Preparation and Isolation of a Chiral Methandiide and Its Application as Cooperative Ligand in Bond Activation

Article abstract of DOI:10.1002/chem.201504724

The activation of element-hydrogen bonds by means of metal-ligand cooperation has received increasing attention as alternative to classical activation processes, which exclusively occur at the metal center. Carbene complexes derived from methandiide precu

Full text of DOI:10.1002/chem.201504724

DOI: 10.1002/chem.201504724
Communication
&
Carbene Complexes
Preparation and Isolation of a Chiral Methandiide and Its
Application as Cooperative Ligand in Bond Activation
Kai-Stephan Feichtner, Simon Englert, and Viktoria H. Gessner*[a]
bene ligands actively participate in bond activation reactions
Abstract: The activation of element–hydrogen bonds by
means of metal–ligand cooperation has received increas-
ing attention as alternative to classical activation process-
es, which exclusively occur at the metal center. Carbene
complexes derived from methandiide precursors have
been applied in this chemistry enabling the activation of
a series of EÀH bonds by addition reactions across the M=
C bond. However, no chiral carbene complexes have been
applied to realize stereoselective transformations to date.
Herein, we report the isolation and structure elucidation
of an enantiomerically pure dilithiomethane, which could
be prepared by direct double deprotonation. The ob-
tained dilithium salt was used for the preparation of the
first chiral methandiide-derived carbene complex, which
was applied in stereoselective cooperative SÀH bond acti-
vation.
by means of metal–ligand cooperation involving a transition
from a M=C double to a MÀC single bond.[5] Particularly, nucle-
ophilic carbene complexes with late transition metals have
proven to be applicable in this chemistry.[6] One way to access
these unusual carbene species is the use of methandiide pre-
cursors, such as A–C (Figure 1a).[7] These ligands allowed the
The selective formation of chiral compounds by asymmetric
catalysis fascinates chemists since many years. This field of
chemistry has grown rapidly due to its importance in a variety
of synthetic routes in the chemical and pharmaceutical indus-
try, as well as for the preparation of agrochemicals, fragrances,
or flavors.[1] The transfer of stereoinformation in stereoselective
processes normally relies on the use of chiral ligands. Thus, the
development of new asymmetric transformations often de-
pends on the design of new ligand systems. Most of the metal
complexes employed make use of chiral phosphine ligands as
source of stereoinformation.[2] However, in the past years,
phosphines have often been replaced by carbene ligands. Be-
cause of their strong donor ability, carbenes were found to be
excellent ligands to support robust metal complexes active in
a variety of different reactions,[3] including asymmetric transfor-
mations.[4]
Figure 1. a) Methandiides used in the synthesis of carbene complexes and
b) H2 activation by a carbene complex by metal–ligand cooperation.
isolation of a series of unique complexes with metal–carbon
bonds varying between predominantly electrostatic interac-
tions and “real” metal–carbon double bonds.[8–10] This flexibility
facilitated bond-activation reactions by active participation of
the M=C linkage.[11] For example, carbene complexes derived
from methandiide C could also be applied in reversible bond-
activation (e.g., D to E; Figure 1b) and transfer hydrogenation
reactions.[12,13] Because of this application of methandiides as
cooperating ligands, the preparation of chiral derivatives and
metal complexes thereof would be desirable. Until today, no
chiral carbene complex based on a methandiide ligand has
been reported, which is mainly due to synthetic difficulties in-
volved in the preparation and isolation of such dianionic com-
pounds. Given the stability and accessibility of the sulfonyl
substituted system C, we assumed that the analogous sulfoxi-
mine 1 might be a suitable precursor to selectively access
chiral carbene complexes.[14] Herein, we report the preparation
and isolation of the enantiomerically pure dilithiomethane 1,
as well as its use as carbene ligand and in the cooperative, ste-
reoselective activation of SÀH bonds.
Typically, carbene ligands act as so-called spectator ligands,
solely controlling transformations at the metal center by ma-
nipulating its electronic and steric properties. However, in the
last years, a number of reports have appeared, in which car-
[a] K.-S. Feichtner, S. Englert, Dr. V. H. Gessner
Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: vgessner@uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/forschungs-
gruppen/dr_v_gessner/
For the preparation of the optically pure methandiide 1, the
sulfoximines (S)-2 and (R)-2 were synthesized by a multistep
procedure from thioanisole following literature procedures
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504724.
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(see the Supporting Information for experimental details).[15]
Racemic resolution with camphor sulfonic acid selectively gave
way to both enantiomers in pure form. The sulfoximine was
then converted to 3 by deprotonation at the sulfur bound
methyl group, treatment with Ph2PCl and oxidation with ele-
mental sulfur (Scheme 1). Work-up gave 3 as colorless solid in
equivalents of tBuLi in THF were applied. Although the later
combination could only be used in situ,[17] the PMDTA adduct
could be isolated as highly air- and moisture-sensitive, pale
yellow solid in 52% yield. Unfortunately, all attempts to grow
single crystals of the PMDTA adduct failed, yet NMR data sug-
gest the formation of a 2:1 adduct [(S)-1·(PMDTA)2], which DFT
studies indicated to be monomeric (see the Supporting Infor-
mation for computational details). [D8]Toluene solution of [(S)-
1·(PMDTA)2] exhibited only broadened signals in the 1H, 7Li,
and 31P{1H} NMR spectra at ambient temperature. Variable-tem-
perature (VT) NMR studies showed a sharpening of these sig-
nals upon cooling to À408C, thus indicating exchange process-
es or fluxional behavior at ambient temperature. At À408C,
[(S)-1·(PMDTA)2] is characterized by the absence of any methyl-
ene proton and a singlet at dP =22.4 ppm in the 31P{1H} NMR
7
spectrum. The Li NMR spectrum exhibited two singlets at dLi =
0.60 and 1.28 ppm, which is consistent with the NMR data re-
ported for a monomeric bis(iminophosphinoyl)methandiide.[7d]
Compound [(S)-1·(PMDTA)2] is stable as a solid and can be
stored at ambient temperature for weeks.
Scheme 1. Preparation of the chiral dilithiomethane (S)-1. Molecular struc-
ture of (S)-3. Selected bond lengths [Š] and angles [8]: CÀS 1.824(2), CÀP
1.836(2), PÀS 1.946(1), SÀO 1.457(1), SÀN 1.507(2), SÀCPh 1.762(2), PÀCPh
1.815(2); P-C-S 118.6(1); see the Supporting Information for data of the R-
enantiomer.
The molecular structure of dilithiomethane (S)-1 is depicted
¯
in Figure 2 (triclinic space group P1). The asymmetric unit con-
tains four dimers and additional THF and diethyl ether solvent
molecules. The two subunits of the pseudo-C2 symmetric
dimers consist of one molecule of (S)-1, Me3SiOLi, and two co-
ordinating THF molecules. In the structure, the three lithium
atoms form a Li3 triangle, on top of which the silanolate is co-
ordinating. The metalated carbon atoms exhibit two contacts
to the lithium atoms with bond lengths between 2.153(6) and
2.311(5) Š.[18] The PÀC and SÀC bond lengths within the P-C-S
backbone are considerably contracted compared to the dipro-
tonated precursor 3, the PÀC by 11% and SÀC bond 7%. This
is well in line with other dilithiomethanes[7] and can be attrib-
uted to strong electrostatic interactions involved with the neg-
ative charge on the central carbon atom. Negative hyperconju-
gation effects additionally lead to an elongation of the PÀS, SÀ
O, SÀN, and P/SÀCPh bonds. All lithium atoms are four coordi-
nate with LiÀN, LiÀO, and LiÀS distances in the range of those
reported for related compounds.[19]
57% yield. Compound 3 was characterized by multinuclear
NMR spectroscopy, elemental analysis, and single-crystal X-ray
diffraction analysis. The phosphorus signal appears at dP =
33.1 ppm in the 31P{1H} NMR spectrum, the PCH2S protons as
characteristic AB system at dH =4.38 and 4.71 ppm (JAB
=
2
1
15.2 Hz, JPH =10.3 Hz) in the H NMR spectrum. The molecular
structures of both enantiomers (Scheme 1 and the Supporting
Information) confirmed the configuration at the stereogenic
sulfur atom with bond lengths being in the expected regions.
20
Measurements of the specific rotation gave a value of [a]D
=
63.3Æ1.58 for the R-enantiomer, and a value of [a]D20 =59.6Æ
1.68 for the S-enantiomer.
The direct double deprotonation of 3 was first studied by
using the S-enantiomer. Lithiation experiments were conduct-
ed with different organolithium bases in combination with ni-
trogen ligands for their activation. The conversions were fol-
lowed by 31P{1H} NMR spectroscopy. First attempts with two e-
quivalents of methyllithium or n-butyllithium in THF (both
combinations have successfully been used for the synthesis of
methandiide C) showed only incomplete conversion to the de-
sired methandiide 1 (dP =19.1 ppm). Removal of the solvent
and dissolving in C6D6 or [D8]THF always led to product mix-
tures of 1 and considerable amounts of the monolithiated spe-
cies (dP =29.7 ppm). This was also the case when using an
excess of organolithium reagent. Nevertheless, single crystals
of (S)-1 could be obtained by slow diffusion of pentane into
a THF/Et2O solution of (S)-1 with three equivalents of methyl-
lithium. Under these conditions, the excessive base resulted in
the decomposition of silicone grease and the incorporation of
one equivalent of Me3SiOLi into the crystalline product (see
below).[16] However, for a more convenient preparation of (S)-
1 and its further application, either a 1:1 mixture of tBuLi and
N,N,N’,N’,N’’-pentamethyldiethylenetriamine (PMDTA) or two
With successful isolation of the chiral dilithiomethane, we
next addressed its application as ligand in carbene complexes
to probe its use in stereoselective bond-activation reactions by
means of metal–ligand cooperation. Hence, a THF solution of
(R)-1 was treated with [(p-cymene)RuCl2]2 at À788C, which
upon warming to room temperature resulted in a color
change to blue and the formation of a single new product, as
was evidenced by a new signal at dP =41.3 ppm in the
31P{1H} NMR spectrum. Work-up to remove the formed LiCl af-
forded the desired carbene complex (R)-4 as blue solid in 64%
yield (Scheme 2). Most characteristic for the formation of (R)-4
is the low-field shifted signal of the carbene carbon atom,
which appears as doublet at dC =129.8 ppm with a coupling
constant of 1JPC =59.3 Hz in the 13C{1H} NMR spectrum. This
signal is thus in the range of other nucleophilic carbene com-
plexes.[9] The proton and carbon atoms of the phosphorus-
bound phenyl groups and the cymene ligand are diastereotop-
ic and thus give rise to two sets of signals in the 1H and
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13C{1H} NMR spectrum. Single crystals of (R)-4 were grown by
cooling of a saturated toluene solution to À308C. The molecu-
lar structure confirms the three-coordinate, planar carbon
atom (sum of angles: 356.9(3)8) with a short RuÀC distance of
1.962(6) Š (Figure 3). This bond length is in the range of other
Figure 3. Molecular structures of the chiral carbene complex (RS)-4 (left) and
the activation product (RS,SC,RRu)-5 (right). Hydrogen atoms (except for H1 at
C1 in 5) and benzene solvent are omitted for clarity; displacement ellipsoids
are drawn at the 50% probability level. Selected bond lengths [Š] and
angles [8]: (RS)-4. RuÀC1 1.962(6), RuÀN 2.065(6), RuÀS2 2.7067(17), S2ÀO
1.445(4), S1ÀP 1.977(2), S2ÀN 1.583(5), PÀC1 1.757(7), PÀC2 1.801(7); S2-C1-P
123.5(4), S2-C1-Ru 95.5(3), P-C1-Ru 137.9(3); 5: RuÀN 2.144(2), RuÀC1
2.160(3), RuÀS3, S1ÀP 1.9520(10), S2ÀN 1.553(2), 2.4291(8), S2ÀC1 1.756(2),
PÀC1 1.812(3); S2-C1-P 114.38(13).
known Ru=C double bonds.[13a,20] The PÀC and SÀC distances
are longer than in dilithiomethane 1 and thus reflect an effi-
cient charge transfer from carbon to the metal when going
from the methandiide to the carbene complex. The M=C
double bond character in (R)-4 was also confirmed by natural
bond orbital (NBO) analysis showing s and p interactions,
which are almost equally distributed between both atoms
(61.1 and 63.8% at C) yet with a slight polarization towards
the carbon atom (qCNBO =À0.95). The Wiberg bond index of
WBIRuÀC =1.04 is in the range of other nucleophilic carbene
complexes with late transition metals (see the Supporting In-
formation for details).[9] Contrary to D (Figure 1), the ligand in
(R)-4 selectively coordinates to the metal through the sulfoxi-
mine nitrogen and not through the thiophosphinoyl moiety
due to stronger coordination ability of nitrogen. It is interest-
ing to note, that the p-cymene ligand coordinates to the metal
in that way, that its iPr moiety arranges trans to the phenyl
group of the sulfoximine and thus minimizes repulsion be-
tween the two groups. The diastereotopic splitting of the sig-
nals of the cymene ligand in the NMR spectra suggests that
this arrangement is also present in solution with no fluxional
behavior at ambient temperature.
Figure 2. Top: molecular structure of the chiral dilithiomethane (S)-1 crystal-
lized with Me3SiOLi [((S)-1·Me3SiOLi·THF)2]. Bottom: its monomeric subunit.
Further molecules of 1 and additional solvent molecules are omitted for
clarity; displacement ellipsoids are drawn at the 50% probability. Selected
bond lengths [Š] and angles [8] (’ corresponds to further dimers in the asym-
metric unit): P1ÀC1 1.719(3), P2ÀC21 1.721(3), P1’ÀC1’ 1.722(3), P2’ÀC21’
1.715(3), P1’’ÀC1’’ 1.705(3), P2’’ÀC21’’ 1.710(3), S2ÀC1 1.622(3), S4ÀC21
1.620(3), S2’ÀC1’ 1.619(3), S4’ÀC21’ 1.627(3), S1ÀP1 2.0030(11), S3ÀP2
2.010(1), S1’ÀP1’ 2.0040(10), S3’ÀP2’ 2.0070(10), S2ÀO1 1.492(2), S2ÀN1
1.589(2), S4ÀO2 1.495(2), S4ÀN2 1.586(2), S2’ÀO1’ 1.494(2), S2’ÀN1’ 1.583(2),
S4’ÀO2’ 1.500(2), S4’ÀN2’ 1.580(2), C1ÀLi3 2.180(6), C1ÀLi1 2.315(6), C21ÀLi6
2.179(6), C21ÀLi5 2.311(5), C1’ÀLi3’ 2.190(5), C1’ÀLi1’ 2.295(5), C21’ÀLi6’
2.202(6), C21’ÀLi5’ 2.289(5); S2-C1-P1 117.86(16), S4-C21-P2 117.35(16), S2’-
C1’-P1’ 116.1(2), S4’-C21’-P2’ 116.8(2).
With successful isolation of the first chiral methandiide de-
rived carbene complex, its application in stereoselective bond-
activation chemistry by means of metal–ligand cooperation
was probed. Treatment of a solution of (RS)-4 in toluene at
À788C with tert-butylthiol resulted in an immediate color
change from blue to red and the selective formation of
a single new product (dP =42.4 ppm). Isolation gave the SÀH
Scheme 2. Preparation of carbene complex (RS)-4 and its use in stereoselec-
tive cooperative SÀH activation.
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activation product (RS,SC,RRu)-5, which was isolated as red solid
in 95% yield. The NMR spectra of 5 (as well as of the crude re-
action product) showed only one set of signals, thus being in
line with the stereoselective bond activation and the formation
of a single diastereoisomer via addition of the SÀH bond from
only one side of the former Ru=C double bond. X-ray crystal-
lography revealed the formation of the (RS,SC,RRu)-isomer of the
thiolato complex (Figure 3, right). Thereby, this isomer is
formed by the selective attack of the thiol from the side of the
Ru=C bond trans to the iPr group of the p-cymene ligand. In
the molecular structure (orthorhombic space group P212121),
the RuÀC1 bond is elongated (2.160(3) Š) compared to the car-
bene complex, thus being in line with a transition from a Ru=C
double bond to a single bond. Overall, the activation occurred
through a metal–ligand cooperation with the carbene ligand
serving as proton acceptor. To the best of our knowledge, such
a stereoselective cooperative bond-activation reaction by a car-
bene ligand has never been reported before.
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In conclusion, we have reported the preparation and isola-
tion of an enantiomerically pure dilithiomethane and its use as
ligand for the preparation of the first chiral carbene complex
of its kind. The complex allowed the activation of the SÀH
bond in tBuSH by metal–ligand cooperation, which resulted in
the stereoselective formation of the thiolato complex. This has
been the first example of the use of a methandiide-derived
carbene complex in stereoselective transformations. Future
studies are focusing on the application in further activation re-
actions, as well as stoichiometric and catalytic asymmetric
transformations.
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Acknowledgements
We thank the DFG (Emmy Noether grant), the Dr.-Otto-Rçhm-
Gedächtnisstiftung, and the Fonds der Chemischen Industrie
for financial support. We also thank Rockwood Lithium GmbH
for the supply of chemicals.
Keywords: alkali metals · bond activation · carbenes · lithium ·
structure elucidation
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Received: November 24, 2015
Published online on December 8, 2015
Chem. Eur. J. 2016, 22, 506 – 510
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