On the reactivity of silylboranes toward lewis bases: Heterolytic B-Si cleavage vs. adduct formation

Article abstract of DOI:10.1002/ejic.201300119

Silylboranes are important reagents in a variety of catalytic silylation and silaboration reactions. While transition-metal-catalyzed reactions are well established, organo-/Lewis base-catalyzed reactions of silylboranes have only recently emerged. For both catalytic processes the reactivity of silylboranes toward Lewis bases is of relevance. While for organo-catalyzed reactions Lewis base activation of the silylborane has been proposed, transition-metal- and especially copper-catalyzed reactions also frequently require the presence of Lewis basic alkali metal alkoxides. In the present study we explore the reaction of K(18-crown-6) tert-butoxide and the NHC 1,3-diisopropyl-4,5-dimethyl- imidazol-2-ylidene as exemplary Lewis bases with the two silylboranes pinB-SiMe₂Ph and pinB-SiPh₃ (pin = OCMe ₂CMe₂O). The reaction with K(18-crown-6) tert-butoxide results in activation of the boron-silicon bond. The isolated product of this activation is either the potassium silyl complex [K(18-crown-6)SiPh₃] or [K(18-crown-6)(thf)₂][pinB(SiMe₂Ph)₂], the formal Lewis acid/base adduct of [K(18-crown-6)SiMe₂Ph] with pinB-SiMe₂Ph. Both complexes react essentially as sources of nucleophilic silyl moieties in reactions with exemplary electrophiles. In contrast, usage of the carbene leads to the formation of isolable Lewis acid/base adducts of the type (NHC)pinB-SiR₃, which do not react as sources of nucleophilic silyl moieties. The identification and characterization of these species appears of relevance for the mechanistic understanding and further development of Lewis base/organo- as well as transition-metal-catalyzed silyl transfer reactions. Copyright

Full text of DOI:10.1002/ejic.201300119

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DOI:10.1002/ejic.201300119
On the Reactivity of Silylboranes toward Lewis Bases:
Heterolytic B–Si Cleavage vs. Adduct Formation
Christian Kleeberg*[a] and Corinna Borner[a]
Keywords: Silicon / Boranes / Lewis bases / Heterocycles / Carbenes / B–Si activation
Silylboranes are important reagents in a variety of catalytic
silylation and silaboration reactions. While transition-metal-
catalyzed reactions are well established, organo-/Lewis
base-catalyzed reactions of silylboranes have only recently
emerged. For both catalytic processes the reactivity of silyl-
boranes toward Lewis bases is of relevance. While for or-
gano-catalyzed reactions Lewis base activation of the silyl-
borane has been proposed, transition-metal- and especially
copper-catalyzed reactions also frequently require the pres-
ence of Lewis basic alkali metal alkoxides. In the present
study we explore the reaction of K(18-crown-6) tert-butoxide
and the NHC 1,3-diisopropyl-4,5-dimethyl-imidazol-2-
ylidene as exemplary Lewis bases with the two silylboranes
pinB-SiMe2Ph and pinB-SiPh3 (pin = OCMe2CMe2O). The
reaction with K(18-crown-6) tert-butoxide results in acti-
vation of the boron–silicon bond. The isolated product of this
activation is either the potassium silyl complex [K(18-crown-
6)SiPh3] or [K(18-crown-6)(thf)2][pinB(SiMe2Ph)2], the formal
Lewis acid/base adduct of [K(18-crown-6)SiMe2Ph] with
pinB-SiMe2Ph. Both complexes react essentially as sources
of nucleophilic silyl moieties in reactions with exemplary
electrophiles. In contrast, usage of the carbene leads to the
formation of isolable Lewis acid/base adducts of the type
(NHC)pinB-SiR3, which do not react as sources of nucleo-
philic silyl moieties. The identification and characterization
of these species appears of relevance for the mechanistic un-
derstanding and further development of Lewis base/organo-
as well as transition-metal-catalyzed silyl transfer reactions.
in the cesium fluoride-promoted carboxymethylation of cer-
tain α-amido sulfones.[7] Very recently, Ito and co-workers
reported the borylation of aryl, alkenyl, and alkyl halides
with 1 in the presence of KOMe as the Lewis base
(Scheme 1). Again, the respective Lewis acid/base adduct
(type II) has been proposed as the central intermediate.[6b]
Moreover, the insertion of certain Lewis bases into the
B–Si bond as well as the generation of silyl anions by B–Si
bond cleavage by metal organyls, presumably also involving
the initial formation of a Lewis acid/base adduct, have been
reported.[8,9,10] Especially, the formation of a nucleophilic
silyl species by activation of the silylborane pinB-SiPh3 (2)
with a stoichiometric amount of KOtBu suggested by Ka-
wachi et al. in 2001 appears relevant. In this case, a nucleo-
philic silyl species was trapped by reaction with Me3SiCl
(Scheme 1, bottom).[9] The formation of the silyl anion as
the nucleophilic species was concluded by analogy with the
reaction of 2 with MeLi where the silyl anion was unambig-
uously identified by in situ 29Si NMR experiments.[9]
More recently, a series of CuI-catalyzed silylation reac-
tions employing the silylborane 1 have been reported that
require the presence of stoichiometric or catalytic amounts
of an alkali metal alkoxide. It is believed that the alkoxide
is essential to form a copper alkoxido complex, which acti-
vates the Si–B bond with formation of the catalytically
active copper silyl complex.[2] However, considering reports
on the activation of silylboranes by Lewis bases this mode
of activation may also be relevant here either as part of the
Introduction
The use of silylboranes, in particular pinB-SiMe2Ph (1)
(pin = OCMe2CMe2O), in transition-metal- (e.g., Rh, Pd)
and especially CuI-catalyzed transformations is an estab-
lished but still developing field of research.[1,2,3] However,
the first Lewis base-catalyzed transition-metal-free silyl-
ation, silaboration, and borylation reactions employing sil-
ylboranes have only recently been reported.[4–7] In 2011 Ho-
veyda and co-worker described the enantioselective metal-
free silylation of α,β-unsaturated carbonyl compounds with
1 in the presence of catalytic amounts of an in situ-gener-
ated chiral N-heterocyclic carbene (NHC) as shown in
Scheme 1.[5] Moreover, Ito et al. reported a KOtBu-cata-
lyzed procedure for the silaboration of aromatic alkenes by
1.[6a] For both reactions it has been proposed on the basis
of 11B NMR spectroscopic data that an intermolecular
Lewis acid/base adduct (of the type I and II) formed by
1 and the Lewis base is of crucial mechanistic relevance
(Scheme 1).[5,6a] A related Lewis acid/base adduct of 1 and
a fluoride anion has also been proposed as an intermediate
[a] Institut für Anorganische und Analytische Chemie, Technische
Universität Carolo-Wilhelmina zu Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-391-5387
E-mail: ch.kleeberg@tu-braunschweig.de
Homepage: www.tu-braunschweig.de/iaac/personal/kleeberg
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300119.
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studied in some detail.[12] Compound 4 shows close spectro-
scopic and structural similarities to these complexes. Crys-
tals of 4 were obtained from PhMe/n-pentane and an X-ray
diffraction study was conducted (Figure 1, a). In the crystal
single molecules of 4 are connected by intermolecular π-
cation interactions between one of the phenyl rings and an
adjacent potassium atom to form chains along a screw-axis
in a space group of the type P21/c.[13]
Scheme 2. Heterolytic cleavage of the silylboranes 1 and 2 in the
presence of 3.
Scheme 1. Examples of Lewis base-catalyzed reactions of silylbor-
anes and proposed intermediate Lewis acid/base adduct types.[5,6,9]
catalytic process or as a side reaction, especially as it was
noted during the studies on CuI-catalyzed reactions that 1
shows decomposition in the presence of stoichiometric
amounts of an alkoxide, but the decomposition products
were not analyzed.[2b] Furthermore, a CuII-catalyzed silyl
conjugate addition to α,β-unsaturated carbonyl compounds
with silylboranes in the presence of an aqueous amine was
recently reported.[3] Here, the relevance of an oxygen Lewis
base (H2O or OH–) in the B–Si activation step has been
emphasized.
Figure 1. Molecular structures of 4 (a) and the [pinB(SiMe2Ph)2]–
anion in 5 (b). Selected bond lengths [Å] and angles [°]: 4: Si1–K1
3.5404(5); 5: Si1–B1 2.094(3), O1–B1 1.492(4), Si1–B1–Si1Ј
107.5(2), Si1–B1–O1 108.14(8), Si1–B1–O1Ј 114.62(8), O1–B1–O1Ј
104.1(4).
It is apparent that an understanding of the reactivity of
silylboranes toward Lewis bases is of central importance to
a detailed understanding of the currently emerging field of
Lewis base as well as transition-metal-catalyzed reactions
In contrast to this the reaction of 1 with 3 in THF results
of silylboranes. In this report we present a comparative in the isolation of the borate [K(18-C-6)(thf)2][pinB(Si-
study on the reactions of the silylboranes 1 and 2 with po- Me2Ph)2] [5(thf)2]. The formation of this compound may be
tassium(18-crown-6) tert-butoxide and a nucleophilic NHC, rationalized by the initial formation of a formal SiMe2Ph–
respectively, in order to gain first insights into the activation anion, in analogy to the reaction of 2, and its subsequent
of B–Si bonds with these catalytically relevant classes of coordination to unreacted 1. All attempts to isolate a potas-
Lewis bases.
sium complex of the free silyl anion Me2PhSi– under vari-
ous reaction conditions (solvent, temperature, and stoichi-
ometry) have been unsuccessful. Employing optimized con-
ditions 5 was isolated in 69% yield. The THF solvate
Results and Discussion
Treating 2 with potassium(18-crown-6) tert-butoxide 5(thf)2 (space group C2/c) crystallizes from a THF/n-pent-
{[K(18-C-6)OtBu] (3)} leads to the formation of the silyl ane mixture with the [pinB(SiMe2Ph)2]– anions located on
anion as suggested by Kawachi et al. for KOtBu (vide su- the twofold axis (Figure 1, b). The structural parameters at
pra).[9,11] The silyl anion was isolated as [K(18-C-6)SiPh3] the tetrahedral sp3-boron atom are comparable to those
(4), a stable, microcrystalline solid in 88% yield under opti- found in related species [Si–B in pinB(sp2)Si: 2.04 Å (av.),
mized conditions (Scheme 2). While compound 4 has not vide infra].[10b–10d,14] The 11B NMR spectrum in [D8]THF
been isolated previously, complexes of the type [Li(L)n- shows a single resonance at 9.2 ppm (Δw1/2 = 80 Hz).
1
SiPh3] and [K(L)nSiPh3] (L = donor solvent) have been Chemical shift and line width as well as H and 13C NMR
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spectroscopic data are in agreement with the data expected not thoroughly structurally or spectroscopically charac-
for a tetrahedrally coordinated boron species containing a terized.[5]
Bpin moiety (vide infra).[14] The 29Si NMR shift of δ =
Both compounds 7 and 8 were isolated as well-developed
–29.6 ppm is significantly different to the shift expected for crystals from PhMe solutions and characterized both in
a
silyl potassium complex (KSiMe2Ph in [D8]THF: solution and in the solid state. Similar to compound
–22.7 ppm)[12b] and more similar to the value observed for 5(thf)2 single resonances at 6.0 ppm (7) (Δw1/2 = 180 Hz)
the parent silylborane 1 of δ = –28.0 ppm. The solvate and 6.5 ppm (8) (Δw1/2 = 59 Hz) are observed in the 11B
5(thf)2 is susceptible to solvent loss upon drying in vacuo NMR spectrum. The significant increased line width of 7
followed by partial decomposition. One of the decomposi- indicates, in agreement with the broadening of certain reso-
1
tion products was identified by NMR spectroscopy as the nances in the H NMR spectrum, the presence of dynamic
disilane (PhMe2Si)2.[13]
processes.[16] A certain, though less significant broadening
The crown ether complex 3 was used instead of KOtBu of some 1H NMR resonances is also observed for 8.[13] The
in order to improve the crystallization properties, as em- NMR spectroscopic data of 7 and 8 resemble the data re-
ploying KOtBu in reactions with 1 and 2 did not lead to ported for the related NHC adduct of the diboron com-
the isolation of uniform materials.[13] In situ NMR spectro- pound B2pin2.[14] All attempts to obtain 29Si NMR spectro-
scopic experiments were carried out for the reactions of 1 scopic data of 7 and 8 were unsuccessful due to the unfavor-
and 2 with 3 and KOtBu, respectively. In the reaction of 2 able situation of the presence of dynamic processes as well
with the Lewis bases 3 and KOtBu, respectively, the forma- as an adjacent quadrupolar boron atom leading to excessive
tion of the boric acid ester pinBOtBu is detected. This indi- line broadening.
cates the occurrence of B–Si bond cleavage under these con-
The molecular structures of 7, 8, and 5 are very similar:
ditions.[13] The data for the reaction of 1 with 3 and KOtBu, the boron atom shows distorted tetrahedral coordination
respectively, are in agreement with the report of Ito et al. with generally comparable bond length and angles (Fig-
proposing the formation of an adduct of type II as the ure 2). However, the increased steric bulk of the silyl moiety
major product (vide supra).[6a,13] But notably, in the case of in 8 may account for the differences, especially the signifi-
3, and contrary to the respective reaction with KOtBu, the cantly increased Si1–B1 distance compared to 7. The B–Si
formation of pinBOtBu indicates the occurrence of some distances are comparable to 5 and slightly longer than re-
B–Si bond cleavage.[13] However, the principally reversible ported for B(sp2)-Si compounds (vide supra), this has to
nature of all reaction steps involved should be emphasized be attributed to the change in hybridization at the boron
and may account for the influence of the crown ether, the atom.[10b–10d,14] The molecular structures of 7 and 8 are
solvent, and the crystallization conditions on the isolation comparable to the mentioned NHC adduct of B2pin2 [O–
of certain species. This is supported by the fact that B(sp3) 1.490(2)/1.483(2) Å, B–C 1.673(2) Å, O–B(sp3)–O
[K(thf){pinB(OtBu)2}] was characterized crystallographi- 104.6(1)°, O–B–C 107.6(1)°/111.7(1)°].[14]
cally as one of the reaction products of the reaction of 1
with KOtBu in THF giving evidence for the occurrence of
B–Si bond cleavage to a certain extent in the absence of
crown ether.[13] However, a detailed understanding of these
reactions and especially of the species present in solution
remains the subject of further studies.
The use of the NHC 1,3-diisopropyl-4,5-dimethyl-imid-
azol-2-ylidene (6) as a Lewis base in reactions with 1 and
2, respectively, results in the formation of the Lewis acid/
base adducts 7 and 8 of type I (Scheme 3), whereas no indi-
cations for B–Si bond cleavage have been detected.[13,15]
While an analogous adduct of 1 with 1,3-bis(mesityl)imid-
azol-2-ylidene was suggested as the active species in the
NHC-catalyzed silyl transfer reaction (vide supra) it was
Figure 2. Molecular structures of 7 (a) and 8 (b). Selected bond
lengths [Å] and angles [°]: 7: Si1–B1 2.105(1), O1–B1 1.475(1), O2–
B1 1.482(2), C1–B1 1.671(2), Si1–B1–C1 105.48(8), Si1–B1–O1
110.27(8), Si1–B1–O2 114.05(8), O1–B1–O2 105.2(1), C1–B1–O1
114.43(9), C1–B1–O2 107.6(1); 8: Si1–B1 2.123(1), O1–B1 1.474(2),
O2–B1 1.462(2), C1–B1 1.668(2), Si1–B1–C1 105.37(8), Si1–B1–O1
114.01(9), Si1–B1–O2 107.43(9), O1–B1–O2 106.0(1), C1–B1–O1
109.9(1), C1–B1–O2 114.3(1).
After isolation of compounds 4, 5(thf)2, 7, and 8 we
studied their reactions toward benzyl bromide, methyl iod-
ide, and TMS chloride as exemplary electrophiles by in situ
NMR spectroscopy and GC/MS analysis. The triphenylsilyl
Scheme 3. Adduct formation in reactions of 1 and 2 with 6.
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source of nucleophilic silyl groups. The substitution prod- PhMe2Si-Bn/1/Bn2/(PhMe2Si)2 8:11:2:3]. Furthermore, in
ucts Ph3SiMe and Ph3Si–SiMe3, identified by comparison agreement with the report of Ito and co-workers the
with authentic samples, were obtained virtually quantita- borylation product pinB-Bn was detected in small amounts
tively as detected by in situ 1H NMR spectroscopy by GC/MS analysis of the reaction mixture (ratio pinB-Bn/
(Scheme 4).[13]
PhMe2Si-Bn Ͻ 1:10).[6b,13,17]
The NHC adducts 7 and 8 react with excess MeI and
BnBr, respectively, contrary to 4 and 5(thf)2, not as sources
of nucleophilic silyl moieties, but as a source of a nucleo-
philic/basic NHC moiety. However, no significant conver-
sion was observed between TMS-Cl and 7 and 8, respec-
tively, at comparable conditions and time scale. This is pre-
sumably because of the sterically more demanding nature
of this electrophile (Scheme 6).
Scheme 4. Reaction of 4 with exemplary electrophiles. Note that in
the reaction with BnBr additional and so far unidentified products
are formed.
1
According to in situ H NMR experiments the reaction
with benzyl bromide led to Ph3Si-CH2Ph as the major
product (57% isolated yield). However, small amounts of a
so far uncharacterized by-product containing a Ph3Si moi-
ety were also detected.[13] In addition the formation of bi-
benzyl was observed, suggesting the presence of an ad-
ditional (radical) reaction pathway (ratio Ph3Si-CH2Ph/
bibenzyl = 4:1).
During the reactions with the three different electrophiles
the formation of a colorless precipitate was noted. Al-
though the identity of this material was not investigated
in every case, for the reaction with MeI [K(18-C-6)I] was
identified by single-crystal X-ray diffraction, suggesting
that the precipitate is generally the corresponding potas-
sium salt [K(18-C-6)X] (X = Cl, Br, I).[13]
Scheme 6. Reaction of 7 and 8 with exemplary electrophiles. Note
that in the reaction with BnBr additional and so far unidentified
products are formed.
The reactions of 7 and 8 with excess MeI were studied
in detail. It is evident from the in situ 1H/11B NMR spectro-
scopic data that the parent silylboranes 1 and 2, respec-
tively, are formed as the only boron-containing products. In
addition three NHC-derived products were unambiguously
The borate 5(thf)2 reacts with the electrophiles MeI,
TMS-Cl, and BnBr as a source of one nucleophilic PhMe2Si
1
moiety (Scheme 5) as is evident from the in situ H NMR
spectroscopic data.[13] Hence, along with the formation of
the respective substitution product the silylborane 1 is re-
generated (Scheme 5).[13] During this reaction again the for-
mation of a colorless precipitate, presumably [K(18-C-6)X]
(X = Cl, Br, I), was observed. However, the reactions are
complicated, compared to the respective reactions of 4, by
the formation of (PhMe2Si)2 as a side product in appreci-
able amounts. However, it has to be emphasized that
(PhMe2Si)2 is also formed as a decomposition product of
5(thf)2 in the absence of any electrophile (vide supra).[13]
1
identified by combined in situ 1D/2D H/13C NMR spec-
troscopy and high-resolution ESI mass spectrometry as the
imidazolium cations 9a–c (Scheme 6, Figure 3 exemplarily
for 7).[13]
Scheme 5. Reaction of 5(thf)2 with exemplary electrophiles.[13]
In the case of the reaction of 5(thf)2 with BnBr the iden-
tification of bibenzyl as an additional product suggests
1
Figure 3. Details of in situ H NMR spectra of the reaction of 7
with excess MeI (600 MHz, [D8]THF, * represents a residual sol-
again the presence of additional (radical) processes [ratio vent signal).
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The formation of these three imidazolium cations may sulting boric acid ester pinB-OtBu in the case of the alk-
be rationalized assuming the initial partial formation of 9a oxide. Conversely, the unfavorable charge separation and
by methylation of the NHC with MeI. Some 9a is then de- the formation of the highly Lewis acidic cation in a poten-
protonated by free NHC at the 2-Me group leading directly tial ion pair of the type [pinB(6)]+[SiPhR2]– may account
to the protonation product 9b and, after the methylation, for the stability of the NHC adducts. This is demonstrated
to 9c.[18] It has to be emphasized that the amounts of the in reactions of 4, 5, 7, and 8 with exemplary electrophiles
individual compounds 9a–c vary between 7 and 8 but also (MeI, TMS-Cl, BnBr). Here, the silyl complex 4 and the
between different runs and appear to depend greatly on the silyl anion adduct 5(thf)2 act as sources of nucleophilic silyl
exact reaction conditions. Quantification is further compli- groups while the NHC adducts act as a source of the nu-
cated by the precipitation of different amounts of 9a–c (pre- cleophilic NHC. The activation of silylboranes by Lewis
dominantly 9b) during the reaction.
bases, either within the productive reaction pathway or as a
The use of BnBr as the electrophile was not studied in possible side reaction, is of major relevance to the currently
detail, but results in the regeneration of the parent silyl- flourishing field of Lewis base- and transition-metal (espe-
borane.[13] While for 7 quantitative reaction with BnBr is cially Cu)-mediated (or catalyzed) reactions of silylboranes.
observed within 4 h, for 8 the reaction is not complete In the present study potentially important intermediates are
within 20 h at room temperature. As an NHC-derived prod- isolated for the first time, and thoroughly characterized and
uct only the imidazolium salt 9b was identified by NMR studied concerning their reactivity toward exemplary elec-
spectroscopy in the precipitate formed during the reac- trophiles. The present study may improve the understanding
tion.[13,18]
of the crucial B–Si activation step and trigger further devel-
These results show clearly that the NHC adducts 7 and opment in the field. Further studies toward a detailed
8 are not reacting as activated silylboranes, hence, sources mechanistic understanding of Lewis base-promoted B–Si
of nucleophilic silyl moieties, but as simple Lewis acid/base bond activation of silylboranes, especially concerning the
adducts of the nucleophilic NHC. Noteworthy is that species present in solution, are currently ongoing.
the reported NHC-catalyzed silylation of α,β-unsaturated
carbonyl compounds performed best in the presence of
Experimental Section
General Considerations: PinB–SiMe2Ph (1),[19] pinB–SiPh3 (2),[19]
water.[5] While the high levels of enantioselectivity observed
indicate clearly the importance of the chiral NHC for the
reaction, the potential role of H2O or HO– for the B–Si [K(18-crown-6)OtBu] (3)[11]
, and 1,3-diisopropyl-4,5-dimethyl-
imidazol-2-ylidene (6)[20] were prepared according to literature pro-
cedures. All other compounds were commercially available and
used as received. All solvents were dried using MBraun solvent
purification systems, deoxygenated using the freeze-pump-thaw
method, and stored under purified dinitrogen. All manipulations
were performed using standard Schlenk techniques under an atmo-
sphere of purified dinitrogen or in a dinitrogen-filled glove box
(MBraun Labstar). Flash column chromatography was performed
on silica gel 60 (Macherey–Nagel, 40–63 μm, 230–400 mesh,
ASTM) using the indicated solvents. Technical-grade solvents for
activation should also be considered.
Conclusions
The Lewis bases [K(18-C-6)OtBu] (3) and NHC 6 react
with the silylboranes 1 and 2. While different final products
are obtained it can be assumed that the initial step is in all
cases the coordination of the Lewis base to the Lewis acidic
boron atom. This adduct is either isolable (for 6) or decom- extraction and chromatography were distilled prior to use. NMR
spectra were recorded with Bruker Avance II 300, Bruker Avance
400, Bruker DRX 400, or Bruker Avance II 600 spectrometers.
Chemical shifts (δ) are given in ppm, using the resonance of the
(residual protio-) solvent (C6D6: Eurisotop, 99.5% deuteration,
dried with potassium/benzophenone and degassed; [D8]THF:
Eurisotop, 99.5% deuteration, dried with potassium/benzophenone
and degassed. CDCl3: Eurisotop, 99.8% deuteration) as internal
reference (C6D6: 1H NMR: 7.16 ppm, 13C NMR: 128.06 ppm; [D8]-
THF: 1H NMR: 1.72 ppm, 13C NMR: 25.31 ppm. CDCl3: 1H
NMR: 7.26 ppm, 13C NMR: 77.16 ppm).[21] 11B and 29Si chemical
poses under B–Si bond cleavage forming a (formal) silyl
anion (for 3). In the case of the Ph3Si– anion (from 2) the
resulting potassium silyl complex 4 was isolated. In contrast
the Me2PhSi– anion formed from 1 is not isolated as such,
but as the Lewis acid/base adduct 5(thf)2 of the parent silyl-
borane 1. This may be rationalized by the different steric
properties of the Me2PhSi and the Ph3Si moiety. For the
reaction of 1 and 2 with KOtBu as the Lewis base in THF
no uniform products were obtained, but the NMR spectro-
scopic data are in agreement with the occurrence of B–Si shifts are reported relative to external BF3·Et2O and TMS, respec-
tively. 29Si NMR spectra were recorded employing direct detection
bond cleavage in the case of 2 and the formation of the
respective adduct in the case of 1 as proposed by Ito et
and inverse-gated decoupling. 11B NMR spectra were processed ap-
al.[6a] At present it is not clear if the differences in the out-
plying a back linear prediction in order to suppress the broad back-
ground signal due to the borosilicate glass in the NMR tube and
come of the reaction with KOtBu and [K(18-C-6)OtBu] (3),
respectively, is because of the changed Lewis basic proper-
ties or the effect of the crown ether on the crystallization
properties. The distinct reactivity of the silylboranes 1 and
2 toward the alkoxide and the NHC Lewis bases, respec-
tively, may be explained by the formation of the very favor-
a Lorentz-type window function [LB (line broadening) = 10 Hz];
the spectra were carefully evaluated to ensure that no genuinely
broad signals of the sample were suppressed. Air-sensitive NMR
samples were measured in flame-sealed NMR tubes or NMR tubes
equipped with screw caps (WILMAD). Melting points were deter-
mined with a Büchi 530 apparatus in flame-sealed capillaries under
able O–B bond and the reduced Lewis acidity of the re- dinitrogen and are not corrected. Elemental analyses were per-
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formed with an Elementar vario MICRO cube instrument at the
Institut für Anorganische und Analytische Chemie of the Techni-
sche Universität Carolo-Wilhelmina zu Braunschweig. ESI mass
spectra were obtained with a Finnigan LCQ Deca or a Finnigan
MAT95 XL Trap instrument. The samples were prepared under
inert conditions, but strictly inert conditions could not be main-
tained during the entire measurement. GC/MS analyses were per-
formed with an Agilent 6890 gas chromatograph coupled to a JMS-
crystals suitable for X-ray diffraction after 16 h. The solution was
decanted, the residue washed with n-pentane (2ϫ2 mL) and dried
in vacuo to give 51 mg (91 μmol, 88%) of a yellow crystalline pow-
der, m.p. 126°–130 °C. 1H NMR (400 MHz, [D8]THF, room temp.):
δ = 7.38–7.34 (m, 6 H, o-CHAr), 6.90–6.85 (m, 6 H, m-CHAr), 6.77–
6.71 (m, 3 H, p-CHAr), 3.54 (s, 24 H, OCH2) ppm. 13C{1H} NMR
(100 MHz, [D8]THF, room temp.): δ = 160.8 (ipso-CAr), 131.1 (o-
CHAr), 126.4 (m-CHAr), 123.0 (p-CHAr), 71.0 (OCH2) ppm. 1H
T100GC (GCAccuTOF, JEOL) time-of-flight mass spectrometer NMR (300 MHz, C6D6, room temp.): δ = 8.08–8.03 (m, 6 H, o-
or a Shimadzu GCMS-QP2010SE instrument operating in EI mode
(70 eV).
CHAr), 7.40–7.32 (m, 6 H, m-CHAr), 7.23–7.16 (m, 3 H, p-CHAr),
3.08 (s, 24 H, OCH2) ppm. 29Si{1H} NMR (79.5 MHz, [D8]THF,
room temp.): δ = –7.1 (s) ppm. C30H39KO6Si (562.82): calcd. C
64.02, H 6.98; found C 63.96, H 6.93.
X-ray Crystal Structure Determinations: All crystals were mounted
in inert perfluoroether oil either at room temperature on top of a
human hair {7, 8, and [K(thf)(pinB(OtBu)2)]} or at low tempera-
ture using an XTEMP-2 apparatus on top of a glass fiber (4 and
5) and transferred to the cold nitrogen gas stream of the dif-
fractometer.[22a–22c] The data sets of 4, 5, and 7 were collected with
an Oxford Diffraction Xcalibur E instrument using monochro-
mated Mo-Kα radiation at 100(2) K; the data set for 8 and
[K(thf){pinB(OtBu)2}] was collected with an Oxford Diffraction
Nova A instrument, using mirror-focused Cu-Kα radiation at
100(2) K. The data were integrated and a multiscan absorption cor-
rection performed employing the CrysAlisPro software.[22d] All
structures were solved by direct methods {4, 5, [K(thf)(pinB-
(OtBu)2)][13], and [K(18-C-6)I]:[13] SIR-92;[22e] 7 and 8: SHELXS-
97[22f]} and refined anisotropically for all non-hydrogen atoms by
full-matrix least-squares on F2 using SHELXL-97.[22f] All hydrogen
atoms were refined employing a riding model. The crystals of 8
and [K(18-C-6)I] were racemic twins and treated accordingly; the
twin factors refined to 0.45(2) and 0.41(2), respectively. During re-
finement and analysis of the crystallographic data the programs
WinGX, PLATON, and ORTEP-III were used.[22g–22i] Further de-
tails are given in the deposited crystallographic data.
[K(18-C-6)thf2][pinB(SiMe2Ph)2] [5(thf)2]: In a nitrogen-filled glove
box 3 (30 mg, 80 μmol, 1.0 equiv.) was dissolved in THF (3 mL)
and 1 (70 mg, 267 μmol, 3.3 equiv.) was added. After 1 h at room
temp. the yellow solution was layered with n-pentane (5 mL) and
crystallized at –30 °C giving pale yellow crystals suitable for X-ray
diffraction after 16 h. The bulk material was isolated by decanting
and washing with n-pentane (2ϫ2 mL). For reactions and NMR
spectroscopy the material was used after careful drying in a stream
of nitrogen; thorough drying in vacuo leads to loss of co-crys-
tallized THF and subsequent decomposition.[13] However, after
brief drying in vacuo 39 mg (56 μmol, 69%) of analytically satisfac-
torily pure [K(18-C-6)][pinB(SiMe2Ph)2] (5, C34H58O8KBSi2) were
obtained, m.p. decomp. Ͼ 50 °C. 1H NMR (300 MHz, [D8]THF,
room temp.): δ = 7.60–7.55 (m, 4 H, o-CHAr), 7.01–6.94 (m, 4 H,
m-CHAr), 6.92–6.85 (m, 2 H, p-CHAr), 3.64–3.59 [m, 8 H,
(CH2CH2)2O], 3.59 (s, 24 H, OCH2), 1.79–1.73 [m, 8 H, (CH2-
CH2)2O], 0.79 [s, 12 H, OC(CH3)2], –0.06 [s, 12 H, Si(CH3)2] ppm.
13C{1H} NMR (75 MHz, [D8]THF, room temp.): δ = 135.5 (o/m-
CHAr), 131.3 (br. s, ipso-CHAr), 126.1 (o/m-CHAr), 125.0 (p-CHAr),
77.0 [OC(CH3)2], 71.0 (OCH2), 68.0 [(CH2CH2)2O], 27.1 [OC-
(CH3)2], 26.1 [(CH2CH2)2O], 1.1 [Si(CH3)2] ppm. 11B{1H} NMR
(96 MHz, [D8]THF, room temp.): δ = 9.2 (s, Δw1/2 = 80 Hz) ppm.
29Si{1H} NMR (79.5 MHz, [D8]THF, room temp.): δ = –29.6 (br.
s) ppm. ESI-MS (positive ion mode, MeCN): m/z: 303 [K(18-
Crystallographic data (excluding structure factors) for the struc-
tures reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
numbers CCDC-893777 (for 4), -893778 (for 5), -893779 (for 7),
-893780 (for 8), 903888 (for [K(thf){pinB(OtBu)2}]), and 919508
(for [K(18-C-6)I]). Copies of the data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
C-6)]+, (negative ion mode, MeCN): m/z: 413 [pinB(SiMe2Ph)2
+
O]–. C34H58BKO8Si2 (700.91): calcd. C 58.26, H 8.34; found C
57.88, H 8.44.
[pinB-SiMe2Ph(6)] (7): In a nitrogen-filled glove box 6 (90 mg,
499 μmol, 1.0 equiv.) was dissolved in toluene (3 mL) and 1
(130 mg, 496 μmol, 1.0 equiv.) was added. After 6 h at room temp.
the solution was concentrated in vacuo (to ca. 0.8 mL) and stored
at –20 °C. After a few hours large, well-developed colorless crystals
suitable for X-ray diffraction had separated. The supernatant solu-
tion was decanted; the crystals were washed with cooled n-pentane
(2ϫ1 mL, –20 °C) and dried in vacuo to give 47 mg (106 μmol,
21%) of 7, m.p. 95°–98 °C. 1H NMR (400 MHz, [D8]THF, room
temp.): δ = 7.48–7.44 (m, 2 H, CHAr), 7.14–7.04 (m, 3 H, CHAr),
5.85 [br. s, 2 H, CH(CH3)2], 2.17 [s, 6 H, NC(CH3)], 1.30 [br. s, 12
H, CH(CH3)2], 1.01 [br. s, 12 H, OC(CH3)2], 0.02 [s, 6 H,
Si(CH3)2] ppm. 13C{1H} NMR (100 MHz, [D8]THF, room temp.):
δ = 135.2 (CHAr), 127.5 (CHAr), 127.4 (CHAr), 124.8 [NC(CH3)],
79.1 [OC(CH3)2], 49.1 [CH(CH3)2], 26.0 [OC(CH3)2], 22.6
[CH(CH3)2], 10.4 [NC(CH3)], –0.6 [Si(CH3)2] ppm, the carbene-C
and the ipso-C atom were not detected. 11B{1H} NMR (96 MHz,
[pinB–SiMe2Ph] (1):[19a] Additional NMR spectroscopic data: 1H
NMR (300 MHz, [D8]THF, room temp.): δ = 7.56–7.50 (m, 2 H,
CHAr), 7.27–7.22 (m, 3 H, CHAr), 1.21 {s, 12 H, [OC(CH3)2]2},
0.25 [s, 6 H, Si(CH3)2] ppm. 11B{1H} NMR (96 MHz, [D8]THF,
room temp.): δ = 35.1 (s) ppm. 11B{1H} NMR (96 MHz, C6D6,
room temp.): δ = 35.1 (s) ppm. 29Si{1H} NMR (79.5 MHz, C6D6,
room temp.): δ = –28.0 (br. s) ppm.
[pinB–SiPh3] (2):[19a] Additional NMR spectroscopic data: 1H
NMR (300 MHz, [D8]THF, room temp.): δ = 7.55–7.48 (m, 6 H,
CHAr), 7.34–7.22 (m,
9 H, CHAr), 1.29 {s, 12 H, [OC-
(CH3)2]2} ppm. 11B{1H} NMR (96 MHz, [D8]THF, room temp.): δ
= 34.9 (s) ppm. 1H NMR (300 MHz, C6D6, room temp.): δ = 7.87–
7.82 (m, 6 H, CHAr), 7.22–7.13 (m, 9 H, CHAr), 1.01 {s, 12 H,
[OC(CH3)2]2} ppm. 11B{1H} NMR (96 MHz, C6D6, room temp.):
δ = 34.8 (s) ppm. 29Si{1H} NMR (79.5 MHz, C6D6, room temp.):
δ = –27.8 (br. s) ppm.
1
[D8]THF, room temp.): δ = 6.0 (s, Δw1/2 = 180 Hz) ppm. H NMR
[K(18-C-6)SiPh3] (4): In a nitrogen-filled glove box 3 (39 mg, (400 MHz, [D8]THF, –10 °C): δ = 7.46–7.42 (m, 2 H, CHAr), 7.12–
104 μmol, 1.0 equiv.) and 2 (40 mg, 104 μmol, 1.0 equiv.) each dis-
solved in toluene (2 mL) were mixed in a Schlenk tube. After a few
minutes the mixture was layered with n-pentane (ca. 3 mL) and
7.05 (m, 3 H, CHAr), 5.92 [sept., J = 7 Hz, 2 H, CH(CH3)2], 2.19
[s, 6 H, NC(CH3)], 1.39 [d, J = 7 Hz, 6 H, CH(CH3)2], 1.13 [d, J
= 7 Hz, 6 H, CH(CH3)2], 1.03 [s, 6 H, OC(CH3)2], 0.92 [s, 6 H,
crystallized at room temp. giving yellow-orange well-developed OC(CH3)2], 0.00 [s, 6 H, Si(CH3)2] ppm. 13C{1H} NMR (100 MHz,
Eur. J. Inorg. Chem. 2013, 2799–2806
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[D8]THF, –10 °C): δ = 166.4 (BCN2), 147.5 (ipso-CAr), 135.1
(CHAr), 127.4 (CHAr), 127.2 (CHAr), 125.2 [NC(CH3)], 78.4
[OC(CH3)2], 49.2 [CH(CH3)2], 26.5 [OC(CH3)2], 25.8 [OC(CH3)2],
22.5 [CH(CH3)2], 21.5 [CH(CH3)2], 10.6 [NC(CH3)], –0.3
[Si(CH3)2] ppm. C25H43BN2O2Si (442.52): calcd. C 67.85, H 9.79,
N 6.33; found C 67.86, H 9.75, N 6.57. All attempts to obtain a
29Si NMR spectrum were unsuccessful due to the dynamic pro-
cesses and the adjacent quadrupolar boron atom.
[pinB-SiPh3(6)] (8): As described for 7; 6 (68 mg, 378 μmol,
1.0 equiv.) and 2 (147 mg, 380 μmol, 1.0 equiv.) were used to give
141 mg (249 μmol, 66%) of the product. Colorless crystals suitable
for X-ray diffraction were obtained upon cooling a hot solution in
PhMe to room temp, m.p. decomp. Ͼ 172 °C. 1H NMR (400 MHz,
[D8]THF, room temp.): δ = 7.35–7.27 (m, 6 H, CHAr), 7.16–7.06
(m, 9 H, CHAr), 6.09 [sept., J = 7 Hz, 2 H, CH(CH3)2], 2.17 [s, 6
H, NC(CH3)], 1.33 [d, J = 7 Hz, 6 H, CH(CH3)2], 0.99 [s, 6 H,
OC(CH3)2], 0.90 [s, 6 H, OC(CH3)2], 0.53 [d, J = 7 Hz, 6 H,
CH(CH3)2] ppm. 13C{1H} NMR (100 MHz, [D8]THF, room
temp.): δ = 164.3 (br. s, BCN2), 142.9 (ipso-CAr), 137.7 (CHAr),
127.8 (CHAr), 127.4 (CHAr), 125.5 [NC(CH3)], 78.9 [OC(CH3)2],
49.3 [CH(CH3)2], 26.2 [br. s, OC(CH3)2], 22.9 [CH(CH3)2], 20.0
[CH(CH3)2], 10.6 [NC(CH3)] ppm. 11B{1H} NMR (96 MHz, [D8]-
THF, room temp.): δ = 6.5 (s, Δw1/2 = 59 Hz) ppm. 1H NMR
(300 MHz, C6D6, room temp.): δ = 7.35–7.27 (m, 6 H, CHAr), 7.27–
7.12 (m, 9 H, CHAr), 6.27 [sept., J = 7 Hz, 2 H, CH(CH3)2], 1.57
[s, 6 H, NC(CH3)], 1.26 [s, 6 H, OC(CH3)2], 1.21–1.11 [m, 12 H,
C(CH3)2, CH(CH3)2 (overlapping)], 0.49 [d, J = 7 Hz, 6 H,
CH(CH3)2] ppm. 11B{1H} NMR (96 MHz, C6D6, room temp.): δ
= 6.4 (s, Δw1/2 = 53 Hz) ppm. C35H47BN2O2Si (566.66): calcd. C
74.19, H 8.36, N 4.94; found C 74.28, H 8.17, N 4.72. All attempts
to obtain a 29Si NMR spectrum were unsuccessful due to the dy-
namic processes and the adjacent quadrupolar boron atom.
[3]
[4]
[5]
[6]
Supporting Information (see footnote on the first page of this arti-
cle): The Supporting Information contains experimental details and
in situ NMR spectra of the reactions of 1 and 2 with 3 and KOtBu,
respectively, as well as of the reactions of 4, 5(thf)2, 7, and 8 with
MeI, TMS-Cl, and BnBr, respectively. In addition the crystal struc-
tures of [K(thf){pinB(OtBu)2}] and [K(18-C-6)I] as well as the de-
composition of 5(thf)2 upon evacuation is discussed.
[7]
[8]
Acknowledgments
Support of this research through a Liebig-Stipendium of the Fonds
der Chemischen Industrie (to C. K.) and a Doktoranden-Sti-
pendium (to C. B.) is gratefully acknowledged. Additional funding
was kindly provided by the Deutsche Forschungsgemeinschaft
(DFG). The authors thank Frouke Aal and Prof. Dr. Matthias
Tamm for supplying the NHC 6 and Evgenia Feldmann for valu-
able help in the laboratory.
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[16] The free enthalpy of activation was estimated from the peak
separation at slow chemical exchange {128 Hz [CH(CH3)2],
50 Hz [OC(CH3), 400 MHz] at –80 °C} and the coalescence
temperature (18 °C) to be 57–60 kJmol–1. J. Sandström, Dy-
namic NMR Spectroscopy, Academic Press Inc., London, 1982.
[17] Note also that a mixture of equimolar amounts of 1 and
KOtBu {presumably forming the [pinB(OtBu)(SiMe2Ph)]– ad-
duct proposed by Ito et al.}[5a] was treated with excess BnBr
and resulted, according to 1H NMR spectroscopic data, in the
formation of PhMe2Si-Bn, Bn2, and (PhMe2Si)2 along with un-
characterized products, while small amounts of pinB-Bn were
detected by GC/MS analysis.[13]
Received: January 25, 2013
Published Online: April 5, 2013
Eur. J. Inorg. Chem. 2013, 2799–2806
2806
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim