Regiocontrol in the cobalt-catalyzed hydrosilylation of alkynes

Article abstract of DOI:10.1039/C8CC07267A

Hydrofunctionalizations of unsaturated hydrocarbons are key strategies for the synthesis of functionalized building blocks. Here, we report highly versatile cobalt-catalyzed hydrosilylations of alkynes that operate with minute amounts of the inexpensive, bench-stable pre-catalyst Co(OAc)₂·4H₂O under mild conditions (0.1-1 mol%, THF, r.t., 1 h). Near-perfect regiocontrol/stereocontrol was induced by the choice of the ligand: bidentate phosphines afforded (E)-β-vinylsilanes; α-vinylsilanes formed with bipyridine ligands.

Full text of DOI:10.1039/C8CC07267A

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Regiocontrol in the cobalt-catalyzed
hydrosilylation of alkynes†
Cite this: Chem. Commun., 2018,
4, 12322
5
Guojiao Wu, Uttam Chakraborty and Axel Jacobi von Wangelin
*
Received 7th September 2018,
Accepted 4th October 2018
DOI: 10.1039/c8cc07267a
rsc.li/chemcomm
Hydrofunctionalizations of unsaturated hydrocarbons are key
strategies for the synthesis of functionalized building blocks. Here,
we report highly versatile cobalt-catalyzed hydrosilylations of alkynes
that operate with minute amounts of the inexpensive, bench-stable
pre-catalyst Co(OAc)
2
ꢀ4H
2
O under mild conditions (0.1–1 mol%, THF,
r.t., 1 h). Near-perfect regiocontrol/stereocontrol was induced by the
choice of the ligand: bidentate phosphines afforded (E)-b-vinylsilanes;
a-vinylsilanes formed with bipyridine ligands.
Alkenylsilanes constitute versatile building blocks in the realm
of fine chemicals and materials synthesis by virtue of their
1
dense poly-functionalization. The combination of a polarized
alkene moiety, a Si center, and the substituents at alkene and
silicon offer ample opportunities for post-synthesis manipulations.
Hydro-silylations of alkynes enable a most straightforward and
atom-economic synthesis of alkenylsilanes in the presence of the
2–4
noble metal catalysts Ru, Rh, and Pt. Only few protocols rely on
the use of inexpensive and environmentally benign 3d base metal
5
catalysts. Despite the recent progress in the field, the precise
Scheme 1 Cobalt-catalyzed hydrosilylations of alkynes.
control of regioselectivity and stereoselectivity remains a challenge
of utmost importance. Cobalt catalysts were demonstrated to
exhibit especially high activity and tolerance of functional groups substrate scope involving terminal and internal alkynes and
6
in hydrosilylations of alkenes. Much less attention has been trihydrosilanes. Documented herein are the benefits of a versatile
directed towards cobalt-catalyzed hydrosilylations of alkynes which regiodivergent and stereoselective hydrosilylation of alkynes in the
often require high catalyst loadings, complex ligands, and harsh presence of only 0.1–1 mol% Co(OAc) ꢀ4H O and commercial
2
2
conditions or showed poor regio/stereocontrol or a limited sub- phosphine or bipyridine ligands (Scheme 1, bottom).
7–12
strate scope with regard to alkynes and silanes (Scheme 1, top).
An initial evaluation of parameters in the model reaction
Very recently, Ge et al. reported Co-catalyzed hydrosilylations to between phenylacetylene (1a) and phenylsilane (2a) in the
1
3
give (Z)-vinylsilanes in the presence of pyridine-2,6-diimines. We presence of the bench-stable and inexpensive Co(OAc)
2
2
ꢀ4H O
believed that a most user-friendly protocol would combine the revealed very good regioselectivity and stereoselectivity toward
following criteria: (i) high catalytic activity of a commercial catalyst (E)-styrylsilane with various commercial phosphine ligands
system under very mild conditions; (ii) control of regioselectivity (Table 1, entries 1–7). With only 0.1 mol% Co(OAc)
and stereoselectivity by the choice of the ligand, and (iii) a wide an isolated yield of 86% was obtained with very high stereo-
2
ꢀ4H
2
O/dppb,
selectivity (450/1 E/Z) and regiocontrol (1/49 a/b). A complete
reversal of regioselectivity was observed upon employment of
bipyridine ligands (up to 25/1 a/b, entries 8–12). These results
Department of Chemistry, University of Hamburg Martin Luther King Pl. 6,
2
0146 Hamburg, Germany. E-mail: axel.jacobi@uni-hamburg.de
are a significant extension of previous reports with N,N,N-ligands
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
12
c8cc07267a
that resulted in poor regio-selectivity with PhSiH
3
.
The strict
1
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a
Table 1 Optimization of reaction conditions
b
b
Entry
Ligand
Yield [%]
E/Z/a
1
2
3
4
5
6
n = 1
n = 2
n = 3
72
81
95
92
90
89
94 (86)
75
66
87/0/13
97/0/3
98/0/2
97/0/3
98/0/2
87/2/11
98/0/2
10/0/90
4/0/96
5/0/95
5/0/95
10/0/90
—
Dppf
DPEphos
Xantphos
Dppb
c
d
7
0
8
9
2,2 -Bipyridine (bipy)
4
4
4
-OMe
bipy
bipy
bipy
Scheme 2 Hydrosilylation of terminal aryl alkynes. Conditions: 1 (0.4 mmol),
-Me
d
10
11
12
13
14
70 (60)
2
2
a (0.48 mmol), Co(OAc)
0 1C, under N , 1 h. Isolated yields are given. E/a ratios were determined by
quantitative GC-FID vs. internal n-dodecane. 3 h. Co(OAc)
2
ꢀ4H
2
O (0.1 mol%), dppb (0.1 mol%), 0.5 mL THF,
-tBu
64
61
0
2
1,10-Phenanthroline
L1 or L2 or L3
a
b
2 2
ꢀ4H O (0.5 mol%),
c
d
0
0
00
dppb (0.5 mol%). Co(OAc)
2
ꢀ4H
2
O (1 mol%), dppb (1 mol%). Xantphos.
2,2 ;6 ,2 -Terpyridine
0
—
a
Conditions: 1a (0.40 mmol), 2a (0.48 mmol), Co(OAc)
(1 h w/PR
ligands). Yield and product ratios from quantitative GC-FID vs. internal Unsymmetrical alkynes engaged in the regioselective addition of
2
ꢀ4H
2
O (0.5 mol%),
, 3 h with N,N-
ligand (0.5 mol%), 0.5 mL THF, 20 1C, under N
2
3
b
c
n-dodecaned. 0.1 mol% Co(OAc)
2
2
ꢀ4H O (in 10 mL methanol), 0.1 mol%
the silyl moiety to the less bulky C atom (Ph, i-Pr vs. alkyl; alkyl vs.
Me). This is also manifested in the series of 2-alkyl phenylacetylenes
with increasing regioselectivities in the order Me o Et o nBu (5p, 5q,
dppb, 1 h. Isolated yields in parentheses.
ligand control of this protocol is further documented by the lack of and 5r). The conjugated enyne 2-methylhex-1-ene-3-yne cleanly
catalytic activity in the presence of other N,N-ligands such as afforded the desired 3-silyl product 5s. The hydrosilylation of the
(
(
pyridin-2-yl)methanimine, butane-2,3-diimine, and terpyridine sterically rather unbiased 2-pentyne gave impressive regio-selectivity
entries 13 and 14). (9/1) and stereoselectivity. Ph SiH , HSi(OEt) , and HSiMe(OEt) fared
Various arylacetylenes underwent clean formation of (E)-alkenyl- equally well. Steric silanes (HSiEt and HSi(OiPr)Me ) gave complex
2
2
3
2
3
2
silanes in good yields, very high stereoselectivities (450/1 E/Z) and product mixtures, possibly from rapid alkyne (cyclo)oligomerizations.
regioselectivities (418/1 b/a) with only 0.1 mol% Co(OAc)
and dppb at r.t. (Scheme 2). Diverse substitution patterns (incl. (entries 8–12, Table 1). The Co/2,2 -bipyridine catalysts enabled a
2
ꢀ4H
2
O
We further explored the regioselective a-silylation of terminal alkynes
0
ortho-substituents) and functional groups (OH, NH
aldehyde, thiophene, and pyridine) were tolerated. No dehalogena- which constitute important synthetic building blocks (Scheme 4).
tion was observed with halides (Br, Cl, and F). The same conditions The reaction displayed compatibility with Br, NH , ester, nitrile, and
2
, nitrile, ester, reversal of regio-selectivity to cleanly afford 1-phenylvinyl silanes
1
2
were successfully applied to hydrosilylations with monohydrosilanes free OH functional groups. 2-Ethynyl-6-methoxynaphthalene,
and dihydrosilanes (i.e. (EtO) SiH and Ph SiH ). An extension of the 3-ethynylthiophene, and 3-ethynylpyridine gave slightly lower
3
2
2
methodology to terminal and internal alkyl alkynes was realized conversions (6n, 6o, and 6p). Internal alkynes reacted poorly
with cobalt/diphosphine catalyst systems (entries 5 and 6 in Table 1 (B10% yield, low regiocontrol).
and ESI†). Terminal alkynes exhibited the highest reactivities, very
high stereoselectivities (450/1 E/Z), and very high regioselectivities ulations is exemplified in Scheme 5. Sequential hydro-silylations
up to 99/1 b/a) toward (E)-alkenyl-silanes in the presence of 1 mol% afforded a divinylsilane (87/13 b/a) via the alkenyl-silane 3a. Sub-
Co(OAc) O and DPEphos (Scheme 3, top). Silylethers, halides, stitution of the hydride at Si with Grignard reagents is a robust
The versatility of the derived alkenylsilanes for further manip-
(
2
ꢀ4H
2
nitriles, and ester moieties were tolerated. Free OH groups inhibited method of silane functionalization. With PhMgBr, tertiary silane 3a
the conversion. The protocol was also applied to hydrosilylation was obtained in 74% yield. Tamao oxidation of vinylsilane 6a gave
with diphenylsilane (5m).
the corresponding phenone in 81% yield. Further alkenylsilane
Internal alkynes successfully reacted under slightly modified reactions of high utility include electrophilic and nucleophilic olefin
conditions with Xantphos as a ligand (Scheme 3, bottom). Highly additions, silyl substitutions, oxidations, cross-couplings, hydro-
1
selective syn-hydrosilylation was operative with all substrates. functionalizations and polymerizations.
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Scheme 5 Post-synthesis manipulations of (E)-alkenyl silanes.
In extension of literature precedents and our own preliminary
mechanistic data derived from the optimization experiments,
substrate scope, and regiochemical and stereochemical course of
the cobalt-catalyzed hydrosilylation reactions, we performed key
mechanistic studies on the nature of catalytic intermediates
6,10–15
6g
(Scheme 6).
In full accord with the literature, the reduction
of Co(OAc) by the silane in the presence of diphosphine ligands
2
(
L = P ) was observed by MS and NMR. We postulate the formation
2
of the monohydrido species L CoH (A-I). Indeed, the penta-
n
2
coordinate complex (dppb) CoH was observed in LIFDI-MS spectra
1
(
m/z 912.27) and showed a characteristic H NMR resonance at
ꢁ
14.5 ppm (see the ESI† for details). Coordination of the alkyne
A-II) and migratory insertion into the Co–H bond constitute the
(
Scheme 3 Hydrosilylation of alkynes. Conditions: 4 (0.4 mmol), 2a
elemental steps that govern the regioselectivity and stereoselectivity
of the reaction. The preferential addition of the cobalt complex to
the less hindered side of the alkyne bearing the smaller substituent
Rs affords the more stable alkenylcobalt species A-III. Formal trans-
metalation to Si results in the formation of the (E)-alkenylsilane and
regeneration of the active species A-I. The reaction is first order in
(
0.48 mmol), Co(OAc)
solvent, 1 h, under N
determined by quantitative GC-FID vs. internal n-dodecane. Xantphos.
2
ꢀ4H
2
O (0.1 or 1 mol%), ligand (0.1 or 1 mol%), 0.5 mL
2
. Isolated yields are given. Product ratios E/a were
a
b
c
d
Co(OAc)
2
ꢀ4H
2
O (0.5 mol%). Co(OAc)
2
ꢀ4H
2
O (1 mol%). Co(OAc)
2
ꢀ4H
2
O
(0.5 mol%), 0.5 mL MeCN, 60 1C, 2 h.
Scheme 4 Markovnikov hydrosilylation of arylacetylenes. Conditions: 1
(
1
0.4 mmol), 2a (0.48 mmol), Co(OAc)
mol%), 0.5 mL solvent for 1 h under N
2
ꢀ4H
2
O (0.1 or 1 mol%), ligand (0.1 or
. Isolated yields are given. The
2
a
b
c
product ratios [a/E] were determined by GC analysis. 50 1C. 6 h. 2a
(
Scheme 6 Proposed reaction mechanisms of hydrosilylation catalysed by
(diphosphine)cobalt complexes (top) and (bipy)cobalt complexes (bottom).
d 4-OMe
1.5 equiv.).
bipy as a ligand (0.5 mol%).
1
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the cobalt catalyst and zero order in phenylacetylene and silane (see
the ESI†), which suggests that the alkyne insertion into Co–H is rate-
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c,6f,11 1
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4Me
Co(OAc)
2
ꢀ4H
2
O/ bipy with PhSiH (1:1:10) documented the
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3
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2
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5
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(
d) J. P. Morales-Cer ´o n, P. Lara, J. L ´o pez-Serrano, L. L. Santos, V. Salazar,
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´
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4-Me
formed by the reaction of equimolar Co(OAc)
2
ꢀ4H
2
O and
bipy
4
5
6
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the addition of LiAlH
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3
. The dehydrocoupling could be reversed by
(2.5 equiv. per [Co]) which resulted in the
(see the ESI† for details). The silylcobalt
(
(
4
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3
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syn-alkenylcobalt complex B-III releases a-alkenylsilane upon reac-
tion with PhSiH . This hydrosilylation reaction is first order in [Co]
3
and silane and zero order in phenylacetylene (see the ESI†). This
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-Me
Co(OAc)
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ꢀ4H
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O and the ligand dppb or
bipy. The mild condi-
tions allow a wide substrate scope (terminal and internal alkynes,
various silanes) and the tolerance of sensitive functional groups
2
(halides, aldehydes, esters, nitriles, NH , and OH). Key mechanistic
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4-Me
effected by the ligand
bipy which selectively delivers Markovnikov
products. The former pathway involves the formation of hydrido-
cobalt catalyst species, while the latter mode of reactivity is most
likely based on silylcobalt species. The high functional group
tolerance and mild reaction conditions make these protocol
highly attractive for complex molecule synthesis with great utility
for medicinal and materials chemistry endeavours.
8
9
1
This work was supported by the Deutsche Forschungsge-
meinschaft (JA 1107/6-1) and the European Research Council
1
(CoG 683150).
1
1
1
2 A. Rivera-Hern ´a ndez, B. J. Fallon, S. Ventre, C. Simon, M.-H. Tremblay,
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Conflicts of interest
5
There are no conflicts to declare.
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