(Me3N)Mo(CO)5-catalyzed reduction of DMF by disiloxane and disilane moieties: Fate of the silicon-containing fragments

Article abstract of DOI:10.1021/om301206c

The use of HSiMe₂OSiMe₂H (1) and various hydrodisilanes, R₃SiSiMe₂H (2; R = alkyl, aryl), as reductants for N,N-dimethylformamide (DMF) in the presence of (Me ₃N)Mo(CO)₅ as a catalyst led to the formation of a series of novel and structurally interesting siloxanes as well as trimethylamine. In the case of 1 the cyclic poly(dimethylsiloxanes) D₄ and D₆ are obtained, and for 2 the products are bis(disilyl) ethers, (R ₃SiSiMe₂)₂O. Siloxymethylamine intermediates resulting from an initial hydrosilylation of DMF, (Me₂NCH ₂OSiMe₂)₂O (3) and R₃SiSiMe ₂OCH₂NMe₂ (4; R = Me, Ph), from the reactions of 1 and 2, respectively, can be observed and, in the case of 3, isolated and purified. In the presence of the respective starting silanes and the catalyst the intermediates readily react to form the appropriate siloxane materials and trimethylamine. Compound 3 was functionalized by reaction with R₃ECl (E = Si, Ge, R = Me, Ph) to provide group 14 containing products (R ₃EOSiMe₂)₂O (R = Me, E = Si (5a), Ge (6a); R = Ph, E = Si (5b), Ge (6b)). Reactions of Me₃SiSiMe₂OCH ₂NMe₂ (4a) with R₃ECl produced Me ₃SiSiMe₂OER₃ (R = Me, E = Si (7), R = Ph, E = Ge, 8). The crystal structure of (Ph₃SiSiMe₂)₂O (9c) is reported and exhibits an Si-O-Si angle of 165 and the longest Si-Si bond length (2.376(2) A) for such bis(disilyl) ethers. The new (Ph ₃EOSiMe₂)₂O derivatives 5b and 6b have been structurally characterized and exhibit distinct conformations about the central SiOSi fragment. In the case of the Ph₃Si compound 5b the dihedral angle between the two end groups is 180 with completely staggered SiMe groups on the central Si atoms, whereas for the Ge congener it is 55.7 and the structure exhibits eclipsed SiMe groups. The distinction seems to be due to both intra- and intermolecular phenyl group π stacking in 6b stabilizing this formally higher energy conformation.

Full text of DOI:10.1021/om301206c

Article
pubs.acs.org/Organometallics
(Me3N)Mo(CO)5‑Catalyzed Reduction of DMF by Disiloxane and
Disilane Moieties: Fate of the Silicon-Containing Fragments
Hemant K. Sharma,† Renzo Arias-Ugarte,† David Tomlinson,† Rie Gappa,†,‡ Alejandro J. Metta-Magana,†
̃
Haruhiko Ito,‡ and Keith H. Pannell*,†
†Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968-0513, United States
‡Department of Chemistry, Nagaoka University of Technology, Nagaoka 940-2188, Japan
S
* Supporting Information
ABSTRACT: The use of HSiMe2OSiMe2H (1) and various
hydrodisilanes, R3SiSiMe2H (2; R = alkyl, aryl), as reductants
for N,N-dimethylformamide (DMF) in the presence of
(Me3N)Mo(CO)5 as a catalyst led to the formation of a
series of novel and structurally interesting siloxanes as well as
trimethylamine. In the case of 1 the cyclic poly-
(dimethylsiloxanes) D4 and D6 are obtained, and for 2 the
products are bis(disilyl) ethers, (R3SiSiMe2)2O. Siloxymethyl-
amine intermediates resulting from an initial hydrosilylation of DMF, (Me2NCH2OSiMe2)2O (3) and R3SiSiMe2OCH2NMe2 (4;
R = Me, Ph), from the reactions of 1 and 2, respectively, can be observed and, in the case of 3, isolated and purified. In the
presence of the respective starting silanes and the catalyst the intermediates readily react to form the appropriate siloxane
materials and trimethylamine. Compound 3 was functionalized by reaction with R3ECl (E = Si, Ge, R = Me, Ph) to provide
group 14 containing products (R3EOSiMe2)2O (R = Me, E = Si (5a), Ge (6a); R = Ph, E = Si (5b), Ge (6b)). Reactions of
Me3SiSiMe2OCH2NMe2 (4a) with R3ECl produced Me3SiSiMe2OER3 (R = Me, E = Si (7), R = Ph, E = Ge, 8). The crystal
structure of (Ph3SiSiMe2)2O (9c) is reported and exhibits an Si−O−Si angle of 165° and the longest Si−Si bond length
(2.376(2) Å) for such bis(disilyl) ethers. The new (Ph3EOSiMe2)2O derivatives 5b and 6b have been structurally characterized
and exhibit distinct conformations about the central SiOSi fragment. In the case of the Ph3Si compound 5b the dihedral angle
between the two end groups is 180° with completely staggered SiMe groups on the central Si atoms, whereas for the Ge
congener it is 55.7° and the structure exhibits eclipsed SiMe groups. The distinction seems to be due to both intra- and
intermolecular phenyl group π stacking in 6b stabilizing this formally higher energy conformation.
R3SiH
INTRODUCTION
■
HC(O)NR′2 + R3SiH → R3SiOCH2NR′2 ⎯⎯⎯⎯⎯→
The metal-catalyzed reduction of amides by hydrosilanes is now
a well-established procedure effected by a range of both
CH3NR′2 + R3SiOSiR3
(1)
transition and main-group metals with significant application in
the pharmaceutical industry.1 Although the chemistry was first
also resulting in the formation of bis(triethylsilyl) thioether,
noted in 1985 for the reduction of DMF along with the
formation of disiloxanes,2 only recently has it developed into a
using FpMe as catalyst, was more recently reported involving
the intermediacy of an interesting iron aminocarbene
reaction of choice due to its mildness and selectivity in
intermediate.5b
comparison to classical methods.3 We recently reported the
reduction of simple formamides to amines using silanes,
germanes, and stannanes with concomitant formation of
disiloxanes, digermoxanes, and distannoxanes using Mo(CO)6,
(Me3N)Mo(CO)5, (η5-C5H5)Fe(CO)2CH3 (FpMe), (η5-
C5H5)Mo(CO)3CH3 (MpMe), and (η5-C5H5)Mn(CO)3 as
catalysts.4 For the reactions using Mo(CO)6 and (Me3N)Mo-
(CO)5 we proposed a stepwise reduction mechanism based
upon our first-time observation and isolation of the initial
hydrosilylation products, i.e. siloxymethylamines, as intermedi-
ates (eq 1).4b
The disiloxane 1,1,3,3-tetramethyldisiloxane (HSi-
Me2OSiMe2H, 1) has been reported as a useful amide
reductant as well as an alkene hydrosilylation reagent.6,7
Indeed, it has been suggested that organosilanes bearing two
proximate SiH units allow the reduction of tertiary amides to
take place under mild conditions, the so-called “dual SiH
effect”.6 These reactions employed platinum catalysts and were
noted to be amide-specific, and interestingly, there was no
reaction when monosilanes bearing a single Si−H bond were
used. However, during the use of 1 as an amide reductant no
siloxymethylamine intermediates were noted and indeed such
We also reported the related reduction of dimethylthiofor-
mamide by silanes to produce Me3N and disilyl thioethers,
R3SiSSiR3; however, in that case no initial hydrosilylation
intermediates were observed.5a A related reduction with Et3SiH
Received: December 13, 2012
© XXXX American Chemical Society
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potential transients resulting from an initial hydrosilylation of
the amide, e.g. (Me2NCH2OSiMe2)2O (3), are unknown.
Furthermore, in the articles describing the use of 1 as an amide
reductant, the ultimate fate of the silicon-containing species was
described simply as “silicone waste”.6
As a continuation of our interest in the reactions of the Si−H
bond in the presence of transition metals, we have now
explored the reduction of DMF employing 1, attempting to
observe possible siloxymethylamine transients using this
reagent. Furthermore, we have extended the range of disilicon
compounds useful as reductants by utilizing R3SiSiR2H (2; R =
aryl, alkyl mixtures) for the metal-catalyzed reduction of DMF,
with particular attention to the fate of the Si-containing species.
Since the transition-metal-catalyzed activation of the Si−Si
bond has been demonstrated, involving rearrangements, Si−Si
bond rupture, etc.,8 the ability to perform amide reduction
using disilanes without cleavage of the Si−Si bond in the
presence of transition-metal catalysts is an open and interesting
question.
pressure provided the product 3 in 72% yield. The new material
exhibits three 13C NMR resonances at −0.31 (SiMe2), 41.5
(NMe2), and 82.3 ppm (CH2) and a single 29Si signal at −14.0
ppm, data in accord with those we previously reported for
siloxymethylamines.4
We have previously shown that, using (Me3N)Mo(CO)5 as a
catalyst, the siloxymethylamine intermediates formed by initial
hydrosilylation of DMF by monosilanes, R3SiOCH2NMe2 (e.g.,
R3 = Et3, PhMe2 etc.), further react with R3SiH in the presence
of excess DMF and/or the catalyst to form R3SiOSiR3 and
Me3N.4 In the present case a similar chemistry can also be
noted, as shown by the 13C NMR monitoring of the reaction
between DMF and 1 (1:3 molar ratio) (Figure 1). Such a
sequence clearly denotes the intermediacy (appearance and
disappearance) of 3 in the reduction process.
RESULTS AND DISCUSSION
■
For the DMF reduction experiments we utilized (Me3N)Mo-
(CO)5, 2−3 mol %, as the catalyst under thermal conditions on
the basis of our previous studies, which showed its excellent
solubility in the solvent of choice for monitoring these
reactions, C6D6. Thus, all reactions were performed thermally
in an oil bath set at 90 °C unless specifically noted otherwise.
The disiloxane and disilanes were allowed to react with a 3-fold
molar excess of dimethylformamide (DMF) (relative to the
active Si−H unit) and the progress of the reactions was
monitored by 13C and 29Si NMR spectroscopy.
Use of Tetramethyldisiloxane (HMe2SiOSiMe2H, 1).
Despite the previous use of 1 as an amide-reducing agent, there
is no record of the fate of the Si species subsequent to its use as
a reductant other than the claim of “SiO” polymeric materials.6
Additionally, there has been neither observation nor mention of
an initial hydrosilylation product. Using the reaction conditions
described above, 90 °C for ∼12 h, along with the formation of
Me3N we observed and isolated the cyclic polysiloxanes
octamethylcyclotetrasiloxane (D4) and dodecamethylcyclohex-
asiloxane (D6) in 35 and 16% yields, respectively, as the sole Si-
containing materials (eq 2). The identity of these compounds
Figure 1. 13C NMR monitoring of the catalyzed reaction between 1
and DMF, exhibiting the intermediacy of 3 with resonances at −0.31,
41.5, and 82.3 ppm and formation of Me3N at ∼47 ppm: (a) 0 h; (b)
0.5 h; (c) 1 h; (d) 5 h; (e) 16 h; (f) 19 h. For a−c the temperature was
40 °C; for d−f the temperature was raised to 80 °C.
Mironov et al.10 and ourselves4 have reported the
functionalization of siloxymethylamines, R3SiOCH2NMe2,
with R3ECl (E = Si,4,10 Ge,4 Sn4) to form disiloxanes,
siloxygermanes, and siloxystannanes, respectively. Siloxanes as
materials have a history of a wide range of potential
applications,11 and furthermore the introduction of metal-
(loid)s to the structure of organosilicone compounds may
result in materials with improved thermal, catalytic, and
conducting properties; therefore, antimony-,12a arsenic-,12b
boron-,12c tin-,12d and possibly lead-containing12e siloxanes
have been investigated in this regard, although not always with
success. With the high-yield isolation of compound 3 we have
attempted extension of the Si−O−Si chain by reacting it with a
variety of R3ECl group 14 reagents (R3E = Me3Si, Ph3Si,
Me3Ge, Ph3Ge, Bu3Sn, Ph3Sn). In the case of the Si and Ge
compounds the reaction led to the formation of
(R3EOSiMe2)2O (E = Si, (5), Ge (6); eq 4). The reactions
of 3 with organotin chlorides led to the direct formation of only
the cyclic polysiloxanes D4 and D6.
was confirmed unambiguously by mass spectrometry and NMR
spectroscopy in comparison to those data for authentic
samples.9 During the monitoring process we noted transient
formation of the intermediate material 3, but under the reaction
conditions it could not be isolated.
However, by performing and monitoring the reaction
between 1 and DMF at room temperature, we could observe
the high-yield formation of the initial hydrosilylated compound,
3 (eq 3). Optimal conditions for the high-yield preparation of
this material involved using the reactant ratio [SiH]:[DMF] =
1:2 in the presence of 1−2 mol % of (Me3N)Mo(CO)5 and
allowing the mixture to stand at ambient temperatures
overnight (12−14 h). Subsequent distillation under reduced
The spectroscopic and analytical data for 5 and 6 are in total
accord with their proposed structures, and the data are
collected in the Experimental Section. Compounds 5a and 5b
have been reported previously;13,14 however, no spectroscopic
B
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data were reported for 5b, and the Ge analogues 6 are
unreported. Both 5b and 6b are solid materials that we were
able to crystallize from hexanes, and their structures are
illustrated in Figures 2 and 3, respectively.
The bond angles and bond lengths of 5b and 6b are
unremarkable. The internal Si−O−Si bond angles for 5b and
6b are 140 and 155°, respectively. In 5b the external Si−O−Si
angles are wider at 160.5°, whereas in 6b the corresponding
exterior Ge−O−Si angles are significantly reduced at ∼138°.
The reduction of the Ge−O−Si bond angle in comparison to
the Si−O−Si angle is expected, on the basis of the limited
Figure 3. Molecular structure for 6b. Ellipsoids are drawn at the 30%
probability level. Selected bond distances (Å) and angles (deg): Ge1−
O1 = 1.788(3), Ge2−O3 = 1.777(3), Si1−O1 = 1.619(3), Si2−O3 =
1.622(4), Si1−O2 = 1.624(3), Si2−O2 = 1.617(3); Ge1O1Si1 =
138.5(2), Ge2O3Si2 = 137.6(2), Si1O2Si2 = 155.5(3). The hydrogen
atoms are omitted for clarity.
published data.15 Thus, the 180° Si−O−Si angle in
15a
Ph3SiOSiPh3
is dramatically reduced to 142.5° for the
mixed germyl silyl ether Ph3GeOSiPh3,15b and a further
reduction to 135.7° has been reported for the all-germanium
ether Ph3GeOGePh3.15c These trends can be associated with
the lesser ability of the oxygen lone pairs to be delocalized onto
the germanium atom.
phenyl-1,3,5,7-tetrasiloxane, (Ph2(OH)SiOSiMe2)2O, which
also exhibits a similar dihedral angle of 54.4°.17 However, in
this case, the hydroxyl groups play an important role in
stabilizing this more energetic structure containing eclipsed Si−
methyl groups by an intramolecular hydrogen-bonding
interaction. It seems that in the solid state of 6b even the
very long inter- and intramolecular π-stacking phenyl groups
have the capacity to provide some stabilization to the eclipsed
methyl group conformation.
Attempts to synthesize the tin congeners of 5 and 6 were not
successful. Only the cyclic polysiloxanes D4 and D6 were
obtained, and the fate of the tin materials is unknown; however,
in these reactions an insoluble material (SnO?) also was
formed. The sole formation of cyclic polysiloxanes is
presumably related to the use of such organotins as catalysts
The dihedral angle X−(SiOSi)−X for 5b is 180°, similar to
that noted in the published structure of the related tetrasiloxane
(Ph2MeSiOSiMe2)2O.16 However, for the germanium com-
pound 6b this angle is dramatically reduced to only 55.7(4)°. In
the latter material this small angle results in the phenyl rings
forming intramolecular (4.48 Å) and intermolecular (4.55 Å)
close contacts. Although these distances are longer than the
usually accepted limit for π−π interactions, they seem
important, since such a conformation results in eclipsed methyl
groups at the Si atoms for 6b, as shown in Figure 4. In contrast,
5b with a dihedral angle of 180°, exhibits completely staggered
silicon−methyl groups (Figure 5).
There is an analogue for the structural arrangement of 6b in
the compound 1,7-dihydroxy-1,1,7,7-tetramethyl-3,3,5,5-tetra-
Figure 2. Molecular structure for 5b. Thermal ellipsoids are drawn at the 35% probability level. Selected bond distances (Å) and angles (deg): Si1−
O1 = 1.622(2), Si2−O1 = 1.598(2), Si2−O2 = 1.545(5); Si1O1Si2 = 160.5(2), Si2O2Si2#1 = 140.4(3). The hydrogen atoms are omitted for clarity.
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HC(O)NMe2 + R3SiSiMe2H
C6D6, 90 ◦C
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ [R3SiSiMe2OCH2NMe2]
5% (Me3N)MO(CO)5
4
4 + R3SiSiMe2H → R3SiSiMe2OSiMe2SiR3 + NMe3
R3 = Me3 (9a), Ph2Me (9b), Ph3 (9c)
(5)
In the case of the reductions using 2a and 2c transient
siloxymethylamines (4a and 4c) could be noted via NMR
spectroscopy, as in the case of the reductions using 1. However,
in the case of 2b at the “normal” reaction temperature of 90 °C
we were unable to observe the intermediacy of the appropriate
siloxymethylamine; the reaction was too fast. However, by
performing the reaction at room temperature the intermediate
4b was observed, illustrating the delicate aspects of sterics/
electronics at play in this chemistry. In all reactions trimethyl-
amine (Me3N) was produced, as identified in the 13C NMR as a
resonance at ∼47.0 ppm.
Siloxane 9a, (Me3SiMe2Si)2O, is a well-known material20 and
our observed spectral data, recorded in the Experimental
Section, are in accord with the proposed structure. Both 9b and
9c were characterized by NMR spectral data and HRMS and, in
the case of 9c, by single-crystal X-ray diffraction analysis; its
structure is illustrated in Figure 6.
Figure 4. Structure of 6b showing the eclipsed conformation along the
Si−O−Si bond.
A search of the CCDC revealed only three structures which
share the bis(pentaalkyl(aryl)disilyl) ether backbone:
2,2,3,3,5,5,6,6-octamethyl-2,3,5,6-tetrasila-4-oxa-1(1,8)-
anthracenacyclohexaphane (INIWEI),21 6,7:8,9-dibenzo-3-oxa-
1,2,2,4,4,5-hexaphenyl-1,2,4,5-tetrasilabicyclo(3.2.2)-
nonadiene−benzene solvate (RURKIY),22 and 1,5-dihydroxy-
1,1,2,2,4,4,5,5-octaphenyl-3-oxa-1,2,4,5-tetrasilapentane−ben-
zene solvate (OFUPIO).23 Both INIWEI and RURKIY are
cyclic siloxanes (Figure 7), and thus the Si−O−Si bond angles
(∼153 and 145°, respectively) are significantly smaller than for
the acyclic OFUPIO (Ph2Si(OH)SiPh2)2O; average ∼168°)
and 9c (165°) due to ring strain. The Si−Si bond length in 9c
(2.376(2) Å; Table 1) is statistically longer than the others due
to the bulky Ph3Si substituents, and for example, in the other
acyclic OFUPIO, the Si−Si bond distance is shorter because
one of the Ph groups of 9c is replaced by a smaller hydroxyl
group. The relationships between smaller SiOSi angles and
longer Si−O bonds is maintained in this set of molecules.24
The presence of the hydroxyl groups also has a significant
impact upon the Si−Si···Si−Si torsional angle for OFUPIO due
to an intramolecular H bond between the two groups, as can be
seen in Figures 8 and 9 and Table 1. For the two cyclic
molecules the torsional angle is ∼5°, for OFUPIO it averages
∼60°, and for 9c it is ∼151°.
Figure 5. Structure of 5b showing the staggered conformation along
the Si−O−Si bond.
for the condensation of siloxane diols and functional silanes,
leading to useful polysiloxanes under mild conditions.18
Certainly many cyclic stannasiloxanes have been reported and
have a thermodynamic and kinetic stability suggesting that
under our reaction conditions they should be observed.19 At
present we have not further investigated this aspect of the new
chemistry.
Use of Disilanes R3SiSiMe2H (R3 = Me3 (2a), Ph2Me
(2b), Ph3 (2c)). Since transition-metal complexes are well-
established to facilitate rupture of Si−Si bonds,8 the use of 2 as
an efficient amide reductant was not obvious. However, the
reaction between 2 and DMF in the presence of a catalytic
amount of (Me3N)Mo(CO)5 resulted in the efficient formation
of Me3N and transformation of the specific disilane to the
corresponding disiloxane 9a−c, via the appropriate transient
disiloxymethylamines 4 (eq 5).
As noted above, we were able to observe the transient
formation of the disiloxymethylamines R3SiSiMe2OCH2NMe2
(4) during the reduction reactions; however, attempts to isolate
these materials were unsuccessful. They are very sensitive to
moisture, and our attempts to distill them from reaction
mixtures were unsuccessful. Despite these difficulties, it was
possible to use them as transient reagents, and we have treated
Me3SiSiMe2OCH2NMe2 (4a) with R3ECl (R = Me, E = Si; R =
Ph, E = Ge) to synthesize the corresponding ethers (eq 6).
RT
4a + R3ECl ⎯⎯⎯⎯→ Me3SiSiMe2OER3 + ClCH2Me2
C6H6
R = Me, E = Si (7); R = Ph, E = Ge (8)
(6)
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Figure 6. Molecular structure of compound 9c, with ellipsoids at the 30% probability level. Selected bond distances (Å) and angles (deg): Si−O =
1.618(2), Si−Si = 2.376(2); Si−O−Si = 165.2(4). The hydrogen atoms are omitted for clarity.
Figure 7. Cyclic structures of bis(disilyl) siloxanes.
Table 1. Selected Structural Data for 9c in Comparison with
Published Analogues
Figure 9. Torsion angle views of 9c (left) and OFUPIO (right).
9c
INIWEI21
RURKIY22
OFUPIO23
Si−Si (Å)
Si−O (Å)
2.376(2) 2.351(1),
2.355(1)
2.362(2) 2.352(1),
2.353(1)
a
molar ratio in THF.25 Compound 8 is a new material, and we
have characterized it by HRMS and NMR, as reported in the
Experimental Section. In each case we were unable to remove
trace amounts of the corresponding bis(disilyl) ether
(Me3SiMe2Si)2O (9a).20
1.618(2) 1.637(2),
1.640(2)
1.644(3) 1.628(1),
1.626(1)
Si−O−Si
165.2(4) 154.3(1),
152.8(1)
144.5(2) 161.5(1),
175.7(1)
(deg)
Si−Si·Si−Si
150.9
3.4, 6.8
5.20
82.3, 43.0
b
(deg)
a
When there is more than one isomer in the asymmetric unit, more
b
CONCLUSIONS
than one value is listed. The torsion angles are posted as absolute
■
values.
The widely used transition-metal-catalyzed reduction of amides
to amines using silanes (R3SiH) was first observed by the
Voronkov group in a seminal but unrecognized and generally
uncited 1985 report.2 We have now further demonstrated that
this reaction leads initially to the hydrosilylation product
R3SiOCH2NMe2 when (Me3N)Mo(CO)5 is the catalyst. Using
the popular tetramethyldisiloxane HSiMe2SiOSiMe2H (1) as a
reductant, we have observed the corresponding hydrosilylated
transient Me2NCH2OSiMe2OSiMe2OCH2NMe2 for the first
time. Furthermore, we have demonstrated that this inter-
mediate can be isolated and further functionalized in the
presence of chlorosilanes and -germanes to form siloxy
derivatives of the appropriate group 14 element,
R3EOSiMe2OSiMe2OER3. In the absence of such trapping
and/or isolation, the ultimate fate is the formation of cyclic
dimethylsiloxanes D4 and D6. Using disilanes R3SiSiMe2H (2)
as reductants, the ultimate silicon-containing products are the
corresponding bis(disilyl) siloxanes (R3SiSiMe2)2O.
Figure 8. Torsion angle views for cyclic siloxanes INIWEI (left) and
RURKIY (right).
Compound 7 mixed with other siloxane materials was
previously synthesized from the cross-coupling reaction
between Me3SiOSiMe2Cl and Me3SiCl with lithium in a 1:2
E
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vacuum at 88 °C/18 mmHg; yield 0.31 g (57%). NMR (ppm, C6D6):
1H, 0.25 (SiCH3), 0.34 (GeMe3); 13C, 2.25 (SiMe3), 2.74 (GeMe3);
29Si, −20.1. HRMS (ESI): calcd 401.0249 [M + 1]+, found 401.0113.
Anal. Calcd for C10H30Ge2O3Si2: C, 30.05; H, 7.57. Found: C, 29.96;
H, 7.60.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out under a
nitrogen atmosphere using Schlenk or vacuum line techniques. THF
and diethyl ether were distilled under nitrogen from benzophenone
ketyl prior to use. Other solvents, hexanes, benzene, and toluene, were
dried over sodium metal and distilled prior to use. DMF was distilled
over BaO. The complex (Me3N)Mo(CO)5 was prepared according to
the reported method using Mo(CO)6 purchased from Strem
Chemicals.26 Silyl hydrides were purchased either from Aldrich or
Gelest; NMR spectra were recorded on a 300 MHz Bruker or 600
MHz JEOL spectrometer in C6D6 or CDCl3, and data are singlets
unless otherwise stated; HRMS were recorded on a JEOL AccuTOF
JMS-T100LC instrument with DART access unit. All column
chromatographic separations were performed using silica gel (Aldrich,
700−230 mesh, 60 Å, pore volume 0.75 cm3/g). The crystal structures
of 5b (CCDC 935196), 6b (CCDC 935197), and 9c (CCDC
915315) were determined on a Bruker Apex CCD diffractometer. The
crystallographic data and structural refinement details are presented in
Table S1 of the Supporting Information, and selected bond lengths
and angles of 9c are given in Table 1.
Reaction of 3 with Ph3GeCl To Form (Ph3GeOSiMe2)2O (6b).
A 50 mL Schlenk flask was charged with Ph3GeCl (0.475 g, 1.40
mmol) in 10 mL of benzene, and to this was added 8 mL of a benzene
solution of 3 (0.2 g, 0.71 mmol) via syringe under an Ar atmosphere at
room temperature. After the addition of Ph3GeCl the solution became
turbid and the solution was maintained at 40 °C in an oil bath for 20 h.
The solution was filtered through Celite to remove white precipitates
of ClCH2NMe2, and the solvent was removed under vacuum. The
product (Ph3GeOSiMe2)2O (6b) was repeatedly recrystallized from
hexanes: yield 0.18 g (33%); mp 68−69 °C. NMR (ppm, CDCl3): 1H,
−0.21 (SiCH3), 7.26−7.36, 7.51−7.55 (m, Ph); 13C, 1.71 (SiMe3),
128.1, 129.7, 134.2, 136.5 (ipso) (Ph); 29Si, −17.9. Anal. Calcd for
C40H42Ge2O3Si2: C, 62.22; H, 5.48. Found: C, 62.44; H, 5.18.
Reaction of Me3SiSiMe2H (2a) with DMF. In a typical
experiment, an NMR tube was charged with Me3SiSiMe2H (0.33 g,
0.5 mmol), DMF (0.11 g, 1.5 mmol), 2−5 mol % of (Me3N)Mo-
(CO)5, and 0.5 mL of C6D6. The tube was sealed under vacuum and
heated in an oil bath at 90 °C. When the reaction was complete, as
noted by crude 13C and/or 29Si NMR spectroscopic analysis, the
volatiles and DMF were removed by flash distillation (into liquid
nitrogen cooled receptacles) at ambient temperature (0.5 mm of Hg)
to leave the desired product, which was further purified by distillation
under vacuum (59−60 °C, 0.5 mmHg) to give 0.39 g (60% yield) of
Large-Scale Preparation of (Me2NCH2OSiMe2)2O (3). To a
mixture of 1,1,3,3-tetramethyl-1,3-disiloxane (1; 2.69 g, 20.0 mmol)
and N,N-dimethylformamide (3.20 g, 43.8 mmol) in benzene (12 mL)
was added (Me3N)Mo(CO)5 (0.18 g, 2 mol %) in a Schlenk tube. The
resulting solution was degassed, closed, and left standing overnight for
approximately 14 h. The solvent was removed in vacuo and the crude
liquid product distilled to provide the title compound as a clear,
colorless liquid: yield 4.03 g, 72%; bp 70−71 °C/0.5 mmHg. NMR
(Me3SiSiMe2)2O (9a).20 NMR (ppm, C6D6): H, 0.34 (SiMe2), 0.23
1
1
(ppm, C6D6): H, 0.18 (SiMe), 2.38 (NMe), 4.48 (CH2); 13C, −0.31
(SiMe3); 13C, 2.7 (SiMe), −1.9 (SiMe); 29Si, −23.2 (SiMe3), 5.3
(SiMe2O). HRMS: calcd 279.1374 [M + 1]+, found 279.1369.
Using the same methodology the reactions of Ph2MeSiSiMe2H and
Ph3SiSiMe2H with DMF led to the following products.
(SiMe), 41.5 (NMe), 82.3 (CH2); 29Si, −14.0. Anal. Calcd for
C10H28N2O3Si2: C, 42.82; H, 10.06. Found: C, 41.82; H, 10.38.
Thermal Reaction of 3 with HSiMe2OSiMe2H (1) in C6D6 with
1 mol % of (Me3N)Mo(CO)5. A 5 mm Pyrex NMR tube was charged
with a solution of HSiMe2OSiMe2H (0.095 g, 0.7 mmol) in 0.4 mL of
C6D6, 0.10 g (0.35 mmol) of 3, Me2NCH2OSiMe2OSiMe2O-
CH2NMe2, and 1 mol % of the catalyst (Me3N)Mo(CO)5. The tube
was degassed twice using pump−thaw cycles and flame-sealed under
vacuum. The tube was immersed in an oil bath maintained at 90 °C.
The progress of the reaction was periodically monitored by 13C and
29Si NMR spectroscopy. After 40 h of thermolysis, the NMR analysis
(Ph2MeSiSiMe2)2O (9b): purified by column chromatography with
1
hexanes as eluent; 33% yield as an oil. NMR (ppm, C6D6): H, 0.33
(SiMe2), 0.67 (SiMe), 7.29, 7.68 (ArH); 13C, −4.33 (SiMe), 3.22
(SiMe), 129.6, 134.8, 135.8, 137.6 (ArH); 29Si, −26.1 (Ph2MeSi), 5.6
(SiMe2O). HRMS: calcd 526.2000, found 526.2010; Anal. Calcd for
C30H38OSi4: C, 68.38; H, 7.24. Found: C, 68.00; H, 7.07.
(Ph3SiSiMe2)2O (9c): recrystallized from hexanes; 32% yield; mp
94−95 °C. NMR (ppm, C6D6): 1H, 0.33 (SiMe2), 7.68 (m, ArH); 13C,
3.7 (SiMe), 128.5, 129.9, 135.7, 136.8 (Ar); 29Si, −25.1 (Ph3Si), 5.8
(SiMe2O).
indicated the formation of Me3N, D4, and D6 along with traces of
other oligomeric siloxanes and starting materials.
Reaction of 3 with Ph3SiCl To Produce (Ph3SiOSiMe2)2O (5b).
A 50 mL Schlenk flask was charged with Ph3SiCl (0.42 g, 1.42 mmol)
in 10 mL of benzene. A benzene solution (8 mL) of 3 (0.2 g, 0.71
mmol) was added via a syringe under an Ar atmosphere at room
temperature, and the solution became turbid. The solution was
maintained at 40 °C in an oil bath for 20 h. The solution was filtered
through Celite to remove white precipitates of ClCH2NMe2, and the
solvent was removed under vacuum. The product, (Ph3SiOSiMe2)2O,
was twice recrystallized from hexanes: yield 0.22 g (45%); mp 98−99
°C (lit.14 mp 102−103 °C). NMR (ppm, CDCl3): 1H, −0.03 (SiCH3),
7.31, 7.56−7.58 (m, Ph); 13C, 1.23 (SiMe3), 127.8, 129.8, 135.1, 136.0
(ipso) (Ph); 29Si, −19.3, −21.2.
Reaction of Me3SiMe2SiOCH2NMe2 (4a), with Ph3GeCl. A
Schlenk tube was charged with pentamethyldisilane (2a; 0.662 g, 5.00
mmol), DMF (0.548 g, 7.50 mmol), and (Me3N)Mo(CO)5 (0.045,
0.15 mmol) dissolved in benzene (12 mL), and the tube was degassed
and allowed to stand overnight at ambient temperature. After this
time, with complete loss of the disilane, conversion to the transient
siloxymethylamine 4a was observed to be approximately 90% complete
(based on 1H NMR). Characteristic NMR resonances (ppm) for
Me3SiSiMe2OCH2NMe2 (4a) were observed in situ: 13C, 83.3
(OCH2N), 41.5 (NMe2), 2.40 (SiMe), −0.15 (SiMe); 29Si, 11.7
(SiOCH2NMe2), −22.4 (SiMe3). DMF was removed by flash
distillation (into liquid nitrogen cooled receptacles) at ambient
temperature (0.5 mmHg). Isolation of the pure 4a proved difficult,
and the reaction mixture with traces of 9a was used directly and
treated with 1 equiv of Ph3GeCl in benzene (5 mL) in a Schlenk flask
maintained at 50 °C for 3 days. A white precipitate of ClCH2NMe2
was formed and removed by filtration over Celite, and the solvents
were evaporated under vacuum. Unreacted Ph3GeCl was removed by
repeated recrystallization from hexanes. The residue showed the
Reaction of 3 with Me3SiCl To Produce (Me3SiOSiMe2)2O
(5a). Using the above procedure, the reaction of 3 with Me3SiCl led to
the formation of 5a: yield 86% as a colorless oil; NMR (ppm, C6D6):
1H, 0.03 (SiMe2), 0.08 (SiMe3); 13C, 1.8 (SiMe2), 2.3 (SiMe3); 29Si,
−21.1 (OSiMe2), 7.6 (OSiMe3). Data are in accord with those
reported.13
Reaction of 3 with Me3GeCl To Produce (Me3GeOSiMe2)2O
(6a). In a 50 mL Schlenk flask under an Ar atmosphere at room
temperature was charged 3 (0.38 g, 1.35 mmol) in 8 mL of benzene
and, via syringe, Me3GeCl (0.415 g, 2.7 mmol) in 10 mL of benzene.
The reaction was exothermic, and after the addition of Me3GeCl the
solution became turbid. The solution was heated to 40 °C for 15 h.
The solution was filtered through Celite to remove a white precipitate
of ClCH2NMe2, and the solvent was removed under vacuum at 20
mmHg. The product (Me3GeOSiMe2)2O (6a) was distilled under
1
formation of 7 with traces of 9a. The H, 13C, and 29Si NMR and
HRMS data recorded below are illustrated in the Supporting
Information.
Ph3GeOSiMe2SiMe3 (8): yield 0.17 g, 20%. NMR (ppm, CDCl3):
1H, 0.15 (SiMe3) 0.25 (SiMe2), 7.59 (m, ArH); 13C, −2.3 (SiMe), 2.7
(SiMe), 128.6, 130.5, 136.2 (Ar); 29Si, −22.7 (SiMe2O), 10.1 (SiMe3).
HRMS: calcd 453.1004 [M + 1]+, found 453.1129.
F
dx.doi.org/10.1021/om301206c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Me3SiOSiMe2SiMe3 (7). Using the procedure used in the synthesis
of 8, the reaction of the crude Me3SiSiMe2OCH2NMe2 (4a) with 1
equiv of Me3SiCl in benzene produced Me3SiOSiMe2SiMe3 (7) in
60% yield. NMR (ppm, C6D6): 13C, −2.5 (SiMe3), 1.8 (OSiMe2), 2.01
(OSiMe3); 29Si, −23.1 (Me3Si), 4.4 (Me2SiO), 7.3 (M3SiO). The
spectroscopic data are in agreement with the reported data.25
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Figures giving H, 13C, and 29Si NMR spectra for 3, 8, 9b, and
9c and HRMS spectra for 8, Table S1 giving crystallographic
data and structural refinement details for 5b, 6b, and 9c, and
CIF files giving crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.H.P.: kpannell@utep.edu.
Notes
■
(13) Kurfurst, M.; Blechta, V.; Schraml, J. Magn. Reson. Chem. 2011,
̈
The authors declare no competing financial interest.
49, 492−501.
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(b) Borisov, S. N.; Sviridova, N. G.; Orlova, V. Zh. Obshch. Khim.
1966, 36, 687−92.
ACKNOWLEDGMENTS
■
K.H.P. wishes to dedicate this article to the memory of his
friend Mikhail Yu Antipin (1951−2013). In so doing he also
wishes to salute those Russian chemists, past and present, who
have made such a seminal contribution to organosilicon
chemistry. We thank the Welch Foundation, Houston, TX,
for support of this research via Grant AH-0456. We
acknowledge Andrew L. C. Morris, Ph.D., for preliminary
experiments associated with this chemistry. K.H.P. wishes to
dedicate this article to the memory of his friend Mikhail Yu
Antipin (1951−2013). In so-doing he also wishes to salute
those Russian chemists, past and present, who have made such
a seminal contribution to organosilicon chemistry.
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dx.doi.org/10.1021/om301206c | Organometallics XXXX, XXX, XXX−XXX