Efficient in situ palladium nano catalysis for Z-selective semi transfer hydrogenation of internal alkynes using safer 1, 4-butanediol

Article abstract of DOI:10.1016/j.tetlet.2019.151395

Simple and efficient in situ generated palladium nanoparticles (PdNPs) in PEG-4OO catalyzed semi transfer hydrogenation of internal alkynes to Z-alkenes with excellent selectivity along with the formation of beneficial γ-butyrolactone as a byproduct using low quantity of safer and attractive 1, 4-butanediol as a hydrogen source was described.

Full text of DOI:10.1016/j.tetlet.2019.151395

Journal Pre-proofs
Efficient in situ palladium nano catalysis for Z-selective semi transfer hydro-
genation of internal alkynes using safer 1, 4-butanediol
Siva Kumar Rapeti, Krishna Chaitanya Kasina, Prasad Gundepaka, Saritha
Birudaraju, B.B.V. Sailaja
PII:
S0040-4039(19)31186-4
DOI:
https://doi.org/10.1016/j.tetlet.2019.151395
TETL 151395
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 September 2019
11 November 2019
12 November 2019
Please cite this article as: Rapeti, S.K., Kasina, K.C., Gundepaka, P., Birudaraju, S., Sailaja, B.B.V., Efficient in
situ palladium nano catalysis for Z-selective semi transfer hydrogenation of internal alkynes using safer 1, 4-
butanediol, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.2019.151395
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Graphical Abstract
Efficient in situ palladium nano catalysis for
Z-selective semi transfer hydrogenation of
internal alkynes using 1, 4-butanediol
Leave this area blank for abstract info.
a,b
a
c
a*
b*
Siva Kumar Rapeti , Krishna Chaitanya Kasina , Prasad Gundepaka , Saritha B , Sailaja B. B.V

1
Tetrahedron Letters
Efficient in situ palladium nano catalysis for Z-selective semi transfer
hydrogenation of internal alkynes using safer 1, 4-butanediol
a,b
a
c
a*
b*
Siva Kumar Rapeti , Krishna Chaitanya Kasina , Prasad Gundepaka , Saritha Birudaraju , Sailaja B.B.V
aGVK Biosciences Pvt Ltd, Hyderabad, T.S. India; Email :sarithakbirudaraju@gmail.com
bDepartment of Inorganic & Analytical Chemistry, Andhra University, Visakhapatnam, Andhra Pradesh, India,
Email : sailajabbv.chemistry@gmail.com
c
Centre for Pharmaceutical Science, Institute of Science and Technology, JNTU, Hyderabad, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Simple and efficient in situ generated palladium nanoparticles (PdNPs) in PEG-4OO catalyzed
semi transfer hydrogenation of internal alkynes to Z-alkenes with excellent selectivity along
with the formation of beneficial γ-butyrolactone as a byproduct using low quantity of safer and
attractive 1, 4-butanediol as a hydrogen source was described.
Available online
Keywords:
Nano palladium
Semi transfer hydrogenation
Internal alkynes
2
009 Elsevier Ltd. All rights reserved.
Safer and attractive 1, 4-butanediol
Constructing the Z-alkenes having versatile applications in
pharmacologically active constituents, materials and organic
synthesis [1], through the selective semi hydrogenation of
alkynes has always been an attracting and motivating assignment
to the scientific and industrial community. Though, the Lindlar’s
catalyst is a common and good selection for this persistence till
today [2], few limitations including (i) the use of a toxic Pb salt
1, 4-Butanediol is a proficient and benign hydrogen source
that readily accessible as of renewable biomass derivatives [14].
The loss of two equivalents of molecular hydrogen from 1, 4-
butanediol upon its dehydrogenation yields γ-butyrolactone
(GBL) as a sole product [15]. Owing to the substantial role of
lactones in natural products and polymers [16], their synthesis by
catalytic transfer dehydrogenation of diols was considered as an
eco-friendly method [17]. Remarkably, the irreversible nature of
the 1, 4-butanediol as understood by resulting lactone formation
is a significant benefit and thus 1, 4-butanediol have become a
good alternative to formic acid in transfer hydrogenations.
Consequently, several metal catalyzed transfer hydrogenations of
nitrile [18], nitro [19], carbonyl [20-21], olefin [21] and imine
[21] compounds were successively accomplished by means of 1,
4-butanediol. To the best of our knowledge, there is no
precedence of metal catalyzed selective semi transfer
hydrogenation of alkynes using 1, 4-butanediol as a hydrogen
source. Herein, we first time describe the catalytic ability of
(
ii) excess of quinoline to suppress the over-hydrogenation, (iii)
low substrate tolerance (iv) isomerization of the (Z)-olefins to the
E)-olefines and (v) less stability concerns have stimulated the
(
chemists to develop other alternative V [3], Cr [4],Fe [5], Co [6],
Ni [7], Cu [8], Nb [9], Ru [10], Rh [11] and Au [12] catalysts.
Specifically, palladium has been recognized as the most
active and appropriate catalyst, due to the stronger adsorption of
the C≡C (compare to C=C group) on Pd metal surface permits
the inhibition of the over-reduction of alkene. Accordingly,
numerous heterogeneous nano Pd-catalysts [13] were developed
for this perseverance. Nonetheless, all these Pd-catalysts have
shown high activity and selectivity, several reasonable
shortcomings were found, such as slightly complicated and
tedious materials preparation [13a-n & 13p], none [13h-k, 13m
2
simple and efficient in situ generated PdNPs [(Pd(OAc) /PEG-
400)] system for the selective conversion of internal alkynes to
Z-alkenes via transfer hydrogenation pathway using attractive
and safer 1.4-butanediol along with the formation of beneficial λ-
butyrolactone as a bi-product.
&
13o-p] or little recyclability [13e-g & 13l] and the use of
external H gas [13a-o] that consist of special handling and
2
storage matters. Therefore, the development of easily accessible
nano Pd-catalysis for the selective semi transfer hydrogenation of
alkynes using more attractive and safer hydrogenating agent is a
desirable objective.
At the outset, we have concentrated on the selection of
suitable Pd-catalysts for the semi transfer hydrogenation (STH),
choosing diphenylacetylene (1a) and 1, 4-butanediol (2a) are
model substrates. As shown in table 1, using 3 mol % of
homogeneous Pd-catalysts like PdCl2, Pd(OAC)
2
and

2
Tetrahedron Letters
d
PdCl2(PPh
3
)
2
in THF solvent at reflux temperature, low yields
8
Pd(OAc)
PEG-200
2
(1)/
(1)/
6
4
74/0
92/0
Trace
59/0
92/0
88/12
(17-31%) of Stilbene (3a) that associated with low to moderate
d
9
Pd(OAc)
PEG-600
2
94/6
-
selectivity (58-72%) of Z-stilbene (Z-3a) was found, even after 8
hours (Table 1, entries 1-3). The effect of ionic liquids combined
10
Pd(OAc) (1)/
2
12
8
PEG-400
with PdCl
2
(PPh
, entries 4). Interestingly, the addition of polyethylene glycol-
00 (PEG-400) solvent to the 1 mol% of PdCl2, Pd(OAC) and
have greatly allowed the formation of high yield
3 2
) catalyst was also none in the STH of 1a (Table
d,e
1
1
1
1
1
2
3
4
Pd(OAc)
00
2
(1)/PEG-
(1) /PEG-
86/14
1
4
4
d,f
2
Pd(OAc)
400
6
94/6
92/8
2
3 2
PdCl2(PPh )
g
Ex situ PdNPs
7
6
83/0
trace
(84-92%) of 3a with excellent selectivities (89-94%) of Z-3a, at
o
6
5 C after 4 hour only (Table 1, entries 5-7). However, the
h
Pd(OAc)
00
The reaction was conducted in 1 mL of THF at reflux temperature; Yields
2
(1) /PEG-
-
PdCl
2
(PPh
3
)
2
in PEG-400 system has been found to best (Table 1,
for the further
4
entry 7), we have interested to select Pd(OAC)
optimization study, since it does not require any additives such as
base and phosphine ligand etc., (Table 1, entry 6). Further, the
2
a
b
1
c
1
d
were determined by H NMR; Z/E ratio was Determined by H NMR; The
reaction was carried out in 2 mL of corresponding PEG at 65 C; Reaction
o
e
f
O
g
was carried out at RT; Reaction was carried out at 45 C; Pre-prepared
PdNPs; 100 mmol of Hg was added.
h
combination of Pd(OAc)
2
and PEG-200 system has decreased the
yield of 3a and selectivity of Z-3a to little extent. On the other
hand, no change in the yield of the 3a and selectivity of Z-3a was
obtained, when we used Pd(OAC) +PEG-600 system (Table 1,
2
entry 9). Particularly, in the presence of PEGs, the color of the
crude reaction mixture was reddish brown, that indicated us the
formation of possible in situ nano-palladium particles (Table 1,
entries 5-11). Previously, the use poly ethylene glycols (PEGs) as
efficient synthetic and stabilizing agents for the PdNPs has been
well established [22].The TEM study revealed that the size of in
Using the above optimized catalytic system, we then
investigated the hydrogenation capability of few mono and di-
hydric alcohols. The findings suggested that the transfer
hydrogenation of 1a using excess (3-5 equivalents) of
isopropanol and 2-butanol was not fruitful even after 12 hours
(
Table 2, entries 1-4). Surprisingly, all the 1 equivalent diols used
in the STH of 1a, have afforded the moderate to high yields (47-
9
9
1
2%) of desired 3a, where excellent selectivities of Z-3a (91-
4%) were also found (Table 2, entries 5-8). Among all the diols,
, 4-butanediol has provided appreciable results in the present
2
situ generated Pd nanoparticles [1 mol% Pd(OAC) + PEG-400]
system in the range of 2.5-3.0 nm (Figure S1, ESI). The high
yield of 3a and excellent selectivity of Z-3a may be attributed to
the formation of very small size PdNPs and their subsequent
steric stabilization by PEG-400 (Table 1, entry 6). Besides, the
approach (Table 2, entry 6). The efficacy of low quantity of 1, 4-
butanediol as an efficient hydrogen source in several transfer
hydrogenation reactions, because of possible equilibrium
determined through the formation of GBL was well elucidated
o
reaction was not considerable at room temperature and 45 C
18-21
previously.
(
Table 1, entry 10-11). There was also found that increasing in
Table 2. Selection of suitable hydrogen-source
the reaction time up to 6 hours, the yield of 3a and selectivity of
Z-3a remains intact (Table 1, entry 12). We have also prepared
ex situ PdNPs according to literature procedure 23 and then
directly utilized for the model reaction under the optimal
conditions to afford 83% yield of 3a (Table 1, entry 13) which
indicates that in situ formed PdNPs are highly active catalytic
Entry
H-source(eq)
Time
Yield 3a
Selectivity
b
(Z.3a/E-3a)c
(
%)
11
16
17
22
92
47
58
1
2
3
4
5
6
7
Iso-propanol (3)
2-butanol (3)
Iso-propanol (5)
2-butanol (5)
1,4-butanediol (1)
1,6-hexanediol (1)
1,5-pentanediol (1)
12
12
12
12
4
71/29
79/21
78/22
83/17
94/6
.
species as compared to ex situ PdNPs. Finally the addition of Hg
(100 equiv to Pd) to model reaction under the optimal conditions
has led to completely prevent the STH, indicating that the actual
active catalytic system is likely to be nanopalladium (Table 1,
entry 14).
4
4
91/9
93/7
Table 1. Selection of suitable Pd-catalyst
a
O
The reaction was conducted using 2 mL of PEG-400 at 65 C temperature;
Yields were determined by H NMR; Z/E ratio was Determined by
NMR;
b
1
c
1
H
The above optimized conditions in our hand, we further
extended our protocol to examine the compatibility of various
internal alkynes. From the table 3, it was understood that all
internal alkynes used were found to well suited in the present
protocol, to give excellent selectivities (>90%) of corresponding
Z-alkenes. Among all internal alkynes, the diaryl alkynes were
more reactive and provided high yields of corresponding olefins
(Table 3, entries 2-11). The nature of substituents (electron
releasing/electron withdrawing) on aryl internal alkynes has not
much effective in the present STH. Conversely, with increasing
in the aliphatic nature of internal alkynes, the yields of alkenes
have been decreased progressively (Table 3, entries 12-15).
c
Entry
Catalyst (mol%)
Time
3a/4a
Yield(%)
Selectivity (%)
Z-(3a)/E-(3a)
b
d
1
2
3
4
PdCl
2
(3)
8
8
8
8
17/12
23/11
33/9
58/42
63/37
d
Pd(OAc)
2
(3)
d
PdCl
PdCl
2
(PPh
(PPh
3
)
)
2
(3)
(3)/b
72//28
72//28
d
2
3
2
33/9
mimBF
PdCl (1)/PEG-400
Pd(OAc) (1)/
PEG-400
PdCl (PPh
EG-400
4
d
5
6
2
4
4
84/0
92/0
89/11
94/6
d
2
d
7
2
)
3 2
(1)/P
4
94/0
95/5

3
Based on the controlled experiments and from the literature
17a, 18, 22f & 24] we proposed a plausible mechanism (Scheme
Noticeably, any reduction of halogens, nitriles and carbonyls
functionalities/groups present on the diaryl alkynes was not
accomplished, indicating that our present approach could
selectively reduce the C≡C alkynes only, under the chosen
optimized conditions (table 3, entries 5-10).
[
2
). Initially, 1, 4-butanediol reacts with active PdNP catalytic
species to form Pd-H species (A) by yielding tetrahydrofuran-2-
2
ol (B). Next, the addition of A to the internal alkyne C gives σ-
vinyl palladium adduct E through the formation of activated
intermediate D. Finally, the adduct E undergoes reductive
elimination to give desired Z-olefin with the regeneration of
active PdNPs. These PdNPs further react with B to form another
Table 3. Substrate scope
Pd-H
2
species (A) along with the formation of GBL (G)
species (A)
byproduct. At this juncture, the newly formed Pd-H
2
continues the semi transfer hydrogenation of new internal alkyne
molecule.
a
2
The reaction was conducted using 1 mol% Pd(OAc) and 2 mL of PEG-400
o
b
1
c
solvent at 65 C for 4 h; Yields were determined by H NMR; Z/E ratio
was determined by H NMR; Reaction time was 4.5 h; The reaction time
was 5 h.
1
d
e
To understand the possible transfer hydrogenation path, we
have carried out some controlled (1A-1D, scheme 1). Though,
the PEG-400 polymer is a polar protic solvent, no reduction of 1a
was achieved indicating that PEG-400 couldn’t serve as
hydrogen source in the present PdNPS catalyzed STH reaction
(1A, scheme 1). Besides, the transfer hydrogenation of 1a using
1
, 4-butanediol without using any catalyst was also unsuccessful,
Sc
which specify that the Pd-catalyst is necessary for the activation
of 1, 4-butanediol to allow its two H-atoms for the semi-
hydrogenation of 1a (1B, scheme 1). Furthermore, in situ PdNPS
catalyzed STH of 1a by means of 1, 4-butanediol, has provided
the 92% of 3a, that contains 94% of Z-3a, along with the
formation of γ-butyro lactone (GBL) byproduct confirmed that
Sheme 2. Proposed reaction pathway
In order to demonstrate the competence of this
procedure, recyclability of the established in situ generated
PdNPs catalytic system was studied using 1a as a substrate
Figure 1). After first investigation, the reaction mixture was
extracted with diethyl ether (4 X 10 mL) and solidified
Pd(OAc) /PEG 400 was further allowed to a second catalytic run
using same substrate. We have detected that transformation is
faster, after the first catalytic run, by giving high yields of 3a
could be due to the generation of in situ PdNPs. Interestingly, no
appreciable loss in the yield of 3a was observed, using the
present PdNPs catalytic system up to six recycle runs.
(
1
,4-butane diol acts as hydrogenating agent in the present STH
2
approach (1C, scheme 1). The formation of (GBL) bi-product
1
was analyzed H NMR spectrum (ESI) and consistent with
2
3
previous data. The addition of radical scavenger 2, 2, 6, 6-
Tetramethyl-piperidin-1-yl)oxyl (TEMPO) to the reaction
mixture has not altered yield of 3a as well as selectivity of Z-3a,
that indicate our transformation don’t proceed through radical
intermediates (1D, scheme 1).
1
9
00
0
2a Yield (% )
8
0
7
0
6
0
5
0
4
0
3
0
20
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6 Cycle 7
2
Figure 1. Recyclability of Pd(OAc) + PEG-400 catalytic system
In conclusion, we first time established a simple and
efficient palladium nanoparticles catalyzed semi transfer
hydrogenation of alkynes to achieve highly selective Z-alkenes in
good to excellent yields using stoichiometric amount of safer 1,4-
Scheme 1. Some supporting studies

4
Tetrahedron Letters
butanediol as a hydrogen source. The co-formation of GBL
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(
during the synthesis of Z-olefins makes our protocol attractive.
Furthermore, the established catalytic system worked well up to 6
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5
Highlights
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Operationally simple catalysis
Safer and attractive H -source
2
Z-selective semi transfer hydrogenation
Valuable byproduct formation