Structural influence of steric factors and donor functions on lithium silylamides in non-coordinating solvents

Article abstract of DOI:10.1039/c1dt11550b

We herein present the synthesis and crystallographic characterisation of lithium silylamides displaying different coordination numbers and aggregation states according to the number of N- and O-donor functions in the starting material, (aminomethyl) substituted silazane ligands. The dimeric dimethyl-(N-lithio-tert-butylamino)-piperidinomethyl)-silane and dimethyl-(N-lithio-iso-propylamino)-piperidinomethyl)-silane, with three-coordinate lithium centres, were prepared by deprotonation of the corresponding silazane with ⁿBuLi. Using the tridentate silazane (1R,2R)-N¹-[{(tert-butylamino)-dimethylsilyl}methyl]-N ¹,N²,N²-trimethylcyclohexane-1,2-diamine a mixed "dimer" of lithium silylamide and lithium silanolate with four-coordinate lithium centres was obtained. Additionally, a monomeric lithium silylamide was synthesised using the tridentate [bis(methoxyethyl)aminomethyl] side arm.

Full text of DOI:10.1039/c1dt11550b

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PAPER
Structural influence of steric factors and donor functions on lithium
silylamides in non-coordinating solvents†
Victoria P. Colquhoun and Carsten Strohmann*
Received 17th August 2011, Accepted 7th November 2011
DOI: 10.1039/c1dt11550b
We herein present the synthesis and crystallographic characterisation of lithium silylamides displaying
different coordination numbers and aggregation states according to the number of N- and O-donor
functions in the starting material, (aminomethyl) substituted silazane ligands. The dimeric
dimethyl-(N-lithio-tert-butylamino)-piperidinomethyl)-silane and dimethyl-(N-lithio-iso-
propylamino)-piperidinomethyl)-silane, with three-coordinate lithium centres, were prepared by
deprotonation of the corresponding silazane with nBuLi. Using the tridentate silazane
(1R,2R)-N1-[{(tert-butylamino)-dimethylsilyl}methyl]-N1,N2,N2-trimethylcyclohexane-1,2-diamine a
mixed “dimer” of lithium silylamide and lithium silanolate with four-coordinate lithium centres was
obtained. Additionally, a monomeric lithium silylamide was synthesised using the tridentate
[bis(methoxyethyl)aminomethyl] side arm.
solvents (hexane, benzene) in order to prevent solvation effects.
Introduction
The studies were performed on lithium silylamides derived
from (aminomethyl)silazanes. Providing a suitable model ligand,
(aminomethyl)silazanes featuring a coordinating sidearm can
easily be modified to incorporate different N- and O-donor
functions, resulting in bi-, tri- and tetradentate ligands.
In modern synthetic chemistry, lithium amides are widely used
as deprotonation reagents, especially for asymmetric syntheses,
or for metathesis reactions.1 Compared to common alkyl lithium
reagents lithium amides exhibit reasonably high basicity combined
with low nucleophilicity. Therefore, major investigations of lithium
amides have been conducted aiming to improve our knowledge
of the reactive species in solution. Former research thereby has
focused mainly on structural aspects and their influence on the
reactivity of lithium amides, with lithiumdiisopropylamide (LDA)
being the most prominent target of these investigations.2,3 Hereby,
the aggregation state and solvent effects have been identified as
the most important characteristics to influence the reactivity of
lithium amides in solution. Using coordinating solvents and a
wide selection of differing sterically demanding ligand systems, a
variety of aggregates can be prepared selectively, including small
aggregates of lithium amides.4 As demonstrated by Collum et al.,
a dimeric aggregate is the most reactive LDA-species in non-
coordinating solvents.3
Results and discussion
Dimeric lithium silylamides
The deprotonation of both silazanes 1a and 1b featuring a
(piperidinomethyl) sidearm and hence one additional donor
function with nBuLi in hexane leads to colourless crystals of
the dimeric lithium silylamides 2a,b (Scheme 1). Since even 2a,
with a sterically demanding tert-butyl group is dimeric, differing
sterically demanding groups do not result in a reduced aggregation
state in non-coordinating solvents.
To further expand our knowledge on the reactivity and the
structural pattern of lithium amides, we investigated the influ-
ence of additional donor atoms and steric hindrance on the
aggregation state and reactivity/stability of lithium silylamides.
These investigations were conducted mainly in non-coordinating
Scheme 1 Synthesis of compounds 2a,b.
Anorganische Chemie, Technische Universita¨t Dortmund, 44227, Dortmund,
Germany. E-mail: mail@carsten-strohmann.de; Fax: 0049 231 755 7062;
Tel: 0049 231 755 3807
† Electronic supplementary information (ESI) available: Total energies and
Cartesian coordinates for all computed structures. Specific sections of
NMR spectra concerning the dynamic process of lithium amide dimers.
CCDC reference numbers 840418–840421. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt11550b
Compound 2a crystallises in the triclinic crystal system in the
¯
space group P1 and compound 2b in the monoclinic crystal system
in the space group C2/c (Fig. 1 and Table 1). Both dimers
display C2 or pseudo-C2 symmetry. Due to the sterically de-
manding tert-butyl group, the N1–Li–N3 angles [108.71(18) A,
˚
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Table 1 Crystal data and structure refinements for 2a, 2b, 4 and 6
2a
2b
4
6
Formula
Fw/g mol-1
T/K
C24H54Li2N4Si2
468.77
173(2)
C22H50Li2N4Si2
440.72
193(2)
0.71073
Monoclinic
C28H63Li2N5OSi2
555.89
173(2)
0.71073
Monoclinic
C13H31LiN2O2Si
282.43
173(2)
˚
Wavelength/A
Crystal system
Space group
0.71073
0.71073
Triclinic
Triclinic
¯
¯
P1
C2/c
P21
P1
˚
a/A
11.1011(16)
11.1191(19)
14.389(2)
81.72(3)
22.239(4)
8.340(2)
17.941(4)
90
8.3553(4)
17.7254(6)
11.8129(5)
90
8.4867(10)
9.1182(10)
12.9775(12)
87.737(8)
77.741(9)
64.186(11)
881.82(16)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
71.41(3)
63.21(3)
1502.7(4)
2
1.036
0.135
520
122.498(18)
90
2806.8(10)
4
1.043
0.141
99.415(4)
90
1725.94(12)
2
1.070
0.129
3
˚
V/A
Z
rc/g cm-3
1.064
0.133
312
m/mm-1
F(000)
976
616
Crystal size/mm
q range [◦]
Limiting indices
0.30 ¥ 0.30 ¥ 0.20
1.49–26.00
-13 < h < 13
-13 < k < 13
-17 < l < 17
0.0855
26667
5912
5912/0/322
1.029
0.0539, 0.1130
0.0822, 0.1271
0.40 ¥ 0.40 ¥ 0.30
2.17–26.00
-27 < h < 27
-10 < k < 10
-22 < l < 22
0.0582
13062
2757
2757/0/140
1.025
0.0611, 0.1620
0.0690, 0.1705
0.50 ¥ 0.30 ¥ 0.20
2.47–26.00
-10 < h < 9
-21 < k < 21
-12 < l < 14
0.0235
10408
6373
6373/1/356
1.019
0.0329, 0.0654
0.0414, 0.0665
0.05(7)
0.50 ¥ 0.30 ¥ 0.20
2.49–24.99
-10 < h < 10
-10 < k < 10
-12 < l < 15
0.0359
5587
3085
3085/0/179
0.959
0.0441, 0.0495
0.0897, 0.0519
Rint
Reflections collected
Independent reflections
Data/restraints/parameters
GooF on F2
R1, wR2 indices [I > 2s(I)]
R1, wR2 (all data)
Absolute structure parameter
-3
˚
Largest diff peak and hole/e A
0.250 and -0.252
0.500 and -0.428
0.224 and -0.225
0.257 and -0.259
longer dative Li–N bond of the coordinating (piperidinomethyl)
˚
sidearm is observed for both compounds [2a: 2.064(4) A and
˚
˚
2.076(4) A, 2b: 2.117(3) A]. The three-coordinate lithium centres
show a trigonal pyramidal geometry with angle sums of 349◦ (2a)
and 341◦ (2b). Due to an increase in ionic bond character caused
by the formation of the lithium amide, the Si–N bond length is
˚
˚
shortened to 1.687(2) A and 1.698(2) A for compounds 2a and 2b
respectively as compared to the Si–N bond of a free silazane.6
Both compounds assemble to dimers exhibiting rotational
symmetry rather than inversion symmetry. Hence the relative
energy differences of both possible configuration isomers, dimers
with C2 and Ci symmetry, were calculated for a tert-butyl and
methyl substituted derivative [B3LYP/6-31+G(d)]. Both deriva-
tives favour the C2 isomer, although the energy difference is much
smaller (DE = 6 kJ mol-1) for the dimers with the sterically less
demanding methyl group. With DE = 46 kJ mol-1 the calculated
energy difference for the tert-butyl substituted dimers is much
higher.
Fig. 1 Molecular structures of the crystals of compounds 2a (left) and 2b
˚
(right, symmetry operation: -x + 1, y, -z + 3/2). Selected bond lengths (A)
and angles (◦) of 2a: Li1–N3 2.009(4), Li1–N1 2.037(4), Li1–N2 2.064(4),
N1–Si1 1.687(2), C3–N1 1.479(3), N3–Li1–N1 108.71(18), N3–Li1–N2
134.6(2), N1–Li1–N2 105.20(17), Si1–N1–Li1 97.86(13), Li2–N1–Li1
71.21(15); 2b: C3–N1 1.481(2), Li–N1 2.021(4), Li–N1¢ 2.055(3), Li–N2¢
2.117(3), N1–Si 1.698(2), N1–Li–N1¢ 107.61(15), N1–Li–N2¢ 130.43(17),
N1¢–Li–N2¢ 102.58(14), Si–N1–Li¢ 101.74(11), Li–N1–Li¢ 70.72(15).
When comparing the NMR spectra of both compounds,
different behaviour of dynamic processes in solution is visible.
While the 1H and 13C NMR spectra of compound 2a exhibit
two separate broad signals assigned to the silicon bonded methyl
groups, there is only one peak visible in the spectra of compound
2b. The same effect is observed for the methylene groups in the
a- and b-position of the nitrogen atom in the piperidine moiety.
Since the splitting of those signals for compound 2a disappears
at a higher temperature (350 K), this effect has been attributed
to dynamic processes in solution, mainly the removal of the
coordinating (piperidinomethyl) sidearm.
˚
˚
108.20(18) A] of 2a are wider than the N1–Li–N1¢ angle
[107.61(15) A] in 2b.
The slight influence of steric hindrance in compound 2a is also
observed in the bending of the central Li–N–Li–N-cycle. The
bridging nitrogen centre of 2b displays an angle of 16.6(1)◦ to
the plane described by the other atoms in this central cycle while
in compound 2a this angle has reduced to only 3.9(3)◦. The Li–NSi
˚
˚
bonds in both compounds vary from 2.009(4) A to 2.055(3) A,
which is comparable to other known lithium amide structures.5 A
1898 | Dalton Trans., 2012, 41, 1897–1902
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◦
˚
Scheme 2 Synthesis and molecular structure of the crystal of compound 4. Selected bond lengths (A) and angles ( ) of 4: C3–N1 1.471(2), Li1–O 1.813(3),
Li1–N1 2.102(3), Li1–N2 2.101(3), Li1–N3 2.196(3), Li2–O 1.893(3), Li2–N1 2.148(3), Li2–N5 2.205(3), Li2–N4 2.472(3), N1–Si1 1.681(2), O–Si2
1.576(1), O–Li1–N2 113.88(16), O–Li1–N1 101.35(14), N2–Li1–N1 99.62(13), O–Li1–N3 118.14(16), N2–Li1–N3 83.62(12), N1–Li1–N3 135.28(16),
O–Li2–N1 97.11(14), O–Li2–N5 135.73(18), N1–Li2–N5 114.72(15), O–Li2–N4 89.20(13), N1–Li2–N4 154.85(17), N5–Li2–N4 75.02(10), Si1–N1–Li1
99.84(11), Li1–N1–Li2 73.00(12), C8–N2–C7 110.85(14), C8–N2–C9 113.80(15), C7–N2–C9 112.38(14), C8–N2–Li1 107.87(14), C7–N2–Li1 103.82(13),
C9–N2–Li1 107.47(13), Si2–O–Li1 154.87(12), Si2–O–Li2 117.90(12), Li1–O–Li2 86.02(14).
Mixed “dimer” of lithium silylamide and lithium silanolate
R-configuration. This effect has been observed in similar structures
based on N,N,O-ligands derived from (R,R)-TMCDA.9
Since the aggregation state of the lithium silylamides 2a,b could
not be reduced solely by steric factors, our aim was to include
another N-donor function in the coordinating sidearm. This could
be achieved in a one-pot-synthesis using dimethoxydimethylsi-
lane as a starting material (Scheme 2). In the first step, one
methoxy group is substituted by lithiated (R,R)-N,N,N¢,N¢-
tetramethylcyclohexane-1,2-diamine (TMCDA), then lithiated
tert-butylamine substitutes the second methoxy function resulting
in the silazane 3. The preparation of aminomethyl substituted
silazanes via this route is limited to amines which can be lithiated
in the a-position of the nitrogen centre.
Deprotonation of 3 with MeLi in diethylether and subsequent
storage over three months, which lead to the slow hydrolysis of
the silazane bond forming an intermediate silanol, resulted in
colourless crystals of 4, a mixed “dimer” (Scheme 2). With the
exception of the recently published mixed compounds by Leznoff
et al. this is the only example of a combined lithium silylamide
and lithium silanolate.7
Due to dissociation into monomers of lithium silylamide and
lithium silanolate and a number of possible stereoisomers in
1
solution, the H and 13C NMR spectra of compound 4 show a
complex set of signals, which could not be assigned properly. A set
of three signals in the 29Si and 7Li NMR spectra further supports
this assumption of a dynamic process.10
Monomeric lithium silylamide
Starting from compound 5, the substitution of a dihydrofuryl
group with lithiated tert-butylamine leads to the monomeric
lithium silylamide 6 (Scheme 3), which is not stabilised by
additional coordinating solvents.
Compound 4 crystallises in the monoclinic crystal system in the
˚
space group P21. Due to the lithiation, both the Si–N [1.681(2) A]
˚
and the Si–O [1.576(1) A] bond lengths are shortened compared
Scheme 3 Synthesis of compound 6.
to the protonated equivalents.
While the bridging oxygen centre has a trigonal planar geom-
etry, the bridging nitrogen centre is tetrahedral and bent out of
the Li–O–Li plane by 16.6(1)◦, which is comparable to the iso-
propyl substituted dimer 2b. The coordination of an additional
nitrogen atom and hence a four-coordinate lithium centre lead to
Compound 6 crystallises in the triclinic crystal system in the
space group P1. The geometry at the lithium centre is strongly
distorted tetrahedral. The lithium centre, the nitrogen N1 and
the coordinating oxygen atoms O1 and O2 are positioned in one
plane. The amine nitrogen atom N2 coordinates to the lithium
centre from one side of this plane. The Li1–N1 bond length of
¯
˚
rather long Li–N distances [2.101(3)–2.196(3) A], especially the
˚
˚
weak dative Li2–N4 and Li2–N5 bonds with lengths of 2.472(3) A
1.913(4) A is slightly shorter than the bridging lithium amide
˚
and 2.205(3) A respectively. This comparatively weak coordinating
bond in the dimeric compounds 2a,b. Caused by steric repulsion
of the tert-butyl group and the methyl groups of the coordinating
bond is caused by the steric repulsion of the tert-butyl group and
the methyl groups at the nitrogen atom N5.8 While the stereogenic
nitrogen centre N2 adopts an S-configuration by placing the
sterically demanding substituent in the pseudo equatorial position
and away from the cyclohexane backbone, the opposite effect is
observed for the stereogenic nitrogen centre N4, which adopts an
methoxy function, the Li–O bond lengths are comparatively long
11
˚
˚
at 2.045(4) A and 2.028(4) A. Since the silicon bonded nitrogen
centre adopts a trigonal planar geometry, the ionic bond character
of Si1–N1 is increased resulting in the short bond length of
˚
1.6597(15) A.
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Compound 6 proved to be remarkably stable and could be
distilled with a kugelrohr apparatus at 150 ◦C and 1.6 ¥ 10-1 mbar.
A geometry optimisation on the B3LYP/6-31+G(d) level was
calculated. As can be seen in the space-filling model of this
structure the lithium ion is mostly shielded by the ligand (Fig.
2c). Additionally, the four-coordinate lithium ion shows no
concentrated partial positive charge necessary for a nucleophilic
attack (Fig. 2b). In agreement with investigations by Collum et
al.3 small aggregates like this monomeric lithium amide are not
necessarily the most reactive species, as has been shown previously
with a comparatively unreactive [tBuLi·(-)-sparteine] adduct.12
spectra were recorded on the Bruker Avance-500, Bruker DRX
500, Bruker Avance-400, Bruker DRX 400, Bruker DPX 300,
Bruker DRX 300 and Varian Inova 500 spectrometers. 1H NMR
spectra are referenced to the internal standards C6D5H at 7.16 ppm
or CHCl3 at 7.27 ppm, 13C NMR spectra are referenced to C6D5H
at 128.39 ppm or CHCl3 at 77.00 ppm (external standard: TMS).
The 29Si NMR spectra are referenced to the external standard
TMS (d = 0.0). The 7Li NMR spectra are referenced to the
internal standard LiCl in D2O (d = 0.0), which was added to
the solution in a sealed capillary. The GC-MS spectra were
measured with the gas chromatography column HP 6890 [J. &
W. Scientific; 25 m length, ID 0.2 mm, helium gas, 2.06 bar;
◦
temperature progam: 50 ◦C (1 min) - 40 C min-1 - 300 ◦C (5
min)] and a HP Mass Selective Detector 5973 (EI(+)-MS, 70 eV).
The elemental analysis was conducted on a Leco CHNS–932/O
VTF-900 analyzer. Compounds 1a,b were prepared according
to literature procedures.13 The enantiomerically pure compound
TMCDA was isolated according to the literature procedure.14 All
chemicals were used as purchased unless otherwise mentioned.
Crystal structure determination of compounds 2a, 2b, 4, 6
The crystals of all compounds were mounted in an inert
oil (perfluoropolyalkylether) at -60 ◦C (N2 stream), using the
X-TEMP 2 device.15 2a: Bruker Apex CCD diffractometer; pro-
grams used for data collection, cell determination and refinement:
Smart V. 5622 (Bruker AXS, 2001), integration: SaintPlus V. 6.02
(Bruker AXS, 1999), empirical absorption correction: Sadabs V.
2.01 (Bruker AXS, 1999). 2b: Stoe IPDS diffractometer; data
collection: Expose in IPDS (Stoe & Cie, 1999), cell determination
and refinement: Cell in IPDS (Stoe & Cie, 1999), integration:
Integrate in IPDS (Stoe & Cie, 1999), numerical absorption
correction: Faceit in IPDS (Stoe & Cie, 1999). 4 and 6: Oxford
Diffraction Xcalibur S, programs used for data collection, cell
determination and refinement: CrysAlis (Oxford, 2008); CrysAlis
RED (Oxford, 2008), empirical absorption correction: Scale3
Abspack (Oxford, 2008). The structures were solved using direct
methods (SHELXS90), structural refinement was done with
SHELXL97.
Fig. 2 a) Molecular structure of compound 6. Selected bond lengths
◦
˚
(A) and angles ( ) of 6: C3–N1 1.434(3), Li1–N1 1.913(4), Li1–O2
2.028(4), Li1–N2 2.033(4), Li1–O1 2.045(4), N1–Si1 1.660(2), N1–Li1–O2
121.67(18), N1–Li1–N2 99.61(15), O2–Li1–N2 85.93(16), N1–Li1–O1
134.71(19), O2–Li1–O1 103.62(17), N2–Li1–O1 82.67(16), C3–N1–Si1
128.72(14), C3–N1–Li1 119.50(17), Si1–N1–Li1 109.16(15), C9–O1–C10
112.22(16), C9–O1–Li1 109.67(14), C10–O1–Li1 129.99(18), C12–O2–C13
112.43(18); b) Fast surface of compound 6 showing the electrostatic
potential (Vmin = -0.128; Vmax = 0.069);16 c) Space-filling model of
molecular structure.
Conclusions
Lithium amides are widely used reagents in organic synthesis
and therefore a fundamental understanding of their structural
features is essential. In this work lithium silylamides with different
structural features have been synthesised and characterised via
X-ray crystallography. The structure of these compounds is
controlled by the number of donor functions and steric restrictions
of the ligand system. Using a coordinating side arm with one
donor functionality leads to dimeric lithium silylamides. Including
two donor atoms in the coordinating side arm resulted in a
mixed dimeric aggregate. At last a ligand with three donor
functions in an (aminomethyl) side arm produced a monomeric
compound. Additionally, the high stability of this monomeric
lithium silylamide 6, a small aggregate, is explained by the shielding
effects of the ligand, which are illustrated on the basis of quantum
chemical calculations.
Synthesis
Lithiumsilylamide 2a. To 142 mg (0.62 mmol) of compound
1a, which was cooled to -60 ◦C, 0.39 mL of nBuLi (0.62 mmol, 1.6
M in hexane) were added. The reaction mixture was allowed to
warm to 0 ◦C and stored for 20 h, resulting in the formation
of colourless crystals (111 mg, 0.24 mmol, 76%). 1H NMR
(300.1 MHz, C6D6): d = 0.13 [s(br), 3H], 0.54 [s(br), 3H;
Si(CH3)2], 0.86–1.06 [m, 1H; NCH2CH2CH2], 1.24–1.80 [m, 8H;
NCH2CH2CH2, SiCH2N], 1.52 [s, 9H; NC(CH3)3], 2.15 + not
2
visible [AB-system, 1H, JHH = 14.25 Hz; SiCH2N], 3.06–3.13
[m, 1H; NCH2CH2CH2], 3.40–3.48 [m, 1H; NCH2CH2CH2].
13C NMR (125.8 MHz, C6D6): d = 7.2 + 7.9 [2C, Si(CH3)2],
24.2 [1C, NCH2CH2CH2], 26.5 [1C, NCH2CH2CH2], 26.6 [2C,
NCH2CH2CH2], 36.9 [3C, NC(CH3)3], 53.3 [1C, NC(CH3)3],
54.0 [1C, SiCH2N], 56.1 [1C, NCH2CH2CH2], 60.9 [1C,
Experimental
General procedures
7
All reactions were carried out under argon using a standard
Schlenk line. All solvents were dried by standard methods. NMR
NCH2CH2CH2]. 29Si NMR (59.6 MHz, C6D6): d = -15.7. Li
NMR (116.6 MHz, pentane, D2O): d = 1.8.
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Lithiumsilylamide 2b. To 123 mg (0.57 mmol) of compound
1b, which was cooled to -60 ◦C, 0.36 mL of nBuLi (0.57 mmol, 1.6
M in hexane) were added. The reaction mixture was allowed to
warm to room temperature and stored for 20 h, resulting in the for-
mation of colourless crystals (75 mg, 0.17 mmol, 60%). 1H NMR
(400.1 MHz, C6D6): d = 0.26 [s, 6H; Si(CH3)2], 0.86–0.99 [m, 1H;
NCH2CH2CH2], 1.35–1.59 [m, 5H; NCH2CH2CH2], 1.40 [d, 6H,
3JHH = 6.0 Hz; NCH(CH3)2], 1.65–1.71 [m, 2H; NCH2CH2CH2],
1.78 [s, 2H; SiCH2N], 3.13–3.15 [m, 2H; NCH2CH2CH2], 3.54
with 50 mL of pentane. The solvent of the filtrate was removed
and the residue dissolved in 150 mL of pentane and filtered a
second time. The solvent of the filtrate was removed and the crude
residue of (4,5-dihydrofuran-2-yl)(iodomethyl)dimethylsilane
◦
was purified via distillation at 44 C and 4.8 ¥ 10-2 mbar
(7.7 g, 29 mmol, 84%). 1H NMR (300.1 MHz, CDCl3):
d = 0.29 [s, 6H; Si(CH3)2], 2.10 [s, 2H; SiCH2I], 2.61 [td, 2H,
3
3JHH = 9.7 Hz, JHH = 2.6 Hz; SiCCHCH2CH2O], 4.28 (t, 2H,
3
3JHH = 9.7 Hz; SiCCHCH2CH2O), 5.31 (t, 1H, JHH = 2.6 Hz;
[sept, 1H, JHH = 6.2 Hz; NCH(CH3)2]. 13C NMR (125.8 MHz,
SiCCHCH2CH2O).13C NMR (75.5 MHz, CDCl3): d = -15.3 (1C,
SiCH2I), -3.7 [2C, Si(CH3)2], 30.7 [1C, SiCCHCH2CH2O], 70.5
[1C, SiCCHCH2CH2O], 113.1 [1C, SiCCHCH2CH2O], 159.0
[1C, SiCCHCH2CH2O].29Si NMR (59.6 MHz, CDCl3): d = -8.2.
GC/El(+)-MS: tR = 4.71 min; m/z [%]: 268 (82) [M+], 171 (66)
[M+ - DHF], 141 (40) [M+ - I], 97 (100) [(DHF)Si+]. Anal. calcd
for C7H13IOSi: C, 31.35; H, 4.89. Found: C, 31.4; H, 5.0.
3
C6D6): d = 4.7 [2C, Si(CH3)2], 24.1 [1C, NCH2CH2CH2],
26.9 [2C, NCH2CH2CH2], 32.4 [2C, NCH(CH3)2], 48.7 [1C,
NCH(CH3)2], 54.0 [1C, SiCH2N], 58.6 [2C, NCH2CH2CH2].29Si
NMR (59.6 MHz, C6D6): d = -11.7.7Li NMR (116.6 MHz,
pentane, D2O): d = 1.9.
(1R,2R)-N1 -[{(Tert-butylamino)dimethylsilyl}methyl]-N1 ,N2,
N2-trimethylcyclohexane-1,2-diamine (3). To a solution of 3.4 g
TMCDA in 25 mL of pentane, which was cooled to -80 ◦C,
4.5 mL (3.3 g, 32 mmol) triethylamine and 4.5 mL (4.1 g,
31 mmol) bis(methoxyethyl)amine were added to 8.0 g (30 mmol)
(4,5-dihydrofuran-2-yl)(iodomethyl)dimethylsilane dissolved in
150 mL toluene. The reaction mixture was stirred under reflux for
30 h and then filtered to remove some oily brown residue which
was washed with 200 mL pentane. The solvent of the filtrate was
removed and the crude product purified via kugelrohr distillation
(140 ◦C, 1 ¥ 10-2 mbar) to give 6.1 g (22 mmol, 73%) of compound
5.1H NMR (300.1 MHz, CDCl3): d = 0.17 [s, 6H; Si(CH3)2], 2.18
t
10.0 mL (20 mmol) of BuLi (1.9 M in pentane) were added.
The reaction mixture was allowed to warm to room temperature
and stirred for 4 h. Afterwards this solution was slowly added to
2.7 g (22 mmol) of dimethyldimethoxysilane in 10 mL of pentane
at 0 ◦C and stirred for 1 h at room temperature (suspension
n
1). Meanwhile 5.8 mL (15 mmol) of BuLi (2.5 M in hexane)
were added to a solution of 3.2 mL (1.8 g, 30 mmol) of tert-
3
3
[s, 2H; SiCH2N], 2.55 [td, 2H, JHH = 9.7 Hz, JHH = 2.6 Hz;
SiCCHCH2CH2O], 2.65 [t, 4H, 3JHH = 6.4 Hz; NCH2CH2O], 3.30
[s, 6H; NCH2CH2OCH3], 3.42 [t, 4H, 3JHH = 6.2 Hz; NCH2CH2O],
◦
butylamine in 25 mL of pentane at -80 C. This suspension was
allowed to warm to room temperature and stirred for 20 min
(suspension 2). After removing the solvent of suspension 1 in
vacuo and adding 50 mL of pentane, suspension 2 was added at
3
4.23 [t, 2H, JHH = 9.5 Hz; SiCCHCH2CH2O], 5.22 [t, 1H,
3JHH = 2.6 Hz; SiCCHCH2CH2O].13C NMR (75.5 MHz, CDCl3):
d = -4.0 [2C, Si(CH3)2], 30.6 [1C, SiCCHCH2CH2O], 44.9 [1C,
SiCH2N], 57.0 [2C, NCH2CH2O], 58.7 [2C, NCH2CH2OCH3],
70.2 [1C, SiCCHCH2CH2O], 71.0 [2C, NCH2CH2O], 112.1
[1C, SiCCHCH2CH2O], 161.0 [1C, SiCCHCH2CH2O].29Si NMR
(59.6 MHz, CDCl3): d = -13.0. GC-El(+)MS: tR = 5.55 min; m/z
[%]: 273 (3) [M+], 228 (100) [M+ - 3 Me, MH+ - CH2NC4H8O2Me2],
146 (51) [(CH2NC4H8O2Me2)+]. Anal. calcd: C, 57.10; H, 9.95; N,
5.12. Found: C, 56.9; H, 9.9; N 4.9.
0
◦C and the mixture was stirred for 1 h at room temperature.
After removing the solvent in vacuo, 80 mL of pentane were
added and the suspension was filtered through celite under an inert
atmosphere. The solvent of the filtrate was removed and the crude
product purified by kugelrohr distillation (125 ◦C, 5.6 ¥ 10-2 mbar)
1
giving 2.3 g (8 mmol, 53%) of 3. H NMR (300.1 MHz, C6D6):
d = 0.27 [s, 3H], 0.31 [s, 3H; Si(CH3)2], 0.97–1.06 [m, 4H;
NCHCH2CH2], 1.24 [s, 9H; SiNHC(CH3)3], 1.59–1.84 [m, 5H;
NCHCH2CH2, SiNHC(CH3)3], 1.78 + 2.10 [AB-System, 2H,
2JHH = 14.2 Hz; SiCH2N], 2.23–2.31 [m, 2H; NCHCH2CH2],
2.28 [s, 6H; N(CH3)2], 2.33 [s, 3H; NCH3].13C NMR (75.5 MHz,
C6D6): d = 3.1 + 3.4 [2C, Si(CH3)2], 24.4 [1C, NCHCH2CH2],
24.6 [1C, NCHCH2CH2], 26.5 [2C, NCHCH2CH2], 34.4 [3C,
SiNHC(CH3)3], 40.8 [2C, N(CH3)2], 41.2 [1C, NCH3], 44.4 [1C,
SiCH2N], 49.4 [1C, SiNHC(CH3)3], 64.7 [1C, NCHCH2CH2], 67.6
[1C, NCHCH2CH2].29Si NMR (59.6 MHz, C6D6): d = -6.0.
Lithiumsilylamide 6. 1.6 mL (1.1 g, 15 mmol) of tert-
butylamine were dissolved in 5 mL of pentane and cooled to
-60 ◦C. Then 2.2 mL (5.5 mmol) of nBuLi (2.5 M in hexane) were
added. This reaction mixture was allowed to warm to 0 ◦C, then
1.5 g (5.5 mmol) of compound 5 were added and the resulting
solution was stirred for 30 min. Afterwards, the solvent was
removed and the residue dissolved in 5 mL of pentane and stored
at -78 ◦C. Crude brown crystals of 6 were formed after 5 h, which
were purified by fivefold recrystallisation from 5 mL of pentane
at -78 ◦C yielding 0.8 g (2.8 mmol, 52%) of colourless crystals
Lithiumamide 4. 0.4 mL (0.64 mmol) of MeLi (1.6 M in
diethylether) were added to 193 mg (0.64 mmol) of 3 and cooled to
-70 ◦C. The reaction mixture was stored at -78 ◦C over a period
of three months and another two weeks at room temperature
resulting in 51 mg (0.10 mmol, 31%) of colourless crystals of 4. 29Si
NMR (59.6 MHz, C6D6): d = -12.8 (br), -21.3, -24.8.7Li NMR
(116.6 MHz, 1 : 4 pentane/toluene, D2O): d = 1.0, 1.4, 1.6.
◦
1
of 6. Bp 150 C, 1.6 ¥ 10-1 mbar; H NMR (300.1 MHz, C6D6):
d = 0.51 [s, 6H; Si(CH3)2], 1.63 [s, 9H; NC(CH3)3], 1.89 [s, 2H;
SiCH2N], 2.03–2.07 [m, 2H; NCH2CH2OCH3], 2.24–2.28 [m, 2H;
NCH2CH2OCH3], 2.75–2.78 [m, 2H; NCH2CH2OCH3], 2.95 [s,
6H; NCH2CH2OCH3], 2.96–2.99 [m, 2H; NCH2CH2OCH3].13C
NMR (125.8 MHz, C6D6): d = 7.6 [2C, Si(CH3)2], 39.5 [3C,
NC(CH3)3], 50.2 [1C, SiCH2N], 52.7 [1C, NC(CH3)3], 54.4
[2C, NCH2CH2OCH3], 58.9 [2C, NCH2CH2OCH3], 70.5 [2C,
NCH2CH2OCH3].29Si NMR (59.6 MHz, C6D6): d = -26.6.7Li
NMR (116.6 MHz, pentane, D2O): d = 2.2.
N-[{(4,5-Dihydrofuran-2-yl)dimethylsilyl}methyl]-2-methoxy-
N-(2-methoxyethyl)ethanamine (5). 6.0 g (34 mmol) of (4,5-
dihydrofuran-2-yl)(chloromethyl)dimethylsilane13 and 12
g
(80 mmol) of NaI were dissolved in 150 mL of acetone and heated
under reflux for 6 h. The resulting solids were filtered and washed
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J. Fuller, Aidan T. Harrison and David B. Collum, J. Org. Chem., 1991,
56, 4435.
Acknowledgements
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We thank the Deutsche Forschungsgemeinschaft, the Fonds der
Chemischen Industrie and Wacker Chemie AG for their support.
Additionally the authors wish to express their gratitude towards
Jana Becker and Jonathan O. Bauer.
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10 The distinction of the “dimeric” species, producing a broad signal,
from the monomeric species is tentatively possible since the signal for
monomeric species is shifted upfield in the 29Si NMR spectrum. This
assignment is based on the chemical shift observed for the dimeric
compounds 2a,b and the monomeric compound 6.
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1902 | Dalton Trans., 2012, 41, 1897–1902
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