The synthesis of bromo and iodo trifunctionalised tribenzosilatranes

Article abstract of DOI:10.1016/j.tetlet.2004.11.034

A reliable synthetic route towards selective derivatisation at the 6′,6″ and 6? positions of tribenzosilatranes with bromo or iodo groups has been developed. As these groups can be reacted further, this extends the chemistry associated with tribenzosilatranes. In this report we present a direct synthesis of bromo 8 and iodo 9 trisubstituted tribenzosilatranes. Commercially available o-anisidine 1 was transformed into the triarylamine 3 in a two-step sequence, which was halogenated to furnish the tribromo 4 or the triiodo 5 substituted triamines, respectively. Subsequent deprotection of the methyl ethers furnished the novel tripod ligands 6 and 7. A transilylation reaction in the last step led to the synthesis of the desired para-halogenated tribenzosilatranes 8 and 9.

Full text of DOI:10.1016/j.tetlet.2004.11.034

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 67–68
The synthesis of bromo and iodo trifunctionalised tribenzosilatranes
*
´
Ruben Alvarez and Georg H. Mehl
Department of Chemistry, University of Hull, Hull HU6 7RX, UK
Received 13 September 2004; revised 28 October 2004; accepted 5 November 2004
Abstract—In this report we present a direct synthesis of bromo 8 and iodo 9 trisubstituted tribenzosilatranes. Commercially avail-
able o-anisidine 1 was transformed into the triarylamine 3 in a two-step sequence, which was halogenated to furnish the tribromo 4
or the triiodo 5 substituted triamines, respectively. Subsequent deprotection of the methyl ethers furnished the novel tripod ligands 6
and 7. A transilylation reaction in the last step led to the synthesis of the desired para-halogenated tribenzosilatranes 8 and 9.
Ó 2004 Elsevier Ltd. All rights reserved.
The effect of hypervalency of organic compounds con-
taining elements of group 14 has been known and stud-
ied for more than three decades. Silatranes, which are of
interest due to their biological and physicochemical
properties, are a good example of such hypercoordina-
tion of silicon. Five substituents are bonded to the sili-
con atom in a quasi trigonal bipyramidal geometry.
Tribenzosilatranes, where three benzene rings are fused
together have received much attention in fundamental
research and in the context of materials science.1–3
which takes into account that reactivity in large aro-
matic systems is difficult to predict.
Different approaches were attempted towards the syn-
thesis of bromo 8 and iodo 9 substituted tribenzosilatr-
anes. The best results were obtained when the
triarylamine 3 was halogenated leading to new materials
4 and 5. Deprotection of the trimethoxyamines 4 and 5
led to 6 and 7, which could be used as tripod ligands for
elements of groups 13, 14 and 15 or by substituting the
halogen atoms for other molecules with more diverse
functionalities.
In the main, research has focused on functionalisation
of the apex position of the silicon atom or substitution
of the three equatorial oxygen atoms for other hetero-
atoms (such as N, S, C, etc).3,4 Tribenzosilatrane mole-
cules where the aromatic systems substituted with
aliphatic chains were used, yield materials, which dis-
play liquid crystal properties.5,6 Arguably the design of
a route to trifunctionalised tribenzosilatranes, where
the silatrane group can be functionalised further would
enhance access to this class of materials.
The synthetic route to halogenated tribenzosilatranes 8
and 9 was achieved in four steps beginning with com-
mercially available o-anisidine as shown in Scheme 1.
Diazotisation and subsequent iodination of o-anisidine
1 led to the formation of o-iodoanisole 2 in a moderate
yield. The combination of 1 and 2 in a copper catalysed
reaction furnished the triarylamine 3,8 which was bromi-
nated selectively at the para-positions at room tempera-
ture to give 4, which bears three bromine atoms in its
structure. The synthesis of the triiodobenzeneamine 5
was achieved successfully by iodination of 3 with
silver(I) trifluoroacetate and molecular iodine in
chloroform.
A very efficient manner to build up extended aromatic
ring systems, attractive in materials science (i.e., fluores-
cence, light emission properties) is the use of cross-cou-
pling reactions. Many of these require halogenated,
preferably brominated groups. Moreover iodinated
materials are also important due their biological activ-
ity.7 This requires reactions with high regioselectivity,
Bromination of the arylamine 3 proceeded successfully
under mild conditions.9 A solution of bromine in chlo-
roform was added dropwise at ꢀ78°C giving as the
major product the para-substituted tribromo derivative
4 with minor by-products, which could be removed
readily by column chromatography. For the insertion of
iodine groups a different approach had to be used. A
range of reagents and conditions such as I2/chloroform,
Keywords: Tribromotribenzosilatrane; Triiodotribenzosilatrane; Tri-
benzosilatrane.
*
Corresponding author. E-mail: g.h.mehl@hull.ac.uk
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.11.034

68
R. Alvarez, G. H. Mehl / Tetrahedron Letters 46 (2005) 67–68
groups can be reacted further, this extends the chemistry
associated with tribenzosilatranes.
O
O
I
NH2
i
Supplementary data
1
2
R
Experimental details and analytical data are available.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2004.11.034.
ii
O
O
iii
iv
O
O
N
O
N
O
References and notes
R
R
3
4 (R= Br)
5 (R=I)
1. Gevorgyan, V.; Borisova, L.; Vyater, A.; Ryabova, V.;
Lukevics, E. J. Organomet. Chem. 1997, 548, 149, and
references cited therein.
2. Frye, C. L.; Vincent, G. A.; Hauschildt, G. L. J. Am.
Chem. Soc. 1966, 88, 2727.
3. Verkade, J. G. Coord. Chem. Rev. 1994, 137, 233.
4. Voronkov, M. G. J. Organomet. Chem. 1982, 233, 1.
5. (a) Norvez, S.; Simon, J. J. Chem. Soc., Chem. Commun.
1993, 1398; (b) Bassoul, P.; Simon, J.; Soulie, C. J. Phys.
Chem. 1996, 100, 3131.
6. Soulie, C. Tetrahedron 2001, 57, 1035.
7. Seevers, R. H.; Counsell, R. E. Chem. Rev. 1982, 82,
575.
v
R
O
O
Si
R
O
N
HO
OH
R
vi
N
8 (R= Br)
9 (R=I)
OH
R
R
R
6 (R= Br)
7 (R=I)
8. Alvarez, R.; Mehl, G. H.; 20th International Liquid
Crystal Conference, Ljubljana, Slovenia, 4-9/07/2004;
Alvarez, R.; Mehl, G. H., submitted for publication.
9. Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. J.
Mater. Chem. 1997, 2343.
Scheme 1. Reagents and conditions: (i) H2SO4, NaNO2, KI; (ii)
K2CO3, Cu powder, 18-crown-6 ether, (1,2)-dichlorobenzene; (iii) Br2,
dichloromethane; (iv) I2, silver(I) trifluoroacetate, CHCl3, (v) BBr3,
ꢀ78°C, CH2Cl2; (vi) vinyltrichlorosilane, dry di-n-butyl ether.
10. Zembower, D. E.; Zhang, H. J. Org. Chem. 1995, 63, 9300.
´
11. Lulinski, P.; Skulski, L. Bull. Chem. Soc. Jpn. 1997, 70,
1665.
I2/AcOH,10 I2/AcOH/CrO3,11 and I2/HIO3/AcOH,12
were investigated but gave unsatisfactory yields. At-
tempts using I2/HgO,13 gave selective para-iodination
but the results were not always reproducible and the
yields tended to be poor. The best results for para-sub-
stitution were achieved using I2/silver(I) trifluoroace-
tate,14 leading to the desired triiodoarylamine in
reproducible yields of 15%.
12. Bushby, R. J.; Lu, Z. Synthesis 2001, 763.
13. Orito, K.; Hatakeyama, T.; Takeo, M.; Suginome, H.
Synthesis 1995, 1273.
14. Merkushev, E. B. Synthesis 1998, 923.
15. Spectroscopic data for 8: mp = 183°C, TLC (SiO2,
CH2Cl2): Rf = 0.74, 1H NMR (CDCl3) d: 6.18 (3H, m),
7.09 (3H, dd, J = 8.4, 2.2), 7.23 (3H, d, J = 2.2), 7.51 (3H,
d, J = 8.4). 13C NMR (CDCl3) d: 121.67, 122.51, 125.62,
127.19, 130.64, 134.83, 135.84, 153.81. 29Si NMR DEPT
(CDCl3) d: ꢀ71.25. Anal. Calcd for C20H12Br3NO3Si
(582.12): C, 41.27; H, 2.08; N, 2.41. Found: C, 41.29; H,
2.28; N, 2.42. MS (MALDI-TOF) = 583.94 [M]+, 582.95
[M]+.
16. Spectroscopic data for 9: mp = 245°C, TLC [SiO2,
CH2Cl2/hexane (1:1)]: Rf = 0.51, 1H NMR (CDCl3) d:
6.06 (3H, m), 7.19 (3H, dd, J = 8.2, 1.8), 7.27 (3H, d,
J = 8.2), 7.32(3H, d, J = 1.8). 13C NMR (CDCl3) d: 93.77,
127.48, 127.51, 130.67, 131.63, 135.49, 135.80, 153.64. 29Si
NMR DEPT (CDCl3) d: ꢀ71.75. Anal. Calcd for
C20H12I3NO3 (723.11): C, 33.22; H, 1.67; N, 1.94. Found:
C, 33.48; H, 1.32; N, 1.92.
The removal of the three methyl ethers with BBr3 at
ꢀ78°C was achieved in one step for both iodo 5 and
bromo 4 triarylamines leading to the corresponding
trihydroxy derivatives 6 and 7, respectively, which were
refluxed in the presence of trichlorovinylsilane and dry
di-n-butyl ether to give the desired trihalosubstituted
tribenzosilatranes 8 and 9 as white solids.15,16
A reliable synthetic route towards the selective derivati-
sation of tribenzosilatranes at the 60,600 and 6000 positions
with bromo or iodo groups has been developed. As these