The smaller, the better? How the aggregate size affects the reactivity of (trimethylsilyl)methyllithium

Article abstract of DOI:10.1039/c9dt02182e

Weighting both the basicity and nucleophilicity of an organolithium compound is crucial for an effective use of these reagents in syntheses. To achieve this, an aggregate of optimal size and reactivity has to be formed by adding suitable donating agents. Against usual expectations, this is not inevitably the smallest possible aggregate. In this work, we show that the monomeric complex of (trimethylsilyl)methyllithium stabilized by the bidentate ligand (R,R)-TMCDA shows no significant reactivity. In contrast, two dimeric aggregates stabilized by monodentate quinuclidine were obtained, exhibiting enhanced reactivity compared to the parent compound and to the monomeric complex.

Full text of DOI:10.1039/c9dt02182e

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DOI: 10.1039/C9DT02182E
ARTICLE
The smaller, the better? How aggregate size affects the reactivity
of (trimethylsilyl)methyllithium
Lena Knauer, Jonathan Wattenberg, Ulrike Kroesen and Carsten Strohmann*
Received 00th January 20xx,
Accepted 00th January 20xx
Weighting both the basicity and the nucleophilicity of an organolithium compound is crucial for an effective use of these
reagents in syntheses. To achieve this, an aggregate of optimal size and reactivity has to be formed by adding suitable
donating agents. Against usual expectations, this is not inevitably the smallest possible aggregate. In this work, we show
that the monomeric complex of (trimethylsilyl)methyllithium stabilized by the bidentate ligand (R,R)-TMCDA shows no
significant reactivity. On contrary, two dimeric aggregates stabilized by monodentate quinuclidine were obtained, exhibiting
enhanced reactivity compared to the parent compound and to the monomeric complex.
DOI: 10.1039/x0xx00000x
Introduction
The reactivity of alkyllithium compounds is strongly related to
1
their structure. Depending on steric demands and electronic
2
effects, a variety of structural motifs can be observed. In most
synthetic applications of these reagents, the admixture of
Lewis-basic additives aims to stabilize smaller and therefore –
3
as assumed in literature – more reactive aggregates, aspiring
the formation of the supposedly ideal case of a monomeric
species of a primary organolithium compound. However, our
work presented here will give a more differentiated view on this
relationship. In general, the steric demand of the additive as
well as the size of the alkyl moiety determine the degree of
deaggregation (see Figure 1 top). According to this, the addition
of sterically demanding (
−
)-sparteine enables the formation of
4
monomeric tert-butyllithium, whereas the smaller primary
alkyllithium compound n-butyllithium forms tetramers in
presence of the sterically less demanding additive tetrahydro-
5
furane (thf). The heteroatom-stabilized primary (trimethyl-
3 2
silyl)methyllithium, Me SiCH Li, combines both tendencies.
Whereas it is – identical to likewise primary n-butyllithium –
hexameric in the absence of donating agents, the addition of
the bidentate ligands (−)-sparteine or N,N,N’,N’-tetramethyl-
6
Figure 1: Top: Deaggregation of alkyllithium compounds by donor bases.
Bottom: Formation of small aggregates of Me SiCH Li by the addition of
bidentate (R,R)-TMCDA (5) and monodentate quinuclidine (6).
3
2
ethylenediamine (tmeda) leads to the formation of dimeric ag-
7
7
gregates. An interesting octameric coordination mode was ob- N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (pmdta). In con-
tained by the group of Stalke upon the addition of 1,4-Diazabi- trast, no monomeric aggregate is known to date for n-butyl-
cyclo[2.2.2]octane (dabco). Instead of the formation of a coordi- lithium. The knowledge of these principles is crucial for an effec-
nation polymer, a molecular assembly of four dimeric units tive use of the different commercially available organolithium
linked by dabco is observed, leading to an increased reactivity reagents. When planning a synthesis, a compromise of cost and
8
compared to the parent hexameric aggregate. A monomeric use can be accomplished when considering the known struc-
compound can be obtained by addition of the tridentate ligand tures of the most widely used alkyllithium reagents. Neverthe-
less, unexpected reactivities are encountered in some synthe-
ses, where the use of the most reactive base system does not
lead to the best results. This is, among others, observed for
Institute for Inorganic Chemistry, TU Dortmund University, Otto-Hahn-Str. 6/6a,
4
4227 Dortmund, Germany.
E-mail: carsten.strohmann@tu-dortmund.de
Electronic Supplementary Information (ESI) available: Experimental details, NMR
spectra, crystallographic data and details of DFT calculations. CCDC 1908867,
3 2
Me SiCH Li. For instance, Stalke et al. observed the formation
of pure crystalline benzyllithium by using Me
3 2
SiCH Li for the de-
9
1
908868, 1908869. For ESI and crystallographic data see DOI: 10.1039/x0xx00000x
protonation of toluene. Here, the use of other – presumably
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Figure 2: Monomeric Me
3
SiCH Li stabilized by (R,R)-TMCDA (compound 2). Left: Molecular structure of 2 in the crystal (top view). Selected bond lengths [Å]
2
and angles [°]:C1-Li1 2.069(3), N1-Li1 2.050(2), N2-Li1 2.049(4), Si1-C1 1.8075(17), Si1-C2 1.887(2), Si1-C3 1.8958(18), Si1-C4 1.887(2); Si1-C1-Li1 114.64(12),
C1-Si1-C2 110.44(10), C1-Si1-C3 117.89(9), C1-Si1-C4 110.37(10), C2-Si1-C3 105.09(12), C2-Si1-C4 107.60(10), C3-Si1-C4 104.84(11). Right: Connolly surface of
compound 2, calculated with a probe radius of 1.4 Å (side view).
more reactive – alkyllithium reagents leads to undesired side re- coordination mode might lead to the formation of small
actions like ortho or meta dilithations. Avoiding side reactions aggregates. The second one was the strained monodentate
and reagent degradation when using Me
other alkyllithiums was also achieved by the group of Gros, who properties due to its strongly directed lone pair. Herein we
established a base system formed by Me SiCH Li and DMAE to present solid state structures of small aggregates of Me SiCH Li
3 2
SiCH Li instead of ligand quinuclidine (6), which offers increased donating
3
2
3
2
give a reagent capable of functionalizing bromo- and chloro- stabilized by these donating agents. The stabilizing effects as
pyridines selectively under mild conditions.10 They encountered well as the synthetic impact of these systems are discussed,
another advantage of Me SiCH Li since it exhibits a strongly de- based on experimental as well as theoretical findings.
3 2
creased nucleophilicity compared to other primary organoli-
thium compounds. Especially in the conversion of multi-functio- Deaggregation of (trimethylsilyl)methyllithium by bidentate (R,R)-
nalized substrates a consideration of the basicity and nucleo- TMCDA (5)
philicity of alkyllithium compounds is crucial since it determines
In the presence of the chiral bidentate ligand (R,R)-TMCDA (
5
),
the regioselectivity of the organolithium reagent’s attack. Ne-
vertheless, a decreased nucleophilicity mostly results in
increased deprotonation barriers. It might therefore be
desirable to increase this reactivity of Me SiCH Li by the
3 2
addition of a suitable donating agent to give a potent base
system. While fundamental works on the structural chemistry
we observed a monomeric complex of Me SiCH Li, the structure
3 2
of which is shown in Figure 2. It adds to the small number of
monomeric complexes of primary alkyllithium compounds.7
Compound
space group P2
2
crystallizes in the monoclinic crystal system in the
. The lithium cation is coordinated by both
1
nitrogen atoms of the (R,R)-TMCDA ligand, resulting in an
incomplete coordination sphere with a coordination number of
three. To the best of our knowledge, based on a structure
search in the CCDC database, this is the first example of a
primary organolithium compound exhibiting such a low coor-
dination number in a monomeric complex.
The formation of a monomeric species of a primary alkyllithium
compound is an unusual observation. For the most common of
these, n-butyllithium, the smallest aggregate known to date is a
dimer, since the lack of stabilizing substituents at the carb-
anionic center results in a higher degree of aggregation. In
of Me
3
SiCH
2
Li combined with common chelating agents was
7
previously performed by the group of Stalke, we focused on
the interrelation of structure and reactivity. A detailed
knowledge of the reagent’s structure is pivotal for
understanding reaction pathways which proceed via transition
states closely related to the starting material. Regarding
alkyllithium reagents, significant reactivity can be observed for
compounds in which the lithium centers are accessible for a
donating site of the substrate. In our work presented here we
were able to show that this can be achieved not in a monomeric
complex of (trimethylsilyl)methyllithium, but in dimeric
aggregates. This allows deprotonation reactions on the
substrate to take place more easily, which is referred to in this
work as “reactivity”.
3 2
contrast, Me SiCH Li benefits from the additional stabilizing
effect of the neighboring silicon atom, which can stabilize an
adjacent negative charge via the α-effect. This is often explained
by negative hyperconjugation of the negative charge into the
anti-binding molecular orbital of the Si
−C bonds to an antiperi-
planar oriented substituent, resulting in a weakening and
Results and Discussion
1
1
elongation of this contact. This is not observed for complex
C contacts of the trimethylsilyl group show no
Li. The first one significant differences [Si1
C2 1.887(2) Å, Si1 C3 1.8958(18) Å,
2,
We tested two different nitrogen-based donor ligands on their in which the Si
ability to form reactive aggregates of Me SiCH
was the bidentate (R,R)-TMCDA ), whose chelating Si1
−
3
2
−
−
(
5
−
C4 1.887(2) Å]. Additional quantum chemical calculations
2
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3 2
Figure 3: Dimeric aggregates of Me SiCH
Li stabilized by two (left, aggregate 3, a = 1−x, y, 1.5−z) and three (right, aggregate 4, half a molecule of co-crystallized quinuclidine omitted
for clarity) molecules of quinuclidine (6). Selected bond lengths [Å]: Aggregate 3: Si1-C1 1.8306(10), Si1-C2 1.8878(11), Si1-C3 1.8855(11), Si1-C4 1.8971(11), N1-Li1 2.0822(19), C1-
Li1 2.185(2), Si1-C1-Li1 94.79(6), N1-Li1-C1 123.67(9), C2-Si1-C4 107.80(5), C2-Si1-C3 104.87(5), C1-Si1-C3 117.29(5); aggregate 4 Si1-C8 1.887(2), Si1-C9 1.885(2), Si1-C10 1.885(2),
Si1-C11 1.837(2), Si2-C12 1.8331(19), Si2-C13 1.885(2), Si2-C14 1.883(2), Si2-C15 1.896(2), N1-Li1 2.123(4), N2-Li2 2.141(4), N3-Li2 2.164(4), C11-Li1 2.169(4), C11-Li2 2.280(4), C12-
Li1 2.181(4), C12-Li2 2.284(4), Li1-C11-Li2 65.84(14), Li1-C12-Li2 65.60(14), C11-Li1-C12 117.95(17), C11-Li2-C12 109.55(15), N2-Li2-N3 109.14(16), N1-Li1-C11 121.00(18), N1-Li1-
C12 115.71(17), N2-Li2-C11 108.15(16), N2-Li2-C12 106.86(16).
[
B3LYP, 6-31+G(d), gd3 dispersion correction] also revealed no of the ligand
elongation of the Si C bonds. This hints on a stabilization of the center prohibit the approach of a substrate to the lithium
negative charge by polarization of the silicon center, which is cation, which is assumed to be the first step in a deprotonation
further stressed by a short Si1 C1 distance of 1.8075(17) Å. reaction of an organolithium reagent. Due to its chelating
2
5 as well as the CH protons of the carbanionic
−
−
In contrast to our expectations and to the usual principles of character, the bidentate ligand is not expected to release a
organolithium compounds, this monomeric species with a single coordination site. To give space for a reaction at the
threefold coordinated Li+ exhibits no significant reactivity. carbanionic center, a full detachment of the ligand would be
Although the carbanionic center has only one contact to the necessary, leading to the formation of a “naked” monomeric
adjacent lithium cation and might therefore be accessible to alkyllithium compound and therefore an unreachable high
substrates, as it can be observed for monomeric sec-butyl- energy demand. As a consequence, no reasonable reaction path
lithium stabilized by (R,R)-TMCDA,12 complex
2 was not capable for a deprotonation reaction can be modeled, wherefore we
of deprotonating any of the model compounds we employed in performed no calculations of any activation energies for these
test reactions, as benzene, toluene, and N,N-dimethylbenzyl- reactions. As this example shows, the detailed knowledge of the
amine (7). Motivated by these experimental findings we con- structure and the related reactivity of an organolithium base
ducted quantum chemical calculations to obtain information on system is essential to ensure an effective use in synthetic
the aggregate’s sterical accessibility, which can be described by applications. Within this, the use of the smallest aggregate does
the so-called Connolly surface. It can be understood as the not inevitably lead to the most satisfying results.
distance to which a probe body of a defined radius can approach
the molecule (see Figure 2, right). In accordance with the experi-
mental results, the simulation of the surface clarifies the steric
shielding of the lithium cation of complex 2. The methyl groups
3 2
Scheme 1: Deprotonation reactions of N,N-dimethylbenzylamine by Me SiCH Li in the absence (left) and presence (right) of quinuclidine (6).
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Scheme 2: Calculated isodesmic reaction of the replacement of one donating quinuclidine (6) by substrate 7 as a first step to a deprotonation of 7 [B3LYP/6-31+G(d)/gd3].
Deaggregation of (trimethylsilyl)methyllithium by monodentate
quinuclidine (6)
addition of quinuclidine (6) leads to an isolated yield of 82% of
the trapping product after reacting for three hours in refluxing
heptane. The use of non-polar solvents contributes to a distinct
control of the base system’s reactivity by avoiding the occu-
pation of reactive sites by donating solvent molecules.
This finding is also stressed by our examinations of two dimeric
aggregates of Me
dentate ligand quinuclidine (
3
SiCH
2
Li stabilized by the strained mono-
). Within these aggregates, the
6
Again, quantum chemical calculations were used to substan-
tiate these experimental findings. Our first goal was to deter-
mine the quantum chemical method which describes the ex-
perimental findings best. Previous benchmarking work in our
own group revealed that the functional M062X gives the
smallest deviations of experimental bond lengths in gas phase
number of stabilizing donor base molecules varies from two to
three (see Figure 3). According to the increased number of
ligands, the N
Whereas aggregate
of 2.0822(19) Å, aggregate
nation of the Li cations. Li1 is coordinated by one molecule of
at a distance of 2.123(4) Å. At Li2, two Li N contacts are ob-
served, with lengths of 2.141(4) Å for Li2 N2 and of 2.164(4) Å
for Li2 N3, respectively. This hints on a relatively weak
−
Li bond lengths differ in both structures.
is a symmetric dimer with N Li distances
exhibits an unsymmetrical coordi-
3
−
4
6
1
4
calculations of organolithium compounds. In the case of
aggregates and , the Li N distances obtained by calculations
−
3
4
−
−
on the M062X level of theory and the basis set 6-31+G(d) were
slightly too small (see supplementary information for a detailed
comparison of the theoretical and experimental results).
Whereas bond lengths determined on the B3LYP level of theory
showed to be slightly elongated, the experimental results were
reflected best by an additional gd3 dispersion correction on the
B3LYP functional. In the following calculations, this method was
used in combination with the basis set 6-31+G(d).
−
coordination of the third ligand molecule, enabling the coor-
dination of a substrate to be deprotonated and therefore giving
rise to a synthetic application of the dimeric aggregates. We
probed this in the synthetically relevant deprotonation reaction
of (N,N)-dimethylbenzylamine (7) with subsequent trapping
13
with trimethylchlorosilane (see Scheme 1). Whereas no
reaction was observed in the absence of donating agents, the
C
Si
Li
9-TS
N
−
1
96 kJ·mol
9
−
1
0
kJ·mol
Reaction coordinate
Figure 4: Relative energies (kJ·mol−1) and optimized geometries of the assumed mixed aggregate 9 and the transition state 9-TS of its deprotonation by quinuclidine-stabilized
Me SiCH Li [B3LYP/6-31+G(d)/gd3].
3
2
4
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On the basis of the varying number of quinuclidine molecules in Quantumchemical Calculations
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DOI: 10.1039/C9DT02182E
the solid state, we assumed that the replacement of one donor
All quantumchemical calculation were performed using
Gaussian09, Revision E.01 on the B3LYP/6-31+G(d) level of
base by the substrate is the first step in the deprotonation of
N,N)-dimethylbenzylamine ( ) by Me SiCH Li (see Scheme 2).
1
5
(
7
3
2
theory with additional gd3 empirical dispersion correction
Calculations of the underlying isodesmic reaction revealed an
energy need of only 6 kJ/mol for the replacement, hinting on an
equilibrium of both aggregates. Starting from the assumed
mixed aggregate 9, the transition state 9-TS of the deproto-
nation reaction was calculated to give an energy barrier of 96
16
according to Grimme. The starting structures were modeled
using the GaussView interface and their energies were
optimized. A subsequent frequency calculation confirmed
ground structures by showing no imaginary frequencies. In the
case of transition states, one imaginary frequency was
observed. For the comparison of energies, zero-point corrected
energies were used, enabling the discussion of ΔH values. The
visualization of the optimized structures was aimed by the
kJ/mol (see Figure 4). The addition of both processes – building
aggregate
9 and its deprotonation – gives a total activation
energy of 102 kJ/mol (see Supporting Information for further
details on the performed calculations). This explains the need of
high temperatures for the deprotonation reaction since it was
conducted in refluxing heptane. In the transition state 9-TS, two
lithium centers are involved in stabilizing the carbanionic
17
software Molekel V4.3. Connolly surfaces were calculated by
Molekel V4.3, representing the surface to which a probe body
with a radius of 1.4 Å can approach the molecule. For further
details and complete results including absolute energies in
Hartree, see Supporting Information.
species, stemming from the two Me
pervious dimeric aggregate . Upon deprotonation of substrate
, one of these lithium centers establishes a stabilizing contact
to the deprotonated ortho carbanionic center. Its proton is
abstracted by one of the Me SiCH
formation of tetramethylsilane and releasing the formerly 0.170 g (1 mmol, 1 eq.) (R,R)-TMCDA in 3 mL dry n-pentane was
coordinated lithium cation. This cooperative effect of both cooled to 30 °C. After the addition of 1 mL (1 mmol, 1 eq.) of a
Me SiCH Li moieties, which might be crucial for the observed 1 M Solution of Me SiCH Li in pentane, the mixture was stored
reactivity, cannot take place in a monomeric complex. This at
3 2
SiCH Li moieties of the
4
7
Experimental Details
2
2 3 2
carbanions, leading to the Crystallization of Me SiCH Li·(R,R)-TMCDA (2). A solution of
−
3
2
3
2
−
80 °C for crystallization. After one week colorless needles of
1
again stresses the impact of the organolithium’s structure on its compound
ability to be involved in any chemical conversions. Although 1.54 (s, 2H, SiCH
dimers and are not the smallest possible aggregates of 4 H; (CH NCHCH
Me SiCH Li, our experimental and supplementary theoretical 1.79 – 1.81 [m, 2 H; (CH
observations show that they exhibit reactivity towards N,N- ppm. { H} C NMR (100.64 MHz, C
dimethybenzylamine ( ), in contrast to the monomeric complex 6.6 [3C; (CH )SiCH ], 22.0 [2C; (CH
formed in presence of (R,R)-TMCDA, underlining the impor- (CH
tance of a detailed knowledge of the structure of the organo- (CH
2
were obtained. H NMR (400.25 MHz, C
6
D
6
): δ =
], 0.58 – 0.70 [m,
], 1.33 – 1.43 [m, 4 H; (CH NCHCH CH ],
NCHCH CH ], 1.93 [s, 12 H; (CH N]
): δ = 5.7 [1C; (CH )SiCH ],
NCHCH CH ], 25.3 [2C;
(CH N], 64.2 [2C;
): δ = 2.8
): δ =
−
2
), 0.44 [s, 12 H; CH
CH
2 3 3
Si(CH )
3
2
4
3
)
2
2
2
3
)
2
2
2
3
3
)
2
2
2
3 2
)
1
13
6
D
6
−
3
2
7
3
2
3
)
2
2
2
2
3
)
)
2
NCHCH
NCHCH
2
CH
CH
2
], 40.5
[4C;
3 2
)
7
3
2
2
2
] ppm. Li NMR (155.55 MHz, C
6
D
6
1
29
lithium base to achieve sufficient reactivity and best efficiency [1Li; (CH
3
)SiCH
3.6 [1 Si; (CH )SiCH
Deprotonation reactions utilizing Me
Solutions of 0.170 g (1 mmol, 1 eq.) (R,R)-TMCDA (
mmol of the substrate [0.170 g (N,N)-dimethylbenzylamine (
2
Li] ppm. { H} Si NMR (79.52 MHz, C
6 6
D
of a deprotonation reaction.
−
3
2
Li] ppm.
3
SiCH
2
Li·(R,R)-TMCDA (2)
) and 1
),
.08 mL benzene, 0.10 mL toluene] in dry n-pentane were
.
5
Experimental Section
General Remarks
7
0
cooled to
−
30 °C and each added 1.0 mL (1 mmol, 1 eq.) of a 1M
SiCH Li in pentane. The reaction mixtures were
warmed to rt and stirred for 3 h. After cooling to 30 °C, 0.13
mL (1 mmol, 1 eq.) Me SiCl was added each. The mixtures were
warmed to rt again and stirred for 1 h. After quenching with 5
mL aqueous NH Cl, the layers were separated and the organic
All experiments were performed under an atmosphere of argon
by using standard Schlenk techniques. n-heptane was dried by
refluxing over sodium and distilled under an atmosphere of
argon prior to use. All reagents were used as commercial
products without further purification. NMR spectra were
recorded on a Bruker AV Avance III HD spectrometer. All NMR
solution of Me
3
2
−
3
4
layers were analyzed by GC/EI-MS. No reaction products could
be observed in all reactions.
1
spectra were recorded at room temperature (ca. 22 °C). H NMR
1
13
and { H} C NMR chemical shifts (δ) are reported in parts per
million (ppm) and referred to tetramethylsilane (TMS, δ = 0.0
ppm) with the deuterium signal of the solvent serving as
internal lock and residual solvent signals as an additional
Crystallization of (Me
A solution of 0.111 g (1 mmol, 1 eq.) quinuclidine in 3 mL dry
n-pentane was cooled to 50 °C. At this temperature, 1 mL
1 mmol, 1 eq.) of a 1 M solution of Me SiCH Li in pentane was
added. Upon warming to 20 °C, crystallization began. The
solution was stored at –80 °C. After two weeks, colorless
diamond-shaped plates of
obtained. H NMR (400.25 MHz, C
.35 [s, 9H; CH
1.38 (m, 1H; NCH
3 2 2
SiCH ) · n quinuclidine (3: n=2; 4: n=3).
−
(
3
2
1
29
reference. { H} Si NMR chemical shifts are reported in ppm,
referenced to an external standard of TMS (δ = 0.0 ppm), and
measured via the INEPT puls sequence. For the assignment of
the multiplicities the following abbreviations were used: s =
singlet, m = multiplet.
−
3
and hexagonal plates of
): δ = 1.87 (s, 2H; SiCH
], 1.12 – 1.17 (m, 6H; NCH CH CH), 1.35
CH CH), 2.69 – 2.73 (m, 6H; NCH CH CH)
4
were
1
6
D
6
−
2
),
0
–
2
Si(CH
3
)
3
2
2
2
2
2
2
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1
13
ppm. { H} C NMR (100.64 MHz, C
6
D
6
): δ =
−
5.8 (1C; SiCH
2
), 5.5
CH),
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft
View Article Online
DOI: 10.1039/C9DT02182E
[
4
[
3C; CH
8.0 (3C; NCH
1Li; (CH SiCH
1.22 [1Si; (CH
Deprotonation of N,N-Dimethylbenzylamine (7). A solution of
2
Si(CH
3
)
3
], 20.9 (1C; NCH
2
CH
2
CH), 26.3 (3C; NCH
2
CH
2
7
2
CH
2
CH) ppm. Li NMR (155.55 MHz, C
Li] ppm. { H} Si NMR (79.52 MHz, C
SiCH Li] ppm.
6
D
6
): δ = 2.7
6 6
D ): δ =
(DFG) for financial support. LK and UK thank the Fonds der
1
29
3
)
3
2
Chemischen Industrie (FCI) for doctoral fellowships.
−
)
3 3
2
0
1
.83 g (7.5 mmol, 1.5 eq.) quinuclidine and 0.75 mL (5 mmol,
eq.) N,N-dimethylbenzylamine ( ) in 40 mL dry n-heptane was
Notes and references
7
1
V. H. Gessner, C. Däschlein and C. Strohmann, Chem. Eur.
J., 2009, 15, 3320.
cooled to 0 °C and added 7.5 mL (7.5 mmol, 1.5 eq.) of a 1 M
solution of Me SiCH Li in pentane. After warming to rt the
solution was stirred for 1 h and then refluxed for 4 h. After
cooling to 50 °C, 0.65 mL (5 mmol, 1 eq.) Me SiCl was added.
The solution was left to warm to rt and stirred overnight. The
reaction was quenched by the addition of 40 mL of 2 M aqueous
HCl and the phases were separated. The aqueous layer was
cooled by the addition of ice and brought to pH 14 by the
addition of KOH. It was then extracted with 3x 40 mL Et
combined organic layers were dried over Na SO and the
volatiles were removed under reduced pressure. The residue
was distilled by Kugelrohr distillation at 40 °C and 9.8·10−1 mbar
to give 3-trimethylsilyl-N,N-dimethylbenzylamine (8) as a
3
2
2
a) H. J. Reich, Chem. Rev., 2013, 113, 7130; b) T. Stey and
D. Stalke, in The Chemistry of Organolithium Compounds,
ed. Z. Rappoport and I. Marek, John Wiley & Sons, Ltd,
Chichester, UK, 2004, pp. 47–120; c) K. Gregory, P. v. R.
Schleyer and R. Snaith, Adv. Inorg. Chem., 1991, 37, 47; d)
G. Boche, Angew. Chem., 1989, 101, 286; e) W. N. Setzer
and P. v. R. Schleyer, in Advances in organometallic
chemistry, ed. F. G. A. Stone and R. West, Academic Press,
Orlando, Fla., 1985, vol. 24, pp. 353–451; f) P. v. R.
Schleyer, Pure Appl. Chem., 1984, 56, 151;
−
3
2
O. The
2
4
3
a) C. Strohmann and V. H. Gessner, Angew. Chem., 2007,
yellowish oil (0.85 g, 4.1 mmol, 82%). Spectral data was
consistent with previously reported data.13
1
19, 4650; Angew. Chem. Int. Ed., 2007, 46, 4566; b) C.
Strohmann and V. H. Gessner, J. Am. Chem. Soc., 2008,
30, 11719; c) C. Strohmann, V. H. Gessner and A. Damme,
1
Chem. Comm., 2008, 3381; d) C. Strohmann, K. Strohfeldt
and D. Schildbach, J. Am. Chem. Soc., 2003, 125, 13672; e)
C. Strohmann, K. Strohfeldt, D. Schildbach, M. J. McGrath
and P. O'Brien, Organometallics, 2004, 23, 5389;
Conclusions
To conclude, we herein presented three different solid state
structures of small aggregates of Me SiCH Li, an organolithium
3 2
compound offering advantages due to decreased nucleo-
philicity and simplified handling,18 stabilized by nitrogen donors.
Although the monomeric species 2 is formed in the presence of
the bidentate ligand (R,R)-TMCDA, no significant reactivity is
observed for this complex. This is in contrast to expectations
based on the usual principles of organolithium compounds,
since the remarkably low coordination number of three at the
Li center would suggest notable reactivity. Quantum chemical
calculations confirmed the hindered reactivity due to steric
overload. In contrast to this, the quinuclidine-stabilized dimeric
4
5
6
7
8
9
C. Strohmann, T. Seibel and K. Strohfeldt, Angew. Chem.,
2
003, 115, 4669; Angew. Chem. Int. Ed., 2003, 42, 4531.
M. A. Nichols and P. G. Williard, J. Am. Chem. Soc., 1993,
15, 1568.
1
B. Tecle', A.F.M. Maqsudur Rahman and J. P. Oliver, J.
Organomet. Chem., 1986, 317, 267.
T. Tatic, H. Ott and D. Stalke, Eur. J. Inorg. Chem., 2008,
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T. Tatić, S. Hermann and D. Stalke, Organometallics, 2012,
31, 5615.
T. Tatic, S. Hermann, M. John, A. Loquet, A. Lange and D.
Stalke, Angew. Chem., 2011, 123, 6796; Angew. Chem. Int.
Ed., 2011, 50, 6666.
aggregates
3 and 4 exhibit reaction capabilities, which can be
traced back to a weak coordination of one donor molecule,
foreshadowing the coordination of a substrate to be deproto-
nated. By employing this base system in non-polar solvents, the
occupation of reactive sites by donating solvent molecules can
be avoided. By combining experimental and theoretical results
we were able to stress the importance of a detailed knowledge
of a reagent’s structure to achieve optimal reactivity,19 which is
not inevitably reached by the smallest possible aggregate.
1
1
1
1
1
1
0 A. Doudouh, P. C. Gros, Y. Fort and C. Woltermann,
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1 W. Scherer, P. Sirsch, D. Shorokhov, G. S. McGrady, S. A.
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29, 8952.
3 C. T. Viswanathan and C. A. Wilkie, J. Organomet. Chem.,
973, 54, 1.
8, 2324.
1
Conflicts of interest
There are no conflicts to declare.
1
4 V. H. Gessner, S. G. Koller, C. Strohmann, A.-M. Hogan and
D. F. O'Shea, Chem. Eur. J., 2011, 17, 2996.
5 Gaussian 09, Revision E.01, Gaussian Inc., Wallingfort CT,
2
013.
6
| J. Name., 2012, 00, 1-3
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Pleas De ad l too nn oT tr aa nd sj au cs t ti omn sa rgins
Journal Name
ARTICLE
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DOI: 10.1039/C9DT02182E
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7 P. Flükinger, H. P. Lüthi, S. Portmann and J. Weber,
Molekel 4.3, Swiss Center for Scientific Computing,
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8 a) K. L. Lee, Angew. Chem., 2017, 129, 3719; Angew.
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Dalton Transactions
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DOI: 10.1039/C9DT02182E
An unexpected influence of the nature of stabilizing additives to alkyllithium compounds on an
aggregate’s reactivity was examined experimentally and theoretically.