Regioselective hydrosilylation of epoxides catalysed by nickel(II) hydrido complexes

Article abstract of DOI:10.1039/C7CC01655G

Bench-stable nickel fluoride complexes bearing NNN pincer ligands have been employed as precursors for the regioselective hydrosilylation of epoxides at room temperature. A nickel hydride assisted epoxide opening is followed by the cleavage of the newly formed nickel oxygen bond by σ-bond metathesis with a silane.

Full text of DOI:10.1039/C7CC01655G

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC01655G
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Regioselective Hydrosilylation of Epoxides Catalysed by Nickel(II)
Hydrido Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Jan Wenz,a Hubert Wadepohla and Lutz. H. Gadea,*
www.rsc.org/
2,5-bis(2-oxazolinyldimethylmethyl)pyrroles (PdmBox) which
stabilize Ni-H species12 as compared to previous studies
employing the non-methylated PmBOX pincer in which the Ni
hydride underwent bimolecular transformation to T-shaped
Ni(I) complexes.13 The latter gave rise to characteristic radical
reactivity. The new PdmBox nickel pincer complexes were
found to react readily with PhSiH3 or Ph2SiH2 to generate nickel
hydrido complexes 2a and 2b from fluoridonickel(II) precursors
1a and 1b (Scheme 1) which are bench-stable in the solid state
and can be prepared on a multi-gram scale (for structural
details see ESI).12
Upon treatment of a solution of hydrido complex 2a with
an equimolar amount of styrene oxide or of 2,2-
diphenyloxirane at room temperature, the exclusive formation
of the anti Markovnikov insertion products 3a and 4a occured
(Scheme 2). Complex 3a was also prepared independently by
salt metathesis of [(PdmBox)NiCl] with PhCH2CH2ONa (ESI),
both synthetic routes giving rise to the same reaction product.
Bench-stable nickel fluoride complexes bearing NNN pincer
ligands have been employed as precursors for the regioselective
hydrosilylation of epoxides at room temperature. A nickel hydride
assisted epoxide opening is followed by the cleavage of the newly
formed nickel oxygen bond by -bond metathesis with a silane.
Hydrosilylation of alkenes, alkynes, aldehydes, or ketones is
typically carried out with platinum group catalysts, especially
complexes of Ru,1 Rh,2 or Ir.3 Costs, toxicity and low
abundance recently has led to the development of catalyst
systems based on 3d metals such as iron,4 cobalt,5 or nickel.6
Nickel hydrido complexes are key intermediates in a variety
of chemical transformations.7 They are either generated in situ
in stoichiometric reaction sequences of catalytic cycles or may
be synthesized by substitution of another anionic ligand as in
the cleavage of a nickel-X bond (X = F, OR) by a silane.7 This
transformation is
a key step in the nickel-catalysed
hydrosilylation of aldehydes or ketones,6c,6e,6g-i,8 while nickel
hydride catalysts have also been employed for the
hydrosilylation of olefins in recent years.6a,6b,6d,6f,9 There have
been very few reports of transition metal-catalysed
hydrosilylations of epoxides with concomitant ring opening.10
Scheme 2 Synthesis of nickel alkoxido complexes 3a and 3b.
The molecular structure of 3a was determined by single crystal
X-ray structure analysis of 3a Ph(CH2)2OH (Figure 1), confirming the
pincer-type coordination of the spectator ligand and the occupation
of the fourth coordination site by the alkoxide ligand resulting from
the epoxide insertion into the Ni-H bond. The helical twist [helix
= 29.6(1)°] between the coordination plane of the d8-Ni(II) and the
Scheme 1 Synthesis of (PdmBox)nickel hydrido complexes 2a and 2b.
Although there are some examples for catalytic and
stoichiometric activations of epoxides with nickel
compounds11, there is no report of a catalytic hydrosilylation. plane of the pyrrole ring results from the structural flexibility of the
In this communication, we present the first nickel-based
catalyst for a regioselective hydrosilylation of epoxides.
We recently reported a new type of NNN pincer ligands
ligand backbone, which allows the donor functions to adapt to the
size of the central metal atom. The greater trans influence of the
hydrido ligand in 2a12 compared to the alkoxido ligand in 3a is
reflected in the Ni-N(2) bond lengths [2a: 1.910(2); 3a: 1.865(3) Å].
a.Anorganisch-Chemisches Institut, University of Heidelberg, Im Neuenheimer Feld
270, 69120 Heidelberg (Germany); E-mail: lutz.gade@uni-heidelberg.de †
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Syntheses, spectroscopic
data, crystal data and structure refinement]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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dihydroanthracene had a significant impact on the reactivity
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DOI: 10.1039/C7CC01655G
observed.
Table 1 Substrate scope of nickel-catalysed hydrosilylation of styrene oxide derivatives.
Reaction
Yielda [%]
Entry
1
Substrate
Product
conditions
(NMR Yield)b
2a 94 (95)
2b 91 (95)
Figure 1 Molecular structure of 3a. One molecule of Ph(CH2)2OH, hydrogen bonded to
O3 and Hydrogen atoms are omitted for clarity; thermal ellipsoids displayed at 50%
probability. Selected bond lengths [Å] and angles [°]: 3a Ni-O(3) 1.862(2); Ni-N(1)
1.887(3), Ni-N(2) 1.865(3), Ni-N(3) 1.874(2), O(3)-Ni-N(1) 90.29(11); O(3)-Ni-N(2)
177.85(12); O(3)-Ni-N(3) 91.00(11); N(2)-Ni-N(1) 90.08(12).
rt, 16 h
rt, 16 h
2a 93 (95)
2b 90 (95)
2
3
To regenerate the hydride 2a from the alkoxide resulting from
the insertion, complex 3a was reacted with various hydrosilanes.
PhSiH3 and Ph2SiH2 were identified as good silyl transfer reagents,
leading to complete regeneration of 2a and release of the silyl ether
products within a few minutes (Scheme 3). Other silanes such as
Et3SiH, (EtO)3SiH and poly(methylhydrosiloxane) (PMHS)
regenerated hydride 2a only in poor yields. They required elevated
temperatures, or were found to be completely unreactive (for an
overview of the silane screening see ESI).
2a 95 (95)
2b 90 (95)
rt, 16 h
2a 95 (95)
2b 95 (95)
4
5
rt, 16 h
rt, 16 h
2a 74 (95)
2a 64 (70)
6
7
8
rt, 36 h
rt, 16 h
rt, 24 h
2a 37 (40)
2b 32 (38)
2a 89 (95)
2b 89 (95)
Scheme 3 Cleavage of nickel alkoxides by silanes.
Having established the protocol of stoichiometric epoxide
insertion and hydride regeneration, we set out to investigate
the catalytic activity of hydrido complex 2a in the catalytic
hydrosilylation of styrene oxide. Using PhSiH3 as reducing
agent, hydrido complex 2a catalysed the hydrosilylation of
styrene oxide. The reaction was complete within 16 h at room
temperature with catalyst loadings of 5 mol% on a 1 mmol
scale (Scheme 4). After alkaline cleavage of the silyl ether, the
"anti-Markovnikov" alcohol could be isolated in good yield.
Both NMR data and GCMS analysis of the reaction mixture and
of the crude products after alkaline hydrolysis give no evidence
of the formation of the other possible regioisomer.
2a 55c (95)
2b 45c (95)
9
rt, 24 h
rt, 16 h
2a 90 (95)
2b 95 (95)
10
2a 95 (95)
2b 90 (95)
11
12
rt, 16 h
rt, 72 h
2a 80 (85)
Scheme 4 Catalytic hydrosilylation of styrene oxide using 1a as catalyst.
50 °C,
24 h
2a 80, rac.d
2b 70, rac.d
13
1
Upon monitoring the catalytic transformation by H NMR,
the nickel hydrido complex 2a was identified as the resting
state during the entire reaction while the presence of nickel
alkoxido species was not observed (ESI). A control experiment
in the absence of the nickel hydride 2a showed no significant
conversion, even at 80 °C for 5 days. Furthermore the mercury
test had no influence on the outcome of the reaction,
rendering colloidal nickel as active catalyst unlikely. Finally, we
also probed the presence of radicals during the reaction by
addition of radical traps: Neither triphenylmethane nor 9,10-
aAlcohol after workup determined by 1H-NMR using 1,4-dimethoxybenzene as internal
standard. bNMR yield of the corresponding silylprotected alcohol of the crude reaction
NMR using 1,4-dimethoxybenzene as internal standard. cK2CO3/MeOH was used for
workup. ddetermined by chiral HPLC.
The scope of the catalytic system was investigated using
5 mol% of nickel fluoride complex 1a or 1b as precatalysts to
generate the catalytically active hydrido species 2a or 2b in
situ in the presence of a excess of PhSiH3 (Scheme 1). The
2 | J. Name., 2012, 00, 1-3
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reaction products were isolated as alcohols following basic
hydrolysis of the silyl ethers. After a reaction time of 16 to 36 h
at room temperature, complete conversion of substituted
styrene oxide derivatives (Table 1, entries 1−5; 8-11) was
observed. In those cases, the formation of the corresponding
silyl-protected alcohol was observed in high yields. The
differing electronic nature of each substrate had no influence
on the regioselectivity.
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DOI: 10.1039/C7CC01655G
Notably, halide substitution on the aromatic system was at
least in part tolerated (entries 6 and 7). Nevertheless,
hydrodehalogenation was observed as a slow side reaction
yielding the catalytically inactive nickel chloride or bromide
species. Although the p-acetoxy-derivative in entry 9 was
converted to the corresponding silyl ether in high yield, the
low isolated yields are due to the basic work up which led to
partial ester hydrolysis. In contrast to the other substrates, cis-
or trans-stilbene oxide required elevated temperatures (50 °C)
to react and, after workup, yielded racemic 1,2-
diphenylethanol. Finally, we were able quantitatively to
convert 4 mmol (500 mg) of styrene oxide with very low
catalyst loading (1 mol%) in four hours under solvent-free
version conditions. Solvent free conditions were also found to
be possible for solid epoxides as substrates using a ball mill14
(for details see ESI). We have found that only aryl-substituted
epoxides, can be converted to an acceptable degree under
catalytic conditions. The use of aliphatic epoxides leads to no
reaction at room temperature and raising the reaction
temperature led to the partial decomposition of the
catalytically active nickel hydride species, resulting in very low
conversions for alkyl-substituted epoxides (< 5%).
The stereocenter of the racemic epoxides is lost during the
reaction. However, in order to determine whether our chiral
catalyst system is capable of distinguishing the isomers, the
reaction was stopped at 50% conversion and the configuration
of the unreacted starting material was investigated. No kinetic
resolution was observed (i.e. the unreacted epoxide was
racemic). In the case of styrene oxide, both enantiomerically
pure starting materials are commercially available. The use of
the enantiomeric pure compounds had no effect on the results
or reaction rates compared to the racemic starting material.
In order to obtain insight into the reaction mechanism, we
carried out deuterium labelling experiments. Hydrosilylation of
styrene oxide using PhSiD3 or Ph2SiD2 yielded the
Scheme
5 Synthesis of β-deuterated alcohols using 1a as a catalyst (a).
Competition experiment to determine the kinetic isotopic effect (b).
Stoichiometric conversion of Styreneoxide with equivmolar amounts of
nickelhydride 2a and deuterosilane (c).
A H/D kinetic isotope effect (KIE) of kH/kD = 1.5 was
determined by H and 13C NMR employing equal amounts of
1
hydrosilane and deuterosilane in the nickel catalysed
hydrosilylation and workup of the product with NaOH/MeOH
[Scheme 5 (b)].
The deuterium-labeled silane was found not to undergo
H/D exchange with the metal hydride within 24 hours.
Therefore, the origin of the hydride in the product could be
studied by stoichiometric reaction of styrene oxide, Ph2SiD2
and the nickel hydrido complex.8a,16 No incorporation of
deuterium into the product alcohol was detected by NMR after
alkaline hydrolysis while the formation of the nickel deuteride
complex occurred concomitantly [Scheme 5 (c)]. The nickel
hydride 2a was thus unambiguously identified as the hydride-
transfer reagent and a potential non-hydride mechanism for
the catalytic reaction could be excluded.
Furthermore, a kinetic study of the reaction was carried
out by 19F NMR using 4'-fluorostyrene oxide as a substrate
under the conditions employed previously for the substrate
screening (see Table 1). Notably, the reaction profiles of the
conversions displayed no induction periods (ESI), i.e. the nickel
hydride, which is initially formed, reacts directly. The reaction
kinetics were determined by the initial rate method in which
the initial concentrations of either substrate, phenylsilane, or
precatalyst 1a were varied while the other two were kept
constant. The initial rate was found to display first-order
dependence on substrate concentration and on nickel
concentration, but observed to be independent of the
concentration of the silane (zeroth order dependence, see
Figure 2 and ESI).
corresponding silyl ether and after hydrolysis the
monodeuterated alcohol exclusively [Scheme 5 (a)].
-
This contrasts with the observation made by Brookhart et
al. for a cationic η-1-silane iridium(III) complex that catalysed
the regioselective hydrosilylation of epoxides, in which
deuteration of the
-position in the resulting alcohol was
observed.10b The Lewis acid catalysed ("Meinwald") epoxide
rearrangement to aldehydes or ketones is well known.15 Figure 2 Initial rate kinetics by varying the catalyst and substrate concentration.
Followed by hydrosilylation the deuterium incorporation at the
From these kinetic data, the following rate law for the nickel-
catalysed hydrosilylation on epoxides is derived.

position can be explained. Since the nickel catalyst system is
inactive for the conversion carbonyl functions, an epoxide
isomerization followed by a hydrosilylation step can be
excluded.
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A. M. Tondreau, C. C. H. Atienza, K. J. Weller, S. A. Nye, K. M. Lewis, J. G.
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propose
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catalytic cycle for the nickel-catalysed
DOI: 10.1039/C7CC01655G
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hydrosilylation of epoxides which is similar to a mechanism
proposed by Guan et al. for the nickel-catalysed hydrosilylation
of aldehydes and ketones.6h Initially, the precatalyst 1a or 1b is
rapidly transformed into the nickel hydride 2a or 2b by
reaction with the silane. The apparent rate law, the low H/D
kinetic isotope effect, as well as the observation of NiH as the
resting state suggests that the rate-limiting step in this
hydrosilylation of epoxides is the insertion of the epoxide into
the nickel hydride bond. The resulting alkoxido complex then
reacts rapidly with the silane in a
-bond metathesis to
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Scheme 5 Mechanistic proposal for the nickel- catalysed hydrosilylation of
epoxides.
In conclusion, we have developed the first protocol for
nickel-catalysed anti-Markovnikov hydrosilylation of epoxides.
Such primary alcohols, which are of considerable synthetic
value, are usually obtained by hydroboration and subsequent
oxidation. We point out that our catalytic system also provides
a useful tool for the selective preparation of β-deuterated
alcohols, which can be difficult to obtain by alternative
catalytic methods.17
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We acknowledge funding by the Deutsche Forschungs-
gemeinschaft (Ga 488/9-1) as well as the University of
Heidelberg. We thank Prof. A.S.K Hashmi for allowing us to use
his ball mill.
12. J. Wenz, A. Kochan, H. Wadepohl and L. H. Gade, Inorg. Chem,
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