Ligand-free copper-catalyzed C-S coupling of aryl iodides and thiols

Article abstract of DOI:10.1021/jo800491k

(Chemical Equation Presented) A protocol for the copper-catalyzed aryl-sulfur bond formation between aryl iodides and thiophenols is reported. The reaction is catalyzed by a low amount (1-2.5 mol %) of readily available and ligand-free copper iodide salt. A variety of diaryl thioethers are synthesized under relatively mild reaction conditions with good chemoselectivity and functional group tolerance.

Full text of DOI:10.1021/jo800491k

bond formation is a much less studied transformation than the
corresponding C-N and C-O bond formations. The synthetic
reaction involving sulfur-containing compounds poses special
requirements because the sulfur functionality is known to be
reactive and may act as a poison for metal-based catalysts
because of its strong coordinative properties, often making the
catalytic reaction ineffective.1b In the last decades, transition-
metal-catalyzed organosulfur chemistry received particular
interest, which brought about important progress in the field.
One of the first reports involving the coupling between aryl
halides and thiols refers to Migita’s system, using Pd(PPh3)4 as
catalyst.5 Other efficient palladium catalysts are based on
bidentate phosphines or diverse organophosphane derivatives.6
Nevertheless, these systems still suffer from some limitations
because of the need to prepare and use environmentally
unfriendly PR3 ligands. Recently, the application of other metals
in the catalytic carbon-sulfur bond formation resulted in
synthetic protocols based on nickel7 and cobalt,8 but these were
fraught with common problems such as metal toxicity, low
turnover numbers, and reagents needed in excess. Consequently,
there still is interest in further development of the classical
Ullmann’s coupling reaction,1 applying cheap metals (e.g.,
copper) for the preparation of the diaryl thioether functionality.
The major drawbacks related to this transformation are the use
of large amounts of metal catalysts, their short lifetime (and
hence low turnover numbers), harsh reaction conditions, and
often the narrow scope. Therefore, different approaches have
been studied in order to develop a general and more efficient
system for the preparation of diaryl thioethers. Examples of
attractive copper-catalyzed processes have recently been reported
by Palomo,9 Buchwald,10 Venkataraman,11 and others12 and
more recently also by Dom´ınquez13 and Verma,14 mainly using
copper halide salts as the metal source together with a suitable
ligand. The general approach for the C-S bond coupling
Ligand-Free Copper-Catalyzed C-S Coupling of
Aryl Iodides and Thiols
Elena Sperotto,† Gerard P. M. van Klink,‡,§ Johannes G. de
Vries,‡ and Gerard van Koten*,†
Chemical Biology and Organic Chemistry, Debye Institute
for Nanomaterials Science, Faculty of Science, Utrecht
UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands,
and DSM Pharmaceutical Products, AdVanced Synthesis,
Catalysis and DeVelopment, P.O. Box 18,
6160 MD Geleen, The Netherlands
g.Vankoten@uu.nl
ReceiVed March 17, 2008
A protocol for the copper-catalyzed aryl-sulfur bond forma-
tion between aryl iodides and thiophenols is reported. The
reaction is catalyzed by a low amount (1-2.5 mol %) of
readily available and ligand-free copper iodide salt. A variety
of diaryl thioethers are synthesized under relatively mild
reaction conditions with good chemoselectivity and func-
tional group tolerance.
The formation of aryl-sulfur bonds represents a key step in
the synthesis of many molecules that are of biological,
pharmaceutical, and materials interest.1 For example, a large
variety of aryl sulfides are in use for diverse clinical applications
such as the treatment of Alzheimer’s and Parkinson’s diseases,2
treatment of cancer,3 and treatment of human immunodeficiency
virus4 diseases. However, transition-metal-mediated C(aryl)-S
Muesing, M. A.; Patick, A. K.; Reich, S. H.; Su, K. S.; Tatlock, J. H. J. Med.
Chem. 1997, 40, 3979–3985.
(5) Kosugi, M.; Ogata, T.; Terada, M.; Sano, H.; Migita, T. Bull. Chem.
Soc. Jpn. 1985, 58, 3657–3658.
(6) For selected examples, see: (a) Murata, M.; Buchwald, S. L. Tetrahedron
2004, 60, 7397–7403. (b) Ferna´ndez-Rodr´ıguez, M. A.; Shen, Q.; Hartwig, J. F.
Chem.—Eur. J. 2006, 12, 7782–7796, and references therein. (c) Ferna´ndez-
Rodr´ıguez, M. A.; Shen, Q.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 2180–
2181. (d) Itoh, T.; Mase, T. Org. Lett. 2004, 6, 4587–4590.
(7) (a) Zhang, Y.; Ngeow, K. N.; Ying, J. Y. Org. Lett. 2007, 9, 3495–3499.
(b) Saxena, A.; Kumar, A.; Mozumdar, S. Appl. Catal. A 2007, 317, 210–215.
(c) Jammi, S.; Barua, P.; Rout, L.; Saha, P.; Punniyamurthy, T. Tetrahedron
Lett. 2008, 49, 1484–1487.
† Utrecht University.
‡ DSM Pharmaceutical Products.
§ Present address: DelftChemTech, Faculty of Applied Sciences, Delft
University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
(1) For reviews on C-S coupling, see: (a) Procter, D. J. J. Chem. Soc., Perkin
Trans. 1 2001, 335–354. (b) Kondo, T.; Mitsudo, T.-A. Chem. ReV. 2000, 100,
3205–3220, references therein. (c) Ley, S. V.; Thomas, A. W. Angew. Chem.,
Int. Ed. 2003, 43, 5400–5449. (d) Lindley, J. Tetrahedron 1984, 40, 1433–1456.
For a recent example on ligand-free copper catalysis, see: (e) Correa, A.; Bolm,
C. AdV. Synth. Catal. 2007, 349–2673.
(8) Wong, Y.-C.; Jayanth, T. T.; Cheng, C.-H. Org. Lett. 2006, 8, 5613–
5616.
(9) Palomo, C.; Oiarbide, M.; Lo´pez, R.; Go´mez-Bengoa, E. Tetrahedron
Lett. 2000, 41, 1283–1286.
(10) Yee Kwong, F.; Buchwald, S. L. Org. Lett. 2002, 4, 3517–3520.
(11) Bates, C. G.; Gujadhur, R. K.; Venkataraman, D. Org. Lett. 2002, 4,
2803–2806.
(2) (a) Wang, Y.; Chackalamannil, S.; Hu, Z.; Clader, J. W.; Greenlee, W.;
Billard, W.; Binch, H.; Crosby, G.; Ruperto, V.; Duffy, R. A.; McQuade, R.;
Lachowicz, J. E. Bioorg. Med. Chem. Lett. 2000, 10, 2247–2250. (b) Nielsen,
S. F.; Nielsen, E. O.; Olsen, G. M.; Liljefors, T.; Peters, D. J. Med. Chem. 2000,
43, 2217–2226.
(12) (a) Ranu, B. C.; Saha, A.; Jana, R. AdV. Synth. Catal. 2007, 349, 2690–
2696. (b) Zhu, D.; Xu, L.; Wu, F.; Wan, B. Tetrahedron Lett. 2006, 47, 5781–
5784. (c) Rout, L.; Sen, T. K.; Punniyamurty, T. Angew. Chem., Int. Ed. 2007,
46, 5583–5586. (d) Savarin, C.; Srogl, J.; Liebeskind, L. S. Org. Lett. 2002, 4,
4309–4312. (e) Rout, L.; Saha, P.; Jammi, S.; Punniyamurthy, T. Eur. J. Org.
Chem. 2008, 4, 640–643. (f) Lv, X.; Bao, W. J. Org. Chem. 2007, 72, 3863–
3867.
(3) De Martino, G.; Edler, M. C.; La Regina, G.; Cosuccia, A.; Barbera,
M. C.; Barrow, D.; Nicholson, R. I.; Chiosis, G.; Brancale, A.; Hamel, E.; Artico,
M.; Silvestri, R. J. Med. Chem. 2006, 49, 947–954.
(13) Carril, M.; SanMartin, R; Dom´ınguez, E.; Tellitu, I. Chem.—Eur. J.
2007, 13, 5100–5105.
(4) Kadlor, S. W.; Kalish, V. J.; Davies, J. F.; Shetty, B. V.; Fritz, J. E.;
Appelt, K.; Burgess, J. A.; Campanale, K. M.; Chirgadze, N. Y.; Clawson, D. K.;
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7199–7202.
10.1021/jo800491k CCC: $40.75
Published on Web 06/21/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5625–5628 5625

TABLE 1. Screening of Solvents for the Copper-Catalyzed C-S
Coupling (reaction conditions: thiophenol (6.5 mmol), iodobenzene
(5 mmol), K2CO3 (5.5 mmol), CuI (2.5 mol %), solvent (1 mL), 100
°C, 16 h)
SCHEME 1. Model Reaction for CuI-Catalyzed C-S
Coupling
entry
solvent
yield (%)a
1
2
3
4
5
6
7
8
toluene
DMSO
DMF
DMA
NMP
H2O
11
75
99
99
procedures is the employment of a catalytic amount of copper,
usually between 5 and 10 mol %, always in the presence of a
ligand (10-20 mol %) and a base (1.5-2.5 equiv), under
relatively mild conditions (80-110 °C for 18-24 h).
Prompted by our current interest in carbon-carbon15–17 and
carbon-heteroatom coupling reactions,18 we decided to inves-
tigate the copper-catalyzed carbon-sulfur bond formation via
the utilization of commercially available copper(I) halide salts.
In this paper, we report a simple, inexpensive, and fast
catalytic system for the synthesis of substituted diaryl thioethers
under mild, ligand-free reaction conditions.
The influence of copper(I) iodide as catalyst on C-S bond
formation was first investigated for the cross-coupling reaction
of iodobenzene with thiophenol (Scheme 1). CuI was the chosen
halide salt because of its stability to air. Catalytic tests performed
with CuCl and CuBr, freshly prepared before use, gave the same
results as obtained with commercially available CuI salt. Cu(II)
salts tested (CuBr2, CuSO4·5H2O, CuCl2) were found to be less
efficient as catalysts (yields 25-35%) than the Cu(I) salts. A
low amount of 2.5 mol % of copper iodide salt was employed
in these initial reactions.
99
60c
10b
98c
a Determined by GC using 100 µL of dihexyl ether as external standard.
b K2CO3 as base. c NEt3 (5.5 mmol) as base.
TABLE 2. Screening of Bases for the Copper Iodide-Catalyzed
C-S Coupling (reaction conditions: thiophenol (6.5 mmol),
iodobenzene (5 mmol), base (5.5 mmol), CuI (2.5 mol %), NMP (1
mL, 10.4 mmol), 100 °C, 16 h)
entry
base
yield (%)a
1
2
3
4
5
6
7
K2CO3
NEt3
DIPEA
pyridine
2,6-lutidine
Cs2CO3
99
98
97
37
64
85
5
a Determined by GC using 100 µL of dihexyl ether as external stan-
dard.
Our first goal was to optimize reaction conditions and to
achieve information about the role of additives and solvent
polarity.
Since in previous studies of copper-catalyzed C-N and C-O
coupling reactions18 N-methylpyrrolidinone (NMP) and potas-
sium carbonate were found to be a preferred solvent and base,
respectively, we used these conditions first.
It appeared that, by applying NMP and potassium carbonate,
diphenyl thioether was obtained in quantitative yield using
copper iodide as such, that is, without the use of added ligands.
The need to use a polar solvent is highlighted by the screening
results presented in Table 1, which show that, unlike toluene,
DMF and DMA are excellent solvents for this coupling reaction
(entries 3-5). Besides polarity, a weak coordination of these
solvents to copper could also be a reason for their excellent
performance.
Remarkably, apart from results in water, these findings also
pointed to the possibility of performing the coupling reaction
under neat conditions, that is, without using a solvent (Table 1,
entry 8) but working with a homogeneous system.
Similarly, a series of bases were screened. Among inorganic
bases, potassium carbonate gave quantitative results for the
coupling reaction to diphenyl thioethers (Table 2), while, among
organic bases, triethyl amine and Hunig’s base (DIPEA) gave
excellent results (entries 2 and 3) in a homogeneous reaction
mixture. Notably, the required amount for the base is only 1.1
equiv (based on the aryl halide), while in common reports this
usually is between 1.5 and 2.5 equiv.9–14
Both Tables 1 and 2 refer to overnight reactions and a reaction
temperature of 100 °C. Further experiments were performed to
find the optimal reaction temperature and reaction time. An
excellent yield of 95% was obtained after 6 h of reaction (see
Supporting Information, Chart 1). It was noted that a small
decrease in temperature of only 15 °C already caused a sig-
nificant decrease in diaryl thioether yield to 75% (Supporting
Information, Chart 2). Accordingly, at 100 °C, the reaction of
iodobenzene with thiophenol is relatively fast (65 and 95% after
2 and 6 h, respectively).
Subsequently, different phenyl halides were tested. It appeared
that under the above developed conditions fluoro-, chloro-, and
bromobenzene hardly showed any reactivity, giving only traces
(4-7%) of the C-S coupling product (see Table 3), whereas
reactions of the corresponding iodo derivative gave rise to almost
quantitative formation of diphenyl thioether.
It is noteworthy that an interesting yield of 60% was obtained
by using water as solvent and NEt3 as base (Table 1, entry 6).
(15) (a) Persson, E. S. M.; van Klaveren, M.; Grove, D. M.; Ba¨ckvall, J.-E.;
van Koten, G. Chem.—Eur. J. 1995, 1, 351–359. (b) van Klaveren, M.; Persson,
E. S. M.; del Villar, A.; Grove, D. M.; Ba¨ckvall, J.-E.; van Koten, G. Tetrahedron
Lett. 1995, 36, 3059–3062. (c) Meuzelaar, G. J.; Karlstro¨m, A. S. E.; van
Klaveren, M.; Persson, E. S. M.; del Villar, A.; van Koten, G.; Ba¨ckvall, J.-E.
Tetrahedron 2000, 56, 2895–2903. (d) van Klaveren, M.; Persson, E. S. M.;
Grove, D. M.; Ba¨ckvall, J.-E.; van Koten, G. Tetrahedron Lett. 1994, 35, 5931–
5934.
(16) (a) Arink, A. M.; Braam, T. W.; Keeris, R.; Jastrzebski, J. T. B. H.;
Benhaim, C.; Rosset, S.; Alexakis, A.; van Koten, G. Org. Lett. 2004, 6, 1959–
1962. (b) van Klaveren, M.; Lambert, F.; Eijkelkamp, D. J. F. M.; Grove, D. M.;
van Koten, G. Tetrahedron Lett. 1994, 35, 6135–6138. (c) Lambert, F.; Knotter,
D. M.; Janssen, M. D.; van Klaveren, M.; Boersma, J.; van Koten, G.
Tetrahedron: Asymmetry 1991, 2, 1097. (d) Knotter, D. M.; Grove, D. M.;
Smeets, W. J. J.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1992, 114,
3400.
Finally, the influence of the amount of CuI catalyst was
evaluated. As shown in Table 4, lowering the amount of CuI to
1.5 mol % did not affect the efficiency of the coupling reaction,
while only a slight decrease in yield from 99 to 96% was noticed
when the amount of CuI was further lowered to 1 mol %.
(17) Haubrich, A.; van Klaveren, M.; van Koten, G.; Handke, G.; Krause,
N. J. Org. Chem. 1993, 58, 5849–5852.
(18) (a) Jerphagnon, T.; van Klink, G. P. M.; de Vries, J. G.; van Koten, G.
Org. Lett. 2005, 7, 5241–5244. (b) Sperotto, E.; van Klink, G. P. M.; de Vries,
J. G.; van Koten, G. Tetrahedron Lett. 2007, 48, 7366–7370.
5626 J. Org. Chem. Vol. 73, No. 14, 2008

TABLE 3. Aryl Halide Screening for C-S Coupling (reaction
conditions: thiophenol (6.5 mmol), phenyl halide (5 mmol), K2CO3
(5.5 mmol), CuI (2.5 mol %), NMP (1 mL, 10.4 mmol), 100 °C,
16 h)
TABLE 5. Reaction of Aryl Iodides with Thiophenol (reaction
conditions: thiophenol (6.5 mmol), aryl iodide (5 mmol), K2CO3 (5.5
mmol), CuI (2.5 mol%), NMP (1 mL, 10.4 mmol), 100 °C, 16 h)
entry
phenyl halide
yield (%)a
1
2
3
4
fluorobenzene
chlorobenzene
bromobenzene
iodobenzene
7
4
5
99
a Determined by GC using 100 µL of dihexyl ether as external stan-
dard.
TABLE 4. Copper Iodide Amount Screening for the C-S
Coupling (Scheme 1) (reaction conditions: thiophenol (6.5 mmol),
iodobenzene (5 mmol), K2CO3 (5.5 mmol), CuI, NMP (1 mL, 10.4
mmol), 100 °C, 16 h)
entry
Cu (mol %)
yield (%)a
1
2
3
4
5
6
0
0.5
1
1.5
2
2.5
2
76
96
99
99
99
a Determined by GC using 100 µL of dihexyl ether as external stan-
dard.
Remarkable turnover numbers have been obtained (67-100)
using 1-1.5 mol % of CuI only, but the turnover frequency
was still low (6-13 h-1, catalyst load 2.5 mol %).
As a summary of the above-discussed results, the optimized
system presented here involves the reaction between iodoben-
zene and thiophenol in a polar solvent (NMP, DMF, DMA) or
under neat conditions at 100 °C, in the presence of 1.1 equiv
of K2CO3 (or NEt3) and only 1.5-2.5 mol % of CuI.
a Determined by GC using 100 µL dihexyl ether as external standard.
b Using 2.6 equiv of thiophenol and 2.2 equiv of base.
To determine the scope of the catalytic system, the present
protocol was applied to reactions of a range of commercially
available aryl iodides and thiophenols (Tables 5 and 6). As
shown in Table 5, the coupling of thiophenol with different aryl
iodide moieties was successful, leading to the desired products
in good yields. The protocol is tolerant to electron-withdrawing
and -donating functional groups and also to the presence of a
functional group in the ortho-position of the aryl iodide (entries
5 and 6). Moreover, the reactions show interesting chemose-
lectivity; see entry 6 for preferred C-S coupling in the presence
of an -NH2 grouping. Since reaction conditions for the already
optimized C-N and C-O bond formations are similar18 and
these couplings present good chemoselectivity toward bromo
derivatives, 1-bromo-4-iodobenzene is a suitable substrate for
further development of one-pot N- and S- or O- and S-arylations.
Full selectivity toward the iodo derivative was found (entry
7), leading to 4-bromophenyl phenyl thioether as the only
product through a clean and selective reaction. Results presented
here also include an example of a double C-S bond formation
reaction in good yield, starting from 1,3-diiodobenzene (entry
8). This coupling reaction could be an interesting option for
applications in the field of material science and supramolecular
chemistry for the preparation of poly(phenylene sulfide) oligo-
mers and polymers.19
TABLE 6. Reaction of Iodobenzene with Various Thiols (reaction
conditions: thiol (6.5 mmol), iodobenzene (5 mmol), K2CO3 (5.5
mmol), CuI (2.5 mol %), NMP (1 mL, 10.4 mmol), 100 °C, 16 h)
a Determined by GC using 100 µL of dihexyl ether as external stan-
dard.
The carbon-sulfur bond formation reaction was also tested
employing diverse commercially available thiols (Table 6),
providing the corresponding diaryl thioethers in good to
excellent yields. Iodobenzene was maintained as arylating
substrate, in order to analyze the influence of the different
(19) Pinchat, A.; Dallaire, C.; Gingras, M. Tetrahedron Lett. 1998, 39, 543–
546.
J. Org. Chem. Vol. 73, No. 14, 2008 5627

substitution patterns on the reactivity of the aryl thiol. Interest-
ingly, satisfactory results also with electron-deficient thiols and
even with butanethiol were obtained.
mind, the observation that in the present study diphenylthioether
was formed in 60% yield in the reaction of iodobenzene and
thiophenol in water warrants further study.
In summary, a selective and efficient copper-catalyzed C-S
bond-forming reaction of aryl iodides and various thiophenols
was developed. This catalytic procedure offers general ap-
plicability and simplicity, avoiding the expensive and time-
consuming preparation of suitable ligands and activated sub-
strates. Interestingly, the experimental conditions proposed for
the coupling reaction facilitate the easy workup of the reaction
mixtures and isolation of the desired product. Because of these
issues, we believe that this protocol could find large application
in organic synthesis.
The preparation of 2-[(dimethylamino)methyl]phenyl phenyl
thioether (Table 6, entry 5, 85% yield) could also be achieved
through reaction between bromobenzene and 2-[(dimethylami-
no)methyl]thiophenol but in much lower yield (30%). Neverthe-
less, since the halide screening (Table 3) showed no reactivity
for bromobenzene in the model reaction, the result obtained here
underlines a possible cooperative role played in the catalytic
reaction by the ortho-NMe2 group, which is not yet understood.
All the desired products here reported were formed in a clean
and selective coupling reaction. The formation of only minute
amounts (<2%) of the undesired symmetrical disulfides6b,20 was
detected. Traces of diphenyldisulfides found at the end of the
reaction were most likely due to the fact that the starting
thiophenol was used in excess.
The present optimized catalytic process provides the arylation
of thiophenols with aryl iodides, in the presence of K2CO3 or
NEt3 as a base in a polar solvent (NMP, DMF, or DMA) or
under neat conditions. The use of the solvent can be avoided
by using triethylamine, which can act both as solvent and as
base for the system. Copper(I) iodide, in amounts between 1
and 2.5 mol %, is used as catalyst. To the best of our knowledge,
this is the first report about aryl-sulfur bond formation in which
a simple, commercially available copper halide salt is used
without additional ligands.
Experimental Section
General Procedure. A glass tube was charged with inorganic
base (5.5 mmol) and solid substrate, if present. Liquid reagents
(phenyl halide, 5 mmol; thiophenol, 6.5 mmol) and solvent (1 mL)
were then added and finally the copper(I) salt. The reaction vessel,
prepared under air, was closed and placed under stirring in a
preheated oil bath at 100 °C. Subsequently, the reaction mixture
was allowed to cool to room temperature and diluted with
acetonitrile (5 mL), and dihexyl ether (100 µL, 0.425 mmol) was
added as external standard. Samples were analyzed by gas chro-
matography. The reaction mixture was filtered through a plug of
Celite and the solvent removed in vacuo to yield the crude product,
which was purified by silica gel chromatography.
4-Phenylsulfanylacetophenone (Table 5, entry 1). Column chro-
The scope of the reaction is quite broad, ranging from
electron-withdrawing to electron-donating substituents on both
the thiophenol and aryl iodide. The present C-S coupling
reactions are chemoselective toward the -SH functionality in
the presence of an -NH2 group. The chemoselectivity and
tolerance of functional groups lead to an attractive opportunity
to catalyze a variety of C-X coupling reactions in a tandem
fashion by the same copper(I) catalyst.
In recent years, research efforts have been directed toward
the design of more sustainable chemical processes, via the use
of nontoxic chemicals and environmentally friendly solvents.21
In this respect, water seems to be a first choice for the
development of “green” chemical protocols. With this idea in
matography (hexane/ethyl acetate 9:1), isolated as a yellow solid
(89%): H NMR (CDCl3, 400 MHz) δ 2.55 (s, 3H), 7.19 (dt, J )
7.00, 1.75 Hz, 2H), 7.39-7.41 (m, 3H), 7.48-7.50 (m, 2H), 7.82
(dt, J ) 7.00, 1.75 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ 26.9,
127.7, 127.8, 129.0, 129.3, 129.9, 132.4, 134.1, 145.1, 197.3; HRMS
(ES+) calcd for C14H12OS ([M + H]+) 229.0687, found 229.0654.
1
Acknowledgment. We thank DSM, the Ministry of Economic
Affairs, NWO/CW, NRSC-C, and Chemspeed Technologies for
the financial support given to the CW/CombiChem programme.
C. Versluis and Prof. A. J. R. Heck are kindly acknowledged
for the HRMS analysis.
Supporting Information Available: Representative experi-
mental procedures with compound characterization data and
additional charts. This material is available free of charge via
the Internet at http://pubs.acs.org.
(20) (a) Takagi, K. Chem. Lett. 1987, 2221–2224. (b) Correa, A.; Carril,
M.; Bolm, C. Angew. Chem., Int. Ed. 2008, 47, 2880.
(21) For examples on organic reactions in water, see: (a) Li, C.-J.; Chen, L.
Chem. Soc. ReV. 2006, 35, 68–82. (b) Li, C.-J. Chem. ReV. 2005, 105, 3095–
3165.
JO800491K
5628 J. Org. Chem. Vol. 73, No. 14, 2008