Catalytic carbene insertion into an aminoporphyrin and formation of a new chiral supramolecular porphyrin system

Article abstract of DOI:10.1016/j.tetlet.2011.06.120

The catalytic insertion of ethyl diazoacetate into a porphyrin derivative bearing an NH₂ substituent, in the presence of a Rh-based catalyst, was investigated. The X-ray analysis of one of the isolated products reveals the formation of a chiral

Full text of DOI:10.1016/j.tetlet.2011.06.120

Tetrahedron Letters 52 (2011) 4741–4744
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalytic carbene insertion into an aminoporphyrin and formation
of a new chiral supramolecular porphyrin system
Ana T. P. C. Gomes a, Filipe A. Almeida Paz b, Maria G. P. M. S. Neves a, , Augusto C. Tomé a, Artur M. S. Silva a,
⇑
Maria Cecília B. V. de Souza c, Vítor F. Ferreira c, José A. S. Cavaleiro a,
⇑
a Department of Chemistry and QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal
b Department of Chemistry and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
c Departamento de Química Orgânica, Universidade Federal Fluminense, 24020-150 Niterói, RJ, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic insertion of ethyl diazoacetate into a porphyrin derivative bearing an NH2 substituent, in the
presence of a Rh-based catalyst, was investigated. The X-ray analysis of one of the isolated products
reveals the formation of a chiral supramolecular system.
Received 12 May 2011
Revised 27 June 2011
Accepted 28 June 2011
Available online 13 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Porphyrin
Ethyl diazoacetate
Insertion reaction
Carbene
Researchers in organic synthesis, mainly those working in the
area of tetrapyrrolic macrocycles, are constantly looking for new
methodologies leading to molecules that might exhibit specific
properties.1 In fact, many synthetic approaches lead to porphyrin
derivatives that are used in several applications like photodynamic
therapy of cancer cells (PDT),2 catalysis,3 electronics or solar cells
production. Nowadays, functional self-assembled materials with
well-defined three dimensional architectures have attracted much
attention owing to their application in electronics, photonics, light-
energy conversion and catalysis.4 Porphyrins and their analogues
have already shown their ability to form supramolecular systems
to be used in those areas. In fact, even in Nature, porphyrins and
chlorophylls are often self-assembled into nanoscale superstruc-
tures to perform many essential functions, such as light harvesting
and electron transport.4
zation of porphyrins by catalytic insertion of a-diazocarbonyl com-
pounds into X–H bonds are scarce.12,13 In 2008, we reported that
Rh2(OAc)4 is able to catalyse the insertion of the carbene generated
from ethyl diazoacetate (EDA) into peripheral positions of the zinc
complexes of meso-tetraarylporphyrins giving rise to cyclopropa-
nation, CH insertion and Büchner ring expansion products.13
Following our interest on the development of new synthetic ap-
proaches to prepare novel porphyrin derivatives with interesting
properties,14–17 it was decided to investigate the insertion reaction
of EDA into porphyrins with N–H bonds in the presence of a Rh(II)-
based catalyst. For this study the [5-(4-aminophenyl)-10,15,20-tri-
phenylporphyrinato]zinc(II) 1 was selected; the use of the zinc
complex avoids the undesired insertion of the carbene into the
inner N–H bonds if a free-base macrocycle would be used. Consid-
ering the substituents present in the expected new derivatives it
was also aimed at looking at potential intermolecular interactions
between the product molecules.
The use of diazo compounds, namely diazocarbonyl com-
pounds, has attracted much attention due to their versatility in
several synthetically useful transformations.5,6 In particular, the
The insertion reaction with porphyrin derivative 1 was per-
catalytic insertion of
a-diazocarbonyl compounds into X–H bonds
formed with EDA, a very reactive commercially available a-diazo-
(X@C, N, O, S) is a versatile organic transformation for preparing
complex target molecules.7–10 The concept of metal-carbenoid
insertions into X–H bonds (X@C, N, O, S) has been known for more
than three decades,11 but documented studies on the functionali-
carbonyl compound, in the presence of a catalytic amount of
Rh2(OAc)4 (0.1 equiv). The reaction was carried out in refluxing
dichloromethane and the EDA (diluted in dichloromethane) was
slowly injected (for 10 h) using a syringe pump.18 The slow addi-
tion of the diazo compound minimizes the formation of carbene
dimers (Scheme 1).13 TLC of the reaction mixture shows the forma-
tion of three new compounds, one less polar (compound 2) and
other two which are more polar than the starting porphyrin
⇑
Corresponding authors. Tel.: +351 234 370 717; fax: +351 234 370 084.
E-mail addresses: gneves@ua.pt (M.G.P.M.S. Neves), jcavaleiro@ua.pt (J.A.S.
Cavaleiro).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.120

4742
A. T. P. C. Gomes et al. / Tetrahedron Letters 52 (2011) 4741–4744
NH2
4 the CH2-10 protons do not correlate with any carbon of the 5-phe-
nyl ring, indicating that compound 4 is also an amide derivative. In
fact, the resonance of the NH proton of the amide group was de-
tected as a singlet at 8.93 ppm.
Ph
Ph
N
Zn
N
N
N
O
The formation of the unexpected amides 3 and 4 seems to imply
the formation of ethyl glycolate by the reaction of EDA with a
water molecule present in the reaction medium, as suggested by
Afonso et al.20 Assuming the presence of ethyl glycolate in the
reaction medium, its reaction with porphyrin 1 can explain the for-
mation of amide 4. Therefore, an O–H insertion reaction of EDA in
this hydroxyamide can easily justify the formation of compound 3.
To confirm this hypothesis, we performed two complementary
experiments (Scheme 2). The first one involved the reaction of
porphyrin 1 with commercially available ethyl glycolate. As ex-
pected, amide 4 was formed. The second experiment involved
the reaction of amide 4 with EDA in the presence of Rh2(OAc)4.
Compound 3 was obtained, and its formation supports the pro-
posed mechanism.
Ph
1
OEt
(10 equiv.)
N2
Rh2(OAc)4 (0.1 equiv.)
CH2Cl2, 40 ºC
CO2Et
CO2Et
1'
O
O
O
O
OH
OEt
HN
HN
N
1'
2'
1'
Ph
Ph
Ph
Ph
5
5
5
N
Zn
N
N
N
Zn
N
Zn
N
N
N
N
N
+
N
N
+
Ph
Ph
Ph
Ph
Ph
Amide derivative 3 was successfully crystallised as small red
prisms by layering hexane over a solution in dichloromethane, at
room temperature and over a period of two months. The crystallo-
graphic details were unveiled from single-crystal X-ray diffraction,
showing that compound 3 crystallises at 150 K in the non-centro-
symmetric, chiral P21 space group (see Table 1, Supplementary
data). The asymmetric unit is composed of a whole molecular unit
of 3 as depicted in Figure 2a. The substituent pendant group does
not seem to impose a significant steric pressure on the central por-
phyrin ring, with all the four crystallographically independent pyr-
role units lying approximately on the same average plane: the
mean planes of each pair of consecutive pyrrole rings subtend
mutual dihedral angles which only vary between ca. 2.6° and
11.3° (Fig. 2a). These results agree well with the geometrical fea-
tures usually observed for this type of compounds as revealed by
a systematic search in the Cambridge Structural Database (CSD,
Version 5.31, November 2009 with four updates until August
2010)21: from 3335 crystal determinations of porphyrin deriva-
tives, nearly 82% exhibit analogous dihedral plane angles, all smal-
ler than ca. 12.2°. The central Zn2+ cation exhibits a slightly
distorted {ZnN4O} square pyramidal coordination environment.
The basal plane is formed by the four N-atoms of the porphyrin
ring [Zn–N bond lengths ranging from 2.043(4) to 2.069(4) Å; see
Table 2 in the Supplementary data], with the Zn2+ cation being
raised by only ca. 0.24 Å from the average plane containing the
N-pyrrole units. The apical position is occupied by a carbonyl O-
atom from the pendant group of a neighbouring molecular unit
[Zn–O bond length of 2.152(4) Å] (Fig. 2b). The internal cis-angles
of the coordination polyhedron vary between 88.63(16)° and
99.36(16)°, with the latter angle being significantly larger than
the remaining ones (see Fig. 3b and also Table 2 in the Supplemen-
tary data) mainly because of the polymeric nature of the com-
2
3
4
Scheme 1. Reaction of porphyrin complex 1 with EDA in the presence of Rh2(OAc)4.
(compounds 3 and 4). Compounds 2, 3 and 4 were isolated in 14%,
12% and 16% yield, respectively.19
The structure of compound 2 corresponds to the insertion of
two carbene units, generated from EDA, into the N–H function of
porphyrin 1. Its molecular formula was confirmed by ESI-HRMS
(m/z 863.2445 (M+)) and its structure was elucidated by NMR. In
the 1H NMR spectrum the integration of the signals due to the
inserted EDA moieties also confirmed the double insertion. The
proton resonances of the ethyl groups appear as a quartet and as
a triplet at, respectively, 4.33 and 1.37 ppm (J = 7.1 Hz). The signal
due to the CH2-10 protons appears as a singlet at 4.38 ppm. It is
important to notice that from the HMBC spectrum of 2 connectiv-
ities were found between the CH2-10 protons and the carbons of the
carbonyl group and also with the para-carbon of the 5-phenyl
group (d 147.3 ppm) (Fig. 1a).
The molecular formula of compound 3 was confirmed by its ESI-
HRMS where a peak at m/z 853.2131 (M+) is observed. The aliphatic
region of the 1H NMR spectrum shows the typical profile due to the
resonances of the ethyl protons (a quartet at 4.30 ppm and a triplet
at 1.34 ppm) and also two singlets due to the resonances of two
0
´
protons each at 3.79 ppm (CH2-1 ) and at 4.16 ppm (CH2-2),
unequivocally assigned through the HMBC correlations (Fig. 1b).
Contrary to what is observed for compound 2, (Fig. 1a), this 2D
NMR spectrum did not show any correlation between CH2-10 and
the carbons of the 5-phenyl group. This fact confirms that we are
in the presence of an amide derivative and not in the presence of
an N-substituted amine derivative. In the aromatic region of the
spectrum it is observed the resonance due to the NH proton of
the amide group, as a singlet at 9.13 ppm, together with the reso-
nances of the b-pyrrolic and of the meso-aryl protons.
O
O
OH
N
N
N
N
The ESI-HRMS spectrum of 4 shows a peak at m/z 749.1764 (M+)
in agreement with its molecular formula. In the HMBC spectrum of
EtO
OH
Zn
Zn
NH2
N
Rh2(OAc)4
CH2Cl2,
40 ºC
H
4
1
Rh2(OAc)4
CH2Cl2,
40 ºC
EDA
(a)
(b)
CH
CH3
O
2
Ho Hm
Ho Hm
N
N
N
CH2
O
H
O
O
O
Zn
N
N
Zn
N
CH2
1'
O
O
CH2
O
N
CH3
CH2
Zn
OEt
N
H
Ho Hm
Ho Hm
N
O
CH2
2'
2
CH3
CH2
3
3
O
O
Scheme 2. Reaction of porphyrin 1 with ethyl glycolate and O–H insertion reaction
of EDA into porphyrin 4 to afford compound 3.
Figure 1. (a) and) (b) Correlations observed in the HMBC spectra of 2 and 3.

A. T. P. C. Gomes et al. / Tetrahedron Letters 52 (2011) 4741–4744
4743
Figure 2. (a) Schematic representation of the molecular unit of metalloporphyrin 3.
(b) Detailed view of the slightly distorted square pyramidal {ZnN4O} coordination
environment of the central Zn2+ cation, showing the labelling scheme for all atoms
composing the coordination sphere. In both representations, while non-hydrogen
atoms are represented as thermal ellipsoids drawn at the 50% probability level
(except for the terminal –CH3 moiety whose carbon atom is drawn as a sphere),
hydrogen atoms are instead represented as small spheres with an arbitrary radius.
For bond lengths (in Å) and angles (in degrees) for the {ZnN4O} coordination
environment see Table 2 in the Supplementary data.
pound. Indeed, because the carbonyl group of an adjacent molecu-
lar unit coordinates to Zn2+, this creates a small steric pressure on
the resulting polymer which is minimized by a small tilt of the
coordination polyhedron; this occurrence is ultimately reflected
in a slightly larger N–Zn–O angle. Even though five-coordinated
Zn2+ cations are relatively common among metalloporphyrins, only
a handful are polymeric and, usually, a solvent molecule occupies
the fifth coordinative position. The exception to date is the struc-
ture reported by Senge and Smith in the early nineties, in which
the apical position of Zn2+ is occupied by a nitro group.22
Figure 3. Schematic representation of the one-dimensional, chiral supramolecular
metalloporphyrin chain running parallel to the b-axis.
literature revealed the existence of only a handful of chiral poly-
27
meric metalloporphyrins, usually determined in the P21
P212121 space groups.
or
28
The crystal structure of 3 can be essentially described by the
close packing in the ac plane of the aforementioned chiral 1D poly-
mers mediated by van der Waals contacts (see Fig. S1 in the Sup-
plementary data). Indeed, the zigzag nature of the polymer and
the absence of a significant number of donors to promote a net-
work of structure-directing hydrogen bonding interactions lead
to a simple geometrical packing. The only relevant hydrogen bond
is of intramolecular nature within the pendant substituent group
(not shown): there is a N–HꢀꢀꢀO interaction connecting the N–H
moiety of the amide to the ester group [dDꢀꢀꢀA of 3.114(7) Å, and
<(DHA) of ca. 144°].
In summary, it is reported here the first study on the function-
alization of porphyrins by catalytic insertion of ethyl diazoacetate
into N–H bonds. The porphyrin with the NH2 group 1 affords the
insertion product 2 plus the amide derivatives 3 and 4. In the solid
state, amide 3 forms an unusual chiral supramolecular metallopor-
phyrin chain, forming a right-handed helix and being an enantio-
meric pure aggregate. This fact allows us to conclude that the
catalytic insertions of diazocarbonyl compounds can be considered
a new synthetic approach to new supramolecular systems based
on porphyrinic arrays.
The most remarkable structural feature of 3 concerns the pres-
ence of a 1D coordination polymer running parallel to the b-axis, as
depicted in Figure 3. A search in the literature and in the CSD re-
veals that polymeric metalloporphyrins are not abundant, being
less than ca. 9% of all known crystal structures of this family of
compounds. The encapsulation of metallic centres into the porphy-
rin central ring leads to a maximum of two available coordinating
exo-positions, which may allow the rational design of supramolec-
ular architectures,23 many of which exhibit interesting proper-
ties,24 such as conductivity.25 Most of this design is focused in
the use of the metalloporphyrins with an auxiliary bridging ligand
capable of effectively establishing connections between adjacent
units, leading to ‘‘shish-kebab’’ coordination polymers. Recent
notable examples come from the Sanders group.26 Compound 3
is clearly distinct from the ‘‘shish-kebab’’ family of compounds be-
cause the bridges between units are instead ensured by the substi-
tuent pendant group of the organic moiety: as described above, the
carbonyl of the amide group coordinates to the Zn2+ centre of an
adjacent molecule, with this being in the genesis of the 1D polymer
(Fig. 3a). In 3 this arrangement occurs in a spiral fashion, leading to
the formation of a right-handed chiral helix (Fig. 3b) with a pitch of
ca. 10.4 Å running parallel to the b-axis of the unit cell (Fig. 3a and
c). It is also worth noting that the crystal structure determination
clearly ensured the presence of an enantiomerically pure com-
pound (see the Supplementary data). Remarkably, a search in the
Acknowledgements
Thanks are due to the University of Aveiro, Fundação para a
Ciência e a Tecnologia (FCT), FEDER and the FCT-CAPES collaborative

4744
A. T. P. C. Gomes et al. / Tetrahedron Letters 52 (2011) 4741–4744
Alonso, C. M. A.; Serra, V. I. V.; Neves, M. G. P. M. S.; Tomé, A. C.; Silva, A. M. S.;
Paz, F. A. A.; Cavaleiro, J. A. S. Org. Lett. 2007, 9, 2305–2308; (c) Cunha, A. C.;
Gomes, A. T. P. C.; Ferreira, V. F.; Souza, M. C. B. V.; Neves, M. G. P. M. S.; Tomé,
A. C.; Silva, A. M. S.; Cavaleiro, J. A. S. Synthesis 2010, 510–514; (d) Carvalho, C.
M. B.; Neves, M. G. P. M. S.; Tomé, A. C.; Paz, F. A. A.; Silva, A. M. S.; Cavaleiro, J.
A. S. Org. Lett. 2011, 13, 130–133.
program for funding this work. Ana T.P.C. Gomes thanks FCT for her
PhD Grant (SFRH/BD/38528/2007). Thanks are also due to FCT for
specific funding towards the purchase of the single-crystal diffrac-
tometer and the Portuguese National NMR Network.
18. Reaction of porphyrin 1 with EDA: To a dry dichloromethane (6 mL) solution of
Supplementary data
porphyrin 1 (50 mg, 72.0
40 °C, under a nitrogen atmosphere, was added dropwise a solution of EDA
(75.7 L, 0.72 mmol, 10 equiv) in dichloromethane (5 mL) during 10 h using a
lmol) and Rh2(OAc)4 (3.2 mg, 7.2 lmol, 0.1 equiv), at
l
Supplementary data (crystal structure of compound 3 in CIF
format and the corresponding check CIF report are presented) asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.06.120.
syringe pump. The reaction mixture was further stirred at the same
temperature for 8 h. The solvent was then evaporated under reduced
pressure and the residue was fractionated by column chromatography (silica
gel) using CH2Cl2 as the eluent. The obtained fractions were further purified by
preparative TLC using a mixture of hexane and ethyl acetate (4:1) as the eluent.
19. Compound 2: 1H NMR (300 MHz, CDCl3): d 9.06 and 8.97 (AB, J = 4.7 Hz, 4H, H-
2,3 and H-7,8), 8.93 (br s, 4H, H-12,13 and H-17,18), 8.23–8.21 (m, 6H, Ho-Ph-
10,15,20), 8.07 (d, J = 8.6 Hz, 2H, Ho-Ph-5), 7.77–7.71 (m, 9H, Hm,p-Ph-10,15,
20), 6.97 (d, J = 8.6 Hz, 2H, Hm-Ph-5), 4.38 (s, 4H, CH2-10), 4.33 (quart,
J = 7.1 Hz, 4H, OCH2CH3), 1.37 (t, J = 7.1 Hz, 6H, OCH2CH3). 13C NMR (75 MHz,
CDCl3): d 171.1 (C@O), 150.7, 150.1, 150.0, 147.3 (Cp-Ph-5), 142.9, 135.4 (Co-
Ph-5), 134.4 (Co-Ph-10,15,20), 132.7, 132.2 and 131.88 and 131.86 and 131.7
(b-C), 127.4 and 126.5 (Cm,p-Ph-10,15,20), 121.5, 120.9, 120.7, 110.7 (Cm-Ph-
References and notes
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5), 61.3 (CH2-10), 53.7 (OCH2CH3), 14.3 (OCH2CH3). UV–vis (CHCl3): kmax (log
e)
420 (5.14), 505 (3.39), 548 (3.86), 587 (3.35) nm. HRMS-ESI: m/z 863.2445.
Calcd. for C52H41N5O4Zn (M+) 863.2450.
Compound 3: 1H NMR (300 MHz, CDCl3): d 9.13 (s, 1H, NH), 9.95–8.93 (m, 8H,
H-2,3,7,8,12,13,17,18), 8.24–8.21 (m, 6H, Ho-Ph-10,15,20), 8.13 (d, J = 8.4 Hz,
2H, Ho-Ph-5), 7.77–7.72 (m, 9H, Hm,p-Ph-10,15,20), 7.69 (d, J = 8.4 Hz, 2H, Hm-
Ph-5), 4.30 (quart, J = 7.1 Hz, 2H, OCH2CH3), 4.16 (s, 2H, CH2-20), 3.79 (s, 2H,
CH2-10), 1.34 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C NMR (75 MHz, CDCl3): d 170.3
(CO2Et), 167.1 (CONH), 150.2, 150.1, 142.8, 138.9, 136.5, 134.8 (Co-Ph-5), 134.4
(Co-Ph-10,15,20), 132.0 (b-C), 131.9 (b-C), 127.4 and 126.5 (Cm,p-Ph-10,15,20),
121.1, 121.0, 120.4, 117.8, 117.7 (Cm-Ph-5), 71.7 (CH2-10), 69.3 (CH2-20), 61.6
(OCH2), 14.2 (OCH2CH3). UV–vis (CHCl3): kmax (log e) 423 (5.59), 517 (3.47),
552 (4.26), 595 (3.72) nm. HRMS-ESI: m/z 835.2131. Calcd. for C50H37N5O4Zn
(M+) 835.2137.
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H-2,3,7,8,12,13,17,18), 8.24–8.19 (m, 6H, Ho-Ph-10,15,20), 7.97 (d, J = 7.9 Hz,
2H, Ho-Ph-5), 7.79–7.70 (m, 11H, Hm,p-Ph-5,10,15,20), 4.30 (s, 2H, CH2-10). 13C
NMR (126 MHz, CDCl3): d 167.7 (C@O), 150.3, 150.3, 150.2, 150.1, 142.8, 135.0,
134.4, 131.9, 131.9, 127.4, 127.4, 127.4, 126.5, 121.1, 121.1, 29.7 (CH2-10). UV–
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