Silicon–carbon bond cleavage of organosilicon amines
MenN[CH2Si(OCH2CH2)3N]3–n (n ؍
1, 2) by phenols
Nataliya F. Lazareva,* Esfir I. Brodskaya and Gennadii V. Ratovsky
2
Institute of Chemistry, Siberian Division, Russian Academy of Sciences, 1 Faforsky Street,
Irkutsk 664033, Russian Federation. E-mail: nata@irioch.irk.ru
Received (in Cambridge, UK) 23rd July 2002, Accepted 18th September 2002
First published as an Advance Article on the web 25th October 2002
Anomalously high basicity of organosilicon amines MenN[CH2Si(OCH2CH2)3N]3–n determines the ease of nucleo-
philic cleavage of the Si–C bond by phenols even at room temperature. The conversion of silatrane increases both
with phenol acidity and basicity of the exocyclic nitrogen atom.
trimethylsilatranyl and phenyl groups and the appearance of
Introduction
new bands at 2490 and 2590 cmϪ1 which can be assigned to the
ammonium cation Nϩ–H.17 It could be assumed that there is
some interaction between phenol and silatrane MeN[CH2Si-
(OCHMeCH2)3N]2.
It is known that the Si–C bond in the silatranes RSi(OCH2-
CH2)3N (R = Me, Ph, Vin) is easily cleaved by electrophiles
such as bromosuccinimide, 3-chloroperbenzoic acid, bromine
or iodine monochloride, and mercury salts.1–4 The only example
of nucleophilic cleavage of the Si–C bond is the reaction of
1-iodomethylsilatrane with N,N-dimethylaminoethanol.5 How-
ever the cleavage of the Si–C bond by nucleophiles is observed
in the case of acyclic RSi(ORЈ)3 and monocyclic R2Si(OCH2-
CH2)2NRЈ silatrane analogs with tetracoordinated silicon
atoms. For example, the intramolecular ring closure of 2,2-
diorganyl-6-(hydroxyethyl)-1,3-dioxa-6-aza-2-silacyclooctanes
in bicyclic silatranes proceeds via cleavage of a Si–C bond by a
fragment HOCH2CH2N.6–8
Now we report our detailed study of the reaction of phenol
and p-nitrophenol with N-methyl-N,N-bis(1-silatranylmethyl)-
amine MeN[CH2Si(OCH2CH2)3N]2
1 and N,N-dimethyl-
(1-silatranylmethyl)amine Me2NCH2Si(OCH2CH2)3N 2.
Results and discussion
We found that the compounds 1 and 2 react with phenol and
4-nitrophenol, and cleavage of the Si–C bond takes place,
similarly to the previously investigated N-methyl-N,N-bis(tri-
ethoxysilylmethyl)amine MeN[CH2Si(OEt)3]2:
Recently in studies of hydrogen bonding of organosilicon
amines, we found that N-methyl-N,N-bis(triethoxysilylmethyl)-
amine MeN[CH2Si(OEt)3]2 reacts with phenol in a heptane
solution. Transetherification of the ethoxy groups by phenol in
the initial stage, following the cleavage of a Si–C bond, takes
place and results in the formation of tetraphenoxysilane and
trimethylamine.9
As was shown previously, phenol and 1-substituted organyl-
silatranes, RCH Si(OCHRЈCH ) N (R = H, CH ᎐CH, Cl, EtS;
᎐
2
2
2
3
RЈ = H, Me) form only one PhO–H ؒ ؒ ؒ O–(Si) hydrogen
bond both in non-polar and polar solvents without Si–C bond
cleavage.10 Phenol is coordinated by the nitrogen atom of
1-piperidinomethyl-3,7,10-trimethylsilatrane, (CH2)5NCH2Si-
(OCHMeCH2)3N, in heptane solution this interaction being
much stronger in comparison with triethylamine (Keq 1100 and
52 dm3 molϪ1, respectively).11,12 The enhanced basicity of the
piperidine nitrogen atom is due to the super electron-donor
inductive effect of the silatranyl methyl group (σ* = Ϫ2.24)13–15
in comparison with other organosilicon groups CH2SiR3 (SiR3
= SiAlk3, Si(OAlk)3, σ* ≈Ϫ0.6). It is to be expected that the
basicity of exocyclic nitrogen atoms in organosilicon amines
R3–nN[CH2Si(OCHRCH2)3N]n increase with increasing n.
Enhancement of the electron density on the exocyclic nitrogen
is also confirmed by the considerable low frequency shift of the
CH3(N) stretching vibration (2730 cmϪ1) in the IR spectra
of the methylbis(1-silatranylmethyl)amines, MeN[CH2Si-
(OCHRCH2)3N]2 (R = H, Me) as compared to methylalkyl-
amines (2780–2830 cmϪ1).16 When we tried to involve MeN-
[CH2Si(OCHRCH2)3N]2 in intermolecular hydrogen bonding
with phenol, a white precipitate was formed immediately when
the phenol and MeN[CH2Si(OCHMeCH2)3N]2 were mixed in
equimolar ratio in heptane solution. The IR spectrum of the
isolated product (A) shows absorption bands of 3,7,10-
1
The IR, UV and H NMR data of silatrane 3 are consistent
with those of the compound prepared in accordance to ref. 18.
Thus, in the IR spectra of the product 3 the group frequencies
assigned to the silatranyl (585, 634, 782, 800, 920, 940, 1017,
19
1090, 1115 cmϪ1
)
are retained, and the CH3(N) vibrations
observed at 2700–2800 cmϪ1 (ref. 20) are absent. The assign-
ment of the new bands at 500, 694, 767, 886, 1492, 1575, 1592,
3010, 3030, 3050, 3070 cmϪ1 to the C6H5 modes is quite clear.
1
The H NMR spectra of compound 3 in CD3CN show the
proton signals of the silatranyl group CH2N (t, 2.93 ppm),
CH2O (t, 3.92 ppm) as well as those for the C6H5O group (m,
6.68 and m, 7.82 ppm). By treatment of silatranes 1 and 2 with
p-nitrophenol the Si–C bond cleavage is also observed, with the
formation of 1-(p-nitrophenoxy)silatrane 4. The IR, UV and 1H
NMR spectral data are in good agreement with those for the
compound prepared in accordance with ref. 18.
1H NMR monitoring of the reaction, performed in CD3CN
solution at room temperature in a sealed NMR ampoule for
6 h, shows the presence in the reaction mixture of initial
compounds 1 or 2, phenol in use and generated 1-aroxy-
silatrane only. The conversion was determined by integration
of the intensities of the CH2N and CH2O proton signals of
the silatrane ring in the initial and the formed silatranes. The
DOI: 10.1039/b207208d
J. Chem. Soc., Perkin Trans. 2, 2002, 2083–2086
This journal is © The Royal Society of Chemistry 2002
2083