Quinine-catalyzed enantioselective michael addition of diphenyl phosphite to nitroolefins: Synthesis of chiral precursors of α-substituted β-aminophosphonates

Article abstract of DOI:10.1002/adsc.200600429

A quinine-promoted, enantioselective Michael addition reaction of diphenyl phosphite with nitroalkenes has been developed. This methodology affords a facile access to enantiomerically enriched β-nitrophosphates, precursors for the preparation of synthetic

Full text of DOI:10.1002/adsc.200600429

COMMUNICATIONS
DOI: 10.1002/adsc.200600429
Quinine-Catalyzed Enantioselective Michael Addition of
Diphenyl Phosphite to Nitroolefins: Synthesis of Chiral
Precursors of a-Substituted b-Aminophosphonates
Jian Wang,a Lora D. Heikkinen,a Hao Li,a Liansuo Zu,a Wei Jiang,a Hexin Xie,a
and Wei Wanga,*
a
Department of Chemistry, University of New Mexico, MSC03 2060, Albuquerque, NM 87131-0001, USA
Fax : (+1)-505-277-2609; e-mail: wwang@unm.edu
Received: August 22, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A quinine-promoted, enantioselective Mi-
chael addition reaction of diphenyl phosphite with
nitroalkenes has been developed. This methodology
affords a facile access to enantiomerically enriched
b-nitrophosphates, precursors for the preparation of
synthetically and biologically useful b-amino-
phosphonates.
The Michael addition of phosphorus compounds to
nitroolefins is a convenient method for synthesis of b-
nitrophosphonates, precursors for preparation of b-
aminophosphonic acids. The non-asymmetric conjuga-
tion addition processes were first reported by Pudovik
et al.[8] Yoshimura described an improved procedure
for the Michael addition reaction by employing di-
alkyl phosphites as Michael donors.[9] The asymmetric
version of the process was recently disclosed by
Enders and co-workers using TADDOL as chiral aux-
iliary for stereocontrol.[10] However, a catalytic, enan-
tioselective approach for the process has not yet been
reported. The study we present here showed that it
was possible to develop such a process in the presence
Keywords: amino acids; asymmetric catalysis; Mi-
chael addition; organic catalysis
In the recent past, the enantioselective synthesis of a- of an organocatalyst.[11,12]
and b-aminophosphonic acids has received considera-
In the exploratory study, 10 bifunctional organoca-
ble attention as a result of their increasing applica- talysts, including l-proline I,[13] l-pyrrolidinol II,
tions in peptide and medicinal chemistry.[1,2] They (1R,2S)-ephedrine III, amine thioureas IV[14] and V,[15]
serve as the surrogates of a- and b-amino acids in Cinchona alkaloids VI–X,[16] were screened for their
peptides and peptidomimetics with significantly im- catalytic ability to promote the Michael addition reac-
proved bioactivities and stabilities and as therapeutic tion of diphenyl phosphite 1a with trans-b-nitrostyr-
agents with a broad spectrum of biological activi- ene 2a (Figure 1 and Table 1) since they had been
ties.[1,2] A great deal of effort has been directed demonstrated for the activation of nitroolefins in or-
toward the development of asymmetric methods for ganocatalysis. The initial reactions were performed by
synthesis of a-aminophosphonic acids.[1,3] In contrast, using 10 mol% of the catalyst at room tmperature in
the approaches for enantioselective synthesis of b- Et2O. Examination of the results revealed that their
aminophosphonic acids are sparse.[2] Among the small catalytic activities varied significantly (Table 1). For
number of non-racemic strategies described thus far example, processes promoted by l-pyrrolidinol II and
are those relying on the use of chiral precursors[4] or (1R,2S)-ephedrine III exhibited good activity (1 h and
auxiliaries,[5] and enzymatic resolutions.[6] To our 1.5 h, respectively), but with low enantioselectivities
knowledge, only one single enantioselective, catalytic (18% and 0% ee, respectively, Table 1, entries 2 and
method has been disclosed using the Sharpless asym- 3). Under the same reaction conditions, poor results
metric aminohydroxylation of a,b-unsaturated phos- were obtained for l-proline I and amine thioureas IV
phonates.[7] Herein, we report a new organocatalytic, and V (entries 1, 4 and 5). A promising result came
enantioselective Michael addition reactions of phos- from the study of natural product Cinchona alkaloid
phites with nitroolefins to afford b-nitrophosphonates. quinine VI (entry 6). By using catalyst VI, reaction of
The Michael adducts can be conveniently transformed diphenyl phosphite 1a with trans-b-nitrostyrene 2a
to chiral a-substituted b-aminophosphonic acids.
took place to form adduct 3a more rapidly (5 min)
1052
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1052 – 1056

Enantioselective Michael Addition of Diphenyl Phosphite to Nitroolefins
Table 2. Optimization of phosphites 1.[a]
COMMUNICATIONS
Entry
R
t
Yield [%][b]
% ee[c]
1
2
3
4
5
Ph, 1a
5 min
48 h
48 h
48 h
48 h
88
37
<5
<5
<5
23
19
2-Cl-C6H4, 1b
2-Me-C6H4, 1c
3,5-Me2-C6H3, 1d
Et, 1e
n.d.[d]
n.d.[d]
n.d.[d]
[a]
Unless specified, see Experimental Section.
Isolated yields.
Determined by chiral HPLC analysis (Chiralpak AS-H).
Not determined.
[b]
[c]
[d]
albeit with poor enantioselectivity (23% ee). The en-
couraging outcome prompted us to carry out a more
detailed investigation aimed at improving the enantio-
selectivity of the Michael reaction with a focus on the
modification of quinine. First, a benzyl group was in-
troduced at the position C-9 in catalyst VIII to restrict
the rotation of C-4’/C-9 bond, however, no gain was
observed (entry 8). Second, the results turned out to
be disappointing when an acidic and a possible addi-
tional H-bonding donor C-6’-OH was incorporated
into catalyst IX (entry 9), indicating an extra hydro-
gen bond donor affecting the reaction rate. The obser-
vation was further confirmed by cinchonidine VII cat-
alyzed the process (entry 7). Finally, a more structur-
ally rigid Cinchona alkaloid dimer X was surveyed
with an unsatisfactory result observed as well
(Table 1, entry 10). As a result, the organocatalyst VI
proved to be of the choice for further investigation.
To further improve the enantioselectivity of the VI-
catalyzed Michael reaction, we have studied the effect
of nucleophilic phosphites 1 (Table 2). Among the di-
alkyl and diphenyl phosphites examined, diphenyl
phosphite (1a) still afforded the best result (5 min,
88% yield, 23% ee, entry 1). If substituents (methyl
group or chloride) were introduced to the 2- or 3,5-
positions on the phenyl rings, the reaction rates signif-
icantly decreased (entries 2–4). When the diethyl
phosphite (1e) was used, the reaction also proceeded
very slowly (48 h, <5% yield, entry 5).
Figure 1. Structures of chiral organocatalysts.
Table 1. Results of exploratory studies of catalytic asymmet-
ric Michael addition reaction of diphenyl phosphite (1a)
with trans-b-nitrostyrene (2a).[a]
Entry
Catalyst
t
Yield [%][b]
% ee[c]
1
2
3
4
I
II
III
IV
24 h
1 h
29
93
87
38
21
88
91
86
42
<5
À13
À18
rac.[d]
11
1.5 h
24 h
24 h
5 min
5 min
2 h
À8
A survey of solvents revealed that the reaction
media had a significant effect on this process. For ex-
ample, the reaction carried out in xylenes gave the
highest enantioselectivity (42% ee, Table 3, entry 3).
Lower enantioselectivities were observed when other
solvents were used in the processes (Table 3, entries 1,
2 and 7–10). By lowering the reaction temperature,
the ee was improved in xylenes (entries 3–6). At
À558C, the enantioselectivity was increased to 70%
ee (entry 6). Therefore, xylenes were selected as the
6
7
8
9
VI
23
VII
VIII
IX
16
À17
24 h
24 h
15
10
X
n.d.[e]
[a]
Reaction conditions: see Experimental Section.
Isolated yields.
Determined by chiral HPLC analysis (Chiralpak AS-H).
Racemic.
[b]
[c]
[d]
[e]
Not determined.
Adv. Synth. Catal. 2007, 349, 1052 – 1056
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1053

Jian Wang et al.
COMMUNICATIONS
Table 3. Solvent effect on VI-catalyzed asymmetric Michael
addition of diphenyl phosphite (1a) with trans-b-nitrostyrene
(2a).[a]
Table 4. The scope of VI-catalyzed asymmetric Michael ad-
dition of diphenyl phosphite (1a) to trans-b-nitrostyrene
(2a).[a]
Entry
Solvent
t
Yield [%][b]
% ee[c]
Entry
R
t
Yield
[%][b]
%
[days]
ee[c]
1
2
3
Et2O
5 min
5 min
5 min
45 min
8 h
6 days
30 min
8 h
88
95
93
90
86
82
82
88
<5
85
23
30
42
52
53
70
30
toluene
xylenes
xylenes
xylenes
xylenes
CH2Cl2
CH3NO2
MeOH
DMF
1
2
3
4
5
6
Ph, 3a
4-F-C6H4, 3b
4-Cl-C6H4, 3c
6
6
6
6
6.5
7
7
6
5
5
7
6
4
4
5
5
82
85
82
83
83
78
77
78
82
78
79
67
77
68
62
60
70
77
72
66
80
75
64
71
82
81
88
72
45
63
60
55
4[f]
5 g]
6 h]
7
4-Br-C6H4, 3d
4-Me-C6H4, 3e
4-MeO-C6H4, 3f
2-MeO-C6H4, 3g
2,4-(MeO)2-C6H3, 3h
3-BnO-4-MeO-C6H3, 3i
3,4-(OCH2O)-C6H3, 3j
2-thiophene, 3k
2-furan, 3l
8
9
10
22
7[d]
8[d]
9[d]
10[d]
11
36 h
5 min
n.d.[d]
rac.[e]
[a]
Unless specified, see Experimental Section.
Isolated yields.
ee determined by chiral HPLC analysis (Chiralpak AS-
[b]
[c]
12[d]
13[d]
14[d]
15[d]
16[d]
Me2CHCH2, 3m
PhCH2CH2, 3n
n-C5H11, 3o
H).
[d]
[e]
[f]
Not determined.
Racemic mixture.
Reaction at 08C.
Reaction at À208C.
Reaction at À558C.
n-C6H13, 3p
[g]
[h]
[a]
Unless specified, see Experimental Section for reaction
conditions.
Isolated yields after flash chromatography.
Determined by chiral HPLC analysis (Chiralpak AS-H).
Reaction performed at À208C.
[b]
[c]
[d]
reaction medium for the processes to probe the scope
of the asymmetric processes at À558C.
To demonstrate the generality of the VI-catalyzed
Michael addition processes, reactions of a variety of
In a study, we demonstrated that the chiral b-nitro-
nitroalkenes 2 with diphenyl phosphite 1a in xylenes phosphonate 3c could be conveniently converted to
were explored (Table 4). As the data show, the pro- corresponding b-aminophosphonic acid 4 in a two-
cesses took place smoothly to give adducts 3 in mod- step transformation (Scheme 1).
erate to good yields (60–85% yield) with moderate to
high ee (45–88% ee). Analysis of these data reveals
that the nature of the electronic and substitution pat-
tern of the substituents on the aromatic rings has an
impact on enantioselectivity. Reactions of nitroolefins
with electron-withdrawing groups at the para-position
(entries 2–4) or electron-donating substituents at the
ortho-position of the phenyl ring (entries 7 and 8) oc-
curred with relatively lower enantioselectivities (64–
77% ee). However, the nitrostyrenes bearing elec-
tron-donating groups at the para-positions gave the
higher ee (75–88% ee, entries 5, 6, 9 and 10). The re-
action is also applicable to heterocyclic nitroolefins
(entries 11 and 12) with good yields (67% and 79%,
respectively) and good to high enantioselectivities
(72% and 88% ee, respectively). Relatively low ee
values were also observed for aliphatic nitroalkenes
(45–63% ee, entries 13–16). The absolute configura-
tion of 3k prepared under the conditions was deter-
mined by X-ray crystallography to be R (Figure 2).[17] Figure 2. X-ray crystal structure of 3k.
1054
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1052 – 1056

Enantioselective Michael Addition of Diphenyl Phosphite to Nitroolefins
COMMUNICATIONS
Scheme 1. Synthesis of b-aminophosphonic acid 4 from b-nitrophosphonate 3c.
AS-H, 2-propanol/hexane=40/60, flow rate 0.5 mLminÀ1,
l=254 nm): tminor =12.6 min, tmajor =17.2 min, ee=70%; [a]D25
(major):=À9.7 (c 1.0 in CHCl3).
Acknowledgements
The financial support for this research by the Department of
Chemistry and the Research Allocation Committee, Universi-
ty of New Mexico, NIH-INBRE (P20 RR016480) and the
American Chemical Society-PRF (G1 type) is gratefully ac-
knowledged.
Figure 3. Proposed a transition state model.
Our preliminary understanding of this enantioselec-
tive Michael addition reaction invokes H-bonding in-
teraction between both substrates and the catalyst VI
(Figure 3). In this proposed simple transition state
(TS) model, the trans-b-nitrostyrene is activated by
the OH group of the catalyst. Meanwhile, the amino
moiety via the second H-bonding interaction activates
and directs the phosphite group for si face attack of
the trans-b-nitrostyrene, which affords the observed
(S) product.
In summary, the first small molecule quinine VI
catalyzed enantioselective conjugate addition of di-
phenyl phosphite to nitroolefins has been developed.
This methodology provides a general and convenient
access to a wide range of good to high enantiomeri-
cally enriched b-nitrophosphates, precursors for prep-
aration of synthetically and biologically useful b-ami-
nophosphonates. Further investigation of the full
scope of the Michael reaction, its mechanism and ap-
plications are underway in our laboratory and the re-
sults will be reported in due course.
References
[1] For reviews of syntheses and applications of a-amino-
phosphonic acids, see: a) S. Hannour, A. Ryglowski,
M. L. Roumestant, P. Viallefont, J. Martinez, F. Ouaz-
zani, A. El Hallaoui, Phosphorus, Sulfur Silicon 1998,
419; b) I. M. Lefebvre, S. A. Evans, Phosphorus, Sulfur
Silicon 1999, 397; c) H. Groger, B. Hammer, Chem.
Eur. J. 2000, 6, 943.
[2] For reviews of syntheses and applications of b-amino-
phosphonic acids, see: a) F. Palacios, C. Alonso, J. M.
De Los Santos, in: Enantioselective Synthesis of b-
Amino Acids, 2nd edn., (Eds.: E. Juaristi, V. A. Solo-
shonok), John Wiley & Sons, Inc., Hoboken, N. J.,
2005, p 277; b) F. Palacios, C. Alonso, J. M. De Los
Santos, Chem. Rev. 2005, 105, 899.
[3] For recent selected examples of asymmetic synthesis of
a-aminophosphonic acids, see: a) A. B. Smith,
III,
K. M. Yager, C. M. Taylor, J. Am. Chem. Soc. 1995,
117, 10879; b) F. A. Davis, S. Lee, H. Yan, D. D. Titus,
Org. Lett. 2001, 3, 1757; c) G. D. Joly, E. N. Jacobsen, J.
Am. Chem. Soc. 2004, 126, 4102; d) S. Kobayashi, H.
Kiyohara, Y. Nakamura, R. Matsubara, J. Am. Chem.
Soc. 2004, 126, 6558; e) T. Akiyama, H. Morita, J. Itoh,
K. Fuchibe, Org. Lett. 2005, 7, 2583.
Experimental Section
[4] a) M. M. Campbell, N. I. Carruthers, S. J. Mickel, Tetra-
hedron 1982, 38, 2513; b) M. E. Duggan, D. S. Kara-
newsky, Tetrahedron Lett. 1983, 43, 2935; c) L. D.
Arnold, T. Kalantar, J. D. Vederas, J. Am. Chem. Soc.
1985, 107, 7105; d) T. Yokomatsu, T. Yamagishi, S. Shi-
buya, Tetrahedron: Asymmetry 1993, 4, 1401; e) F. Pala-
cios, A. M. Ochoa de Retana, J. I. Gil, Tetrahedron
Lett. 2000, 41, 5363; f) C. de Risi, A. Dondoni, D. Per-
rone, G. P. Pollini, Tetrahedron Lett. 2001, 42, 3033;
g) F. Palacios, D. Aparicio, A. M. Ochoa de Retana,
J. M. De Los Santos, J. I. Gil, R. Lopez de Munain, Tet-
rahedron: Asymmetry 2003, 14, 689.
Typical Procedure (Table 4, entry 1)
Catalyst quinine (VI) (8 mg, 0.025 mmol) was added to a
vial containing xylenes (3 mL), diphenyl phosphite (1a)
(52.5 mL, 0.25 mmol) and trans-b-nitrostyrene (2a) (38 mg,
0.25 mmol) at À558C. After 6 days of stirring, TLC analysis
indicated completion of the reaction. The reaction mixture
was concentrated under vacuum. The residue was purified
by flash silica gel chromatography (ethyl acetate/hexane=
1:15 to 1:4) to afford the adduct 3a as a white solid; yield:
79 mg (82%) with 70% ee, determined by HPLC (Chiralpak
Adv. Synth. Catal. 2007, 349, 1052 – 1056
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1055

Jian Wang et al.
COMMUNICATIONS
[5] a) V. A. Soloshonok, Y. N. Belokon, N. A. Kuzmina,
V. I. Maleev, N. Y. Svistunova, V. A. Solodenko, V. P.
Kukhar, J. Chem. Soc., Perkin Trans. 1 1992, 1525; b) S.
Hanessian, Y. L. Bennani, Y. HervØ, S ynlett 1993, 35;
c) F. A. Davis, T. Ramachandarf, Y.-D. Wu, J. Org.
Chem. 2003, 68, 6894.
[6] C. Yuan, C. Xu, Y. Zhang, Tetrahedron 2003, 59, 6095.
[7] A. A. Thomas, K. B. Sharpless, J. Org. Chem. 1999, 64,
8379.
C. Weckbecker, K. Huthmacher, Eur. J. Org. Chem.
2005, 4995; k) S. H. McCooey, S. J. Connon, Angew.
Chem. 2005, 117, 6525; S. H. McCooey, S. J. Connon,
Angew. Chem. Int. Ed. 2005, 44, 6367; l) J. Ye, D. J.
Dixon, P. S. Hynes, Chem. Commun. 2005, 4481; m) H.
Li, Y. Wang, L. Tang, F. Wu, X. Liu, C. Guo, B. M.
Foxman, Li. Deng, Angew. Chem. 2005, 117, 107; H. Li,
Y. Wang, L. Tang, F. Wu, X. Liu, C. Guo, B. M.
Foxman, Li. Deng, Angew. Chem. Int. Ed. 2005, 44,
105; n) Y.-M. Xu, W.-B. Zou. H. Sunden, I. Ibrahem,
A. Cꢁrdova, Adv. Synth. Catal. 2006, 348, 418; o) W.
Wang, J. Wang, H. Li, Angew. Chem. 2005, 117, 1393;
W. Wang, J. Wang, H. Li, Angew. Chem. Int. Ed. 2005,
44, 1369 and J. Wang, H. Li, B.-S. Lou, L.-S. Zu, H.
Guo, W. Wang, Chem. Eur. J. 2006, 12, 4321.
[8] A. N. Pudovik, F. N. Siditikova, Dokl. Akad. Nauk.
SSSR 1959, 125, 826.
[9] C. Shin, Y. Yonezawa, K. Katayama, J. Yoshimura,
Bull. Chem. Soc. Jpn. 1973, 46, 1727.
[10] D. Enders, L. Tedeschi, J. W. Bats, Angew. Chem. 2000,
39, 4605; D. Enders, L. Tedeschi, J. W. Bats, Angew.
Chem. Int. Ed. 2000, 39, 4605.
[13] B. List, Tetrahedron 2002, 58, 5573.
[11] For recent reviews of asymmetric Michael addition re-
actions, see: a) M. Sibi, S. Manyem, Tetrahedron 2001,
56, 8033; b) N. Krause, A. Hoffmann-Rçder, Synthesis
2001, 171; c) O. M. Berner, L. Tedeschi, D. Enders, Eur.
J. Org. Chem. 2002, 1877; d) J. Christoffers, A. Baro,
Angew. Chem. 2003, 115, 1726; J. Christoffers, A. Baro,
Angew. Chem. Int. Ed. 2003, 42, 1688; e) P. I. Dalko, L.
Moisan, Angew. Chem. 2004, 116, 5248; P. I. Dalko, L.
Moisan, Angew. Chem. Int. Ed. 2004, 43, 5138;.
[12] For selected examples of organocatalyzed asymmetric
Michael addition of nitroolefins, see: a) B. List, P. Po-
jarliev, H. J. Martin, Org. Lett. 2001, 3, 2423; b) D.
Enders, A. Seki, Synlett 2002, 26; c) O. Andrey, A.
Alexakis, G. Bernardinelli, Org. Lett. 2003, 5, 2559;
d) N. Mase, R. Thayumanavan, F. Tanaka, C. F.
Barbas, III, Org. Lett. 2004, 6, 2527; e) J. M. Betan-
cort, K. Sakthivel, R. Thayumanavan, F. Tanaka, C. F.
Barbas, III, Synthesis 2004, 1509; f) T. Ishii, S. Fiujio-
ka, Y. Sekiguchi, H. Kotsuki, J. Am. Chem. Soc. 2004,
126, 9558; g) A. J. A. Cobb, D. A. Longbottom, D. M.
Shaw, S. V. Ley, Chem. Commun. 2004, 1808; h) Y.
Hayashi, T. Gotoh, T. Hayasji, M. Shoji, Angew. Chem.
2005, 117, 4284; Y. Hayashi, T. Gotoh, T. Hayasji, M.
Shoji, Angew. Chem. Int. Ed. 2005, 44, 4212; i) R. P.
Herrera, V. Sgarzani, L. Bernardi, A. Ricci, Angew.
Chem. 2005, 117, 6734; R. P. Herrera, V. Sgarzani, L.
Bernardi, A. Ricci, Angew. Chem. Int. Ed. 2005, 44,
6576; j) S. B. Tsogoevam, D. A. Yalalov, M. J. Hateley,
[14] For recent reviews of thioureas catalyzed reactions,
see: a) P. R. Schreiner, Chem. Soc. Rev. 2003, 32, 289;
b) Y. Takemoto, Org. Biomol. Chem. 2005, 3, 4299;
c) M. S. Taylor, E. N. Jacobsen, Angew. Chem. Int. Ed.
2006, 45, 1520; d) B. List, Chem. Commun. 2006, 819;
e) T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal.
2006, 348, 999; f) S. J. Connon, Chem. Eur. J. 2006, 12,
5418; g) T. Marcelli, J. H. van Maarseveen, H. Hiem-
stra, Angew. Chem. Int. Ed. 2006, 45, 7496; examples of
thioureas catalyzing reactions, see: h) T. Okino, Y.
Hoashi, Y. Takemoto, J. Am. Chem. Soc. 2003, 125,
12672; i) T. Okino, S. Nakamura, T. Furukawa, Y. Take-
moto, Org. Lett. 2004, 6, 625.
[15] a) B. Vakulya, S. Varga, A. Csampai, T. Soꢁs, Org. Lett.
2005, 7, 1967; b) L. Bernardi, F. Fini, R. P. Herrera, A.
Ricci, V. Sgarzani, Tetrahedron 2006, 62, 375.
[16] A review of Cinchona alkaloids catalyzing reactions,
see: a) S.-K. Tian, Y.-G. Chen, J.-F. Hang, L. Tang, P.
McDaid, L. Deng, Acc. Chem. Res. 2004, 37, 621;
recent selected examples of cinchona alkaloids catalyz-
ing reactions, see: b) S. Lou, B. M. Tkoka, M. Ting,
S. E. Schaus, J. Am. Chem. Soc. 2005, 127, 11256;
c) A. L. Tillman, J. Ye, D. J. Dixon, Chem. Commun.
2006, 1191; d) J. Song, Y. Wang, L. Deng, J. Am. Chem.
Soc. 2006, 128, 6049.
[17] CCDC-618770 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk.
1056
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1052 – 1056