Platinum-catalyzed reduction of DMF by 1,1,3,3-tetramethyldisiloxane, HMeSi2OSiMe2H: New intermediates HSiMe 2OSiMe2OCH2NMe2 and HSiMe 2(OSiMe2)3OCH2</

Article abstract of DOI:10.1021/om500439p

The use of Karstedt's catalyst to study the reduction of Me₂NCHO (DMF) by the popular "dual SiH"-containing tetramethyldisiloxane, HMe₂SiOSiMe₂H (1), has revealed that the first step in the process involves an initial sing

Full text of DOI:10.1021/om500439p

Communication
pubs.acs.org/Organometallics
Open Access on 06/11/2015
Platinum-Catalyzed Reduction of DMF by 1,1,3,3-
Tetramethyldisiloxane, HMeSi2OSiMe2H: New Intermediates
HSiMe2OSiMe2OCH2NMe2 and HSiMe2(OSiMe2)3OCH2NMe2 and
Their Further Chemical Reactivity
Jorge L. Martinez, Hemant K. Sharma, Renzo Arias-Ugarte, and Keith H. Pannell*
Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968-0513, United States
S
* Supporting Information
ABSTRACT: The use of Karstedt’s catalyst to study the reduction
of Me2NCHO (DMF) by the popular “dual SiH”-containing
tetramethyldisiloxane, HMe2SiOSiMe2H (1), has revealed that the
first step in the process involves an initial single hydrosilylation to
form HSiMe2OSiMe2OCH2NMe2 (3). This intermediate is readily
isolated and purified via distillation. In the continued presence of the catalyst, 3 transforms to the transient tetrasiloxane
HMe2SiOSiMe2OSiMe2OSiMe2OCH2NMe2 (4), along with the formation of Me3N. The tetrasiloxane 4 itself transforms to
Me3N and (Me2SiO)n (n = 4−6). Despite the demonstrated reactivity of 3, it can also be used to perform the expected metal-
catalyzed hydrosilylation chemistry of the SiH group as well as reactions of the SiOCH2NMe2 functionality involving siloxane
chain extension and is thus an important new reagent for siloxane chemistry.
ince the initial, and generally uncited, report by the Voronkov
group that silanes, R3SiH, could effectively reduce DMF to
this arena we have now used Karstedt’s catalyst to study the
reaction of 1 with DMF, monitoring the reaction with 29Si, 13C,
and H NMR spectroscopy. Typical monitoring sequences of
S
Me3N with concomitant formation of disiloxanes,1 their use as
reducing agents to transform amides to amines has been well-
studied and widely used due to the generally mild reaction condi-
tions employed.2 Using monosilanes, R3SiH, to reduce DMF as
a model amide, we recently demonstrated that such reductions
proceed via an initial hydrosilylation reaction to produce siloxy-
methylamines R3SiOCH2NMe2 (O-silylated hemi-aminals).3 This
class of compounds can further react with silanes in the presence
of catalysts, and also in excess DMF, to form the amine and
appropriate disiloxane (eq 1).
1
such a reaction in C6D6 at room temperature (∼295 K) are
presented in Figures 1 (29Si) and 2 (13C). Along with the
Figure 1. 29Si NMR monitoring of the reaction between
HSiMe2OSiMe2H (−4.6 ppm) and DMF (1:5 molar ratio) catalyzed
by 1 mol % of Karstedt’s catalyst, illustrating the formation of 3 (−6.4
and −12.2 ppm).
disappearance of the 29Si resonance at −4.6 ppm due to 1, there
is a growth of two resonances at −6.4 and −12.2 ppm, and under
the reaction conditions used, no further significant chemistry
takes place.
A particularly versatile silane for such reductions is 1,1,3,3-
tetramethyldisiloxane, HSiMe2OSiMe2H (1), and using a range
of catalysts, including Karstedt’s catalyst (bis[1,3-bis(η2-ethenyl)-
1,1,3,3-tetramethyldisiloxane]platinum), this reagent possesses
some unique reduction characteristics.4−6 For example, it has
been demonstrated that 1 can effectively reduce amides under
conditions where other silanes are ineffective and this special
property has been associated with a “dual SiH effect”.5a
Furthermore, the same disiloxane has been noted as a poor
reagent for other reactions.5c We have previously reported
that using 1 in the presence of (Me3N)Mo(CO)5 as catalyst,
for the reduction of DMF, a double hydrosilylation occurs to
form Me2NCH2OSiMe2OSiMe2OCH2NMe2, (2), which can be
isolated and characterized.7 As a continuation of our studies in
The related 13C spectral sequence (Figure 2) exhibits the ap-
pearance of new resonances at 81.7, 41.0, 0.84, and −0.89 ppm
typical of the SiOCH2NMe2 group and two new Me2Si units.
The new material is the single hydrosilylation product,
Me2NCH2OSiMe2OSiMe2H (3; eq 2).
The same chemistry takes place on a larger preparative scale
using either hexane or benzene as solvent, and we have been
Received: April 26, 2014
Published: June 11, 2014
© 2014 American Chemical Society
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Figure 2. 13C NMR monitoring of the reaction between
HSiMe2OSiMe2H (0.1 ppm) and DMF (1:5 molar ratio) catalyzed
by 1 mol % of Karstedt’s catalyst, illustrating the formation of 3 (81.7
(CH2), 41.0 (NMe2), 0.84 and −0.89 ppm (SiMe)). Me resonances
for DMF appear at 30.6 and 35.2 ppm.
Figure 4. Transformation of 4 (81.8 ppm (CH2); 41.1 ppm (Me2N)
to Me3N (47.5 ppm) and D4−6 monitored by 13C NMR. The minor
resonances at ∼20 ppm are associated with the xylene solvent of the
catalyst solution.
Me3N and 4, which further eliminates Me3N along with
formation of Dn, both reactions demonstrating the reactivity of
the siloxymethylamines with SiH species involving formation of
Me3N and siloxanes (Scheme 1).3 This is a further illustration
Scheme 1. Sequence of Reactions Leading to Production of
Me3N and Dn
able to isolate this material by distillation at 49 °C/15 mmHg in
excellent yield. Compound 3 is relatively stable at room
temperature; however, when it is subjected to the presence of
Karstedt’s catalyst the 29Si resonances at −6.4 and −12.2 ppm
transform to four new signals at −6.7, −13.9, −19.8, and
−21.5 ppm and via 13C NMR we observe the concurrent
formation of Me3N. The new silicon-containing material is the
tetrasiloxane HSiMe2OSiMe2OSiMe2OSiMe2OCH2NMe2 (4).
While we have been unable to obtain 4 as an analytically
pure material, we have been able to prepare and isolate it in
∼95% purity and study its further chemistry, as described in
the Supporting Information. Continued exposure of 4 to the
catalytic conditions results in formation of more Me3N and a
mixture of cyclic dimethylpolysiloxanes, (Me2SiO)n (n = 4, D4;
n = 5, D5; n = 6, D6), as noted by comparison of their 29Si
NMR and GC/MS spectra with those of known materials
(Figures S-5−S-8, respectively (Supporting Information)). The
29Si and 13C monitoring of this transformation is presented in
Figures 3 and 4, respectively.
of the capacity of hydrosilanes to reduce siloxymethylamines
(O-silylated hemi-aminals), as illustrated in eq 1, and the more
general capacity to reduce aminals.9
The transformation of 4 to Me3N poses the question as to
whether this is an intramolecular elimination reaction or
involves a bimolecular reaction with a second SiH functionality.
Since the silicon-containing compound formed is predominantly
D4 and we see no significant amounts of long-chain poly-
siloxanes, we favor the intramolecular amine elimination. We
attempted to answer that question by D-labeling experiments
using deuteriotetramethyldisiloxane, DSiMe2OSiMe2D10 (1D).
Use of 1D to reduce DMF resulted, as expected, in the forma-
tion of Me2NCHD2. However, using an equimolar mixture of 1
and 1D resulted predominantly in the formation of Me2NCH2D
and Me3N and smaller amounts of Me2NCHD2: i.e., a signifi-
cant H/D scrambling had occurred (Figure S-4 (Supporting
Information)). Separate experiments mixing 1D and Et3SiH
resulted in a very rapidly established equilibrium illustrating the
metal-catalyzed H/D exchange in hydrosilanes,11 thus ruling out
the mechanistic clarification we sought.
As we previously reported, the use of (Me3N)Mo(CO)5 as
catalyst for the reduction of DMF by 1 proceeds via a different
route, involving the intermediacy of the double-hydrosilylation
intermediate (Me2NCH2OSiMe2)2O.7 We have treated the new
intermediate 3 with DMF in the presence of the molybdenum
catalyst and observed the rapid formation of (Me2NCH2OSiMe2)2O
(2) with no initial Me3N formation, illustrated in Figure 5. The
two catalysts clearly have distinctive properties in this chemistry,
Figure 3. Transformation of 4 (−6.7, −13.9, −19.8, and −21.5 ppm)
to predominantly D4 (−19.1 ppm) and D6 (−21.6 ppm) monitored
by 29Si NMR.
To prove the structure and formulation, we have reacted 4
with Me3SiCl. This procedure results in the formation and
high-yield isolation of the expected pentasiloxane HSi-
Me2OSiMe2OSiMe2OSiMe2OSiMe3 (5), a known compound.8
Overall during the reduction of DMF by 1, the initially
formed hydrosilylation product 3 reacts with itself to liberate
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the amine elimination/amide reduction product (eq 2). Hence,
a competition between hydrosilylation and amine elimina-
tion was anticipated. However, in a test hydrosilylation reac-
tion we treated 3 with Me3SiCHCH2 in the presence of
Karstedt’s catalyst and obtained a high yield of the “expected”
hydrosilylation product 7 (eq 6). No amine elimination
chemistry was observed and the hydrosilylation appears to be
regiospecific.
Figure 5. 29Si NMR monitoring of the reaction between DMF and
HSiMe2OSiMe2OCH2NMe2 (3) catalyzed by 1 mol % of (Me3N)-
Mo(CO)5 leading to Me2NCH2OSiMe2OSiMe2OCH2NMe2, (2) at
−13.9 ppm.
catalyst
3a ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Me3SiCH2CH2SiMe2OSiMe2OCH2NMe2
Me3SiCHCH2
7
(6)
and studies to tease out these distinctions, along with those of
other catalysts, are in progress.
Compound 7 can be readily reacted with chlorosilanes for
further siloxane chain extension (eq 7).
Treatment of 3 under varying conditions of the functional
group reactivity was performed to illustrate that the two terminal
silyl groups (SiH and SiOCH2NMe2) retain their established
chemistry in the presence of each other, even in the presence of
metal catalysts. Thus, the reaction of 3 with Me3ECl (E = Si, Ge)
led to the high-yield formation of the siloxane chain extension
products, resulting in either trisiloxane 6a or disiloxygermoxane
6b (eq 5).
7 ⎯M⎯⎯e⎯3⎯S⎯⎯i→Cl Me3SiCH2CH2SiMe2OSiMe2OSiMe3
(7)
8
Thus, the newly observed and isolated material 3 has three
distinct and very useful modes of chemistry: elimination of the
reduced amide as the amine, hydrosilylation using the SiH
group, and siloxane chain extension using the SiOCH2NMe2
functionality (Scheme 2).
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving experimental details and spectroscopic/
analytical data for all compounds prepared in this paper. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The trisiloxane 6a is a useful, commercially available reagent,12
whereas the germanium analogue is unreported. All analytical
and spectroscopic data are in accord with either the published
data or those expected. For example, the 29Si NMR data for 6a
exhibits three resonances at 7.4, −6.9, and −19.4 ppm for the
Me3SiO, HMe2SiO, and Me2SiO silicon atoms, respectively. The
Ge analogue 6b exhibits 29Si resonances at −8.2 and −17.6 ppm,
in accord with expectation.
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.H.P.: kpannell@utep.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The chemistry of the SiH functionality of 3 was initially
expected to be complicated, since the hydrosilylation reaction
normally needs a catalytic species similar to that required for
We thank the Welch Foundation (Grant No. AH-0546) and
the NIH-MARC program (Grant No. 2T34GM008048) for
support of this research.
a
Scheme 2. Reactivity of 3
a
Legend: (a) Me3SiCHCH2/Karstedt’s catalyst; (b) (Me3N)Mo(CO)5/DMF; (c) Me3ECl, E = Si, Ge; (d) Karstedt’s catalyst.
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