Room Temperature Carbonylation of (Hetero) Aryl Pentafluorobenzenesulfonates and Triflates using Palladium-Cobalt Bimetallic Catalyst: Dual Role of Cobalt Carbonyl

Article abstract of DOI:10.1002/adsc.201600736

An efficient method for the carbonylation of (hetero) aryl pentafluorobenzenesulfonates and triflates under exceptionally mild conditions using palladium/dicobalt octacarbonyl [Pd/Co₂(CO)₈] has been developed. Besides acting as carbon monoxide (CO) source, Co₂(CO)₈enhances the reaction rate by accelerating the CO insertion through an in situ generated bimetallic palladium cobalt tetracarbonyl [Pd-Co(CO)₄] complex. Under the optimized reaction condition, carbonylation of a wide range of activated and deactivated, as well as sterically hindered and heteroaromatic, substrates proceeded efficiently at room temperature. The high chemoselectivity and improved synthesis of biologically relevant Isoguvacine and Lazabemide intermediates highlights its scope as a valuable synthetic method. The generality of this protocol was further extended to other electrophiles (bromides, chlorides and tosylates). (Figure presented.).

Full text of DOI:10.1002/adsc.201600736

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DOI: 10.1002/adsc.201600736
Very Important Publication
Room Temperature Carbonylation of (Hetero) Aryl Pentafluoro-
benzenesulfonates and Triflates using Palladium-Cobalt Bimetallic
Catalyst: Dual Role of Cobalt Carbonyl
Jayan T. Joseph,a,b Ayyiliath M. Sajith,b Revanna C. Ningegowda,b
and Sheena Shashikantha,*
a
Department of Studies in Chemistry, Manasagangothri, University of Mysore, Mysore – 570006, India
Fax : (+91)-821-254-7279; e-mail: shashis1956@gmail.com
Syngene International Ltd., Biocon Park, Bommasandra IV Phase, Jigani Link Road, Bangalore – 560099, India
b
Received: July 10, 2016; Revised: November 21, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600736.
Abstract: An efficient method for the carbonylation
of (hetero) aryl pentafluorobenzenesulfonates and
triflates under exceptionally mild conditions using
palladium/dicobalt octacarbonyl [Pd/Co2(CO)8] has
been developed. Besides acting as carbon monoxide
(CO) source, Co2(CO)8 enhances the reaction rate
by accelerating the CO insertion through an in situ
generated bimetallic palladium cobalt tetracarbonyl
[Pd-Co(CO)4] complex. Under the optimized reac-
tion condition, carbonylation of a wide range of ac-
tivated and deactivated, as well as sterically hin-
dered and heteroaromatic, substrates proceeded ef-
ficiently at room temperature. The high chemose-
lectivity and improved synthesis of biologically rele-
vant Isoguvacine and Lazabemide intermediates
highlights its scope as a valuable synthetic method.
The generality of this protocol was further extended
to other electrophiles (bromides, chlorides and tosy-
lates).
sponding aryl halides in view of the fact that the hy-
droxy functional group can be used as a directing
group to access a wide range of synthetic intermedia-
tes.[3b] Although carbon monoxide, CO, still remains
as effective carbonyl source for many industrial pro-
cesses, several attempts to use other CO surrogates
which generate CO gas in situ or ex situ have been ex-
plored in various laboratories.[4] Such CO releasing
molecules (CORMs) include formates,[5] forma-
mides,[6] formic acid,[7] silacarboxylic acids,[8] acid
chlorides,[9] metal carbonyls,[10] and others.[11]
As a part of our research towards developing effi-
cient palladium-catalyzed synthetic protocols,[12] we
were interested in the use of easily handled (hetero)
aryl pentafluorobenzenesulfonates as electrophiles for
carbonylation under mild conditions. Combining the
facts that both oxidative addition of Pd to Ar-OPFBs/
triflates[12] and generation of CO gas from Co2(CO)8
by homomolecular disproportionation [Eqs. (1) and
(2)] occur at room temperature,[13] prompted us to ex-
plore a possible carbonylation using metal carbonyls
at room temperature. We envisioned that these
milder conditions would obviate the possible catalyst
poisoning by high CO pressure and temperature[14]
thereby increasing the rate of the reaction and turn-
over numbers (TON).
Keywords: bimetallic catalysis; carbonylation; che-
moselectivity; cobalt; palladium
Metal-catalyzed carbonylation chemistry has evolved
as an indispensable synthetic tool both in academic
and industrial processes.[1] Successful carbonylations
employing aryl/heteroaryl bromides, iodides and tri-
flates under moderate conditions are well-document-
ed in the literature.[2] Notable advances have been
made in achieving carbonylation of challenging sub-
Our reaction optimization study was focused on the
strates such as chlorides, tosylates and mesylates, al- alkoxycarbonylation of deactivated 4-methoxyphenyl
though high temperature or CO pressure with de- OPFBs with ethanol to give the ester (Table 1), using
manding catalytic systems are required.[3] The use of Co2(CO)8 as the carbonyl source. Initial screening of
pseudohalides can be advantageous over the corre- reaction with Pd(OAc)2/XantPhos as the catalyst
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Table 1. Optimization of the reaction conditions.[a]
Entry
Catalyst (mol%)
Ligand (mol%)
Base
Solvent
CO Source
Temp. [oC]
Yield [%]
1
2
3
4
5
6
7
8
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
XantPhos (5)
XantPhos (5)
XantPhos (5)
dppf (5)
dppf (5)
–
–
–
BINAP (5)
dppe (5)
dppp (5)
dppb (5)
DPEPhos (5)
–
PPh3 (5)
XPhos (5)
P(Cy)3 (5)
P(t-Bu)3 (5)
–
–
–
–
–
–
DMAP
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
DBU
THF/EtOH
THF/EtOH
EtOH
EtOH
ACN/EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Co2(CO)8
Mo(CO)6
Cr(CO)6
50
50
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
50
50
50
50
80
80
80
50
50
r.t.
r.t.
r.t.
r.t.
80
80
80
80
50
50
76
88
94
93
78
96
92
87
83
81
79
84
78
46
27
53
57
61
56
62
92
77
84
68
61
14
33
18
Pd(OAc)2 (3)
PdCl2dppf·CH2Cl2 (3)
PdCl2dppf·CH2Cl2 (1)
PdCl2dppf·CH2Cl2 (5)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(PPh3)2Cl2 (5)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Pd(OAc)2 (3)
Pd(OAc)2 (3)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
PdCl2dppf·CH2Cl2 (5)
Pd(OAc)2 (3)
Et3N
NaOAc
K2CO3
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
–
–
–
W(CO)6
Fe(CO)5
Co2(CO)8
CO (1 atm)
Co2(CO)8 (5)
dppf (5)
[a]
Reaction conditions: Ar-OPFBs (0.5 mmol), catalyst, base, solvent (0.50M).
system, DMAP as the base and THF as the solvent at But, the use of Co2(CO)8, with a Pd catalyst or
508C, resulted in 76% product yield (Table 1, a ligand alone gave poor yields (Table 1, entries 27
entry 1). A 12% improvement in the yield was ob- and 28). The standard conditions (Table 1, entry 6)
served (Table 1, entry 2), when KOAc was used as the were then further investigated with other metal car-
base. Furthermore, use of ethanol as solvent, proved bonyls as potential CO sources (Table 1, entries 23–
very effective giving a 94% yield at room temperature 26). All of them remained unreactive at room temper-
(Table 1, entry 3). The optimum amount of Co2(CO)8 ature which was evident from the pressure readings of
was found to be 0.3 to 0.4 equiv. Among the bases the tinyclave used for the reactions. Mo(CO)6 gave
screened [DMAP, KOAc, DBU, Et3N, NaOAc and a good yield at 808C, while Cr(CO)6 and W(CO)6 re-
K2CO3], inorganic bases such as KOAc and NaOAc sulted in moderate yields at 1008C. However,
provided the best results (Table 1, entries 1, 3 and 19– Fe(CO)5 was inactive even at this temperature with
22).
Next, a set of bidentate ligands together with
a poor carbonylation yield.
Having established low temperature carbonylation
Pd(OAc)2 and KOAc was examined, of which dppf conditions, we set out to explore the scope of this pro-
and XantPhos (Table 1, entries 3, 4 and 9–13) gave tocol as shown in Scheme 1. Our initial screening of
good yields. Surprisingly, the commercially available nucleophiles revealed that this reaction tolerates the
complex, the PdCl2dppf·CH2Cl2 adduct, gave the best synthesis of acids as well as esters including secondary
result with 96% conversion (Table 1, entry 6). Even alkyl, benzyl and phenyl derivatives (Scheme 1, en-
monodentate ligands were efficient at higher temper- tries 1–7 and 28). Also, the conditions developed were
atures with moderate yields (Table 1, entries 14–18). shown to be general for a variety of electronically di-
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Scheme 1. Alkoxy- and hydroxycarbonylation of Ar-OPFBs/OTf; isolated yields and average of two runs. Ar-OPFBs/OTf
(1.0 mmol), 0.50M.
verse (hetero) aryl OPFBs and triflates. In the case of
From the industrial perspective, the carbonylation
activated systems, the use of triflates provided the de- of heteroaromatic systems is of potential importance,
sired product in lower yield as compared to analogous since the resulting products are valuable building
Ar-OPFBs, probably due to the competing detrifla- blocks for the synthesis of agrochemicals, herbicides
tion (Scheme 1, entries 11–20). A diverse range of and many APIs.[1] Heteroaromatic systems (Scheme 1,
functional groups (halide, ether, ester, amide, alde- entries 32–41 and 46) also yielded the corresponding
hyde, nitro, cyano and methanesulfonyl) on the elec- esters and acids in excellent yields. However, the 2-
trophile was well tolerated. Notably, a set of ortho- pyridyl substrate gave a modest yield (77%) due to
substituted substrates underwent smooth carbonyla- the competing hydrolysis to form the undesired
tion at room temperature (Scheme 1, entries (19–22 phenol (Scheme 1, entry 32 and 33). Improved synthe-
and 27–30). However, 2,6-dimethylphenyl OPFBs and sis of boc Isoguvacine, a GABA receptor agonist in-
triflate, which remained recalcitrant at room tempera- termediate under mild condition (at low CO pressure
ture (Scheme 1, entries 23 and 24), required thermal and temperature) in gram scale with excellent yield
activation (508C) or a more demanding catalytic highlights the synthetic utility of this carbonylation
system [Pd(OAc)2/JosiPhos].
protocol (Scheme 1, entry 42).[3a] Chemoselective syn-
thesis of ester-containing (hetero) aromatic acids con-
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Scheme 2. Chemoselective carbonylation of Ar-OPFBs/OTf.
Isolated yields and average of two runs. Ar-OPFBs/OTf
(1.0 mmol), 0.50M.
tinues to be a key challenge in advanced organic syn-
thesis. With our improved protocol, hydroxycarbony-
lation of ester derivatives was effortlessly achieved
with very good yields (Scheme 1, entries 43–46).
Chemoselective monofunctionalization can be
a powerful tool in the area of organic synthesis.[15]
Next we tried to evaluate the difference in reactivity
of halides and other pseudohalides with respect to
Ar-OPFBs/triflates toward carbonylation (Scheme 2).
Scheme 3. Aminocarbonylation of Ar-OPFBs/OTf. Isolated
yields and average of two runs. Ar-OPFBs/OTf (1.0 mmol),
0.50M.
We have established with our studies that, using examine the synergistic effect of Co2(CO)8 in the re-
a PdCl2·dppf catalytic system, it is possible to carbon- action pathway. The conversion of 4-methoxyphenyl
ylate a difunctionalized arene chemoselectively. The OPFBs into ester 3 was examined at different temper-
use of excess Co2(CO)8 in these reactions did not atures ranging from 10 to 808C, with five different
yield the dicarbonylated products at room tempera- conditions of CO loading, keeping other parameters
ture, even for highly activated systems (Scheme 2, en- intact (Figure 1).
tries 47–53).
When Co2(CO)8 was used either as a stoichiometric
We next examined the aminocarbonylation of reagent (A) or as an additive (B), it resulted in excel-
Ar-OPFBs/triflates under the standard conditions lent carbonylation at room temperature. Surprisingly,
(Scheme 3). While triflates produced various amides an alcoholic solution of KCo(CO)4, when added to
in excellent yields, Ar-OPFBs gave only moderate the reaction flask under a CO atmosphere(C), also re-
yields due to the undesirable SNAr displacement of sulted in excellent conversion. On the contrary, the
fluorine on the pentafluorobenzene ring, mainly with use of CO gas in the absence of Co2(CO)8 and
more nucleophilic alkylamines (Scheme 3, entries 54– KCo(CO)4 required higher temperatures (D). Even
61).
the rapid CO gas generation by the reaction of
The enhanced reaction rates and facile carbonyla- Co2(CO)8 using a CO gen apparatus (E) could not fa-
tion of even sterically demanding substrates led us to cilitate the carbonylation under milder conditions.
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Figure 1. Comparison of reaction yields with different CO
sources at different temperatures. GC yields, Ar-OPFBs
(1.0 mmol), 0.50M.
Scheme 4. Mechanistic proposal
tures of Ph(OPFBs)Pd(dppf) and KCo(CO)4 in THF
at room temperature, and formation of bimetallic in-
These experimental observations clearly indicate the termediate (III) was confirmed by NMR, IR and
cooperative effect of Co, possibly interfering in the mass spectroscopy. Also, stirring of III in CDCl3
catalytic cycle either as an activator of Pd(II) to Pd(0) under a CO atmosphere confirmed the formation of
or as a catalyst species itself.
acylpalladium complex (V). However, our attempts to
The series of studies on hydroformylation by isolate this intermediate for further characterization
Komiya et al.[16a] and Hidai et al.[16b] revealed a signifi- were not successful due to the partial reductive elimi-
cant rate enhancement of CO insertion with the bi- nation.
metallic (dppe)MePd-Co(CO)4 complex, wherein the
From our experimental results and observations, we
bent, semibridging carbonyl on Co facilitates the are forced to believe that the initial oxidative addition
rapid CO insertion. They also confirmed with isotopic would be the rate-limiting step as the low energy
studies that CO in Co(CO)4 inserts into the Ar–Pd pathway involving the Pd–Co binuclear intermediate
bond. With the similar results observed in our present (IV) facilitates the rapid CO insertion. To illustrate
study, we propose a plausible mechanism involving this, we sought to perform carbonylation reactions
the (dppf)ArPd·Co(CO)4 bimetallic catalytic species using demanding ligands like JosiPhos which facilitate
for the carbonylation reaction (Scheme 4).
the oxidative addition for challenging substrates
After the initial catalyst activation to form the true under milder conditions (Scheme 5). Successful CO
catalytic species (I), oxidative addition of the ligated gas-free carbonylation was achieved for substrates
Pd(0) species into the carbon-pseudohalide bond such as 2,6-dimethylbromobenzene (Scheme 5, 23), 4-
occurs to give the cationic complex (II). This would bromoaniline (62), 3-chloropyridine (36), 4-chloroani-
be followed by the nucleophilic displacement of pseu- sole (64) and 4-methoxyphenyl tosylate (3) at lower
dohalide by [Co(CO)4]À to form the bimetallic cata- reaction temperatures than the existing protocols.[1e,f]
lytic species (III). The facile migration of the bent, Similarly, synthesis of Boc lazabemide intermediate
semibridging CO ligand from Co to Pd and its subse- (63),
a
reversible, selective MAOB inhibitor[17]
quent insertion into the Ar–Pd bond results in the for- through a room temperature aminocarbonylation of
mation of acylpalladium complex (V). The CO gas 2,5-dichloropyridine and an aldehyde (65) through
generated in the reaction medium by the decomposi- a reductive carbonylation was accomplished, high-
tion of excess Co2(CO)8 enters the catalytic cycle to lighting the scope of this method, both in academic
regenerate the Pd–Co(CO)4 intermediate. The acyl and industrial applications.
complex (V) then reacts with a nucleophile such as al-
cohol, water or amine followed by reductive elimina- cient carbonylation of triflates and pentafluorobenz-
tion to yield the ester, acid or amide products. enesulfonates at room temperature has been devel-
In conclusion, a generalized protocol for the effi-
AHCTUNGTRENNUNG
To provide support for the potential intermediacy oped using Co2(CO)8 as the stoichiometric CO surro-
of III, a reaction was performed with equimolar mix- gate. The wide substrate scope, high chemoselectivity,
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Experimental Section
General Procedure for the Carbonylation Reaction
To a Schlenk tube equipped with a magnetic stirring bar and
a Teflon septum was charged KOAc (3.0 mmol, 3 equiv.),
Ar-OPFBs/OTf (1.0 mmol, 1.0 equiv.) and PdCl2dppf
(0.03 mmol, 3 mol%). Under an inert atmosphere, alcohol
(2 mL) was added followed by Co2(CO)8 (0.3 mmol,
0.3 equiv.). The tube was immediately closed with a Teflon
screw cap and was stirred at room temperature for the time
indicated. The reaction mixture was diluted with EtOAc
(5 mL) and filtered through a celite bed. The organic layer
was concentrated under reduced pressure. The residue was
purified through silica gel column chromatography using
ethyl acetate in petroleum ether to afford the product.
Acknowledgements
The authors are thankful to Dr. Sathya Shanker, Dr. Sridevi
Bashyam and Dr. Rajesh Sankaranarayanan, Syngene Intl.
ltd. for their valuable suggestions and support.
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8 Room Temperature Carbonylation of (Hetero) Aryl
Pentafluorobenzenesulfonates and Triflates using Palladium-
Cobalt Bimetallic Catalyst: Dual Role of Cobalt Carbonyl
Adv. Synth. Catal. 2017, 359, 1 – 8
Jayan T. Joseph, Ayyiliath M. Sajith,
Revanna C. Ningegowda, Sheena Shashikanth*
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