Preferential formation of cyclic trimers by palladium-catalyzed oxidative coupling reactions of 2,18-diethynylporphyrins

Article abstract of DOI:10.1002/anie.201207763

And palladium makes three: In contrast to the formation of cyclic dimers in the Cu-mediated reaction, Pd-catalyzed oxidative coupling of 2,18-diethynylporphyrins preferentially produced cyclic trimers (see scheme). A porphyrin hexamer with a doubly 1,3-butadiyne-bridged conjugated trimeric core and directly meso-appended peripheral porphyrin substituents was also synthesized. Copyright

Full text of DOI:10.1002/anie.201207763

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Chemie
DOI: 10.1002/anie.201207763
Porphyrin Macrocycles
Preferential Formation of Cyclic Trimers by Palladium-Catalyzed
Oxidative Coupling Reactions of 2,18-Diethynylporphyrins**
Sumito Tokuji, Hideki Yorimitsu,* and Atsuhiro Osuka*
Inspired by the gigantic cyclic arrangements of bacteriochlor-
ophylls and chlorophylls in photosynthetic light-harvesting
antenna, considerable efforts have been devoted to the
synthesis of cyclic porphyrin oligomers (CPOs).[1] Among
these, directly 1,3-butadiyne-bridged conjugated CPOs have
a special place owing to their intriguing structural, electronic,
and optical properties. Usually, effective macrocyclizations
require synthetic tricks. Sugiura et al. used L-shaped 5,10-
diaryl-15,20-diethynylporphyrin as a key corner part for the
synthesis of square meso-to-meso butadiyne-bridged planar
CPOs.[2] Anderson and co-workers developed an elegant
template-directed synthesis of meso-to-meso butadiyne-
bridged circular CPOs using linear 5,15-diaryl-10,20-diethy-
nylporphyrin precursors,[3] in which the structure of the
template–substrate complex dictates the major CPO product.
This approach has evolved into a conceptually new Vernier-
template synthesis.[3d,f] We have also reported Cu-mediated
oxidative dimerization reaction of 5,10,15-triaryl-2,18-dieth-
ynylporphyrins 1M that provided b-to-b 1,3-butadiyne-
bridged cyclic porphyrin dimers 2M in good yields.[4]
Scheme 1. Proposed metal acetylide intermediates in Cu-mediated
(left) and Pd-catalyzed (right) reactions.
preferential formation of trimers 3M by Pd-catalyzed cou-
pling reactions of 1M.
A solution of 1Ni in toluene and diisopropylamine was
stirred at room temperature in the presence of catalytic
amounts of PdCl2(PPh3)2 and CuI and two equivalents of 1,4-
benzoquinone as the oxidant. Separation by preparative gel
permeation chromatography provided 2Ni[4] and 3Ni and in
9.3 and 72% yields, respectively (Scheme 2).[10] The MALDI-
TOF mass spectrum of 3Ni displayed a parent ion peak at
2933.39 (calcd for C198H210N12Ni3, m/z 2933.49 [M]+). The
It has been recognized that Pd-catalyzed oxidative
coupling of acetylene can produce different products from
the usual Cu-mediated reaction. As a representative example,
Haley and co-workers have reported metal-dependent selec-
tive synthesis of two different dehydrobenzoannulenes by the
intramolecular coupling reactions of single precursors, which
is owing to the difference of their reaction mechanisms.[5]
While still under debate, the mechanism of Cu-mediated
oxidative butadiyne synthesis is proposed to involve a dimeric
Cu acetylide arranged in a pseudo-trans configuration prior to
reductive elimination (Scheme 1).[6] On the other hand, in Pd-
catalyzed reactions, two ethynyl groups are proposed to be
arranged in a cis geometry for reductive elimination.[7] In
contrast to Haley and co-workers intramolecular reactions,
metal-catalyzed intermolecular oxidative butadiyne forma-
tion is more difficult to control.[8,9] Herein, we report
[*] S. Tokuji, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science,
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
[**] This work was supported by MEXT through Grants-in-Aid (Nos.
22245006 (A), 20108001 “pi-Space”, and 24106721 “Reaction
Integration”). S.T. acknowledges a JSPS Fellowship for Young
Scientists. We thank Prof. Dr. H. Maeda and Y. Bando of
Ritsumeikan University for MALDI-TOF MS measurement.
Supporting information for this article is (experimental details)
available on the WWW under http://dx.doi.org/10.1002/anie.
201207763.
Scheme 2. Chemical formulas of 1M, 2M, 3M, and 4Ni. Ar=3,5-di-
tert-butylphenyl. M=Ni, Zn, 2H.
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1H NMR spectrum of 3Ni in CDCl3 revealed a singlet signal
at 10.35 ppm owing to the meso proton, reflecting its highly
symmetric structure. Under similar concentrations, treatment
of 1Ni with Cu(OAc)2 in pyridine/tetrahydrofuran (THF)
provided 2Ni in 82% yield with a trace amount of 3Ni.[4]
Similar results were obtained for a ZnII porphyrin substrate
1Zn; the Pd-catalyzed reaction provided 2Zn in 24% yield
and 3Zn in 60% yield, while the Cu-mediated reaction
yielded 2Zn in 78% yield with a trace amount of 3Zn.
Demetalation of 3Zn with hydrochloric acid provided 3H in
54% yield.
The triangular structure of 3Ni has been confirmed by X-
ray crystallographic analysis (Figure 1).[11] The average devi-
ation from the mean planes composed of twenty-four atoms
of porphyrin cores is 0.21 ꢀ. The averaged bond angles of
2
À
ꢀ
ꢀ À
C(sp ) Ca Cb and Ca Cb Cb (q1 and q2) of six non-equivalent
ꢀ
C C units are 170.08 and 172.58, respectively, hence showing
a distinct deviation from the linear structure, and the average
À
À
Ca Ca and Cb Cb distances (da and db) are 5.50 and 6.31 ꢀ,
respectively. These data indicate that the structural distortion
is concentrated at the 1,3-butadiyne bridges in 3Ni. We have
also determined the X-ray crystal structures of 1Ni and
2Ni.[11] The distances da and db in 2Ni are 5.29 ꢀ and 5.37 ꢀ,
respectively, which are close to those in 1Ni (da = 5.21 ꢀ and
db = 5.34 ꢀ). The averaged bond angles (q1 and q2) are 173.78
and 176.88 in 2Ni, respectively, indicating there is less
structural distortion.
Interestingly, trimer 3Ni can be used to prepare the 2,5-
thienylene-bridged triporphyrin 4Ni. Simple heating of 3Ni
in the presence of Na2S·9H2O in p-xylene/2-methoxyethanol
gave 4Ni in 77% yield as the sole product (Scheme 2).[12]
Notably, the free base trimer 4H was prepared by Suzuki–
Miyaura coupling of b,b’-diborylporphyrin with 2,5-dibromo-
thiophene, in 10% yield along with many other porphyrin
macrocycles.[13,14]
Figure 2 shows the UV/Vis absorption and fluorescence
spectra of 1Zn–3Zn measured in CH2Cl2. 3Zn displays
a highly perturbed absorption spectrum, which is similar to
the case of 2Zn. The Soret band of 3Zn is split into two peaks
at 452 and 487 nm and Q-bands are observed at 534, 566, 580
and 603 nm with a shoulder around 620 nm. This shoulder
peak of 3Zn is red-shifted as compared with that of 1Zn
(around 600 nm) but blue-shifted from that of 2Zn (around
645 nm). The fluorescence spectrum of 3Zn shows peak
maxima at 630 and 689 nm, which are blue shifted by 27 and
43 nm, respectively, as compared with those of 2Zn (Fig-
ure 2b). Interestingly, the fluorescence quantum yield FF of
3Zn was determined by a sphere photon-counting apparatus
to be 0.056, larger than those of 1Zn (FF = 0.025) and 2Zn
(FF = 0.009).[4] The blue shifts in the absorption and fluores-
cent spectra of 3Zn may be ascribed to its less effective
p conjugation as compared with 2Zn, probably due to its
distorted 1,3-butadiyne structure.
Figure 1. X-ray crystal structures. a) Top view and b) side view of 3Ni,
c) top view and d) side view of 2Ni, e) top view and f) side view of
1Ni. Thermal ellipsoids are shown at the 30% probability level.
Solvent molecules and 3,5-di-tert-butylphenyl groups are omitted for
clarity. Light gray spheres=Ni atoms.
reactions owing to difficulty in forming the requisite cis
geometry, which would drive the intermolecular coupling
reaction of 2M_AC with another 1M, to give the intermedi-
ate 3M_AC. Then, the macrocyclic ring closure of 3M_AC
can proceed better, probably because 3M_AC allows for-
mation of the cis geometry that is key for the reductive
elimination.[16] To confirm the stepwise mechanism, 2Ni_AC
was prepared separately (see Supporting Information) and its
reactions were examined. While the Cu-mediated oxidative
coupling of 2Ni_AC provided 2Ni in 74%, the yield of 2Ni
These Cu-mediated and Pd-catalyzed reactions are
believed to proceed in a stepwise manner via an acyclic
intermediate 2M_AC or its related species (Scheme 3).[15]
Intramolecular cyclization of 2M_AC should be smooth in
the Cu-mediated reactions through an easily available
pseudo-trans geometry, but might be slow in the Pd-catalyzed
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Figure 2. a) UV/Vis absorption spectra and b) fluorescence spectra of
1Zn, 2Zn, and 3Zn in CH2Cl2. e=molar extinction coefficient.
Scheme 4. Chemical formulas of 5Zn, 6Zn, and 7Zn. Ar=3,5-di-tert-
butylphenyl.
harvesting antenna in terms of possible excitation energy
transfer from the peripheral porphyrins to the trimeric
porphyrin core. The absorption spectra of 6Zn and 7Zn are
roughly similar to those of 2Zn and 3Zn with additional
contributions of the excitonically coupled meso-linked
peripheral porphyrins (Figure 3a). The fluorescence spec-
Scheme 3. Plausible reaction mechanisms of the Cu-mediated and Pd-
catalyzed reactions. Ar=3,5-di-tert-butylphenyl. M=Ni, Zn.
dropped to 44% with various polymeric products in the Pd-
catalyzed reaction. The Pd-catalyzed reaction of 1Ni with an
equimolar amount of 2Ni_AC gave 2Ni and 3Ni in 20% and
45% yield, respectively.[17] These results are consistent with
the stepwise mechanism.
Finally, this strategy was applied to a meso–meso directly
linked diporphyrin 5Zn. The Cu-mediated reaction of 5Zn
produced predominantly 6Zn in 86% yield, while the Pd-
catalyzed reaction of 5Zn provided the porphyrin hexamer
7Zn preferentially in 54% yield along with 6Zn in 19% yield
(Scheme 4). These contrasting results indicate the generality
of this synthetic method. The hexamer 7Zn is attractive
because of its functional similarity to the natural light-
Figure 3. a) UV/Vis absorption spectra and b) fluorescence spectra of
5Zn, 6Zn, and 7Zn in CH2Cl2.
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which are distinctly blue-shifted as compared with those of
6Zn (674 and 747 nm), a similar trend as was observed for
2Zn and 3Zn. The fluorescence quantum yields are 0.038,
0.036, and 0.06 for 5Zn, 6Zn, and 7Zn, respectively, again
indicating that the trimeric coupling product is the most
fluorescent. Collectively, these data indicate that the elec-
tronic conjugation along the 1,3-butadiyne bridges is stronger
than the exciton coupling in meso–meso-linked diporphyrin
segments.
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In summary, the Pd-catalyzed oxidative coupling of 2,18-
diethynylporphyrins 1M led to the preferential formation of
1,3-butadiyne-bridged porphyrin trimers 3M, which is com-
plementary to Cu-mediated macrocyclization. The applica-
tion of this synthetic strategy to 5Zn provided hexaporphyrin
7Zn as a novel light-harvesting antenna model. This strategy
may be applied to other diethynylated compounds, which are
actively being studied in our laboratory.
Received: September 26, 2012
[10] Use of a catalyst bearing a cis-bidentate ligand PdCl2(dppf)led to
decreased yields of 3Ni (14%) and 2Ni (18%) along with
polymerized products (see Supporting Information).
Published online: November 9, 2012
Keywords: alkyne coupling · butadiyne bridge · palladium ·
.
[11] Crystallographic data for 3Ni: C198H210N12Ni3, Mw = 2933.91,
monoclinic, space group C2/c (No. 15), a = 65.51(3), b =
24.309(7), c = 31.96(2) ꢀ, b = 113.67(5)8, V= 46614(43) ꢀ3, Z =
porphyrinoids · triporphyrin synthesis
8,
D , T= 93(2) K, R1 = 0.1121 (I > 2.0s(I)),
calc = 0.836 gcmÀ3
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