Palladium-catalyzed thiocarbonylation of iodoarenes with thiols in phosphonium salt ionic liquids

Article abstract of DOI:10.1021/jo800287s

(Chemical Equation Presented) Trihexyl(tetradecyl)phosphonium hexafluorophosphate, a phosphonium salt ionic liquid (PSIL) is a particularly effective general reaction media for the Pd-catalyzed carbonylation reaction of iodoarenes and thiols to form thioesters. Recycling of the ionic liquid containing active Pd-catalyst was also demonstrated.

Full text of DOI:10.1021/jo800287s

Palladium-Catalyzed Thiocarbonylation of Iodoarenes with Thiols in
Phosphonium Salt Ionic Liquids
Hong Cao, Laura McNamee, and Howard Alper*
Centre for Catalysis Research and InnoVation, Department of Chemistry, UniVersity of Ottawa,
10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
howard.alper@uottawa.ca
ReceiVed February 4, 2008
Trihexyl(tetradecyl)phosphonium hexafluorophosphate, a phosphonium salt ionic liquid (PSIL) is a
particularly effective general reaction media for the Pd-catalyzed carbonylation reaction of iodoarenes
and thiols to form thioesters. Recycling of the ionic liquid containing active Pd-catalyst was also
demonstrated.
Introduction
and thiophenols, still represents a challenging subject since the
strong thiophilicity of transition metals3 may make catalytic
reactions ineffective.4 Within the past few years, we and other
groups have developed a series of thiocarbonylation reactions
using transition metal catalysts, especially palladium catalysts.5
These reactions demonstrate the utility of transition metal
catalysts for the reactions of sulfur compounds.
Recently, the combination of task specific ionic liquids
(TSILs) as versatile and novel reaction media, with transition
metal complexes as catalysts, has resulted in many diverse and
flexible “platforms” to establish highly effective and easily
separable catalytic systems.6 Because of their unique properties
such as nonvolatility, nonflammability, excellent chemical and
thermal stability, and recyclability,7 ionic liquids were also
successfully applied to transition-metal-catalyzed carbonylation
reactions.8 While most ionic liquid research is conducted in
Transitional-metal-catalyzed carbonylation chemistry is widely
used in organic synthesis in both academia and industry. Among
numerous methods for the introduction of a carbonyl moiety
into an organic molecule, this direct functionalization of a
substrate using carbon monoxide has attracted a great deal of
attention.1 Although a large number of catalytic systems have
been developed for the carbonylation of a wide range of
compounds, carbonylation chemistry has still not achieved its
full potential. The search for next generation carbonylation
technology continues, with the goal of increasing the diversity
of possible substrates and recycling often expensive catalysts.2
The development of transition-metal-catalyzed carbonylation
reactions employing organo-sulfur compounds, especially thiols
(1) (a) Colquhoun, H. M.; Thompson, D. J.; Twigg, M. V. Carbony-lation:
Direct Synthesis of Carbonyl Compounds; Plenum Press: New York, 1991. (b)
Tsuji, J. Palladium Reagents and Catalysts; Wiley: New York, 1995. (c)
Khumtaveeporn, K.; Alper, H. Acc. Chem. Res. 1995, 28, 414. (d) Ishii, Y.;
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1987, 28, 1987. (i) Zhu, J.; Lu, X. Chem. Commun. 1987, 1318. (j) Marshall,
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M.; Mastsumoto, Y.; Hayashi, T. Tetrahedron Lett. 1987, 28, 4849. (n) Lu, X.;
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3530 J. Org. Chem. 2008, 73, 3530–3534
10.1021/jo800287s CCC: $40.75 2008 American Chemical Society
Published on Web 04/03/2008

Palladium-Catalyzed Thiocarbonylation of Iodoarenes
TABLE 1. Palladium-Catalyzed Thiocarbonylation of Aryl Iodides with Thiols in Various PSILsa
entry
anion (X-)
isolated yield (%)b
1
2
3
4
5
6
7
Cl-
55
84
56
60
70
82
91
Br-
decanoate
bis(2,4,4-trimethylpentyl)phosphinate
dicyanamide
NTf2
PF6
a Reaction conditions: 1a (1.2 mmol), 2a (1 mmol), CO 200 psi, triethylamine (2 mmol), Pd(OAc)2 (0.05 mmol), PPh3 (0.20 mmol), PSIL (1.5 g),
18 h, 100 °C. b Isolated yield based on thiophenol.
nitrogen-based solvents,9 McNulty and other groups recently
investigated the application of phosphonium salt ionic liquids
(PSILs) in general and specifically for palladium-catalyzed
processes.10 Efficient recovery and recycling of the palladium
catalysts was demonstrated. When compared to their ammonium
counterparts, PSILs displayed increased stability toward thermal
and chemical degradation, making them ideal for high temper-
atures or in processes in which products can be removed by
distillation.11 PSILs are also nonvolatile, economical, and
available on an industrial scale. As a new replacement of
traditional organic solvents, PSILs are an area of significant
promise.
Results and Discussion
Initial studies focused on examining the feasibility of the
thiocarbonylation reactions and optimizing reaction conditions
that could be applied to a variety of thiols and aryl halides.
The reaction of iodobenzene 1a and thiophenol 2a with carbon
monoxide was chosen as a model reaction. This reaction was
run on a 1 mmol scale, using 5 mol % of Pd(OAc)2 as catalyst,
20 mol % of PPh3 as ligand, 1.2 equiv of iodobenzene, and 2
equiv of triethylamine in 1.5 g of the PSIL, trihexyl(tetrade-
cyl)phosphonium bromide, at 100 °C, under a pressure of 200
psi carbon monoxide for 18 h and gave 3a in 84% isolated yield.
The same reaction in THF provided 3a in only 29% yield (eq 1).
The palladium-catalyzed carbonylation of aryl halides and
their derivatives with nucleophiles such as alcohols, amines,
and carbon nucleophiles is a powerful method for the synthesis
of many aromatic compounds, especially carboxylic acids and
their derivatives.12 Although this reaction has been well
established, much less attention has been paid to reactions with
organic sulfur compounds.13 To our knowledge, there have been
no reports of transition-metal-catalyzed carbonylation reactions
of aryl halides and thiols. Based on our previous work on
thiocarbonylation and the application of ionic liquids in
transitional-metal-catalyzed carbonylation reactions, we now
report the first examples of palladium-catalyzed thiocarbony-
lation of aryl iodides with thiols in phosphonium ionic liquids.
A variety of PSILs consisting of the trihexyl(tetradecyl)phos-
phonium cation with a range of common anions were screened
for the carbonylation reaction of iodobenzene and thiolphenol
using Pd(OAc)2/PPh3 as the catalyst and triethylamine as the
base (Table 1). The results presented in Table 1 reveal that most
of these PSILs are effective as reaction solvents. The best PSILs
are THP-Br, THP-NTf2, and THP-PF6, which furnished the
product 3a in 84%, 82%, and 91% isolated yield, respectively
(5) (a) Shim, S. C.; Antebi, S.; Alper, H. J. Org. Chem. 1985, 50, 147. (b)
Antebi, S.; Alper, H. Tetrahedron Lett. 1985, 26, 2609. (c) Antebi, S.; Alper,
H. Tetrahedron Lett. 1985, 26, 1935. (d) Ogawa, A.; Takeba, M.; Kawakami,
J.; Ryu, I.; Kambe, N.; Sonoda, N. J. Am. Chem. Soc. 1995, 117, 7564. For the
palladium-catalyzed thiocarbonylation, see: (e) Xiao, W.-J.; Alper, H. J. Org.
Chem. 1997, 62, 3422. (f) Xiao, W.-J.; Vasapollo, G.; Alper, H. J. Org. Chem.
1998, 63, 2609. (g) Xiao, W.-J.; Alper, H. J. Org. Chem. 1998, 63, 7939. (h)
Xiao, W.-J.; Vasapollo, G.; Alper, H. J. Org. Chem. 1999, 64, 2080. (i) Xiao,
W.-J.; Alper, H. J. Org. Chem. 1999, 64, 9646. (j) Xiao, W.-J.; Vasapollo, G.;
Alper, H. J. Org. Chem. 2000, 65, 4138. (k) Xiao, W.-J.; Alper, H. J. Org.
Chem. 2001, 66, 6229. (l) Xiao, W.-J.; Alper, H. J. Org. Chem. 2005, 70, 1802.
(m) Cao, H.; Xiao, W.-J.; Alper, H. AdV. Synth. Catal. 2006, 348, 1807. For the
rhodium- or platinum-catalyzed thiocarbonylation, see: (n) Ogawa, A.; Kawakami,
J; Mihara, M.; Ikeda, T.; Sonoda, N.; Hirao, T. J. Am. Chem. Soc. 1997, 119,
12380. (o) Ogawa, A.; Kuniyasu, H.; Sonoda, N.; Hirao, T. J. Org. Chem. 1997,
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J. Org. Chem. Vol. 73, No. 9, 2008 3531

Cao et al.
TABLE 2. Influence of Palladium Catalysts, Pressure, and Temperature on the Thiocarbonylation of 1a with Thiophenol and COa
entry
catalyst
temp. (°C)
CO (psi)
isolated yield (%)b
1
2
3
4
5
6
7
8
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/dppb
Pd(OAc)2/dppp
Pd2(dba)3 ·CHCl3
Pd(OAc)2
100
60
200
200
100
400
200
200
200
200
91
77
53
96
76
21
14
NR
100
100
100
100
100
100
a Reaction conditions: 1a (1.2 mmol), 2a (1 mmol), catalyst (0.05 mmol), ligand PPh3 0.20 mmol), dppb (0.10 mmol) or dppp (0.10 mmol),
triethylamine (2 mmol), PSIL (1.5 g). b Isolated yield based on thiophenol.
(Table 1, entries 2, 6, 7). While the results for the three ionic
liquids are comparable, THP-PF6 demonstrated itself to be much
easier to deal with during the purification stage. Addition of
hexane to the reaction mixture containing THP-PF6 allowed the
separation of two phases with the palladium remaining in the
ionic liquid layer and the carbonylated product in the organic
layer. Other PSILs needed extra H2O/MeOH to form three layers
for extraction.
Further optimization of the reaction conditions is shown
in Table 2. The nature of the ligands employed plays an
important role in this transformation, with PPh3 being a
superior ligand. In particular, the use of Pd(OAc)2/4PPh3 was
the most effective catalyst system for the formation of 3a in
91% yield (Table 2, entry 1). Although 1,4-bis-(diphe-
nylphosphino)butane (dppb) was also useful as a ligand for
this palladium-catalyzed reaction, the result was inferior to
PPh3 (Table 1, entry 5). Compared to the other bidentate
phosphine dppb, 1,3-bis(diphenylphosphino)propane (dppp)
was almost ineffective (Table 1, entry 3).
reaction (Table 2, entries 1–4). Under the same conditions,
the reaction proceeded more slowly at lower pressure (53%
of 3a at 100 psi, Table 2, entry 3) and lower temperature
(77% of 3a after 18 h at 60 °C, Table 2, entry 2).
The study of the scope of the palladium-catalyzed thio-
carbonylation reaction in the PSIL was extended to a series
of aryl halides, reacting with a range of thiols (Table 2). The
results show that trihexyl(tetradecyl)phosphonium hexafluo-
rophosphate is an excellent ionic liquid for most of the
substrates. Thiophenol affords a higher yield of 3 than
substituted thiophenols, while electron-withdrawing substitu-
tion on the phenol group provided better yields of 3 than
analogues containing an electron-donating group (Table 3,
entries 1–7). 2-Naphthalenethiol also reacts with 1a, and the
yield is a little lower than that using thiophenol (Table 3,
entry 8). Aliphatic thiols also provided 3 in good yields
(Table 3, entries 9–13). Other aryl iodides were also effective
partners in this thiocarbonylation reaction. Electron-with-
drawing groups enhance the reactivity, as expected, and yields
decreased when there was electron-donating substitution on
the phenyl group (Table 3, entries 14–18). 1-Iodonaphthalene
1g and the heteroaromatic iodide 1 h reacted with 1a,
affording the thioester products in 85% and 43% yields,
respectively (Table 3, entries 18–19). We also found that
under these conditions the chloroiodoarene 1e and bromo
iodoarene 1f underwent the thiocarbonylation reaction, giving
3p and 3q in 95% and 88% yields, respectively (Table 3,
entries 16, 17).
Neither Pd(OAc)2 nor Pd2(dba)3 is effective for this
reaction in the absence of a ligand. It was found that the CO
pressure and temperature also affected the thiocarbonylation
(10) (a) McNulty, J.; Capretta, A.; Wilson, J.; Dyck, J.; Adjabeng, G.;
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J. J.; Robertson, A. J. Org. Lett. 2007, 9, 4575.
As a new replacement for volatile organic solvents, PSILs
have another significant attribute, the recyclability—the
recycling of an active IL-soluble catalyst. Catalyst reuse was
also investigated for this thiocarbonylation reaction. After
the reaction, the ionic liquid was partitioned with hexane,
and the ester product was extracted into the hexane phase.
The initial run gave complete conversion, and the thioester
3a was isolated in 91% yield. The IL phase containing active
Pd-catalyst was dried under vacuum and recharged with
starting materials. Complete conversion of the thiol was again
observed after 48 h, and the ester was isolated in 91% yield
indicationg that a viable, recyclable catalyst is present in the
PSIL.
(11) (a) Bradaric, C. J.; Downard, A.; Kennedy, C.; Robertson, A. J.; Zhou,
Y. Green Chem. 2003, 5, 143. (b) Kim, Y. J.; Varma, R. S. J. Org. Chem. 2005,
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UK, 2004; p 265. (b) Skoda-Foldes, R.; Kollar, L. Curr. Org. Chem. 2002, 6,
1097. (c) Mori, M. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley-Interscience: New York, 2002; Vol. 2, p 2663.
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M. Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed.;
Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2002;
Vol. 1, p 145. (f) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpaintner, C. W.
J. Mol. Catal. 1995, 104, 17. (g) Trost, B. M. Science 1991, 25, 4–1471. (h)
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(13) (a) Sawada, N.; Itoh, T.; Yasuda, N. Tetrahedron Lett. 2006, 47 (37),
6595. (b) J. Chem. Res. Synopses 2000, 8, 372. (c) You, J.-Z.; Chen, Z.-C.;
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In conclusion, the PSIL, trihexyl(tetradecyl)phosphonium
hexafluorophosphate, is a particularly effective general reaction
media for thiocarbonylation reactions of iodoarenes with thiols.
3532 J. Org. Chem. Vol. 73, No. 9, 2008

Palladium-Catalyzed Thiocarbonylation of Iodoarenes
TABLE 3. Palladium-Catalyzed Thiocarbonylation of Aryl Iodides with Thiols in PSIL Trihexyl(tetradecyl)phosphonium
Hexafluorophosphatea b c
,
,
a Reaction conditions: iodides (1.2 mmol), thiols (1 mmol), CO 200 psi, triethylamine (2 mmol), Pd(OAc)2 (0.05 mmol), PPh3 (0.20 mmol), PSIL (1.5
g), 100 °C, 18 h. b Based on the iodide employed. c Under 500 psi CO.
100 °C for 18 h. Excess CO was discharged at room temperature.
The resulting solution was extracted with hexane and concen-
trated under reduced pressure. The residue was purified by flash
chromatography on silica gel (gradient from hexane to hexane/
EtOAc 20:1) to give 3 (43–97%). For recovery and reuse of the
ionic liquid containing the palladium catalyst, fresh iodide, thiol,
and triethylamine were added to the remaining ionic liquid for
the next run.
Recyclability of the PSIL and active Pd catalyst was also
demonstrated.
Experimental Section
General Procedure for the Thiocarbonylation Reactions of
Iodoarenes with Thiols. A mixture of the iodoarene (1.2 mmol),
thiol (1.0 mmol), triethylamine (2.0 mmol), Pd(OAc)2 (0.05mmol),
PPh3 (0.2 mmol), and PSIL (1.5 g) was added to the autoclave.
The autoclave was closed, purged three times with carbon
monoxide, pressurized with 200 psi of CO, and then heated at
S-Phenyl Benzothioate.3a.13a 1H NMR (300 MHz, CDCl3): δ
7.53–8.16 (m, 10H). 13C NMR (75 MHz, CDCl3): δ 127.90, 128.00,
129.30, 129.79, 130.05, 134.22, 135.64, 137.12, 190.50.
J. Org. Chem. Vol. 73, No. 9, 2008 3533

Cao et al.
S-Furan-2-ylmethyl Benzothioate. 3l. IR: 1659 cm-1 (CdO).
1H NMR (300 MHz, CDCl3): δ 4.39 (s, 2H), 6.34 (m, 2H),
7.37–8.01 (m, 8H). 13C NMR (75 MHz, CDCl3): δ 26.18, 108.60,
111.12, 127.75, 129.09, 134.00, 137.05, 142.71, 150.88, 191.09.
MS (EI) m/z: 218 (M+, 26). HRMS (EI) m/z: calcd for C12H10O2S
(M+) 218.0402, found 218.0388.
Supporting Information Available: Full experiment details,
characterization for all compounds, and copies of NMR spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We are indebted to Cytec Canada, Inc.
and NSERC (CRD) for financial support of this work.
JO800287S
3534 J. Org. Chem. Vol. 73, No. 9, 2008