Metal-catalyzed direct heteroarylation of C-H (: Meso) bonds in porphyrins: Facile synthesis and photophysical properties of novel meso -heteroaromatic appended porphyrins

Article abstract of DOI:10.1039/c8ob00174j

A simple and rapid microwave-assisted synthesis of heteroaromatic appended porphyrins using the Pd/Cu-catalyzed C-C coupling of meso-bromoporphyrins with various five- and six-membered heteroaromatic azoles has been successfully developed. The prepared he

Full text of DOI:10.1039/c8ob00174j

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Metal-catalyzed direct heteroarylation of C–H
(meso) bonds in porphyrins: facile synthesis and
photophysical properties of novel meso-
heteroaromatic appended porphyrins†
Cite this: DOI: 10.1039/c8ob00174j
Santosh B. Khandagale, Meenakshi Pilania, V. Arun and Dalip Kumar
*
A simple and rapid microwave-assisted synthesis of heteroaromatic appended porphyrins using the Pd/
Cu-catalyzed C–C coupling of meso-bromoporphyrins with various five- and six-membered hetero-
aromatic azoles has been successfully developed. The prepared heteroaromatic porphyrins 7a-lNi were
found to exhibit slightly red-shifted (∼5–10 nm) Soret and Q bands. The developed reaction conditions
are useful for the preparation of diversely substituted heteroaromatic porphyrins (A3B- and A2B2-types).
Received 20th January 2018,
Accepted 27th February 2018
DOI: 10.1039/c8ob00174j
rsc.li/obc
PEPPSI-IPent produced carbazole appended porphyrins 1d.
Therien et al. elaborated bipyridyl substituted porphyrins 1c
Introduction
Porphyrins and metalloporphyrins are a distinct class of com- via the metal-mediated coupling reaction of haloporphyrin
pounds utilized in various fields such as electron transport, and dimethylbipyridyl systems.8 Gryko and Lindsey prepared
catalysis, photodynamic therapy, optoelectronic devices and meso-heteroaromatic substituted porphyrins utilizing the TFA
building blocks for supramolecular assembly.1 Due to the catalyzed condensation of heteroaryl-substituted dipyrro-
interesting photophysical and electrochemical properties of methane and dipyrromethane–dicarbinol.9 Subsequently,
metalloporphyrins, researchers have been making continuous Gryko and Tasior reported an efficient synthesis of trans-A2B2-
efforts to modify donor–acceptor systems in porphyrins by porphyrins from the reaction of dipyrrane with appropriate
using highly conjugated peripheral substituents at reactive pyridine–carboxaldehyde.10
meso- or β-positions.2 For the modulation of the photophysical
properties of parent porphyrins, various extended π-systems or
conjugated with heteroaromatic moieties have been explored.1b
In some of the reported porphyrinoids, structural modifi-
cations at the meso-position led to the absorption region
(∼400–800 nm) with the enhanced fluorescence profile
(650–900 nm) and redox potential.3 In recent years, a large
number of novel porphyrins have been successfully syn-
thesized by using transition metal-catalyzed cross-coupling
reactions of halogenated porphyrins4 as depicted in Scheme 1.
For example, Osuka et al. identified highly fluorescence tri-
meric 5-azaindolylporphyrins 1a through the Pd and Cu-cata-
lyzed intramolecular cyclization of meso-(4-aminopyrid-3-yl)
ethynyl porphyrins.5 Very recently, Sessler’s group reported
porphyrin–oligothiophene conjugates 1b involving the Suzuki
reaction between bromothiophene and boronate porphyrins.6
Similarly, the Ullmann coupling7 of bromoporphyrin with car-
bazole or amine in the presence of special ligands like Pd-
Scheme 1 Existing and present methods to prepare heteroaromatic
porphyrins. Reaction conditions: (a)(i) N-ethyl-3-ethynylpyridin-4-
amine, Pd(PPh3)4, CuI, THF, 17 h, reflux and (ii) Cu(OAc)2, 1,4-dioxane,
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031,
Rajasthan, India. E-mail: dalipk@pilani.bits-pilani.ac.in
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ob00174j
reflux,
4
days; (b) meso-bromoporphyrin, bromoterthiophene,
Pd(PPh3)4, Cs2CO3, toluene, 120 °C; (c) meso-bromoporphyrin, Bu3Sn
[4-(CH2)-4’-(CH2-bPy)], Pd(PPh3)4, THF, 80 °C, 48 h; (d) meso-bromo-
porphyrin, carbazole (10 equiv.), Pd-PEPPSI-IPent, THF, reflux, 42 h.
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Generally, mixed condensation of two or more aldehydes was metallated with Ni(acac)2 under refluxing conditions to
with pyrrole generates isomeric heteroaromatic porphyrins, afford 5aNi in 92% yield.23
thus necessitating chromatographic purification.2c Nowadays,
To achieve 7aNi, our investigation started with 5aNi and
various C–C and C–N bond coupling reactions involving halo- benzoxazole 6a as model substrates to screen different reaction
porphyrins and special ligands like Pd-PEPPSI-IPent, xantphos conditions. Initially, the Pd(OAc)2 catalyzed reaction of 5aNi
or DPEPhos are being frequently explored in the preparation of and 6a in the presence of K3PO4 and PivOH under conventional
heteroarene linked porphyrins.11 Despite the development of a as well as microwave (MW) irradiation at 140 °C in DMF
few simplified protocols, the synthesis of heteroaromatic por- exclusively generated dehalogenated porphyrin 5 (Table 1,
phyrins is a challenging task and warrants the development of entries 1 and 2). Next, the combination of Pd(OAc)2 and PPh3
a facile and efficient method. In the recent past, metal-cata- delivered only a trace amount of 7aNi (Table 1, entry 3). The
lyzed direct arylation has been pursued with increasing inter- use of Cu(OAc)2 with Cs2CO3 also did not afford 7aNi (Table 1,
est to prepare functionalized heterocyclic frameworks. For entry 4). To our delight, the combination of CuI and Pd(OAc)2
instance, the arylation of azoles12 and benzazoles including in the presence of Cs2CO3 in DMF at 140 °C (24 h) delivered
thiophene,13 oxazole,14 oxadiazole,15 benzoxazole16 and 7aNi in 55% yield (Table 1, entry 5). To improve the product
imidazo[1,2-a]pyridine17 has been achieved from the reaction yield and efficiency, the reaction of 5aNi and 6a was explored
of heteroarenes with appropriate aryl halides in the presence
of transition metal catalysts such as Pd,18 Ru19 and Rh.20
Given the appealing photophysical and electrochemical pro-
perties of porphyrin derivatives, it is desirable to prepare novel
Table 1 Optimization of the reaction conditions
azole and benzazole annulated porphyrin frameworks. In con-
tinuation of our efforts to identify novel porphyrins with an
improved absorption band, recently we prepared thiazolyl and
oxadiazolyl porphyrins endowed with efficient DNA photo-
nuclease activity.21 Although the arylation of five/six-membered
heterocycles has been known, however, direct heteroarylation
of porphyrins involving haloporphyrins and azoles or benza-
zoles remains unexplored. To provide a facile route for novel
Catalyst
additive
Temp
[°C]
Time
[min]
Yieldc
[%]
heteroaromatic substituted porphyrin systems, we have devel-
oped an efficient microwave-assisted synthetic protocol for the
preparation of diverse heteroaromatic appended porphyrins.
Entry
1
Base
Solvent
DMF
K3PO4
Pd(OAc)2
PivOH
Pd(OAc)2
PivOH
Pd(OAc)2
PPh3
Cu(OAc)2
Pd(OAc)2
CuI
Pd(OAc)2
CuI
Pd(OAc)2
CuBr
Pd(OAc)2
CuI
Pd(OAc)2
CuI
Pd(OAc)2
CuI
Pd(OAc)2
CuI
140
140
120
1440
10
NRa
2
3
K3PO4
K3PO4
DMF
DMF
NRb
1440
Trace
Results and discussion
Synthesis
4
5
Cs2CO3
Cs2CO3
DMF
DMF
120
140
1440
1440
NRa
55a
6
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
Toluene
DMSO
THF
120
120
80
10
10
10
10
10
10
10
10
15
10
73b
The preparation of 5-(benzoxazol-2′-yl)-10,15,20-triphenylpor-
phyrinato-nickel(II) 7aNi involves the use of readily available
5-bromo-10,15,20-triphenylporphyrinato-nickel(II) 5aNi and
benzoxazole 6a. The key intermediate 5aNi was achieved by the
synthetic route as illustrated in Scheme 2. Firstly, in a modi-
fied Lindsey method,22 the reaction of dipyrrane 1 and freshly
distilled benzaldehyde 2 in dry DCM with a catalytic amount
of trifluoroacetic acid (TFA) produced 5,15-diphenylporphyrin
3. The treatment of 3 with phenyllithium and DDQ afforded
purple coloured meso-5,10,15-triphenylporphyrin 4 in 80%
yield. A further reaction of 4 with N-bromosuccinimide (NBS)
afforded free-base meso-bromoporphyrin (70% yield) which
7
64b
8
Trace
Traceb
40d
9
140
120
120
120
90
10
11
12
13
14
15
Traceb
Traceb
NRb
Pd(OAc)2
CuI
Pd(OAc)2
CuI
Pd(OAc)2
CuI
Pd(OAc)2
CuI
DMF
DMF
120
120
60b
Na2CO3
Traceb
a Reaction conditions: 5aNi (0.0693 mmol), 6a (0.1386 mmol),
Pd(OAc)2 (10 mol%), additive (10 mol%), base (2.0 equiv.), DMF
(2.5 mL) 140 °C, 24 h. b Reaction performed under MW irradiation
(100 W), 120 °C, 10 min. NR = no reaction. c Isolated yield. d 5bNi
(0.0693 mmol), DMF (2.5 mL), 6a (0.1386 mmol), Pd(OAc)2 (10 mol%),
CuI (10 mol%), Cs2CO3 (2.0 equiv.), MW (100 W), 120 °C, 10 min.
Scheme 2 Synthesis of bromoporphyrin 5aNi.
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Table 2 Preparation of meso-heteroaromatic appended porphyrins
7a-kNi
with focused MW. The usefulness of MW irradiation in
organic transformations is very popular due to its beneficial
features such as efficient heating, less energy consumption,
shorter reaction time and high product yields.24 The reaction
of 5aNi and 6a in DMF under MW irradiation at 120 °C for
10 min afforded 7aNi in 73% yield (Table 1, entry 6). Changing
the additive from CuI to CuBr afforded 7aNi with a reduced
yield (64%) and lowering the temperature (80 °C) failed to
afford the desired product (Table 1, entries 7 and 8). The reac-
tion yield dropped at an elevated temperature (140 °C) with
the formation of unidentified side products (Table 1, entry 9).
Under similar reaction conditions, the use of (5-iodo-10,15,20-
triphenylporphyrinato)nickel(II) 5bNi instead of 5aNi afforded
7aNi in low yield (Table 1, entry 10). Attempts to conduct the
reaction in solvents like toluene, DMSO and THF were failed
(Table 1, entries 11–13) to produce 7aNi in good yield.
Similarly, the use of K2CO3 in DMF afforded 7aNi in moderate
yield (60%), whereas Na2CO3 offered 7aNi in a trace amount
(Table 1, entries 14 and 15).
Finally, we found that the optimal conditions include the
use of Pd(OAc)2 (10 mol%), CuI (10 mol%) and Cs2CO3 (2
equiv.) in DMF at 120 °C under MW irradiation for 10 min for
the synthesis of 7aNi from the reaction of 5aNi with 6a. HRMS
of the porphyrin 7aNi ionized by MALDI-TOF showed a mole-
cular ion peak for C45H28N5NiO at m/z 712.1392 (calc. m/z
712.1617).
Having the optimized reaction conditions in hand, we per-
formed the reaction of 5aNi with 6-methylbenzoxazole 6b to
obtain 7bNi in 70% yield. Next, the reaction of 5aNi with
N-substituted benzimidazole 6c furnished 7cNi in 65% yield
(Table 2). It is noteworthy to mention that we successfully
extended the protocol to couple porphyrin 5aNi with a biologi-
cally active molecule, caffeine 6d,16c and furnished 7dNi in
68% yield (Table 2). The 1H NMR spectrum of 7dNi showed
characteristic singlets at 3.61 ppm and 3.88 ppm for CONCH3
and NCH3, respectively. The MALDI-MS spectrum of 7dNi dis-
played a molecular ion peak corresponding to C46H32N8NiO2
appearing at m/z 786.1514 (calc. m/z 786.2002).
a Reaction conditions: 5aNi (0.0693 mmol), 6a–b or 6g–j
(0.1386 mmol), Pd(OAc)2 (10 mol%), Cs2CO3 (2.0 equiv.), CuI
(10 mol%), DMF (2.5 mL), MW (100 W), 120 °C. b 5aNi (0.0693 mmol),
6c–f (0.1732 mmol), Pd(OAc)2 (10 mol%), K2CO3 (2.5 equiv.), CuI
(10 mol%), DMF (3 mL), MW (100 W), 120 °C, 10 min.
Furthermore, the reaction of 5aNi with imidazo[1,2-a]pyri-
dine 6e, afforded 7eNi in moderate yield (58%). But the use of
K2CO3 as the base with the extended reaction time (10 min)
enhanced the reactivity of 5aNi with 6e and 6f to afford por-
phyrins 7eNi (70%) and 7fNi (61%), respectively (Table 2).
Similarly, the coupling reactions of 5aNi with oxadiazole 6g
and oxazoles 6h–j were also smoothly proceeded to furnish 7g-
jNi in good yields (60–75%). Under standard reaction con-
ditions, the thienyl group was also incorporated into porphyrin
7kNi in 62% yield.
Scheme 3 Synthesis of [bis(benzoxazolyl)porphyrinato]nickel(II) 7lNi.
absorption peaks of 7a-lNi exhibited a bathochromic shift
The scope of the reaction was further expanded by prepar- when compared to those of tetraphenylporphyrin (H2TPP). In
ing [bis(benzoxazolyl)porphyrinato]nickel(II) 7lNi (64% yield) particular, porphyrins 7dNi, 7fNi and 7jNi exhibited 5–10 nm
from the reaction of 5,15-dibromo-10,20-diphenylporphyrin 5c red-shifted Soret bands ranging 416–427 nm probably due to
with 2.5 equivalents of 6a (Scheme 3).
electron-rich (metalloporphyrin) and electron-deficient units
which may act as push–pull type architectures stabilized by
charge-transfer.25 In the case of porphyrin 7dNi with caffeine
Photophysical studies
Next, we studied the UV-vis absorption spectra of porphyrins nucleus, a Soret band was observed at 420 nm with increased
7a-lNi using analytical grade chloroform (Fig. 1). Interestingly, intensity indicating an efficient energy transfer among the
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Conclusions
In summary, we have developed a MW-assisted rapid and ver-
satile synthesis of diverse meso-heteroaromatic appended por-
phyrins 7a-lNi by the coupling of meso-bromoporphyrins with
various azoles 6a–k in the presence of the Pd–Cu catalytic
system. Identified reaction conditions were also extended to
assemble bis-heteroarylporphyrins and could be useful for the
preparation of A3B and A2B2-types of heteroaromatic appended
porphyrins. Porphyrins 7a-lNi displayed slightly red-shifted
(5–10 nm) Soret and Q bands when compared to parent por-
phyrins. Overall, we have effectively used operationally simple
catalytic systems to access π-extended porphyrins possessing
impressive electron donor–acceptor chromophoric systems.
The developed protocol is believed to be useful in the design
and synthesis of new porphyrinoids in fewer steps. Further
applications in structural modifications and functionalization
of meso-heteroaromatic appended porphyrins are presently
under progress.
Fig. 1 UV–visible absorption spectra (chloroform) of porphyrins 7a-jNi.
singlet states of porphyrin and the caffeine moiety. Intense
broadening in the Q bands of 7dNi was observed in the visible
region (λmax 500–650 nm) as illustrated in Fig. 1. The incorpor-
ation of conjugated imidazo[1,2-a]pyridine with porphyrin
7fNi induced systematic changes both in the Soret and Q
bands. Increased π-conjugation in the porphyrin core may
cause the splitting of the π–π* level, thereby reducing the
HOMO–LUMO gap to induce a bathochromic shift and broad-
ening in the Soret and Q bands.26 Interestingly, A2B2-type por-
phyrin 7lNi also showed an intense absorption peak at
421 nm.
Experimental sections
Materials
All chemicals and reagents were purchased from commercially
available sources like Merck, Sigma-Aldrich and Spectrochem.
The key intermediate 5-bromo-10,15,20-triphenylporphyrin
5aNi and 5-iodo-10,15,20-triphenylporphyrin 5bNi were pre-
pared by a literature procedure.28 Reactions were performed
using CEM Discover focused microwave apparatus in a 10 mL
microwave tube, and monitored by thin layer chromatography
(TLC) performed using a silica precoated alumina sheet
ordered from Merck. The crude products were purified by
column chromatography using silica gel of 100–200 mesh size.
1H NMR and 13C NMR spectra were recorded on a Bruker
Advance II 400 MHz spectrometer in CDCl3 using tetramethyl
silane (TMS) as the internal standard. 1H and 13C NMR chemi-
cal shifts are expressed in parts per million (ppm) relative to
TMS (0 ppm) and analysis performed at 20 °C. Mass analysis
carried out on a MALDI–TOF MS spectrometer was performed
using 2,5-dihyrdoxybenzoic acid (DHB) as the matrix. The UV-
vis spectra of the synthesized porphyrins derivatives were
recorded by using a steady state absorption spectrophotometer
(Jasco V-650) in UV spectroscopy grade chloroform.
Based on the experimental and the literature reports,18a,27
a
plausible mechanism for the formation of 7a-lNi is depicted in
Scheme 4. Initially, oxidative addition of Pd(0) to bromopor-
phyrin 5a is believed to produce the porphyrin Pd(II) complex
(A). Reactive azole–copper species B, in situ generated from
azole 6 and Cu-catalyst, undergoes transmetallation with A to
furnish Pd(II) complex C/D. Finally, reductive elimination of
C/D may lead to porphyrins 7 with concomitant regeneration
of the active catalyst.
(a) Synthesis of (5,15-dibromo-10,20-diphenylporphyri-
nato)nickel(II) 5cNi. To a solution of 5,15-diphenylporphyrin 3
(200 mg) in chloroform (250 mL) was added pyridine (4 mL).
After stirring the contents for
5 min, a solution of
N-bromosuccinimide (2.2 equivalent) in chloroform (100 mL)
was added dropwise over a period of 30 min. The reaction
mixture was quenched with acetone (50 mL) and concentrated
under reduced pressure. The residue thus obtained was puri-
fied by column chromatography on silica gel (100–200 mesh)
and eluted with hexane/chloroform to isolate free-base 5,15-
dibromoporphyrin in 80% yield. Next, Ni(acac)2 (5 mmol) was
added to a solution of 5,15-bromoporphyrin (1 mmol) in
Scheme 4 Plausible pathway for the formation of porphyrins 7.
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toluene (30 mL) and the mixture was heated at 110 °C for 3 h. and DMF (3 mL). The reaction mixture was irradiated with
After the completion of metallation, toluene was evaporated focused MW (100 W) at 120 °C for 10 min. The progress of the
and extracted with chloroform (200 mL). The organic layer was reaction was monitored by TLC in 50% hexane/chloroform.
washed with water (250 mL), dried over sodium sulphate and After completion of the reaction, the mixture was filtered over
distilled at reduced pressure. The crude product was purified silica gel and washed with chloroform (2 × 50 mL). The filtrate
by column chromatography using silica (100–200 mesh) and was concentrated and the residue was purified by column
chloroform : hexane (1 : 9) as the eluent to afford 5cNi. Yield chromatography using chloroform : hexane : TEA (20 : 80 : 1 to
1
85% H NMR (400 MHz, CDCl3) δ 9.45 (d, J = 5.0 Hz, 4H), 8.74 70 : 30 : 1) as an eluent to afford a pure purple coloured solid.
(d, J = 5.0 Hz, 4H), 7.95 (dd, J = 7.8, 6.3 Hz, 4H), 7.84–7.62 (m, Yield 65% (36.5 mg), mp > 300 °C; UV/vis (CHCl3): λmax (ε,
1
6H). UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 417 L Mol−1 cm−1) nm = 417 (380 550), 533 (265 400) nm; H NMR
(457 000), 534 (32 600) nm; the spectral data of 5cNi were in (400 MHz, CDCl3) δ 8.80 (t, J = 4.3 Hz, 4H), 8.78 (d, J = 1.9 Hz,
good agreement with the literature.29
1H), 8.76 (s, 1H), 8.71 (d, J = 4.9 Hz, 2H), 8.26 (d, J = 8.0 Hz,
(b) Synthesis of [5-(benzoxazol-2′-yl)-10,15,20-triphenylpor- 1H), 8.01 (d, J = 6.0 Hz, 2H), 7.84 (d, J = 7.8 Hz, 1H), 7.76–7.68
phyrinato]nickel(II) 7aNi. A microwave vial containing a mag- (m, 10H), 7.57 (d, J = 6.7 Hz, 1H), 7.50–7.47 (m, 2H), 7.43 (dd,
netic stir bar was charged with 5aNi (0.069 mmol, 46.5 mg), 6a J = 6.0, 3.5 Hz, 2H), 7.25 (d, J = 7.7 Hz, 1H), 6.71 (t, J = 7.6 Hz,
(0.083 mmol, 9.88 mg), cesium carbonate (0.1386 mmol, 2H), 6.32 (d, J = 8.4 Hz, 2H), 4.84 (s, 2H) ppm; 13C NMR
45 mg), Pd(OAc)2 (10 mol%), CuI (10 mol%) and DMF (100 MHz, CDCl3) δ 153.2, 151.7, 143.5, 142.8, 142.6, 140.7,
(2.5 mL). The reaction mixture was irradiated with focused 140.5, 136.1, 135.0, 133.8, 133.7, 132.8, 132.4, 131.9, 130.8,
MW (100 W) at 120 °C for 10 min. The progress of the reaction 128.4, 128.1, 128.0, 127.0, 126.9, 119.9, 116.0, 115.1, 110.4,
was monitored by TLC. Upon completion of the reaction, the 103.7, 99.1, 48.1 ppm; IR (neat, ATR) νmax (cm−1) = 3010, 2919,
contents were filtered over silica gel and washed with chloro- 2376, 2376, 2291, 1751, 1696, 1620, 1620, 1535, 1450, 1450,
form (50 mL). The filtrate was concentrated using a rotary 1342, 1249, 1157, 1095, 1010, 802, 702, 646, 470; MALDI-MS
evaporator and the residue was purified by column chromato- m/z calcd for C52H34ClN6Ni [M + 2H]+: 836.1665; found
graphy using chloroform : hexane : triethylamine (20 : 80 : 1 to 836.5734.
50 : 50 : 1) as an eluent to afford the pure product 7aNi. Yield
[5-(1′,3′,7′-Trimethyl-xanthin-8′yl)-10,15,20-triphenylporphyri-
73%. mp > 300 °C; UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = nato]nickel(II) 7dNi. Purple colour solid, yield 68%; mp >
418 (397 000), 535 (35 000) nm; 1H NMR (400 MHz, CDCl3) 300 °C; UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 420
δ 9.44 (d, J = 5.1 Hz, 2H), 8.84 (d, J = 5.1 Hz, 2H), 8.72 (d, J = (402 700), 532 (35 150), 559 (17 500) and 550–600 nm boarding
1
4.9 Hz, 2H), 8.68 (d, J = 4.9 Hz, 2H), 8.14–8.12 (m, 1H), of the Q-band; H NMR (400 MHz, CDCl3) δ 9.96 (s, 1H), 9.20
8.03–7.95 (m, 6H), 7.82–7.81 (m, 1H), 7.72–7.67 (m, 9H), (d, J = 4.4 Hz, 2H), 8.97–8.89 (m, 4H), 8.77 (d, J = 4.9 Hz, 3H),
7.58–7.50 (m, 2H) ppm; 13C NMR (100 MHz, CDCl3) δ 164.4, 7.78 (br, 10H), 7.56 (d, J = 8.5 Hz, 1H), 7.37 (d, J = 2.0 Hz, 1H),
152.2, 143.5, 142.8, 142.3, 142.2, 140.5, 140.4, 133.9, 133.7, 7.31 (s, 1H), 3.88 (s, 3H), 3.61 (s, 6H) ppm; 13C NMR
133.6, 132.9, 132.2, 132.1, 128.0, 127.0, 126.9, 120.9, 119.9, (100 MHz, CDCl3) δ 154.8, 151.4, 150.6, 147.1, 142.6, 141.9,
111.0, 101.6 ppm; IR (neat, ATR) νmax (cm−1) = 3160, 3020, 141.7, 141.6, 139.4, 132.9, 132.7, 131.8, 131.8, 127.00, 125.9,
2956, 2360, 2260, 1951, 1720, 1597, 1437, 1720, 1597, 1437, 118.6, 107.6, 105.6, 101.1, 32.9, 28.9 ppm; IR (neat, ATR) νmax
1360, 1273, 995, 794, 694, 563; MALDI-MS m/z calcd for (cm−1) = 3120, 2930, 2885, 2500, 2144, 2013, 1751, 1697, 1620,
C45H28N5NiO [M + H]+: 712.1647; found 712.1392.
1522, 1396, 1219, 1096, 1026, 948, 810, 771, 632; MALDI-MS
[5-(6′-Methylbenzoxazol-2′-yl)-10,15,20-triphenylporphyrinato] m/z calcd for C46H33N8NiO2 [M + H]+: 787.2080; found
nickel(II) 7bNi. Prepared by following the same procedure as 787.1574.
for 7aNi and 7bNi was isolated as a purple solid. Yield 70%;
[5-(Imidazo[1,2-a]pyridin-3′yl)-10,15,20-triphenylporphyrinato]
mp > 300 °C; UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 419 nickel(II) 7eNi. Prepared by following the same procedure as for
1
(377 500), 534 (27 500) nm; H NMR (400 MHz, CDCl3) δ 9.40 7cNi and 7eNi was isolated as a purple solid. Yield 70%; mp >
(d, J = 5.1 Hz, 2H), 8.84 (d, 5.0 Hz, 2H), 8.78–8.70 (m, 2H), 300 °C; UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 418
8.68 (d, J = 4.9 Hz, 2H), 8.00–7.98 (m, 7H), 7.71–7.62 (m, 10 H), (377 450), 532 (35 000), 559 (17 000) nm; 1H NMR (400 MHz,
7.36 (d, J = 8.2 Hz, 1H), 2.61 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ 9.94 (s, 1H), 9.59 (s, 1H), 9.21 (dd, J = 4.8 Hz, 2H),
CDCl3) δ 163.7, 143.5, 142.8, 142.2, 140.4, 136.0, 133.6, 132.8, 9.16 (s, 1H), 8.99 (d, J = 4.8 Hz, 1H), 8.91 (d, 4.8 Hz, 1H), 8.95
132.1, 130.9, 128.8, 127.9, 126.9, 121.1, 120.2, 119.8, 111.1, (d, J = 4.7 Hz, 1H), 8.85 (d, J = 4.8 Hz, 1H), 8.13–8.00 (m, 5H),
101.8, 21.9 ppm; IR (neat, ATR) νmax (cm−1) = 3165, 3023, 2959, 7.93 (d, J = 9.1 Hz, 1H), 7.82 (dd, J = 8.1, 1.7 Hz, 2H), 7.78–7.66
2361, 2265, 1960, 1725, 1600, 1440, 1597, 1437, 1375, (m, 6H), 7.36–7.32 (m, 1H), 7.07–6.93 (m, 3H), 6.72–6.69 (m,
1273, 998, 795, 691, 562; MALDI-MS m/z calcd for 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 161.9, 143.5, 143.4,
C46H30N5NiO [M + H]+: 726.1804; found 726.1558.
143.2, 143.0, 142.9, 142.7, 141.8, 140.7, 140.6, 140.3, 134.2,
[5-(1-(4-Chlorobenzyl)-benzo[d]imidazole-2′-yl)-10,15,20-tri- 133.9, 133.8, 133.7, 132.9, 132.8, 132.7, 132.6, 132.5, 132.4,
phenylporphyrinato]nickel(II) 7cNi. A microwave vial contain- 132.3, 128.3, 127.9, 127.8, 127.5, 127.1, 126.9, 125.7, 124.2,
ing a magnetic stir bar was charged with 5aNi (45.6 mg, 118.9, 118.6, 117.6, 115.4, 105.3, 104.1 ppm; IR (neat, ATR)
0.0693 mmol), 6c (84 mg, 0.345 mmol), potassium carbonate νmax (cm−1) = 3012, 2954, 2916, 2846, 2384, 2291, 2074, 1743,
(23.90 mg, 0.1732 mmol), Pd(OAc)2 (10 mol%), CuI (10 mol%) 1697, 1635, 1535, 1460, 1342, 1257, 1010, 981, 802, 632;
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MALDI-MS m/z calcd for C45H29N6Ni [M + H]+: 711.1807; IR (neat, ATR) νmax (cm−1) = 3161, 2951, 2916, 2846, 1735, 1612,
found 711.1891. 1573, 1465, 1315, 1148, 1080, 1010, 964, 802, 604; MALDI-MS
[5-(2′-Phenylimidazo[1,2-a]pyridin-3′-yl)-10,15,20-triphenyl- m/z calcd for C48H33N5NiO [M]+: 751.1882; found 751.2351.
porphyrinato]nickel(II) 7fNi. Purple solid: yield 61%; mp >
[5-{5′-(2-Nitrophenyl)-oxazol-2′-yl}-10,15,20-triphenylporphyr-
300 °C; UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 418 inato]nickel(II). Faint green solid: yield 60%; mp > 300 °C;
(366 000), 533 (41 500), 556 (42 500) nm; 1H NMR (400 MHz, UV/vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 418 (361 667), 530
1
CDCl3) δ 9.82 (s, 1H), 9.11 (d, J = 4.7 Hz, 2H), 8.88 (d, J = (26 000) nm; H NMR (400 MHz, CDCl3) δ 9.85 (s, 1H), 9.39 (d,
4.7 Hz, 2H), 8.81 (d, J = 4.8 Hz, 2H), 8.77–8.72 (m, 2H), J = 3.9 Hz, 2H), 9.14 (d, J = 3.7 Hz, 2H), 8.94 (dd, J = 14.1, 4.3
8.77–8.64 (m, 2H), 8.46 (br, 1H), 7.99 (br, 5H), 7.76–7.68 (m, Hz, 2H), 8.87 (d, J = 4.0 Hz, 2H), 8.05 (d, J = 5.8 Hz, 2H), 7.98
10H), 7.49–7.47 (m, 2H), 7.38–7.33 (m, 2H), 6.62 (t, J = 6.7 Hz, (d, J = 8.0 Hz, 2H), 7.88 (d, J = 8.0 Hz, 2H), 7.77–7.66 (m, 8H),
1H) ppm; 13C NMR (100 MHz, CDCl3) δ 170.8, 143.5, 143.4, 7.58–7.52 (m, 3H), 7.32 (d, J = 2.4 Hz, 1H), 7.09 (dd, J = 8.2, 2.4
143.3, 142.9, 142.8, 140.5, 140.4, 133.8, 133.7, 133.0, 132.9, Hz, 1H), 6.62 (d, J = 8.2 Hz, 1H) ppm; 13C NMR (100 MHz,
130.8, 127.0, 119.9, 119.8, 119.4, 117.3, 115.6, 113.4, 106.1, CDCl3) δ 147.6, 143.4, 142.6, 140.5, 139.3, 134.0, 133.9, 133.8,
100.0, 99.6 ppm; IR (neat, ATR) νmax (cm−1) = 3012, 2954, 2916, 133.7, 132.8, 132.7, 132.6, 132.6, 131.7, 130.07, 129.97, 129.87,
2384, 2291, 2074, 1743, 1697, 1635, 1535, 1460, 1342, 1010, 128.1, 128.0, 127.9, 127.0, 126.9, 124.6, 124.7, 124.5, 123.5,
981, 802, 632; MALDI-MS m/z calcd for C51H33N6Ni [M + H]+: 121.9, 119.4, 115.6, 114.1 ppm; IR (neat, ATR) νmax (cm−1)
=
787.2120; found 787.1453.
3363, 2985, 2831, 2250, 1728, 1540, 1458, 1373, 1249, 1103,
[5-(2′-Phenyl-1,3,4-oxadiazol-5′-yl)-10,15,20-triphenylporphyr- 817, 748, 663; MALDI-MS m/z calcd for C47H28N6NiO3 [M]+
inato]nickel(II) 7gNi. Purple solid: yield 75%; mp > 300 °C; UV/ 782.1576; found 782.9997
vis (CHCl3): λmax (ε, L Mol−1 cm−1) nm = 418 (402 950), 535
[5-(2-Thienyl)-10,15,20-triphenylporphyrinato]nickel(II) 7kNi.
1
(35 050) nm; H NMR (400 MHz, CDCl3) δ 9.39 (d, J = 5.1 Hz, Purple solid; yield 62%; mp > 300 °C; UV/vis (CHCl3): λmax
1
2H), 8.86 (d, J = 5.1 Hz, 2H), 8.74 (d, J = 4.9 Hz, 2H), 8.70 (d, J = (relative intensity) = 421 (0.631), 545 (0.065), 588 nm; H NMR
4.9 Hz, 2H), 8.31 (d, J = 7.4, 2H), 8.02–7.98 (m, 6H), 7.70 (m, (400 MHz, CDCl3) δ 9.31 (d, J = 4.9 Hz, 1H), 9.07 (d, J = 5.0 Hz,
8H), 7.58–7.60 (m, 4H) ppm; 13C NMR (100 MHz, CDCl3) 2H), 8.82–8.80 (m, 3H), 8.54–8.56 (m, 6H), 8.08–806. (m, 1H),
δ 166.5, 166.0, 143.6, 142.5, 142.3, 140.3, 137.6, 134.2, 133.7, 8.03 (d, J = 5.1 Hz, 1H), 7.85–7.78 (m, 1H), 7.71–7.61 (m, 8H),
133.6, 133.0, 132.4, 132.0, 131.5, 130.9, 130.0, 129.3, 129.1, 7.47–7.49 (d, J = 1.8 Hz, 1H) ppm;13C NMR (100 MHz, CDCl3)
128.9, 128.3, 128.0, 127.2, 127.1, 127.0, 126.9, 124.1, 123.5, δ 163.7, 143.5, 142.8, 142.2, 140.4, 136.0, 133.6, 132.8, 132.1,
121.6, 120.1, 97.4 ppm; IR (neat, ATR) νmax (cm−1) = 3120, 130.9, 128.8, 127.9, 126.9, 126.1, 121.1, 120.2, 119.8, 111.1,
3110, 2956, 2360, 1951, 1805, 1597, 1442, 1327, 1257, 995, 101.8 ppm; IR (neat, ATR) νmax (cm−1): 3445, 2925, 2362, 1630,
1027, 995, 884, 786, 748, 694, 104; MALDI-MS m/z calcd for 1520, 1480, 1430, 790, 1011; MALDI-MS m/z calcd for
C46H29N6NiO [M + H]+: 739.1756; found 739.1260.
C42H26N4NiS [M]+: 676.1232; found 676.0805.
[5-{(5′-Phenyl)-oxazol-2′-yl}-10,15,20-triphenylporphyrinato]-
(C) Synthesis of [5,15-bis(benzoxazolyl)-10,20-diphenylpor-
nickel(II) 7hNi. Purple solid: yield 63%; mp > 300 °C; UV/vis phyrinato]nickel(II) 7lNi. A microwave vial containing a mag-
(CHCl3): λmax (ε, L Mol−1 cm−1) nm = 417 (458 667), 540 netic stir bar was charged with 5cNi (50 mg, 0.073 mmol), 6a
1
(36 000) nm; H NMR (400 MHz, CDCl3) δ 8.77 (d, J = 4.9 Hz, (21.97 mg, 0.184 mmol), cesium carbonate (45 mg,
2H), 8.66 (d, J = 4.9 Hz, 2H), 8.02 (t, J = 7.2 Hz, 3H), 7.85–7.63 0.184 mmol), Pd(OAc)2 (20 mol%), CuI (10 mol%) and DMF
(m, 5H), 7.65–7.49 (m, 5H), 7.53–7.42 (m, 3H), 7.41–7.33 (m, (2.5 mL). The reaction mixture was irradiated with focused
2H), 7.20–7.25 (m, 2H), 7.08–6.82 (m, 4H), 6.57–6.62 (m, 1H) MW (100 W) at 120 °C for 15 min. The progress of the reaction
ppm; 13C NMR (100 MHz, CDCl3) δ 155.2, 143.2, 142.9, 142.6, was monitored by TLC. Upon completion of the reaction, the
142.0, 140.9, 136.5, 133.8, 132.9, 132.4, 132.3, 131.2, 130.8, contents were filtered over silica gel and washed with chloro-
130.4, 128.3, 127.8, 126.7, 125.7, 118.9, 114.4, 114.2, 114.1, form (50 mL). The filtrate was concentrated using a rotary
113.4, 110.7, 105.7 ppm; IR (neat, ATR) νmax (cm−1) = 3363, evaporator and the obtained residue was purified by column
2985, 2831, 2250, 1728, 1458, 1373, 1219, 1103, 817, 748, 663; chromatography using chloroform : hexane (9 : 1) as an eluent
MALDI-MS m/z calcd for C47H30N5NiO [M]+: 737.1724; found to afford pure 7lNi. Yield 64%, mp > 300 °C; UV/vis (CHCl3):
737.1238.
λmax (ε, L Mol−1 cm−1) nm = 420 (270 000), 535 (35 000) nm; 1H
[5-{(5′-p-Tolyl)-oxazol-2′-yl}-10,15,20-triphenylporphyrinato]- NMR (400 MHz, CDCl3) δ 9.44 (d, J = 5.1 Hz, 2H), 8.84 (d, J =
nickel(II) 7iNi. Purple solid, yield 65%, mp > 300 °C; UV/vis 5.1 Hz, 2H), 8.72 (d, J = 4.9 Hz, 2H), 8.68 (d, J = 4.9 Hz, 2H),
(CHCl3): λmax (ε, L Mol−1 cm−1) nm = 417 (305 667), 543 (2500) 8.14–8.12 (m, 1H), 8.03–7.95 (m, 4H), 7.82–7.81 (m, 1H),
nm; 1H NMR (400 MHz, CDCl3) δ 9.86 (s, 1H), 9.44 (d, J = 7.72–7.67 (m, 6H), 7.58–7.50 (m, 2H) ppm; 13C NMR (100 MHz,
4.7 Hz, 2H), 9.15 (d, J = 4.4 Hz, 2H), 8.95–8.85 (m, 4H), 8.06 (d, CDCl3) δ 164.4, 152.2, 143.5, 142.8, 142.3, 142.2, 140.5, 140.4,
J = 6.2 Hz, 3H), 7.94 (d, J = 7.7 Hz, 2H), 7.82–7.66 (m, 10H), 133.9, 133.7, 133.6, 132.9, 132.2, 132.1, 128.0, 127.0, 126.9,
7.54 (d, J = 7.4 Hz, 2H), 7.32 (d, J = 7.8 Hz, 3H), 2.44 (s, 3H) 120.9, 119.9, 111.0, 101.6 ppm; IR (neat, ATR) νmax (cm−1) =
ppm; 13C NMR (100 MHz, CDCl3) δ 154.9, 143.1, 142.8, 142.5, 3160, 3023, 2960, 2362, 1955, 1720, 1597, 1725, 1598, 1437,
142.3, 140.9, 140.8, 136.5, 133.8, 133.7, 133.0, 132.3, 132.0, 1363, 1273, 995, 794, 694, 563; MALDI-MS m/z calcd for
131.4, 130.9, 130.3, 128.3, 127.8, 127.7, 127.7, 126.9, 125.7, C48H28N8NiO2 [M + H]+: 753.2333; found m/z (calcd for m/z
120.0, 119.2, 114.4, 114.2, 114.0, 112.9, 110.7, 29.7 ppm; 753.4474).
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Conflicts of interest
There are no conflicts to declare.
8 (a) S. G. DiMagno, V. S. Lin and M. J. Therien, J. Am. Chem.
Soc., 1993, 115, 2513–2515; (b) S. G. DiMagno, V. S. Lin and
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Acknowledgements
S. B. K. gratefully acknowledges the financial support from the
Birla Institute of Technology and Science, Pilani. We also 10 D. T. Gryko and M. Tasior, Tetrahedron Lett., 2003, 44,
acknowledge the financial support provided by the 3317–3321.
Department of Science and Technology, New Delhi, for a 11 (a) G.-Y. Gao, A. J. Colvin, Y. Chen and X. P. Zhang, J. Org.
research grant (DST/INT/JSPS/P-185/2014).
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