Sequential copper(I)-catalyzed reaction of amines with o-acetylenyl- substituted phenyldiazoacetates

Article abstract of DOI:10.1002/adsc.200800249

A highly efficient, tetrakis(acetonitrile)-copper(I) hexafluorophosphate [Cu(MeCN)₄PF₆]-catalyzed tandem Cu(I)-carbene N-H insertion/Cu(I)-catalyzed hydroamination of alkynes, which leads to sequential formation of two C-N bonds to yield isoindole derivatives, has been developed.

Full text of DOI:10.1002/adsc.200800249

FULL PAPERS
DOI: 10.1002/adsc.200800249
Sequential Copper(I)-Catalyzed Reaction of Amines with
o-Acetylenyl-Substituted Phenyldiazoacetates
Cheng Peng,a,b Jiajia Cheng,a,b and Jianbo Wanga,b,*
a
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and
Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, Peopleꢀs
Republic of China
Fax : (+86)-10-6275-1708; e-mail: wangjb@pku.edu.cn
State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, Peopleꢀs Republic
b
of China
Received: April 26, 2008; Published online: September 9, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800249.
À
Abstract: A highly efficient, tetrakis(acetonitrile)- leads to sequential formation of two C N bonds to
copper(I) hexafluorophosphate [Cu(MeCN)4PF6]- yield isoindole derivatives, has been developed.
À
A
catalyzed tandem Cu(I)-carbene N H insertion/
Cu(I)-catalyzed hydroamination of alkynes, which Keywords: alkynes; carbenoids; copper; diazo com-
pounds; homogeneous catalysis; insertion
À
Introduction
metal-catalyzed C N bond formations. Among these
catalytic C N bond formations, metal carbene N H
À
À
Transition metal catalysis has become indispensable bond insertion[5,6] and hydroamination of alkynes[7]
in modern organic synthesis. While efficiency and se- are particularly attractive due to their high efficiency.
lectivity are the major concerns in these metal-cata- For metal carbene N H insertions, a-diazocarbonyl
À
lyzed reactions, an emerging area of research in this compounds normally serve as metal carbene precur-
field is to develop reaction systems in which a single sors, and Cu(I) and Rh(II) complexes are the most
catalyst mediates two or more different reactions in a widely used catalysts. While for hydroamination of al-
selective manner.[1–4] Such a kind of sequential or con- kynes, transition metal complexes of Ti,[8] Pd,[9] Pt,[10]
current catalysis is particularly attractive in organic Ru,[11] Au,[12] Rh,[13] Ir,[14] La,[7f,15] Zn and Cd,[16] Hg,[17]
synthesis in view of ecological and economical con- Zr[18] and Ac[19] have been widely utilized. Although it
cerns in the fine chemical industries. Because there is less common, copper-catalyzed hydroamination of
are many different reactions that can be catalyzed by alkynes has also been reported.[20] Significant progress
the same or similar transition metal catalysts, a great has been made in both intra- and intermolecular hy-
potential exists to link these reactions in sequence. droaminations of alkynes by utilizing these catalytic
Recently, a number of efficient sequential catalytic systems. We thought that it might be possible to com-
À
processes, such as Ru-catalyzed RCM/hydrogena- bine the metal carbene N H insertion and hydroami-
tion,[2] Rh-catalyzed allylic alkylation/Pauson–Khand nation of alkynes into a sequential catalytic process
reaction,[3] Pd-catalyzed aryl alkylation/cyanation re- with only one catalyst. Here we report our study
action,[4] have been developed.
C N bond formation is important in organic syn-
thesis, and there have been many reports on transition
based on this concept (Scheme 1).
À
À
Scheme 1. Two C N bond formations catalyzed by single catalyst.
Adv. Synth. Catal. 2008, 350, 2359 – 2364
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2359

Cheng Peng et al.
FULL PAPERS
Results and Discussion
of the substituent on the amine seems to have no
effect on the reaction. Both p-NO2- and p-MeO-sub-
o-Acetylenyl-substituted phenyldiazoacetate 1 was se- stituted anilines gave the corresponding isoindole
lected as the substrate and it was subjected to copper products in high yields (entries 3 and 4). However, for
catalysts in the presence of aniline (Table 1). the reactions with benzylamine and the aliphatic
Cu(MeCN)4PF6 was found to effectively catalyze amine t-BuNH2 as substrates, only low yields or trace
G
À
diazo decomposition, and the subsequent N H inser- amounts of isoindole products could be identified (en-
tion and alkyne hydroamination (entry 1). The reac- tries 7 and 8). The reaction with tosylamide gave only
tion gave isoindole 3a through 5-exo-dig cyclization. the carbene dimerization product (entry 9). The struc-
No products from 6-endo-dig cyclization could be de- ture of isoindole 3d was unambiguously confirmed by
tected. Other copper catalysts were also examined. single crystal X-ray analysis (Figure 1).[21]
Cu
be effective for the reaction, but with lower yields as was prepared and their reactions with p-MeC6H4NH2
compared with the Cu(MeCN)4PF6-catalyzed reaction were investigated (Table 3). The reactions all gave
A
Next, a series of substituted diazo compounds 4a–h
AHCTREUNG
(entries 2–4). Catalyst loading and reaction tempera- isoindole derivatives as major products. In contrast to
ture had only maginal effects on the reaction (en- the reaction of 1, various amounts of dihydroquino-
tries 5 and 6).
line derivatives 6a–h were also isolated in these cases.
A series of substituted anilines was then subjected It was noted that the reactions with 4a–h all took
to this sequential catalytic reaction with diazoacetate longer time as compared to the reaction with 1.
substrate 1. As shown in Table 2, the reactions with
A proposed mechanism is shown in Scheme 2.
substituted anilines all gave the corresponding isoin- Cu(I) carbene 7 is generated upon treatment of 1a
À
dole derivatives in high yields. The electronic nature with Cu
A
Table 1. Reaction of 1 and aniline 2a with various Cu catalysts.
Entry
Catalyst (mol%)
CuPF6(MeCN)4 (10)
2 (mol%)
Temperature [oC]
Time
Yield [%][a]
1
2
3
4
5
6
A
120
150
150
150
150
150
35
35
35
35
35
60
10 min
10 h
24 h
30 min
10 min
10 min
87
56
74
89
91
90
Cu(acac)2 (5)
CuI (5)
(CuOTf)2·C6H6 (2.5)
ACHTREUNG
G
Cu (MeCN)4PF6 (10)
Cu (MeCN)4PF6 (5)
[a]
Isolated yield after separation with column chromatography.
Table 2. Reaction of substrate 1 with substituted anilines.
Entry
2, R
3, Yield [%][a]
Entry
2, R
3, Yield [%][a]
1
2
3
4
5
2a, C6H5
3a, 90
3b, 95
3c, 93
3d, 90
3e, 97
6
7
8
9
2f, p-BrC6H4
2g, C6H5CH2
2h, tBu
3f, 92
3g, 28
2b, p-MeC6H4
2c, p-MeO-C6H4
2d, p-NO2C6H4
2e, m,p-Cl2C6H3
3h, trace
2i, p-MeC6H4SO2
3i, –[b]
[a]
Isolated yield after separation with column chromatography.
The reaction gave the dimerization product.
[b]
2360
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2359 – 2364

Sequential Copper(I)-Catalyzed Reaction of Amines
FULL PAPERS
Table 3. Cu(I)-catalyzed reaction of 4a–h with p-MeC6H4NH2.
Entry
4, R
Time [h]
5, Yield [%][a]
6, Yield [%][a]
1
2
3
4
5
6
7
8
4a, C6H5
23
6
6
5
5
6
24
23
5a, 85
5b, 94
5c, 78
5d, 87
5e, 87
5f, 93
5g, 57
5h, 66
6a, 11
6b, 4
6c, 16
6d, <1
6e, <1
6f, 7
4b, p-BrC6H4
4c, p-MeC6H4
4d, o-ClC6H4
4e, o-BrC6H4
4f, m-MeC6H4
4g, p-MeOC6H4
4h, Me
6g, 33
6h, 17
[a]
Isolated yield after separation with column chromatography.
To substantiate the proposed mechanism, we tried
À
to isolate the N H insertion intermediate such as 8.
The reaction of 1 with amine was so fast that isolation
À
of the N H insertion intermediate proved to be diffi-
cult. Gratifyingly, the reaction of 4a with p-
MeC6H4NH2 catalyzed by Rh2
N H insertion product 11a (Table 4, entry 1). Product
A
À
11a was then subjected to further catalysis with
Cu(MeCN)4PF6. However, contrary to our expecta-
A
tion isoindole 5a was isolated in only 6% yield. The
major product was dihydroisoquinoline derivative 6a,
which was isolated in 76% yield (entry 1). The forma-
tion of 6a is due to 6-endo-dig attack of the amino
group to the Cu(I)-activated triple bond.
Figure 1. X-ray structure of 3d.
À
A series of substituted N H insertion products
with aniline leads to amine 8. Subsequently, the 11b–h was then prepared by similar reactions cata-
À
amino nitrogen attacks the activated triple bond, lyzed by Rh
which is coordinated with Cu(I) complex, in a 5-exo- were subsequently subjected to catalysis by
A
dig manner, to give intermediate 10. Compound 10 Cu(MeCN)4PF6. The reactions with the substrates
G
quickly isomerizes to isoindole 3a (path a). If 6-endo- 11b, c, f and g, which bear para- and meta-aryl sub-
dig cyclization from 9 occurs, dihydroisoquinoline 6a stituents in the triple bond moiety, all afforded the di-
is generated (path b).
hydroisoquinoline derivative as the major products
Scheme 2. Mechanistic proposal.
Adv. Synth. Catal. 2008, 350, 2359 – 2364
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2361

Cheng Peng et al.
FULL PAPERS
Table 4. Stepwise reaction of 4a–h and p-MeC6H4NH2 with two catalysts.[a]
Entry
4, R
Time [h]
11, Yield [%][b]
Time [h]
5, Yield [%][b]
6, Yield [%][b]
1
2
3
4
5
6
7
8
4a, C6H5
8
7
11a, 92
11b, 99
11c, 90
11d, 70
11e, 85
11f, 72
11g, 84
11h, 90
12
5
5
4
5
6
4
4
5a, 6
5b, 19
5c, 4
5d, 85
5e, >99
5f, 8
6a, 76
6b, 81
6c, 95
6d, 9
6e, <1
6f, 89
6g, 91
6h, 85
4b, p-BrC6H4
4c, p-MeC6H4
4d, o-ClC6H4
4e, o-BrC6H4
4f, m-MeC6H4
4g, p-MeOC6H4
4h, Me
20
6
25
11
5
5g, 1
5h, 3
23
[a]
The two reactions were carried out separately with two different catalysts.
Isolated yield after separation with column chromatography.
[b]
(entries 2, 3, 6, 7). The corresponding substrates 11d
and 11e, which bear ortho-substituents, gave isoin-
doles as the major products (entries 4 and 5). This se-
lectivity might be due to the steric effect of the ortho-
substituent, which raise the energy of the transition
state of 6-endo-dig cyclizaiton. The aliphatic substitu-
ent also gave the dihydroisoquinoline derivative 6h as
the major product (entry 8).
Table 5. Cu(I)-catalyzed reaction of 11a with additives.
Entry Additive
(mol%)
Time 5, Yield 6, Yield
[h]
Compared with the one-catalyst reactions summar-
ized in Table 3, the regioselective hydroamination ob-
served in the stepwise reaction was unexpected. The
A
[%][a]
[%][a]
1
2
3
4
5
6
7
8
9
none
12
4
4
4
4
4
4
12
24
6
90
56
68
86
64
76
9
4
2
4
34
p-MeC6H4NH2 (10)[b]
pyridine (50)
conditions of Cu(MeCN)4PF6-catalyzed reaction were
N
essentially identical, except that in one-catalyst reac-
tions an excess amount of amine (1.5 equiv.) was ap-
plied. Was the excess amine responsible for the ob-
served selective formation of isoindole derivatives in
the case of one-catalyst reactions? With this hypothe-
sis in mind, we investigated the effect of organic
bases on the Cu(I)-catalyzed reaction of 11a
(Table 5). Adding 10 mol% of p-tolylamine indeed re-
versed the selectivity of the reaction, leading to 5a as
the major product. Other bases, such as pyridine, mor-
pholine and 1H-pyrrole also enchanced the selectivity
for 5a, albeit with less efficiency as compared to p-tol-
ylamine. When 2,2’-bipyridine or 1H-imidazole was
used as additive, the reaction was completely shut
down. Although further study is needed to fully un-
derstand this remarkable additive effect, we consider
pyridine (20)
pyridine (10)
1H-pyrrole (10)
morpholine (10)
2,2’-bipyridine (10)
1H-imidazole (10)
74
12
N.R.[c]
N.R.[c]
N.R.[c]
N.R.[c]
[a]
[b]
[c]
Isolated yield after separation with column chromatogra-
phy.
The number in paranthesis refers to the molar ratio of
the amine additive relative to 11a.
N.R.: no reaction. Starting material was recovered.
that the steric effect of the additive, which coordi- alyzed by a single Cu(I) catalyst. This tandem process
nates with Cu(I) catalyst, is responsible for the ob- provides a novel and straightforward way to synthe-
served change of reaction pathway.[22]
size isoindole and dihydroisoquinoline derivatives.[23]
Moreover, a mechanistic study reveals a remarkable
effect of amine in directing the regioselectivity of
Cu(I)-catalyzed intramolecular hydroamination of al-
kynes. This discovery opens a possibility to control
Conclusions
In summary, we have developed a highly efficient the regioselectivity of the hydroamination by addi-
À
tandem N H insertion/hydroamination of alkyne cat- tives to the catalysts.
2362
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2359 – 2364

Sequential Copper(I)-Catalyzed Reaction of Amines
FULL PAPERS
[4] B. Mariampillai, D. Alberico, V. Bidau, M. Lautens, J.
Am. Chem. Soc. 2006, 128, 14436.
Experimental Section
[5] For reviews, see: a) M. P. Doyle, M. A. McKervey, T.
Ye, Modern Catalytic Methods for Organic Synthesis
with Diazo Compounds, Wiley-Interscience, New York,
1998; b) T. Ye, M. A. McKervey, Chem. Rev. 1994, 94,
1091.
General Information
For chromatography, 200–300 mesh silica gel (Qingdao,
China) was employed. 1H and 13C NMR spectra were re-
corded at 200 and 50 MHz with a Varian Mercury 200 spec-
trometer, or 300 and 75 MHz with a Varian Mercury 300
spectrometer. Chemical shifts are reported in ppm using tet-
ramethylsilane as internal standard. Mass spectra were ob-
tained on a VG ZAB-HS mass spectrometer.
For the preparation of the diazo substrates and the char-
acterization data of all new compounds, see the Supporting
Information.
À
[6] For selected recent examples of metal carbene N H in-
sertions, see: a) S. H. Lee, K. Yoshida, H. Matsushita,
B. Clapham, G. Koch, J. Zimmermann, K. D. Janda, J.
Org. Chem. 2004, 69, 8829; b) J. R. Davies, P. D. Kane,
C. J. Moody, J. Org. Chem. 2005, 70, 7305; c) F. A.
Davis, H. Xu, Y. Wu, J. Zhang, Org. Lett. 2006, 8, 2273.
[7] For recent reviews, see: a) A. R. Muci, S. L. Buchwald,
Top. Curr. Chem. 2002, 219, 131; b) J. F. Hartwig, in:
Handbook of Organopalladium Chemistry for Organic
Synthesis, (Ed.: E. Negishi), Wiley-Interscience, New
York, 2002, p 1051; c) T. E. Müller, M. Beller, Chem.
Rev. 1998, 98, 675; d) F. Alonso, I. P. Beletskaya, M.
Yus, Chem. Rev. 2004, 104, 3079; e) F. Pohlki, S. Doye,
Chem. Soc. Rev. 2003, 32, 104; f) M. Nobis, B. Drieben-
Hçlscher, Angew. Chem. Int. Ed. 2001, 40, 3983; g) M.
Beller, J. Seayad, A. Tillack, H. Jiao, Angew. Chem.
Int. Ed. 2004, 43, 3368; h) S. Hong, T. J. Marks, Acc.
Chem. Res. 2004, 37, 673.
À
Typical Procedure for Rh2
Insertion Reaction
(OAc)4-Catalyzed N H
To a solution of 4a (138 mg, 0.5 mmol) and Rh2
(2 mg, 0.005 mmol) was added p-MeC6H4NH2 (0.75 mmol,
80 mg) at 508C. After the reaction has finished, the solvent
was evaporated under vacuum. Then purification by column
chromatography of the mixture gave the pure 11a as a pale
yellow solid; yield: 163 mg (92%).
(OAc)4
General Procedure for the Cu(I)-Catalyzed Tandem
Reaction
[8] a) E. Haak, I. Bytschkov, S. Doye, Angew. Chem. Int.
Ed. 1999, 38, 3389; b) E. Haak, H. Siebeneicher, S.
Doye, Org. Lett. 2000, 2, 1935; c) F. Pohlki, S. Doye,
Angew. Chem. Int. Ed. 2001, 40, 2305; d) J. S. Johnson,
R. G. Bergman, J. Am. Chem. Soc. 2001, 123, 2923;
e) B. F. Straub, R. G. Bergman, Angew. Chem. Int. Ed.
2001, 40, 4632; f) Y. Shi, J. T. Ciszewski, A. L. Odom,
Organometallics 2001, 20, 3967; g) C. Cao, J. T. Ciszew-
ski, A. L. Odom, Organometallics 2001, 20, 5011; h) C.
Cao, Y. Shi, A. L. Odom, Org. Lett. 2002, 4, 2853; i) A.
Tillack, I. G. Castro, C. G. Hartung, M. Beller, Angew.
Chem. Int. Ed. 2002, 41, 2541; j) I. Bytschkov, S. Doye,
Eur. J. Org. Chem. 2003, 6, 935; k) Y. Shi, Y. Li, A. L.
Odom, J. Am. Chem. Soc. 2004, 126, 1794; l) C. Lorber,
R. Choukroun, L. Vendier, Organometallics 2004, 23,
1845.
To a solution of 1 (100 mg, 0.5 mmol) and CuPF6(MeCN)4
G
(9 mg, 0.025 mmol) was added PhNH2 2a (70 mg,
0.75 mmol) at room temperature. After completion of the
reaction as monitored by TLC, the solvent was evaporated
under vacuum. Then purification by column chromatogra-
phy of the mixture gave the pure 3a as a pale yellow oil;
yield: 119 mg (90%).
The procedure of the reaction of 11a catalyzed by Cu(I) is
the same as that for the Cu(I)-catalyzed tandem reaction.
Acknowledgements
The project is generously supported by Natural Science Foun-
dation of China (Grant No. 20572002, 20521202, 20772003)
and the Ministry of Education of China (Cheung Kong
Scholars Program).
[9] a) I. Kadota, A. Shibuya, L. M. Lutete, Y. Yamamoto,
J. Org. Chem. 1999, 64, 4570; b) T. Shimada, Y. Yama-
moto, J. Am. Chem. Soc. 2002, 124, 12670; c) G. B. Baj-
racharya, Z. Huo, Y. Yamamoto, J. Org. Chem. 2005,
70, 4883.
[10] J. J. Brunet, N. C. Chu, O. Diallo, S. Vincendeau, J.
Mol. Catal. A: Chem. 2005, 240, 245.
[11] a) M. Tokunaga, M. Eckert, Y. Wakatsuki, Angew.
Chem. Int. Ed. 1999, 38, 3222; b) Y. Uchimaru, Chem.
Commun. 1999, 1133.
[12] For recent reviews, see: a) R. A. Widenhoefer, X. Han,
Eur. J. Org. Chem. 2006, 4555; b) N. T. Patil, Y. Yama-
moto, Arkivoc 2007, x, 121; for recent reports, see:
c) D. Kadzimirsz, D. Hildebrandt, K. Merz, G. Dyker,
Chem. Commun. 2006, 661; d) E. Mizushima, T. Haya-
shi, M. Tanaka, Org. Lett. 2003, 5, 3349.
References
[1] For recent reviews of tandem catalysis, see: a) R. A.
Bunce, Tetrahedron 1995, 51, 13103; b) P. J. Parsons,
C. S. Penkett, A. J. Shell, Chem. Rev. 1996, 96, 195;
c) D. E. Fogg, E. N. dos Santos, Coord. Chem. Rev.
2004, 248, 2365; d) J. C. Wasilke, S. J. Obrey, R. T.
Baker, G. C. Bazan, Chem. Rev. 2005, 105, 1001; e) A.
Ajamian, J. L. Gleason, Angew. Chem. Int. Ed. 2004,
43, 3754.
[13] a) C. G. Hartung, A. Tillack, H. Trauthwein, M. Beller,
J. Org. Chem. 2001, 66, 6339; b) M. Beller, C. M.
Breindl Eichberger, C. G. Hartung, J. Seayad, O. R.
Thiel, A. Tillack, H. Trauthwein, Synlett 2002,1579.
[2] J. Louie, C. W. Bielawski, R. H. Grubbs, J. Am. Chem.
Soc. 2001, 123, 11312.
[3] P. A. Evans, J. E. Robinson, J. Am. Chem. Soc. 2001,
123, 4609.
Adv. Synth. Catal. 2008, 350, 2359 – 2364
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2363

Cheng Peng et al.
FULL PAPERS
[14] X. Li, A. Chianese, Y. Vogel, R. H. Crabtree, Org. Lett.
2005, 7, 5437.
[15] a) Y. Li, T. J. Marks, Organometallics 1996, 15, 3770;
b) Y. Li, T. J. Marks, J. Am. Chem. Soc. 1998, 120, 1757;
c) J. S. Ryu, G. Y. Li, T. J. Marks, J. Am. Chem. Soc.
2003, 125, 12584.
[16] a) C. W. Kruse, R. F. Kleinschmidt, J. Am. Chem. Soc.
1961, 83, 213; b) C. W. Kruse, R. F. Kleinschmidt, J.
Am. Chem. Soc. 1961, 83, 216.
[17] a) J. Barluenga, F. Aznar, R. Liz, R. Rodes, J. Chem.
Soc. Perkin Trans. 1 1980, 2732; b) J. Barluenga, F.
Aznar, Synthesis 1977, 195.
[18] a) P. J. Walsh, A. M. Baranger, R. G. Bergman, J. Am.
Chem. Soc. 1992, 114, 1708; b) A. M. Baranger, P. J.
Walsh, R. G. Bergman, J. Am. Chem. Soc. 1993, 115,
2753.
[19] a) A. Haskel, T. Straub, M. S. Eisen, Organometallics
1996, 15, 3773; b) T. Straub, A. Haskel, T. G. Neyroud,
M. Kapon, M. Botoshansky, M. S. Eisen, Organometal-
lics 2001, 20, 5017.
[20] a) L. Xu, I. R. Lewis, S. K. Davidsen, J. B. Summers,
Tetrahedron Lett. 1998, 39, 5159; b) T. E. Müller, M.
Grosche, E. Herdtweck, A. K. Pleier, E. Walter, Y. K.
Yan, Organometallics 2000, 19, 170.
[21] CCDC 692612 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[22] Very recently, Wu and co-workers have reported selec-
tive synthesis of isoindole and dihydroisoquinoline de-
rivatives by using different transition metal catalysts,
see: Q. Ding, Y. Ye, R. Fan, J. Wu, J. Org. Chem. 2007,
72, 5439.
[23] For recent reports on the synthesis and application of
isoindole derivatives, see: a) N. Dieltiens, C. V. Stevens,
Org. Lett. 2007, 9, 465; b) T. Mitsumori, M. Bendikov,
O. Dautel, F. Wudl, T. Shioya, H. Sato, Y. Sato, J. Am.
Chem. Soc. 2004, 126, 16793.
2364
asc.wiley-vch.de
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2359 – 2364