Triptycene as a rigid, 120° orienting, three-pronged, covalent scaffold for porphyrin arrays

Article abstract of DOI:10.1016/j.tetlet.2008.07.008

The synthesis of novel porphyrin trimers covalently linked by one central, rigid triptycene unit is described. Reaction of 2,6,14-triiodotriptycene, generated in a three-step synthesis from triptycene, with borylated porphyrins under Suzuki cross-coupling conditions afforded porphyrin trimers. In addition, Sonogashira cross-coupling conditions could be successfully applied for the synthesis of trimeric porphyrin arrays as well.

Full text of DOI:10.1016/j.tetlet.2008.07.008

Tetrahedron Letters 49 (2008) 5397–5399
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Triptycene as a rigid, 120° orienting, three-pronged,
covalent scaffold for porphyrin arrays
*
Katja Dahms, Mathias O. Senge
School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of novel porphyrin trimers covalently linked by one central, rigid triptycene unit is
described. Reaction of 2,6,14-triiodotriptycene, generated in a three-step synthesis from triptycene, with
borylated porphyrins under Suzuki cross-coupling conditions afforded porphyrin trimers. In addition,
Sonogashira cross-coupling conditions could be successfully applied for the synthesis of trimeric porphy-
rin arrays as well.
Received 28 May 2008
Revised 21 June 2008
Accepted 1 July 2008
Available online 4 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Porphyrin assemblies with highly conjugated
p
systems have
covalently linked to one central triptycene residue via Suzuki or
Sonogashira cross-coupling reactions.9b
attracted considerable attention as promising organic materials
for electronic devices,1,2 and there has been increasing interest in
the design and construction of porphyrin arrays with well-defined
geometries for this purpose.3 Multi-porphyrin architectures are
desirable systems as they provide rigid molecular machinery
frameworks via relatively straightforward synthetic pathways. In
nature, the antenna chlorophylls in photosynthetic bacteria are
arranged as macrorings to absorb and transfer solar energy
efficiently.4 This biological importance of circularly arranged
multi-porphyrin arrays makes them an attractive research target.
Whilst significant advances in this area have been made with
regard to synthetic methodology, the current focus is more on
rational building strategies and the use of appropriate scaffold
units that define the orientation of the chromophores in space,
and can be used as components for molecular machineries. In this
context, the triptycene unit is a convenient molecular scaffold to
construct rotationally oriented light harvesting devices.
First, commercially available triptycene 1 was reacted with
nitric acid under reflux conditions to give 2,6,14-trinitrotriptycene
2 in 81% yield according to a procedure by Zhang and Chen10 The
nitro residues were subsequently reduced to amino groups to
afford 2,6,14-triaminotriptycene 3. Finally, Sandmeyer reaction of
3 resulted in the formation of 2,6,14-triiodotriptycene 4 in an over-
all yield of 61% (Scheme 1).
In the past, Suzuki cross-coupling reactions with borylated
porphyrins as synthons had been successfully carried out.11–14
The requisite borylated porphyrins 8–10 were prepared from
bromoporphyrins 5 to 715 via Pd-catalysed reaction with 4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (pinacolborane). Debromination
of the starting material was a competitive reaction, thus a ten-fold
excess of pinacolborane was used. The borylated porphyrins 8–10
were obtained in 51%, 29% and 48% yields, respectively.16 In the
The first porphyrins with triptycene substituents were reported
by Wasielewski and Niemczyk in 1984.5 A quinone substituent was
incorporated into the triptycene group, such that the functional-
ised molecules possessed a porphyrin electron donor and a
quinone electron acceptor. The resulting porphyrins were used
for the study of photoinduced electron transfer reactions, as they
represented model systems with well-defined donor–acceptor dis-
tances and orientations.6–8 Osuka et al. later described the first
porphyrin oligomer in which three porphyrin units were attached
to one benzene ring.9a The porphyrin trimers were obtained after
an 11-step synthesis in overall yields of 13%.
O2N
HNO3
O2N
1
2 81%
NO2
Pd/C,
NaBH4
I
H2N
HCl,
NaNO2,
KI
Herein, we describe a straightforward synthetic pathway for the
preparation of porphyrin trimers in which each porphyrin unit is
I
H2N
4 61%
3
I
NH2
* Corresponding author. Tel.: +353 1 896 8537; fax: +353 1 896 8536.
E-mail address: sengem@tcd.ie (M. O. Senge).
Scheme 1. Three-step synthesis of 2,6,14-triiodotriptycene 4.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.008

5398
K. Dahms, M. O. Senge / Tetrahedron Letters 49 (2008) 5397–5399
Br
M
N
N
N
N
R1
R1
R2
R2
N
5 M = Ni(II), R1 = R2 = Phenyl
6 M = Zn(II), R1 = R2 = Phenyl
7 M = 2H, R1 = Tolyl, R2 = Butyl
N
R1
M
4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
PdCl2(PPh3)2
R1
N
N
O
O
I
B
R1
R1
N
N
Pd(PPh3)4
K3PO4
N
N
N
I
R1
M
R1
M
N
+
R2
N
M
N
N
N
N
4
I
R1
R2
N
R1
R2
8 M = Ni(II), R1 = R2 = Phenyl 51%
9 M = Zn(II), R1 = R2 = Phenyl 29%
11 M = Ni(II), R1 = R2 = Phenyl 22%
12 M = Zn(II), R1 = R2 = Phenyl 18%
10 M = 2H, R1 = Tolyl, R2 = Butyl 48%
13 M = 2H, R1 = Tolyl, R2 = Butyl 16%
Scheme 2. Synthesis of triptycenyl-linked porphyrins via Suzuki cross-coupling reaction with 2,6,14-triiodotriptycene.
next step, compounds 8–10 were reacted with 2,6,14-triiodotripty-
cene 4 under Suzuki cross-coupling conditions. Following work-up,
the desired porphyrin trimers 11–13 were isolated in 22%, 18% and
16% yields, respectively17 (Scheme 2). The yield of the free base tri-
mer is comparable to that of the metallated trimers, which implies
that this reaction is generally applicable to different porphyrin
systems.
As a second pathway for the synthesis of porphyrin trimers
covalently linked by one triptycene unit, Sonogashira cross-
coupling conditions were employed. Sonogashira reactions with
porphyrins have been applied widely, and proved successful here
as well.18–21 The ethynylporphyrins 14 and 16 were used as
porphyrin units15 and reacted with 2,6,14-triiodotriptycene 4.
The reactions were carried out with Pd2dba3 as catalyst in dry
THF at reflux under an argon atmosphere (Schemes 3 and 4). The
desired porphyrin trimers 15 and 17 were both isolated in 22%
yield.22
I
I
N
N
N
N
I
+
Ni
N
N
N
N
I
+
Ni
4
I
4
I
Pd2dba3
THF
14
R
N
Pd dba
2
3
R
THF
N
Ni
N
16
R
N
N
R
N
Ni
N
N
R
R
N
N
N
N
R
Ni
R
N
N
R
N
R
N
N
Ni
Ni
15 R = Tolyl
N
N
Ni
N
17 R = Tolyl
R
N
N
N
22%
22%
N
R
R
R
R
Scheme 3. Synthesis of porphyrin trimer 15 via Sonogashira cross-coupling.
Scheme 4. Synthesis of porphyrin trimer 17 via Sonogashira cross-coupling.

K. Dahms, M. O. Senge / Tetrahedron Letters 49 (2008) 5397–5399
150.09, 154.10 ppm; UV–vis (CH2Cl2): kmax (lg
5399
e
) = 417 nm (5.13), 546 (4.27),
The results indicate that it is not important for the outcome of
the reaction how close the ethynyl group is to the porphyrin moi-
ety. It also shows the potential utility of this approach to prepare
even larger porphyrin arrays using this method by attaching oli-
goporphyrins to the triptycene unit.
581 (3.57); HRMS (ES+) [C44H35BN4O2Zn+H] calcd 727.2223, found 727.2206.
For compound 10: Yield: 207.3 mg (0.308 mmol, 48%); mp 260 °C; Rf = 0.37
(CH2Cl2/n-hexane, 1:1, v/v); 1H NMR (400 MHz, CDCl3,20 °C): d = 1.14 (t, 3 H,
3J = 7.40 Hz, CH2–CH3), 1.84 (s, 14H, CH3 + CH2–CH3), 2.55 (m, 2H, CH2–CH2–
CH2), 2.76 (s, 6H, C6H4–CH3), 5.04 (t, 2H, 3J = 7.91 Hz, CH2–CH2–CH2–CH3), 7.59
(d, 4H, 3J = 7.78 Hz, ArH), 8.10 (d, 4H, 3J = 7.78 Hz, ArH), 8.92 (d, 2H, 3J = 4.77 Hz,
Hb), 8.95 (d, 2H, 3J = 4.77 Hz, Hb), 9.49 (d, 2H, 3J = 4.77 Hz, Hb), 9.82 (d, 2H,
3J = 4.77 Hz, Hb) ppm; 13C NMR (100.6 MHz, CDCl3, 20 °C): d = 14.24, 21.58,
23.71, 25.33, 35.28, 40.94, 85.06, 119.58, 122.50, 127.29, 134.45, 137.24,
In summary, 2,6,14-triiodotriptycene 4 was generated in a
three-step synthesis from commercially available triptycene.
Treatment of the borylated porphyrins 8–10 with 4 under Suzuki
cross-coupling conditions afforded the desired porphyrin trimers
11–13 in unoptimised yields of 16–22%. Likewise, the reaction of
the ethynylporphyrins 14 and 16 under Sonogashira conditions
gave the target compounds 15 and 17 in unoptimised yields of
22%. These synthetic pathways therefore provide a straightforward
approach for the design of rigid porphyrin trimers, and have
opened up access to a new class of porphyrin arrays. We are cur-
rently optimising the reactions, extending them to the coupling
of oligoporphyrins and the use of such units in dendritic porphyrin
arrays.
139.67 ppm; UV–vis (CH2Cl2): kmax (lg
(3.79), 589 (3.77), 644 (3.53); HRMS (ES+) [C44H45BN4O2+H]: calcd 673.3714,
found 673.3685.; potential rational for the initially low yields is the
e) = 417 nm (5.35), 517 (4.22), 549
A
reduction of the bromoporphyrin as noted by Balaban et al.: (b) Balaban, T. S.;
Bhise, A. D.; Fischer, M.; Schaetzel-Linke, M.; Roussel, C.; Vanthuyne, N. Angew.
Chem., Int. Ed. 2003, 42, 2140–2144; (c) Balaban, T. S.; Goddard, R.; Linke-
Schaetzel, M.; Lehn, J. M. J. Am. Chem. Soc. 2003, 125, 4233–4239.
17. (a) All new compounds reported herein showed spectral data consistent with
the assigned structures. Selected data: For compound 11: Yield: 20.6 mg
(0.01 mmol, 22%); mp >310 °C; Rf = 0.47 (CH2Cl2/n-hexane, 1:1, v/v); 1H NMR
(400 MHz, CDCl3, 20 °C): d = 5.96 (s, 1H, CH), 6.10 (s, 1H, CH), 7.71 (m, 30H,
ArH), 8.05 (m, 21H, ArH), 8.18 (s, 1H, ArH), 8.25 (s, 1H, ArH), 8.33 (s, 1H, ArH),
8.78 (m, 16H, Hb), 8.92 (m, 8H, Hb) ppm; 13C NMR (150.9 MHz, CDCl3, 20 °C):
d = 13.99, 22.56, 29.24, 29.58, 31.80, 53.97, 54.01, 118.83, 118.84, 118.87,
118.91, 118.96, 122.43, 126.69, 126.71, 127.56, 129.43, 131.16, 132.02, 132.06,
132.29, 132.36. 132.42, 133.57, 138.02, 140.80 ppm; UV–vis (CH2Cl2): kmax (lg
Acknowledgement
e) = 418 nm (5.54), 528 (5.06); HRMS (MALDI LD+) [C134H80N12Ni3]: calcd
2030.4689, found 2030.4669. For compound 12: Yield: 18.3 mg (0.009 mmol,
18%); mp >310 °C; Rf = 0.09 (CH2Cl2/n-hexane, 2:1, v/v); 1H NMR (400 MHz,
CDCl3, 20 °C): d = 6.16 (s, 1H, CH), 6.31 (s, 1H, CH), 7.81 (m, 30H, ArH), 8.30 (m,
21H, ArH), 8.51 (s, 1H, ArH), 8.59 (s, 1H, ArH), 8.69 (s, 1H, ArH), 9.03 (m, 18H, Hb),
9.22 (m, 6H, Hb) ppm; 13C NMR (150.9 MHz, CDCl3, 20 °C): d = 14.01, 22.59,
29.26, 29.56, 29.60, 31.83, 36.26, 121.01, 121.07, 121.12, 122.27, 126.47,
127.40, 130.53, 131.89, 131.92, 132.00, 132.29, 134.35, 134.38, 140.04, 142.79,
This work was generously supported by a Science Foundation
Ireland Research Professorship Award (SFI 04/RP1/B482).
References and notes
144.09, 144.92 ppm; UV–vis (CH2Cl2): kmax (lg e) = 424 nm (5.49), 549 (4.92),
1. Winnischofer, H.; Toma, H. E.; Araki, K. J. Nanosci. Nanotechnol. 2006, 6, 1701–
1709.
587 (4.32). For compound 13: Yield: 8.9 mg (0.005 mmol, 16%); mp 237 °C;
Rf = 0.13 (CH2Cl2/n-hexane, 1:1, v/v); 1H NMR (600 MHz, CDCl3, 20 °C):
d = À2.64 (s, 2H, NH), À2.61 (s, 2H, NH), À2.57 (s, 2H, NH), 1.16 (t, 9H,
3J = 7.31 Hz, CH3), 1.85 (m, 6H, CH2–CH3), 2.57 (m, 6H, CH2–CH2–CH2), 2.77 (m,
18H, C6H5–CH3), 5.06 (m, 6H, CH2–CH2–CH2–CH3), 6.13 (s, 1H, CH), 6.26 (s, 1H,
CH), 7.61 (m, 12H, ArH), 8.14 (m, 18H, ArH), 8.48 (s, 1H, ArH), 8.53 (s, 1H, ArH),
2. Linke-Schaetzel, M.; Anson, C. E.; Powell, A. K.; Buth, G.; Palomares, E.; Durrant,
J. D.; Balaban, T. S.; Lehn, J.-M. Chem. Eur. J. 2006, 12, 1931–1940.
3. Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. Rev. 2001, 101.
4. McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornwaite-Lawless, A. M.; Papiz,
M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517–521.
5. Wasielewski, M. R.; Niemczyk, M. P. J. Am. Chem. Soc. 1984, 106, 5043–5045.
6. Wiehe, A.; Senge, M. O.; Kurreck, H. Liebigs Ann. Recl. 1997, 1951–1963.
7. Korth, O.; Wiehe, A.; Kurreck, H.; Röder, B. Chem. Phys. 1999, 246, 363–372.
8. Wiehe, A.; Senge, M. O.; Schäfer, A.; Speck, M.; Tannert, S.; Kurreck, H.; Röder, B.
Tetrahedron 2001, 57, 10089–10110.
9. (a) Osuka, A.; Liu, B.-L.; Maruyama, K. J. Org. Chem. 1993, 58, 3582–3585; (b)
Arnold, D. P.; Nitschinsk, L. J. Tetrahedron Lett. 1993, 34, 693–696.
10. Zhang, C.; Chen, C.-F. J. Org. Chem. 2006, 71, 6626–6629.
11. Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J. J. Am. Chem. Soc. 1998,
120, 12676–12677.
8.60 (s, 1H, ArH), 8.91 (m, 5H, Hb), 8.99 (m, 13H, Hb), 9.51 (m, 6H, Hb) ppm; 13
C
NMR (150.9 MHz, CDCl3, 20 °C): d = 13.97, 14.06, 21.41, 22.54, 23.51, 29.21,
29.56, 31.79, 35.11, 40.75, 54.17, 119.25, 119.46, 119.53, 120.29, 122.34,
127.19, 128.69, 130.47, 130.75, 132.04, 134.33, 137.05, 137.08, 137.15, 139.30,
139.43, 144.11, 144.87 ppm; UV–vis (CH2Cl2): kmax (lg e) = 422 nm (5.40), 446
(5.01), 518 (4.44), 554 (4.25), 593 (4.04), 649 (4.05); HRMS (ES+)
[C134H110N12+2H]: calcd 1888.9133, found 1888.9158.
18. Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63, 4034–4038.
19. Tong, L. H.; Pascu, S. I.; Jarrosson, T.; Sanders, J. K. M. Chem. Commun. 2006,
1085–1087.
20. Kuo, M.-C.; Li, L.-A.; Yen, W.-N.; Lo, S.-S.; Lee, C.-W.; Yeh, C.-Y. Dalton Trans.
2007, 1433–1439.
12. Aratani, N.; Osuka, A. Org. Lett. 2001, 3, 4213–4216.
13. Chng, L. L.; Chang, C. J.; Nocera, D. G. Org. Lett. 2003, 5, 2421–2424.
14. (a) Yamane, O.; Sugiura, K.; Miyasaka, H.; Nakamura, K.; Fujimoto, T.;
Nakamura, K.; Kaneda, T.; Sakata, Y.; Yamashita, M. Chem. Lett. 2004, 33, 40–
41; (b) Sergeeva, N. N.; Lopez Pablo, V.; Senge, M. O. J. Organomet. Chem. 2008,
693, 2637–2640.
15. (a) Horn, S. unpublished results, The University of Dublin, Trinity College, 2007.
Prepared via synthesis of 5,10,15-tritolylporphyrin,15b metallation,
bromination, insertion of a –CꢀC–TMS group and deprotection to the alkynyl
derivative; (b) Senge, M. O.; Feng, X. J. Chem. Soc., Perkin Trans. 1 2000, 3615–
3621.
16. (a) All new compounds reported herein showed spectral data consistent with
the assigned structures. Selected data: For compound 8: Yield: 201.5 mg
(0.28 mmol, 51%); mp 230 °C; Rf = 0.53 (CH2Cl2/n-hexane, 1:1, v/v); 1H NMR
(400 MHz, CDCl3, 20 °C): d = 1.72 (s, 12H, CH3), 7.71 (m, 9H, ArH), 8.02 (m, 6H,
ArH), 8.70 (d, 2H, 3J = 4.9 Hz, Hb), 8.74 (d, 2H, 3J = 4.9 Hz, Hb), 8.86 (d, 2H,
3J = 5.1 Hz, Hb), 9.79 (d, 2H, 3J = 5.1 Hz, Hb) ppm; 13C NMR (100.6 MHz, CDCl3,
20 °C): d = 24.73, 84.39, 118.25, 126.38, 127.23, 127.29, 131.12, 131.86, 132.61,
133.18, 133.26, 133.54, 140.33, 140.44, 141.25, 141.45, 142.48, 146.44 ppm;
21. Fazekas, M.; Pintea, M.; Senge, M. O.; Zawadzka, M. Tetrahedron Lett. 2008,
2236–2239.
22. All new compounds reported herein showed spectral data consistent with the
assigned structures. Selected data: For compound 15: Yield: 7.2 mg
(0.003 mmol, 22%); mp >310 °C; Rf = 0.45 (CH2Cl2/n-hexane, 1:1, v/v); 1H
NMR (400 MHz, CDCl3, 20 °C): d = 2.96 (s, 18H, CH3), 5.61 (s, 1H, CH), 5.62 (s,
1H, CH), 7.42 (d, 4H, 3J = 7.65 Hz, ArH), 7.53 (d, 15H, 3J = 7.56 Hz, ArH), 7.79 (m,
2H, ArH), 7.86 (d, 6H, 3J = 7.83 Hz, ArH), 7.95 (d, 12H, 3J = 7.56 Hz, ArH), 8.04 (d,
6H, 3J = 7.91 Hz, ArH), 8.80 (m, 6H, Hb), 8.85 (m, 6H, Hb), 8.95 (m, 6H, Hb), 9.16
(m, 6H, Hb), 9.86 (s, 3H, Hmeso) ppm; 13C NMR (150.9 MHz, CDCl3, 20 °C):
d = 21.06, 29.27, 88.35, 90.05, 104.21, 118.01, 118.38, 120.03, 122.33, 123.56,
126.63, 127.18, 128.92, 129.62, 131.27, 131.65, 131.84, 132.23, 133.26, 133.30,
137.01, 137.49, 140.75, 141.59, 142.35, 142.48, 142.54, 144.18, 144.28 ppm;
UV–vis (CH2Cl2): kmax (lg e) = 410 nm (5.14), 522 (4.17), 553 (3.68); HRMS
(MALDI LD+) [C146H92N12Ni3]: calcd 2186.5628, found 2186.5596. For
compound 17: Yield: 4.8 mg (0.002 mmol, 22%); mp >310 °C; Rf = 0.61
(CH2Cl2/n-hexane, 1:1, v/v); 1H NMR (400 MHz, CDCl3, 20 °C): d = 2.65 (s, 9H,
CH3), 2.67 (s, 18H, CH3), 5.82 (s, 1H, CH), 5.87 (s, 1H, CH), 7.51 (m, 20H, ArH),
7.73 (m, 4H, ArH), 7.89 (m, 18H, ArH), 8.14 (s, 1H, ArH), 8.16 (s, 1H, ArH), 8.19 (s,
1H, ArH), 8.70 (m, 12H, Hb), 8.84 (m, 6H, Hb), 9.59 (m, 6H, Hb) ppm; 13C NMR
(150.9 MHz, CDCl3, 20 °C): d = 21.05, 29.27, 89.29, 96.52, 98.41, 119.35, 119.95,
127.21, 130.91, 131.62, 131.84, 132.51, 133.15, 137.05, 137.22, 137.30, 141.95,
UV–vis (CH2Cl2): kmax (lg e) = 413 nm (5.27), 528 (4.21), 569 (3.78); HRMS
(ES+) [C44H35BN4NiO2+H]: calcd 721.2285, found 721.2272. For compound 9:
Yield: 108.7 mg (0.15 mmol, 29%); mp 148 °C; Rf = 0.17 (CH2Cl2/n-hexane, 1:1,
v/v); 1H NMR (400 MHz, CDCl3, 20 °C): d = 1.83 (s, 12H, CH3), 7.72 (m, 9H, ArH),
8.17 (m, 6H, ArH), 8.83 (d, 2H, 3J = 4.7 Hz, Hb) 8.85 (d, 2H, 3J = 4.9 Hz, Hb), 8.99
(d, 2H, 3J = 4.7 Hz, Hb), 9.83 (d, 2H, 3J = 4.7 Hz, Hb) ppm; 13C NMR (100.6 MHz,
CDCl3, 20 °C): d = 25.02 ppm, 84.91, 120.64, 122.30, 126.20, 127.13, 127.19,
131.24, 131.85, 132.56, 132.69, 134.02, 134.17, 142.44, 142.55, 149.02, 149.69,
142.25, 142.79, 144.24, 144.43 ppm; UV–vis (CH2Cl2): kmax (lg e) = 437 nm
(5.68), 545 (4.84), 584 (4.84); HRMS (MALDI LD+) [C149H98N12Ni3]:
calcd2228.6098, found 2228.6174.