[(NHC)Yb{N(SiMe3)2}2]-catalyzed cross-dehydrogenative coupling of silanes with amines

Article abstract of DOI:10.1002/anie.201205317

Top cat: [(NHC)Yb{N(SiMe₃)₂}₂] adducts (NHC=N-heterocyclic carbene) are efficient catalysts for catalytic cross-dehydrogenative coupling of silanes with a range of primary and secondary amines to yield silylamines in high

Full text of DOI:10.1002/anie.201205317

Angewandte
Chemie
DOI: 10.1002/anie.201205317
Rare-Earth Catalysis
[(NHC)Yb{N(SiMe3)2}2]-Catalyzed Cross-Dehydrogenative Coupling
of Silanes with Amines**
Weilong Xie, Hongfan Hu, and Chunming Cui*
Silylamines are important silicon compounds that have been
used as silylation and coupling agents, ligands for organome-
tallic compounds, and precursors for Si, N polymeric materi-
als.[1] Catalytic cross-dehydrogenative coupling [CDC;
Eq. (1)] of amines with silanes presents the most attractive
and atom-economic synthetic approach for silylamines
because the only by-product is hydrogen. Several well-
defined metal complexes including metal carbonyls,[2] f-
element, early-transition-metal,[3] and magnesium com-
plexes[4] have been successfully employed for catalytic Si–N
coupling reactions. However, the control of the selectivity for
Si–N coupling has proved to be problematic because the
reactions can lead to several products with different Si/N
ratios, including polysilazanes, as a result of multiple dehy-
drogenative-coupling steps; this issue is especially common
for the coupling of primary silanes and amines. Thus, the
development of highly reactive and selective catalytic systems
for reactions of various amines and silanes is highly desirable.
been reported,[10] there are only a couple of reports on NHC
adducts of rare-earth amides.[11] We recently reported the
dehydrosilylation reaction of [(thf)2Yb{N(SiMe3)2}2] (1) with
ꢀ
an aminosilane ArNHSiH3; in this reaction the Yb N bond
undergoes s-bond metathesis with the silane under mild
reaction conditions, thus leading to the Si–N coupling and the
formation of the first ytterbium silaimine complex.[12] The
results suggested that ytterbium silylamide 1 might be
a suitable catalyst for Si–N coupling reactions. Herein, we
report the catalytic cross-dehydrogenative-coupling reactions
of silanes with amines by [(NHC)Yb{N(SiMe3)2}2] adducts.
Remarkably, the catalytic activity and selectivity can be
modulated by NHCs.
Initially, the stoimetric reaction of 1 with PhSiH3 in C6D6
at room temperature was investigated. 1H NMR analysis
disclosed the almost quantitative formation of PhSiH2N-
(SiMe3)2, thus indicating a clean Si–N coupling. Therefore, we
conducted the reactions of PhSiH3 with equimolar amounts of
three secondary amines (HN(SiMe3)2, HN(iPr)2, and HNEt2)
and 5 mol% loading of catalyst 1. However, the reactions of
the two bulky amines yielded the expected 1:1 coupling
products only in very low conversions of 24% and 3%,
respectively, whereas the reaction of HNEt2 gave the two
products PhSiH2NEt2 and PhSiH(NEt2)2 with an almost
quantitative conversion. These preliminary studies indicate
that 1 is not a satisfactory catalyst for the coupling reactions
and some modifications to the catalyst should be made to
improve the activity and selectivity. It was found that the
combination of 1 with a suitable N-heterocyclic carbene
constitutes an efficient catalytic system for the Si–N coupling
reactions. Thus, it was essential to study the reactions of 1 with
NHCs to probe the roles of NHCs in the catalytic reaction. As
we expected, the reactions of 1 with one equivalent of 1,3-
diisopropyl-4,5-dimethyl-imidazol-2-ylidene (IiPr) and 1,3-
bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes) yielded
the NHC adducts 2 and 3, respectively, in high yield
(Scheme 1). Adducts 2 and 3 were fully characterized by
1H NMR, 13C NMR, and IR spectroscopy, and elemental
analysis. The 31C NMR spectra of 2 and 3 display CNHC
resonances at d = 197.9 and 205.4 ppm, respectively, which
are very similar to those for the reported NHC adducts of
cyclopentadienyl ytterbium complexes.[13] The molecular
structure of 3 was determined by single-crystal X-ray analysis.
Single crystals of 3 suitable for X-ray single-crystal
analysis were obtained from toluene solution at ꢀ408C, and
the structure is shown in Figure 1 together with selected bond
distances and angles. Complex 3 is monomeric with a three-
cat:
PhSiH3 þ HNR2
PhH SiꢀNR
ð1Þ
ƒƒ!
2
2
ꢀH2
Homoleptic
rare-earth-metal
silylamides
[Ln{N-
(SiMe3)2}n] (n = 2,3) are readily available in large amounts
ꢀ
and feature reactive Ln N bonds for s-bond metathesis and
insertion reactions.[5] Therefore, they are ideal choice for
catalytic applications and have been successfully employed as
catalysts for ring-opening polymerization reactions of lac-
tones, intramolecular hydroamination, hydroxygenation reac-
tions of unsaturated hydrocarbons, as well as several others.[6]
To modify the catalytic performance and extend the catalytic
applications of rare-earth-metal amides, various bulky anionic
ligands have been designed for the preparation of ligand-
supported amides.[7] Immobilization of these amide catalysts
has also been studied by the research groups of Anwander
and Montreux.[8] Given the excellent catalytic performance of
ubiquitous N-heterocyclic carbenes (NHCs) in transition
metal catalysis,[9] it is surprising that the catalytic applications
of NHC adducts of rare-earth amides have been virtually
unexplored. Despite the fact that a large number of rare-earth
complexes having NHC-functionalized anionic ligands have
[*] W.-L. Xie, H.-F. Hu, Prof. Dr. C.-M. Cui
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin, 300071 (China)
E-mail: cmcui@nankai.edu.cn
Homepage: skleoc.nankai.edu.cn
[**] We are grateful to the National Natural Science Foundation of China
and 973 Program (Grant No. 2012CB821600) for the support of this
work.
ꢀ
coordinate ytterbium atom. The Yb CNHC bond length of
2.600(3) ꢀ in 3 is only marginally longer than those observed
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205317.
in NHC adducts of substituted ytterbocenes (2.552(4) and
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silylamide 1 and steric factors of NHC have significant
influence on the activity and selectivity for the catalytic
reactions.
As complex 3 gave the best results for the Si–N coupling
of the above-mentioned reactions, the coupling reactions of
several primary and secondary amines with PhSiH3 catalyzed
by 3 (5 mol%) were investigated. In general, 3 is highly active
for the reactions of the selected amines, as the reactions
showed very high conversions after 1 h. Notably, no efficient
catalysts for the coupling of the bulky amines HN(SiMe3)2,
HNiPr2, and DippNH2 with silanes have been reported
previously. The reaction products are largely dependent on
the steric factors of amines. The reactions with bulky primary
amines can be controlled by changing the molar ratios of the
two reactants to give a single product (Table 2). The reaction
with 4 equivalents of iPrNH2 afforded the tris(amino)silane
PhSi(NHiPr)3 (Table 2, entry 4), whereas reactions with the
more-bulky amines, tBuNH2 and DippNH2 (Dipp = 2,6-
iPr2C6H3), yielded bis(amino)silanes (Table 2, entries 5 and
6). Conversely, reactions of these amines with 2.2 equivalents
of PhSiH3 yielded disilazanes (Table 2, entries 7–9). Reac-
tions of the bulky secondary amines (SiMe3)2NH and iPr2NH
catalyzed by 3 gave mono(amino)silanes in nearly quantita-
tive yield (Table 2, entries 1 and 2), whereas when 3 equiv-
alents of Et2NH was used the bis(amino)silane PhSiH(NEt2)2
was obtained as single product in excellent yield. However,
the reaction of the aryl-substituted secondary amine Ph2NH
with PhSiH3 catalyzed by 3 proved to be sluggish. Instead, this
Scheme 1. Synthesis of 2 and 3. Mes=2,4,6-Me3C6H2.
Figure 1. Ortep drawing of 3 with thermal ellipsoids set at 30%
probability. Hydrogen atoms have been omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Yb1–N4 2.317(3), Yb1–N3 2.323(2),
Yb1–C1 2.600(3); N4-Yb1-N3 122.21(9), N4-Yb1-C1 114.24(9),
N3-Yb1-C1 117.28(10).[15]
Table 1: Comparison of activity and selectivity of catalysts 1, 2, and 3 for
the coupling of PhSiH3 with (SiMe3)2NH, iPr2NH, and HNEt2.
2.598(3) ꢀ).[13] Adduct 3 is thermally stable in solution even at
1208C but 2 slowly decomposed at this temperature.
The NHC Ytterbium silylamide complexes 2 and 3 were
examined as catalysts for the dehydrogenative-coupling
reactions of the three secondary amines HN(SiMe3)2, HN-
(iPr)2, and HNEt2 with one equivalent of PhSiH3 (Scheme 2)
Cat.[a]
R=SiMe3
R=iPr
R=Et
product [%]
product [%]
product [%]
1
2
3
A (24)
A (58)
A (98)
A (3)
A (13)
A (100)
A (11) + B (89)
A (50) + B (50)
A (0) + B (100)
[a] Reaction conditions: amine (0.18 mmol, 1.2 equiv), silane
(0.15 mmol, 1.0 equiv), catalyst (5.0 mol% ), C6D6 (0.5 mL), room
temperature, 1 h. Conversions were obtained on the basis of the
consumption of PhSiH3 from integration of signals in the 1H NMR
spectra.
Scheme 2. Dehydrogenative coupling catalyzed by 1, 2, and 3.
Table 2: Reactions of various amines with PhSiH3 catalyzed by 3
(5 mol%).[a]
to evaluate the steric effects of NHCs on the catalytic
reactions. These results were compared with those for the
reaction with 1 as the catalyst under the same conditions
(Table 1). For the coupling of PhSiH3 with the two bulky
secondary amines HN(SiMe3)2 and HN(iPr)2, the NHC
complexes 2 and 3 show increased catalytic activity compared
to that of 1, and complex 3 with the bulky N-Mes-substituted
NHC displays the highest activity with almost complete
conversion in 1 hour (Table 1). For HNEt2, the three catalysts
showed comparable activity but only catalyst 3 produced the
diaminosilane B as a sole product. Furthermore, in the cases
with 1 and 2 as catalysts, noticeable amounts of Ph2SiH2 and
SiH4 were detected by NMR analysis. These results indicate
that the NHC-containing complexes are far superior to
Entry PhSiH3 (equiv) Amine
(equiv)
Product
Yield [%][b]
1
1
(SiMe3)2NH
iPr2NH
Et2NH (3.0)
iPrNH2 (4.0)
tBuNH2 (4.0)
PhSiH2N(SiMe3)2 98 (85)
PhSiH2NiPr2
PhSiH(NEt2)2
PhSi(NHiPr)3
PhSiH(NHtBu)2
2
1
100 (89)
100 (90)
99 (83)
3
1
1
1
1
2.2
2.2
2.2
4[c]
5
100 (94)
6[d]
7
DippNH2 (4.0) PhSiH(NHDipp)2 95 (59)
iPrNH2
tBuNH2
DippNH2
(PhSiH2)2NiPr
(PhSiH2)2NtBu
(PhSiH2)2NDipp 90 (64)
95 (75)
100 (77)
8
9[d]
[a] Reaction conditions: 3 (5 mol%) in benzene or C6D6. [b] NMR yields,
the values in paentheses refer to the yield of isolated product.
[c] Reaction time: 6 h. [d] Reaction time: 24 h.
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reaction can be catalyzed by 2 at 408C in C6D6 (NMR scale) to
give the aminosilane Ph2NSiH2Ph in 61% yield in 15 h.
The reactions of the secondary silanes Ph2SiH2 and
PhMeSiH2 with primary amines and secondary amines
catalyzed by 3 (5 mol%) gave 1:1 coupling products, exclu-
sively, in high yields (Table 3). The reaction of the bulky
primary silane MesSiH3 (Mes = 2,4,6-Me3C6H2) with tBuNH2
gave the 1:1 coupling product MesSiH2(NHtBu) (Table 3,
entry 9). However, 3 is not an effective catalyst for the
coupling reactions of the secondary silanes with the bulky
amines, that is, (SiMe3)2NH and iPr2NH. Interestingly, the
reaction of Ph2SiH2 with the bulky secondary amine iPr2NH
Table 3: Reactions of bulky silanes with amines catalyzed by 3
Figure 2. Rate of the formation of silylamine (u) versus the concen-
tration of the catalyst 3. This plot shows the first-order dependence of
the reaction on the concentration of 3 (C6D6, 298 K).
(5 mol%).[a]
Entry
Silane
Amine
Product
Yield [%][b]
1
2
Ph2SiH2
PhMeSiH2
Ph2SiH2
Ph2SiH2
PhMeSiH2
Ph2SiH2
PhMeSiH2
Ph2SiH2
MesSiH3
Et2NH
Et2NH
Ph2Si(H)NEt2
PhMeSi(H)NEt2
Ph2Si(H)NiPr2
Ph2SiH(NHiPr)
PhMeSiH(NHiPr)
Ph2SiH(NHtBu)
PhMeSiH(NHtBu)
Ph2SiH(NHDipp)
MesSiH2(NHtBu)
100 (85)
87 (86)
82 (73)
100 (87)
100 (84)
100 (78)
100 (90)
99 (64)
3[c]
4
iPr2NH
iPrNH2
iPrNH2
tBuNH2
tBuNH2
DippNH2
tBuNH2
(SiMe3)2 (Si/N ratios 1: ꢁ 6, Table S3) significantly suppresses
the reaction to give very low or no conversions within
30 minutes.
5
6
7
The mechanism of the reaction has yet to be investigated.
However, it is possible that the key intermediate for the
coupling reaction might be a NHC-stabilized ytterbium
hydride formed by s-bond metathesis of 3 with silanes. The
stoichiometric reaction of 3 with PhSiH3 at different concen-
trations in C6D6 was monitored by 1H NMR analysis and the
formation of PhSiH2N(SiMe3)2 was observed. However, the
117Yb NMR spectra showed no peaks corresponding to a new
product in these cases probably because of the instability of
the intermediate under the experimental conditions. The
reactions shown in Table 2 (entries 1, 2 and 3) were per-
formed with catalyst by 1 in the presence two equivalents of
IMes, and similar results were obtained as with 3 as the
catalyst. The reaction of (SiMe3)2NH with PhSiH3 catalyzed
8[d]
9
100 (86)
[a] Reaction conditions: 3 (5 mol%) in benzene or C6D6. [b] NMR yields,
the values in parentheses refer to the yields of isolated product. [c] 2 (10
mol%), 1008C, 24 h. [d] Reaction time: 12 h.
can catalyzed by 2 to give the 1:1 coupling product in high
yield (Table 3, entry 3). The results indicated that the steric
factors of both substrates and the NHCs have significant
effects on the coupling products and reaction conditions that
were required. The coupling of PhSiH3 with primary amines
proved to be complicated, and yielded a mixture if the ratios
of the two reactants were not optimized. However, polymeric
silazanes were not detected in these reactions.
by
1
in the presence of the bidentate donor
The experimental results demonstrated that 2 and 3 are
efficient catalysts for reactions of a wide range of amines and
silanes, even for bulky secondary amines, thus indicating that
the coordination of NHCs to the ytterbium effectively
modulates the catalytic activity. To isolate possible inter-
mediates, the reaction of 3 with PhSiH3 was investigated
under various reaction conditions. Unfortunately, attempts to
isolate the putative (NHC)YbH2 have been unsuccessful to
date. As the reaction of PhSiH3 with (SiMe3)2NH catalyzed by
3 is clean and yielded PhSiH2N(SiMe3)2 as the sole product
when a range amine/silane ratios were used, the influence of
the amount of 3 on the reaction rate and conversions was
studied by NMR spectroscopy in C6D6 at room temperature.
The amount of the catalyst has a substantial influence on the
reaction rate and conversions. The reaction rate increases
linearly with an increase of the amount of catalyst (Figure 2),
and the conversions also increased when the amount of the
catalyst was increased (see the Supporting Information,
Table S1). The use of an excess of PhSiH3 (Si/N ratios from
1:1 to 6:1; see the Supporting Information, Table S2) has
a very limited effect on the conversions in 30 minutes,
whereas the use of a large excess amount of amine HN-
Me2NCH2CH2NMe2 resulted in a very low conversion (ca.
4%) in 1 h. These experiments indicate that the NHC plays
a key role in the catalytic reactions probably as a result of the
strong s-donor properties of NHC ligands. It is known that
homoleptic rare-earth-metal hydrides tend to oligomerize as
precipitates in solution.[14] The presence of an NHC on the
ytterbium center may prevent rapid oligomerization of the
hydride in solution and allow it survive for a long enough time
for the subsequent hydrogen elimination reaction with amines
to occur, thus significantly improve the catalytic performance.
The steric effects of NHCs on the catalytic reactions shown in
Table 1 further demonstrated the important role of NHCs in
the catalytic reactions.
In summary, we have shown that NHC adducts of
a ytterbium amide catalyze cross-dehydrogenative-coupling
of amines and silanes to yield various coupling products. The
rare-earth-metal NHC silylamide complexes are highly reac-
tive catalysts for the coupling reactions, as indicated by the
unprecedented high yielding coupling of several bulky amines
with silanes. In addition, the selective formation of a single
coupling product can be realized by changing the molar ratios
of the two substrates in the case of the primary silane PhSiH3.
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rare-earth NHC silylamide adducts are currently in progress.
Experimental Section
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Preparation of 3: A solution of 1,3-bis(2,4,6-trimethylphenyl)imid-
azol-2-ylidene (0.91 g, 3.0 mmol) in toluene (30 mL) was added to
a solution of Yb[N(SiMe3)2]2(THF)2 (1.91 g, 3.0 mmol) in toluene.
The resulting solution was stirred for 12 h at room temperature and
then concentrated under vacuum. After storage at ꢀ408C overnight 3
was obtained as reddish black crystals (2.1 g, 88%). m.p: 2488C.
1
ꢀ
H NMR (C6D6, 400 MHz): d = 0.21 (s, 36H, SiMe3), 2.01 (s, 12H, Ar
ꢀ
=
Me), 2.12 (s, 6H, Ar Me), 6.05 (s, 2H, NCH CHN), 6.81 ppm (s, 4H,
Ar H); C NMR (C6D6, 100.64 MHz): d = 5.69 ( SiMe3), 12.28 (Ar
Me), 20.88 (Ar Me), 122.0 (NCH CHN), 130.2, 134.9, 135.4, 139.7
(aromatic carbons), 205.4 ppm (NCN). Elemental anal. calcd for
C33H60N4Si4Yb: C 49.65, H 7.58, N 7.02; found: C 50.21, H 7.52, N,
6.31.
13
ꢀ
ꢀ
ꢀ
=
Further details of the synthesis and characterization of 2 and 3,
catalytic processes, and characterization of the resulting silylamines
are given in the Supporting Information.
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Received: July 5, 2012
Published online: October 4, 2012
Keywords: amines · cross-coupling · N-heterocyclic carbenes ·
.
silanes · ytterbium
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