Synthesis and characterization of the first examples of ferrocene and [η5-CpCo(η4-C4Ph4)] derived 2-pyridones

Article abstract of DOI:10.1016/j.inoche.2013.07.012

Reactions of ferrocene and (η⁵-cyclopentadienyl) (η⁴-tetraphenylcyclobutadiene)cobalt derived alkynylaldehydes with p-anisidine gave the corresponding metal sandwich based alkynylimines, 4-methoxy-N-(3-ferrocenylprop-2-ynylidene)anil

Full text of DOI:10.1016/j.inoche.2013.07.012

Inorganic Chemistry Communications 35 (2013) 346–350
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of the first examples of ferrocene and
[η5-CpCo(η4-C4Ph4)] derived 2-pyridones
⁎
Karunesh Keshav, Anil J. Elias
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Reactions of ferrocene and (η5-cyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt derived alkynylaldehydes
with p-anisidine gave the corresponding metal sandwich based alkynylimines, 4-methoxy-N-(3-ferrocenylprop-
2-ynylidene)aniline and 4-methoxy-N-[3-(η5-cyclopentadienyl)(η4-tetra-phenylcyclobutadiene)cobalt prop-2-
ynylidene]aniline. The 1,4-nucleophilic addition of sodium methylmalonate with these alkynylimines gave the
first examples of metal sandwich substituted 2-pyridones, [methyl 1-(4-methoxyphenyl)-2-oxo-4-ferrocenyl
1,2-dihydropyridine-3-carboxylate and methyl 1-(4-methoxyphenyl) 2-oxo-4-(η5-cyclopentadienyl)(η4-
tetra-phenylcyclobutadiene)cobalt 1,2-dihydropyridine-3-carboxylate in good yields. Reaction of sodium
ethylmalonate with the ferrocene derived alkylimine gave the stable ferrocene derived 2-pyridone, ethyl 1-(4-
methoxyphenyl)-2-oxo-4-ferrocenyl 1,2-dihydropyridine-3-carboxylate which was also structurally characterized.
© 2013 Elsevier B.V. All rights reserved.
Article history:
Received 10 May 2013
Accepted 20 July 2013
Available online 1 August 2013
Keywords:
Ethynyl ferrrocene
Pyridone
Alkynyl aldehyde
Cobalt sandwich
A major development in the area of bio-organometallic chemistry
has been the introduction stable organometallic units onto a drug mol-
ecule thereby enhancing its activity many-fold. The most well known
examples for such compounds are ferroquine and ferrocifen wherein
ferrocenyl replaces an alkyl or aryl unit of a known drug. Ferroquine,
an organometallic variant of the antimalarial drug chloroquine, wherein
an alkyl fragment of chloroquine has been replaced by ferrocene was
found to be very efficient even against chloroquine resistant malaria.
Ferrocifen, another drug molecule where ferrocenyl group replaces an
aryl group of the well known breast cancer drug tamoxifen has shown
enhanced activity than the latter [1,3]. 2-Pyridones, a well known
class of organic heterocycles are an important constituent of many
drugs and biologically active natural products [4]. Due to their dual
properties as an aromatic compound and an amide, they have the ten-
dency to interact and bind with many biological sites [2]. Fig. 1 lists a
few of the well known biologically active 2-pyridone derivatives
which are multidrug resistance (MDR) modulators [4], antileukemic
and antitumor agents [6,7] as well as acetyl cholinesterate inhibitors
[8]. The effectiveness of improving the properties and activity of many
drugs as well as the use of such molecules as organometallic tags
prompted us to attempt the synthesis of air and moisture stable organ-
ometallic sandwich derivatives of 2-pyridones.
(η5-cyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt, η5-CpCo
(η4-C4Ph4) prompted us to use these sandwich compounds in this
study. Unlike ferrocene, direct derivatization of the sterically bulky
η5-CpCo(η4-C4Ph4) has been found to be difficult [9]. However, by
using substituted sodium cyclopentadienyls as precursors in the sand-
wich synthesis, we as well as others have reported methods to prepare
a range of potential derivatives of η5-CpCo(η4-C4Ph4) [10–13]. As ethy-
nyl ferrocene and ethynyl derived η5-CpCo(η4-C4Ph4) are air and water
stable compounds which can be synthesized in a few steps [14,15], we
were keen to see the possibility of using these compounds as precursors
for realizing metal sandwich derived alkynyl imines.
A reaction of aryl alkynyl carboxaldehyde with p-anisidine has been
found to form a Schiff's base which when further treated with dialkyl
malonate under basic conditions yielded a substituted 2-pyridone [5].
Here 1,4- addition reaction of dialkyl sodiomalonate to alkynylimine
generates an allenamine which in turn isomerizes to an enamine and fi-
nally undergoes cyclization to give the 2-pyridone (Fig. 2). We were keen
to see the possibility of using this procedure to prepare ferrocene and
the bulky cobalt sandwich compound, η5-CpCo(η4-C4Ph4) substituted
2-pyridones.
Reaction of ferrocenyl(formyl) acetylene with a p-anisidine gave a
carbinolamine intermediate which underwent dehydration to give a
substituted imine 1 in 52% yield. A similar reaction of the cobalt sand-
wich derivative of alkynyl carboxaldehyde with p-anisidine gave the
substituted alkynyl imine 2 in 67% yield (Scheme 1).
A reaction of the in-situ generated dimethyl sodiomalonate with
ethynylferrocene substituted imine 1 gave the 2-pyridone derivative 3
in 50% yield. A similar reaction of the dimethyl sodiomalonate derivative
with ethynyl cobalt sandwich derived imine 2 gave an orange-yellow
Although examples of 2-pyridones with a variety of alkyl and aryl
substituents have been reported [4,5], no examples of metal sandwich
substituted 2-pyridones are known till to date. The very high air
and moisture stability of ferrocene and the relatively lesser known
⁎
Corresponding author. Tel.: +91 11 26591504; fax: +91 11 26581102.
E-mail address: eliasanil@gmail.com (A.J. Elias).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.07.012

K. Keshav, A.J. Elias / Inorganic Chemistry Communications 35 (2013) 346–350
347
derivative. The cause of this variation is the substitution of a more elec-
tronegative carbonyl group by a less electronegative aldimine group.
Similar variations in the νC`C infrared stretching bands were observed
in the case of formation of the aldimine derivative of phenyl
propionaldehyde. While phenyl propionaldehyde shows a νC`C infra-
red stretching band at 2190 cm−1, the same has been found to shift to
2200 cm−1 after aldimine formation [5].
O
N
N
N
O
O
CN
O
1H NMR spectra of the compounds 1, 3 and 5 exhibited three sets of
peaks for the ferrocene moiety. The protons attached to the substituted
cyclopentadienyl units in these compounds were found to resonate in
the range of 4.24–4.51 ppm. A deshielded peak at 9.22 and 8.74 in
case of 3-ferrocenyl-2-propynal and [η5-C5H4(C`C)CHO]Co(η4-C4Ph4)
which was assigned as aldehydic protons, disappeared after formation
of their respective alkynyl imines 1 and 2. The proton signals for the
cyclopentadienyl units of the cobalt sandwich compounds 2, and 4
were observed as two signals in the range of 4.74–5.21 ppm. The chem-
ical shifts for the two pyridone protons in case of 5 were observed as
doublets at 6.46 ppm and 7.29 ppm, the latter overlapping with other
phenyl proton signals. Similarly, the two pyridone protons in case of
3 were observed as doublets at 6.39 ppm and at 7.23 ppm. This chemi-
cal shift is consistent with that of the analogous phenyl substituted
2-pyridone, 1-phenyl-3-methyl-4-phenyl-2(1H)-pyridone, where both
the protons on the pyridone ring resonate at 6.20 and 7.25 ppm [5]. In
contrast, to that of 3 and 5 in which one of the pyridone hydrogens over-
lap with that of the phenyl signal, the corresponding hydrogen in case of
the cobalt sandwich substituted pyridone 4 was observed as two sepa-
rate doublets at 6.66 and 6.98 ppm.
N-benzyl 2-pyridones
Perampanel
O
N
Me
H
N
N
O
O
Me
H2N
Et
O
OH
HuperzineA
Camptothecin
Fig. 1. Biologically active natural products and drugs having 2-pyridone rings.
O
O
In the 13C NMR the carbon atoms of the cyclopentadiene units of the
cobalt sandwich compounds, 2 and 4 were observed as three signals in
the range of 86.2 to 87.4 ppm. Carbon atoms of the pyridone moiety
were found to resonate in the range of 114.9 to 159.4 in case of
ferrocenyl derived pyridones 3 and 5, while it was found to resonate
in between 118.3 and 148.1 ppm in case of the cobalt sandwich
substituted 2-pyridone, 4.
NAn
Na
RO2C
Ph
RO2C
+
N
CO2R
Ph
Fig. 2. Synthesis of aryl substituted 2-pyridones by 1,4 addition reaction.
colored 2-pyridone, 4 in 48% yield (Scheme 1) [16]. However, repeated
efforts to obtain suitable crystals of 3 and 4 for single crystal X-ray struc-
tural characterization were unsuccessful. We therefore attempted the
synthesis of an analogous ethyl derivative starting from ethyl malonate
in place of methyl malonate. A 1:1 reaction of diethylmalonate with so-
dium hydride in THF as a solvent gave the diethyl sodiomalonate. Reac-
tion of this compound with the imine 1 under basic conditions gave the
cyclized 2-pyridone 5 in 54% yield (Scheme 2) [17].
Interestingly, the νC`C infrared stretching bands of the alkynyl units
of compounds 1 and 2 and its parent (formyl) acetylene derivative were
found to be highly dependent on the type of substitution around the
alkynyl unit. For example the ferrocenyl(formyl)acetylene showed a
band at 2179 cm−1 while the same vibrational band was found at
2191 cm−1 for its alkynylimine derivative 1. Similar variation for
ν(C`C) bands was also observed on formation of the cobalt sandwich
A CCDC search for structures of analogous 2-pyridones to compound
5 indicated that no similar pyridones have been structurally character-
ized so far. The only reported structure of a 2-pyridone whose features
are close to that of 5 is that of compound A (Fig. 3) in which a phenyl
group occupies the position of ferrocene of 5, a methyl group at the
sixth position replaces the ester group at the third position and the N
is bonded to a 4-methoxy phenyl group instead of a benzyl group [18].
Average C\C and C\N bond distances in the pyridone moiety of
5 were found to be 1.393(3) and 1.388(3) Å which are close to the
corresponding bond distances in case of the phenyl based pyridone
6-methyl-4-phenyl-1-(phenylmethyl)-2(1H)-pyridone where it was
observed at 1.392(2) and 1.388(2) Å [18]. The C\O bond distance of
the carbonyl group of the pyridone was found to be 1.226(3) Å which
is close to 1.243(2) Å observed in the case of the structure reported
for the phenyl based pyridone [18]. The dihedral angle between the
plane defined by all six atoms of pyridone 5 makes an angle of
substituted alkynylimine
2
from its parent (formyl) acetylene
O
N
O
CH2Cl2, r.t.
1h
O
H
R
R
+
NH2
Ph
Co
Ph
Ph
R =
Fe
Ph
1
2
Scheme 1. Synthesis of ferrocene and cobalt sandwich derived alkynyl imines 1 and 2.

348
K. Keshav, A.J. Elias / Inorganic Chemistry Communications 35 (2013) 346–350
O
O
O
R
O
R'
NaH, THF
reflux, 7 h
R'O2C
N
+
O
N
CO2R'
R
Ph
Co
Ph
Ph
Fe
Fe
R=
Ph
CH3
CH3
R'=
CH2CH3
3
4
5
Scheme 2. Synthesis of ferrocene and cobalt sandwich based pyridones 3 and 4 and 5.
the 2-pyridone ring in both these compounds is comparable. The
crystal packing diagram of the compound 5 indicated that both
oxygen atoms of carbonyl group of the pyridone and ester moiety
take part in hydrogen bonding resulting in a sheet-like motif (see
supplementary data).
In conclusion, the first examples of air and moisture stable metal
sandwich derived 2-pyridones have been synthesized by 1,4-nucleophilic
addition of dialkyl sodiomalonates with the metal sandwich based
alkynyl imines followed by cyclization. The novel organometallic
2-pyridones have been structurally characterized [19].
O
O
N
H
O
N
O
H
3CO
H
H
3C
Fe
H
H
Compound 5
Compound A
Fig. 3. Comparison of compound A with analogous compound 5.
Acknowledgments
62.1(1)° with the phenyl group in 5. The dihedral angle between the
plane defined by all five atoms of cyclopentadiene ring of the ferrocene
unit of 5 make an angle of 42.7(1)° with the plane passing through all
six ring atom of pyridone ring (Fig. 4).
This is in contrast to the structure reported for the phenyl substituted
2-pyridone where both phenyl and the pyridone units were found
to be almost planar making an angle of 13.3 (1)°. The planarity of
The authors thank the Department of Science and Technology (DST),
India and CSIR India for financial assistance in the form of research
grants to A.J.E. [DST SR/SI/IC-43/2010 and CSIR 01(2693)/12/EMR-II].
KK thanks CSIR India for the research fellowship. We thank DST-FIST
and IITD for funding of the single crystal X-ray diffraction and HRMS
facilities at IIT Delhi.
Fig. 4. X-ray structure of the compound 5. Thermal ellipsoids are drawn at the 30% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg)
with esd's in parenthesis. C(1)–C(11) 1.474(3); C(11)–C(12) 1.423(3); C(12)–C(13) 1.343(4); C(14)–O(3) 1.228(3); C(13)–N(1) 1.364(3); C(15)–C(11)–C(12) 118.1(2); C(11)–C(12)–
C(13) 119.6(2); C(13)–N(1)–C(14) 121.7(2).

K. Keshav, A.J. Elias / Inorganic Chemistry Communications 35 (2013) 346–350
349
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information. CCDC 936287 contains the supplementary crystallographic
data for 5. These data can be obtained free of charge from the Cambridge
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[16] Synthesis of 3: To a solution of dimethyl malonate (0.13 g, 1.25 mmol) in 2.0 ml of
tetrahydrofuran, 60% NaH suspension (0.03 g, 1.00 mmol) was added with constant
stirring. Then a solution of ferrocenyl alkynyl imine (0.17 g, 0.50 mmol) in 2 ml was
added to it. The reaction mixture was stirred under reflux for 7 hours and then brine
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(ν, cm−1): 1645 (C_O). Mp: 176–178 °C (dec.). Found: C, 65.11; H, 4.72; N, 3.18.
Calcd. for C24H21FeNO4: C, 65.03; H, 4.78; N, 3.16. 1H NMR (δ, 300 MHz, CDCl3):
3.76 (3H, s, CH3), 3.78 (3H, s, CH3), 4.10 (5H, s, Cp), 4.37 (2H, s, Cp), 4.51 (2H, s,
Cp), 6.38–6.40 (1H, d), 6.88–6.91 (2H, d, Ph), 7.20–7.27(3H, m). 13C{1H} NMR: δ
52.4 (OCH3), 55.5 (OCH3), 68.5, 70.1, 70.8, 71.9 (Cp), 114.3, 127.1, 136.7, 127.4,
149.4 (Ph), 117.1, 133.0, 159.3, 159.5, (C_C), 166.8 (C_O). HRMS: Calcd. for
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189–200.
worked up in a similar manner using dimethyl malonate (0.40 g, 2.50 mmol) and
the cobalt sandwich substituted alkynyl imine (0.63 g, 1.00 mmol). Yield: 0.35 g,
48 % IR (ν, cm−1): 1643 (C_O). Mp: 195–197 °C (dec.) Found: C, 76.54; H, 5.06;
N, 1.92. Calcd. for C47H36CoNO4: C, 76.52; H, 4.92; N, 1.90.1H NMR (δ, 300 MHz,
CDCl3): 3.71 (3H, s, CH3), 3.86 (3H, s, OCH3), 4.77 (2H, s, Cp), 5.11 (2H, s, Cp),
6.65–6.68 (1H, d), 6.96–6.99 (1H, d), 7.19–7.28 (20H, m, Ar), 7.40–7.43 (8H, m,
Ph). 13C{1H} NMR: δ 52.3 (OCH3), 54.5 (OCH3), 77.6 (CbC), 86.5, 86.2, 87.1 (Cp),
114.5, 115.2, 135.6, 151.7 (C6H4OCH3) 126.4, 127.9, 128.8, 134.2 (CbPh), 118.3,
131.0, 142.5, 132.4, 148.1 (C_C), 171.3 (C_O). HRMS: Calcd. for C47H36CoNO4
Na: 760.1874, Found: 760.1856.
[10] (a) N. Singh, A.J. Elias, Cyclopentadienyl 1,2- and 1,3-disubstituted cobalt sandwich
compounds {η5-[MeOC(O)]2C5H3}Co(η4-C4Ph4): Precursors for sterically hin-
dered bidentate chiral and achiral ligands, Organometallics 31 (2012)
2059–2065;
(b) N. Singh, A.J. Elias, Palladacycles of novel bisoxazoline chelating ligands based
on the dimeric cyclobutadiene linked cobalt sandwich compound [(η5-Cp)
Co(η4-C4Ph3)]2, Dalton Trans. 40 (2011) 4882–4891;
(c) R. Jana, M.S. Kumar, N. Singh, in: A.J. Elias (Ed.), Synthesis, reactivity and struc-
tural studies of (η5-methylcyclopentadienyl)(η4-tetraphenylcyclobutadiene)
cobalt and its derivatives, J. Organomet. Chem., 693, 2008, pp. 3780–3786;
(d) M.S. Kumar, S. Upreti, A.J. Elias, Synthesis and characterization of novel
fluorophosphazene-derived cobaltacyclopentadienyl metallacycles: Reagents
[17] Synthesis of 5: To a solution of diethyl malonate (0.16 g, 2.50 mmol) in 5.0 ml of
tetrahydrofuran a 60 % NaH (0.08 g, 2.00 mmol) was added with constant stirring.
Then a solution of the alkynyl imine (0.34 g, 1.00 mmol) in 5 ml of tetrahydrofuran
was added to it. The reaction mixture was stirred under reflux for 7 hours and then
brine was added. The mixture was extracted with dichloromethane and the organic

350
K. Keshav, A.J. Elias / Inorganic Chemistry Communications 35 (2013) 346–350
layer was dried over sodium sulphate. The solvents were evaporated off and the re-
[18] H. Imase, K. Noguchi, M. Hirano, K. Tanaka, Convergent and rapid assembly of
substituted 2-pyridones through formation of N-alkenyl alkynylamides followed
by gold-catalyzed cycloisomerization, Org. Lett. 10 (2008) 3563–3566.
[19] (a) SMART: Bruker Molecular Analysis Research Tools, version 5.618, Bruker
Analytical X-ray Systems, 2000. (Madison, WI);
sultant residue was purified over silica gel to give a yellow colored crystalline solid
which was characterized as the ferrocenyl substituted 2-pyridone 5. Yield: 0.24 g,
54 %. IR (ν, cm−1): 1650 (C_O). Mp: 183–185 °C (dec.). Found: C, 65.71; H, 5.03;
N, 3.14. Calcd. for C25H23FeNO4: C, 65.66; H, 5.07; N, 3.06.1H NMR (δ, 300 MHz,
CDCl3): 1.30 (3H, s, CH3), 3.82 (2H, s, CH2), 4.17 (5H, s, Cp), 4.33 (2H, q, -OCH3),
4.43 (2H, s, Cp), 4.49 (2H, s, Cp), 6.45–6.47 (2H, s), 6.95–6.97 (2H, d), 7.26–7.33
(3H, m). 13C{1H} NMR: δ 13.9 (CH3), 55.4 (OCH2), 61.5, 68.6, 70.0, 70.6 (Cp),
114.2, 127.4, 136.6, 149.1, (Ph), 106.9, 122.4, 159.4, 159.2 (C_C), 167.3 (C_O).
HRMS: Calcd. for C25H23FeNO4: 457.0976, Found: 457.0996.
(b) G.M. Sheldrick, SAINT-NT, version 6.04; Bruker Analytical X-ray Systems, 2001.
(Madison, WI);
(c) G.M. Sheldrick, SHELXTL-NT, version 6.10; Bruker Analytical X-ray Systems,
2000. (Madison, WI);
(d) B. Klaus, DIAMOND, version 2.1 c, University of Bonn, Bonn, Germany, 1999.