Diverse reactivity of 2-formylphenylboronic acid with secondary amines: Synthesis of 3-amino-substituted benzoxaboroles

Article abstract of DOI:10.1016/j.tetlet.2010.09.091

In the reaction of 2-formylphenylboronic acid with secondary amines different products are formed depending on the structure of the amines. For aliphatic amines the reaction proceeds with the formation of 3-amino-substituted benzoxaboroles or with the for

Full text of DOI:10.1016/j.tetlet.2010.09.091

Tetrahedron Letters 51 (2010) 6181–6185
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Diverse reactivity of 2-formylphenylboronic acid with secondary amines:
synthesis of 3-amino-substituted benzoxaboroles
a
a
b
a,b,
⇑
´
´
Agnieszka Adamczyk-Wozniak , Izabela Madura , Aldrik H. Velders , Andrzej Sporzynski
a Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
b Laboratory of Supramolecular Chemistry and Technology, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente,
PO Box 217, 7500 AE Enschede, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2010
Revised 25 August 2010
Accepted 20 September 2010
Available online 25 September 2010
In the reaction of 2-formylphenylboronic acid with secondary amines different products are formed
depending on the structure of the amines. For aliphatic amines the reaction proceeds with the formation
of 3-amino-substituted benzoxaboroles or with the formation of complexed boroxins. A crystal structure
of the benzoxaborole with a thiomorpholinyl substituent was determined.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Boronic acids
Benzoxaboroles
Boroxins
Secondary amines
Crystal structure
1. Introduction
A comprehensive review on the chemistry, structure, and appli-
cations of benzoxaboroles has been published.9 The main methods
Boronic acids, RB(OH)2, are compounds of great interest due to
their numerous applications in organic synthesis, catalysis, supra-
molecular chemistry, biology, and medicine.1,2 Only recently has
growing attention been paid to their derivatives, benzoxaboroles
I, due to their new medical applications as potent antifungal
drugs.3 They have also been tested for the treatment of periodontal
disease,4 psoriasis,5 and malaria together with other parasites.6
Moreover, chiral (S)-3-aminomethylbenzoxaborole was found to
be efficacious against Gram negative bacteria.6
for their synthesis (Scheme 1) are based on the formation of the
appropriate ortho-boronobenzyl alcohols by the introduction of a
hydroxymethyl group into the boronic acid molecule (method A),
by the reaction of 2-formylboronic acid with nucleophiles (method
B), or by the introduction of a boronic group into benzyl alcohol
(method C). Depending on the particular systems, appropriate pro-
tection of functional groups in the substrates can be applied.9
Recently, 3-amido-substituted benzoxaboroles were obtained in
high yields as the products of the reaction of 2-formylphenylbo-
ronic acid with isonitriles (Scheme 1, method B, X: CONHR).10
Reactions with activated olefins can lead to benzoxaboroles with
olefinic substituents (Scheme 1, method B, X: C(@CH2)R and
CH2C(CH2)R).11
The properties of a benzoxaborole strongly depend on the sub-
stituents. The influence of the substituents on the phenyl ring has
been widely investigated from the point of view of the biological
activity of the compounds.6 The introduction of a substituent to
an oxaborole ring, that is, at position 3 has influence on the geom-
etry of the whole molecule, which was proved by a combination of
experimental and theoretical methods.12,13 However, in compari-
son with the numerous examples of molecular structures known
for compounds with a carbon substituent at position 3,9 there is
only one example of a 3-amino-substituted compound.14
HO
1
B
O2
3
7
6
R
4
5
X
I
Benzoxaboroles reveal high receptor activity toward polyols
due to the formation of anionic form of cyclic esters derived from
saccharides in neutral or slightly alkaline media.7 Another very
important application of benzoxaboroles is their use in Suzuki cou-
pling to give ortho-substituted benzyl alcohols.8
⇑
2-Formylphenylboronic acid reacts with primary amines
yielding ortho-iminomethylphenylboronic acids, which can be
Corresponding author. Tel.: +48 22 2345737; fax: +48 22 6282741.
E-mail address: spor@ch.pw.edu.pl (A. Sporzyn
´ ski).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.091

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 51 (2010) 6181–6185
6182
HO
HO
OH
OH
HO
OH
Br
HO
HO
OH
OH
B
B
B
B
B
O
NBS
H2O
R
R
R
R
HO
Method A
B
O
CHO
X
+ nucleophile
X: CH2COOH, CN, CH2NO2,
NR1R2, CONHR, C(=CH2)R,
CH2C(=CH2)R
Method B
HO
Br
OH
R
B
O
1. BuLi, 2. B(OR)3, 3. H3O+
R
Method C
Scheme 1. The main methods for the synthesis of benzoxaboroles: from o-(bromomethyl)phenylboronic acid (A), from o-formylphenylboronic acid (B) or from a benzyl
alcohol (C).
subsequently reduced to aminomethyl derivatives.15 Amino-
methylarylboronic acids are currently the established standards
for the recognition of sugars, so their synthesis, characterization,
and receptor activity have been widely investigated.1,16 Methods
for the synthesis of these compounds have been reviewed.15 Re-
cently, self-assembling systems containing 2-formylphenylboron-
ic acids, primary amines, and diols were described.17,18
The aim of the present work was to investigate a new method
for the synthesis of 3-amino-substituted benzoxaboroles and the
properties of the compounds obtained.
The reaction of 2-formylphenylboronic acid with secondary
amines was investigated in solution (acetone-d6 or acetonitrile-
d3) by 1H, 13C, and 11B NMR spectroscopy. The results are collected
in Table 1. On the basis of these results, one can divide the reactiv-
ity of the systems investigated into three groups (Scheme 2).
For the reactions with morpholine, thiomorpholine, piperidine,
pyrrolidine, N-ethylbenzylamine, and dibenzylamine, the signal of
the aldehyde proton completely disappeared (entries a–d, h, and i),
whereas new signals at ca. d 3 (broad) and at d 5.8–6 appeared. The
1H signal at ca. d 6 originates from the CH protons in the oxaborole
ring and at ca. d 3 from water. In the 13C NMR spectra, new signals
appeared at ca. d 96 and d 153, while the signal at d 196 due to the
aldehyde carbon was no longer present. Assignment of the signals
in the 13C spectra can be done on the basis of HSQC and HMBC cor-
relation spectra which have been reported for several benzoxabo-
roles (Table 2).12 In some cases a 13C signal at d 77 was observed,
which can be attributed to the hemiaminal 10. Due to the presence
of the boronic acid group, hemiaminals can be stabilized by the for-
mation of intramolecular hydrogen bonds.20 This signal slowly dis-
appears in time, while that at d 95 increases. The products were
identified as 3-amino-substituted benzoxaboroles 1. The presence
of only one signal due to the three-coordinated boron atom in
the 11B NMR spectra confirms the proposed reaction (pathway I).
Several aminobenzoxaboroles were obtained in this way by the
preparative scale reactions.21
signals corresponding to the benzoxaborole unit (at ca. d 96 and
d 153) were not observed and those of an aldehyde carbon were
present. In the 11B NMR spectra, signals corresponding to three-
and four-coordinate boron atoms were observed. The most proba-
ble explanation is the formation of a boroxin 2 complexed by the
amine molecule (pathway II). This type of reaction has been
described as a ‘ligand facilitated trimerization’22 Complexes of
boroxins with amines are dynamic systems,23 and the signals
observed are usually averages due to three- and tetra-coordinate
boron atoms.24 However, in our investigated systems separate sig-
nals were observed. A possible explanation is that the exchange is
slowed down by the additional interactions between the carbonyl
and amine groups resulting in broadening of the signals in the 11B
NMR spectra. Additional signals in the 11B NMR spectra may also
originate from hemiaminals or imino derivatives,25 the formation
of which cannot be excluded.
Numerous examples of the crystal structures of such complexes
have been described in the literature for several phenylboronic
acids.24,26 Attempts to obtain crystals of the complexes of 2-form-
ylphenylboronic anhydride with amines for X-ray determination
failed, but the products obtained in the preparative reactions21
were identified by elemental analysis and NMR.
The third group of reagents were aromatic secondary amines
(entries j and k), for which only signals due to unreacted substrates
were observed (pathway III). The reaction of 2-formylphenylbo-
ronic acid with N-ethylaniline has recently been investigated
and the unexpected product of a C-addition was obtained (3,
Scheme 2). However, it was obtained in low yield in methanol.12
In acetone only traces of this product were found (Table 1, entry j).
On the basis of the above results, the reaction course can be
described as outlined in Scheme 2. For sufficiently strong nucleo-
philes the hemiaminal 10 is formed, which can react to form a
benzoxaborole or a complex of boroxin. Formation of 10 was
proved by the presence of a 13C signal at d 77 (C–OH) in the reac-
tion mixture, which disappeared upon the formation of benzox-
aborole. For the basic amines, a boroxin complex is formed in
solution, while for weaker amines the formation of a benzoxabo-
role is observed. Aminobenzoxaboroles are stable in the solid
state but in solution slowly decompose with the precipitation of
For the reaction of 2-formylphenylboronic acid with diethyl-
amine, di-isopropylamine, and dicyclohexylamine (entries e–g) a
new 1H NMR signal at ca. d 3.8 appeared, while the signal of the
aldehyde proton moved to higher values and split into two. The
signals of the aromatic protons were broadened. The signal at d
3.8 originates from the hydroxy groups in water. This was proved
by the addition of water to a sample, which increased the intensity
of this signal with a small shift to ca. d 3.5. In the 13C NMR spectra,
a
complexed boroxin. This was observed during attempted
crystallization of compounds 1: the products after the crystal-
lization contained an increased amount of complexed boroxin.
Weak bases (aromatic amines) form neither benzoxaboroles nor

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 51 (2010) 6181–6185
6183
Table 1
NMR signals (d, ppm) for the spectra of the reaction mixture of 2-formylphenylboronic acid and secondary amine (1:1 molar ratio) in acetone-d6 at rta
Entry
Amine (pKa)19
1H NMRb
10–11 ppm region
13C NMR (amine signals are underlined)
11B NMR
3–6 ppm region
—
0
None
10.2
130, 132, 133, 135, 141, 196
130, 133, 134, 135, 140, 196c
30.5
10.1c
30.7c
48, 67,
O
S
NH
NH
NH
a
b
c
3.1 (broad), 5.8
3.2 (broad), 5.8
3.1 (broad), 5.8
6.0c
—
—
—
—
32.1
32.7
32.3
96, 123, 129, 130, 131, 153
(8.39)
(9.13)
28, 50,
98, 123, 128, 130, 131, 153
25, 27, 47,
77, 97, 121, 127, 130, 131, 153
(11.12)
d
e
9.7, 28.9c
NH
25, 47, 93, 123, 128, 130, 153c
(11.27)
Et2NH (10.98)
4.1, 6.0d
3.8, 6.0c
11.1d (weak)
11.0c
16.5, 32.1c
14, 15, 42, 44, 77, 95, 122, 128, 130, 131, 155d
15, 43, 44, 97, 124, 127, 129, 131, 132, 156c
f
iPr2NH (10.96)
3.8
11.0, 11.1
8.6, 21.2, 30.5
8.3, 20.2, 30.0c
22, 46,
126, 129, 132, 135, 142, 196
25, 26, 33, 53, 117, 126, 133, 135, 142, 196c
g
CHx2NH (10.40)
3.7
11.1, 11.2
9.1, 21.0, 30.6
3.7c
h
i
PhCH2N(Et)H (9.65)
(PhCH2)2NH (8.76)
4.4 (broad) 6.0,
10.2 (trace)
—
32.7
32.8
13, 15, 43, 52, 54, 77, 94, 122, 123, 127, 128, 141, 154
3.1 (broad), 5.8
53, 77, 93, 122, 123, 127, 128, 141, 157
j
k
PhN(Et)H (5.12)
Ph2NH (1.78)
6.1 (trace)
—
10.2
10.2
n.i.e
n.i.e
30.6
30.6
a
Frequencies: 300, 96.2 and 128.3 MHz for 1H, 13C and 11B, respectively. Number of decimal places in chemical shift values was reduced for clarity, exact values are given in
Supplementary data.
b
Signals of the amine groups and aromatic protons not shown.
In acetonitrile-d3.
Immediately after mixing of the reagents.
Not investigated.
c
d
e
HO
HO
OH
HO
OH
B
O
B
B
OH
Pathway I
NR1R2
-H2O
CHO
NR1R2
1
2
+
HNR R
H
1'
1
Pathway II
-H2O
CHO
-H2O
-H2O
Pathway III:
NHR1R2
no reaction or
HO
B
B
O
CHO O
B
O
B
O
NHR2
2
OHC
(for R1 = Ph)
3
Scheme 2. Reactions of 2-formylphenylboronic acid with secondary amines.
complexed boroxins, but can undergo electrophilic aromatic sub-
stitution (pathway III, Scheme 2).
Compound 1b crystallizes in the monoclinic system in the P21/n
space group. The main structural feature is a centrosymmetric di-
mer formed via relatively strong O–Hꢀ ꢀ ꢀO hydrogen bonds (Fig. 1).
This dimeric motif can be described with an R22 (8) graph set28 and
is found to be typical for most of the previous structurally charac-
terized benzoxaboroles.9 The fused six- (phenyl) and five- (borole)
membered rings are flat with the largest deviation from the least
square plane being only 0.033(1) Å. The boron center shows ideal
trigonal coordination (the sum of the angles at the boron atom is
2. Crystal structure
The crystal structure of 1,3-dihydro-1-hydroxy-3-(thiomorpho-
lin-4-yl)-2,1-benzoxaborole 1b was determined by the single crys-
tal X-ray diffraction.27 The molecular structure of this compound
with the atom numbering scheme used is shown in Figure 1.

´
6184
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 51 (2010) 6181–6185
Table 2
Acknowledgments
Assignments of the signals in 13C NMR spectra
Carbon atom R = Ph (Ref. 12)a R = NR1R2 (this work)
This work has been supported by the European Union in the
framework of the European Social Fund through the Warsaw
University of Technology Development Programme and by
scholarships awarded by the Center of Advanced Studies for And-
1a
3
3a
4
5
6
Not detected
83.61
HO
1
B
O2
93–98
158.48
123.22
131.87
128.21
131.24
153–157
121–126
128–132
1a
3a
´
rzej Sporzynski at the University of Twente and for Agnieszka
3
7
6
R
Adamczyk-Woz´niak.
4
7
Supplementary data
5
a
In CDCl3, 125.1 MHz.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.09.091.
S(1)
References and notes
C(9)
C(8)
1. Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine; Hall,
D. G., Ed.; Wiley-VCH: Weinheim, Germany, 2005.
2. Severin, K. Dalton Trans. 2009, 5254.
C(10)
C(11)
3. (a) Baker, S. J.; Hui, X.; Maibach, H. I. Ann. Rep. Med. Chem. 2005, 40, 323; (b)
Hui, X.; Baker, S. J.; Wester, R. C.; Barbadillo, S.; Cashmore, A. K.; Sanders, V.;
Hold, K. M.; Akama, T.; Zhang, Y.-K.; Plattner, J. J.; Maibach, H. I. J. Pharm. Sci.
2007, 96, 2622; (c) Baker, S. J.; Zhang, Y.-K.; Akama, T.; Lau, A.; Zhou, H.;
Hernandez, V.; Mao, W.; Alley, M. R. K.; Sanders, V.; Plattner, J. J. J. Med. Chem.
2006, 49, 4447; (d) Rock, F. L.; Mao, W.; Yaremchuk, A.; Tukalo, M.; Crepin, T.;
Zhou, H.; Zhang, Y.-K.; Hernandez, V.; Akama, T.; Baker, S. J.; Plattner, J. J.;
Shapiro, L.; Martinis, S. A.; Benkovic, S. J.; Cusack, S.; Alley, M. R. K. Science 2007,
316, 1759; (e) Alley, M. R. K.; Baker, S. J.; Beutner, K. R.; Plattner, J. Expert Opin.
Invest. Drugs 2007, 16, 157.
N(1)
C(6)
C(4)
C(3)
C(5)
C(7)
O(1)i
O(2)
B(1)i
B(1)
C(2)
C(1)
O(2)i
4. Sanders, V.; Maples, K. R.; Plattner, J. J.; Bellinger-Kawahara, C. PCT Int. Appl.
WO2007146965; Chem. Abstr. 2007, 148, 61302.
O(1)
5. (a) Anacor Pharm., CA, USA . Expert Rev. Clin. Immunol. 2008, 4, 309; (b) Akama,
T.; Baker, S. J.; Zhang, Y.-K.; Hernandez, V.; Zhou, H.; Sanders, V.; Freund, Y.;
Kimura, R.; Maples, K. R.; Plattner, J. J. Bioorg. Med. Chem. 2009, 19, 2129.
6. Freund, Y. R.; Plattner, J.; Ding, C.; Zhang, Y. K.; Liu, L.; Wu, A.; Bu, W.; Easom, E.;
Gut, J.; Rosenthal, P. J. The American Society of Tropical Medicine and Hygiene
Conference, Washington, DC, Nov. 20, 2009. http://www.anacor.com/
conference_posters.php.
7. (a) Dowlut, M.; Hall, D. G. J. Am. Chem. Soc. 2006, 128, 4226; (b) Bérubé, M.;
Dowlut, M.; Hall, D. G. J. Org. Chem. 2008, 73, 6471.
8. Gunasekera, D. S.; Gerold, D. J.; Aalderks, N. S.; Chandra, J. S.; Maanu, C. A.;
Kiprof, P.; Zhdankin, V. V.; Reddy, M. V. R. Tetrahedron 2007, 63, 9401.
_
9. Adamczyk-Woz´niak, A.; Cyran´ ski, M. K.; Zubrowska, A.; Sporzyn´ ski, A. J.
Organomet. Chem. 2009, 694, 3533.
10. Kumar, J. S.; Jonnalagadda, S. C.; Mereddy, V. R. Tetrahedron Lett. 2010, 51,
779.
11. Kumar, J. S.; Bashian, C. M.; Corsello, M. A.; Jonnalagadda, S. C.; Mereddy, V. R.
Figure 1. The hydrogen bonded dimeric structure of 1,3-dihydro-1-hydroxy-3-(4-
thiomorpholinyl)-2,1-benzoxaborole (1b). Thermal ellipsoids are drawn with 50%
probability. The intermolecular O–Hꢀ ꢀ ꢀO hydrogen bonds are depicted with dashes.
Tetrahedron Lett. 2010, 51, 4482.
12. Adamczyk-Woz´niak, A.; Cyran´ ski, M. K.; Jakubczyk, M.; Klimentowska, P.; Koll,
_
_
A.; Kołodziejczak, J.; Pojmaj, G.; Zubrowska, A.; Zukowska, G. Z.; Sporzyn´ ski, A.
J. Phys. Chem. A 2010, 114, 2324.
13. Jezierska, A.; Panek, J. J.; Zukowska, G. Z.; Sporzyn´ ski, A. J. Phys. Org. Chem.
360.0(2)°) and the O(1)–H(1) group is in plane with the fused ring
system. In contrast to morpholine derivative 1a, where the hydro-
gen-bonded dimer is described as involving interacting parallel
planes of benzoxaborole units shifted by ca. 0.5 Å,9,14 in the case
of 1b, the H-bonded dimer is flat with the largest deviation from
planarity around 0.05 Å. This decreases significantly the hydrogen
bond length to 2.728(2) Å for the Oꢀ ꢀ ꢀO separation from that ob-
served for 1a [2.797(2) Å]. The comparison between the relevant
bonds around the boron atom shows that in the case of 1b all
the bonds are significantly shorter than in 1a. Similar to 1a, the thi-
omorpholine ring in 1b adopts a chair conformation and is almost
perpendicular to the plane of the central dimeric ring. The ob-
served discrepancies in the geometry of a molecule of 1b compared
to that of 1a indicate that the borole fragment is extremely sensi-
tive to modifications introduced by substituents.9 This example
shows that both steric and electronic factors are important but a
weak interaction should not be precluded.
In conclusion, on the basis of NMR results, we have described
the diverse reactivity of 2-formylphenylboronic acid with second-
ary amines as a function of their basicity and structure. Substituted
benzoxaboroles are the main products of the reactions with amines
of moderate basicity, while with strong Lewis bases complexes of
boroxin are formed. With aromatic amines, the formation of
C-alkylated products is the dominant reaction.
_
2010, 23, 451.
14. Sporzyn
´ ski, A.; Lewandowski, M.; Rogowska, P.; Cyran´ ski, M. K. Appl.
Organometal. Chem. 2005, 19, 1202.
_
´
´
15. Sporzynski, A.; Zubrowska, A.; Adamczyk-Wozniak, A. Synthesis of Boronic
Acids—Molecular Receptors for Sugars. In Synthetic Receptors in Molecular
Recognition; Rybachenko, V. I., Ed.; Schidnyj Wydawnyczyj Dim: Donetsk,
Ukraine, 2007; pp 51–87.
16. James, T. D.; Phillips, M. D.; Shinkai, S. In Boronic Acids in Saccharide Recognition;
RSC Publishing: Cambridge, UK, 2006.
17. Hutin, M.; Bernardinelli, G.; Nitschke, J. R. Chem. Eur. J. 2008, 14, 4585.
18. Galbraith, E.; Kelly, A. M.; Fossey, J. S.; Kociok-Köhn, G.; Davidson, M. G.; Bull, S.
D.; James, T. D. New J. Chem. 2009, 33, 181.
19. (a) Perrin, D. D. In Dissociation Constants of Organic Bases in Aqueous Solutions;
Butterworths: London, 1965; (b) Williams, R. pKa Data Compiled, http://
research.chem.psu.edu/brpgroup/pKa_compilation.pdf, 2004.
20. (a) Suni, V.; Kurup, M. R. P.; Nethaji, M. J. Mol. Struct. 2005, 749, 177; (b)
Velazquez, M.; Salgado-Zamora, H.; Perez, C.; Campos-A, M. E.; Mendoza, P.;
Jimenez, H.; Jimenez, R. J. Mol. Struct. 2010, 979, 56.
21. (a) Typical procedure for the synthesis of aminobenzoxaboroles 1: Na2SO4 (5.5 g)
was added to a solution of 2-formylphenylboronic acid (1.0 g, 6.7 mmol) and
morpholine (0.58 g, 6.7 mmol) in Et2O (50 mL). The reaction mixture was
stirred for 24 h at room temperature. After this time, the Na2SO4 was removed
by filtration and another portion of Na2SO4 (5.5 g, 40 mmol) was added to the
resulting clear solution. The reaction mixture was stirred for an additional
24 h. After this time the Na2SO4 was removed by filtration and the organic
filtrate was slowly evaporated to dryness. The resulting yellowish crystalline
solid was ground and was washed with hexane (2 mL) to wash out the
remaining morpholine. The hexane was decanted and the solid was dried to
give the desired product 1a as a cream powder (1.32 g, 89%); (b) Typical

´
A. Adamczyk-Wozniak et al. / Tetrahedron Letters 51 (2010) 6181–6185
6185
procedure for the synthesis of complexes 2: Na2SO4 (3.0 g) was added to a
solution of 2-formylphenylboronic acid (0.75 g, 5.0 mmol) and diethylamine
(0.12 g, 1.7 mmol) in Et2O (50 mL). The reaction mixture was treated as above
to give 0.73 g (90%) of 2e as an oil.
S.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K. M. A. J. Organomet. Chem. 1997, 535,
33; (f) Beckett, M. A.; Brassington, D. S.; Owen, P.; Hursthouse, M. B.; Light, M.
E.; Malik, K. M. A.; Varma, K. S. J. Organomet. Chem. 1999, 585, 7; (g) Wu, Q. G.;
Wu, G.; Brancaleon, L.; Wang, S. Organometallics 1999, 18, 2553; (h) Beckmann,
J.; Dakternieks, D.; Duthie, A.; Lim, A. E. K.; Tiekink, E. R. T. J. Organomet. Chem.
2001, 633, 149; (i) Sanchez, M.; Sanchez, O.; Höpfl, H.; Ochoa, M.-E.; Castillo,
D.; Farfan, N.; Rojas-Lima, S. J. Organomet. Chem. 2004, 689, 811; (j) McKinley,
N. F.; O’Shea, D. F. J. Org. Chem. 2004, 69, 5087; (k) Frost, B. J.; Mebi, C. A.;
Gingrich, P. W. Eur. J. Inorg. Chem. 2006, 1182; (l) Pearson, W. H.; Lin, S.; Iovine,
P. M. Acta Cryst. E 2008, 64, o235.
22. Iovine, P. M.; Fletcher, M. N.; Lin, S. Macromolecules 2006, 39, 6324.
23. Korich, A. L.; Iovine, P. M. Dalton Trans. 2010, 39, 1423.
´
24. Sporzynski, A.; Lewandowski, M.; Zarychta, B.; Zaleski, J. Pol. J. Chem. 2005, 79,
1099.
25. Duggan, P. J.; Houston, T. A.; Kiefel, M. J.; Levonis, S. M.; Smith, B. D.; Szydzik,
M. L. Tetrahedron 2008, 64, 7122.
26. (a) Yalpani, M.; Boese, R. Chem. Ber. 1983, 116, 3347; (b) Ferguson, G.; Lough, A.
J.; Sheenan, J. P.; Spalding, T. R. Acta Crystallogr., Sect. C 1990, 46, 2390; (c)
Beckett, M. A.; Hibbs, D. E.; Hursthouse, M. B.; Oven, P.; Malik, K. M. A.; Varma,
K. S. Main Group Chem. 1998, 2, 251; (d) Beckett, M. A.; Strickland, G. C.; Varma,
K. S. Polyhedron 1995, 14, 2623; (e) Beckett, M. A.; Strickland, G. C.; Varma, K.
27. Crystallographic data for compound 1b have been deposited with the
Cambridge Crystallographic Data Centre (CCDC 782356) and can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44(0) 1223 336033 or deposit@ccdc.cam.ac.uk).
28. Etter, M. C. Acc. Chem. Res. 1990, 23, 120.