Chiral calcium organophosphate-catalyzed enantioselective electrophilic amination of enamides

Article abstract of DOI:10.1021/ol102625s

Highly enantioselective direct amination of enamides catalyzed by chiral nonracemic calcium bis(phosphate) complex 3g afforded optically active 1,2-hydrazinoimines 4. Following a subsequent in situ hydrolysis or reduction, 2-hydrazinoketones 5 or syn-1,2-

Full text of DOI:10.1021/ol102625s

ORGANIC
LETTERS
2011
Vol. 13, No. 1
94-97
Chiral Calcium
Organophosphate-Catalyzed
Enantioselective Electrophilic Amination
of Enamides
Fleur Drouet,† Claudia Lalli,† Hua Liu,† Ge´raldine Masson,*,† and Jieping Zhu*,†,‡
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS,
91198 Gif-sur-YVette Cedex, France, Institute of Chemical Sciences and Engineering,
Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), EPFL-SB-ISIC-LSPN,
CH-1015 Lausanne, Switzerland
masson@icsn.cnrs-gif.fr; jieping.zhu@epfl.ch
Received October 29, 2010
ABSTRACT
Highly enantioselective direct amination of enamides catalyzed by chiral nonracemic calcium bis(phosphate) complex 3g afforded optically
active 1,2-hydrazinoimines 4. Following a subsequent in situ hydrolysis or reduction, 2-hydrazinoketones 5 or syn-1,2-disubstituted 1,2-
diamines 6 were obtained in high yields and excellent enantiomeric excess.
The importance of enamides (enecarbamates)1,2 as useful
nucleophiles in enantioselective addition reactions has been
growing enormously ever since Kobayashi’s seminal report
in 2004.2b The Lewis acid and Brønsted acid catalyzed
enantioselective C-C bond forming processes involving
enamides and carbon-centered electrophiles2-6 are now
well established. However, reports on nucleophilic addition
of enamides to electrophilic nitrogen atom remained scarce,
in sharp contrast to the enamine chemistry.7-11 Kobayashi
et al. reported the first examples of enantioselective amination
of (E)-enecarbamates using chiral diamines-Cu(OTf)2 com-
plexes as catalysts.4d,12 Very recently, Feng et al. detailed a
chiral N,N-dioxide-Cu(OTf)2 complex-catalyzed asymmetric
R-amination of (Z)-enamides.13 To the best of our knowl-
edge, no example of Brønsted acid catalyzed asymmetric
R-aminations of enamides (enecarbamates) with azodicar-
boxylates has been described.14 In connection with our
studies on small organic molecule-catalyzed transformation
(3) For selected examples with imines as electrophiles, see: (a) Mat-
subara, R.; Nakamura, Y.; Kobayashi, S. Angew. Chem., Int. Ed. 2004, 43,
1679. (b) Kiyohara, H.; Matsubara, R.; Kobayashi, S. Org. Lett. 2006, 8,
5333. (c) Terada, M.; Machioka, K.; Sorimachi, K. Angew. Chem., Int. Ed.
2006, 45, 2254. (d) Matsubara, R.; Doko, T.; Uetake, R.; Kobayashi, S.
Angew. Chem., Int. Ed. 2007, 46, 3047. (e) Terada, K.; Machioka, K.;
Sorimachi, K. J. Am. Chem. Soc. 2007, 129, 10336. (f) Baudequin, C.;
Zamfir, A.; Tsogoeva, S. B. Chem. Commun. 2008, 4637. (g) Terada, M.;
Machioka, K.; Sorimachi, K. Angew. Chem., Int. Ed. 2009, 48, 2553. (h)
Liu, H.; Dagousset, G.; Masson, G.; Retailleau, P.; Zhu, J. J. Am. Chem.
Soc. 2009, 131, 4598. (i) Dagousset, G.; Drouet, F.; Masson, G.; Zhu, J.
Org. Lett. 2009, 11, 5546.
† Centre de Recherche de Gif, Institut de Chimie des Substances
Naturelles.
‡ Institute of Chemical Sciences and Engineering, Ecole Polytechnique
Fe´de´rale de Lausanne.
(1) For review see: (a) Evano, G.; Blanchard, N.; Toumi, M. Chem.
ReV. 2008, 108, 3054. (b) Nugenta, T. C.; El-Shazlya, M. AdV. Synth. Catal.
2010, 352, 753
.
(2) (a) Carbery, D. R. Org. Biomol. Chem. 2008, 6, 3455. (b) Matsubara,
R.; Kobayashi, S. Acc. Chem. Res. 2008, 41, 292
.
10.1021/ol102625s  2011 American Chemical Society
Published on Web 12/01/2010

of enecarbamates3g,h and our ongoing project on catalytic
asymmetric synthesis,15 we were interested in examining a
chiral phosphoric acid-catalyzed amination of enamides using
azodicarboxylate as an electrophilic partner.
to provide an asymmetric environment for the desired
enantioselective amination process (Scheme 1).
Chiral BINOL-derived phosphoric acids, pioneered by
Akiyama et al. and Terada et al., are now well-established
bifunctional organocatalysts that are particularly effective
in catalyzing the addition of nucleophiles to imines.16,17
Therefore, we reasoned that chiral phosphoric acids might
be able to activate both enamides 1 and azodicarboxylates 2
Scheme 1. Catalytic Enantioselective R-Amination of Enamides
(4) For selected examples with aldehydes and ketones as electrophiles,
see: (a) Matsubara, R.; Nakamura, Y.; Kobayashi, S. Angew. Chem., Int.
Ed. 2004, 43, 3258. (b) Fossey, J. S.; Matsubara, R.; Vital, P.; Kobayashi,
S. Org. Biomol. Chem. 2005, 3, 2910. (c) Matsubara, R.; Kawai, N.;
Kobayashi, S. Angew. Chem., Int. Ed. 2006, 45, 3814. (d) Terada, M.; Soga,
K.; Momiyama, N. Angew. Chem., Int. Ed. 2008, 47, 4122. (e) Yang, L.;
Wang, D.-X.; Huang, Z.-T.; Wang, M. X. J. Am. Chem. Soc. 2009, 131,
10390.
To validate our hypothesis, we initially examined the
reaction of (E)-N-(1-phenylprop-1-en-1-yl)acetamide (1a)
with diisopropyl azodicarboxylate (2a) in the presence of
10 mol % of chiral phosphoric acid 3a in DCM at -35 °C.
This resulted in the formation of the desired 1,2-hydrazi-
noimine 4a, accompanied by the 1,2-hydrazinoketone 5a.
Although addition of molecular sieves in the reaction
prevented the hydrolysis of the unstable N-acylimine 4a, the
enantioselectivities and yields were determined for compound
5a, obtained by in situ hydrolysis of 4a under acidic
conditions (EtOH and 33% HBr in AcOH, v/v ) 1/10).
Phosphoric acids with different steric environment (3a-f,
Figure 1) were next examined. It was found that the less
(5) For selected examples with Michael acceptors as electrophiles, see:
(a) Berthiol, F.; Matsubara, R.; Kawai, N.; Kobayashi, S. Angew. Chem.,
Int. Ed. 2007, 46, 7803. (b) Hayashi, Y.; Gotoh, H.; Masui, R.; Ishikawa,
H. Angew. Chem., Int. Ed. 2010, 47, 4012. (c) Zu, L.; Xie, H.; Li, H.;
Wang, J.; Yu, X.; Wang, W. Chem.sEur. J. 2008, 14, 6333.
(6) Guo, Q.-X.; Peng, Y.-G.; Zhang, J.-W.; Song, L.; Feng, Z.; Gong,
L.-Z. Org. Lett. 2009, 11, 4620.
(7) For selected pioneering work on asymetric R-amination using
azodicarboxylate as electrophilic nitrogen, see: Evans, D. A.; Nelson, S. G.
J. Am. Chem. Soc. 1997, 119, 6452.
(8) For general reviews of R-amination, see: (a) Bøgevig, A.; Juhl, K.;
Kumaragurubaran, N.; Zhuang, W.; Jørgensen, K. A. Angew. Chem., Int.
Ed. 2002, 41, 1790. (b) Erdik, E. Tetrahedron 2004, 60, 8747. (c) Greck,
C.; Drouillat, B.; Thomassigny, C. Eur. J. Org. Chem. 2004, 1377. (d)
Marigo, M.; Jørgensen, K. A. Chem. Commun. 2006, 2001. (e) Na´jera, C.;
Sansano, J. M. Chem. ReV. 2007, 107, 4584. (f) Vilaivan, T.; Bhanthmnavin,
W. Molecules 2010, 15, 917.
(9) For selected reviews on enamine catalysis, see: (a) Dalko, P. I.
EnantioselectiVe Organocatalysis; Wiley-VCH: Weinheim, 2007. (b)
Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. ReV. 2007, 107,
5471. (c) MacMillan, D. W. C. Nature 2008, 455, 304. (d) Melchiorre, P.;
Marigo, M.; Carlone, A.; Bartoli, G. Angew. Chem., Int. Ed. 2008, 47, 6138.
(e) Bertelsen, S.; Jørgensen, K. A. Chem. Soc. ReV. 2009, 38, 2178.
(10) For selected examples of proline-catalyzed R-amination of aldehydes
and ketones with dialkylazodicarboxylate, see: (a) List, B. J. Am. Chem.
Soc. 2002, 124, 5656. (b) Kumaragurubaran, N.; Juhl, K.; Zhuang, W.;
Bøgevig, A.; Jørgensen, K. A. J. Am. Chem. Soc. 2002, 124, 6254. (c) Vogt,
H.; Vanderheriden, S.; Bro¨se, S. Chem. Commun. 2003, 2448. (d) Chowdari,
N. S.; Ramachary, D. B.; Barbas, C. F., III Org. Lett. 2003, 5, 1685. (e)
Dahlin, N.; Bøgevig, A.; Adolfsson, H. AdV. Synth. Catal. 2004, 346, 1101.
(f) Suri, J. T.; Steiner, D.; Barbas, C. F., III Org. Lett. 2005, 7, 3885. (g)
Franze´n, J.; Marigo, M.; Fielenbach, D.; Wabnitz, T.; Kjærsgaard, A.;
Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 18296. (h) Baumann, T.;
Vogt, H.; Bra¨se, S. Eur. J. Org. Chem. 2007, 266. (i) Kotkar, S. P.; Chavan,
V. B.; Sudalai, A. Org. Lett. 2007, 9, 1001. (j) Dine´r, P.; Kjærsgaard, A.;
Lie, M. A.; Jørgensen, K. A. Chem.sEur. J. 2008, 14, 122. (k) Hayashi,
Y.; Aratake, S.; Imai, Y.; Hibino, K.; Chen, Q.-Y.; Yamaguchi, J.; Uchimaru,
T. Chem. Asian J. 2008, 3, 225. (l) Baumann, T.; Ba¨chle, M.; Hartmann,
C.; Bra¨se, S. Eur. J. Org. Chem. 2008, 2207. (m) Liu, P.-M.; Magar, D. R.;
Chen, K. Eur. J. Org. Chem. 2010, 5705.
Figure 1. List of phosphoric acids examined.
hindered phosphoric acid 3a was the most effective in terms
of both yield and ee of the product (entries 2-7, Table 1).
During this study we noticed that enantioselectivities (from
78% to 89%) and yields (from 45% to quantitative) under
the optimal conditions varied considerably depending on the
batch of 3a used. Following Ding’s observation,18 3a washed
with HCl was used as catalyst that led indeed to the
(11) For selected examples of Cinchona-catalyzed R-amination of
ketones with dialkylazodicarboxylate, see: Liu, T.-Y.; Cui, H.-L.; Zhang,
Y.; Jiang, K.; Du, W.; He, Z.-Q.; Chen, Y. C. Org. Lett. 2007, 9, 3671.
(12) Matsubara, R.; Kobayashi, S. Angew. Chem., Int. Ed. 2006, 45,
7993.
(13) Chang, L.; Kuang, Y.; Qin, B.; Zhou, X.; Liu, X.; Lin, L.; Feng,
X. Org. Lett. 2010, 12, 2214.
(14) During preparation of this manuscript, Zhong reported a chiral
phosphoric acid-catalyzed asymmetric R-aminoxylation of enecarbamates;
see: Lu, M.; Lu, Y.; Zhu, D.; Zeng, X.; Li, X.; Zhong, G. Angew. Chem.,
Int. Ed. 2010, 49, 8588.
(16) For recent reviews on chiral phosphoric acid catalysis, see: (a)
Akiyama, T.; Itoh, J.; Fuchibe, K. AdV. Synth. Catal. 2006, 348, 999. (b)
Akiyama, T. Chem. ReV. 2007, 107, 5744. (c) Terada, M. Chem. Commun.
2008, 4097. (d) Terada, M. Synthesis 2010, 1929.
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2007, 9, 3615. (b) Wang, S.-X.; Wang, M.-X.; Wang, D.-X.; Zhu, J. Eur.
J. Org. Chem. 2007, 4076. (c) Wang, S.-X.; Wang, M.-X.; Wang, D.-X.;
Zhu, J. Angew. Chem., Int. Ed. 2008, 47, 388. (d) Yue, T.; Wang, M.-X.;
Wang, D.-X.; Zhu, J. Angew. Chem., Int. Ed. 2008, 47, 9554. (e) Yue, T.;
Wang, M.-X.; Wang, D.-X.; Masson, G.; Zhu, J. Angew. Chem., Int. Ed.
2009, 48, 6717.
(17) For recent reviews on Brønsted acid catalysis, see: (a) Doyle, A. G.;
Jacobsen, E. N. Chem. ReV. 2007, 107, 5713. (b) Yu, X.; Wang, W. Chem.
Asian J. 2008, 3, 516. (c) Kampen, D.; Reisinger, C. M.; List, B. Top.
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Int. Ed. 2008, 47, 2840.
Org. Lett., Vol. 13, No. 1, 2011
95

produced whether 3a or 3g was used as a catalyst and the
absolute configuration of 5a was determined to be (S) by
comparison of the sign of its optical rotation with that of
the literature data.13
Table 1. Synthesis of 1,2-Hydrazinoketones: A Survey of
Reaction Conditions
We have also examined the effect of the alkene geometry
of enamides on the reaction outcome. It was observed that
the (Z)-isomer 1b (entry 11, Table 1) was much less reactive
and no reaction took place at -35 °C. Performing the
reaction at room temperature afforded the desired product
5a in low yield with no enantioselectivity. These results are
therefore complementary to Feng’s catalytic system wherein
the (Z)-enamides were the preferred substrates for the similar
amination reaction.13
yield of yield of
entry
1
additive
3
4a (%)e 5a (%)e ee (%)f,g
1a
E-1a
E-1a
E-1a
E-1a
E-1a
E-1a
E-1a
E-1a
E-1a
E-1a
Z-1a
no
3ac
3ac
3bc
3cc
3dc
3ec
3fc
35
29 (5a)
99 (5a)
56 (5a)
66 (5a)
57 (5a)
44 (5a)
59 (5a)
92 (5a)
75 (5a)
91 (5a)
23 (5a)
80
89
71
18
8
88
78
85h
95
89
0
2b
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
MS4Å
3b
4b
Having identified the optimum conditions, we explored
the reaction scope using different (E)-N-(1-arylprop-1-en-
1-yl)acetamides 1. Results are summarized in Table 2.
5b
6b
7b
8b
3ad
3g
9b
10b
11b
3h
3g
a Experimental conditions: 1a/2a/3 ) 1.0/5.0/0.1 in CH2Cl2 (c ) 0.1)
at -35 °C. b General conditions: 1a/2a/3 ) 1.0/5.0/0.1 in CH2Cl2 (c )
0.1) at -35 °C followed by acid hydrolysis with EtOH and 33% HBr in
AcOH (v:v ) 1/10). c Purified on silica gel. d Washed with HCl after
purification on silica gel. e Yields refer to chromatographically pure products.
f Enantiomeric excess was determined by chiral HPLC analysis. g For the
determination of the absolute configuration, see the Supporting Information.
h An additional experiment was performed with 20 mol % of phosphoric
acid 3a leading to 5a in similar yield and ee.
Table 2. Scope of the Enantioselective Ca-Phosphate-Catalyzed
Synthesis of 1,2-Hydrazinoketones.a
entry
R1
R2
5
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
3-Cl
4-Cl
3-F
4-Br
4-Me
4-OMe
4-CF3
H
Me
Me
Me
Me
Me
Me
Me
Et
5b
5c
5d
5e
5f
5g
5h
5i
80
97
77
76
83
73
75
91
94
90
85
90
93
88
91
93
89d
94
reproducible ee values and yields. However, a slightly lower
enantioselectivity was obtained with this acid-washed catalyst
(entry 8). Inspired by the work of Ishihara,19,20 the catalytic
efficiency of chiral calcium phosphates 3g and 3h derived
from 3a and 3f, respectively, were tested (entries 9 and 10,
Table 1). Gratefully, with catalyst 3g, compound 5a was
produced in 75% yield with 95% ee and the result was
perfectly reproducible.21-24 The same enantiomer of 5a was
H
n-Pr
5j
a General conditions: 1a/2a/3 ) 1.1/5.0/0.1 in CH2Cl2 (c ) 0.1) at -35
°C followed by acid hydrolysis with EtOH and 33% HBr in AcOH (v/v )
1/10). b Yields refer to chromatographically pure products. c Enantiomeric
excess was determined by chiral HPLC analysis. d With 10 mol % of 3a.
(19) (a) Hatano, M.; Moriyama, K.; Maki, T.; Ishihara, K. Angew. Chem.,
Int. Ed. 2010, 49, 3823. (b) Klussmann, M.; Ratjen, L.; Hoffmann, S.;
Wakchaure, V.; Goddard, R.; List, B. Synlett 2010, 2189. (c) For an example
of phosphoramide/calcium complex, see: Rueping, M.; Theissmann, T.;
Kuenkel, A.; Koenigs, R. M. Angew. Chem., Int. Ed. 2008, 47, 6798
.
Enamides bearing electron-neutral, -rich, and -poor aromatic
substituents at R-position react smoothly with 2 to give, after
in situ hydrolysis, 2-hydrazinoketones 5 in good yields
(75-97%) and excellent enantioselectivities (85-97% ee).
In addition, enamide having a longer alkyl chain at the
ꢀ-position, which gave low enantioselectivity in a previous
report,13 are suitable substrates (entries 8 and 9). With (E)-
N-(1-phenylpent-1-en-1-yl)acetamides (1j, R1 ) H, R2 )
n-Pr), the corresponding hydrazinoketone 5j was obtained
in 94% yield with 97% ee.
With this transformation in hand, we next turned our
attention to the in situ reduction of hydrazinoimines to 1,2-
diamines which are very useful chiral ligands, auxiliaries,
and building blocks in the synthesis of natural and bioactive
products.25 Reduction of 4a with NaBH4 (-78 to -45 °C,
MeOH) according to Kobayashi et al. provided the syn-1,2-
(20) (a) Hatano, M.; Ikeno, T.; Matsumura, T.; Torii, S.; Ishihara, K.
AdV. Synth. Catal. 2008, 350, 1776. For a full account, see: (b) Hatano,
M.; Ishihara, K. Synthesis 2010, 3785
.
(21) For selected examples of phosphate ligands, see: (a) Suzuki, S.;
Furono, H.; Yokoyama, Y.; Inanaga, J. Tetrahedron: Asymmetry 2006, 17,
504. (b) Yue, T.; Wang, M.-X.; Wang, D.-X.; Masson, G.; Zhu, J. J. Org.
Chem. 2009, 74, 8396. (c) Yang, L.; Zhu, Q.; Guo, S.; Qian, B.; Xia, C.;
Huang, H. Chem.sEur. J. 2010, 16, 1638
.
(22) For selected examples using combined chiral Brønsted acids and
transition metals, see: (a) Komanduri, V.; Krische, M. J. J. Am. Chem. Soc.
2006, 128, 16448. (b) Hamilton, G. L.; Kang, E. J.; Mba, M.; Toste, F. D.
Science 2007, 317, 496. (c) Mukherjee, S.; List, B. J. Am. Chem. Soc. 2007,
129, 11336. (d) Hu, W.; Xu, X.; Zhou, J.; Liu, W.-J.; Huang, H.; Hu, J.;
Yang, L.; Gong, L.-Z. J. Am. Chem. Soc. 2008, 130, 7782. (e) Li, C.; Wang,
C.; Villa-Marcos, B.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 14450. (f) Xu,
X.; Zhou, J.; Yang, L.; Hu, W. Chem. Commun. 2008, 6564. (g) Guo, Z.;
Shi, T.; Jiang, J.; Yang, L.; Hu, W. Org. Biomol. Chem. 2009, 5028. (h)
Li, C.; Villa-Marcos, B.; Xiao, J. J. Am. Chem. Soc. 2009, 131, 6967. (i)
Tetrade, M.; Toda, Y. J. Am. Chem. Soc. 2009, 131, 6354. (j) Lu, Y.;
Johnstone, T. C.; Arndtsen, B. A. J. Am. Chem. Soc. 2009, 131, 11284. (k)
Klussmann, M. Angew. Chem., Int. Ed. 2009, 48, 7124. (l) Liu, X.-Y.; Che,
C.-M. Org. Lett. 2009, 11, 4204. (m) Garcia-Garcia, P.; Lay, F.; Garcia-
Garcia, P.; Rabalakos, C.; List, B. Angew. Chem., Int. Ed. 2009, 48, 4363.
(n) Shen, K.; Liu, X.; Cai, Y.; Lin, L.; Feng, X. Chem.sEur. J. 2009, 15,
6008. (o) Yazaki, R.; Kumagai, N.; Shibasaki, M. J. Am. Chem. Soc. 2010,
(23) The combination of chiral Brønsted acid and MgF2: Lv, J.; Li, X.;
132, 10275
.
Zhong, L.; Luo, S.; Cheng, J.-P. Org. Lett. 2010, 12, 1096
.
96
Org. Lett., Vol. 13, No. 1, 2011

diamine26 6a in high diastereoselectivity (>95:5 dr).27 This
reduction step was found to be compatible with the amination
process. Indeed, a one-pot amination/reduction process
furnished syn-1,2-diamine 6a in 84% yield with 92% ee
(entry 1, Table 3). Other representative chiral 1,2-diamines
Scheme 2. Ternary Complex, a Hypothetic Intermediate
Table 3. Scope of the Enantioselective Ca-Phosphate-Catalyzed
One-Step Synthesis of 1,2-Hydrazinoaminesa
entry
R1
R2
6
yield (%)b
ee (%)c
1
2
3
4
H
Me
Me
Me
n-Pr
6a
6b
6c
6d
84
88
84
99
92
88
93
96
3-F
4-CF3
H
of enamide 1 and azodicarboxylate 2 to 3g could generate a
monometallic complex with concurrent formation of an
intermediate of type 9.19a,28 A pseudointramolecular Si-face
attack of enamide 1 onto azodicarboxylate 2 would then take
place to afford the observed (S)-hydrazinoimines 4 (Scheme
2).
a General conditions: 1a/2a/3 ) 1.0/5.0/0.1 in CH2Cl2 (c ) 0.1) at -35
°C followed by addition of NaBH4 in MeOH. b Yields refer to chromato-
graphically pure products. c Enantiomeric excess was determined by chiral
HPLC analysis.
In summary, we demonstrated that a chiral nonracemic
calcium bis(phosphate) complex 3g is capable of catalyzing
an enantioselective nucleophilic addition of enamides to
azodicarboxylate providing chiral enantio-enriched 1,2-
hydrazinoimines. Subsequent in situ hydrolysis or diaste-
reoselective reduction of imine function led to 2-hydrazi-
noketones and 1,2 diamines, respectively, in excellent yields
and enantioselectivities. Further investigation to understand
the reaction mechanism is underway.
prepared were enlisted in Table 3. In general, diamines 6
were isolated in slightly higher yields than hydrazinoketones
5. As expected, the ee of products 5 and 6 obtained by these
two different one-pot processes were almost identical,
reflecting directly the enantioselectivity of the amination step.
The precise structure of the catalyst 3g is actually not
known; however, we believe that divalent-Ca is able to react
with Brønsted acids to form an oligomeric calcium complex
as predicted by Ishihara et al.18,28-30 The observed 31P NMR
chemical shift of 3g agreed well with the chemical shifts
observed for the oligomeric form reported previously (see
the Supporting Information).19a Moreover, the catalyst 3a
gave a MALDI-TOF spectrum that exhibits [M + H] peaks
at m/z 1039 and 2077, corresponding to monometallic and
dimetallic species 7 and 8, respectively (Scheme 2). In light
of Ishihara’s recent observation, we assumed that addition
Acknowledgment. Financial support from CNRS is grate-
fully acknowledged. F.D., C.L., and H.L. thank ICSN for
doctoral and postdoctoral fellowships.
Supporting Information Available: Catalysis optimiza-
tion, spectroscopic data, and ee measurement. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL102625S
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