A Fourfold Benzodehydroannuleno-Fused Porphyrazine

Article abstract of DOI:10.1055/s-0034-1378710

Four benzodehydroannulene subunits were implemented on a dibenzoquinoxalinopoprhyrazine core using efficient palladium-mediated alkyne coupling and oxidative acetylene coupling on phenanthrene dione precursors. Despite the significant amount of strain in system, as evidenced by X-ray analysis of an immediate precursor, porphyrazine assembly leads to an extended model for N-doped graphyne with photosensitizing properties.

Full text of DOI:10.1055/s-0034-1378710

SYNLETT
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9
3
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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 1620–1624
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A Fourfold Benzodehydroannuleno-Fused Porphyrazine
Fabian Körte
Clemens Bruhn
Rüdiger Faust*
Universität Kassel, Institute for Chemistry and CINSaT – Center for
Interdisciplinary Nanostructure Science and Technology,
Heinrich-Plett-Str. 40, 34132 Kassel, Germany
r.faust@uni-kassel.de
Dedicated to Professor K. Peter C. Vollhardt – eminent chemist and
inspirational character
Received: 20.03.2015
Accepted after revision: 11.05.2015
Published online: 11.06.2015
During our recent work on peripherally functionalized
porphyrazines,10 substituted phenanthrene diones have
emerged as a valuable starting material for soluble dibenzo-
quinoxalinoporphyrazines (Scheme 1). Of particular impor-
tance for successful manipulations of expanded porphyra-
zines and their precursors are the tri(p-tolyl)propinyl
groups in the 2,7-positions of the phenanthrene core of 2
that assist to suppress aggregation of the larger cyclic struc-
tures to be formed during the course of the synthesis.
The 3,6-bromine substituents, on the other hand, pro-
vide convenient starting points for alkyne attachment via
routine palladium-mediated cross-coupling methods. For
the assembly of the target molecule, the diacetal-protected
phenanthrene dione 210a was thus reacted with a differen-
tially alkynylated veratrol derivative 3, a compound pre-
pared in three steps starting from the known 1-bromo-2-
iodo-4,5-dimethoxybenzene 811,12 as shown in Scheme 2.13
The methoxy substituents implemented in 3 not only
facilitate the synthesis of the required terminal alkyne, but
are also intended to allow for the future conversion to an-
choring groups for the attachment of the target molecule 1
to surfaces.
With tetraalkynyl phenanthrene dione 4 in hand, the
preparation of the BDA follows the established protocols of
protodesilylation with tetra-n-butylammonium fluoride
(TBAF) to yield 5 featuring two opposing terminal alkyne
units. Subsequent copper-mediated oxidative acetylene ho-
mocoupling using copper(II) acetate in pyridine established
the butadiynyl-linked BDA 614 in remarkably high yield. The
porphyrazine assembly is then initiated by liberating the la-
tent quinone functionality in 6 with p-toluenesulfonic acid,
followed by the condensation of the intermittently formed
dione with diamino maleodinitrile to yield the dicyano-
functionalized dibenzoquinoxaline 7 as a deep red solid.15
DOI: 10.1055/s-0034-1378710; Art ID: st-2015-b0201-l
Abstract Four benzodehydroannulene subunits were implemented on
a dibenzoquinoxalinopoprhyrazine core using efficient palladium-medi-
ated alkyne coupling and oxidative acetylene coupling on phenan-
threne dione precursors. Despite the significant amount of strain in sys-
tem, as evidenced by X-ray analysis of an immediate precursor,
porphyrazine assembly leads to an extended model for N-doped gra-
phyne with photosensitizing properties.
Key words acetylene coupling, cycloalkynes, dehydroannulenes,
phthalocyanines, singlet oxygen
The juxtaposition of mutually interfering π-electron pe-
rimeters has been a longstanding theme in physical organic
chemistry as highlighted by the (meanwhile) classical mile-
stones1 kekulene2 and Vollhardt’s triangular [4]phenylene,3
among many others. Benzodehydroannulenes (BDA) feature
prominently as structural motifs in this area and have been
explored as structural alternatives to benzenoid graphene
(‘graphyne’)4 and – again by Vollhardt and coworkers – as
an explosive source for carbon nanotubes.5 Previous work
in our group on N-heterocyclic BDA6,7 has revealed interest-
ing molecular electronic properties emanating from the in-
teraction between benzenoid heterocycle and cycloalkyne
moiety. Noteworthy in this respect are the dehydroannu-
lene-fused phthalocyanines prepared independently by
Torres and Cook.8 Given the current excitement surround-
ing N-doped graphene as a promising material for catalysis
and/or energy storage technology,9 we have decided to ex-
plore 1 (Figure 1) as a new structural motif related to N-
doped graphyne and present here a dibenzoquinoxalino-
porphyrazine core fused to four strained BDA.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1620–1624

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MeO
OMe
MeO
OMe
(p-Tol)3C
C(p-Tol)3
C(p-Tol)3
C(p-Tol)3
MeO
MeO
OMe
OMe
N
N
N
N
N
Mg
N
N
N
N
N
N
N
N
N
MeO
MeO
OMe
OMe
N
N
C(p-Tol)3
C(p-Tol)3
C(p-Tol)3
(p-Tol)3C
MeO
OMe
MeO
OMe
1
Figure 1 Target structure 1 featuring four benzodehydroannulene units around a dibenzoquinoxalinoporphyrazine core
We were able to obtain single crystals of 7 suitable for
analysis by X-ray diffraction methods (Figure 2).16 The mol-
ecule crystallizes in the monoclinic space group P21/n and
has no internal symmetry. Two features in the solid state
structure of 1 are remarkable: For one, the spatial require-
ments of the tri(p-tolyl)propynyl groups are substantial,
underlining the efficiency with which these groups can
suppress π–π stacking if attached to porphyrazine macrocy-
cles and their precursors.10 Secondly, there is a considerable
degree of strain in the BDA portion of the molecule as evi-
denced by the distortion of all bond angles around the
alkyne units in 7. This is most notably illustrated by the
bond angle between carbons C24, C25, and C26 of the buta-
diynyl group that is compressed by nine degrees to only
171(1)°. Hence, the butadiynyl substructure is significantly
bent towards the center of the BDA. The bond angle around
the internal ethynyl unit, namely C12, C36, and C35, is re-
duced by the same amount, revealing that this subunit is, in
contrast, bent slightly outwards.
Remarkable in view of the strained butadiynyl-bridged
BDA subunits is the fact that the conclusion of the synthesis
of 1 by based-induced cyclotetramerization of 7 with mag-
nesium butanolate in o-dichlorobenzene (o-DCB) at 160 °C
proceeds relatively smoothly to give the target compound 1
as a olive-brown solid in a yield of 15% after GPC purifica-
tion (Scheme 1).
Figure 2 Structure of 7 in the solid state as determined by analysis of
the X-ray diffraction pattern of a single crystal of 7. N shown in blue, O
in red, C, H in grey.
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OMe
OMe
C(p-Tol)3
C(p-Tol)3
C(p-Tol)3
MeO
MeO
TIPS
MeO
MeO
Br
Br
O
O
O
O
O
O
O
O
O
O
O
O
TBAF, THF–MeOH (5:1),
3
50 °C, 16 h
TIPS
TIPS
89%
Pd(PPh3)4, CuI, dioxane,
diisopropylamine, 85 °C, 5 h
70%
MeO
MeO
C(p-Tol)3
C(p-Tol)3
C(p-Tol)3
OMe
OMe
5
2
4
C(p-Tol)3
C(p-Tol)3
MeO
MeO
MeO
MeO
i) AcOH, o-DCB, PTSA,
140 °C, 2–3 h
ii) DAMN, 70 °C, 3 h
Mg(n-BuO)2,
o-DCB,
160 °C, 2 h
Cu(OAc)2,
pyridine,
60 °C, 2 h
O
O
O
O
N
CN
CN
1
82%
15%
90%
N
MeO
MeO
MeO
MeO
C(p-Tol)3
C(p-Tol)3
6
7
Scheme 1 Synthesis of target molecule 1
The identity of 1 is inferred mainly from mass spectral
and photophysical data.13 Due to the dimensions of the
molecule, meaningful NMR spectra could not be obtained,
as the slow tumbling of the molecule in solution at room
temperature with respect to the NMR timescale leads to
very broad signals. In addition, the compound is apprecia-
bly aggregated in concentrated solutions suitable for NMR
measurements. The MALDI-TOF mass spectrum, acquired
using a dithranol matrix, shows a single signal peaking at
m/z = 5078, which deviates only marginally from the molec-
ular mass calculated for 1 (C360H248MgN16O16 with 5078.22
g mol–1). Solutions of 1 in organic solvents are olive-green,
an appearance that is also testified by the UV/vis absorp-
tion spectrum of 1 (Figure 3).
TMS
PdCl2(PPh3)2,
CuI, THF, Et3N, r.t., 16 h
MeO
MeO
Br
I
MeO
MeO
Br
76%
TMS
8
9
TIPS
Pd(PPh3)4,
CuI, dioxane,
piperidine,
TMS
TIPS
K2CO3,
THF–MeOH,
50 °C, 3 h
MeO
MeO
MeO
MeO
90 °C, 16 h
88%
83%
TIPS
3
10
Scheme 2 Synthesis of veratrol derivative 3 featuring one free terminal and one silyl-protected alkynyl substituent
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1620–1624

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(1) Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH: Wein-
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(2) Diederich, F.; Staab, H. A. Angew. Chem., Int. Ed. Engl. 1978, 17,
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(3) (a) Diercks, R.; Armstrong, J. D.; Boese, R. Angew. Chem., Int. Ed.
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Advances in Strain in Organic Chemistry; Vol. 5; Halton, B., Ed.;
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(4) Haley, M. M. Pure Appl. Chem. 2008, 80, 519.
(5) (a) Boese, R.; Matzger, A. J.; Vollhardt, K. P. C. J. Am. Chem. Soc.
1997, 119, 2052. (b) Faust, R. Angew. Chem. Int. Ed. 1998, 37,
2825.
(6) Ott, S.; Faust, R. Chem. Commun. 2004, 388.
(7) Ott, S.; Faust, R. Synlett 2004, 1509.
(8) (a) Garcia-Frutos, E. M.; Fernandez-Lazaro, F.; Maya, E. M.;
Vazquez, P.; Torres, T. J. Org. Chem. 2000, 65, 6841. (b) Cook, M.
J.; Heeney, M. J. Chem. Commun. 2000, 969.
(9) See, for example: Wang, L.; Sofer, Z.; Luxa, L.; Pumera, M.
J. Mater. Chem. C 2014, 2, 2887.
Figure 3 Electronic absorption spectra of 1 (green) and 7 (red) at r.t. in
CHCl3
(10) (a) Engelhardt, V.; Kuhri, S.; Fleischhauer, J.; Garcia-Iglesias, M.;
Gonzalez-Rodriguez, D.; Bottari, G.; Torres, T.; Guldi, D. M.;
Faust, R. Chem. Sci. 2013, 4, 3888. (b) Kuhri, S.; Engelhardt, V.;
Faust, R.; Guldi, D. M. Chem. Sci. 2014, 5, 2580.
(11) Buu-Hoï, N. P. Liebigs Ann. Chem. 1944, 1, 1.
(12) Yadav, J. S.; Reddy, B. V. S.; Reddy, P. S. R.; Basak, A. K.; Narsaiah,
A. V. Adv. Synth. Catal. 2004, 346, 77.
The UV/vis/NIR absorption spectrum of 1 features a sig-
nal pattern characteristic for porphyrazine macrocycles
with an intensive high-energy B band around λ = 400 nm
and a low-energy Q band peaking at λ = 721 nm. The broad
shape of the Q band can be attributed to aggregation phe-
nomena of the expansive, largely planar chromophore of 1
in solution. The immediate precursor of 1, namely 7, shows
a longest wavelength absorption maximum around λ = 420
nm. Noteworthy is the almost superimposable region be-
tween λ = 300 and 450 nm in both spectra, a region which
is dominated by transitions associated with the dehydroan-
nluene and the dibenzoquinoxaline portions of the mole-
cules. In addition, we were able to detect the diagnostic sin-
glet-oxygen phosphorescence at λ = 1270 nm (not shown)13
upon irradiating aerated CHCl3 solutions of 1 with light
from a quartz-tungsten lamp. This finding lends additional
credibility to the formation of the tetrapyrrolic macrocycle
1, as dibenzoquinoxalinoporphyrazines are established as
efficient photosensitizers for 1O2 formation.17
(13) For details, see the Supporting Information.
(14) Compound 6
A solution of 5 (500 mg, 0.39 mmol) and Cu(OAc)2 in pyridine
(10 mL) was stirred for 2 h at 60 °C. After cooling to r.t. CHCl3
(50 mL) was added, and the mixture was washed successively
with 1 N HCl (50 mL), H2O (2 × 50 mL), and sat. aq NaCl (50 mL).
The organic layer was separated, dried over Na2SO4, filtered, and
excess solvent was removed under reduced pressure. The
residue was purified by column chromatography on silica gel
with CHCl3 to yield a bright yellow solid (450 mg, 0.35 mmol,
90%). 1H NMR (400 MHz, CDCl3): δ = 8.51 (s, 2 H, CHphenan), 7.74
3
(s, 2 H,, CHphenan), 7.35 (d, JHH = 8.2 Hz, 12 H, CHarom), 7.10 (d,
3JHH = 8.2 Hz, 12 H, CHarom), 7.08 (s, 2 H, CHarom), 6.42 (s, 2 H, CHa-
rom), 4.19 (br s, 4 H, OCH2), 3.91 (s, 6 H, OCH3), 3.69 (br s, 4 H,
OCH2), 3.39(s, 6 H, OCH3), 2.32 (s, 18 H, CH3) ppm. 13C NMR (126
MHz, CDCl3): δ = 149.69, 149.32, 142.56, 136.28, 132.79, 130.93,
130.24, 129.06, 128.99, 128.72, 128.24, 124.42, 118.43, 118.30,
116.33, 113.54, 101.14, 93.10, 92.46, 92.42, 83.76, 82.46, 56.03,
55.72, 55.46, 20.95 ppm. MALDI-MS (DCTB, positive ion mode):
m/z = 1279.55 [M + H]+.
In summary, we have developed the synthetic route to a
fourfold BDA-fused porphyrazine 1 that may serve as a
structural motif in the rational design of extended frag-
ments of N-doped graphyne. The strain inherent in the BDA
substructure of 1 may trigger additional reactivity that is as
yet unexplored. Further developments focus on the deposi-
tion of 1 on surfaces.
(15) Compound 7
A solution of 6 (300 mg, 0.23 mmol) and PTSA·H2O (134 mg, 0.7
mmol) in glacial AcOH and o-dichlorobenzene was heated at
140 °C for 2 h until the starting material was completely hydro-
lyzed (TLC control). After cooling to 70 °C diaminomaleonitrile
(40 mg, 0.35 mmol) was added, and the solution was stirred for
3 h. The solution was cooled to r.t. and slowly added to MeOH
(50 mL). The resulting brownish red precipitate was collected by
filtration. The residue was purified by column chromatography
on silica gel with PE–EtOAc (2:1) to yield a red solid (243 mg,
0.19 mmol, 82%).1H NMR (400 MHz, CDCl3): δ = 9.10 (s, 2 H,
CHphenan),8.50 (s, 2 H, CHphenan), 7.35 (d, 3JHH = 8.2 Hz, 12 H, CHa-
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1378710.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
3
rom), 7.11 (d, JHH = 8.2 Hz, 12 H, CHarom), 6.83 (s, 2 H, CHarom),
6.21 (s, 2 H, CHarom), 3.69 (s, 6 H, OCH3), 3.36 (s, 6 H, OCH3), 2.36
(s, 18 H, CH3) ppm. 13C NMR (126 MHz, CDCl3): δ = 149.69,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1620–1624

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149.32, 142.56, 136.28, 132.79, 130.93, 130.24, 129.06, 128.99,
128.72, 128.24, 124.42, 118.43, 118.30, 116.33, 113.54, 101.14,
93.10, 92.46, 92.42, 83.76, 82.46, 56.03, 55.72, 55.46, 20.95
ppm. ESI-HRMS: m/z calcd for [C90H62N4O4 + Ag]+ = 1369.3817;
(16) Crystallographic data for compound 7 have been deposited with
the accession number CCDC 1054333, and can be obtained free
of charge from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; Fax +44(0)1223
found: 1369.3836 [M + Ag]+. UV/vis (CHCl3): λmax (ε/105 M–1 cm–1
)
336033;
Email:
deposit@ccdc.cam.ac.uk;
web
site:
= 312 (1.06), 351 (0.66), 374 (0.48), 409 (0.53) nm.
www.ccdc.cam.ac.uk/conts/retrieving.html.
(17) Löser, P.; Winzenburg, A.; Faust, R. Chem. Commun. 2013, 49,
9413.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1620–1624