Lithiated sulfoxides: α-Sulfinyl functionalized carbanions

Article abstract of DOI:10.1039/c4dt02238f

Reactions of alkyl aryl sulfoxides H-CRR′S(O)Ar with n-BuLi-TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) afforded α-sulfinyl functionalized alkyl aryl lithium compounds of the type [Li₂{CRR′S(O)Ar}₂(TMEDA)₂] (1, R/

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Lithiated sulfoxides: α-sulfinyl functionalized
carbanions†
Cite this: DOI: 10.1039/c4dt02238f
a
b
a
a
b
Gerd Ludwig, Tobias Rüffer, André Hoppe, Till Walther, Heinrich Lang,
a
a
Stefan G. Ebbinghaus and Dirk Steinborn*
Reactions of alkyl aryl sulfoxides H–CRR’S(O)Ar with n-BuLi–TMEDA (TMEDA = N,N,N’,N’-tetramethyl-
ethylenediamine) afforded α-sulfinyl functionalized alkyl aryl lithium compounds of the type [Li {CRR’S-
O)Ar} (TMEDA) ] (1, R/R’ = H/H, Ar = Ph; 2, R/R’ = H/H, Ar = p-Tol; 3, R/R’ = Me/Me, Ar = Ph; 4, R/R’ =
H/Ph, Ar = Ph; 5, R/R’ = Me/Ph, Ar = Ph). The compounds were characterized by H, C and Li NMR
spectroscopy and, except for 5, by single-crystal X-ray diffraction analyses. In crystals of 1, 2, 3 and 4·Et
dinuclear molecules with four-membered Li rings were found. There are no Li⋯C contacts, thus,
free” carbanions are the main structural feature. Reactions of 1–6 (6, R/R’ = H/Me, Ar = Ph) with benz-
aldehyde and benzophenone afforded the corresponding sulfoxides of the type ArS(O)CRR’CHPhOH
1a–6a) and ArS(O)CRR’CPh OH (1b–6b), respectively. The reactions yielding 1a/3a and 4b/6b proceeded
with high diastereoselectivities. By X-ray diffractometry it has been shown that in the case of 3a and 4b
the diastereomers consisting of the two enantiomers S and R were formed.
2
(
2
2
1
13
7
2
O
2
O
2
α
“
Received 23rd July 2014,
Accepted 25th September 2014
(
2
DOI: 10.1039/c4dt02238f
www.rsc.org/dalton
R
S C
S C
S
2
0–31
findings,
it was stated that the diastereofacial differen-
Introduction
tiation in reactions of α-sulfinyl functionalized alkyl aryl
Sulfur is one of the most employed elements in organic lithium compounds is determined by the chiral sulfinyl group
1
–4
synthesis and three typical sulfur containing functionalities as follows: electrophiles with an oxygen-containing group like
5
–7
are sulfides, sulfoxides and sulfones. An important property D O, benzaldehyde, benzophenone or CO tend to attack the
2
2
of the sulfur atom is its ability to activate an α-hydrogen atom anionic C atom on the side of the S–O bond due to an attrac-
+
of an attached alkyl group, with an increased activation in the tive interaction of the electrophile with the countercation Li .
order –S– < –S(O)– < –S(O) –. Thus, sulfanyl-, sulfinyl-, and sul- In the case of CH I the electrophile approaches from the oppo-
2
3
fonyl alkyl carbanions can be readily generated by lithiation of site side, because of the lack of an attractive interaction with
+
32,33
the corresponding sulfides, sulfoxides and sulfones, the countercation Li as shown in Fig. 1.
respectively.
On that basis and
8
–11
α-Sulfinyl functionalized alkyl aryl lithium under consideration of quantum chemical calculations of
compounds of the type Li[CRR′S(O)Ar] are of special interest, Wolfe et al., the “ion-pair model” for α-sulfinyl functionalized
because on the one hand they possess a Lewis-basic hetero- alkyl aryl lithium compounds of the type Li[CRR′S(O)Ar] was
7
,34,35
atom (nonbonding electron pair at the sulfur atom) and on the established (cf. Fig. 1).
The first X-ray crystal structure
1
2
other hand a dipole stabilized heteroatomic center. Gene- analysis of
a
lithiated sulfoxide, [Li {CMePhS(O)Ph}
rally, sulfinyl functionalized alkyl aryl lithium compounds are (TMEDA) ], was reported by Boche et al. in 1986. The lithium
2
2
-
3
6
2
good synthons for enantioselective and diastereoselective compound crystallized in a dimer fashion with a Li O four-
2
2
1
3–17
syntheses.
Thus, Durst et al. found that methylation and membered ring as main structural feature. Furthermore, this
deuteration of lithiated sulfoxides proceeded with good dia- lithium compound is characterized by a “free” carbanion,
1
8,19
stereoselectivities.
On the basis of these and further
a
Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-
Straße 2, D-06120 Halle, Germany. E-mail: dirk.steinborn@chemie.uni-halle.de
b
Institute of Chemistry, Chemnitz University of Technology, Straße der Nationen 62,
D-09107 Chemnitz, Germany
†
Electronic supplementary information (ESI) available: Protocols of the
quantum chemical calculations, structural data and Wiberg bond indices of 1–4
and 1*–4*, respectively, tables and figures of molecular structures of 3a′, 4a′ and
Fig. 1 Diastereofacial differentiation for reactions of lithiated sulfoxides
(S atom is hidden by the α-C atom) with electrophiles according to the
“ion-pair model” (adapted from ref. 7).
4b′. CCDC 1014086–1014092. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt02238f
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+
meaning that there are no interactions between Li and the (–0.25 to 0.55 ppm). The virtually identical TMEDA resonances
carbanionic C atom. Thus, the formerly postulated “ion-pair in 1–5 might indicate that in solution the TMEDA ligand is
model” was found to be in accordance with the crystal struc- partially cleaved off (dynamic coordination/decoordination) as
3
7
ture determination of Boche et al. Since then no further crystal also observed in other cases.
structure analyses of lithiated sulfoxides have been described.
Solid-state structures of lithiated alkyl aryl sulfoxides
Here, we report on the synthesis, characterization and solid
state structures of lithiated sulfoxides of the type [Li
S(O)Ar} (TMEDA) ]. Furthermore, the stereoselective C–C bond Tol} (TMEDA) ] (2), [Li {CMe S(O)Ph} (TMEDA) ] (3) and
2
{CRR′- Crystals of [Li {CH S(O)Ph} (TMEDA) ] (1), [Li {CH S(O)p-
2
2
2
2
2
2
2
2
2
2
2
2
2
2
formation of the lithiated sulfoxides with benzaldehyde and [Li {CHPhS(O)Ph} (TMEDA) ]·Et O (4·Et O) suitable for X-ray
2
2
2
2
2
benzophenone was investigated under the aspect of a diastereo- diffraction analyses were obtained from diethyl ether solutions
facial differentiation.
at −7 °C. All compounds crystallized as isolated dinuclear
molecules without unusual intermolecular interactions (short-
est distance between non-hydrogen atoms: 3.606(2) Å,
C14⋯C17′, 1; 3.003(6) Å, C19⋯C9′, 2; 3.646(3) Å, C9⋯C9′, 3;
Results and discussion
Synthesis of lithiated alkyl aryl sulfoxides
2
3.362(2) Å, C32⋯C14B′, 4·Et O). The asymmetric unit of 1 con-
tains two symmetrically independent molecules with very
similar structures. The dinuclear compound 3 exhibits crystal-
lographically imposed C symmetry. The molecular structures
2
Lithiated alkyl aryl sulfoxides of the type [Li {CRR′S(O)-
Ar} (TMEDA) ] (1–5) were prepared by metallation of the
2 2
2
are presented in Fig. 2–5. Selected structural parameters are
given in the respective figure captions, although, in respect of
the quality of the structure solutions, this discussion must not
be exaggerated.
As for the structure described by Boche, the central build-
ing blocks of the dinuclear molecules of 1–4 are four-mem-
corresponding sulfoxide with n-BuLi/TMEDA in diethyl ether/
n-pentane according to Scheme 1. The products were obtained
as strongly moisture- and oxygen-sensitive yellowish crystals in
yields between 61 and 84%. All complexes were characterized
3
6
1
13
7
by NMR spectroscopy ( H, C, Li) and, with exception of 5, by
single-crystal X-ray structure analyses. Selected NMR spectro-
scopic parameters of complexes 1–5 are given in Table 1. The
bered Li O rings with Li–O bond lengths between 1.843(1) Å
2
2
1
13
and 1.917(9) Å and Li1⋯Li2 (respectively, Li1⋯Li1′ for 3)
distances between 2.558(1) and 2.646(8) Å. The dihedral angles
O1–Li1⋯Li2–O2 (respectively, O1–Li1⋯Li1′–O1′ for 3) between
H and C NMR spectra of 1–5 give proof of the selective
metallation of the α-C atoms of the alkyl aryl sulfoxides. Thus,
an ortho-metallation of the aryl rings can be fully excluded.
13
159.1(5)° and 163.9(6)° show that the rings are folded. Thus,
The C chemical shifts of the α-C atoms are in direct relation
to their substitution pattern: the resonances of the unsubsti-
tuted α-C atoms in 1 and 2 were found at about 35 ppm,
whereas those of the substituted ones (3–5) are located
between 54 and 65 ppm. On the other hand, in all lithiated
the Li rings in 1–4 are structurally similar to those in other
2 2
O
lithium compounds (Li–O: median 1.917 Å, lower/upper quar-
tile 1.851/1.984 Å; Li⋯Li′: median 2.607 Å, lower/upper quar-
7
compounds 1–5 the Li resonances are in a narrow range
Fig. 2 Structure of one of the two symmetrically independent mole-
Scheme 1 Synthesis of lithiated sulfoxides [Li
1–5).
2 2 2
{CRR’S(O)Ar} (TMEDA) ]
cules [Li
2 2 2 2
{CH S(O)Ph} (TMEDA) ] in crystals of 1 (thermal ellipsoids at
(
30%; only the major occupied position of disordered atoms is displayed).
For reasons of clarity, only the hydrogen atoms of the α-C atoms are
shown. Selected distances (in Å) and angles (in °) (values of the two sym-
metrically independent molecules are given separated by a slash): Li1–
O1 1.878(8)/1.909(8), Li2–O2 1.882(8)/1.900(8), Li1⋯Li2 2.614(1)/
Table 1 Selected NMR spectroscopic data (δ in ppm) of lithiated sulfox-
ides of the type [Li
2
2 2
{CRR’S(O)Ar} (TMEDA) ](1–5)
2
1
.634(1), S1–O1 1.576(3)/1.568(3), S2–O2 1.574(3)/1.573(3), S1–C1
.660(5)/1.611(8), S2–C8 1.606(7)/1.645(6), S1–C2 1.809(5)/1.811(5), S2–
C9 1.817(5)/1.819(5), Li1⋯C8 3.775(1)/3.907(1), Li2⋯C1 3.889(9)/3.898(1),
0.55 O1–S1–C1 116.9(3)/118.3(4), O2–S2–C8 117.9(3)/117.3(3), O1–S1–C2
R/R′
Ar
δ
α-H
δ
α-C
δ
Li
1
2
3
4
5
H/H
H/H
Me/Me
H/Ph
Me/Ph
Ph
p-Tol
Ph
Ph
Ph
1.96
1.88
—
3.77
—
34.7
36.3
53.8
65.3
62.2
0.10
−0.10 98.6(2)/99.0(2), O2–S2–C9 97.9(2)/98.0(2), C1–S1–C2 101.3(2)/101.6(4),
−0.25 C8–S2–C9 102.0(3)/102.1(3), O1–Li1–O2 90.9(3)/90.8(3), O1–Li1⋯Li2–
−0.20
O2 159.1(5)/160.0(5).
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tile 2.481/2.759 Å; O–Li⋯Li′–O′: median 170.5°, lower/upper
3
8
quartile 163.4/174.9°, n = 398). Two nitrogen atoms of the
chelating TMEDA ligands complete the distorted coordination
tetrahedrons around the lithium atoms. The shortest Li⋯C
α
distance (3.769(7) Å) was found in 4·Et O. Thus, in no com-
2
pound is an evidence for an intramolecular interaction
+
between Li and the carbanionic α-C atom.
For a more detailed description of the molecular structures
of the dinuclear lithium compounds 1–4 quantum-chemical
calculations on the DFT level of theory were performed using
the B3LYP functional and high-quality basis sets for all atoms
2 2 2 2
Fig. 3 Molecular structure of [Li {CH S(O)p-Tol} (TMEDA) ] in crystals
(
details see Experimental). In all calculations solvent effects
of 2 (thermal ellipsoids at 30%; only the major occupied position of dis-
ordered atoms is displayed). For reasons of clarity, only the hydrogen
atoms of the α-C atoms are shown. Selected distances (in Å) and angles
(
THF) were considered according to Tomasi’s polarized conti-
3
9–41
nuum model.
Comparison of the solid-state structures
(TMEDA) ] (1–4) with the analogous calcu-
(
in °): Li1–O1 1.843(1), Li2–O2 1.878(9), Li1⋯Li2 2.558(1), S1–O1 1.583(4), [Li
2
{CRR′S(O)Ar}
2
2
S2–O2 1.581(4), S1–C1 1.572(1), S2–C9 1.665(7), S1–C2 1.844(4), S2– lated structures 1*–4*‡ revealed a good agreement including
C10 1.821(3), Li1⋯C9 4.007(1), Li2⋯C1 4.039(2), O1–S1–C1 120.9(7),
O1–S1–C2 97.2(2), C1–S1–C2 103.4(6), O1–Li1–O2 93.9(5), O1–
Li1⋯Li2–O2 163.9(6).
the conformation of the Li O rings and the Li⋯C distances
2
2
α
(
cf. Table S1†). Of special interest is the degree of pyramidaliza-
tion of the carbanionic α-C atoms, which can be evaluated by
the sum of angles around the α-C atom and its deviation Δd
from the plane spanned by its substituents (S, C/H, C/H) (cf.
Table S2†). For 1*–3* (R/R′ = H/H, Me/Me) slightly pyramida-
lized α-C atoms were found (sum of angles: 347.0–354.7°; Δd =
0.210–0.260 Å), as also found in crystals of 3 (sum of angles:
355.0°; Δd = 0.208 Å). On the other hand, the sulfinyl substi-
tuted benzyl carbanion in 4* (R/R′ = H/Ph) is almost trigonal-
planar (sum of angles: 359.5°; Δd = 0.059 Å), as also in the
compound with R/R′ = Me/Ph (sum of angles: 357.0°; Δd =
3
6
0
.120 Å). This planarization can be traced back to a π bond
between the p orbitals of the carbanions and the phenyl rings.
1
3
Similar conclusions were drawn from C NMR spectroscopic
data of Li[CHPhS(O)Me], Li[CHPhS(O)t-Bu] and Li[CHPh-
Fig. 4 Molecular structure of [Li
(thermal ellipsoids at 30%). H atoms have been omitted for clarity. S(O)
Selected distances (in Å) and angles (in °): C1–C2 1.499(3), C1–C3
.499(3), Li1–O1 1.890(4), Li1⋯Li1’ 2.609(7), S1–O1 1.583(1), S1–C1 1.646(2),
S1–C4 1.809(2), Li1⋯C1 4.144(4), S1–C1–C2 115.2(1), S1–C1–C3 123.5(1),
C2–C1–C3 116.4(2), O1–S1–C1 119.6(9), O1–S1–C4 99.0(8), C1–S1–C4
2 2 2 2
{CMe S(O)Ph} (TMEDA) ] in crystals of
4
2,43
3
2
t-Bu].
Furthermore, in all compounds the S–C
α
bond lengths,
1
both the experimental and the calculated values, were found
to be significantly shorter (1.572(1)–1.666(5)/1.677–1.687 Å,
1
1
–4/1*–4*) than the S–C
.826–1.834 Å, 1–4/1*–4*), albeit the C atoms are clearly sp
i
bond lengths (1.795(3)–1.844(4)/
102.4(1), O1–Li1–O1’ 92.0(2), O1–Li1⋯Li1’–O1’ 162.4(2).
2
i
hybridized. An analysis of the Wiberg bond indices (WBI) of
the S–C bonds in the lithiated sulfoxides 1*–4* and the corres-
ponding nonlithiated (neutral) sulfoxides H–CRR′S(O)Ar
exhibited – as expected – extraordinarily strong S–C
α
bonds in
1*–4* (WBI = 1.24–1.35) compared to those in the neutral sulf-
oxides (WBI = 0.85–0.91) and also to the S–C bonds (1*–4*:
i
WBI = 0.89–0.91; neutral sulfoxides: WBI = 0.90), cf. Table S3.†
The strengthening of the S–C bonds can be attributed to a sta-
α
bilizing interaction between the nonbonding orbital on the
α-carbon atoms and the S–O antibonding σ* orbital in the
Fig. 5 Molecular structure of [Li
tals of 4·Et O (thermal ellipsoids at 30%; only the major occupied posi-
tion of disordered atoms is displayed). For reasons of clarity, only the
2 2 2
{CHPhS(O)Ph} (TMEDA) ] (4) in crys-
3
1,44
sense of a negative hyperconjugation.
The negative hyper-
2
conjugation can be reasonably assumed because the nonbond-
hydrogen atoms of the α-C atoms are shown. Selected distances (in Å) ing electron pairs at the α-C atoms are bent only 16.4 to 18.4°
and angles (in °): C1–C2 1.447(4), C14A–C15A 1.448(7), Li1–O1 1.903(6),
Li1–O2 1.902(6), Li1⋯Li2 2.646(8), S1–O1 1.565(2), S2A–O2 1.573(2), S1–
C1 1.641(3), S2A–C14A 1.666(5), S1–C8 1.795(3), S2A–C21A 1.826(5),
Li1⋯C1 3.888(6), Li2⋯C14A 3.769(7), S1–C1–C2 122.2(2), S2A–C15A–
C14A 123.3(4), O1–S1–C1 115.7(1), O1–S1–C8 100.8(1), C1–S1–C8
away from the antiperiplanar position to the S–O bond (cf.
Table S2†).
1
02.3(1), O1–Li1–O2 91.0(3), O1–Li1⋯Li2–O2 162.7(3).
‡Here and in the following calculated structures are marked with a star.
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Reactions of lithiated alkyl aryl sulfoxides with benzaldehyde
and benzophenone
molecular O2–H⋯O1′ hydrogen bonds (O2⋯O1′ 2.727(1) Å,
O2–H⋯O1′ 170°), resulting in formation of six-membered
chains (graph set: C(6)). In crystals of 4a′ the molecules are
4
5
Reactions of the racemic lithiated sulfoxides 1–5 and of
connected by O1–H⋯O2′ hydrogen bonds (O1⋯O2′ 2.710(1) Å,
[
Li {CHMeS(O)Ph} (TMEDA) ] (6; prepared as described above
2 2 2
O1–H⋯O2′ 170°) such that centrosymmetric dinuclear units
but without isolation) with benzaldehyde and benzophenone
generated the corresponding sulfoxides of the type ArS(O)CRR′
CHPhOH (1a–6a; Scheme 2, pathway A) and ArS(O)CRR′
2
2
are formed (graph set: R
(12)). In contrast to 3a′ and 4a′, in
molecules of 4b′ intramolecular O1–H⋯O2 hydrogen bonds
O1⋯O2 2.710(1) Å, O1–H⋯O2 158°) exist thus forming six-
membered rings (graph set: S(6)). According to the distance
(
CPh
2
OH (1b–6b; Scheme 2, pathway B), respectively. The pro-
ducts were isolated as colorless crystals in yields between 61
4
6,47
1
13
criterion
moderately strong. Since 3a′, 4a′ and 4b′ crystallize in the
centrosymmetric space group P2 /c the crystals are racemic
and, hence, two enantiomers (S and R , 3a′; R and
S R R , 4a′; S R and R S , 4b′) are present.
all these hydrogen bonds can be characterized as
and 89% and characterized by NMR spectroscopy ( H, C)
and, in part, by single-crystal X-ray structure analyses. Most of
the products obtained exist as diastereomers; the diastereo-
meric excess (de) of the reactions ranged between 14 and 94%
1
S
R
C
S
S
C
S C C
S S
S
C
C
S
C
S C
(
cf. Table 2).§ Recrystallization from diethyl ether–n-pentane of
the diastereomeric mixtures of 3a (>94% de), 4a (60% de) and
b (>94% de) resulted in crystals of PhS(O)CMe CHPhOH (3a′),
4
2
About the diastereoselectivity
2
PhS(O)CHPhCHPhOH (4a′) and PhS(O)CHPhCPh OH (4b′)
suitable for X-ray diffraction analyses. The molecular struc- Regarding the diastereoselection in the asymmetric C–C bond
tures of 3a′, 4a′ and 4b′ are presented in Fig. S1–S3,† selected formation of the lithium compounds 1–6 with benzaldehyde
structural parameters are given in the respective figure cap- and benzophenone, two reaction types have to be distin-
tions. In crystals of 3a′ the molecules are connected by inter- guished, namely that of an achiral carbanion center with a pro-
chiral electrophile (type I) and vice versa (type II). The
following discussion is restricted to the highly diastereoselec-
tive reactions where the configuration of the products is
known from single-crystal X-ray measurements, see the for-
mation of 3a and 4b for a type I and type II reaction, respect-
ively (Scheme 3). Exemplarily, for 4b it has been shown that
the single crystal measured is identical with the bulk material
by measuring an X-ray powder diffractogram of it and compar-
ing it with the pattern simulated from the X-ray single crystal
data, see Fig. S4.† The nearly perfect match of the two diffrac-
Scheme 2 Reaction of lithiated sulfoxides with benzaldehyde (A) and
benzophenone (B).
Table 2 Diastereoselectivities of reactions of lithiated sulfoxides 1–6
with benzaldehyde and benzophenone
Benzaldehyde
1a–6a)
Benzophenone
(1b–6b)
(
a
a
R/R′
Ar
no. c.c.
de in %
no. c.c.
de in %
b
1a/b
2a/b
3a/b
4a/b
5a/b
6a/b
H/H
H/H
Me/Me
H/Ph
Me/Ph
H/Me
Ph
p-Tol
Ph
Ph
Ph
2
2
2
3
3
3
>94
20
>94
60
14
62
1
1
1
2
2
2
n/a
n/a
n/a
c>94
Ph
>94
a
b
c
Number of chiral centers in the product. n/a = not applicable. For
unknown reasons, isolation of 5b as desired product failed.
Scheme 3 Nucleophilic addition of [Li
the prochiral benzaldehyde (type I) and of [Li
(4) to benzophenone (type II), resulting in the formation of diastereo-
S C
merically pure 3a-S /R and 4b-S /R (shown with the S R
enantiomers as example), respectively. The six-membered cyclic tran-
sition states ts1 and ts2 are explicitly shown.
2
{CMe
2
S(O)Ph}
{CHPhS(O)Ph}
2
(TMEDA)
2
] (3) to
2
2
(TMEDA) ]
2
1
§
de Values were determined by H NMR spectroscopy (% de = % major dia-
stereomer – % minor diastereomer). The signal-to-noise ratios allowed to detect
impurities ≥3%. With the assumption that this amount of the minor diastereo-
mer remains to be undetected, an upper limit for de values of 94% is given.
S
R
C
S
S
C
S
R
C
S
S
C
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tograms gives proof that only the 4b-S
formed in the reaction.
S
R
C
/R
S
S
C
diastereomer is Na/benzophenone and freshly distilled prior to use. NMR
1
13
7
spectra ( H, C, Li) were recorded, if not otherwise stated, at
1
To understand the preferential formation of the diastereo- 27 °C on Varian Gemini 200 and VXR 400 spectrometers. H
1
3
mers (3a-S
S C S C S C S C 8
R /R S and 4b-S R /R S ) in these reactions, two and C chemical shifts are relative to solvent signals (THF-d ,
assumptions have to be made: first, according to the “ion-pair δH 1.72, δ 67.21; CDCl , δH 7.26, δC 77.16) as internal refer-
C
3
7
7
model” (Fig. 1) for these two electrophiles (PhCHO and ences; Li NMR spectra were referenced to a solution of LiCl in
Ph CO) a precoordination of the carbonyl O atom at the O (external). The preparative centrifugally accelerated thin
lithium center can be assumed. Second, in accordance with layer chromatography was performed using a Chromatotron
2
D
2
2
4,48–50
other reactions of lithiated sulfoxides
aldol reactions
and also with (Harrison Research). Sulfoxides were prepared according to lit-
5
1,52
53
the formation of six-membered cyclic tran- erature procedures.
Preparation of [Li {CRR′S(O)Ar} (TMEDA) ] (1–5). At room
sition states as given in Scheme 3 can be assumed. Based on
2
2
2
the experimental findings, the diastereoselectivities observed temperature to a solution of the respective racemic sulfoxide
in this work can be explained in the following way: the for- (1.0 mmol) in diethyl ether (5 mL) a solution of n-BuLi/TMEDA
mation of the S R - and R S -configured products 3a and (1.0 mmol; 1.6 M in n-hexane) in n-pentane (2 mL) was added
S
C
S C
4
b suggests that the two reactions proceed via the transition while stirring. After two hours the volume of the solution was
state ts2.
reduced in vacuum to about 1.5 mL. After 6–12 h, yellowish
crystals precipitated which were filtered off, washed with
n-pentane (3 × 5 mL) and dried in vacuo.
1
1
(R/R′ = H/H, Ar = Ph). Yield: 220 mg (84%). H NMR
Conclusions and summary
(
400 MHz, THF-d ): δ 1.96 (s, br, 2H, LiCH ), 2.16 (s, 12H, 2 ×
8
2
Lithiated sulfoxides of the type [Li
2
{CRR′S(O)Ar}
2
(TMEDA)
2
]
N(CH
3
)
2
, TMEDA), 2.31 (s, 4H, 2 × CH
2
, TMEDA), 7.18–7.22 (m,
(
1–5) were prepared and their constitution was unequivocally 1H, p-H, SOPh), 7.28–7.32 (m, 2H, m-H, SOPh), 7.92–7.94 (m,
1
3
confirmed by NMR and single crystal X-ray diffraction studies. 2H, o-H, SOPh). C NMR (100 MHz, THF-d ): δ 34.7 (s, LiCH ),
8
2
The lithium compounds crystallize in a dimer fashion with 46.2 (s, N(CH
Li four-membered rings and “free” carbanions as main p-C, SOPh), 127.9 (s, o-C, SOPh), 128.0 (s, m-C, SOPh), 155.0 (s,
structural features. As additionally revealed by quantum- i-C, SOPh). Li NMR (194 MHz, THF-d ): δ 0.1 (s, Li).
3 2 2
) , TMEDA), 58.8 (s, CH N, TMEDA), 127.8 (s,
2 2
O
7
8
1
chemical calculations the carbanionic center is stabilized by a
2 (R/R′ = H/H, Ar = p-Tol). Yield: 188 mg (68%). H NMR
and remains to be slightly pyra- (400 MHz, THF-d ): δ 1.88 (s, br, 1H, LiCH ), 2.14 (s, 12H, 2 ×
midalized with exception of the benzylic one in 4. N(CH ) , TMEDA), 2.29 (s, 3H, CH ), 2.31 (s, 4H, 2 × CH ,
C–C bond formation in reactions of 1–3 having an achiral TMEDA), 7.09–7.11 (m, 2H, m-H, SOTol), 7.80–7.82 (m, 2H, o-
3
1,44
negative hyperconjugation
8
2
3
2
3
2
1
3
α-C atom with a prochiral electrophile (PhCHO, type I) and of H, SOPh). C NMR (100 MHz, THF-d
8 3
): δ 22.0 (s, CH ), 36.3 (s,
4
and 6 having a chiral α-C atom with an achiral electrophile LiCH ), 47.2 (s, N(CH ) , TMEDA), 59.7 (s, CH N, TMEDA),
2
3 2
2
(
2
Ph CO, type II) proved to proceed with high diastereoselectiv- 126.9 (s, p-C, SOTol), 129.1 (s, o-C, SOTol), 129.7 (s, m-C,
7
8
ities (de >94%) with one exception (formation of 2a). In two SOTol), 138.3 (s, i-C, SOTol). Li NMR (194 MHz, THF-d ):
selected cases (type I: 3 + PhCHO; type II: 4 + Ph CO) it has δ 0.55 (s, Li).
2
1
been shown that only the S
formed and not the S /R diastereomers. The stereoselec- (400 MHz, THF-d
tion of analogous reactions (lithiated methyl 1-naphtyl sulfox- N(CH ) , TMEDA), 2.30 (s, 4H, 2 × NCH , TMEDA), 7.25–7.80
S
C
R /R
S
S
C
diastereomers were
3 (R/R′ = Me/Me, Ar = Ph). Yield: 200 mg (69%). H NMR
S
S
C
S
R
C
8 3
): δ 1.11 (m, 6H, 2 × CH ), 2.15 (s, 12H, 2 ×
3
2
2
1
3
ide with aromatic ketones) has been traced back to stabilizing (m, 5H, SOPh). C NMR (100 MHz, THF-d
8
): δ 12.1/15.5 (s/s, 2
, TMEDA), 53.8 (s, C(CH ), 57.9 (s,
2
6
π⋯π interactions between the aromatic rings which might × CH
3
), 45.2 (s, N(CH
3
)
2
3 2
)
also be the case in the systems described here. On the other CH N, TMEDA), 124.5 (s, p-C, SOPh), 128.4 (s, m-C, SOPh),
2
7
hand, the combination of both a chiral α-C atom and a chiral 130.1 (s, o-C, SOPh), 143.8 (s, i-C, SOPh). Li NMR (194 MHz,
electrophile led to the formation of three stereogenic centers THF-d
and to de values between 14 and 62% only. 4 (R/R′ = H/Ph, Ar = Ph). Yield: 206 mg (61%). H NMR
Thus, the present investigations allow a deeper insight into (400 MHz, THF-d ): δ 2.15 (s, 12H, 2 × N(CH , TMEDA), 2.31
the molecular structures of lithiated sulfoxides and their reac- (s, 4H, 2 × NCH , TMEDA), 3.77 (s, 1H, LiCHPh), 6.15 (m, 1H,
tivity against electrophiles in asymmetric C–C bond formation p-H, CHPh), 6.70 (m, 4H, o-/m-H, CHPh), 7.20 (t, J
8
): δ −0.1 (s, Li).
1
8
3 2
)
2
3
= 7.1 Hz,
H,H
3
reactions.
1H p-H, SOPh), 7.28 (t, JH,H = 7.4 Hz, 2H, m-H, SOPh), 7.77 (d,
3
13
J
H,H = 7.6 Hz, 2H, o-H, SOPh). C NMR (100 MHz, THF-d
8
):
δ 45.2 (s, N(CH ) , TMEDA), 57.9 (s, CH N, TMEDA), 65.3 (s,
3
2
2
LiCHPh), 112.5/115.2 (s/s, 2 × p-C, SOPh + CHPh), 125.4–127.5
s/s/s/s, 2 × o-C, 2 × m-C, SOPh + CHPh), 148.4/149.5 (s/s, 2 ×
Experimental part
General comments
(
7
i-C, SOPh + CHPh). Li NMR (194 MHz, THF-d ): δ −0.25 (s, Li).
8
1
Organolithium compounds were prepared and handled under
5 (R/R′ = Me/Ph; Ar = Ph). Yield: 254 mg (72%). H NMR
purified argon using standard Schlenk techniques. Solvents (400 MHz, THF-d
8
): δ 1.46 (s, 3H, CH
3
), 2.15 (s, 12H,
(
diethyl ether, n-pentane, tetrahydrofuran) were dried over 2 × N(CH ) , TMEDA), 2.31 (s, 4H, 2 × NCH , TMEDA),
3
2
2
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Dalton Transactions
3
3
1
6
.15 (t, JH,H = 7.0 Hz, 1H, p-H, CCH
3
Ph), 6.79 (t, JH,H = 7.6
Minor diastereomer B (7%): H NMR (400 MHz, CDCl
3
):
= 2.7 Hz, 1H, CHSO), 5.70 (d, J = 2.7 Hz,
H,H
3
3
3
Hz, 2H, m-H, CCH Ph), 6.93 (d, JH,H = 7.5 Hz, 2H, o-H, δ 3.65 (d, J
C), 7.20 (t, JH,H = 7.1 Hz, 1H p-H, SOPh), 7.31 (t, JH,H
.4 Hz, 2H, m-H, SOPh), 7.62 (d, JH,H = 7.5 Hz, 2H, o-H, SOPh).
3
H,H
3
3
C
6
H
5
=
1H, CHOH), 6.72 (m, 1H, COH), 6.95–7.54 (m, 15H, HPh).
3
7
5a (R/R′ = Me/Ph, Ar = Ph). Yield: 259 mg (77%). Major dia-
1
3
1
C NMR (100 MHz, THF-d ): δ 11.9 (s, CH ), 45.2 (s, N(CH ) , stereomer (57%): H NMR (400 MHz, CDCl ): δ 0.89 (s, 3H,
8
3
3 2
3
TMEDA), 57.8 (s, CH
2
N, TMEDA), 62.2 (s, CCH
3
Ph), 112.2/ CH
3
), 5.01 (d, 1H, CH), 6.42, (m, 1H, COH), 6.75–7.38 (m, 15H,
1
3
1
15.5 (s/s, 2 × p-C, SOPh + CCH
3
Ph), 126.6–127.5 (s/s/s/s, 2 ×
H
Ph). C NMR (100 MHz, CDCl
3
): δ 12.9 (s, CH
3
), 72.0 (s,
o-C, 2 × m-C, SOPh + CCH Ph), 148.6/149.7 (s/s, 2 × i-C, SOPh + CCH ), 75.5 (s, COH), 125.9–141.6 (CPh).
3
3
7
1
CCH
3
Ph). Li NMR (194 MHz, THF-d
8
): δ −0.2 (s, Li).
Reaction of lithiated sulfoxides with benzaldehyde
2
At −78 °C, to stirred suspension of [Li {CRR′S(O)-
Minor diastereomer (43%): H NMR (400 MHz, CDCl
δ 1.27 (s, 3H, CH ), 5.69 (d, 1H, CH), 6.41, (m, 1H, COH),
.75–7.38 (m, 15H, HPh).
6a (R/R′ = H/Me, Ar = Ph). Yield: 174 mg (67%). Major dia-
3
):
3
6
a
1
Ar} (TMEDA) ] (1–6; 0.5 mmol; 6, R/R′ = H/Me; Ar = Ph, has stereomer (81%): H NMR (400 MHz, CDCl
been prepared as 1–5 but without isolation) in diethyl ether
10 mL), PhCHO (1.0 mmol) was added rapidly. Then, the reac- (s, 1H, CHOH), 7.16–7.72 (m, 10H, HPh). C NMR (100 MHz,
tion mixture was slowly warmed up to room temperature and CDCl ): δ 3.4 (s, CH ), 64.7 (s, CH), 73.9 (s, CHOH), 124.3/
stirred for another hour. At 0 °C, an aqueous solution of 125.9/127.8/128.5/129.1/130.9 (s/s/s/s/s/s, 2 × p-C, 2 × m-C, 2 ×
NH Cl (ca. 25 mL; 30%) was added slowly. After phase separ- o-C, SOPh + Ph), 141.2/141.3 (s/s, 2 × i-C, SOPh + Ph).
ation, the aqueous phase was extracted with diethyl ether. The
combined organic phases were dried (Na SO ) and the solvents δ 0.97 (m, 3H, CH ), 2.72 (m, 1H, CH), 3.46 (br, 1H, OH), 5.34,
2
2
3
): δ 0.97 (d, 3H,
3
JH,H = 6.9 Hz, CH ), 2.81 (m, 1H, CH), 3.46 (br, 1H, OH), 5.45,
3
1
3
(
3
3
4
1
3
Minor diastereomer (19%): H NMR (400 MHz, CDCl ):
2
4
3
were removed in vacuo. The crude product was purified by cen- (s, 1H, CHOH), 7.16–7.72 (m, 10H, HPh).
trifugally accelerated thin layer chromatography (eluent:
diethyl ether–n-hexane 1/1) yielding pure colorless crystals of
Reaction of lithiated sulfoxides with benzophenone
ArS(O)CRR′CHPhOH.
The reactions of the lithiated sulfoxides (0.5 mmol) with
CO (1.0 mmol) were performed analogously to the reac-
stereomer (>94%): H NMR (400 MHz, CDCl ): δ 2.97–3.40 (m, tions with PhCHO (section 4.3), to afford colorless crystals of
1
a (R/R′ = H/H, Ar = Ph). Yield: 150 mg (61%). Major dia- Ph
2
1
3
2
H, CH
2
), 3.75 (s, 1H, OH), 5.72, (s, 1H, CHOH), 7.21–7.82 (m, ArS(O)CRR′CPh
2
OH.
1
3
1
1
0H, HPh). C NMR (100 MHz, CDCl
3
2
): δ 64.2 (s, CH ), 75.6 (s,
1b (R/R′ = H/H, Ar = Ph). Yield: 238 mg (74%). H NMR
COH), 124.1/127.2/127.7/128.7/129.9/130.9, (s/s/s/s/s/s, 2 × p-C, (400 MHz, CDCl ): δ 3.33–3.57 (m, 2H, CH ), 5.72 (s, 1H, OH),
3
2
2
× m-C, 2 × o-C, SOPh + Ph), 142.0/145.3 (s/s, 2 × i-C, SOPh + 7.14–7.67 (m, 15H, HPh).
1
3
Ph). Minor diastereomer not detectable.
C NMR (100 MHz, CDCl
3
2
): δ 68.8 (s, CH ), 81.4 (s,
2
a (R/R′ = H/H, Ar = p-Tol). Yield: 166 mg (64%). Major dia- CPh OH), 123.9/125.5/126.6/127.6/129.5/131.5/144.0/145.9 (s/s/
2
1
stereomer (60%): H NMR (400 MHz, CDCl
CH ), 2.83 (m, 1H, CH ), 3.22 (m, 1H, CH ), 4.05 (br, 1H, OH),
.26 (s, 1H, CHOH), 7.23–7.56 (m, 9H, HAr).
100 MHz, CDCl ): δ 21.4 (s, CH ), 63.4 (s, CH
24.0/125.6/127.9/128.6/130.1/139.4, (s/s/s/s/s/s, 2 × p-C, 2 × m- CDCl
124.5–147.0 (CAr).
3 2
): δ 2.42 (s, 3H, s/s/s/s/s/s, 2 × p-C, 2 × m-C, 2 × o-C, 2 × i-C, SOPh + CPh OH).
1
3
2
2
2b (R/R′ = H/H, Ar = p-Tol). Yield: 231 mg (69%). H NMR
1
3
5
(
1
C NMR (400 MHz, CDCl ): δ 2.35 (s, 3H, CH ), 3.92 (m, 2H, CH ), 6.32
2
), 68.9 (s, COH), (s, 1H, OH), 7.16–7.55 (m, 14H, HAr). C NMR (100 MHz,
3
3
2
1
3
3
3
3
): δ 21.3 (s, CH
3
), 70.4 (s, CH
2
2
), 76.8 (s, CPh OH),
C, 2 × o-C, SOTol + Ph), 142.0/142.1 (s/s, 2 × i-C, SOTol + Ph).
1
1
Minor diastereomer (40%): H NMR (400 MHz, CDCl
.42 (s, 3H, CH ), 2.93 (m, 1H, CH ), 3.22 (m, 1H, CH ), 4.05 (400 MHz, CDCl
br, 1H, OH), 5.36 (s, 1H, CHOH), 7.23–7.56 (m, 9H, HAr). (s, 1H, OH), 7.18–7.95 (m, 15H, HPh). C NMR (100 MHz,
a (R/R′ = Me/Me, Ar = Ph). Yield: 244 mg (89%). Major dia- CDCl ): δ 16.9 (s, CH ), 21.6 (s, CH ), 64.7 (s, CSO), 83.3 (s,
OH), 126.9–145.1 (CPh).
4b (R/R′ = H/Ph, Ar = Ph). Yield: 271 mg (68%). Major dia-
3
): δ
3b (R/R′ = Me/Me, Ar = Ph). Yield: 280 mg (80%). H NMR
2
(
3
2
2
3 3 3
): δ 1.20 (s, 3H, CH ), 3.57 (s, 3H, CH ), 5.95
1
3
3
3
3
3
1
stereomer (>94%): H NMR (400 MHz, CDCl
3
): δ 0.82/1.13 (s/s, CPh
2
3
1
H, CH ), 4.20 (s, 1H, OH), 5.13 (s, 1H, CHOH), 7.26–7.69 (m,
3
1
3
1
0H, SOPh + Ph). C NMR (100 MHz, CDCl
/CH ), 62.7 (s, C(CH ), 78.8 (s, CHOH), 126.5–131.7 (m, CHPh), 5.97 (s, 1H, COH), 6.93–7.96 (m, 20H, HPh). C NMR
× p-C, 2 × o-C, 2 × m-C, SOPh + Ph), 138.9/139.1 (s/s, 2 × i-C, (125 MHz, CDCl ): δ 74.4 (s, CHPh), 82.0 (s, COH), 124.2–146.3
3 3
): δ 12.3/18.3 (s/s, stereomer (>94%): H NMR (400 MHz, CDCl ): δ 4.63 (s, 1H,
1
3
CH
3
3
3 2
)
2
2
3
× i-C, SOPh + Ph). Minor diastereomer not detectable.
a (R/R′ = H/Ph, Ar = Ph). Yield: 222 mg (69%). Major dia-
(CPh). Minor diastereomer not detectable.
4
6b (R/R′ = H/Me, Ar = Ph). Yield: 252 mg (75%). Major dia-
1
3
1
stereomer (80%): H NMR (400 MHz, CDCl ): δ 3.92 (d, J
.6 Hz, 1H, CHSO), 5.66 (d, JH,H = 9.6 Hz, 1H, CHOH), 6.72
m, 1H, COH), 6.95–7.54 (m, 15H, HPh). C NMR (100 MHz, CHCH ), 4.85 (s, 1H, OH), 7.14–7.74 (m, 15H, HPh). C NMR
=
stereomer (>94%): H NMR (400 MHz, CDCl ): δ 0.99 (d,
3
H,H
3
3
3
3
9
(
J
H,H = 6.9 Hz, 3H, CHCH
3
), 3.65 (q, JH,H = 6.8 Hz 1H,
1
3
13
3
CDCl ): δ 77.6 (s, CHSO), 78.1 (s, CHOH), 124.8–142.0 (m, CPh). (125 MHz, CDCl ): δ 5.4 (s, CH ), 64.4 (s, CHCH ), 80.5 (s,
3
3
3
3
1
Minor diastereomer A (13%): H NMR (400 MHz, CDCl
δ 3.59 (d, JH,H = 8.3 Hz, 1H, CHSO), 5.52 (d, JH,H = 8.3 Hz, SOPh), 126.9/127.3 (s/s, 2 × p-C, CPh
3
): COH), 124.2/124.9 (s/s, 2 × m-C, CPh
2
OH), 125.8 (s, m-C,
OH), 128.4/128.6 (s/s, 2 ×
3
3
2
1
H, CHOH), 6.72 (m, 1H, COH), 6.95–7.54 (m, 15H, HPh).
o-C, CPh OH), 129.2 (s, o-C, SOPh), 130.8 (s, p-C, SOPh), 141.1
2
Dalton Trans.
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(
2
s, i-C, SOPh), 145.1/145.9 (s/s, 2 × i-C, CPh OH). Minor diaster-
References
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2
3
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X-ray crystallography
Data for X-ray diffraction analyses of single crystals were col-
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3
(
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4
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5
6
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7
8
5
4,55
the PLATON program package
Tmin/Tmax: 0.59/1.49, 1; 0.74/
0
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6
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9
5
7
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2
58
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4
DFT calculations were performed with the Gaussian09
6
0
61
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d,p) basis sets as implemented in Gaussian09 were employed
1
(
for all atoms. All systems were fully optimized without any
symmetry restrictions. The resulting geometries were charac-
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Acknowledgements
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Graduate Scholarship of the State Saxony-Anhalt.
1045.
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