Screening of a modular sugar-based phosphite-oxazoline ligand library in asymmetric Pd-catalyzed Heck reactions

Article abstract of DOI:10.1002/chem.200601714

We have synthesised a library of phosphite-oxazoline ligands derived from readily available D-glucosamine. These ligands have been successfully screened in the palladium-catalysed Heck reaction of several substrates with high regio- (up to 99%) and enanti

Full text of DOI:10.1002/chem.200601714

FULL PAPER
DOI: 10.1002/chem.200601714
3296
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Chem. Eur. J. 2007, 13, 3296 – 3304

FULL PAPER
Screening of a Modular Sugar-Based Phosphite–Oxazoline Ligand Library in
Asymmetric Pd-Catalyzed Heck Reactions
Yvette Mata, Oscar Pàmies,* and Montserrat DiØguez*[a]
Abstract: We have synthesised
a
ties under standard thermal conditions.
The results indicate that the catalytic
performance is highly affected by the
oxazoline and biarylphosphite substitu-
ents and the axial chirality of the biaryl
moiety of the ligand. The Heck reac-
tions were also performed under mi-
crowave irradiation conditions, allow-
ACHTREUNG
derived from readily available d-glu-
cosamine. These ligands have been suc-
cessfully screened in the palladium-cat-
alysed Heck reaction of several sub-
strates with high regio- (up to 99%)
and enantioselectivities (eeꢁs up to
99%) as well as with improved activi-
ing
a considerably shorter reaction
time (full conversion in minutes) main-
taining the excellent regio- and enan-
tioselectivities.
Keywords: asymmetric catalysis
·
Heck reaction · P,N ligands · palla-
dium
Introduction
Catalytic asymmetric carbon–carbon bond formation is one
of the most actively pursued areas of research in the field of
asymmetric catalysis. In this respect, the asymmetric Pd-cat-
alysed Heck reaction, that is, the coupling of an aryl or al-
kenyl halide or triflate to an alkene, is a powerful and
highly versatile procedure because it tolerates several func-
tional groups.[1] Chiral bidentate phosphine ligands have
played a key role in the success of this process.[1] However,
in the intermolecular Heck reaction, regioselectivity is often
a problem. So, for example, in the Heck reaction of 2,3-di-
hydrofuran S1 with phenyl triflate, a mixture of two prod-
ucts is obtained—the expected product 2-phenyl-2,5-dihy-
drofuran (1) and 2-phenyl-2,3-dihydrofuran (2; Scheme 1).
The latter is formed due to an isomerisation process.[1]
Scheme 1. Model Pd-catalysed Heck reaction of 2,3-dihydrofurane S1.
gands for the intermolecular Heck reaction.[2] Two of the
most representative examples of this type of ligands are the
phosphanyloxazoline PHOX ligands developed by Pfaltz[2a,b]
and co-workers and the phosphanyloxazoline based on keto-
pinic acid developed by Gilbertson and co-workers.[2e] De-
spite these successes, ligands that provide good regio- and
enantioselectivities usually have two considerable draw-
backs: 1) reaction times are usually long and 2) they are
prepared from expensive chiral synthons or in tedious syn-
thetic steps (Scheme 2). Therefore, it is very important to
develop ligands that induce higher rates and selectivities
(regio- and enantioselectivities) based on simple starting
materials in this reaction. Carbohydrates are particularly ad-
vantageous for this purpose due to their low price and easy
modular construction.
In this context, to further expand the range of ligands and
to improve the performance of these asymmetric Pd-cata-
lysed Heck reactions, we designed a library of chiral phos-
phite–oxazoline ligands L1–L4a–g. These ligands are de-
rived from natural d-glucosamine and have the advantages
of carbohydrate and phosphite ligands, such as availability
at a low price from readily available alcohols and facile
modular constructions.[3] Furthermore, they are less sensitive
to air than typical phosphanes, which are widely used as li-
In the last few years, a class of heterodonor ligands—the
phosphanyloxazoline ligands—have emerged as suitable li-
[a] Y. Mata, Dr. O. Pàmies, Dr. M. DiØguez
Departament de Química Física i Inorgànica
Universitat Rovira i Virgili. Campus Sescelades
C/Marcel·lí Domingo, s/n. 43007 Tarragona (Spain)
Fax : (+34)977-559-563
E-mail: oscar.pamies@urv.net
montserrat.dieguez@urv.net
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. It contains the
results for the catalytic arylation and cyclohexenylation of S1 and S2
and the temperature, power and pressure versus time profiles for the
microwave experiments are included.
Chem. Eur. J. 2007, 13, 3296 – 3304ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3297

M. DiØguez, O. Pàmies, and Y. Mata
Results and Discussion
Synthesis of ligands: Ligands L1–L4a–g were efficiently syn-
thesised in one step through the reaction of the correspond-
ing sugar oxazoline alcohols (3–6) with one equivalent of
the corresponding phosphorochloridite (a–g) in the presence
of pyridine (Scheme 3). Oxazoline alcohols 3–6 are easily
Scheme 2. Summary of the best results using the most representative
ligand families developed for the Pd-catalysed Heck reactions (reactions
usually carried out with 3–5 mol% of Pd).
Scheme 3. Synthesis of phosphite-oxazoline ligand library L1– L4a-g. a)
Ref. 7. b) 1 equiv of ClP(OR)2/Toluene/Py at 808C.
gands in asymmetric catalysis. In addition, the introduction
of a phosphite moiety in the ligand design has proved to be
highly advantageous in terms of activity because of its great-
er p-acceptor ability.[4] All these favourable features have
enabled us to synthesise and screen a series of chiral ligands
in the search for high activity and selectivity. Although car-
bohydrate-based ligands have been successfully used in
other enantioselective reactions,[3] there are only two reports
on the enantioselective palladium-catalysed asymmetric
Heck reaction using this type of ligand.[5]
In this paper, we report on the design of a library of 28
potential sugar-based chiral phosphite–oxazoline ligands and
screen their use in the Pd-catalysed asymmetric Heck reac-
tion of several substrates and triflate sources.[6] The synthesis
and screening of the library were performed by using a
series of parallel reactors each equipped with 12 different
positions. With this library we fully investigated the effects
of systematically varying the electronic and steric properties
of the oxazoline substituents (L1–L4) and different substitu-
ents/configurations in the biaryl phosphite moiety (a–g). By
carefully selecting these elements, we achieved high selectiv-
ities (regio- and enantioselectivities) and activities in differ-
ent substrate types and aryl sources.
prepared on a large scale from d-glucosamine.[7] All the li-
gands were stable during purification on neutral alumina
under an atmosphere of argon and were isolated as white
solids. They are stable at room temperature and very stable
to hydrolysis. The elemental analyses were in agreement
1
with the assigned structures. The H and 13C NMR spectra
agree with those expected for these C1 ligands. One singlet
was observed in the 31P NMR spectrum. Rapid ring inver-
sions (atropoisomerisation) in the biphenyl-phosphorus moi-
eties occurred on the NMR timescale, since the expected
diastereoisomers were not detected by low-temperature
phosphorus NMR spectroscopy.[8]
Asymmetric Heck reactions under thermal conditions—
Heck reaction of 2,3-dihydrofuran S1: In this section, we
report the use of the chiral phosphite–oxazoline ligand li-
brary L1–L4a–g in the Pd-catalysed asymmetric Heck reac-
tion of 2,3-dihydrofuran S1 [Eq. (1)] by using several tri-
flates with different electronic and steric properties: phenyl
triflate, 1-naphthyl triflate, toluyl triflate, para-nitrophenyl
triflate and cyclohexenyl triflate. In all cases, the catalysts
were generated in situ by mixing [Pd
corresponding chiral ligand.
A
Heck reaction of 2,3-dihydrofuran S1 with phenyl triflate
[Eq. (1)]: For an initial evaluation of this new type of ligand
in the palladium-catalysed asymmetric Heck reaction, we
chose the phenylation of S1 [Eq (1); R=C6H5]. As this re-
action has been carried out with a variety of ligands carrying
different donor groups, it is possible to directly compare the
efficacy of different ligand systems.
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Asymmetric Catalysis
FULL PAPER
In a first set of experiments, we determined the optimal
reaction conditions by conducting a series of experiments in
which we varied the solvent, temperature and base. In all
cases, the formation of the desired product 2-phenyl-2,5-di-
hydrofuran 1 was favoured over the formation of 2-phenyl-
2,3-dihydrofuran 2.
We first studied the effect of the solvent. Four solvents
(tetrahydrofuran (THF), benzene, toluene and dimethylfor-
mamide (DMF)) and four ligands (L1a, L2a, L3a and L4a)
were tested. The results show that the efficiency of the pro-
cess strongly depended on the nature of the solvent
(Figure 1). The best activity and selectivity (regio- and enan-
tioselectivity) was achieved with tetrahydrofurane as the sol-
vent.
We next studied the effect of varying the temperature.
The conversion and selectivity when THF was used as a sol-
vent and ligands L1a and L4a are shown in Table 1 (similar
tendencies were observed with other solvents and ligands).
We observed that this parameter affected both activity and
selectivity. Increasing the temperature from 508C to 758C
had a negative effect on regio- and enantioselectivity (en-
tries 1 and 4versus 2 and 5). Lowering the temperature to
258C hardly affected regio- and enantioselectivity, but activ-
ities dropped considerably (entry 1 and 3 versus 4and 6).
The best trade-off between activity and selectivity was
therefore achieved at 508C.
We then studied the effect of several bases. The conver-
sion and selectivity when THF was used as a solvent with li-
gands L1a and L4a are shown in Table 2 (similar trends
were observed for the other solvents and ligands). Although
the activities and regioselectivities obtained with proton
sponge (PS) and diisopropylethylamine are comparable, the
use of the latter base provides slightly better enantioselec-
tivities (entries 1 and 9 versus 2 and 10). On the other hand,
sodium acetate yielded the highest regioselectivity, but its
activities and enantioselectivities are lower than those of
diisopropylethylamine (entries 1 and 9 versus 3 and 11). The
remainder of the bases tested provided lower activities and
selectivities than those obtained with diisopropylamine (en-
tries 1 and 9 versus 4–8, 12 and 13). In conclusion we chose
diisopropylethylamine as our base.
In the end we found that, the optimum trade-off between
selectivities and reaction rates was achieved with tetrahydro-
furan, a temperature of 508C and diisopropylethylamine as
a base. These optimal conditions were then used to test the
catalytic performance of the complete series of ligands. The
results, which are summarised in Table 3, indicate that the
catalytic performance (activity and selectivity) is highly af-
fected by the substituents at both oxazoline and phosphite
moiety and by the axial chirality of the biaryl phosphite
moiety. In general, high activities, regio- (up to 98%) and
enantioselectivities (eeꢁs up to 99%) were obtained in the
phenylation of S1.
Figure 1. Results of the catalytic phenylation of S1 by using ligands L1a,
L2a, L3a and L4a in four solvents at 508C and using iPr2NEt as base. a)
Conversions after 24h. b) Regioselectivities in product 1. c) Enantiose-
lectivities of product 1. Positive numbers refer to the formation of the R-
isomer in excess.
group on the oxazoline decreased, the regio- and enantiose-
lectivity of the catalyst increased (i.e., Me>Ph>iPr>tBu).
This contrasts with the oxazoline-substituent effect observed
for phosphanyloxazoline PHOX ligands, the enantioselecti-
vites of which are higher when bulky tert-butyl groups are
present.[2a,b] In terms of activity, this is mainly affected by
the bulkiness of the oxazoline group. Therefore, activity de-
creases when bulky substituents are present (Table 3, en-
The effect of the oxazoline substituent was studied with li-
gands L1a, L2a, L3a, L4a (Table 3, entries 1, 8–10). We
found that these substituents affected both activities and se-
lectivities. Therefore, we observed that when the size of the
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3299

M. DiØguez, O. Pàmies, and Y. Mata
Table 1. Selected results for the Pd-catalysed enantioselective phenyla-
The effects of phosphite moieties were studied using li-
gands L1a–g (Table 3, entries 1–7). We found that this
moiety affected both activity and selectivity. Bulky substitu-
ents in the ortho positions of the biphenyl moiety are
needed for high activities as well as high regio- and enantio-
selectivities. Thus, ligands L1a–c with bulky substituents at
the ortho positions of the biphenyl moiety provided higher
activities and selectivities than ligands L1d and L1e with
small substituents in these positions (Table 3, entries 1–3
versus 4–5). However, substituents in the para positions also
play a small but crucial role. Therefore, the best activities
and selectivities were obtained by using ligand L1c, which
contains bulky trimethylsilyl groups at the ortho positions
and a hydrogen atom at the para positions of the biphenyl
moiety. To further investigate how enantioselectivity was in-
fluenced by the groups attached to the biaryl moiety, ligands
L1 f and L1g containing different enantiomerically pure bi-
napthyl moieties were also tested (Table 3, entries 6 and 7).
The results indicate that there is a cooperative effect be-
tween the configuration of the biaryl moiety and the config-
urations of the ligand backbone on enantioselectivity that
results in a matched combination for ligand L1g, which con-
tains an (R)-binaphtyl moiety.
To sum up, the best result was obtained with ligand L4c,
which contains the optimal combination of substituents in
the oxazoline and in the biaryl phosphite moieties
(entry 11). These results clearly show the efficiency of using
highly modular scaffolds in the ligand design. In addition,
comparing these excellent results with the activities ob-
tained with Pfaltzꢁs and Gilberstonꢁs ligand Pd-systems
(Scheme 2) in the phenylation of S1 we can conclude that
the presence of a phosphite moiety in ligands L1–L4a-g has
been highly advantageous. These results are among the best
reported so far.[2a,b,e]
tion of S1 using ligands L1a and L4a. Effect of temperature.[a]
Ligand
T [8C]
conv [%] (1:2)[b]
ee [%] 1[c]
ee [%] 2[c]
1
2
3
4
5
6
L1a
L1a
L1a
L4a
L4a
L4a
50
75
25
50
75
25
98 (87:13)
100 (80:20)
28 (88:12)
94(97:3)
100 (91:9)
29 (97:3)
97 (R)
93 (R)
98 (R)
99 ( R)
94( R)
99 (R)
88 (R)
87 (R)
88 (R)
nd[d]
91 (R)
nd[d]
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), phenyl triflate
H
(0.5 mmol), Ligand (2.8”10À2 mmol), THF (3 mL), iPr2NEt (1 mmol),
t=24h. [b] Conversion percentages determined by GC. [c] Enantiomeric
excesses measured by GC. [d] Not determined.
Table 2. Selected results for the Pd-catalysed enantioselective phenyla-
tion of 2,3-dihydrofuran S1 using ligands L1a and L4a. Effect of the
base.[a]
Ligand
Base
conv [%] (1:2)[b]
ee [%] 1[c]
ee [%] 2[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
L1a
L1a
L1a
L1a
L1a
L1a
L1a
L1a
L4a
L4a
L4a
L4a
L4a
iPr2NEt
PS
NaOAc
NEt3
KOAc
K2CO3
Li2CO3
DBU
iPr2NEt
PS
98 (87:13)
99 (87:13)
91 (91:9)
97 (82:18)
53 (82:18)
89 (86:14)
92 (68:32)
6 (72:28)
94(97:3)
97 (R)
95 (R)
91 (R)
87 (R)
43 (R)
88 (R)
95 (R)
65 (R)
99 ( R)
98 ( R)
92 (R)
95 (R)
57 (R)
88 (R)
87 (R)
56 (R)
84 R)
(
4
R) 7 (
27 (R)
92 (R)
4
R) 3 (
nd[d]
94(96:4)
nd[d]
NaOAc
NEt3
KOAc
89 (98:2)
93 (88:12)
62 (84:16)
nd[d]
89 (R)
38 (R)
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), phenyl triflate
C
(0.5 mmol), Ligand (2.8”10À2 mmol), THF (3 mL), base (1 mmol),
T=508C, t=24h. [b] Conversion percentages determined by GC. [c] En-
antiomeric excesses measured by GC. [d] Not determined.
Table 3. Selected results for the Pd-catalysed enantioselective phenyla-
tion of 2,3-dihydrofuran S1 by using phosphite–oxazoline ligand library
L1–L4a–g.[a]
Heck reaction of 2,3-dihydrofuran S1 with other aryl triflate
[Eq. (1)]: To further study the effects of electronic and
steric properties of the aryl triflate source on the product
outcome, we tested these new ligands in the Pd-catalysed
Heck reaction of S1 with several aryl triflates, in which
these properties were systematically varied [Eq. (1),
R=1-naphthyl, p-CH3-C6H4, p-NO2-C6H4]. The most note-
worthy results are shown in Table 4. They followed the same
trend in terms of the effect of the oxazoline and phosphite
moieties as the phenylation of S1 (see Supporting Informa-
tion). In general, high activities, regio- (up to 99%) and
enantioselectivities (eeꢁs up to 99%) were also obtained in
the arylation of S1. The results indicate that both steric and
electronic parameters on the triflate affected catalytic per-
formance. Thus, enantioselectivities are best for 1-naphthyl-
and phenyltriflate (Table 4, entries 1, 2, 5, 6, 9 and 10). On
the other hand regioselectivities are better when electron-
withdrawing aryl triflates are used (Table 4, entries 4, 8, 12).
Ligand
conv [%] (1:2)[b]
ee [%] 1[c]
ee [%] 2[c]
1
2
L1a
L1b
L1c
L1d
L1e
L1 f
L1g
L2a
L3a
L4a
L4c
98 (87:13)
86 (85:15)
100 (97:3)
42 (72:28)
45 (60:40)
32 (58:42)
28 (55:45)
80 (71:29)
12 (65:35)
94(97:3)
97 (R)
97 (R)
99 (R)
25 (R)
80 (R)
6 (R)
73 (R)
84( R)
83 (R)
99 ( R)
99 (R)
88 (R)
89 (R)
nd[e]
16 (R)
69 (R)
19 (R)
3[d]
4
5
6
7
8
9
10
11
4
R)
8 (
90 (R)
23 (R)
nd[e]
100 (98:2)
nd[e]
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), phenyl triflate
G
(0.5 mmol), Ligand (2.8”10À2 mmol), solvent (3 mL), iPr2NEt (1 mmol),
T=508C, t=24h. [b] Conversion percentages determined by GC. [c] En-
antiomeric excesses measured by GC. [d] t=15 h. [e] Not determined.
tries 8 and 9 versus 1 and 10). However, the electronic prop-
erties of the oxazoline substituent have a slight, but impor-
tant effect on activity. Activities are therefore highest when
a phenyl oxazoline moiety is present (Table 3, entries 1
versus 10).
Heck reaction of 2,3-dihydrofuran S1 with cyclohexenyl tri-
flate [Eq. (1)]: We also evaluated the ligand library in the
Heck reaction of S1 with cyclohexenyl triflate. The prelimi-
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Asymmetric Catalysis
FULL PAPER
Table 4. Selected results for Pd-catalysed enantioselective arylation of
modular scaffolds in the ligand design.
2,3-dihydrofuran S1 using ligands L1a, L1c and L4a.[a]
Regarding the effect of the oxazoline substituents, again
the presence of bulky substituents in this position considera-
bly decreased activities and selectivities (Table 5, entries 1,
8–10). However, in contrast to the arylation of S1, the elec-
tronic effect was more important. Therefore, selectivities are
higher when a phenyl substituent is present (Table 5, entry 1
versus 10).
Concerning the effect of the phosphite moiety on catalytic
performance, again the presence of bulky substituents in the
ortho positions of the biphenyl moiety was necessary for
high selectivities. However, the effect of the type of substitu-
ents at the para positions in selectivity is more pronounced
than for the arylation of S1. Thus, whereas good selectivities
are obtained with ligands L1a and L1c, regioselectivity
dropped considerably for ligand L1b, which contains me-
thoxy groups at the para positions (Table 5, entries 1 and 3
versus 2).
Ligand
R
conv [%] (1:2)[b] ee [%] 1[c] ee [%] 2[c]
1
2
3
4
L1a
L1a
L1a
L1a
C6H5
98 (87:13)
88 (86:14)
99 (82:18)
94(95:5)
97 (R)
97 (R)
94( R)
88 ( R)
99 (R)
99 (R)[d]
96 ( R)
90 (R)
99 ( R)
99 (R)
96 (R)
91 (R)
88 (R)
89 (R)
91 (R)
nd[f]
1-naphthyl
p-CH3-C6H4
p-NO2-C6H4
C6H5
1-naphthyl
p-CH3-C6H4
p-NO2-C6H4
C6H5
5[e] L1c
6[e] L1c
7[e] L1c
8[e] L1c
100 (97:3)
89 (95:5)
94(85:15)
89 (>99:1)
94(97:3)
85 (95:5)
95 (87:13)
87 (>99:1)
nd[f]
93 (R)
93 (R)
nd[f]
9
10
11
12
L4a
L4a
L4a
L4a
nd[f]
1-naphthyl
p-CH3-C6H4
p-NO2-C6H4
nd[f]
89 (R)
nd[f]
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), aryl triflate
G
(0.5 mmol), Ligand (2.8”10À2 mmol), THF (3 mL), iPr2NEt (1 mmol),
T=508C, t=24h. [b] Conversion percentages determined by GC. [c] En-
antiomeric excesses measured by GC. [d] Enantiomeric excesses mea-
sured by HPLC. [e] t=15 h. [f] Not determined.
Finally, the effect of the configuration of the biaryl phos-
phite moiety follows a similar trend as those with the previ-
ous arylation of S1 (Table 5, entries 6 and 7).
nary investigations performed on the solvent and base effect
revealed a different trend regarding the base effect than
those with the previously tested aryl triflates. Therefore, the
best selectivities (regio- and enantioselectivities) and reac-
tion rates were obtained when THF and proton sponge were
used as a solvent and base, respectively (see Supporting In-
formation).
The results of using ligands L1–L4a--g under the opti-
mised conditions are shown in Table 5. In general, high ac-
tivities, regio- (up to 98%) and enantioselectivities (eeꢁs up
to 98%) were also obtained in this case. Again enantioselec-
tivity was affected by the substituents at both oxazoline and
phosphite moiety and the cooperative effect between stereo-
centers. However, the effect of these parameters was differ-
ent from those observed in the arylation of S1. Thus, regio-
and enantioselectivities were
Asymmetric Heck reactions under thermal conditions—
Heck reaction of cyclopentene S2 [Eq. (2)]: We also
screened the phosphite–oxazoline ligand library in the phe-
nylation and alkenylation of cyclopentene S2 [Eq. (2)]. Se-
lectivity for S2 is more difficult to control than for function-
alised alkenes such as S1, due to extensive double-bond mi-
gration.[1] Moreover, in addition to the desired product 7, re-
gioisomer 8 and the achiral product 9 can also be obtained.
Therefore, to date only high regio- (regioselectivity up to
96% in product 7) and enantioselectivities (eeꢁs up to 91%)
have been obtained with the phosphanyloxazoline PHOX
ligands developed by Pfaltz and co-workers.[2a,b]
best with ligand L1a (regiose-
lectivity up to 98%, eeꢁs up to
98%). These results clearly
show the importance of using
Table 5. Selected results for the Pd-catalysed enantioselective cyclohexe-
In this section, we report that the chiral phosphite–oxazo-
nylation of 2,3-dihydrofuran S1 using ligands L1–L4a–g.[a]
line ligands L1–L4a–g applied in the previous section to the
Pd-catalysed arylation and alkenylation of substrate S1, can
also be used for unfunctionalised alkene substrate S2. In this
case, two triflate sources were used [Eq. (2)]: phenyl triflate
and cyclohexenyl triflate. In general, high activities and se-
lectivities (regioselectivity up to 94% and eeꢁs up to 96%)
were obtained in the phenylation and cyclohexenylation of
S2. Interestingly, the formation of achiral product 9 did not
take place. These results compete favourably with the best
reported in the literature.[2a,b]
Ligand
conv [%] (1:2)[b]
ee [%] 1[c]
ee [%] 2[c]
1
2
3
4
5
6
7
8
9
L1a
L1b
L1c
L1d
L1e
L1 f
L1g
L2a
L3a
L4a
100 (98:2)
88 (80:20)
100 (92:8)
99 (74:26)
57 (58:42)
42 (55:45)
38 (53:47)
94(84:16)
98 (R)
96 (R)
97 (R)
82 (R)
45 (R)
12 (R)
38 (R)
98 ( R)
77 (R)
95 (R)
nd[d]
36 (R)
52 (R)
76 (R)
32 (R)
9 (R)
9 (R)
4
R)
5 (
44 (51:49)
100 (88:12)
5 (R)
56 (R)
10
Preliminary investigations into the solvent and base ef-
fects revealed the same trends as those with the previously
tested substrate S1 with aryltriflate. The trade-off between
selectivities and reaction rates was optimum with THF as
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), cyclohexyl triflate
C
Chem. Eur. J. 2007, 13, 3296 – 3304ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3301

M. DiØguez, O. Pàmies, and Y. Mata
solvent and diisopropylthylamine as base (see Supporting
Information).
The results of using the phosphite–oxazoline ligand li-
brary under the optimised conditions are shown in Table 6.
In general, they follow the same trends as for the alkenyla-
Interestingly, under non-optimised conditions, our new li-
gands also proved to be highly efficient in terms of activity
and enantioselectivity in the phenylation of S3 (Table 7).
Table 7. Selected results for the Pd-catalysed enantioselective phenyla-
tion of S3 using ligands L1–L4a–g.[a]
Table 6. Selected results for Pd-catalysed enantioselective phenylation
Ligand
conv [%][b]
% ee [%] 10[c]
and cycloalkenylation of cyclopentene S2 using ligands L1–L4a-g.[a]
1
2
L1a
L1b
L1c
L2a
L3a
L4a
100
98
100
95
5461 (
100
92 (R)
88 (R)
92 (R)
75 (R)
R)
Ligand
R
conv [%] (7:8)[b]
ee [%] 7[c]
ee [%] 8[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
L1a
L1b
L1c
L1d
L1e
L1 f
L1g
L2a
L3a
L4a
L1a
L1b
L1c
L1d
L1e
L1 f
L1g
L2a
L3a
L4a
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H9
C6H9
C6H9
C6H9
C6H9
C6H9
C6H9
C6H9
C6H9
C6H9
100 (94:6)
98 (85:15)
100 (92:8)
93 (79:21)
47 (51:49)
42 (45:55)
44 (52:48)
94(86:14)
95 (R)
94( R)
95 (R)
75 (R)
44 (R)
12 (R)
45 (R)
87 ( R)
59 ( R)
94( R)
96 (R)
95 ( R)
96 (R)
82 (R)
51 (R)
11 (R)
49 (R)
93 (R)
61 (R)
94( R)
nd[d]
3[d]
4
66 (R)
82 (R)
57 (R)
34 R)
9 (R)
5
6
90 (R)
(
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S3 (2.0 mmol), phenyl triflate
G
(0.5 mmol), Ligand (2.8”10À2 mmol), THF (3 mL), iPr2NEt (1 mmol),
T=708C, t=2.5 days. [b] Conversion percentages determined by GC.
[c] Enantiomeric excesses measured by GC. [d] t=2 days.
4
R) 2 (
68 (R)
11 (R)
86 (R)
nd[d]
67 (R)
78 (R)
79 (R)
60 (R)
5 (R)
16 (R)
89 (R)
18 (R)
89 (R)
34(49:51)
100 (93:7)
100 (95:5)
94(83:17)
100 (93:7)
100 (78:22)
45 (54:46)
41 (52:48)
47 (53:47)
98 (85:15)
56 (63:37)
100 (92:8)
The results indicated that enantioselectivities and activities
are mainly affected by the steric properties of the oxazoline
substituents. We found that when the size of the group on
the oxazoline increased, activity and enantioselectivity of
the catalyst decreased (entries 1 versus 4and 5). The highest
enantioselectivities (eeꢁs up to 92%) and activities were ob-
tained using ligands L1a and L1c. In addition, comparing
these excellent results with the activities obtained with
Pfaltzꢁs and Gilbertsonꢁs ligand Pd-systems (Scheme 2) in
the phenylation of S3, we can conclude that the presence of
a phosphite moiety has been highly advantageous. These re-
sults are among the best reported so far.[2a,b,e]
[a] [Pd2
Ligand
G
(2.8”10À2 mmol),
T=708C, t=48 h. [b] Conversion percentages determined by GC. [c] En-
antiomeric excesses measured by GC. [d] Not determined.
tion of S1. Although, as expected, the activities were lower
than in the alkenylation of S1, they were much higher than
those obtained with the most successful ligand system.[2a,b]
Again, the presence of a phosphite moiety in the ligand
design has been highly advantageous in terms of activity and
enantioselectivity.
Microwave-assisted asymmetric Heck reactions: The bene-
fits of microwave irradiation, including reduction of reaction
rates and electricity costs, have already been reported in
[10]
À
several C C coupling reactions. Therefore, we decided to
use the advantages of microwave irradiation in the asym-
metric Pd-catalysed Heck reactions using the ligand library
L1–L4a–g. To the best of our knowledge there is only one
report on the use of microwave irradiation for the enantio-
selective Heck reactions by using Pfaltzꢁs PHOX and
BINAP ligands. Under optimal reaction conditions they con-
siderably shortened reaction times (from 4days to 1 hour)
but enantioselectivities were lower compared to those ob-
tained under thermal conditions.[11]
Asymmetric Heck reactions under thermal conditions—
Heck reaction of 4,7-dihydro-1,3-dioxepin S3 [Eq. (3)]:
A
tion and alkenylation of 2,3-dihydrofuran S1 and cyclopen-
tene S2, we also examined the phenylation of 4,7-dihydro-
1,3-dioxepin S3 [Eq. (3)]. This substrate is of great impor-
tance, since the resulting enol ethers 10 are easily converted
to chiral b-aryl-g-butyrolactones, which are useful synthetic
intermediates.[9] Despite this interesting characteristic, there
are only few reports that study this substrate.[2a,b,e,h] An im-
portant drawback for this substrate is that the catalysts de-
veloped to date proceed at low reaction rates (i.e., the reac-
tion takes typically 5 to 7 days for full conversion).[2a,b,e,h]
As an initial evaluation we studied the Pd-catalysed asym-
metric Heck reaction of substrate S1 using two different tri-
flate sources (phenyl triflate and cyclohexenyl triflate) with
ligands L1a and L1c (Table 8). After studying three differ-
ent temperatures, we found that the optimal temperature
was 708C. At lower temperatures, activities and selectivities
decreased (entries 3 and 6 versus 1, 2, 4and 5).
It is interesting to note that under microwave irradiation,
reaction times have been dramatically improved (from 15 h
to 10 minutes) while maintaining the excellent regio- (up to
98%) and enantioselectivities (eeꢁs up to 99%) obtained
under thermal conditions.
3302
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 3296 – 3304

Asymmetric Catalysis
FULL PAPER
Table 8. Microwave-assisted Pd-catalysed enantioselective arylation and
correctly choosing ligand components (phosphite and oxazo-
line substituents). Excellent activities (up to 100% conver-
sion in 10 minutes), and regio- (up to >99%) and enantio-
selectivities (eeꢁs up to 99%) were obtained in a wide range
of substrates and triflate sources. These results compete fa-
vourably with the most successful ligands developed for this
reaction.[1] Note also that these ligands provided higher ac-
tivities than those for other successful ligands. The use of
microwave irradiation conditions allowed a considerably
shorter reaction times (full conversion in few minutes) main-
taining excellent regio- and enantioselectivities. These re-
sults open up a new class of ligands for the highly active and
enantioselective Pd-catalysed Heck reaction, which will be
of great practical interest.
alkenylation of 2,3-dihydrofuran S1 using ligands L1a and L1c.[a]
Ligand
R
T [8C]
t [min]
conv [%] (1:2)[b]
ee [%] 1[c]
1
2
3
4
5
6
7
L1a
L1a
L1a
L1c
L1a
L1a
L1c
C6H5
C6H5
C6H5
C6H5
C6H9
C6H9
C6H9
50
40
70
70
50
70
70
15
15
10
10
15
10
10
81 (96:4)
12 (94:6)
99 (96:4)
100 (98:2)
82 (93:7)
100 (95:5)
100 (93:7)
93 (R)
91 (R)
96 (R)
99 (R)
89 (R)
93 (R)
97 (R)
[a] [Pd2
(dba)3]·dba (1.25”10À2 mmol), S1 (2.0 mmol), triflate (0.5 mmol),
G
Ligand (2.8”10À2 mmol), THF (3 mL), iPr2NEt (1 mmol). [b] Conversion
percentages determined by GC. [c] Enantiomeric excesses measured by
GC.
Encouraged by these excellent results, we also studied the
phenylation of cyclopentene S2 and 4,7-dihydro-1,3-dioxepin
S3, which required longer reaction times under thermal con-
ditions than substrate S1 (Scheme 4). Again, the use of mi-
Experimental Section
General considerations: All syntheses were performed by using standard
Schlenk techniques under argon atmosphere. Solvents were purified by
standard procedures. Compounds 3–6 were prepared by previously de-
scribed methods.[7] Phosphorochloridites were prepared as previously de-
scribed.[12] Ligands L1a–c, L1e, L2a, L3a, L4a have been previously syn-
thesised.[13] All other reagents were used as commercially available. 1H,
13C{1H} and 31P{1H} NMR spectra were recorded on a Varian Gemini
400 MHz spectrometer. The chemical shifts are referenced to tetrame-
thylsilane (1H and 13C) as internal standard or H3PO4 (31P) as external
1
standard. The H and 13C NMR spectral assignments were determined by
1H-1H and 1H-13C correlation spectra. Microwave experiments were car-
ried out by using a CEM Explorer, in which the temperature is controlled
by a non-contact infrared sensor that is located beneath the cavity floor
and “looks” up at the bottom of the vessel.
General procedure for the preparation of ligands L1–L4: The corre-
sponding phosphorochloridite (3.0 mmol), produced in situ, was dissolved
in toluene (12.5 mL); pyridine (1.14mL, 14mmol) was added. Hydrox-
yoxazoline (2.8 mmol) was azeotropically dried with toluene (3”2 mL)
and then dissolved in toluene (12.5 mL) to which pyridine (1.14mL,
14mmol) was added. The oxazoline solution was transferred slowly at
08C to the solution of phosphorochloridite. The reaction mixture was stir-
red overnight at 808C, and the pyridine salts were removed by filtration.
Evaporation of the solvent gave a white foam, which was purified by
flash chromatography (toluene/NEt3 =100:1) to produce the correspond-
ing ligand as a white solid.
Ligand L1d: Yield: 0.17 g, 28%; 31P NMR (400 MHz, C6D6, 258C,
H3PO4): d=150.3 ppm (s); 1H NMR (400 MHz, C6D6, 258C, TMS):
d=2.27 (s, 3H; CH3), 2.30 (s, 3H; CH3), 2.31 (s, 3H; CH3), 2.42 (s, 3H;
CH3), 3.50 (m, 1H; H-6’), 3.67 (m, 2H; H-4, H-5), 4.20 (m, 1H; H-6),
4.31 (m, 1H; H-2), 4.81 (m, 1H; H-3), 5.47 (s, 1H; H-7), 5.70 (d, 2J-
Scheme 4. Pd-catalysed Heck reactions of S2 and S3 using ligand L1a
under microwave irradiation.
crowaves was highly advantageous, providing excellent
regio- and enantioselectivities with much lower reaction
times (from 2 days to 45 minutes). It should be noted that
for substrate S2, the use of microwave irradiation also im-
proved regio- and enantioselectivity. Therefore, the reaction
of cyclopentene S2 and phenyltriflate at 708C gave the cou-
pling product 7 with 97% ee in 99% regioselectivity.
AHCTREUNG
Conclusion
AHCTREUNG
A library of readily available phosphite–oxazoline ligands
has been synthesised and applied in Pd-catalysed asymmet-
ric Heck reactions of several substrates and triflates under
thermal and microwave conditions. These ligands have the
advantage that they are easily prepared in a few steps from
commercial d-glucosamine as an inexpensive natural chiral
source. In addition, they can be easily tuned in the oxazoline
and biarylphosphite moieties, so that their effect on catalytic
performance can be explored. We found that the degree of
isomerisation and the effectiveness in transferring the chiral
information in the product and the activity can be tuned by
4), 102.2 (C-7), 103.5 (C-1), 126.0 (CH=), 127.2 (CH=), 129.0 (CH=),
129.3 (CH=), 129.4(CH =), 129.6 (CH=), 131.0 (C), 131.6 (CH=), 131.7
(CH=), 132.3 (CH=), 134.3 (CH=), 138.2 (C), 138.31 (C), 139.3 (C), 146.9
(C), 164.1 ppm (C); elemental analysis calcd (%) for C36H34NO7P: C
69.33, H 5.50, N 2.25; found: C 69.39, H 5.52, N 2.23.
Ligand L1 f: Yield: 0.1 g, 16%; 31P NMR (400 MHz, C6D6, 258C, H3PO4):
d=154.7 ppm (s); 1H NMR (400 MHz, C6D6, 258C, TMS): d=3.40 (m,
1H; H-6’), 3.53 (m, 2H; H-4, H-5), 4.06 (m, 2H; H-2, H-6), 4.66 (m, 1H;
H-3), 5.44 (d, 2J
(H1,H2)=7.8 Hz, 1H; H-1), 5.51 (s, 1H; H-7), 6.90–
A
8.15 ppm (m, 22H; CH=); 13C NMR (400 MHz, C6D6, 258C, TMS):
2
d=63.7 (C-5), 68.8 (C-6), 69.3 (C-2), 78.7 (d, J
A
(C-4), 101.9 (C-7), 103.5 (C-1), 122.6 (CH=), 123.0 (CH=), 125.5 (CH=),
126.9 (CH=), 127.0 (CH=), 127.8 (CH=), 127.9 (CH=), 128.9 (CH=),
Chem. Eur. J. 2007, 13, 3296 – 3304ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3303

M. DiØguez, O. Pàmies, and Y. Mata
129.0 (CH=), 129.1 (CH=), 129.6 (CH=), 130.2 (CH=), 131.1 (CH=),
132.0 (C), 132.3 (CH=), 132.4(C), 133.7 (C), 133.9 (C), 138.3 (C), 148.7
(C), 148.8 (C), 164.1 ppm (C); elemental analysis calcd (%) for
C40H30NO7P: C 71.96, H 4.53, N 2.10; found: C 71.59, H 4.59, N 2.07.
X. L. Hou, Acc. Chem. Res. 2003, 36, 659–667; c) C. Bolm, J. P. Hil-
debrand, K. MuÇiz, N. Hermanns, Angew. Chem. 2001, 113, 3382–
3407; Angew. Chem. Int. Ed. 2001, 40, 3284–3308; d) M. Shibasaki,
E. M. Vogl in Comprehensive Asymmetric Catalysis (Eds.: E. N. Ja-
cobsen, A. Pfaltz, H. Yamamoto), Springer, Heidelberg, 1999,
pp. 457–487; e) O. Loiseleur, M. Hayashi, M. Keenan, N. Schemees,
A. Pfaltz, J. Organomet. Chem. 1999, 576, 16–22; f) M. Beller, T. H.
Riermeier, G. Stark in Transition Metals for Organic Synthesis (Eds.:
M. Beller, C. Bolm), Wiley-VCH, Weinheim, 1998, pp. 208–240.
[2] See for instance: a) O. Loiseleur, P. Meier, A.; Pfaltz, Angew. Chem.
1996, 108, 218–220; Angew. Chem. Int. Ed. Engl. 1996, 35, 200–202;
b) O. Loiseleur, M. Hayashi, N. Schmees, A. Pfaltz, Synthesis 1997,
1338–1345; c) T. Tu, X. L. Hou, L. X. Dai, Org. Lett. 2003, 5, 3651–
3653; d) S. R. Gilbertson, D. Xie, Z. Fu, J. Org. Chem. 2001, 66,
7240–7246; e) S. R. Gilbertson, Z. Fu, Org. Lett. 2001, 3, 161–164;
f) T. Tu, W. P. Deng, X. L. Hou, L. X. Dai, X. C. Dong, Chem. Eur.
J. 2003, 9, 3073–3081; g) S. R. Gilbertson, D. G. Genov, A. L. Rhein-
gold, Org. Lett. 2000, 2, 2885–2888; h) Y. Hashimoto, Y. Horie, M.
Hayashi, K. Saigo, Tetrahedron: Asymmetry 2000, 11, 2205–2210;
i) X. L. Hou, D. X. Dong, K. Yuan, Tetrahedron: Asymmetry 2004,
15, 2189–2191; j) D. Liu, Q. Dai, X. Zhang, Tetrahedron 2005, 61,
6460–6471.
[3] See for instance: a) M. DiØguez, O. Pàmies, C. Claver, Chem. Rev.
2004, 104, 3189–3215; b) M. DiØguez, O. Pàmies, A. Ruiz, Y. Díaz,
S. Castillón, C. Claver, Coord. Chem. Rev. 2004, 248, 2165–2192;
c) M. DiØguez, A. Ruiz, C. Claver, Dalton Trans. 2003, 2957–2963;
d) O. Pàmies, M. DiØguez, A. Ruiz, C. Claver, Chem. Today 2004,
12–14; e) M. DiØguez, O. Pàmies, A. Ruiz, C. Claver, in Methodolo-
gies in Asymmetric Catalysis (Ed.: S. V. Malhotra), American Chem-
ical Society, Washington DC, 2004, pp. 161–173. f) M. DiØguez, O.
Pàmies, C. Claver, Tetrahedron: Asymmetry 2004, 15, 2113–2122.
[4] a) G. P. F. van Strijdonck, M. D. K. Boele, P. C. J. Kamer, J. G. de
Ligand L1 g: Yield: 0.08 g, 12%; 31P NMR (400 MHz, C6D6, 258C,
H3PO4): d=154.2 ppm (s); 1H NMR (400 MHz, C6D6, 258C, TMS):
d=3.27 (m, 1H; H-6’), 3.45 (m, 2H; H-4, H-5), 4.02 (m, 1H; H-6), 4.14
(dd, 2J
5.29 (s, 1H; H-7), 5.55 (d, 2J
(m, 22H; CH=); 13C NMR (400 MHz, C6D6, 258C, TMS): d=63.1 (C-5),
68.7 (C-6), 69.2 (C-2), 78.3 (d, 2J
(C,P)=19.0 Hz, C-3), 78.7 (C-4), 101.7
A
ACHTREUNG
AHCTREUNG
AHCTREUNG
(C-7), 103.5 (C-1), 122.5 (CH=), 123.5 (CH=), 125.5 (CH=), 125.6 (CH=),
126.9 (CH=), 127.0 (CH=), 127.1 (CH=), 127.8 (CH=), 127.9 (CH=),
128.8 (C), 128.9 (CH=), 129.0 (CH=), 129.1 (CH=), 129.5 (CH=), 129.8
(C), 130.4(CH =), 131.1 (CH=), 132.1 (C), 132.4(CH =), 133.6 (C), 133.8
(CH=), 138.5 (CH=), 147.8 (C), 147.9 (C), 164.2 ppm (C); elemental anal-
ysis calcd (%) for C40H30NO7P: C 71.96, H 4.53, N 2.10; found: C 71.74,
H 4.56, N 2.12.
Ligand L4c: Yield: 0.22 g, 30%; 31P NMR (400 MHz, C6D6, 258C,
H3PO4): d=150.2 ppm (s); 1H NMR (400 MHz, C6D6, 258C, TMS):
d=0.42 (s, 9H; CH3-Si), 0.50 (s, 9H; CH3-Si), 1.55 (d, 3H, J
1.6 Hz; CH3), 3.37 (m, 1H; H-6’), 3.52 (m, 1H; H-5), 3.84(dd, 3J-
(H4,H3)=10 Hz, 3J
(H4,H5)=7.6 Hz, 1H; H-4), 4.05 (m, 1H; H-2), 4.10
(H,P)=
A
ACHTREUNG
(m, 1H; H-6), 4.91 (m, 1H; H-3), 5.37 (d, 1H, 2J
(H1,H2)=7.6 Hz; H-1),
A
5.40 (s, 1H; H-7), 7.0–7.6 ppm (m, 11H; CH=); 13C NMR (400 MHz,
C6D6, 258C, TMS): d=0.8 (CH3-Si), 14.2 (CH3), 63.4(C-5), 69.4(C-6),
70.2 (C-2), 72.2 (d, 2J
(C,P)=13.7 Hz, C-3), 80.9 (C-4), 101.9 (C-1), 102.4
N
(C-7), 125.6 (CH=), 125.8 (CH=), 127.4(CH =), 128.9 (CH=), 129.6 (CH=
), 129.9 (C), 132.3 (C), 132.4(C), 133.3 (CH =), 134.4 (C), 136.0 (CH=),
136.1 (CH=), 137.9 (C), 138.2 (C), 153.8 (C), 155.1 (C), 164.0 ppm (C);
elemental analysis calcd (%) for C43H56NO7P: C 70.76, H 7.73, N 1.92;
found: C 70.79, H 7.74, N 1.95.
A
General procedure for the Pd-catalysed enantioselective Heck reactions:
1076; b) O. Pàmies, M. DiØguez, C. Claver, C. J. Am. Chem. Soc.
2005, 127, 3646–3647.
A mixture of [Pd2(dba)3]·dba (12 mg, 1.25”10–2 mmol) and the corre-
C
sponding chiral ligand (2.8”10–2 mmol) in dry degassed solvent (3.0 mL)
was stirred under argon at room temperature for 15 min. The correspond-
ing olefin (2.0 mmol), triflate (0.50 mmol) and base (1.0 mmol) were
added to the catalyst solution. The solution was stirred at the desired
temperature under argon. After the desired reaction time, the mixture
was diluted with additional diethyl ether and washed with water, dried
over MgSO4 and evaporated. For compounds 2-(1-naphthyl)-2,5-dihydro-
furan and 2-(4-nitrophenyl)-2,5-dihydrofuran conversion was measured
by 1H NMR spectroscopy and selectivity was measured by HPLC.[2b] For
the rest of compounds, conversion and selectivity were determined by
GC.[2e]
[5] a) K. Yonehara, K. Mori, T. Hashizume, K. G. Chung, K. Ohe, S.
Uemura, J. Organomet. Chem. 2000, 603, 40–49; b) R. Imbos, A. J.
Minnaard, B. L. Feringa, J. Am. Chem. Soc. 2002, 124, 184–185.
[6] The preliminary Heck results were partly reported in Y. Mata, M.
DiØguez, O. Pàmies, C. Claver, Org. Lett. 2005, 7, 5597–5599. They
represented the first application of phosphite–oxazoline ligands in
this process.
[7] K. Yonehara, T. Hashizume, K. Mori, K. Ohe, S. Uemura, J. Org.
Chem. 1999, 64, 9374–9380.
[8] O. Pàmies, M. DiØguez, G. Net, A. Ruiz, C. Claver, Organometallics
2000, 19, 1488–1496.
[9] S. Takano, K. Dsmizu, K. Ogasawara, Synlett 1993, 393–394.
[10] See for instance: Microwave Assisted Organic Synthesis (Eds.: J. P.
Tierney, P. Lidstrçm), Blackwell, 2005, and references therein.
[11] P. Nilsson, H. Gold, M. Larhed, A. Hallberg, Synthesis 2002, 1611–
1614.
Acknowledgements
[12] a) G. J. H. Buisman, P. C. J. Kamer, P. W. N. M. van Leeuwen, Tetra-
hedron: Asymmetry 1993, 4, 1625–1634; b) G. J. H. Buisman, L. A.
van der Veen, A. Klootwijk, W. G. J. de Lange, P. C. J. Kamer,
P. W. N. M. van Leeuwen, D. Vogt, Organometallics 1997, 16, 2929–
2939.
We thank the Spanish Government (Consolider Ingenio CSD2006–0003,
CTQ2004–04412/BQU and Ramon y Cajal fellowship to O.P.) and the
Generalitat de Catalunya (2005SGR007777 and Distinction to M.D.) for
financial support. We thank ICIQ (Institud Català d’Investigació Quími-
ca) for technical support on the microwave experiments.
[13] Y. Mata, M. DiØguez, O. Pàmies, C. Claver, Adv. Synth. Catal. 2005,
347, 1943–1947.
[1] For recent reviews, see: a) L. T. Tietze, H. Ila, H. P. Bell, Chem. Rev.
2004, 104, 3453–3516; b) L. X. Dai, T. Tu, S. L. You, W. P. Deng,
Received: November 30, 2006
Published online: February 9, 2007
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