Organocatalytic enantioselective domino synthesis of highly functionalized cyclohexanes with an all-carbon quaternary stereocenter

Article abstract of DOI:10.1016/j.tetlet.2009.02.209

A highly enantioselective organocatalytic domino Michael/aldol reaction is presented. The reaction is catalyzed by chiral amines and gives access to highly functionalized cyclohexanes with one all-carbon quaternary stereocenter and multiple chiral stereoc

Full text of DOI:10.1016/j.tetlet.2009.02.209

Tetrahedron Letters 50 (2009) 3458–3462
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Organocatalytic enantioselective domino synthesis of highly functionalized
cyclohexanes with an all-carbon quaternary stereocenter
Gui-Ling Zhao a, Pawel Dziedzic a, Farman Ullah a, Lars Eriksson b, Armando Córdova a,
*
a Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
b Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly enantioselective organocatalytic domino Michael/aldol reaction is presented. The reaction is cat-
alyzed by chiral amines and gives access to highly functionalized cyclohexanes with one all-carbon qua-
ternary stereocenter and multiple chiral stereocenters in high yields and 83–98% ee.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 18 December 2008
Revised 13 February 2009
Accepted 26 February 2009
Available online 4 March 2009
Domino or cascade reactions, that involve the formation of mul-
tiple carbon-carbon bonds and stereocenters in one-pot, is a rap-
idly growing research field within the synthesis of small
molecules with complex molecular scaffolds.1 Domino reactions
are advantageous since they include important ‘green’ chemistry
factors such as atom economy,2 reduction of synthetic steps, and
minimization of solvents and waste.3 Another important factor
that adds to the concept of domino reactions is asymmetric catal-
ysis. In this context, the development of organocatalytic asymmet-
ric one-pot domino reactions is an actively growing research
area.4,5
The stereoselective synthesis of all-carbon quaternary stereo-
genic centers in complex organic molecules is very important.
However, this is a difficult task due to the inherent steric bias in
the carbon–carbon bond-forming step.6 Thus, the development of
enantioselective catalytic domino reactions that give access to
small molecules containing all-carbon quaternary stereogenic cen-
ters is considered very challenging.7 Our group,8a as well as
Melchiorre and co-workers,7b has recently reported the amine-cat-
alyzed asymmetric conjugate addition of aldehydes to Michael
acceptors derived from malonates and cyanoacetic acid, respec-
tively.7 Prior to this, the amine-catalyzed addition of ketones to
alkylidene malonates was reported.8c,d Notably, Hayashi and co-
workers have shown that glutaraldehyde can be used as a sub-
strate in the construction of optically active cyclohexanes.9 They
subsequently developed a catalytic domino Michael/nitro-aldol
reaction. Based on these findings and our research interest in
‘green’ asymmetric catalysis,10 we envisioned a route to highly
functionalized molecules with one quaternary all-carbon stereo-
center and three additional stereocenters by a possible amine-cat-
alyzed enantioselective domino Michael/aldol reaction (Eq. 1):
OH
X*
O
O
EWG1
EWG2
N
H
EWG1
EWG2
*
*
+
R
ð1Þ
H
H
*
*
R
O
H
Herein, we demonstrate a highly enantioselective domino Mi-
chael/aldol one-pot synthesis of cyclohexane derivatives with a
quaternary all-carbon stereocenter and cyano, formyl, hydroxy,
and ester functional groups in 83–98% ee.
In an initial catalyst and solvent screening for the reaction be-
tween glutaraldehyde 1 and alkylidene malonate 2a (Scheme 1),
we found that of the investigated chiral amines, only protected
diphenylprolinol 411 catalyzed the asymmetric formation of cyclo-
hexane derivative 3a with a high level of stereoselectivity (>25:1 dr
and 97% ee).
Encouraged by this result, we decided to investigate chiral
amine 4 as a catalyst for the reaction between 1 and 2-cyanocin-
namic acid ester 2b, since a cyclohexane with an asymmetric qua-
ternary stereocenter would be generated (Table 1). This type of
compound is
manipulations.
a
valuable synthon for further synthetic
This transformation resulted in the formation of the corre-
sponding cyclohexane 3b in high yields but with poor to high lev-
els of diastereoselectivity (3b:3b0) and 37–79% ee at room
temperature in all the solvents investigated (entries 1–4). The
absolute stereochemistry of the product was switched in CH3CN
(entry 2). The switch in the enantioselectivity of the reaction in
CH3CN has also been observed in the chiral amine 4-catalyzed
a-
selenylation of aldehydes.12 Notably, increasing the reaction
temperature improved the enantioselectivity significantly (entries
6–8) whereas decreasing it did not (entry 5). To our delight,
decreasing the catalyst loading further improved the enantioselec-
tivity in CHCl3 and gave the corresponding cyclohexane 3b with an
* Corresponding author. Tel.: +46 8 162479; fax: +46 8 154908.
E-mail addresses: acordova@organ.su.se, acordova1a@netscape.net (A. Córdova).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.209

G.-L. Zhao et al. / Tetrahedron Letters 50 (2009) 3458–3462
3459
Ph
Ph
OTMS
N
H
OH
CO2Me
CO2Me
R
4
O
O
(20 mol%)
CO2Me
CO2Me
2a: R = 4-NO2-C6H4
+
R
H
H
CH3CN
rt, 112 h
O
H
1
3a: 21% yield, >25:1 dr, 97% ee
Scheme 1. Catalytic domino reaction between 1 and 2a.
Table 1
Optimization of the conditions for the reaction between 1 and ester 2ba
OH
OH
CO2Me
CN
CO2Me
CN
4
O
O
(20 mol%)
CO2Me
CN
2b: R = 4-NO2-C6H4
Temp (°C)
+
R
R
R
+
H
H
Solvent
O
H
O
H
1
3b
3b'
Entry
Solvent
Time (h)
Yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
CHCl3
23
23
23
23
4
45
40
45
3
80
94
51
70
70
87
96
94f
7:1
79
37e
38
70
41
96
87
98f
CH3CN
CH2Cl2
Toluene
CHCl3
CHCl3
CH2Cl2
CHCl3
24
21
5
40
0.5
0.3
0.8
24:1
12:1
1:1
7:1
6:1
6:1
6:1f
a
Experimental conditions: A mixture of 1 (0.5 mmol), 2b (0.25 mmol), and catalyst (20 mol %) in 1 mL of solvent was stirred at the temperature and for the time indicated.
Isolated yield of 3b after silica gel column chromatography.
b
c
d
e
f
Diastereomeric ratio of 3b:3b0 determined by 1H NMR analysis of the crude reaction mixture.
Ee of 3b determined by chiral-phase HPLC analysis.
The opposite enantiomer ent-3b was formed.
10 mol % of catalyst 4 was used at all stereocenters.
all-carbon quaternary stereocenter in 94% yield, a 6:1 dr and 98%
ee (entry 8).
NOE
Based on these results, we decided to investigate the chiral
amine 4-catalyzed enantioselective domino reactions between
dialdehyde 1 and various 2-cyanocinnamic acid esters 2 in CHCl3
at 45 °C (Table 2).13
HO
H
4 OH
H
4 CN
CO2Me
CO2Me
CN
1
1
2
2
3
H2
H2
3
4
4
The organocatalytic asymmetric domino reactions gave the cor-
responding cyclohexanes 3b–i in high yields (72–98%) as the pre-
dominant diastereoisomers (5:1–10:1 dr, 3:30), which could be
separated readily from the minor diastereoisomers 3b0–i0 by silica
gel column chromatography. The enantioselectivity of the reac-
tions was high (83–98% ee). Thus, the one-pot procedure allows
for the construction of four stereocenters with very high stereocon-
trol. The diastereoselectivity of this reaction is lower compared to
when nitrostyrenes are used as substrates.9 This is due to the fact
that a quaternary stereocenter is formed and that a higher temper-
ature is employed compared to the reactions of nitrostyrenes at
room temperature. This reaction was accelerated and the diastere-
oselectivity was slightly improved by the use of microwave irradi-
ation without significantly affecting the enantioselectivity (entry
9).14 This could be due to a combination of the increased temper-
ature and pressure. The corresponding acids 5, diol 6, and tri-ester
7 were synthesized in high yields from cyclohexanes 3 (Scheme 2).
The relative stereochemistry of 3c15 was established by NOE
experiments, which confirmed the relative configuration between
all the neighboring substituents. The relative stereochemistry of
the minor diastereoisomer was established by NOE experiments
O
H1
O
H1
Cl
Cl
H
H
H
H
3c
3c'
NOE
NOE
This was also confirmed by the X-ray analysis (Fig. 1) of acid 5c
(1R,2R,3S,4R).17
Based on these results and the chiral amine 4-catalyzed alde-
hyde additions to Michael acceptors 2,9 we propose the following
domino reaction mechanism to account for the observed stereo-
chemistry of the reaction (Scheme 3).
Thus, efficient shielding of the Si-face of the chiral enamine
intermediate I by the bulky aryl groups of 4 leads to stereoselective
Re-facial nucleophilic conjugate attack. Next, the in situ generated
anion of the chiral iminium intermediate II undergoes a six-exo-
trig intramolecular aldol addition on the aldehyde moiety to form
iminium intermediate III. Hydrolysis gives the corresponding
cyclohexane 3 and releases the chiral amine catalyst. The intermo-
lecular aldol addition step was very fast since no free Michael
16
on compound 3c0 (93% ee).

3460
G.-L. Zhao et al. / Tetrahedron Letters 50 (2009) 3458–3462
Table 2
Catalytic asymmetric domino reactions between dialdehyde 1 and esters 2a
OH
OH
CO2Me
CN
CO2Me
CN
4
O
O
(10 mol%)
CO2Me
CN
2b
+
+
R
R
R
H
H
CHCl3, 45 °C
O
H
O
H
3
3'
1
Entry
R
Product
Time (h)
Yieldb (%)
drc
eed (%)
1
3b
3c
3d
3e
3f
0.8
2
94
98
95
82
78
6:1
6:1
9:1
8:1
5:1
98
92
85
92
93
O2N
Cl
2
3
2
Br
4e
5e
2
2.5
6e
7e
8e
3g
3h
3i
1
2
2
1
96
89
72
97
8:1
6:1
10:1
7:1
83
95
89
89
Cl
O
9f
3c
Cl
a
Experimental conditions: A mixture of 1 (0.50 mmol), 2a (0.25 mmol), and catalyst (10 mol %) in 1 mL of CHCl3 was stirred at 45 °C for the time displayed.
Isolated yield of 3 after silica gel column chromatography.
b
c
d
e
f
Diastereomeric ratio of 3:30 determined by 1H NMR analysis of the crude reaction mixture.
ee of 3 determined by chiral-phase HPLC analysis.
20 mol % of catalyst 4 was used.
The reaction was performed in a sealed tube under microwave irradiation using a microwave synthesizer from personal systems (Biotage AB).
OH
OH
OH
CO2Me
CN
CO2Me
CN
CO2Me
CN
(b)
(a)
90%
R
R
R
HO
O
OH
O
H
6h: R = n-Pr
5c: R = 4-ClC6H4, 85%
5d: R = 4-BrC6H4, 88%
3
89%
O
(b), (c)
Ph
O
CO2Me
CN
R
7h: R = n-Pr
O
O
Ph
Scheme 2. Reagents and conditions: (a) NaClO2, KH2PO4, (CH3)2C@CHCH3, t-BuOH/H2O (5:1); (b) NaCNBH3, THF, AcOH; (c) BzCl, CH2Cl2, Et3N.

G.-L. Zhao et al. / Tetrahedron Letters 50 (2009) 3458–3462
3461
be performed under microwave irradiation and the corresponding
products can be used as scaffolds for further diversification. Fur-
ther development of catalytic domino reactions that generate opti-
cally active molecules with an all carbon stereocenter is underway
in our laboratory.
Acknowledgments
We gratefully acknowledge the Swedish National Research
Council and Carl-Trygger Foundation for financial support.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.02.209.
References and notes
1. For reviews on this concept see: (a) Tietze, L. F.; Brasche, G.; Gericke, K. Domino
Reactions in Organic Synthesis; Wiley-VCH: Weinheim, 2006. p 672; (b) Tietze, L.
F. Chem. Rev. 1996, 96, 115; (c) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G.
Angew. Chem., Int. Ed. 2006, 45, 7134; (d) Guo, H.; Ma, J. Angew. Chem., Int. Ed.
2006, 45, 354; (e) Pellieser, H. Tetrahedron 2006, 62, 2143; (f) Ramon, D. J.; Yus,
M. Angew. Chem., Int. Ed. 2005, 44, 1602.
Figure 1. ORTEP picture of compound 5c.
2. (a) Trost, B. M. Science 1991, 254, 1471; (b) Trost, B. M. Angew. Chem., Int. Ed.
Engl. 1995, 107, 258.
3. (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford
University Press: Oxford, 2000. p 135; For an excellent paper on combining
one-pot, step reduction and atom economy in organic chemistry see: (b) Clarke,
P. A.; Santos, S.; Martin, W. H. C. Green Chem. 2007, 9, 438.
4. For excellent reviews on organocatalytic domino reactions see: (a) Enders, D.;
Grondal, C.; Hüttl, M. R. M. Angew. Chem., Int. Ed. 2007, 46, 1570; (b) Yu, X.;
Wang, W. Org. Biomol. Chem. 2008, 6, 2037.
5. For selected examples of organocatalytic asymmetric domino and cascade
reactions see: (a) Halland, N.; Aburell, P. S.; Jørgensen, K. A. Angew. Chem., Int.
Ed. 2004, 43, 1272; (b) Huang, Y.; Walji, A. M.; Larsen, C. H.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2005, 127, 15051; (c) Yang, J. W.; Hechavarria Fonseca, M. T.;
intermediate, which would have been formed by hydrolysis of
iminium intermediate II, was observed. Moreover, this rapid equi-
librium was also responsible for the time dependent conversion of
the minor diastereoisomer 30 to the more thermodynamically sta-
ble major diastereoisomer 3.
In summary, we have developed a highly diastereo- and enan-
tioselective one-pot domino Michael/aldol reaction for the synthe-
sis of cyano, formyl, hydroxy, and ester functionalized
cyclohexanes with one all-carbon stereocenter and three addi-
tional stereocenters. The reaction was accelerated by heating, can
Ph
Ph
+
MeO2C
CN
R
Ph
Ph
N
N
NC
R
OTMS
OTMS
-
MeO2C
I
II
O
O
H2O
H+
O
H2O
1
O
Ph
+
Ph
Ph
R
N
NC
MeO2C
OTMS
Ph
N
H
OTMS
HO
III
R
O
NC
MeO2C
HO
3
Scheme 3. Proposed reaction pathway.

3462
G.-L. Zhao et al. / Tetrahedron Letters 50 (2009) 3458–3462
List, B. J. Am. Chem. Soc. 2005, 127, 15036; (d) Marigo, M.; Schulte, T.; Franzén,
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13. To a stirred solution of the catalyst 4 (10 or 20 mol %) in CHCl3 (1 mL) were
added glutaraldehyde (100 mg, 0.5 mmol, 40% aqueous solution) and ester 2
(0.25 mmol) at 45 °C. Next, the reaction mixture was stirred vigorously and
monitored by TLC. After the time shown in Table 2, the reaction mixture was
allowed to cool and then extracted with EtOAc (3 ꢀ 15 mL). The organic layer
was combined, dried over Na2SO4, and the solvent was removed. The residue
was purified by silica gel column chromatography (pentane/ethyl
acetate = 4:1–1:1) to give the corresponding product 3. (1S,2R,3R,6R)-Methyl
J.; Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 15710; (e) Yamamoto, Y.;
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Govender, T.; Hojabri, L.; Moghaddam, F. M.; Arvidsson, P. I. Tetrahedron:
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M.; Runsink, J.; Raabe, G.; Wendt, B. Angew. Chem., Int. Ed. 2007, 46, 467; (r)
Enders, D.; Hüttl, M. R. M.; Raabe, G.; Bats, J. W. Adv. Synth. Catal. 2008, 350,
267; (s) Reyes, E.; Jiang, H.; Milelli, A.; Elsner, P.; Hazell, R. G.; Jørgensen, K. A.
Angew. Chem., Int. Ed. 2007, 46, 9202; (t) Sundén, H.; Rios, R.; Xu, Y.; Eriksson,
L.; Córdova, A. Adv. Synth. Catal. 2007, 349, 2549; (u) Hong, B.-C.; Nimje, R. Y.;
Sadani, A. A.; Liao, J.-H. Org. Lett. 2008, 10, 2345; (v) Tan, B.; Cua, P. J.; Li, Y.;
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D.; Koshino, D.; Hayashi, Y. Org. Lett. 2008, 10, 1445.
1-cyano-3-formyl-6-hydroxy-2-(4-nitrophenyl)cyclohexanecarboxylate
3b:
½ ꢁ
a 2D5
+ 16.1 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) d 9.46 (d, J = 2.0 Hz,
1H), 8.20 (d, J = 8.8 Hz, 2H), 7.51 (d, J = 8.8 Hz, 2H), 4.24 (dd, J = 4.0, 12.0 Hz,
1H), 3.59 (s, 3H), 3.50 (d, J = 12.4 Hz, 1H), 3.30 (ddt, J = 2.0, 4.0, 12.4 Hz, 1H),
2.44 (br s, 1H), 2.35–2.26 (m, 2H), 2.06–1.97 (m, 1H), 1.62–1.52 (m, 1H); 13C
NMR (100 MHz, CDCl3) d 199.4, 167.5, 148.1, 143.0, 129.5, 124.2, 115.7, 73.7,
61.0, 53.9, 49.7, 47.9, 30.2, 24.3. The enantiomeric excess was determined by
HPLC with an AD column (n-hexane/i-PrOH = 90:10, k = 205 nm), 0.5 mL/min;
tR = minor enantiomer 88.4 min, major enantiomer 99.7 min. HRMS (ESI): calcd
for [M+Na]+ (C16H16N2O6Na) requires m/z 355.0901, found 355.0897.
14. Similar effects have been observed in organocatalytic Mannich-type
reactions see: (a) Westermann, B.; Neuhaus, C. Angew. Chem., Int. Ed. 2005,
44, 4077; (b) Rodriguez, B.; Bolm, C. J. Org. Chem. 2006, 71, 2888; (c)
Ibrahem, I.; Dziedzic, P.; Córdova, A. Synthesis 2006, 4060; (d) Hosseini, M.;
Stiasni, N.; Barbieri, V.; Kappe, O. C. J. Org. Chem. 2007, 72, 1417.
6. For selected reviews on quaternary stereocenters see: (a) Christoffers, J.; Mann,
A. Angew. Chem., Int. Ed. 2001, 40, 4591; (b) Trost, B. M.; Jiang, C. Synthesis 2006,
369; (c) Christoffers, J.; Baron, A. Quaternary Stereocenters: Challenges and
Solutions for Organic Synthesis; Wiley-VCH: Weinheim, 2006; (d) Cozzi, P. G.;
Hilgraf, R.; Zimmermann, N. Eur. J. Org. Chem. 2007, 5969.
7. To the best of our knowledge, there are only a few previous recent examples of
the formation of an all-carbon stereocenter in chiral amine-catalyzed domino
reactions see: Ref. 5w and (a) Rios, R.; Sundén, H.; Vesely, J.; Zhao, G.-L.;
Dziedzic, P.; Córdova, A. Adv. Synth. Catal. 2007, 349, 1028; (b) Penon, O.;
Carlone, A.; Mazzanati, A.; Locatelli, M.; Sambri, L.; Bartoli, G.; Melchiorre, P.
Chem. Eur. J. 2008, 14, 4788; (c) Enders, D.; Wang, C.; Bats, D. W. Angew. Chem.,
Int. Ed. 2008, 47, 3579.
8. (a) Zhao, G.-L.; Vesely, J.; Sun, J.; Christensen, K. E.; Bonneau, C.; Córdova, A.
Adv. Synth. Catal. 2008, 350, 657; For an excellent review on organocatalytic
Michael additions see: (b) Sulzer Mossé, S.; Alexakis, A. Chem. Commun. 2007,
3123; For conjugate addition of ketones see: (c) Betancort, J. M.; Saktihvel, K.;
Thayumanavan, R.; Barbas, C. F., III Tetrahedron Lett. 2001, 42, 4441; (d) Cao, C.-
L.; Sun, X. L.; Zhou, J.-L.; Tang, Y. J. Org. Chem. 2007, 72, 4073.
15. Compound 3c: ½a D25
ꢁ
ꢂ6.4 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) d 9.43 (d,
J = 2.0 Hz, 1H), 7.34 (d, J = 8.8 Hz, 2H), 7.29–7.27 (m, 2H), 4.24 (d, J = 12.0 Hz,
1H), 3.61 (s, 3H), 3.35 (d, J = 12.0 Hz, 1H), 3.21 (ddt, J = 2.0, 4.0, 12.0 Hz, 1H),
2.40 (br s, 1H), 2.32–2.26 (m, 1H), 2.23–2.17 (m, 1H), 2.03–1.93 (m, 1H), 1.68–
1.59 (m, 1H); 13C NMR (100 MHz, CDCl3) d 200.4, 167.7, 135.0, 133.8, 129.8,
129.5, 116.0, 73.6, 61.5, 53.8, 49.5, 48.0, 30.1, 24.2. The enantiomeric excess
was determined by HPLC with an AD column (n-hexane/i-PrOH = 90:10,
k = 205 nm), 0.5 mL/min; tR = minor enantiomer 39.7 min, major enantiomer
44.1 min. HRMS (ESI): calcd for [M+Na]+ (C16H16ClNO4Na) requires m/z
344.0660, found 344.0657.
16. Compound 3c0: ½a D25
ꢁ
+ 4.7 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) d 9.39
(d, J = 2.0 Hz, 1H), 7.33–7.28 (m, 4H), 4.40 (t, J = 2.4 Hz, 1H), 3.93 (br s, 1H),
3.82 (d, J = 12.4 Hz, 1H), 3.55 (s, 3H), 3.26–3.18 (m, 1H), 2.18–2.12 (m, 2H),
2.06–1.97 (m, 1H), 1.92–1.87 (m, 1H); 13C NMR (100 MHz, CDCl3) d 200.4,
169.3, 134.8, 134.7, 130.1, 129.3, 116.6, 69.1, 55.1, 53.9, 50.4, 41.9, 27.7,
19.7. The enantiomeric excess was determined by HPLC with an AD column
(n-hexane/i-PrOH = 90:10, k = 205 nm), 0.5 mL/min; tR = minor enantiomer
22.5 min, major enantiomer 29.4 min. HRMS (ESI): calcd for [M+Na]+
(C16H16ClNO4Na) requires m/z 344.0660, found 344.0658.
9. Hayashi, Y.; Okano, T.; Aratake, S.; Hazeland, D. Angew. Chem., Int. Ed. 2007, 46,
4922.
10. (a) Ibrahem, I.; Córdova, A. Angew. Chem., Int. Ed. 2006, 45, 1952; (b) Zhao, G.-L.;
Vesely, J.; Rios, R.; Ibrahem, I.; Sundén, H.; Córdova, A. Adv. Synth. Catal. 2008,
350, 237; (c) Vesely, J.; Ibrahem, I.; Zhao, G. -L.; Rios, R.; Cordova, A. Angew.
Chem., Int. Ed. 2007, 46, 778; (d) Sundén, H.; Rios, R.; Ibrahem, I.; Zhao, G.-L.;
Eriksson, L.; Córdova, A. Adv. Synth. Catal. 2007, 349, 827; (e) Vesely, J.; Rios, R.;
Ibrahem, I.; Zhao, G.-L.; Eriksson, L.; Córdova, A. Chem. Eur. J. 2008, 14, 2693.
17. CCDC 704580 contains the supplementary crystallographic data for this
Letter. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.