Unexpected catalytic activity of simple triethylborohydrides in the hydrosilylation of alkenes

Article abstract of DOI:10.1039/c7cc01531c

The first example of sodium triethylborohydride-catalysed hydrosilylation of alkenes is reported. The hydrosilylation of certain alkenes, in particular styrenes, vinylsilanes and allyl glycidyl ether, with aromatic hydrosilanes proceeded in a highly regioselective manner to give Markovnikov products. It is significant that several protocols use NaHBEt₃ as a reducing agent generating active catalysts in situ of other hydrosilylation reactions. An anionic mechanism of hydrosilylation is proposed.

Full text of DOI:10.1039/c7cc01531c

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Unexpected catalytic activity of simple triethylborohydride in
hydrosilylation of alkenes
M. Zaranek,†* S. Witomska,† V. Patroniak, and P. Pawluć*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first example of sodium triethylborohydride-catalysed although not only on them.2–10 It is also possible to obtain
hydrosilylation of alkenes is reported. The hydrosilylation of certain products of formal substitution of terminal hydrogen through a
alkenes, in particular styrenes, vinylsilanes and allyl glycidyl ether, process of dehydrosilylation which is known to occur over
with aromatic hydrosilanes proceeded in a highly regioselective transition metal catalysts, especially ruthenium ones.1,11
manner to give Markovnikov products. It is significant that several Since its discovery by Sommer et al. in 1947,12 hydrosilylation
protocols use NaHBEt3 as reducing agent generating active has come a long way towards its contemporary popularity. The
catalysts in situ of other hydrosilylation reactions. An anionic first milestone in the development of hydrosilylation was the
mechanism of hydrosilylation is proposed.
discovery of a platinum(IV) catalyst, known as the Speier’s
catalyst.13 The second, and the truly ground-breaking one, was
the discovery of the Karstedt’s catalyst based on a platinum(0)
complex.14 Its use facilitated a significant decrease in the
catalyst loading, down to couple dozens of ppms, thus enabling
implementation in industrial synthesis. Although very active,
platinum-based catalysts still make a significant component of
the final product’s price, mainly due to very high price of this
precious metal and its complete non-recoverability. This fact
combined with the invariable trends of trying to replace every
platinum group metal (PGM), pushes the research towards non-
precious metal and even non-metal catalysis. There has been a
plenty of attempts to make a first-row transition-metal olefin
hydrosilylation catalyst, of which the most successful ones were
those by the groups of Chirik,15–17 Deng,18 Huang,19 Holland,20
Hu,21, Lu,22 Nakazawa23,24, Nikonov,25 and Thomas.26 It is worth
noting that many of them used sodium triethylborohydride to
reduce the metal precatalyst.8,15,22,23,25 There are even examples
of main-group metals’ hydrides and hydrido complexes
catalysing hydrosilylation,7,27 and a perfluorinated borane,
B(C6F5)3, has been also successful.28,29 Significantly,
triethylborohydrides of lithium and sodium were found to be
good catalysts for hydrosilylation of C=O and C=N bonds.30 On
the other hand, they have never been used alone as catalysts of
hydrosilylation of alkenes. Only LiAlH4 has been found a catalyst
of hydrosilylation of ethene and 1-hexene with SiH4,31 and it has
also been active in catalysing dehydrogenative coupling of
terminal alkynes with this silane.32
The hydrosilylation of alkenes (Scheme 1) is one of the most
widely used homogeneous catalytic processes. Thanks to its
ultimate atom economy and robustness, this reaction is now
successfully employed in a large-scale synthesis of various
industrially-applicable chemicals, e.g. coatings and adhesives,
heat transfer media, separation membranes, lubricants,
surfactants, etc.1 It can also be a step in more complex organic
syntheses, e.g., as a mean of enantioselective reduction of
carbonyl and imine compounds.1
Scheme 1. Possible products of hydrosilylation
Addition of Si-H bond to an olefin can lead to two main products
of hydrosilylation – 1,2-addition yields products often referred
to as anti-Markovnikov or linear ones and is facilitated by most
of known catalysts, whereas 2,1-addition gives Markovnikov or
branched products and has been a matter of a limited number
of reports, mostly on early transition metals and lanthanides,
Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89C,
61-614 Poznań, Poland
† These authors contributed equally.
* piotr.pawluc@amu.edu.pl, m.zaranek@amu.edu.pl
Electronic Supplementary Information (ESI) available: experimental procedures
analytical data of isolated compounds. See DOI: 10.1039/x0xx00000x
We were trying out a cobalt catalyst which was activated by in
situ reduction with sodium triethylborohydride.33 In that study
we followed a previously published procedure23 that is
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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generally accepted in examining first-row transition metal reactions giving secondary products, or catVaielwysAirsticleuOsnilninge
complexes needing reduction. Bearing in mind the reports on sophisticated catalysts. Formation of 4 DwOaI:s10n.1o0t39o/Cb7sCeCrv0e15d31iCn
catalytic activity of NaHBEt3, we decided to perform most cases. It is worth noting that lithium triethylborohydride,
experiments without Co complex, which enabled us to state while being far better in hydrosilylation of ketones and imines,30
that the borohydride was not catalytically active in this exhibited activity of only a quarter of that of NaHBEt3 in the
transformation. In fact, the substrates used were anything but same conditions (entries
5
and 8).
Potassium
similar to those used in the literature report on hydrosilylation triethylborohydride led to full conversion of phenylsilane,
with NaHBEt3 catalyst, as they involved imines, ketones, however, with selectivity towards bis-hydrosilylation (entry 9).
aldehydes, etc.30 However, further examination of the process In the reaction of equimolar amounts of
1 and 2, KHBEt3 led to
and change of substrates led to a conclusion that NaHBEt3 can formation of both mono- and bis-hydrosilylation products in
indeed convert phenylsilane and styrene. The results of almost equal amounts (entry 10). It should be emphasised that
preliminary catalytic reactions are presented in Table 1.
a stochastic mixture of all three isomers of 4 was formed. A
reaction tried out with triethylborane in order to exclude action
similar to this of perfluorinated boranes29 did not proceed
(entry 11). Switching the solvent to 1,4-dioxane caused a drop
in conversion (to 68%; entry 12) and polymerisation of styrene.
No better were also THF (entry 13) and bis(2-methoxyethyl)
ether (diglyme; entry 14), additionally giving a bunch of
unidentified and higher-order side products of multiple
hydrosilylation. The use of twofold excess of styrene leads to a
higher PhSiH3 conversion, and interestingly, the reaction
proceeds almost exclusively towards the single hydrosilylation
product. Apparently, the mono-hydrosilylation product is less
reactive than the starting PhSiH3 under these conditions,
resulting in high selectivity towards monoaddition products.
Then, knowing the optimal conditions for hydrosilylation of
styrene with phenylsilane and some substrate scope limitations,
Table 1. Screening of the reaction conditions.
Entry
[1]/[2]
Catalytic
system[a]
t/h
T/°C
Conv
Selecti-
vity[c]
of 2[b]
1
10% NaHBEt3
10% NaHBEt3
10% NaHBEt3
10% NaHBEt3
10% NaHBEt3
5% NaHBEt3
10% NaHBEt3
10% LiHBEt3
10% KHBEt3
10% KHBEt3
10% BEt3
24
24
2
60
16%
43%
28%
52%
100%
87%
74%
27%
100%
68%
4%
>99% 3
>99% 3
>99% 3
>99% 3
>99% 3
2
2
2
2
2
2
1
2
2
1
2
2
2
2
2
80
3
100
100
100
100
100
100
100
100
100
100
60
4
8
5
24
24
24
24
24
24
24
24
24
24
we
performed
more
syntheses
using
sodium
6
92% 3
8% 4
triethylborohydride as catalyst (Table 2). We decided to use
consistently 2 equivalents of olefins to provide better insight
into their reactivity by excluding concentration effects. Its
excess posed no threat to the reaction, as it mostly polymerised.
The results depict that NaHBEt3 can be applied as a catalyst to
hydrosilylation of fairly various alkenes, e.g., styrenes, allyl
ethers, and vinylsilanes, with hydrosilanes bearing
preferentially aromatic substituents. The representative
products of reactions that proceeded with good yields were
isolated and they were proven to be Markovnikov-type isomers.
Simple styrene reacted with phenylsilane and diphenylsilane to
7
97% 3
3% 4
8
>99% 3
9
2% 3
98% 4
10
11
12
13
14
51% 3
49% 4
-
10% NaHBEt3
in 1,4-dioxane
68%
87%
26%
92% 3
8% 4
10% NaHBEt3
in THF
73% 3
form selectively and quantitatively
3
and
5
. Substituting the α
10% NaHBEt3
in diglyme
100
18% 3;
6% 3b
72% 4
hydrogen with a methyl group resulted in
6
obtained with only
a moderate yield. What is noticeable, introduction of electron-
donating groups into the phenyl ring also causes a drop in
[a] Reaction as 0.5 M solution of 2 in toluene unless stated otherwise; [b] Determined
by GC; [c] taking into account all actual products, determined by GC-MS and 1H NMR
of isolated 3;
conversion, no matter if they are weak donors (products
) or stronger ones (products 25 and 26). Triphenylsilane and
dimethylphenylsilane were less satisfyingly efficient
hydrosilylating agents either for styrene (products 23 and 24
7 and
8
,
Sodium triethylborohydride occurred to be an efficient catalyst
of hydrosilylation of styrene, nonetheless, it failed completely
to catalyse hydrosilylation of 1-hexene, nor was it successful in
a reaction of styrene with triethylsilane, in which case a
complete polymerisation of the former reagent was observed.
However, the positive results were encouraging to explore the
resp.), or allyl glycidyl ether (products 21 and 22, resp.) than
phenyl- or diphenylsilane. Similar relation was observed for 1,1-
diphenylethene and vinylsilanes. Dimethylphenylvinylsilane
used as an olefin led to 15 and 16 with moderate yield, whereas
hydrosilylations of triphenylvinylsilane went to completion to
give 17 and 18. So did also the reaction of 2-vinylnaphthalene
capabilities of NaHBEt3 further, especially as product
3 was
with phenylsilane leading to a product identified as 12
To our great surprise, it was possible to obtain a group of silyl
derivatives of allyl glycidyl ether 19 22
.
formed quantitatively in Markovnikov manner (entry 5). To
stress again, such a selectivity is not frequently observed in
hydrosilylation of alkenes, being mostly a result of side
-
.
2 | J. Name., 2012, 00, 1-3
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Table 2. Results of alkene hydrosilylation using NaHBEt3 as catalyst.[a]2.
detected. Silanes with more than one Si-H bond wVeierweAgrteicnleeOrnalilnlye
subject to only a single addition, whenever vinyl aromatics were
DOI: 10.1039/C7CC01531C
used, which remains in a good accordance with the observation
that alkyl-substituted silanes are less reactive or completely
inactive in this system.A general observation is that the
reactivity decreases in the sequence PhSiH3 ≈ Ph2SiH2 >> Ph3SiH.
However, when
triphenylvinylsilane is a better hydrosilylation substrate than
dimethylphenylvinylsilane. Interestingly, product of
a
vinylsilane is used as alkene,
9
hydrosilylation of styrene with triethoxysilane was obtained
with very good yield. Trans-1-phenylbuta-1,3-diene was
subjected to hydrosilylation with phenylsilane and
diphenylsilane to determine regioselectivity of the catalytic
system for two potential addition sites. Product 13 was found
to constitute 93% of the products mixture with a reaction
completed in 3 hours, whereas product 14 was formed less
selectively (73% out of mixture of isomers) over a longer time
(20 h).
To examine the reactivity of other allyl ether, THP-protected 2-
allyloxyethanol was prepared and reacted with triphenylsilane.
Unfortunately, it gave (2-(allyloxy)ethoxy)triphenylsilane as the
only silylated product, and similarly did also tert-butyl acrylate
with diphenylsilane, giving diphenyl(tert-butoxy)silane. Further,
it came as no surprise that a reaction of cinnamaldehyde with
phenylsilane proved carbonyl reduction highly preferred over
C=C hydrosilylation. An attempt to hydrosilylate styrene with
1,1,3,3-tetramethyldisiloxane led to a mixture made up mostly
of undefined siloxanes containing dimethylsiloxy and
dimethyl(hydro)siloxy units, and similarly failed hydrosilylation
of triethoxyvinylsilane, leading to its condensation.
Scheme 2. Proposed mechanism of NaHBEt3-catalysed hydrosilylation of styrene with
phenylsilane
[a]
Reaction conditions: 0.5 M silane in toluene, [Si]:[C=C]:[NaHBEt3] = 1:2:0.1;
100 °C, all yields in relation to silanes, for description of GC yield determination,
see SI; [b] [Si]:[C=C] = 1:3;
The observations made in the course of our investigation made
it possible to propose a mechanism of hydrosilylation catalysed
by sodium triethylborohydride (Scheme 2). It has to be stressed
that there is no direct evidence for this catalytic cycle, however,
some characteristic observations are backed by literature data.
On the basis of the work of Brown and Kim, it is justified to
assume that in the first step the addition of Na-H bond to an
alkene takes place.34 Although their work was focused on
hydroboration by lithium borohydrides, they indeed mentioned
It turned out that products of hydrosilylation of allyl glycidyl
ether are more active in hydrosilylation than starting
phenylsilane. If allyl glycidyl ether was used in a stoichiometric
amount relative to Si-H (i.e., 3 eq relative to PhSiH3), the
hydrosilylation weas to completed in 15 minutes yielding
quantitatively 19. Otherwise, free phenylsilane was left
unreacted and no single or double addition products were
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N. Komine, M. Abe, R. Suda and M. Hirano, Organometallics,
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7
8
9
that sodium triethylborohydride was not successful in this
process. The assumption on the initial step of the catalytic cycle
is further supported by the fact that the addition of
triethylborane slows the reaction down, which can be explained
in terms of the dissociation equilibrium of triethylborohydride.
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borohydrides.35 What is more, the observation by Brown and
Kim of α-lithiation of 1,1-diphenylethylene yielding a red colour
of the reaction mixture is in line with the very same observation
made by us using NaHBEt3.34 This step of the reaction can also
clarify the origin of its enormous selectivity as the formal
benzylic carbanion is far more favourable providing essential
charge stabilisation by resonance, and this approach has also
been proposed by Kobayashi et al. in their original paper.31
Next, it would explain a better reactivity of phenylvinylsilanes
than aliphatic alkenes since silylene bridges appear to be
capable of transferring electronic charge, which has been
shown in spectroscopic measurements of silylene-containing
polymers.36 In a further step, it is reasonable to consider the
formation of a pentacoordinate silylate species, however, a
concerted mechanism involving silane-to-borane hydride
transfer simultaneous to the formation of Si-C bond is also
possible. Observation of the formation of a product of siloxyl
substitution when using hydrosiloxane promotes rather the
former possibility (see SI). In the final step, a hydride is
abstracted from the former species by free triethylborane
molecule through a transition state similar to a frustrated Lewis
pair reported by Piers et al.37. The abstraction can be potentially
obstructed by solvent coordination to BEt3, which can explain
worse performance of hydrosilylation in THF and other donor
solvents. Overall, the anionic mechanism proposed by us is
different from the one proposed by Harder et al.,7 as we
assumed no insertion-elimination steps.
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with aromatic silanes that does not provide pathways for
dehydrogenative processes. However, the substrate scope is
inherently limited to those unsusceptible to nucleophilic
addition of hydrides and those without acidic protons. It is
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used for in-situ reduction of other hydrosilylation precatalysts.
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Financial support from the National Science Centre (Poland),
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DOI: 10.1039/C7CC01531C
Highly regioselective hydrosilylation of olefins with aryl- and alkoxysilanes has been developed using
a simple sodium triethylborohydride.