β-Enaminone Synthesis from 1,3-Dicarbonyl Compounds and Aliphatic and Aromatic Amines Catalyzed by Iron Complexes of Fused Bicyclic Imidazo[1,5-a]pyridine Derived N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/ejic.201800906

A series of Fe–NHC complexes (1–2)c of the fused bicyclic imidazo[1,5-a]pyridine framework of the type [CpFe(2-R-imidazo[1,5-a]pyridin-3-ylidene)(CO)₂]BF₄ {R = mesityl (1c), nPr (2c)} successfully carried out the synthesis of β-enamino ketones (3–10) and (17–27) and β-enamino esters (11–16) and (28–36) by the condensation of acyclic and cyclic 1,3-dicarbonyl compounds and various aliphatic and aromatic amines in the presence of light irradiation. Quite significantly, the catalytically relevant substrate adduct species of the type [CpFe(NHC)(acac)] (2e) and the product adduct species of the type [CpFe(NHC)(β-enaminone)] (2f) of the Fe–NHC precatalyst (2c) have been detected by mass spectrometry study. The [CpFe(2-R-imidazo[1,5-a]pyridin-3-ylidene)(CO)₂]BF₄ {R = mesityl (1c), nPr (2c)} complexes were obtained from their respective N–heterocyclic carbene precursors namely, the 2-R-imidazo[1,5-a]pyridin-2-ium chloride {R = mesityl (1a), nPr (2a)} by the reaction with CpFe(CO)₂I in the presence of KN(SiMe₃)₂ followed by the salt metathesis reaction with AgBF₄.

Full text of DOI:10.1002/ejic.201800906

A Journal of
Accepted Article
Title: ￿ ꢀ-EnamEꢁEꢂ SyEthꢂsms frꢁa 1,3ꢀDmcnrbꢁEyl CꢁapꢁuEds nEd
Almphntmc nEd Arꢁantmc AamEꢂs Cntnlyzꢂd by IrꢁE cꢁaplꢂxꢂs ꢁf
Fusꢂd Bmcyclmc Iamdnzꢁ[1,5ꢀn]pyrmdmEꢂ Dꢂrmvꢂd NHCs
Authors: A P Prnknshna, MnEꢁj Kuanr GnEgwnr, nEd PrnsꢂEjmt Ghꢁsh
Thms anEuscrmpt hns bꢂꢂE nccꢂptꢂd nftꢂr pꢂꢂr rꢂvmꢂw nEd nppꢂnrs ns nE
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publmshꢂd ꢁElmEꢂ mE -nrly Vmꢂw ns sꢁꢁE ns pꢁssmblꢂ nEd any bꢂ dmffꢂrꢂEt
tꢁ thms Accꢂptꢂd Artmclꢂ ns n rꢂsult ꢁf ꢂdmtmEg. Rꢂndꢂrs shꢁuld ꢁbtnmE
thꢂ VꢁR frꢁa thꢂ jꢁurEnl wꢂbsmtꢂ shꢁwE bꢂlꢁw whꢂE mt ms publmshꢂd
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cꢁEtꢂEt ꢁf thms Accꢂptꢂd Artmclꢂ.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ꢂjmc.201800906
Link to VoR: http://dx.dꢁm.ꢁrg/10.1002/ꢂjmc.201800906

European Journal of Inorganic Chemistry
10.1002/ejic.201800906

-Enaminone Synthesis from 1,3-Dicarbonyl Compounds and
Aliphatic and Aromatic Amines Catalyzed by Iron complexes of
Fused Bicyclic Imidazo[1,5-a]pyridine Derived N−Heterocyclic
Carbenes
†
†
,†
Prakasham. A. P. Manoj Kumar Gangwar and Prasenjit Ghosh*
†Department of Chemistry
Indian Institute of Technology Bombay
Powai, Mumbai 400 076, India
Email: pghosh@chem.iitb.ac.in
Fax: +91 22 2572 3480
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Keywords
Iron complexes ● N−heterocyclic carbenes ● imidazo[1,5-a]pyridine ligands ● -enaminone
reaction ● homogeneous catalysis ● 1,3-dicarbonyl compounds ● aliphatic amine ● aromatic
amines
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Highlights
The iron N−heterocyclic carbene complexes derived from fused bicyclic imidazo[1,5-a]pyridine
based systems were synthesized and structurally characterized.
These Fe−NHC complexes successfully carried out the condensation reaction between 1,3-
dicarbonyl compounds and various aliphatic and aromatic amines giving -enaminones.
The catalytically relevant substrate adduct specie and the product adduct specie have been
detected by mass spectrometry study.
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Abstract: A series of Fe−NHC complexes (1−2)c of the fused bicyclic imidazo[1,5-a]pyridine
framework of the type [CpFe(2-R-imidazo[1,5-a]pyridin-3-ylidene)(CO) ]BF [R = mesityl (1c),
2
4
n-Pr (2c)] successfully carried out the synthesis of -enamino ketones (3−10) and (17−27) and -
enamino esters (11−16) and (28−36) by the condensation of acyclic and cyclic 1,3-dicarbonyl
compounds and various aliphatic and aromatic amines in presence of light irradiation. Quite
significantly, the catalytically relevant substrate adduct specie of the type [CpFe(NHC)(acac)]
(
2e) and the product adduct specie of the type [CpFe(NHC)(-enaminone)] (2f) of the Fe−NHC
precatalyst (2c) have been detected by mass spectrometry study. The [CpFe(2-R-imidazo[1,5-
a]pyridin-3-ylidene)(CO) ]BF [R = mesityl (1c), n-Pr (2c)] complexes were obtained from their
respective N−heterocyclic carbene precursors namely, the 2-R-imidazo[1,5-a]pyridin-2-ium
chloride [R = mesityl (1a), n-Pr (2a)] by the reaction with CpFe(CO) I in presence of
KN(SiMe followed by the salt metathesis reaction with AgBF
2
4
2
3
)
2
4
.
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Introduction
The -enaminones are important intermediates in the synthesis of a variety of natural products
[
1]
[2]
[3]
like the biologically essential alkaloids i.e, Anibamine, Asparagamine A, Norallosedamine
and Koumine[4] and are also of interest for their antibacterial,[5] anti-convulsant[6] and anti-
[
7]
inflammatory properties. They are essential to the synthesis of the N- containing heterocyclic
compounds like, the pyridines, pyrroles, pyrazoles, pyrimidines,[11] indoles[12] and various
other N-heterocycles.[13] Thus, because of their wide spread utility in the synthesis of natural
products and also for their biological properties, convenient access routes to these -enaminone
scaffolds are of considerable interest.
[
8]
[9]
[10]
One conventional approach for the synthesis of the -enaminone involves the condensation of
1
,3-dicarbonyl compounds with amines. The reaction has been extensively studied with simple
transition metal salts,[14-16] an s-block metal salt,[17] the p-block metal salts,[18, 19] and even the f-
block metal salts.[20, 21] Apart from these, the usage of simple acids,[22] organocatalysts[23] and
the heterogeneous catalysts[24-26] have been reported for the synthesis of -enamino compounds.
However, despite the vast literature that exists,[27] the use of well-defined transition metal
catalysts for -enaminone synthesis remains surprisingly overlooked.
From this perspective, we become interested in exploring the utility of well-defined transition
metal N-heterocyclic carbenes as precatalysts for the condensation reaction, in tune with our
larger interest in the application of transition metal N-heterocyclic carbene complexes in
homogeneous catalysis reactions and in biomedical applications.[28] In this regard, we have
recently reported the utility of the coinage metals like the Au(I/III) Nheterocyclic carbene
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
[
29]
complexes for the -enaminone synthesis and proceeding further along the line we set out to
explore the potential of the FeNHC complexes in the synthesis of -enamino compounds. Of
late, there has been a significant interest in iron based catalysis[30] and in this context we have
[
31]
reported the well-defined FeNHC based precatalysts for the hydrosilylation and the Michael
addition reactions.[32]
Hence, in this manuscript we report the synthesis of a series of fused cyclic FeNHC complexes
derived from imidazo[1,5-a]pyridine based N-heterocyclic carbene ligands (Figure 1) and their
catalytic potential in the synthesis of -enamino ketones and -enamino esters through the
condensation of 1,3-dicarbonyl compounds with a variety of aliphatic and aromatic amines.
Figure 1. Synthesized FeNHC complexes (1−2)b and (1−2)c.
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Results and discussion
With the intent of exploring the potential of the iron N−heterocyclic carbene complexes in the
condensation reaction between 1,3-dicarbonyl compounds with amines yielding the -enaminone
compounds, a series of iron N-heterocyclic carbene complexes (1−2)c stabilized over a fused
bicyclic ring system namely, 2-R-imidazo[1,5-a]pyridin-3-ylidene [R = mesityl (1c), n-Pr (2c)]
were synthesized. Specifically, the half-sandwich complexes of the type [CpFe(2-R-
imidazo[1,5-a]pyridin-3-ylidene)(CO)
obtained by the deprotonation of their N−heterocyclic carbene precursors namely, the 2-R-
imidazo[1,5-a]pyridin-2-ium chloride [R = mesityl (1a), n-Pr (2a)] by KN(SiMe followed by
the reaction with the CpFe(CO) I in ca. 29−41 % yield (Scheme 1). The 2-R-imidazo[1,5-
2
]I [R = mesityl (1b), n-Pr (2b)] were conveniently
)
3 2
2
a]pyridin-2-ium chloride salts (1−2)a were synthesised from the one pot reaction of the
respective amines, formaldehyde and picolinaldehyde.[33]
1
The FeNHC complexes (1−2)b were found to be diamagnetic in nature as observed from the H
NMR spectroscopy.
For example, the (1−2)b complexes showed the characteristic
cyclopentadienyl (Cp) resonance appearing as a singlet at 5.39 ppm (1b) and at 5.52 ppm (2b) in
the H NMR spectrum and at 86.9 ppm (1b) and at 87.3 ppm (2b) in the 13C{ H} NMR
1
1
1
3
1
spectrum. Furthermore, the C{ H} NMR spectrum showed the Fe−CO resonance at 209.7 ppm
(
1b) and at 210.9 ppm (2b) and the Fe−Ccarbene resonance at 156.2 ppm (1b) and at 151.7 ppm
2b). As expected, the infrared spectrum displayed two νCO stretching frequencies observed at
(

1
1
1
1
ca. 2039 cm and 1991 cm for (1b) and at 2050 cm and 1991 cm for (2b), and which are in
the range of the νCO stretching frequencies reported previously for the related complexes namely,
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906

1
the {Cp[1,3-di-R
1
-benzimidazol-2-ylidene]Fe(CO)
2
}I type complexes {R = Et [2039 cm and
1

1
1
1
1
1
[31]
1
993 cm ], i-Pr [2039 cm and 1994 cm ] and n-Bu [2039 cm and 1986 cm ]}, and the
imidazole based {Cp[1-benzyl-3-R
2
-imidazol-2-ylidene]Fe(CO)
2
}PF
6
type complexes {R = Me
2

1
1
1
1 [31]
[
2038 cm and 1989 cm ], Et [2037 cm and 1987 cm ] and {Cp[1,3-di-mesityl-imidazol-

1
1
[34]
2
-ylidene]Fe(CO) }I [2050 cm and 2006 cm ]}.
2
Subsequently, the counter anion exchange by salt metathesis reaction was performed by the
treatment of FeNHC complexes (1−2)b with AgBF to obtain the [CpFe(2-R-imidazo[1,5-
a]pyridin-3-ylidene)(CO) ]BF [R = mesityl (1c), n-Pr (2c)] complexes in ca. 87−91 % yield.
These isostructural FeNHC complexes (1−2)c exhibited exactly similar spectroscopic
4
2
4
1
9
1
signatures to that of their related analogues (1−2)b. In addition, the F{ H} NMR spectrum
showed the presence of BF counter ion by observing the peak at −152.1 ppm (1c) and −151.9
4
ppm (2c).
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Scheme 1. Synthetic route to FeNHC complexes (1−2)c.
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
The molecular structures of the (1−2)c complexes showed the iron centre in a tetrahedral
environment exhibiting a “piano stool” type geometry, in which the metal centre was bound to
one cyclopentadienyl (Cp) ligand, one imidazo[1,5-a] pyridine derived NHC ligand and two
carbonyl ligands (Figures 2−3). Of particular interest is the Fe‒Ccarbene distance of 1.969(2) Å in
(1c) and 1.977(7) Å in (2c) complexes and which are slightly shorter than the sum of their
individual covalent radii of low spin Fe and Csp 2.05 Å[35] but are comparable to that in the
2
+
2
other related complexes namely, the benzimidazole based {Cp[1,3-di-R -benzimidazol-2-
1
ylidene]Fe(CO)
Å]},[31] and the imidazole based {Cp[1-benzyl-3-R
= Me [1.9753(19) Å] and Et [1.977(4) Å]}.[31] Similarly, the two metal bound
2
}I type complexes {R = Et [1.972(5) Å], i-Pr [1.972(4) Å] and n-Bu [1.965(5)
1
2
-imidazol-2-ylidene]Fe(CO)
2
}PF
6
type
complexes {R
2
carbonyls showed unequal Fe‒CCO bond distances of 1.779(2) Å and 1.777(3) Å in (1c) and of
.789(8) Å and 1.774(8) Å in (2c), in agreement with that observed in the related analogues
namely, the benzimidazole based {Cp[1,3-di-R -benzimidazol-2-ylidene]Fe(CO) }I type
1
1
2
complexes {R
1.774(5) Å and 1.783(5) Å]}[31] and the imidazole based {Cp[1-benzyl-3-ethyl-imidazol-2-
type complex [1.782(4) Å and 1.777(4) Å].[31] The two carbonyl moieties
1
= Et [1.765(5) Å and 1.780(5) Å], i-Pr [1.780(4) Å and 1.770(4) Å] and n-Bu
[
ylidene]Fe(CO)
2
}PF
6
in the (1-2)c complexes are bound to the metal centre at right angles to each other [∠
C19−Fe1−C20 94.16(11) in (1c) and [∠ C8−Fe1−C9 88.7(4) in (2c)]. The BF
non-bonded in the (1-2)c complexes.
4
anion remains
10
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Figure 2. ORTEP of 1c with thermal ellipsoids shown at the 50 % probability level. Hydrogen
atoms were omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)−C(1) 1.969(2),
Fe(1)−C(19) 1.779(2), Fe(1)−C(20) 1.777(3), C(1)−N(1) 1.361(3), C(1)−N(2) 1.372(3),
B(1)−F(1)
1.403(3),
Fe(1)−C(1)−N(1)
130.69(17),
Fe(1)−C(1)−N(2)
126.02(16),
C(19)−Fe(1)−C(20) 94.16(11), O(1) C(19) Fe(1) 171.7(2), O(2) C(20) Fe(1) 179.6(2).
11
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Figure 3. ORTEP of 2c with thermal ellipsoids shown at the 50 % probability level. Hydrogen
atoms were omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)−C(1) 1.977(7),
Fe(1)−C(8) 1.789(8), Fe(1)−C(9) 1.774(8), C(1)−N(1) 1.349(10), C(1)−N(2) 1.363(10),
B(1)−F(1) 1.340(10), Fe(1)−C(1)−N(1) 130.6(6), Fe(1)−C(1)−N(2) 126.2(6), C(8)−Fe(1)−C(9)
8
8.7(4), O(1) C(8) Fe(1) 175.9(6), O(2) C(9) Fe(1) 178.2(7).
12
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Table 1. X−ray crystallographic data for FeNHC complexes (1−2)c.
Compound
Lattice
(1c)
(2c)
Orthorhombic
BF
Monoclinic
Formula
Formula weight
Space group
a/Å
C
23
H
21FeN
2
O
2
BF
4
C
17
H17FeN
2
O
2
4
500.08
P21/n
423.99
P 21 21 21
7.344(4)
14.813(8)
15.799(8)
90.000
90.000
90.000
1718.7(16)
4
10.189(3)
13.490(4)
16.608(5)
90.000
b/Å
c/Å

/°
/°

97.920(3)
90.000
/°
V/Å3
2261.0(12)
4
Z
Temperature (K)
Radiation (,Å)
ρ(calcd.), g cm-3
100(2)
0.71070
1.469
0.723
25.00
3961
150(2)
0.71075
1.639

(Mo Kα), mm-1
θ max, deg.
0.934
25.00
No. Of data
3018
No. Of parameters
301
245
R
1
0.0441
0.0981
1.047
0.0937
0.2052
1.066
wR
2
GOF
13
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Significantly enough, the FeNHC complexes (1−2)c successfully performed the desired
condensation reaction between a variety of acyclic and cyclic 1,3-dicarbonyl compounds
(Equations 1-3) and aliphatic and aromatic amines yielding -enamino ketones (3−10) (Table 2)
and (17−27) (Table 4) and -enamino esters (11−16) (Table 3) and (28−36) (Table 5) in presence
of light irradiation. More importantly, substantial enhancement of the -enaminone product
yields of ca. 35 % (1c) and of ca. 42 % (2c) were observed for the representative substrates,
acetyl acetone and methyl amine, when compared with the control run performed with
CpFe(CO)
in the -enaminone synthesis reaction can be gauged from the blank experiment that showed 37
yield for the same substrates under analogous reaction conditions (Table S3). The FeNHC
1−2)c complexes successfully performed the condensation reaction between acyclic 1,3-
2
I under analogous reaction conditions. Lastly, the influence of the (1-2)c complexes
%
(
dicarbonyl compound namely, pentane-2,4-dione and a variety of aliphatic amines like, methyl
amine, ethyl amine, n-propyl amine, isopropyl amine, n-butyl amine, pentan-3-amine,
cyclohexanamine and morpholine yielding aliphatic -enamino ketones (3−10), in moderate to
excellent yields (ca. 68−95 %) under ambient condition in presence of UV light irradiation ( =
2
94 nm) at 27 °C (Table 2). The FeNHC (1−2)c complexes also effectively carried out the
condensation reaction between pentane-2,4-dione and a wide variety of aromatic amines namely,
the benzylic ones, phenylmethanamine, 1-phenylethan-1-amine, heterocyclic ones, pyridin-2-
ylmethanamine, furan-2-ylmethanamine, aryl amines like, aniline, 2,6-dimethylaniline, 2,4,6-
trimethylaniline, 2,6-diisopropylaniline, 2-iodoaniline, 2-methoxyaniline and 2-phenoxyaniline
yielding aromatic -enamino ketones (17−27) in good to excellent yields (ca. 71−96 %) in
presence of light irradiation (Table 4). Notably the -enamino ketones (17−20) obtained from
benzylic amines, the catalysis reaction was performed at 27 °C in presence of UV light
14
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
irradiation ( = 294 nm) (Entries 1−4, Table 4) while for the -enamino ketones (21−27)
obtained from the aryl amines were performed at 70 °C in presence of mercury light irradiation
(Entries 5−11, Table 4).
The condensation reaction between the cyclic and acylic 1,3-dicarbonyl compound namely ethyl
-oxocyclopentane-1-carboxylate and ethyl acetoacetate with various aliphatic amines like, ethyl
2
amine, n-propyl amine, isopropyl amine, n-butyl amine and morpholine yielding aliphatic -
enamino esters (11−16) were also observed for the FeNHC complexes (1−2)c in moderate to
high yields (ca. 57−87 %) under ambient condition in presence of UV light irradiation ( = 294
nm) at 27 °C (Table 3). The ethyl acetoacetate with aromatic amines like phenylmethanamine,
1
-phenylethan-1-amine and 2,6-dimethylaniline gave aromatic -enamino esters (28−30) in
yields of ca. 67-78 % (Table 5). In addition, the FeNHC (1−2)c complexes performed the
condensation reaction between ethyl 2-oxocyclopentane-1-carboxylate and various aromatic
amines like the p-toluidine, 2,6-dimethylaniline, 2,4,6-trimethylaniline, 2,6-diisopropylaniline, 2-
iodoaniline and 2-methoxyaniline yielding aromatic -enamino esters (31−36), in good yields
(ca. 72−85 %) in presence of mercury light irradiation at 70 °C (Table 5).
Among the various primary aliphatic amines that underwent condensation with acyclic 1,3-
dicarbonyl compound acetyl acetone, the pentan-3-amine displayed highest yield of 95 % as
catalyzed by FeNHC complex (2c) (Table 2, entry 6). The 2,4,6-trimethylaniline displayed
highest yield of 96 % as catalyzed by FeNHC complex (1c) (Table 4, entry 7) in case of the aryl
15
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
amine. Similarly, for the condensation reaction of the cyclic 1,3-dicarbonyl compound ethyl 2-
oxocyclopentane-1-carboxylate, the ethyl amine showed highest yield of 87 % as catalyzed by
FeNHC complex (2c) (Table 3, entry 1) and 2,6-diisopropylaniline showed highest yield of 85
%
as catalyzed by FeNHC complex (1c) (Table 5, entry 7) in case of the aryl amine. Lastly, the
alkyl substituted FeNHC complex (2c) exhibited superior yields than the aryl analogue (1c).
As there is no prior report of the FeNHC complexes in the -enaminone synthesis reaction, a
comparison of the (12)c complexes is made with simple iron salt like, NH
4
[Fe(NO
3
4
) ], reported
for the same reaction.[36] Notably, for the representative reaction of acetyl acetone and benzyl
amine, NH
4
[Fe(NO
3
4
) ] gave 92 % yield in 12 minutes at 10 mol % catalyst loading whereas the
FeNHC complex (1c) gave 92 % yield at 2 mol % catalyst loading in 6 hours reaction time
(Table 4, entry 1). We are aware of only one other report of the use of the well-defined
1
complexes for the formation of the -enaminones namely the Au(I)NHC complexes [1-(R )-3-
2
1
2
1
2
1
(
R )-imidazol-2-ylidene]AuCl where [R = i-Pr, R = CH
2
Ph; R = mesityl, R = CH
2
Ph; R = i-
2
t
1
2
1
Pr, R = CH
2
CO -Bu, and R = t-Bu, R = CH
2
COt-Bu] and the Au(III)NHC complexes [1-(R )-
2
1
2
1
2
1
3
-(R )-imidazol-2-ylidene]AuBr
3
where [R = i-Pr, R = CH
2
Ph; R = mesityl, R = CH Ph; R =
2
2
1
2
i-Pr, R = CH
2
COt-Bu, and R = t-Bu, R = CH
2
COt-Bu] that carried out the -enaminone
synthesis under ambient conditions.[29] In particular, the FeNHC complexes (12)c exhibited
superior yields (ca. 7786 %) to the Au(I)NHC complexes (ca. 149 %) and are comparable to
the Au(III)NHC complexes (ca. 4099 %) for the condensation of pentane-2,4-dione with
primary amines like methyl amine, ethyl amine, n-propyl amine, isopropyl amine and n-butyl
amine under analogous reaction condition. Interestingly, the FeNHC complexes (12)c gave
16
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
better yields in case of the cyclic ethyl 2-oxocyclopentane-1-carboxylate (ca. 7287 %) when
compared to both of the Au(I)NHC (ca. 620 %) and Au(III)NHC complexes (ca. 2452 %)
in reaction with primary amines like ethyl amine, n-propyl amine and n-butyl amine.[29] The
catalysis results of the FeNHC complexes are promising and would further the cause of iron in
the catalysis.
17
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Equation 1. General equation for the -enamino ketone synthesis from acyclic -diketo compound and various primary aliphatic and
aromatic amines as catalyzed by the FeNHC complexes (1−2)c.
18
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Equation 2. General equation for -enaminone synthesis from acyclic -diketo compounds and secondary amine as catalyzed by the
FeNHC complexes (1−2)c.
19
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European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Equation 3. General equation for the -enamino ester synthesis from acyclic and cyclic -keto ester substrates and various primary
aliphatic and aromatic amines as catalyzed by the FeNHC complexes (1−2)c.
20
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Table 2. Selected results for the formation of -enamino ketones from pentane-2,4-dione and various aliphatic amines as catalyzed by the FeNHC
European Journal of Inorganic Chemistry
10.1002/ejic.201800906
a
complexes (1−2)c .
S. No
1,3-diketone
amine
product
(
2c)
(
1c)
yieldb
yieldb
1
2
3
4
5
70
77
(
(
(
(
(
3)
4)
5)
6)
7)
68
76
82
84
77
86
86
85
6
92
95
(
8)
7
91
93
(
9)
8
89
92
(
10)
(a). Reaction conditions: 1.00 mmol of 1,3-diketone, 4.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 27 °C, irradiated at 294 nm
for 6 h. (b). Isolated yields (%).
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21

Table 3. Selected results for the formation of -enamino esters from cyclic and acyclic -keto esters and various aliphatic amines as catalyzed by the
European Journal of Inorganic Chemistry
FeNHC complexes (1−2)c .
10.1002/ejic.201800906
a

-keto ester
substrate
S. No
amine
product
(
2c)
(
1c)
yieldb
yieldb
1
2
3
83
87
(
(
(
11)
12)
13)
80
76
64
57
4
5
72
71
69
66
(
(
14)
15)
6
69
84
(
16)
(a). Reaction conditions: 1.00 mmol of -keto ester substrate, 4.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 27 °C, irradiated at
2
94 nm for 6 h. (b). Isolated yields (%).
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22

Table 4. Selected results for the formation of -enamino ketones from pentane-2,4-dione and various aryl amines as catalyzed by the FeNHC complexes (1−2)c.
S. No
1,3-diketone
amine
product
European Journal of Inorganic Chemistry
10.1002/ejic.201800906
(
2c)
(
1c)
yieldc
yieldc
1a
92
90
(
17)
2a
90
91
(
(
(
18)
19)
20)
3a
71
88
89
91
4a
5b
6b
7b
72
92
96
74
89
86
(
(
(
21)
22)
23)
8b
82
87
86
91
81
89
89
87
(
(
(
24)
25)
26)
9b
1
0b
1
1b
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(
27)
Reaction conditions: (a). 1.00 mmol of 1,3-diketone, 4.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 27 °C, irradiated at 294 nm for 6 h. (b). 1.00
mmol of ketone, 2.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 70 °C, irradiated under mercury vapor lamp for 12 h. (c) Isolated yields (%).
23

Table 5. Selected results for the formation of -enamino esters from acyclic and cyclic -keto esters and various aryl amines as catalyzed by the FeNHC complexes
(1−2)c.
European Journal of Inorganic Chemistry
10.1002/ejic.201800906

-keto ester
S. No
amine
product
substrate
(
2c)
(
1c)
yieldc
yieldc
1a
74
72
(
28)
2a
67
71
(
29)
3a
78
73
(
(
(
(
30)
31)
32)
33)
4b
75
72
79
78
76
84
5b
6b
7b
85
83
(
34)
8b
79
83
(
35)
9b
76
74
This article is protec(3
t
6
e
)
d by copyright. All rights reserved.
(
h. (b). 1.00 mmol of 1,3-dicarbonyl substrate, 2.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 70 °C, irradiated under mercury vapor lamp for 12 h.
c). Isolated yields (%).
a). Reaction conditions: (a). 1.00 mmol of -keto ester substrate, 4.0 mmol of amine, 2 mol % of catalyst (1−2)c, 2.5 mL of MeOH at 27 °C, irradiated at 294 nm for 6
(
24

European Journal of Inorganic Chemistry
10.1002/ejic.201800906
The mass spectrometry studies were undertaken to get an insight about the mechanism of -
enaminone synthesis as catalyzed by the FeNHC catalyst (2c). The proposed catalytic cycle
for the two representative substrates acetyl acetone and methyl amine initiates with the
formation of the electronically and coordinatively unsaturated 14-electron intermediate
+
specie (2d) of the type [CpFe(NHC)] , that has been detected in the mass spectrometric
+
studies as [M] peak at m/z 281.0737 [Calcd. 281.0736] (Table 6) upon UV light radiation (λ
=
294 nm), similar to that of the related iron carbonyl based catalysts.[37]
The intermediate specie (2d) upon reaction with the 1,3-dicarbonyl compound resulted in the
formation of the substrate adduct specie of the type [CpFe(NHC)(acac)] (2e). Interestingly,
the substrate adduct specie of the type [CpFe(NHC)(acac)] (2e) has been detected in the mass
+
spectrometric studies as [M+H] peak at m/z 381.0760 [Calcd. 381.1260] (Table 6). Further,
the reaction of the substrate adduct specie of the type [CpFe(NHC)(acac)] (2d) with methyl
amine would yield the corresponding product adduct specie of the type [CpFe(NHC)(-
enaminone)] (2f) and which too has been detected in the mass spectrometric studies as
+
[
M+H] peak at m/z 394.8843 [Calcd. 394.1576] (Table 6). Finally, the release of the
product, 4-(methylamino)pent-3-en-2-one, from the product adduct specie of the type
CpFe(NHC)(-enaminone)] (2f) would regenerate back the initial intermediate specie (2d)
thereby completing the catalytic cycle (Scheme 2).
[
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25

European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Scheme 2. Proposed mechanism of the -enaminone formation for the condensation of
acetyl acetone and methyl amine as catalyzed by the Fe−NHC precatalyst (2c).
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26

European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Table 6. Mass data for the intermediate (2d), substrate adduct specie of the type
CpFe(NHC)(acac)] (2e) and the product adduct specie of the type [CpFe(NHC)(-
[
enaminone)] (2f) of the FeNHC precatalyst (2c).
S.N
intermediate
o
mass data
(a)
1
.
(b)
(a)
2
.
(b)
(a)
3
(b)
(a). Experimental mass distribution, (b). Simulated mass distribution.
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27

European Journal of Inorganic Chemistry
10.1002/ejic.201800906
In summary, a series of Fe−NHC complexes (1−2)b and (1−2)c supported over fused bicyclic
imidazo[1,5-a]pyridine scaffold have been successfully synthesized and structurally
characterized. The Fe−NHC complexes (1−2)c efficiently carried out the -enaminone
synthesis by the condensation of 1,3-dicarbonyl compounds and aliphatic and aromatic
amines in good yields in presence of light irradiation. A catalytic cycle has been proposed
based on the detection of the substrate adduct specie of the type [CpFe(NHC)(acac)] (2e) and
the product adduct specie of the type [CpFe(NHC)(-enaminone)] (2f) of the Fe−NHC
precatalyst (2c) by ESIMS study.
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28

European Journal of Inorganic Chemistry
10.1002/ejic.201800906
Experimental Section
General Procedures: All manipulations were carried out using a combination of a glovebox
and standard Schlenk techniques. Solvents were purified and degassed by standard
procedures. KN(SiMe
3
)
2
and [CpFe(CO)
2
]
2
were purchased from Sigma Aldrich (USA) and
used without any further purification. 2-Mesitylimidazo[1,5-a]pyridin-2-ium chloride
(
1a),[33] and CpFe(CO) I,[38] were synthesized according to modified literature procedures.
2
1
13
1
19
1
H, C{ H} and F{ H} NMR spectra were recorded on Bruker 400 MHz and Bruker 500
1
MHz NMR spectrometer. H NMR peaks are labeled as singlet (s), doublet (d), triplet (t),
quartet (quat), quintet, broad (br), triplet of triplets (tt), doublet of doublets (dd), doublet of
triplets (dt), doublet of quartets (dq), multiplet (m), and septet. Infrared spectra were
recorded on a Perkin Elmer Spectrum One FT-IR spectrometer. Mass spectrometry
measurements were done on a Micromass Q-Tof and Bruker Maxis Impact spectrometer.
Elemental analysis was carried out on Thermo Quest FLASH 1112 SERIES (CHNS)
Elemental Analyzer. X-ray diffraction data for compounds (1−2)c were collected on a
Rigaku Hg 724+ diffractometer and crystal data collection and refinement parameters are
summarized in Table 1. The structures were solved by using direct methods and standard
difference map techniques, and were refined by full-matrix least-squares procedures on F2
with SHELXTL (Version 6.10).[39] For the catalysis runs, the GCMS analyses were done
using Agilent Technologies 7890A GC systems with 5975C inert XL EI/CI MSD Triple-Axis
detector.
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29