One-step conversion of methoxysilanes to aminosilanes: A convenient synthetic strategy to N,O-functionalised organosilanes

Article abstract of DOI:10.1039/c2cc32727a

A straightforward substitution of silicon-bonded methoxy groups by a lithium amide leading to aminomethoxysilanes is presented. DFT calculations allow thermodynamic insight into the unusual equilibrium conditions. The applicability of N,O-functionalised organosilanes is exemplified by the convenient synthesis of an unsymmetrical methoxydisiloxane.

Full text of DOI:10.1039/c2cc32727a

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COMMUNICATION
One-step conversion of methoxysilanes to aminosilanes: a convenient
synthetic strategy to N,O-functionalised organosilaneswz
Jonathan O. Bauer and Carsten Strohmann*
Received 17th April 2012, Accepted 25th May 2012
DOI: 10.1039/c2cc32727a
A straightforward substitution of silicon-bonded methoxy groups
by a lithium amide leading to aminomethoxysilanes is presented.
DFT calculations allow thermodynamic insight into the unusual
equilibrium conditions. The applicability of N,O-functionalised
organosilanes is exemplified by the convenient synthesis of an
unsymmetrical methoxydisiloxane.
in organic synthesis7 as well as the search for novel silicon-
based building blocks in materials and polymer science,8 we
herein focus on the one-step conversion of methoxysilanes
to N,O-functionalised organosilicon intermediates from a pre-
parative and theoretical point of view. Following the synthesis of
novel variably substituted aminomethoxysilanes, we elucidate the
unusual and remarkable thermodynamic conditions of this
silicon–nitrogen bond formation for the first time.
Silicon–nitrogen bonds are generally known to be highly
sensitive towards acids and alcohols,1 a finding that is in
accordance with the well-established knowledge about the
high stability of silicon–oxygen bonds (Scheme 1).2
To explore the synthetic scope of the reaction we chose
the methoxysilanes 1a–g as suitable starting compounds. The
secondary lithium amide, prepared via deprotonation of the
free amine with n-butyllithium in pentane, was allowed to
react with the respective methoxysilanes over a period of 20 h
at room temperature (Scheme 2).
Based on this unfavourable equilibrium condition, the
formation of Si–NR2 bonds generally proceeds via the
conversion of halogenosilanes with primary and secondary
amines or alkaline metal amides.3 Very recently, our group
reported on a silicon–carbon bond cleavage by primary lithium
amides under formation of Si–NHR functions.4 In the light of
this background, a one-step substitution of Si–OR by Si–NR2
functions seems to be an exception in silicon chemistry, apparently
in conflict with the common thermodynamic beliefs and is hitherto
unique to only a few examples. First reports on such straight-
forward transformations mediated by alkaline metal amides came
from Schmitz-DuMont et al. and Wannagat et al., but they all
succeed exclusively under harsh or rather particular reaction
conditions, either in liquid ammonia or using stabilised leaving
groups such as alkaline phenolate. This type of reaction was also
already used by Ikeuchi et al., who prepared a selection of alkoxy-
bearing aminosilanes.5
The aminomethoxysilanes 2a–g could be obtained in good
yields up to 86% (Table 1).
These methoxide/amide substitutions reveal a powerful
synthetic tool in silicon–nitrogen chemistry, insofar as this
procedure paves the way to aromatic, aliphatic as well as
a-donor atom containing aminomethoxysilanes. The latter class
of compounds, so called a-silanes, that are patterned with a
donor function in their germinal position with respect to the
silicon atom show interesting reactivities of importance for
commercial applications, for example as polymer cross-linkers.9
The additional presence of a silicon–nitrogen function may be
Considering the use of cyclic aminoalkoxysilanes in
organic chemistry,6 the need for effective protecting groups
Scheme 2 Synthesis of aminosilanes 2a–g via direct conversion of
methoxysilanes with a lithium amide.
Scheme 1 Reaction of silicon–nitrogen bonds with hydroxy functions.
Table 1 Newly formed N,O-functionalised organosilanes 2a–g
Anorganische Chemie, Technische Universitat Dortmund,
¨
Otto-Hahn-Straße 6, D-44227 Dortmund, Germany.
Starting compound Product R1
R2
Yield [%]
E-mail: mail@carsten-strohmann.de; Fax: +49 231 755 3797;
Tel: +49 231 755 3807
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
procedures, crystallographic data and details of the quantum chemical
calculations. CCDC 874664. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2cc32727a
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
Ph
Ph
1-Np
t-Bu
OMe 80
Ph 86
OMe 77
OMe 40
Piperidinomethyl OMe 82
Piperidinomethyl t-Bu 35
Pyrrolidinomethyl OMe 86
1g
2g
c
This journal is The Royal Society of Chemistry 2012
7212 Chem. Commun., 2012, 48, 7212–7214

used to facilitate the construction of complex silicon containing
molecules with diverse reactive functionalities.
Table 2 Calculated reaction enthalpies for an isolated bimolecular
conversion of (methoxy)phenylsilanes to aminosilanes [B3LYP/
6-31+G(d)]
But how can the direction of these substitutions be rationalised.
For a deeper insight into the ongoing processes we performed
DFT calculations on the B3LYP/6-31+G(d)10 level to understand
the reactions from a thermodynamic point of view.
At first view, regarding nitrogen nucleophiles as simplified
naked anions for an attack at a methoxy-bearing silicon
centre, it would even be likely to expect that the presence of
electronegative Si–OR substituents enables the silicon atom to
prefer a higher coordination number rather than undergoing a
substitution reaction, leading to a stable pentacoordinate
compound. This is supported by reports of Corriu et al. and
Holmes et al., who succeeded in isolating potassium salts of
pentacoordinate anionic alkoxy- and aryloxysilicates even in
the absence of crown ethers.11 As elucidated by quantum
chemical calculations using the Polarisable Continuum Model
(PCM), the substitution of a methoxy group in (trimethoxy)-
phenylsilane by a free amide results in an exothermic reaction
of ꢀ52 kJ molꢀ1. The most stable pentacoordinate silicon
species with both the phenyl and the pyrrolidinyl group at
Entry
X
n
R1
R2
DH [kJ molꢀ1
]
1
2
3
4
5
Li
Li
Li
Li
H
1
4
4
4
1
OMe
OMe
Pyrrolidinyl
Pyrrolidinyl
OMe
OMe
OMe
OMe
Pyrrolidinyl
OMe
ꢀ49
ꢀ78
ꢀ61
ꢀ60
+46
the thermodynamic conditions, we also took account of the
first coordination sphere of the lithium cation by considering
tetrameric lithium amide and methoxide aggregates as plau-
sible model systems in apolar solvents.12 The computations
clearly demonstrate that the driving force is significantly
enhanced, compared to the monomeric species, by an energy
gain of 29 kJ molꢀ1 (Table 2, entry 2). In addition to
such inherent structural principles, solubility effects may also
contribute to the experimentally observed direction of the
conversion. It is also worth mentioning that this type of
reaction even renders a successive substitution of silicon-
bonded methoxy groups possible, as shown by entries 3 and 4,
however with a decreased gain in energy. As expected from the
high reactivity of aminosilanes towards hydroxy functions,
calculations using pyrrolidine as a reactant render the Si–OR/
Si–NR2 substitution notably endothermic at +46 kJ molꢀ1
(Table 2, entry 5).
the equatorial position is higher in energy by 7 kJ molꢀ1
,
compared to the substitution products, and thus does not
represent the global minimum of this reaction (Fig. 1, for
further details see the ESIz).
To broaden our thermodynamic discussion on metal ion
effects when changing from free amides to lithiated reactants,
we further investigated the substitution reaction by considering
metal-supported aggregation effects of the amide and the leaving
alkoxide which may facilitate the reverse control of the equili-
brium condition, as already mentioned in Scheme 1 (Table 2).
Indeed, the quantum chemical studies approve a highly
exothermic reaction of ꢀ49 kJ molꢀ1 between (trimethoxy)-
phenylsilane and lithiated pyrrolidine allowing for a substitu-
tion via a single monomer of the nucleophile and the leaving
group respectively (Table 2, entry 1). Astonishingly, PCM
calculations regarding naked anions (Fig. 1) and monomeric
lithium salts (Table 2, entry 1), respectively, come to nearly the
same energetic result. This finding furthermore suggests that
the better stabilisation of the negative charge in the methoxide
anion is an essential factor for the driving force of these exchange
reactions and hence the reason for the observed reverse control of
the chemical equilibrium. To draft a more conclusive picture of
Aminosilanes are applied as sophisticated silylation reagents
for the functionalisation of surface silanol groups on meso-
porous silica that exhibit various advantages over chloro- and
alkoxysilane precursors.13 Furthermore, the controlled synthesis
of organoalkoxysiloxanes with defined structures is an impor-
tant challenge for the development of novel siloxane-based
materials.14 However, synthetic routes have been limited to
date. The aminomethoxysilanes described herein represent
useful intermediates supplying a great many of applicable
substances, due to the chemoselective differentiation of their
two heteroatom substituents in reactions with hydroxy functions.
For example, the conversion of 2b with triphenylsilanol
selectively leads to the unsymmetrical disiloxane 3 with an
additional hydrophilic and further transformable reactive
methoxy group in quantitative yield that is only difficult to
access otherwise (Scheme 3).
3 crystallises from pentane–diethylether in the triclinic
%
crystal system, space group P1, as colorless plates and
concomitantly demonstrates the practicability of the mixed
oxygen–nitrogen patterned organosilanes for the successive
Fig. 1 A simplified thermodynamic view on the formation of a
silicon–nitrogen bond by performing PCM calculations (solvent:
THF) [B3LYP/6-31+G(d)].
Scheme 3 Control of chemoselectivity in N,O-functionalised
organosilanes.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7212–7214 7213

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In conclusion, a not habitual type of Si–NR2 bond formation
has been further investigated, starting from methoxysilanes and
a lithium amide via a one-step conversion. Based on quantum
chemical calculations, we assume that the formation of lithium
methoxide and moreover aggregation effects play a key role as
the driving force of the reaction. The authors wish to attract
attention to a powerful strategy in silicon chemistry to form a
mixed functionalised substitution pattern, giving rise to specific
transformations due to the different reaction rates of Si–OR
and Si–NR2 bonds towards hydroxy functions and metallated
nucleophiles, respectively. We are currently working on
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This publication is dedicated to Prof. Dr. Klaus Jurkschat
on the occasion of his 60th birthday. We are grateful to the
Deutsche Forschungsgemeinschaft for financial support and
to Wacker Chemie AG for providing us with special chemicals.
J.O.B. thanks the Fonds der Chemischen Industrie (FCI) for a
doctoral scholarship. Finally, we would like to express our
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7214 Chem. Commun., 2012, 48, 7212–7214