(Aminomethyl)pyridine Complexes for the Cobalt-Catalyzed Anti-Markovnikov Hydrosilylation of Alkoxy- or Siloxy(vinyl)silanes with Alkoxy- or Siloxyhydrosilanes

Article abstract of DOI:10.1002/anie.201612460

Cobalt-catalyzed anti-Markovnikov reactions that involve siloxy- or alkoxy(vinyl)silanes and siloxy- or alkoxyhydrosilanes are disclosed. More than 25 new cobalt–(aminomethyl)pyridine complexes were developed as catalysts for the hydrosilylation of indust

Full text of DOI:10.1002/anie.201612460

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International Edition: DOI: 10.1002/anie.201612460
German Edition: DOI: 10.1002/ange.201612460
Silicones
(Aminomethyl)pyridine Complexes for the Cobalt-Catalyzed Anti-
Markovnikov Hydrosilylation of Alkoxy- or Siloxy(vinyl)silanes with
Alkoxy- or Siloxyhydrosilanes
Kangsang L. Lee*
Abstract: Cobalt-catalyzed anti-Markovnikov reactions that
involve siloxy- or alkoxy(vinyl)silanes and siloxy- or alkoxy-
hydrosilanes are disclosed. More than 25 new cobalt–(amino-
methyl)pyridine complexes were developed as catalysts for the
hydrosilylation of industry-relevant and challenging siloxy- or
alkoxy-terminated vinylsilanes. These transformations typi-
cally proceed in the presence of 0.25 mol% of the cobalt
complex with 0.75 mol% of the alkylating agent to afford the
desired products in up to > 98% yield with > 98% anti-
Markovnikov selectivity in 30 min. The current protocol shows
a broad substrate scope, delivering more than 25 siloxanes with
siloxy or alkoxy functional groups at both termini, and can also
be applied to polymeric vinyl- and hydrosiloxanes.
H
ydrosilylation is a well-studied and useful reaction with
wide use in the chemical industry.[1] Among others, platinum
complexes such as the Speier[2] and Karstedt[3] catalysts have
been employed to add the silicon–hydrogen bond across
unsaturated bonds and these catalysts are considered the
industrial standard due to their high reactivity and air-
stability. Recently, many efforts have been devoted to the
development of metal-free catalysts[4] and catalysts containing
earth-abundant transition metals to replace the more expen-
sive platinum. In this context, research groups including those
of Chirik, Huang, Holland, Nagashima, and Fout recently
reported significant advancements in Fe- and Co-catalyzed
alkene hydrosilylations (Scheme 1a).[5–7] However, these
existing protocols are mainly suited for vinylalkanes and
vinylarenes with a limited number of alkoxy- or siloxyhydro-
silanes; alkoxy- or siloxy(vinyl)silanes have not been exten-
sively studied.
Scheme 1. Fe- or Co-catalyzed hydrosilylations and project synopsis.
ates for silicone fluid synthesis because they have alkoxy- or
siloxy-terminations on both Si atoms (Scheme 1b).
Hydrosilylation gives the anti-Markovnikov product (A)
and/or the Markovnikov adduct (B) with concomitant
byproducts from dehydrogenative silylation[9] (C) or hydro-
genation (D) (Scheme 1b). However, hydrosilylations of
siloxy- or alkoxy(vinyl)silanes are considered challenging
because they often deliver additional redistribution byprod-
ucts[10] such as E–J and/or complicated hydrosilylation
adducts derived from them (Scheme 1c). To the best of our
knowledge, there is no efficient and anti-Markovnikov-
selective protocol employing reagents with alkoxy or siloxy
terminations and earth-abundant hydrosilylation catalysts.[11]
Herein, we report our efforts to develop cobalt catalysts
for the anti-Markovnikov-selective hydrosilylation of various
alkoxy- or siloxy(vinyl)silanes (primary, secondary, and
tertiary vinylsilanes) with aryl-, alkoxy-, or siloxyhydrosilanes.
The resulting hydrosilylation products, where both sides are
alkoxy- or siloxy-terminated, were obtained in up to > 98%
yields and > 98% anti-Markovnikov selectivity with low
catalyst loading (0.25 mol%). Newly developed cobalt com-
plexes derived from more than 25 (aminomethyl)pyridines[12]
and CoBr2 efficiently catalyze hydrosilylations to afford
Alkoxysilanes and siloxanes are important compounds in
the silicone industry and have been widely used as precursors
for material syntheses.[8] These intermediates are often
prepared by hydrosilylation and incorporated into siloxane
polymers. Although previous methods allowed for the hydro-
silylation of vinylarenes and -alkanes by alkoxy- or siloxyhy-
drosilanes in a few cases (Scheme 1a), from an industrial
perspectives, the hydrosilylation products obtained from
alkoxy- or siloxy-terminated educts are useful as intermedi-
[*] Dr. K. L. Lee
Core R&D, The Dow Chemical Company
2200 West Salzburg Road, Auburn, MI 48611 (USA)
E-mail: kangsang.lee@dowcorning.com
Supporting information and the ORCID identification number for the
author of this article can be found under:
http://dx.doi.org/10.1002/anie.201612460.
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industrially relevant siloxy- or alkoxysilane compounds and
cross-linked silicone polymers.
substituent a (p-methoxyphenyl, L5) increased the yield
(> 98% yield, 228C). L6 with a 2-methylnaphthyl group as c
and phenyl groups as a and b made the hydrosilylation
significantly efficient at 808C as well as at 228C (> 98% anti-
Markovnikov selectivity, > 98% yield). The corresponding
electron-rich ligand (a = p-methoxyphenyl, L7) was equally
effective.
After optimization of the reaction conditions and a brief
initial screening,[13] we identified the core structure of
a potential ligand as (aminomethyl)pyridine and surmised
that steric and electronic modifications of three substituents
(a, b, c at L in Scheme 2) may lead to more reactive and
We then selected L6 to synthesize Co-A, which was
obtained as blue crystals, X-ray crystallographically charac-
terized [Eq. (1)], and used as precatalyst for the hydro-
silylation of various substrates (Scheme 3). Reactions of
HSiMe2(OTMS) with mono-, bis-, and tris(trimethylsiloxy)-
vinylsilanes efficiently proceeded to afford the desired anti-
Markovnikov products in 93 to > 98% yield (90–93% yield of
Scheme 2. Ligand screening for Co-catalyzed hydrosilylations. For the
full ligand screening result, see the Supporting Information. Yields are
1
determined by H NMR analysis.
selective catalysts. As model substrates we selected penta-
methylvinyldisiloxane and HSiMe(OTMS)2; the results are
summarized in Scheme 2. In the presence of the cobalt
complex prepared in situ from CoBr2 and L1 [a = b = Ph, c =
2,6-di(isopropyl)phenyl], hydrosilylation afforded only the
desired product in > 98% yield (> 98% anti-Markovnikov
selectivity). A lower yield (34%) was observed at elevated
temperatures (808C). When a 3,5-dimethylphenyl group was
introduced as a (L2), however, a mixture of A, C, and D
(10:27:15) was obtained. The introduction of a quaternary
carbon center at the connection to b was not effective: no
reaction at 228C for L3. With an electron-withdrawing
substituent a (2,6-difluorophenyl, L4), hydrosilylation was
less efficient (45% yield), whereas an electron-donating
Scheme 3. Co-catalyzed anti-Markovnikov hydrosilylation of siloxy- or
alkoxy(vinyl)silanes with siloxanes; yields determined by H NMR
analysis, yields of isolated products in parentheses. TMSO: trimethyl-
siloxy.
1
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isolated products, A only, P1–P3). These three vinylsiloxanes
also efficiently reacted with HSiMe(OTMS)2 to furnish the
desired products (P4–P6). Various alkoxy(vinyl)silanes such
as mono-, di-, or triethoxy(vinyl)silanes and their correspond-
ing methoxy variants could also be used, resulting in P7–P13
in 86–99% yield. Although trialkyl(vinyl)silanes are less
interesting hydrosilylation substrates in the silicon industry
than siloxy- or alkoxy(vinyl)silanes, to probe the scope of our
protocol, one substrate [triethyl(vinyl)silane] was tested
(91% yield, P14).
Scheme 5. Cross-linking of polymeric vinyl- and hydrosiloxanes.
Next, we examined the efficiency of Co-A in the hydro-
silylation of various siloxy- or alkoxy(vinyl)silanes with
alkoxy- or arylhydrosilanes. As shown in Scheme 4, hydro-
silylation of HSiMe2(OEt) with siloxy(vinyl)silanes (!P15,
used in the silicon industry (Scheme 5). In the presence of Co-
A, both linear (SFD-119, Dow Corning, Midland, MI) and Q-
branched vinyl polymers (2-7753) were efficiently cross-
linked with polymeric hydrosilane 2-7678; with 50 ppm of Co
concentration, cross-linking was accomplished in 3–5 min,
and with 100 ppm concentration, only 15 or 35 s were needed
for gelation. Even faster curing was observed with 200 ppm of
the catalyst (4 s for SFD-199, 20 s for 2-7753). The resulting
cured polymers are slightly yellow[13] compared to the gel
obtained by reaction with the Karstedt catalyst. This result
demonstrates that the current protocol is very comparable to
the one with Pt catalysts.
To get information on the initial catalyst species, Co-A
was treated with TMSCH2Li, followed by quenching by D2O,
which led to a deuterated compound (L6D, > 95% D
incorporation; Scheme 6). We also observed no hydrosilyla-
Scheme 4. Co-catalyzed anti-Markovnikov hydrosilylation of siloxy- or
alkoxy(vinyl)silanes with alkoxy- or arylhydrosilanes; yields determined
as described in Scheme 3. In the case of P19, the ratio A:redistribution
byproduct was 1.4:1, in the case of P20, more than three redistributed
hydrosilylation byproducts were observed.
Scheme 6. Deuterium experiment.
tion with a cobalt complex, prepared in situ from Co-A and
NaEt3BH (instead of TMSCH2Li).[14] These two results
tentatively suggest the formation of a tridentate cobalt
complex (Co-A’, Scheme 6). The formation of this structure
is indirectly supported by two previous reports.[12] For
example, a similar tridentate Hf complex was isolated from
L1 and HfCl4 in the presence of BuLi and MeMgBr.[12a]
Further experiments are required to clarify whether one
P16), triethyl(vinyl)silane (!P17), and mono-, di-, or
trialkoxy(vinyl)silanes (!P18–P20) efficiently proceeded to
afford the desired products in 80–98% yield. It should be
noted that for P19 and P20, we observed some redistributed
hydrosilylation byproducts. Arylsilanes are often incorpo-
rated into optical silicone materials because of their high
refractive index. In the presence of Co-A, PhMe2SiH and
Ph2MeSiH efficiently reacted with various alkoxy(vinyl)-
silanes as well as with siloxy(vinyl)silanes to furnish the
desired products in 90–99% yields (P21–P28).
ꢀ
ꢀ
or both of the Co C and Co N bonds in Co-A’ remain intact
or break, or are modified by the two hydrosilylation reagents
during the reaction (protonation/silylation/alkylation) and
ꢀ
ꢀ
how the participation of the Co C or Co N bond affects the
Co oxidation state (redox- vs. non-redox-active catalytic
cycle). In addition, although some dehydrogenative silylation
byproducts (C) were observed (4–10% yield for P3, P6, P24,
We also tested the feasibility of the current protocol
toward the hydrosilylation of vinyl-terminated polymers with
ꢀ
a cross-linker (i.e., polymeric Si H), which are commonly
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Acknowledgements
The author thanks Matthew Masters and Dr. Chengli Zu for
HR mass spectrometry, Dr. Britt Vanchura for X-ray crys-
tallography, Donald Eldred for NMR analysis, and Drs. Loren
Durfee, Dimitris Katsoulis, Zhanjie Li, and Richard Taylor
for helpful discussions.
Conflict of interest
The Dow Chemical Company including the heritage Dow
Corning Corporation declares following interest on this
research: WO patent Appl.2013043783A2, and 2013043846,
2013.
Keywords: alkoxysilanes · alkoxy(vinyl)silanes · cobalt catalysis ·
hydrosilylation · siloxanes
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4
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Manuscript received: December 23, 2016
Final Article published: && &&, &&&&
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Communications
Silicones
K. L. Lee*
&&&& — &&&&
(Aminomethyl)pyridine Complexes for
the Cobalt-Catalyzed Anti-Markovnikov
Hydrosilylation of Alkoxy- or
Siloxy(vinyl)silanes with Alkoxy- or
Siloxyhydrosilanes
More than 25 siloxanes with siloxy or
alkoxy functional groups at both termini
are obtained in high yields using a cobalt
catalyst derived from (aminomethyl)pyr-
idine ligands and CoBr2 (see Scheme).
This protocol can also be applied for the
hydrosilylation of polymeric vinylsilanes
by hydrosilanes to prepare silicones.
6
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