Palladium-catalyzed electrochemical carbonylation of alkynes under very mild conditions

Article abstract of DOI:10.1081/SCC-120002698

Terminal alkynes were carbonylated under very mild conditions to yield acetylenecarboxylates under atmospheric pressure of carbon monoxide at room temperature using palladium(II) catalyst in combination with its anodic recycling at a graphite electrodes.

Full text of DOI:10.1081/SCC-120002698

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
PALLADIUM-CATALYZED ELECTROCHEMICAL
CARBONYLATION OF ALKYNES UNDER VERY MILD
CONDITIONS
I. Chiarotto & I. Carelli
To cite this article: I. Chiarotto & I. Carelli (2002) PALLADIUM-CATALYZED ELECTROCHEMICAL
CARBONYLATION OF ALKYNES UNDER VERY MILD CONDITIONS, Synthetic Communications,
32:6, 881-886, DOI: 10.1081/SCC-120002698
To link to this article: http://dx.doi.org/10.1081/SCC-120002698
Published online: 16 Aug 2006.
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Date: 30 December 2015, At: 08:39

SYNTHETIC COMMUNICATIONS, 32(6), 881–886 (2002)
PALLADIUM-CATALYZED
ELECTROCHEMICAL CARBONYLATION
OF ALKYNES UNDER VERY MILD
CONDITIONS
I. Chiarotto* and I. Carelli
Universita degli Studi ‘‘La Sapienza,’’
`
Dipartimento di Ingegneria Chimica dei Materiali delle
Materie Prime e Metallurgia,
via Castro Laurenziano 7, I 00185 Rome, Italy
ABSTRACT
Terminal alkynes were carbonylated under very mild condi-
tions to yield acetylenecarboxylates under atmospheric pres-
sure of carbon monoxide at room temperature using
palladium(II) catalyst in combination with its anodic
recycling at a graphite electrodes.
Palladium(II)-catalyzed carbonylation of terminal alkynes in
methanol gives methyl acetylenecarboxylates. These reactions are carried
out using PdCl as catalyst and stoichiometric amounts of CuCl2 as
2
1
reoxidant. However, from a synthetic and environmental point of view,
it is important to develop a halogen-free and copper-free reoxidant
system. Recently, Sakurai et al. reported a new oxidation system,
*
Corresponding author.
8
81
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

8
82
CHIAROTTO AND CARELLI
Pd(OAc) /chlorohydroquinone/ molybdovanadophosphate (NPMoV), to
2
2
render the reaction catalytic in palladium(II). However, a high pressure
of carbon monoxide and dioxygen are still required. The electrochemical
reoxidation of Pd(0) in the carbonylation of acetylenic compounds has also
been reported, but none of these methods occur without reducing the triple
3
,4
bonds and acetylenecarboxylates are not produces.
In this study, we wish to report a new procedure for the electro-
chemical synthesis of methyl acetylenecarboxylates from alkynes, under
mild conditions, using a palladium complex as catalyst, carbon monoxide
(p CO¼1 atm) and methanol at room temperature. The process is outlined
in the following reaction (Equation 1).
(1)
The reaction proceeds under atmospheric pressure of carbon monox-
ꢀ
ide at 25 C in a CH CN/CH OH solvent in the presence of bases (NaOAc
3
3
or NEt ) using a catalytic amount of a palladium(II) complex regenerated
3
by oxidation of an intermediate Pd(0) complex at the anode. The results for
the electrolyses using various palladium complexes are summarized in
Table 1. Accordingly, phenylacetylene 1a was recovered in 90% yield when
it was reacted in CH CN/CH OH (7: 3) solution containing n-Bu NBF
4
3
3
4
À3
0.2 mol dm ) in the presence of 10 mol% of Pd(OAc) (PPh ) , and two
2 3 2
(
equivalents of base, under one atmosphere of carbon monoxide, without
Table 1. Carbonylation of Phenylacetylene (1a) to Methyl Acetylenecarboxylate
2a) in the Presence of Palladium(II) Complexes
(
Recovered 1a
(%)
Yield 2a
a
(%)
Current
Efficiency (%)
b
Catalyst
Pd(PPh ) (OAc)
2
23
20
48
22
56 (73)
57( 71 )
22 (42)
45 (58)
93
95
85
77
3
2
Pd(PPh
PdCl
Pd(PPh3)4
3
)
2
Cl
2
2
a
Yields are relative to the initial 1a and determinated by GC analyses. Yields in
b
parentheses are relative to the disappearance of 1a. Current efficiency was calculated
in comparison with theoretic coulomb (2F/mol).

CARBONYLATION OF ALKYNES
883
any electrochemical oxidation; methyl phenylacetylenecarboxylate 2a was
obtained in only 8% yield.
The Pd(OAc) (PPh ) /electrode system is an efficient catalyst, as good
2
3 2
as the PdCl (PPh ) /electrode system. It is of interest that PdCl (PPh )
3 2
2
3 2
2
and Pd(OAc) (PPh ) gave higher yields of methyl phenylacetylenecarbox-
2
3 2
ylates than PdCl (Table 1). The PPh ligand stabilizes the intermediate
2
3
Pd(0) complex. Pd(OAc) (PPh ) may generate a Pd(0) complex in a slow
2
3 2
5
reaction; however, this latter will be oxidized at the anode back to
Pd(OAc) (PPh ) . The electrochemical reoxidation of Pd(0) under atmos-
2
3 2
pheric pressure of CO appears to be an interesting alternative to the
PdCl /CuCl or Pd(II)/HQ-Cl/NPMoV procedures working under high
pressure of carbon monoxide and dioxygen.
2
2
As proposed by Heck, for the carbonylation of olefins and acetylenes,
the electrocarbonylation of terminal alkynes is considered to proceed via
6
a similar mechanism (Scheme 1).
Scheme 1.
0
Because of the syn addition of L (OAc)PdCOOR on the alkyne,
2
the formation of HPd(OAc)(PPh ) by a b-hydride syn elimination is impos-
3
2
sible. The presence of the base is essential as a proton trapper by elimination
þ
7
of the H from the b position.
Table 2 shows the results for the carbonylation of various terminal
alkynes by using Pd(OAc) (PPh ) as catalyst and NaOAc or NEt as
3 2
2
3

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84
CHIAROTTO AND CARELLI
Table 2. Carbonylation of Various Terminal Alkynes
a
Acetylenecarboxylate 2
1
to Methyl
bases. It seems that the use of NEt as a base is in favour of a more efficient
3
5
catalytic cycle (compare Entries 1–5 and 2–6, Table 2). When NEt was
3
used as base, the reaction occurs in shorter times suggesting that the Pd(II)
catalyst is more efficiently recycled in the presence of NEt3 than with
NaOAc.
þ
The reaction is considered to proceed by the elimination of H in the b
7
position in complex [A], induced by the base (Scheme 1). NEt is probably
3
À
more efficient than AcO in this reaction. The Pd(0) is then generated in

CARBONYLATION OF ALKYNES
885
higher concentrations from complex [A] in the presence of NEt and can be
3
reoxidized to Pd(II) at the anode. Moreover, when acetate was used as
base, acetic acid is formed from complex [A] concomitantly with the Pd(0)
complex. We have reported that Pd(0) undergoes oxidative addition with
8
acetic acid to form a cationic palladium(II) hydride (Equation 2).
(2)
Under such conditions the Pd(0) complex generated from [A]
þ
(
tion which is in competition with its reoxidation at the anode.
Scheme 1) would be partially stored under the form of HPd(II)L , a reac-
2
We are presently using the electrochemical oxidation of Pd(0) com-
plexes at the anode in other catalytic carbonylation reactions. This method
is promising as an efficient and mild system to reoxidize Pd(0) to Pd(II) in
reactions catalyzed by palladium. We will report those results shortly.
EXPERIMENTAL
General
A potentiostat AMEL model 552 equipped with an integrator was
used in controlled potential electrolysis. GC measurements were carried
out on a Supelco SPB-5 30 m ꢁ 0.25 mm capillary column using a Hewlett
Packard 5890 gaschromatography instrument.
Alkynes 1a–d, palladium complexes and all other reagents are com-
mercially available and were used as received.
Electrolysis of alkynes with palladium complexes. General procedure.
À3
The electrolysis was carried out on solution of alkyne (1 mmol) in 0.03 dm
À3
of CH CN/CH OH (7: 3) containing n-Bu NBF (0.2 mol dm ) as sup-
4
3
3
4
porting electrolyte, in the presence of palladium catalyst, (0.1 mmol) and
NaOAc or NEt (2 mmol) under one atmosphere of CO at þ 0.4 V vs SCE at
3
ꢀ
2
5 C. A graphite electrode, of apparent area 3 cm , was used as the working
2
electrode. The counter-electrode was a Pt wire and the reference was a
saturated calomel electrode (SCE). At the end of the electrolyses, yields
were calculated by GC analysis of crude reaction mixtures by comparison
with authentic samples of standards.

8
86
CHIAROTTO AND CARELLI
ACKNOWLEDGMENTS
This work has been supported by the Consiglio Nazionale Delle
Ricerche (CNR, Italy), the Ministero dell’Universita e della Ricerca
`
Scientifica e Technologica (MURST, Italy). We want to thank Dr. Anny
Jutand for helpful discussions and Mr. Marco Di Pilato for his technical
contribution.
REFERENCES
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2
3
4
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Received in the Netherlands March 23, 2001