Catalytic asymmetric synthesis of epoxides from aldehydes using sulfur ylides with in situ generation of diazocompounds

Article abstract of DOI:10.1002/1521-3773(20010417)40:8<1430::AID-ANIE1430>3.0.CO;2-W

A practical, general, and convergent to epoxides with control of the relative and absolute stereochemistry has been achieved by generating the reactive intermediate (the diazo compound) in situ from tosylhydrazone salts (see scheme, PTC = phase-transfer catalyst, Ts = toluene-4-sulfonyl). High yields (58-82%), high d.r. (88:12-98:2), and high ee values (87-94%) have been obtained using a new class of stable chiral sulfides at low catalyst loading (5 mol%) and [Rh₂(OAc)₄] (0.5 mol%).

Full text of DOI:10.1002/1521-3773(20010417)40:8<1430::AID-ANIE1430>3.0.CO;2-W

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coupling of two different aldehydes to form epoxides with
control over the relative and absolute stereochemistry.
We focused our efforts on the use of the Bamford ± Stevens
reaction for generating the diazo compound,[5] and after some
experimentation found that warming a suspension of the
tosylhydrazone salt 2 (tosyl ˆ Ts ˆ toluene-4-sulfonyl) in the
presence of a phase-transfer catalyst[6] (PTC; to aid the
passage from the solid to the liquid phase), allowed the
generation of the diazo compound at moderate temper-
ature.[7] This modified protocol was also compatible with our
established epoxidation process and was remarkably efficient
(Scheme 2, Table 1, entry 1). Furthermore, these new condi-
tions were found to be general (Table 1). All the aromatic,
heteroaromatic, and unsaturated aldehydes investigated fur-
nished the corresponding epoxide in high yields and with high
trans selectivities (entries 1 ± 7). Aliphatic aldehydes also
worked well, although they gave lower yields and selectivities
(entries 8 and 9).
Catalytic Asymmetric Synthesis of Epoxides
from Aldehydes Using Sulfur Ylides with
In Situ Generation of Diazocompounds**
Varinder K. Aggarwal,* Emma Alonso, George Hynd,
Kevin M. Lydon, Matthew J. Palmer,
Marina Porcelloni, and John R. Studley
The development of new methods for catalytic epoxidation
continues to attract considerable attention. Whilst most
methods have concentrated on alkene oxidation,[1] we have
focused our efforts on the epoxidation of carbonyl com-
pounds.[2] Recently, we reported a catalytic and asymmetric
process for converting carbonyl compounds directly into
epoxides which operated under neutral conditions and
employed sub-stoichiometric amounts of sulfide 1 and metal
catalyst (Scheme 1).[3]
Following this success we decided to investigate a more
diverse range of tosylhydrazone salts in the epoxidation
process. Initially, we utilized a variety of substituted aryl
tosylhydrazone salts (2a ± g, Table 2). The yields of the
epoxides obtained were excellent, except where a sterically
hindered reagent was employed (Table 2, entry 6). In general
trans epoxides were obtained exclusively, but tosylhydrazone
O
Me
S
+
R'
_
Rh2(OAc)4
N2CHR'
RCHO
O
R'
N2
Rh=CHR'
(20 mol%)
R
85-94% ee
O
Me
S
1
Scheme 1. Catalytic cycle for epoxidation.
However, a limitation of the original protocol was
the need to synthesize and handle diazocompounds,
which, since they are potentially explosive,[4] severely
limits the practicality of the process. We therefore
considered the possibility of generating the diazo
compound in situ and coupling this reaction to our
established epoxidation process. Herein we describe
our success in achieving this aim and beyondÐto the
development of a unique process for the direct
Scheme 2. One-pot coupling of a carbonyl compound with either a tosylhydrazone
salt or a second carbonyl compound. Ts ˆ toluene-4-sulfonyl.
Table 1. Yields and ratios of epoxides formed from aldehydes and
tosylhydrazone salt 2 using 0.2 equivalents of tetrahydrothiophene.
[*] Prof. V. K. Aggarwal,[‡] Dr. E. Alonso, Dr. G. Hynd, Dr. K. M. Lydon,
Dr. M. J. Palmer, M. Porcelloni, Dr. J. R. Studley
Department of Chemistry
+
1 mol% Rh2(OAc)4
20 mol% tetrahydrothiophene
20 mol% BnEt3N+Cl–
MeCN, 40 oC, 3 h
O
Na
O
Ph
_
N
+
R
H
Ph
N
2
Ts
R
1 equiv
University of Sheffield
Brook Hill, Sheffield, S3 7HF (UK)
1.5 equiv
‡
[ ] Current address:
Entry
Aldehyde
Yield [%][a]
95
94
86
98
71
97[c]
78[c]
69
trans:cis[b]
School of Chemistry
Bristol University
1
2
3
4
5
6
7
8
9
benzaldehyde
> 98:2
> 98:2
> 98:2
> 98:2
> 98:2
> 98:2
91:9
p-nitrobenzaldehyde
p-chlorobenzaldehyde
p-methoxybenzaldehyde
3-pyridinecarboxaldehyde
trans-cinnamaldehyde
trans-crotonaldehyde
cyclohexanecarboxaldehyde
valeraldehyde
Cantockꢁs Close, Bristol BS8 1TS (UK)
Fax : (‡44)117-9298611
E-mail: V.Aggarwal@bristol.ac.uk
[**] We thank the EPSRC (K.M.L., M.J.P., J.R.S.), Avecia for the support
of a studentship (M.P.), the EU for a Marie Curie Fellowship (E.A.;
HPMF-CT-1999-00076), and Sheffield University for financial sup-
port. We thank Dr. J. Blacker (Avecia), Dr. R. V. H. Jones (Zeneca
Agrochemicals), and Dr. R. Fieldhouse (Zeneca Agrochemicals) for
their interest in this work.
65:35
70:30
59
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy.
[c] Crude yield determined by 1H NMR spectroscopy (epoxides are
unstable to silica gel). Bn ˆ benzyl.
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Table 2. Yields and ratios of epoxides formed from benzaldehyde and
tosylhydrazone salts using 0.2 equivalents of tetrahydrothiophene.
Table 4. Yields and ratios of epoxides formed from aldehydes and
tosylhydrazone salt 2 (generated in situ) using 0.2 equivalents of tetrahy-
drothiophene.
+
1 mol% Rh2(OAc)4
Na
R'
'R
Ar
20 mol% tetrahydrothiophene
a) 1.05 equiv PhCHO, 1,4-dioxane, RT, 0.5 h
b) 1.1 equiv NaH, RT, 1 h
_
N
O
PhCHO
1 equiv
O
+
Ar
Ph
Ph
20 mol% BnEt3N+Cl–
TsNHNH2
N
Ts
MeCN, 40 oC, 3 h
c) 1 mol% Rh2(OAc)4
R
1.1 equiv
20 mol% tetrahydrothiophene
20 mol% BnEt3N+Cl–
Entry
Ar
p-ClC6H4
p-MeOC6H4
p-MeC6H4
o-MeC6H4
p-CNC6H4
R'
Salt
Yield [%][a]
trans:cis[b]
1 equiv RCHO, 40 oC, 6 h
1
2
3
4
5
6
7
H
H
H
H
H
H
Me
2a
2b
2c
2d
2e
2 f
2g[c]
95
96
73
86
89
17
86
> 98:2
67:33
80:20
> 98:2
> 98:2
> 98:2
> 98:2
Entry
Aldehyde (RCHO)
Yield [%][a]
trans:cis[b]
1
2
3
4
5
6
benzaldehyde
70
81
70
40
30
43
> 98:2
> 98:2
> 98:2
> 98:2
70:30
p-nitrobenzaldehyde
p-methoxybenzaldehyde
pyridine 3-carboxaldehyde
cyclohexanecarboxaldehyde
valeraldehyde
2,4,6-Me3C6H2
C6H5
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy.
[c] In situ salt formation from 2,4,6-triisopropylbenzenesulfonyl hydrazone
using NaHMDS (HMDS ˆ 1,1,1,3,3,3,-hexamethyldisilazane) as the base.
80:20
[a] Yield of isolated product. [b] Determined by 1H NMR spectroscopy.
epoxides were obtained. We therefore embarked on the
synthesis of a range of stable, mono and bicyclic chiral sulfides
based on thiolanes, thianes, and 1,4-oxathianes, and from this
extensive study we found that the bridged bicyclic sulfides
6a, b performed exceptionally well.
The [2.2.1] bicyclic sulfide 6a was prepared in four steps
from camphorsulfonyl chloride in 48% overall yield
(Scheme 3). The key step in the synthesis was the cyclo-
salts bearing electron-donating substituents led to lower
diastereoselectivity (Table 2, entries 2 and 3). Tosylhydrazone
salts derived from aryl ketones were also successfully
employed in the process (Table 2, entry 7). We were also able
to couple alkenylhydrazones 3 ± 5 with benzaldehyde to
furnish vinyl epoxides (Table 3). Although high yields were
obtained, the diastereoselectivity was poor and this problem is
currently being addressed.
Table 3. Yields and ratios of epoxides formed from benzaldehyde and
alkenyl-substituted sulfonylhydrazones using 1.0 equivalents of tetrahy-
drothiophene.
R'
H
a) NaHMDS, THF, -78 oC, 10-60 min
N
N
Ts
R
O
b) 1 mol% Rh2(OAc)4
R'
R
20-100 mol% tetrahydrothiophene
20 mol% BnEt3N+Cl–
Ph
1 equiv PhCHO, 40 oC, 16 h
1.5 equiv
Entry
R
R'
Hydrazone
Yield [%][a]
trans:cis[b]
1
2[c]
3
CH3
Ph
H
CH3
Ph
SiMe3
3
4
5
> 90
> 90
76[d]
50:50
67:33
50:50
Scheme 3. Reagents and conditions: a) PPh3, 1,4-dioxane:H2O (4:1), 1 h,
reflux, 92%; b) PhCOCH2Cl, K2CO3, THF, RT, 20 h, 82%; c) sun lamp
(hn), CH2Cl2, 208C, 6 h, cyclopentadiene !9a, 76% or cyclo-
hexadiene !9b, 40%; d) H2, Pd-S/C EtOH, RT, 3 h, 6a, 84%; 6b, 82%.
[a] Crude yield determined by 1H NMR spectroscopy (epoxides are
unstable to silica gel). [b] Determined by 1H NMR spectroscopy. [c] Using
0.2 equiv of tetrahydrothiophene. [d] Yield of isolated pure epoxide (stable
to silica gel).
addition of thioaldehyde
8 (generated photochemically
from phenacyl sulfide 7) with cyclopentadiene.[8] This reaction
was not only highly endo selective, but also showed very
high levels of diastereoselectivity. A single diastereomeric
cycloadduct was obtained in 76% yield. Hydrogenation
and recrystallization gave analytically pure sulfide 6a
whose structure was confirmed by X-ray analysis. The
[2.2.2] bicyclic sulfide 6b was prepared in an analogous
manner (Scheme 3).
The conditions for epoxidation were optimized with sulfide
6a and it was found that the sulfide loading could be reduced
from 20 mol% to just 5 mol% without significantly affecting
the yield or ee value (Table 5, entry 1). Furthermore we were
also able to recover and reuse the sulfide without any loss of
Having demonstrated that we could generate a diazo
compound in situ, we considered the possibility of generating
the hydrazone itself in situ. Remarkably, this worked (Ta-
ble 4) and provided a one-pot, atom-economical method for
coupling two different aldehydes to give epoxides. Thus, we
have developed a general, user-friendly catalytic process for
preparing epoxides by coupling together a carbonyl com-
pound with either a tosylhydrazone salt or a second carbonyl
compound (Scheme 2).
Our efforts to render these processes asymmetric using
thioacetal 1, which had worked well in the epoxidation
process when preformed phenyldiazomethane was used,[3]
were not successful: only low yields of the corresponding
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pair reacts to form a single sulfonium ylide 13.[12] The ylide can
adopt conformations 13A or 13B, but 13B should be strongly
favored because of the 1,4-steric interactions present in 13A
(Scheme 5). The face selectivity of the ylide is then controlled
Table 5. Yields, enantioselectivities, and diastereoselectivities of epoxides
formed from aldehydes and tosylhydrazone salt 2 using 0.05 equivalents of
sulfide 6a.[a]
Entry
Aldehyde
Yield [%][b]
d.r.[c]
ee [%]
1
2
3
4
5
6
7
8
9
benzaldehyde
82
82
75
80
68
84
68
70[c]
58
60[c]
77
> 98:2
> 98:2
> 98:2
> 98:2
> 98:2
> 98:2
> 98:2
98:2
94
92
92
91
92
90
90
87
90
91
92
benzaldehyde[d]
H
p-nitrobenzaldehyde
p-chlorobenzaldehyde
p-methoxybenzaldehyde
p-tolualdehyde
H
H
Ph
H
H
(R,R)
S
Ph
S
H
O
O
(S,S)
o-tolualdehyde
trans-cinnamaldehyde
cyclohexanecarboxaldehyde
2-furylaldehyde
88:12
98:2
98:2
13A
13B
10
11
3-furylaldehyde
Scheme 5. Model to account for the enantioselectivity.
[a] Aldehyde (1.0 mmol), tosylhydrazone sodium salt 2 (2.0 mmol), sulfide
6a (0.05 mmol), phase-transfer catalyst (0.05 mmol), Rh2(OAc)4
(0.01 mmol), and CH3CN (0.8 mL). [b] Yield of isolated product. [c] De-
termined by 1H NMR spectroscopy (epoxides are unstable to silica gel).
[d] Using 6b in place of 6a.
by the bulky camphor group which blocks attack from the
Si face, thus leading to the R,R epoxide. To achieve high
enantioselectivity requires control of the conformation and
high face selectivity of the ylide. The control in the ylide
conformation arises from the conformational rigidity of
sulfides 6a, b, which means that ylide conformer 13A cannot
be easily accommodated through small changes in bond
angles around the sulfur atom. This effect leads to a significant
difference in the energy between the two ylide conformers
13A, B, which in conjunction with the high face selectivity
imposed by the bulky camphor moiety results in the high
enantioselectivity observed.
In summary, we have developed a new class of sulfides
which, at 5 mol% loading, gives high diastereoselectivities,
high enantioselectivities, and high yields in the coupling of a
range of different aldehydes with tosylhydrazone salts. The
process has been scaled up and now represents a practical way
of converting carbonyl compounds directly into epoxides with
control of relative and absolute stereochemistry.
asymmetric induction. Sulfide 6b gave a similarly high yield
and enantioselectivity (Table 5, entry 2), but as its synthesis
was lower yielding further studies were conducted with 6a.
The new procedure was applied to a range of aromatic,
heteroaromatic, unsaturated, and aliphatic aldehydes and was
found to be general (Table 5) and gave high yields, high
diastereoselectivities, and high enantioselectivities in all cases.
The process shown in Table 5, entry 1 has been scaled up to
a 50-mmol scale and found to give similar yields and
enantioselectivities (78 and 92%, respectively). Further
reductions in catalyst loading of Rh2(OAc)4 and the phase-
transfer catalyst have been achieved upon scale-up. Conduct-
ing work on this scale would not have been contemplated
using the ex situ process.[4]
Finally, the synthetic utility of the process is demonstrated
by the conversion of the furaldehyde-derived epoxides into
glycidic esters with high enantiomeric excess values
(Scheme 4). Glycidic esters are versatile intermediates and
Experimental Section
Epoxidation of benzaldehyde (50-mmol scale): Compound 6a (62.5 mg,
2.5 mmol), anhydrous acetonitrile (50 mL), rhodium(ii) acetate dimer
(110 mg, 0.25 mmol), benzyl triethylammonium chloride (228 mg,
1.0 mmol), benzaldehyde (5.1 mL, 50 mmol), and tosylhydrazone sodium
salt 2 (22.3 g, 75 mmol) were added sequentially to a 100-mL 2-neck flask.
The reaction mixture was stirred at 408C under a static nitrogen pressure
for 2 d using an overhead stirrer. The reaction was quenched by the
addition of water (25 mL) and ethyl acetate (25 mL), and the phases were
separated. The aqueous layer was washed with ethyl acetate (2 Â 25 mL)
and the combined organic phases dried (MgSO4), filtered, and concen-
trated in vacuo. The crude product was analyzed by 1H NMR spectroscopy
to determine the diastereomeric ratio and then purified by flash column
chromatography to afford stilbene oxide (7.6 g, 78%).
Scheme 4. Reagents and conditions: a) RuCl3 (2 mol%), NaIO4 (4 equiv),
MeCN:CCl4:H2O, RT, 3 h; b) CH2N2, Et2O, RT.
Received: November 22, 2000
Revised: January 23, 2001 [Z16153]
have found widespread use in synthesis. Thus, oxidation of 10/
11 with RuCl3/NaIO4 followed by esterification[9] furnished
the glycidic esters in good yields. The absolute stereochem-
istry of the glycidic ester was determined by comparison of the
[a]D value with the literature value,[10] and this provided
additional proof of the stereochemical assignment of the
furaldehyde-derived epoxides.
[1] a) T. Katsuki in Comprehensive Asymmetric Catalysis II (Eds.: E. N.
Jacobsen, A. Pfaltz, H. Yamamoto), Springer, Berlin, 1999, pp. 621 ±
648; b) E. N. Jacobsen, M. H. Wu in Comprehensive Asymmetric
Catalysis II (Eds.: E. N. Jacobsen, A. Pfaltz, H. Yamamoto), Springer,
Berlin, 1999, pp. 649 ± 677.
[2] a) V. K. Aggarwal in Comprehensive Asymmetric Catalysis II (Eds.:
E. N. Jacobsen, A. Pfaltz, H. Yamamoto), Springer, Berlin, 1999,
pp. 679 ± 693; b) A. H. Li, L. X. Dai, V. K. Aggarwal, Chem. Rev. 1997,
97, 2341 ± 2372.
The following model is proposed to account for the high
enantioselectivity.[11] Of the two lone pairs only the exo lone
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COMMUNICATIONS
[3] a) V. K. Aggarwal, J. G. Ford, S. Fonquerna, H. Adams, R. V. H. Jones,
R. Fieldhouse, J. Am. Chem. Soc. 1998, 120, 8328 ± 8339; b) V. K.
Aggarwal, J. G. Ford, A. Thompson, R. V. H. Jones, M. Standen, J.
Am. Chem. Soc. 1996, 118, 7004 ± 7005; c) V. K. Aggarwal, Synlett
1998, 329 ± 336.
epoxides with high enantioselectivity. Herein we describe the
extension of this work to the asymmetric aziridination of
imines[1] and to the asymmetric cyclopropanation of electron-
deficient alkenes.[2]
[4] M. Regitz, G. Maas, Diazo Compounds: Properties and Synthesis,
Academic Press, London, 1996. Over a period of five years we
experienced three explosions when distilling phenyldiazomethane and
this encouraged us to seek alternative protocols. Because of these
problems, we did not contemplate scaling up the reaction beyond
1 mmol.
We previously reported that 1,3-oxathiane 1 gave good
yields and high enantioselectivity in aziridination[3] and
cyclopropanation[4] reactions. However, these processes are
[5] For related work, see W. R. Bamford, T. S. Stevens, J. Chem. Soc. 1952,
4735 ± 4740.
[6] For related work, see S. Wulfman, S. Yoousefian, J. M. White, Synth.
Commun. 1978, 8, 569 ± 572.
[7] We have also used this procedure for homologation of aldehydes:
V. K. Aggarwal, J. De Vicente, B. Pelotier, I. P. Holmes, R. V. Bonnert,
Tetrahedron Lett. 2000, 41, 10327 ± 10331; see also: S. R. Angle, M. L.
Neitzel, J. Org. Chem. 2000, 65, 6458 ± 6461.
S
S
O
O
O
S
1
2a
2b
[8] a) E. Vedejs, T. H. Eberlein, D. J. Mazur, C. K. McClure, D. A. Perry,
R. Ruggeri, E. Schwartz, J. S. Stuls, D. L. Varie, R. G. Wilde, S.
Wittemberger, J. Org. Chem. 1986, 51, 1556 ± 1562; b) E. Vedejs, J. S.
Stuls, R. G. Wilde, J. Am. Chem. Soc. 1988, 110, 5452 ± 5460.
[9] a) Y. Tokoro, Y. Kobayashi, Chem. Commun. 1999, 807 ± 808; b) J.-N.
potentially hazardous, cannot be easily scaled up, and the
sulfide cannot be fully recovered. We successfully solved these
problems in the epoxidation reaction by generating the diazo
compound in situ and by developing a new class of chiral
sulfides 2,[5] which were completely stable to the reaction
conditions. We were keen to examine whether these new
conditions and new sulfides were compatible with the
aziridination and cyclopropanation processes.
Optimization of the conditions for aziridination of the N-
SES-activated imine derived from benzaldehyde[6] (this imine
is easily prepared and provides a readily cleavable group) with
sulfide 2a revealed that 1,4-dioxane was the best solvent. A
broad study of different activating groups on the nitrogen
atom showed that sulfonylimines[7] led to aziridines in good
yield, high enantioselectivities, but low diastereoselectivities
(Table 1, entries 1 ± 3). A notable example is the naphthylsul-
fonylimine, which gave excellent results and this group is
considerably easier to deprotect[8] than the toluene-4-sulfonyl
(tosyl) group. Improved diastereoselectivity was observed
Ä
Denis, A. E. Greene, A. Aaraa Serra, M.-J. Luche, J. Org. Chem. 1986,
51, 46 ± 50.
[10] L. He, H. S. Byun, R. Bittman, Tetrahedron Lett. 1998, 39, 2071 ±
2074.
[11] For a discussion of the diastereoselectivity, see V. K. Aggarwal, S.
Calamai, J. G. Ford, J. Chem. Soc. Perkin Trans. 1 1997, 593 ± 599.
[12] Alkylation of sulfide 6 with benzyl bromide gave a single sulfonium
salt whose stereochemistry was determined by X-ray analysis. Full
details will be published elsewhere.
Application of Chiral Sulfides to Catalytic
Asymmetric Aziridination and
Cyclopropanation with In Situ Generation of
the Diazo Compound**
Varinder K. Aggarwal,* Emma Alonso, Guangyu Fang,
Marco Ferrara, George Hynd, and Marina Porcelloni
Table 1. Effect of the nitrogen substituent on the yield, diastereoselectiv-
ity, and enantioselectivity.[a]
In the preceding paper we described a highly efficient
catalytic process for converting carbonyl compounds into
+
R
N
Na
R
+
_
Rh2(OAc)4 (1 mol%)
BnEt3N+Cl– (10 mol%)
N
N
Ph
[*] Prof. V. K. Aggarwal,[‡] Dr. E. Alonso, G. Fang, M. Ferrara,
Dr. G. Hynd, M. Porcelloni
Ph
N
Ts
Ph
Ph
1,4-dioxane,40 °C
sulfide 2a (20 mol%)
1.5 equiv
Department of Chemistry
University of Sheffield
Brook Hill, Sheffield, S3 7HF (UK)
Entry
R
Yield [%][b]
d.r.[c]
(trans:cis)
ee [%][d]
‡
[ ] Current address:
1
2
3
4
5
6[g]
SES
Ts
SO2C10H7
Boc
TcBoc
SES
75
68
70
33[e,f]
71
2.5:1
2.5:1
3:1
8:1
6:1
94
98
97
89
90
95
School of Chemistry
Bristol University
Cantockꢁs Close, Bristol, BS8 1TS (UK)
Fax : (‡44)117-9298611
E-mail: V.Aggarwal@bristol.ac.uk
66
2.5:1
[**] We thank Avecia (M.P.), the EPSRC (M.F.), the EU for a Marie Curie
Fellowship (E.A.; HPMF-CT-1999-00076), Luꢁan Teacherꢁs College
and the Education Minister of The Peoples Republic of China (G.F.),
and Sheffield University for financial support. We thank Dr. J. Blacker
(Avecia), Dr. R. V. H. Jones (Zeneca Agrochemicals), and Dr. R.
Fieldhouse (Zeneca Agrochemicals) for their interest and support of
this work.
[a] Tosylhydrazone salt (1.5 equiv), imine (1.0 equiv), phase-transfer cata-
lyst (PTC, 0.1 equiv), Rh2(OAc)4 (0.01 equiv), 1,4-dioxane (0.33m), chiral
sulfide 2a (0.2 equiv), 408C. [b] Yield of isolated product. [c] The trans:cis
ratio was determined by 1H NMR spectroscopy. [d] Enantiomeric excess
values were determined on a Chiralcel OD column; the absolute config-
uration was 1R,2R. [e] 0.05 equiv of PTC were used. [f] trans-stilbene oxide
was obtained as the main side product. [g] 5 mol% of sulfide was used.
Ts ˆ tosyl ˆ toluene-sulfonyl.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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