Selective catalytic synthesis of amino-silanes at part-per million catalyst loadings

Article abstract of DOI:10.1039/c7cc08530c

Platinum(ii) complex [Pt(I^tBu′)(I^tBu)][BAr^F₄] (1a) is a highly active and selective catalyst in the dehydrocoupling of amines and silanes at part-per-million catalyst loadings (up to 10 ppm, 0.001 mol%), achievi

Full text of DOI:10.1039/c7cc08530c

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Selective Catalytic Synthesis of Amino-Silanes at Part-per Million
Catalyst Loadings
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
Pablo Ríos, Marta Roselló-Merino, Orestes Rivada-Wheelaghan, Javier Borge, Joaquín López-
a
a,
Serrano, Salvador Conejero *
DOI: 10.1039/x0xx00000x
www.rsc.org/
t
t
F
4
Platinum(II) complex [Pt(I Bu’)(I Bu)][BAr ] (1a) is a highly active using an electron-deficient platinum(II) catalyst, stabilized by
and selective catalyst in the dehydrocoupling of amines and N-heterocyclic carbenes (Scheme 1), that exhibits high
silanes at part-per-million catalyst loadings (up to 10 ppm, 0.001 efficiency at part-per million catalyst loadings with an
mol%), achieving the highest TON and TOF numbers reported in exceptional selectivity toward the formation of mono-
the literature (up to 1 mmol scale). NMR studies suggest a process aminosilanes from primary silanes. Low temperature NMR
taking place through electrophilic activation of the silane by the studies suggest that transfer of a hydride atom from the silane
platinum species, assisted by an amine.
to the platinum center (through σ-SiH complex intermediates)
occurs during the catalytic cycle.
Catalytic dehydrocoupling processes leading to the formation
of main-group E-E’ bonds (E = B, Si; E’ = O, N) are gaining
importance as a tool for cleaner, atom economic, synthetic
We have reported that the coordinatively unsaturated
t
t
t
F
6
4
complex [Pt(I Bu’)(I Bu)][BAr ] (1a) (where I Bu is 1,3-di-tert-
t
F
4
butylimidazolylidene and I Bu’ its cyclometalated form, BAr is
1
methods. In this sense, the catalytic synthesis of Si−N bonds
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (Scheme 1), is
an efficient catalyst in dehydrocoupling processes leading to
7
amino and diaminoboranes. Mechanistic studies support a
from silanes and amines is particularly attractive since the only
by-product is dihydrogen. Amino-silanes are useful molecules
for the synthesis of silicon-based bases, polymers and
reaction that proceeds through nucleophilic attack of free
dimethylamine to the boron atom in a σ-BH intermediate
1
c
ceramics that have been traditionally prepared from halo-
silanes, amines and bases which is less convenient than the
direct dehydrocoupling. During the last years some groups
have reported efficient catalytic systems based on main group
elements (Li, Na, K, Mg, Ca, Sr, Al), lanthanides and actinides
1
HBH ·NHMe , exhibiting a η -BH coordination
complex [Pt]
←
2
2
mode (Shimoi type). The mechanistic similarities of this
process and the dehydrocoupling of silanes and alcohols by
8
iridium species prompted us to study the dehydrocoupling of
2
-4
(
Yb, U) and late transition metals. These examples required
silanes and amines. Recently, we have reported that complex
9
can interact with silanes to form σ-SiH complexes which,
4
4
catalyst loadings of 1-5 mol% (10 −5x10 ppm) to achieve good
conversions and reaction rates under mild reaction conditions.
Most recently, it has been possible to decrease the catalyst
loading (up to 0.25 mol%, 2500 ppm) by using a barium
1
a
1
according to DFT calculations, exhibit a η -SiH coordination
mode similar to that reported by Brookhart in an iridium
1
0
complex.
5
complex. Another challenge associated with this type of
In this work, we first investigated the dehydrocoupling of
t
phenylsilane (PhSiH ) and tert-butylamine ( BuNH ) (1 to 1
transformation is to control the selectivity of the reaction.
3
2
Particularly, primary silanes
(RSiH3)
can produce
ratio) using a catalyst loading of 0.5 mol% (5000 ppm) (Scheme
monosilazanes (RSiH (NRR’)) together with di- and trisilazanes
2
1). Under these conditions a rather violent reaction led to the
t
exclusive formation of PhSiH (N BuH) within a few seconds.
(RSiH(NRR’) and RSi(NRR’) ).
2 3
2
Herein, we report the catalytic synthesis of amino-silanes
This selectivity is comparable to that reported using a Mg
BArF
PhSiH
+
3
N
N
N
N
[
Pt]
2
PhSiH (NRR')
CH
2
Cl
2
/ r.t.
[Pt] =
Pt
NRR'H
-
H
2
1a
Scheme 1. Catalytic dehydrocoupling of primary silanes an amines.
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t
t
F
4
Table1. Cross-dehydrocoupling of silanes and amines catalyzed by complex [Pt(I Bu’)(I Bu)][BAr ] (1a).
a
b
-1
Entry
Silane
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
Amine (equiv)
mol % (ppm) cat
0.005 (50)
0.003 (30)
0.2 (2000)
0.005 (50)
0.001 (10)
0.3 (3000)
0.01 (100)
0.005 (50)
0.2 (2000)
0.3 (3000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.005 (50)
0.2 (2000)
0.005 (50)
0.2 (2000)
0.005 (50)
0.2 (2000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
0.1 (1000)
product
% yield (isolated)
TON
TOF (h )
t
t
4
4
1
2
3
4
5
6
7
8
9
3
BuNH
2
(1)
(1)
(2)
PhSiH
2
( BuNH)
99(90)
99
2 x 10
9.6 x 10
t
t
4
4
4
3
BuNH
2
PhSiH
2
( BuNH)
3.3 x 10
920
5.7 x 10
t
t
3
BuNH
2
PhSiH( BuNH)
2
92(90)
99(88)
98
540
4
5
3
NEt
NEt
NEt
2
H (1)
2
H (1)
2
H (2)
PhSiH
PhSiH
2
(NEt
(NEt
2
)
)
2
2 x 10
3.3 x 10
4
3
2
2
9.8 x 10
2.9 x 10
3
PhSiH(NEt
PhSiH
PhSiH
2
)
99(86)
99
666
595
4
4
3
(CH
2
2
2
)
4
NH (1)
NH (1)
NH (2)
2
[N(CH
2
)
4
4
]
]
]
2
1 x 10
6.4 x 10
4
4
3
(CH
)
4
2
[N(CH
2
)
99
2 x 10
3.4 x 10
3
(CH
)
4
PhSiH[N(CH
2
)
4
99(96)
99(93)
99
1000
333
8450
1600
667
i
i
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
3
Pr
Pr
Pr
2
NH (1)
NH (1)
NH (4)
PhSiH
PhSiH
PhSiH
PhSiH
PhSiH
2
(N Pr
2
)
i
i
i
3
2
2
(N Pr
2
)
1000
1000
950
i
4
3
2
2
(N Pr
2
)
99
1.7 x 10
3
MesNH
2
(2)
(4)
2
(NHMes)
95
840
3
MesNH
2
2
(NHMes)
99
1000
1000
1000
1000
2300
2000
1300
1330
t
t
Ph
Ph
2
2
2
SiH
SiH
SiH
2
2
2
BuNH
H (1)
NH (1)
2
(1)
Ph
Ph
2
2
2
SiH( BuNH)
99(97)
99(91)
99(92)
99(90)
96(91)
99(88)
99(89)
NEt
2
SiH(NEt
2
)
Ph
(CH
2
)
4
Ph
SiH[N(CH ) ]
2 4
n
t
n
t
4
5
BuSiH
BuSiH
BuSiH
BuSiH
BuSiH
BuSiH
BuSiH
3
3
3
3
3
3
3
BuNH
2
(1)
(2)
BuSiH
2
( BuNH)
2 x 10
1.3 x 10
n
n
n
n
n
n
t
n
t
4
BuNH
2
BuSiH( BuNH)
2
1000
1.9 x 10
n
4
5
NEt
NEt
2
H (1)
H (2)
BuSiH
2
(NEt
2
)
2 x 10
1.2 x 10
n
3
2
BuSiH(NEt
2
)
2
1000
5.7 x 10
n
c
4
4
(CH
2
)
4
NH (1)
NH (2)
BuSiH
2
[N(CH
2
)
4
]
99(91)
2 x 10
3 x 10
n
4
(CH
2
)
4
BuSiH[N(CH
2
)
4
]
2
99(95)
96(86)
99(98)
99(96)
99(98)
1000
1000
1000
1000
1000
1.3 x 10
i
n
i
4
Pr
2
NH (1)
BuNH (1)
NEt H (1)
(CH NH (1)
BuSiH
2
(N Pr
2
)
3.5 x 10
t
t
5
Et
Et
Et
2
2
2
SiH
SiH
SiH
2
2
Et
2
Et
2
Et
2
SiH( BuNH)
3.3 x 10
4
2
2
2
SiH(NEt
2
)
5.3 x 10
3
2
)
4
SiH[N(CH
2
)
4
]
9.4 x 10
a
b
c
n
With respect to silane. Determined by NMR. ca. 5% of diaminosilane BuSiH[N(CH
2
)
4
]
2
has been detected in this case (see ESI).
2
a
4
catalyst, for which longer reaction times (24 h) and higher numbers (TON) and turnover frequencies (TOF) of 2 x 10 and
4
catalyst loadings (5 mol%, 5x10 ppm) were required. Other 3.3 x 10 h , and 9.6 x 10 and 5.7 x 10 h , respectively (cf.
4
-1
4
4
-1
-1
main-group catalysts provide mixtures of mono- and TON and TOF values of 167 and 24 h , respectively, reported
3
a
bis(dehydrogenated) phenylsilane with, again, longer reaction by Eisen). The reaction with NEt
2
H (using 0.005 mol%, 50
times and catalyst loadings of 5 mol%. In addition, at the end ppm, of 1a) is faster (Table 1, entry 4) with full conversion of
of the reaction 1a evolved to its hydrogenated form the phenylsilane into the mono(amino)silane PhSiH (NEt ) in
2
b
2
2
5
t
F
11
] (1b).
4
-1
[
PtH(I Bu)
2
][BAr
4
3.6 min, leading to TONs and TOFs of 2 x 10 and 3.3 x 10 h
The encouraging results obtained with complex 1a respectively, the highest reported to the best of our
-1
prompted us to decrease the catalyst loading. Thus, when the knowledge (cf. TON and TOF values of 20 and 0.83-1.6 h ,
2
a,d
reaction was carried out with 0.005 mol% (50 ppm) of 1a all respectively, in previous reports).
In this case the amount
the silane was consumed in ca. 12 min (Table 1, entry 1), of catalyst can be reduced to 0.001 mol% (10 ppm) (Table 1,
whereas catalyst loadings of 0.003 mol% (30 ppm) (Table 1, entry 5) leading to 98% conversion of the silane in 180 min.
entry 2) required 35 min, which correspond to turnover Interestingly, pyrrolidine, one of the most challenging
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[
1a] (0.1 %)
substrates in terms of selectivity, can be converted with
complete selectivity into PhSiH [N(CH ] using 0.005 mol% (50
ppm) of the catalyst (Table 1, entry 8) in 35 min. However,
R
2
SiH
2
+
R'R''NH
R HSi NR'R''
2
CH
2
-
Cl
2
/ r.t.
2
2 4
)
H
2
i
R = Ph, Et
2 2
bulkier (HN Pr ) or less nucleophilic amines (MesNH ) required
t
R' = Bu, R'' = H; R' = R'' = Et; R' = R'' = (CH
2
)
2
higher catalyst loadings (0.1 mol%, 1000 ppm) to achieve
comparable reaction times (45 min) and complete conversions
4
Table 1, entries 10-14) (cf. 5%, 5x10 ppm, of catalysts with
Scheme 2. Dehydrocoupling of secondary silanes.
(
-1
3b
TON = 20 and TOF = 20 h , reported by Cui).
n
Primary alkylsilanes such as BuSiH
3
are also efficiently
converted into the corresponding mono(amino)silanes (Table
, entries 18, 20, 22 and 24) using typically 0.005 mol% (50
1
ppm) as catalyst loading. The rates of the reactions are
i
comparable to those found for PhSiH , except when Pr NH
3
2
was used, for which an increase of the reaction rate of one
order of magnitude was observed (compare table 1, entries 11
and 24). Secondary silanes can also be dehydrocoupled
t
2 3 2
Figure 1. Kinetic profile (H evolution) of the reaction of PhSiH with BuNH ,
i
NEt H and Pr NH (catalyst loadings of 0.005, 0.005 and 0.3 mol%, respectively).
t
2
2
efficiently. Thus, Ph SiH reacts with NEt NH, BuNH and
2
2
2
2
(CH ) NH but catalyst loadings of at least 0.1 mol% (1000 ppm)
2 4
are necessary to form the desired mono(amino)silanes (Figure 1). The different kinetic profiles are probably related
Ph SiH(NR ) at good rates (Scheme 2 and Table 1, entries 15- to a more efficient coordination of the amine to 1a following
2
2
4
t
i
1
7) (cf. 5%, 5x10 ppm of catalyst loadings to achieve TON and the sequence BuNH > NEt H > Pr NH. In fact, when 1a is
2 2 2
-1
2b,3b,5b
TOF values of ca. 20 and 9-2822 h , respectively).
The mixed with an excess of the less hindered amines a decrease in
i
bulkier Pr NH does not undergo reaction at room the PtH coupling constant of the Pt−CH in the H NMR spectra
1
2
2
o
temperature, while heating at 60 C for 4 days yields mixtures (reduction of 15 and 7 Hz in the presence of 5 equiv of BuNH
t
2
of products. Significantly, the rates at which Et SiH2 is or NEt H, respectively) serves as an indication of this
2
2
i
dehydrocoupled under the same reaction conditions are very interaction, whereas it remain unaltered with Pr NH. Thus,
13
2
high (Table 1, entries 25-27), achieving TOF numbers up to 3.3 de-coordination of the less-hindered amines is a prerequisite
5
x 10 h , that is up to two orders of magnitude higher in for the reaction to proceed, which causes the induction period
-1
1
comparison to Ph SiH (Table 1, entries 15-17). Finally, the observed at early reaction times and as the concentration of
4
2
2
bulkier silane Ph SiH does not react at all with any amine.
3
amine decreases the reaction becomes faster giving rise to the
1
5,16
If an amine:primary-silane molar ratio of 2:1 is used for sigmoidal profile.
BuNH2, NEt H and (CH ) NH, the bis(amino)silanes
t
1
2
2 4
Low temperature (-30
°C)
H NMR studies of a
RSiH(NR’R’’) are formed but catalyst loadings above 0.2 mol% stoichiometric reaction of Ph SiH , NEt H and 1a conducted in
2
2
2
2
8
2000 ppm) are required (Table 1, entries 3, 6, 9, 19, 21, and THF-d (a solvent in which the reaction takes place at slower
(
4
3) (5%, 5x10 ppm, catalyst loadings have been previously reaction rates) indicate the formation of the neutral platinum
2
-1
7a
3
reported leading to TON and TOF values of ca. 40 and 40 h , hydride
3
coexisting with 1a (Scheme 3). Very likely, complex
b;5a
-
respectively,
1
cf. TONs of 666-1000 and TOFs of 540-8450 h
shown in Table 1, entries 3, 6 and 9). The second the platinum atom (preceded by formation of a η -SiH,
3 is formed through transfer of a hydride atom from Ph SiH to
1
2
2
n
9
dehydrogenation process is considerably faster when BuSiH3 complex), assisted by the amine in a similar way to the
is used instead of PhSiH (compare for example TOF numbers transfer of
hydride from amine-boranes during
3
a
in Table 1, entries 19 and 23 vs 3 and 9, respectively). No dehydrocoupling processes induced by 1a. In addition, the H
7
1
i
bis(amino)silanes have been detected in reactions with Pr NH NMR spectra revealed the formation of aminosilane
or MesNH even with high catalyst loadings.
2
2
Ph SiH(NEt ) together with another species (
2
2
A
) that contains
At this point it should be noted that when an excess of the an NEt fragment. This species exists in a 1:1 ratio with respect
2
amine is used (up to 8 equiv) in reactions with primary silanes, to hydride complex
3
. As the temperature increases the
and decreases concurrently, until
ppm), the rate of the reaction increases or decreases they disappear completely at r. t. According to H, Si-HMBC
depending on the nature of the amine (Figures S111-114). The NMR experiments, species
has no silicon atom and,
NH resulted in slower furthermore, we have not detected any other silicon-
reaction rates, whereas a significant increase of the speed of containing species other than Ph SiH2 and aminosilane
working at low catalyst loadings (below 0.01 mol%, 100 concentration of both
3
A
1
2
1
29
A
t
2 2 2 4
use of an excess of BuNH , NEt H, (CH )
2
i
the process takes place in the case of the bulkier Pr
the less nucleophilic MesNH This is related to the fragment in species
2
NH and Ph SiH(NEt ). Additionally, the CH groups of the NEt2
2
2
2
2
.
A
correlate in a COSY experiment with
1
7
coordination ability of the less hindered amines to the another broad signal at ca. 7.6 ppm. All this information
platinum atom. The kinetic profile of the reaction of PhSiH
points at species being the ammonium cation
NH, obtained by [NEt H ][BAr ], presumably formed by deprotonation of a
3
A
t
i
F
4
and the amines BuNH
measuring the amount of dihydrogen released in the process transient, unobserved, silylium cation “Ph SiH(NEt H) ” by free
2 2 2
, NEt H and Pr
2
2
+
2
2
1
15
in a closed system, depends on the bulkiness of the amine
2
NEt H. Moreover, H and N NMR chemical shifts of A match
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Journal Name
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Scheme 3. Low temperature NMR studies.
5
F
4
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2
2
2
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2
4
2
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reaction of 1a with Ph SiH and NEt H was carried out using 2
2
6
7
2
2
eq of NEt H (1a : Ph SiH : NEt H stoichiometry 1 : 1 : 2) under
2
2
2
2
identical reactions conditions, we observed complex
3
together with Ph SiH(NEt ) and species A as major products
2
2
J. Rama, J. Díez and S. Conejero, Chem. Commun., 2016, 52
8389.
,
but, importantly, no Ph SiH2 nor 1a are observed (in
2
agreement with the need of two equiv of amine per silane).
8
9
1
(a) X.-L. Luo and R. H. Crabtree, J. Am. Chem. Soc., 1989, 111
527; (b) G. J. Kubas, Adv. Inorg. Chem., 2004, 56, 127.
P. Ríos, J. Díez, J. López-Serrano, A. Rodríguez and S.
Conejero, Chem.—Eur., J. 2016, 22, 16791.
0 J. Y. Yang, P. S. White, C. K. Schauer and M. Brookhart,
Angew. Chem., Int. Ed., 2008, 47, 4141.
,
2
The final step of the process is the protonation of the platinum
F
hydride
3
by the ammonium salt [NEt H ][BAr ], releasing H
2
2
4
2
1
8
and complex 1a that is hydrogenated leading to 1b
.
In summary, we have developed a very selective and
efficient catalytic system for the generation of amino-silanes 11 O. Rivada-Wheelaghan, M. Roselló-Merino, M. A. Ortuño, P.
Vidossich, E. Gutiérrez-Puebla, A. Lledós and S. Conejero,
Inorg. Chem., 2014, 53, 4257.
(up to 1 mmol scale) using a highly electrophilic, robust, Pt(II)
complex that can operate at ppm catalyst loadings.
Experimental evidence hints at a process involving the transfer
of a hydride from the Si−H bond to plaꢀnum without an
1
2 At catalyst loadings below 0.01% no bis(amino)silanes are
produced in the presence of an excess of amines. We are
currently investigating the reasons for this effect.
oxidative addition pathway. Thus, the cationic platinum(II) 13 M. A. Ortuño, S. Conejero and A. Lledós, Beilstein J. Org.
Chem., 2013, 9, 1352.
complex 1a can be viewed as a new example of Lewis acid
catalyst in which their electrophilicity is transmitted to a silicon
atom upon coordination of the silane leading to a σ-SiH
1
1
4 Similar induction periods, associated to the need to generate
a vacant site, have been observed during dehydrocoupling
process of amino-boranes by Ruthenium complexes: A. E. W.
Ledger, C. E. Ellul, M. F. Mahon, J. M. J. Williams and M. K.
Whittlesey, Chem.—Eur. J., 2011, 17, 8704.
5 Poisoning experiments using a large excess of Hg (1000
equiv) proceeded at the same rate with identical kinetic
sigmoid profile, ruling out the formation nanoparticles or
colloids during the reaction.
1
9,19
complex (with a η coordination mode).
Current work is
geared at determining the mechanism of the dehydrocoupling
process.
We are grateful for financial support from the Junta de
Andalucía (FQM-2126) and Spanish MINECO (CTQ2013-40591-
P, CTQ2016-81797-REDC and CTQ2016-76267-P, FEDER
support) and Principado de Asturias (FC-15-GRUPIN14-006). P.
R. thanks the Junta de Andalucía for a research grant.
1
1
6 Kinetic profiles in all reactions with Ph
exhibit exponential shapes (see ESI for details).
7 Overlapping of this signal with those of the phenyl groups in
the Ph SiH and Ph SiH(NEt ) precluded integration of this
signal. However, in the reaction of Et SiH and NEt H the
relative integral of this signal is 2 with respect to the Et
fragments. Significantly, the chemical shifts of species
both experiments are identical, with independence of the
silane used, hinting at being the same in nature.
8 We have previously proved that ammonium salts can
protonate complex producing H and 1a (see ref. 7a).
2 2 2 2
SiH (and Et SiH )
2
2
2
2
2
2
2
Conflicts of interest
There are no conflicts to declare.
A
in
A
1
1
Notes and references
3
2
1
(a) G. Alcaraz and S. Sabo-Etienne, Angew. Chem., Int. Ed.,
010, 49, 7170; (b) H. C. Johnson, T. N. Hooper and A. S.
9 (a) M. C. Lipke, A. L. Liberman-Martin, T. D. Tilley, Angew.
Chem., Int. Ed., 2017, 56, 2260; (b) J. Y. Corey, Chem. Rev.
2
2
016, 116, 11291.
4
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