Palladium(II), silver(I), and gold(I) complexes of a new class of chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbenes (NHCs): Synthesis, structure and application studies

Article abstract of DOI:10.1016/j.poly.2020.115011

A new class of chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbene (NHC) ligands was synthesised in its enantiopure form from commercially available, cheap amino acid without undertaking any chiral resolution. In particular, the bicycli

Full text of DOI:10.1016/j.poly.2020.115011

Polyhedron 197 (2021) 115011
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Palladium(II), silver(I), and gold(I) complexes of a new class of chiral
bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbenes (NHCs):
Synthesis, structure and application studies
1
2
⇑
Manoj Kumar Gangwar , Shreyata Dey, Prakasham. A. P , 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:
A new class of chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbene (NHC) ligands was
synthesised in its enantiopure form from commercially available, cheap amino acid without undertaking
any chiral resolution. In particular, the bicyclic N-heterocyclic carbene precursor, (S)-7-benzyl-6,6-(R1)2–
2-R2-[1,2,3]-triazolooxazinium iodide [R1 = R2 = Me (1a); R1 = H, R2 = Me (2a); R1 = H, R2 = Et (3a)] salts
were conveniently prepared by the N-alkylation reactions of the corresponding [1,2,3]- triazolooxazine
derivatives with alkyl iodides in ca. 37–73% yields. The copper mediated [3 + 2] cycloaddition reaction
of the PhCH2CH(N3)CR2OH (R = H, Me) azido alcohol compounds and propargyl bromide gave the desired
[1,2,3]-triazolooxazines. These chiral bicyclic [1,2,3]- triazolooxazine derived N-heterocyclic carbene
ligands were characterised in the form of its silver (NHC)AgCl (1–3)b, gold (NHC)AuCl (1–3)c, and the pal-
ladium (NHC)2PdCl2 (1–3)d and the PEPPSI type (NHC)PdI2(NC5H5) (1–3)e complexes (NHC = (S)-7-ben-
zyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-ylidene; R1 = H, Me and R2 = Me, Et; PEPPSI = Pyridine
Enhanced Precatalyst Preparation Stabilisation and Initiation).
Received 5 November 2020
Accepted 26 December 2020
Available online 31 December 2020
Keywords:
Palladium complexes
Silver complexes
Gold complexes
Chiral N-heterocyclic carbene (NHC)
Bicyclic triazolooxazine derived ligands
Transmetallation reactions
Homogeneous catalysis
Asymmetric Michael
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
nes of these bicyclic and tricyclic ring systems [11], only a handful
of examples report the use of well-defined transition metal based
The boundaries of conventional singlet N-heterocyclic carbenes
(NHCs), first reported by Arduengo in 1991 [1,2] and primarily of
the imidazole framework [3–6], are melting and expanding to var-
ious exotic and complex architectures having specialized attri-
singlet carbene complexes of these ligands for asymmetric cataly-
sis. Notable are the gold complexes of an axially chiral bicyclic
[1,2,4]- triazole based N-heterocyclic carbene ligand, namely,
[2-(adamantyl)-5-(2-R-naphthalen-1-yl)-[1,2,4]-triazoloisoquino-
lin-3-ylidene]AuCl [R = Me, cyclohexyl] that carried out enantios-
elective Diels–Alder cycloaddition reactions in high yields
(48–88%) and enantioselectivities (63–99% ee) [12]. Similarly, a
series of chiral bicyclic [2-(R1)-5-(R2)-pyrrolo-imidazol-2-yli-
butes. Consequently, in the last decade, there has been
a
tremendous explosion in the report of various kinds of singlet car-
bene ligands ranging from charge separated mesoionic ones (MICs)
[7,8] to their neutral counterparts, stabilised over a variety of multi
cyclic ring structures to the heteroatom stabilised acyclic deriva-
tives [9,10]. In this regard, exciting frameworks containing bicyclic
and tricyclic N-heterocyclic carbenes with chiral substituents hav-
ing restricted rotation have been developed [11] with the intent of
higher asymmetric induction in chiral catalysis, a field that
remains largely unexplored even though the achiral version has
been studied exhaustively. While much of the literature involves
organocatalytic applications of the in situ generated singlet carbe-
dene](COD)IrCl [R1
=
CH2Ph, CHPh2, CH(C6H4OCH3)2, CH
(C6H4CF3)2; CH(C6H4CH3)2; R2 = Ph, 1,3,5-(CH3)3C6H2, 2,6-(i-Pr)2C6-
H3, 2,4,6-(C6H11)3C6H2] type catalysts performed the asymmetric
transfer hydrogenation of ketones in presence of t-BuOK in i-pro-
panol at a low catalyst loading of 0.1 mol % in high yields (ca.
55–98%) and enantioselectivities (ca. 77–97% ee) at 70 °C in 1 h
of reaction time [13,14]. Despite high asymmetric induction, a
major drawback of these aforementioned catalyst systems arises
from their synthetic protocol that calls for a tedious chiral resolu-
tion of the racemic ligand for asymmetric catalysis applications.
In this regard, we became interested in developing a variety of
multi-cyclic singlet carbene platforms having restricted rotation of
the chiral auxiliary with the intent of achieving higher asymmetric
1
Present address: KAUST Catalysis Center (KCC) King Abdullah University of
Science and Technology, Kingdom of Saudi Arabia
2
Present address: School of Chemistry, Tel Aviv University, Israel
⇑
Corresponding author.
E-mail address: pghosh@chem.iitb.ac.in (P. Ghosh).
https://doi.org/10.1016/j.poly.2020.115011
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
induction. Hence we have reported the rhodium(I) and iridium(I)
complexes of the chiral bicyclic oxazolidine-fused N-heterocyclic
carbene ligands, namely, [3-R-6-methyl-7-phenyl-imidazooxazol-
5-ylidene]M(COD)Cl (R = s-Bu, i-Bu, i-Pr; M = Rh, Ir) [15] and the
rhodium(I) and iridium(I) complexes of the C2-symmetric chiral
tricyclic bioxazoline fused imidazole derived N-heterocyclic car-
bene ligand, namely, [3,7-di-R-7-dioxazoloimidazol-5-ylidene]M
(COD)Cl [R = s-Bu, i-Bu, i-Pr; M = Rh [16], Ir [17]], which were syn-
thesised in a multistep sequence in their enantiopure forms start-
ing from the commercially available cheap chiral amino acids
without requiring any chiral optical resolution. Continuing further
along the line, we also set out to explore the catalytic potential of
other late transition metals mainly silver, gold and palladium in
asymmetric catalysis using the new chiral bicyclic N-heterocyclic
carbene platforms we are designing.
In connection with this, notable are our efforts in performing
Michael addition reaction, particularly under base-free bifunc-
tional catalysis condition using a variety of transition metal car-
bene complexes, namely that of nickel and iron, both in achiral
[18–20] and chiral fashions [21–23]. Michael addition provides a
classical platform for the construction of CAC bonds in organic
synthesis [24–26]. Hence building up on our earlier results, we
decided to explore the utility of palladium chiral bicyclic [1,2,3]-
triazolooxazine derived N-heterocyclic carbene (NHC) complexes
in asymmetric Michael addition reaction.
and d 3.28 ppm) and in 3a at (d 5.25 ppm and d 5.17 ppm), (d
4.22 ppm and d 4.13 ppm) and (d 3.51 ppm and d 3.18 ppm).
Further corroboration of the presence of the [1,2,3]-triazoloox-
azine ring in (1–3)a came from the IR studies that showed strong
m
C–O (ether) stretching band at ca. 1080 cmꢀ1 [Supporting Informa-
tion Table S1]. Additionally, the stretching mC–H (cmꢀ1) vibration
assignable to the CH2 moieties of the [1,2,3]-triazolooxazine ring
appeared at ca. 2900–2800 cmꢀ1, while the two benzyl ring C@C
stretching bands were observed at ~ 1600 and 1450 cmꢀ1 with
medium to weak intensities. As expected similar IR spectra were
observed for all of the respective silver(I) (1–3)b, gold(I) (1–3)c,
palladium(II) (1–3)d and (1–3)e complexes [Supporting Informa-
tion Table S1].
Both of the dimethyl [1,2,3]-triazolooxazine and the dihydro
[1,2,3]-triazolooxazines were synthesized by using the Cu medi-
ated ‘‘Click reaction” involving a [3 + 2] cyclocaddition reaction
between the corresponding azido alcohol PhCH2CH(N3)CR2OH
(R = H, Me) and propargyl bromide [27–29].
This new class of chiral bicyclic [1,2,3]-triazolooxazoline
derived N-heterocyclic carbene ligands bearing dimethyl and dihy-
dro substituents at the 6,6 position of the [1,2,3]-triazolooxazoline,
were characterised in its metal complexes through a variety of
metalation studies. In this regard, the following silver complexes,
[(S)-7-benzyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-ylidene]AgCl
[R1 = R2 = Me (1b); R1 = H, R2 = Me (2b); R1 = H, R2 = Et (3b)], were
obtained from the reaction of the corresponding [1,2,3]-triazoloox-
azin-3-ilium iodide salts (1–3)a with Ag2O in the presence of one
equivalent of NaCl in CH2Cl2 at room temperature in ca. 91%
(1b), ca. 68% (2b) and ca. 65% (3b) yields respectively (Scheme 2).
The 1H NMR spectra of the silver (1–3)b complexes showed the
absence of the diagnostic [1,2,3]- triazolooxazin-3-ilium (C2HN3)
resonances appearing at d 9.38 ppm (1a), d 9.29 ppm (2a) and d
9.30 ppm (3a) in the NHC precursors and thereby attesting to the
formation of the desired silver (1–3)b complexes. Interestingly,
the diastereotopic OCH2 moieties of silver complexes (1–3)b
appeared at higher frequency region in comparison to its corre-
sponding ligand (1–3)a, while for the benzyl CH2 group the differ-
ences were less significant. For example, the OCH2 resonance
appeared upfield shifted at d (4.99 ppm and 4.88 ppm) from the
corresponding value of d (5.15 ppm and 5.05 ppm) for 1a, while
the diastereotopic benzyl CH2 resonances of 1b appeared at d
(3.25 ppm and 3.17 ppm) with respect to that of d (3.26 ppm
and 3.16 ppm) for 1a. Along the same line of observation, the 1H
NMR spectra of the 2b and 3b complexes also showed upfield shifts
Here in this manuscript we report a new class of chiral bicyclic
[1,2,3]-triazolooxazine derived N-heterocyclic carbene (NHC)
ligands along with their silver(I) (1–3)b, gold(I) (1–3)c, palladium
(II) (1–3)d and (1–3)e complexes (Fig. 1). The palladium (2–3)d
and (2–3)e complexes were explored for asymmetric Michael addi-
tion reaction.
2. Results and discussions
A new class of chiral bicyclic [1,2,3]-triazolooxazoline based N-
heterocyclic carbene namely, (S)-7-benzyl-6,6-(R1)2–2-R2-[1,2,3]-
triazolooxazin-3-ylidene [R1 = H, Me; R2 = Me, Et] was designed
with the intent of probing its utility in homogenous asymmetric
catalysis and was synthesised by a multi-step sequence (Scheme 1)
starting from commercially available cheap chiral amino acid pre-
cursor. The strategy involved preparation of these chiral bicyclic
[1,2,3]-triazolooxazoline based N-heterocyclic carbene ligands in
their enantiopure form without any tedious chiral resolution
protocol.
(Dd) of ca. 0.30 ppm and ca. 0.20 ppm for the OCH2 moieties as
The dimethyl N-heterocyclic carbene precursor 1a and its
dihydro counterparts (2–3)a were synthesised from the dimethyl
(S)-7-benzyl-6, 6-(Me)2-[1,2,3]-triazolooxazine and dihydro (S)-7-
benzyl-6, 6-(H)2-[1,2,3]-triazolooxazine respectively. The reaction
of these [1,2,3]-triazolooxazines with methyl or ethyl iodides in
refluxing CH3CN produced the desired N-heterocyclic carbene
precursors, 1a (61%), 2a (73%) and 3a (37%) in respective yields.
Notable are the [1,2,3]-triazolooxazin-3-ilium (C2HN3) reso-
nances that appeared at d 9.38 ppm (1a), d 9.29 ppm (2a) and
d 9.30 ppm (3a) in the 1H NMR and at d 135.7 ppm (1a), d
137.5 ppm (2a) and d 137.0 ppm (3a) in the 13C{1H} NMR spec-
tra. The assignments of the 1H and 13C{1H} NMR spectra for a
representative compound (3a) are shown in Fig. 2 and Fig. 3
respectively.
Another interesting feature is the observation of two sets of
geminal couplings (2JHH = ca. 14–17 Hz) for the two diastereotopic
CH2 moieties present in 1a that appeared at (d 5.15 ppm and d
5.05 ppm) and (d 3.26 ppm and d 3.16 ppm) and the three sets of
such geminal couplings (2JHH = ca. 13–16 Hz) for the three diaster-
eotopic CH2 moieties present in 2a that appeared at (d 5.25 ppm
and d 5.18 ppm), (d 4.20 ppm and d 4.13 ppm) and (d 3.58 ppm
compared to that in 2a and 3a respectively.
The molecular structures of the silver complexes (1b and 3b)
were determined by the single crystal X-ray crystallography stud-
ies [Fig. 4 (1b), Fig. 5 (3b) and Supporting Information Tables S2
and S4]. The X-ray diffraction studies showed that the chiral bicyc-
lic triazolooxazine derived NHC moiety was bound to the metal
center in a linear fashion with a chloride moiety at the opposite
end. The geometry at the metal center is consistent with the obser-
vation of linear \ C1 ꢀ Ag1 ꢀ Cl1 angle of \ 174.48(17)° in 1b and
of \ 176.95(11)° in 3b. The Ccarbene–Ag bond distances in 1b [2.075
(5) Å] and 3b [2.063(4) Å], were comparable to the sum of the
covalent radii of Ag and C (2.18 Å) [30].
Significantly enough the silver complexes 1b and 3b repre-
sented the only structurally characterised example of the chiral
bicyclic [1,2,3]-triazolooxazoline derived N-heterocyclic carbene
ligands. In this context, the closest structurally characterised
examples were those of a handful of silver complexes reported
for the family of the [1,2,3]-triazole derived N-heterocyclic carbene
analogues (Table 1). The Ccarbene–Ag distances of 2.075(5) Å in 1b
and 2.063(4) Å in 3b were comparable to 2.064(12) Å in {[(4-cobal-
toceniumyl-1-ferrocenyl-3-methyltriazolylidene)]AgCl}OTf [31],
2

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
R1
R1
R1
O
R1
O
N
N
N
Ag Cl
Au Cl
Ph
N
Ph
N
N
R2
R2
1-3 b
1-3 c
(
)
(
)
1
2
1 a-e
R = R = Me [ ( )]
1
2
2 a-e
R = H, R = Me [ ( )]
1
2
3 a-e
R = H, R = Et [ ( )]
R1
O
R1
R1
R1
R1
R1
O
O
Ph
Ph
N
N
N
N
N
I
N
N
Pd
I
N
Ph
N
R2
N
R2
R2
Cl
Cl
1-3 d
1-3 e
)
(
)
(
Fig. 1. Pd(II) (1–3)d and (1–3)e, Ag(I) (1–3)b, and Au(I) (1–3)c complexes of the chiral bicyclic triazolooxazine derived N-heterocyclic carbene (NHC) ligands.
Scheme 1. Synthetic routes to the chiral bicyclic triazolooxazine derived N-heterocyclic carbene (NHC) ligand precursors (1–3)a are shown.
2.075(4) Å in {[(1,3-(2,6-i-Pr2C6H3)2–5-CH2-4-ylidene]2NH}Ag2Cl2
[32] and 2.084(4) Å in [(1-methyl-2-phenyl-4-tolyl-[1,2,3]-tria-
zol-5-ylidene)]AgCl [33].
lic [1,2,4]-triazolooxazoline derived N-heterocyclic carbene com-
plexes of silver, a handful of structurally characterised [1,2,4]-
triazolium based N-heterocyclic carbene complexes are known,
that exhibit bond distances of 2.085(10) Å and 2.086(11) Å in a
dimeric {[(S)-(5-(fluorodiphenylmethyl)-2-phenyl-pyrrolo-[1,2,4]-
triazol-3-ylidene)]AgCl}2 [35] and of 2.079(3) Å in [1-(tert-butyl)-
In the absence of any chiral bicyclic [1,2,3]-triazolooxazoline
derived N-heterocyclic carbene complexes of silver, a comparison
of the Ccarbene–Ag distances in 1b and 3b is made with another type
of chiral dicationic and tricationic silver complexes of chiral
bicyclic [1,2,4]-triazolooxazoline derived N-heterocyclic carbene
platforms namely, {2,20-[5-R-[1,2,4]-triazolooxazin-3-ylidene]2-
C6H4]2Ag2}(X)2 and {2,20-[5-R-[1,2,4]-triazolooxazin-3-ylidene]2-
C6H4]3Ag3}(X)3 (R = Ph, CH2Ph, i-Pr; X = PF6, BF4, Cl) [34]. For
example, the Ccarbene–Ag distances of 2.044(12) Å and 2.085(12)
Å in representative dicationic {2,20-[5-Ph-[1,2,4]-triazolooxazin-3-
ylidene]2-C6H4]2Ag2}(PF6)2 and of 2.083(5) Å and 2.078(5) Å in
representative tricationic {2,20-[5-CH2Ph-[1,2,4]-triazolooxazin-3-
ylidene]2-C6H4]3Ag3}(PF6)3 complexes are comparable to that of
2.075(5) Å and 2.063(4) Å in the covalent silver complexes 1b
and 3b respectively. Other than the above mentioned chiral bicyc-
3-phenyl-4-(p-tolyl)-[1,2,4]-triazol-2-ylidene)]AgCl
[36].
The
above examples clearly depict the rarity of the silver complexes
of the chiral bicyclic [1,2,3]-triazoloxazine derived N-heterocyclic
carbene ligands, and for which the complexes 1b and 3b represents
the only structurally characterised examples known till date.
The silver complexes (1–3)b were conveniently transmetallated
with (SMe2)AuCl at room temperature to obtain the following gold
complexes, [(S)-7-benzyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-
ylidene]AuCl [R1 = R2 = Me (1c); R1 = H, R2 = Me (2c); R1 = H,
R2 = Et (3c)], in ca. 71% (1c), ca. 68% (2c) and ca. 85% (3c) yields
respectively (Scheme 2). Quite expectedly, the 1H NMR spectra of
the gold complexes (1–3)c closely resembled the related silver
3

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
O
3
5
7
1
4
6
N
+
Ph
I
8
N
N
Ar
2
3a
)
(
1
8
2
Ar
5
3
4
7
4
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
ppm
Fig. 2. 1H NMR assignments of a representative compound 3a.
O
3
6
5
7
4
N
+
Ph
I
8
N
N
Ar
2
3a
( )
1
Fig. 3. 13C{1H} NMR assignments of a representative compound 3a.
analogues (1–3)b, indicating their structural homology as an out-
come of similar electronic environment prevalent in these metal
complexes. For example, the NCH3 resonance of the gold 1c com-
plex appeared at d 3.93 ppm in close agreement with the value
of d 4.03 ppm, observed in case of the silver 1b complex. Further-
more, for the gold 1c complex, the two doublets arising out of the
diastereotopic OCH2 protons appeared at d 4.92 ppm and d
4.80 ppm and are similar to that of d 4.99 ppm and d 4.88 ppm
observed for the silver 1b complex. In the 13C{1H} NMR, the diag-
nostic peak for the carbene–Au resonance appeared at
153.1 ppm for 1c, d 154.1 ppm for 2c and d 159.1 ppm for 3c
respectively and are similar to the related [1,2,3]-triazole based
N-heterocyclic carbene gold complexes. For instance, the corre-
sponding Ccarbene–Au resonances appeared at d 156.2 ppm for
[1,4-bis(ferrocenyl)-3-methyl-[1,2,3]-triazol-5-ylidene)]AuCl [37],
C
d
d
157.0 ppm for [(1,4-(n-Bu)2–3-methyl-[1,2,3]-triazol-5-yli-
4

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
Scheme 2. Synthetic routes to the Pd(II) (1–3)d and (1–3)e, Ag(I) (1–3)b, and Au(I) (1–3)c complexes of the chiral bicyclic triazolooxazine derived N-heterocyclic carbene
(NHC) ligands are shown.
Fig. 5. ORTEP diagram of 3b with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Ag1 ꢀ C1 2.063(4),
Ag1 ꢀ Cl1 2.3161(10), N3 ꢀ C1 1.354(5), C1 ꢀ Ag1 ꢀ Cl1 176.95(11), N3 ꢀ C1 ꢀ Ag1
125.6(3), N3 ꢀ N2 ꢀ N1 102.3(3), N2 ꢀ N3 ꢀ C1 115.7(3).
Fig. 4. ORTEP diagram of 1b with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Ag1 ꢀ C1 2.075(5),
Ag1 ꢀ Cl1 2.3338(17), N3 ꢀ C1 1.377(7), C1 ꢀ Ag1 ꢀ Cl1 174.48(17), N3 ꢀ C1 ꢀ Ag1
1c [1.985(6) Å], 2c [2.000(12) Å] and 3c [1.981(7) Å], are indeed
smaller than the corresponding Ccarbene–Ag bond distances in 1b
[2.075(5) Å] and 3b [2.063(4) Å] by ca. 0.08–0.09 Å. For the com-
126.7(4), N1 ꢀ N2 ꢀ N3 102.2(4), N2 ꢀ N3 ꢀ C1 115.6(4).
parison purpose the Ccarbene–Au distances in {[(1,4-dicobaltoceni-
umyl-3-methyl-[1,2,3]-triazol-5-ylidene)]AuCl}(PF6)(OTf) [1.982
(4) Å] [31] and [(1,3-(2,6-i-Pr2C6H3)2–4-phenyl-[1,2,3]-triazol-5-
ylidene)]AuCl [1.991(6) Å] [39] are also in good agreement with
that observed in the gold (1–3)c complexes. In this context, it is
worth noting that though a related chiral bicyclic [1,2,4]-tria-
zolooxazine derived N-heterocyclic carbene complex of gold
namely, [(S)-8-benzyl-2-mesityl-[1,2,4]-triazolooxazin-3-ylidene]
AuCl [40], has been prepared in situ, no spectroscopic as well as
X-ray crystallography data had been reported for the same.
Further characterization of the [1,2,3]-triazolooxazoline derived
N-heterocyclic carbene ligands (1–3)a was carried out, by prepar-
ing two types of palladium complexes, i.e. the (NHC)2PdCl2 type
dene)]AuCl [38] and d 156.6 ppm for [1-(n-Bu)-3-methyl-4-(CMe2-
OAc)-[1,2,3]-triazol-5-ylidene)]AuCl [38].
In concurrence with the similar NMR data observed for the sil-
ver (1–3)b and the gold (1–3)c complexes, the single crystal X-ray
diffraction study revealed analogous molecular structures for these
complexes (Figs. 4–7 and Supporting Information Fig. S1 and
Tables S1–S3). All of the silver 1b and 3b and the gold (1–3)c com-
plexes exhibited a linear geometry at the metal centre consistent
with that of the d10 configuration of the metal ion, with the singlet
chiral bicyclic [1,2,3]-triazolooxazin-3-ylidene ligand and a chlo-
ride moieties occupying the diametrically opposite sites. Further-
more, consistent with a shorter covalent radius (rcov / Å) of Au as
compared to Ag by 0.09 Å [30], the Ccarbene–Au bond distances in
5

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
Table 1
A comparison of the metrical data of the structurally characterised silver complexes of the chiral bicyclic triazolooxazoline derived N-heterocyclic carbenes with the closely
related [1,2,3]-triazole and [1,2,4]-triazole based N-heterocyclic carbene analogues.
S.
complex
d(Ccarbene–Ag)/ (Å)
d(Ag–Cl)/ (Å)
Reference
No.
1
2.064(12)
2.345(3)
Chem. Eur. J. 2018, 24, 3742–3753
2
2.075(4)
2.324(3)
ChemistrySelect 2018, 3(38), 10706–
10710
3
2.084(4)
2.335(1)
Inorg. Chem. 2013, 52, 6142–6152
4
2.044(12) and 2.085
(12)
N.A
Chem. Sci., 2012,3, 359–363
5
2.083(5) and 2.078
(5)
N.A
Chem. Sci., 2012,3, 359–363
6

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
Table 1 (continued)
S.
complex
d(Ccarbene–Ag)/ (Å)
d(Ag–Cl)/ (Å)
Reference
No.
6
2.085(10) and 2.086
(11)
2.334(2) and 2.326 Organometallics, 2010, 29, 20, 4424–
(3)
4427
7
2.079(3)
2.3601(6)
Dalton Trans., 2014,43, 15465–15474
8
2.075(5)
2.3338(17)
This work
9
2.063(4)
2.3161(10)
This work
(1–3)d and the PEPPSI type (NHC)PdI2(NC5H5) (1–3)e complexes
NMR spectra can serve as a diagnostic tool for them. The Ccarbene–
(NHC
=
(S)-7-benzyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-yli-
Pd resonances appeared at d 143.9 ppm (1d), d 137.5 ppm (2d)
and d 143.6 ppm (3d) in the respective 13C{1H} NMR spectra, which
were significantly upfield shifted from the related [1,2,3]-triazoloox-
azine based trans-(NHC)2PdCl2 type complexes. For example, the
corresponding Ccarbene–Pd resonances appeared at d 159.0 ppm for
[1-ethyl-3-methyl-4-phenyl-[1,2,3]-triazol-5-ylidene)]2PdCl2 [41]
and d 157.1 ppm for [1-(n-Bu)-3-methyl-4-(CMe2OMe)-[1,2,3]-tria-
zol-5-ylidene)]2PdCl2 [42].
dene; R1 = H, Me and R2 = Me, Et). The silver complexes (1–3)b
reacted with (COD)PdCl2 at room temperature to yield the [(S)-7-
benzyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-ylidene]2PdCl2 [R1
=
R2 = Me (1d); R1 = H, R2 = Me (2d); R1 = H, R2 = Et (3d)] complexes
in 28–67% yields (Scheme 2). Quite expectedly, the 1H NMR spectra
of the palladium (1–3)d complexes, mirrored that of its analogous
silver (1–3)b and gold (1–3)c complexes, so the analysis of 13C{1H}
7

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
Fig. 6. ORTEP diagram of 1c with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Au1 ꢀ C1 1.985(6),
Au1 ꢀ Cl1 2.2936(16), N3 ꢀ C1 1.370(6), C1 ꢀ Au1 ꢀ Cl1 178.4(3), N3 ꢀ C1 ꢀ Au1
125.1(4), N3 ꢀ N2 ꢀ N1 103.2(4), N2 ꢀ N3 ꢀ C1 115.1(5).
Fig. 8. ORTEP diagram of 1d with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Pd1 ꢀ C1 1.975(10),
Pd1 ꢀ Cl1 2.374(2), N1 ꢀ C1 ꢀ Pd1 126.4(6), C1 ꢀ Pd1 ꢀ C1i 92.0(5), C1 ꢀ Pd1 ꢀ Cl1
86.6(3), Cl1 ꢀ Pd1 ꢀ Cl1i 94.77(12), C1i ꢀ Pd1 ꢀ Cl1i 86.6(3), C1 ꢀ Pd1 ꢀ Cl1i 178.5
(3).
Fig. 7. ORTEP diagram of 3c with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Au1 ꢀ C1 1.981(7),
Au1 ꢀ Cl1 2.296(2), N3 ꢀ C1 1.350(9), C1 ꢀ Au1 ꢀ Cl1 178.5(2), N1 ꢀ C1 ꢀ Au1
126.6(6), N3 ꢀ N2 ꢀ N1 102.7(6), N2 ꢀ N3 ꢀ C1 115.3(7).
In contrast to the conventional wisdom of trans form being
more stable than the cis one, the molecular structures of palladium
complexes (1–3)d [Fig. 8 (1d), Supporting information Fig. S2 (2d),
Fig. 9 (3d) and Supporting Information Tables S2–S4] as deter-
mined by the single crystal X-ray crystallography, demonstrated
that the two chiral bicyclic [1,2,3]- triazolooxazoline derived N-
heterocyclic carbene ligands were in unusual cis disposition
around the square planer d8 palladium metal centre, with the other
two sites occupied by the chloride moiety. The observation of the
square planer geometry at the metal centre is consistent with
the value of \ Ccarbene ꢀ Pd ꢀ Ccarbene = 92.0(5)° (1d), 90.6(4)°
(2d), and 89.75(13)° (3d) and \ Cl1 ꢀ Pd ꢀ Cl2 = 94.77(12)° (1d),
93.58(9)° (2d) and 93.43(3)° (3d). In the absence of any struc-
turally characterised example of chiral bicyclic [1,2,3]-triazolooxa-
zoline derived N-heterocyclic carbene ligand based palladium
complexes, a comparison of the Ccarbene ꢀ Pd bond distances in
1d [1.975(10) Å], 2d [1.970(10) Å and 1.984(10) Å] and 3d [1.980
(3) Å and 1.984(10) Å] was made with that of the only two struc-
turally characterised racemic bicyclic [1,2,3]-triazolooxazine
derived N-heterocyclic carbene based PEPPSI type (NHC)PdI2(NC5-
H5) complexes, namely [3-(R)-5a,6,7,8,9,9a-hexahydro-benzo-
Fig. 9. ORTEP diagram of 3d with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Pd1 ꢀ C1 1.980(3),
Pd1 ꢀ C13 1.982(3), Pd1 ꢀ Cl1 2.3902(8), Pd1 ꢀ Cl2 2.3757(8), N3 ꢀ C1 ꢀ Pd1 129.7
(2), N6 ꢀ C13 ꢀ Pd1 126.2(2), C1 ꢀ Pd1 ꢀ C13 89.75(13), C1 ꢀ Pd1 ꢀ Cl1 88.61(9),
C1 ꢀ Pd1 ꢀ Cl2 177.74(9), C13 ꢀ Pd1 ꢀ Cl1 178.04(9), C13 ꢀ Pd1 ꢀ Cl2 88.19(9),
Cl2 ꢀ Pd1 ꢀ Cl1 93.43(3).
The most interesting feature about the PEPPSI (1–3)e type com-
plexes is the metal-bound ‘‘throwaway” pyridine moiety, which
functions as a labile ligand by giving way to the incoming substrate
in catalytic cycle. The metal-bound pyridine moiety appeared as
distinct resonances in both the 1H and 13C{1H} NMR spectra with
regard to that of the free pyridine. For example, the o-NC5H5 reso-
nances in 1H NMR appeared at d 9.03 ppm ꢀ 9.01 ppm (1e), d
9.03 ppm ꢀ9.01 ppm (2e) and d 8.99 ppm ꢀ 8.97 ppm (3e) in com-
parison to the value of d 8.58 ppm in free pyridine [44]. Similarly in
the 13C{1H} NMR the o-NC5H5 resonances appeared significantly
downfield shifted at d 154.1 ppm (1e), d 153.1 ppm (2e) and d
153.8 ppm (3e) from the value of d 149.5 ppm for free pyridine
[44]. Furthermore, the characteristic Ccarbene–Pd resonances at d
137.6 ppm (1e), d 138.5 ppm (2e) and d 137.6 ppm (3e) in the
respective 13C{1H} NMR spectra were considerably downfield
shifted with respect to the corresponding values, associated with
the only two structurally characterised racemic bicyclic [1,2,3]- tri-
[1,2,3]-triazolooxazin-4-ylidene]PdI2(NC5H5) [R = Me, d/ Ccarbene
-
ꢀ Pd = 1.989(14) Å; R = Et, d/ Ccarbene ꢀ Pd = 1.961(6) Å] [43].
The Pd–Cl distances were found to be [2.374(2) Å] in 1d, [2.396
(3) Å and 2 2.366(3) Å] in 2d and [2.3902(8) Å and 2.3757(8) Å]
in 3d respectively.
The metallation of the [1,2,3]-triazolooxazin-3-ilium iodide
salts (1–3)a with PdCl2 in pyridine, using K2CO3 as base and KI
yielded the PEPPSI type (NHC)PdI2(NC5H5) complexes (1–3)e in
32–46% yields (Scheme 2). Notably, the palladium (1–3)e com-
plexes were purified by column chromatography in a mixed sol-
vent medium of petroleum ether : EtOAc (4:1 v/v) as the eluent.
8

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
Fig. 11. ORTEP diagram of 3e with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Pd1 ꢀ C1 1.968(6),
Pd1 ꢀ N4 2.110(5), Pd1 ꢀ I1 2.5964(8), Pd1 ꢀ I2 2.6219(8), N3 ꢀ C1 ꢀ Pd1 124.3(5),
C1 ꢀ Pd1 ꢀ N4 175.1(3), C1 ꢀ Pd1 ꢀ I1 86.39(19), C1 ꢀ Pd1 ꢀ I2 89.05(19),
N4 ꢀ Pd1 ꢀ I1 91.75(16), N4 ꢀ Pd1 ꢀ I2 93.04(16), I1 ꢀ Pd1 ꢀ I2 174.48(2).
Fig. 10. ORTEP diagram of 1e with thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (Å) and angles (°): Pd1 ꢀ C1 1.974(8),
Pd1 ꢀ N4 2.078(7), Pd1 ꢀ I1 2.6038(9), Pd1 ꢀ I2 2.6004(10), N3 ꢀ C1 ꢀ Pd1 125.5
(6), C1 ꢀ Pd1 ꢀ N4 176.7(3), C1 ꢀ Pd1 ꢀ I1 88.0(2), C1 ꢀ Pd1 ꢀ I2 93.8(2),
N4 ꢀ Pd1 ꢀ I1 88.64(19), N4 ꢀ Pd1 ꢀ I2 89.59(19), I2 ꢀ Pd1 ꢀ I1 177.32(3).
2- oxocyclopentane-1-carboxylate and a Michael acceptor sub-
strate, methyl vinyl ketone, the palladium (2–3)d and (2–3)e com-
plexes efficiently produced the desired product ethyl-2-oxo-1-(3-
oxobutyl)cyclopentane-1-carboxylate (4) (Equation 1) under ambi-
ent temperature at 1 mol % of catalyst loading and 8 h of reaction
time, exhibiting near quantitative yield [85% (2d), 91% (2e), 88%
(3d), 89% (3e)] with negligible chiral induction [ee 1–14%] (Table 2).
Among the low enantioselectivities of the palladium (2–3)d and
(2–3)e complexes, the (NHC)2PdCl2 type (3d) complex showed
the maximum enantiomeric excess (ee) of 14%.
The observation of low asymmetric induction may be attributed
to the far away disposition of the chiral centre from the catalyti-
cally active metal centre, partaking the catalysis and, thereby
pointed towards subdued asymmetric transfer for this type of the
chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic car-
bene ligands, (S)-7-benzyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-
3-ylidene [R1 = H, Me; R2 = Me, Et]. Despite the observation of
the low ee values, all of the palladium (2–3)d and (2–3)e com-
plexes exhibited near equal yield for the representative substrates,
implying the catalytic pocket arising out of the ligand dispositions
around the metal centre remaining similar. Furthermore, no varia-
tion in product yields were observed between that of the (NHC)2-
PdCl2 type (2–3)d and the PEPPSI type (NHC)PdI2(NC5H5) (2–3)e
complexes.
azolooxazine derived N-heterocyclic carbene based PEPPSI type
(NHC)PdI2(NC5H5) complexes, namely [3-(R)-5a,6,7,8,9,9a-hexahy-
dro-benzo-[1,2,3]-triazolooxazin-4-ylidene]PdI2(NC5H5) [R = Me (d
129.0 ppm); R = Et (d 127.4 ppm)] [43].
The molecular structures of the palladium (1–3)e complexes as
determined by the single crystal X-ray crystallography [Fig. 10
(1e), Supporting information Fig. S3 (2e), Fig. 11 (3e) and Support-
ing Information Tables S1–S3] were also in corroboration with the
presence of a metal coordinated pyridine moiety. The Pd ꢀ Npyridine
bond distances in 1e [1.974(8) Å], 2e [1.974(9) Å] and 3e [1.968(6)
Å] were comparable to the sum of the covalent radii of Pd and N
(2.10 Å) [30]. Furthermore, the Pd ꢀ Npyridine bond distances in
(1–3)e were in good agreement with that of the only two struc-
turally characterised racemic bicyclic [1,2,3]-triazolooxazine
derived N-heterocyclic carbene based PEPPSI type (NHC)PdI2(NC5-
H5) complexes, namely [3-(R)-5a,6,7,8,9,9a-hexahydro-benzo-
[1,2,3]-triazolooxazin-4-ylidene]PdI2(NC5H5) [R
=
Me, d/
Pd ꢀ Npyridine = 2.090(11) Å; R = Et, d/Pd ꢀ Npyridine = 2.096(5) Å].
In the same line of comparison the Ccarbene ꢀ Pd bond distances
in 1e [1.974(8) Å], 2e [1.974(9) Å] and 3e [1.968(6) Å] and Pd ꢀ I
bond distances in 1e [2.6038(9) Å and 2.6004(10) Å], 2e [2.5989
(10) Å and 2.6116(10) Å] and 3e [2.5964(8) Å and 2.6219(8) Å]
were in similar range with the aforementioned complexes, [3-
(R)-5a,6,7,8,9,9a-hexahydro-benzo-[1,2,3]-triazolooxazin-4-yli-
Equation 1. Asymmetric Michael addition of ethyl-2- oxocy-
clopentane-1-carboxylate with methyl vinyl ketone using chiral
Pd complexes (2d/2e/3d/3e) of the chiral bicyclic triazolooxazine
derived N-heterocyclic carbene (NHC) ligands is shown.
In this regard, important is the comparison of the catalytic
activities of our palladium (NHC)2PdCl2 type (2–3)d and the PEPPSI
type (NHC)PdI2(NC5H5) (2–3)e complexes with that of the other
related palladium analogous for the asymmetric Michael addition
reaction of similar substrates. For example, the chiral cationic C2-
symmetric N-heterocyclic carbene complex of the type (NHC)Pd
(OH2)2(OTf)2 [NHC = 2,20-bis(3-R-benzo-imidazol-2-ylidene)-1,10-
dene]PdI2(NC5H5) [R = Me, d/ Ccarbene ꢀ Pd = 1.989(14) Å, d/
Pd ꢀ I = 2.6187(17) Å and 2.5904(17) Å; R = Et, d/ Ccarbene
-
ꢀ Pd = 1.961(6) Å, d/Pd ꢀ I = 2.6123(10) Å and 2.6031(10) Å] [43].
The asymmetric induction ability of the chiral bicyclic [1,2,3]-
triazolooxazine derived N-heterocyclic carbene ligands, (S)-7-ben-
zyl-6,6-(R1)2–2-R2-[1,2,3]-triazolooxazin-3-ylidene [R1 = H, Me;
R2 = Me, Et] was probed in asymmetric Michael addition reaction
using the archetypal pair of palladium (NHC)2PdCl2 type (2–3)d
and the PEPPSI type (NHC)PdI2(NC5H5) (2–3)e complexes. Specifi-
cally, for a representative pair of Michael donor substrate, ethyl-
9

Table 2
Selected results for the asymmetric Michael addition of ethyl-2- oxocyclopentane-1-carboxylate with methyl vinyl ketone as catalyzed using Pd ꢀ NHC (2d/2e/3d/3e) complexes.
yielda
85
eeb
yielda
91
eeb yielda
88
eeb
yielda
89
eeb
1
1
14
3
Reaction conditions: 1.00 mmol of Michael donor (ethyl-2- oxocyclopentane-1-carboxylate), 3.00 mmol of Michael acceptor (methyl vinyl ketone), 3.00 mmol of Et3N, 1 mol % of catalyst (2d/2e/3d/3e) in 5 mL of solvent at room
temperature for 8 h. (a) isolated yields (b) enantiomeric excess was determined by chiral GC using CP-Chirasil-Dex-CB column.

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
binaphthalene); R = Me, CH2Ph], with 5 mol % of catalyst loading
together with 4 A° molecular sieves produced the Michael addition
adduct, t-Bu-2-oxo-1-(3-oxobutyl)cyclopentane-1-carboxylate, in
high yield (ca. 98%) and high enantioselectivity (ca. 71% ee) at room
temperature for the representative substrates, t-Bu-2- oxocy-
clopentane-1-carboxylate and methyl vinyl ketone [45]. Similarly,
[Pd{(R)-tol-BINAP}(OH2)2](OTf)2, type complex catalysed the
Michael addition reaction at 5 mol % of catalyst loading at –20 °C
in ca. 92% yield and ca. 92% enantioselectivity for the representa-
tive substrates, t-Bu-2- oxocyclopentane-1-carboxylate and
methyl vinyl ketone [46]. Significantly enough, despite the above
comparisons, the palladium (2–3)d and (2–3)e complexes repre-
sents the only examples of structurally characterised molecular
catalysts for the asymmetric Michael addition reaction. Further-
more, the palladium (2–3)d and (2–3)e complexes operated at a
much lower catalyst loading of 1 mol % as opposed to 5 mol %
for the above catalysts, and also exhibited lower reaction time of
8 h as opposed to 24 h for the above mentioned literature
examples.
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. X-ray
diffraction data for all compounds were collected on a Rigaku Hg
724 + diffractometer. Crystal data collection and refinement
parameters were summarized in Supporting Information Tables
S2–S4. CCDC-1011069 (for 1b), CCDC-1012239 (for 1c), CCDC-
1021594 (for 1d), CCDC-1024176 (for 1e), CCDC-1011070 (for
2c), CCDC-1059753 (for 2d), CCDC-1024225 (for 2e), CCDC-
1023266 (for 3b), CCDC-1024174 (for 3c) and CCDC-1012271 (for
3d), CCDC-1040518 (for 3e) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data centre via
www.ccdc.cam.ac.uk/data_request/cif.
3.1.2. Synthesis of
L
-phenylalanine methyl ester hydrochloride [47]
-phenylalanine (5.00 g, 30.3 mmol) in CH3-
To a suspension of
L
OH (ca.75 mL), SOCl2 (5.20 mL, 43.7 mmol) was added dropwise at
0 °C, over a period of 30 min and the resulting reaction mixture
was stirred for overnight at room temperature. Then all the vola-
tiles were removed in vacuo, the crude compound was triturated
with Et2O (ca. 30 mL) followed by filtration and drying under vac-
uum to give the product as a white solid (3.60 g, 55%). 1H NMR
(D2O, 500 MHz, 25 °C), d 7.35–7.29 (m, 3H, (CH2C6H5), 7.20 (d,
3. Conclusion
In summary, a new class of chiral bicyclic [1,2,3]-triazoloox-
azine derived N-heterocyclic carbene (NHC) ligands (1–3)a was
synthesised and characterised by preparing its silver (NHC)AgCl
(1–3)b, gold (NHC)AuCl (1–3)c, palladium (NHC)2PdCl2 (1–3)d
and the PEPPSI type (NHC)PdI2(NC5H5) (1–3)e complexes. Signifi-
cantly enough, the silver (1b and 3b), gold (1–3)c, and the palla-
dium (1–3)d and (1–3)e complexes represent the only
structurally characterized examples known till date of the chiral
bicyclic [1,2,3]-triazoloxazine based N-heterocyclic carbene
ligands. These new class of chiral bicyclic [1,2,3]-triazolooxazine
derived N-heterocyclic carbene (NHC) ligands successfully sta-
bilised monomeric transition metal complexes as observed from
the molecular structures of silver (1b and 3b), gold (1–3)c, and
the palladium (1–3)d and (1–3)e complexes. The palladium (2–3)
d and (2–3)e complexes efficiently catalysed the asymmetric
Michael addition reaction in high yields and low enantioselectivi-
ties. The lack of any meaningful asymmetric induction in this catal-
ysis has been attributed to the distant location of the chiral
auxiliary from the catalytically active metal centre.
3
3
2H, JHH = 7 Hz, CH2C6H5), 4.34 (t, 1H, JHH = 7 Hz, CHNH2), 3.74
2
3
(s, 3H, OCH3), 3.25 (dd, 1H, JHH = 14 Hz, JHH = 5 Hz, CH2C6H5),
2
3
3.12 (dd, 1H, JHH = 14 Hz, JHH = 7 Hz, CH2C6H5). HRMS (ESI): m/
z 180.1017, [M + H]+, Calcd. 180.1019. Anal. Calcd. for C10H14ClNO2:
C, 55.69; H, 6.54; N, 6.49, Found: C, 55.52; H, 6.40; N, 6.63%.
3.1.3. Synthesis of (S)-3-amino-2-methyl-4-phenylbutan-2-ol [47]
L
-phenylalanine methyl ester hydrochloride (8.00 g, 37.1 mmol)
was added in portions, over a period of 30 min, to a solution of
CH3MgI [prepared by CH3I (13.8 mL, 223 mmol) and Mg (5.38 g,
223 mmol) in Et2O (ca. 250 mL)]. The resulting heterogeneous mix-
ture was heated to reflux for 6 h. A saturated aqueous solution of
NH4Cl (ca. 60 mL) was added dropwise at 0 °C with vigorous stir-
ring. The reaction mixture was filtered by celite and the organic
layer was separated. The aqueous layer was basified with aqueous
NH3 and extracted with Et2O (ca. 3 ꢁ 30 mL). The organic layer was
dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure to give crude amino alcohol, which was further
purified by vacuum distillation at 220 °C, to give amino alcohol
as colorless oil (1.99 g, 30%). 1H NMR (CDCl3, 500 MHz, 25 °C), d
7.33 (t, 2H, 3JHH = 7 Hz, CH2C6H5), 7.24 (t, 1H, 3JHH = 7 Hz, CH2C6H5),
3.1. Experimental section
3
3
3
3.1.1. General procedures
All manipulations were carried out using standard Schlenk
techniques. Solvents were purified and degassed by standard pro-
7.21 (d, 2H, JHH = 7 Hz, CH2C6H5), 3.03 (dd, 1H, JHH = 13 Hz, -
2
3
JHH = 2 Hz, CHNH2), 2.82 (dd, 1H, JHH = 11 Hz, JHH = 2 Hz, CH2C6-
H5), 2.28 (dd, 1H, 2JHH = 11 Hz, 3JHH = 12 Hz, CH2C6H5), 2.24 (br, 3H,
OH and NH2), 1.31 (s, 3H, CH3), 1.22 (s, 3H, CH3).
cedures. L–phenylalanine, triflic anhydride, sodium azide, methyl
vinyl ketone were purchased from Spectrochem Chemicals and
propargyl bromide, ethyl-2- oxocyclopentane-1-carboxylate were
purchased from Sigma Aldrich Chemicals and used without any
further purification. 1H NMR and 13C{1H} NMR spectra were
recorded on Bruker 400 MHz and Bruker 500 MHz NMR spectrom-
eter. 1H NMR peaks are labeled as singlet (s), doublet (d), triplet (t),
quartet (q), doublet of doublet (dd), doublet of triplets (dt), triplets
of triplets (tt), multiplet (m). The (S)-3-amino-2-methyl-4-
phenylbutan-2-ol [47] and (S)-7-benzyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazine [27–29] were synthesized by modifica-
tion of procedures reported in literature. High-resolution mass
spectrometry measurements were done on a Micromass Q–Tof
spectrometer and a Bruker maxis impact spectrometer. Infrared
spectra were recorded on a Perkin Elmer Spectrum One FT-IR spec-
trometer. Elemental Analysis was carried out on Thermo Quest
FLASH 1112 SERIES (CHNS) Elemental Analyser. For the catalysis
runs, the GCMS analyses were done using Agilent Technologies
3.1.4. Synthesis of (S)-3-azido-2-methyl-4-phenylbutan-2-ol
A mixture of (S)-3-amino-2-methyl-4-phenylbutan-2-ol (1.24 g,
6.91 mmol), 4-dimethylaminopyridine (0.562 g, 4.42 mmol) and
CuSO4ꢂ5H2O (0.077 g, 0.311 mmol) were dissolved in CH2Cl2 (ca.
120 mL). The CF3SO2N3 solution in CH2Cl2 (ca. 90 mL) [prepared
by dropwise addition of triflic anhydride (4.60 mL, 27.3 mmol) in
a mixture of NaN3 (7.48 g, 115 mmol), distilled water (ca. 25 mL)
and CH2Cl2 (ca. 75 mL) at 0 °C, followed by the extraction of the
CH2Cl2 layer after stirring 2 h] was added dropwise at room tem-
perature. The reaction mixture was then stirred vigorously for
2 h at room temperature, after which the color changed from blue
to green. The organic layer was washed with 10% aqueous citric
acid solution (ca. 3 ꢁ 50 mL), with saturated aqueous solution of
NaHCO3 (ca. 3 ꢁ 60 mL) and saturated aqueous solution of NaCl
(ca. 3 ꢁ 80 mL), dried over anhydrous Na2SO4, filtered and evapo-
rated in vaccuo to give as a crude azido alcohol (0.795 g, 56%). 1H
11

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
3
2
3
NMR (CDCl3, 400 MHz, 25 °C): d 7.33–7.23 (m, 5H, CH2C6H5), 3.42
(dd, 1H, 3JHH = 11 Hz, 3JHH = 3 Hz, CHN3), 3.02 (dd, 1H, 2JHH = 14 Hz,
3JHH = 3 Hz, CH2C6H5), 2.63 (dd, 1H, 2JHH = 14 Hz, 3JHH = 11 Hz, CH2-
C6H5), 1.91 (br, 1H, OH), 1.31 (s, 3H, CH3), 1.28 (s, 3H, CH3). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): 138.4 (CH2C6H5), 129.3 (CH2C6H5),
128.8 (CH2C6H5), 126.9 (CH2C6H5), 74.1 (CHN3), 73.3 (CH2C6H5),
36.5 (C(CH3)2), 26.6 (CH3), 25.6 (CH3).
Hz, JHH = 5 Hz, CH2C6H5), 3.17 (dd, 1H, JHH = 14 Hz, JHH = 9 Hz,
CH2C6H5), 1.46 (s, 3H, CH3), 1.30 (s, 3H, CH3). IR data (cmꢀ1) KBr pel-
let: 3016 (w), 2979 (w), 2925 (w), 1632 (m), 1454 (m), 1315 (w),
1269 (w), 1249 (w), 1210 (m), 1146 (w), 1071 (s), 833 (m), 761
(m), 710 (w), 558 (w), 510 (w). HRMS (ESI): m/z 364.0581, [MꢀCl]+,
Calcd. 364.0574. Anal. Calcd. for C15H19N3OAgCl: C, 44.97; H, 4.78;
N, 10.49. Found: C, 45.27; H, 3.84; N, 10.18.
3.1.5. Synthesis of (S)-7-benzyl-6,6-dimethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazine
3.1.8. Synthesis of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}AuCl (1c)
NaH (0.224 g, 9.36 mmol) was added to a solution of (S)-3-
azido-2-methyl-4-phenylbutan-2-ol (1.28 g, 6.24 mmol) in dry
THF (ca. 50 mL), after which propargyl bromide (0.965 g,
8.11 mmol) was added dropwise over a period of 20 min at 0 °C.
The reaction mixture was stirred for 24 h at room temperature
and then the solvent was removed in vacuo. The residue was
extracted with EtOAc (ca. 3 ꢁ 60 mL), washed with brine, dried
over anhydrous Na2SO4, filtered and evaporated in vacuum to give
the product as a yellow oil (1.15 g, 76%). 1H NMR (CDCl3, 400 MHz,
25 °C): d 7.44 (s, 1H, C2HN3 of C5H4N3O), 7.28–7.21 (m, 3H, CH2C6-
A
mixture of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4] oxazin-3-ylidene}AgCl (1b) (0.179 g,
0.447 mmol) and (Me2S)AuCl (0.132 g, 0.447 mmol) in CH3CN
(ca. 50 mL) was stirred overnight at room temperature. The reac-
tion mixture was filtered; the filtrate was collected and was dried
under vacuum to give the product 1c as a light yellow solid
(0.157 g, 71%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 7.26–7.19 (m,
3
2
3H, CH2C6H5), 6.95 (d, 2H, JHH = 7 Hz ,CH2C6H5), 4.92 (d, 1H,
-
2
JHH = 17 Hz, OCH2 of C5H3N3O), 4.80 (d, 1H, JHH = 17 Hz, OCH2 of
3
H5), 7.10–7.08 (d, 2H, JHH = 7 Hz, CH2C6H5), 4.86 (s, 2H, OCH2 of
3
3
C5H3N3O), 4.40 (dd, 1H, JHH = 9 Hz, JHH = 5 Hz, CHCH2C6H5),
3.93 (s, 3H, CH3), 3.20 (dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5),
3.10 (dd, 1H, 2JHH = 14 Hz, 3JHH = 9 Hz, CH2C6H5), 1.41 (s, 3H, CH3),
1.22 (s, 3H, CH3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): 153.4 (Au–
C), 138.9 (C2N3 of C5H3N3O), 136.1 (CH2C6H5), 129.1 (CH2C6H5),
128.9 (CH2C6H5), 127.6 (CH2C6H5), 74.0 (CHCH2C6H5), 66.8 (OCH2
of C5H3N3O), 58.1 (CH2C6H5), 42.0 (C(CH3)2 of C5H3N3O), 37.1
(NCH3), 24.0 (CH3), 23.6 (CH3). IR data (cmꢀ1) KBr pellet: 3032
(w), 2966 (w), 2925 (w), 2857 (w), 1533 (m), 1491 (w), 1454
(m), 1386 (w), 1374 (m), 1312 (m), 1269 (w), 1250 (w), 1213
(m), 1145 (m), 1080 (s), 954 (w), 923 (w), 893 (w), 833 (s), 757
(m), 738 (m), 704 (s), 651 (w), 599 (w), 558 (w), 509 (w). LRMS
3
2
3
C5H4N3O), 4.58 (t, 1H, JHH = 6 Hz, CHCH2C6H5), 3.40 (dd, 1H,
-
-
3
2
JHH = 14 Hz, JHH = 6 Hz, CH2C6H5), 3.18 (dd, 1H, JHH = 14 Hz,
JHH = 6 Hz, CH2C6H5), 1.43 (s, 3H, CH3), 1.21 (s, 3H, CH3).
3.1.6. Synthesis of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-2-ium iodide} (1a)
A mixture of (S)-7-benzyl-6,6-dimethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazine (1.10 g, 4.52 mmol) and methyl iodide
(6.42 g, 45.2 mmol) was stirred at 80 °C in CH3CN (ca. 100 mL)
for 24 h, after which, the volatiles were removed under vacuum.
The yellow solid thus obtained was further purified by column
chromatography using silica gel as a stationary phase and eluted
with MeOH:CHCl3 (1:9 v/v) to give the product 1a as a light yellow
solid (1.06 g, 61%). 1H NMR (CDCl3, 500 MHz, 25 °C): d 9.38 (s, 1H,
C2HN3 of C5H4N3O), 7.26–7.19 (m, 3H, CH2C6H5), 7.00–6.98 (m, 2H,
CH2C6H5), 5.15 (d, 1H, 2JHH = 17 Hz, OCH2 of C5H4N3O), 5.05 (d, 1H,
2JHH = 17 Hz, OCH2 of C5H4N3O), 4.60 (dd, 1H, 3JHH = 9 Hz, 3JHH = 5-
(ESI): m/z 495, [MꢀCl + CH3CN]+, Calcd. 495. Anal. Calcd. for C15
-
H
19N3OAuClꢂH2O: C, 35.48; H, 4.17; N, 8.28. Found: C, 34.55; H,
25
3.16; N, 8.27. [
a]
ꢀ 90.2 (c 1.00 in CHCl3).
D
2
3
Hz, CHCH2C6H5), 4.23 (s, 3H, NCH3), 3.26 (dd, 1H, JHH = 14 Hz,
-
3.1.9. Synthesis of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}2PdCl2 (1d)
2
3
JHH = 5 Hz, CH2C6H5), 3.16 (dd, 1H, JHH = 14 Hz, JHH = 9 Hz,
CH2C6H5), 1.46 (s, 3H, CH3), 1.31 (s, 3H, CH3). 13C{1H} NMR (CDCl3,
125 MHz, 25 °C): 135.7 (C2HN3 of C5H4N3O), 135.4 (C2HN3 of
C5H4N3O), 129.2 (CH2C6H5), 129.1 (CH2C6H5), 128.6 (CH2C6H5),
127.8 (CH2C6H5), 74.2 (CHCH2C6H5), 68.0 (OCH2 of C5H4N3O),
57.3 (CH2C6H5), 41.4 (C(CH3)2 of C5H4N3O), 36.9 (NCH3), 24.4
(CH3), 23.5 (CH3). IR data (cmꢀ1) KBr pellet: 3159 (w), 3111 (w),
3067 (w), 3037 (w), 2969 (w), 2925 (m), 2850 (w), 1644 (m),
1432 (w), 1329 (m), 1274 (w), 1247 (w), 1210 (w), 1142 (m),
1080 (s), 954 (w), 926 (w), 895 (w), 804 (m), 702 (w), 548 (w),
507 (w), 468 (w). HRMS (ESI): m/z 258.1596, [MꢀI]+, Calcd.
A
mixture of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4] oxazin-3-ylidene}AgCl (1b) (0.200 g,
0.499 mmol) and (COD)PdCl2 (0.071 g, 0.249 mmol) in CH3CN
(ca. 50 mL) was stirred at room temperature, until the formation
of an off-white AgCl precipitate was observed. The reaction mix-
ture was filtered and solvent was removed under vacuum to give
the product 1d as light yellow solid (0.187 g, 67%). 1H NMR (CDCl3,
400 MHz, 25 °C): d 7.28–7.26 (m, 3H, CH2C6H5), 6.95–6.94 (m, 2H,
CH2C6H5), 4.90 (d, 1H, 2JHH = 13 Hz, OCH2 of C5H3N3O), 4.69 (br, 1H,
OCH2 of C5H3N3O), 4.40 (br, 1H, CHCH2C6H5), 4.16 (s, 3H, CH3), 3.19
(dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5), 3.04 (d, 1H, 2JHH = 14 Hz,
3JHH = 9 Hz, CH2C6H5), 1.42 (s, 3H, CH3), 1.24 (s, 3H, CH3). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): 143.9 (Pd–C), 136.2 (C2N3 of
C5H3N3O), 135.9 (CH2C6H5), 129.0 (CH2C6H5), 128.9 (CH2C6H5),
127.6 (CH2C6H5), 74.1 (CHCH2C6H5), 66.4 (OCH2 of C5H3N3O),
58.3 (CH2C6H5), 42.0 (C(CH3)2 of C5H3N3O), 37.1 (NCH3), 23.9
(CH3), 23.6 (CH3). IR data (cmꢀ1) KBr pellet: 3650 (w), 3475 (s),
3276 (w), 3052 (w), 3028 (w), 3005 (w), 2974 (w), 2914 (w),
2853 (w), 1649 (s), 1604 (w), 1524 (w), 1497 (w), 1458 (m),
1390 (w), 1374 (w), 1341 (w), 1324 (w), 1286 (w), 1268 (m),
1246 (w), 1214 (w), 1166 (w), 1137 (m), 1087 (s), 1059 (w),
1039 (w), 1005 (w), 905 (w), 879 (w), 831 (m), 797 (w), 744 (m),
708 (m), 696 (m), 650 (w), 562 (w), 542 (w), 510 (w), 487 (w).
HRMS (ESI): m/z 657.1744 [MꢀCl]+, calcd. 655.1782. Anal. Calcd.
258.1601. Anal. Calcd. for C15H20IN3O: C, 46.77; H, 5.23; N, 10.91.
25
Found: C, 46.58; H, 5.34; N, 10.15. [
a
]
ꢀ 52.9 (c 1.00 in CHCl3).
D
3.1.7. Synthesis of {(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}AgCl (1b)
A
mixture of (S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-2-ium iodide (1a) (0.200 g,
0.519 mmol), Ag2O (0.120 g, 0.519 mmol) and NaCl (0.030 g,
0.519 mmol) in CH2Cl2 (ca. 50 mL) was stirred overnight at room
temperature. The reaction mixture was filtered through a pad of
celite and volatiles were removed under vacuum to give the pro-
duct 1b as a light brown solid (0.189 g, 91%). 1H NMR (CDCl3,
500 MHz, 25 °C): d 7.30–7.28 (m, 3H, CH2C6H5), 7.02–7.00 (d, 2H, 3-
2
JHH = 7 Hz, CH2C6H5), 4.99 (d, 1H, JHH = 17 Hz, OCH2 of C5H3N3O),
4.88 (d, 1H, 2JHH = 17 Hz, OCH2 of C5H3N3O), 4.47 (dd, 1H, 3JHH = 9 Hz,
3JHH = 5 Hz, CHCH2C6H5), 4.03 (s, 3H, NCH3), 3.25 (dd, 1H, 2JHH = 14-
for C30H38N6O2PdCl2ꢂH2O: C, 50.75; H, 5.68; N, 11.84. Found: C,
25
50.07; H, 4.93; N, 11.71. [
a]
ꢀ 97.2 (c 1.00 in CHCl3).
D
12

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
3.1.10. Synthesis of trans-{(S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-
4H-[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}PdI2(pyridine) (1e)
Cl2 (ca. 50 mL) was stirred for overnight at room temperature. The
reaction mixture was filtered through a pad of celite and volatiles
were removed under vacuum to give the product 2b as a light
brown solid (0.429 g, 68%). 1H NMR (CDCl3, 500 MHz, 25 °C): d
A
mixture of (S)-7-benzyl-2,6,6-trimethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-2-ium iodide (1a) (0.438 g,
1.13 mmol), PdCl2 (0.201 g, 1.13 mmol), K2CO3 (1.25 g, 9.10 mmol)
and NaI (0.852 g, 5.68 mmol) was refluxed in pyridine (5 mL,
63 mmol) for 16 h. The reaction mixture was cooled to room tem-
perature, diluted with CHCl3 (ca. 100 mL) and subsequently
washed with saturated aqueous CuSO4 solution (ca. 3 ꢁ 50 mL).
The organic layer was separated and dried over anhydrous Na2SO4
and filtered. The filtrate was concentrated under vacuum to give a
sticky, brown residue. The residue thus obtained was further puri-
fied by column chromatography using silica gel as a stationary
phase and eluted with EtOAc:petroleum ether (1:4 v/v) to give
the product 1e as a yellow solid (0.256 g, 32%). 1H NMR (CDCl3,
400 MHz, 25 °C): d 9.03–9.01 (m, 2H, NC5H5), 7.72 (t, 1H, 3JHH = 8 Hz,
3
7.38–7.32 (m, 3H, CH2C6H5), 7.21 (d, 2H, JHH = 8 Hz,CH2C6H5),
4.95 (d, 1H, 2JHH = 16 Hz, OCH2 of C5H5N3O), 4.86 (d, 1H, 2JHH = 16-
Hz, OCH2 of C5H5N3O), 4.65–4.61 (m,1H, CHCH2C6H5), 4.24 (s, 3H,
2
3
NCH3), 3.97 (dd, 1H, JHH = 13 Hz, JHH = 5 Hz, OCH2 of C5H5N3O),
2
3
3.92 (dd, 1H, JHH = 13 Hz, JHH = 5 Hz, OCH2 of C5H5N3O), 3.48
(dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5), 3.12 (dd, 1H, 2JHH = 14-
3
Hz, JHH = 10 Hz, CH2C6H5). Anal. Calcd. for C13H15AgClN3O: C,
41.91; H, 4.06; N, 11.28. Found: C, 41.85; H, 3.73; N, 10.96.%.
3.1.13. Synthesis of {(S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}AuCl (2c)
A mixture of {(S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-
3
NC5H5), 7.31–7.21 (m, 5H, CH2C6H5), 7.02 (d, 2H, JHH = 8 Hz,
triazolo[5,1-c][1,4]
oxazin-3-ylidene}AgCl
(2b)
(0.200
g,
NC5H5), 5.18 (d, 1H, 2JHH = 16 Hz, OCH2 of C5H3N3O), 5.08 (d, 1H, 2-
JHH = 16 Hz, OCH2 of C5H3N3O), 4.28 (dd, 1H, 3JHH = 8 Hz, 3JHH = 6 Hz,
CHCH2C6H5), 4.13 (s, 3H, NCH3), 3.18 (dd, 1H, 2JHH = 14 Hz, 3JHH = 6-
Hz, CH2C6H5), 3.12 (dd, 1H, 2JHH = 14 Hz, 3JHH = 8 Hz, CH2C6H5), 1.57
(s, 3H, CH3), 1.25 (s, 3H, CH3). 13C{1H} NMR (CDCl3, 125 MHz,
25 °C): d 154.1 (NC5H5), 137.6 (Pd–C), 136.2 (NC5H5), 136.2 (C2N3
of C5H3N3O), 129.3 (NC5H5), 128.9 (CH2C6H5), 128.3 (CH2C6H5),
127.3 (CH2C6H5), 124.5 (CH2C6H5), 73.9 (CHCH2C6H5), 66.3 (OCH2
of C5H3N3O), 59.4 (CH2C6H5), 42.7 (C(CH3)2 of C5H3N3O), 36.8
(NCH3), 23.9 (CH3), 23.6 (CH3). IR data (cmꢀ1) KBr pellet: 2923
(m), 2853 (w), 1601 (w), 1449 (m), 1264 (w), 1241 (w), 1211
(m), 1142 (w), 1084 (s), 830 (m), 760 (w), 737(w), 696 (s), 666
(w), 539 (w), 509 (w). HRMS (ESI): m/z 569.0052, [MꢀI]+, Calcd.
0.539 mmol) and (Me2S)AuCl (0.159 g, 0.539 mmol) in CH3CN
(ca. 50 mL) was stirred overnight at room temperature. The reac-
tion mixture was filtered; the filtrate was collected and was dried
under vacuum to give the product 2c as a light yellow solid
(0.186 g, 75%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 7.38–7.31 (m,
3
2
3H, CH2C6H5), 7.21 (d, 2H, JHH = 8 Hz,CH2C6H5), 4.95 (d, 1H,
-
2
JHH = 16 Hz, OCH2 of C5H5N3O), 4.86 (d, 1H, JHH = 16 Hz, OCH2 of
C5H5N3O), 4.66–4.61 (m, 1H, CHCH2C6H5), 4.20 (s, 3H, NCH3),
2
3
3.97 (dd, 1H, JHH = 13 Hz, JHH = 5 Hz, OCH2 of C5H5N3O), 3.92
2
3
(dd, 1H, JHH = 13 Hz, JHH = 5 Hz, OCH2 of C5H5N3O), 3.48 (dd,
1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5), 3.12 (dd, 1H, 2JHH = 14 Hz,
3JHH = 10 Hz, CH2C6H5). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C):
154.1 (Au–C), 140.3 (C2N3 of C5H5N3O), 134.5 (CH2C6H5), 129.4
(CH2C6H5), 129.3 (CH2C6H5), 128.0 (CH2C6H5), 65.9 (CHCH2C6H5),
63.3 (OCH2 of C5H5N3O), 58.8 (CH2C6H5), 42.3 (OCH2 of C5H5N3O),
38.4 (NCH3). IR data (cmꢀ1) KBr pellet: 3542 (w), 3057 (w), 3027
(w), 2928 (m), 2854 (w), 2359 (w), 2318 (w), 2247 (w), 1639 (w),
1603 (w), 1530 (w), 1454 (m), 1440 (m), 1317 (m), 1249 (m),
1224 (m), 1112 (m), 1088 (m), 1061 (m), 1027 (w), 971 (m), 927
(w), 902 (m), 815 (m), 780 (w), 751 (s), 707 (s), 666 (w), 616 (w),
569.0032. Anal. Calcd. for C20H24I2N4OPd: C, 34.48; H, 3.47; N,
25
8.04. Found: C, 34.72; H, 3.00; N, 7.90. [
CHCl3).
a]
ꢀ 110.8 (c 1.00 in
D
3.1.11. Synthesis of {(S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-2-ium iodide} (2a)
A mixture of (S)-7-benzyl-6,7-dihydro-4H-[1,2,3]-triazolo[5,1-
c][1,4]oxazine (1.00 g, 4.65 mmol) and methyl iodide (6.60 g,
46.5 mmol) was stirred at 80 °C in CH3CN (ca. 50 mL) for 24 h, after
which, the volatiles were removed under vacuum. The yellow solid
thus obtained was further purified by column chromatography
using silica gel as a stationary phase and eluted with MeOH:CHCl3
(1:9 v/v) to give the product 2a as a light yellow solid (1.20 g, 73%).
1H NMR (CDCl3, 400 MHz, 25 °C): d 9.29 (s, 1H, C2HN3 of C5H6N3O),
558 (w), 522 (w), 499 (w). Anal. Calcd. for C13H15N3OAuCl: C,
25
33.82; H, 3.27; N, 9.10. Found: C, 34.65; H, 3.17; N, 8.24. [a]
D
38.1 (c 1.00 in CHCl3).
3.1.14. Synthesis of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}2PdCl2 (2d)
2
7.34–7.23 (m, 5H, CH2C6H5), 5.25 (d, 1H, JHH = 16 Hz, OCH2 of
A mixture of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-tria-
zolo[5,1-c][1,4] oxazin-3-ylidene}AgCl (2b) (0.496 g, 0.133 mmol)
and (COD)PdCl2 (0.190 g, 0.666 mmol) in CH3CN (ca. 50 mL) was
stirred at room temperature, until the formation of an off-white
AgCl precipitate was observed. The reaction mixture was filtered
and solvent was removed under vacuum to give the product 2d
as light yellow solid (0.403 g, 47%). 1H NMR (CDCl3, 400 MHz,
25 °C): d 7.37–7.30 (m, 3H, CH2C6H5), 7.17 (d, 2H, 3JHH = 8 Hz,CH2-
C6H5), 4.85 (br, 2H, OCH2 of C5H5N3O), 4.53–4.50 (m, 1H, CHCH2C6-
H5), 4.37 (s, 3H, NCH3), 3.90 (dq, 2H, 2JHH = 13 Hz, 3JHH = 5 Hz, OCH2
of C5H5N3O), 3.44 (dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5), 3.06
(dd, 1H, 2JHH = 14 Hz, 3JHH = 10 Hz, CH2C6H5). 13C{1H} NMR (CDCl3,
100 MHz, 25 °C): 137.5 (Pd–C), 134.1 (C2N3 of C5H5N3O), 139.5
(CH2C6H5), 129.3 (CH2C6H5), 128.4 (CH2C6H5), 128.0 (CH2C6H5),
65.5 (CHCH2C6H5), 62.3 (OCH2 of C5H5N3O), 59.8 (CH2C6H5), 41.5
(OCH2 of C5H5N3O), 38.2 (NCH3). IR data (cmꢀ1) KBr pellet: 3698
(w), 3023 (w), 2923 (s), 2852 (m), 2363 (s), 2343 (m), 1617 (s),
1450 (m), 1321 (m), 1265 (w), 1221 (m), 1156 (w), 1097 (s), 936
(w), 817 (m), 741 (m), 704 (s), 557 (w). HRMS (ESI): m/z
599.1156, [M]+, Calcd. 599.1155. Anal. Calcd. for C26H30N6O2PdCl2:
2
C5H6N3O), 5.18 (d, 1H, JHH = 16 Hz, OCH2 of C5H6N3O), 4.92–4.86
(m, 1H, CHCH2C6H5), 4.50 (s, 3H, NCH3), 4.20 (dd, 1H, 2JHH = 13 Hz,
3JHH = 4 Hz, OCH2 of C5H6N3O), 4.13 (dd, 1H, 2JHH = 13 Hz, 3JHH = 4-
Hz, OCH2 of C5H6N3O), 3.54 (dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2-
2
3
C6H5), 3.16 (dd, 1H, JHH = 14 Hz, JHH = 10 Hz, CH2C6H5). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): 137.5 (C2HN3 of C5H6N3O), 134.1
(C2HN3 of C5H6N3O), 129.4 (CH2C6H5), 129.2 (CH2C6H5), 128.3
(CH2C6H5), 127.9 (CH2C6H5), 65.4 (CHCH2C6H5), 62.3 (OCH2 of
C5H6N3O), 59.8 (CH2C6H5), 41.5 (OCH2 of C5H6N3O), 38.1(NCH3).
IR data (cmꢀ1) KBr pellet: 3153 (w), 3036 (s), 3009 (s), 2911 (s),
2750 (w), 2362 (m), 2343 (m), 2250 (w), 1601 (s), 1489 (w),
1450 (s), 1388 (w), 1334 (m), 1260 (w), 1235 (m), 1200 (w),
1177 (m), 1106 (s), 1087 (s), 1064 (s), 970 (w), 935 (w), 904 (m),
853 (w), 831 (w), 805 (m), 751 (m), 703 (m), 677 (w), 553 (w),
541 (w), 499 (w), 477 (w). LRMS (ESI): m/z 230, [MꢀI]+, Calcd. 230.
3.1.12. Synthesis of {(S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}AgCl (2b)
A mixture of (S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-tri-
azolo[5,1-c][1,4]oxazin-2-ium iodide (2a) (0.605 g, 1.69 mmol),
Ag2O (0.393 g, 1.69 mmol) and NaCl (0.098 g, 1.69 mmol) in CH2-
C, 44.99; H, 4.47; N, 11.66. Found: C, 44.93; H, 4.08; N, 12.48.%.
25
[a
]
ꢀ 65.9 (c 1.00 in CHCl3).
D
13

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
3.1.15. Synthesis of trans-{(S)-7-benzyl-2-methyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}PdI2(pyridine) (2e)
3.1.17. Synthesis of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}AgCl (3b)
A mixture of (S)-7-benzyl-2-methyl-6,7-dihydro-4H-[1,2,3]-tri-
azolo[5,1-c][1,4]oxazin-2-ium iodide (2a) (0.231 g, 0.649 mmol),
PdCl2 (0.115 g, 0.649 mmol), K2CO3 (0.717 g, 5.19 mmol) and KI
(0.862 g, 5.19 mmol) was refluxed in pyridine (5 mL, 63 mmol)
for 16 h. The reaction mixture was cooled to room temperature,
diluted with CHCl3 (ca. 100 mL) and subsequently washed with
saturated aqueous CuSO4 solution (ca. 3 ꢁ 50 mL). The organic
layer was separated and dried over anhydrous Na2SO4 and filtered.
The filtrate was concentrated under vacuum to give a sticky, brown
residue. The residue thus obtained was further purified by column
chromatography using silica gel as a stationary phase and eluted
with EtOAc:petroleum ether (1:4 v/v) to give the product 2e as a
yellow solid (0.186 g, 43%). 1H NMR (CDCl3, 400 MHz, 25 °C): d
A mixture of (S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-tria-
zolo[5,1-c][1,4]oxazin-2-ium iodide (3a) (1.27 g, 3.44 mmol),
Ag2O (0.799 g, 3.44 mmol) and NaCl (0.201 g, 3.44 mmol) in CH2-
Cl2 (ca. 50 mL) was stirred overnight at room temperature. The
reaction mixture was filtered through a pad of celite and volatiles
were removed under vacuum to give the product 3b as a light
brown solid (0.869 g, 65%). 1H NMR (CDCl3, 400 MHz, 25 °C): d
3
7.37–7.28 (m, 3H, CH2C6H5), 7.20 (d, 2H, JHH = 8 Hz, CH2C6H5),
4.93 (d, 1H, 2JHH = 16 Hz, OCH2 of C5H5N3O), 4.84 (d, 1H, 2JHH = 16-
Hz, OCH2 of C5H5N3O), 4.66–4.61 (m, 1H, CHCH2C6H5), 4.51 (q, 2H,
3JHH = 8 Hz, NCH2CH3), 3.97 (dd, 1H, 2JHH = 13 Hz, 3JHH = 4 Hz, OCH2
2
3
of C5H5N3O), 3.93 (dd, 1H, JHH = 13 Hz, JHH = 4 Hz, OCH2 of
2
3
C5H5N3O), 3.47 (dd, 1H, JHH = 14 Hz, JHH = 5 Hz, CH2C6H5), 3.14
(dd, 1H, 2JHH = 14 Hz, 3JHH = 10 Hz, CH2C6H5), 1.59 (t, 3H, 3JHH = 8 Hz
, NCH2CH3). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): 159.1 (Ag–C),
141.4 (C2N3 of C5H5N3O), 134.7(CH2C6H5), 129.3 (CH2C6H5), 129.0
(CH2C6H5), 127.6 (CH2C6H5), 66.0 (CHCH2C6H5), 63.8 (OCH2 of
C5H5N3O), 58.8 (CH2C6H5), 51.7 (OCH2 of C5H5N3O), 38.3(NCH2-
CH3), 16.2(NCH2CH3). IR data (cmꢀ1) KBr pellet: 3083 (w), 3055
(w), 2979 (w), 2935 (m), 2875 (w), 1601 (s), 1491 (w), 1452 (s),
1438 (s), 1365 (m), 1320 (m), 1306 (m), 1243 (w), 1218 (w),
1187 (w), 1155 (w), 1109 (s), 1091 (s), 1063 (s), 1028 (w), 969
(m), 936 (w), 902 (m), 831 (w), 797 (m), 747 (m), 702 (s), 610
(w), 555 (w), 499 (m). LRMS (ESI): m/z 350, [MꢀCl]+, Calcd. 350.
3
9.03–9.01 (m, 2H, NC5H5), 7.72 (t, 1H, JHH = 8 Hz, NC5H5), 7.31–
3
7.21 (m, 5H, CH2C6H5), 7.02 (d, 2H, JHH = 8 Hz, NC5H5), 5.18 (d,
2
2
1H, JHH = 16 Hz, OCH2 of C5H5N3O), 5.08 (d, 1H, JHH = 16 Hz,
OCH2 of C5H5N3O), 4.28 (dd, 1H, 3JHH = 8 Hz, 3JHH = 6 Hz, CHCH2C6-
H5), 4.13 (s, 3H, NCH3), 3.97 (dd, 1H, 2JHH = 12 Hz, 3JHH = 4 Hz, OCH2
2
3
of C5H5N3O), 3.92 (dd, 1H, JHH = 12 Hz, JHH = 4 Hz, OCH2 of
C5H5N3O), 3.18 (dd, 1H, JHH = 14 Hz, JHH = 6 Hz, CH2C6H5), 3.12
2
3
(dd, 1H, JHH = 14 Hz, JHH = 8 Hz, CH2C6H5). 13C{1H} NMR (CDCl3,
100 MHz, 25 °C): d 153.1 (NC5H5), 138.5 (Pd–C), 137.4 (NC5H5),
135.5 (C2N3 of C5H5N3O), 129.5 (NC5H5), 128.8 (CH2C6H5), 128.3
(CH2C6H5), 127.3 (CH2C6H5), 125.5 (CH2C6H5), 65.9 (CHCH2C6H5),
63.9 (OCH2 of C5H5N3O), 57.7 (CH2C6H5), 42.8 (OCH2 of C5H5N3O),
37.5 (NCH3). IR data (cmꢀ1) KBr pellet: 2979 (w), 2928 (m), 2847
(w), 2358 (m), 1601 (s), 1446 (s), 1359 (w), 1309 (s), 1241 (w),
1198 (w), 1149 (w), 1091 (s), 1069 (s), 974 (w), 932 (w), 884 (w),
789 (w), 757 (m), 730 (w), 703 (m), 694 (s), 642 (w), 596 (w),
559 (w). LRMS (ESI): m/z 540.9717, [M]+, Calcd. 540.5187. Anal.
2
3
Anal. Calcd. for C14H17AgClN3O: C, 43.49; H, 4.43; N, 10.87. Found:
25
C, 43.59; H, 3.71; N, 10.41. [
a]
38.06 (c 1.00 in CHCl3).
D
3.1.18. Synthesis of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}AuCl (3c)
A mixture of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-tria-
zolo[5,1-c][1,4] oxazin-3-ylidene}AgCl (3b) (0.206 g, 0.533 mmol)
and (Me2S)AuCl (0.157 g, 0.533 mmol) in CH3CN (ca. 50 mL) was
stirred overnight at room temperature. The reaction mixture was
filtered the filtrate was collected and was dried under vacuum to
give the product 3c as a light yellow solid (0.215 g, 85%). 1H
NMR (CDCl3, 400 MHz, 25 °C): d 7.37–7.26 (m, 3H, CH2C6H5),
Calcd. for C18H20N4OPdI2: C, 32.34; H, 3.02; N, 8.38. Found: C,
25
32.14; H, 2.78; N, 7.82.%. [
a
]
37.2 (c 1.00 in CHCl3).
D
3.1.16. Synthesis of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-2-ium iodide} (3a)
3
2
A mixture of (S)-7-benzyl-6,7-dihydro-4H-[1,2,3]-triazolo[5,1-
c][1,4]oxazine (0.407 g, 1.90 mmol) and ethyl iodide (2.98 g,
19.2 mmol) was stirred at 80 °C in CH3CN (ca. 50 mL) for 24 h, after
which, the volatiles were removed under vacuum. The yellow solid
thus obtained was further purified by column chromatography
using silica gel as a stationary phase and eluted with MeOH:CHCl3
(1:9 v/v) to give the product 3a as a light yellow solid (0.262 g,
37%). 1H NMR (CDCl3, 400 MHz, 25 °C): d 9.30 (s, 1H, C2HN3 of
C5H6N3O), 7.35–7.29 (m, 3H, CH2C6H5), 7.22–7.20 (m, 2H, CH2C6-
7.20 (d, 2H, JHH = 8 Hz, CH2C6H5), 4.95 (d, 1H, JHH = 16 Hz,
2
OCH2 of C5H5N3O), 4.85 (d, 1H, JHH = 16 Hz, OCH2 of C5H5N3O),
4.67–4.62 (m, 1H, CHCH2C6H5), 4.54 (q, 2H, 3JHH = 8 Hz, NCH2CH3),
3.98 (dd, 1H, 2JHH = 13 Hz, 3JHH = 4 Hz, OCH2 of C5H5N3O), 3.94 (dd,
2
3
2
3
1H, JHH = 13 Hz, JHH = 4 Hz, OCH2 of C5H5N3O), 3.47 (dd, 1H,
-
-
3
2
JHH = 14 Hz, JHH = 5 Hz, CH2C6H5), 3.15 (dd, 1H, JHH = 14 Hz,
JHH = 10 Hz, CH2C6H5), 1.62 (t, 3H, JHH = 8 Hz , NCH2CH3). 13C
{1H} NMR (CDCl3, 125 MHz, 25 °C): 152.5 (Au–C), 140.1 (C2N3 of
C5H5N3O), 134.7(CH2C6H5), 129.4 (CH2C6H5), 129.2 (CH2C6H5),
127.8 (CH2C6H5), 66.0 (CHCH2C6H5), 63.3 (OCH2 of C5H5N3O),
58.8 (CH2C6H5), 51.0 (OCH2 of C5H5N3O), 38.3(NCH2CH3), 15.8
(NCH2CH3). IR data (cmꢀ1) KBr pellet: 3054 (w), 3022 (w), 2924
(s), 2874 (m), 2853 (m), 2362 (w), 2343 (w), 1961 (w), 1627 (m),
1601 (m), 1527 (w), 1495 (w), 1453 (s), 1438 (s), 1363 (w), 1353
(w), 1306 (s), 1243 (w), 1217 (w), 1192 (w), 1088 (s), 1064 (s),
1028 (w), 970 (m), 923 (w), 902 (m), 880 (w), 837 (w), 797 (m),
779 (w), 747 (s), 702 (s), 613 (w), 599 (w), 549 (w), 540 (w), 498
3
2
2
H5), 5.25 (d, 1H, JHH = 16 Hz, OCH2 of C5H6N3O), 5.17 (d, 1H,
JHH = 16 Hz, OCH2 of C5H6N3O), 4.91–4.85 (m, 1H, CHCH2C6H5),
-
3
2
3
4.78 (q, 2H, JHH = 8 Hz, NCH2CH3), 4.22 (dd, 1H, JHH = 13 Hz,
-
JHH = 4 Hz, OCH2 of C5H6N3O), 4.13 (dd, 1H, 2JHH = 13 Hz, 3JHH = 4 Hz,
OCH2 of C5H6N3O), 3.51 (dd, 1H, 2JHH = 14 Hz, 3JHH = 5 Hz, CH2C6H5),
2
3
3
3.16 (dd, 1H, JHH = 14 Hz, JHH = 10 Hz, CH2C6H5), 1.67 (t, 3H,
-
JHH = 8 Hz , NCH2CH3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C):
137.0 (C2HN3 of C5H6N3O), 134.1 (C2HN3 of C5H6N3O), 129.3 (CH2-
C6H5), 128.9 (CH2C6H5), 128.9 (CH2C6H5), 127.5 (CH2C6H5), 65.3
(CHCH2C6H5), 62.0 (OCH2 of C5H6N3O), 59.5 (CH2C6H5), 50.1
(OCH2 of C5H6N3O), 37.9(NCH2CH3), 14.5(NCH2CH3). IR data
(cmꢀ1) KBr pellet: 3027 (m), 2926 (s), 2873 (m), 2438 (w), 2250
(w), 2118 (w), 1981 (w), 1614 (s), 1453 (s), 1331 (s), 1530 (w),
1227 (m), 1203 (w), 1156 (m), 1104 (s), 1029 (w), 969 (m), 937
(m), 903 (w), 878 (w), 834 (m), 758 (m), 754 (m), 740 (m), 705
(s), 543 (m), 501 (m). HRMS (ESI): m/z 244.1448, [MꢀI]+, Calcd.
(m), 433 (w). Anal. Calcd. for C14H17AuClN3OꢂH2O: C, 34.06; H,
25
3.88; N, 8.51. Found: C, 34.55; H, 3.16; N, 8.27. [
1.00 in CHCl3).
a]
14.96 (c
D
3.1.19. Synthesis of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-
triazolo[5,1-c][1,4]oxazin-3-ylidene}2PdCl2 (3d)
A mixture of {(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-tria-
zolo[5,1-c][1,4] oxazin-3-ylidene}AgCl (3b) (0.276 g, 0.718 mmol)
and (COD)PdCl2 (0.102 g, 0.359 mmol) in CH3CN (ca. 50 mL) was
stirred at room temperature, until the formation of an off-white
AgCl precipitate was observed. The reaction mixture was filtered
244.1444. Anal. Calcd. for C14H18N3IOꢂH2O: C, 43.20; H, 5.18; N,
25
10.80. Found: C, 43.59; H, 3.71; N, 10.41. [
CHCl3).
a
]
13.8 (c 1.00 in
D
14

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
and solvent was removed under vacuum to give the product 3d as
light yellow solid (0.129 g, 28%). 1H NMR (CDCl3, 400 MHz, 25 °C):
d 7.37–7.29 (m, 3H, CH2C6H5), 719–7.17 (m, 3H, CH2C6H5), 5.32 (d,
1H, 2JHH = 16 Hz, OCH2 of C5H5N3O), 5.19 (d, 1H, 2JHH = 16 Hz, OCH2
of C5H5N3O), 4.93 (q, 2H, 3JHH = 8 Hz, NCH2CH3), 4.52–4.49 (m, 1H,
8 h at room temperature. The volatiles were then removed in vacuo
and the crude product was purified by column chromatography
using silica as a stationary phase and eluting with a mixed medium
of petroleum ether:EtOAc (v/v 80:20 to 70:30) to give the product
ethyl-2-oxo-1-(3-oxobutyl)cyclopentane-1-carboxylate (4) as
a
2
3
CHCH2C6H5), 3.95 (dd, 1H, JHH = 13 Hz, JHH = 4 Hz, OCH2 of
colorless liquid. The ee was determined by chiral GC with a CP-Chi-
rasil-Dex CB column, GC conditions: injection temperature: 250 °C,
detector temperature: 300 °C, column temperature: initial temper-
ature 60 °C, ramp 3 °C/min to 120 °C then hold 5 min, 3 °C/min to
130 °C then hold 20 min.
2
3
C5H5N3O), 3.89 (dd, 1H, JHH = 13 Hz, JHH = 4 Hz, OCH2 of
2
3
C5H5N3O), 3.42 (dd, 1H, JHH = 14 Hz, JHH = 5 Hz, CH2C6H5), 3.09
(dd, 1H, 2JHH = 14 Hz, 3JHH = 10 Hz, CH2C6H5), 1.82 (t, 3H, 3JHH = 8 Hz
, NCH2CH3). 13C{1H} NMR (CDCl3, 125 MHz, 25 °C): 143.6 (Pd–C),
137.5 (C2N3 of C5H5N3O), 134.5(CH2C6H5), 129.3 (CH2C6H5), 129.0
(CH2C6H5), 127.6 (CH2C6H5), 65.9 (CHCH2C6H5), 63.6 (OCH2 of
C5H5N3O), 58.5 (CH2C6H5), 50.6 (OCH2 of C5H5N3O), 38.1(NCH2-
CH3), 15.1(NCH2CH3). IR data (cmꢀ1) KBr pellet: 3030 (w), 3976
(w), 2927 (w), 2871 (w), 1636 (m), 1495 (w), 1455 (s), 1381 (w),
1357 (w), 1301 (s), 1261 (w), 1236 (w), 1202 (w), 1180 (w), 1100
(s), 1069 (m), 1031 (w), 1002 (w), 935 (m), 881 (w), 789 (m),
756 (w), 729 (w), 702 (s), 644 (w), 588 (w), 556 (w), 514 (w). Anal.
3.1.22. Synthesis of ethyl-2-oxo-1-(3-oxobutyl)cyclopentane-1-
carboxylate (4) [48]
Calcd. for C28H34N6O2PdCl2: C, 50.65; H, 5.16; N, 12.66. Found: C,
25
50.23; H, 4.71; N, 11.92. [
a
]
37.2 (c 1.00 in CHCl3).
D
3.1.20. Synthesis of trans-{(S)-7-benzyl-2-ethyl-6,7-dihydro-4H-
[1,2,3]-triazolo[5,1-c][1,4]oxazin-3-ylidene}PdI2(pyridine) (3e)
A mixture of (S)-7-benzyl-2-ethyl-6,7-dihydro-4H-[1,2,3]-tria-
zolo[5,1-c][1,4]oxazin-2-ium iodide (3a) (0.320 g, 0.862 mmol),
PdCl2 (0.152 g, 0.862 mmol), K2CO3 (0.952 g, 6.90 mmol) and KI
(1.14 g, 6.90 mmol) was refluxed in pyridine (5 mL, 63 mmol)
for 16 h. The reaction mixture was cooled to room temperature,
diluted with CHCl3 (ca. 100 mL) and subsequently washed with
saturated aqueous CuSO4 solution (ca. 3 ꢁ 50 mL). The organic
layer was separated and dried over anhydrous Na2SO4 and fil-
tered. The filtrate was concentrated under vacuum to give a
sticky, brown residue. The residue thus obtained was further
purified by column chromatography using silica gel as a station-
ary phase and eluted with EtOAc:petroleum ether (1:4 v/v) to
give the product 3e as a yellow solid (0.195 g, 46%). 1H NMR
(CDCl3, 500 MHz, 25 °C): d 8.99–8.97 (m, 2H, NC5H5), 7.72–7.68
(m, 1H, NC5H5), 7.34–7.27 (m, 5H, CH2C6H5), 7.16–7.15 (m, 2H,
Yields: 0.192 g, 85% (2d), 0.206 g, 91% (2e), 0.198 g, 88% (3d),
0.201 g, 89% (3e).
1H NMR (CDCl3, 400 MHz, 25 °C): d 4.12 (dq, 2H, 3JHH = 7 Hz and
2 Hz, CH2CH3), 2.69–2.63 (m, 1H, C(O)CH2CH2C), 2.46–2.23 (m, 4H,
C(O)CH2CH2C and C(O)(CH2)3C), 2.09 (s, 3H, CH3CO), 2.08–2.03 (m,
3
1H, C(O)(CH2)3C), 2.00–1.81 (m, 4H, C(O)(CH2)3C), 1.21 (t, 3H,
-
JHH = 7 Hz, CH2CH3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): d
215.0 (CO), 207.9 (CH3CO), 171.5 (CO2CH2CH3), 61.5 (CH2CH3),
59.1 (C(O)CC(O)CH2), 39.0 (C(O)CH2CH2CH2C), 38.1 (C(O)CH2CH2C),
34.5 (C(O)CH2CH2C), 30.0 (CH3CO), 27.1 (C(O)CH2CH2CH2C), 19.7 (C
(O)CH2CH2CH2C), 14.2 (CH2CH3). GC–MS (ESI): m/z = 226 [M] + .
Anal. Calcd. for C12H18O4: C, 63.70; H, 8.02; Found: C, 63.63; H,
7.59%. GC [CP-Chirasil-Dex CB, column temperature = 60 °C (initial)
, inject temperature = 250 °C, detector temperature = 300 °C]:
tR = 40.6 min, tR = 44.1 min.
2
NC5H5), 5.14 (d, 1H, JHH = 16 Hz, OCH2 of C5H5N3O), 5.02 (d,
2
3
1H, JHH = 16 Hz, OCH2 of C5H5N3O), 4.79 (q, 2H, JHH = 8 Hz,
NCH2CH3), 4.53–4.48 (m, 1H, CHCH2C6H5), 3.93 (dd, 1H, 2JHH = 13-
3
2
3
Hz, JHH = 4 Hz, OCH2 of C5H5N3O), 3.90 (dd, 1H, JHH = 13 Hz,
-
2
CRediT authorship contribution statement
JHH = 4 Hz, OCH2 of C5H5N3O), 3.43 (dd, 1H, JHH = 14 Hz,
3JHH = 5 Hz, CH2C6H5), 3.11 (dd, 1H, JHH = 14 Hz, JHH = 10 Hz,
2
3
CH2C6H5), 1.70 (t, 3H, JHH = 8 Hz , NCH2CH3). 13C{1H} NMR
3
Manoj Kumar Gangwar: Methodology, Investigation, Data
curation. Shreyata Dey: Methodology, Investigation, Data curation,
Writing - original draft. Prakasham. A. P: Visualization, Software.
Prasenjit Ghosh: Conceptualization, Supervision, Funding acquisi-
tion, Project administration, Writing - review & editing.
(CDCl3, 125 MHz, 25 °C): d 153.8 (NC5H5), 137.6 (Pd–C), 136.9
(NC5H5), 134.9 (C2N3 of C5H5N3O), 129.4 (NC5H5), 129.1 (CH2C6-
H5), 127.5 (CH2C6H5), 124.4 (CH2C6H5), 116.5 (CH2C6H5), 65.8
(CHCH2C6H5), 64.2 (OCH2 of C5H3N3O), 58.4 (CH2C6H5), 51.4
(OCH2 of C5H5N3O), 38.1(NCH2CH3), 14.6(NCH2CH3). IR data
(cmꢀ1) KBr pellet: 3019 (w), 2979 (w), 2928 (s), 2847 (w), 2368
(w), 1601 (m), 1446 (s), 1359 (w), 1327 (w), 1309 (s), 1288
(m), 1241 (w), 1198 (w), 1149 (w), 1091 (s), 1069 (s), 974 (w),
932 (m), 884 (w), 789 (m), 757 (m), 729 (w), 703 (m), 694 (m),
641 (w), 596 (w), 559 (w), 513 (w). HRMS (ESI): m/z 554.9869,
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[MꢀI]+, Calcd. 554.9875. Anal. Calcd. for C19H22I2N4OPd: C,
25
33.43; H, 3.25; N, 8.21. Found: C, 33.48; H, 2.92; N, 7.91. [
a
]
Acknowledgments
D
37.3 (c 1.00 in CHCl3).
We thank the Department of Science and Technology (Grant
No: SR/S1/IC-50/2011, EMR/2014/000254, CRG/2019/000029)
New Delhi, India, for financial support of this research. We grate-
fully acknowledge the Single Crystal X-ray Diffraction Facility,
Department of Chemistry, IIT Bombay, Mumbai, India, for the crys-
tallographic characterization data. M.K.G, A.P.P thank CSIR, New
Delhi, India, and S.R.D thanks IIT Bombay, Mumbai, India for
research fellowships.
3.1.21. General procedure for asymmetric Michael addition reaction of
ethyl-2-oxocyclopentane-1-carboxylate with methyl vinyl ketone
(Table 2)
A mixture of catalyst (2d/2e/3d/3e) (0.01 mmol, 1 mol %), ethyl-
2-oxocyclopentane-1-carboxylate (0.156 g, 1.00 mmol), methyl
vinyl ketone (0.210 g, 3.00 mmol) Et3N (0.304 g, 3.00 mmol) was
added in 5 mL CHCl3 and the resultant solution was stirred for
15

M. Kumar Gangwar, S. Dey, Prakasham. A. P et al.
Polyhedron 197 (2021) 115011
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Appendix A. Supplementary data
CCDC-1011069 (for 1b), CCDC-1012239 (for 1c), CCDC-1021594
(for 1d), CCDC-1024176 (for 1e), CCDC-1011070 (for 2c), CCDC-
1059753 (for 2d), CCDC-1024225 (for 2e), CCDC-1023266 (for
3b), CCDC-1024174 (for 3c) and CCDC-1012271 (for 3d), CCDC-
1040518 (for 3e) contain the supplementary crystallographic data.
These data can be obtained free of charge via http://www.ccdc.-
cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data (1H NMR, 13C{1H} NMR, IR, HRMS, CHNS) of
the compounds (1ꢀ3)a, (1ꢀ3)b, (1ꢀ3)c, (1ꢀ3)d and (1ꢀ3)e, and
(1H NMR, 13C{1H} NMR, GC and GCꢀMS) of the catalysis product
4 (PDF) can be found with this article. Supplementary data to this
article can be found online at https://doi.org/10.1016/j.poly.2020.
115011.
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