Isolation and racemization mechanism of optically active benzylmethylphenyltelluronium salts

Article abstract of DOI:10.1002/ejoc.201000893

Diastereomeric benzylmethylphenyltelluronium (1S)-(+)- and (1R)-(-)-camphor-10-sulfonates were synthesized and resolved by fractional recrystallization. The absolute configurations of the isomers were determined by X-ray crystallographic analysis. Enantio

Full text of DOI:10.1002/ejoc.201000893

FULL PAPER
DOI: 10.1002/ejoc.201000893
Isolation and Racemization Mechanism of Optically Active
Benzylmethylphenyltelluronium Salts
Toshio Shimizu,*[a] Ryuuya Sakurai,[a] Yoko Azami,[a] Kazunori Hirabayashi,[a] and
Nobumasa Kamigata[a]
Keywords: Chirality / Configuration determination / Circular dichroism / Racemization mechanism / Tellurium
Diastereomeric benzylmethylphenyltelluronium (1S)-(+)-
and (1R)-(–)-camphor-10-sulfonates were synthesized and re-
solved by fractional recrystallization. The absolute configura-
tions of the isomers were determined by X-ray crystallo-
graphic analysis. Enantiomerically pure telluronium salts
with achiral counteranions, such as tetrafluoroborate, 4-chlo-
robenzenesulfonate, bromide, iodide, and tetraphenylborate,
were also obtained by anion-exchange reactions of the sepa-
rated diastereomers. The optically active telluronium salts
possessing counteranions with a high nucleophilicity, such as
bromide and iodide, were found to racemize in solution. The
racemization of the telluronium salts was found to proceed
either through the formation of a tellurane by the nucleo-
philic addition of a counteranion to the tellurium atom fol-
lowed by decomposition into methyl phenyl telluride and
benzyl halide or by several pseudorotations of the tellurane.
Introduction
Results and Discussion
Optical Resolution of Diastereomeric
Benzylmethylphenyltelluronium Salts
Optically active tricoordinate chalcogen compounds have
been the subject of considerable interest, particularly re-
garding their synthesis, stability, and role as intermediates
in organic synthesis. Chiral sulfur compounds, in particular,
have been widely investigated.[1,2] We have been interested
in the isolation and stereochemistry of optically active sele-
nium and tellurium compounds. Some optically active sele-
nium and tellurium compounds have been synthesized and
their stereochemistry has been studied.[2–4] Among the chi-
ral chalcogen compounds, optically active telluronium salts
were isolated for the first time as early as 1929,[5] and the
next paper appeared in 1945.[6] In those papers, it was de-
scribed that the chiral telluronium salts decomposed or ra-
cemized in solution. Previously, we isolated optically active
dialkylaryltelluronium salts but observed no racemization
even in solution.[7–9] Koizumi and co-workers also isolated
stable optically active dialkyl- or alkylaryl(2-exo-hydroxy-
10-bornyl)telluronium salts.[10] Recently, we synthesized
benzylmethylphenyltelluronium salts and isolated their op-
tically active isomers with several counteranions. We also
found that some isomers racemized in solution. We report
herein the isolation, stereochemistry, and racemization of
optically active benzylmethylphenyltelluronium salts, and
clarify the racemization mechanism.[11]
A racemic sample of benzylmethylphenyltelluronium
bromide was prepared by reacting methyl phenyl telluride
with benzyl bromide. A diastereomeric mixture of benzyl-
methylphenyltelluronium
(1S)-(+)-camphor-10-sulfonate
(dia.-1) {[α]2D5 = +15.8 (c = 1.00, MeOH). [α]2D5 = +33.5 (c
= 1.00, CHCl3)} was synthesized by counteranion-exchange
reaction of the bromide.[11] The diastereomeric mixture was
separated roughly into acetone-soluble and -insoluble frac-
tions. Repeated recrystallization (four times) of the acetone-
soluble fraction from acetone/hexane afforded the dia-
stereomerically pure (+)-telluronium salt (+)-1 as colorless
needles. In the case of the previously synthesized dialkylar-
yltelluronium salt, ethylmethylphenyltelluronium (1S)-(+)-
camphor-10-sulfonate, the (R)-isomer was dextrorotatory in
methanol and levorotatory in chloroform,[8] whereas (+)-1
showed dextrorotatory rotation due to the chirality of the
tellurium atom in both methanol and chloroform {[α]D25
=
+80.0 (c = 1.00, MeOH). [α]2D5 = +88.8 (c = 1.00, CHCl3)}.
Recrystallization (once) of the acetone-insoluble fraction
from dichloromethane/acetone yielded the diastereo-
merically pure (–)-telluronium salt (–)-1 {[α]2D5 = –57.3 (c =
1.00, MeOH). [α]2D5 = –21.2 (c = 1.00, CHCl3)} as colorless
1
prisms. The optical purities were determined by H NMR
spectroscopic measurements (the methyl signals of the
telluronium cations for (+)-1 and (–)-1 showed different
chemical shifts). (+)-Benzylmethylphenyltelluronium (1R)-
(–)-camphor-10-sulfonate possessing an enantiomeric
counteranion was also prepared in a similar manner as its
[a] Department of Chemistry, Graduate School of Science and
Engineering, Tokyo Metropolitan University,
Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
Fax: +81-42-677-2525
E-mail: shimizu-toshio@tmu.ac.jp
6556
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6556–6562

Optically Active Benzylmethylphenyltelluronium Salts
enantiomer (–)-1, by counteranion-exchange of the racemic obtained showed opposite signs of the specific rotations in
telluronium bromide followed by recrystallization of the methanol and chloroform.[8] However, the chiral benzyl-
acetone-insoluble fraction from dichloromethane/acetone.
methylphenyltelluronium salt 3 with the same counteranion
did not show such a phenomenon.
Table 1. Anion exchange reactions of (R)-(+)-1 and (S)-(–)-1 and
their specific rotations.
Absolute Configuration of Diastereomeric Telluronium Salts
The absolute configuration around the tellurium atom of
(–)-1 was determined to be S by X-ray crystallographic
analysis based on the known configuration of (1S)-(+)-cam-
phor-10-sulfonate as an internal standard.[11] Therefore, the
absolute configuration of (+)-1 was assigned to be R
around the tellurium atom. The tellurium atom of (S)-(–)-
1 was also found to interact with two oxygen atoms of the
different sulfonate anions in the crystalline state, and the
distances between the tellurium atom and the oxygen atoms
were 2.84 and 3.01 Å, respectively (Figure 1). Through
these tellurium–oxygen interactions, the salts are arranged
in a zigzag manner.
[a]
Compound
Yield [%]
[α]D
in MeOH
in CHCl3
(R)-(+)-2
(S)-(–)-2
(R)-(+)-3
(S)-(–)-3
(R)-(+)-4
(S)-(–)-4
(R)-(+)-5
(S)-(–)-5
(R)-(+)-6
(S)-(–)-6
60
65
90
94
63
73
67
80
79
89
+96.3
–94.1
+80.3
+143.5
–158.3
+71.4
–81.8
–74.9
+64.9[b]
–66.5
+93.8[b]
–96.5
+76.5[b]
–77.3
+105.6[b]
–107.4
+22.5
+58.0[b]
–30.5[c]
–14.0
[a] Specific rotations were measured at 25 °C and c = 1.00. [b] c =
0.50. [c] c = 0.67.
Figure 1. Crystal structure of (S)-(–)-1 with cation–anion interac-
tions. Hydrogen atoms are omitted for clarity.
Circular Dichroism Spectra of Optically Active Telluronium
Salts
Diastereomerically pure telluronium salt (R)-(+)-1
showed a positive first Cotton effect at 298 nm and a posi-
tive second Cotton effect at 247 nm in the circular dichro-
Transformation of Diastereomerically Pure Telluronium
Salts into Enantiomerically Pure Telluronium Salts
Enantiomeric (R)- and (S)-benzylmethylphenyltellur- ism spectrum in methanol (Figure 2). Telluronium salt (S)-
onium tetrafluoroborates (R)-(+)-2 and (S)-(–)-2 were ob- (–)-1 also showed a similar positive first Cotton effect at
tained in 60 and 65% yields, respectively, by the anion-ex- 297 nm, but the corresponding second Cotton effect was
change reactions of diastereomerically pure telluronium negative. Therefore, the observed first Cotton effects of (R)-
salts (R)-(+)-1 and (S)-(–)-1 with sodium tetrafluoroborate (+)-1 and (S)-(–)-1 were found to be derived from the com-
(Table 1). The optical purities of (R)-(+)-2 and (S)-(–)-2 mon chiral counteranion, (1S)-(+)-camphor-10-sulfonate
were determined to be 100% ee on the basis of the 1H bearing a carbonyl group, and the second Cotton effect
NMR spectra by using (S)-(–)-1,1Ј-bi-2-naphthol as the chi- must be caused by the chirality on the tellurium atoms. In
ral shift reagent; singlet signals of methyl groups for (R)- chloroform, both diastereomers gave Cotton effects with
(+)-2 and (S)-(–)-2 were observed at different chemical the same respective signs as those in methanol, but the sec-
shifts in the presence of the chiral reagent. Optically active ond Cotton effects shifted to 268 nm. Enantiomerically
(R)- and (S)-benzylmethylphenyltelluronium p-chlorobenz- pure telluronium salt (R)-(+)-2 showed a positive Cotton
enesulfonates (R)-(+)-3 and (S)-(–)-3, bromides (R)-(+)-4 effect at 247 nm in methanol similar to that of (R)-(+)-1,
and (S)-(–)-4, iodides (R)-(+)-5 and (S)-(–)-5, and tetra- and no Cotton effect around 300 nm, as observed in the
phenylborates (R)-(+)-6 and (S)-(–)-6 were also obtained by cases of 1. Telluronium salt (S)-(–)-2 showed a symmetrical
similar anion-exchange reactions of (R)-(+)-1 and (S)-(–)-1. spectrum to that of (R)-(+)-2. Other enantiomeric tellur-
The telluronium salts (R)- and (S)-3–5 were also found to onium salts with R-configuration, (R)-(+)-3–6, also showed
be optically pure, but the optical purities of (R)- and (S)-6 positive Cotton effects. The circular dichroism spectra of
1
could not be determined because the split of the H NMR (R)-(+)-2–6 looked very similar to one another (the maxima
signals of the racemic sample was not observed in the pres- of the Cotton effects appeared at 246–247 nm) in methanol
ence of several chiral shift reagents. The specific rotations (compounds 2–6 showed absorption maxima or shoulder in
are also summarized in Table 1. The chiral ethylmeth- the range of 237–249 nm in methanol corresponding to π–
ylphenyltelluronium p-chlorobenzenesulfonate previously π* transitions), while the corresponding Cotton effects in
Eur. J. Org. Chem. 2010, 6556–6562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6557

T. Shimizu, R. Sakurai, Y. Azami, K. Hirabayashi, N. Kamigata
FULL PAPER
chloroform were observed in the range of 268–276 nm. served in a narrow region, as shown in Figure 3, indicating
These results indicate that the Cotton effects in methanol that the interaction between the tellurium atom of the salts
are not affected by the different counteranions, possibly due and bromide or iodide is stronger than those with the other
to the strong solvation by methanol, and the Cotton effects counteranions. These experimental findings may indicate
in methanol are mainly derived from the telluronium cation the occurrence of nucleophilic attack of the counteranion
moiety.
on the positive tellurium atom to yield a tellurane.
Table 2. First-order rate constants and half-lives for racemization
of enantiomeric telluronium salts (S)-4 and (S)-5.[a]
Solvent
kϫ105 [s–1] (t1/2 [h])
(S)-4
(S)-5
CHCl3
EtOH
MeOH
DMSO
10
1.1
0.49
0.26
(1.9)
(17)
(39)
(75)
440
31
7.4
5.7
(0.044)
(0.63)
(2.6)
(3.4)
[a] Racemizations were measured at 25 °C.
Table 3. 125Te NMR chemical shifts of telluronium salts 1–6.
Solvent
Chemical shift [ppm]
(S)-1
2
3
4
5
6
[a]
CDCl3
[D6]acetone
[D6]DMSO
CD3OD
629
654
660
655
626
647
656
654
631
656
658
655
608
616
634
638
585
626
646
640
–
633
655
[a]
–
[a] Insoluble.
Figure 2. Circular dichroism spectra of optically active telluronium
salts 1–6 in methanol.
Configurational Stability and Racemization Mechanism of
Optically Active Telluronium Salts
Diastereomerically pure telluronium salt (S)-1 was stable
both in the crystalline state and in chloroform at room tem-
perature. However, in boiling chloroform, the racemization
of (S)-1 was observed together with the formation of trace
amounts of methyl phenyl telluride and benzyl (1S)-(+)-
camphor-10-sulfonate. The racemization obeyed first-order
Figure 3. 125Te NMR spectra of telluronium salts 1–5.
Two plausible mechanisms for the racemization of the
kinetics, and the rate constant k and the half-life t1/2 were telluronium salts having a benzyl group are considered, as
4.46ϫ10–5 s–1 and 4.32 h, respectively. In the case of enan- shown in Scheme 1. A pyramidal inversion mechanism for
tiomerically pure telluronium salts, (S)-4 and (S)-5 were the racemization of many telluronium species has been
found to racemize in several solvents, such as chloroform, ruled out by theoretical studies at least at room tempera-
ethanol, methanol, and DMSO, even at room temperature, ture.[12,13] The first mechanism is as follows. The
whereas salts (S)-2, (S)-3, and (S)-6 were stable in the counteranion attacks the positive tellurium atom of the tell-
above-mentioned solvents. In addition, all of the tellu- uronium salt to give a tellurane, which yields achiral methyl
ronium salts were stable in MeOH/H2O (4:1) solution or in phenyl telluride and benzyl halide by the ligand-coupling
the crystalline state. The results indicate that telluronium reaction, then the telluronium salt is reproduced. The sec-
salts possessing counteranions with a high nucleophilicity ond mechanism involves Berry pseudorotation[14] of the tel-
racemize in solution. The first-order rate constants and lurane (5 pseudorotations (Φ) result in the enantiomeric tel-
half-lives for the racemization of (S)-4 and (S)-5 at 25 °C lurane), which is known to occur easily. When the reaction
are summarized in Table 2. The results show that the race- of racemic benzylmethylphenyltelluronium bromide 4 with
mization proceeds more rapidly in less polar solvents. To p-methylbenzyl bromide was carried out in chloroform at
examine the cation-anion interaction of the telluronium room temperature, methyl(p-methylbenzyl)phenyltellur-
salts, 125Te NMR spectra were measured in several solvents onium bromide 7 was obtained together with benzyl bro-
(Table 3). The 125Te NMR signals of 4 and 5 appeared at a mide and a small amount of methyl phenyl telluride (Fig-
higher field than those of telluronium salts 1–3 in chloro- ure 4). The reverse reaction was also observed in the reac-
form, whereas in methanol, the signals of 1–5 were ob- tion that used telluronium salt 7 and benzyl bromide. These
6558
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6556–6562

Optically Active Benzylmethylphenyltelluronium Salts
results strongly support the existence of the first mecha- ray analysis of a diastereomerically pure compound, spe-
nism. These ligand-exchange reactions, however, are slow cific rotations, and circular dichroism spectra. Telluronium
compared to the racemization of 4 in chloroform. There- salts possessing counteranions with a high nucleophilicity
fore, the Berry pseudorotation of the tellurane is considered were found to racemize in solution, and the racemization
to take place concurrently.
of telluronium salts with a benzyl substituent was found to
proceed through two pathways involving the nucleophilic
addition of a counteranion to the tellurium atom.
Experimental Section
Synthesis of Benzylmethylphenyltelluronium (1S)-(+)-Camphor-10-
sulfonate (dia.-1): A mixture of methyl phenyl telluride[15] (6.5 g,
30 mmol) and benzyl bromide (4.6 mL, 39 mmol) was stirred for
1 d under nitrogen. The resulting precipitate was collected by fil-
tration and washed with hexane to yield benzylmethylphenyltellur-
onium bromide (rac.-4) (10.2 g, 87%). Silver (1S)-(+)-camphor-10-
sulfonate (8.5 g, 25 mmol) was added to a dichloromethane solu-
tion (250 mL) of the telluronium bromide (9.0 g, 23 mmol), and the
mixture was stirred for 1 d. Precipitated silver bromide was filtered
off, and the filtrate was concentrated. The residue was subjected to
chromatography on a short alumina column (MeOH/acetone = 1:5)
to give a diastereomeric mixture of benzylmethylphenyltelluronium
(1S)-(+)-camphor-10-sulfonate (dia.-1) (11.7 g, 94%).
Scheme 1. Plausible racemization mechanism of benzylmethylphen-
yltelluronium salts.
Benzylmethylphenyltelluronium Bromide (rac.-4): M.p. 105–106 °C
1
(decomp, white powder). H NMR (500 MHz, CDCl3, 25 °C): δ =
2.19 (s, 3 H, Me), 4.46 (d, J = 11.7 Hz, 1 H, CH2), 4.68 (d, J =
11.6 Hz, 1 H, CH2), 6.86–6.88 (m, 2 H, Ar), 7.09–7.10 (m, 3 H,
Ar), 7.25–7.28 (m, 2 H, Ar), 7.33–7.36 (m, 1 H, Ar), 7.89 (d, J =
7.5 Hz, 2 H, Ar) ppm. 13C NMR (125 MHz, CDCl3): δ = 12.8,
37.7, 122.9, 127.7, 128.1, 129.3, 129.8, 130.3 ppm. 133.1, 134.6.
125Te NMR (158 MHz, CDCl ): δ = 607 ppm. IR (KBr): ν
=
˜
3
max
3028, 2359, 2340, 1480, 1454, 1437, 1058, 998, 844, 739 cm–1. UV
(MeOH): λmax (ε, Lmol–1 cm–1) = 201 (3.0ϫ104), 241 (1.1ϫ104)
nm. C14H15BrTe (390.78): calcd. C 43.03, H 3.87; found C 42.87,
H 3.86.
Benzylmethylphenyltelluronium
(1S)-(+)-Camphor-10-sulfonate
(dia.-1): M.p. 97–99 °C (pale yellow powder, decomp). 1H NMR
(500 MHz, CDCl3, 25 °C): δ = 0.80 (s, 3 H, Me), 1.06 (s, 3 H, Me),
1.30–1.34 (m, 1 H, CH2), 1.61–1.67 (m, 1 H, CH2), 1.84 (d, J =
18.3 Hz, 1 H, CH2), 1.93–1.99 (m, 1 H, CH), 2.00–2.03 (m, 1 H,
CH2), 2.27–2.33 (m, 1 H, CH2), 2.41 (s, 3/2 H, CH2), 2.42 (s, 3/2
H, CH2), 2.63–2.70 (m, 1 H, CH2), 2.75 (d, J = 14.7 Hz, 1 H, CH2),
3.28 (d, J = 14.7 Hz, 1 H, CH2), 4.50 (d, J = 11.0 Hz, 1/2 H, CH2),
4.52 (d, J = 11.0 Hz, 1/2 H, CH2), 4.65 (d, J = 11.3 Hz, 1 H, CH2),
6.96–6.98 (m, 2 H, Ar), 7.17–7.18 (m, 3 H, Ar), 7.36–7.42 (m, 4 H,
Ar), 7.45–7.48 (m, 1 H, Ar) ppm. 13C NMR (125 MHz, CDCl3): δ
= 8.7, 19.6, 19.8, 24.3, 26.9, 35.5, 42.4, 42.7, 47.3, 47.7, 58.4, 121.6,
127.9, 128.2, 129.6, 130.1, 131.1, 131.9, 133.7, 216.6 ppm. 125Te
Figure 4. Ligand exchange reactions of telluronium salts 4 and 7.
The mutarotation observed in chiral 2-p-chlorophenacyl-
telluroisochromanium salts having a benzyl moiety, which
was previously reported,[6] is speculated to be due to race-
mization by a similar mechanism.
NMR (158 MHz, CDCl ): δ = 630.8, 631.2 ppm. IR (KBr): ν
=
˜
3
max
2962, 2359, 2341, 1737, 1192, 1036, 868, 740 cm–1. UV (MeOH):
λmax (ε, Lmol–1 cm–1) = 201 (3.6ϫ104), 241 (sh, 1.4ϫ104) nm. UV
(CHCl3): λmax (ε, Lmol–1 cm–1) = 235 (1.3ϫ104), 254 (1.3ϫ104)
nm. [α]2D5 = +15.8 (c = 1.00, MeOH). [α]2D5 = +18.5 (c = 1.00,
EtOH). [α]2D5 = +21.3 (c = 1.00, AcOEt). [α]2D5 = +22.2 (c = 1.00,
MeCN). [α]2D5 = +23.2 (c = 1.00, acetone). [α]2D5 = +29.5 (c = 1.00,
CH2Cl2). [α]2D5 = +33.5 (c = 1.00, CHCl3). CD (MeOH): λ = 296
([θ] +6.5ϫ103) nm. CD (CHCl3): λ = 297 ([θ] +1.1ϫ104) nm.
C24H30O4STe (542.17): calcd. C 53.17, H 5.58; found C 53.18, H
5.51.
Conclusions
Through the optical resolution of diastereomeric benzyl-
methylphenyltelluronium salts, followed by anion-exchange
reactions, several enantiomeric benzylmethylphenyltellur-
onium salts could be isolated in an optically pure form. The
Optical Resolution of Benzylmethylphenyltelluronium (1S)-(+)-Cam-
absolute configurations were determined on the basis of X- phor-10-sulfonate (dia.-1): Diastereomeric telluronium salt dia.-1
Eur. J. Org. Chem. 2010, 6556–6562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6559

T. Shimizu, R. Sakurai, Y. Azami, K. Hirabayashi, N. Kamigata
FULL PAPER
(22.7 g, 42 mmol) was added to acetone (10 mL); a portion dis-
solved and a portion was insoluble. The insoluble fraction (13.5 g)
was collected by filtration and recrystallized from CH2Cl2/acetone
to give diastereomerically pure (S)-(–)-1 (5.2 g, 9.6 mmol) as color-
less prisms. The acetone-soluble fraction (8.9 g) was recrystallized
from acetone/hexane, and the resulting crystals were recrystallized
a further three times to yield diastereomerically pure (R)-(+)-1
(0.68 g, 1.3 mmol) as colorless needles.
(R)-(+)-Benzylmethylphenyltelluronium (1R)-(–)-Camphor-10-sulf-
onate: 100% de; m.p. 122–124 °C (decomp). H NMR (500 MHz,
1
CDCl3, 25 °C): δ = 0.81 (s, 3 H, Me), 1.07 (s, 3 H, Me), 1.31–1.37
(m, 1 H, CH2), 1.65–1.71 (m, 1 H, CH2), 1.86 (d, J = 18.0 Hz, 1
H, CH2), 1.94–2.00 (m, 1 H, CH), 2.02–2.03 (m, 1 H, CH2), 2.28–
2.33 (m, 1 H, CH2), 2.43 (s, 3 H, Me), 2.66–2.72 (m, 1 H, CH2),
2.78 (d, J = 14.7 Hz, 1 H, CH2), 3.30 (d, J = 14.7 Hz, 1 H, CH2),
4.53 (d, J = 11.3 Hz, 1 H, CH2), 4.65 (d, J = 11.3 Hz, 1 H, CH2),
6.97–6.99 (m, 2 H, Ar), 7.19–7.20 (m, 3 H, Ar), 7.36–7.41 (m, 4 H,
Ar), 7.48 (t, J = 7.2 Hz, 1 H, Ar) ppm. 13C NMR (125 MHz,
CDCl3): δ = 9.0, 19.8, 20.0, 24.5, 27.0, 36.2, 42.6, 42.9, 47.5, 47.9,
58.6, 121.7, 128.2, 128.5, 129.9, 130.3, 131.4, 131.7, 133.7, 216.9
(R)-(+)-Benzylmethylphenyltelluronium (1S)-(+)-Camphor-10-sulf-
1
onate {(R)-(+)-1}: 100% de; m.p. 113–115 °C (decomp). H NMR
(500 MHz, CDCl3, 25 °C): δ = 0.85 (s, 3 H, Me), 1.11 (s, 3 H, Me),
1.36–1.41 (m, 1 H, CH2), 1.74–1.78 (m, 1 H, CH2), 1.89 (d, J =
18.3 Hz, 1 H, CH2), 1.99–2.06 (m, 1 H, CH), 2.31–2.35 (m, 1 H,
CH2), 2.46 (s, 3 H, Me), 2.73–2.78 (m, 1 H, CH2), 2.86 (d, J =
14.7 Hz, 1 H, CH2), 3.36 (d, J = 14.7 Hz, 1 H, CH2), 4.48 (d, J =
11.3 Hz, 1 H, CH2), 4.69 (d, J = 11.3 Hz, 1 H, CH2), 4.69 (d, J =
11.3 Hz, 1 H, CH2), 6.97–6.98 (m, 2 H, Ar), 7.20–7.22 (m, 3 H,
Ar), 7.26–7.28 (m, 2 H, Ar), 7.40 (t, J = 7.7 Hz, 2 H, Ar), 7.52 (t,
J = 7.5 Hz, 1 H, Ar) ppm. 13C NMR (125 MHz, CDCl3): δ = 8.9,
19.8, 19.9, 24.5, 27.0, 35.8, 42.6, 42.9, 47.4, 47.8, 58.5, 121.6, 128.1,
128.4, 129.8, 130.2, 131.3, 131.7, 133.8, 216.8 ppm. 125Te NMR
ppm. 125Te NMR (158 MHz, CDCl ): δ = 632 ppm. IR (KBr): ν
˜
3
max
= 2966, 1737, 1191, 1035, 866, 738 cm–1. UV (MeOH): λmax (ε,
Lmol–1 cm–1) = 199 (3.3ϫ104), 244 (sh, 1.3ϫ104) nm. [α]2D5
=
+51.0 (c = 1.00, MeOH). CD (MeOH): λ = 247 ([θ] +4.0ϫ104),
298 ([θ] +6.5ϫ103) nm. C24H30O4STe (542.17): calcd. C 53.17, H
5.58; found C 52.99, H 5.55.
General Procedure for Transformation of Diastereomerically Pure
Telluronium Salts into Enantiomerically Pure Telluronium Salts: An
aqueous solution (13 mL) of diastereomerically pure telluronium
salt (R)-(+)-1 or (S)-(–)-1 (0.41 g, 0.75 mmol) and sodium tetra-
fluoroborate, sodium 4-chlorobenzenesulfonate, potassium bro-
mide, potassium iodide, or sodium tetraphenylborate (10 equiv.)
was stirred for 2 h. When using sodium tetrafluoroborate, sodium
4-chlorobenzenesulfonate, or sodium tetraphenylborate, the prod-
ucts were extracted with dichloromethane followed by recrystalli-
zation from CH2Cl2/diethyl ether. When using potassium bromide
and potassium iodide, the resulting precipitates were collected by
filtration.
(158 MHz, CDCl ): δ = 627 ppm. IR (KBr): ν
= 2956, 1735,
˜
3
max
1180, 1039, 736, 700 cm–1. UV (MeOH): λmax (ε, Lmol–1 cm–1) =
199 (3.4ϫ104), 244 (sh, 1.3ϫ104) nm. UV (CHCl3): λmax (ε,
Lmol–1 cm–1) = 236 (1.3ϫ104), 256 (1.2ϫ104) nm. [α]2D5 = +80.0
(c = 1.00, MeOH). [α]2D5 = +88.8 (c = 1.00, CHCl3). CD (MeOH):
λ = 247 ([θ] +3.7ϫ104), 298 ([θ] +5.6ϫ103) nm. CD (CHCl3): λ =
268 ([θ] +2.3ϫ104), 294 ([θ] +1.1ϫ104) nm. C24H30O4STe (542.17):
calcd. C 53.17, H 5.58; found C 53.40, H 5.53.
(S)-(–)-Benzylmethylphenyltelluronium (1S)-(+)-Camphor-10-sulf-
(R)-(+)-Benzylmethylphenyltelluronium Tetrafluoroborate {(R)-(+)-
2}: 60%; 100% ee; m.p. 101–103 °C (decomp, colorless needles). 1H
NMR (500 MHz, CDCl3, 25 °C): δ = 2.33 (s, 3 H, Me), 4.46 (d, J
= 11.6 Hz, 1 H, CH2), 4.54 (d, J = 11.6 Hz, 1 H, CH2), 6.90 (d, J
= 7.3 Hz, 2 H, Ar), 7.17–7.24 (m, 3 H, Ar), 7.36–7.43 (m, 4 H, Ar),
7.51–7.54 (m, 1 H, Ar) ppm. 13C NMR (125 MHz, CDCl3): δ =
6.0, 34.3, 117.2, 128.7, 128.8, 130.3, 130.36, 130.42, 132.3, 133.7
1
onate {(S)-(–)-1}: 100% de; m.p. 125–126 °C (decomp). H NMR
(500 MHz, CDCl3, 25 °C): δ = 0.80 (s, 3 H, Me), 1.05 (s, 3 H, Me),
1.30–1.34 (m, 1 H, CH2), 1.60–1.66 (m, 1 H, CH2), 1.85 (d, J =
18.3 Hz, 1 H, CH2), 1.91–1.97 (m, 1 H, CH), 2.00–2.02 (m, 1 H,
CH2), 2.27–2.32 (m, 1 H, CH2), 2.42 (s, 3 H, Me), 2.63–2.69 (m, 1
H, CH2), 2.73 (d, J = 14.7 Hz, 1 H, CH2), 3.27 (d, J = 14.7 Hz, 1
H, CH2), 4.52 (d, J = 11.3 Hz, 1 H, CH2), 4.62 (d, J = 11.3 Hz, 1
H, CH2), 6.96–6.98 (m, 2 H, Ar), 7.17–7.19 (m, 3 H, Ar), 7.37–
7.42 (m, 4 H, Ar), 7.46 (t, J = 7.0 Hz, 1 H, Ar) ppm. 13C NMR
(125 MHz, CDCl3): δ = 8.8, 19.7, 19.9, 24.4, 26.9, 35.6, 42.5, 42.8,
47.4, 47.7, 58.4, 121.6, 128.0, 128.3, 129.7, 130.2, 131.2, 131.9,
133.8, 216.7 ppm. 125Te NMR (158 MHz, CDCl3): δ = 629 ppm.
ppm. 125Te NMR (158 MHz, CDCl ): δ = 626 ppm. IR (KBr): ν
˜
3
max
= 3032, 2359, 1701, 1577, 1494, 1483, 1437, 1020, 871, 737, 702,
689 cm–1. UV (MeOH): λmax (ε, Lmol–1 cm–1) = 199 (3.1ϫ104),
245 (sh, 1.3ϫ104) nm. UV (CHCl3): λmax (ε, Lmol–1 cm–1) = 237
(1.3ϫ104), 260 (1.4ϫ104) nm. [α]2D5 = +96.3 (c = 1.00, MeOH).
[α]2D5 = +143.5 (c = 1.00, CHCl3). CD (MeOH): λ = 247 ([θ]
+3.7ϫ104) nm. CD (CHCl3): λ = 274 ([θ] +3.7ϫ104) nm.
C14H15BF4Te (397.68): calcd. C 42.28, H 3.80; found C 42.07, H
3.78.
IR (KBr): ν
= 2968, 1740, 1192, 1035, 868, 740 cm–1. UV
˜
max
(MeOH): λmax (ε, Lmol–1 cm–1) = 200 (3.3ϫ104), 253 (sh, 1.7ϫ104)
nm. UV (CHCl3): λmax (ε, Lmol–1 cm–1) = 237 (1.2ϫ104), 255
(1.2ϫ104) nm. [α]2D5 = –57.3 (c = 1.00, MeOH). [α]2D5 = –52.9 (c =
1.00, EtOH). [α]2D5 = –17.1 (c = 1.00, MeCN). [α]2D5 = –23.8 (c =
1.00, acetone). [α]2D5 = –8.5 (c = 1.00, CH2Cl2). [α]2D5 = –21.2 (c =
1.00, CHCl3). CD (MeOH): λ = 247 ([θ] –3.9ϫ104), 297 ([θ]
+5.7ϫ103) nm. CD (CHCl3): λ = 268 ([θ] –4.1ϫ104), 297 ([θ]
+1.0ϫ104) nm. C24H30O4STe (542.17): calcd. C 53.17, H 5.58;
found C 53.03, H 5.51.
(S)-(–)-Benzylmethylphenyltelluronium Tetrafluoroborate {(S)-(–)-
2}: 65%; 100% ee; m.p. 102–104 °C (decomp, colorless prisms).
[α]2D5 = –94.1 (c = 1.00, MeOH). [α]2D5 = –93.2 (c = 1.00, EtOH).
[α]2D5 = –87.9 (c = 1.00, MeCN). [α]2D5 = –110.5 (c = 1.00, acetone).
[α]2D5 = –108.1 (c = 1.00, CH2Cl2). [α]2D5 = –158.3 (c = 1.00, CHCl3).
CD (MeOH): λ = 247 ([θ] –4.1ϫ104) nm. CD (CHCl3): λ = 275
([θ] –3.1ϫ104) nm. C14H15BF4Te (397.68): calcd. C 42.28, H 3.80;
Optical Resolution of Benzylmethylphenyltelluronium (1R)-(–)-Cam-
phor-10-sulfonate: A diastereomeric mixture of benzylmethylphen-
yltelluronium (1R)-(–)-camphor-10-sulfonate (13.3 g, 25 mmol),
prepared in a similar way to dia.-1 by the reaction of rac.-4 with
silver (1R)-(–)-camphor-10-sulfonate, was added to hot acetone
(10 mL), a portion dissolved and a portion was insoluble. The in-
soluble fraction was collected by filtration and recrystallized from
CH2Cl2/acetone to give diastereomerically pure (R)-(+)-benzyl-
methylphenyltelluronium (1R)-(–)-camphor-10-sulfonate (1.47 g,
2.7 mmol) as colorless prisms.
1
found C 42.01, H 3.76. The H NMR and 13C NMR spectra were
almost the same as those for (R)-(+)-2.
(R)-(+)-Benzylmethylphenyltelluronium 4-Chlorobenzenesulfonate
{(R)-(+)-3}: 90%; 100% ee; m.p. 125–127 °C (decomp, colorless
needles). 1H NMR (500 MHz, CDCl3, 25 °C): δ = 2.34 (s, 3 H,
Me), 4.47 (d, J = 11.3 Hz, 1 H, CH2), 4.60 (d, J = 11.3 Hz, 1 H,
CH2), 6.85 (d, J = 7.1 Hz, 2 H, Ar), 7.11–7.19 (m, 3 H, Ar), 7.25
(d, J = 8.4 Hz, 2 H, Ar), 7.31 (t, J = 7.4 Hz, 2 H, Ar), 7.36 (d, J
= 7.4 Hz, 2 H, Ar), 7.44 (d, J = 7.4 Hz, 1 H, Ar), 7.71 (d, J =
6560
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6556–6562

Optically Active Benzylmethylphenyltelluronium Salts
8.4 Hz, 2 H, Ar) ppm. 13C NMR (125 MHz, CDCl3): δ = 8.3, 35.2,
NMR (158 MHz, [D ]acetone): δ = 638 ppm. IR (KBr): ν
=
˜
6
max
120.7, 127.5, 128.17, 128.23, 128.4, 129.8, 130.1, 131.4, 131.6, 3054, 1579, 1479, 1437, 1427, 1160, 1131, 856, 744, 731, 709 cm–1.
133.7, 135.5, 144.2 ppm. 125Te NMR (158 MHz, CDCl3): δ = 630
ppm. IR (KBr): ν = 2359, 1474, 1438, 1224, 1178, 1120, 1085,
UV (MeOH): λmax (ε, Lmol–1 cm–1) = 201 (1.3ϫ104) nm. UV
(CHCl3): λmax (ε, Lmol–1 cm–1) = 237 (3.8ϫ104) nm. [α]2D5 = +58.0
(c = 0.50, MeOH). [α]2D5 = +22.5 (c = 1.00, CHCl3). CD (MeOH):
λ = 246 ([θ] +4.1ϫ104) nm. CD (CHCl3): λ = 276 ([θ] +1.6ϫ104),
305 ([θ] –6.1ϫ103) nm. C38H35BTe (630.11): calcd. C 72.43, H 5.60;
found C 72.31, H 5.66.
˜
max
1030, 1006, 832, 755 cm–1. UV (MeOH): λmax (ε, Lmol–1 cm–1) =
223 (2.9ϫ104), 249 (1.2ϫ104) nm. UV (CHCl3): λmax (ε,
Lmol–1 cm–1) = 236 (1.5ϫ104), 260 (1.3ϫ104) nm. [α]2D5 = +80.3
(c = 1.00, MeOH). [α]2D5 = +71.4 (c = 1.00, CHCl3). CD (MeOH):
λ = 246 ([θ] +4.3ϫ104) nm. CD (CHCl3): λ = 268 ([θ] +4.2ϫ104)
nm. C20H19ClO3STe (502.49): calcd. C 47.81, H 3.81; found C
47.58, H 3.80.
(S)-(–)-Benzylmethylphenyltelluronium Tetraphenylborate {(S)-(–)-
6}: 89%; m.p. 132–133 °C (decomp, colorless prisms). [α]2D5 = –30.5
(c = 0.67, MeOH). [α]2D5 = –30.5 (c = 1.67, MeOH). [α]2D5 = –17.6
(c = 0.50, EtOH). [α]2D5 = –52.8 (c = 1.00, MeCN). [α]2D5 = –61.9 (c
= 1.00, acetone). [α]2D5 = –34.4 (c = 1.00, CH2Cl2). [α]2D5 = –14.0 (c
= 1.00, CHCl3). CD (MeOH): λ = 246 ([θ] –4.5ϫ104) nm. CD
(CHCl3): λ = 276 ([θ] +1.0ϫ104), 301 ([θ] +4.0ϫ103) nm.
C38H35BTe (630.11): calcd. C 72.43, H 5.60; found C 72.13, H 5.65.
The 1H NMR and 13C NMR spectra were almost the same as those
for (R)-(+)-6.
(S)-(–)-Benzylmethylphenyltelluronium
4-Chlorobenzenesulfonate
{(S)-(–)-3}: 94%; 100% ee; m.p. 123–125 °C (decomp, colorless
needles). [α]2D5 = –81.8 (c = 1.00, MeOH). [α]2D5 = –82.4 (c = 1.00,
EtOH). [α]2D5 = –59.8 (c = 1.00, MeCN). [α]2D5 = –65.2 (c = 1.00,
acetone). [α]2D5 = –50.8 (c = 1.00, CH2Cl2). [α]2D5 = –74.9 (c = 1.00,
CHCl3). CD (MeOH): λ = 248 ([θ] –4.5ϫ104) nm. CD (CHCl3): λ
= 269 ([θ] –4.1ϫ104) nm. C20H19ClO3STe (502.49): calcd. C 47.81,
H 3.81; found C 48.03, H 3.92. The 1H NMR and 13C NMR spec-
tra were almost the same as those for (R)-(+)-3.
Reaction of Benzylmethylphenyltelluronium Bromide (4) with p-
Methylbenzyl Bromide: In an NMR tube, racemic benzylmeth-
ylphenyltelluronium bromide (4) (20 mg, 0.051 mmol) and p-meth-
ylbenzyl bromide (9.5 mg, 0.051 mmol) were dissolved in CDCl3
(0.8 mL), and the sample was left at room temperature and moni-
tored by 1H NMR spectroscopy. The reaction of methyl(p-meth-
ylbenzyl)phenyltelluronium bromide (7) with benzyl bromide was
carried out similarly.
(R)-(+)-Benzylmethylphenyltelluronium Bromide {(R)-(+)-4}: 63%;
100% ee; m.p. 101–103 °C (decomp, white powder). [α]2D5 = +64.9
(c = 0.50, MeOH). [α]2D5 = +93.8 (c = 0.50, CHCl3). CD (MeOH):
λ = 247 ([θ] +3.3ϫ104) nm. CD (CHCl3): λ = 275 ([θ] +2.6ϫ104)
nm. C14H15BrTe (390.78): calcd. C 43.03, H 3.87; found C 42.98,
H 3.88. The 1H NMR and 13C NMR spectra were almost the same
as those for rac.-4.
Methyl(p-methylbenzyl)phenyltelluronium Bromide (rac.-7): M.p.
141–142 °C (decomp, colorless powder). 1H NMR (500 MHz,
CDCl3, 25 °C): δ = 2.24 (s, 3 H, Me), 2.27 (s, 3 H, Me), 4.51 (d, J
= 11.6 Hz, 1 H, CH2), 4.71 (d, J = 11.6 Hz, 1 H, CH2), 6.81 (d, J
= 7.7 Hz, 2 H, Ar), 6.94 (d, J = 7.7 Hz, 2 H, Ar), 7.29–7.40 (m, 3
H, Ar), 7.76 (d, J = 7.7 Hz, 2 H, Ar) ppm. 13C NMR (125 MHz,
CDCl3): δ = 12.7, 21.1, 37.2, 123.0, 128.9, 129.4, 129.6, 129.8,
130.4, 134.5, 137.7 ppm. 125Te NMR (158 MHz, CDCl3): δ = 620
(S)-(–)-Benzylmethylphenyltelluronium Bromide {(S)-(–)-4}: 73%;
100% ee; m.p. 108–110 °C (decomp, white powder). [α]2D5 = –66.5
(c = 1.00, MeOH). [α]2D5 = –96.5 (c = 1.00, CHCl3). CD (MeOH):
λ = 249 ([θ] –3.1ϫ104) nm. CD (CHCl3): λ = 275 ([θ] –2.7ϫ104)
nm. C14H15BrTe (390.78): calcd. C 43.03, H 3.87; found C 42.97,
H 4.14. The 1H NMR and 13C NMR spectra were almost the same
as those for rac.-4.
ppm. IR (KBr): ν
= 3051, 1575, 1510, 1480, 1436, 1404, 999,
˜
max
(R)-(+)-Benzylmethylphenyltelluronium Iodide {(R)-(+)-5}: 67%;
100% ee; m.p. 78–80 °C (decomp, pale yellow powder). H NMR
824, 734, 689, 565, 456 cm–1. UV (MeOH): λmax (ε, Lmol–1 cm–1)
= 253 (1.0ϫ104), 241 (1.1ϫ104) nm. C15H17BrTe (404.80): calcd.
C 44.51, H 4.23; found C 44.75, H 4.25.
1
(500 MHz, CDCl3, 25 °C): δ = 2.20 (s, 3 H, Me), 4.46 (s, 2 H, CH2),
7.18–7.21 (m, 2 H, Ar), 7.23–7.26 (m, 2 H, Ar), 7.28–7.30 (m, 2 H,
Ar), 7.37–7.38 (m, 2 H, Ar), 7.63–7.65 (m, 2 H, Ar) ppm. 13C NMR
(125 MHz, CDCl3): δ = 12.8, 38.7, 121.2, 127.9, 128.1, 129.1, 129.6,
129.9 ppm. 134.7, 136.7. 125Te NMR (158 MHz, CDCl3): δ = 586
[1] For books see: a) P. Metzner, A. Tuillier in Sulfur Reagents in
Organic Synthesis, Academic Press, London, 1993; b) S. Patai,
Z. Rappoport, C. J. M. Stirling (Eds.), The Chemistry of
Sulphones and Sulphoxides, Wiley, New York, 1988; c) M. Mi-
kolajczyk, J. Drabowicz in Topics in Stereochemistry (Eds.:
E. L. Allinger, E. L. Eliel, S. H. Wilen), Wiley, New York, 1982,
vol. 13, p. 333; d) C. J. M. Stirling, S. Patai (Eds.), The Chemis-
try of the Sulfonium Group, Wiley, New York, 1981.
[2] For a book see: T. Shimizu, N. Kamigata in Handbook of Chal-
cogen Chemistry: New Perspectives in Sulfur, Selenium and Tel-
lurium (Ed.: F. A. Devillanova), RSC, Cambridge, 2007, p. 577.
[3] For books see: a) S. Patai, Z. Rappoport (Eds.), The Chemistry
of Organic Selenium and Tellurium Compounds, Wiley, New
York, 1987, vol. 2; b) C. Paulmier in Selenium Reagents and
Intermediates in Organic Synthesis, Pergamon Press, Oxford,
1986.
ppm. IR (KBr): ν
= 3027, 1598, 1573, 1492, 1479, 1453, 1435,
˜
max
1059, 997, 853, 760, 738, 700 cm–1. UV (MeOH): λmax (ε,
Lmol–1 cm–1) = 199 (5.2ϫ104), 217 (3.7ϫ104), 249 (sh, 2.2ϫ104)
nm. [α]2D5 = +76.5 (c = 0.50, MeOH). [α]2D5 = +105.6 (c = 0.50,
CHCl3). CD (MeOH): λ = 247 ([θ] +4.7ϫ104) nm. C14H15ITe
(437.78): calcd. C 38.41, H 3.45; found C 38.51, H 3.47.
(S)-(–)-Benzylmethylphenyltelluronium Iodide {(S)-(–)-5}: 80%;
100% ee; m.p. 77–78 °C (decomp, pale yellow powder). [α]2D5
=
–77.3 (c = 1.00, MeOH). [α]2D5 = –107.4 (c = 1.00, CHCl3). CD
(MeOH): λ = 249 ([θ] –4.9ϫ104) nm. C14H15ITe (437.78): calcd. C
38.41, H 3.45; found C 38.34, H 3.42. The 1H NMR and 13C NMR
spectra were almost the same as those for (R)-(+)-5.
[4] For reviews see: a) T. Shimizu, N. Kamigata, Rev. Heteroat.
Chem. 1998, 18, 11–35; b) T. Shimizu, N. Kamigata, Org. Prep.
Proced. Int. 1997, 29, 603–629.
[5] T. M. Lowry, F. L. Gilbert, J. Chem. Soc. 1929, 2867–2876.
[6] F. G. Holliman, F. G. Mann, J. Chem. Soc. 1945, 37–44.
[7] T. Shimizu, T. Urakubo, N. Kamigata, Chem. Lett. 1996, 297–
298.
(R)-(+)-Benzylmethylphenyltelluronium Tetraphenylborate {(R)-(+)-
6}: 79%; m.p. 131–133 °C (decomp, colorless prisms). 1H NMR
(500 MHz, [D6]acetone, 25 °C): δ = 2.40 (s, 3 H, Me), 4.51 (s, 2 H,
CH2), 6.78 (t, J = 7.3 Hz, 4 H, Ar), 6.93 (t, J = 7.3 Hz, 8 H, Ar),
7.01–7.03 (m, 2 H, Ar), 7.23–7.27 (m, 3 H, Ar), 7.36–7.37 (m, 8 H,
Ar), 7.49–7.54 (m, 4 H, Ar), 7.60–7.63 (m, 1 H, Ar) ppm. 13C NMR
(125 MHz, [D6]acetone): δ = 7.7, 34.8, 119.9, 122.3, 126.05, 126.09,
129.4, 129.6, 130.8, 131.2, 132.2, 133.1, 134.6, 137.0 ppm. 125Te
[8] T. Shimizu, T. Urakubo, N. Kamigata, J. Org. Chem. 1996, 61,
8032–8038.
Eur. J. Org. Chem. 2010, 6556–6562
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6561

T. Shimizu, R. Sakurai, Y. Azami, K. Hirabayashi, N. Kamigata
FULL PAPER
[9] T. Shimizu, T. Urakubo, P. Jin, M. Kondo, S. Kitagawa, N.
Kamigata, J. Organomet. Chem. 1997, 539, 171–175.
[10] J. Zhang, S. Saito, T. Koizumi, J. Org. Chem. 1998, 63, 5423–
5429.
[13] Telluronium ylide: a) T. Shimizu, A. Matsuhisa, N. Kamigata,
S. Ikuta, J. Chem. Soc. Perkin Trans. 2 1995, 1805–1808; tellu-
ronium imide: b) T. Shimizu, N. Kamigata, S. Ikuta, J. Chem.
Soc. Perkin Trans. 2 1999, 1469–1473.
[11] Preliminary communication: T. Shimizu, Y. Azami, R. Sakurai, [14] R. S. Berry, J. Chem. Phys. 1960, 32, 933–938.
K. Hirabayashi, N. Kamigata, Chem. Lett. 2009, 38, 1070– [15] E. G. Hope, T. Kemmitt, W. Levason, Organometallics 1988, 7,
1071.
78–83.
[12] Activation energies for pyramidal inversion of Me3Ch+ calcu-
lated by MP3/d95**(Te:10s8p4d/16s11p6d) level was
42.0 kcalmol–1 (unpublished result: T. Shimizu, T. Urakubo, N.
Kamigata, S. Ikuta).
Received: June 21, 2010
Published Online: October 28, 2010
6562
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6556–6562