Enantiomerically enriched 'carbanions': Studies on the stereochemical course of selective transformations of metal alkyls

Article abstract of DOI:10.1016/S0022-328X(02)01825-9

Two (aminomethyl)(lithiomethyl)silanes 1-[(S)-2-(methoxymethyl)pyrrolidinyl]} [(R,S)-17] are presented including their solid state structures, the first one non-chiral, the latter highly diastereomerically enriched. By metathesis reactions with metal(II) halides (metal = Mg, Ga, Pd, Cd and Hg), the corresponding bis{[(aminomethyl)silyl]methyl}metal(II) compounds or the [(aminomethyl)silyl]methylmetal(II) halides were obtained. In the case of highly diastereomerically enriched (aminomethyl)(lithiomethyl)silane (R,S)-17, the 'carbanionic' fragment could be transferred with high stereoselectivities on the metals Hg and Pd. For all of the compounds, the solid state molecular structures were determined.

Full text of DOI:10.1016/S0022-328X(02)01825-9

Journal of Organometallic Chemistry 661 (2002) 149ꢃ158
/
www.elsevier.com/locate/jorganchem
Enantiomerically enriched ‘carbanions’:
Studies on the stereochemical course of selective transformations of
metal alkyls
Carsten Strohmann a,*, Bors C. Abele a, Klaus Lehmen a, Fernando Villafane b,
˜
Luisa Sierra b, Susana Mart´ın-Barrios b, Daniel Schildbach a
a Institut fur Anorganische Chemie, Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
¨
¨
¨
¨
b Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain
Received 2 May 2002; accepted 19 July 2002
Dedicated to Professor Helmut Werner in recognition of his lifetime achievements in the field of organometallic chemistry
Abstract
Two (aminomethyl)(lithiomethyl)silanes Ph2Si(CH2Li)(CH2NC5H10) (NC5H10
[CHLiPh]}(CH2SMP) {SMPꢀ1-[(S)-2-(methoxymethyl)pyrrolidinyl]} [(R,S)-17] are presented including their solid state struc-
tures, the first one non-chiral, the latter highly diastereomerically enriched. By metathesis reactions with metal(II) halides (metalꢀ
ꢀ1-piperidinyl) (2) and Me2Si{[R]-
/
/
/
Mg, Ga, Pd, Cd and Hg), the corresponding bis{[(aminomethyl)silyl]methyl}metal(II) compounds or the [(aminomethyl)si-
lyl]methylmetal(II) halides were obtained. In the case of highly diastereomerically enriched (aminomethyl)(lithiomethyl)silane
(R,S)-17, the ‘carbanionic’ fragment could be transferred with high stereoselectivities on the metals Hg and Pd. For all of the
compounds, the solid state molecular structures were determined.
# 2002 Published by Elsevier Science B.V.
Keywords: Organolithium; Organometallic chemistry; Solid state structures; Carbanion; Diastereomerically enriched
1. Introduction
on the stereochemical course of transmetallations and
metathesis reactions. Our recent results on this will be
summarized in the following.
Stereogenic metalated carbon centers are usually
generated starting from the corresponding lithium
alkyls. These are mostly synthesized by deprotonation
using alkyllithium bases, e.g. butyllithium [1]. The
stereochemical information is either introduced inter-
The synthesis of enantiomerically1 enriched metal
alkyls with a stereogenic metalated carbon center has
been intensively studied for the last 20 years [1]. In
selective transformations, these types of compounds can
act as chiral alkyl transfer reagents, the stereogenic
metalated carbon center has been transferred to organic
molecules and hetero element centers. But furthermore,
compounds of this type are valuable ‘tools’ for studies
molecularly by chiral auxiliaries, like (ꢁ)-sparteine [2,3],
/
or intramolecularly by side-arm donation [4] of chiral
substituents [5], like the [(S)-2-(methoxymethyl)pyrroli-
dinomethyl] substituent (CH2SMP). Moreover, the
stereochemical information can also be introduced prior
to the deprotonation reaction by using enantiomerically
* Corresponding author. Tel.: ꢂ
8884605
E-mail
Strohmann).
/
49-931-8884613; fax: ꢂ
/
49-931-
address:
c.strohmann@mail.uni-wuerzburg.de
(C.
enriched CÃ
/
H acidic precursors bearing only one proton
1
Remark: In general, we speak of enantiomerically enriched metal
alkyls, when we focus our interest on the stereogenic metalated carbon
center. In the real case, these molecules are almost always
diastereomerically enriched metal alkyls, due to the presence of more
stereogenic centers than only the metalated one.
at the stereogenic carbon center [1b,2a].
The latter chiral (aminomethyl) substituent is an
important part of our concept for the synthesis of
enantiomerically enriched metal alkyls: (amino-
0022-328X/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 8 2 5 - 9

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C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ158
/
2. Non-chiral metal alkyls
The starting lithium alkyl for the synthesis of the non-
chiral metal alkyls, (aminomethyl)(lithiomethyl)silane
(2), is generated in n-pentane at low temperatures, using
tert-butyllithium (Scheme 2). After warming up to room
temperature, single crystals of 2 were obtained, suitable
for X-ray diffraction studies on the solid state structure
of the lithium compound [7].
Fig. 1. Stabilization effects in (aminomethyl)(metallomethyl)silanes on
their metalated centers.
Compound 2 crystallized from n-pentane in the
monoclinic crystal system, space group P21/n. It forms
a dimer in the solid state with a central four-membered
methyl)(metallomethyl)silanes (Fig. 1). The side-arm
donor SMP fulfills two important tasks. First of all, it
introduces the stereochemical information into the
coordination sphere of the metal fragment by coordina-
tion to the metal. Secondly, also by coordination, it
stabilizes the absolute configuration of the stereogenic
carbon center by fixing the metal fragment to the
metalated carbon center. Without this ‘trick’, unstabi-
lized stereogenic metal centers would lose their stereo-
chemical information even at low temperatures due to
the small barrier of inversion of metalated carbon
centers (‘carbanions’ are isoelectronic to amines, which
are known to have an unstable absolute configuration).
The silicon center of the (aminomethyl)(metallo-
methyl)silane also has two important functions. It
stabilizes a-metalated carbon centers by polarization
effects [6]. Moreover, it prevents the b-elimination,
which is often observed for polar organo metal com-
pounds.
In the field of enantiomerically enriched metal alkyls,
the determination of the absolute configuration at the
metalated stereogenic center and the clarification of the
stereochemical course of further transformations are
most important. However, this is often difficult to
realize without speculation. In many cases, stereoche-
mical assumptions have been made that are not always
generally applicable. Furthermore, many aspects of the
reaction mechanisms have not been understood yet.
In order to optimize the synthetic methods and to
understand the structure/reactivity patterns, the aggre-
gation behavior and the involved reaction mechanisms,
it is important to start with lithium alkyls which have a
defined structure in solution and the solid state. This is
why we studied the influence of different metals and
substituents or ligands with non-chiral (amino-
methyl)(metallomethyl)silanes of type B first (Scheme
1).
CÃ
/
LiÃ
/
CÃ
/
Li ring (Fig. 2).
Compared to non-metalated silanes, the bond SiÃ
/
˚
C(1)ꢀ1.806(2) A is shortened, indicating the stabilizing
/
effect of the silicon center on the a-metalated carbon
center [6]. Thus, this bonding situation is an indicator on
the polarity of the C(1)Ã
by an NÃLi contact of 2.049(4) A.
Starting from lithium alkyl 2, the bis{[diphenyl(piper-
idinomethyl)silyl]methyl}metal compounds 3ꢃ
[metalꢀmagnesium (3) [9], cadmium (4) [9] and mer-
/Li contact, which is underlined
˚
/
/
5
/
cury (5)] where synthesized in different solvents (toluene,
THF) by metathesis reactions, using the corresponding
metal(II) halides (Scheme 3) [10].
At a temperature of ꢁ90 8C, a cooled solution of
/
two equivalents of the lithium alkyl 2 was added to the
cooled solution (suspension) of the metal(II) halide and
the reaction mixture was warmed to room temperature.
After separation of the lithium halide and removing the
solvent, single crystals of compounds 3, 4 and 5 were
Scheme 2.
Fig. 2. Molecular structure and numbering scheme of compound 2 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃC(1) 1.806(2), LiÃC(1) 2.207(4), LiÃC(1)? 2.178(4), NÃLi
2.049(4), C(1)ÃSiÃC(14) 108.36(10), C(1)?ÃLiÃC(1) 113.76(15).
/
/
/
/
Scheme 1.
/
/
/
/

C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ
/
158
151
Scheme 3.
Fig. 4. Molecular structure and numbering scheme of compound 4 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃ
/
C(1) 1.831(3), CdÃ
/
C(1) 2.202(3), CdÃ
/
N 2.537(2), C(1)Ã
/
SiÃ
/
C(14) 108.63(13), C(1)Ã
/
CdÃ
/C(1)? 140.58(15).
metal center is different (Fig. 5). The two coordinating
nitrogen atoms are no longer equivalent, the HgÃ
/
N(1)
N(2) distance is 3.643(7)
˚
distance is 3.026(6) A, the HgÃ
/
˚
A. Thus, no symmetry elements can be found for the
molecular structure of compound 5 in the solid state.
Moreover, the structure is mainly determined by the T-
shape coordination geometry formed by two mercury
carbon bonds and one stronger mercury nitrogen
contact, typical for mercury compounds of this type.
Fig. 3. Molecular structure and numbering scheme of compound 3 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃC(1) 1.820(2), MgÃC(1) 2.181(2), MgÃN 2.254(2), C(1)ÃSiÃ
C(14) 106.78(8), C(1)?ÃMgÃC(1) 123.29(11).
/
/
/
/
/
/
/
obtained, suitable for single crystal X-ray diffraction
analyses.
The weak HgÃ
plete the coordination sphere of the central metal. With
the angle C(20)ÃHgÃC(1)ꢀ177.1(3)8, the mercury cen-
/
N(2) interaction seems to merely com-
Monomeric magnesium compound 3 crystallized
from toluene in the monoclinic crystal system, space
group C2/c [9]. The coordination situation at the
magnesium center of a distorted tetrahedron, resulting
in a C2 symmetry of the molecule, is formed by two
/
/
/
ter is almost linearly coordinated by the two carbon
centers.
When one equivalent of the lithium compound 2 is
reacted with mercury(II) chloride in toluene at low
temperatures, formally only one chloride is substituted
to give {[diphenyl(piperidinomethyl)silyl]methyl}mer-
magnesium carbon contacts and two magnesium nitro-
˚
C(1)ꢀ1.820(2) A bond
gen contacts (Fig. 3). The SiÃ
/
/
length is again shortened in this molecule, the MgÃ
/
N
C(1) angle of
˚
distance is of 2.254(2) A. The C(1)?Ã
/
MgÃ
/
123.3(1)8 lies within the normal range for compounds of
this type [10].
Monomeric cadmium compound 4 crystallized from
tolueneꢃn-pentane in the monoclinic crystal system,
/
space group C2/c [9]. Like in magnesium compound 3,
the coordination situation is described by a distorted
tetrahedron and the molecule is of C2 symmetry (Fig. 4).
The decreased polarity of the cadmium carbon bond is
˚
C(1) distance of 1.831(3) A and a
represented by an SiÃ
/
˚
N distance of 2.537(2) A. To the best of our
CdÃ
/
knowledge, the C(1)Ã
/
CdÃC(1)? angle of 140.6(2)8 is
/
unusually small and the shortest angle, that has been
observed by X-ray diffraction methods for this type of
compounds [10].
Fig. 5. Molecular structure and numbering scheme of compound 5 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
Monomeric mercury compound 5 crystallized from
tolueneꢃn-pentane in the monoclinic crystal system,
/
(8): Si(1)Ã
C(20) 2.087(9), HgÃ
C(14) 109.6(4), C(20)Ã
/
C(1) 1.858(8), Si(2)Ã
N(1) 3.026(6), HgÃ
Si(2)ÃC(33) 111.1(4), C(20)Ã
/
C(20) 1.852(8), HgÃ
N(2) 3.643(7), C(1)Ã
HgÃC(1) 177.1(3).
/
C(1) 2.095(7), HgÃ
/
space group P21/c. Compared to compounds 3 and 4,
/
/
/
Si(1)Ã
/
the situation in compound 5 with mercury(II) as the
/
/
/
/

152
C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ158
/
Monomeric compound 7 crystallized from n-pentane
¯
in the triclinic crystal system, space group P1: The
coordination geometry at the gallium center is of a
distorted tetrahedron. As only one of the {[(amino-
methyl)diphenylsilyl]methyl} substituents acts as a che-
Scheme 4.
lating ligand, only one GaÃ
/
N contact GaÃ
/
N(1)ꢀ
/
˚
2.198(6) A is of interest, comparable to that of
magnesium alkyl 3. Nevertheless, two contacts between
silicon and a metalated carbon center can be found:
Si(1)Ã
/
C(1)ꢀ
/
1.840(8) and Si(2)Ã
/
C(20)ꢀ1.853(8). Both
/
distances are in the range of cadmium compound 4 and
mercury compounds 5 and 6, indicators for a more
covalent CÃ/Ga bond in this molecule.
As a conclusion, the {[diphenyl(piperidinomethyl)si-
lyl]methyl} fragment was successfully transferred to the
metals magnesium, gallium, cadmium and mercury.
Since the nitrogen center of the (aminomethyl) substi-
tuent is intramolecularly coordinating the metal center,
it is strongly determining the molecular structures. Thus,
no additional solvent molecules are found intermolecu-
larly coordinating to the central metal. As a regularity,
Fig. 6. Molecular structure and numbering scheme of compound 6 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃ
C(14) 109.1(3), C(1)Ã
/
C(1) 1.859(8), HgÃ
/
C(1) 2.066(7), HgÃ
/
N 2.778(6), C(1)Ã
/
SiÃ
/
/
HgÃCl 170.5(2).
/
cury(II) chloride (6) (Scheme 4). After separation of
lithium chloride and removing the solvent, single
crystals of compound 6 were obtained, suitable for
single crystal X-ray diffraction analysis.
Monomeric compound 6 crystallized from acetone in
the monoclinic crystal system, space group P21/n. Like
in compound 5, the mercury center is almost linearly
the SiÃ
of the CÃ
shorter is the SiÃ
metalated carbon center. Finally, some exciting struc-
/
C bond length is in accordance with the polarity
M bond: the more polar the CÃM bond is, the
C bond between the silicon and the
/
/
/
tural features could be observed [e.g. the small C(1)Ã
/
CdÃ
/
C(1)? angle in cadmium compound 4].
coordinated by C(1) and Cl with an angle C(1)Ã
/
HgÃ
/
Cl
˚
N contact of 2.778(6) A, the T-
of 170.5(2)8. By the HgÃ
/
shape coordination geometry, typical for mercury com-
pounds, is obtained (Fig. 6). The more covalent
character of the metal carbon bond in both mercury
alkyls is underlined by the length of the bond SiÃ
/
C(1) of
˚
˚
both compounds [1.858(8) A for 5 and 1.859(8) A for 6].
Compared to lithium compound 2 or magnesium
compound 3, these distances are of almost the same
value than in non-metalated (aminomethyl)silanes due
to minimal stabilizing effects by the vicinal silicon
center.
Scheme 5.
The
carbanionic
{[(aminomethyl)diphenylsilyl]-
methyl} fragment of lithium alkyl 2 can also be
transferred to Group 13 main group elements (e.g.
gallium), shown in Scheme 5 [11].
Two equivalents of 2 react with di{[bis(trimethylsi-
lyl)methyl](m-acetato)}digallan in n-pentane at low tem-
peratures, yielding the unsymmetrically substituted
triorgano gallium compound 7 (Scheme 5). In this
compound with a coordination number of four at the
gallium center, one of the {[bis(trimethylsilyl)methyl]
groups of the starting material is left. Moreover, two
{[(aminomethyl)diphenylsilyl]methyl} substituents are
found at the gallium center, only one coordinating it
Fig. 7. Molecular structure and numbering scheme of compound 7 in
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): Si(1)Ã
C(20) 2.015(9), GaÃ
C(14) 104.1(4), C(20)Ã
115.9(3), Si(1)ÃC(33)ÃN(2) 119.4(3).
/
C(1) 1.840(8), Si(2)Ã
N(1) 2.198(6), GaÃ
Si(2)ÃC(33) 114.7(4), C(20)Ã
/
C(20) 1.853(8), GaÃ
C(39) 2.041(6), C(1)Ã
GaÃ
/
C(1) 2.018(7), GaÃ
Si(1)Ã
C(1)
/
/
/
/
/
in a chelating manner by an additional GaÃ
/
N contact,
/
/
/
/
as the crystal structure of 7 reveals (Fig. 7).
/
/

C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ
/
158
153
˚
3. Synthesis and reactivity of non-chiral palladium alkyls
The PdÃ
/
C distance of 2.034(2) A is in the typical
C(sp3) single bonds (from 2.0 to 2.1
A) [13]. The difference in the PdÃCl distances of
range found for PdÃ
/
˚
Since palladium was the first metal that we used for
enantiomerically enriched metal alkyls, reactions of
lithium alkyl 2 with palladium(II) compounds were
intensively studied [12]. The following questions had to
be answered: (1) Can the {[diphenyl(piperidinomethyl)-
silyl]methyl} fragment be transferred to palladium? (2)
/
˚
2.491(1) and 2.329(1) A reflects the strong electron
donating properties of the (silylmethyl) ligand, being an
indicator for a high trans influence.
The reactivity of complex 9 was studied in order to
determine the behavior of the [(aminomethyl)(silyl-
methyl)] ligand coordinated to palladium. A wide
variety of mono- and bidentate ligands of different
geometric and electronic characteristics, neutral or
anionic, were used.
Is the PdÃ/C bond stable towards ligand exchange
reactions? (3) Is the coordination of the (aminomethyl)
substituent a structure-determining factor?
The reaction of lithium alkyl
2 with trans-
Monodentate ligands cleave the chloro bridges to give
compounds of type Pd[CH2SiPh2(CH2NC5H10)]ClL
[PdCl2(SMe2)2] directly leads to dimeric {Pd[CH2SiPh2-
(CH2NC5H10)](m-Cl)}2 (9), where the [CH2SiPh2(CH2-
NC5H10)] ligand forms a palladacycle (Scheme 6). When
trans-[PdCl2(tht)2] is used, the corresponding mono-
meric (aminomethyl)(palladiomethyl)silane 8 with one
tht ligand coordinated to the palladium center can be
isolated. The chloro bridged dimer 9 crystallizes as the
[Lꢀ
PPh3 (10), PMe3 (11), CNtBu (12), 4-methylpyr-
/
idine (4-MePy) (13)], where the incoming ligand and the
carbon center of the cyclometalated ligand are coordi-
nated cis (Scheme 7). One of these compounds (10) was
crystallographically examined by X-ray diffraction stu-
dies.
trans isomer from CH2Cl2ꢃn-hexane (monoclinic crys-
/
As an example, the molecular structure of PPh3
complex 10 is shown in Fig. 9. The compound crystal-
tal system, space group P21/n), as both carbon donor
atoms or both nitrogen donor atoms are at opposite
sides of the molecule (Fig. 8). However, the compound
exists as a mixture of the cis isomer and the trans isomer
in solution.
lized from CH2Cl2ꢃn-hexane in the orthorhombic
/
crystal system, space group P212121. The coordination
geometry at the palladium center is square planar with
the PPh3 ligand in cis position to the metalated carbon
center. As can be seen, the five-membered palladacycle
remains intact during the reaction with the phosphine.
Scheme 7.
Scheme 6.
Fig. 8. Molecular structure and numbering scheme of compound 9 in
Fig. 9. Molecular structure and numbering scheme of compound 10 in
˚
˚
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃC(1) 1.855(3), PdÃC(1) 2.034(2), PdÃN 2.126(2), C(1)ÃSiÃ
C(14) 104.40(11).
the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): PdÃC(1) 2.067(5), SiÃC(1) 1.824(4), PdÃN 2.212(4), C(1)ÃSiÃ
C(14) 100.4(2).
/
/
/
/
/
/
/
/
/
/

154
C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ158
/
The cationic complex {Pd[CH2SiPh2(CH2NC5H10)]-
(4-MePy)2}BF4 (14) with two 4-MePy ligands at the
palladium center is obtained by treating the starting
dimer 9 with TlBF4 and 4-MePy in a redox reaction
(Scheme 8). The solid state structure of 14 was
determined.
The molecular structure of the 4-MePy complex 14 is
shown in Fig. 10. The compound crystallized from
CH2Cl2ꢃ
group P21/n. Two different PdÃ
found in the crystal [PdÃN(2)ꢀ2.180(4), PdÃ
/
Et2O in the monoclinic crystal system, space
N(sp2) distances are
N(3)ꢀ
/
/
/
/
/
2.046(3)], indicating a 4-MePy ligand more covalently
bound and one with a more dative bond to the
palladium center.
A chelate complex Pd[CH2SiPh2(CH2NC5H10)](S2C-
NEt2) (15) is obtained when reacting the starting chloro
bridged dimer 9 with NaS2CNEt2 (Scheme 9). The solid
state structure of compound 15 could be determined.
Fig. 10. Molecular structure and numbering scheme of compound 14
˚
in the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃ
/
C(1) 1.846(5), PdÃ
/
C(1) 2.050(5), PdÃ
/
N(1) 2.124(3), PdÃN(2)
/
2.180(4), PdÃ
/
N(3) 2.046(3),C(1)Ã
/
SiÃC(14) 102.8(2).
/
Compound 15 crystallized from CH2Cl2ꢃn-hexane in
/
quirements on structure and reactivity have been met.
All the synthesized non-chiral metal alkyls are well-
defined compounds in the solid state. By reaction of a
metal halide with a highly diastereomerically enriched
alkyl lithium compound, diastereomerically enriched
metal alkyls should be accessible. The high diastereos-
electivities of lithiated (aminomethyl)benzylsilane
(R,S)-17 in trapping reactions with organic electrophiles
[14] (e.g. alkyl halides) and organotinhalides made us
choose this molecule for alkyl transfer reactions on
other metals. This alkyl lithium compound is again
obtained by deprotonation of the corresponding (ami-
nomethyl)benzylsilane 16 with tert-butyllithium in non-
polar solvents at low temperatures (Scheme 10).
the monoclinic crystal system space group P21/c. The
coordination geometry at the palladium center is again
square planar, but with some greater distortion than in
the monodentate complexes 10ꢃ
is found for S(1)ÃPdÃS(2), 104.35(6)8 for N(1)Ã
(Fig. 11).
The reactions of lithium alkyl
/
14. A value of 75.12(3)8
/
/
/
PdÃS(2)
/
2
with trans-
[PdCl2(SMe2)2] show that the {[diphenyl(piperidino-
methyl)silyl]methyl} fragment can also be transferred
to palladium and that in all of the reactions a five-
membered palladacycle is formed. This palladacycle
remains intact during all of the reactions, where bridging
chloro ligands are cleaved by neutral ligands or sub-
stituted by anionic bidentate ligands. However, SiÃ
/
C
and PdÃN cleavages are detected during the attempts to
/
synthesize cationic complexes, the complexes with 4-
methylpyridine being the only ones to be cleanly
obtained. X-ray structural analyses of each type of
compound show the strong trans influence of the
[(piperidinomethyl)silylmethyl] fragment, regardless of
neutral or cationic complexes. Again, the nitrogen
centers of the (aminomethyl) substituent can be re-
garded as strongly structure determining.
Scheme 9.
4. Enantiomerically enriched metal alkyls
Summarizing the results on the transformations with
non-chiral (aminomethyl)(lithiomethyl)silane 2, the re-
Fig. 11. Molecular structure and numbering scheme of compound 15
˚
in the crystal (Schakal plot) [8]. Selected bond distances (A) and angles
(8): SiÃC(1) 1.843(2), PdÃC(1) 2.074(2), PdÃN(1) 2.155(2), C(1)ÃSiÃ
C(14) 105.19(10).
/
/
/
/
/
Scheme 8.

C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ
/
158
155
reaction remained unclear and was based on stereo-
chemical speculations due to the absence of knowledge
of the solid state structure for (R,S)-17.
Having determined the molecular structure in the
solid state and the absolute configuration at the
metalated a-carbon center, we experimentally examined
and explained the stereochemical course of an integral
sequence of transformations, starting from the unmeta-
lated silane 16 (Scheme 10). The absolute configuration
at the a-carbon center of compound (S,S)-18 was
determined by conversion into the ammonium iodide
(S,S)-18×
/
HI and subsequent determination of the crys-
tal structure. The selectivity of the SiÃ
/C cleavage
Scheme 10.
reaction and the absolute configuration of benzyl
alcohol (S)-19 could be determined by NMR methods
with the help of chiral lanthanide shift reagents [Er(tfc)3]
[15].
The amazingly high selectivity of reactions of lithium
alkyl (R,S)-17, which was first investigated by Chan et
al. [14a], and the apparent stability of configuration
make a closer look at the stereochemical course of its
transformations necessary. Thus, it was of great interest
to determine the molecular structure and the absolute
configuration of lithium alkyl (R,S)-17 in the crystal,
since both had not been known yet [15].
Compound (R,S)-17 crystallized from toluene in the
hexagonal crystal system, space group P32. Three
crystallographically independent molecules are found
in the asymmetric unit, each building up infinite chain
structures of (R,S)-17 in the solid state. These chains are
formed by p-interactions between a lithium center and
the phenyl group of an adjoining silane molecule (Fig.
12). The metalated stereogenic carbon center C(3) is
In the literature, both the (R,S) and the (S,S)
diastereomer of lithium alkyl 17 have been proposed
as the major diastereomer [5,14]. Based on these
predictions, both retention and inversion of the config-
uration at C(3) have been postulated for the reactions of
(R,S)-17 with alkyl halides [14]. Our X-ray structural
analysis of (R,S)-17 shows that (R) configuration at the
metalated carbon center and inversion of configuration
[for the reaction of (R,S)-17 with methyl iodide in
toluene] are in fact the case.
A look in the literature reveals that both retention and
inversion of the configuration have been observed for
the reactions of lithiated benzylic and related systems
with various electrophiles [16]. In most cases no solid
state structure of the corresponding lithium alkyl could
be determined to confirm the absolute configuration of
the stereogenic metalated carbon center. The stereoche-
mical course of the reaction of our lithium alkyl with the
electrophile MeI in toluene can be understood on the
basis of the solid state structure of compound (R,S)-17,
which was crystallized from the same solvent used in the
substitution reaction with the alkyl iodide, in the
combination with computational studies.
planar but is nevertheless*
/
due to the metalÃ
/carbon
contact Li(1)ÃC(3)*a stereogenic center. The sum of
/
/
the angles around C(3) is 360(3)8, regarding only the
‘carbanionic’ moiety [15].
The reaction sequence of the metalation, followed by
methylation and direct SiÃ/C cleavage reaction to the
correspending benzyl alcohol (S)-19 was already studied
by Chan et al. [14a]. The stereochemical pathway of this
The modelling of monomeric (R,S)-17 [B3LYP/6-
31ꢂG(d)] indicates that the highest occupied molecular
/
orbital (HOMO) is chiefly located at the metalated
carbon center and the aromatic ring system (Fig. 13). It
can be deduced from the calculated orbital coefficients
that both inversion and retention of configuration are
almost equally likely to result from electrophilic attack.
Only the fact that the site opposite to the lithium center
is sterically accessible to attack by electrophiles (the
coordination polymer of the solid state structure should
be broken up in solution) makes it possible for (R,S)-17
to react selectively with inversion of configuration at
C(3) under kinetic control in non-polar solvents.
Fig. 12. Molecular structure and numbering scheme of compound
(R,S)-17 (molecule A) in the crystal (Schakal plot) [8]. Selected bond
When lithiated benzyl silane (R,S)-17 is reacted with
one equivalent of HgCl2 in the solvent diethyl ether,
mercury compound (S,S)-20 is obtained mainly with
˚
distances (A) and angles (8): Si(1)Ã
/
C(3) 1.797(7), Li(1)Ã
/
C(3) 2.269(14),
C(3)Ã
/
C(4) 1.419(9), N(1)Ã
/
Li(1) 2.183(12), C(3)ÃSi(1)Ã
/
/C(10) 107.9(3).

156
C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ158
/
Scheme 12.
was isolated in a diastereomeric ratio of d.r.]98:2. In
/
this compound, one labile SMe2 ligand of the starting
material is left cis to the metalated carbon center, as well
as the intramolecularly coordinating nitrogen center of
the SMP substituent. In trans position to the metalated
carbon center is the substitutable chloro ligand. Upon
Fig. 13. B3LYP/6-31ꢂG(d)-optimized structure of monomeric (R,S)-
/
17 and visualization of the highest occupied molecular orbital
(HOMO) (Molekel plot [17]; numbering scheme adopted from Fig.
12).
warming (trans,S,S,R)-21 in vacuo, cis ꢃtrans isomer-
/
ization and inversion at the nitrogen center takes place,
the chloro substituent now in cis position to the
metalated carbon center and both donor centers of the
amine substituent intramolecularly coordinating to the
metal center of palladium compound (cis,S,S,S)-22.
Also this compound can be isolated in a diastereomeric
inversion of configuration at the metalated carbon
center in a diastereomeric ratio of d.r.[(S,S):(R,S)]ꢀ
/
90:10 (Scheme 11).
The molecular structure of (S,S)-20 in the solid state
could be determined and associated with the corre-
sponding NMR data (Fig. 14). The compound crystal-
ratio of d.r.]98:2.
/
Since the nitrogen center has a coordination number
of four in both palladium compounds (Scheme 12, Figs.
15 and 16), it is a stereogenic center with inverted
absolute configuration for compound (trans,S,S,R)-21
and (cis,S,S,S)-22. By analysis of the NMR spectra of
the crude product, it is assumed that the molecule is
formed as mixture of epimers with different absolute
configurations at the nitrogen centers. In one of these
epimers, intramolecular OMe coordination is possible,
followed by conversion into palladium compound
(cis,S,S,S)-22. In the other epimer, an intramolecular
OMe coordination is not possible, what probably makes
the compound crystallize as this stereoisomer (Fig. 15).
Compound (trans,S,S,R)-21 crystallized from
lized from tolueneꢃn-pentane in the orthorhombic
/
crystal system, space group P212121. The molecular
structure of (S,S)-20 is similar to that of non-chiral
mercury compound 6. Only the nitrogen donor center of
the SMP ligand is coordinating to the Group 12 metal
center, the oxygen center does not act as a ligand.
By the reaction of lithium alkyl (R,S)-17 with one
equivalent of trans-[PdCl2(SMe2)2], two interesting
compounds were isolated in high d.r. values from the
reaction mixture (Scheme 12). In the first step at low
temperatures, palladium compound (trans,S,S,R)-21
CH2Cl2ꢃn-pentane in the orthorhombic crystal system,
/
space group P212121. The absolute configuration at the
metalated carbon center C(3) is (S). Thus, the stereo-
chemical course of the metathesis reaction with the
palladium(II) halide is inversion of configuration.
Scheme 11.
Fig. 14. Molecular structure and numbering scheme of compound
˚
(S,S)-20 in the crystal (Schakal plot) [8]. Selected bond distances (A)
Fig. 15. Molecular structure and numbering scheme of compound
(trans,S,S,R)-21 in the crystal (Schakal plot) [8]. Selected bond
˚
and angles (8): SiÃ
C(3)ÃSiÃC(10) 109.8(4), C(3)Ã
/
C(3) 1.875(8), HgÃ
/
C(3) 2.105(6), HgÃ
/N 2.905(6),
distances (A) and angles (8): SiÃ
/
C(3) 1.872(5), PdÃ
/
C(3) 2.117(4),
/
/
/
HgÃCl 173.8(2).
/
PdÃ
/
N 2.183(3), C(3)Ã
/
C(4) 1.501(6), C(3)ÃSiÃC(10) 103.7(2).
/
/

C. Strohmann et al. / Journal of Organometallic Chemistry 661 (2002) 149ꢃ
/
158
157
exchange reactions and neutral or cationic palladium
complexes can be obtained. When the ‘carbanionic’
fragment of lithium alkyl (R,S)-17 is transferred to
palladium, two reactive diastereomerically enriched
palladium alkyls were isolated that bear a stereogenic
carbon center directly bound to an active metal (labile
ligands at the palladium center). We are hopeful to
observe stereoselective reactions at the active metal site
or at the PdÃ
/
C bond of palladium compounds
Fig. 16. Molecular structure and numbering scheme of compound
(cis,S,S,S)-22 in the crystal (Schakal plot) [8]. Selected bond distances
(trans,S,S,R)-21 and (cis,S,S,S)-22.
˚
(A) and angles (8): SiÃ
/
C(3) 1.907(8), PdÃ
/
C(3) 2.071(7), PdÃN 2.112(6),
/
C(3)Ã
/
C(4) 1.521(9), C(3)ÃSiÃC(10) 104.5(3).
/
/
Acknowledgements
Compound (cis,S,S,S)-22 crystallized from CH2Cl2ꢃ
/
n-pentane in the orthorhombic crystal system, space
group P212121 (Fig. 16). When compound
(trans,S,S,R)-21 is transformed into compound
(cis,S,S,S)-22, the absolute configuration at C(3) does
not change, as can be seen from Fig. 16. Thus, the
isomerization at the palladium center does not influence
the stereochemistry at the metalated carbon center.
Both palladium compounds (trans,S,S,R)-21 and
(cis,S,S,S)-22 are reactive molecules (labile ligands in
either cis or trans position) that bear a stereogenic
carbon center in a-position to the active metal fragment.
Very selective reactions at either the metal center or the
We are grateful to the Deutsche Forschungsge-
meinschaft (DFG), especially the Sonderforschungsber-
eich SFB 347, the Fonds der Chemischen Industrie (FCI)
and the Institut fur Anorganische Chemie der Universitat
¨
the grant of a doctoral scholarship.
¨
Wurzburg for financial support. D.S. thanks the FCI for
¨
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/