Eco-friendly Suzuki-Miyaura coupling of arylboronic acids to aromatic ketones catalyzed by the oxime-palladacycle in biosolvent 2-MeTHF

Article abstract of DOI:10.1039/c5nj02734a

Oxime-palladacycle catalyzed Suzuki-Miyaura cross-coupling reaction of acid chlorides and arylboronic acids to yield aryl ketones was developed. The remarkable benefit of this method is the use of water immiscible (practically) 2-MeTHF as solvent, which provides easy isolation of the crude reaction mixture just by separation of 2-MeTHF-water layers, and then evaporation of 2-MeTHF. Moreover, the use of relatively equal proportion of substrates and less generation of side products make the method highly atom economic.

Full text of DOI:10.1039/c5nj02734a

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Eco-friendly Suzuki–Miyaura coupling of
arylboronic acids to aromatic ketones catalyzed
Cite this:DOI: 10.1039/c5nj02734a
by the oxime-palladacycle in biosolvent 2-MeTHF†
Manoj Mondala and Utpal Bora*ab
Oxime-palladacycle catalyzed Suzuki–Miyaura cross-coupling reaction of acid chlorides and arylboronic
acids to yield aryl ketones was developed. The remarkable benefit of this method is the use of water
immiscible (practically) 2-MeTHF as solvent, which provides easy isolation of the crude reaction mixture
just by separation of 2-MeTHF–water layers, and then evaporation of 2-MeTHF. Moreover, the use of
relatively equal proportion of substrates and less generation of side products make the method highly
atom economic.
Received (in Montpellier, France)
6th October 2015,
Accepted 29th January 2016
DOI: 10.1039/c5nj02734a
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Introduction
Ketones are ubiquitous structural motifs found across various
natural products, pharmaceuticals and agrochemicals.1 They
also represent a wide range of ideal starting materials for the
synthesis of cyanohydrins, oximes, carbazones, acetals, pinacols,
etc. Conventional routes to synthesize ketone are the Friedel–Crafts
Scheme 1 Synthesis of ketones.
acylation2 and Suzuki–Miyaura cross-coupling of aryl halides with less reactive acid anhydride or carboxylic acid in an aqueous
organometallic reagents under a carbon monoxide atmosphere.3 system needing high substrate loading;6b (ii) formation of emul-
However, the formation of tertiary alcohols and isomeric mixtures sions, rag layers at the phase interface and poor extraction yields;
due to limited regioselectivity,4 and the use of corrosive AlCl3 and (iii) the long and troublesome synthetic procedures for ionic
and toxic carbon monoxide5 are certain difficulties associated liquids.17 Overall, it is estimated that around 80% of the chemical
with these methods.
waste produced from drug synthesis corresponds to the organic
The palladium catalyzed Suzuki–Miyaura cross-coupling of solvent.18 Thus, while designing the acid halide-based reactions
acid derivatives with organoborons represents another powerful particular care should be taken in the choice of the solvent.
tool to access ketones (Scheme 1).6 This methodology has gained
Recently, Pace and co-workers reported the excellent preparation
immense attention as it relies on the use of easily accessible, of Weinreb amides starting from acid halides in a biphasic system
widely functionalized, relatively low-toxic,7 heat and moisture water–2-methyltetrahydrofuran (2-MeTHF).19 Importantly, they
stable organoborons. Moreover, the use of active and commercially obtained a range of pure products by simple removal of 2-MeTHF
available acid derivatives presents wide scope to various ketone in good yields and purity, and did not use extra solvent during the
derivatives. In recent years, numerous significant outcomes have work-up process. Interestingly, 2-MeTHF resembles toluene or
been achieved using diverse palladium catalysts viz. PdCl2,8 THF in terms of physical properties, and when used in organo-
Pd(OAc)2,9 Pd/C,6b Pd(dba)2,10 Pd(PPh3)4,11 PdCl2(dppf),12 POPd,13 metallic reactions it offers both economical and environmentally
Pd2dba3,14 imidazolium chloride tagged Pd(II) complex15 and friendly advantages over common aprotic solvents.20,21 In reactions
(BeDABCO)2Pd2Cl6,16 under reaction systems, like toluene, THF, including Grignard reagent,22 organopalladium, organozinc
water, water–acetone, ionic liquid or hydrated base, etc. In fact, and LiAlH4 reductions,23 2-MeTHF has found widespread
synthetic methods based on the use of these solvents present application. 2-MeTHF has been reported to work like THF
the following disadvantages: (i) the possibility of generation of in nickel24 and copper25 catalyzed couplings. Pharmaceutical
process development researchers have found that the use of
2-MeTHF provides superior diastereoselectivity when compared
a Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India.
with other solvents.26
E-mail: utbora@yahoo.co.in, ubora@tezu.ernet.in
b Department of Chemical Sciences, Tezpur University, Tezpur 784028, Assam, India
The growing interest towards 2-MeTHF is also due to its
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02734a adherence with several Green Chemistry principles.27 Among
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Table 1 Effects of catalyst loading, temperature and timea
Table 2 Screening solvents and basesa
Entry
Solvent
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
2-MeTHF
Acetone
Toluene
DMF
THF
i-PrOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
Na3PO4Á12H2O
KOH
4
97
84
Entry Catalyst
(mol% Pd) Temp. (1C) Time. (h) Yieldb (%)
12
5.5
8
6.5
4
6
6
5.5
6
6
7.5
5
5
95
1
2
3
4
5
6
7
8
9
10
1
1
1
1
1
1
1
1
80
80
80
80
80
50
27
80
80
80
4
4
98
97
43
71
97
50
32
42
39
31
77c
88
0.5
0.1
0.25
0.4
0.4
0.4
12
12
4
12
12
6
28d
83
Acetonitrile
DCM
73
9
PEG-400
2-MeTHF–H2O
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
87e
65 f
74g
87
10
11
12
13
Pd(OAc)2 0.4
PdCl2
Pd/C
0.4
0.4
6
6
91
89
58
49
43
Trace
a
Reaction conditions: 2a (0.5 mmol), 3a (0.55 mmol), 2-MeTHF (4 mL), 14
b
15
16
17
18
6
6
6
6
K2CO3 (1 mmol). Isolated yield.
NaOH
Et3N
—
them, the 3rd (less hazardous chemical synthesis – permitted
daily exposure of 2-MeTHF in human is upto 6.2 mg dayÀ1),28
5th (safer solvents and auxiliaries), 7th (use of renewable
feedstocks – it can be obtained from biomass feedstock
like furfural or levulinic acid derived from corncobs and
sugarcane)20a and the 10th (design for degradation – it degrades
by factors like air and sunlight, probably via oxidation and ring-
opening)29 principles refer it as a suitable green solvent for
both academia and industries.
In recent years, many workers have focused their research
interests on the application of oxime-palladacycles, which have
exhibited high catalytic activity in various cross-coupling reac-
tions.30 They are stable to heat, air and moisture, and easily
accessible from low-cost starting materials. However, to the
best of our knowledge, oxime-palladacycles have never been
studied for the coupling of acid chlorides and arylboronic
acids. As part of our ongoing efforts to develop transition-
metal-catalyzed reactions,6b,31 we herein report the use of a
combination of oxime-palladacycle 1 and 2-MeTHF in the
coupling of acid chlorides (Table 1). The present method
provides easy isolation of the reaction mixture without the use
of additional organic solvent, which would enhance the applica-
tion scope of the acylation coupling to a more acceptable level.
a
Reaction conditions: 2a (0.5 mmol), 3a (0.55 mmol), 1 (0.4 mol% Pd),
b
c
solvent (4 mL), base (1 mmol). Isolated yield. Treated at 120 1C.
d
f
e
Isopropyl benzoate was formed 36% (GC yield). Treated at 80 1C.
g
(3 : 1) ratio. N2 (1 atm).
comparably lower yield under present reaction conditions
(Table 1, entries 8–10).
Encouraged by the highest yield in the presence of 0.4 mol%
Pd catalyst 1, we further optimized the effect of different solvents
and bases (Table 2). It has been shown that the reaction proceeded
with 84% isolated yield within 12 hours in acetone (Table 2,
entry 2). Screening of other solvents indicated that toluene
(Table 2, entry 3) was optimal; whereas DMF, acetonitrile, dichloro-
methane and PEG-400 gave yields lower than 87% (Table 2, entries 4,
7, 8 and 9, respectively). The use of isopropanol produces isopropyl
benzoate in high amounts (Table 2, entry 6). The use of co-solvent
2-MeTHF–H2O (3:1) afforded 4aa in 65% yield (Table 2, entry 10).
This may be due to the hydrolysis of benzoyl chloride or the
formation of the aryloxy anion intermediate from arylboronic acid.
From these results it seems that various organic solvents could
be employed for the acylation reaction with oxime-palladacycle;
however, we decided to examine 2-MeTHF as the solvent for
further studies. The reason for our decision was that 2-MeTHF is
non-carcinogenic and is practically not miscible with water
(4 g/100 mL),32 which provides easy isolation during work-up step
without needing to add any noxious volatile solvent [see Experi-
Results and discussion
The investigation was initiated by using a model reaction between mental procedure]. Reaction under a N2 atmosphere provides
benzoyl chloride 2a and phenylboronic acid 3a in the presence slightly lower yield of the coupling product (Table 2, entry 11).
of palladacycle 1 (1 mol% Pd) and K2CO3 (2 equiv.) under open This suggests that the presence of dissolved air in 2-MeTHF
air conditions in 2-MeTHF at reflux. The results are summarized accelerates the catalytic cycle by enhancing the rate of activation
in Table 1. The desired benzophenone 4aa was formed in 98% of arylboronic acid to aryl ion species, vital for catalytic-
isolated yield within 4 h (Table 1, entry 1). Upon optimizing the transmetalation step.
reaction conditions with different catalyst loading, best results
We next examined the effect of different bases, and found
have been obtained by using catalyst 1 (0.4 mol% Pd) in 2-MeTHF that the inorganic bases like Na2CO3, Cs2CO3 and Na3PO4Á12H2O
at reflux (Table 1, entry 5). Cross-coupling using commonly have considerable effect on the reaction efficiency (Table 2,
employed palladium source like Pd(OAc)2, PdCl2 and Pd/C gave entries 12–14). Alkali metal hydroxides afforded the desired
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Table 3 Scope of acid chloridea
substrate. Thus, a Pd(0) species can possibly be poisoned by
adding excess Hg(0) (molar ratio to [Pd] B 400) to the reaction
mixture during the reaction. This suppression of the reaction is
generally considered as evidence for the presence of Pd(0) as the
active species. For the Hg-poisoning test, we have performed a
coupling reaction between benzoyl chloride 2a and phenylboronic
acid 3a using catalyst 1 (0.4 mol% Pd). After 1 h of reaction (41%
conversions vide GC-MS) excess Hg(0) was added to the reaction
mixture and stirred for next 6 h. It should be noted that the
catalytic activity apparently gets hindered even after 6 h of constant
stirring (indicated by GC-MS). This finding suggests that the true
active catalyst during the reaction is Pd(0) species. The formation
of palladium nanoparticles during the reaction is also demon-
strated by TEM analysis (see Fig. S1, ESI†).
Entry
R1, 2
R2, 3
4
Yieldb (%)
1
2
3
4
5
6
7
8
C6H5, 2a
C6H5, 2a
C6H5, 2a
C6H5, 2a
C6H5, 2a
C6H5, 2a
4-NO2C6H4, 2b
4-ClC6H4, 2c
4-MeOC6H4, 2d
4-MeOC6H4, 2d
4-ClC6H4, 2c
4-MeOC6H4, 2d
CH3, 2e
C6H5, 3a
4aa
4ab
4ac
4ad
4ae
4af
4ba
4ca
4da
4dd
4cg
4dg
4ea
4fa
97
96
93
91
90
94
87
91
95
89
91
94
0c
4-MeC6H4, 3b
4-t-BuC6H4, 3c
4-ClC6H4, 3d
4-FC6H4, 3e
4-CNC6H4, 3f
C6H5, 3a
C6H5, 3a
C6H5, 3a
4-ClC6H4, 3d
4-MeOC6H4, 3g
4-MeOC6H4, 3g
C6H5, 3a
9
10
11
12
13
14
On the basis of the literature reports,10,35 the reaction mechanism
of this acylation reaction was hypothesized as shown in Scheme 2.
Initially, Pd(0) is formed by the reduction of the Pd–C bond of
complex 1, and then the oxidative addition of acid chloride 2 to Pd(0)
produces the acylpalladium(II) chloride intermediate A. Under
alkaline conditions, subsequent transmetallation of ArB(OH)2 3
to acylpalladium intermediate A takes place at the Pd–Cl bond
C3H7, 2f
C6H5, 3a
0c
a
Reaction conditions: 2a–g (0.5 mmol), 3a–g (0.55 mmol), 1 (0.4 mol% Pd),
K2CO3 (1 mmol), 2-MeTHF (4 mL) at 80 1C under air for 6 h. b Isolated yield.
c
Treated for 12 h.
product within 49–58% yield (Table 2, entries 15 and 16). to generate (acyl)(aryl)palladium(II) species B. Finally, the reductive
Organic base (Et3N) gave much lower yield of the product elimination of species B delivers the oxidative coupling product
(Table 2, entry 17). However, in the absence of any base no 4, regenerating the active Pd(0) species and resuming the
traceable amount of product was observed (Table 2, entry 18). catalytic cycle.
With the standard reaction conditions, we next explored the
The cross-coupling reactions are implemented on a regular
substrate scope and limitations of the reaction by employing a basis in large scale chemical industries due to the wide accessibility
variety of acid chlorides 2a–d and arylboronic acids 3a–g and functionality of organometallic derivatives, and mild reaction
(Table 3). The standard reaction conditions were found to be conditions. Thus, we were keen to explore the scope for scale-up to
compatible with a wide range of electronically diverse substrates. prepare grams instead of milligrams of biaryl ketones. With the
Though, slight electronic interventions due to the substituent optimized reaction conditions and easy work-up procedure on a
were also observed in certain cases. It is found that both neutral small scale, we next investigated their performance upon scale-up.
and electron rich acid chloride reacts more efficiently with both Thus, the reaction of excess benzoyl chloride 2a (5 mmol), phenyl-
electron-rich and -poor arylboronic acids (Table 3, entries 1–6 boronic acid 3a (5.5 mmol), K2CO3 (10 mmol), complex 1 (4 mol%
and 10–12). It is also noteworthy that the reaction conditions Pd) and 2-MeTHF (20 mL) was performed at 80 1C for 4 h. We were
tolerate the halogen substituent in both substrates, which are able to isolate 0.8987 grams (98.7%) of benzophenone 4aa.
usually used in various cross-coupling reactions, and thus
Although most aqueous protocols described in the literature
exhibits further synthetic utility. Unfortunately, the aliphatic acid do use some amount of water as solvent, they often require
chlorides did not respond to the present reaction conditions, even excessive amounts of toxic organic solvents during product
after 12 h (Table 3, entries 13 and 14).
extraction from the water medium. Thus, making the total
It is well established from the literature that the oxime-derived
palladacycles are catalytically inactive species which supplies active
Pd(0) to the reaction mixture.30 Thus, they act as a reservoir of
highly active Pd(0) species, which are generated by the reaction of
palladacycle with one or more components of the reaction mixture
and are usually released into the catalytic cycle at a slower rate
than that of the catalytic reaction. This slow releasing property
prevents unwanted inactivation processes such as nucleation
and growth of large inactive palladium particles within the
reaction system.
Catalyst poisons that could selectively stop the catalytic
activity of Pd-catalysts can be used to determine the involved
active catalytically species. To date, the Hg(0) poisoning test has
been recognized as the most powerful in metal-catalyzed reactions.33
Hg(0) amalgamates strongly with Pd(0), Rh(0) and Ni(0)-based
complexes,34 thereby blocking the access of the active site to the Scheme 2 Probable reaction pathways.
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Table 4 Comparison of palladium catalysts for the Suzuki–Miyuara coupling reaction
Entry
Conditions
Catalyst (mol%)
Yield, 4aa
Ref.
1
2
3
4
5
6
2a (1.0 mmol), 3a (1.2 mmol), Pd/C, K2CO3, acetone–H2O, 60 1C
3
5
0.4
3.3
5
93
80
96
90
68
97
6b
11a
16
8b
2a (1.0 mmol), 3a (1.2 mmol), Pd(PPh3)4, Cs2CO3, toluene, N2, 100 1C, 16 h
2a (0.75 mmol), 3a (0.5 mmol), (BeDABCO)2Pd2Cl6, K2CO3, toluene, rt, 1 h
2a (1.5 mmol), 3a (1 mmol), PdCl2, Na2CO3, grinding, 25–30 1C, 5 min
2a (1 mmol), 3a (0.5 mmol), Pd(PPh3)4, Cs2CO3, toluene, MW, 10 min
2a (0.5 mmol), 3a (0.55 mmol), catalyst 1, K2CO3, 2-MeTHF, 80 1C, 4 h
14
0.4
Present work
relative amount of noxious solvent in the entire process much dried over anhydrous Na2SO4, filtered and evaporated under
higher, negating the main objective of green chemistry. Importantly, reduced pressure. The resultant residue was subjected to silica
by using 2-MeTHF the crude mixture of cross-coupling products was gel column chromatography (eluent, 10–15%: ethyl acetate/hexane)
simply extracted by quenching with water followed by the separation to afford the desired product.
of the resulting 2-MeTHF–water phases and drying, without the
Benzophenone (4aa):36 white solid, yield: 97%, 88.2 mg;
need to use additional organic solvent during the entire workup m.p. 48–49 1C (literature,37 47–48.5 1C), H NMR (CDCl3, 400
procedure. Thus, the present method is an excellent example MHz, ppm) d: 7.81–7.79 (m, 4H), 7.57 (t, J = 7.7 Hz, 2H), 7.48
of real green chemistry. Moreover, upon comparing with the (t, J = 7.7 Hz, 4H); 13C NMR (CDCl3, 100 MHz, ppm) d: 196.9, 137.6,
previously reported catalyst system, our approach shows significant 132.5, 130.1, 128.3; FT-IR (KBr, cmÀ1): 1659 (nCQO); GC-MS m/z:
1
advantages (Table 4).
182.1 (M+, 100).
Conclusion
Acknowledgements
In conclusion, the oxime-palladacycle catalyzed Suzuki-type M. Mondal is thankful to UGC, New Delhi, for financial
carbonylation reaction via the cleavage of the C–Cl bond of assistance in the form of UGC-BSR (RFSMS) Fellowship.
acid chlorides to yield aryl ketones has been described for the
first time. Overall, this protocol offers a mild and efficient alternative
to the existing methods, as (i) the oxime-palladacycle pre-catalysts
Notes and references
are easily accessible and highly efficient, (ii) the solvent is
biologically obtainable, and (iii) is immiscible in water, and
this property minimizes the dissolution of moisture in the
solvent surface, and avoids the need to maintain inert conditions,
(iv) the crude reaction mixture was isolated just by separation of
2-MeTHF–water layers and then evaporation of 2-MeTHF, without
the use of additional organic solvent, and (v) high atom economy
(64.2%) and E-factor (39.3) due to the relatively equal proportion of
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Typical experimental procedure for Suzuki–Miyaura cross-coupling
of arylboronic acids
A 50 mL branched test tube was charged with a mixture of
K2CO3 (138 mg, 1.0 mmol), complex 1 (0.4 mol% Pd), 2-MeTHF
(4 mL) under open air. The solution was heated to 80 1C on a
Process Station Personal Synthesizer with continuous stirring,
and added with phenylboronic acid 3a (0.55 mmol) and benzoyl
chloride 2a (0.5 mmol). The mixture was stirred at 80 1C for 4 h
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