π-allylic azidation in water with an amphiphilic resin-supported palladium-phosphine complex

Article abstract of DOI:10.1055/s-2006-948189

π-Allylic azidation of allyl esters was performed with an amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-supported phosphine-palladium complex in water under hetero-geneous conditions to give allyl azides in good to high yields. The polymeric palladium catalyst can be readily recovered and recycled. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-948189

LETTER
2109
p-Allylic Azidation in Water with an Amphiphilic Resin-Supported
Palladium–Phosphine Complex
p-Allylic
Azidatio
a
n in
W
ater suhiro Uozumi,* Toshimasa Suzuka, Ray Kawade, Hiroe Takenaka
Institute for Molecular Science (IMS), The Graduate University for Advanced Studies, CREST, Higashiyama 5-1, Myodaiji,
Okazaki 444-8787, Japan
Fax +81(564)595574; E-mail: uo@ims.ac.jp
Received 20 June 2006
demonstrating that the allylic azidation of various allyl
esters with sodium azide proceeds in water in the presence
of a palladium complex of an amphiphilic PS-PEG resin-
bound triarylphosphine ligand (Scheme 1, Figure 1).
Abstract: p-Allylic azidation of allyl esters was performed with an
amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-
supported phosphine–palladium complex in water under hetero-
geneous conditions to give allyl azides in good to high yields. The
polymeric palladium catalyst can be readily recovered and recycled.
Key words: p-allylpalladium, azide, aqueous media, polymer sup-
port, palladium catalyst
Pd
1
OR
N3
Pd
NaN3
in H2O
Organic azides have found widespread utility as synthetic
precursors of various nitrogen functional groups.1 With
the advent of ‘click chemistry’,2 the chemistry of alkyl
azides has seen an increase in interest over the past few
years. The most commonly applied method of preparing
alkyl azides is nucleophilic substitution of labile function-
al groups (e.g. halides, pseudo-halides) by sodium azide.
Palladium-catalyzed substitution of allyl esters via p-al-
lylpalladium intermediates, the so-called Tsuji–Trost re-
action, is a powerful synthetic means for forming carbon–
carbon as well as carbon–nitrogen bonds. While extensive
research has been devoted to the p-allylic alkylation and
amination, research on p-allylic azidation has been limit-
ed to isolated reports.3 Thus, for example, Murahashi and
co-workers have examined the p-allylic azidation of allyl
acetates with sodium azide in the presence of Pd(PPh3)4 to
give the corresponding allyl azides where the reactions
were carried out in THF–H2O (5:2) because of the insolu-
bility of sodium azide in organic solvents. If p-allylic azi-
dation took place with sodium azide in water with
recyclable palladium catalysts, where neither aqueous–or-
ganic solvent wastes nor metal-contaminated wastes were
produced, this would go a long way to meeting green
chemical requirements.
O
C
Ph2
Pd
1
PS
O
O
N
P
n
H
Pd
1
Cl
Scheme 1 p-Allylic azidation.
Figure 1 Microscopic images of 1: (a) optical microscope; (b) scan-
ning-electron microscope (SEM).
Reaction of methyl cinnamyl carbonate (2a) with sodium
azide was found to take place smoothly in water at 25 °C
in the presence of 1 mol% palladium of the PS-PEG
(TentaGel) resin-supported p-allylpalladium–triarylphos-
phine complex (1, Figure 1, 1% DVB cross-linked, dia-
meter 130 mm, Pd loading = 0.27 mmol/g). After
completion of the reaction in 24 hours (GC monitoring),
the reaction mixture was filtered and the recovered resin
beads were extracted with ethyl acetate to give 96% yield
of cinnamyl azide (3a) (Table 1, run 1). To demonstrate
the extent of substrate tolerance in this reaction system,
various allylic carbonates or acetates were used for the p-
allylic azidation in water using the same procedure as em-
ployed for run 1. Representative results are summarized in
Table 1.
We have recently developed amphiphilic polystyrene-
poly(ethylene glycol) (PS-PEG) resin-supported palladi-
um catalysts which promote various catalytic trans-
formations, including p-allylic substitutions, smoothly in
water4 under heterogeneous conditions5,6 with high re-
cyclability.7,8 Our continuing interest in the catalytic
utility of PS-PEG resin-supported palladium complexes
led us to examine the p-allylic azidation in water with the
PS-PEG–Pd complexes.9 We report herein our results
The benzyl carbonate 2b, the allylic isomer of cinnamyl
carbonate 2a, gave cinnamyl azide (3a), exclusively, un-
der similar conditions (run 2), and the benzylic azide 3b
was not detected on GC. The azidation of benzyl esters
SYNLETT 2006, No. 13, pp 2109–2113
Advanced online publication: 09.08.2006
DOI: 10.1055/s-2006-948189; Art ID: D17306ST
© Georg Thieme Verlag Stuttgart · New York
1
7
.0
8
.2
0
0
6

2110
Y. Uozumi et al.
LETTER
Table 1 p-Allylic Azidation of Allyl Carbonates 2 in H2Oa
Run
1
Allyl ester 2
Product 3
Yield (%)
96
OCOOMe
N3
3a
3a
2a
OCOOMe
2
3
88
92
2b
OCOOMe
N3
F3C
F3C
3c
2c
OCOOMe
N3
H3C
4
92
H3C
3d
2d
OCOOMe
N3
H3CO
5
6
95
89
H3CO
3e
2e
OCOMe
N3
2f
3f
N3
OCOOMe
N3
7
81
2g
3g
3i
(92:8)
3g/3i (91:9)
8
9
90
95
OCOOMe
2h
3g/3i (90:10)
OCOOMe
2i
a All reactions were carried out in water at 25 °C for 24 h. The ratio of 2 (mol)/NaN3 (mol)/Pd (mol)/H2O (L) = 1:1.5:0.01:3.
N3
N3
3h
3b
Synlett 2006, No. 13, 2109–2113 © Thieme Stuttgart · New York

LETTER
p-Allylic Azidation in Water
Scheme 3 Recycling experiments.
2111
2c–e bearing electron-donating and -withdrawing sub-
stituents at their para-positions gave the cinnamyl azides
3c, 3d, and 3e in 92%, 92%, and 95% yield, respectively
(runs 3, 4, and 5). The diphenylpropenyl ester 2f also un-
derwent the azidation to give 3f in 89% yield (run 6). Ger-
anyl, neryl, and linalyl carbonates 2g, 2h, and 2i reacted
with sodium azide under similar conditions to give 81%,
90%, and 95% yields of a mixture of regioisomeric azida-
tion products 3g and 3i (runs 7, 8, and 9). The ratios of
geranyl azide 3g and linalyl azide 3i obtained from the
reactions of 2g, 2h, and 2i were almost the same (3g/
3i = 9:1) and neryl azide 3h was not observed at all. These
results demonstrate that the reactions of 2g–i proceeded
by way of the same p-allylpalladium intermediate.
COOMe
COOMe
MeOOC
COOMe
N
N
3a
100 °C, 60 min, in H2O
N
8 (84% yield)
Scheme 4 [3+2] Triazole cycloaddition in H2O.
The synthetic utility of the allyl azides is demonstrated by
the [3+2] triazole synthesis, which has recently attracted
significant interest owing to its utility in ‘Click Chemis-
try’. Preliminary results are shown in Scheme 4. Thus, the
[3+2] triazole cycloaddition of cinnamyl azide (3a) with
dimethyl acetylenedicarboxylate took place smoothly in
emulsive aqueous conditions at 100 °C for 60 minutes to
give 84% isolated yield of the N-cinnamyl triazole (8).
The azidation of cis-5-carbomethoxy-2-cyclohexenyl
methyl carbonate (4) was also catalyzed by the PS-PEG–
palladium 1 in water at 25 °C to afford the cis-5-car-
bomethoxy-2-cyclohexenyl azide (5) in 87% yield
(Scheme 2). The exclusive formation of the cycloalkenyl
azide 5 having the cis-configuration from the cis-allylic
ester 4 revealed that this p-allylic azidation proceeds via a
double-inversion pathway (p-allylpalladium formation
and nucleophilic attack with an azide anion) in water un-
der these conditions. The tetrahydropyridyl carbonate 6
reacted with sodium azide under similar reaction condi-
tions to afford the tetrahydropyridyl azide 7 in 70% yield.
In summary, we have developed a practical protocol for
azidation of allylic esters with sodium azide using a PS-
PEG resin-supported palladium catalyst to give allylic
azides including cycloalkenyl derivatives. The p-allylic
azidation and the [3+2] triazole cycloaddition were
carried out in water without any organic solvents to meet
green chemical requirements.
Experimental: Palladium-Catalyzed Allylic Azidation (Table 1)
A typical procedure is given for the reaction of cinnamyl carbonate
2a (run 1). To a mixture of catalyst 1 (18.5 mg, 0.005 mmol) and
cinnamyl carbonate (2a, 96.0 mg, 0.5 mmol) in H2O (3.0 mL) was
added NaN3 (48.7 mg, 0.75 mmol) at 25 °C. The mixture was
shaken 25 °C for 24 h and filtered. The recovered resin beads were
extracted with EtOAc and methyl tert-butyl ether (MTBE). The
combined extracts were washed with aq NaCl and dried over anhyd
MgSO4. The solvent was evaporated and the residue was chromato-
graphed on silica gel (hexane–EtOAc, 10:1) to give 76.3 mg (96%
yield) of cinnamyl azide (3a).
Pd
1
(1 mol% Pd)
OCOOMe
N3
NaN3 (1.5 equiv)
25 °C, 24 h, in H2O
COOMe
COOMe
4
5 (87%)
PS-
Pd
PEG
–
N3
COOMe
Spectral and Analytical Data
Cinnamyl Azide (3a)
Pd
1
(5 mol% Pd)
OCOOMe
N3
CAS registry number: 76024-91-4. 1H NMR (CDCl3): d = 7.39 (d,
J = 7.3 Hz, 2 H), 7.32 (t, J = 7.3 Hz, 2 H), 7.26 (t, J = 7.3 Hz, 1H),
6.63 (d, J = 15.8 Hz 1 H), 6.23 (td, J = 6.7, 15.8 Hz, 1 H), 3.92 (d,
J = 6.7 Hz, 2 H). 13C NMR (CDCl3): d = 135.9, 134.4, 128.6, 128.1,
126.5, 122.3, 52.9.
NaN3 (1.5 equiv)
40 °C, 24 h, in H2O
N
N
COO
COO
7 (70%)
6
Scheme 2 Preparation of cycloalkenyl azides.
3-(o-Trifluoromethylphenyl)allyl Azide (3c)
CAS registry number: 164363-74-0. 1H NMR (CDCl3): d = 7.58 (d,
J = 7.9 Hz, 2 H), 7.48 (d, J = 7.9 Hz, 2 H), 6.67 (d, J = 15.8 Hz, 1
Recycling experiments were examined for azidation of
the cinnamyl carbonate (2a). After the first use of the
polymeric palladium catalyst 1 (Table 1, run 1) to give
87% yield of cinnamyl azide (3a), the recovered catalyst
beads were taken on to two reuses and exhibited stable
catalytic activity (Scheme 3).
H), 6.32 (td, J = 6.1, 15.8 Hz, 1 H), 3.98 (d, J = 6.1 Hz, 2 H). 13
C
NMR (CDCl3): d = 139.4, 132.3, 129.9 (q, J = 33.0 Hz), 126.7,
125.6 (q, J = 4.1 Hz), 125.2, 124.0 (q, J = 270 Hz), 52.6. MS (EI):
m/z (%) = 227 (2) [M+], 198 (64), 172 (bp), 151 (44). IR (ATR):
n = 2099, 1321 cm–1. Anal. Calcd for C10H9N3F3: C, 52.87; H, 3.55;
N, 18.50. Found: C, 52.72; H, 3.56; N, 18.38.
Pd
1
(5 mol% Pd)
3-(o-Tolyl)allyl Azide (3d)
1st use: 87% yield
3a 2nd use: 97% yield
3rd use: 92% yield
1H NMR (CDCl3): d = 7.29 (d, J = 8.0 Hz, 2 H), 7.14 (d, J = 8.0 Hz,
2 H), 6.61 (d, J = 15.8 Hz, 1 H), 6.18 (td, J = 6.7, 15.2 Hz, 1 H), 3.92
(d, J = 6.1 Hz, 2 H). 13C NMR (CDCl3): d = 138.1, 134.5, 133.2,
129.3, 126.5, 121.2, 53.1, 21.2.
2a
NaN3 (2a/NaN3 = 3:2)
25 °C, 6 h, in H2O
Synlett 2006, No. 13, 2109–2113 © Thieme Stuttgart · New York

2112
Y. Uozumi et al.
LETTER
3-(o-Methoxyphenyl)allyl Azide (3e)
References and Notes
CAS registry number: 164363-75-1. 1H NMR (CDCl3): d = 7.29 (d,
J = 8.0 Hz, 2 H), 7.14 (d, J = 8.0 Hz, 2 H), 6.61 (d, J = 15.8 Hz, 1
(1) For reviews, see: (a) Sandler, S. R.; Karo, W. Organic
Functional Group Preparations, 2nd ed.; Academic Press:
Orlando, 1986, 323–352. (b) The Chemistry of the Azido
Group; Patai, S., Ed.; Interscience Publishers (J. Wiley and
Sons): London, 1971.
H), 6.18 (td, J = 6.7, 15.2 Hz, 1 H), 3.92 (d, J = 6.1 Hz, 2 H). 13
NMR (CDCl3): d = 138.1, 134.5, 133.2, 129.3, 126.5, 121.2, 53.1,
21.2.
C
(2) For a review, see: Kolb, H. C.; Finn, M. G.; Sharpless, K. B.
Angew. Chem. Int. Ed. 2001, 40, 2004.
1,3-Diphenyl-2-propenyl Azide (3f)
CAS registry number: 120990-01-4. H NMR (CDCl3): d = 7.41–
1
(3) (a) Murahashi, S.-I.; Tanigawa, Y.; Imada, Y.; Taniguchi, Y.
Tetrahedron Lett. 1986, 27, 227. (b) Murahashi, S.-I.;
Taniguchi, Y.; Imada, Y.; Tanigawa, Y. J. Org. Chem. 1989,
54, 3292. (c) Blart, E.; Genêt, J. P.; Safi, M.; Savignac, M.;
Sinou, D. Tetrahedron 1994, 50, 505. (d) Deardorff, D. R.;
Taniguchi, C. M.; Tafti, S. A.; Kim, H. Y.; Choi, S. Y.;
Downey, K. J.; Nguyen, T. V. J. Org. Chem. 2001, 66,
7191. (e) Liao, M.-C.; Duan, X.-H.; Liang, Y.-M.
Tetrahedron Lett. 2005, 46, 3469.
7.23 (m, 10 H), 6.71 (d, J = 15.6 Hz, 1 H), 6.28 (dd, J = 15.6, 7.3
Hz, 1 H), 5.20 (d, J = 7.3 Hz, 1 H). 13C NMR (CDCl3): d = 138.6,
135.9, 133.0, 128.8, 128.7, 128.3, 128.2, 127.1, 126.9, 126.8, 67.2.
Geranyl Azide (3g)
CAS registry number: 84457-88-5. H NMR (CDCl3): d = 5.34–
1
5.31 (m, 1 H), 5.09–5.08 (m, 1 H), 3.76 (t, J = 7.9 Hz, 2 H), 2.13–
2.10 (m, 4 H), 1.70 (br s, 3 H), 1.68 (s, 3 H), 1.61 (s, 3 H). 13C NMR
(CDCl3): d = 123.5, 123.4, 117.9, 117.0, 48.0, 39.5, 32.7, 26.6, 26.3,
25.6.
(4) For reviews on aqueous-switching of organic
transformations, see: (a) Li, C.-J.; Chan, T.-H. Organic
Reactions in Aqueous Media; Wiley-VCH: New York,
1997. (b) Grieco, P. A. Organic Synthesis in Water; Kluwer
Academic Publishers: Dordrecht, 1997. (c) Herrmann, W.
A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993,
32, 1524. (d) Lindström, U. M. Chem. Rev. 2002, 102, 2751.
(5) For reviews on heterogeneous-switching of organic
transformations, see: (a) Bailey, D. C.; Langer, S. H. Chem.
Rev. 1981, 81, 109. (b) Shuttleworth, S. J.; Allin, S. M.;
Sharma, P. K. Synthesis 1997, 1217. (c) Shuttleworth, S. J.;
Allin, S. M.; Wilson, R. D.; Nasturica, D. Synthesis 2000,
1035. (d) Dörwald, F. Z. Organic Synthesis on Solid Phase;
Wiley-VCH: Weinheim, 2000. (e) Leadbeater, N. E.;
Marco, M. Chem. Rev. 2002, 102, 3217. (f) Ley, S. V.;
Baxendale, I. R.; Bream, R. N.; Jackson, P. S.; Leach, A. G.;
Longbottom, D. A.; Nesi, M.; Scott, J. S.; Storer, R. I.;
Taylor, S. J. J. Chem. Soc., Perkin Trans. 1 2000, 3815.
(g) McNamara, C. A.; Dixon, M. J.; Bradley, M. Chem. Rev.
2002, 102, 3275. (h) Chiral Catalyst Immobilization and
Recycling; De Vos, D. E.; Vankelecom, I. F. J.; Jacobs, P.
A., Eds.; Wiley-VCH: Weinheim, 2000. (i) Fan, Q.-H.; Li,
Y.-M.; Chan, A. S. C. Chem. Rev. 2002, 102, 3385.
(6) For a recent review of solid-phase reactions using palladium
catalysts, see: (a) Uozumi, Y.; Hayashi, T. Solid-Phase
Palladium Catalysis for High-Throughput Organic
Synthesis, In Handbook of Combinatorial Chemistry;
Nicolaou, K. C.; Hanko, R.; Hartwig, W., Eds.; Wiley-VCH:
Weinheim, 2002, Chap. 19. (b) Uozumi, Y. Top. Curr.
Chem. 2004, 242, 77.
Linaryl Azide (3i)
CAS registry number: 128318-87-6. H NMR (CDCl3): d = 5.78
(dd, J = 17.9, 10.9 Hz, 1 H), 5.23 (d, J = 17.9 Hz, 1 H), 5.20 (d,
J = 10.9 Hz, 1 H), 5.09–5.08 (m, 1 H), 2.07 (m, 4 H), 1.79–1.35 (m,
9 H).
1
cis-3-Methoxycarbonyl-2-cyclohexenyl Azide (5)
CAS registry number: 120990-27-4. H NMR (CDCl3): d = 5.93–
5.91 (m, 1 H), 5.67 (d, J = 10.3 Hz, 1 H), 3.99 (br s, 1 H), 3.72 (s, 3
H), 2.67 (dddd, J = 12.2, 9.7, 6.4, 2.4 Hz, 1 H), 2.42–2.24 (m, 3 H),
1.72 (ddd, J = 12.2, 12.2, 10.9 Hz, 1 H). 13C NMR (CDCl3): d =
174.4, 129.5, 125.8, 56.9, 51.9, 38.1, 30.7, 27.1.
1
tert-Butyl 3-Azido-3,6-dihydropyridine-1(2H)-carboxylate (7)
1
CAS N/A. H NMR (CDCl3, 60 °C): d = 6.02 (br d, J = 9.7 Hz, 1
H), 5.86–5.84 (m, 1 H), 4.08 (br d, J = 18.5 Hz, 1 H), 3.80–3.73 (m,
3 H), 3.54–3.51 (m, 1 H), 1.48 (s, 9 H). 13C NMR (CDCl3, –60 °C,
mixture of rotamers): d = 154.6, 154,2, 130.7, 130.0, 121.7, 121.4,
80.4, 80.3, 53.1, 53.0, 45.8, 44.2, 43.2, 42.2, 28.1, 28.1. MS (EI):
m/z (%) = 196 (0.8) [M+ – Me2], 140 (7), 94 (80), 41 (bp). IR (ATR):
n = 2976, 2931, 2359, 2341, 2094, 1695 cm–1. Anal. Calcd for
C10H16N4O2: C, 53.56; H, 7.19; N, 24.98. Found: C, 53.35; H, 7.16;
N, 24.65.
Compound 8 ([3+2] Addition of Cinnamyl azide (3a) with
Dimethyl Acetylenecarboxylate; Scheme 4)
To a mixture of cinnamyl azide (3a, 63.6 mg, 0.4 mmol) in H2O (3
mL) was added dimethyl acetylenecarboxylate (71.1 mg, 0.5 mmol)
and the mixture was stirred at 100 °C for 1 h. The reaction mixture
was extracted with MTBA and the extract was dried over MgSO4,
filtered and concentrated to afford a yellow residue. Purification by
silica gel chromatography (hexane–EtOAc, 1:1) gave 105 mg (84%
(7) For studies on organic p-allylic transformations with
polymer-supported complex catalysts in water, see:
(a) Uozumi, Y.; Danjo, H.; Hayashi, T. Tetrahedron Lett.
1997, 38, 3557. (b) Uozumi, Y.; Danjo, H.; Hayashi, T.
Tetrahedron Lett. 1998, 39, 8303. (c) Danjo, H.; Tanaka,
D.; Hayashi, T.; Uozumi, Y. Tetrahedron 1999, 55, 14341.
(d) Uozumi, Y.; Shibatomi, K. J. Am. Chem. Soc. 2001, 123,
2919. (e) Uozumi, Y.; Tanaka, H.; Shibatomi, K. Org. Lett.
2004, 6, 281. (f) Hocke, H.; Uozumi, Y. Tetrahedron 2004,
60, 9297. (g) Nakai, Y.; Uozumi, Y. Org. Lett. 2005, 7, 291.
(h) Uozumi, Y.; Kimura, M. Tetrahedron: Asymmetry 2006,
17, 161.
1
yield) of 1-cinnamyl-4,5-dimethoxycarbonyltriazole (8). H NMR
(CDCl3): d = 7.36–7.27 (m, 5 H), 6.65 (d, J = 15.8 Hz, 1 H), 6.29
(td, J = 6.1, 15.8 Hz, 2 H), 5.38 (d, J = 6.1 Hz, 1 H), 3.97 (s, 3 H),
3.95 (s, 3 H). 13C NMR (CDCl3): d = 160.3, 158.8, 139.8, 135.8,
135.2, 129.7, 128.5, 128.4, 126.5, 120.9, 53.2, 52.5, 52.3. MS (EI):
m/z (%) = 301 (1.7) [M+], 183 (32), 115 (bp), 91 (44). IR (ATR):
n = 2360, 2341, 1729, 1219 cm–1. FAB-HRMS: m/e calcd for
C15H15N3O4: 302.1062; found: 302.1144 [M+ +1].
(8) For examples of other processes with polymer-supported
catalysts in water, see: (a) Cross-coupling: Uozumi, Y.;
Danjo, H.; Hayashi, T. J. Org. Chem. 1999, 64, 3384.
(b) Carbonylation reaction: Uozumi, Y.; Watanabe, T. J.
Org. Chem. 1999, 64, 6921. (c) Michael addition:
Shibatomi, K.; Nakahashi, T.; Uozumi, Y. Synlett 2000,
1643. (d) Suzuki–Miyaura coupling: Uozumi, Y.; Nakai, Y.
Org. Lett. 2002, 4, 2997. (e) Heck reaction: Uozumi, Y.;
Acknowledgment
This work was supported by the CREST program, sponsored by
the JST. We also thank the JSPS (GRANT-in-AID for Scientific
Research, No.15205015) and the MEXT (Scientific Research on
Priority Areas, No. 420) for partial financial support of this work.
Synlett 2006, No. 13, 2109–2113 © Thieme Stuttgart · New York

LETTER
Kimura, T. Synlett 2002, 2045. (f) Rhodium catalysis:
p-Allylic Azidation in Water
2113
2003, 42, 194; Angew. Chem. 2003, 115, 204. (k) Suzuki–
Miyaura coupling: Uozumi, Y.; Kikuchi, M. Synlett 2005,
1775. (l) Reduction: Nakao, R.; Rhee, H.; Uozumi, Y. Org.
Lett. 2005, 7, 163. (m) Alkylation: Yamada, Y. M. A.;
Uozumi, Y. Org. Lett. 2006, 1375.
Uozumi, Y.; Nakazono, M. Adv. Synth. Catal. 2002, 344,
274. (g) Wacker cyclization: Hocke, H.; Uozumi, Y. Synlett
2002, 2049. (h) See also: Hocke, H.; Uozumi, Y.
Tetrahedron 2003, 59, 619. (i) Sonogashira reaction:
Uozumi, Y.; Kobayashi, Y. Heterocycles 2003, 59, 71.
(j) Oxidation: Uozumi, Y.; Nakao, R. Angew. Chem. Int. Ed.
(9) One preliminary example (Table 1, run 6) has been reported
in ref. 7c.
Synlett 2006, No. 13, 2109–2113 © Thieme Stuttgart · New York