Titanium and zirconium complexes with helical bis(phenolato) ligands as hydroamination catalysts

Article abstract of DOI:10.1055/s-2007-986651

A racemic bisphenolato (OSSO)-type ligand that contains a trans-1,2-cyclohexanediyl backbone can be obtained in two steps from commercially available starting materials. In situ combination of this ligand with Ti(NMe₂)₄ or Zr(NMe

Full text of DOI:10.1055/s-2007-986651

2564
LETTER
Titanium and Zirconium Complexes with Helical Bis(phenolato) Ligands as
Hydroamination Catalysts
H
ydroamination C
l
atalystsaudia Marcseková,a Christian Loos,a Frank Rominger,a Sven Doye*b
a
Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
b
Institut für Reine und Angewandte Chemie, Universität Oldenburg, Carl-von-Ossietzky-Str. 9–11, 26111 Oldenburg, Germany
Fax +49(441)7983329; E-mail: doye@uni-oldenburg.de
Received 4 July 2007
related but sterically more demanding chiral bisphenolato
(OSSO)-ligand and its use for initial Ti- and Zr-catalyzed
hydroamination reactions.
Abstract: A racemic bisphenolato (OSSO)-type ligand that con-
tains a trans-1,2-cyclohexanediyl backbone can be obtained in two
steps from commercially available starting materials. In situ combi-
nation of this ligand with Ti(NMe2)4 or Zr(NMe2)4 results in the for-
mation of bis(phenolato) complexes that catalyze hydroaminations
of alkynes and alkenes.
The synthesis of the desired ligand rac-3 (Scheme 1)
started with the reaction of commercially available phenol
1 with S2Cl2 in the presence of TiCl4 which gave the
disulfide 2 in 55% yield on a 50-gram scale.7,8 Subse-
quently, 2 was directly reacted with cyclohexene in the
presence of BF3·OEt2 to give rac-3 in 96% yield as a crys-
talline compound.9,10 Interestingly, the 500 MHz 1H NMR
spectrum of rac-3 in CD2Cl2 showed only two broad sin-
glets for the two pairs of diastereotopic methyl groups at
d = 1.64 (12 H) and 1.70 (12 H) ppm. The desired trans
configuration of the ligand rac-3 was confirmed by X-ray
crystallographic analysis (Figure 1).11
Key words: alkynes, aminations, amines, homogenous catalysis,
titanium
During the last few years, many metal complexes that cat-
alyze the addition of N–H across carbon–carbon multiple
bonds have been identified.1 In particular, neutral Ti- and
Zr-complexes have been shown to catalyze the hydro-
amination of alkynes2 and alkenes3 with primary amines.
Recently, corresponding cyclizations of aminoalkenes
have been achieved in an enantioselective fashion4 with ee
values up to 93% in the presence of a chiral zirconium
amidate complex.4b Inspired by a report from the Okuda
group5 that described the use of scandium complexes with
dithiaalkanediyl-bridged bisphenolato (OSSO)-type
ligands for the ring-opening polymerization of lactide we
decided to investigate Ti- and Zr-complexes with corre-
sponding chiral bisphenolato ligands as potential catalysts
for enantioselective hydroamination reactions. However,
in this context, it must be mentioned that during the course
of our study, the Okuda group reported the synthesis of an
enantiomerically pure chiral Ti-complex with an (OSSO)-
ligand that contains a trans-1,2-cyclohexanediyl back-
bone and its use for the polymerization of styrene.6 In this
letter we describe a short racemic synthesis of a closely
Since our original idea was to use a dimethyl titanium
complex as the catalyst for the planned hydroamination
reactions ligand rac-3 was initially treated with TiCl4 to
give the bis(phenolato)titanium dichloro complex rac-4
as a dark red crystalline compound (Scheme 2).12 In con-
trast to the free ligand, the 500 MHz 1H NMR spectrum of
rac-4 in CD2Cl2 showed four sharp singlets for the two
pairs of diastereotopic methyl groups at d = 1.65 (6 H),
1.70 (6 H), 1.73 (6 H) and 1.85 (6 H) ppm. To confirm the
monomeric structure of complex rac-4 an X-ray crystallo-
graphic analysis was carried out (Figure 2).13 As observed
before,5,6 the helical arrangement of the ligand around the
metal center causes an additional stereogenic unit. How-
ever, rac-4 was obtained as a single diastereomer with dis-
torted octahedral geometry around the titanium center.
PhMe2C
CMe2Ph
OH
cyclohexene
BF3·OEt2
MeNO2, CH2Cl2
25 °C, 72 h
S2Cl2
2.0 mol% TiCl4
toluene, 25 °C, 72 h
OH
OH
OH
PhMe2C
PhMe2C
S
S
CMe2Ph
S
S
rac-3
96%
55%
CMe2Ph
CMe2Ph
CMe2Ph
OH
1
2
PhMe2C
CMe2Ph
Scheme 1 Two-step synthesis of bis(phenolato) ligand rac-3
SYNLETT 2007, No. 16, pp 2564–2568
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1
.1
0
.2
0
0
7
Advanced online publication: 12.09.2007
DOI: 10.1055/s-2007-986651; Art ID: G19807ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Hydroamination Catalysts
2565
PhMe2C
CMe2Ph
OH
PhMe2C
CMe2Ph
TiCl4
toluene
25 °C, 3 h
S
S
O
Ti
O
S
Cl
Cl
S
99%
OH
CMe2Ph
PhMe2C
PhMe2C
CMe2Ph
rac-3
rac-4
Scheme 2 Synthesis of titanium complex rac-4
Figure 1 X-ray crystal structure of rac-311
Due to the relatively strong binding chloro ligands rac-4
showed only poor activity as a hydroamination catalyst.
For example, in the presence of 5 mol% of rac-4, the
reaction between 1-phenylpropyne and 4-methylaniline
gave only trace amounts of hydroamination products after
48 hours at 105 °C. Consequently, we tried to exchange
the chloro ligands with more labile methyl groups. How-
ever, our attempts to convert rac-4 into the corresponding
dimethyl complex using methyllithium in diethyl ether at
temperatures between –20 °C and –80 °C failed. For that
reason, we turned our attention to an in situ generation of
the hydroamination catalyst from the combination of
equimolar amounts of the ligand rac-3 and Ti(NMe2)4.4a,14
First of all, we tried to observe the involved amine elimi-
Figure 2 X-ray crystal structure of rac-413
strates were added and all resulting mixtures were heated
to 105 °C for 24 hours (reaction times have not been
minimized). After subsequent reduction with NaBH3CN
in the presence of ZnCl2, secondary amines were obtained
from most of the test reaction sequences.15
1
nation reaction by H NMR spectroscopy. Upon mixing
Ti(NMe2)4 and an equimolar amount of rac-3 in CD2Cl2
at 25 °C the singlet for the methyl groups of Ti(NMe2)4 at
d = 3.04 ppm as well as the broad signals for the phenolic
H-atoms (d = 6.83 ppm) and the methyl groups of rac-3
(vide supra) disappeared completely. On the other hand,
four new sharp singlets for the diastereotopic methyl
groups of the in situ formed Ti-complex appeared at d =
1.66 (6 H), 1.68 (6 H), 1.70 (6 H) and 1.83 (6 H) ppm
(compare with rac-4). Furthermore, a new dimethylamide
signal at d = 2.80 ppm (12 H) proved that a monomeric Ti-
species comparable to the dichloro complex rac-4 was
present in solution. Additionally, a doublet at d = 2.37
ppm caused by free dimethylamine was observed.
As can be seen from Table 1, 4-methylaniline and cyclo-
pentylamine underwent smooth addition reactions with 1-
phenylpropyne (entries 1 and 2) to give the corresponding
anti-Markovnikov products with excellent regioselec-
tivity. Interestingly, sterically more demanding tert-butyl-
amine did not undergo a hydroamination reaction with 1-
phenylpropyne (entry 3). A similar behavior was observed
with the terminal alkyne 1-dodecyne (entries 7–9). Again,
no conversion was observed with tert-butylamine while
4-methylaniline and cyclopentylamine underwent a
hydroamination reaction. Interestingly, 1-dodecyne and
cyclopentylamine were selectively converted into the
Markovnikov product (entry 8). Formation of the
Markovnikov product was also favored with 4-methyl-
aniline (entry 7) but in this case, the selectivity was only
6:1. anti-Markovnikov-selective additions to 4-methoxy-
phenylacetylene could be realized with 4-methylaniline,
With this result in hand, we turned our attention to some
intermolecular hydroaminations of alkynes (Table 1). The
in situ formation of the catalyst was always carried out by
stirring 5 mol% of the ligand rac-3 and an equimolar
amount of Ti(NMe2)4 in toluene for 30 minutes at room
temperature. Subsequently, the alkyne and amine sub-
Synlett 2007, No. 16, 2564–2568 © Thieme Stuttgart · New York

2566
K. Marcseková et al.
LETTER
Table 1 Intermolecular Hydroamination of Alkynes15
1) 5 mol% Ti(NMe2)4
5 mol% rac-3
toluene, 105 °C, 24 h
R1
NHR3
R2
R2
R1
R1
R2
+
H2N R3
+
NHR3
2) NaBH3CN, ZnCl2
MeOH, 25 °C, 20 h
a
b
Entry
R1
R2
Me
Me
Me
H
R3
tolyl
Yield a + b (%)a
Ratio a/bb
1
2
3
4
5
6
7
8
9
Ph
79
62
–
>99:1
98:2
Ph
cyclopentyl
t-Bu
Ph
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
n-C10H21
n-C10H21
n-C10H21
tolyl
51
64
28
99
58
–
2.4:1
H
cyclopentyl
t-Bu
7:1
>99:1
1:6
H
H
tolyl
H
cyclopentyl
t-Bu
< 1:99
H
a Reaction conditions: (1) alkyne (2.40 mmol), amine (2.64 mmol), Ti(NMe2)4 (0.12 mmol, 5 mol%), rac-3 (0.12 mmol, 5 mol%), toluene
(1.0 mL), 105 °C, 24 h; (2) NaBH3CN (4.80 mmol), ZnCl2 (2.40 mmol), MeOH (10 mL), 25 °C, 20 h. Yields refer to isolated compounds.
b Determined by GC–MS.
cyclopentylamine and tert-butylamine (entries 4–6). Al- In summary, we have shown for the first time that a di-
though the best regioselectivity was achieved with tert- thiaalkanediyl-bridged bisphenolato (OSSO)-type ligand
butylamine the yield was only 28%. Increased yields that contains a trans-1,2-cyclohexanediyl backbone can
(51% and 64%) but decreased regioselectivities (2.4:1 and be used for various group IV metal-catalyzed hydroami-
7:1) were observed with 4-methylaniline and cyclopentyl- nation reactions. The helical arrangement of the chiral
amine. Additionally, it must be mentioned that reactions ligand around the metal center suggests that correspond-
of 3-hexyne with various amines did not result in the for- ing optically pure ligands6 can be used for enantioselec-
mation of any hydroamination products. However, the re- tive hydroamination reactions. Further studies in this area
sults summarized in Table 1 clearly indicate that an in situ are currently underway in our laboratories.
generated Ti-complex with an (OSSO)-type ligand that
contains a trans-1,2-cyclohexanediyl backbone can
principally be used as a catalyst for the intermolecular
Acknowledgment
We thank the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie for financial support of our research.
hydroamination of alkynes.
Finally, we performed two initial hydroamination experi-
ments with aminoalkene 5 using an in situ generated Ti-
or Zr-catalyst (Scheme 3). In both cases, formation of the
catalyst was carried out by stirring 5 mol% of the ligand
rac-3 and an equimolar amount of either Ti(NMe2)4 or
Zr(NMe2)4 in toluene for 30 minutes at room temperature.
Subsequent addition of aminoalkene 5 and heating to
105 °C for 24 hours resulted in the formation of the
desired cyclization product 6 in 76% and 82% yields,
respectively.
References and Notes
(1) For reviews, see: (a) Chekulaeva, I. A.; Kondratjeva, L. V.
Russ. Chem. Rev. (Engl. Transl.) 1965, 34, 669.
(b) Steinborn, D.; Taube, R. Z. Chem. 1986, 26, 349.
(c) Taube, R. In Applied Homogeneous Catalysis with
Organometallic Compounds; Cornils, B.; Herrmann, W. A.,
Eds.; Wiley-VCH: Weinheim, 1996, 507. (d) Müller, T. E.;
Beller, M. Chem. Rev. 1998, 98, 675. (e) Müller, T. E.;
Beller, M. In Transition Metals for Organic Synthesis, Vol.
2; Beller, M.; Bolm, C., Eds.; Wiley-VCH: Weinheim, 1998,
316. (f) Haak, E.; Doye, S. Chem. Unserer Zeit 1999, 33,
296. (g) Nobis, M.; Drießen-Hölscher, B. Angew. Chem. Int.
Ed. 2001, 40, 3983; Angew. Chem. 2001, 113, 4105.
(h) Brunet, J. J.; Neibecker, D. In Catalytic
5 mol% M(NMe2)4
5 mol% rac-3
toluene, 105 °C, 24 h
Ph
Ph
Ph
NH2
Ph
N
H
M = Ti 76%
M = Zr 82%
Heterofunctionalization; Togni, A.; Grützmacher, H., Eds.;
Wiley-VCH: Weinheim, 2001, 91. (i) Roesky, P. W.;
Müller, T. E. Angew. Chem. Int. Ed. 2003, 42, 2708; Angew.
Chem. 2003, 115, 2812. (j) Bytschkov, I.; Doye, S. Eur. J.
5
6
Scheme 3 Intramolecular hydroamination of alkenes
Synlett 2007, No. 16, 2564–2568 © Thieme Stuttgart · New York

LETTER
Org. Chem. 2003, 935. (k) Hartwig, J. F. Pure Appl. Chem.
Hydroamination Catalysts
2567
25.42°, radiation Mo–Ka, l = 0.71073 Å, 0.3° w-scans with
CCD area detector, covering a whole sphere in reciprocal
space, 21011 reflections measured, 8802 unique [R(int) =
0.0883], 4564 observed [I >2s(I)], intensities were corrected
for Lorentz and polarization effects, an empirical absorption
correction was applied using SADABS16 based on the Laue
2004, 76, 507. (l) Alonso, F.; Beletskaya, I. P.; Yus, M.
Chem. Rev. 2004, 104, 3079. (m) Doye, S. Synlett 2004,
1653. (n) Odom, A. L. Dalton Trans. 2005, 225.
(o) Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367.
(p) Hultzsch, K. C. Org. Biomol. Chem. 2005, 3, 1819.
(2) For reviews, see: (a) Pohlki, F.; Doye, S. Chem. Soc. Rev.
2003, 32, 104. (b) Severin, R.; Doye, S. Chem. Soc. Rev.
2007, 36, 1407.
symmetry of the reciprocal space, m = 0.15 mm–1, Tmin
=
0.94, Tmax = 0.98, structure solved by direct methods and
refined against F2 with a full-matrix least-squares algorithm
using the SHELXTL-PLUS16 software package, 577
parameters refined, hydrogen atoms were treated using
appropriate riding models, except for H1 and H2 at the
oxygen atoms, which were refined isotropically, goodness
of fit 0.98 for observed reflections, final residual values
R1(F) = 0.062, wR(F2) = 0.115 for observed reflections,
residual electron density –0.30 eÅ–3 to 0.24 eÅ–3. CCDC
number 652550 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(3) (a) Bexrud, J. A.; Beard, J. D.; Leitch, D. C.; Schafer, L. L.
Org. Lett. 2005, 7, 1959. (b) Kim, H.; Lee, P. H.;
Livinghouse, T. Chem. Commun. 2005, 5205. (c) Müller,
C.; Loos, C.; Schulenberg, N.; Doye, S. Eur. J. Org. Chem.
2006, 2499. (d) Thomson, R. K.; Bexrud, J. A.; Schafer, L.
L. Organometallics 2006, 25, 4069. (e) Lee, A. V.; Schafer,
L. L. Organometallics 2006, 25, 5249. (f) Kim, H.; Kim, Y.
K.; Shim, J. H.; Kim, M.; Han, M.; Livinghouse, T.; Lee, P.
H. Adv. Synth. Catal. 2006, 348, 2609. (g) Stubbert, B. D.;
Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149.
(4) (a) Watson, D. A.; Chiu, M.; Bergman, R. G.
Organometallics 2006, 25, 4731. (b) Wood, M. C.; Leitch,
D. C.; Yeung, C. S.; Kozak, J. A.; Schafer, L. L. Angew.
Chem. Int. Ed. 2007, 46, 354; Angew. Chem. 2007, 119, 358.
(5) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem. Int. Ed.
2006, 45, 7818; Angew. Chem. 2006, 118, 7982.
(12) Experimental Procedure: TiCl4 (95 mg, 0.5 mmol) was
added to a solution of rac-3 (402 mg, 0.5 mmol) in toluene
(3 mL) at r.t. After the mixture had been stirred for 3 h at r.t.,
all volatiles (toluene, HCl) were removed under reduced
pressure. The residue was washed with hexane.
(6) Beckerle, K.; Manivannan, R.; Lian, B.; Meppelder, G.-J.
M.; Raabe, G.; Spaniol, T. P.; Ebeling, H.; Pelascini, F.;
Mülhaupt, R.; Okuda, J. Angew. Chem. Int. Ed. 2007, 46,
4790; Angew. Chem. 2007, 119, 4874.
(7) Pastor, S. D.; Denney, D. Z. J. Heterocycl. Chem. 1988, 25,
681.
Crystallization from toluene gave complex rac-4 (456 mg,
0.49 mmol, 99%) as red-brown crystals. 1H NMR (500 MHz,
CD2Cl2): d = 0.64–0.72 (m, 2 H), 0.87–0.93 (m, 2 H), 1.46–
1.48 (m, 2 H), 1.65 (s, 6 H), 1.68–1.69 (m, 2 H), 1.70 (s, 6
H), 1.73 (s, 6 H), 1.79–1.81 (m, 2 H), 1.85 (s, 6 H), 6.80 (d,
J = 2.2 Hz, 2 H), 7.12–7.32 (m, 20 H), 7.56 (d, J = 2.1 Hz, 2
H). In order to avoid the formation of solid byproducts the
reaction was performed in the absence of a base.
(8) Experimental Procedure: S2Cl2 (16.88 g, 125 mmol) was
added dropwise to a solution of 2,4-bis(a,a-dimethyl-
benzyl)phenol (1, 82.62 g, 250 mmol) and TiCl4 (0.474 g,
2.5 mmol, 2.0 mol%) in toluene (180 mL) at –5 °C. The
resulting mixture was stirred at –5 °C for 30 min and at r.t.
for 72 h. Then, the mixture was washed with aq HCl (2 × 250
mL, c = 19%), sat. aq Na2CO3 solution and H2O. The organic
layer was dried with MgSO4 and concentrated under
vacuum. Crystallization from MeCN (1800 mL) gave
analytically pure disulfide 2 (49.88 g, 69 mmol, 55%). 1H
NMR (300 MHz, CD2Cl2): d = 0.97 (s, 12 H), 1.04 (s, 12 H),
6.49–6.62 (m, 24 H). Anal. Calcd for C48H50O2S2 (723.0): C,
79.73; H, 6.97; S, 8.87. Found: C, 79.70; H, 6.93; S, 8.96.
(9) Caserio, M. C.; Fisher, C. L.; Kim, J. K. J. Org. Chem. 1985,
50, 4390.
(10) Experimental Procedure: Cyclohexene (4.6 g, 56 mmol)
and BF3·OEt2 (0.5 mL) were added to a solution of disulfide
2 (20 g, 28 mmol) in a mixture of nitromethane (15 mL) and
CH2Cl2 (15 mL) at –10 °C. The resulting mixture was stirred
at –10 °C for 3 h and at r.t. for 72 h. Then, the mixture was
washed with sat. aq NaHCO3 solution. The organic layer was
dried with MgSO4 and concentrated under vacuum.
Crystallization from a 4:1-mixture of MeCN and acetone
(200 mL) gave analytically pure bis(phenolato) ligand rac-3
(21.68 g, 27 mmol, 96%). 1H NMR (500 MHz, CD2Cl2): d =
1.02–1.06 (m, 4 H), 1.54–1.58 (m, 2 H), 1.64 (s, 12 H), 1.70
(s, 12 H), 1.70–1.74 (m, 2 H), 2.47–2.51 (m, 2 H), 6.83 (s, 2
H), 7.10–7.14 (m, 6 H), 7.15–7.22 (m, 8 H), 7.25–7.30 (m, 8
H), 7.37 (d, J = 2.1 Hz, 2 H). Anal. Calcd for C54H60O2S2
(805.2): C, 80.55; H, 7.51; S, 7.96. Found: C, 80.38; H, 7.51;
S, 7.97.
(13) Red-brown crystal(polyhedron), dimensions 0.20 × 0.16 ×
0.05 mm3, crystal system monoclinic, space group C2/c, Z =
4, a = 28.177(3) Å, b = 11.1949(13) Å, c = 19.892(2) Å, b =
105.392(3)°, V = 6049.5(12) Å3, r = 1.215 g/cm3, T = 200(2)
K, Qmax = 28.40°, radiation Mo–Ka, l = 0.71073 Å, 0.3° w-
scans with CCD area detector, covering a whole sphere in
reciprocal space, 31295 reflections measured, 7545 unique
[R(int) = 0.0531], 5283 observed [I >2s(I)], intensities were
corrected for Lorentz and polarization effects, an empirical
absorption correction was applied using SADABS16 based
on the Laue symmetry of the reciprocal space, m = 0.34
mm–1, Tmin = 0.93, Tmax = 0.98, structure solved by direct
methods and refined against F2 with a full-matrix least-
squares algorithm using the SHELXTL-PLUS16 software
package, 488 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 1.03
for observed reflections, final residual values R1(F) = 0.057,
wR(F2) = 0.117 for observed reflections, residual electron
density –0.20 eÅ–3 to 0.38 eÅ–3. CCDC number 652551
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(14) (a) Khedkar, V.; Tillack, A.; Beller, M. Org. Lett. 2003, 5,
4767. (b) Tillack, A.; Khedkar, V.; Beller, M. Tetrahedron
Lett. 2004, 45, 8875. (c) Tillack, A.; Khedkar, V.; Jiao, H.;
Beller, M. Eur. J. Org. Chem. 2005, 5001.
(15) General Procedure: A Schlenk tube equipped with a Teflon
stopcock and a magnetic stirring bar was charged with
Ti(NMe2)4 (27 mg, 0.12 mmol, 5.0 mol%), rac-3 (97 mg,
0.12 mmol, 5.0 mol%) and toluene (1.0 mL). After this
mixture had been stirred for 30 min at r.t., the alkyne (2.40
mmol) and the amine (2.64 mmol) were added and the
resulting mixture was heated to 105 °C for 24 h. Then the
(11) Colorless crystal(polyhedron), dimensions 0.42 × 0.18 ×
0.10 mm3, crystal system triclinic, space group P1, Z = 2,
a = 8.5820(2) Å, b = 16.3662(4) Å, c = 18.3078(5) Å, a =
73.1710(10)°, b = 83.3570(10)°, g = 77.9890(10)°, V =
2403.18(10) Å3, r = 1.169 g/cm3, T = 200(2) K, Qmax
=
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2568
K. Marcseková et al.
LETTER
mixture was cooled to r.t. and a mixture of NaBH3CN (302
mg, 4.80 mmol) and ZnCl2 (326 mg, 2.40 mmol) in MeOH
(10 mL) was added. After this mixture had been stirred at
25 °C for 20 h, CH2Cl2 (50 mL) and sat. Na2CO3 solution
(20 mL) were added. The resulting mixture was filtered and
the solid residue was washed with CH2Cl2 (50 mL). After
extraction, the organic layer was separated. The aqueous
layer was extracted with CH2Cl2 (6 × 50 mL). The combined
organic layers were dried with Na2SO4. After concentration
under vacuum, the residue was purified by flash chromatog-
raphy (SiO2). All compounds were identified by comparison
of the obtained 1H and 13C NMR spectra with those reported
in the literature.17
(16) Sheldrick, G. M. SHELXTL-PLUS; Bruker Analytical X-ray
Division: Madison Wisconsin, 2001.
(17) Heutling, A.; Pohlki, F.; Doye, S. Chem. Eur. J. 2004, 10,
3059.
Synlett 2007, No. 16, 2564–2568 © Thieme Stuttgart · New York