Carboxylic acid addition to terminal alkynes utilizing ammonium tagged Hoveyda-Grubbs catalyst supported on magnetically separable core/shell silica: A highly reusable and air compatible catalytic system

Article abstract of DOI:10.1016/j.jorganchem.2019.01.005

In this study, the performance of ammonium tagged Hoveyda-Grubbs catalyst supported on magnetically separable core/shell silica gel was tested on carboxylic acid addition reactions to terminal alkynes using a variety of carboxylic acid derivatives under air atmosphere. The catalytic system was found to be compatible with air atmosphere and can tolerate even non-degassed solvents. The reaction parameters such as temperature, substrate/catalyst ratio and the effect of carboxylic acid on the selectivity and yield of the reaction were investigated in details. The reaction of arylacetylenes with acetic acid yielded the corresponding E-isomer with conversion values up to 99% with a catalytic loading of 1% Ru. The reusability of the catalyst was tested using acetic acid/benzoic acid and phenylacetylene in toluene at 85 °C under air atmosphere. The catalyst was found to be highly reusable and maintained its activity up to 11th run, reaching a conversion value of 83% with minimum ruthenium leaching.

Full text of DOI:10.1016/j.jorganchem.2019.01.005

Accepted Manuscript
Carboxylic acid addition to terminal alkynes utilizing ammonium tagged Hoveyda-
Grubbs catalyst supported on magnetically separable core/shell silica: A highly
reusable and air compatible catalytic system
Bengi Özgün Öztürk, Didar Gürcü, Solmaz Karabulut Şehitoğlu
PII:
S0022-328X(18)30939-2
DOI:
https://doi.org/10.1016/j.jorganchem.2019.01.005
JOM 20683
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 24 October 2018
Revised Date: 13 December 2018
Accepted Date: 10 January 2019
Please cite this article as: Bengi.Öü. Öztürk, D. Gürcü, S.K. Şehitoğlu, Carboxylic acid addition to
terminal alkynes utilizing ammonium tagged Hoveyda-Grubbs catalyst supported on magnetically
separable core/shell silica: A highly reusable and air compatible catalytic system, Journal of
Organometallic Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2019.01.005.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Carboxylic Acid Addition to Terminal Alkynes Utilizing Ammonium Tagged Hoveyda-Grubbs
Catalyst Supported on Magnetically Separable Core/Shell Silica: A Highly Reusable and Air
Compatible Catalytic System
Bengi Özgün ÖZTÜRK*, Didar GÜRCÜ, Solmaz KARABULUT ŞEHİTOĞLU*
Hacettepe University, Faculty of Science, Chemistry Department, 06800, Beytepe-Ankara, TURKEY
*Corresponding author
E-mail: bengi04@hacettepe.edu.tr
ABSTRACT
In this study, the performance of ammonium tagged Hoveyda-Grubbs catalyst supported on
magnetically separable core/shell silica gel was tested on carboxylic acid addition reactions to terminal
alkynes using a variety of carboxylic acid derivatives under air atmosphere. The catalytic system was
found to be compatible with air atmosphere and can tolerate even non-degassed solvents. The reaction
parameters such as temperature, substrate/catalyst ratio and the effect of carboxylic acid on the
selectivity and yield of the reaction were investigated in details. The reaction of arylacetylenes with
acetic acid yielded the corresponding E-isomer with conversion values up to 99 % with a catalytic
loading of 1 % Ru. The reusability of the catalyst was tested using acetic acid/benzoic acid and
phenylacetylene in toluene at 85 °C under air atmosphere. The catalyst was found to be highly
reusable and maintained its activity up to 11th run, reaching a conversion value of 83 % with minimum
ruthenium leaching.
Keywords: Carboxylic acid, alkyne, ruthenium, reusable catalyst, enol-ester
INTRODUCTION
Carboxylic acid addition reactions to terminal alkynes are of great importance owing to their ability to
form unique C-O bonds through the enol-ester formation in an efficient manner. Different transition
metals such as Ru, Rh, Ir, Re, Pd, and Au are used efficiently in enol-ester synthesis [1-2]. Among
these transition metal-based catalysts, ruthenium complexes were commonly used as highly active and
selective catalysts in carboxylic acid addition to terminal alkynes [3-5] (Scheme 1).
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Scheme 1. Ruthenium based catalysts for carboxylic acid addition to terminal alkynes
The first example of carboxylic acid addition to terminal alkyne reaction was reported by Shvo and
Rotem utilizing Ru3(CO)12 as an efficient catalyst [6]. Dixneuf and coworkers demonstrated the effect
of the catalyst structure on the selectivity of the addition reaction using RuCl2(η6-arene)(PR3)] and
[{Ru(µ-O2CH)-(CO)2(PPh3)}2 to obtain Marknovnikov product. On the other hand, [Ru(η3-2-
C3H4Me)2{κ2(P,P)-Ph2P(CH2)nPPh2}] yielded mainly Z-isomer of the addition product. [7-9]. Recently
Stark et al. reported a cationic ruthenium catalyst: [RuCl(2,6-diacetylpyridine)(PPh3)2]BArF giving
predominantly gem isomer in carboxylic acid addition reactions to alkynes [10]. The dimeric
bis(allyl)ruthenium (IV) complex and mononuclear trans-[RuCl2(η3:η3-C10H16)PPh3] were found to be
efficient catalysts for addition reactions of aromatic/aliphatic carboxylic acids to a variety of alkynes
such as enynes, diynes and terminal alkynes [11]. [Ru(dppe)2(CH3CN)Cl][BF4] complex was reported
as an anti-Markovnikov selective catalyst for carboxylic acid addition reactions to terminal alkynes
[12]. In 2016, a highly regioselective protocol (up to 99 % Markovnikov addition product) utilizing
[Ru(CO)2(P(p-C6H4-X)3)2(O2CPh)2] (1a, X = CF3; 1b, X = Cl; 1c, X = H; 1d, X =Me; 1e, X = OMe)
for carboxylic acid addition to terminal alkyne reactions was reported [13]. In addition to metathesis
reactions, Grubbs type catalysts can be used in various non-metathetical catalytic transformation
reactions [14]. The thermolysed Grubbs catalyst was found to be an efficient catalyst for carboxylic
acid addition reactions to alkynes giving Markovnikov addition products and the same study also
demonstrated the pH-dependent selectivity of the reaction [15]. Our research group reported the
activity of Grubbs 2nd generation analog on carboxylic acid addition and terminal alkyne dimerization
reactions [16]. The analog of Grubbs catalyst bearing N-heterocyclic carbene ligands showed high Z
selectivity yielding Markovnikov products in carboxylic acid addition reactions and can even catalyze
the carboxylic acid addition to internal alkynes. A limited amount of study was focused on the
carboxylic acid addition to internal alkynes which are known as challenging substrates [17-20].
Although a variety of different catalysts were employed in carboxylic acid addition reactions, only a
few catalytic systems were found to be reusable [21-23]. A reusable heterogeneous catalyst system
based on phosphine modified Ru/CeO2 for carboxylic acid addition reactions to terminal alkynes was
reported by Nishiumi et al. [22]. Recently our research group has reported a novel support material
based on core/shell structured magnetically separable silica gel for ammonium tagged Hoveyda-
Grubbs catalysts [24] as a robust and highly reusable catalytic system for ring-closing metathesis
reactions [25]. Here we report the activity of ammonium tagged Hoveyda-Grubbs catalysts
immobilized on magnetically separable core/shell silica supports for the carboxylic acid addition
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reactions to arylacetylenes. The homogenous ammonium tagged Hoveyda-Grubbs catalyst and its
supported analog was compared in means of selectivity. The supported catalyst Ru-1@SiO2 catalyzed
the acetic acid/benzoic acid addition reactions to phenylacetylene up to 11th run without any
significant decrease in both activity and selectivity.
EXPERIMENTAL
Otherwise noted all chemicals were bought from Sigma-Aldrich and used as received. Toluene was
dried over Na wire/benzophenone and distilled under a nitrogen atmosphere. Ammonium tagged
Hoveyda-Grubbs catalyst; (1,3-bis(mesityl)imidazolidin-2-ylidene)(2-((1-(methoxy(methyl)amino)-1-
oxopropan-2-yl)oxy)benzylidene)ruthenium(VI) chloride were bought from Strem and used as
received. Core/Shell structured magnetically separable silica gels and the supported catalyst; Ru-
1@SiO2-magn. were synthesized according to the literature [25]. Gas chromatography-mass
spectrometry (GC-MS) analyses were performed with a Shimadzu GC-MS 2010Plus using a
Restek Rxi-5Sil column (30 m × 0.25 mm × 0.25 µm) and temperature range of 50–320 °C
1
13
with a constant helium flow rate of 1 ml/min. H and C NMR spectra were recorded at 25
°C with a Bruker GmbH 400-MHz high performance digital FT-NMR spectrometer using
CDCl3 as the solvent.
The representative protocol for carboxylic acid addition to terminal alkynes
Alkyne (0.18 mmol) and carboxylic acid (1.8 mmol) was added to the Schlenk reactor. Toluene (1
mL) and Ru-1@SiO2-magn. (20 mg, 0.0018 mmol Ru) were added to the reaction medium and
reaction mixture were stirred at 85 °C under air atmosphere. Aliquots taken from the reaction mixture
was diluted with methanol and analyzed by GC-MS using n-tetradecane as an internal standard. Once
the reaction was finished, the catalyst was separated from the reaction mixture with the aid of a
magnet. The volatiles were removed by high vacuum. The remaining solution was diluted by
dichloromethane and extracted with saturated NaHCO3 solution followed by extraction with water.
The organic layer was dried with MgSO4 and then volatiles were removed by high vacuum. The
resulting final product was characterized by means of 1H NMR spectroscopy and GC-MS analysis.
The representative protocol for reusability tests
Alkyne (0.18 mmol) and carboxylic acid (1.8 mmol) were added to the Schlenk reactor. Toluene (1
mL) and Ru-1@SiO2-mgn (20 mg, 0.0018 mmol Ru) were added to the reaction medium and reaction
mixture were stirred at 85 °C under air atmosphere. Aliquots taken from the reaction mixture was
diluted with methanol and analyzed by GC-MS using n-tetradecane as the internal standard. Once the
reaction was finished, the catalyst was separated from the reaction mixture with the aid of a magnet.
The catalyst was kept in the reactor while the solution phase was taken out of the reactor via Pasteur
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pipet. The catalyst was washed with toluene and dichloromethane and then reused in carboxylic acid
addition reactions.
RESULTS&DISCUSSION
Magnetic mesoporous core/shell silica gels and Ru-1@SiO2-magn. were synthesized according to the
literature (Figure 1). [25]. Ru-1 (commercial name; Aquamet) was supported on core/shell silica gel
(surface area: 630 m2/g; pore size/volume; 2.88 nm/0.393 cm3) in CH2Cl2 medium. The ruthenium
content of the Ru-1@SiO2-magn. was found to be 0.090 mmol Ru/g as determined by ICP-MS.
Figure 1. Ru-1 and Ru-1@SiO2-magn. catalysts
First tests were carried out using phenylacetylene and acetic acid with a catalytic loading of 1 % Ru in
toluene at 85 °C. In the first trial, acetic acid/phenylacetylene (mol/mol) ratio was varied between 1/1,
2/1, 5/1 and 10/1 and results were given in Table 1.
Table 1. The effect of acid/alkyne ratio on carboxylic acid addition reaction.
Entrya Acid/Alkyne Conversionb Additionc
2a
2b
2c Dimerizationc
%
Product
(%)
36
(%)
(%) (%) Product (%) Z/E/Gem
(gem)d (Z)d (E)d
1/1
2/1
90
95
99
99
15
16
15
11
23
22
22
26
62
62
63
63
54
29
15
-
60/40/0
55/45/0
53/47/0
-
1
2
3
4
66
5/1
84
10/1
99
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a: Reaction time: 24 h; b: Determined by GC-MS using n-tetradecane as internal standard. c: Determined by GC-MS d:
İsomer ratios was determined using integration values of GC peaks of corresponding isomers. Alternatively, isomer ratio was
also determined by 1H NMR using the integration value of olefinic peaks of E, Z, gem isomers. E % = (Integration
E/(Integration E+ Integration Z+ Integration gem))x100
As it can be seen in Table 1, alkyne dimerization reaction can be suppressed by increasing acid/alkyne
ratio. The amount of dimerization product had significantly decreased when acid/alkyne ratio was
increased from 1/1 to 10/1. Dimerization reaction was completely suppressed at the acid/alkyne ratio
of 10/1. On the next trial, the effect of solvent on carboxylic acid addition reactions was tested using
both homogeneous and supported catalysts; Ru-1 and Ru-1@SiO2-magn. in water, methanol,
methanol/water (v/v: 1/1), toluene and no solvent media. As it can be seen in Table 2, the reaction
exhibited no selectivity in means of gem, Z and E isomers of addition products in both water and
water/methanol mixture. Gem/Z/E products at ratios of 16/41/43 (74 % conversion in water) and
20/42/38 (60 % conversion in methanol/water) were obtained using Ru-1@SiO2-magn. The reaction
proceeded more selectively in both toluene and no solvent media, reaching conversion values up to 99
%. The reaction yielded E isomer as the main product (Table 2, Entry 6, 63 % E) using Ru-1@SiO2-
magn. Homogenous catalyst Ru-1 exhibited low selectivity (gem/Z/E %; 44/26/30 %) in addition
reactions
when
compared
toRu-1@SiO2-mag.
Figure 2. 1H NMR spectrum of 2a/2b/2c in CDCl3
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1H NMR spectrum of the reaction product also confirmed the presence of both E, Z and gem isomers
(Figure 2). Olefinic proton signals of E and Z isomer appeared at 7.84, 7.49 and 6.33, 5.69 ppm as
doublets. Gem signals were observed at 5.40 and 4.95 ppm as singlets. CH3COO- signals of E, Z and
gem isomers appeared at 2.20, 2.27 and 2.36 ppm as singlets. Aromatic signals of both isomers were
overlapped at 7.08-7.38ppm region.
Table 2. Comparison of Ru-1 and Ru-1@SiO2 on acetic acid addition to phenylacetylene using
different solvents
Entrya
Catalyst
Solvent
Conversion %b
gem %
Z %
E %
Ru-1
water
94
38
47
15
1
2
3
Ru-1@SiO2-magn.
Ru-1
water
74
70
16
27
41
40
43
33
MeOH/water
(v/v: 1/1)
Ru-1@SiO2-magn.
MeOH/water
(v/v: 1/1)
60
20
42
38
4
Ru-1
Toluene
95
99
94
99
44
11
36
10
26
26
27
28
30
63
37
62
5
6
7
8
Ru-1@SiO2-magn.
Ru-1
Toluene
No solvent
No solvent
Ru-1@SiO2-magn.
a: The reaction conditions are as follows: A reactor was charged with 1 % Ru (0.018 mmol),
phenylacetylene (1.8 mmol) and acetic acid (18 mmol) in toluene ( 1 mL) and reacted under air
atmosphere at 85 °C for 24 h. b) No dimerization product was observed.
The performance of the catalytic system was tested on both air and nitrogen atmosphere using
degassed dried toluene and non-degassed neat toluene at 85 °C and results were given in Table 3. No
significant difference in means of conversion value and selectivity was observed when the reaction
was carried out under air and nitrogen atmosphere. On the other hand, when a trace amount of water
was present in the reaction media, acetophenone formation was inevitable.
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Table 3. Acetic acid addition to phenylacetylene under different reaction atmospheres
Entrya
1
Atmosphere
Solvent
Degassed
Conversion
%
Addition%
(gem/Z/E)
100
Acetophenone %
b
N2
99
99
99
90
-
dry toluene
Non-degassed
toluene
(11/26/63)
2
3
4
N2
Air
Air
100
(11/27/62)
100
-
-
Non-degassed
toluene
(11/26/63)
84
Non-degassed
toluene + trace
amount of water
16
(14/26/60)
a: The reaction conditions are as follows: 1 % Ru (0.018 mmol), phenylacetylene (1.8 mmol) and acetic acid (18
mmol) in solvent (2 mL) under air atmosphere at 85 °C for 24 h.
Table 4. Carboxylic acid addition to arylacetylenes
Entrya Alkyne
Temperature
Acid
Conversion %b
Gem
%
12
11
12
10
7
Z % E %
( °C)
50
1a
1a
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
20
99
70
90
99
99
99
99
99
98
99
99
25
26
30
23
24
28
27
29
26
22
28
27
63
63
58
67
69
61
64
62
59
55
62
64
1
2
85
Acetic acid
(Ac1)
110
85
3
4
85
5
85
11
9
6
85
7
85
9
8
85
Benzoic
acid
15
23
10
9
9
85
10
11
12
85
(Ac2)
85
a: Please see experimental section for the synthesis protocol. b: No dimerization product was observed.
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Phenylacetylene (1a), 2-ethynyltoluene (1b), 4-ethynyltoluene (1c), 2-ethynylanisole (1d) and
4-ethynylanisole (1e) were reacted with acetic acid (Ac1) and benzoic acid (Ac2) in toluene at 85 °C
with a catalytic loading of 1 % Ru (Table 4). No significant selectivity difference was observed when
using acetic acid and benzoic acid. In all cases, higher conversion values were obtained even using
sterically hindered ortho-substituted alkynes (1b and 1d; 90 % and 99 %). E-isomer was formed as the
major product during addition reaction. No trace amount of dimerization product was observed during
the reaction. To further investigate the compatibility of the reaction with different acids; trans-
cinnamic acid, trans-2-octenoic acid, 4-cyclooctene carboxylic acid were used on addition reactions of
phenylacetylene under same reaction conditions and also configuration retentions of carboxylic acids
were investigated. All addition reactions were reached conversion values up to 99 % under pre-
determined reaction conditions (Table 5). Trans-2-octenoic acid, and trans-cinnamic acid retained their
1
trans configurations during the reaction as confirmed by H NMR analysis (supporting information,
Figure S3-4). Although cyclooctene derivatives can undergo ring opening metathesis polymerization
reactions in the presence of Grubbs type ruthenium alkylidene complexes, 4-cyclooctenoic acid didn’t
undergo any ring opening reaction during the carboxylic acid addition reactions (supporting
information (SI), Figure S5). The selectivity of the reaction was increased in the case of 4-cyclooctene
carboxylic acid and trans-2-cinnamic acid.
Table 5. The addition reaction of phenylacetylene with different carboxylic acids
Alkynea
Acids
Conversion %b
gem/Z/E (%)b
99
9/17/74
99
95
3/19/78
7/23/70
8

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a: The reaction conditions as follows: A glass reactor was charged with 1 % Ru (0.018 mmol), phenylacetylene
(1.8 mmol) and corresponding carboxylic acid (18.0 mmol) in toluene (2 ml) under air atmosphere and stirred for
24 h at 85 °C. b: Determined by GC-MS using n-tetradecane as internal standard.
Reusability of the catalyst was tested on both benzoic acid and acetic acid addition reactions to
phenylacetylene under pre-determined reaction conditions. The catalyst was separated by applying
magnetic field with an aid of a magnet. The catalyst was washed with toluene twice and then used for
the reaction. The reaction was terminated when no increase in conversion values was observed. As it
can be seen on both Graph 1 and 2, Ru-1@SiO2-magn. is highly usable in both acetic and benzoic
acid addition to phenylacetylene (Graph 1 and 2). No significant activity loss and catalyst leaching
were observed up to 11th catalytic turn in both reactions. Ruthenium leaching was found to be 1 % Ru
after 11th run as determined by ICP-MS.
Graph 1. Reusability of Ru-1@SiO2-magn on acetic acid addition to phenylacetylene
Graph 2. Reusability of Ru-1@SiO2-magn. on benzoic acid addition to phenylacetylene
9

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CONCLUSION
The ammonium tagged Hoveyda-Grubbs catalyst (Ru-1) supported on magnetically separable
core/shell silica gels was found to be an efficient, robust and air tolerant catalytic system for
carboxylic acid addition to terminal alkynes. The reaction showed high E selectivity using
arylacetylene derivatives with different carboxylic acids such as acetic acid, benzoic acid, trans-
cinnamic acid, trans-2-octenoic acid and 4-cyclooctenecarboxylic acid. The olefinic groups of trans-
cinnamic acid, trans-2-octenoic acid and 4-cyclooctenecarboxylic acid remained unreacted and
preserved trans configuration during the addition reaction. The effect of support material on the
selectivity of the reaction was investigated by comparing the selectivity of both homogenous catalyst
Ru-1 and its supported analog Ru-1@SiO2-magn, the latter one was found to be an E-selective catalyst
for addition reactions. Ru-1@SiO2-magn. catalyzed the addition reactions of acetic acid and benzoic
acid to phenylacetylene up to 11th run without any significant activity loss.
ACKNOWLEDGMENTS
We would like to thank Hacettepe University- Scientific Research Coordination Unit (Project No:
FHD-2017-14926) for the financial support.
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HIGHLIGHTS
•
•
•
•
Ammonium tagged Hoveyda-Grubbs catalyst was immobilized on core/shell silica gels
Carboxylic acid addition to alkynes proceeded with high E selectivity
The catalyst was found to be air compatible
The catalyst is reusable up to 11th run