Synthesis of N-aryl-aza-crown ethers via Pd-catalyzed amination reactions of aryl chlorides with aza-crown ethers

Article abstract of DOI:10.1016/j.tet.2004.09.100

The Pd₂(dba)₃/P(i-BuNCH₂CH ₂)₃N (1) catalyst system effectively catalyzes the coupling of aza-crown ethers with electronically diverse aryl chlorides, affording N-aryl-aza-crown ethers in good yields. The Pd₂(dba) ₃/P(i-BuNCH₂)₃CMe (2) catalyst system containing the more constrained bicyclic triaminophosphine is useful for aryl chlorides possessing base-sensitive ester, nitro, and nitrile functional groups. Graphical Abstract

Full text of DOI:10.1016/j.tet.2004.09.100

Tetrahedron 60 (2004) 11837–11842
Synthesis of N-aryl-aza-crown ethers via Pd-catalyzed amination
reactions of aryl chlorides with aza-crown ethers
*
Sameer Urgaonkar and John G. Verkade
Department of Chemistry, 1275 Gilman Hall, Iowa State University, Ames, IA 50011, USA
Received 30 August 2004; revised 26 September 2004; accepted 28 September 2004
Available online 27 October 2004
Abstract—The Pd2(dba)3/P(i-BuNCH2CH2)3N (1) catalyst system effectively catalyzes the coupling of aza-crown ethers with electronically
diverse aryl chlorides, affording N-aryl-aza-crown ethers in good yields. The Pd2(dba)3/P(i-BuNCH2)3CMe (2) catalyst system containing
the more constrained bicyclic triaminophosphine is useful for aryl chlorides possessing base-sensitive ester, nitro, and nitrile functional
groups.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
However, this method was limited to electron-poor aryl and
heteroaryl bromides. Interestingly, the use of P(t-Bu)3,7 a
popular ligand for Pd-catalyzed amination reactions, gave
inferior results probably because of its steric bulk.
The chemistry of N-aryl-aza-crown ether derivatives is
attracting significant interest because of the utility of these
compounds in synthesizing fluoroionophores in which, for
example, a fluorescent aryl moiety is covalently linked to
the nitrogen of an aza-crown ether.1 These molecules can
serve as sensitive and selective sensors of cations by binding
them in the crown ether, thereby modifying the intensity
and/or the energy of the signal of the fluorophore. Pyridine-
functionalized aza-crown ethers have also been used as a
scaffold for self-assembly in supramolecular chemistry.2
Traditional approaches to the preparation of N-aryl-aza-
crown ethers include nucleophilic aromatic substitution of
activated aryl halides with aza-crown ethers under high
pressure conditions,3 or manipulation of functional groups
on aniline precursors.4 However, these approaches suffer
from one or more of the following problems that impede
accessibility to this important class of compounds: stringent
conditions, multiple step syntheses, low yields, and limited
substrate scope.
An improvement to the above protocol was described by
Zhang and Buchwald9 who achieved cross-coupling of aryl
bromides with aza-crown ethers using a catalyst system
comprised of Pd2(dba)3 and biphenyl-based monophosphine
ligands with NaO-t-Bu as the base. Although electronically
diverse and also ortho-substituted aryl bromides could be
employed in these reactions, limitations still existed. For
example, the authors noted that poor yields of N-aryl-aza-
crown ethers were obtained when weak bases such as
Cs2CO3 or K3PO4 were used in place of NaO-t-Bu, thus
precluding the introduction of various base-sensitive
functional groups into the aryl substrate. A particularly
important apparent limitation was that no examples
employing aryl chlorides as the coupling partner were
reported. Aryl chlorides are cheaper and are available in
wider diversity than bromides or iodides, and their
applicability in coupling with aza-crown ethers would
constitute a significant advance, especially since aza-crown
ethers are currently quite expensive. We report here a
general and efficient method for the synthesis of N-aryl-aza-
crown ethers via a palladium-catalyzed amination reaction
of aryl chlorides that occurs in the presence of bicyclic
triaminophosphine ligands 1 and 2.
In recent years, palladium-catalyzed Buchwald–Hartwig
amination reactions of aryl halides with amines have
emerged as a method of choice for C–N bond forming
processes.5–7 In this respect, Witulski et al.8 have developed a
Pd/PPh3 and a Pd/P(o-tol)3 catalyst system for the coupling
of aryl and heteroaryl bromides with aza-crown ethers.
Our recent explorations in palladium-catalyzed cross
coupling reactions have established that electron-rich and
commercially available proazaphosphatrane 1 (Fig. 1), first
synthesized in our laboratories, serves as an excellent ligand
Keywords: Proazaphosphatrane; N-Aryl-aza-crown ether; Buchwald–
Hartwig amination; Palladium; Bicyclic triaminophosphine.
* Corresponding author. Tel.: C1 515 2945023; fax: C1 515 2940105;
e-mail: jverkade@iastate.edu
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.09.100

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S. Urgaonkar, J. G. Verkade / Tetrahedron 60 (2004) 11837–11842
The potential and scope of this methodology is illustrated in
Table 1 by the reaction of a variety of aryl chlorides and
bromides with 1-aza-15-crown-5. It is seen in this table that
electronically diverse aryl chlorides as well as bromides can
be coupled successfully in good to excellent yields with the
aza-crown ether. Using the Pd2(dba)3/1 catalyst system
(2 mol% Pd), electron-poor 4-chlorobenzotrifluoride
afforded the desired product in 86% yield (entry 1).
Electron-neutral 4-chlorotoluene also reacted efficiently
(80% product yield, entry 5). Electron-rich 4-chloroanisole,
a more challenging substrate, also functioned as a substrate,
providing the desired N-aryl-aza-crown ether in moderate
yield (50%, entry 7). The meta-substituted aryl chloride,
3-chloroanisole also coupled, giving a 66% product yield
(entry 6). Notably, 2-chloropyridine and less reactive 3-
chloropyridine were also successfully coupled (entries 8 and
9). The coupling shown in entry 9 is particularly impressive,
because to date, no catalyst system has been reported for the
coupling of 3-halopyridines with aza-crown ethers. Under
these conditions, electron-neutral and electron-rich aryl
bromides also participated in the process (entries 11–13).
Figure 1. Bicyclic triaminophosphine ligands.
in Suzuki,10 Buchwald–Hartwig amination,11 Stille,12 and
alpha-arylation13 reactions. Notoriously unreactive aryl
chlorides can also be employed in these transformations.
We have also developed a new bicyclic triaminophosphine
ligand 2 (Fig. 1) for which we have demonstrated utility in
Buchwald–Hartwig amination reactions.14 It was noted that
ligand 2 is especially useful for substrates with function-
alities that require a weak base such as Cs2CO3.
As mentioned earlier, the Pd2(dba)3/2 catalyst system in the
presence of the weak base Cs2CO3 was employed (4 mol%
Pd) for substrates with base-sensitive functional groups.
Thus, methyl-3-chlorobenzoate (73%, entry 2), 4-chloro-
nitrobenzene (81%, entry 3), 4-chlorobenzonitrile (56%,
entry 4), and 4-bromonitrobenzene (87%, entry 10) were
all aminated under our standard conditions. For the reaction
of 4-bromonitrobenzene, however, 2 mol% of Pd was
sufficient for the coupling to occur in high yield.
2. Results and discussion
We initially investigated the coupling of commercially
available 1-aza-15-crown-5 with aryl chlorides. After brief
experimentation, we established that a variety of aryl
chlorides can be coupled with 1-aza-15-crown-5 using
1 mol% Pd2(dba)3 and 4 mol% ligand 1 in toluene at 100 8C
(Scheme 1) in the presence of NaO-t-Bu as the base.
Although ligands 1 and 2 are slightly air- and moisture-
sensitive, they can be easily handled without the need for a
glove-box using standard Schlenk techniques. Because of
the moisture sensitivity of NaO-t-Bu and Cs2CO3, these
reagents were stored and weighed inside the glove-box.
However, we have established that the weighing of the
aforementioned reagents inside the glove-box is not an
absolute requirement. Thus when a sample of one of these
reagents was taken from material stored inside the glove-
box and weighed outside the glove-box with manipulations
carried out using Schlenk techniques, amination reactions
proceeded with almost equal efficiency (see parenthesized
yields in entries 3, 6, and 11 of Table 1). The same
procedure was also applied to other ingredients, that is,
Pd2(dba)3, aryl chloride, toluene, and aza-crown ether.
Scheme 1. Conditions for Pd2(dba)3/1-catalyzed synthesis of N-aryl-aza-
crown ethers.
Not surprisingly, aryl chlorides possessing an ester, nitro or
nitrile group did not fare well in this approach. For these
substrates, however, conditions developed by us utilizing 2
as the ligand in the presence of the mild base Cs2CO3 proved
to be gratifyingly efficacious (Scheme 2).
The present methodology is not without its limitations,
however. For example, ortho-substituted aryl chlorides did
not couple with 1-aza-15-crown-5 to an appreciable extent
and ortho-substituted aryl bromides provided only trace
amounts of products.
We have also applied a biphenyl based aminophosphine
ligand 3 (Buchwald’s ligand) to synthesize the target
compounds from aryl chlorides (Table 2). Using the
protocol of Buchwald [i.e., 1 mol% of Pd2(dba)3 and
6 mol% of 3 (3L/Pd)], 4-chloroanisole and 4-chlorotoluene
efficiently reacted with 1-aza-15-crown-5, affording the
desired products in 61% (entry 1, Table 2) and 79% yields
(entry 2, Table 2), respectively. However, when aryl
chlorides with functional groups such as nitro and ester
Scheme 2. Conditions for Pd2(dba)3/2-catalyzed synthesis of N-aryl-aza-
crown ethers possessing base-sensitive functional groups.

S. Urgaonkar, J. G. Verkade / Tetrahedron 60 (2004) 11837–11842
Table 1. Pd2(dba)3/1 or 2-catalyzed synthesis of N-aryl-aza-crown ethers from aryl chlorides
11839
Entry
1
Aryl halide
Ligand
mol% Pd
2
Base
Yield (%)a
1
NaO-t-Bu
80b
2
2
4
Cs2CO3
73c
3
4
5
2
2
1
4
4
2
Cs2CO3
Cs2CO3
NaO-t-Bu
81c (79)d
56c
80b
6
1
2
NaO-t-Bu
66b (61)d
7
1
1
1
2
2
2
2
2
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
Cs2CO3
50b
76b
51b
87c
8
9
10
11
1
2
NaO-t-Bu
82b (81)d
12
13
1
1
2
2
NaO-t-Bu
NaO-t-Bu
68b
60b
a Isolated yields (average of two runs).
b For reaction conditions, see Scheme 1.
c For reaction conditions, see Scheme 2.
d Yields in parenthesis refer to the same reaction performed without the use of glove-box (see text).
were employed in the presence of Cs2CO3 as the base, the
corresponding products were obtained in only poor yields
3. Conclusions
The synthesis of various N-aryl-aza-crown ethers was
readily achieved via palladium-catalyzed amination of
aryl chlorides, bromides, and iodides in which the catalyst
system consists of Pd2(dba)3 and one of the bicyclic
triaminophosphine ligands 1 or 2, the choice depending on
the nature of the aryl substrate. Using this approach, the
reaction is tolerant of a variety of functional groups. To the
best of our knowledge, our protocol is the first reported for
coupling aryl chlorides with aza-crown ethers. We have
also demonstrated the utility of Buchwald’s ligand in the
reactions involving aryl chlorides.
(entries 3 and 4, Table 2). As demonstrated above, the
coupling of these substrates can be best carried out using
ligand 2.
We have extended our methodology based on the
Pd2(dba)3/1 catalyst system to the arylation of a second
aza-crown ether, namely, 1-aza-18-crown-6 and the results
are summarized in Table 3. Using unactivated and
deactivated aryl chlorides, bromides, and iodides, yields
obtained were in the range of 51–55% (entries 1–4). For an
aryl iodide, only 1 mol% of Pd was used.

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S. Urgaonkar, J. G. Verkade / Tetrahedron 60 (2004) 11837–11842
Table 2. Synthesis of N-aryl-aza-crown ethers from aryl chlorides using Buchwald’s protocol
Entry
R
Yield (%)a
1
2
3
4
4-OMe
4-Me
4-NO2
3-CO2Me
61
79
44b,c
26b,c
a Isolated yields (average of two runs).
b 2 mol% of Pd2(dba)3 and 12 mol% of 3 were used.
c Cs2CO3 was used as the base.
Table 3. Pd2(dba)3/1-catalyzed coupling of aryl halides with 1-aza-18-crown-6
Entry
1
Aryl halide
mol% Pd
2
Yield (%)a
51
2
3
4
2
1.8
1
52
54
55
a Isolated yields (average of two runs).
4. Experimental
4.1. General methods
were performed by Desert Analytics (Tucson, Arizona,
USA). Mass spectra were recorded on a Kratos MS 50
instrument. The yields reported are isolated yields and are
the average of two runs.
Pd2(dba)3, NaO-t-Bu, and Cs2CO3 were purchased from
Aldrich and used without further purification. Toluene was
collected from a Grubbs type solvent purification system.
All other reagents were commercially available and are
used as received. Ligands 115 and 214 were prepared
according to previously reported procedures, although 1 is
commercially available from Aldrich and Strem Chemicals.
For convenience, stock solutions of 1 and 2 in toluene
(2 mM) were prepared and stored under argon outside
the glove-box. All reactions were performed under an
4.2. General procedure for the coupling of aryl halides
with aza-crown ethers using the Pd2(dba)3/1 or
Pd2(dba)3/2 catalyst system (Tables 1 and 3)
An oven-dried Schlenk flask equipped with a magnetic
stirring bar was charged with Pd2(dba)3 (0.5–2 mol%, see
Tables 1 and 3), an appropriate aza-crown ether (1.2 mmol),
and NaO-t-Bu (1.4 mmol) or Cs2CO3 (1.5 mmol) inside a
glovebox. If the aryl halide (1.0 mmol) was a solid, it was
also added at this time. The flask was capped with a rubber
septum and removed from the glove box. Ligand 1 or 2
(2–8 mol%) was then added via syringe from a stock solution
1
atmosphere of argon in oven-dried glassware. H and 13C
NMR spectra were recorded at 300 and 75.5 MHz,
respectively, unless otherwise noted. Elemental analyses

S. Urgaonkar, J. G. Verkade / Tetrahedron 60 (2004) 11837–11842
11841
(2 mM in toluene). Aryl halide (if a liquid, 1.0 mmol) and
toluene (3 mL) were then successively added via syringe.
The reaction mixture was heated at the temperature indicated
(see Tables 1 and 3) for 24 h. The mixture was then cooled to
room temperature, adsorbed onto silica gel and then purified
by column chromatography using initially 10% ethyl acetate/
hexanes and then ethyl acetate as eluents.
entry 8). 1H NMR (300 MHz, CDCl3): d 8.05 (d, JZ
3.5 Hz, 1H), 7.34 (t, JZ7.1 Hz, 1H), 6.49–6.42 (m, 2H),
3.72–3.57 (m, 20H). 13C NMR (75 MHz, CDCl3): d 157.8,
148.0, 137.2, 111.6, 106.0, 71.4, 70.4, 70.2, 69.3, 51.2.
HRMS m/z Calcd for C15H24N2O4: 296.17361. Found:
296.17410. Anal. Calcd for C15H24N2O4: C, 60.81; H, 8.11.
Found: C, 60.67; H, 8.31.
4.3. General procedure for the coupling of aryl chlorides
with 1-aza-15-crown-5 using Buchwald’s catalyst system
(Table 2)
4.4.9. N-(3-Pyridinyl)-1-aza-15-crown-5 (Table 1, entry
9). 1H NMR (400 MHz, CDCl3): d 8.08–8.05 (m, 1H), 7.90
(d, JZ4.3 Hz, 1H), 7.09–7.06 (m, 1H), 6.94–6.92 (m, 1H),
3.74–3.55 (m, 20H). 13C NMR (100 MHz, CDCl3): d 143.7,
137.3, 134.2, 123.8, 118.2, 71.5, 70.5, 70.2, 68.4, 52.5.
HRMS m/z Calcd for C15H24N2O4: 296.17361. Found:
296.17410. Anal. Calcd for C15H24N2O4: C, 60.81; H, 8.11.
Found: C, 60.98; H, 7.97.
Inside a glovebox, an oven-dried Schlenk flask equipped
with a magnetic stirring bar was charged with Pd2(dba)3
(1 mol%), 1-aza-15-crown-5 (1.2 mmol), ligand
3
(6 mol%), and NaO-t-Bu (1.4 mmol). The flask was capped
with a rubber septum and removed from the glove box. Then
aryl chloride (1.0 mmol) and toluene (3 mL) were succes-
sively added via syringe and the reaction mixture was
heated at 100 8C for 24 h. The mixture was cooled to room
temperature, adsorbed onto silica gel and then purified by
column chromatography using initially 10% ethyl acetate/
hexanes, followed by ethyl acetate as eluents.
4.4.10. N-(3,5-Dimethylphenyl)-1-aza-15-crown-5 (Table
1, entry 11). The 1H and 13C NMR spectra were in
accordance with those described in the literature.9
4.4.11. N-(4-Methoxyphenyl)-1-aza-18-crown-6 (Table 3,
1
entries 1, 3, and 4). H NMR (300 MHz, CDCl3): d 6.82–
6.70 (m, 4H), 3.74–3.56 (m, 27H). 13C NMR (75 MHz,
CDCl3): d 151.8, 142.4, 115.1, 114.4, 71.03, 70.99, 70.8,
69.0, 56.0, 52.4. HRMS m/z Calcd for C19H31NO6:
369.21514. Found: 369.21580. Anal. Calcd for
C19H31NO6: C, 61.79; H, 8.40. Found: C, 61.63; H, 8.33.
4.4. References for known compounds and spectroscopic
data for unknown compounds
4.4.1. N-(4-Trifluoromethylphenyl)-1-aza-15-crown-5
1
(Table 1, entry 1). The H and 13C NMR spectra were in
accordance with those described in the literature.9
4.4.12. N-(4-Methylphenyl)-1-aza-18-crown-6 (Table 3,
entry 2). 1H NMR (300 MHz, CDCl3): d 7.02 (d, JZ8.4 Hz,
2H), 6.62 (d, JZ8.5 Hz, 2H), 3.70–3.56 (m, 24H), 2.23 (s,
3H). 13C NMR (75 MHz, CDCl3): d 146.0, 130.0, 125.2,
112.1, 71.1, 71.08, 71.0, 70.9, 69.1, 51.7, 20.4. HRMS m/z
Calcd for C19H31NO5: 353.22022. Found: 353.22100. Anal.
Calcd for C19H31NO5: C, 64.59; H, 8.78. Found: C, 64.67;
H, 8.67.
4.4.2. N-(3-Carbomethoxyphenyl)-1-aza-15-crown-5
(Table 1, entry 2). H NMR (300 MHz, CDCl3): d 7.30–
1
7.19 (m, 3H), 6.85–6.82 (m, 1H), 3.86 (s, 3H), 3.76–3.58
(m, 20H). 13C NMR (75 MHz, CDCl3): d 167.9, 147.8,
131.3, 129.4, 117.0, 116.0, 112.3, 71.5, 70.4, 70.3, 68.6,
52.7, 52.2. HRMS m/z Calcd for C18H27NO6: 353.18384.
Found: 353.18430. Anal. Calcd for C18H27NO6: C, 61.19;
H, 7.65. Found: C, 61.34; H, 7.81.
Acknowledgements
4.4.3. N-(4-Nitrophenyl)-1-aza-15-crown-5 (Table 1,
1
entries 3 and 10). The H and 13C NMR spectra were in
We thank the Aldrich Chemical Co. for their generous
support of this study by supplying research samples. The
National Science Foundation is gratefully acknowledged for
financial support of this work in the form of a grant.
accordance with those described in the literature.8a
4.4.4. N-(4-Cyanophenyl)-1-aza-15-crown-5 (Table 1,
entry 4). The 1H and 13C NMR spectra were in accordance
with those described in the literature.16
4.4.5. N-(4-Methylphenyl)-1-aza-15-crown-5 (Table 1,
1
entry 5 and Table 2, entry 2). The H and 13C NMR
spectra were in accordance with those described in the
References and notes
literature.17
1. (a) Lohr, H.-G.; Vogtle, F. Acc. Chem. Res. 1985, 18, 65. (b) de
Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem.
Rev. 1997, 97, 1515.
4.4.6. N-(3-Methoxyphenyl)-1-aza-15-crown-5 (Table 1,
entries 6 and 12). The H and 13C NMR spectra were in
1
accordance with those described in the literature.9
2. Chi, K.-W.; Addicott, C.; Stang, P. J. J. Org. Chem. 2004, 69,
2910.
4.4.7. N-(4-Methoxyphenyl)-1-aza-15-crown-5 (Table 1,
3. (a) Matsumoto, K.; Hashimoto, M.; Toda, M.; Tsukube, H.
J. Chem. Soc., Perkin Trans. 1 1995, 2497. (b) Ciobanu, M.;
Matsumoto, K. Liebigs Ann. 1997, 623.
1
entries 7 and 13, and Table 2, entry 1). The H and 13C
NMR spectra were in accordance with those described in the
literature.9
4. (a) Crochet, P.; Malval, J.-P.; Lapouyade, R. Chem. Commun.
2000, 289. (b) Hirano, T.; Kikuchi, K.; Urano, Y.; Higuchi, T.;
Nagano, T. Angew. Chem., Int. Ed. 2000, 39, 1052. (c) Lednev,
4.4.8. N-(2-Pyridinyl)-1-aza-15-crown-5 (Table 1,

11842
S. Urgaonkar, J. G. Verkade / Tetrahedron 60 (2004) 11837–11842
I. K.; Hester, R. E.; Moore, J. N. J. Chem. Soc., Faraday Trans.
9. Zhang, X.-X.; Buchwald, S. L. J. Org. Chem. 2000, 65, 8027.
10. Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. Tetrahedron
Lett. 2002, 43, 8921.
1997, 93, 1551. (d) Liu, R. C. W.; Fung, P.-S.; Xue, F.; Mak,
T. C. W.; Ng, D. K. P. J. Chem. Res. (S) 1998, 414.
5. (a) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219,
131. (b) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem.
1999, 576, 125. (c) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.;
Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805.
11. (a) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. J. Org. Chem.
2003, 68, 452. (b) Urgaonkar, S.; Nagarajan, M.; Verkade,
J. G. Org. Lett. 2003, 5, 815. (c) Urgaonkar, S.; Verkade, J. G.
Adv. Synth. Catal. 2004, 346, 611.
6. (a) Hartwig, J. F. In Modern Amination Methods; Ricci, A.,
Ed.; Wiley: Weinheim, Germany, 2000. (b) Hartwig, J. F. Acc.
Chem. Res. 1998, 31, 852. (c) Hartwig, J. F. Angew. Chem.,
Int. Ed. 1998, 37, 2046.
12. (a) Su, W.; Urgaonkar, S.; Verkade, J. G. Org. Lett. 2004, 6,
1421. (b) Su, W.; Urgaonkar, S.; McLaughlin, P. A.; Verkade,
J. G. J. Am. Chem. Soc., in press.
13. (a) You, J.; Verkade, J. G. Angew. Chem., Int. Ed. 2003, 42,
5051. (b) You, J.; Verkade, J. G. J. Org. Chem. 2003, 68, 8003.
14. Urgaonkar, S.; Xu, J.-H.; Verkade, J. G. J. Org. Chem. 2003,
68, 8416.
7. (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy,
K. H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575.
(b) Nishiyama, M.; Yamamoto, T.; Koie, Y. Tetrahedron Lett.
1998, 39, 617.
15. Kisanga, P. B.; Verkade, J. G. Tetrahedron 2001, 57, 467.
16. Letard, J. F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem.
1993, 65, 1705.
8. (a) Witulski, B. Synlett 1999, 1223. (b) Witulski, B.;
Zimmerman, Y.; Dacros, V.; Desvergne, J.-P.; Bassani,
D. M.; Bouas-Laurent, H. Tetrahedron Lett. 1998, 39, 4807.
(c) Witulski, B.; Weber, M.; Bergstrasser, U.; Desvergne, J.-P.;
Bassani, D. M.; Bouas-Laurent, H. Org. Lett. 2001, 3, 1467.
17. Lu, X.; Zhong, R.; Liu, S.; Liu, Y. Polyhedron 1997, 16, 3865.