Intramolecularly sulfur-stabilized silicon cations as Lewis acid catalysts

Article abstract of DOI:10.1021/om500570d

The synthesis and spectroscopic characterization of previously unprecedented sulfur-stabilized silicon cations are reported. Several 1,3-dithiolan-2-yl- and 1,3-dithian-2-yl-substituted silanes were prepared and successfully transformed into the corresponding silicon cations by hydride abstraction. The silicon-sulfur interaction creates three consecutive stereocenters at three different elements. It is remarkable that the present stereocenter at the silicon atom determines the stereochemical outcome at the formerly prochiral sulfur and carbon atoms with excellent diastereoselectivity. All sulfur-stabilized silicon cations are shown to be potent catalysts in a challenging Diels-Alder reaction. Moreover, structurally related oxazoline-stabilized silicon cations were generated and characterized but found to be unreactive.

Full text of DOI:10.1021/om500570d

Article
pubs.acs.org/Organometallics
Intramolecularly Sulfur-Stabilized Silicon Cations as Lewis Acid
Catalysts
Volker H. G. Rohde, Phillip Pommerening, Hendrik F. T. Klare, and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The synthesis and spectroscopic characterization
of previously unprecedented sulfur-stabilized silicon cations are
reported. Several 1,3-dithiolan-2-yl- and 1,3-dithian-2-yl-substi-
tuted silanes were prepared and successfully transformed into the
corresponding silicon cations by hydride abstraction. The
silicon−sulfur interaction creates three consecutive stereocenters
at three different elements. It is remarkable that the present
stereocenter at the silicon atom determines the stereochemical
outcome at the formerly prochiral sulfur and carbon atoms with excellent diastereoselectivity. All sulfur-stabilized silicon cations
are shown to be potent catalysts in a challenging Diels−Alder reaction. Moreover, structurally related oxazoline-stabilized silicon
cations were generated and characterized but found to be unreactive.
introduced Me3Si[B(OTf)4], which was found not to be
ionized.12 Hence, the obvious next step was to use ionized
silicon Lewis acids where the silicon cation is separated from
the weakly coordinating counteranion. Sterically accessible
tricoordinate silicon cations (silicenium or silylium ions) are
not available, as a consequence of their extreme electron
deficiency.13 However, inter- and intramolecularly stabilized
systems have been developed,2,13 and arene-stabilized [Et3Si-
INTRODUCTION
■
Tetracoordinate silicon is usually weakly Lewis acidic,1 and new
strategies to boost that acidity continue to be relevant to
synthetic chemistry.2 The coordinating ability of the X group in
R3SiX influences that property, and the degree of deshielding of
the silicon nucleus in the 29Si NMR spectrum is a qualitative
measure of its Lewis acidity (Figure 1).3 Trends seen in the 29Si
14
(toluene)]+[B(C6F5)4]− as well as ferrocene-stabilized [FcSi-
(tBu)Me]+[B(C6F5)4]−, the latter reported by our laborator-
y,6b,15 have been applied in Lewis acid catalysis with
considerable success.6 Again, the excellent performance of
these silicon cations as catalysts is in agreement with their 29Si
NMR chemical shifts.16
The exceptional Lewis acidity of ionized silicon acids is
beneficial in one way or the other but makes these reactive
intermediates seek stabilization either inter- or intramolecularly.
This entails, for example, the formation of arenium ions that
potentially act as proton sources and that, in turn, contains the
risk of hidden proton catalysis.6c For us, the question was
whether the pronounced and at the same time problematic
Lewis acidity of reported silicon cations is really necessary for
achieving high or even unprecedented catalytic activity. We
therefore asked ourselves if stabilization of a tricoordinate
silicon cation by a heteroatom donor would render the
resulting Lewis adduct more “well behaved” while maintaining
sufficient Lewis acidity. Judicious choice of the donor group
would then allow us to engineer the Lewis acidity in a targeted
way.
Figure 1. Silicon Lewis acids and their 29Si NMR chemical shifts.
NMR analysis often correlate with experimental findings.4−6
The substituents at the silicon atom, typically alkyl and/or aryl
groups, exert a minor effect on the electronic situation at the
silicon atom, with (Me3Si)3SiNTf2 being an exception,7 but
steric congestion around the silicon atom is a major parameter.8
In addition, the Lewis acidity increases when the silicon atom is
incorporated into a strained ring.9 The electron deficiency at
the silicon atom in R3SiX further increases by interaction with
another Lewis acid.10 On the basis of seminal work by Olah and
co-workers using boron Lewis acids,11 Davis and co-workers
Aside from the spectroscopic and structural characterization
of a few heteroatom-stabilized silicon cations, i.e., silylated
Received: May 28, 2014
Published: July 2, 2014
© 2014 American Chemical Society
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Article
onium ions,13,17 the use of such adducts in Lewis acid catalysis
is essentially not documented. A famous example is the chiral
acetonitrile adduct introduced by Helmchen and Jørgensen,
which promoted a Diels−Alder reaction with little asymmetric
induction.18,19 The 29Si NMR chemical shift of δ 34.0 ppm in
acetonitrile-d3 is typical for silylnitrilium ions,17a−c indicating
reduced Lewis acidity in comparison to conventional R3SiX
(Figure 1, left). On the basis of this precedence, one portion of
the present work focuses on the preparation and character-
ization of oxazoline-stabilized silicon cations 1 and 2 (Figure 2,
elements (silicon, sulfur, and carbon) are generated in one step.
The catalytic activity of this new class of silicon Lewis acids will
be highlighted in Diels−Alder reactions.
RESULTS AND DISCUSSION
■
Oxazoline-Stabilized Silicon Cations. We anticipated
that silicon cations bearing oxazolin-2-yl units would be
stabilized by either the oxygen or the nitrogen atom of the
heterocycle. As a result, the ferrocene backbone usually
incorporated in our silicon cations6b,c,15 (cf. Figure 1, right)
would not be necessary. For that reason, the corresponding
benzene-based silane 5 was synthesized in addition to the
ferrocene-based silane 6 (Scheme 1). Oxazolin-2-yl phenyl
Scheme 1. Preparation of Silicon Cation Precursors with an
Oxazolin-2-yl Unit
Figure 2. Heteroatom-stabilized silicon cations investigated in this
work.
top), and we evaluate their ability to catalyze representative
(enantioselective) Diels−Alder reactions.19 The oxazoline unit
is chiral, and we investigate the effect of the existing
stereocenter on the stereogenicity at the silicon atom.
silane 5 was accessible in moderate yield through ortho-
directed metalation of the literature-known phenyl oxazoline
723 and subsequent trapping of the anion with racemic
tBuMeSi(H)Cl (Scheme 1, top). The silylation showed a
small preference for one diastereomer with diastereomeric
ratios ranging from 59:41 to 69:31. The corresponding
ferrocenyl-substituted silane 6 was synthesized in high yield
in a similar manner from ferrocenyl oxazoline 824 (Scheme 1,
bottom). The planar chirality is fully controlled by the chiral
oxazolin-2-yl unit in the metalation step.25 The diastereose-
lectivity seen in electrophilic substitution with tBuMeSi(H)Cl
hence refers to the configuration at the silicon atom relative to
the existing central and planar chirality of the backbone; the
diastereomeric ratios obtained again ranged from 61:39 to
67:33.
The generation of oxazoline-stabilized silicon cations 1 and 2
was achieved by hydride abstraction from 5 and 6 with trityl
tetrakis(pentafluorophenyl)borate, [Ph3C]+[B(C6F5)4]−
(Scheme 2). However, cation formation was much slower
than that of non-heteroatom-bearing ferrocenyl-substituted
silanes.6b,c,15b As a result, significantly longer reaction times
were required (30 min vs 1 min). We explain this observation
by competitive, yet reversible, coordination of the trityl cation
to either of the heteroatoms prior to hydride abstraction. In
comparison with ferrocene-stabilized [FcSi(tBu)Me]+[B-
(C6F5)4]− (δ 114.6 ppm),6b,15b the 29Si NMR spectra of the
pairs of diastereomers of 1 and 2 revealed pronounced upfield
shifts at δ 24.2/24.7 and 25.4/26.4 ppm, respectively. We judge
these chemical shifts as an indication of the heteroatom
Another donor atom attracted our attention. Our laboratory
recently disclosed the cooperative activation of Si−H bonds at
the Ru−S bond of a coordinatively unsaturated ruthenium(II)
thiolate complex.20 A σ-bond metathesis splits the Si−H bond
heterolytically into a ruthenium(II) hydride and sulfur-
stabilized silicon cation (not shown). The [Si−S]+ motif of
this metallasulfonium ion is an excellent silicon electrophile that
readily transfers onto various nucleophiles.20 We were thrilled
to learn that related sulfonium ions are elusive and that Olah
and co-workers had only been able to prepare [(Me3Si)3S]+ and
[(Me3Si)2SMe]+ but not [Me3SiSMe2]+ (all with [B(C6F5)4]−
as counteranion).21 It is worthy of note, though, that Jutzi and
co-workers had prepared a silylium ion with two aryl
substituents each equipped with a thiomethoxymethyl group
in the ortho position; the dynamic coordination equilibria
between tetra- and pentacoordinate cations were analyzed by
NMR spectroscopy.22 Intrigued by this fascinating insight, we
decided to systematically elaborate the chemistry of sulfur-
stabilized silicon cations that are expected to be rather
electrophilic adducts. Disproportionation of the [Me3SiSMe2]+
Lewis pair had thwarted its synthesis,21 which is why we
employ 1,3-dithiolanes (n = 1) and 1,3-dithianes (n = 2) as
donor groups, again using benzene and ferrocene as platforms
(3 and 4, Figure 2, bottom). The stereochemical consequence
of the silicon−sulfur interaction in 3 and 4 is particularly
interesting, as three consecutive stereocenters at three different
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On comparison of the 29Si NMR chemical shifts of 1 (δ
24.7/24.2 ppm) and 2 (δ 26.4/25.4 ppm) with that of the
known silicon Lewis acid Me3SiOTf (δ 43.4 ppm), weaker
Lewis acidity and, hence, lower catalytic activity are to be
expected.4b Conversely, 1 and 2 are cationic, whereas
Me3SiOTf is neutral (not dissociated), and coordination of
the weak donor acetonitrile is possible (vide supra). However,
utilizing 1 and 2 as catalysts in various Diels−Alder reactions
revealed that neither one is catalytically active. Even for diene/
dienophile combinations with a relatively low activation barrier,
poor yields were obtained after prolonged reaction times with
no stereoinduction. Consequently, we must conclude that these
oxazoline-stabilized silicon cations are an inappropriate choice
for the development of chiral silicon Lewis acids.
Scheme 2. Generation of Oxazoline-Stabilized Silicon
Cations and Equilibration of Diastereomers
1,3-Dithiolane-/1,3-Dithiane-Stabilized Silicon Cati-
ons. With the nitrogen atom being too strong of a donor,
we decided to replace it with a sulfur atom (cf. Intro-
duction).20−22 Hence, the structurally related silanes 9a−9d
and 10 with benzene and ferrocene backbones emerged as new
targets (Schemes 3 and 4). Cyclic dithioacetals were chosen not
Scheme 3. Preparation of Silicon Cation Precursors with 1,3-
Dithiolan-2-yl-Substituted (n = 1) and 1,3-Dithian-2-yl-
Substituted (n = 2) Phenyl Platforms and Silicon Cation
Generation
stabilization of the silicon cation by the adjacent oxazoline
donor, while the ferrocenyl substituent in 2 is nothing but a
spectator substituent. To distinguish between the oxygen and
1
nitrogen atoms acting as the donor, H,15N HMQC NMR
experiments of precursors 5 and 6 as well as cations 1 and 2
were performed. We observed that the 15N NMR resonances of
1 and 2 were markedly shifted to lower frequencies in
comparison to those of silanes 5 and 6 (15N NMR Δδ ≈ 80
ppm; for details see the Supporting Information).26 Moreover,
a correlation of the nitrogen nucleus to the methyl group was
detected, and that is only possible for an Si−N bond. These
findings strongly indicate that it is the nitrogen donor that
interacts with the empty orbital at the silicon atom.27 This
result is in agreement with previous studies where an sp3-
hybridized nitrogen atom was shown to be a better donor for a
tricoordinate silicon cation than an sp3-hybridized oxygen
atom.22
With the nitrogen atom as the stabilizing heteroatom, the
tert-butyl group of the oxazolin-2-yl unit is brought into close
proximity of the silicon fragment. To our surprise, the
diastereomeric ratio changed little during cation generation
despite steric congestion: 59:41 → 54:46 for 5 → 1 and 61:39
→ 72:28 for 6 → 2 (Scheme 2). Substantial amounts of the
thermodynamically disfavored syn isomer were present. We,
therefore, assume that the hydride abstraction is assisted by the
nucleophilic nitrogen atom or even accompanied by simulta-
neous Si−N bond formation. The strength of the intra-
molecular Si−N adduct is demonstrated by our failed attempts
to thermally enrich the thermodynamically more stable anti
isomer (not shown). As an alternative, we anticipated that an
external weak donor such as acetonitrile would promote
epimerization at the silicon atom through reversible coordina-
tion. To our delight, acetonitrile (1.0 equiv) indeed trans-
formed the syn into the anti isomer, and both 1 (dr = 98:2) and
2 (dr = 97:3) were obtained in almost diastereomerically pure
form. The anti relative configuration was verified by NOE
experiments.
only because of their accessibility but also for their tolerance
against harsh conditions. Several 1,3-dithiolan-2-yl- and 1,3-
dithian-2-yl-substituted systems 9a−9d were prepared by
starting from the corresponding bromides 11a−11d in
moderate to good yields (Scheme 3, top). Except for 11b (n
= 1, R = Me), bromides 11a,c,d are known compounds28 but
were synthesized according to a different procedure.29 Variation
of the ring size and the substitution pattern at C2 of the cyclic
dithioacetals would allow for an evaluation of the influence of
these structural features on stabilizing the silicon cation. Indeed,
distinct differences in the 29Si NMR chemical shifts were
observed for silicon cations 3a−3d (Scheme 3, bottom). The
1,3-dithiolan-2-yl-substituted cations 3a (δ 56.7 ppm) and 3b
(δ 51.7 ppm) were shifted to higher frequencies in comparison
to their 1,3-dithian-2-yl-substituted homologues 3c (δ 51.3
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To test the catalytic activity of sulfur-stabilized silicon cations
3a−3d and 4, the Diels−Alder cycloaddition of cyclohexa-1,3-
diene (14) and chalcone (15) to adduct 16 was chosen as a
model reaction. We have already used this reaction in our
previous studies, and this allows us to establish a relation
between the performance of the new silicon cations and that of
known Lewis acids.6c,8b Moreover, this rather difficult Diels−
Alder reaction is catalyzed neither in the absence of any Lewis
acid catalyst nor by protons.6c,34 As was done before, a
comparison of the 29Si NMR shifts of 3a−3d and 4 with
Ghosez’s results for Me3SiOTf (δ 43.4 ppm) and Me3SiNTf2 (δ
56.2 ppm)4b enables a qualitative estimation of their Lewis
acidity.3 The deshielding of the sulfur-stabilized cations is more
pronounced than that of Me3SiOTf and more than that of
Me3SiNTf2 for 3a and two diastereomers of ferrocene-based 4.
Hence, successful catalyses were assumed at least for the latter.
To our delight, these new silicon Lewis acids were all capable of
catalyzing this challenging reaction6c even at 0 °C; 3a−3d
performed equally well (Table 1, entries 1−4). Somewhat
Scheme 4. Preparation of a Silicon Cation Precursor with
1,3-Dithiolan-2-yl-Substituted Ferrocenyl Platform and
Silicon Cation Generation
Table 1. Model Diels−Alder Reaction Catalyzed by Sulfur-
a b
,
Stabilized Silicon Cations
ppm) and 3d (δ 42.3/48.6 ppm). The greater deshielding of
the silicon atom in 3a,b is likely due to a less favorable n(S) →
3p(Si) orbital overlap in the [3.3.0] bicyclic chelate as opposed
to the [4.3.0] ring system in 3c,d. The influence of the angular
methyl group in these bicycles (i.e., substitution at C2 of the
dithioacetals) is not fully understood but could be electronic in
nature, thereby increasing the donor ability of the sulfur atom.
The stereochemical outcome of the cation formation is
particularly striking. As a consequence of the Si−S interaction,
additional stereogenicity is generated at the coordinating sulfur
atom and at C2 of the (formerly prochiral) dithioacetal unit.
We find it remarkable that the formation of one diastereomer is
highly favored. The selectivity is a result of efficient
diastereotopic group selection induced by the stereochemical
information at the silicon atom. By this, three stereocenters at
three different elements are formed with high diastereoselec-
tivity in a single transformation. The relative configuration was
assigned on the basis of NOE experiments, which indicate a syn
relationsship of the tert-butyl group at the silicon atom and the
R group at C2 of the dithioacetal.
On the basis of our experience with the structural design of
benzene-based cations 3a−3d, we prepared planar chiral
ferrocene-based 10 starting from the literature-known aldehyde
12 (Scheme 4, top).30 Formation of the dithiolane 13 and
subsequent substitution of the adjacent sulfoxide afforded 10 in
reasonable yield and with good diastereoselectivity with respect
to the silicon atom. The 1,3-dithiolan-2-yl unit with R = H was
used, since this ring size with no substitution at C2 had shown
the highest degree of deshielding for the cations with the
benzene platform (Scheme 3, bottom). The diastereomeric
ratio of 92:8 did not translate into high diastereoenrichment of
the corresponding cation (10 → 4); 4 was obtained as a
mixture of four stereoisomers (Scheme 4, bottom). Attempts to
accumulate one isomer in the same way as for oxazoline-
stabilized silicon cations 1 and 2 were unsuccessful; addition of
acetonitrile resulted in a complex mixture of compounds.
Si
conv.
c
yield
d
entry cation
29Si NMR (δ/ppm)
n
R
(%)
(%)
1
2
3
4
5
3a
3b
3c
3d
4
56.7
1
1
2
2
H
78
59
73
64
59
78
66
62
65
51.7
Me
H
51.3
42.3/48.6
Me
e
50.4/51.3/57.3/57.8
46
a
All reactions were performed according to General Procedure 4 at a
concentration of 0.5 M of the dienophile. Diastereomeric (trans/cis)
and endo/exo ratios determined by GLC analysis of the reaction
mixture prior to purification. Determined by GLC analysis of the
reaction mixture after 1 h using triphenylmethane as internal standard.
Yield after 5 h of analytically pure product after flash column
chromatography on silica gel. 0% ee.
b
c
d
e
unexpectedly, 4, with the Lewis acid attached to the ferrocene
backbone, showed the lowest catalytic activity in terms of
isolated yield (Table 1, entry 5). The increased steric
congestion around the Lewis acid center in 4 might result in
decreased accessibility. Moreover, no asymmetric induction was
seen. At lower temperatures (−40 °C), reaction rates were
significantly lower (<5% conversion after 22 h with 4), still not
providing any stereoinduction.
CONCLUSION
■
In summary, the generation of several novel heteroatom-
stabilized silicon cations was accomplished. Notably, silicon
Lewis acids reversibly coordinated by a sulfur donor were
successfully generated and spectroscopically characterized. As a
consequence of the Si−S interaction in these cations, three
consecutive stereocenters at three different elements (silicon,
sulfur, and carbon) were established, resulting in highly
diastereomerically enriched Lewis acids. The deshielding of
the silicon atom in the 29Si NMR spectra is in part determined
by the effectiveness of the n(S) → 3p(Si) orbital overlap that is,
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3
1
the JH,Si coupling. The peak intensities in the H,29Si HMQC NMR
in turn, dependent on the ring size of the sulfur-donor-
containing acetals. Moreover, methyl substitution at C2 of the
1,3-dithiolan-2-yl and 1,3-dithian-2-yl groups led to a shift of
the 29Si NMR resonance to lower frequencies. Qualitative
comparison of these 29Si NMR chemical shifts (δ ∼50 ppm)
with those of known silicon Lewis acids (cf. Figure 1) enabled
us to predict substantial Lewis acidity, hence rendering sulfur-
stabilized silicon cations suitable canditates for catalysis.
Gratifyingly, all of them were catalytically active in a
representative difficult Diels−Alder reaction at 0 °C. While
no stereoinduction was seen with 4, the present findings
nevertheless pave the way for the development of chiral cationic
silicon-based Lewis acids.
spectra cannot be correlated to the amount of the compound. H,15N
HMQC NMR spectra were measured with a coupling constant of 3.0
Hz for the JH,N coupling. Infrared (IR) spectra were recorded on an
1
3
Agilent Technologies Cary 630 FTIR spectrophotometer equipped
with an ATR unit and are reported as wavenumbers (cm−1) (w =
weak, m = medium, s = strong). Gas−liquid chromatography (GLC)
was performed on an Agilent Technologies 7820A gas chromatograph
equipped with an HP-5 capillary column (30 m × 0.32 mm, 0.25 μm
film thickness) or on a Shimadzu GC-17A gas chromatograph
equipped with a FS-SE-54 capillary column (30 m × 0.32 mm, 0.25
μm film thickness) using the following program: N2 carrier gas;
column flow 1.74 mL min−1; injection temperature 250 °C; detector
temperature 300 °C; temperature program start temperature 40 °C,
heating rate 10 °C min−1, end temperature 280 °C for 10 min. Melting
points (mp) were determined with a Stuart Scientific SMP20 melting
point apparatus and are not corrected. Optical rotations were
measured on a Schmidt & Haensch Polatronic H532 polarimeter
with [α]D values reported in deg 10−1 cm2 g−1; the concentration c is
in g 100−1 mL−1. Enantiomeric excesses were determined by analytical
high-performance liquid chromatography (HPLC) analysis on an
Agilent Technologies 1290 Infinity instrument with a chiral stationary
phase using a Daicel Chiralcel OD-H column (n-heptane/i-PrOH
mixtures as solvents). Mass spectrometry (MS) was performed by the
The structurally related oxazoline-stabilized silicon cations
were also generated, but these were shown to be not sufficiently
reactive to act as Lewis acid catalysts. The stabilization by the
nitrogen atom results in a significantly stronger shielding of the
silicon atom than in the case of the sulfur analogues.
EXPERIMENTAL SECTION
■
General Remarks. All reactions were performed in flame-dried
glassware using an MBraun glovebox (O2 <0.5 ppm, H2O <0.5 ppm)
or conventional Schlenk techniques under a static pressure of argon or
nitrogen. Liquids and solutions were transferred with syringes.
Solvents (THF, toluene, n-hexane, diethyl ether, acetonitrile, DMF,
CHCl3, CH2Cl2, and 1,2-Cl2C6H4,) were dried and purified following
standard procedures. Technical grade solvents for extraction or
chromatography (tert-butyl methyl ether, CH2Cl2, cyclohexane, and
ethyl acetate) were distilled prior to use. C6D6 and CDCl3 (purchased
from Eurisotop) were dried over 4 Å molecular sieves. 1,2-Cl2C6D4
(purchased from Eurisotop) was dried over CaH2, distilled, and stored
under argon. Potassium tert-butoxide, tert-butyllithium (1.48−1.76 M
in n-pentane; see respective experiment for the exact concentration),
benzoyl chloride, ferrocene, ethyl chloroformate, thionyl chloride,
triethylamine, trimethylaluminum (2.0 M in toluene), 2-bromoben-
zaldehyde, 2′-bromoacetophenone, ethane-1,2-dithiol, propane-1,3-
dithiol, iodine, n-butyllithium (1.38−1.54 M in hexane fractions; see
respective experiment for the exact concentration), 4-toluenesulfonyl
chloride, tetramethylethylenediamine (TMEDA), (1R,2S,5R)-
(−)-menthyl (S)-p-toluenesulfinate, and diisopropylamine were
obtained from commercial sources and used without further
purification. Cyclohexa-1,3-diene (14) was distilled from NaBH4,
and (E)-chalcone (15) was recrystallized from ethanol and dried by
azeotropic distillation with benzene prior to use. (S)-2-Amino-3,3-
dimethylbutan-1-ol31 and tert-butylchloro(methyl)silane6b were pre-
pared according to reported procedures. Trityl tetrakis-
(pentafluorophenyl)borate ([Ph3C]+[B(C6F5)4]−) was prepared fol-
lowing a reported procedure,32 recrystallized from CH2Cl2/n-pentane,
and stored in a glovebox. Analytical thin-layer chromatography (TLC)
was performed on silica gel 60 F254 glass plates by Merck. Flash
column chromatography was performed on silica gel LC60 Å (40−63
μm) by Davisil using the indicated solvents. 1H, 13C, 11B, 15N, 19F, and
29Si NMR spectra were recorded in CDCl3, C6D6, or 1,2-Cl2C6D4 on
Bruker AV700, Bruker AV500, and Bruker AV400 instruments.
Chemical shifts are reported in parts per million (ppm) and are
calibrated to the residual solvent resonance as the internal standard
(CHCl3, δ 7.26 for 1H NMR; CDCl3, δ 77.16 for 13C NMR; C6D5H, δ
Analytical Facility of the Institut fur Chemie, Technische Universitat
Berlin.
̈
̈
General Procedure for the Preparation of 1,3-Dithiolan-2-yl-
and 1,3-Dithian-2-yl-Substituted Phenyl Bromides 11 (GP1). In
analogy to a reported procedure by Firouzabadi and Iranpoor,29
ethane-1,2-dithiol (1.2 equiv) or propane-1,3-dithiol (1.2−2.0 equiv)
and iodine (0.1 equiv) were added to a solution of the corresponding
carbonyl compound (1.00 equiv) in CHCl3 (0.20−0.23 M), and the
resulting mixture was stirred at room temperature (2−15 h). The
reaction was quenched by the addition of aqueous NaOH solution (2
N), the phases are separated, and the aqueous phase was extracted
with tert-butyl methyl ether (3×). The combined organic phases were
washed with saturated aqueous NaCl solution (1×), dried over
Na2SO4, and concentrated under reduced pressure. The crude product
was purified by flash column chromatography on silica gel using
cyclohexane/ethyl acetate mixtures as eluent or by recrystallization
from ethanol, respectively.
General Procedure for the Preparation of 1,3-Dithiolan-2-yl-
and 1,3-Dithian-2-yl-Substituted Phenylsilanes 9 (GP2). To a
solution of the corresponding bromide 11 (1.00 equiv) in THF (0.10−
0.11 M) cooled to −78 °C was added nBuLi (1.51−1.54 M in hexane
fractions, 1.29−1.30 equiv) dropwise. After an additional 1 h at −78
°C, a solution of tert-butylchloro(methyl)silane (1.83−2.00 equiv) in
THF (0.36−0.51 M) was added, and the resulting mixture was
warmed to room temperature (2−15 h). After addition of water and
dilution with tert-butyl methyl ether, the phases were separated, and
the aqueous phase was extracted with tert-butyl methyl ether (3×).
The combined organic phases were washed with saturated aqueous
NaCl solution (1×), dried over Na2SO4, and concentrated under
reduced pressure. The crude product was purified by flash column
chromatography on silica gel using cyclohexane/ethyl acetate mixtures
as eluent.
For the generation of the corresponding silicon cations, all silanes
were azeotropically dried by addition and subsequent evaporation of
dry benzene under vacuum (3×).
General Procedure for the Preparation of Intramolecularly
Donor Stabilized Silicon Cations 1, 2, 3a−3d, and 4 (GP3). In a
glovebox, a solution of the corresponding silane (1.00 equiv) in 1,2-
Cl2C6D4 (0.4 mL) was added to a suspension of [Ph3C]+[B(C6F5)4]−
(1.00−1.10 equiv) in 1,2-Cl2C6D4 (0.2 mL) in an 8 mL vial equipped
with a magnetic stir bar. The resulting mixture was stirred for 30 min,
transferred to an NMR tube, and directly subjected to NMR
spectroscopic analysis.
1
7.16 for H NMR; C6D6, δ 128.06 for 13C NMR; 1,2-Cl2C6D3H, δ
6.94 and 7.20 for 1H NMR; 1,2-Cl2C6D4, δ 127.1, 130.1, and 132.5 for
13C NMR). All other nuclei (11B, 13C, 15N, 19F, and 29Si) are
referenced in compliance with the unified scale for NMR chemical
shifts as recommended by IUPAC.33 For 15N NMR chemical shifts,
liquid NH3 was used as the standard. Data are reported as follows:
chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet, br m = broad multiplet, mc = centrosymmetric
General Procedure for the Model Diels−Alder Reaction
Catalyzed by Sulfur-Stabilized Silicon Cations (GP4). In a
glovebox, a flame-dried Schlenk tube equipped with a magnetic stir bar
1
multiplet), coupling constants (Hz), and integration. H,29Si HMQC
NMR spectra were measured with a coupling constant of 7.0 Hz for
3622
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Organometallics
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was charged with [Ph3C]+[B(C6F5)4]− (5.00 mol %). The Schlenk
tube was transferred to a fume hood and connected to a nitrogen−
vacuum manifold. After addition of 1,2-Cl2C6H4 (0.2 mL), a solution
of the corresponding silane (5.50 mol %) in 1,2-Cl2C6H4 (0.4 mL) was
added, and the resulting mixture was stirred for 30 min at room
temperature.34 The thus obtained catalyst solution was cooled to 0 °C,
and a solution of cyclohexa-1,3-diene (14, 2.00 equiv) and (E)-
chalcone (15, 1.00 equiv) in 1,2-Cl2C6H4 (0.4 mL) was added
dropwise. The reaction mixture was stirred at 0 °C for 5 h, followed by
addition of saturated aqueous NaHCO3 solution. The phases were
separated, and the aqueous phase was extracted with tert-butyl methyl
ether (3×). The combined organic phases were washed with saturated
aqueous NaCl solution, dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by flash column
chromatography on silica gel using cyclohexane/ethyl acetate (50/1)
as eluent, affording analytically pure Diels−Alder product 16 as a white
solid.
(S)-4-tert-Butyl-2-phenyl-4,5-dihydrooxazole (7). To a sol-
ution of benzoyl chloride (1.00 g, 7.11 mmol, 1.00 equiv) in CH2Cl2
(20 mL) was added a solution of triethylamine (0.99 mL, 0.72 g, 7.1
mmol, 1.0 equiv) and (S)-tert-leucinol (835 mg, 7.15 mmol, 1.01
equiv, >99% ee) in CH2Cl2 (5 mL) dropwise at 0 °C. The resulting
mixture was slowly warmed to room temperature over a period of 12
h. After removal of all volatiles under vacuum, thionyl chloride (1.55
mL, 2.54 g, 21.3 mmol, 3.00 equiv) was added, and the reaction
mixture was stirred for a further 3.5 h at room temperature. The
mixture was then cooled to 0 °C, diluted with tert-butyl methyl ether
(10 mL), and carefully neutralized with aqueous NaOH solution (2 N,
approximately 20 mL). The phases were separated, and the aqueous
phase was extracted with tert-butyl methyl ether (2 × 15 mL). The
combined organic phases were washed with saturated aqueous NaCl
solution (40 mL), dried over Na2SO4, and concentrated under reduced
pressure. The residue was purified by bulb-to-bulb distillation,
affording phenyl oxazoline 7 (914 mg, 4.50 mmol, 63%, >99% ee)
as a colorless solid. Mp: 33.1 °C (benzene). GLC (HP-5): tR = 13.5
spectroscopic data for the major diastereomer are as follows. 1H NMR
(400 MHz, C6D6): δ 0.52 (d, J = 3.8 Hz, 3H), 0.88 (s, 9H), 1.12 (s,
9H), 3.80−3.92 (m, 3H), 4.63 (q, J = 3.7 Hz, 1H), 7.10−7.15 (m,
2H), 7.72−7.75 (m, 1H), 8.12−8.17 (m, 1H). 13C NMR (126 MHz,
C6D6): δ −4.9, 18.0, 26.2, 28.7, 33.9, 68.4, 77.1, 129.4, 129.8, 129.9,
135.0, 137.1, 137.9, 164.6. 1H,15N HMQC NMR (700/71 MHz,
C6D6): δ 232.9. 29Si DEPT NMR (99 MHz, C6D6): δ 2.1. NMR
spectroscopic data for the minor diastereomer are as follows. 1H NMR
(400 MHz, C6D6): δ 0.40 (d, J = 3.8 Hz, 3H), 0.89 (s, 9H), 1.13 (s,
9H), 3.80−3.92 (m, 3H), 4.78 (q, J = 3.8 Hz, 1H), 7.10−7.15 (m,
2H), 7.64−7.68 (m, 1H), 8.12−8.17 (m, 1H). 13C NMR (126 MHz,
C6D6): δ −5.4, 18.0, 26.2, 28.6, 33.9, 68.4, 77.2, 129.2, 129.7, 130.0,
135.0, 137.1, 137.9, 164.3. 1H,15N HMQC NMR (700/71 MHz,
C6D6): δ 234.2. 29Si DEPT NMR (99 MHz, C6D6): δ −1.6.
Ethyl Ferrocenecarboxylate. Ferrocene (500 mg, 2.69 mmol,
1.00 equiv) and KOtBu (30 mg, 0.27 mmol, 0.10 equiv) were
suspended in THF (3 mL), and the suspension was cooled to −78 °C.
tBuLi (1.76 M in n-pentane, 1.53 mL, 2.69 mmol, 1.00 equiv) was
added dropwise, and the resulting mixture was stirred for 1 h at −78
°C and 1 h at room temperature. After the mixture was cooled to −78
°C, ethyl chloroformate (0.38 mL, 0.44 g, 4.0 mmol, 1.5 equiv) was
added. The reaction mixture was warmed to room temperature and
stirred overnight. After addition of saturated aqueous NH4Cl solution
(10 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 50 mL). The combined
organic phases were washed with saturated aqueous NaCl solution
(120 mL), dried over MgSO4, and concentrated under reduced
pressure. The residue was purified by flash column chromatography on
silica gel using cyclohexane/ethyl acetate (20/1) as eluent, affording
ethyl ferrocenecarboxylate (554 mg, 2.15 mmol, 80%) as a brown oil.
Rf = 0.57 (cyclohexane/ethyl acetate 20/1). GLC (HP-5): tR = 16.0
̃
min. IR (ATR): ν 3110 (w), 2961 (w), 1691 (s), 1455 (m), 1374 (m),
1273 (s), 1169 (w), 1129 (s), 999 (m), 915 (w), 821 (m), 778 (m)
cm−1. HRMS (ESI): calculated for C13H15FeO2 [M + H]+, 259.0416;
1
found, 259.0415. H NMR (400 MHz, C6D6): δ 1.07 (t, J = 7.1 Hz,
min. IR (ATR): ν
̃
2954 (m), 2901 (w), 2867 (w), 1648 (s), 1578 (m),
3H), 4.01 (s, 5H), 4.02 (mc, 2H), 4.14 (q, J = 7.1 Hz, 2H), 4.88 (mc,
2H). 13C NMR (126 MHz, C6D6): δ 14.7, 60.0, 70.0, 70.6, 71.3, 72.6,
170.9. The analytical and spectroscopic data are in accordance with
those reported.24
1492 (m), 1449 (m), 1355 (s), 1340 (s), 1258 (m), 1083 (m), 1053
(m), 1021 (s), 966 (s), 929 (w), 898 (m), 782 (m), 723 (m), 692 (s)
cm−1. HRMS (ESI): calculated for C13H18NO [M + H]+, 204.1383;
1
found, 204.1373. H NMR (500 MHz, C6D6): δ 0.88 (s, 9H), 3.83−
(S)-N-(1-Hydroxy-3,3-dimethylbutan-2-yl)ferrocenyl Carbox-
amide. To a solution of (S)-tert-leucinol (1.31 g, 11.2 mmol, 1.21
equiv) in toluene (25 mL) at 0 °C was added trimethylaluminum
(2.00 M in toluene, 10.2 mL, 20.4 mmol, 2.20 equiv) dropwise. The
cooling bath was removed, and a solution of ethyl ferrocenecarboxylate
(2.40 g, 9.29 mmol, 1.00 equiv) in toluene (25 mL) was added
dropwise. After complete addition, the resulting mixture was heated to
reflux for 14 h. The reaction mixture was diluted with CH2Cl2 (30 mL)
at room temperature, and an aqueous potassium sodium tartrate
solution (1.3 M, 40 mL) was added. The phases were separated, and
the aqueous phase was extracted with CH2Cl2 (3 × 25 mL). The
combined organic phases were washed with saturated aqueous NaCl
solution (2 × 20 mL), dried over MgSO4, and concentrated under
reduced pressure. The residue was purified by flash column
chromatography on silica gel using ethyl acetate/cyclohexane (2/1)
as eluent, affording (S)-N-(1-hydroxy-3,3-dimethylbutan-2-yl)-
ferrocenyl carboxamide (2.28 g, 6.93 mmol, 76%) as orange needles.
Mp: 174.6 °C (benzene). Rf = 0.23 (ethyl acetate/cyclohexane 2/1).
3.94 (m, 3H), 7.06−7.12 (m, 3H), 8.19−8.26 (m, 2H). 13C NMR
(126 MHz, C6D6): δ 26.0, 34.0, 68.6, 76.7, 128.5, 128.7, 129.0, 131.2,
20
163.1. Specific rotation [α]D = −9.4 (c = 0.26, CHCl3, >99% ee).
The analytical and spectroscopic data are in accordance with those
reported.23
(SiS*,S)-tert-Butyl(2-(4-tert-butyloxazolin-2-yl)phenyl)-
methylsilane (5). To a solution of phenyl oxazoline 7 (0.55 g, 2.7
mmol, 1.0 equiv) in diethyl ether (22 mL) was added nBuLi (1.51 M
in hexane fractions, 2.69 mL, 4.06 mmol, 1.50 equiv) dropwise at 0 °C.
After an additional 1 h at 0 °C, tert-butylchloro(methyl)silane (444
mg, 3.25 mmol, 1.20 equiv) was added dropwise, and the reaction
mixture was warmed to room temperature (12 h). Water (10 mL) was
added, the phases were separated, and the aqueous phase was extracted
with tert-butyl methyl ether (3 × 30 mL). The combined organic
phases were washed with saturated aqueous NaCl solution (50 mL),
dried over Na2SO4, and concentrated under reduced pressure. The
residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (100/1) as eluent, affording silane 5
(0.30 g, 0.99 mmol, 37%, dr = 59:41) as a slightly yellow oil. For the
generation of the corresponding silicon cation, the silane was
azeotropically dried by addition and subsequent evaporation of dry
benzene under vacuum (3×). Rf = 0.15 (cyclohexane/ethyl acetate 50/
1). GLC (HP-5): tR = 17.8 min (major diastereomer), tR = 17.7 min
̃
GLC (HP-5): tR = 19.4 min. IR (ATR): ν 3227 (br), 3083 (w), 2946
(m), 1609 (m), 1549 (s), 1465 (w), 1367 (w), 1305 (s), 1256 (w),
1183 (w), 1105 (w), 1053 (m), 999 (m), 892 (w), 806 (m), 670 (w)
cm−1. HRMS (ESI): calculated for C17H24FeNO2 [M + H]+, 330.1151;
1
found, 330.1149. H NMR (400 MHz, CDCl3): δ 1.04 (s, 9H), 2.69
(mc, 1H), 3.60−3.69 (m, 1H), 3.91−3.99 (m, 2H), 4.23 (s, 5H), 4.37
(minor diastereomer). IR (ATR): ν
̃
2954 (w), 2277 (w), 1653 (w),
(mc, 2H), 4.67 (mc, 1H), 4.70 (mc, 1H), 5.86 (d, J = 8.2 Hz, 1H). 13
C
1543 (m), 1471 (w), 1358 (s), 1173 (w), 1062 (s), 1023 (s), 848 (w),
735 (w), 706 (s) cm−1. HRMS (ESI): calculated for C18H30NOSi [M
+ H]+, 304.2091; found, 304.2079. Specific rotation [α]D20 = 41.3 (c =
0.96, CHCl3). For NMR analysis, a diastereoenriched sample (dr =
72:28) was used, which was obtained by chromatographic separation
of the diastereomers on silica gel using cyclohexane as eluent. NMR
NMR (126 MHz, CDCl3): δ 27.2, 33.6, 59.8, 63.7, 68.0, 68.6, 69.9,
20
70.66, 70.71, 76.1, 172.0. Specific rotation [α]D = −21.8 (c = 0.80,
CHCl3). The analytical and spectroscopic data are in accordance with
those reported.24
(S)-4-tert-Butyl-2-ferrocenyl-4,5-dihydrooxazole (8). To a
solution of (S)-N-(1-hydroxy-3,3-dimethylbutan-2-yl)ferrocenyl car-
3623
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boxamide (1.50 g, 4.57 mmol, 1.00 equiv) in CH2Cl2 (15 mL) at 0 °C
were added successively tosyl chloride (1.13 g, 5.94 mmol, 1.30 equiv)
and triethylamine (1.39 mL, 1.01 g, 10.0 mmol, 2.20 equiv). The
cooling bath was removed, and the reaction mixture was stirred for 20
h at room temperature. After addition of saturated aqueous NH4Cl
solution (15 mL), the phases were separated, and the aqueous phase
was extracted with CH2Cl2 (3 × 20 mL). The combined organic
phases were washed with saturated aqueous NaCl solution (50 mL),
dried over Na2SO4, and concentrated under reduced pressure. The
residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (10/1) as eluent, affording ferrocenyl
oxazoline 8 (1.04 g, 3.34 mmol, 73%) as a brown solid. Mp: 138.7 °C
(benzene). Rf = 0.06 (cyclohexane/ethyl acetate 10/1). GLC (HP-5):
1,2-Cl2C6D4): δ 0.61 (s, 3H), 0.77 (s, 9H), 0.81 (s, 9H), 4.08 (dd, J =
10.1, 7.8 Hz, 1H), 4.86 (dd, J = 10.0, 8.0 Hz, 1H), 5.00 (dd, J = 10.2,
10.4 Hz, 1H), 7.36−7.42 (m, 1H), 7.42−7.57 (m, 2H), 7.62−7.66 (m,
1H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −5.1, 18.7, 25.28, 25.32,
33.9, 71.1, 79.1, 124.6 (br m), 126.4, 132.4, 133.5, 134.2, 136.8 (d, J =
238.1 Hz), 137.2, 138.7 (d, J = 241.8 Hz), 140.9, 148.8 (d, J = 241.8
Hz), 181.8. 1H,15N HMQC NMR (700/71 MHz, 1,2-Cl2C6D4): δ
151.2. 1H,29Si HMQC NMR (500/99 MHz, 1,2-Cl2C6D4): δ 24.7.
1
NMR spectroscopic data for (SiR,S)-1 are as follows. H NMR (500
MHz, 1,2-Cl2C6D4): δ 0.51 (s, 3H), 0.85 (s, 9H), 0.89 (s, 9H), 4.04
(dd, J = 12.6, 9.6 Hz, 1H), 4.64 (dd, J = 12.7, 9.6 Hz, 1H), 4.92 (dd, J
= 9.6, 9.6 Hz, 1H), 7.36−7.42 (m, 1H), 7.42−7.57 (m, 2H), 7.57−7.61
(m, 1H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −8.6, 20.2, 25.97,
25.99, 32.0, 73.0, 77.7, 124.6 (br m), 125.8, 127.7, 133.5, 134.2, 136.8
(d, J = 238.1 Hz), 137.3, 138.7 (d, J = 241.8 Hz), 142.3, 148.8 (d, J =
241.8 Hz), 182.9. 1H,15N HMQC NMR (700/71 MHz, 1,2-Cl2C6D4):
̃
tR = 19.4 min. IR (ATR): ν 3092 (w), 2954 (s), 1658 (s), 1473 (m),
1356 (m), 1307 (m), 1265 (m), 1195 (m), 1103 (s), 1017 (s), 967 (s),
811 (s) cm−1. HRMS (ESI): calculated for C17H22FeNO [M + H]+,
1
δ 151.3. H,29Si HMQC NMR (500/99 MHz, 1,2-Cl2C6D4): δ 24.2.
1
312.1045; found, 312.1053. H NMR (500 MHz, C6D6): δ 0.91 (s,
11B NMR (160 MHz, 1,2-Cl2C6D4): −16.2. 19F NMR (471 MHz, 1,2-
9H), 3.78 (dd, J = 10.2, 8.2 Hz, 1H), 3.85−3.93 (m, 2H), 4.02 (mc,
2H), 4.09 (s, 5H), 4.90 (mc, 1H), 4.94 (mc, 1H). 13C NMR (126 MHz,
C6D6): δ 26.2, 33.6, 68.2, 69.6, 69.8, 70.28, 70.35, 72.0, 76.8, 165.2.
Cl2C6D4): −166.1, −162.3, −131.7.
(SiS*,S,Sp)-tert-Butyl(2-(4-tert-butyloxazolin-2-yl)ferrocenyl)-
methylsilylium Tetrakis(pentafluorophenyl)borate (2). This was
prepared from (SiS*,S,Sp)-tert-butyl(2-(4-tert-butyloxazolin-2-yl)-
ferrocenyl)methylsilane (6; 20.6 mg, 50.1 μmol, 1.00 equiv) and
[Ph3C]+[B(C6F5)4]− (50.7 mg, 55.0 mmol, 1.10 equiv) according to
GP3. The silicon cation 2 was obtained along with small amounts of
unreacted silane 6. NMR spectroscopic data for (SiS,S,Sp)-2 are as
20
Specific rotation [α]D = −183.8 (c = 0.86, CHCl3). The analytical
and spectroscopic data are in accordance with those reported.24
(SiS*,S,Sp)-tert-Butyl(2-(4-tert-butyloxazolin-2-yl)ferrocenyl)-
methylsilane (6). To a solution of ferrocenyl oxazoline 8 (141 mg,
0.452 mmol, 1.00 equiv) and TMEDA (0.89 mL, 68 mg, 0.59 mmol,
1.3 equiv) in n-hexane (10 mL) cooled to −78 °C was added nBuLi
(1.4 M in hexane fractions, 0.43 mL, 0.59 mmol, 1.3 equiv) dropwise,
and the resulting mixture was stirred for 2 h at −78 °C and warmed to
0 °C over a period of 1 h. tert-Butylchloro(methyl)silane (86.5 mg,
0.633 mmol, 1.40 equiv) was added dropwise, and the reaction mixture
was warmed to room temperature over a period of 20 h. After dilution
with tert-butyl methyl ether (15 mL) and addition of saturated
aqueous sodium bicarbonate solution (20 mL), the phases were
separated, and the aqueous phase was extracted with tert-butyl methyl
ether (3 × 10 mL). The combined organic phases were washed with
saturated aqueous NaCl solution (15 mL), dried over Na2SO4, and
concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel using cyclohexane/ethyl
acetate (20/1) as eluent, affording silane 6 (171 mg, 0.416 mmol, 92%,
dr = 61:39) as a brown oil. For the generation of the corresponding
silicon cation, the silane was azeotropically dried by addition and
subsequent evaporation of dry benzene under vacuum (3×). Rf = 0.17
(cyclohexane/ethyl acetate 20/1). GLC (HP-5): tR = 22.2 min (major
1
follows. H NMR (500 MHz, 1,2-Cl2C6D4): δ 0.71 (s, 9H), 0.79 (s,
3H), 0.81 (s, 9H), 3.79 (dd, J = 9.8, 8.1 Hz, 1H), 4.17 (s, 5H), 4.58−
4.64 (m, 1H), 4.71−4.80 (m, 2H), 4.82−4.88 (m, 2H). 13C NMR
(126 MHz, 1,2-Cl2C6D4): δ −2.8, 18.9, 25.0, 25.5, 33.3, 66.9, 69.5,
71.8, 72.3, 74.4, 77.9, 78.8, 80.6, 124.7 (br m), 136.8 (d, 241.8 Hz),
138.7 (d, J = 239.2 Hz), 148.8 (d, J = 244.5 Hz), 188.1. 1H,15N
HMQC NMR (700/71 MHz, 1,2-Cl2C6D4): δ 147.7. 29Si DEPT NMR
(99 MHz, 1,2-Cl2C6D4): δ 26.4. NMR spectroscopic data for
(SiR,S,Sp)-2 are as follows. 1H NMR (500 MHz, 1,2-Cl2C6D4): δ
0.22 (s, 3H), 0.82 (s, 9H), 1.17 (s, 9H), 3.62−3.68 (m, 1H), 4.19 (s,
5H), 4.65−4.70 (m, 1H), 4.72−4.76 (m, 1H), 4.76 (m, 1H), 4.82−
4.88 (m, 2H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −4.5, 18.9, 21.0,
24.9, 26.8, 66.5, 69.5, 72.2, 72.3, 75.7, 78.2, 79.4, 80.7, 124.7 (br m),
136.8 (d, 241.8 Hz), 138.7 (d, J = 239.2 Hz), 148.8 (d, J = 244.5 Hz),
1
188.6. H,15N HMQC NMR (700/71 MHz, 1,2-Cl2C6D4): δ 152.5.
29Si DEPT NMR (99 MHz, 1,2-Cl2C6D4): δ 25.4. 11B NMR (160
MHz, 1,2-Cl2C6D4): −16.0. 19F NMR (471 MHz, 1,2-Cl2C6D4):
−165.7, −161.9, −131.4.
̃
diastereomer), tR = 22.1 min (minor diastereomer). IR (ATR): ν 3096
(w), 2951 (m), 2852 (m), 2139 (w), 1637 (m), 1547 (w), 1462 (m),
1360 (m), 1248 (m), 1139 (m), 1056 (w), 996 (m), 870 (m), 813 (s),
719 (m) cm−1. HRMS (ESI): calculated for C22H34FeNOSi [M + H]+:
412.1754; found, 412.1738. Specific rotation [α]D20 = 160.0 (c = 0.58,
CHCl3). NMR spectroscopic data for the major diastereomer are as
follows. 1H NMR (500 MHz, C6D6): δ 0.38 (d, J = 3.8 Hz, 3H), 0.92
(s, 9H), 1.13 (s, 9H), 3.71 (dd, J = 10.2, 8.2 Hz, 1H). 3.81−3.89 (m,
2H), 4.05 (mc, 1H), 4.10 (s, 5H), 4.19−4.22 (m, 1H), 4.94 (q, J = 3.8
Hz, 1H), 5.05 (mc, 1H). 13C NMR (126 MHz, C6D6): δ −6.5, 18.0,
26.2, 28.1, 33.5, 68.0, 68.3,† 70.3, 72.8, 73.7, 76.9, 77.4, 79.7, 165.2 [†
2-(2-Bromophenyl)-1,3-dithiolane (11a). According to GP1,
ethane-1,2-dithiol (0.91 mL, 1.0 g, 11 mmol, 2.0 equiv) and iodine
(0.14 g, 0.54 mmol, 0.10 equiv) were added to a solution of 2-
bromobenzaldehyde (0.63 mL, 1.0 g, 5.4 mmol, 1.0 equiv) in CHCl3
(23 mL), and the resulting mixture was stirred at room temperature
for 15 h. After addition of aqueous NaOH solution (2 N, 20 mL), the
phases were separated, and the aqueous phase was extracted with tert-
butyl methyl ether (3 × 10 mL). The combined organic phases were
washed with saturated aqueous NaCl solution (10 mL), dried over
Na2SO4, and concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel using
cyclohexane as eluent, affording dithiolane 11a (1.14 g, 4.36 mmol,
81%) as a colorless oil. Rf = 0.47 (cyclohexane/ethyl acetate 30/1).
1
from the HMBC experiment]. H,15N HMQC NMR (700/71 MHz,
C6D6): δ 226.4. 29Si DEPT NMR (99 MHz, C6D6): δ −6.1. NMR
spectroscopic data for the minor diastereomer are as follows. 1H NMR
(500 MHz, C6D6): δ 0.78 (d, J = 3.7 Hz, 3H), 0.90 (s, 9H), 1.15 (s,
9H), 3.68 (dd, J = 9.7, 8.5 Hz, 1H), 3.75−3.83 (m, 2H), 4.14 (s, 5H),
4.19−4.22 (m, 1H), 4.31 (mc, 1H), 4.47 (q, J = 3.6 Hz, 1H), 5.05 (mc,
1H). 13C NMR (126 MHz, C6D6): δ −5.0, 18.0, 26.2, 28.2, 33.6, 68.0,
̃
GLC (HP-5): tR = 17.7 min. IR (ATR): ν 3053 (w), 2918 (w), 2826
(w), 1458 (m), 1435 (m), 1273 (m), 1239 (w), 1158 (w), 1111 (w),
1018 (s), 843 (m), 729 (s), 683 (m) cm−1. HRMS (ESI): calculated
1
for C9H9BrS2 [M]+, 261.9309; found, 261.9264. H NMR (500 MHz,
1
68.3, 70.2, 72.5, 74.1, 77.0, 77.2, 79.7, 165.2. H,15N HMQC NMR
C6D6): δ 2.64−2.71 (m, 2H), 2.74−2.81 (m, 2H), 6.15 (s, 1H), 6.60
(mc, 1H), 6.93 (mc, 1H), 7.26 (mc, 1H), 7.85 (mc, 1H). 13C NMR (126
MHz, C6D6): δ 39.6, 55.6, 124.5, 127.7, 129.2, 129.8, 133.0, 141.0.
2-(2-Bromophenyl)-2-methyl-1,3-dithiolane (11b). According
to GP1, ethane-1,2-dithiol (0.17 mL, 0.19 g, 2.0 mmol, 2.0 equiv) and
iodine (25.6 mg, 0.101 mmol, 0.101 equiv) were added to a solution of
2′-bromoacetophenone (0.14 mL, 0.20 g, 1.0 mmol, 1.0 equiv) in
CHCl3 (4.3 mL), and the resulting mixture was stirred at room
temperature for 5 h. After addition of aqueous NaOH solution (2 N,
(700/71 MHz, C6D6): δ 224.3. 29Si DEPT NMR (99 MHz, C6D6): δ
0.8.
(SiS*,S)-tert-Butyl(2-(4-tert-butyloxazolin-2-yl)phenyl)-
methylsilylium Tetrakis(pentafluorophenyl)borate (1). This was
prepared from (SiS*,S)-tert-butyl(2-(4-tert-butyloxazolin-2-yl)phenyl)-
methylsilane (5; 23.9 mg, 78.7 μmol, 1.00 equiv) and [Ph3C]+[B-
(C6F5)4]− (77.0 mg, 83.5 μmol, 1.06 equiv) according to GP3. NMR
1
spectroscopic data for (SiS,S)-1 are as follows. H NMR (500 MHz,
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10 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 20 mL). The combined
organic phases were washed with saturated aqueous NaCl solution (10
mL), dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (20/1) as eluent, affording dithiolane
11b (208 mg, 0.755 mmol, 75%) as a colorless oil. Rf = 0.45
(cyclohexane/ethyl acetate 10/1). GLC (HP-5): tR = 17.9 min. IR
organic phases were washed with saturated aqueous NaCl solution (20
mL), dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (50/1) as eluent, affording silane 9a
(132 mg, 0.467 mmol, 61%) as a slightly yellow oil. For the generation
of the corresponding silicon cation, the silane was azeotropically dried
by addition and subsequent evaporation of dry benzene under vacuum
(3×). Rf = 0.35 (cyclohexane/ethyl acetate 50/1). GLC (HP-5): tR =
(ATR): ν
̃
3054 (w), 2965 (m), 2917 (m), 2854 (m), 2719 (w), 2368
19.0 min. IR (ATR): ν 3051 (w), 2923 (s), 2852 (s), 2111 (s), 1587
̃
(w), 2281 (w), 2097 (w), 1922 (w), 1804 (w), 1454 (s), 1418 (s),
1369 (m), 1274 (m), 1184 (m), 1123 (w), 1051 (m), 1016 (s), 947
(m), 848 (m), 804 (w), 756 (s), 730 (s), 663 (s) cm−1. HRMS (ESI):
(w), 1463 (m), 1427 (m), 1360 (w), 1253 (m), 1119 (m), 1067 (w),
1007 (m), 887 (s), 822 (s), 725 (s) cm−1. HRMS (ESI): calculated for
1
C14H22S2Si [M]+, 283.0966; found, 283.1010. H NMR (500 MHz,
calculated for C10H12BrS2 [M + H]+, 276.9538; found, 276.9538. H
1
CDCl3): δ 0.39 (d, J = 3.8 Hz, 3H), 0.98 (s, 9H), 3.33−3.41 (m, 2H),
3.51−3.61 (m, 2H), 4.44 (q, J = 3.7 Hz, 1H), 5.98 (s, 1H), 7.23 (mc,
1H), 7.37−7.43 (m, 2H), 7.94 (mc, 1H). 13C NMR (126 MHz,
CDCl3): δ −7.1, 17.2, 27.5, 40.7, 40.8, 56.5, 127.1, 128.9, 130.2, 134.5,
135.3, 146.3. 29Si DEPT NMR (99 MHz, CDCl3): δ −7.5.
NMR (500 MHz, C6D6): δ 2.34 (s, 3H), 2.60−2.67 (m, 2H), 2.76−
2.83 (m, 2H), 6.63 (mc, 1H), 6.91 (mc, 1H), 7.44 (dd, J = 7.9, 1.4 Hz,
1H), 8.21 (dd, J = 8.0, 1.7 Hz, 1H). 13C NMR (126 MHz, C6D6): δ
31.3, 40.0, 70.4, 123.8, 127.2, 128.7, 136.0, 144.8.
2-(2-Bromophenyl)-1,3-dithiane (11c). According to GP1,
propane-1,3-dithiol (1.1 mL, 1.2 g, 11 mmol, 2.0 equiv) and iodine
(0.14 g, 0.54 mmol, 0.10 equiv) were added to a solution of 2-
bromobenzaldehyde (0.63 mL, 1.0 g, 5.4 mmol, 1.0 equiv) in CHCl3
(23 mL), and the resulting mixture was stirred at room temperature
for 15 h. After addition of aqueous NaOH solution (2 N, 20 mL), the
phases were separated, and the aqueous phase was extracted with tert-
butyl methyl ether (3 × 10 mL). The combined organic phases were
washed with saturated aqueous NaCl solution (10 mL), dried over
Na2SO4, and concentrated under reduced pressure. The residue was
purified by recrystallization from EtOH, affording dithiane 11c (1.18 g,
4.29 mmol, 79%) as a colorless solid. Mp: 98.3 °C (benzene). GLC
tert-Butylmethyl(2-(2-methyl-1,3-dithiolan-2-yl)phenyl)-
silane (9b). According to GP2, nBuLi (1.5 M in hexane fractions, 0.92
mL, 1.4 mmol, 1.3 equiv) was added dropwise to a solution of bromide
11b (0.30 g, 1.1 mmol, 1.0 equiv) in THF (10 mL) at −78 °C. After
an additional 1 h at −78 °C, a solution of tert-butylchloro(methyl)-
silane (298 mg, 2.18 mmol, 1.98 equiv) in THF (5 mL) was added,
and the resulting mixture was warmed to room temperature over a
period of 15 h. After addition of water (20 mL) and dilution with tert-
butyl methyl ether (15 mL), the phases were separated, and the
aqueous phase was extracted with tert-butyl methyl ether (3 × 20 mL).
The combined organic phases were washed with saturated aqueous
NaCl solution (35 mL), dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by flash column
chromatography on silica gel using cyclohexane/ethyl acetate (50/1)
as eluent, affording silane 9b (153 mg, 0.516 mmol, 47%) as a slightly
red oil. For the generation of the corresponding silicon cation, the
silane was azeotropically dried by addition and subsequent evaporation
of dry benzene under vacuum (3×). Rf = 0.45 (cyclohexane/ethyl
̃
(HP-5): tR = 18.5 min. IR (ATR): ν 3071 (w), 2929 (m), 2896 (m),
2831 (m), 1465 (m), 1417 (s), 1274 (s), 1175 (s), 1111 (w), 910 (m),
875 (m), 741 (s), 667 (s) cm−1. HRMS (ESI): calculated for
1
C10H11BrS2 [M]+, 275.9465; found, 275.9600. H NMR (500 MHz,
C6D6): δ 1.31−1.39 (m, 1H), 1.60 (mc, 1H), 2.26−2.33 (m, 2H),
2.51−2.60 (m, 2H), 5.81 (s, 1H), 6.56 (mc, 1H), 6.84 (mc, 1H), 7.26
(mc, 1H), 7.91 (mc, 1H). 13C NMR (126 MHz, C6D6): δ 25.2, 32.1,
51.3, 123.4, 128.3, 129.9, 130.4, 133.2, 139.2. The analytical and
spectroscopic data are in accordance with those reported.28b
̃
acetate 50/1). GLC (FS-SE-54): tR = 19.5 min. IR (ATR): ν 3048 (w),
2923 (s), 2852 (s), 2146 (s), 1927 (w), 1580 (w), 1458 (s), 1440 (m),
1361 (m), 1247 (s), 1089 (m), 1065 (m), 1008 (m), 842 (s), 716 (s)
cm−1. HRMS (ESI): calculated for C15H24S2Si [M]+: 297.1122; found,
2-(2-Bromophenyl)-2-methyl-1,3-dithiane (11d). According to
GP1, propane-1,3-dithiol (0.18 mL, 0.20 g, 1.8 mmol, 1.2 equiv) and
iodine (38.3 mg, 0.151 mmol, 0.100 equiv) were added to a solution of
2′-bromoacetophenone (0.20 mL, 0.30 g, 1.5 mmol, 1.0 equiv) in
CHCl3 (7.5 mL), and the resulting mixture was stirred at room
temperature for 2 h. After addition of aqueous NaOH solution (2 N,
10 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 10 mL). The combined
organic phases were washed with saturated aqueous NaCl solution (15
mL), dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (10/1) as eluent, affording dithiane
11d (252 mg, 0.871 mmol, 49%) as a colorless oil. Rf = 0.50
(cyclohexane/ethyl acetate 10/1). GLC (FS-SE-54): tR = 19.0 min. IR
1
297.1162. H NMR (500 MHz, C6D6): δ 0.41 (d, J = 3.6 Hz, 3H),
1.08 (s, 9H), 2.32 (s, 3H), 259−2.69 (m, 2H), 2.75−2.87 (m, 2H),
5.15 (q, J = 3.5 Hz, 1H), 7.04 (mc, 1H), 7.12 (mc, 1H), 7.58 (mc, 1H),
8.28 (mc, 1H). 13C NMR (126 MHz, C6D6): δ −5.5, 18.1, 28.9, 34.5,
38.5, 39.5, 72.2, 125.9, 126.7, 128.4, 136.50, 136.51, 153.2. 29Si DEPT
NMR (99 MHz, C6D6): δ −8.2.
tert-Butyl(2-(1,3-dithian-2-yl)phenyl)methylsilane (9c). Ac-
cording to GP2, nBuLi (1.5 M in hexane fractions, 0.63 mL, 0.95
mmol, 1.3 equiv) was added dropwise to a solution of bromide 11c
(0.20 g, 0.73 mmol, 1.0 equiv) in THF (7 mL) at −78 °C. After an
additional 1 h at −78 °C, a solution of tert-butylchloro(methyl)silane
(199 mg, 1.45 mmol, 1.99 equiv) in THF (3 mL) was added, and the
resulting mixture was warmed to room temperature over a period of
15 h. After addition of water (10 mL) and dilution with tert-butyl
methyl ether (10 mL), the phases were separated, and the aqueous
phase was extracted with tert-butyl methyl ether (3 × 15 mL). The
combined organic phases were washed with saturated aqueous NaCl
solution (25 mL), dried over Na2SO4, and concentrated under reduced
pressure. The residue was purified by flash column chromatography on
silica gel using cyclohexane/ethyl acetate (30/1) as eluent, affording
silane 9c (80.7 mg, 0.272 mmol, 37%) as a colorless oil. For the
generation of the corresponding silicon cation, the silane was
azeotropically dried by addition and subsequent evaporation of dry
benzene under vacuum (3×). Rf = 0.30 (cyclohexane/ethyl acetate 50/
̃
(ATR): ν 3055 (w), 2899 (m), 2231 (w), 2087 (w), 1922 (w), 1811
(w), 1579 (w), 1419 (s), 1259 (m), 1181 (m), 1120 (m), 1056 (m),
1015 (s), 907 (m), 735 (s), 662 (s) cm−1. HRMS (ESI): calculated for
1
C11H13BrS2 [M]+, 289.9622; found, 289.9572. H NMR (500 MHz,
C6D6): δ 1.25−1.32 (m, 1H), 1.55−1.65 (m, 1H), 2.11 (s, 3H), 2.15−
2.22 (m, 2H), 2.31−2.40 (m, 2H), 6.67 (mc, 1H), 6.98 (mc, 1H), 7.54
(mc, 1H), 8.24 (mc, 1H). 13C NMR (126 MHz, C6D6): δ 24.4, 28.7,
29.1, 54.1, 123.3, 127.0, 128.8, 132.6, 137.3, 141.4.
tert-Butyl(2-(1,3-dithiolan-2-yl)phenyl)methylsilane (9a). Ac-
cording to GP2, nBuLi (1.5 M in hexane fractions, 0.66 mL, 1.0 mmol,
1.3 equiv) was added dropwise to a solution of bromide 11a (0.20 g,
0.77 mmol, 1.0 equiv) in THF (7 mL) at −78 °C. After an additional 1
h at −78 °C, a solution of tert-butylchloro(methyl)silane (209 mg,
1.53 mmol, 1.99 equiv) in THF (3 mL) was added, and the resulting
mixture was warmed to room temperature over a period of 2 h. After
addition of water (10 mL) and dilution with tert-butyl methyl ether
(10 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 7 mL). The combined
̃
1). GLC (HP-5): tR = 19.8 min. IR (ATR): ν 3053 (w), 2925 (m),
2891 (m), 2852 (m), 2109 (s), 1586 (w), 1465 (m), 1421 (m), 1273
(m), 1170 (w), 1121 (m), 1066 (w), 1006 (w), 885 (s), 820 (s), 730
(s), 674 (m) cm−1. HRMS (ESI): calculated for C15H24S2Si [M]+,
297.1122; found, 297.1169. 1H NMR (500 MHz, CDCl3): δ 0.41 (d, J
= 3.8 Hz, 3H), 1.00 (s, 9H), 1.95 (mc, 1H), 2.14−2.21 (m, 1H), 2.86−
2.95 (m, 2H), 3.03 (mc, 2H), 4.38 (q, J = 3.6 Hz, 1H), 5.36 (s, 1H),
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7.24−7.29 (m, 1H), 7.40 (mc, 1H), 7.45 (mc, 1H), 7.69 (mc, 1H). 13
NMR (126 MHz, CDCl3): δ −7.2, 17.3, 25.3, 27.4, 32.5, 32.8, 52.7,
127.5, 128.3, 130.2, 133.8, 136.1, 145.1. 29Si DEPT NMR (99 MHz,
CDCl3): δ −6.6.
C
Cl2C6D4): δ −9.1, 21.0, 22.7, 22.9, 24.9, 31.5, 59.3, 124.7 (br m),
128.1, 130.1, 134.1, 134.8, 136.8 (d, J = 243.1 Hz), 138.7 (d, J = 240.2
Hz), 140.2, 142.5, 148.8 (d, J = 240.2 Hz). 11B NMR (160 MHz, 1,2-
Cl2C6D4): δ −16.0. 19F NMR (471 MHz, 1,2-Cl2C6D4): δ −166.0,
−162.2, −131.7. 29Si DEPT NMR (99 MHz, 1,2-Cl2C6D4): δ 51.3.
tert-Butylmethyl(2-(2-methyl-1,3-dithian-2-yl)phenyl)-
silylium Tetrakis(pentafluorophenyl)borate (3d). This was
prepared from tert-butylmethyl(2-(2-methyl-1,3-dithian-2-yl)phenyl)-
silane (9d; 24.9 mg, 80.2 μmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]−
tert-Butylmethyl(2-(2-methyl-1,3-dithian-2-yl)phenyl)silane
(9d). According to GP2, nBuLi (1.54 M in hexane fractions, 1.17 mL,
1.80 mmol, 1.30 equiv) was added dropwise to a solution of bromide
11d (0.40 g, 1.4 mmol, 1.0 equiv) in THF (13 mL) at −78 °C. After
an additional 1 h at −78 °C, a solution of tert-butylchloro(methyl)-
silane (345 mg, 2.53 mmol, 1.80 equiv) in THF (7 mL) was added,
and the resulting mixture was warmed to room temperature over a
period of 15 h. After addition of water (15 mL) and dilution with tert-
butyl methyl ether (10 mL), the phases were separated, and the
aqueous phase was extracted with tert-butyl methyl ether (3 × 20 mL).
The combined organic phases were washed with saturated aqueous
NaCl solution (30 mL), dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by flash column
chromatography on silica gel using cyclohexane/ethyl acetate (50/1)
as eluent, affording silane 9d (193 mg, 0.621 mmol, 45%) as a white
solid. For the generation of the corresponding silicon cation, the silane
was azeotropically dried by addition and subsequent evaporation of
dry benzene under vacuum (3×). Mp: 75.9 °C (cyclohexane/ethyl
acetate). Rf = 0.45 (cyclohexane/ethyl acetate 20/1). GLC (FS-SE-
1
(73.8 mg, 80.0 μmol, 1.00 equiv) according to GP3. H NMR (500
MHz, 1,2-Cl2C6D4): δ 0.59 (s, 3H), 0.92 (s, 9H), 1.56−1.63 (m, 1H),
1.70−179 (m, 1H), 1.76 (s, 3H), 2.27 (ddd, J = 13.7, 8.1, 4.8 Hz, 1H),
2.35−2.43 (m, 1H), 2.64 (ddd, J = 12.7, 10.0, 2.7 Hz, 1H), 3.00 (ddd, J
= 11.9, 7.6, 2.4 Hz, 1H), 7.26−7.31 (m, 1H), 7.31−7.34 (m, 1H),
7.42−7.48 (m, 2H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −6.1, 19.7,
20.1, 25.2, 26.3, 33.6, 34.9, 73.8, 125.0 (br m), 126.0, 128.1, 130.3,
134.0, 134.2, 136.8 (d, J = 246.0 Hz), 138.7 (d, J = 237.4 Hz), 147.7,
148.8 (d, J = 240.2 Hz). 11B NMR (225 MHz, 1,2-Cl2C6D4): δ −16.0.
19F NMR (659 MHz, 1,2-Cl2C6D4): δ −165.7, −161.9, −131.5. 29Si
DEPT NMR (139 MHz, 1,2-Cl2C6D4): δ 42.3, 48.6.
(SS)-Ferrocenyl p-Tolyl Sulfoxide. Ferrocene (0.50 g, 2.7 mmol,
1.0 equiv) and KOtBu (30 mg, 0.27 mmol, 0.10 equiv) were
suspended in THF (8 mL), and the suspension was cooled to −78 °C.
tBuLi (1.48 M in n-pentane, 1.73 mL, 2.55 mmol, 0.944 equiv) was
added dropwise. The resulting mixture was stirred for 30 min at −78
°C and 30 min at room temperature and then added dropwise to a
solution of (1R,2S,5R)-(−)-menthyl (S)-p-toluenesulfinate (0.791 g,
2.69 mmol, 1.00 equiv) in THF (8 mL) at −78 °C. The reaction
mixture was warmed to room temperature and stirred for a further 16
h. After addition of water (15 mL), the phases were separated, and the
aqueous phase was extracted with tert-butyl methyl ether (3 × 10 mL).
The combined organic phases were washed with saturated aqueous
NaCl solution (25 mL), dried over Na2SO4, and concentrated under
reduced pressure. The residue was purified by flash column
chromatography on silica gel using cyclohexane/ethyl acetate (2/1)
as eluent, affording (SS)-ferrocenyl p-tolyl sulfoxide (585 mg, 1.80
mmol, 67%, 93% ee) as an orange solid. Mp: 141.8 °C (cyclohexane/
ethyl acetate). Rf = 0.40 (cyclohexane/ethyl acetate 1/1). GLC (FS-
̃
54): tR = 20.5 min. IR (ATR): ν 2930 (s), 2851 (s), 2197 (s), 2111
(w), 1582 (w), 1449 (m), 1244 (m), 1125 (w), 1058 (s), 1003 (w),
818 (s), 713 (s) cm−1. HRMS (ESI): calculated for C16H26S2Si [M]+,
311.1279; found, 311.1320. 1H NMR (500 MHz, C6D6): δ 0.37 (d, J =
3.5 Hz, 3H), 1.10 (s, 9H), 1.34−1.41 (m, 1H), 1.61 (mc, 1H), 2.09 (s,
3H), 2.15−2.30 (m, 3H), 2.41 (mc, 1H), 5.35 (q, J = 3.4 Hz, 1H), 7.07
(mc, 1H), 7.14−7.19 (m, 1H), 7.68 (mc, 1H), 8.19 (mc, 1H). 13C NMR
(126 MHz, C6D6): δ −5.2, 18.5, 24.3, 28.3, 28.8, 29.1, 32.4, 55.9,
125.9, 128.5, 129.3, 137.1, 137.8, 150.3. 29Si DEPT NMR (99 MHz,
C6D6): δ −8.7.
tert-Butyl(2-(1,3-dithiolan-2-yl)phenyl)methylsilylium
Tetrakis(pentafluorophenyl)borate (3a). This was prepared from
tert-butyl(2-(1,3-dithiolan-2-yl)phenyl)methylsilane (9a; 14.1 mg, 49.9
μmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]− (50.7 mg, 55.0 μmol, 1.10
1
equiv) according to GP3. H NMR (500 MHz, 1,2-Cl2C6D4): δ 0.67
̃
SE-54): tR = 24.1 min. IR (ATR): ν 3075 (m), 3029 (w), 2915 (w),
(s, 3H), 0.81 (s, 9H), 2.39 (ddd, J = 13.3, 10.1, 5.7 Hz, 1H), 3.20 (ddd,
J = 13.3, 5.2, 3.1 Hz, 1H), 3.44 (ddd, J = 11.9, 10.2, 5.2 Hz, 1H), 3.49
(ddd, J = 11.9, 5.7, 3.1 Hz, 1H), 6.20 (s, 1H), 7.18−7.22 (m, 1H), 7.25
(mc, 1H), 7.31−7.35 (m, 1H), 7.39 (mc, 1H). 13C NMR (126 MHz,
1,2-Cl2C6D4): δ −6.2, 20.0, 24.7, 36.8, 46.1, 71.8, 124.7 (br m), 127.2,
128.1, 131.8, 133.8, 136.8 (d, J = 240.2 Hz), 138.7 (d, J = 243.1 Hz),
142.5, 143.0, 148.8 (d, J = 243.1 Hz). 11B NMR (160 MHz, 1,2-
Cl2C6D4): δ −16.2. 19F NMR (471 MHz, 1,2-Cl2C6D4): δ −166.0,
−162.1, −131.8. 29Si DEPT NMR (99 MHz, 1,2-Cl2C6D4): δ 56.7.
tert-Butylmethyl(2-(2-methyl-1,3-dithiolan-2-yl)phenyl)-
silylium Tetrakis(pentafluorophenyl)borate (3b). This was
prepared from tert-butylmethyl(2-(2-methyl-1,3-dithiolan-2-yl)-
phenyl)silane (9b; 23.7 mg, 79.9 μmol, 1.00 equiv) and [Ph3C]+[B-
2161 (w), 2074 (w), 1918 (w), 1653 (w), 1595 (w), 1491 (m), 1446
(w), 1402 (m), 1159 (m), 1083 (m), 1043 (s), 890 (w), 808 (s), 704
(m) cm−1. HRMS (ESI): calculated for C17H17FeOS [M + H]+,
1
325.0305; found, 325.0341. H NMR (500 MHz, CDCl3): δ 2.37 (s,
3H), 4.32 (mc, 1H), 4.36 (mc, 2H), 4.37 (s, 5H), 4.61 (mc, 1H), 7.25
(d, J = 8.2 Hz, 2H), 7.52 (d, J = 8.2 Hz, 2H). 13C NMR (126 MHz,
CDCl3): δ 21.5, 65.4, 68.0, 70.0, 70.1, 124.5, 129.8, 141.1, 143.1† (the
dagger (†) denotes a signal from the HMBC experiment). HPLC
(Daicel Chiralcel OD-H, 20 °C, n-heptane/i-PrOH 90/10, flow rate
0.70 mL/min, λ = 230 nm): tR = 18.7 min [(SR)-ferrocenyl(p-
tolyl)sulfoxide], tR = 22.1 min [(SS)-ferrocenyl(p-tolyl)sulfoxide].
Specific rotation [α]D20 = 284.4 (c = 0.93, CHCl3). The analytical and
spectroscopic data are in accordance with those reported.30a
1
(C6F5)4]− (73.8 mg, 80.0 μmol, 1.00 equiv) according to GP3. H
NMR (500 MHz, 1,2-Cl2C6D4): δ 0.62 (s, 3H), 0.85 (s, 9H), 2.04 (s,
3H), 2.36 (ddd, J = 13.5, 11.6, 4.8 Hz, 1H), 3.18 (ddd, J = 13.5, 5.0,
1.6 Hz, 1H), 3.49 (ddd, J = 12.0, 3.6, 1.4 Hz, 1H), 3.63 (ddd, J = 11.8,
5.0, 5.0 Hz, 1H), 7.18−7.20 (m, 1H), 7.25 (mc, 1H), 7.31−7.35 (m,
1H), 7.41 (mc, 1H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −6.1, 19.6,
25.1, 29.1, 38.5, 48.4, 92.6, 124.5 (br m), 125.8, 128.1, 131.6, 134.0,
136.8 (d, J = 248.9 Hz), 138.7 (d, J = 240.2 Hz), 142.5, 148.8 (d, J =
243.1 Hz), 149.6. 11B NMR (225 MHz, 1,2-Cl2C6D4): δ −16.0. 19F
NMR (471 MHz, 1,2-Cl2C6D4): δ −166.1, −162.1, −132.0. 29Si DEPT
NMR (99 MHz, 1,2-Cl2C6D4): δ 51.7.
(SS,Sp)-1-Formyl-2-((4-methylphenyl)sulfinyl)ferrocene (12).
To a solution of (SS)-ferrocenyl(p-tolyl)sulfoxide (0.50 g, 1.5 mmol,
1.0 equiv, 93% ee) in THF (7.7 mL) cooled to −78 °C was added
freshly prepared lithium diisopropylamide (0.88 M in THF, 2.3 mL,
2.0 mmol, 1.3 equiv), and the resulting mixture was stirred for 30 min
at −78 °C. DMF (0.30 mL, 0.28 g, 3.9 mmol, 2.5 equiv) was added,
and the reaction mixture was stirred for an additional 30 min at −78
°C and 1.5 h at 0 °C. After addition of aqueous NaOH solution (2 N,
15 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 15 mL). The combined
organic phases were washed with saturated aqueous NaCl solution (25
mL), dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (1/1) as eluent, affording aldehyde 12
(305 mg, 0.865 mmol, 56%, 96% ee) as a dark brown solid. Mp: 147.9
°C (cyclohexane/ethyl acetate). Rf = 0.20 (cyclohexane/ethyl acetate
tert-Butyl(2-(1,3-dithian-2-yl)phenyl)methylsilylium
Tetrakis(pentafluorophenyl)borate (3c). This was prepared from
tert-butyl(2-(1,3-dithian-2-yl)phenyl)methylsilane (9c; 14.9 mg, 50.2
μmol, 1.00 equiv) and [Ph3C]+[B(C6F5)4]− (51.0 mg, 55.3 μmol, 1.10
1
equiv) according to GP3. H NMR (500 MHz, 1,2-Cl2C6D4): δ 0.53
(s, 3H), 0.81 (s, 9H), 1.79−1.87 (m, 2H), 2.19−2.32 (m, 2H), 2.42−
2.50 (m, 1H), 2.67 (mc, 1H), 5.44 (s, 1H), 7.30−7.34 (m, 1H), 7.37−
7.40 (m, 1H), 7.50 (mc, 1H), 7.60 (mc, 1H). 13C NMR (126 MHz, 1,2-
1/1). IR (ATR): ν 3320 (w), 3092 (w), 2818 (w), 2320 (w), 1667 (s),
̃
1594 (w), 1434 (m), 1225 (m), 1162 (m), 1029 (s), 814 (s), 752 (s)
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Organometallics
Article
cm−1. HRMS (ESI): calculated for C18H17FeOS [M + H]+, 353.0254;
found, 353.0294. H NMR (500 MHz, CDCl3): δ 2.37 (s, 3H), 4.49
(m, 2H), 3.86 (mc, 1H), 4.17 (s, 5H), 4.24 (mc, 1H), 4.49 (q, J = 3.4
Hz, 1H), 4.84 (mc, 1H), 5.89 (s, 1H). 29Si DEPT NMR (99 MHz,
C6D6): δ −8.1.
1
(s, 5H), 4.71 (mc, 1H), 4.76 (mc, 1H), 5.02 (mc, 1H), 7.25 (d, J = 6.6
Hz, 2H), 7.50 (d, J = 7.5 Hz, 2H), 10.49 (s, 1H). 13C NMR (126
MHz, CDCl3): δ 21.5, 70.7, 71.8, 73.1, 74.2, 78.7, 97.4, 124.3, 130.0,
141.4, 142.4, 192.9. HPLC (Daicel Chiralcel OD-H, 20 °C, n-heptane/
(Sp )-tert-Butyl(2-(1,3-dithiolan-2-yl)ferrocenyl)-
methylsilylium Tetrakis(pentafluorophenyl)borate (4). This was
prepared from (Sp)-tert-butyl(2-(1,3-dithiolan-2-yl)ferrocenyl)-
methylsilane (10; 31.2 mg, 79.9 μmol, 1.00 equiv) and [Ph3C]+[B-
i-PrOH 80/20, flow rate 0.70 mL/min, λ = 210 nm): tR = 21.4 min
1
(C6F5)4]− (73.8 mg, 80.0 μmol, 1.00 equiv) according to GP3. H
20
[(SS,Sp)-12], tR = 28.8 min [(SR,Rp)-12]. Specific rotation [α]D
=
NMR (500 MHz, 1,2-Cl2C6D4): δ 0.53 (s, 2H, SiMe), 0.54 (s, 3H,
SiMe), 0.80 (s, 6H, SitBu), 0.86 (s, 9H, SitBu), 0.87 (s, 2H, SiMe),
0.90, (s, 3H, SiMe), 0.98 (s, 6H, SitBu), 1.09 (s, 10H, SitBu), 2.98−
2.95 (mc, 3H), 3.14−3.21 (mc, 1H), 3.23−3.33 (m, 3H), 3.34−3.43
(m, 2H), 3.45−3.48 (m, 1H), 3.48−3.51 (m, 2H), 3.51−3.59 (m, 5H),
3.59−3.65 (2H), 4.03−4.04 (m 1H), 4.05 (s, 5H), 4.06 (s, 5H), 4.10−
4.11 (m, 1H), 4.14 (s, 3H), 4.17 (s, 1H), 4.34−4.41 (m, 3H), 4.47−
4.51 (m, 3H), 4.52−4.55 (m, 2H), 5.66 (s, 1H), 6.00 (s, 1H), 6.07 (s,
1H). 13C NMR (126 MHz, 1,2-Cl2C6D4): δ −3.0, −2.2, −0.62, −0.60,
19.9, 20.3, 21.3, 22.8, 24.9, 26.01, 26.03, 26.2, 26.4, 26.5, 37.3, 41.4,
42.0, 42.4, 42.7, 42.9, 43.1, 44.4, 63.6, 64.6, 68.6, 69.3, 69.6, 70.8, 70.9,
71.11, 71.14, 71.3, 71.5, 71.8, 74.1, 76.7, 77.5, 78.2, 78.5, 78.7, 80.0,
85.1, 85.8, 87.8, 96.7, 124.6 (br m), 136.7 (d, J = 243.1 Hz), 138.6 (d, J
= 243.1 Hz), 148.7 (d, J = 237.4 Hz). 11B NMR (225 MHz, 1,2-
Cl2C6D4): δ −16.4. 19F NMR (659 MHz, 1,2-Cl2C6D4): δ −166.1,
−162.2, −131.9. 1H,29Si HMQC NMR (500/99 MHz, 1,2-Cl2C6D4): δ
50.4, 51.3, 57.3, 57.8.
223.5 (c = 0.74, CHCl3). The analytical and spectroscopic data are in
accordance with those reported.30c
(SS,Sp)-1-(1,3-Dithiolan-2-yl)-2-((4-methylphenyl)sulfinyl)-
ferrocene (13). Ethane-1,2-dithiol (75 μL, 80 mg, 0.85 mmol, 1.2
equiv) and iodine (18 mg, 0.071 mmol, 0.10 equiv) were added to a
solution of aldehyde 12 (0.25 g, 0.71 mmol, 1.0 equiv, 96% ee) in
CHCl3 (3.5 mL), and the resulting mixture was stirred at room
temperature for 3.5 h. After addition of aqueous NaOH solution (2 N,
8 mL), the phases were separated, and the aqueous phase was
extracted with tert-butyl methyl ether (3 × 8 mL). The combined
organic phases were washed with saturated aqueous NaCl solution (10
mL), dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica gel
using cyclohexane/ethyl acetate (2/1) as eluent, affording dithiolane
13 (241 mg, 0.563 mmol, 79%, 97% ee) as an orange solid. Mp: 143.5
°C (cyclohexane/ethyl acetate). Rf = 0.50 (cyclohexane/ethyl acetate
̃
1/1). GLC (FS-SE-54): tR = 32.1 min. IR (ATR): ν 3037 (w), 2919
(w), 2299 (w), 2064 (w), 1893 (w), 1717 (w), 1491 (m), 1408 (m),
1276 (w), 1229 (m), 1180 (w), 1083 (m), 1034 (s), 804 (s), 701 (m)
cm−1. HRMS (ESI): calculated for C20H20FeOS3 [M]+, 429.0059;
ASSOCIATED CONTENT
* Supporting Information
■
S
1
found, 429.0095. H NMR (500 MHz, C6D6): δ 1.98 (s, 3H), 2.50−
Figures giving NMR spectra of the compounds synthesized in
this paper. This material is available free of charge via the
Internet at http://pubs.acs.org.
2.57 (m, 1H), 2.64 (mc, 2H), 2.73−2.80 (m, 1H), 3.95 (mc, 1H), 4.24
(mc, 1H), 4.44 (s, 5H), 4.53 (mc, 1H), 6.70 (s, 1H), 6.96 (d, J = 8.0
Hz, 2H), 7.64 (d, J = 8.3 Hz, 2H). 13C NMR (126 MHz, C6D6): δ
21.2, 39.4, 40.0, 49.6, 69.3, 70.1, 70.6, 71.6, 90.0, 92.1, 124.9, 129.8,
140.1, 143.9. HPLC (Daicel Chiralcel OD-H, 20 °C, n-heptane/i-
PrOH 80/20, flow rate 0.70 mL/min, λ = 254 nm): tR = 19.3 min
[(SS,Sp)-13], 50.9 min [(SR,Rp)-13]. Specific rotation [α]D20 = 455.1 (c
= 0.89, CHCl3).
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.O.: martin.oestreich@tu-berlin.de.
■
Notes
(SiS*,Sp)-tert-Butyl(2-(1,3-dithiolan-2-yl)ferrocenyl)-
methylsilane (10). To a solution of dithiolane 13 (0.25 g, 0.58 mmol,
1.0 equiv, 97% ee) in THF (3 mL) cooled to −78 °C was added tBuLi
(1.5 M in n-pentane, 0.43 mL, 0.64 mmol, 1.1 equiv) dropwise, and
the resulting mixture was stirred for an additional 30 min at −78 °C. A
solution of tert-butylchloro(methyl)silane (95.7 mg, 0.700 mmol, 1.21
equiv) in THF (4 mL) was added, and the reaction mixture was
warmed to room temperature over a period of 20 h. After addition of
aqueous NaOH solution (2 N, 20 mL), the phases were separated, and
the aqueous phase was extracted with tert-butyl methyl ether (3 × 15
mL). The combined organic phases were washed with saturated
aqueous NaCl solution (25 mL), dried over Na2SO4, and concentrated
under reduced pressure. The residue was purified by flash column
chromatography on silica gel using cyclohexane/CH2Cl2 (10/1) as
eluent, affording silane 10 (114 mg, 0.291 mmol, 50%, 97% ee, dr =
92:8) as a brown oil. For the generation of the corresponding silicon
cation, the silane was azeotropically dried by addition and subsequent
evaporation of dry benzene under vacuum (3×). Rf = 0.30
(cyclohexane/CH2Cl2 10/1). GLC (FS-SE-54): tR = 24.3 min
(major diastereomer), tR = 24.6 min (minor diastereomer). IR
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.H.G.R. thanks the Fonds der Chemischen Industrie for a
predoctoral fellowship (2012−2014), and M.O. is indebted to
the Einstein Foundation (Berlin) for an endowed professorship.
We are grateful to Dr. Sebastian Kemper for expert advice with
the NMR measurements (TU Berlin).
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