product was dissolved in
CHCl3 (100 mL) and washed with water. The organic layer
was dried (Na2SO4), and the solvent was removed. The residue
was purified by chromatography [silica, CHCl3/ethyl acetate
(1:1)], affording a yellow solid (4.01 g). Recrystallization
(CHCl3/hexanes) afforded a slightly yellow solid (3.80 g,
Exp er im en ta l Section
1
Gen er a l. H NMR spectra (300 or 500 MHz) and 13C NMR
spectra (75 MHz) were collected in CDCl3. Absorption spectra
and fluorescence spectra (corrected) were collected in toluene
at room temperature. Mass spectra of porphyrins were ob-
tained via laser desorption mass spectrometry (LD-MS) in the
absence of an added matrix,53 or by high-resolution fast atom
bombardment mass spectrometry (FAB-MS). All reagents were
obtained from Aldrich and were used as received. The known
compounds 2,33 Zn -3,6 4,6 9,42 10,43 11,43 13b,44 13c,44 14b,44
15b,44 Zn -16b,48 17,41 18,47 19,42 3-iodobenzoyl chloride,44 and
S-2-pyridyl 3-iodobenzothioate46 were prepared according to
literature procedures.
Solven ts. Solvents were dried by standard methods. Di-
ethylamine, triethylamine and toluene were distilled from
CaH2. The solvents CH3CN, CH2Cl2, and CHCl3 (stabilized
with 0.8% ethanol) were reagent-grade and were used as
received from Fisher.
1
92%): mp 113-114 °C; H NMR δ 8.54 (d, J ) 5.1 Hz, 2H),
7.25 (d, J ) 5.1 Hz, 2H), 3.60 (br s, 1H), 1.63 (s, 6H); 13C NMR
δ 149.3, 131.4, 125.7, 99.2, 79.3, 65.1, 31.2. Anal. Calcd for
C10H11NO: C, 74.50; H, 6.88; N, 8.69. Found: C, 74.46; H, 6.96;
N, 8.71.
4-Eth yn ylp yr id in e (6). A solution of 5 (3.00 g, 18.6 mmol)
in toluene (90 mL) was treated with finely powdered NaOH
(0.80 g, 20 mmol), and the mixture was refluxed for 3 h. TLC
[silica, ethyl acetate/CHCl3 (1:1)] indicated the reaction was
complete. The reaction mixture was cooled to room tempera-
ture and filtered. The filtrate was concentrated taking care to
minimize the loss of the product, which is quite volatile.
Ch r om a togr a p h y. Adsorption column chromatography
was performed using flash silica gel (Baker, 60-200 mesh) or
alumina (alumina basic, Brockman activity I, Fisher). Pre-
parative-scale size exclusion chromatography (SEC) was per-
formed using BioRad Bio-beads SX-1 in THF (Fisher HPLC
(54) Seybold, P. G.; Gouterman, M. J . Mol. Spectrosc. 1969, 31,
1-13.
(53) (a) Srinivasan, N.; Haney, C. A.; Lindsey, J . S.; Zhang, W.;
Chait, B. T. J . Porphyrins Phthalocyanines 1999, 3, 283-291. (b)
Fenyo, D.; Chait, B. T.; J ohnson, T. E.; Lindsey, J . S. J . Porphyrins
Phthalocyanines 1997, 1, 93-99.
(55) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467-4470. (b) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.;
Hagihara, N. Synthesis 1980, 627-630.
(56) Ames, D. E.; Bull, D.; Takundwa, C. Synthesis 1981, 364-365.

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7415
Kugelrohr distillation (0.01 mmHg, 30 °C) afforded a white
solid (1.22 g, 64%): mp 96-97 °C; 1H NMR δ 8.60 (d, J ) 5.9
Hz, 2H), 7.35 (d, J ) 5.9 Hz, 2H), 3.30 (s, 1H); 13C NMR δ
149.6, 130.1, 125.9, 81.8, 80.7; HRMS (EI) obsd 103.0418, calcd
103.0422 (C7H5N).
124.0, 123.7, 103.3, 96.1, 0.3; FAB-MS obsd 312.0898, calcd
312.0878 (C17H17NOSSi). Anal. Calcd: C, 65.55; H, 5.50; N,
4.50. Found: C, 65.53; H, 5.55; N, 4.45.
5-Mesit yl-1-{3-[2-(t r im et h ylsilyl)et h yn yl]b en zoyl}d i-
p yr r om eth a n e (14a ). Following a published procedure,44
EtMgBr (44.0 mL, 44.0 mmol, 1.0 M in THF) was added to a
solution of 9 (5.28 g, 20.0 mmol) in dry THF (20 mL) at room
temperature under argon. The mixture was stirred for 10 min
at room temperature and then cooled to -78 °C. A solution of
13a (6.22 g, 20.0 mmol) in THF (30 mL) was then added. The
mixture was stirred at -78 °C for 10 min and then allowed to
warm to room temperature. TLC analysis [silica gel, CH2Cl2/
ethyl acetate (98:2)] showed complete consumption of 13a after
40 min. The reaction mixture was poured into a mixture of
CH2Cl2 (200 mL) and saturated aqueous NH4Cl (100 mL). The
organic phase was separated, washed with brine and water,
dried (Na2SO4), and concentrated to give a dark foamlike solid.
Chromatography [silica, CH2Cl2/ethyl acetate (98:2)] afforded
an amorphous yellow solid (7.16 g, 77%): mp 95-97 °C; 1H
NMR δ 9.21 (s, br, 1H), 7.92 (m, 1H), 7.84 (s, br, 1H), 7.77 (d,
J ) 8.1 Hz, 1H), 7.62 (d, J ) 8.1 Hz, 1H), 7.40 (t, J ) 8.1 Hz,
1H), 6.90 (s, 2H), 6.84-6.82 (m, 1H), 6.70-6.68 (m, 1H), 6.24-
6.21 (m, 1H), 6.15-6.12 (m, 2H), 5.96 (s, 1H), 2.30 (s, 3H), 2.09
(s, 6H), 0.26 (s, 9H); 13C NMR δ 183.0, 141.3, 138.5, 137.4,
137.1, 134.7, 132.9, 132.1, 130.5, 129.5, 128.9, 128.6, 128.2,
123.2, 120.8, 116.9, 110.1, 108.8, 107.1, 104.1, 95.1, 38.5, 20.7,
20.6, 0.2; FAB-MS obsd 464.2311, calcd 464.2284 (C30H32N2-
OSi). Anal. Calcd: C, 77.54; H, 6.94; N, 6.03. Found: C, 77.39;
H, 6.83; N, 5.91.
1-(4-Br om ob en zoyl)-5-m esit yld ip yr r om et h a n e (14c).
Following the procedure for the preparation of 14a , the
reaction of 9 (1.85 g, 7.00 mmol) in THF (10 mL) with EtMgBr
(15.4 mL, 15.4 mmol, 1.0 M in THF) followed by S-2-pyridyl
4-bromobenzenethioate44 (2.06 g, 7.00 mmol) afforded, upon
standard workup and chromatography (silica, CH2Cl2), a
yellow foamlike solid (2.46 g, 78%): mp 120-122 °C; 1H NMR
δ 9.24 (s, br, 1H), 7.84 (s, br, 1H), 7.72 (d, J ) 8.1 Hz, 2H),
7.60 (d, J ) 8.1 Hz, 2H), 7.00 (s, 2H), 6.80-6.78 (m, 1H), 6.69-
6.68 (m, 1H), 6.23-6.20 (m, 1H), 6.15-6.12 (m, 2H), 5.95 (s,
1H), 2.29 (s, 3H), 2.09 (s, 6H); 13C NMR δ 182.7, 141.3, 137.4,
137.2, 137.1, 132.9, 131.5, 130.5, 130.3, 129.5, 128.9, 126.3,
120.5, 116.9, 110.2, 108.9, 107.2, 38.6, 20.7; FAB-MS obsd
446.0979, calcd 446.0994 (C25H23BrN2O). Anal. Calcd: C,
67.12; H, 5.18; N, 6.26. Found: C, 67.11; H, 5.23; N, 6.23.
1-(3-Iod oben zoyl)-5-m esityl-9-{3-[2-(tr im eth ylsilyl)eth -
yn yl]ben zoyl}d ip yr r om eth a n e (15a ). Following a standard
procedure,44 a solution of monoacyldipyrromethane 14a (7.03
g, 14.2 mmol) in toluene (60 mL) was treated with EtMgBr
(28.4 mL, 28.4 mmol, 1.0 M in THF) under argon. After stirring
at room temperature for 5 min, a solution of 3-iodobenzoyl
chloride (3.78 g, 15.1 mmol) in toluene (2 mL) was added under
argon. Stirring was continued at room temperature for 10 min.
EtMgBr (28.4 mL, 28.4 mmol) was again added followed by
stirring for 5 min, and then 3-iodobenzoyl chloride (3.78 g, 14.2
mmol) in toluene (2 mL) was added. After 15 min, TLC
analysis [silica, CH2Cl2/ethyl acetate (98:2)] indicated complete
consumption of 14a . The reaction mixture was poured into a
mixture of saturated aqueous NH4Cl (200 mL) and ethyl
acetate (300 mL). The organic phase was separated, washed
with brine and water, dried (Na2SO4), and concentrated.
Chromatography (silica, CH2Cl2) followed by recrystalliza-
tion (CH2Cl2/methanol) afforded a brown solid (6.76 g, 64%):
mp 100-102 °C; 1H NMR δ 10.18 (s, br, 2H), 8.08 (m, 1H),
7.86-7.83 (m, 2H), 7.75-7.69 (m, 2H), 7.59 (d, J ) 8.1 Hz,
1H), 7.36 (t, J ) 8.1 Hz, 1H), 7.17 (t, J ) 8.1 Hz, 1H), 6.94
(s, 2H), 6.77-6.72 (m, 2H), 6.12 (s, 1H), 6.10 (s, 2H), 2.32 (s,
3H), 2.19 (s, 6H), 0.25 (s, 9H); 13C NMR δ 182.5, 181.7, 140.8,
140.5, 140.1, 139.8, 138.4, 138.0, 137.4, 137.0, 134.0, 133.1,
133.0, 130.2, 130.0, 129.8, 129.6, 129.1, 128.2, 127.9, 122.9,
121.5, 121.4, 110.8, 110.7, 104.2, 94.8, 93.5, 39.2, 20.9, 0.2;
FAB-MS obsd 694.1531, calcd 694.1513 (C37H35IN2O2Si). Anal.
Calcd: C, 63.97; H, 5.08; N, 4.03. Found: C, 63.81; H, 5.18;
N, 3.98.
1,3,5-Tr is(4-iod op h en yl)ben zen e (7). 4-Iodoacetophenone
(25.0 g, 0.10 mol) and trimethyl orthoformate (17.1 g, 0.16 mol)
were dissolved in CHCl3 (200 mL) in a three-necked round-
bottomed flask fitted with a gas inlet and a gas outlet; the
latter was connected to an aqueous sodium hydroxide trap.
The solution was purged with HCl gas for 30 min at 0 °C and
then was stirred overnight at room temperature. TLC [silica,
CHCl3/hexanes (1:3)] showed complete consumption of 4-iodo-
acetophenone. The solvent was removed, and the residue was
purified by chromatography [silica, 7.5 × 30 cm, CHCl3/
hexanes (1:3)]. The first band (yellow) was the desired product
followed by the byproduct 8 as the second band. Recrystalli-
zation (CHCl3/methanol) afforded a slightly yellow solid (8.6
g, 37%): mp >260 °C; 1H NMR δ 7.78 (d, J ) 8.1 Hz, 6H),
7.67 (s, 3H), 7.39 (d, J ) 8.1 Hz, 6H); 13C NMR δ 141.6, 140.1,
138.0, 129.1, 124.9, 93.6. Anal. Calcd for C24H15I3: C, 42.12;
H, 2.21. Found: C, 42.13; H, 2.20. Data for compound 8 (2.98
1
g, 12%): mp 96-98 °C; H NMR δ 7.78 (d, J ) 8.1 Hz, 2H),
7.74 (d, J ) 8.8 Hz, 2H), 7.67 (d, J ) 8.1 Hz, 2H), 7.29 (d, J )
8.8 Hz, 2H), 7.06 (d, J ) 1.5 Hz, 1H), 2.54 (s, 3H); 13C NMR δ
190.4, 154.4, 141.8, 138.2, 137.7, 137.6, 129.5, 128.1, 121.4,
100.5, 95.4, 18.6. Anal. Calcd for C16H12OI2: C, 40.54; H, 2.55.
Found: C, 40.55; H, 2.50.
5,15-Bis(3-et h yn ylp h en yl)-10,20-d im esit ylp or p h yr in
(12). Following an early method for preparing 11 at one-fifth
the scale,43 a solution of 10 (955 mg, 4.73 mmol) and 5-mesi-
tyldipyrromethane (9) (1.25 g, 4.73 mmol) in CHCl3 (625 mL)
was treated with BF3‚O(Et)2 (260 µL, 3.3 mM). After 1 h, DDQ
(1.68 g, 7.40 mmol) was added, and the mixture was stirred
for another 1 h at room temperature. The reaction mixture
was filtered through a pad of alumina, washed with CHCl3,
and the porphyrin fraction was collected. Chromatography
[silica, CH2Cl2/hexanes (1:1)] gave TMS-protected porphyrin
1
11 as a purple solid (1.25 g, 58%). H NMR and LD-MS data
were identical to those previously reported.43 A solution of 11
(1.20 g, 1.34 mmol) in CHCl3 (150 mL) was treated with TBAF
on silica gel (2.68 g, 2.68 mmol, 1.0-1.5 mmol F-/g resin) at
room temperature for 2 h. The mixture was washed with 10%
aqueous NaHCO3 and water. The organic layer was dried (Na2-
SO4) and concentrated. Chromatography [silica, CHCl3/hex-
anes (1:1)] afforded a purple solid (0.84 g, 84%): 1H NMR δ
8.76 (d, J ) 5.1 Hz, 4H), 8.70 (d, J ) 5.1 Hz, 4H), 8.35 (d, J )
1.5 Hz, 2H), 8.20 (d, J ) 7.5 Hz, 2H), 7.91 (d, J ) 8.1 Hz, 2H),
7.70 (d, J ) 8.1 Hz, 2H), 7.28 (s, 4H), 3.16 (s, 2H), 2.63 (s,
6H), 1.83 (s, 12H), -2.68 (s, br, 2H); LD-MS obsd 747.7; FAB-
MS obsd 746.3442, calcd 746.3409 (C54H42N4); λabs 420, 514,
548, 591, 647 nm; λem (λex ) 520 nm) 649, 717 nm.
S-2-P yr idyl 3-[2-(tr im eth ylsilyl)eth yn yl]ben zen eth ioate
(13a ). Following a refined procedure,44 samples of S-2-pyridyl
3-iodobenzenethioate46 (17.1 g, 50.1 mmol), Pd2(dba)3 (0.83 g,
0.90 mmol), PPh3 (1.97 g, 7.51 mmol), and CuI (0.95 g, 5.0
mmol) were weighed into a 250 mL Schlenk flask. The flask
was pump-purged with argon three times, and then THF/
triethylamine (140 mL, 3:1) and trimethylsilylacetylene (11.0
mL, 76.9 mmol) were added. The flask was sealed tightly. The
mixture was stirred for 2 h at 50 °C, at which point TLC
analysis [silica, hexanes/ethyl acetate (4:1)] showed incomplete
consumption of S-2-pyridyl 3-iodobenzenethioate. The flask
was cooled to room temperature and additional trimethylsi-
lylacetylene (7.00 mL, 49.6 mmol) was added. Stirring was
continued at 50 °C for another 1 h, affording complete reaction.
The mixture was filtered, and the filtered material was washed
with ethyl acetate. The filtrate was concentrated and purified
by chromatography [silica, hexanes/ethyl acetate (4:1)], af-
fording a slightly brown solid. Recrystallization from hexanes/
ethyl acetate gave pale yellow crystals (12.58 g, 81%): mp 90-
92 °C; 1H NMR δ 8.70-8.68 (m, 1H), 8.10 (m, 1H), 7.96-7.93
(m, 1H), 7.84-7.78 (m, 2H), 7.73-7.68 (m, 2H), 7.44 (t, J )
8.1 Hz, 1H), 7.38-7.33 (m, 1H), 0.27 (s, 9H); 13C NMR δ 188.6,
150.9, 150.5, 137.1, 136.8, 136.5, 130.8, 130.7, 128.7, 127.2,
1-(4-Br om oben zoyl)-9-(4-iod oben zoyl)-5-m esityld ip yr -
r om eth a n e (15c). Following the procedure for the synthesis

7416 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
of 15a , a solution of monoacyldipyrromethane 14c (2.46 g, 5.50
mmol) in toluene (25 mL) was treated with EtMgBr (11.0 mL,
11.0 mmol, 1.0 M in THF) under argon for 5 min at room
temperature. Then 4-iodobenzoyl chloride (1.47 g, 5.50 mmol)
was added under argon, and the mixture was stirred for 10
min at room temperature. This procedure was repeated twice
[EtMgBr (11.0 mL, 11.0 mmol), 4-iodobenzoyl chloride (1.47
g, 5.50 mmol); EtMgBr (5.5 mL, 5.5 mmol), 4-iodobenzoyl
chloride (0.74 g, 2.78 mmol)]. The mixture was stirred at room
temperature for 30 min, followed by standard workup and
chromatography [silica, CH2Cl2/ethyl acetate (98:2)]. Precipita-
tion of the product from CH2Cl2/methanol afforded a yellow
solid (2.21 g, 59%): mp 233-235 °C (dec); 1H NMR δ 10.35 (s,
br, 2H), 7.78 (d, J ) 8.7 Hz, 2H), 7.63 (d, J ) 8.7 Hz, 2H), 7.56
(d, J ) 8.7 Hz, 2H), 7.48 (d, J ) 8.7 Hz, 2H), 6.93 (s, 2H),
6.70-6.68 (m, 2H), 6.10-6.07 (m, 3H), 2,32 (s, 3H), 2.18 (s,
6H); 13C NMR δ 182.7, 182.4, 140.5, 137.4, 137.2, 136.9, 132.9,
131.2, 130.8, 130.3, 130.0, 126.4, 121.1, 110.7, 98.9, 39.2, 20.8;
FAB-MS obsd 676.0245, calcd 676.0222 (C32H26BrIN2O2). Anal.
Calcd: C, 56.74; H, 3.87; N, 4.14. Found: C, 56.96; H, 4.08;
N, 3.94.
(1.90 g, 2.80 mmol) in THF/methanol (160 mL, 3:1) was treated
with NaBH4 (5.18 g, 137 mmol). Standard workup and
condensation with 5-mesityldipyrromethane (9) (0.74 g, 2.80
mmol) in CH3CN (1120 mL) using TFA (2.60 mL, 30 mM) was
followed by oxidation with DDQ (1.93 g, 8.5 mmol). The
standard workup, including chromatography (silica, CH2Cl2)
followed by sonication with methanol afforded a purple solid
(613 mg, 24%): 1H NMR δ 8.78-8.76 (m, 4H), 8.70 (d, J ) 4.5
Hz, 4H), 8.08 (d, J ) 8.1 Hz, 4H), 7.95 (d, J ) 8.7 Hz, 2H),
7.88 (d, J ) 8.1 Hz, 2H), 7.29 (s, 4H), 2.63 (s, 6H), 1.82 (s,
12H), -2.68 (s, br, 2H); LD-MS obsd 902.8; FAB-MS obsd
902.1478, calcd 902.1481 (C50H40BrIN4); λabs 420, 515, 548, 592,
648 nm; λem (λex ) 520 nm), 650, 719 nm.
Zn (II)-5-(4-Br om op h en yl)-15-(4-iod op h en yl)-10,20-d i-
m esitylp or p h yr in (Zn -16c). A solution of porphyrin 16c (506
mg, 0.56 mmol) in CHCl3 (140 mL) was treated with Zn(OAc)2‚
2H2O (613 mg, 2.80 mmol) overnight at room temperature.
Standard workup and sonication with methanol afforded a
purple solid (515 mg, 95%): 1H NMR δ 8.86 (d, J ) 4.5 Hz,
2H), 8.85 (d, J ) 4.5 Hz, 2H), 8.78 (d, J ) 4.5 Hz, 4H), 8.10 (d,
J ) 6.6 Hz, 2H), 8.07 (d, J ) 6.6 Hz, 2H), 7.97 (d, J ) 8.1 Hz,
2H), 7.87 (d, J ) 8.1 Hz, 2H), 7.28 (s, 4H), 2.63 (s, 6H), 1.82
(s, 12H); LD-MS obsd 964.0; FAB-MS obsd 964.0646, calcd
5-(3-Iod op h en yl)-10,20-d im esit yl-15-{3-[2-(t r im et h yl-
silyl)eth yn yl]p h en yl}p or p h yr in (16a ). Following a stan-
dard procedure,44 a solution of diacyldipyrromethane 15a (2.78
g, 4.00 mmol) in dry THF/methanol (176 mL, 10:1) was treated
with NaBH4 (3.02 g, 80.0 mmol) in portions, and the mixture
was stirred at room temperature. TLC analysis [alumina, ethyl
acetate/CH2Cl2 (1:1)] showed that the reduction was complete
after 30 min. The reaction mixture was poured into a mixture
of saturated aqueous NH4Cl (200 mL) and CH2Cl2 (300 mL).
The organic phase was washed with brine and water, dried
(Na2SO4), and concentrated. The resulting residue was dried
under vacuum for 15 min. To the freshly prepared dipyr-
romethanedicarbinol (4.00 mmol) were added 9 (1.06 g, 4.00
mmol) and CH3CN (1600 mL) at room temperature. TFA (3.68
mL, 48.0 mmol, 30 mM) was added. The solution instantly
turned dark. After 3.5 min, the spectroscopic yield of porphyrin
had leveled off. Then DDQ (2.72 g, 12.0 mmol) was added and
the mixture was stirred for 1 h at room temperature. Triethyl-
amine (6.80 mL, 48.0 mmol) was then added. The mixture was
filtered through a pad of alumina (eluted with CH2Cl2).
Removal of the solvent gave a dark solid which was redissolved
in CH2Cl2 (30 mL) and chromatographed (silica, CH2Cl2),
964.0616 (C50H38BrIN4Zn); λabs 423, 549, 589 nm; λem (λex
550 nm) 594, 645 nm.
)
Mg(II)-5,15-Bis(4-iod op h en yl)-10,20-d im esitylp or p h y-
r in (Mg-17). Following a standard procedure,38 a solution of
5,15-bis(4-iodophenyl)-10,20-dimesitylporphyrin41 (190 mg, 0.20
mmol) in CHCl3 (15 mL) was treated with N,N-diisopropyl-
ethylamine (695 µL, 4.0 mmol) and MgI2 (556 mg, 2.0 mmol)
at room temperature. Fluorescence excitation spectroscopy
indicated complete metalation after 20 min. The mixture was
washed with 10% aqueous NaHCO3, dried (Na2SO4), and
concentrated to afford a purple residue. Chromatography
(alumina, CHCl3) afforded a purple solid (174 mg, 89%): 1H
NMR δ 8.76 (d, J ) 4.5 Hz, 4H), 8.69 (d, J ) 4.5 Hz, 4H), 8.05
(d, J ) 8.1 Hz, 4H), 7.95 (d, J ) 8.1 Hz, 4H), 7.27 (s, 4H), 2.63
(s, 6H), 1.81 (s, 12H); LD-MS obsd 970.8; FAB-MS obsd
972.1030, calcd 972.1036 (C50H38I2MgN4); λabs 406, 428, 526,
564, 605 nm; λem (λex ) 550 nm) 609, 661 nm.
5,15-Bis(4-iod op h en yl)-10,20-d i-p-tolyl-23H-21-oxa p or -
p h yr in (20). To an ice-cooled solution of 1847 (0.59 g, 1.0 mmol)
in THF/methanol (40 mL, 3:1) was added NaBH4 (1.90 g total,
50.0 mmol) in small portions over 10 min. The reduction was
followed by TLC analysis [alumina, CH2Cl2/ethyl acetate (2:
1)] and was complete after stirring for 40 min at room
temperature. The reaction mixture was quenched with water
and extracted with CH2Cl2. The organic phase was dried (Na2-
SO4), and the solvent was removed affording a foamlike solid.
The freshly prepared furylpyrromethane-dicarbinol was then
condensed with 19 (0.35 g, 1.0 mmol) in CH3CN (180 mL)
containing TFA (920 µL, 30 mM) at room temperature. After
3.5 min the spectroscopic yield of porphyrin had stopped
increasing. DDQ (0.68 g, 3.0 mmol) was added, the reaction
mixture was stirred at room temperature for 1 h, and then
triethylamine (2.40 mL) was added with stirring for 10 min.
The reaction mixture was then combined with water and
extracted with CHCl3. The organic phase was dried (Na2SO4)
and concentrated. Chromatography [silica, THF/CH2Cl2 (1:9)]
removed a fast-moving black band (non-porphyrin species)
from the slowly eluting porphyrin. The column was eluted until
no red fluorescence was observed in the eluent using a UV
lamp (365 nm). Further chromatography (basic alumina,
Brockman activity I, CH2Cl2) afforded a light blue band (non-
porphyrin species), and then elution with CH2Cl2/ethyl acetate
(98:2) gave the desired porphyrin as a slow-moving green band.
Removal of the solvent and washing the residue with ethanol
afforded a black-purple solid (130 mg, 14%): 1H NMR δ 9.21
(d, J ) 4.2 Hz, 1H), 9.14 (d, J ) 4.2 Hz, 1H), 8.91 (d, J ) 5.1
Hz, 1H), 8.84 (d, J ) 4.2 Hz, 1H), 8.64 (d, J ) 4.5 Hz, 1H),
8.57 (s, 2H), 8.50 (d, J ) 4.2 Hz, 1H), 8.10-8.03 (m, 8H), 7.90
(dd, J 1 ) 8.1 Hz, J 2 ) 1.5 Hz, 4H), 7.55 (d, J ) 7.2 Hz, 4H),
2.70 (s, 3H), 2.69 (s, 3H); LD-MS obsd 896.5; FAB-MS obsd
896.0650, calcd 896.0635 (C46H31I2N3O); λabs, 423, 509, 541,
613, 673 nm; λem (λex ) 550 nm) 679, 750 nm.
1
affording a purple solid (0.75 g, 20%). The H NMR data were
identical to those reported for the product obtained from a
mixed aldehyde condensation.43
Zn (II)-5-(3-Iod op h en yl)-10,20-d im esit yl-15-{3-[2-(t r i-
m eth ylsilyl)eth yn yl]p h en yl}p or p h yr in (Zn -16a ). A solu-
tion of 16a (0.71 g, 0.77 mmol) in CHCl3 (150 mL) was treated
with a solution of Zn(OAc)2‚2H2O (0.85 g, 3.88 mmol) in
methanol (5 mL) at room temperature for 3 h. The standard
workup afforded a purple solid (0.72 g, 95%): 1H NMR δ 8.86-
8.83 (m, 4H), 8.80-8.77 (m, 4H), 8.60-8.59 (m, 1H), 8.35 (m,
1H), 8.22-8.16 (m, 2H), 8.11 (d, J ) 8.7 Hz, 1H), 7.88 (d, J )
8.1 Hz, 1H), 7.67 (t, J ) 8.1 Hz, 1H), 7.47 (t, J ) 8.1 Hz, 1H),
7.29 (s, 4H), 2.64 (s, 6H), 1.82 (m, 12H), 0.25 (s, 9H); LD-MS
obsd 985.9; FAB-MS obsd 982.1916, calcd 982.1906 (C55H47
-
IN4SiZn); λabs 424, 550, 590 nm; λem (λex ) 550 nm) 645, 595
nm.
5-(4-Iod op h en yl)-10,20-d im esit yl-15-{4-[2-(t r im et h yl-
silyl)eth yn yl]p h en yl}p or p h yr in (16b). Following the pro-
cedure for preparing 16a , a solution of diacyldipyrromethane
15b (2.30 g, 3.31 mmol) in THF/methanol (146 mL, 10:1) was
treated with NaBH4 (2.51 g, 66.4 mmol). Standard workup and
condensation with 9 (0.874 g, 3.31 mmol) in CH3CN (1300 mL)
at room temperature using TFA (3.05 mL, 39.7 mmol, 30 mM)
was followed by oxidation with DDQ (2.25 g, 9.93 mmol).
Treatment with triethylamine (5.60 mL, 39.7 mmol), filtration
through a pad of alumina (eluted with CH2Cl2) and chroma-
tography (silica, CH2Cl2) afforded a purple solid (0.665 g, 22%).
The 1H NMR data were identical to those reported for the
product obtained from a mixed aldehyde condensation.43
5-(4-Br om op h en yl)-15-(4-iod op h en yl)-10,20-d im esityl-
p or p h yr in (16c). Following the procedure described for the
preparation of 16a , a solution of diacyldipyrromethane 15c

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7417
5,15-Bis{4-[2-(4-pyr idyl)eth yn yl]ph en yl}-10,20-di-p-tolyl-
23H-21-oxa p or p h yr in (21). Following a standard method for
Pd-mediated coupling,48 samples of N3O-porphyrin 20 (90.0
mg, 0.10 mmol), 4-ethynylpyridine 6 (41.6 mg, 0.40 mmol), Pd2-
(dba)3 (55.1 mg, 0.060 mmol), and P(o-tol)3 (146.2 mg, 0.48
mmol) were added to a 50 mL Schlenk flask. The flask was
evacuated and purged with argon three times, taking care to
avoid the loss of 4-ethynylpyridine by vacuum sublimation.
Degassed toluene/triethylamine (24 mL, 5:1) was added, and
the mixture was stirred at 35 °C. The reaction was followed
by LD-MS and TLC analysis [alumina, CHCl3/methanol (98:
2)]. After 4 h, the starting porphyrin had been consumed.
Chromatography [basic alumina, Brockman activity I, packed
with CHCl3; eluted with CHCl3/ethyl acetate (98:2)] gave a
trace of the mono-coupled byproduct (confirmed by LD-MS
analysis), followed by the desired product as the second band.
The solvent was removed, and the residue was placed on top
of a cotton plug in a Pasteur pipet. After washing with
methanol, the purple material on top of the cotton plug was
dissolved in CHCl3, concentrated and dried, affording a purple
solid (30.2 mg, 36%): 1H NMR δ 9.24 (d, J ) 4.2 Hz, 1H), 9.16
(d, J ) 5.1 Hz, 1H), 8.93 (d, J ) 5.1 Hz, 1H), 8.86 (d, J ) 4.5
Hz, 1H), 8.69 (d, J ) 6.0 Hz, 4H), 8.66 (d, J ) 4.2 Hz, 1H),
8.60 (s, 2H), 8.52 (d, J ) 4.5 Hz, 1H), 8.21 (d, J ) 8.1 Hz, 4H),
8.06 (d, J ) 6.6 Hz, 4H), 7.96-7.93 (m, 4H), 7.57-7.52 (m,
8H), 2.71 (s, 3H), 2.70 (s, 3H); LD-MS obsd 847.6; FAB-MS
obsd 846.3248, calcd 846.3233 (C60H39N5O); λabs (log ꢀ), 425
(5.47), 510 (4.44), 542 (3.90), 616 (3.52), 675 (3.71) nm; λem (λex
) 510 nm) 680, 751 nm (Φf ) 0.060).
Zn F bZn -m /m -CCTMS. Following the refined Pd-coupling
procedure for the preparation of multiporphyrin arrays,48
samples of 3 (243 mg, 0.32 mmol), Zn -16a (640 mg, 0.65
mmol), Pd2(dba)3 (90 mg, 0.098 mmol), and P(o-tol)3 (241 mg,
0.78 mmol) were weighed into a 250 mL Schlenk flask which
was then pump-purged three times with argon. Toluene/
triethylamine (132 mL, 5:1) was added, and the flask was
stirred at 35 °C. Monitoring by analytical SEC and LD-MS
showed the reaction had leveled off after 7 h. The solvent was
removed, and the residue was chromatographed [silica, hex-
anes/CHCl3 (1:2)] affording unreacted porphyrin monomers
followed by a mixture of mono-coupled dimer, desired trimer
and high molecular weight materials (HMWM). The mixture
of porphyrins was concentrated to dryness, dissolved in 20 mL
of THF, and chromatographed in two equal portions (SEC,
THF). Gravity elution afforded four major components (in
order of elution): HMWM, desired trimer, mono-coupled
byproduct (dimer, LD-MS obsd m/z at 1604.5, calcd 1604.0 for
with TBAF on silica gel (1.20 g, 1.0-1.5 mmol F-/g resin), and
the mixture was stirred at room temperature. After 2 h, LD-
MS and TLC analysis [silica, CHCl3/hexanes (1:1)] showed the
deprotection was complete. The reaction mixture was washed
with water and dried (Na2SO4), and the solvent was removed.
Chromatography (silica, CHCl3/hexanes 1:1) afforded a purple
solid, which was washed (by sonication) with methanol,
filtered, and dried, affording a purple solid (312 mg, 90%): 1H
NMR δ 8.96 (d, J ) 4.5 Hz, 4H), 8.86-8.83 (m, 8H), 8.80-
8.78 (m, 8H), 8.68 (d, J ) 4.5 Hz, 4H), 8.56 (s, 2H), 8.38 (d, J
) 7.0 Hz, 2H), 8.28 (d, J ) 7.0 Hz, 2H), 8.23 (t, J ) 7.0 Hz,
2H), 8.19 (d, J ) 8.1 Hz, 4H), 8.08 (d, J ) 7.8 Hz, 2H), 7.93 (d,
J ) 8.1 Hz, 4H), 7.90 (d, J ) 7.8 Hz, 2H), 7.81 (m, 2H), 7.71
(m, 2H), 7.30 (s, 8H), 7.25 (s, 4H), 3.15 (s, 2H), 2.65 (s, 12H),
2.60 (s, 6H), 1.85 (s, 24H), 1.80 (s, 12H), -2.65 (s, br, 2H);
LD-MS obsd 2315.8; FAB-MS obsd 2314.85, calcd 2314.82
(C158H118N12Zn2); λabs 421, 515, 549, 592, 648 nm; λem (λex
550 nm) 652, 720 nm.
)
Zn 3-m /m -CCH. A solution of Zn F bZn -m /m -CCH (119 mg,
0.051 mmol) in CHCl3 (10 mL) was treated with a solution of
Zn(OAc)2‚2H2O (56.0 mg, 0.26 mmol) in methanol (0.5 mL)
overnight at room temperature. Standard workup and chro-
matography [silica, CHCl3/hexanes (2:1)] afforded a purple
solid (110 mg, 91%): 1H NMR δ 8.97 (d, J ) 5.1 Hz, 4H), 8.88-
8.83 (m, 12H), 8.80 (d, J ) 5.4 Hz, 4H), 8.76 (d, J ) 5.4 Hz,
4H), 8.57 (m, 2H), 8.39-8.37 (m, 2H), 8.27 (d, J ) 8.7 Hz, 4H),
8.20 (d, J ) 8.1 Hz, 4H), 8.08 (d, J ) 8.1 Hz, 2H), 7.95-7.89
(m, 6H), 7.81 (m, 2H), 7.71 (m, 2H), 7.30 (s, 12H), 3.14 (s, 2H),
2.64 (s, 12H), 2.60 (s, 6H), 1.85 (s, 24H), 1.79 (s, 12H); LD-MS
obsd 2376.8, calcd avg mass 2378.9 (C158H116N12Zn3); λabs 422,
550, 590 nm, λem (λex ) 550 nm) 603, 650 nm.
Zn 2F bZn 2-p/p-CCTMS. Following the procedure described
for the preparation of Zn F b Zn -m /m -CCTMS, samples of
Zn F bZn -m /m -CCH (280 mg, 0.12 mmol) and Zn -16b (238 mg,
0.24 mmol) were coupled using Pd2(dba)3 (34 mg, 0.037 mmol)
and P(o-tol)3 (91.2 mg, 0.30 mmol) in toluene/triethylamine (48
mL, 5:1) at 35 °C under argon. Analytical SEC and LD-MS
analysis showed a leveling off of the pentamer yield after 5 h.
Chromatography (silica, CHCl3) removed the Pd species and
afforded a mixture of porphyrins. Further chromatography in
two equal portions (SEC, THF) afforded four major components
(in order of elution): HMWM, the desired pentamer, a trace
amount of mono-coupled byproduct (tetramer LD-MS obsd at
3176.5, calcd for C213H164N16SiZn3 m/z ) 3171.9) and unreacted
monomeric porphyrins. The pentamer-containing fractions
were combined and chromatographed [silica, hexanes/CHCl3
(1:2)], affording a purple solid (201 mg, 41%): 1H NMR δ 8.98-
8.97 (m, 8H), 8.88-8.82 (m, 16H), 8.80-8.75 (m, 12H), 8.69-
8.66 (m, 4H), 8.58 (d, J ) 6.5 Hz, 4H), 8.30-8.28 (m, 4H),
8.23-8.15 (m, 12H), 8.09 (d, J ) 7.5 Hz, 4H), 7.95-7.93 (m,
8H), 7.86-7.80 (m, 8H), 7.32 (s, 8H), 7.27-7.24 (m, 12H), 2.66
(s, 12H), 2.62 (s, 12H), 2.61 (s, 6H), 1.89 (s, 24H), 1.82 (s, 24H),
1.80 (s, 12H), 0.37 (s, 18H), -2.66 (s, br, 2H); LD-MS obsd
4023.4; calcd avg mass 4028.4 (C268H210N20Si2Zn4); λabs 422 (br),
515, 550, 592, 648 nm; λem (λex ) 550 nm) 599, 653, 720 nm
Zn 2F bZn 2-p/p-CCH. Following the procedure described for
the synthesis of Zn F bZn -m /m -CCH, a mixture of Zn 2F bZn 2-
p/p-CCTMS (201 mg, 0.050 mmol) and TBAF on silica gel (400
mg, 1.0-1.5 mmol F-/g resin) in CHCl3/THF (33 mL, 10:1) was
stirred at room temperature for 2 h. The standard workup
followed by chromatography [silica, CHCl3/hexanes (1:1)] gave
a purple solid, which was washed (sonicated) with methanol,
filtered, and dried, affording a purple solid (166 mg, 85%): 1H
NMR δ 9.00-8.97 (m, 8H), 8.90-8.83 (m, 16H), 8.79-8.76 (m,
12H), 8.70-8.66 (m, 4H), 8.59-8.57 (m, 4H), 8.30-8.28 (m,
4H), 8.24-8.17 (m, 12H), 8.10 (d, J ) 7.5 Hz, 4H), 7.94 (d, J
) 7.2 Hz, 8H), 7.88-7.80 (m, 8H), 7.32 (s, 8H), 7.27-7.25 (m,
12H), 3.30 (s, 2H), 2.66 (s, 12H), 2.62 (s, 12H), 2.60 (s, 6H),
1.88 (s, 24H), 1.81 (s, 24H), 1.80 (s, 12H), -2.66 (s, br, 2H);
LD-MS obsd 3882.5; calcd avg mass 3884.0 (C262H194N20Zn4);
λabs 422 (br), 516, 551, 592, 649 nm; λem (λex ) 550 nm) 600,
653, 721 nm.
C
109H88N8SiZn) and unreacted monomeric porphyrins. The
trimer-containing fractions were combined and chromato-
graphed [silica, hexanes/CHCl3 (1:1)], affording the title
compound as a purple solid (373 mg, 47%). Similar purification
of the dimer fraction gave the byproduct dimer as a purple
solid (33 mg, 6%). Data for the title compound: 1H NMR δ
8.97 (d, J ) 4.5 Hz, 4H), 8.85-8.83 (m, 8H), 8.80-8.78 (m,
8H), 8.68 (d, J ) 4.5 Hz, 4H), 8.56 (d, J ) 5.5 Hz, 2H), 8.35 (d,
J ) 7.0 Hz, 2H), 8.27-8.26 (m, 2H), 8.20-8.17 (m, 2H), 8.08
(d, J ) 7.8 Hz, 2H), 7.93 (d, J ) 8.1 Hz, 4H), 7.88 (d, J ) 8.1
Hz, 2H), 7.80 (m, 2H), 7.68 (m, 2H), 7.30 (s, 12H), 2.64 (s, 12H),
2.60 (s, 6H), 1.85 (s, 24H), 1.80 (s, 12H), 0.25 (s, 18H), -2.65
(s, br, 2H); LD-MS obsd 2459.6; calcd avg mass 2459.9
(C164H134N12Si2Zn2); λabs 423, 514, 550, 589, 648 nm; λem (λex
)
550 nm) 653, 720 nm. Data for the dimer (byproduct): 1H NMR
δ 8.96 (d, J ) 4.7 Hz, 2H), 8.84-8.82 (m, 4H), 8.80-8.78 (m,
4H), 8.76 (d, J ) 4.5 Hz, 2H), 8.69 (d, J ) 4.7 Hz, 4H), 8.56 (d,
J ) 6.0 Hz, 1H), 8.36 (d, J ) 6.0 Hz, 1H), 8.28-8.25 (m, 1H),
8.20 (d, J ) 8.1 Hz, 2H), 8.18 (d, J ) 7.9 Hz, 2H), 8.08 (d, J )
7.9 Hz, 1H), 7.94 (d, J ) 7.9 Hz, 2H), 7.89-7.86 (m, 4H), 7.80
(m, 1H), 7.67 (m, 1H), 7.30 (s, 4H), 7.26 (s, 4H), 3.30 (s, 1H),
2.64 (s, 6H), 2.61 (s, 6H), 1.86 (s, 12H), 1.82 (s, 12H), 0.26 (s,
9H), -2.65 (s, br, 2H); LD-MS obsd 1602.6; FAB-MS obsd
1600.63, calcd 1600.62 (C109H88N8SiZn); λabs 421, 515, 549, 592,
648 nm; λem (λex ) 550 nm) 652, 720 nm.
Zn F bZn -m /m -CCH. A solution of Zn F bZn -m /m -CCTMS
(370 mg, 0.15 mmol) in CHCl3/THF (44 mL, 10:1) was treated
Zn 5-p/p-CCH. A solution of Zn 2F bZn 2-p/p-CCH (66.2 mg,
17.0 µmol) in CHCl3 (10 mL) was treated with a solution of

7418 J . Org. Chem., Vol. 66, No. 22, 2001
Yu and Lindsey
Zn(OAc)2‚2H2O (37.3 mg, 170 µmol) in methanol (0.5 mL), and
the mixture was stirred overnight at room temperature under
argon. The reaction mixture was diluted with CHCl3, washed
with 10% aqueous NaHCO3, H2O, dried (Na2SO4), filtered, and
concentrated to give a purple solid (63.7 mg, 95%): 1H NMR
δ 8.98 (d, J ) 5.4 Hz, 8H), 8.89-8.82 (m, 20H), 8.78-8.74 (m,
12H), 8.58 (d, J ) 5.1 Hz, 4H), 8.28-8.27 (m, 4H), 8.22-8.16
(m, 12H), 8.09 (d, J ) 7.8 Hz, 4H), 7.94 (d, J ) 7.5 Hz, 8H),
7.88-7.79 (m, 8H), 7.32 (s, 8H), 7.25 (s, 12H), 3.29 (s, 2H), 2.65
(s, 12H), 2.62 (s, 12H), 2.60 (s, 6H), 1.88 (s, 24H), 1.80 (s, 24H),
1.79 (s, 12H); LD-MS obsd 3946.1; calcd avg mass 3947.4
(C262H192N20Zn5); λabs 424, 432 (br), 551, 592 nm; λem (λex ) 550
nm) 599, 646 nm.
Cyclo-Zn 5F bU via th e 5 + 1 Rou te. Samples of Zn 5-p/p-
CCH (63.7 mg, 16.1 µmol), 4 (18.4 mg, 19.4 µmol), tripyridyl
template 1 (9.8 mg, 16.1 µmol), Pd2(dba)3 (4.5 mg, 4.8 µmol),
and P(o-tol)3 (11.8 mg, 38.6 µmol) were added to a 25 mL
Schlenk flask. The flask was evacuated and purged with argon
three times, and then deaerated toluene/triethylamine (9 mL,
5:1) was added. The flask was immersed in an oil bath at 35
°C. The reaction was followed by analytical SEC and found to
be complete after 2.5 h. Chromatography (silica, CHCl3)
removed the Pd species and the tripyridyl template, affording
a mixture of the monomeric porphyrin, the desired hexamer,
and HMWM as the first band. Further chromatography (SEC,
5 × 65 cm, THF) afforded the desired hexamer as the second
band contaminated with HMWM. Four repetitive chromatog-
raphy (SEC, THF) runs afforded fractions containing the pure
hexamer, which were combined and chromatographed [silica,
CHCl3/hexanes (2:1)]. The hexamer fraction was collected, and
the solvent was removed. The residue was placed on top of a
cotton plug in a Pasteur pipet and washed with methanol. The
purple material on top of the cotton plug was dissolved in
CHCl3, concentrated, and dried, affording a purple solid (9.8
mg, 13%): 1H NMR δ 8.97 (d, J ) 4.5 Hz, 8H), 8.88 (d, J ) 5.1
Hz, 4H), 8.84 (d, J ) 5.1 Hz, 20H), 8.76 (d, J ) 5.1 Hz, 4H),
8.73 (d, J ) 5.1 Hz, 12H), 8.53-8.50 (m, 4H), 8.32 (d, J ) 7.5
Hz, 6H), 8.18 (d, J ) 8.7 Hz, 12H), 8.08 (d, J ) 7.5 Hz, 6H),
7.91 (d, J ) 7.8 Hz, 12H), 7.82 (m, 8H), 7.31 (s, br, 12H), 7.22
(s, br, 12H), 2.65 (s, br, 12H), 2.58 (s, 24H), 1.88 (s, 12H), 1.86
(s, 12H), 1.76 (s, 48H), -2.61 (s, br, 2H); LD-MS obsd 4645.4,
calcd avg mass 4642.3 (C312H230N24Zn5); λabs (log ꢀ) 428 (6.45),
512 (4.71), 550 (5.23), 591 (4.47), 649 nm; λem (λex ) 550 nm)
599, 651, 719 nm (Φf ) 0.085).
triethylamine (9.0 mL, 5:1) at 80 °C under argon. Analytical
SEC and LD-MS analysis showed the reaction was complete
after 2 h. The reaction mixture was loaded directly on a short
SEC column (THF), and all the porphyrin-containing fractions
were collected. Chromatography (silica, CHCl3) gave a por-
phyrin mixture, which was purified by preparative SEC (THF),
removing most of the HMWM. Repetitive preparative SEC
(three columns) gave a purple solid. Final chromatography
[silica, CHCl3/hexanes (4:1)] and washing with methanol gave
a purple solid (9.5 mg, 13.6%). LD-MS, 1H NMR, UV-vis, and
fluorescence spectral data were identical with those obtained
by the 5 + 1 route.
Zn F bZn -p/p-CCTMS. Following the procedure described
for the preparation of Zn F bZn -m /m -CCTMS, samples of
porphyrin 12 (187 mg, 0.25 mmol) and Zn -16b (512 mg, 0.52
mmol) were coupled using Pd2(dba)3 (75 mg, 0.082 mmol) and
P(o-tol)3 (201 mg, 0.66 mmol) in toluene/triethylamine (110 mL,
5:1) at 35 °C under argon for 5 h. The standard workup and
chromatography [silica, CHCl3/hexanes (2:1)] afforded a mix-
ture of porphyrins, which was purified by SEC in two equal
portions (THF), affording four major bands (in order of
elution): HMWM, desired trimer, mono-coupled byproduct
(dimer), and unreacted monomeric porphyrins. The trimer-
containing fractions were combined and chromatographed
[silica, hexanes/CHCl3 (1:2)], affording the desired trimer as
a purple solid (220 mg, 36%). The same purification gave
byproduct (dimer) as a purple solid (15 mg, 3.7%). Data for
the title compound: 1H NMR δ 8.92-8.87 (m, 8H), 8.85-8.82
(m, 4H), 8.79-8.78 (m, 12H), 8.58-8.56 (d, J ) 6.5 Hz, 2H),
8.29-8.25 (m, 2H), 8.24 (d, J ) 6.6 Hz, 4H), 8.17 (d, J ) 6.6
Hz, 4H), 8.10 (d, J ) 7.5 Hz, 2H), 7.96 (dd, J 1 ) 6.6 Hz, J 2
)
2.6 Hz, 4H), 7.86-7.83 (m, 6H), 7.33 (s, 4H), 7.27 (s, 8H), 2.66
(s, 6H), 2.62 (s, 6H), 2.61 (s, 6H), 1.90 (s, 12H), 1.81 (s, 12H),
1.80 (s, 12H), 0.37 (s, 18H), -2.57 (s, br, 2H); LD-MS obsd
2452.8, calcd avg mass 2459.9 (C164H134N12Si2Zn2); λabs 428,
515, 551, 593, 648 nm; λem (λex ) 550 nm) 600, 649, 718 nm.
Data for byproduct (dimer): 1H NMR δ 8.87 (d, J ) 4.5 Hz,
4H), 8.82 (d, J ) 4.5 Hz, 2H), 8.77-8.73 (m, 8H), 8.65 (m, 4H),
8.54 (m, 1H), 8.22 (d, J ) 8.1 Hz, 4H), 8.17 (d, J ) 8.1 Hz,
2H), 8.09 (d, J ) 7.8 Hz, 1H), 7.95 (d, J ) 8.1 Hz, 2H), 7.84 (d,
J ) 8.1, 2H), 7.80 (m, 1H), 7.30 (s, 4H), 7.27 (m, 6H), 2.64 (s,
6H), 2.62 (s, 6H), 1.87 (s, 12H), 1.81 (s, 12H), 0.37 (s, 9H),
-2.54 (s, 2H); LD-MS obsd 1613.7; FAB-MS obsd 1618.67 calcd
1618.63 (C109H88N8SiZn); λabs 427, 515, 553, 595, 648 nm; λem
(λex ) 550 nm) 604, 649, 719 nm.
Zn F bZn -p/p-Br . Following the procedure for the first step
in the successive iodo/bromo coupling approach,51 samples of
Zn -16c (493 mg, 0.51 mmol) and 12 (174 mg, 0.23 mmol) were
coupled using Pd2(dba)3 (63 mg, 0.069 mmol) and P(o-tol)3 (168
mg, 0.55 mmol) in toluene/triethylamine (96 mL, 5:1) at room
temperature under argon. Analytical SEC and LD-MS analysis
showed the reaction was complete after 3 h. Standard workup
and chromatography (silica, CHCl3) afforded a mixture of
porphyrins. Further purification by chromatography (prepara-
tive SEC, THF) afforded four major bands (in order of
elution): HMWM, desired trimer, mono-coupled byproduct
(dimer) and monomeric species. The trimer-containing fraction
was chromatographed [silica, CHCl3/hexanes (3:1)], affording
a purple solid (195 mg, 35%): 1H NMR δ 8.93 (d, J ) 4.5 Hz,
4H), 8.91 (d, J ) 2.7 Hz, 2H), 8.89 (d, J ) 2.7 Hz, 2H), 8.86 (d,
J ) 2.7 Hz, 2H), 8.85 (d, J ) 2.7 Hz, 2H), 8.81-8.78 (m, 2H),
8.60-8.58 (m, 2H), 8.34-8.28 (m, 2H), 8.26 (d, J ) 2.4 Hz,
2H), 8.23 (d, J ) 2.4 Hz, 2H), 8.13-8.09 (m, 6H), 7.97 (dd, J 1
) 1.5 Hz, J 2 ) 8.1 Hz, 4H), 7.88 (dd, J 1 ) 2.4 Hz, J 2 ) 7.8 Hz,
4H), 7.83 (d, J ) 8.1 Hz, 2H), 7.34 (s, 4H), 7.29 (s, 4H), 7.28
(s, 4H), 2.68 (s, 6H), 2.64 (s, 6H), 2.63 (s, 6H), 1.91 (s, 12H),
1.83 (s, 12H), 1.82 (s, 12H), -2.54 (s, br, 2H); LD-MS obsd
2420.5, calcd avg mass 2425.2 (C154H116Br2N12Zn2); λabs 423,
515, 550, 591, 649 nm; λem (λex ) 550 nm) 601 (w), 649, 718
nm.
Zn F bZn -p/p-CCH. A solution of Zn F bZn -p/p-CCTMS (220
mg, 0.089 mmol) in THF/CHCl3 (12 mL, 1:1) was treated with
TBAF (270 µL, 0.27 mmol, 1.0 M in THF) for 30 min at room
temperature. The solvent was removed, and the resulting
residue was dissolved in CHCl3 (20 mL), washed with 10%
aqueous NaHCO3, and dried (Na2SO4). Chromatography [silica,
CHCl3/hexanes (2:1)] afforded a purple solid (163 mg, 80%):
1H NMR δ 8.92-8.87 (m, 8H), 8.86-8.84 (m, 4H), 8.79-8.76
(m, 12H), 8.57 (d, J ) 6.6 Hz, 2H), 8.29-8.17 (m, 10H), 8.10
(d, J ) 8.1 Hz, 2H), 7.96 (dd, J 1 ) 6.6 Hz, J 2 ) 2.6 Hz, 4H),
7.86 (dd, J 1 ) 8.1 Hz, J 2 ) 2.6 Hz, 4H), 7.81 (d, J ) 7.8 Hz,
2H), 7.32 (s, 4H), 7.27 (s, 8H), 3.30 (s, 2H), 2.66 (s, 6H), 2.62
(s, 6H), 2.61 (s, 6H), 1.90 (s, 12H), 1.81 (s, 12H), 1.80 (s, 12H),
-2.57 (s, br, 2H); LD-MS obsd 2317.0; FAB-MS obsd 2314.85,
calcd 2314.82 (C158H118N12Zn2); λabs 428, 515, 556, 597, 648 nm;
λem (λex ) 550 nm) 604, 649, 717 nm.
Zn 2F bZn 2-m /m -CCTMS. Following the procedure described
for the preparation of Zn F bZn -p/p-CCTMS, samples of Zn -
F bZn -p/p-CCH (160 mg, 0.069 mmol) and Zn -16a (143 mg,
0.14 mmol) were coupled using Pd2(dba)3 (21 mg, 0.022 mmol)
and P(o-tol)3 (56 mg, 0. 18 mmol) in toluene/triethylamine (28
mL, 5:1) for 3 h at 35 °C under argon. The standard workup
and chromatography [silica, CHCl3/hexanes (2:1)] afforded a
mixture of porphyrins, which was purified by SEC in two equal
portions (THF), affording four major bands (in order of
elution): HMWM, desired pentamer, mono-coupled byproduct
(tetramer), and unreacted monomeric porphyrins. The pen-
tamer-containing fractions were combined and chromato-
graphed [silica, hexanes/CHCl3 (1:2)], affording the desired
pentamer as a purple solid (142 mg, 51%): 1H NMR δ 8.97 (d,
Cyclo-Zn 5F bU via th e 3 + 3 Rou te. Following the proce-
dure for the second step in the successive iodo/bromo coupling
approach,51 samples of Zn 3-m /m -CCH (35.7 mg, 0.015 mmol)
and Zn F bZn -p/p-Br (40.0 mg, 0.016 mmol) were coupled using
Pd2(dba)3 (4.6 mg, 0.0050 mmol), P(o-tol)3 (10.9 mg, 0.036
mmol), and template 1 (9.2 mg, 0.015 mmol) in toluene/

Syntheses of Cyclic Hexameric Porphyrin Arrays
J . Org. Chem., Vol. 66, No. 22, 2001 7419
J ) 5.1 Hz, 4H), 8.91-8.83 (m, 20H), 8.80-8.75 (m, 16H), 8.56
(d, J ) 6.5 Hz, 4H), 8.34 (d, J ) 7.1 Hz, 2H), 8.29-8.17 (m,
14H), 8.09 (d, J ) 7.2 Hz, 4H), 7.96-7.92 (m, 8H), 7.88 (d, J
) 8.1 Hz, 2H), 7.84-7.78 (m, 4H), 7.67 (m, 2H), 7.32 (s, 4H),
7.30 (s, 6H), 7.25 (m, 10H), 2.64 (s, 18H), 2.61 (s, 6H), 2.60 (s,
6H), 1.89 (s, 12H), 1.85 (s, 24H), 1.80 (s, 12H), 0.26 (s, 18H),
-2.58 (s, br, 2H); LD-MS obsd 4015.5, calcd avg mass 4028.4
(C268H210N20Si2Zn4); λabs 425, 432, 514, 556, 596, 649 nm; λem
(λex ) 550 nm) 603, 649, 718 nm.
Zn 2F bZn 2-m /m -CCH. Following the procedure described for
the preparation of Zn F bZn -p/p-CCH, a solution of Zn 2F bZn 2-
m /m -CCTMS (140 mg, 0.034 mmol) in THF/CHCl3 (10 mL,
1:1) was treated with TBAF (87 µL, 0.087 mmol, 1.0 M/THF)
for 30 min at room temperature. The solvent was removed,
and the resulting residue was dissolved in CHCl3 (20 mL); the
latter was then washed with 10% aqueous NaHCO3 and dried
(Na2SO4). Chromatography [silica, CHCl3/hexanes (2:1)] af-
forded a purple solid (112 mg, 85%): 1H NMR δ 8.96 (d, J )
5.1 Hz, 4H), 8.90 (d, J ) 5.1 Hz, 4H), 8.88-8.82 (m, 16H),
8.80-8.74 (m, 16H), 8.56 (m, 4H), 8.38 (d, J ) 7.5 Hz, 2H),
8.29-8.20 (m, 14H), 8.11-8.08 (m, 4H), 7.95-7.89 (m, 10H),
7.84-7.79 (m, 4H), 7.70 (m, 2H), 7.32 (s, 4H), 7.30 (s, 4H), 7.25
(m, 12H), 3.15 (s, 2H), 2.64 (s, 18H), 2.61 (s, 6H), 2.60 (s, 6H),
1.89 (s, 12H), 1.85 (s, 24H), 1.80 (s, 12H), 1.79 (s, 12H), -2.58
(s, br, 2H); LD-MS obsd 3883.8, calcd avg mass 3884.0
(C262H194N20Zn4); λabs 425, 432, 516, 556, 597, 650 nm; λem (λex
) 550 nm) 605, 649, 717 nm.
Cyclo-Zn 2F bZn 2F bU. Following the general procedure
described for the preparation of cyclo-Zn 5F bU via the 5 + 1
route, samples of Zn 2F bZn 2-m /m -CCH (67.6 mg, 0.017 mmol),
Mg-17 (18.2 mg, 0.019 mmol) and template 1 (10.4 mg, 0.017
mmol) were coupled using Pd2(dba)3 (4.8 mg, 5.2 µmol) and
P(o-tol)3 (12.4 mg, 0.041 mmol) in toluene/triethylamine (9.6
mL, 5:1) at 35 °C under argon for 1.5 h. LD-MS analysis of
this crude reaction mixture showed an intense peak corre-
sponding to the desired hexamer cyclo-Zn 2F bZn 2MgU at m/z
) 4602.9 (calcd avg mass 4601.2 for C312H230MgN24Zn4). The
solvent was removed, the residue was dissolved in CHCl3 (80
mL), and silica gel (Fisher, 200-425 mesh, 20 g) was added
to demetalate the Mg porphyrin. The mixture was stirred at
room temperature for 2 days. The mixture was then filtered
through a pad of silica gel, and the porphyrin species were
collected and concentrated. Chromatography (silica, CHCl3)
afforded a mixture of porphyrins (monomeric porphyrin, the
desired hexamer, and HMWM). Chromatography (SEC, THF)
afforded the desired demagnesiated hexamer as the second
band contaminated with HMWM. Repetitive chromatography
(SEC, THF) afforded fractions containing the pure hexamer,
which were combined and chromatographed [silica, CHCl3/
hexanes (4:1)], affording a purple solid, which was precipitated
from CHCl3/methanol (7.8 mg, 10%): 1H NMR δ 8.97 (d, J )
4.5 Hz, 4H), 8.91-8.83 (m, 20H), 8.81-8.75 (m, 20H), 8.70 (d,
J ) 7.8 Hz, 4H), 8.56 (m, 4H), 8.39-8.37 (m, 2H), 8.29-8.20
(m, 16H), 8.07 (d, J ) 8.1 Hz, 6H), 7.96-7.89 (m, 12H), 7.81
(m, 4H), 7.71 (m, 4H), 7.31, 7.30, 7.28 (three s, 24H), 2.63, 2.61,
2.60, 2.59 (four s, 36H), 1.89, 1.85, 1.82, 1.79 (four s, 72H),
-2.58 (s br, 2H), -2.69 (s br, 2H); LD-MS obsd 4572.0, calcd
avg mass 4578.9 (C312H232N24Zn4); λabs (log ꢀ) 428 (6.37), 515
(5.02), 550 (5.29), 591 (4.97), 647 (4.01); λem (λex ) 550 nm)
600, 650, 718 nm (Φf ) 0.098).
Ack n ow led gm en t. This research was supported by
a grant from the NSF (CHE9707995). Mass spectra
were obtained at the Mass Spectrometry Laboratory for
Biotechnology at North Carolina State University.
Partial funding for the Facility was obtained from the
North Carolina Biotechnology Center and the NSF.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
all new compounds; 13C NMR spectra for 5-8, 13a , 14a , 14c,
15a , 15c; LD-MS spectra for all porphyrins and porphyrin
arrays; absorption and fluorescence spectra of cyclic hexamers;
SEC traces of cyclic hexamers; a table comparing synthesis
and purification methods in one-flask and stepwise syntheses;
and structural diagrams for each scheme in which compounds
are displayed as icons. This material is available free of charge
via the Internet at http://pubs.acs.org.
J O010742Q