Reactivity of a titanocene pendant Si-H group toward alcohols. Unexpected formation of siloxanes from the reaction of hydrosilanes and Ph3COH catalyzed by B(C6F5)3

Article abstract of DOI:10.1021/om400253g

The reaction of [Cp(η⁵-C₅H₄CH ₂SiMe₂H)TiCl₂] (1; Cp = η⁵- C₅H₅) and methanol in the presence of catalytic amounts of B(C₆F₅)₃ afforded a complex with a pendant silyl ether group, [Cp(η⁵-C₅H₄CH ₂SiMe₂OMe)TiCl₂] (2), in good yield. The analogous reaction of 1 and Ph₃COH resulted in the unexpected formation of [CpTiCl₂{μ-η⁵:η⁵- (C₅H₄)CH₂SiMe₂OSiMe ₂CH₂(C₅H₄)}TiCl₂Cp] (4). The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph₃COH mediated by B(C₆F₅)₃ has a general applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity toward Ph₃COH in comparison with other alcohols can be attributed to the easy generation of the borate salt [Ph₃C]⁺[(C₆F₅)₃B(μ-OH) B(C₆F₅)₃]^- (5) under catalytic conditions. The intramolecular Si-H and Ti-Cl exchange in 1 is catalyzed by B(C₆F₅)₃ in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride, which is either chlorinated to give [Cp(η⁵-C₅H ₄CH₂SiMe₂Cl)TiCl₂] (3) in CD ₂Cl₂ or decomposes into several paramagnetic Ti(III) species in toluene-d₈. Complex 3 was independently synthesized from 1 and Ph₃CCl in a good yield.

Full text of DOI:10.1021/om400253g

Article
pubs.acs.org/Organometallics
Reactivity of a Titanocene Pendant Si−H Group toward Alcohols.
Unexpected Formation of Siloxanes from the Reaction of
Hydrosilanes and Ph3COH Catalyzed by B(C6F5)3
Tomas Strasak,† Jan Sykora,† Martin Lamac,‡ Jirí Kubista,‡ Michal Horacek,‡ Robert Gyepes,‡,§
́
̌
̌
́
́
̌
̌
̌
́
̌
́
and Jirí Pinkas*,‡
̌
†Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, v.v.i., Rozvojova
Czech Republic
‡J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejsk
́
135, 165 02 Prague 6,
́
̌
ova 2155/3, 182 23 Prague 8,
Czech Republic
§J. Selye University, Faculty of Education, Bratislavska
́ ́
cesta 3322, 945 01 Komarno, Slovak Republic
S
* Supporting Information
ABSTRACT: The reaction of [Cp(η5-C5H4CH2SiMe2H)-
TiCl2] (1; Cp = η5-C5H5) and methanol in the presence of
catalytic amounts of B(C6F5)3 afforded a complex with a
pendant silyl ether group, [Cp(η5-C5H4CH2SiMe2OMe)-
TiCl2] (2), in good yield. The analogous reaction of 1 and
Ph3COH resulted in the unexpected formation of [CpTiCl2{μ-
η5:η5-(C5H4)CH2SiMe2OSiMe2CH2(C5H4)}TiCl2Cp] (4).
The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph3COH mediated by B(C6F5)3 has a general
applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom
and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity
toward Ph3COH in comparison with other alcohols can be attributed to the easy generation of the borate salt
[Ph3C]+[(C6F5)3B(μ-OH)B(C6F5)3]− (5) under catalytic conditions. The intramolecular Si−H and Ti−Cl exchange in 1 is
catalyzed by B(C6F5)3 in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride,
which is either chlorinated to give [Cp(η5-C5H4CH2SiMe2Cl)TiCl2] (3) in CD2Cl2 or decomposes into several paramagnetic
Ti(III) species in toluene-d8. Complex 3 was independently synthesized from 1 and Ph3CCl in a good yield.
Scheme 1. Mechanism of the Silylation of Alcohols
Catalyzed by B(C6F5)3
INTRODUCTION
■
The preparation of tris(pentafluorophenyl)borane, B(C6F5)3, a
unique and remarkably strong Lewis acid, was first reported in
1963 by Massey, Stone, and Park.1 However, the story of its
successful application began more than 20 years later, after its
ability to abstract the alkyl group from group 4 metallocene
dialkyls was uncovered. These species, after forming cationic
complexes, exhibited high activity in olefin polymerization.2−4
Since then, B(C6F5)3 as a catalyst in organic, organometallic,
and polymer chemistry has drawn considerable attention,
evidenced also by numerous review articles.5−9 In addition,
catalytic processes utilizing hydrosilanes as reactants have also
become very important in recent years. The main reaction types
include (i) silylation of alcohols (Scheme 1)10,11 and thiols,12
(ii) formation of (poly)siloxanes from silyl hydrides and
alkoxysilanes (known as the Piers−Rubinsztajn reaction),13,14
(iii) hydrosilylation of imines,15 olefins,16 and aldehydes,
ketones, and esters,17 and (iv) activation of C−F bonds in
fluoroalkanes.18 A general feature of all these reactions is the
activation of the Si−H bond via the formation of the silane−
borane adduct R3Siδ+H·Bδ−(C6F5)3 (see Scheme 1). Such an
activated “silylium cation” then attacks the nucleophilic part of
the substrate, which is followed either by the boron-bonded
hydride compensating the carbocation formed (in iii and iv) or
releasing hydrogen or alkane, respectively (in i and ii).
The present work is the result of our long-term interest in
the preparation of group 4 metallocene and related complexes
with pendant functional groups, which are modifiable directly
Received: March 25, 2013
© XXXX American Chemical Society
A
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within the organometallic framework.19−26 The pendant Si−H
group seemed to be a convenient choice for further
transformations, since it is easily introduced at early stages of
the metallocene dichloride synthesis and is also reasonably
stable toward air and moisture under laboratory conditions.
The Si−H group on the metallocene periphery can
subsequently be modified via either catalytic hydrosilylation27,28
or intramolecular dehydrocoupling reactions.29,30
Scheme 3. Methanol Silylation with 1 in CD2Cl2 Catalyzed
by B(C6F5)3
Herein we introduce a new titanocene dichloride bearing a
pendant Si−H group (1) as a convenient precursor for the
preparation of titanocene dichlorides with a pendant silyl ether
functionality using silylation of methanol catalyzed by
B(C6F5)3. This reaction follows the mechanism described in
Scheme 1.10 In contrast, we have found the reaction of
titanocene 1 with Ph3COH and catalyzed by B(C6F5)3 to
proceed in a different way, leading to the formation of a
dinuclear complex having a bis(methylene)tetramethylsiloxane
bridge between its two titanocene units. Extending the reaction
scope to a variety of hydrosilanes showed that formation of
siloxanes from the reaction of 2 equiv of hydrosilanes with
Ph3COH in the presence of a catalytic amount of B(C6F5)3 is of
general applicability. In addition, the reactivity of B(C6F5)3
toward Ph3COH with various stoichiometric ratios was
evaluated.
the signals corresponding to 1 and the formation of a new set of
signals attributable to 2. In addition to the major product 2 (95
mol %), the presence of the byproduct [Cp(η5-
C5H4CH2SiMe2Cl)TiCl2] (3; 5 mol %) was also recognized
(vide infra) in the 1H NMR spectrum. Complex 2 was isolated
in a pure state in a moderate yield of 62% and was characterized
by common spectroscopic methods. The presence of the
methylsiloxy group was confirmed by both NMR in CD2Cl2
solution (SiOMe: δH 3.39 ppm; δC 50.77 ppm) and IR
spectroscopy (ν(C−H) 2832 cm−1, νas(Si−O−C) 1187 and
1089 cm−1; Figure S1 in the Supporting Information). Signals
corresponding to SiMe2 (δH 0.09 ppm, δC −2.58 ppm) and
CH2Si (δH 2.28 ppm, δC 25.94 ppm) appeared as singlets in 1H
NMR spectra due to the disappearance of the Si−H group.
The identity of byproduct 3 was verified by its independent
synthesis following Corey’s work on the chlorination of
hydrosilanes with trityl chloride.31 The reaction of 1 with
Ph3CCl in CDCl3 as a solvent (Scheme 4) ran to completion
RESULTS AND DISCUSSION
■
T h e r e a c t i o n o f l i t h i u m ( d i m e t h y l s i l y l ) -
methylenecyclopentadienide prepared in situ with [CpTiCl3]
in THF (Scheme 2) yielded a titanocene dichloride with a
Scheme 4. Chlorination of 1 with Ph3CCl
Scheme 2. Preparation of 1
within 4 days, affording 3 in 70% yield. The formation of
triphenylmethane as the second reaction product was proved
by NMR spectroscopy (Ph3CH: δH 5.55 ppm; δC 56.92 ppm).
Complex 3 was identified by NMR, IR, and EI-MS.
Replacement of the electron-donating hydride with the
electron-withdrawing chloride led to a significant electron
deshielding on the silicon atom, which resulted in a downfield
shift of the SiMe2 group signals in 29Si NMR (δSi 29.3 ppm) as
pendant (methylene)dimethylsilane group, 1, obtained as a red-
orange solid in good yield (72%) after workup. A characteristic
feature of the complex is the presence of a SiH functionality,
which is easily recognized in the solid state by IR spectroscopy
(ν(Si−H) 2107 cm−1; Figure S1 in the Supporting
Information) and in its CDCl3 solution by NMR spectroscopy
(SiH: δH 3.93 ppm; δSi −11.0 ppm). The presence of an Si−H
group led to a split of signals corresponding to SiMe2 (δH 0.09
ppm) and CH2Si groups (δH 2.32 ppm) into doublets with a
low coupling constant (3JHH = 3.3 Hz), typical for vicinal
coupling through the silicon atom.29,30 The composition of 1
was corroborated by EI-MS, where the [SiMe2H]+ (m/z 59)
fragment was found as the base peak, whereas the abundance of
the molecular ion (m/z 320) was low. Solid 1 was found to be
sufficiently stable toward air and moisture, as no decomposition
was detected by NMR even after it was stored without a
protective atmosphere for a period of several months.
1
well as in H NMR for SiMe2 (δH 0.43 ppm) and CH2Si (δH
2.57 ppm). The presence of pendant chlorosilane was further
supported by IR spectroscopy, where the strong stretching
vibration ν(Si−Cl) could be found at 465 cm−1 (Figure S1 in
the Supporting Information).
The observed formation of byproduct 3 during the reaction
of 1 with methanol (as depicted in Scheme 3) was rather
surprising, since the formation of chlorosilanes has not been
reported for the catalytic silylation of alcohols in chlorinated
solvents.10 Therefore, we have conducted the reaction in the
absence of methanol to evaluate the role of chlorinated solvent
in the formation of 3. The treatment of 1 with ca. 4 mol % of
B(C6F5)3 in CD2Cl2 in an NMR tube led to a complete
consumption of 1 and formation of 3 as the main reaction
product. Furthermore, a quintuplet (2JHD = 1.5 Hz) with a line
The transformation of Si−H into Si−OR in 1 was performed
by following the synthetic protocol for silylation of alcohols.10
The reaction of 1 with methanol in the presence of a catalytic
amount of B(C6F5)3 (ca. 2 mol %) was conducted in CD2Cl2
(Scheme 3). Analysis of the reaction mixture after several hours
by NMR spectroscopy revealed a complete disappearance of
1
ratio of ca. 1:2:3:2:1 positioned at 2.98 ppm in the H NMR
B
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spectrum (Figure S2 in the Supporting Information) can be
assigned to CD2HCl (for comparison, CH3Cl: δH(CDCl3) 3.01
ppm)32 as a product of CD2Cl2 reduction. The reaction
proceeded in the same manner in CDCl3, giving rise to
characteristic signals of CDHCl2 as a triplet (2JHD = 1.0 Hz, ca.
similarly to methanol (with the formation of the corresponding
siloxane) or rather to Ph3CCl. The reaction of 1 with an
equimolar amount of Ph3COH in the presence of 4 mol % of
B(C6F5)3 in CD2Cl2 solution (Scheme 6) afforded the
1
1:1:1 line ratio) at 5.28 ppm in the H NMR spectrum and a
Scheme 6. Reaction of 1 with Ph3COH
triplet (1JCD = 27.5 Hz, 1:1:1 line ratio, Figure S3 in Supporting
Information) at 53.37 ppm in the 13C NMR spectrum in
addition to the signals of 3.
Notably, the reaction of 1 with a catalytic amount of
1
B(C6F5)3 in toluene-d8 led to visible gas evolution and the H
NMR spectrum did not reveal any sufficiently populated
diamagnetic compound. In contrast, the EPR spectrum (Figure
S4 in the Supporting Information) of the sample showed three
signals (centered at g = 1.9886, 1.9767, and ca. 1.98) with the
dominating one being the very broad (ΔH ≈ 20 mT) signal
centered at g ≈ 1.98, which suggested the presence of several
paramagnetic Ti(III) species.
dinuclear complex 4 with a bis(methylene)tetramethylsiloxane
bridge between two titanocene dichloride units as the sole
organometallic product. 1H and 13C NMR spectra of 4 showed
a pattern similar to that for 2 with singlet signals for SiMe2 (δH
0.05 ppm, δC 0.42 ppm) and CH2Si (δH 2.21 ppm, δC 25.28
ppm). The SiMe2 signal observed in 29Si NMR spectra (δSi 5.9
ppm) is shifted considerably upfield in comparison with that for
2 (δSi 15.2 ppm); however, it is close to the value for
hexamethyldisiloxane (δSi 7.4 ppm).38 Further evidence for the
Me2SiOSiMe2 siloxane spacer comes from the presence of a
strong infrared absorption band at 1059 cm−1 assigned to the
νas(Si−O−Si) vibration (Figure S1 in the Supporting
Information).39
We suggest that in the absence of methanol the reaction
proceeds following Scheme 5. The Si−H bond in 1 is polarized
Scheme 5. Proposed Mechanism for Catalytic
Transformations of 1 in the Absence of Methanol
The unexpected formation of 4 led us to investigate the
reaction in detail. We have used PhMe2SiH as a model
hydrosilane for further study in order to simplify the reaction. A
stoichiometric reaction of PhMe2SiH and Ph3COH catalyzed
by B(C6F5)3 (0.5 mol % with respect to the silane) produced a
mixture consisting of approximately equimolar amounts of
(PhMe2Si)2O (tr = 16.1 min, m/z 286), Ph3CH (tr = 19.8 min,
m/z 244) and Ph3COH (tr = 21.7 min, m/z 260), as was
determined by GC-MS. A preparative-scale reaction of 2 equiv
of PhMe2SiH with 1 equiv of Ph3COH catalyzed by B(C6F5)3
(0.5 mol % with respect to the silane) produced (PhMe2Si)2O
and Ph3CH as the reaction products in a ca. 1:1 molar ratio (as
by the formation of the silane−borane adduct [Cp(η5-
C5H4CH2SiMe2H·B(C6F5)3)TiCl2], similarly as known for
Me3SiH·B(C6F5)3 or Et3SiH·B(C6F5)3.9,13 The activated silicon
atom becomes a sufficiently strong electrophile to attack the
negatively charged chlorine ligand and induces an intra-
molecular hydrido chloride exchange between the titanium
and silicon centers yielding a titanocene hydrido chloride with a
pendant chlorosilane arm. A similar intermolecular hydro-
silylation of decamethylscandocene chloride mediated by
B(C6F5)3, leading to the corresponding decamethylscandocene
hydride, was reported recently.33,34
The transiently formed titanocene hydrido chloride complex
is a highly reactive species35 and undergoes chlorination with
CD2Cl2 or CDCl3 to give 3, with concomitant reduction of the
solvent to CD2HCl or CDHCl2, respectively. Indeed, the ability
of titanium hydrides to activate the C−X bond (X = Cl, Br) in
haloaromatic substrates has been well demonstrated.36,37 One
can assume that in a less nucleophilic solvent such as toluene-d8
the titanocene hydrido chloride species reductively decomposes
into paramagnetic Ti(III) species with the concomitant
evolution of molecular hydrogen. An attempt to trap the
transient titanocene hydrido chloride with diphenylacetylene
failed, since the paramagnetic products detected in EPR yielded
a spectrum with a spectral pattern very similar to that obtained
in the previous experiment.
1
determined by GC-MS and H NMR of the reaction mixture
after 30 min). Finally, the siloxane (PhMe2Si)2O was isolated
from the reaction mixture by column chromatography on silica
gel (eluent hexane, Rf = 0.33) in a high yield (84%).
Importantly, the GC-MS analysis of the reaction mixture
showed also a peak with low population (ca. 2 mol % with
respect to (PhMe2Si)2O) at tr = 8.4 min with m/z 152, which
indicated the presence of the silanol PhMe2SiOH. This
observation and the reaction stoichiometry indicate that the
reaction proceeds in two consecutive steps and the silanol
PhMe2SiOH is a crucial intermediate. Indeed, performing the
reaction of silanol PhMe2SiOH and hydrosilane PhMe2SiH
catalyzed by B(C6F5)3 in CD2Cl2 in an NMR tube led to
hydrogen evolution and clean formation of the siloxane
(PhMe2Si)2O (see Figure S5 in the Supporting Information).
On the basis of the above observations we propose the reaction
sequence as depicted in Scheme 7.
To validate the proposed sequence, the reaction was
1
monitored by H NMR in CD2Cl2 at low temperature (see
Figure S6 in the Supporting Information). Immediately after all
components were mixed, formation of Ph3CH was detected (a
characteristic singlet signal of the methine group Ph3CH
appeared at 5.60 ppm), which indicated hydroxyl transfer from
the trityl group to silicon proceeding very quickly. Simulta-
In the next step we have examined the reactivity of 1 toward
triphenylmethanol. We were curious whether it would react
C
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B(C6F5)3·ROH8 does not hold for the particular case of
Ph3COH. Therefore, we have investigated the reactivity of
B(C6F5)3 toward Ph3COH at their different mutual ratios.
As an initial effort, we reacted equimolar amounts of
Ph3COH and B(C6F5)3 in CD2Cl2 (Scheme 8). Mixing both
colorless solids led already to a yellowish mixture, which after
addition of CD2Cl2 dissolved immediately to give a yellow
Scheme 7. Proposed Reaction Sequence for the Reaction of
2 equiv of PhMe2SiH and Ph3COH
1
solution. This solution was investigated by H and 19F NMR
neously, broad singlet signals centered at 3.05 and 2.81 ppm
appeared. The former signal could be attributed to the residual
adduct B(C6F5)3·Ph3COH. As the reaction evolved, the latter
signal shifted downfield to 2.61 ppm, which is close to the value
found for the hydroxyl proton in free PhMe2SiOH (δH 2.48
ppm) (see Figure S5 in the Supporting Information). Within
approximately 10 min, the signal became very broad (in the
range from ca. 2 to 3 ppm), indicating the presence of a rather
complicated mixture of unidentified silanol−borane adducts
and intermediates. The signal of the dihydrogen formed could
be detected at later stages of the reaction (after ca. 4 min), and
its intensity increased slowly as the reaction proceeded.
Concurrently, the intensity of the signal corresponding to the
hydride SiH steadily decreased, which indicated a rather slow
rate of the second step of the reaction depicted in Scheme 7.
The reaction reached completion in approximately 20 min,
affording (PhMe2Si)2O and Ph3CH in the expected stoichio-
metric ratio.
spectroscopy, performed at variable temperature. The 1H NMR
spectrum at −100 °C (Figure S7 in the Supporting
Information) showed two sets of signals: (i) a triplet signal at
6.57 ppm (JHF ≈ 17.5 Hz) and a very broad unresolved signal
centered at ca. 7.8 ppm (ν1/2 ≈ 150 Hz) and (ii) a broad singlet
at 3.29 ppm (ν1/2 ≈ 16 Hz) and a multiplet at 7.09−7.32 ppm.
On the basis of previously published data,40 we have assigned
the former set of signals to a hydroxo-bridged borate anion with
the compensating tritylium cation [Ph3C]+[(C6F5)3B(μ-OH)-
B(C6F5)3]− (5). The assignment of the borate anion was
further supported by 19F NMR spectra (Figure S8 in the
Supporting Information). The second set of signals was slightly
shifted with respect to those of the free triphenylmethanol
(Ph3COH (CD2Cl2): δH 2.80 (s, 1H, OH); 7.20−7.31 (m,
15H, Ph)).41 Moreover, the signal of the hydroxyl group (δH
3.29 ppm at −100 °C) was found to be thermally dependent
(Figure S9 in the Supporting Information). The aforemen-
tioned observations can be tentatively explained by the
formation of the weak adduct [Ph3C]+[(C6F5)3B(μ-OH)B-
(C6F5)3]−·Ph3COH and its equilibrium with free triphenylme-
thanol as depicted on the left side in Scheme 8.
The reaction scope was further extended to three
commercially available tertiary hydrosilanes; the results are
given in Table 1. All hydrosilanes tested were converted cleanly
into the corresponding siloxanes in high yields (78−91%); in all
1
cases Ph3CH was found as a byproduct, as evidenced by H
The reaction between Ph3COH and B(C6F5)3 was
subsequently performed also in a 1:2 ratio to maintain a
proper stoichiometry for the hydroxo-bridged borate salt 5. The
initial formation of a yellow color after mixing both solid
reactants indicated the likely formation of the tritylium cation
already in the solid state. The IR spectrum of the homogenized
solid mixture (Figure S10 in the Supporting Information)
showed a new vibration band for ν(O−H) at 3551 cm−1 (the
starting Ph3COH showed ν(O−H) at 3473 cm−1). This
observed value is closer to the value reported for [Ge-
(Dipp2nacnac)]+[(C6F5)3B(μ-OH)B(C6F5)3]− (where
Dipp2nacnac = {N(C6H3-2,6-i-Pr2)C(Me)}2CH}; ν(O−H)
3540 cm−1)42 than to a range of values known for
hydroxyborate anions in the salt [PPN]+[(C6F5)3B(OH)]−
(where PPN = bis(triphenylphosphoranylidene)ammonium;
ν(O−H) 3680−3700 cm−1);43 therefore, the formation of salt
NMR.
Table 1. Reactions of Tertiary Hydrosilanes with Ph3COH
Catalyzed by B(C6F5)3
a
cat. B(C6F )3
5
R3SiH (2 equiv) + Ph3COH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ R3SiOSiR3
−Ph3CH, −H2
b
run
1
hydrosilane
product
yield, %
91
PhMe2SiH
PhMe2SiH
Et3SiH
(PhMe2Si)2O
(PhMe2Si)2O
(Et3Si)2O
c
d
2
84
3
4
5
88
82
78
Ph3SiH
(Ph3Si)2O
i-Pr3SiH
(i-Pr3Si)2O
a
Reaction conditions: hydrosilane, 0.6 mmol; Ph3COH, 0.3 mmol;
B(C6F5)3, 6 μmol; CH2Cl2, 5 mL; time, 30 min; room temperature.
Yield determined by H NMR. Preparative scale: hydrosilane, 10
b
c
1
1
5 is anticipated even in the solid state. H spectra of CD2Cl2
mmol; B(C6F5)3, 23 μmol; CH2Cl2, 20 mL, time, 30 min, room
temperature. Isolated yield.
d
solution of 5 showed tritylium signals as slightly broadened
peaks in the expected range at all studied temperatures (Figure
S11 in the Supporting Information). The 19F NMR spectra at
−100 °C (Figure S12 in the Supporting Information) showed a
pattern typical for the [(C6F5)3B(μ-OH)B(C6F5)3]− anion,
supportive of the proposed structure. Interestingly, temper-
ature-dependent spectra of 5 did not give any evidence for the
presence of free B(C6F5)3, which indicated that the equilibrium
Reactivity of B(C6F5)3 toward Ph3COH. The above results
indicated a different reactivity of hydrosilanes toward Ph3COH
in the presence of B(C6F5)3, in comparison to other alcohols.
On the basis of these observations one can assume that the
well-documented formation of the borane−alcohol adduct
Scheme 8. Reactivity of B(C6F5)3 toward Ph3COH
D
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proposed by Saverio et al. (eq 1)40 either does not hold at all
for the present case or is at least strongly shifted to the right
side.
reaction could be extended to a variety of hydrosilanes. The
easy generation of the borate salt [Ph3C]+[(C6F5)3B(μ-
OH)B(C6F5)3]− (5) under catalytic conditions is proposed to
be responsible for the catalysis of hydrosilane hydroxylation and
in general for the twist of hydrosilane reactivity toward
Ph3COH in comparison to other alcohols. An intramolecular
mutual Si−H to Ti−Cl group exchange was found in 1 in the
presence of a catalytic amount of B(C6F5)3. The transiently
formed titanocene hydrido chloride was either chlorinated to
give the titanocene dichloride with pendant chlorosilane group
3 in CD2Cl2 solution or decomposed into Ti(III) species in
toluene-d8 solution. This indicates that titanocene hydrides can
be generated in situ by the reaction of the corresponding
metallocene dichlorides with a hydrosilane in the presence of a
catalytic amount of B(C6F5)3, similarly as published recently for
decamethylscandocene.
[(C6F5)3B(OH)]− + B(C6F5)3
⥂[(C6F5)3B(μ‐OH)B(C6F5)3]−
(1)
The formation of 5 could be detected even in a mixture
containing a huge excess of Ph3COH with respect to B(C6F5)3.
The mixing of 50 equiv of Ph3COH with 1 equiv of B(C6F5)3
(i.e., in the ratio at which the catalytic reactions were
performed) gave a yellow solid which dissolved in CD2Cl2,
giving a green solution. The presence of the borate anion
[(C6F5)3B(μ-OH)B(C6F5)3]− could be confirmed by the
appearance of characteristic signals in the 19F NMR spectrum
of the solution at −100 °C (Figure S13 in the Supporting
Information). Moreover, signals of the tritylium cation were
found in the 1H NMR spectrum at −100 °C (Figure S14 in the
Supporting Information).
EXPERIMENTAL SECTION
■
General Considerations. All reactions with moisture- and air-
sensitive compounds were carried out under argon (99.998%) using
standard Schlenk techniques. Solvents (hexane, heptane, toluene,
diethyl ether (Et2O), and tetrahydrofuran (THF)) were dried with
sodium/benzophenone and stored over a sodium mirror under argon.
Chloroform and dichloromethane were dried with CaH2 and stored
over molecular sieves 4 Å. Methanol was dried with sodium and freshly
distilled prior to use. n-BuLi (1.6 M in hexane) was obtained from
Aldrich. [CpTiCl3] (99%) was obtained from Acros Organics and used
as received. Diphenylacetylene (Fluka) was dried under high vacuum
overnight. B(C6F5)3 was obtained from Strem and used as received.
PhMe2SiH, Et3SiH, and i-Pr3SiH (Aldrich) were dried with LiAlH4 and
distilled under vacuum. PhMe2SiOH (Aldrich) was distilled under
vacuum (98−100 °C/10 Torr) prior to use. Ph3COH and Ph3CCl
were obtained from Alfa Aesar and dried under vacuum for 8 h prior to
use. (Dimethylsilyl)methylenecyclopentadiene (a mixture of isomers)
was prepared according to the literature procedure.44 1H, 13C, 19F, and
29Si NMR spectra were recorded on a Varian Mercury 300
spectrometer at 300.0, 75.4, 282.2, and 59.9 MHz, respectively, at
298 K if not otherwise stated. Chemical shifts (δ/ppm) are given
relative to solvent signals (toluene-d8, δH 2.08 ppm, δC 20.43 ppm for
CHD2; CDCl3, δH 7.26 ppm, δC 77.16 ppm; CD2Cl2, δH 5.32 ppm, δC
53.84 ppm). EPR spectra were measured on an ERS-220 spectrometer
(Center for Production of Scientific Instruments, Academy of Sciences
of GDR, Berlin, Germany) operated by a CU-3 unit (Magnettech,
Berlin, Germany) in the X-band. g values were determined using an
Mn2+ standard signal at g = 1.9860 (MI = −1/2 line). EI-MS spectra
were obtained on a VG-7070E mass spectrometer at 70 eV. Crystalline
samples in sealed capillaries were opened and inserted into the direct
inlet under argon. IR spectra of samples either in a Nujol mull or in
KBr were taken on a Nicolet Avatar FTIR spectrometer in the range
400−4000 cm−1. Melting points of samples (under air or in sealed
capillaries under nitrogen) were measured on a Koffler block and were
not corrected. Elemental analyses were carried out on a FLASH 2000
CHN-O Automatic Elemental Analyzer (Thermo Scientific).
From the aforementioned observations we propose that 5 is
the key catalytic species in the B(C6F5)3-catalyzed formation of
silanols from hydrosilanes. A tentatively proposed overall
catalytic cycle is depicted in Scheme 9.
Scheme 9. Proposed Catalytic Cycle for Hydroxylation of
Silanes (Left Cycle) and Subsequent Siloxane Formation
from Silane and in Situ Formed Silanol (Right Cycle)
CONCLUSION
■
This work has demonstrated the ability of a novel titanocene
dichloride bearing a pendant methylenedimethylsilane group, 1,
to achieve methanol silylation in the presence of a catalytic
amount of B(C6F5)3 in a way similar to that known for
commercially available hydrosilanes. A possible extension of the
reaction scope to a variety of alcohols could lead to a direct
modification of the titanocene periphery with various alkoxo
groups in one reaction step. However, Ph3COH reacted with 1
in a different way, and the dimeric titanocene complex 4 with a
bis(methylene)tetramethylsiloxane bridge between cyclopenta-
dienyl rings was formed. A detailed study of the B(C6F5)3-
mediated reaction of Ph3COH with the model hydrosilane
PhMe2SiH showed that the siloxane formation proceeds in two
consecutive steps. The fast initial step consists of hydroxyl
group transfer from the trityl moiety to the silicon atom with
concomitant formation of the silanol PhMe2SiOH and Ph3CH.
In the next step, the in situ formed silanol undergoes a
B(C6F5)3-catalyzed silylation with a second equivalent of
hydrosilane to form a siloxane and H2. The scope of the
[Cp(η5-C5H4CH2SiMe2H)TiCl2] (1). To
a mixture of
(dimethylsilyl)methylenecyclopentadienes (2.00 g, 14.5 mmol) in
THF (20 mL) was slowly added dropwise a solution of n-BuLi in
hexane (9.1 mmol, 1.6 M, 14.5 mmol). The resulting solution was
stirred for 2 h and then slowly dropped into a cold (−78 °C) solution
of [CpTiCl3] (3.17 g, 14.5 mmol) in THF (30 mL). The reaction
mixture was warmed to room temperature and stirred for a further 20
h. The final red mixture was filtered, and the filtrate was evaporated to
dryness under vacuum. The crude product was extracted with a
toluene/CH2Cl2 mixture (1/1 v/v, 10 mL). The volume of the
obtained dark red extract was reduced to a minimum and left in the
freezer (−78 °C) for several days. The formed red-orange solid that
formed was isolated, washed with toluene (5 mL) and hexane (3 × 5
mL), and dried under vacuum. Yield: 3.36 g (72%).
E
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Organometallics
Article
3
Mp: 138 °C. 1H NMR (CDCl3): 0.09 (d, JHH = 3.3 Hz, 6H,
29Si{1H} NMR (CD2Cl2): 29.3 (Me2SiCl). EI-MS, m/z (relative
3
abundance): 354 (M•+, 4); 321 (8); 319 ([M − Cl]+, 10); 295 (9);
294 (8); 293 (28); 292 (19); 291 (78); 290 (25); 289 ([M − Cp]+,
75); 254 ([M − Cp-Cl]•+, 6); 222 (13); 185 (13); 183 ([CpTiCl2]+,
19); 173 (12); 171 ([C5H4CH2SiMe2Cl]+, 27); 150 (21); 148
([CpTiCl]•+, 60); 95 (58); 94 (18); 93 ([SiMe2Cl]+, 100); 83 (22);
65 ([Cp]+, 28). IR (Nujol, cm−1): 3113 (s), 1491 (s); 1444 (s), 1426
(m), 1255 (s), 1170 (m), 1066 (w), 1048 (m), 1023 (w), 1016 (w),
935 (w), 904 (w), 853 (s, sh), 825 (vs), 804 (s), 759 (w), 738 (vw),
660 (w), 591 (vw), 465 (m), 418 (w), 405 (w). Anal. Calcd for
C13H17Cl3SiTi (355.60): C, 43.91; H, 4.82. Found: C, 44.12; H, 5.89.
[CpTiCl2{μ-η5:η5-(C5H4)CH2SiMe2OSiMe2CH2(C5H4)}TiCl2Cp]
(4). To a solid mixture of 1 (17 mg, 53 μmol), Ph3COH (14 mg, 53
μmol), and a catalytic amount of B(C6F5)3 (ca. 1 mg, 2 μmol) was
added CD2Cl2 (0.7 mL). The resulting red solution was stirred for 5
min and transferred into an NMR tube and degassed by a freeze−
pump−thaw process (three times), and the tube was sealed off by
SiMe2); 2.32 (d, JHH = 3.3 Hz, 2H, CH2); 3.93 (m, 1H, SiMe2H);
6.16, 6.41 (2 × pseudo t, 2 × 2H, C5H4); 6.54 (s, 5H, C5H5). 13C{1H}
NMR (CDCl3): −4.31 (SiMe2); 20.74 (CH2); 115.76 (CH, C5H4);
119.57 (C5H5); 121.98 (CH, C5H4); 139.05 (Cipso, C5H4). 29Si{1H}
NMR (CDCl3): −11.0 (Me2SiH). EI-MS, m/z (relative abundance):
322 (17); 321 (11); 320 (M•+, 21); 284 (24); 284 (20); 283 (34); 259
(14); 257 (63); 256 (33); 255 ([M − Cp]+, 75); 253 (36); 244 (23);
242 (32); 224 (28); 223 (18); 222 (64); 191 (24); 190 (46); 185
(24); 183 ([CpTiCl2]+, 45); 163 (21); 161 (23); 150 (75); 149 (69);
148 ([CpTiCl]•+, 85); 137 ([C5H4CH2SiMe2H]+, 37); 85 (18); 83
([TiCl]+, 32); 78 (40); 65 ([Cp]+, 50); 59 ([SiMe2H]+, 100). IR
(Nujol, cm−1): 3107 (m), 2107 (ν(Si−H), vs), 1490 (m), 1248 (m),
1151 (m), 1060 (m), 1026 (m), 1014 (m), 940 (m), 895 (vs), 886
(vs), 860 (m), 838 (w), 824 (s), 777 (w), 459 (w), 418 (w), 409 (vw).
Anal. Calcd for C13H18Cl2SiTi (321.16): C, 48.61; H, 5.65. Found: C,
48.94; H, 5.78.
[Cp(η5-C5H4CH2SiMe2OMe)TiCl2] (2). To a solid mixture of 1 (25
mg, 78 μmol) and a catalytic amount of B(C6F5)3 (ca. 1 mg, 2 μmol)
were added CD2Cl2 (0.7 mL) and methanol (3.2 μL, 78 μmol). The
resulting red solution was stirred for 5 min, transferred into an NMR
tube, and degassed by freeze−pump−thaw process (three times), and
the tube was sealed off by flame. After ca. 1 h, the complete
consumption of 1 and formation of 2 was proved by NMR
spectroscopy. After 1H, 13C, and 29Si spectra were measured, the
tube was opened under argon and its contents were transferred into a
Schlenk flask and layered with heptane (3 mL). Standing of the
mixture for several days in the refrigerator led to the formation of a red
solid. The solid was isolated, washed with heptane (4 × 2 mL), and
dried under vacuum. Yield: 17 mg (62%).
1
flame. After 2 h at room temperature the H, 13C, and 29Si spectra
showed a complete reaction (formation of 4 and Ph3CH). Then, the
tube was opened under argon and its contents were transferred into a
Schlenk flask and layered with heptane (4 mL). Standing of the
mixture for several days in a refrigerator (at 4 °C) led to the formation
of a red microcrystalline solid and a voluminous brownish mud. The
mud was carefully removed by syringe, and the remaining red
microcrystals were washed several times with heptane and dried under
vacuum. Yield: 15 mg (87%).
Mp: 238 °C. 1H NMR (CD2Cl2): 0.05 (s, 6H, SiMe2); 2.21 (s, 2H,
CH2); 6.12 (pseudo t, 2H, C5H4); 6.42 (pseudo t, 2H, C5H4); 6.53 (s,
5H, C5H5). 13C{1H} NMR (CD2Cl2): 0.42 (SiMe2); 25.28 (CH2);
116.32 (CH, C5H4); 119.88 (C5H5); 122.41 (CH, C5H4); 138.67 (Cipso
,
C5H4). 29Si{1H} NMR (CD2Cl2): 5.9 (SiMe2). IR (KBr, cm−1): 3110
(m), 2955 (m), 2922 (m), 1491 (s), 1444 (m), 1426 (w), 1392 (w),
1256 (s), 1168 (m), 1155 (m), 1059 (s), 1017 (m), 937 (w), 840 (vs),
822 (vs), 757 (w), 734 (w), 693 (vw), 669 (vw), 651 (vw), 606 (vw),
522 (vw), 416 (w), 408 (w). Anal. Calcd for C26H34Cl4OSi2Ti2
(656.30): C, 47.58; H, 5.22. Found: C, 47.71; H, 5.27.
Similarly, complex 2 was obtained from 1 (34 mg, 106 μmol),
methanol (4.3 μL, 106 μmol), and B(C6F5)3 (ca. 1 mg, 2 μmol) in a
spectroscopic purity of >90% in toluene-d8 as a solvent.
Mp: 176 °C. 1H NMR (CD2Cl2): 0.09 (s, 6H, SiMe2); 2.28 (s, 2H,
CH2); 3.39 (s, 3H, OMe); 6.16 (pseudo t, 2H, C5H4); 6.42 (pseudo t,
2H, C5H4); 6.52 (s, 5H, C5H5). 1H NMR (toluene-d8): −0.04 (s, 6H,
SiMe2); 2.33 (s, 2H, CH2); 3.19 (s, 3H, OMe); 5.69 (pseudo t, 2H,
C5H4); 5.87 (pseudo t, 2H, C5H4); 6.01 (s, 5H, C5H5). 13C{1H} NMR
(CD2Cl2): −2.58 (SiMe2); 25.94 (CH2); 50.77 (OMe); 116.32 (CH,
C5H4); 119.95 (C5H5); 122.47 (CH, C5H4); 138.33 (Cipso, C5H4).
29Si{1H} NMR (CD2Cl2): 16.0 (SiMe2). 29Si{1H} NMR (toluene-d8):
15.2 (Me2SiOMe). EI-MS, m/z (relative abundance): 350 (M•+, 6);
335 ([M − Me]+, 5); 315 ([M − Cl]+, 8); 300 ([M − MeCl]•+, 5);
291 (18); 289 (38); 287 (88); 286 (49); 285 ([M − Cp]+, 100); 274
(11); 272 (18); 257 (13); 235 (13); 224 (16); 222 (25); 191 (23);
190 (32); 185 (32); 183 ([CpTiCl2 ]+ , 36); 167
([C5H4CH2SiMe2OMe]+, 18); 150 (31); 148 ([CpTiCl]•+, 73); 95
(19); 93 (32); 91 (44); 90 (75); 89 ([SiMe2OMe]+, 86); 65 (28); 59
(98). IR (KBr, cm−1): 3107 (m, sh), 2956 (m), 2899 (m), 2832 (m),
1490 (s), 1443 (w), 1425 (vw), 1392 (vw), 1254 (m), 1187 (vw),
1164 (m), 1089 (s), 1033 (w), 1015 (w), 938 (w), 841 (vs), 826 (vs),
799 (m), 750 (w), 734 (w), 637 (vw), 415 (vw). Anal. Calcd for
C14H20Cl2OSiTi (351.18): C, 47.88; H, 5.74. Found: C, 47.79; H,
5.87.
Note: temperatures above 280 °C were necessary to evaporate the
sample in a mass spectrometer, which obviously caused complex
redistribution/decomposition. The only acceptable ions found in the
spectrum were [CpTiCl2]+ (m/z 183) and [CpTiCl]+ (m/z 148).
NMR Tube Reaction of 1 with a Catalytic Amount of B(C6F5)3
in CD2Cl2 or CDCl3 (Generation of 3). In a J. Young NMR tube, a
solid mixture of 1 (18 mg, 56 μmol) and B(C6F5)3 (1 mg, 2 μmol) was
cooled by liquid nitrogen and CD2Cl2 was distilled into the tube under
vacuum. The mixture was warmed to room temperature, giving a red
solution. 1H and 29Si NMR spectroscopy of the mixture confirmed the
formation of 3 and CD2HCl.
1
2
CD2HCl: H NMR (CD2Cl2) 2.98 (quintuplet, JHD = 1.5 Hz, 1H,
CD2HCl).
The reaction was conducted in an identical manner in CDCl3
starting from 1 (70 mg, 218 μmol) and B(C6F5)3 (4 mg, 8 μmol) and
gave rise to 3 and CDHCl2.
CDHCl2: 1H NMR (CDCl3) 5.28 (t, 2JHD = 1.0 Hz, 1H, CDHCl2);
1
13C{1H} NMR (CDCl3) 53.37 (t, JCD = 27.5 Hz, CDHCl2).
[Cp(η5-C5H4CH2SiMe2Cl)TiCl2] (3). To a mixture of 1 (32 mg, 100
μmol) and Ph3CCl (28 mg, 100 μmol) was added CDCl3 (0.7 mL).
The resulting red solution was transferred into an NMR tube and
degassed by a freeze−pump−thaw process (three times), and the tube
was sealed off by flame. After the reaction was completed (ca. 4 days as
NMR Tube Reaction of 1 with a Catalytic Amount of B(C6F5)3
in Toluene-d8. To a mixture of 1 (17 mg, 53 μmol) and B(C6F5)3
(ca. 1 mg, 2 μmol) was added toluene-d8 (0.7 mL), which caused an
immediate intense gas evolution. The mixture changed color from
intense red to brown within several minutes. After 10 min of stirring,
the mixture was transferred into an NMR tube and degassed by a
freeze−pump−thaw process (three times) and the tube was sealed off
1
determined by H NMR spectroscopy), the NMR tube was opened
and its contents were transferred into a Schlenk flask and then layered
with heptane (4 mL). Standing of the mixture for several days led to a
formation of red solid and a white fluffy powder. The mother liquor
and the white powder were removed, and the residual red solid was
washed with heptane (2 × 2 mL) and dried under vacuum. Yield: 25
mg (70%).
1
by flame. The H NMR of the sample was almost silent.
EPR (toluene-d8, 22 °C; three species): g = 1.9767 (ΔH = 0.53
mT); ca. 1.98 (ΔH ≈ 20 mT); g = 1.9886 (ΔH = 0.35 mT).
Attempted Trapping of the Transient Titanocene Hydrido
Chloride with Diphenylacetylene. To a solution of 1 (38 mg, 118
μmol) with a 5-fold molar excess of diphenylacetylene (105 mg, 590
μmol) in toluene-d8 (0.7 mL) was added a solution of B(C6F5)3 (ca. 2
mg, 4 μmol) in toluene-d8 (0.3 mL). The reaction mixture turned
1
Mp: 195 °C. H NMR (CDCl3): 0.43 (s, 6H, SiMe2); 2.57 (s, 2H,
CH2); 6.22 (pseudo t, 2H, C5H4); 6.42 (pseudo t, 2H, C5H4); 6.55 (s,
5H, C5H5). 13C {1H}(CDCl3): 1.66 (SiMe2); 24.97 (CH2); 114.92
(CH, C5H4); 119.64 (C5H5); 123.11 (CH, C5H4); 135.37 (Cipso, C5H4).
1
brown immediately, and gas evolution was observed. The H NMR
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dx.doi.org/10.1021/om400253g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
spectrum showed diphenylacetylene as the only diamagnetic species.
The EPR spectroscopy revealed the same paramagnetic species as in
the above experiment, although in a slightly different ratio.
−137.9, −140.3 (6 × 2F, CFortho); −157.3, −158.0, −159.3 (3 × 2F,
CFpara); −162.6 (2F, CFmeta); −164.2 (2 × 2F, CFmeta); −164.9,
−165.2, −165.6 (3 × 2F, CFmeta). IR (KBr, cm−1): 3551 (w), 3066
(vw), 3034 (vw), 1647 (s), 1584 (s), 1563 (w), 1520 (vs), 1472 (vs),
1451 (vs), 1380 (s), 1359 (vs), 1321 (m), 1295 (m), 1230 (vw), 1188
(s), 1168 (m), 1111 (m), 1089 (s), 997 (m), 981 (vs), 915 (vw), 844
(vw), 807 (vw), 786 (m), 766 (m), 729 (vw), 705 (m), 670 (m), 623
(w), 609 (vw), 576 (vw), 484 (vw), 472 (vw), 403 (vw).
1 equiv of B(C6F5)3 and 50 equiv of Ph3COH. In a glovebox, a solid
mixture of Ph3COH (717 mg, 2.76 mmol) and B(C6F5)3 (28 mg, 55
μmol) was homogenized with a mortar and pestle until the yellow
color did not appear (ca. 15 min); 80 mg of the mixture was dissolved
in CD2Cl2 (0.7 mL) in a J. Young NMR tube to give a green solution.
The formation of 5 was detected by 19F and 1H NMR spectroscopy at
−100 °C (see the Supporting Information, Figures S13 and S14).
Preparative-Scale Reaction of 2 equiv of PhMe2SiH with
Ph3COH Catalyzed by B(C6F5)3. Solid Ph3COH (1.292 g, 4.97
mmol) and B(C6F5)3 (26 mg, 50 μmol) were mixed in a Schlenk tube,
which caused the formation of a yellow mixture. The mixture was
dissolved in CH2Cl2 (20 mL), and the resulting green solution was
cooled to 0 °C. PhMe2SiH (1.362 g, 10 mmol) was added, and the
cooling bath was removed. In a few minutes gas evolution started and
continued for ca. 10 min. The reaction was finished after an additional
15 min, as was determined by GC-MS (the total reaction time was 30
min). The volatiles were evaporated under vacuum, and the waxy
residue was purified by column chromatography on silica gel with
hexane as eluent (Rf = 0.33). The yield of yellowish liquid was 1.185 g
(84%). The identity of the product was confirmed by NMR and IR
1
spectroscopy. H NMR and IR spectra are given in the Supporting
ASSOCIATED CONTENT
■
Information.
S
* Supporting Information
NMR Tube Reaction of PhMe2SiH with PhMe2SiOH Cata-
lyzed by B(C6F5)3. A solution of PhMe2SiH (82 μL, 540 μmol) and
PhMe2SiOH (84 μL, 540 μmol) in CD2Cl2 (0.7 mL) was added to
solid B(C6F5)3 (3 mg, 6 μmol) in a J. Young NMR tube. Immediate
gas evolution and a slightly exothermic reaction was observed. The
NMR tube was closed with a Teflon valve, and NMR measurements
were performed. The 1H NMR spectrum showed the presence of
(PhMe2Si)2O and H2 (signal at 4.71 ppm).
1
Figures giving the H, 13C, and 19F NMR, EPR, and IR spectra
mentioned in the text. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*J.P.: tel, (+420) 266053735; e-mail, pinkas@jh-inst.cas.cz.
■
1H NMR Monitoring of the Reaction of 2 equiv of PhMe2SiH
with Ph3COH Catalyzed by B(C6F5)3. To a cold (−78 °C) solution
of Ph3COH (78 mg, 300 μmol) and B(C6F5)3 (3 mg, 6 μmol) in
CD2Cl2 (0.5 mL) placed in a J. Young NMR tube was quickly added a
cold (−78 °C) solution of PhMe2SiH (92 μL, 600 μmol). The tube
was shaken quickly, which caused a color change from green to yellow.
The tube was immediately inserted into the NMR probe precooled to
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Academy of Sciences of the Czech
Republic (projects GA203/09/1574 and GAP207/12/2368) is
■
1
−10 °C, and H NMR spectra were periodically acquired (see the
gratefully acknowledged. We thank Dr. Lidmila Petrusova
melting point determinations.
́
for
Supporting Information).
General Procedure for the B(C6F5)-Catalyzed Reaction of
Ph3COH with Hydrosilanes. To a solution of Ph3COH (78 mg, 300
μmol) and B(C6F5)3 (3 mg, 6 μmol) in CH2Cl2 (5 mL) was added
hydrosilane (600 μmol). The mixture was stirred for 30 min, and then
a sample was taken for GC-MS (0.1 mL). The rest of the mixture was
evaporated to dryness, and the residue was dissolved in CDCl3 and
transferred into an NMR tube. THF (30 μL) was added as an internal
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1
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