A Dialkyl Calcium Carbene Adduct: Synthesis, Structure, and Catalytic Cross-Dehydrocoupling of Silanes with Amines

Article abstract of DOI:10.1002/ejic.201900168

Eleven neutral ligands were investigated in promoting the calcium-catalyzed cross-dehydrocoupling reaction between phenylsilane and diisopropylamine. Among them, N-heterocyclic carbene (NHC) ligand IiPr₂Me₂ (1,3-diisopropyl-4,5-dimet

Full text of DOI:10.1002/ejic.201900168

A Journal of
Accepted Article
Title: A Dialkyl Calcium Carbene Adduct: Synthesis, Structure and
Catalytic Cross-Dehydrocoupling of Silanes with Amines
Authors: Nan Li and Bing-Tao Guan
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10.1002/ejic.201900168
European Journal of Inorganic Chemistry
COMMUNICATION
A Dialkyl Calcium Carbene Adduct: Synthesis, Structure and
Catalytic Cross-Dehydrocoupling of Silanes with Amines
Nan Li[a] and Bing-Tao Guan*[a,b]
Abstract: Eleven neutral ligands were investigated in promoting the
calcium-catalyzed cross-dehydrocoupling reaction between
(1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).[6c] Barrett, Hill
and co-workers synthesized the NHC adducts of calcium amide
with carbenes IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene) and IDipp (1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene).[6e] Very recently, Westerhausen achieved the synthesis
phenylsilane and diisopropylamine. Among which, an N-
heterocyclic carbene (NHC) ligand IiPr2Me2 displayed superior activity
and selectivity. The first neutral NHC-supported alkyl calcium complex
{(IiPr2Me2)2Ca[CH(SiMe3)2]2} 4 was subsequently synthesized and
characterized by NMR and X-ray crystallography. Complex 4
displayed high activity and excellent chemoselectivity in the catalytic
cross-dehydrocoupling reactions between bulky amines and silanes
under mild conditions. The asymmetric version was also achieved by
combining Ca[CH(SiMe3)2]2 with a chiral NHC ligand, affording
optically active silicon stereogenic silazane in 26% ee.
of calcium carbene adducts with
a
neutral, tridentate
bis(carbene)-based ligand.[6i] Despite these advances, the
synthesis of NHC adducts of alkyl calcium complexes is
undeveloped and the catalytic application of calcium carbene
adducts still remains limited.
The catalytic silane-amine dehydrogenative coupling reactions
generally represent an efficient method for silazane synthesis.
Various catalysts have been developed based on alkali metals,[9]
alkaline-earth-metals,[10]
early-transition-metals,[11]
metal
carbonyls[12] and other complexes.[13] The calcium catalysts were
firstly investigated by Harder and co-workers,[10a] who disclosed
Introduction
that
the
addition
of
polar
solvent
HMPA
Organocalcium chemistry is recently receiving more attention for
the plentiful reserves and nontoxicity of calcium element. Further
attractive features of calcium complexes comprise Lewis acidity
of calcium(II) cation, basicity and nucleophilicity of the alkyl or
amide anions. However, the low stability, low solubility and fast
ligand exchange of organocalcium complexes plagued its
blossom.[1] To conquer these disadvantages, great efforts have
been made by using bulky and multidentate ligands such as β-
(hexamethylphosphateamide) could significantly improve the
conversion (from 1% to 48% and 49%) in the catalytic
dehydrogenative silylation of aniline and 1-hexyne. Cui[11e]
,
Sadow[11g] and Nembenna[10h] found that the neutral NHC ligands
could accommodate the catalytic performances of ytterbium
amide，ytterbium and samarium dialkyls, and magnesium amide
complexes. Inspired by these fascinating experimental
phenomena, herein we investigate the behaviors of a series of
neutral ligands in the calcium-catalyzed cross-dehydrocoupling
reactions between silanes and amines.
diketiminate,[2]
β-diketonate,[3]
cyclopentadienyl,[4]
tris(pyrazolyl)borate,[2a,c],[5] and functionalization NHCs[6] to tune
the stability, solubility and reactivity. Some coordinating solvents
including tetrahydrofuran, tetrahydropyran, 1,4-dioxane and
pyridine were inventively employed in organocalcium chemistry
not only as solvents but also as neutral ligands.[7] Very recently,
using multidentate neutral Me4TACD and PMDTA ligands, Okuda
and Harder achieved the synthesis of calcium silyl, alkyl, amide
and hydride complexes, and realized their catalytic hydrogenation
reactions.[8] N-Heterocyclic carbenes (NHCs), as strong σ-donor
and poor -acceptor ligands, are widely used in both transition
metal complexes and main group metal complexes. The first
calcium-NHC adduct was synthesized by Arduengo with calcium
metallocene and a carbene IMe4 (tetramethylimidazol-2-ylidene).
Results and Discussion
We commenced our investigation with the cross-
dehydrocoupling reaction between phenylsilane 1a and
diisopropylamine 2a in C6D6 at room temperature (Table 1).
Considering the possible side reactions of calcium amides and
also the better catalytic potential of alkyl calcium complexes, we
preferred Ca[CH(SiMe3)2]2(THF)2 (5 mol%), a THF-coordinated
dialkyl calcium complex, as catalyst. After 8 hours, the desired
silazane product 3aa was obtained in only 10% yield (entry 1).
Considering the reported accelerating effect of HMPA and NHCs
ligands, we tested a series of neutral ligands (10 mol%) in the
calcium-catalyzed dehydrogenative coupling reaction. Neutral O-
donor ligands DME (dimethoxyethane) and triphenylphosphine
oxide, which are commonly used as stabilizers of alkaline-earth-
metal complexes, gave even lower conversions and yields
(entries 2-3). HMPA could improve the yield but not as efficiently
as it worked in Harder’s work (entry 4).[10a] Multidentate O-donor
ligand 18-crown-6 completely converted phenylsilane but gave
the desired product in 63% yield, suggesting that some side
reactions took place (entry 5). We then further carried out the
reaction with neutral N-donor ligands such as DMAP and TMEDA,
[6b]
Schumann and co-workers later synthesized more adducts
with calcium metallocenes and a steric bulkier carbene IiPr2Me2
[a]
[b]
Dr. N. Li, Prof. Dr. B.-T. Guan
State Key Laboratory and Institute of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071 (P.R.China)
E-mail: guan@nankai.edu.cn
http://skleoc.nankai.edu.cn/cms/item/view?table=team&id=638
Prof. Dr. B.-T. Guan
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300071 (China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900168
European Journal of Inorganic Chemistry
COMMUNICATION
but failed to get better yields (9% and 33%, entries 6-7).
Phosphonium ylide was subjected into the reaction as a neutral
ligand, and the desired product was obtained in 63% yield, along
with 72% conversion of phenylsilane (entry 8). Several neutral
NHC ligands were subsequently tested in the reaction. N-Aryl
complex. The ligand substitution reaction between IiPr2Me2 and
Ca[CH(SiMe3)2]2(THF)2 was carried out in toluene at room
temperature (Scheme 1). After removing the solvent, we got a
pale yellow solid (adduct 4). Its 1H NMR spectrum in C6D6 showed
that there was no more THF left, instead of coordinated NHC.
Table 1. Ligand screening[a]
Entry
1
Ligand (10 mol%)
--
Yield (%)[b]
10 (19)
<5 (8)
2
DME (10 mol%)
Ph3PO (10 mol%)
HMPA (10 mol%)
18-crown-6 (10 mol%)
DMAP (10 mol%)
TMEDA (10 mol%)
Ph3PCH2 (10 mol%)
IMes (10 mol%)
IDipp (10 mol%)
IMe4 (10 mol%)
IiPr2Me2 (10 mol%)
IiPr2Me2 (5 mol%)
IiPr2Me2 (15 mol%)
IiPr2Me2 (20 mol%)
--
Scheme 1. Preparation of dialkyl calcium NHC adduct 4.
3
<5 (10)
25 (34)
63 (96)
9 (24)
4
5
6
33 (40)
63 (72)
35 (39)
19 (28)
91 (97)
98 (100)[d]
90 (96)[d]
90 (100)[d]
40 (56)[d]
10 (17)
8 (19)
7
8
9
10
11
12
13
14
15
16[c]
17[c]
IiPr2Me2 (10 mol%)
[a] Reaction conditions: phenylsilane 1a (0.18 mmol), diisopropylamine 2a (0.18
mmol), dialkyl calcium (5 mol%), neutral ligand (10 mol%), hexamethylbenzene
(0.011 mmol), C6D6 (0.4 mL), 25 °C, 8 h. DME: dimethoxyethane; HMPA:
hexamethyl phosphate amide; DMAP: 4-dimethylaminopyridine; TMEDA:
tetramethylethylenediamine. [b] NMR yields (conversions in parentheses)
based on phenylsilane were recorded using hexamethylbenzene as an internal
standard. [c] Ca(HMDS)2 (5 mol%) as catalyst, 80 °C . [d] Reaction time: 12 h.
Figure 1. ORTEP drawing of 4 with 30% probability thermal ellipsoids. Selected
bond lengths [Å]: Ca-C1 2.659(3), Ca-C2 2.650(3), Ca-C3’ 2.484(2), Ca-C3
2.517(1), Ca-C4 2.528(3), Ca-H1 2.849(3), Ca-H1’ 2.813(2).
Both signals of the methylene (δ -1.47 ppm) and methyl groups (δ
0.48 ppm) of the alkyl anions shift slightly downfield compared to
that of the THF-coordinated complex Ca[CH(SiMe3)2]2(THF)2 (δ -
1.62 ppm and 0.36 ppm)[7n]. The 13C NMR signal of the carbene
carbon shifts drastically upfield (δ 193.9 ppm) compared to that of
the free carbene (δ 205.9 ppm). The Ca-NHC adduct 4 is
extremely air- and moisture-sensitive. A colorless crystal suit for
X-ray analysis was obtained by recrystallization from its solution
in toluene/hexane (1:2) at -30 °C in a glovebox (Figure 1). The
molecular structure of adduct 4 reveals a pseudo-tetrahedral
complex containing two alkyl ligands and two NHC ligands. The
Ca-C bond distances of the NHC ligands (2.65-2.66 Å) are in good
agreement with results (2.60-2.67 Å) previously reported in NHC-
calcocene adducts.[6c] The Ca-C bond distances of the alkyl
anions (Ca-C3, Ca-C4, 2.48-2.53 Å) are slightly longer than those
observed in the THF or dioxane-coordinated dialkyl calcium
complexes (2.49 and 2.48 Å)[7a,n], mainly because of the larger
steric hindrance of the carbene ligands. Another notable feature
is the short interatomic distances of Ca−H1 and Ca−H1’ (2.85
and 2.81 Å, respectively) and the small angles of Ca−C4−H1 and
Ca−C3−H1’ (98.89 and 97.17°, respectively), which suggests an
agostic interaction between the calcium center and the C-H bond
NHCs IDipp and IMes gave the product in 19% and 35% yields
(entries 9-10). To our delight, N-alkyl NHCs such as IMe4 and
IiPr2Me2 displayed much better efficiency and selectivity (entries
11-12). In particular, 10 mol% of IiPr2Me2 gave the product in a
high yield up to 98%. Using 5 mol% or 15 mol% of IiPr2Me2,
however, led to lower yields of 90% (entries 13-14). Further
increasing the amount of IiPr2Me2 to 20 mol% resulted in a much
reduced yield (40%, entry 15), suggesting that excess NHC ligand
greatly inhibits the catalytic activity of calcium catalyst. Calcium
amide Ca(HMDS)2, which was used as catalyst in some silane-
amine dehydrogenative coupling reactions[10c], failed to efficiently
catalyze the reaction between phenylsilane and diisopropylamine
even under elevated temperature (entries 16-17). This
comparison demonstrated the unique catalytic activity of the
combination of dialkyl calcium and the neutral NHC ligand.
As the combination of Ca[CH(SiMe3)2]2 with 2 equiv. of IiPr2Me2
displayed the best catalytic activity, we further attempted to
synthesize and isolate the NHC-coordinated dialkyl calcium
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10.1002/ejic.201900168
European Journal of Inorganic Chemistry
COMMUNICATION
of the alkyl group. It is noteworthy that complex 4 is the first
suggested that the participation of NHC ligand could inhibit the
reaction with steric bulky silanes, which could be the key reason
for the nice selectivity of the NHC adduct catalyst.
example of neutral NHC-supported dialkyl calcium complex.
Table 2. Substrates scope[a]
Table 3. Synthesis and ee value determination of compound 5
[a] Isolated yields
[b] Determined by chiral HPLC analysis
Now that the neutral carbene ligands could well coordinate with
dialkyl calcium complex and tune its activity and selectivity，we
would like to further explore the possibility in controlling the
stereoselectivity. Using chiral NHC 6a[14] and 6b[15] as ligands
(Table 3), we carried out the calcium catalyzed cross-
dehydrocoupling reactions between
a
prochiral silane
phenyl(naphthalen-1-yl)silane (1e) and benzylamine. The
reactions took place smoothly at room temperature, and 7 hour
later the desired product 3eh was obtained in high yields. The
moisture sensitive silazane 3eh was allowed to undergo the
boronation reaction to give compound 5, who is stable for the
HPLC analysis.[11h] Using the more rigid chiral carbene 6b as
ligand, compound 5 was obtained in 70% yield with 26% ee value,
which suggested that the chiral carbene ligand did endue the
dialkyl calcium complex with stereoselectivity.
Conclusions
[a] Reaction conditions: silane 1 (0.26 mmol), amine 2 (0.26 mmol), adduct 4 (5
mol%), C6D6 (0.45 mL), 25 °C. Yields were recorded by 1H NMR analysis using
hexamethylbenzene as an internal standard. [b] Amine 2 (0.13 mmol). [c]
Ca[CH(SiMe3)2]2(THF)2 was used as catalyst.
In summary, we systematically evaluated the behavior of a
series of neutral ligands in
dehydrocoupling reaction between
a
calcium-catalyzed cross-
phenylsilane and
With dialkyl Ca-NHC adduct 4 in hand, we then tested its
catalytic activity in silane-amine cross-dehydrocoupling reactions.
In general, the adduct 4 is highly active for the reactions of PhSiH3
with bulky amines such as HN(iPr)2, HNCy2, HN(SiMe3)2, H2N(tBu)
and DippNH2, giving the desired products (Table 2, 3aa-3ae) in
good yields (90-99%) under mild conditions. Moreover, the
reactions of diphenylsilane (1c) with bulky primary or secondary
amines were also applicable, and produced mono(amino)silanes
(3cd-3cg) in nearly quantitative yields (96-99%). Gratifyingly,
some other secondary silanes such as methylphenylsilane (1b),
methyl(naphthalen-1-yl)silane (1d) and phenyl(naphthalen-1-
yl)silane (1e) smoothly underwent the catalytic dehydrogenative
coupling reactions with H2N(tBu) or benzylamine, and afforded
silazanes (3bd, 3dd, 3eh) in excellent yields (98-99%). The cross-
dehydrocoupling reaction between sterically bulky triphenylsilane
(1f) and benzylamine, which was achieved successfully with
Ca(HMDS)2 or Ba(HMDS)2 catalysts[10c], failed to take place when
using adduct 4 as catalyst. However, in the absent of NHC ligands,
dialkyl calcium complex Ca[CH(SiMe3)2]2(THF)2 (5 mol%) itself as
catalyst could smoothly drive on the reaction, and gave the
desired product silazane 3fh in 97% yield. These results
diisopropylamine. We found that N-heterocyclic carbene IiPr2Me2
displayed the best activity and selectivity. Thus, we synthesized
the first neutral NHC-coordinated alkyl calcium complex 4 and
characterized its structure with NMR and X-ray crystallographic
analysis. The dialkyl calcium NHC adduct showed nice catalytic
activity in the cross-dehydrocoupling reactions between silanes
and bulky amines under mild conditions. Furthermore, an
asymmetric version was also realized in this catalytic system by
combining dialkyl calcium catalystwith a chiral NHC ligand. These
results suggested that NHCs could coordinate well with dialkyl
calcium complex and successfully tune its activity,
chemoselectivity and even stereoselectivity.
Experimental Section
In a glovebox, to a solution of Ca[CH(SiMe3)2]2(THF)2 (0.19 mmol, 0.10 g)
in toluene (2 mL) was added a solution of 1,3-diisopropyl-4,5-dimethyl-
imidazol-2-ylidene (0.38 mmol, 0.07 g, 2 equiv.) in toluene (3 mL). The
mixture was stirred at room temperature for 1 h. After removing the
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10.1002/ejic.201900168
European Journal of Inorganic Chemistry
COMMUNICATION
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Acknowledgements
We gratefully acknowledge the State Key Laboratory of
Elemento-Organic Chemistry for generous start-up financial
support. This Project was supported by the NSFC (21202086).
Keywords: calcium • carbene ligands • adduct • cross-
dehydrocoupling • silazane
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Nan Li and Bing-Tao Guan*
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A Dialkyl Calcium Carbene Adduct:
Synthesis, Structure and Catalytic
Cross-Dehydrocoupling of Silanes
with Amines
The first neutral NHC-supported alkyl calcium complex {(IiPr2Me2)2Ca[CH(SiMe3)2]2} was synthesized and structurally characterized.
Thanks to the neutral NHC ligand, the complex displayed excellent reactivity and chemoselectivity on catalytic cross-dehydrocoupling
of silanes with amines under mild conditions.
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