Tandem-reduction of DMF with silanes via necklace-type transition over Pt(0) nanoparticles: Deciphering the dual Si-H effect as an extension of steric effects

Article abstract of DOI:10.1039/c4cc06979j

Dimethylformamide (DMF) undergoes double-reduction to yield trimethylamine as a result of the concerted activation of DMF by two Si-H bonds (from different Si atoms) over Pt(0) nanoparticles as catalytic centers. Sterics on the Si atom govern the reaction and are also decisive for the structure of siloxane products due to potential limitations on the concerted activation. This journal is

Full text of DOI:10.1039/c4cc06979j

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TandemꢀReduction of DMF with Silanes via Necklaceꢀ
type Transition over Pt(0) Nanoparticles: Deciphering
the Dual SiꢀH Effect as an Extension of Steric Effects
Cite this: DOI: 10.1039/x0xx00000x
Vijay P. Taoria and Michael R. Buchmeisera*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
molecule to isolate Oꢀsilylated hemiꢀaminals as singleꢀreduction
intermediates. Although the “dual SiꢀH effect” has been cited to
explain the results, no study has been able to unambiguously explain
such a phenomenon till date. Here, we zeroꢀin on this phenomenon
and the compilation of our results shows that the dual SiꢀH effect is
simply an extension of steric effects; most important, the process we
explain here is without contradictions and fully accounts for all
observed (byꢀ) products and intermediates.
Dimethylformamide (DMF) undergoes “doubleꢀreduction” to
yield trimethylamine as a result of the concerted activation of
DMF by two SiꢀH bonds (from different Si atom) at Pt(0)
nanoparticles as catalytic centers. Sterics on the Si atom
govern the reaction and are also decisive for the structure of
siloxane products due to potential limitations on the
concerted activation.
We chose DMF as a model molecule to observe its reduction by
silanes at room temperature. Because of its resonance structure,
A plethora of homogeneous catalysts has been developed in order to
carry out the reduction of amides, a highly thermodynamically stable
functional group to amines via hydrosilylation1ꢀ10 although most
require elevated temperature1, 4ꢀ7, 10 and produce other Nꢀbased side
products. Amongst these systems a few2, 4, 8ꢀ10 stand out as they show
a unique but unexplained “dual SiꢀH effect”. Ito and coꢀworkers2
were the first to report such a study in which a dihydrosilane
(R2SiH2) was found capable of reducing an amide to an amine in the
DMF is
a
very stable molecule ((CH3)2NꢀC(H)=O
↔
(CH3)2N+=C(H)ꢀOꢀ). Treatment of DMF with a silane does not result
in any reaction. Nonetheless, SiꢀH bonds can be activated by
transition metal complexes.12 Here, we focused on the reduction of
DMF by a simple heterogeneous system consisting of Pt(0)
nanoparticles (NPs) immobilized on a monolithic polymeric support,
whose synthesis and use in both the reduction of CO2 and in the
continuous hydrosilylation of olefins is explained elsewhere13ꢀ16
.
presence of
a Rh catalyst. They further noticed that the
We propose as shown in Scheme 1 that Pt(0) NPs in the catalytic
system can activate DMF and silanes which can lead to a very
specific attack of hydrides on the activated DMF molecule.
monohydrosilane analogue (R3SiH) was ineffective in carrying out
such transformation and a disiloxane molecule (R2HSiOSiHR2) that
was a sideꢀproduct from the original dihydrosilane (R2SiH2) reagent
was also ineffective in further reducing the amide. Nagashima and
10
coꢀworkers4, then published somewhat contradicting reports and
observed that although the Ptꢀbased catalytic systems they used also
did not reduce an amide to an amine in the presence of a
monohydrosilane (R3SiH) such reduction was not viable with
dihydrosilanes (R2SiH2), too. The only reduction of an amide they
observed was with a disiloxane ((R2HSi)2O) type of molecule and
the term “dual SiꢀH effect” was coined, which essentially implied
that with the catalytic systems used, only silane molecules with two
SiꢀH moieties within one molecule could reduce an amide to the
corresponding amine. Later, a plausible mechanism was reported10
but it fell short as it suggested the formation of a fourꢀmembered
siloxane ring in the process of abstracting an oxygen atom from the
amide by the disiloxane molecule, however, no such product was
reported as its formation is energetically disfavored10, 11. Pannell’s
Scheme 1: Proposed activation of a) a monohydrosilane, b) a
dihydrosilane and c) DMF over Pt(0) and d) concerted
activation of DMF and SiꢀH bonds from two different Si atoms
over Pt(0) NPs.
Schemes 1a) and b) differ only as one of the R groups on the Si
atom is replaced by H in case of b) thus reducing the sterics around
the Si atom. Our first assumption here is that only one hydride
attached to a Si atom can be activated by the catalytic system at a
9
group8, also contributed to that topic by using DMF as a model
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given instant. Scheme 1c) shows the lone pair on the N atom in reveals the evolution of two new peaks, one at δ = 4.52 ppm and the
DMF donated to the Pt atom on the catalytic NP surface which can other at δ = 2.09 ppm (also already observed as tiny peaks after 10
make the carbonyl carbon more susceptible to nucleophilic attack minutes).
and finally d) shows the concertedly activated DMF molecule and
SiꢀH bonds from two different Si atoms in a close vicinity engaged
at a Pt NP although the two hydrides may not attack the carbonyl
carbon at the exact same instant and thus this may not be a fully
concerted reaction.
In order to better understand the effect of silanes on DMF reduction
in presence of the Ptꢀbased catalytic system, the following
experiments were designed. First, diethylsilane (Et2SiH2) was chosen
as it is one of the silanes that carries the least sterics around the Si
atom but is also easy to handle. 600 µL of DMFꢀd7 were used as
reagent, reaction and NMR solvent. To this, 50 µL of Et2SiH2 were
added. No reaction in the absence of the catalyst was observed thus
establishing a negative control. In a parallel experiment, the NMR
tube with the aboveꢀmentioned contents was charged with 5 mg of Pt
immobilized monolith (357 ppm Pt; ~9 nanomol of Pt). The molar
ratio of SiꢀH to Pt in this case was ~85700 (1.2.10ꢀ3 molꢀ%). This
1
very small amount of catalyst is in stark contrast to earlier studies
carried out with homogeneous type catalysts where 2 to 10 molꢀ% of
catalyst were used1, 3, 5, 6. It is also important to mention that due to
the excess of DMFꢀd7 the silane was the limiting reagent and a
singleꢀreduction product of DMFꢀd7 could also be expected to form.
However, due to the concerted activation nature of the doubleꢀ
reduction no singleꢀreduction products were observed. The reaction
Figure 1: H NMR of a reaction mixture containing 600 µL of
DMFꢀd7, 5 mg of Pt(0)ꢀcontaining monolith (357 ppm Pt; ~9
nanomole of Pt) and 50 µL of Et2SiH2. 1) Bottom spectrum
1
shows the H NMR taken after 10 minutes. 2) Top spectrum
shows the 1H NMR acquired after 3 days.
1
The resonance at δ = 4.52 ppm shows a clear quintet similar to the
one in Et2SiH2 at δ = 3.62 ppm and is thus assigned to the hydride
resonance in
combination with the proposed reaction Scheme 2 imply the
formation of a (Et2HSi)2O species. This also supports our earlier
stated assumption that only one SiꢀH bond in Et2SiH2 is activated at
any given instance.
was monitored by acquiring H NMR spectra at defined intervals.
Figure 1 (bottom) shows the 1H NMR acquired immediately (10
minutes) after the addition of the reagents and shows a quintet at δ =
3.62 ppm for Et2SiH2 (3JHꢀH = 3.56 Hz). The top spectrum is the
NMR acquired 3 days after the beginning of the reaction and shows
the absence of the original Et2SiH2 peak at δ = 3.62 ppm, which
clearly demonstrates the activation of the SiꢀH bond followed by the
consumption of the Et2SiH2 species shifting the equilibrium in the
forward direction in Scheme 1b). The top spectrum in Figure 1 also
a HꢀSi(Et2)ꢀOꢀ species. These observations in
Scheme 2: Details of the reduction of DMF with silanes in presence of the Pt(0) NPꢀbased catalytic system. Reduction was carried
out at room temperature.
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This given instance however suggests the abstraction of oxygen from 4.52 ppm and concomitantly an increase in the trimethylamineꢀd7
DMFꢀd7 and formation of (CD3)2NCDH2 (trimethylamineꢀd7) for peak at δ = 2.09 ppm is observed. At the end of day 30 when 99% of
which a newly formed triplet (1:1:1 intensity) at δ = 2.09 ppm is the hydrides were consumed the reaction mixture was analysed by
observed. Details of further confirmation, which include
a
GCꢀMS. The analysis shows the formation of both compound 3 and
comparison of the NMR and GCꢀMS data with those of an authentic 4 (R1=R2=Et); SI Figure S6(a) – (e).
sample of (CH3)3N are provided in the supporting information As shown in Table 1, various silanes (50 µL each) were used along
(Figure S1 to Figure S4; SI). Further it can be said in reference to with 600 µL of DMFꢀd7 and 5 mg of Pt(0) NPs loaded monolith (~9
Figure 1 (δ = 2.09 ppm and δ = 4.52 ppm) and Scheme 2 that Stage nanomole of Pt). Table 1, entries 2 and 3 show two other
I of the proposed reaction mechanism through which a Et2SiH2 dihydrosilanes, namely methylphenlsilane (MePhSiH2) and
molecule goes concludes here.
diphenylsilane (Ph2SiH2) that were used in comparison to Et2SiH2.
2* is the key reaction sequence, which leads to the completion of Clearly, silane consumption recorded for the first 3 days goes down
Stage I where clearly SiꢀH bonds from two different molecules of from almost 99% (>50 % SiꢀH) in case of Et2SiH2 to ~11% in case
Et2SiH2 are activated and jointly attack the activated DMF molecule. of MePhSiH2 to ~3% in case of Ph2SiH2 distinctly showing that as
We don’t have unambiguous proof that the reaction is perfectly the sterics on the Si atom increase the reaction dramatically slows
concerted as this would imply the simultaneous breaking of a down.
C(sp2)=O bond, formation of two new C(sp3)ꢀH bonds and
formation of two new SiꢀO bonds to form the disiloxane compound
Table 1: Reduction of DMFꢀd7 with different silanes.
2 (Scheme 2). Such a scenario is depicted in Figure S5(a) (SI). In
Silane
Reduction of
Silane
fact, studies carried out by Pannell’s group8 suggest that their
process first goes through the formation of an Oꢀsilylated hemiꢀ
aminal, which is then reduced to form trimethylamine and
disiloxane. However, in contrast to the Ptꢀbased homogeneous type
Karstedt’s catalyst used in Pannell’s studies, whose use results in a
mixture of various reduction products of DMF (Figure S5(b) SI),
our Pt(0)ꢀbased heterogeneous type catalytic system exclusively
produces trimethylamine as only DMFꢀreduction product. Therefore,
in reference to the 2* reaction sequence and absence of other DMFꢀ
reduction products including Oꢀsilylated hemiꢀaminal we believe
that the catalytic system discussed here in fact shows specificity due
to its unique concerted activation leading to synergy in doubleꢀ
reduction of DMF.
b
DMFꢀd7
Consumptionde
(SiꢀH:Pt)c
molꢀ%
TON
>42900
(77200)f
~4500
~900
~370
~400
<7f
a
1
2º
3º
Et2SiH2
yes (85700)
100
(~99)f
~11
~3
2
3
4
5
6
7
MePhSiH2
Ph2SiH2
(Me2HSi)2O
Me2HSiOSiMe3
Et3SiH
yes (80900)
yes (60000)
yes (62900)
yes (28500)
yes (34700)
no (36200)
~6
~1.3
<0.2f
0f
Me2PhSiH
0f
[a] Arrow represents increase in sterics at the Si atom. [b] Reduction of
DMFꢀd7 to trimethylamineꢀd7 as evidenced by the formation of a triplet
(1:1:1 intensity) at δ = 2.09 ppm in 1H NMR for (CD3)2NCH2D. [c] Molar
ratio. [d] With 9 nmol Pt(0) as catalyst. [e] Based on loss of one hydride from
each molecule for 3 days. [f] Based on the total amount of SiꢀH available for
the reaction over an extended period of time.
Every step indicated by an “*” in Scheme 2 represents such an
event, which involves the aboveꢀmentioned concerted activation and
results in a doubleꢀreduction of DMF to form trimethylamine. After
reaching Stage I the disiloxane molecule (compound 2) can either go
through Stage IIA to follow cycle A or through Stage IIB to follow
cycle B as shown in the Scheme 2. In case of Stage IIA the original
Et2SiH2 can react with compound 2 in a similar fashion than in step
2*. The detailed Stage IIA is on the extreme right of the Scheme 2
and first goes through step 3’*, which leads to the formation of
another siloxane compound 3’. This siloxane compound can then go
through step 3*, which depicts the formation of a “necklaceꢀtype”
transition due to the activation of two terminal SiꢀH bonds over Pt
resulting in the formation of compound 3 that is a cyclotrisiloxane.
Similarly, compound 2 can instead follow cycle B and go through
Stage IIB where two molecules of compound 2 can be activated at
the Pt NP surface. The details of Stage IIB are depicted at the
extreme left of the Scheme 2 and are similar to Stage IIA resulting
in the formation of compound 4 that is a cyclotetrasiloxane, again
via a “necklaceꢀtype” transition, 4*. Another distinction has to be
made here in that in case of Et2SiH2 the reaction via cycle A
diminishes as Et2SiH2 is consumed (>50% SiꢀH bonds) within 3
days (Figure 1). The reaction later slows down dramatically as it
progresses via cycle B as the sterics offered by the siloxane
molecules (compound 2 and 4’) are high as compared to those
offered by Et2SiH2 at the Pt NP. As a result, from day 3 to day 30 the
1H NMRs show a slow but consistent consumption of hydrides at δ =
Reaction with these silanes was monitored via 1H NMR for an
extended period of time. Even if more than 90% of MePhSiH2 were
consumed no formation of compound 3 (R1=Ph, R2=Me) was
observed (unlike in case of Et2SiH2) and GCꢀMS showed the
exclusive formation of compound 4 (Figures S7(a) ꢀ (c), SI). This
can imply that once the mechanism goes through formation of
compound 2 it can still go via cycle A or cycle B but in case of its
journey via cycle A, due to the sterics available (R1=Ph, R2=Me) on
compound 3’ (cycle A) it cannot go through “necklaceꢀtype” unique
step 3* to form a cyclotrisiloxane but now takes a new route, which
involves step (3’→4’)* (Scheme 2) where it prefers cycle B over
cycle A to form compound 4’ and eventually, exclusively compound
4, a cyclotetrasiloxane (Figures S7(a) ꢀ (c), SI). It should be noted
that the formation of cyclotrisiloxanes in case of MePhSiH2 is
usually favoured and is also well supported by our earlier studies on
the reduction of CO2 with MePhSiH2 in presence of a Pt NPꢀbased
catalytic system13 (Figures S8(a) ꢀ (c), SI). CO2 shows singleꢀ
reduction products that is 1st, 2nd and 3rd reduction product of CO2 to
be silylformate, silylacetal and methoxysilane, respectively,
accompanied by the exclusive formation of a cyclotrisiloxane
(R1=Ph, R2=Me). This does not require concerted activation of
terminal SiꢀH bonds on the siloxane molecule thus a sixꢀmembered
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4CC06979J
ring is possible but in case DMF (double) reduction next generation
siloxane ring is formed due to ease in wrapping around Pt NP in
formation of “necklaceꢀtype” transition for concerted activation of
terminal SiꢀH bonds. In case Ph2SiH2 is used, silane consumption
within three days further drops to ~3% and over an extended period
of time a white precipitate is observed to form inside the NMR tube.
GCꢀMS on this reaction mixture after almost 90% of silane
consumption revealed that the formation of both compound 3 and
compound 4 is not observed implying that because of the high sterics
(R1=R2=Ph) on the Si atom the compounds are unable to form a
“necklaceꢀtype” transition and thus form oligomers, which
precipitate from the solvent.
Finally, a series of tertiary silanes were used (Table 1). Entry 4
shows a disiloxane molecule, (Me2HSi)2O, which has two hydrides
but on two different Si atoms within the molecule. As expected,
within 3 days only ~6% of the hydrides were consumed. This
explains the drop in reaction rate in case of Et2SiH2 after 3 days
when all the Et2SiH2 is consumed and the hydrides that were
available for the reduction of DMF came from (Et2HSi)2O, again
stressing on the effect of sterics on the reduction of DMF. GCꢀMS
analysis of the reaction mixture with (Me2HSi)2O shows the
reduction step of DMF involves a concerted activation of
hydrides from two SiꢀH bonds and one DMF molecule at the
catalytic centre that is the Pt(0) NPs. The reduction is strictly
governed by the sterics on the Si atom at the catalytic centre
and goes through
a “necklaceꢀtype” transition prior to
formation of the cyclosiloxane.
Notes and references
a Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring
55, Dꢀ70550 Stuttgart, Germany. Email: michael.buchmeiser@ipoc.uniꢀ
stuttgart.de
†
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/c000000x/
1. S. Zhou, K. Junge, D. Addis, S. Das and M. Beller, Angew. Chem.
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1017ꢀ1020.
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molecules) as compared to a mixture of cyclotetrasiloxanes and
cyclododecasiloxanes in case of Pannell’s study with a Mo based
homogeneous catalyst11. This implies that a necklaceꢀtype transition
results in the formation of the smallest possible cyclosiloxanes as
long as the sterics allows wrapping of the siloxane molecules on the
Pt NP. Furthermore, this also explains the preferred formation of
cyclotrisiloxane with Et2SiH2 over cyclotetrasiloxane mentioned
earlier. Further, the use of Me2HSiOSiMe3 (Table 1, entry 5) shows
that within 3 days only ~1.3% of silane was consumed while using a
monohydrosilane, Et3SiH (analogue of Et2SiH2, which is explained
in details) shows that the reaction for extended time even resulted in
consumption of only 0.2% of silane. Entries 5 and 6 thus provide
strong evidence against the “dual SiꢀH effect” as explained in
previous studies; instead it shows that a double reduction of DMF
also occurs in the presence of a monohydrosilane but the rate of
reaction is extremely slow due to the sterics on the Si atom. Entry 7,
Table 1, shows that Me2PhSiH even over an extended period of time
does not react at all with DMF, which is consistent with the “sterics
effect” theory. The dual SiꢀH effect can thus be redefined as the
presence of two SiꢀH bonds at two different Si atoms, may it be the
terminal Si atoms of siloxanes (unlike in the study by Ito2) or two
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2129ꢀ2131.
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Chem. Soc., 2009, 131, 15032ꢀ15040.
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or two molecules of monohydrosilane (unlike in both studies2, 4, 10),
and their concerted activation together with the amide to form a
tertiary amine. The sterics on the silane decides the fate of the
reaction. The mechanism proposed here accounts for the formation
of an amine by abstraction of the O atom from formamide and also
fully accounts for the particular siloxane molecules formed from
each specific silane used.
292.
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Conclusions
In conclusion, a heterogeneous catalytic system, which contains
Pt(0) NPs immobilized on the surface of a monolithic
polymeric support in the presence of a dihydrosilane allows for
the doubleꢀreduction of DMF to produce trimethylamine and
cyclosiloxanes. The studies here explain that the “dual SiꢀH
effect” phenomenon mentioned in some earlier studies is a
proxy of “sterics effect” and such a reduction is also possible
with a monohydrosilane except the reaction is very slow. The
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Graphical Abstract + Text
The mechanism of the reduction of dimethylformamide to N(CH3)3 by the
action of silanes and a heterogeneous Ptꢀcatalyst is presented.
1.
N
+ R2SiH2
R
R
O
N
Si
O
O
R
R
R
R
2.
O
Si
Si
O
H
H
O
R2SiH2
+
Si
Pt Pt Pt Pt
R2Si SiR2
RT
RT
R
R
+
N
O
O
N
O
- N
R2Si
SiR2
Pt Pt Pt Pt
Pt Pt Pt Pt
O
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