10.1002/chem.201704344
Chemistry - A European Journal
COMMUNICATION
Figure 2.
A series of linear linkers: Cubane-linked porphyrin 10a and
[7]
a) Instability of cubane towards rhodium: L. Cassar, P. E. Eaton, J.
Halpern, J. Am. Chem. Soc. 1970, 92, 3515-3518; b) Instability towards
silver and palladium P. E. Eaton, L. Cassar, J. Halpern, J. Am. Chem.
Soc. 1970, 92, 6366-6368.
molecular structure of precursor 2f (thermal displacement 50%).
The synthesis was achieved with an overall yield of 20 %,
starting from building block 2f with deprotection of the alkynyl
followed by Pd-catalyzed coupling with bromoporphyrin 6e.[32]
Porphyrin 10a is the first example of a cubane-linked porphyrin
array with a further synthetic handle, which represents a
significant step towards using cubane-linked systems as
electron transfer compounds with rigid resistor units.[33] With a
dimension of 9.68 Å from one cubane-carbon (C3) to the end of
the acetylen-unit (C18) and with all carbons in line
(∡C3C11C18 = 175°) this represents an interesting new class of
linker system.
[8]
[9]
S. Plunkett, K. J. Flanagan, B. Twamley, M. O. Senge, Organometallics
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For further details see SI (S4). The attack into the outer π*-orbital of the
redox-active phthalimide moiety further away from the bulky cubane
core seems more promising than the radical generation from the
according cubane-iodines in the Kumada coupling.
[10] a) J. Cornella, J. T. Edwards, T. Qin, S. Kawamura, J. Wang, C. Pan, R.
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In conclusion,
a
modified cubane-aryl coupling via
[11] Full details in SI (S5-S7)
decarboxylative Baran coupling is reported. After systematic
investigations the key obstacle of cubane cross-coupling was
identified and circumvented by altering the electron density of
the transition metal through an adjusted choice of ligand.
Furthermore, for the first time not only the preparation of directly
linked cubane-porphyrin system was achieved but also the
possibility of Sonogashira functionalization of alkynyl arm-
extended cubanes, proving the general compatibility of cubanes
and Pd-catalyzed cross-coupling reactions. These cross-
coupling reactions allow the preparation of new classes of
cubane compounds, important not only for medicinal but also for
materials chemistry. Further investigations on the unique
behavior of cubane during (radical) cross-coupling conditions
and the exact mechanism of decomposition are the topic of
ongoing investigations.
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pyrrole but also nucleophilic additions of lithium cubanes to free meso-
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Acknowledgements
[20] Despite careful control of temperature and concentrations, we observed
a nucleophilic substitution at the meso-position, yielding in an isopropyl
porphyrin. (See SI compound 2p, S20). Similar to the well-developed
nucleophilic substitution of porphyrins with organolithium compounds:
W. W. Kalisch, M. O. Senge, Angew. Chem. Int. Ed. Engl. 1998, 37,
1107-1109; Angew. Chem. 1998, 110, 1156-1159.
This work was supported by grants from the Science Foundation
Ireland (SFI IvP 13/IA/1894) and the Irish Research Council
(GOIPG/2015/3700).
Keywords: Cubanes • Cross-Coupling • Porphyrins •
[21] Alkynylcubanes 3, 8a and 8b were prepared according to reference 24.
For further details see SI (S8).
Sonogashira Coupling • Nickel Catalysis
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[29] For further details see SI (S9)
[30] A determination of the yield from the crude reaction mixture via 1H-NMR
indicated yields comparable to the corresponding arylated-porphyrins
(entry 1-4). Hence, decomposition during the purification is responsible
for the low yields of the dialkyl-porphyrin systems. Only the reaction
with the bisalkynyl 8b achieved the product in good yields again. Due to
the high copper concentration during the reaction conditions, the
insertion of copper was detected as side reaction yielding porphyrin 9i.
[31] Isolation of the pure product from the reaction mixture was not achieved,
either by column chromatography on silica gel, aluminium oxide, celite,
size exclusion chromatography or by iterative recrystallization in various
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