Hydrosilylation of olefins catalyzed by well-Defined cationic aluminum complexes: Lewis acid versus insertion mechanisms

Article abstract of DOI:10.1021/acscatal.6b01694

The cationic aluminum complex [NacNacAlH]⁺ (2; NacNac = CH{C(Me)N(2,6-Prⁱ₂C₆H₃)}₂) can be easily generated from NacNacAlH₂ by hydride abstraction and functions as a catalyst for the hydrosilylation of olefins and alkynes. Mechanistic studies suggest that, although olefin insertion into the Al?H bond is very facile, the catalysis does not proceed by an insertion/metathesis mechanism but likely by Lewis acid activation. Stoichiometric reactions of 2 with alkynes furnished unexpected products of CC addition across the NacNacAl moiety to give tripodal aluminum cations, which are also potent catalysts for the hydrosilylation of alkynes.

Full text of DOI:10.1021/acscatal.6b01694

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Article
Hydrosilylation of olefins catalyzed by well-defined cationic
aluminum complexes: Lewis acid versus insertion mechanisms
Kayla Jakobsson, Terry Chu, and Georgii I. Nikonov
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01694 • Publication Date (Web): 23 Sep 2016
Downloaded from http://pubs.acs.org on September 23, 2016
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Hydrosilylation of olefins catalyzed by well-defined
cationic aluminum complexes: Lewis acid versus
insertion mechanisms
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Kayla Jakobsson, Terry Chu, Georgii I. Nikonov*
Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario,
Canada, L2S
Abstract
+
i
The cationic aluminum complex [NacNacAlH] (2, NacNac = CH{C(Me)N(2,6-Pr C H )} ) can
2
6
3
2
be easily generated from NacNacAlH by hydride abstraction and functions as a catalyst for the
2
hydrosilylation of olefins and alkynes. Mechanistic studies suggest that, although olefin insertion
into the Al–H bond is very facile, the catalysis does not proceed by an insertion/metathesis
mechanism but likely by Lewis acid activation. Stoichiometric reactions of 2 with alkynes
furnished unexpected products of C≡C addition across the NacNacAl moiety to give tripodal
aluminium cations, which are also potent catalysts for the hydrosilylation of alkynes.
Keywords: aluminum, homogeneous catalysis, hydrides, alkene, hydrosilylation
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Introduction
The development of more economical and environmentally benign surrogates for the common
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catalysts based on precious metals is an intensive area of current research. In this regard, main
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group compounds attract significant attention due to the low cost and toxicity of many s and p
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block elements.
At 8.1%, aluminum is the third most abundant element in Earth‘s crust,
which when coupled with its low toxicity makes it a very attractive candidate for catalyst design.
Applications of aluminum in catalysis finds some spectacular classical examples in the
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Meerwein-Ponndorf-Verley (MPV) reduction of carbonyls and Friedel-Crafts alkylation and
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acylation of aromatics. Aluminum complexes have also been applied as catalysts for the ring-
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opening polymerization of cyclic esters. More recently, Berben and co-workers have used
aluminum complexes stabilized by the bisiminopyridine ligand to catalyze the dehydrogenative
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coupling of benzylamine, dehydrogenation of formic acid, and electrocatalytic production of
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hydrogen. The Wright lab have shown that aluminum complexes can catalyze the
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dehydrocoupling reaction of amine–borane as well as amines and silanes while an α-diimine
complex of aluminum was demonstrated by Graves to be a competent catalyst for the
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epoxidation of alkenes. Frustrated Lewis pairs containing aluminum as the Lewis acidic
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partner have been utilized as a polymerization catalyst, phase-transfer catalyst, and a catalyst
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for amine–borane dehydrocoupling. In the field of catalytic reduction, simple aluminum
compounds, in particular AlCl , have been used as catalyts for hydrosilylation since
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978.
Very recently, several well-defined molecular aluminum catalysts have been
developed (Chart 1). Thus Roesky et al. showed that the neutral aluminum hydride
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NacNacAl(OTf)H (NacNac = CH{C(Me)N(2,6-Pr C H )} ) catalyzes the hydroboration of
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aldehydes and ketones with pinacolborane as the reducing agent whereas the related complex
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Et
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NacNacAlH ( NacNac = CH{C(Me)N(2,6-Et C H )} ) catalyzes the hydroboration of
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alkynes. More relevant to the current study is the very small class of cationic aluminum
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complexes represented by Bergman’s tripodal complex [Tp*AlMe] (Tp* = hydro-tris(1,3-
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dimethylpyrazol-1-yl)borate) and Wehmschulte’s cation [Et Al] used in the catalytic
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hydrosilylation of carbonyls (aldehydes, ketones, aldimines, and lactone) and CO2,
respectively. We have recently reported that NacNacZnH (1) is an active catalyst for the
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chemoselective hydrosilylation of aldehydes, ketones, and nitriles. We then supposed that the
+
isolobal aluminum complex [NacNacAlH] (2) could show superior activity due to its increased
Lewis acidity (Chart 2). Therefore, we opted to prepare this compound and study its activity as a
hydrosilylation catalyst. This endeavor resulted in the discovery of aluminum catalyzed
hydrosilylation of alkenes and alkynes which appears to proceed via a Lewis acid mechanism.
This finding is important as hydrosilylation catalyzed by s- and p-block metals is little
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21,22,31,32
studied
and its application to alkenes and alkynes is particularly scarce.
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Chart 1. Well-defined aluminum catalysts for hydrosilylation.
Chart 2. Isolobal relationship between zinc compound 1 and aluminum cation 2.
Results and Discussion
Jordan et al. have previously reported that methyl abstraction from NacNacAlMe by the trityl
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cation leads to the isolable cation [NacNacAlMe] . Similar treatment of dihydride NacNacAlH
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+
F
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
F
with [Ph C] [BAr ] (Ar = C F ) afforded the cation 2. The choice of solvent happened to be
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crucial for this reaction because 2 precipitates in the form of an oil from nonpolar solvents (e.g.
aromatics, hexanes) and decomposes in polar solvents such as CH Cl and THF. Eventually, we
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2
found that 2 can be cleanly generated in chlorobenzene and is stable for at least a day in this
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solvent. Initial substrate screening for catalytic hydrosilylation showed that hard donors, such as
nitriles, pyridines, and ketones, coordinate to the Lewis acidic cation 2 too tightly, resulting in
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1a, b,34
significantly decreased activity in comparison with catalysis by related calcium,
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magnesium, and zinc NacNac complexes. We therefore shifted our goals towards softer
substrates: alkenes and alkynes. Gratifyingly, when 3,3-dimethyl-1-butene was reacted with
methylphenylsilane in the presence of 5 mol% of 2, full conversion of the alkene occurs in less
than 10 min at room temperature. However, a mixture of products was formed due to the very
facile redistribution of the starting and final silane products (Table 1, entry 1). Changing the
silane to dimethylphenylsilane also produced mixtures of products (Table 1, entry 2). Finally, use
of the more robust triethylsilane allowed formation of only one product with the same conversion
in the same time span (Table 1, entry 3), but interestingly this product was not the predicted (3,3-
dimethylbutyl)triethylsilane but rather (2,3-dimethylbutyl)triethylsilane, likely formed as a result
of methyl migration in an intermediate silylated carbocation (vide infra). With this choice of
silane, the conditions were further optimized by varying the catalyst loading from 5% to 0.1%.
While the reaction is still very fast with a 1% catalyst load, it slows down significantly at 0.5%
and stops at 0.3% likely due to catalyst poisoning by impurities in the substrate. To probe the
sensitivity of this catalytic system towards oligomerization of olefins, a control experiment was
conducted. Carrying out the reaction in the absence of silane did result in oligomerization,
however the reaction took much longer (18h; Table 1, entry 8). Thus our optimal conditions were
set at 1 mol% catalyst loading with triethylsilane as the reducing agent.
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Table 1. Optimization of hydrosilylation conditions.
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a
Entry Cat (mol%)
Silane
H SiMePh
2
Product
mixtures
mixtures
Conv. (%)
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b
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>99
HSiMe
Ph
>99
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HSiEt
HSiEt
HSiEt
HSiEt
HSiEt
None
>99
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>99
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d
Oligomerization
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b
Conversions were determined by H NMR spectroscopy. Mixtures of products due to
c
d
parallel redistribution. After 4 days at RT. After 18h at RT, oligomerization occurred.
Next, the scope of the catalysis was explored. Terminal alkenes and cyclohexene all showed
quantitative conversion to the alkylsilanes in less than 10 min at room temperature (Table 2,
entries 1–4). For tri- and tetra-substituted alkenes, a higher catalyst loading (5 mol%) was
required to achieve similar conversions within the same time span (Table 2, entries 5–7). Lower
catalyst loadings resulted in only minor reactivity or none at all. For the tri-substituted cyclic
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alkene, 1-methyl cyclohexene (Table 2, entry 5), H NMR data obtained in chlorobenzene were
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very close to the spectral data obtained in C D Br for the same product prepared in a
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phosphorane-catalyzed hydrosilylation and the relative cis-orientation of the Me and SiEt3
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groups were assigned, consistent with trans hydrosilylation. Trans-addition was also observed
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21b
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previously by Yamamoto and Jung for the AlCl -catalyzed hydrosilylation of cyclic alkenes
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and by Gevorgian for B(C F ) -catalyzed hydrosilylation. The trans-addition is easily
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explained by the stabilization of an intermediate carbocation as a result of conjugation with the
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
-silyl group, which should favor the addition of hydride from the opposite, less sterically
encumbered side of the substrate. Although conversions into hydrosilylation products were high,
the yield of the target silanes was rather moderate due to the formation of various redistribution
3
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products. This fact alone suggests the intermediacy of solvated silylium ions, e.g.
+
[
R Si] *ClPh, as intermediates in the reaction (vide infra). In the case of styrene, partial
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Table 2. Scope of alkene hydrosilylation catalyzed by 2.
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a
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Conversions into hydrosilylation products and yields were determined by H NMR
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spectroscopy using ferrocene as an internal standard. The other hydrosilylation product is
c
(
PhCH CH ) SiEt (21%). Catalyst loading could be decreased to 0.75 mol% to achieve the
2
2 2
2
d
same conversion in 15 min. Catalyst loading of 5 mol%.
We then turned to applying this catalyst towards the hydrosilylation of more robust substrates,
alkynes. Aluminum-catalyzed hydrosilylation of alkynes is known but limited examples are
22a-d
found in the literature.
Heating a reaction mixture with phenylacetylene for multiple days
resulted in a polymeric product (Table 3, entry 1). We hypothesized that the substrate had
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initially undergone hydrosilylation to give a styrene derivative which was then polymerized
under these relatively harsh conditions. This observation limited our choice of substrates to alkyl
substituted acetylenes and internal alkynes without the presence of aromatic groups. We also
chose to use milder conditions, i.e. room temperature. Gratifyingly, we observed the
hydrosilylation of some terminal and internal alkynes. Since the signals of the substrate generally
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overlapped with the signals of the product in the H NMR spectrum, conversions were
determined based on the consumption of silane. For terminal alkynes, 24 h at room temperature
was required to achieve high conversions, albeit with low yields (Table 3, entries 2–3). For
diethyl acetylene, 25 h at room temperature was required to attain quantitative conversion (entry
4
). In the case of cyclohexyl acetylene and cyclohexylmethyl acetylene, hydrosilylation resulted
3
in cis-addition of the Si–H bond evidenced by the observation of large trans J
couplings
H–H
between the alkene hydrogens. Only one major alkenyl silane product for each reaction was
observed, however various other silane products, formed as a result of silane redistribution, were
1
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identified by H- Si HSQC correlation.
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Table 3. Scope of alkyne hydrosilylation mediated by 2.
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Entry
Substrate
Main Product
Polymer
Time
Conv. (%)
Yield (%)
b
1
5 days
>99
-
2
26 h
89
29
3
4
24 h
25 h
69
7
>99
16
a
1
Conversions and yields were determined by H NMR spectroscopy using ferrocene as an
b
internal standard. Reaction heated to 70°C.
Divergent mechanistic scenarios were proposed for the AlCl -catalyzed hydrosilylation of
3
alkenes. Thus, Oertle and Wetter proposed in situ formation of HAlCl which then
2
hydroaluminates olefins to give an alkyl aluminum followed by a Si–H/Al–C metathesis with the
2
1a
21b
21c
hydrosilane. In contrast, Yamamoto and Jung favored the Lewis acid promoted formation
of a silylium ion, its addition to the alkene to give a carbocation culminated by hydride transfer
from the hydrosilane or aluminum hydride, respectively. On the other hand, previous studies on
related NacNac calcium and magnesium-catalyzed reactions suggested substrate insertion into
2
9a, 35
the M–H bond as the key mechanistic event.
Likewise, Roesky et al. suggested substrate
insertion into the Al–H bond followed by metathesis with the B–H bond in the catalytic
24-25
hydroboration mediated by NacNacAlHX (X = OTf, H).
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To elucidate the mechanism of catalysis by the aluminum cation 2, the stoichiometric
reactivity towards alkenes and alkynes was studied. For 1-hexene and 3,3-dimethyl-1-butene, the
products observed were indeed the alkyl derivatives resulting from C=C insertion into the Al–H
bond (Scheme 1), but remarkably for the latter substrate, the alkyl product was the expected
neohexyl derivative 4 and not a 2,3-dimethylbutyl product observed in the catalytic reaction.
This disparity suggested that alkene insertion is not relevant to the actual catalytic process. The
mechanism based on alkene insertion followed by Si–H/Al–C metathesis was ultimately ruled
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out when 3 failed to react with HSiEt even after heating at 70 °C for 24 h.
3
Scheme 1. Reactivity of 2 towards 1-hexene (top) and 3,3-dimethyl-1-butene (bottom).
Interestingly, it was noted that addition of a slight excess of 1-hexene to 2 resulted in
1
broadened signals for 3 in the H NMR spectrum. Cooling down a chlorobenzene solution of 2
with two equivalents of 1-hexene to −21 °C gave well-defined signals for a single product, 5
(Scheme 2). Complex 5 is the product of alkene addition across the aluminum atom and the γ-
carbon in the NacNac ligand of 3, confirmed by the correlation observed between the methylene
(0.11 and −0.64 ppm) and methine (1.73 ppm) protons of 1-hexene and the γ-carbon (60.3 ppm)
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in the H- C HMBC spectrum. As well, the two methyl groups in the ligand framework resolved
into two separate singlets at 1.41 and 1.38 ppm due to the generation of a new chiral center.
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Similar cycloaddition of alkenes with [NacNacAlMe] has been observed previously by Jordan
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and Harder. Warming the sample to room temperature results in complete dissociation of 1-
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hexene and regeneration of 3, in accord with the recent report by the Harder group.
Scheme 2. Reversible reaction between 1-hexene and 3.
Given the fact that hydroalumination of alkenes by 2 was very fast, we deemed that both
+
aluminum cations 2 and [NacNacAlR] could mediate this reactions by means of Lewis acid
catalysis. Indeed, alkyl complex 3 catalyzes the hydrosilylation of 1-hexene as rapidly as 2
(
Scheme 3). Further corroborating this suggestion was the observation that in catalytic (5 mol%)
and stoichiometric reactions of 3 with cyclohexene and HSiEt , the only products observed were
3
(
C H )SiEt and the intact cation 3 (Scheme 4). These labelling experiments allow us to rule out
6 11 3
the alkene insertion/metathesis pathway in the catalytic reactions.
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Scheme 3. Hydrosilylation of 1-hexene mediated by 3.
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Scheme 4. Hydrosilylation of cyclohexene by 3 at 5 mol% (top) and 100 mol% (bottom).
Since mixtures of silane products were invariably formed in catalytic runs, the ability of cation
2
to redistribute silanes was studied in a separate set of experiments. Reactions with both HSiEt3
and H SiEt resulted in fluxional systems containing H Si, H SiEt, H SiEt , and HSiEt . The
2
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exchange could be frozen at about –20°C and the identity of silane products could be established
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in the Si INEPT+ spectrum. While redistribution of silanes is, in general, a well-established
phenomenon, it is usually observed for halo-, alkoxy-, amido-, hydrido- and aryl- substituted
silanes and is relatively rare for alkyl silanes.
These stoichiometric and labelling experiments present a very divergent mechanistic picture.
The lack of skeletal rearrangement of the alkyl group during the formation of 4 and its inability
to regenerate the hydride 2 upon reaction with hydrosilanes allows us to rule out the usual
insertion/bond metathesis pathway suggested for related hydrosilylation and hydroboration
2
9b, 35
reactions catalyzed by Group 2 NacNac complexes.
Formation of the rearranged product,
(2,3-dimethylbutyl)triethylsilane, in the hydrosilylation of 3,3-dimethyl-1-butene and the ease of
silane redistribution by this catalytic system may suggest that these aluminum cations act as
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Lewis acid catalysts. A catalytic cycle promoted by the Lewis acid may proceed through two
mechanisms, distinct from each other by the initial activation of silane or alkene (Scheme 5).
4
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Activation of silanes by electrophilic boranes is well established by Piers and Oestreich and
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the related silane complex [Et Si–H···Al(C F ) ] has been very recently documented by Chen et
3
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al. In the second step of this pathway (Scheme 5a), the alkene attacks the incipient silylium ion
creating a carbocation intermediate which undergoes hydride abstraction from NacNacAlRH to
give the alkyl silane and regenerates the aluminum cation. In the case of 3,3-dimethyl-1-butene,
the corresponding secondary carbocation would rearrange into a more stable tertiary carbocation
which would account for the formation of (2,3-dimethylbutyl)triethylsilane. Analogous reaction
21b, c
pathways have been previously suggested for hydrosilylation reactions catalyzed by AlCl3,
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+
32
B(C F ) and [FP(C F ) ] Lewis acid catalysts.
6
5 3
6 5 3
The alternative mechanism was suggested by Yamamoto et al. This reaction pathway involves
an aluminum cation coordinating to the olefin followed by hydride transfer from the silane
(Scheme 5b), which can account for the structural rearrangement of 3,3-dimethyl-1-butene but
contradicts the chemistry depicted in Scheme 4. In this case, the mechanism predicts the
formation of NacNacAl(Hex)(Cyh) (Hex = hexyl, Cyh = cyclohexyl) which is expected to react
with the incipient silylium ion (or its solvate with chlorobenzene) to give preferentially Et SiHex
3
for steric reasons. In contrast, we observed the exclusive formation of Et SiCyh.
3
Given the facile, reversible formation of the tripodal compound 5, one can argue that it too
could be a catalyst. While its involvement in the catalytic cycle cannot be tested directly at this
point, there are two arguments that it is not the true (or most active) catalyst. The first point is
that 5 features a more shielded, four-coordinate aluminum center, which is likely less
+
electrophilic. Secondly, catalytic hydrosilylation by the tripodal cation [Tp*AlMe] reported by
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Koller and Bergman required much harsher conditions (100 C), suggesting that tripodal
systems are less reactive than 2 and 3, both operating at room temperature.
To summarize, there are strong indications that the catalysis mediated by complexes 2 and 3
proceeds via a variation of Lewis acid activation but the exact mechanistic details are unclear at
this moment. We favor the scenario outlined in Scheme 5a which is consistent with all the
experimental data available to date.
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Scheme 5. Possible Lewis acid catalyzed mechanisms for alkene hydrosilylation by
F
4
[
NacNacAlR][BAr ]: a) Piers-Oestreich type mechanism, b) Yamamoto type mechanism .
Stoichiometric reactivity of alkynes was probed next. Unexpectedly, addition of one equivalent
of phenylacetylene to 2 did not result in insertion into the Al–H bond but rather gave a product of
C≡C addition across the NacNacAl moiety, namely to the methine carbon and aluminum atom,
to form a tripodal product 6 (Scheme 6). Similar reactivity with various unsaturated substrates
3
8-39, 43
has been previously documented for other NacNac complexes.
The diagnostic features of
1
the H NMR spectrum of 6 are a broad signal for the intact Al–H group at δ 3.86 and a signal for
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the backbone CH (5.66 ppm), observed as a doublet ( JH–H = 1.5 Hz) due to coupling to the o-Ph
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signal at 7.13 ppm confirmed by H- H COSY, H- C HSQC, and NOESY spectra.
Stoichiometric reactions of 1-phenyl-1-propyne and diphenylacetylene furnished similar addition
products.
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Scheme 6. Coupling of cation 2 with alkynes.
The catalytic potency of the alkyne bridged product 6 was then investigated in the catalysis of
a reaction between 3-hexyne and HSiEt . Comparable catalytic activity, with 98% conversion
3
within 25 h, was observed which compares well with the >99% conversion achieved for the
reaction catalyzed by 2. When DSiEt was employed, most of the deuterium was found in the
3
product with partial incorporation in the acidic backbone methyls of the NacNac ligand (Scheme
7
). On the other hand, like the cation 2, complex 6 can mediate the redistribution of HSiEt and
3
releases hydrosilylated phenylacetylene very slowly.
Scheme 7. Alkyne labelling experiment with DSiEt and 6 as the catalyst.
3
Conclusion
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In conclusion, we have demonstrated that the cationic aluminum complex
F
4
[
DippNacNacAlH][BAr ] is a catalyst for the hydrosilylation of various alkenes and alkynes.
Stoichiometric reactions between the aluminum complex and alkene and alkyne substrates
revealed divergent reactivity. While alkenes inserted into the Al–H bond to give alkyl products,
labelling experiments ruled out the common insertion/metathesis pathway for this system. On the
other hand, stoichiometric reactions with alkynes revealed addition to the backbone methine
carbon and aluminum to form new bridged tripodal species. Both of these products are
competent catalysts for hydrosilylation. Reactions of hydride complex 2 with trialky- and
tetraalkylsilanes showed facile silane redistribution, accounting for the low yields of the target
hydrosilylation products. These observations suggest a mechanism based on Lewis acid
activation by the cationic aluminum center.
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ASSOCIATED CONTENT
Supporting Information. Details of syntheses, characterization and catalytic studies are
provided. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
Email: gnikonov@brocku.ca
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
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Funding Sources
This research was supported by the Petroleum Research Fund administered by the American
Chemical Society. K.J. further thanks the Natural Sciences and Engineering Research Council of
Canada for an Undergraduate Student Research Award. T.C. is grateful to the Government of
Ontario for an Ontario Graduate Scholarship.
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ACKNOWLEDGMENT
We thank Razvan Simionescu for help with NMR experiments.
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Aluminum cations [NacNacAlR] (R= H or alkyl) catalyze hydrosilylation of alkenes and
alkynes by means of Lewis acid activation of silanes.
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