Carbon tetrabromide-mediated carbon-sulfur bond formation via a sulfenyl bromide intermediate

Article abstract of DOI:10.1021/ol800765s

(Chemical Equation Presented) In the presence of carbon tetrabromide, a variety of dithiocarbamates, xanthates, dithioesters, and thioethers were prepared in one pot by reacting the corresponding dithioic acids or thiols with active methylene compounds/indole derivatives under mild conditions. The formation of a sulfenyl bromide intermediate is proposed as the key step, which initiates the C-S bond formation.

Full text of DOI:10.1021/ol800765s

ORGANIC
LETTERS
2008
Vol. 10, No. 12
2485-2488
Carbon Tetrabromide-Mediated
Carbon-Sulfur Bond Formation via a
Sulfenyl Bromide Intermediate
Jing Tan, Fushun Liang,* Yeming Wang, Xin Cheng, Qun Liu,* and
Hongjuan Yuan
Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, China
liangfs112@nenu.edu.cn
Received April 4, 2008
ABSTRACT
In the presence of carbon tetrabromide, a variety of dithiocarbamates, xanthates, dithioesters, and thioethers were prepared in one pot by
reacting the corresponding dithioic acids or thiols with active methylene compounds/indole derivatives under mild conditions. The formation
of a sulfenyl bromide intermediate is proposed as the key step, which initiates the C-S bond formation.
Carbon-sulfur bond formation is a fundamental approach
to introduce sulfur into organic compounds and has received
much attention due to the occurrence of this bond in many
molecules that are of biological, pharmaceutical, and material
interest.1,2 For C-S cross-coupling, a nucleophilic displace-
ment reaction by a sulfur nucleophile with a carbon elec-
trophile has been widely accepted.3 However, the C-S cross-
coupling between a nucleophilic sulfur species and a
nucleophilic carbon component is generally regarded as
impossible. During our research on ketene dithioacetal
chemistry,4 we found that the C-S cross-coupling of an
active methylene compound with a thiolate, a dithiocarbam-
ate, a xanthate, or a benzodithioate can be achieved in the
presence of carbon tetrabromide and the results are thus
presented in this communication.
Carbon tetrabromide, as a commercially available and
cheap reagent, has found many applications in organic
synthesis,5 for example, the bromination reaction of ben-
zene hydrocarbons,6a terminal acetylenes,6b,c alkanes, or
arylalkanes,5a,6d and active methylenes.6 The known Appel7
and Corey-Fuchs8 reactions represent another typical ap-
(1) Kondo, T.; Mitsudo, T. Chem. ReV. 2000, 100, 3205–3220
.
(2) (a) Nor Norcross, R. D.; Paterson, I. Chem. ReV. 1995, 95, 2041–
2114. (b) Faulkner, D. J. Nat. Prod. Rep. 1995, 12, 223–269. (c) Liu, G.;
Link, J. T.; Pei, Z.; Reilly, E. B.; Leitza, S.; Nguyen, B.; Marsh, K. C.;
Okasinski, G. F.; con Geldern, T. W.; Ormes, M.; Fowler, K.; Gallatin, M.
J. Med. Chem. 2000, 43, 4025–4040. (d) Sawyer, J. S.; Schmittling, E. A.;
Palkowitz, J. A.; Smith, W. J., III J. Org. Chem. 1998, 63, 6338–6343. (e)
Trost, B. M. Chem. ReV. 1978, 78, 363–382. (f) Kita, Y.; Iio, K.; Kawaguchi,
K.; Fukuda, N.; Takeda, Y.; Ueno, H.; Okunaka, R.; Higuchi, K.; Tsujino,
(4) (a) Liang, F.; Zhang, J.; Tan, J.; Liu, Q. AdV. Synth. Catal. 2006,
348, 1986–1990. (b) Kang, J.; Liang, F.; Sun, S.; Liu, Q.; Bi, X. Org. Lett.
2006, 8, 2547–2550. (c) Zhao, L.; Liang, F.; Bi, X.; Sun, S.; Liu, Q. J.
Org. Chem. 2006, 71, 1094–1098. (d) Li, Y.; Liang, F.; Bi, X.; Liu, Q. J.
Org. Chem. 2006, 71, 8006–8010. (e) Liang, F.; Li, D.; Zhang, L.; Gao, J.;
Liu, Q. Org. Lett. 2007, 9, 4845–4848.
T.; Fujioka, H.; Akai, S. Chem.-Eur. J. 2000, 6, 3897–3905
.
(3) Selected examples: (a) Goering, H. L.; Towns, D. L.; Dittmar, B. J.
Org. Chem. 1962, 27, 736–739. (b) Yunoki, S. I.; Takimiya, K.; Aso, Y.;
Otsubo, T. Tetrahedron Lett. 1997, 38, 3017–3021. (c) Herriott, A. W.;
Pickerzb, D. J. Am. Chem. Soc. 1975, 97, 2345–2349. (d) Yin, J. M.;
Pidgeon, C. A. Tetrahedron Lett. 1997, 38, 5953–5954. (e) Shah, S. T. A.;
Khan, K. M.; Heinrich, A. M.; Voelter, W. Tetrahedron Lett. 2002, 43,
8281–8283. (f) Azizi, N.; Aryanasab, F.; Torkiyan, L.; Ziyaei, A.; Saidi,
M. R. J. Org. Chem. 2006, 71, 3634–3635. (g) Rollin, P. Tetrahedron Lett.
1986, 27, 4169–4170. (h) Rollin, P. Synth. Commun. 1986, 16, 611–616.
(5) Selected examples: (a) Schreiner, P. R.; Lauenstein, O.; Kolomitsyn,
I. V.; Nadi, S.; Fokin, A. A. Angew. Chem., Int. Ed. 1998, 37, 1895–1897.
(b) Kharasch, M. S.; Jensen, E. V.; Urry, W. H. J. Am. Chem. Soc. 1946,
68, 154–155. (c) Yadav, J. S.; Reddy, B. V. S.; Harikishan, K.; Madan,
Ch.; Narsaiah, A. V. Synthesis 2005, 2897–2900. (d) Chen, M.-Y.; Lee,
A. S.-Y. J. Org. Chem. 2002, 67, 1384–1387. (e) Zhong, Y.-L.; Lee, J.;
Reamer, R. A.; Askin, D. Org. Lett. 2004, 6, 929–931. (f) Crimmins, M. T.;
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10.1021/ol800765s CCC: $40.75
Published on Web 05/17/2008
2008 American Chemical Society

plication of CBr4 in organic synthesis, which involve the
initial formation of triphenylphosphine bromide (TPPB) or
analogue via a SN2 type attacking of the phosphorus atom
of PPh3 at a bromine atom of CBr4. In this work, the initial
studies were focused on the investigation of the C-S cross-
coupling of a dithiocarbamate, which was derived from the
reaction of an amine and carbon disulfide,9 with an active
methylene compound in the presence of CBr4. The design
was based on the consideration that the C-S coupling might
occur through a transient (dialkylamino(thiocarbonyl))-
sulfenyl bromide intermediate10 which, similar to Appel
agents,7,8 could be formed in situ by the nucleophilic attack
of a dithiocarbamate anion on a bromine atom of CBr4, and
then trapped by an active methylene compound to form an
organic dithiocarbamate.
A model reaction between dimethylamine 1a, carbon
disulfide, acetylacetone 2a and CBr4 was first examined
under various conditions. Among the solvents tested, CH2Cl2
proved to be the most efficient (Table 1, entry 6). After
optimization, in open air at ambient temperature, the mixture
comprising 1a (1.0 equiv), CS2 (1.0 equiv), 2a (1.2 equiv),
CBr4 (1.0 equiv), and triethylamine (1.2 equiv, as the base)
for 2.0 h gave the C-S bond formation product, 2,4-
dioxopentan-3-yl dimethylcarbamodithioate 3aa in 96% yield
(Table 1, entry 9). The structure of 3aa was characterized
on the basis of its spectra and analytical data, as well as
X-ray diffraction (Figure S1, Supporting Information).11
Comparatively, in the absence of CBr4, 3aa could not be
obtained at all (Table 1, entry 1) and catalytic amount of
CBr4 was not enough to drive the reaction to completion
(Table 1, entry 10). The reactivity of CCl4 was also examined
concerning this C-S bond-forming reaction, but proved to
be inactive under otherwise the same conditions (Table 1,
entry 11).
Table 1. Reaction of Dimethylamine 1a with Acetylacetone 2a
under Different Conditions
entry 1a/CS2/2a/CX4 CX4
base
solvent yield (%)a
b
1
2
3
4
5
6
7
8
9
1.0/1.0/7.5/0
CBr4
s
s
s
s
s
s
neat
-
1.0/1.0/1.0/1.0 CBr4
1.0/1.0/1.0/1.0 CBr4
1.0/1.0/1.0/1.0 CBr4
1.0/1.0/1.0/1.0 CBr4
1.0/1.0/1.0/1.0 CBr4
1.0/1.0/1.0/1.0 CBr4 Et3N (1.0) CH2Cl2
1.0/1.0/1.0/1.0 CBr4 Et3N (1.2) CH2Cl2
1.0/1.0/1.2/1.0 CBr4 Et3N (1.2) CH2Cl2
1.0/1.0/1.2/0.5 CBr4 Et3N (1.2) CH2Cl2
1.0/1.0/1.2/1.0 CCl4 Et3N (1.2) CH2Cl2
CH3CN
C2H5OH
THF
DMF
CH2Cl2
18
25
31
36c
50
56
72
96
49
10
11
b
-
a Isolated yields. b No reaction. c Thiuram disulfide was found to be the
main product.
and the corresponding dithiocarbamates 3aa-ea were ob-
tained in high to excellent yields (Table 2, entries 1-5).12
Then the reactions of amines and CS2 with other active
methylenes were examined in the following work. Under
identical conditions as described above, the reactions of
diethylamine 1b, CS2 with ethyl acetoacetate 2b, 3-oxo-N-
phenylbutanamide 2c, N-(4-cholorophenyl)-3-oxobutanamide
2d, 4-chlorobenzoylacetone 2e, and diethyl malonate 2f were
carried out. All reactions proceeded smoothly to afford the
desired products 3bb-bf in high yields (Table 2, entries
6-10).13 Other active methylenes such as malonitrile and
ethyl nitroacetate were examined, but proved to be inefficient
to afford the corresponding C-S bond formation products.
Additionally, the reaction of sodium ethoxide with CS2 and
acetylacetone, and the reaction of dithiobenzoic acid with
acetylacetone were performed. In the presence of CBr4, both
reactions proceeded efficiently, giving xanthate14 4 and
dithioester 5 in 77 and 81% yield, respectively (Scheme 1).
Obviously, this protocol provides a simple and efficient route
to dithiocarbamates 3, xanthates 4, and dithioesters 5. These
compounds are valuable synthetic intermediates15 and have
Next, under the optimized conditions as above (Table 1,
entry 9), a range of reactions of various amines with
acetylacetone and CS2 in the presence of CBr4 were
investigated. The reactions with secondary aliphatic amines,
such as dimethylamine 1a, diethylamine 1b, dibutylamine
1c, morpholine 1d, and piperidine 1e proceeded efficiently
(6) (a) Hunter, W. H.; Edgar, D. E. J. Am. Chem. Soc. 1932, 54, 2025–
2028. (b) Abele, E.; Rubina, K.; Abele, R.; Gaukhman, A.; Lukevics, E.
J. Chem. Res. 1998, 618–619. (c) Abele, E.; Fleisher, M.; Rubina, K.; Abele,
R.; Lukevics, E. J. Mol. Catal. A-Chem. 2001, 165, 121–126. (d) Camps,
X.; Scho¨nberger, H.; Hirsch, A. Chem.-Eur. J. 1997, 3, 561–567.
(7) (a) Appel, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 801–811. (b)
Rabinowitz, R.; Marcus, R. J. Am. Chem. Soc. 1962, 84, 1312–1313.
(8) (a) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 36, 3769–
3772. (b) Mori, M.; Tonogaki, K.; Kinoshita, A. Org. Synth. 2005, 81, 1–
13. (c) Marshall, J. A.; Yanik, M. M.; Adams, N. D.; Ellis, K. C.; Chobanian,
H. R. Org. Synth. 2005, 81, 157–170. (d) Fokin, A. A.; Schreiner, P. R.
AdV. Synth. Cat. 2003, 345, 1035–1052. (e) Donovan, P. M.; Scott, L. T.
J. Am. Chem. Soc. 2004, 126, 3108–3112.
(12) In our experiment, it was found that the secondary aromatic amines
were inert to this C-S bond formation reaction, probably due to their weaker
nucleophilicity. For primary aliphatic amines, the resulting products were
not the desired C-S bond formation products. Instead, thioureas were
obtained based on the spectra data.
(13) In some cases, the S-S coupling products could be detected. In
the absence of the carbon nucleophiles, the thiuram disulfides would be
produced via S-S coupling. The CBr4-promoted S-S coupling reaction
will be reported seperately, along with the results mentioned in ref 12.
(14) For a review on xanthates, see: Zard, S. Z. Angew. Chem., Int. Ed.
1997, 36, 672–685.
(9) Li, G.; Tajima, H.; Ohtani, T. J. Org. Chem. 1997, 62, 4539–4540.
(10) Selected examples for sulfenyl halides, see: (a) Koval′, I. V. Russ.
Chem. ReV. 1995, 64, 731–751. (b) Schroll, A. L.; Eastep, S. J.; Barany,
G. J. Org. Chem. 1990, 55, 1475. (c) Kharasch, N.; Gleason, G. I.; Buess,
C. M. J. Am. Chem. Soc. 1950, 72, 1796–1798. (d) Kharasch, N.; Langford,
R. B. J. Org. Chem. 1963, 28, 1903–1905. (e) Romano, R. M.; Della
Vedova, C. O.; Downs, A. J.; Greene, T. M. J. Am. Chem. Soc. 2001, 123,
5794–5801. (f) Kuhle, E. Synthesis 1970, 561–580. (g) Kato, S.; Komatsu,
Y.; Miyagawa, K.; Ishida, M. Synthesis 1983, 552–553.
(15) (a) Boas, U.; Jakobsen, M. H. J. Chem. Soc., Chem. Commun. 1995,
1995–1996. (b) Elgemeie, G. H.; Sayed, S. H. Synthesis 2001, 1747–1771.
(c) Mukerjee, A. K.; Ashare, R. Chem. ReV. 1991, 91, 1–14. (d) Boas, U.;
Gertz, H.; Christensen, J. B.; Heegaard, P. M. H. Tetrahedron Lett. 2004,
45, 269–272. (e) Grainger, R. S.; Innocenti, P. Angew. Chem., Int. Ed. 2004,
43, 3445–3448. (f) Fabre, S.; Vila, X.; Zard, S. Z. Chem. Commun. 2006,
4964–4966.
(11) X-ray diffraction data for 3aa has been deposited in the Cambridge
Crystallographic Data Centre with supplementary publication number of
CCDC 616702.
2486
Org. Lett., Vol. 10, No. 12, 2008

a
Table 2. Reactions of Amines 1 with CS2 and Active Methylenes 2 in the Presence of CBr4
entry
amine 1
substrate 2
EWG1
EWG2
MeCO
MeCO
MeCO
MeCO
MeCO
EtOCO
PhNHCO
4-ClPhNHCO
4-ClPhCO
EtOCO
base
t (h)
product 3
yield (%)b
1
2
3
4
5
6
7
8
9
dimethylamine (1a)
diethylamine (1b)
dibutylamine (1c)
morpholine (1d)
piperidine (1e)
diethylamine (1b)
diethylamine (1b)
diethylamine (1b)
diethylamine (1b)
diethylamine (1b)
2a
2a
2a
2a
2a
2b
2c
2d
2e
2f
MeCO
MeCO
MeCO
MeCO
MeCO
MeCO
MeCO
MeCO
EtOCO
EtOCO
Et3N
Et3N
Et3N
Et3N
Et3N
NaOH
Et3N
Et3N
NaOH
NaOH
1.5
1.5
1.5
2.5
2.5
2.5
2.5
4.5
5.5
4.5
3aa
3ba
3ca
3da
3ea
3bb
3bc
3bd
3be
3bf
96
92
95
85
93
65
76
81
78
70
10
a Amine (1.0 equiv), CS2 (1.0 equiv), active methylene compounds (1.2 equiv), carbon tetrabromide (1.0 equiv), base (1.2 equiv). b Isolated yields.
shown wide applications as pesticides, fungicides in agri-
culture, sulfur vulcanization in rubber manufacturing,16 and
radical chain transfer agents in the reversible addition
fragmentation chain transfer (RAFT) polymerizations.17
The sulfenylation reaction is one of the particularly
effective methods in the introduction of a thio group into
various organic compounds.18 With the aim to further explore
2-methyl-3-oxo-N-phenylbutanamide also gave satisfactory
result (entry 4). The reaction of 4-methylphenylthiol 6b with
CBr4 and acetyl acetone under the identical conditions
proceeded smoothly, giving 7ba in 70% yield (entry 5). The
reaction of 6b with 3-oxo-N-phenylbutanamide or ethyl
acetoacetate also furnished the sulfides 7ba and 7bc, but the
yields were low (entries 6 and 7). On the basis of the above
results, it was concluded that the reaction is general to a
wide range of thiols or their equivalents.
To clarify the possible mechanism, some supplementary
experiments were carried out. Under otherwise identical
conditions, the mixture of the corresponding disulfides
including 1,2-diphenyldisulfide, 1,2-dibenzyldisulfide, and
1,2-bis(dimethylamino)disulfide gave no reaction, which
ruled out the possible mechanism of first formation of the
disulfides and then the substitution of the disulfides by carbon
nucleophiles.19 On the basis of the above results, a possible
mechanism for the formation of 3-5, and 7 was proposed,
as depicted in Scheme 2. Initially, a nucleophilic attack of a
sulfur anion I (or I′) at a bromine atom of CBr4 leads to the
formation of sulfur-centered electrophile II (or II′).10,20 Then,
the reactive intermediate II (or II′) would be trapped in situ
by a carbon nucleophile, giving the C-S bond-forming
product, dithiolates 3-5 or sulfides 7. Bromoform has been
detected as a byproduct. It is noteworthy, as the key step of
Scheme 1
the scope of the sulfur components in the cross-coupling
reaction, the reaction of benzylthiol 6a with CBr4 and active
methylene compounds was thus carried out at ambient
temperature. To our delight, with NaOH as the base, the
reactions of 6a and a range of active methylene compounds
including acetylacetone, ethyl acetylacetate and 3-oxo-N-4-
chlorophenylbutanamide gave the corresponding C-S bond-
forming products 7aa, 7ab, and 7ag in good to excellent
yields (entries 1-3, Table 3). It should be noted that even
(17) (a) Lai, J. T.; Shea, R. J. Polym. Sci. A: Polym. Chem. 2006, 44,
4298–4316. (b) Dure´ault, A.; Gnanou, Y.; Taton, D.; Destarac, M.; Leising,
F. Angew. Chem., Int. Ed. 2003, 42, 2869–2872. (c) Bathfield, M.; D’Agosto,
F.; Spitz, R.; Charreyre, M.-T.; Delair, T. J. Am. Chem. Soc. 2006, 128,
2546–2547.
(18) Selected examples, see: (a) Scoffone, E.; Fontana, A.; Rocchi, R.
Biochemistry 1968, 7, 971–979. (b) Anzai, K. J. Heterocycl. Chem. 1979,
16, 567–569. (c) Mukaiyama, T.; Kobayashi, S.; Kumamoto, T. Tetrahedron
Lett. 1970, 59, 5115–5118. (d) Behforouz, M.; Kerwood, J. E. J. Org. Chem.
1969, 34, 51–59. (e) Altkinson, J. G.; Hamel, P.; Girard, Y. Synthesis 1988,
480–481. (f) Matsugi, M.; Gotanda, K.; Murata, K.; Kita, Y. Chem.
Commun. 1997, 1387–1388. (g) Marigo, M.; Wabnitz, T. C.; Fielenbach,
D.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2005, 44, 794–797.
(19) (a) Hesselbarth, F.; Wenschuh, E. Heteroatom Chem. 1992, 3, 631–
636. (b) Bergemann, K.; Hesselbarth, F.; Wenschuh, E. Phosphorus, Sulfur,
Silicon 1993, 79, 131–139.
(16) (a) Marinovich, M.; Viviani, B.; Capra, V.; Corsini, E.; Anselmi,
L.; D’Agostino, G.; Nucci, A. D.; Binaglia, M.; Tonini, M.; Galli, C. L.
Chem. Res. Toxicol. 2002, 15, 26–32. (b) Weissmahr, K. W.; Houghton,
C. L.; Sedlak, D. L. Anal. Chem. 1998, 70, 4800–4804. (c) Len, C.;
Boulogne-Merlot, A.-S.; Postel, D.; Ronco, G.; Villa, P.; Goubert, C.;
Jeufrault, E.; Mathon, B.; Simon, H. J. Agric. Food Chem. 1996, 44, 2856–
2858. (d) Bergendorff, O.; Hansson, C. J. Agric. Food Chem. 2002, 50,
1092–1096.
Org. Lett., Vol. 10, No. 12, 2008
2487

Table 3. Reactions of Thiols 1 with Active Methylene Compounds 2 in the Presence of CBr4
entry
substrate 6
R3
Bn
Bn
Bn
R4
EWG1
EWG2
MeCO
t (h)
product 7
yield (%)a
1
2
3
4
5
6
7
6a
6a
6a
6a
6b
6b
6b
H
H
H
Me
H
MeCO
MeCO
MeCO
MeCO
MeCO
MeCO
MeCO
1.2
1.5
1.5
1.5
2.0
2.0
2.5
7aa
7ab
7ad
7ag
7ba
7bb
7bc
95
71
81
72
70
29
28
COOEt
4-ClC6H4NHCO
C6H5NHCO
MeCO
COOEt
C6H5NHCO
Bn
4-MePh
4-MePh
4-MePh
H
H
a Isolated yields.
the new C-S cross-coupling reaction, the original nucleo-
philic sulfur atom is converted into an electrophilic site
assisted by CBr4. The overall process can be understood as
Inspired by Schlosser’s work mentioned above, N-methyl
indole was subjected to the CBr4-mediated C-S coupling
reaction. As a result, 3-benzylthio-N-methylindole 8 was
obtained in 74% yield (eq 1). In this case, heating to reflux
in CH2Cl2 is required to accomplish this reaction. The scope
of the carbon components in this CBr4-mediated C-S cross-
coupling reaction was extended. Further work is ongoing in
our laboratory.
Scheme 2
.
Proposed Mechanism for the C-S Cross-Coupling
Reaction
In conclusion, a new route to dithiocarbamates, xanthates,
dithioesters and thioethers was developed by reacting the
corresponding sulfur components with active methylenes/
indole derivatives. It uses unusual umpolung reactivity
created from the reaction of the thiol and equivalents with
CBr4 which leads to a reactive sulfenyl bromide. Such species
are highly electrophilic and rather unstable, but the advantage
of this chemistry is that they are generated and used in situ.
The C-S bond-forming reaction is characterized as a wide
range of substrates, multicomponent one-pot procedure, mild
conditions, and metal-catalyst-free.
a formal umpolung of sulfur anions. Umpolung strategy has
been demonstrated to facilitate the construction of organic
molecules in unusual ways.21 K. Schlosser et al. reported
the reaction of indole derivatives with N-chlorosucnimide
and thiols, in which a transient formation of the sulfenyl
chloride intermediate was proposed.20b Comparatively, we
succeeded the reaction of acetylacetone with NBS and
benzylthiol. The control experiment indicated that the
mechanism shown in Scheme 2 was reasonable.
(20) Attempts to isolate the sulfur-centered electrophilic species failed.
Also refer to: (a) Tudge, M.; Tamiya, M.; Savarin, C.; Humphrey, G. R.
Org. Lett. 2006, 8, 565–568. (b) Schlosser, K. M.; Krasutsky, A. P.;
Hamilton, H. W.; Reed, J. E.; Sexton, K. Org. Lett. 2004, 6, 819–821. (c)
Goto, K.; Holler, M.; Okazaki, R. Chem. Commun. 1998, 1915–1916.
(21) Representative papers on umpolung, see: (a) Seebach, D. Angew.
Chem., Int. Ed. 1979, 18, 239–258. (b) Fischer, C.; Smith, S. W.; Powell,
D. A.; Fu, G. C. J. Am. Chem. Soc. 2006, 128, 1472–1473. (c) Tseng, H.-
R.; Lee, C.-F.; Yang, L.-M.; Luh, T.-Y. J. Org. Chem. 1999, 64, 8582–
8587. (d) Trost, B. M.; Li, C.-J. J. Am. Chem. Soc. 1994, 116, 3167–3168.
(e) Kieltsch, I.; Eisenberger, P.; Togni, A. Angew. Chem., Int. Ed. 2007,
46, 754–757.
Acknowledgment. Financial support of this research by
NSFC (20672019) is greatly acknowledged.
Supporting Information Available: Experimental details,
characterization data, ORTEP drawing, and CIF data for 3aa.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL800765S
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Org. Lett., Vol. 10, No. 12, 2008