Heavier Carbonyl Olefination: The Sila-Wittig Reaction

Article abstract of DOI:10.1021/jacs.9b09379

The Wittig reaction is one of the most versatile tools in the repertoire of organic chemists. Thus, a broad variety of carbonyl compounds can be converted to tailor-made alkenes with phosphorus ylides under mild conditions. However, no comparable reaction has been reported for silanones, the silicon congeners of ketones. Here, we demonstrate for the first time the successful application of the Wittig olefination to iminosilylsilanone 1. The selective formation of a series of silenes (R₂Sia? CR₂) via the sila-Wittig reaction revealed an unprecedented approach to otherwise elusive compounds. In addition, the highly reactive and zwitterionic nature of 1 was also susceptible to nucleophilic attacks and cycloaddition reactions by and with the phosphorus ylides. Our results therefore make another important contribution to discovering the differences and similarities between carbon and silicon.

Full text of DOI:10.1021/jacs.9b09379

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Article
Heavier Carbonyl Olefination: The Sila-Wittig Reaction
Dominik Reiter, Philipp Frisch, Tibor Szilvási, and Shigeyoshi Inoue
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09379 • Publication Date (Web): 27 Sep 2019
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Heavier Carbonyl Olefination: The Sila-Wittig Reaction
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Dominik Reiter,† Philipp Frisch,† Tibor Szilvási,‡ and Shigeyoshi Inoue*,†
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†
Department of Chemistry, WACKER-Institute of Silicon Chemistry and Catalysis Research Center, Technische
Universität München, Lichtenbergstraße 4, 85748 Garching bei München, Germany
‡
Department of Chemical and Biological Engineering, University of Wisconsin–Madison, 1415 Engineering Drive,
Madison, Wisconsin 53706-1607, United States
Supporting Information Placeholder
ABSTRACT: The Wittig reaction is one of the most versatile
tools in the repertoire of organic chemists. Thus, a broad
SILA-WITTIG REACTION
Dipp
Dipp
variety of carbonyl compounds can be converted to tailormade
alkenes with phosphorus ylides under mild conditions.
However, no comparable reaction has been reported for
silanones, the silicon congeners of ketones. Here, we
demonstrate for the first time the successful application of the
Wittig olefination to iminosilylsilanone 1. The selective
formation of a series of silenes (R2Si=CR2) via the Sila-Wittig reaction revealed an unprecedented approach to otherwise
elusive compounds. In addition, the highly reactive and zwitterionic nature of 1 was also susceptible to nucleophilic attacks
and cycloadditon reactions by and with the phosphorus ylides. Our results therefore make another important contribution
to discovering the differences and similarities between carbon and silicon.
O
C
HR
N
N
Ph3P CHR
– Ph3P
Si
Si
N
SitBu3
O
N
SitBu3
N
N
Dipp
Dipp
(E)/(Z)-silenes
silanone
substituents) results in a rather low stereoselective
outcome.
INTRODUCTION
Since the discovery of alkene synthesis from carbonyl
A major limitation of the traditional Wittig reaction is the
compounds with phosphorus ylides by Staudinger1 and
partially reduced overall efficiency caused by the
further intense developments by Wittig2, this versatile
sometimes challenging removal of the phosphine oxide
chemo- and regioselective reaction has become
by-product.5 Recent investigations addressed this issue
indispensable and surpasses all other known olefination
and targeted the development of catalytic Wittig
methods.3 Therefore, the nowadays widely known Wittig
reactions with reduced phosphine oxide formation.6
reaction has emerged as a powerful synthetic tool with
Despite its discovery a long time ago, the mechanism of
applications both in academia and industry.3 The
the Wittig reaction is not fully elucidated and still
“classical” Wittig reaction occurs either between an
controversially discussed.4a For the lithium-salt free Wittig
aldehyde or ketone and a phosphorus ylide (or ylene; for
reaction, however, the mechanism generally accepted
convenience, the ylene form is used throughout this
today involves an irreversible [2 + 2] cycloaddition
manuscript), providing a mixture of (E)- and (Z)-alkenes
affording a four-membered oxaphosphetane as the only
and the respective phosphine oxide by-product (Scheme
intermediate under kinetic control.4a
1). Over the years, various protocols have been developed
In marked contrast to carbonyl compounds, the
to fine-tune the stereoselectivity and thus the olefin
formation of heavier alkene homologues R2E=CR2 (E = Si
product ratio. In general, the nature of the ylide plays a
(silenes), Ge, Sn, Pb) from carbonyl congeners R2E=O
pivotal role and strongly determines the stereoselectivity
(silanones: E = Si) via the Wittig reaction is not known.
of the reaction.4 While non-stabilized ylides (alkyl
Nevertheless, a Sila-Wittig-type reaction by formation of
a Lewis base-stabilized silene was observed when
substituents) mainly lead to (Z)-alkenes and stabilized
ylides (electron-withdrawing groups (EWG)) to (E)-
alkenes, utilization of semi-stabilized ylides (aryl or alkenyl
mesitylaldehyde was exposed to a stable phosphonium
sila-ylide.7 The ab-
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react with the bases used or the by-products of
Scheme 1. (A) Conversion of an Aldehyde or a Ketone
into Alkenes via the Wittig Reaction. (B) Syntheses of
Silenes from Iminosilylsilanone 1 via the Sila-Wittig
Reaction.
1
2
3
4
5
6
7
8
deprotonation itself. Thus, we pursued a modified
approach and isolated the phosphorus ylides 2 by
reaction of the corresponding phosphonium bromides
with KOtBu in THF, followed by subsequent extraction with
either n-hexane or toluene (Scheme 2, for details see the
Supporting Information). We then treated the isolated
ylides 2 with iminosilylsilanone 1 in benzene at room
temperature. It is worth mentioning that for a successful
Sila-Wittig olefination, the reactions generally have to be
relatively fast, as otherwise 1 slowly and irreversibly
rearranges to its more stable iminosiloxysilylene isomer
(IDippN)(tBu3SiO)Si: (1′) (Scheme 3).13e
(A) Classical Wittig reaction
R4
R3
R4
R3
R3P
ylide
R3P
or
ylene
9
R3 R4
R4 R3
O
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+
R1 R2
R1 R2
R1 R2
aldehyde or ketone
(Z)-alkenes
(E)-alkenes
R3P
O
phosphine oxide
alkyl, aryl, alkoxy
H, alkyl, aryl
R =
R1,R2
R3,R4
=
=
Scheme 2. Synthesis and Structures of Selected
Phosphorus Ylides.
H, alkyl, aryl, vinyl, EWG (C(O)R, CO2R, CN, etc.)
(B) Sila-Wittig reaction (this work)
1) KOtBu (THF)
– KBr
R
Dipp
Ph3P
O
N
R
R
– HOtBu
Si
Br
R
R
N
SitBu3
N
+
Ph3P
Si
Si
Ph3P
SitBu3
IDippN
SitBu3
Dipp
silanone
IDippN
(Z)-silenes
2) Extraction
2
(n-hexane or toluene)
1
(E)-silenes
Ph3P
O
Dipp = 2,6-iPr2-C6H3; IDippN = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-imino
O
R
R
R
Ph3P
Ph3P
semi-stabilized ylides
Ph3P
non-stabilized ylides
sence of further reports is not surprising when one
considers the lack of suitable precursors. It was not until
1976/1981 that Lappert, Brook, West and Yoshifuji
disproved the well-known “double bond rule” with their
seminal work on the isolation of the first distannene8
stabilized ylides
2d
2e
2f-m (
R = H
R = H, Me, Et, Pr, Bu,
Hex, 2-PhEt, 2-BnOEt)
2a
2b
2c
R = Me
R = Ph
R = OMe
R = OiPr
(R2Sn=SnR2),
silene9,
disilene10
(R2Si=SiR2)
and
We started our investigations with stabilized ylides, as
some of them are commercially available. Reaction of
silanone 1 with the ketone-substituted ylide 2a selectively
diphosphene11 (RP=PR), respectively. Despite the rapid
growth in multiply bonded main group compounds12,
including the isolation of the first silanones13, this area of
chemistry remains challenging to this day.14
provided
a
novel product. However, 31P NMR
spectroscopy did not confirm Ph3P=O formation and thus
not indicating a successful Sila-Wittig reaction. Instead a
resonance at δ = –86.9 ppm, characteristic for a penta-
coordinate phosphorus nucleus, was observed.3b
Additional multinuclear/2D NMR spectroscopy and mass
spectrometry identified dioxaphosphasiline 3 as formal
hetero-Diels–Alder reaction product (Scheme 3). The ring
We recently achieved the isolation of the
unprecedented,
acyclic
and
three-coordinate
iminosilylsilanone 1.13e Here, we report for the first time
about the conversion of a silanone into silenes via the Sila-
Wittig reaction (Scheme 1). Because of the distinct nature
of carbonyl compounds and silanones (the higher polarity
and weaker π-bond of the Si=O moiety leads to a
zwitterionic character and hence to instability)12d, both
similarities and differences in reactivity toward Wittig
reagents were observed.
Scheme 3. Reactivity of Silanone 1 Toward Stabilized
Ylides.
RESULTS AND DISCUSSION
Reactivity Studies with Stabilized Ylides. The classical Wittig reaction
typically consists of two steps. First, a suitable phosphorus
ylide is synthesized, usually by in situ deprotonation of the
respective phosphonium salt with strong bases, such as
organolithium compounds, NaH or KOtBu.3b The carbonyl
compound is then added directly to the reaction mixture
and the olefination takes place. In our case, however, the
general method was not applicable since silanones can
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Ph
P
Ph
support a donor-acceptor interaction. The dative Si1–O2
bond is even significantly longer compared to that
observed for cyclourea-stabilized silanone (dSi–
Ph
H2O
HO OH
Si
2a
1
2
3
4
5
6
7
8
9
O
IDippN Si
tBu3Si
IDippN
SitBu3
(C6H6)
(C6H6)
quant.
O
a
2a
–
4
quant.
O = 1.727(2) Å) reported by Driess and colleagues.17
Additionally, a close O1···H41 distance of 2.186 Å (H41 in
calculated position), being much shorter than the sum of
the van der Waals radii of hydrogen and oxygen (2.72 Å)18,
indicates hydrogen bonding. The pronounced downfield
shift of the ylidic α-hydrogen from δ = 3.32 ppm (2c,
3
MeO OH
2b
1
Si
(C6H6)
IDippN
IDippN
SitBu3
– Ph3P=C=C=O
5
94%
OiPr
Si
2
2JPH = 24.3 Hz) to δ = 7.55 ppm (6, JPH = 21.7 Hz) in the
PPh3
OiPr
H
O
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T
O
2c
PPh3
1H NMR spectrum is in
O
Si
(C6H6)
(C6H6)
92%
OSitBu3
H
SitBu3
quant.
IDippN
6
7
Si
IDippN
OSitBu3
2c
+
1'
silicon atom exhibits a signal at δ = –61.0 ppm with a 2JPSi
coupling constant of 20.4 Hz in the 29Si NMR spectrum.
Although we were not able to crystallize 3 and confirm its
molecular structure by single-crystal X-ray diffraction (SC-
XRD), exposure to H2O selectively afforded silanediol 4
and ylide 2a and thus further verified the assumed
structure. The change in coordination number around the
central silicon nucleus of 4 was clearly evident by the
corresponding highfield shift (δ = –44.7 ppm) in the 29Si
NMR spectrum compared to that observed for 1
(δ = 28.8 ppm).13e Furthermore, 4 was also obtained by
direct treatment of 1 with H2O.
As additional confirmation of dioxaphosphasiline 3, we
calculated its structure and NMR chemical shifts using
density functional theory (DFT).15 Accordingly, the
computed 29Si and 31P NMR resonances at δ(29Si) = –
55.9 ppm and δ(31P) = –80.5 ppm are in good agreement
with the experimentally observed values.
Figure 1. Molecular structure of 6 with ellipsoids set at 30%
probability. Hydrogen atoms, except for H41 involved in
hydrogen bonding (visualized by the dashed line), are
omitted for clarity. Selected bond lengths [Å] and
angles [°]: Si1–O1 1.548(3), Si1–O2 1.806(3), Si1–Si2 2.360(5),
Si1–N1 1.707(5), O2–C40 1.299(6); Si2–Si1–O1 109.9(2), Si1–
O2–C40 128.5(3).
line with this observation. Highfield-shifted resonances in
the 29Si (δ = –50.2 ppm) and 31P (δ = 12.9 ppm; 2c:
δ = 18.3 ppm) NMR spectra agree well with the Lewis
base stabilization. Rivard et al. recently reported on the
utilization of Wittig reagents as electron-donors for low
valent group 14 elements.19 Our results revealed donor
abilities apart from the ylidic α-carbon atoms and thus
extend the knowledge of this underdeveloped ligand
class.
Comparable to silanone 1, adduct 6 is not thermally
stable and quantitatively isomerizes slowly in solution.
Multinuclear and 2D NMR spectroscopy indicated
formation of the silyl-substituted phosphorus ylide 7
(Scheme 3). The central silicon nucleus exhibits a doublet
resonance at δ = –58.6 ppm with a coupling constant
2JPSi of 38.6 Hz in the 29Si{1H} NMR spectrum. The
observed 31P NMR shift at δ = 19.9 ppm is characteristic
In contrast, reaction of 1 with the ester-substituted ylide
2b led to the unexpected simultaneous formation of
methoxysilanol
5
(δ(29Sicentral) = –46.6 ppm) and the
Bestmann ylide Ph3P=C=C=O. While 5 is also accessible
via methanolysis of 113e, the Bestmann ylide is usually
prepared by deprotonation of 2b with strong bases such
as NaHMDS.16 Hence, silanone 1 formally served as a base
in this particular reaction.
Astonished at this result, we investigated ester-
substituted ylides with increased steric encumbrance.
Whereas treatment of 1 with Ph3P=CHCOOtBu resulted in
almost no conversion and mainly silanone isomerization,
reaction with 2c selectively afforded 6 as the sole product
in 92% yield. The molecular structure of ylide-stabilized
silanone 6 was unambiguously confirmed by SC-XRD,
which revealed a short Si=O double bond (1.548(3) Å)
being essentially identical to that observed for donor-free
silanone 1 (1.537(3) Å) (Figure 1). The short O2–C40
(1.299(6) Å) and very long Si1–O2 (1.806(3) Å) distances
for
a
four-coordinate phosphorus ylide (cf. 2c:
δ = 18.3 ppm). Additionally, the Si–H moiety was clearly
1
observed in the H NMR spectrum at δ = 4.50 ppm with
coupling constants 1JSiH of 229.8 Hz and 3JPH of 3.1 Hz.
Furthermore, the theoretically calculated 29Si and 31P
NMR chemical shifts of 7 at δ(29Si) = –57.5 ppm and
δ(31P) = 17.2 ppm agree well with the experimentally
observed results and thus support the formation of 7.
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PPh3
2d
The reaction most likely proceeds via the isomerization
HO
Si
1
2
3
(C6H6)
quant.
of ylide-stabilized silanone 6 to iminosiloxysilylene 1′
under dissociation of phosphorus ylide 2c. The
subsequent insertion of 1′ into the ylidic C–H bond of 2c
provides silyl-substituted phosphorus ylide 7. To support
this assumption, we treated 1′ with 2c at ambient
temperature and in fact obtained the same product 7 in
quantitative yield. However, the exact mechanism remains
unclear, since also a concerted reaction pathway is
plausible.
IDippN
SitBu3
8
1
4
5
6
7
Ph
4-electron
electrocyclization
tBu3Si
Ph
Si
IDippN
2e
H
(C6H6)
80%
(C6H6)
Si
IDippN
(Z)/(E)-silene
SitBu3
9
8
9
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The central silicon atom exhibiting a resonance in the
29Si NMR spectrum at δ = –30.4 ppm with a 4JPSi coupling
constant of 3.4 Hz, clearly indicates the change in the
coordination number from three to four. The slight
highfield shift observed in the 31P NMR at δ = 9.6 ppm
compared to ylide 2d (δ = 11.4 ppm) supports the
proposed structure. SC-XRD unambiguously confirmed
the molecular structure of 8 (Figure 2) and revealed short
C=C (1.362(1) Å) and P=C (1.695(1) Å) bonds together
with its absolute configuration. The mechanism of the
reaction presumably involves a nucleophilic attack of the
vinylogous Wittig ylide to the Si=O moiety of 1.
Subsequent hydrogen migration to the oxygen atom and
double bond formation finally provides 8.
Reactivity Studies with Semi-Stabilized Ylides. We then continued our
studies with semi-stabilized ylides (Scheme 2). Whereas
reaction of 1 toward Ph3P=CHPh resulted in a slow and
unselective formation of a complicated mixture of
products, including starting materials, silylene 1′13e and
other unknown products, utilization of the vinyl-
substituted ylide 2d afforded a sole product. Multinuclear
and 2D NMR analyses and mass spectrometry identified
the silanoyl-substituted phosphorus ylide 8 as the product
formed (Scheme 4).
Scheme 4. Reactivity of Silanone 1 Toward Semi-
stabilized Ylides.
In contrast, utilization of the phenyl-substituted
derivative 2e did not lead to similar product formation,
probably for steric reasons.
Figure 2. Molecular structures of 8 (left), 9 (middle) and 11 (right). Ellipsoids are set at 50% probability. Hydrogen atoms, except
for the respective O–H nucleus of ylide 8, and cocrystallized benzene (8) are omitted for clarity. Selected bond lengths [Å] and
angles [°]: 8: Si1–O1 1.668(1), Si1–Si2 2.373(1), Si1–N1 1.698(2), Si1–C40 1.838(2), P1–C42 1.695(2); Si2–Si1–O1 103.4(1), Si1–C40–
C41 126.0(1). 9: Si1–Si2 2.393(1), Si1–N1 1.697(1), Si1–C40 1.874(1), Si1–C42 1.943(1), C40–C41 1.333(2); C40–Si1–C42 74.9(1),
Si1–C40–C41 91.8(1), C40–C41–C42 109.0(1), Si1–C42–C41 83.9(1). 11: Si1–Si2 2.421(1), Si1–N1 1.692(1), Si1–C40 1.917(1), Si1–
C47 1.888(1), C47–C48 1.528(2); Si2–Si1–N1 109.8(1), Si2–Si1–C40 115.9(1), Si2–Si1–C47 108.0(1).
Instead, a successful Sila-Wittig reaction was indicated via
detection of Ph3P=O formation by 31P NMR spectroscopy
(δ = 24.8 ppm). However, further spectroscopic analyses
revealed no resonances for the corresponding silenes, but
pointed out the formation of silacyclobutene 9 as formal
4π–electron electrocyclization product of the transient
silene. 29Si NMR spectroscopy showed a change in
coordination number with an observed upfield-shifted
resonance at δ = –30.2 ppm (cf. 1: δ = 28.8 ppm). 13e Since
initially two different silenes and subsequently two
stereogenic centers are formed upon cycloaddition, there
are up to four different stereoisomers plausible.
Nevertheless, the solid-state structure determined by SC-
XRD revealed the absolute configuration of 9. In addition,
the silacyclobutene moiety was clearly identified with a
sum of bond angles of 359.6° and a short C=C double
bond (1.333(2) Å).
Reactivity Studies with Non-Stabilized Ylides. Motivated by the above-
mentioned results, we investigated non-stabilized ylides
and initially used the simplest, hydrogen-substituted ylide
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2f. Immediate full conversion of silanone 1 and 2f to a and mass spectrometry. Additionally, they are significantly
1
2
3
4
5
6
7
8
mixture of products was detected by multinuclear NMR
spectroscopy. In the 29Si NMR spectrum a signal observed
at δ = 68.1 ppm indicated formation of the half-parent
silene 10a, however it disappeared completely within 3
hours. A comparison to other silenes turned out to be
difficult, because none of this type (iminosilylsilene) have
been reported so far. Nevertheless, the detected 29Si NMR
more stable in solution than 10a. In contrast to the
classical Wittig reaction, however, a removal of the
phosphine oxide by-product was not possible at all, as
decomposition to unidentified product mixtures occurred
upon workup (solvent removal and subsequent extraction
with diethyl ether and/or n-hexane).
Only very recently, Apeloig et al. reported on the
resonance is characteristic for
a
three-coordinate
synthesis
and
isomerization
of
(E)/(Z)-
9
silene.12a-c In addition, concomitant formation of Ph3P=O
was clearly evidenced by its significant 31P NMR resonance
at δ = 24.8 ppm, once again confirming a successful Sila-
Wittig reaction. Presumably, the steric demand of 10a is
not sufficient and fast subsequent decomposition
reactions, such as head-to-tail dimerization are
thermodynamically favored.
(tBu2MeSi)(tBuMe2Si)Si=CH(1-Ad), the
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first-ever isolable silene stereoisomers.20 Thus, we
conducted further DFT calculations to support the
formation of the silenes and especially the assignment of
the (E)/(Z)-isomers. First, we calculated the structure and
the central 29Si NMR chemical shift of 10a. The calculated
resonance at δ = 68.9 ppm is in excellent agreement with
the experimentally measured value (δ = 68.1 ppm) and
therefore confirms the accuracy of the computational
method. Furthermore, the calculated central 29Si NMR
chemical shifts of 10b at δ = 56.5 ppm and 10b′ at
δ = 51.2 ppm (for calculated structures, see Figure 3)
agree well with the spectroscopically observed
resonances (Table 1). These results verify our assignments
for the respective (E)/(Z)-silenes.
Consequently, the increase in the steric encumbrance of
the phosphorus ylides used (2g-m) resulted in
instantaneous, quantitative silene and Ph3P=O formation
(Table 1). In general,
a high selectivity for the
corresponding (Z)-silenes (10b-g) was observed, in
analogy to the classical Wittig reaction, in which the
utilization of triphenylphosphine-derived non-stabilized
phosphorus ylides usually leads to very high Z-
selectivities.4a The stereochemistry assignment was
determined with nuclear Overhauser enhancement
spectroscopy (NOESY) experiments. Interestingly,
conversion of 1 with the
Table 1. 29Si NMR Resonances for the Respective Si=C
Moieties of the Sila-Wittig Reaction Products with
Non-stabilized Ylides.
H
H
R
R
Figure 3. Calculated structures of (Z)-silene 10b (left) and (E)-
silene 10b′ (right) at the HCTH407/6-311+G(d)//B97-D/def2-
SVP level of theory.
Si
Si
Si
IDippN
SitBu3
IDippN
(Z)-silenes
SitBu3
10b-h
IDippN
(E)-silenes
SitBu3
10a
10b'-h'
(Z)-
Silene
δ(29Si) [ppm]
68.1
(E)-
Silene
Stereoselectiv
ity
Ylide
A limitation of the Sila-Wittig reaction is the steric
encumbrance of silanone 1 itself, which is necessary for its
isolation. Therefore, no reaction other than silanone
isomerization was observed already with Ph3P=CMe2.13e
#
R
(Z)/(E) [%]
–
2f
2g
2h
2i
H
Me
Et
Accordingly, formation and isolation of
a
stable
55.9
51.3
52.3
52.0
52.0
53.0
56.3
50.5
43.1
42.8
42.6
42.6
43.3
46.5
~92/8
~94/6
~94/6
~89/11
~85/15
~76/24
~45/55
iminosilylsilene derived from 1 seemed to be rather
challenging.
Pr
N-heterocyclic carbenes (NHCs) are frequently used as
Lewis bases for the stabilization and isolation of reactive
main group compounds.12e Hence we tried to isolate
silenes 10 as NHC adducts. However, reaction of in situ
formed 10b/10b′ with 1,3,4,5-tetramethylimidazolin-2-
ylidene (IMe4) did not afford the corresponding NHC-
stabilized silene, but an intramolecular C–H activation of a
wingtip methyl group occurred and finally silyl-
substituted NHC 11 formed (Scheme 5). Compound 11
was fully characterized and the
2j
Bu
Hex
2k
2l
2-PhEt
2m 2-BnOEt
2-benzyloxyethyl-substituted ylide 2m provided an
approximate 45/55 mixture of (Z)- and (E)-silene
10h/10h′. The silenes formed were sufficiently
characterized by multinuclear and 2D NMR spectroscopy
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Crystallographic data (CCDC 1949650–1949657) (CIF).
Scheme 5. Synthesis of Silyl-Substituted NHC 11.
1
2
3
4
5
AUTHOR INFORMATION
N
IMe4
N
N
N
Si
Corresponding Author
(C6H6)
83%
(C6H6)
IDippN
10b/10b'
SitBu3
Si
Si
IDippN
11
SitBu3
IDippN
SitBu3
*s.inoue@tum.de
6
7
8
9
ORCID
now tetravalent silicon moiety exhibits a resonance in the
29Si NMR spectrum at δ = –21.0 ppm. The divalent carbon
atom was clearly identified in the 13C NMR with a
characteristic down-field shift at δ = 213.2 ppm. SC-XRD
analysis confirmed the molecular structure of the
asymmetric silylcarbene 11 and revealed both a planar
NHC skeleton and the absolute configuration (Figure 2).
To the best of our knowledge, there is only one example
of NHC C–H activation by a low-coordinate silicon
compound reported so far. Driess et al. observed the
formation of a comparable silyl-substituted NHC via
Tibor Szilvási: 0000-0002-4218-1570
Shigeyoshi Inoue: 0000-0001-6685-6352
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are exceptionally grateful to the WACKER Chemie AG and
the European Research Council (SILION 637394) for
continued financial support. We thank Dr. Daniel Wendel for
revising the manuscript and Maximilian Muhr (Prof. R. A.
Fischer) for the LIFDI-MS measurements.
reaction
of
their
β-diketiminato
silylene
{HC[CMeN(Dipp)]2}Si:) with IMe4.21 Our observation
therefore underlines the high reactivity of iminosilylsilenes
10. Nevertheless, the application of the Wittig reaction to
a carbonyl homologue has been further demonstrated
with the isolation of NHC 11, because the former Si=O
double bond was clearly converted into a Si–CH2CH3 bond
via an intermediate Si=CHCH3 double bond.
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new avenues for the isolation of otherwise elusive silenes.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: xx.xxxx/jacs.xxxxxxx.
Experimental details including synthetic, spectroscopic,
crystallographic, and computational data (PDF)
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Journal of the American Chemical Society
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An Isolable Silicon Analogue of a Ketone that Contains an
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SILA-WITTIG REACTION
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Dipp
Dipp
O
C
HR
N
N
Ph3P CHR
– Ph3P
Si
Si
SitBu3
O
N
SitBu3
N
N
N
Dipp
Dipp
(E)/(Z)-silenes
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silanone
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