An alternate and efficient method for preparation of 2,6-diacetamido-4-formylpyridine for formation of porphyrin appended with hydrogen bonding motifs

Article abstract of DOI:10.1080/00397911.2020.1799012

We report here an alternate redesigned five-step synthesis of 2,6-diacetamido-4-formylpyridine. The method described here is more convenient where this aldehyde is prepared in commercially viable quantities from simple and easy-to-use reagents and is a key compound required to prepare 5,10,15,20-Tetrakis(3,5-diacetamido-4-pyridyl)porphyrin (I). This porphyrin can be used as an excellent building block for the construction of supramolecular assemblies and is an interesting compound to study the design, principles, and photonic properties such as the extent of electron and energy transfer.

Full text of DOI:10.1080/00397911.2020.1799012

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
An alternate and efficient method for preparation
of 2,6-diacetamido-4-formylpyridine for formation
of porphyrin appended with hydrogen bonding
motifs
Sunaina Singh , Amit Aggarwal , Phillippe H. Mercier , N. V. S. Dinesh K.
Bhupathiraju , Michael Landress & Keroles Hanna
To cite this article: Sunaina Singh , Amit Aggarwal , Phillippe H. Mercier , N. V. S. Dinesh K.
Bhupathiraju , Michael Landress & Keroles Hanna (2020): An alternate and efficient method for
preparation of 2,6-diacetamido-4-formylpyridine for formation of porphyrin appended with hydrogen
bonding motifs, Synthetic Communications, DOI: 10.1080/00397911.2020.1799012
To link to this article: https://doi.org/10.1080/00397911.2020.1799012
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https://doi.org/10.1080/00397911.2020.1799012
An alternate and efficient method for preparation of 2,6-
diacetamido-4-formylpyridine for formation of porphyrin
appended with hydrogen bonding motifs
Sunaina Singha, Amit Aggarwala, Phillippe H. Merciera, N. V. S. Dinesh K.
Bhupathirajub, Michael Landressa, and Keroles Hannaa
aDepartment of Natural Sciences, LaGuardia Community College of the City University of New York,
b
Long Island City, NY, USA; Department of Chemistry, Hunter College of the City University of New
York, New York, NY, USA
ABSTRACT
ARTICLE HISTORY
Received 3 May 2020
We report here an alternate redesigned five-step synthesis of 2,6-
diacetamido-4-formylpyridine. The method described here is more
convenient where this aldehyde is prepared in commercially viable
quantities from simple and easy-to-use reagents and is a key com-
pound required to prepare 5,10,15,20-Tetrakis(3,5-diacetamido-4-pyri-
dyl)porphyrin (I). This porphyrin can be used as an excellent building
block for the construction of supramolecular assemblies and is an
interesting compound to study the design, principles, and photonic
properties such as the extent of electron and energy transfer.
KEYWORDS
Pyridine; porphyrins;
supramolecular
GRAPHICAL ABSTRACT
Introduction
Photofunctional materials can be fabricated by supramolecular chemistry where the
well-defined molecular assemblies can be constructed either by specific (such as metal
ion coordination and H-bonding) and/or nonspecific (such as van der Waal’s and other
electrostatic forces) intermolecular interactions. These two types of intermolecular inter-
actions are responsible for the formation of their: (a) self-assembled structures that leads
to the formation of discrete, highly ordered systems wherein there is only little tolerance
CONTACT Sunaina Singh
ssingh@lagcc.cuny.edu
Department of Natural Sciences, LaGuardia Community College
of the City University of New York, Long Island City, NY 11101, USA
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

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S. SINGH ET AL.
of error and (b) self-organized structures that are non-discrete, non-directional and are
more tolerant of errors.[1–4] Both of these self-processes are dynamic in nature and
highly relies on the molecular conformation and functional groups attached to the com-
ponent molecules.[4]
The ubiquitous chemical, photophysical and electron transfer properties of the
porphyrins and metalloporphyrins makes them an excellent building blocks for the
construction of their supramolecular assemblies. These properties can further be easily
fine-tuned in several ways. For example, substitution with various electron donating/
withdrawing groups or bulky groups at the macrocycle periphery and/or metalation of
macrocycle distort the otherwise planar structure of the macrocycle for material applica-
tions. The different redox potential of core metal ion may also allow these systems to
be used in electron transport and energy transduction and can have applications in
optoelectronics.[1,3,5–8]
The rigid structure of porphyrins can also be distorted by appending hydrogen-bond-
ing motifs at the meso positions, which can self-assemble into their supramolecular sys-
tems to enhance their optoelectronic properties. There are several reports on the
synthesis and applications of porphyrinoids bearing various H-bonding motifs.[9–11]
However, the substituents bearing N-donor atoms are of great interest as these groups
provide sites for metal coordination, H-bonding, and also to improve the hydrophilicity
of the porphyrin macrocycle.
The tessellation of 2-dimensional arrays mediated by complementary H-bonding
groups on two different porphyrins results in a checkerboard pattern wherein donor
chromophores and acceptor chromophores alternate. These types of systems promote
electron transfer and are of particular interest because of their relevance to light har-
vesting arrays. These porphyrin arrays can be deposited on the glass surface where the
domain size and defect density are strongly influenced by the surface energetics, solvent,
and rate of solvent evaporation.[6,9] By adjusting the number of H-bonds and their rela-
tive orientation, supramolecular assemblies formed by porphyrins through hydrogen
bonds can be tuned.[12–15] For these reasons, 5,10,15,20-Tetrakis(3,5-diacetamido-4-pyri-
dyl)porphyrin (I) (Figure 1) is an interesting compound to study the design, principles,
and photonic properties such as the extent of electron and energy transfer in supra-
molecular systems. However, the synthesis of the diacetamidopyridylporphyrin (I) is
challenging because of the difficulty in preparing the aldehyde precursor, 2,6-diaceta-
mido-4-formylpyridine (6) due to the significant electronic effects at the 4 position.[6,9]
In the previously reported six step method to form 6,[7] the first step involves the con-
version of 4-methylpyridine to 2,6-diamino-4-methylpyridine using tetralin and soda-
mide. This compound was further treated with strong oxidant peracetic acid at elevated
temperature to yield 2,6-diacetamido-4-methylpyridine-1-oxide. Refluxing of this oxide
in acetic anhydride to yield 2-diacetamido-6-acetamido-4-acetoxymethylpyridine, that
was further hydrolyzed using K2CO3 in methanol/water to afford 2,6-diacetamido-4-
hydroxymethylpyridine. This compound was oxidized using PCC under ultrasound con-
ditions. Using this procedure, we were not able to isolate 2,6-diamino-4-methylpyridine
in significant amount despite using freshly dried tetralin, inert atmosphere and pure
reagents thus resulting in very low overall yield of 2,6-diacetamido-4-formylpyridine.
We found this strategy practically not efficient.

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Figure 1. Structure of the rigid coplanar dyad, assembled by three-point hydrogen bonding which
may mediate electron transfer processes, can be formed by mixing 5,10,15,20-Tetrakis(3,5-diaceta-
mido-4-pyridyl)porphyrin (I)and napthalene diimide (NDI) in different aprotic solvents.
Scheme 1. Synthesis of 2,6-diacetamido-4-formylpyridine 6.
So we embarked upon developing a simple method for the synthesis of precursor 2,6-
diacetamido-4-formylpyridine (6) which can be produced in fewer steps, better yield,
prevents the use of peracetic acid at high temperature and is more convenient that can
avoid much of the problems associated with the earlier synthesis.
Results and discussion
The method reported herein (Scheme 1) involves conversion of commercially available cit-
razinic acid 1 to the 2,6-dichloroisonicotinic acid 2 in 77% yield by treatment with phos-
phorous oxychloride and tetramethylammonium chloride.[16,17] Compound 2 was reacted
with ammonia in presence of copper catalyst, at high temperature and pressure (ꢀ180 ꢁC,
20 bar). The copper powder was filtered out from the solution and the compound was pre-
cipitated from the filtrate by treatment with an acid to pH 5. The overall yield of the free

4
S. SINGH ET AL.
Scheme 2. Synthesis of porphyrin I.
acid, 2,6-diaminopyridine-4-carboxylic acid 3 was about 83%.[18] The compound 3 was
then treated with acetic anhydride at refluxing conditions resulting in compound 4 which
was filtered and washed with water several times, to yield 71% of 2,6-diacetamido-4-pyridi-
necarboxylic acid. Compound 4 was then treated with triphosgene to form an acid chlor-
ide, followed by addition of N,O-dimethylhydroxylamine to yield the desired Weinreb
amide 5 in about 79% yield.[19] This is a versatile method for direct conversion of carbox-
ylic acids to the corresponding Weinreb amides. The direct conversion relies on in situ
activation of carboxylic acids with N,O-dimethylhydroxylamine.[20–22] Reduction of
Weinreb amide 5 to aldehyde 6 was obtained in 92% yield by treatment with DIBAL-H
(see ESI).[23] The overall yield of this aldehyde starting from compound 1 is 33% which is
reasonable since the starting compounds are inexpensive and yield is higher than the pre-
viously reported method where the overall yield reported was 17%.[7]
The aldehyde 6 prepared here was not soluble in solvents ideal for Lindsey
method[24–26] conditions, therefore, we only could use Adler method for the preparation
of porphyrin I.[27] Pyrrole and 2,6-diacetamido-4-formylpyridine (6) were added in 1:1
ratio to boiling propionic acid and the reaction mixture was refluxed for 2 h (Scheme 2).
After purification the free base porphyrin I was obtained in a yield of about 29%.
The porphyrin I synthesized and reported here can form a supramolecular assembly
with naphthalene diimide (NDI) (Figure 1). In this system, the porphyrin can act as a
donor and NDI as an acceptor. Having NDI motif directly attached at the meso pos-
ition of a diacetamidoporphyrin provides a suitable through-bond pathway for electron
transfer since it reduces the conformational flexibility of the assembly.[28] Naphthalene
diimides are electron deficient species and possesses unique photophysical, electronic
[29,30]
properties and enhanced solubility
and therefore can be used as an ideal candi-
dates in the design of conducting materials.[31,32]
Experimental procedure
Materials and methods
General
1H and 13C NMR spectra were recorded in a Bru€ker Avance 500 MHz spectrometer.
Electrospray ionization mass spectrometric analyses were performed at the CUNY Mass

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Spectrometry Facility at Hunter College using an Agilent Technologies HP-1100 LC/
MSD instrument. The electrospray ionization was run in methanol, with 0.1% formic
acid. UV-visible spectra were recorded on a Varian Bio3 spectrophotometer. All
reagents were obtained from commercial sources and used without further purification.
Flash column chromatography was performed using silica gel-60, and the analytical
TLC was carried out on precoated sheets with silica gel (0.2 mm thick), both from
Sorbent Technologies.
Synthesis of 5,10,15,20-tetrakis(3, 5-diacetamido-4-pyridyl)porphyrin (I)
Compound 1 was purchased from Sigma Aldrich Co. Compounds 2 (2,6-dichloro-4-
pyridinecarboxylic acid) and 3 (2,6-diamino-4-pyridinecarboxylic acid) were synthesized
by previously described procedures.[16–18]
2,6-Diacetamido-4-pyridinecarboxylic acid (4)
10.0 mL of acetic anhydride was added to 2.0 g (13.0 mmol) of the diamine 3. The
resultant solution was stirred for 4 h at refluxing conditions and 2.2 g (71%) of a brown
1
amorphous solid 4 was collected by filtration, washed with water, mp 195-197 ꢁC. H
NMR (DMSO-d6) d13.53 (s, 1H), 10.28 (s, 2H), 8.21 (s, 2H), 2.13 (s, 6H); 13C NMR
(DMSO-d6) d170.03, 166.64, 151.57, 142.50, 108.77, 24.53; HRMS calcd. for
C10H12N3O4 (M þ H)þ 238.0828, found, 238.0825.
2,6-Diacetamido-4-pyridine-N-methoxy-N-methyl amide (5)
To a solution of the carboxylic acid (4) 0.5 g (2 mmol) in CH2Cl2 (10 mL) at 0 ꢁC were
added triphosgene 0.3 g (1 mmol) and triethylamine 0.15 mL (10 mmol). The N,O-dime-
thylhydroxylamine hydrochloride 0.2 g (2 mmol) was added to the above solution and
the ice bath was removed. The reaction mixture was then stirred at room temperature
and monitored by TLC. After completion of reaction, the triethylamine was removed by
filtration. The filtrate was collected and the solvent was removed under reduced pres-
sure. The solid obtained was subjected to column chromatography on flash silica gel,
eluted with dichloromethane/methanol 9:1, 0.44 g of product was obtained in 79% yield,
1
mp >300 ꢁC. H NMR (MeOD-d4) d7.93 (s, 2H), 3.65 (s, 3H), 3.31 (s, 3H), 2.17 (s,
6H); 13C NMR (MeOD-d4) d170.64, 150.39, 146.17, 106.91, 60.59, 56.07, 22.65; HRMS
calcd. for C12H16N4O4 (Mþ) 280.1176, found 280.1172.
2,6-Diacetamido-4-formylpyridine (6)
To a solution of compound 5 (0.5 g, 1.78 mmol) in CH2Cl2 (65 mL) was added DIBALH
(1 M in THF, 14 ml, 14 mmol) at ꢂ78 ꢁC, and the mixture was stirred at the same tem-
perature for 2 h. After that, methanol was added to quench the DIBALH, the resulting
mixture was partitioned between CHCl3 and 1 N HCl and the organic layer was washed
with brine solution, dried with Na2SO4 and solvent was removed under reduced pres-
sure. The residue was purified by column chromatography (silica gel, ethylacetate/etha-
1
nol (9:1)) to give 360 mg, 92% of the solid, mp 222-224 ꢁC. H NMR (DMSO-d6) d10.33
(s, 2H), 9.96 (s,1H), 8.11 (s, 2H), 2.10 (s, 6H); 13C NMR (DMSO-d6) d194.1, 170.7,

6
S. SINGH ET AL.
152.6, 146.5, 108.9, 25.3; HRMS calcd. for C10H12N3O3, (M þ H)þ 222.0879,
found 222.0876.
5, 10, 15, 20-Tetrakis(3,5-diacetamido-4-pyridyl)porphyrin (I)
Pyrrole (69.5 mL, 1.0 mmol) and 2, 6-diacetamido-4-formylpyridine
6 (221 mg,
1.0 mmol) were added to boiling propionic acid (10.0 mL). The reaction mixture was
refluxed for 2 h and then dried under vacuum. The resulting solid was purified by
column chromatography on silica gel eluting with ethanol/ethylacetate (1:1) to yield
1
70 mg (29%) of porphyrin. H NMR (DMSO-d6) d10.52 (s, 8H), 8.99 (s, 8H), 8.60
(s, 8H), 2.16 (s, 24H), ꢂ3.10 (s, 2H); 13C NMR (DMSO-d6) d170.25, 152.67, 149.50,
130.12, 118.82, 115.80, 24.53. HRMS calcd. for C56H50N16O8 (M þ H)þ1075.4076,
found 1075.4043; UV/visible (DMSO) kmax¼ 422.6, 514.8, 549.0, 587.2 and 642.3 nm
Conclusions
We have developed a new method for the synthesis of 2,6-diacetamido-4-formylpyri-
dine, a key molecule to form porphyrin I bearing rigid H-bonding moieties at the meso
positions. This protocol provides a practical alternative and improved synthetic route to
the previously reported method[7] for the synthesis of this aldehyde and avoids much of
the problems associated with the reported synthetic method. The meso-substituted pyr-
idyl porphyrin can be used for constructing supramolecular structures with molecules
bearing hydrogen bonding motifs through hetero-complementary assembly where por-
phyrin can act as a donor and other molecules can serve as an acceptor. These supra-
molecular materials are expected to have unique optical and physical properties e.g.,
redox and optoelectronic properties. Further studies of this H-bonded porphyrin- naph-
thalene diimide systems for their electron transfer properties are currently under
investigation.
Acknowledgments
AA, SS, ML, and KH acknowledge CUNY Research Scholar Program (CRSP) for their support to
this project.
Funding
AA (PSC-CUNY Award #62251-00 50) and SS (PSC-CUNY Award # 61356-00 49) would like to
thank Professional Staff Congress (PSC) -CUNY Research Award Program to support this work.
Dedication
This work is dedicated to the memory of Dr. Charles Michael Drain for being a wonderful men-
tor, advisor and colleague through the years.

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