Michael addition of cyclic β-oxo ester and α-methyl cyano ester substrates with activated olefins by iron complexes of benzimidazole derived N-heterocyclic carbene ligands

Article abstract of DOI:10.1016/j.jorganchem.2018.01.061

Michael addition reactions of cyclic β-oxo ester and α-methyl cyano ester substrates with acceptor activated olefins were catalyzed by iron N?heterocyclic carbene (NHC) complexes of the type, {Cp[1-benzyl-3-alkyl-benzimidazol-2-ylidene]Fe(CO)₂}

Full text of DOI:10.1016/j.jorganchem.2018.01.061

Journal of Organometallic Chemistry 859 (2018) 106e116
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Michael addition of cyclic b-oxo ester and a-methyl cyano ester
substrates with activated olefins by iron complexes of benzimidazole
derived N-heterocyclic carbene ligands
*
A.P. Prakasham, Manoj Kumar Gangwar, Prasenjit Ghosh
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 November 2017
Received in revised form
Michael addition reactions of cyclic b-oxo ester and a-methyl cyano ester substrates with acceptor
activated olefins were catalyzed by iron Nꢀheterocyclic carbene (NHC) complexes of the type, {Cp[1-
benzyl-3-alkyl-benzimidazol-2-ylidene]Fe(CO)
}I, where [alkyl ¼ i-Pr (1) and Et (2)] in the presence of
-proline as chiral auxiliary under ambient conditions. In case of the cyclic -oxo ester substrates good
yields of up to 68% and enantioselectivities of up to 18% were observed while for the -methyl cyano
ester substrates similar yields of up to 82% and enantioselectivities of up to 8% were observed. Notably,
for the cyclic -oxo ester substrates, the five membered ethyl-2-oxocyclopentane-1-carboxylate gave
superior yields and enantioselectivities over the six membered ethyl-2-oxocyclohexane-1-carboxylate. In
case of -methyl cyano ester substrates, methyl vinyl ketone gave higher yields as compared to the other
acceptor activated olefins namely, ethyl vinyl ketone and acrylonitrile. Based on the mass spectrometry
studies, that detected the presence of -proline adduct species (1e2)A, -proline and substrate adduct
species (1e2)B and -proline and product adduct species (1e2)C in the catalysis reaction mixture, a
2
23 January 2018
L
b
Accepted 31 January 2018
Available online 5 February 2018
a
b
Keywords:
Iron complexes
Nꢀheterocyclic carbenes
Benzimidazole ligands
Michael addition
Asymmetric catalysis
a
L
L
L
L
-proline
Cyclic -oxo ester
-Methyl cyano ester
Mass spectrometry
mechanistic cycle has been proposed for the FeꢀNHC mediated Michael addition reaction.
b
©
2018 Elsevier B.V. All rights reserved.
a
1
. Introduction
Asymmetric Michael addition thus serves as a powerful tool for
preparing optically active compounds of varying interest. Though
transition metal based Nꢀheterocyclic carbene complexes have
been extensively used in homogeneous catalysis their utility in
Michael addition reactions remains sparse [15e20]. In this regard,
we become interested in exploring the asymmetric variants [21]
and the conventional [22e25] Michael addition reaction under
base-free conditions using nickel based bifunctional catalyst
systems.
Proceeding further along the line, we wanted to look in to the
potential of FeeNHC complexes in asymmetric Michael addition
reaction particularly for its economical, environmentally friendly
nature [26e29] and for its broader catalytic application in organic
Michael addition including its asymmetric version is a tradi-
tional method for the construction of CꢀC bonds [1e5] and the
reaction has been successfully employed for synthesizing a various
types of chemical motifs including the formidable ones encoun-
tered in the total synthesis of natural products like 19-
hydroxysarmentogenin [6], (ꢀ)-galanthamine [7], (ꢀ)-lycoramine
[
7], (þ)- -lycorane [8], (þ)-lycorine [8], acromelic acid A and B [9]
a
and claenone [10]. The success of Michael addition reaction arises
from the ready availability of a pool of wide variety of nucleophiles
and activated olefins that provides convenient access to the syn-
thetically useful functionalized organic compounds [11e14].
*
Corresponding author.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
https://doi.org/10.1016/j.jorganchem.2018.01.061
022-328X/© 2018 Elsevier B.V. All rights reserved.
0

A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
107
alkyl-benzimidazolium halide salts [alkyl ¼ i-Pr, X ¼ Br (a) and
I in the pres-
alkyl ¼ Et, X ¼ I (b)] by the treatment with CpFe(CO)
2
ence of KHMDS as a base in moderate yields (29e66%) (Scheme 1)
by following the method first reported by Guerchais [51] and later
applied by Albrecht [52,53], Royo [54,55], Darcel [56], us [57] and
others [58,59]. The formations of the (1e2) complexes were
confirmed by the disappearances of the characteristic NCHN reso-
1
nances at 11.16e11.56 ppm in the H NMR along with the appear-
13
1
ance of FeꢀCcarbene resonances at 182.6e183.2 ppm in the C{ H}
NMR spectroscopy. The characteristic cyclopentadienyl (Cp) reso-
nances appeared at 5.49 ppm for the complex 1 and at 5.45 ppm for
Fig. 1. Synthesized FeꢀNHC complexes (1e2).
1
the complex 2 in the H NMR spectrum. The diamagnetic natures of
1
the iron complexes (1e2) were observed from the H NMR and the
transformations [30]. It is worth noting that only a handful of ex-
13
1
C{ H} NMR spectroscopy. As expected, the infrared spectra
showed two stretching frequencies for the carbonyl moiety (
amples of the transition metal Nꢀheterocyclic carbene complexes
CO
n )
[
21,31,32] have been reported for the asymmetric Michael addition
ꢀ
1
ꢀ1
appearing at 2043 cm and 1996 cm for the complex 1 and at
2
reactions and which do not include any example of an iron
Nꢀheterocyclic carbene complex.
ꢀ
1
ꢀ1
041 cm and 1992 cm for the complex 2.
The molecular structures of the (1e2) complexes, as obtained by
Pioneering work in the area has been done by Christoffers
the single crystal X-ray diffraction studies revealed the iron centers
in piano stool geometries being bound to the cyclopentadienyl (Cp)
ring, two CO ligands and the Nꢀheterocyclic carbene ligands
[
33,34], who extensively studied the Michael addition reaction
using FeCl $6H O staring from classical Michael addition reaction
35,36] to vinylogous Michael reaction [37,38] to intramolecular
Michael reaction [39,40]. Furthermore the FeCl $6H O catalyzed
3
2
[
(
Figs. 2 and 3), similar to the other related structurally character-
3
2
ized examples namely, {Cp[1,3-di-alkyl-benzimidazol-2-ylidene]
Fe(CO)
}I [alkyl ¼ Et, i-Pr and n-Bu] [57] and {Cp[1,3-di-Me-benzi-
midazol-2-ylidene]Fe(CO)(CH CN)}PF
[60]. The FeꢀCCarbene dis-
tance of 1.971(3) (1) and 1.973(3) (2) are similar to the range of
.960(3)ꢀ1.976(4) observed for the structurally characterized
complexes [57,60]. Similarly, the FeꢀCO distance of 1.780(3)ꢀ
.791(3) (1) and 1.771(4)ꢀ1.790(4) (2) are similar to the range of
Michael addition reaction was extended for constructing spirocyclic
carbazoles and acridine-lactams [41] and polymers [42]. The
asymmetric Michael addition catalyzed by FeCl $6H O was ach-
3 2
ieved by using appropriate chiral substrates that yielded up to 73%
ee [43] and 20% de [44] and also by using chiral auxiliaries that
yielded up to 18% ee [45] was obtained. Detailed mechanistic
2
3
6
1
1
1
3 2
studies of the FeCl $6H O catalyzed Michael addition reaction have
.762(3)ꢀ1.780(5) observed for the structurally characterized
also been investigated [46e48]. Additionally, a salene type chiral
iron complex was used in the sulfa-Michael addition reaction of
thiols to acyclic conjugated enones that yielded up to 98% ee [49].
complexes [57,60].
The iron Nꢀheterocyclic carbene complexes (1e2) efficiently
carried out the asymmetric Michael addition reaction of cyclic b-
Asymmetric Michael addition of 4-hydroxycoumarin to
a,
b-unsat-
oxo ester and
fins under ambient conditions (Equations 1 and 2). A variation of
chiral auxiliaries produced the best result with -proline giving a
a-methyl cyano ester substrates with activated ole-
urated ketone was achieved by using Fe(ClO
chiral auxiliary [50].
4 3
) and (R,R)-DPEN as
L
Here in this manuscript, we report the asymmetric Michael
addition reaction of cyclic -oxo ester and -methyl cyano ester
yield of 56% and enantioselectivity of 18% in case of the Michael
addition reaction of ethyl-2-oxocyclopentane-1-carboxylate with
methyl vinyl ketone as catalyzed by 2 mol % of the FeeNHC complex
b
a
substrates under ambient reaction conditions catalyzed by two
FeeNHC complexes (1e2) namely, {Cp[1-benzyl-3-alkyl-benzimi-
3
(1) in presence of Et N as base under ambient conditions in 8 h of
dazol-2-ylidene]Fe(CO)
2
}I, where [alkyl ¼ i-Pr (1) and Et (2)] in
the reaction time (Table 1, entry 5).
presence of -proline as a chiral auxiliary and Et
L
3
N as base (Fig. 1).
A detailed optimization study was undertaken to gauge the
extent of amplification in product yield as obtained by the FeꢀNHC
complexes (1e2) for the asymmetric Michael addition reaction
2
. Result and discussion
performed in the presence of catalytic amount of L-proline. In
Iron complexes of benzimidazole derived Nꢀheterocyclic car-
particular, the blank experiment containing only the substrates
namely, ethyl 2-oxocyclopentane-1-carboxylate and methyl vinyl
ketone gave no product (Supporting Information Table S2, Entry 1).
The catalysis run containing the same two substrates in presence of
bene ligands were chosen for studying the asymmetric Michael
addition reaction. For this purpose, the {Cp[1-benzyl-3-alkyl-ben-
zimidazol-2-ylidene]Fe(CO)
2
}I, [alkyl ¼ i-Pr (1) and Et (2)] com-
plexes were synthesized directly from the respective 1-benzyl-3-
Scheme 1. Synthetic route to FeꢀNHC complexes (1e2).

108
A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
Fig. 2. ORTEP of
Hydrogen atoms and co-crystalized CHCl
bond lengths (Å) and angles (deg): Fe1ꢀC1 1.971(3), Fe1ꢀC20 1.780(3), Fe1ꢀC21
.791(3), C21ꢀO1 1.132(3), C20ꢀO2 1.145(3), N1ꢀC1 1.366(4), N2ꢀC1 1.368(4),
C20ꢀFe1ꢀC21 91.47(13), C20ꢀFe1ꢀC1 97.23(12), C21ꢀFe1ꢀC1 92.09(12), N1ꢀC1ꢀN2
05.9(2).
1
with thermal ellipsoids shown at the 50% probability level.
3
molecules were omitted for clarity. Selected
1
1
3
Et N gave the corresponding product namely, ethyl 2-oxo-1-(3-
oxobutyl)cyclopentane-1-carboxylate (3) in 30% yield (Table S2,
Entry 3). Another control experiment containing these two sub-
Fig. 3. ORTEP of
Hydrogen atoms and co-crystalized H
bond lengths (Å) and angles (deg): Fe2ꢀC1 1.973(3), Fe2ꢀC22 1.790(4), Fe2ꢀC23
2
with thermal ellipsoids shown at the 50% probability level.
2
O molecule were omitted for clarity. Selected
strates and Et
yield and 5% ee (Table S2, Entry 4). The Michael addition reactions
of these two substrates with the CpFe(CO) I complex in the pres-
ence of Et N and -proline gave the product (3) in 49% yield and 0%
ee (Table S2, Entry 5). Finally, the same two substrates in presence
of Et N, -proline and the precatalysts (1) gave the product in 56%
yield and 18% ee (Table S2, Entry 6). Subsequent solvent variation
study too yielded a maximum enantioselectivity of 18% in CHCl
3
N in presence of L-proline gave the product (3) in 46%
1
.771(4), C23ꢀO3 1.144(4), C22ꢀO2 1.136(4), N1ꢀC1 1.367(4), N2ꢀC1 1.358(4),
2
C22ꢀFe2ꢀC23 90.42(17), C23ꢀFe2ꢀC1 93.87(15), C22ꢀFe2ꢀC1 95.24(15), N2ꢀC1ꢀN1
3
L
105.9(3).
L
3
(Table S3, entry 1). The base variation study revealed Et N as the
optimum base for the reaction again based on the enantiose-
lectivity results (Table S4, entry 1).
3
3
,
and which was selected as a medium for further catalysis studies
Equation 1. General equation for the Michael addition reaction of cyclic
b
-oxo ester substrates with activated olefins by FeꢀNHC complexes (1e2).

A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
109
Equation 2. General equation for the Michael addition reaction of
a
-methyl cyano ester substrates with activated olefins catalyzed by FeꢀNHC complexes (1e2).
.
methyl and ethyl vinyl ketones (Table 2). Additionally, the Michael
Having the optimum conditions in hand, a detailed substrate
scope study for the Michael addition reaction was performed on
addition reaction was performed on three a-methyl cyano ester
substrates namely, ethyl 2-cyanopropanoate, isopropyl 2-
cyanopropanoate and t-butyl 2-cyanopropanoate with the acti-
vated olefins methyl vinyl ketone, ethyl vinyl ketone and acrylo-
two cyclic
b-oxo ester substrates namely, the five-membered ethyl-
2
-oxocyclopentane-1-carboxylate and the six-membered ethyl-2-
oxocyclohexane-1-carboxylate with activated olefins namely,
nitrile (Table 2). Interestingly, the a-methyl cyano ester substrates
Table 1
Chiral auxiliary variation study of the Michael addition reaction of ethyl-2-oxocyclopentane-1-carboxylate with methyl vinyl ketone as catalyzed by the FeꢀNHC complex (1).
S.No
1
Chiral auxiliary
FeꢀNHC complex (1)
Without FeꢀNHC
mol %a
Yieldb
43
eec
Yieldb
eec
1
2
53
5
2
1
35
1
38
1
(
continued on next page)

1
10
A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
FeꢀNHC complex (1)
Table 1 (continued )
S.No
Chiral auxiliary
Without FeꢀNHC
Yieldb
mol %a
Yieldb
eec
eec
3
1
46
3
22
1
4
1
2
2
47
56
62
7
26
46
44
8
5
1
5d
18
2
6
Reaction conditions: 1.00 mmol of Michael donor, 1.5 mmol of Michael acceptor, 1.5 mmol of Et
3
N in 5 mL of CHCl
3
at room temperature for 8 h.
a
Mol % of catalyst (1) and mol % of chiral auxiliary used.
Yields are after isolation of the product.
Enantiomeric excess was determined by chiral GC, CP-Chirasil-Dex-CB.
b
c
d
3
1.00 mmol of Michael donor, 3 mmol of Michael acceptor, 3 mmol of Et N.
exhibited better yields (18e82%) and enantioselectivities (1e8%)
while the cyclic -oxo ester substrates that exhibited better enan-
tioselectivities (1e18%) with yields of (15e68%). Between the
methyl cyano ester substrates, isopropyl 2-cyanopropanoate with
methyl vinyl ketone catalyzed by the FeeNHC complex (2) gave the
maximum of 82% yield (Table 2, entry 8) whereas ethyl 2-
cyanopropanoate with ethyl vinyl ketone catalyzed by the
FeeNHC complex (1) gave the maximum of 8% ee (Table 2, entry 6).
and enantioselectivities (1e8%).
b
In the lack of any other report of the FeeNHC complexes for the
Michael addition reaction in general, and for its asymmetric version
in particular, important is the comparison of the FeeNHC com-
plexes (1e2) for the asymmetric Michael addition reaction with
that of our earlier reported chiral NieNHC based bifunctional cat-
alysts under the base-free conditions [21]. For example, under
analogous reaction conditions of the 8 h of reaction time at ambient
temperature and at the same 2 mol % of the catalyst loading, the
FeeNHC complexes (1e2) were slightly inferior in performance
than the bifunctional NieNHC complex {1-(1S)-menthyl-3-[N-
a
-
Among the cyclic
b-oxo ester substrates, the five-membered ethyl-
2
-oxocyclopentane-1-carboxylate gave superior yields (53e68%)
and enantioselectivities (2e18%) over the six-membered ethyl-2-
oxocyclohexane-1-carboxylate that exhibited yields of (15e29%)
2
(phenylacetamido)]imidazol-2-ylidene} Ni [21]. Specifically, for
Table 2
Michael addition of cyclic
b
-oxo ester and
a-methyl cyano ester substrates with activated olefins as catalyzed by the FeꢀNHC complexes (1e2).
S. No.
1
Michael donor
Michael acceptor product
FeꢀNHC complex (1)
FeꢀNHC complex (2)
yielda
56
eeb
18
yielda
eeb
68
4
(
3)
2
53
2
55
3
(
(
(
4)
5)
6)
3
4
15
24
1
7
20
29
1
8

A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
111
Table 2 (continued )
S. No.
Michael donor
Michael acceptor
product
FeꢀNHC complex (1)
FeꢀNHC complex (2)
yielda
eeb
yielda
eeb
5
6
79
1
81
2
(
7)
55
8
71
1
(
(
8)
9)
7
8
36
75
2
7
39
82
2
7
(
10)
9
39
3
65
3
(
(
11)
12)
1
1
0
1
40
39
1
2
44
52
1
2
(
13)
1
1
2
3
19
18
1
1
28
1
1
(
(
14)
15)
26
Reaction conditions: 1.00 mmol of Michael donor, 3 mmol of Michael acceptor, 3 mmol of Et
temperature for 8 h.
3
N, 2 mol % of catalyst (1e2) and 2 mol % of
3
L-proline in 5 mL of CHCl at room
a
Yields are after isolation of the product.
b
Enantiomeric excess was determined by chiral GC, CP-Chirasil-Dex-CB.
the Michael addition reaction of isopropyl 2-cyanopropanoate with
methyl vinyl ketone, the FeeNHC complex (1) yielded the corre-
sponding Michael addition product in 75% yield and 7% ee and the
FeeNHC complex (2) gave 82% yield and 7% ee (Table 2, entry 8),
whereas the bifunctional NieNHC catalyst {1-(1S)-menthyl-3-[N-
Michael addition reaction.
In order to obtain a mechanistic insight, mass spectrometric
studies were undertaken for two representative substrates namely,
ethyl 2-oxocyclopentane-1-carboxylate and methyl vinyl ketone as
catalyzed by the FeꢀNHC complexes (1e2), and based on which a
catalytic cycle is proposed (Scheme 2). In particular, the mass
spectrometric analysis of the catalysis mixture suggested the
dissociation of a CO moiety from the FeꢀNHC precatalysts (1e2) in
(
phenylacetamido)]imidazol-2-ylidene}
ee [21]. Another iron based catalyst, FeCl
tative Michael addition reaction
2
Ni gave 91% yield and 72%
$6H O, for the represen-
between ethyl-2-
3
2
oxocyclopentane-1-carboxylate and methyl vinyl ketone, gave a
better yield of 97% as opposed to that of 56% yield in 18% ee in case
of FeeNHC complex (1) and of 68% yield in 4% ee in case of the
FeeNHC complex (2) [36]. Nonetheless, the very fact that the
FeeNHC complexes (1e2) catalyzed the Michael addition reaction
under ambient reaction conditions is significant as it would pave
way for further study on the utility of such FeeNHC complexes for
presence of L-proline and base to yield the neutral complexes 1A
and 2A (Fig. 4). Significantly enough, the species 1A and 2A have
þ
been detected in the mass spectrometric studies as [MþNa] peak
þ
at m/z 536.1750 for 1A [Calcd. 536.1607] and as [M] peak at m/z
499.2619 for 2A [Calcd. 499.1553]. The subsequent reaction of 1A
and 2A with the Michael donor substrate, ethyl 2-oxocyclopentane-
1-carboxylate yielded the -proline and substrate adduct species 1B
L

112
A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
Scheme 2. Proposed mechanism for the FeꢀNHC catalyzed Michael addition reaction.
þ
and 2B and which too have been detected as [M] peaks at m/z
6
6
the 1C and 2C species have been detected by mass spectrometry as
þ
þ
41.2343 and 627.7621 for 1B [Calcd. 641.2547] and 2B [Calcd.
27.2391] respectively. Lastly, the reaction of 1B and 2B with the
[MþH] peak at m/z 712.3024 for 1C [Calcd. 712.3044] and as [M]
peak at m/z 697.2756 for 2C [Calcd. 697.2809] (Table 3).
Michael acceptor substrate, methyl vinyl ketone yielded the
proline and product adduct species 1C and 2C. Importantly, both of
L
-
In summary, the FeeNHC complexes of benzimidazole derived
Nꢀheterocyclic carbene ligands carried out the asymmetric
Fig. 4. The L-proline adduct species (1e2)A, the L-proline and substrate adduct species (1e2)B and the L-proline and product adduct species (1e2)C of the FeꢀNHC precatalysts
1e2).
(

A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
113
Michael addition reactions of cyclic
b
-oxo ester and
a-methyl
cyano ester substrates under ambient reaction conditions.
L-pro-
line was found to be the best chiral auxiliary among the ones used
in the reaction. Between the two types of Michael donor sub-
strates the
a-methyl cyano ester substrates gave better yields and
cyclic -oxo esters gave better enantioselectivities. Among the
b
Michael acceptor used, the methyl vinyl ketone produced high
yields and enantioselectivities. In addition, key intermediate
species relevant for the FeꢀNHC mediated Michael addition re-
action namely,
substrate adduct species (1e2)B and
L
-proline adduct species (1e2)A,
L
-proline and
-proline and product adduct
species (1e2)C were detected in the mass spectrometry.
L
3
. 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
USA) and used without any further purification. 1-Benzyl-3-
isopropyl-benzimidazolium bromide (a) [61], 1-benzyl-3-ethyl-
benzimidazolium iodide (b) [62], CpFe(CO) I [63,64], ethyl 2-
3 2 2 2
) and [CpFe(CO) ] were purchased from Sigma Aldrich
(
2
cyanopropanoate [21], isopropyl 2-cyanopropanoate [21] and t-
butyl 2-cyanopropanoate [21] were synthesized according to
1
13
1
modified literature procedures. H and C{ H} NMR spectra were
recorded on Varian and Bruker 400 MHz and Bruker 500 MHz
NMR spectrometer. 1H NMR peaks are labeled as singlet (s),
doublet (d), triplet (t), broad (br), triplet of triplets (tt), doublet of
doublets (dd), multiplet (m), and septet (sept). Infrared spectra
were recorded on a Perkin Elmer Spectrum One FT-IR spectrom-
eter. Mass spectrometry measurements were done on a Micro-
mass 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
1e2) were collected on a Rigaku Hg 724 þ diffractometer and
crystal data collection and refinement parameters are summa-
rized in Table S1. The structures were solved by using direct
methods and standard difference map techniques, and were
2
refined by full-matrix least-squares procedures on F with
SHELXTL (Version 6.10) [65,66]. 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 and the GC
analyses were done using Agilent Technologies 7890A GC systems
with CP-Chirasil-Dex CB chiral column.
4
. Synthesis of the Fe¡NHC complexes
4.1. Synthesis of {Cp[1-benzyl-3-isopropyl-benzimidazol-2-
2
ylidene]Fe(CO) }I (1)
To a mixture of 1-benzyl-3-isopropyl-benzimidazolium bro-
mide (a) (0.250 g, 0.755 mmol) and KN(SiMe (0.301 g,
3 2
)
1
.51 mmol), dry THF (ca. 15 mL) was added and the reaction
ꢁ
mixture was cooled to 0 C while stirring. The resultant yellow
solution was vigorously stirred for 1 h at 0 C. To the reaction
ꢁ
2
mixture CpFe(CO) I (0.275 g, 0.906 mmol) was added and further
stirred at room temperature for 10 h. The solvent was then
removed under vacuum and the residue was purified by 230e400
mesh silica gel column chromatography by eluting with a mixed
3
medium of MeOH/CHCl (v/v 10:90) to get the product 1 as an
1
yellow green solid (0.276 g, 66% yield). H NMR (CDCl
3
, 500 MHz,
ꢁ
3
2
C
C
5 C):
d
ppm, 7.68 (d, 1H, JHH ¼ 8 Hz, C
7 4 2
), 7.02 (d, 2H,
H N ), 7.35e7.24 (m, 5H,
3
3
6
H
H
5
), 7.18 (d, 1H,
J
HH ¼ 8 Hz, C
7
H
4
N
2
J
HH ¼ 7 Hz,
7
4
N
2
), 5.94 (s, 2H, CH
2
), 5.49 (s, 5H, C
HH ¼ 5 Hz, CH(CH
5
H
5
), 4.38e4.33 (m, 1H,
3
13
1
CH(CH
3
)
2
), 1.82 (d, 6H,
J
3 2
) ). C{ H} NMR

114
A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
ꢁ
(
C
DMSO-d6, 400 MHz, 25 C):
), 136.5 ipso-(C ), 135.4 (C
), 127.6 (C ), 125.9 (C ), 123.3 (C
13.0 (C ), 112.1 (C ), 87.8 (C
CH(CH ), 20.2 (CH(CH
s) and 1996 (s). HRMS Calcd for the [MꢀI] fragment
23FeN ]: m/z 427.1104; Found m/z 427.1084. Anal. Calcd. for
23FeN I$CHCl : C, 44.58; H, 3.59; N, 4.16; Found: C, 44.85; H,
.00; N, 4.05%.
d
ppm, 210.6 (CO), 182.6 (Fe-NCN of
), 133.4 (C ), 128.6
), 123.3 (C ),
), 54.4 (CH ), 52.4
). IR data (cm ) KBr pellet ( CO): 2043
spectrometry (ESI-MS) study parameters as follows: nebulizer gas
7
H
4
N
2
6
H
5
7
H
4
N
2
7
H
4
N
2
(N
2
, 0.3e0.5 bar), dry gas flow rate (4 lit/min), dry gas temperature
ꢁ
(C
6
H
5
H
7 4
N
2
6
H
5
7
4
H N
2
6
H
5
(180 C), capillary voltage (3700 V), spray shield voltage (500 V), set
charging voltage (2000 V), set APCI heater (0 C) purge flow rate of
200.0
ꢁ
1
7
)
H
4
N
2
7
H
4
N
2
5
H
5
2
ꢀ
1
ꢀ1
(
(
[
3
2
3
)
2
n
m
L h and syringe diameter of 4.610 mm. The instrument
þ
data was acquired within a mass range from m/z 50 to 1500. The
mass spectrometer was calibrated with NaI-CsI mixture. Several
runs were performed at various collision gas pressures and en-
ergies to optimize the formation of the fragment ions and assist in
spectral interpretation.
C
24
H
2 2
O
C
4
24
H
2
O
2
3
4.2. Synthesis of {Cp[1-benzyl-3-ethyl-benzimidazol-2-ylidene]
Fe(CO) }I (2)
2
6.2. General procedure for the direct ionization ESI-MS detection of
FeꢀNHC precatalysts (1e2)
To a mixture of 1-benzyl-3-ethyl-benzimidazolium iodide (b)
(
(
0.365 g, 1.00 mmol) and KN(SiMe
ca. 15 mL) was added and the reaction mixture was cooled to 0 C
3
)
2
(0.400 g, 2.00 mmol), dry THF
The FeꢀNHC complexes {Cp[1-benzyl-3-alkyl-benzimidazol-2-
ꢁ
ylidene]Fe(CO)
solved in CH
diluted with a mixture of CH
2
}I, where [alkyl ¼ i-Pr (1) and Et (2)] were dis-
while stirring. The resultant yellow solution was vigorously stirred
for 1 h at 0 C. To the reaction mixture CpFe(CO)
3
CN (ca. 1 mL) at room temperature and was further
CN: H O (ca. 5 mL). An aliquot of the
ꢁ
2
I (0.365 g,
3
2
1
.20 mmol) was added and further stirred at room temperature for
0 h. The solvent was then removed under vacuum and the residue
was purified by 230e400 mesh silica gel column chromatography
by eluting with a mixed medium of MeOH/CHCl (v/v 10:90) to get
the product 2 as an yellow green solid (0.157 g, 29% yield). H NMR
sample solution was directly injected into the ESI/MS to obtain the
mass spectrometric data for (1e2).
1
3
1
6.3. General procedure for the direct ionization ESI-MS detection of
the intermediate adduct species (1e2) (AꢀB) formed from the
FeꢀNHC precatalysts
ꢁ
3
(
CDCl
3
, 400 MHz, 25 C):
d
ppm, 7.51 (d, 1H,
J
HH ¼ 8 Hz, C
), 7.17 (d, 1H, JHH ¼ 7 Hz, C ), 7.09 (d, 1H,
), 6.97e6.96 (m, 2H, C ), 5.88 (s, 2H, CH ),
HH ¼ 7 Hz, CH CH ), 1.58 (t, 3H,
). C{ H} NMR (DMSO-d6, 400 MHz, 25 C):
ppm, 211.3 (CO),183.2 (Fe-NCN of C ),136.1 ipso-(C ),135.9
),135.6 (C ), 129.1 (C ), 128.0 (C ), 126.4 (C ),
26.4 (C ), 124.1 (C ), 112.1 (C ), 112.0 (C ), 88.4
), 52.7 (CH ), 44.8 (CH CH ), 14.9 (CH CH ). IR data (cm ) KBr
pellet (
CO): 2041 (s) and 1992 (s). HRMS Calcd for the [MꢀI]
fragment [C23 21FeN ]: m/z 413.0947; Found m/z 413.0930. Anal.
Calcd. for C23 21FeN I$2H O: C, 47.94; H, 4.37; N, 4.86; Found: C,
7 4 2
H N ),
3
7
.33e7.22 (m, 4H, C
6
H
5
6 5
H
3
J
HH ¼ 8 Hz, C
7
H
4
N
2
7
H
4
N
2
2
3
In a typical run, performed in air, a 5 mL vial was charged with a
5
.45 (s, 5H, C
5
H
5
), 4.76 (q, 2H,
J
2
3
3
13
1
ꢁ
mixture of the FeꢀNHC precatalysts (1e2) (0.009 mmol),
L
-proline
JHH ¼ 7 Hz, CH
2 3
CH
(
0.009 mmol), the Michael donor substrate, ethyl 2-
d
7
H
4
N
2
6 5
H
3
oxocyclopentane-1-carboxylate (0.09 mmol) and Et N (0.18 mmol)
followed by CHCl (ca. 2 mL) and the resultant mixture was stirred
3
at room temperature for 1 h. The reaction mixture was then diluted
(C
7
4
H N
2
7
H
4
N
2
6
H
5
7
H
4
N
2
6 5
H
1
6
H
5
7
4
H N
2
7
H
4
N
2
7 4 2
H N
ꢀ1
(C
5
H
5
2
2
3
2
3
þ
with ca. 10 mL of CH CN:H O (1:1) mixture and an aliquot of the
n
3
2
sample solution was directly injected into the ESI/MS to obtain the
mass spectrometric data for the adduct species (1e2) (A¡B).
H
2 2
O
H
2
O
2
2
4
7.72; H, 3.95; N, 5.19%.
6
.4. General procedure for the direct ionization ESI-MS detection of
5
. General procedure for asymmetric Michael addition
the ethyl 2-oxo-1-(3-oxobutyl)cyclopentane-1-carboxylate product
adduct species (1e2)C formed from the FeꢀNHC precatalysts
In a typical catalysis run, performed in air, a 20 mL vial was
charged with a mixture of Michael donor:Michael acceptor:base in
a molar ratio of 1:3:3. An iron complex (1e2) (0.02 mmol, 2 mol %)
In a typical run, performed in air, a 5 mL vial was charged with a
mixture of the FeꢀNHC precatalysts (1e2) (0.01 mmol),
L
-proline
and
L
-proline (0.0023 g, 0.02 mmol, 2 mol %) were added to the
(ca. 5 mL) and the resultant solution was
(
1
(
0.1 mmol), the Michael donor substrate, ethyl 2-oxocyclopentane-
mixture followed by CHCl
3
-carboxylate (0.1 mmol), the activated olefin, methyl vinyl ketone
N (0.5 mmol) followed by CHCl (ca. 2 mL) and
the resultant mixture was stirred at room temperature for 4 h. The
reaction mixture was then diluted with ca. 10 mL of CH CN:H
1:1) mixture and an aliquot of the sample solution was directly
stirred at room temperature for 8 h. The volatiles were then
removed in vacuo and the crude product was purified by column
chromatography using silica gel as stationary phase and eluting
with a mixed medium of petroleum ether:EtOAc (v/v 80:20e70:30)
to give the Michael adduct (3e15) as colorless liquid. The ee was
determined by GC with a CP-Chirasil-Dex CB chiral column.
0.2 mmol) and Et
3
3
3
2
O
(
injected into the ESI/MS to obtain the mass spectrometric data for
the final Michael addition product adduct species (1e2)C.
6
. Mass spectrometry
Acknowledgments
6.1. General procedures
We thank the Department of Science and Technology (EMR/
2
014/000254), New Delhi, India, for financial support of this
The molecular weights of the FeꢀNHC precatalysts (1e2), the
L-
research. We gratefully acknowledge the Single Crystal X-ray
Diffraction Facility, Department of Chemistry, IIT Bombay, Mumbai,
India, for the crystallographic characterization data. A.P.P. and
M.K.G thank CSIR, New Delhi, India, for research fellowships.
proline adduct species (1e2)A, the
L-proline and ethyl 2-
oxocyclopentane-1-carboxylate substrate adduct species (1e2)B
and the -proline and ethyl 2-oxo-1-(3-oxobutyl)cyclopentane-1-
L
carboxylate product adduct species (1e2)C formed from the
FeꢀNHC precatalysts were detected by direct-injection electro-
spray mass spectrometry (DIESI-MS) studies performed on a
BRUKER maxis impact. The mass analysis measurements were ac-
quired in positive ion detection modes on an ion trap mass spec-
trometer with an ESI source. The direct-injection electrospray mass
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jorganchem.2018.01.061.

A.P. Prakasham et al. / Journal of Organometallic Chemistry 859 (2018) 106e116
115
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