Diverse Fates of β-Silyl Radical under Manganese Catalysis: Hydrosilylation and Dehydrogenative Silylation of Alkenes

Article abstract of DOI:10.1002/cjoc.201800367

Manganese-catalyzed hydrosilylation of alkenes has been underdeveloped for a long time. Herein, we describe a general, chemo- and regio- selective hydrosilylation of alkenes by using the Mn(CO)₅Br catalyst with ample substrate scopes. Meanwhile, dehydrogenative silylation of aryl olefins can be selectively achieved upon the catalysis of dinuclear Mn₂(CO)₁₀. Mechanistic experiments revealed diverse fates of the common intermediate β-silyl radical, namely, hydrogen atom transfer (HAT) for the hydrosilylation and organometallic β-H elimination for the dehydrogenative silylation of olefins.

Full text of DOI:10.1002/cjoc.201800367

Diverse Fates of β-Silyl Radical under Manganese Catalysis:
Hydrosilylation and Dehydrogenative Silylation of Alkenes
Xiaoxu Yang,a and Congyang Wang*,a
ABSTRACT Manganese-catalyzed hydrosilylation of alkenes has been underdeveloped for a long time. Herein, we describe a general, chemo- and
regio-selective hydrosilylation of alkenes by using the Mn(CO)5Br catalyst with ample substrate scopes. Meanwhile, dehydrogenative silylation of aryl
olefins can be selectively achieved upon the catalysis of dinuclear Mn2(CO)10. Mechanistic experiments revealed diverse fates of the common
intermediate β-silyl radical, namely, hydrogen atom transfer (HAT) for the hydrosilylation and organometallic β-H elimination for the dehydrogenative
silylation of olefins.
KEYWORDS manganese, hydrosilylation, dehydrogenative silylation, alkene, homogenous catalysis
Introduction
Scheme 1 Mn-catalyzed reactions of silanes with alkenes
Organosilanes have wide applications in organic synthesis and
silicon-based materials owing to their stability, non-toxicity, and
versatile transformations.[1] Hydrosilylation reactions are
particular noteworthy methods to straightforwardly access
organosilanes with high atom economy.[2] As an earth-abundant
transition metal, manganese catalyzed hydrosilylation of
carbonyls compounds have made impressive success recently.[3]
In sharp contrast, only sporadic examples of hydrosilylation of C-C
unsaturated bonds were reported (Scheme 1a).[4,5] In 1983,
Faltynek described Mn(CO)5(SiPh3)-catalyzed hydrosilylation of
1-pentene under UV irradiation or heating conditions.[4a] In 1987,
Hilal disclosed Mn2(CO)10 could catalyze hydrosilylation of
1-hexene, unfortunately in low yields.[4b] More than a decade
later, the same group showed hydrosilylation of 1-octene with a
poly(siloxane)-supported dimeric Mn complex.[4c] Recently, the
group of Thomas demonstrated an example of hydrosilylation of
1-octene by the cooperative use of a pincer-Mn(II) complex and
NaOtBu.[5]
Though
elegant,
the
thus-far
reported
manganese-catalyzed hydrosilylation of alkenes was highlighted
with only three substrates of aliphatic olefins.[5b] A general
Mn-catalyzed hydrosilylation of olefins with broad substrate
scopes is still underdeveloped.
As our persistent interest on manganese catalysis,[6,7] we has
recently developed the first manganese-catalyzed hydrosilylation
Results and Discussion
of alkynes.[8] Herein, we report
a general Mn-catalyzed
Results
hydrosilylation of olefins with ample substrate scopes including
aryl, benzyl, alkyl and functionalized ones by using the Mn(CO)5Br
catalyst under mild conditions (Scheme 1b). Meanwhile, the
dehydrogenative silylation of olefins was achieved through the
catalysis of binuclear Mn2(CO)10, which represents the first
Mn-catalyzed dehydrosilylation reaction of olefins. Remarkably,
We commenced our hydrosilylation study with styrene 1a and
benzyldimethylsilane 2a as model substrates and Mn(CO)5Br as a
catalyst (Table 1, entry 1). The hydrosilylation product 3aa was
contaminated with dehydrogenative silylation product 4aa,
formed in 38% and 7% yields respectively. Mn2(CO)10 showed a
decreased chemoselectivity (entry 2). The addition of ligands or
bases gave only inferior results (entries 3-4).[9] In the survey of
solvents, non-polar hexane proved to be the best (entries 5-7).
Gratifyingly, further variations on substrate ratios resulted in an
almost quantitative yield of 3aa and excellent chemoselectivity
(entry 9). Moreover, the reaction temperature could be lowered
to 60 oC with even better outcome (entry 10). In addition, the UV
irradiation could also promote the hydrosilylation reaction at
room temperature in comparable efficiency (entry 11).
mechanistic experiments uncovered
a
radical HAT and
organometallic β-H elimination pathway might account for the
hydrosilylation and dehydrogenative silylation respectively.
a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Molecular Recognition and Function, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, China
E-mail: wangcy@iccas.ac.cn
University of Chinese Academy of Sciences, Beijing 100049, China
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been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800367
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Table 1 Optimization of anti-Markovnikov hydrosilylation a
Table 2 Mn-catalyzed hydrosilylation of alkenes
Yield (%)
3aa 4aa
38
Entry
Cat. [Mn]
Additive 1a:2a
Sol.
3aa:4aa
1
2
Mn(CO)5Br
Mn2(CO)10
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
Mn(CO)5Br
--
1:1
1:1
1:1
1:1
1:1
1:1
1:1
2:1
1:2
1:2
1:2
Toluene
Toluene
Toluene
Toluene
THF
7
5:1
1.7:1
--
--
34
2
20
6
3
AsPh3
4
NaOAc
31
2
8
4:1
5
--
--
--
--
--
--
--
4
--
6
Heptane
Hexane
Hexane
Hexane
Hexane
Hexane
55
70
80
95
98
95
7
8:1
7
6
12:1
4:1
8
21
1
9
>50:1
>100:1
>100:1
10b
<1
<1
11c
a
Reaction conditions unless otherwise noted: 1a (0.2 mmol), 2a (0.2
mmol), cat. (0.01 mmol, 5 mol%), additive (0.04 mmol), hexane (0.2 M),
150 oC, 12 h, N2 atmosphere. Yields were determined by 1H NMR. b 4 h, 60
oC. c 4 h, RT, UV (380 nm).
a 1 (0.5 mmol), 2 (1.0 mmol), Mn(CO)5Br (0.025 mmol), hexane (0.2 M), 60
oC, 4 h, N2 atmosphere. Isolated yields were shown. b 100 oC. c 150 oC. d
1
(0.5 mmol), 2 (0.5 mmol), Mn2(CO)10 (0.025 mmol), hexane (0.2 M), RT, 4 h,
N2 atmosphere, UV (380 nm).
Having the optimized conditions established, we explored the
substrate scope for hydrosilylation of alkenes (Table 2). The
reaction occured smoothly with a wide range of aryl olefins
bearing both electron-withdrawing and -donating groups
(3aa-3la). Various functionalities like halogen and acetoxy groups
remained intact after reaction, which provides handles for further
synthetic transformations (3ca-ea, 3ia, 3ka). Apart from the
phenyl moiety, other aryl groups, such as 2-naphth, 2-py and
pentafluorophenyl were also well tolerated affording 3ma-3oa
successfully. Moreover, aliphatic olefins were amenable to this
method leading to the hydrosilylated products as expected
(3pa-3ra). Electron-biased olefins such as n-butyl acrylate was
also compatible to this protocol (3sa). In addition, variantions on
the silyl groups including aliphatic or aromatic silanes delivered
the corresponding products in good yields (3ab-3ag). Of note, the
use of Mn2(CO)10 under UV irridiation showed an excellent
hydrosilylation reactivity as exeplified by 3af, 3ag.[10]
We next investigated reaction parameters to selectively
access dehydrogenative silylation product 4 with Mn2(CO)10 as the
catalyst (Table 3). The screening of silanes revealed that more
sterically bulky silanes were favored (entries 1-4).[9] A series of
additives was further explored, albeit giving no better results
(entries 5-8).[9] We surmised that styrene 2a might play a crucial
role as a hydrogen acceptor in the dehydrogenative silylation
reaction. Consequently, we enhanced the amount of 2a and, to
our delight, the yield of 4aa increased accordingly (entries 9-11).
Finally, we selected the parameters in entry 10 as the optimized
conditions. Then, the applicability of the dehydrogenative
silylation protocol was investigated (Scheme 2). Aryl alkenes with
varied substituents were well tolerated to deliver the
corresponding products in good yields. Again, the delicate
halogen groups maintained intact under reaction conditions,
which presented reaction sites for further elaborations.
Unfortunately, aliphatic olefins were ineffective in this protocol.
Table 3 Optimization of dehydrogenative silylation a
Yield (%)
Entry
Ligand/Add.
Silane
3: 4
3
4
2a
2b
2c
2f
2f
2f
2f
2f
2f
2f
2f
1:1.4
1.2:1
1:1.3
1:3
1
2
3
4
--
37
55
55
45
--
--
40
18
4
53
56
25
30
44
52
66
81
89
--
AsPh3
AgOAc
LPO
NaOAc
--
5
6
1:6
1:2
15
19
31
16
12
9
1: 2.3
1:1.7
1:4
7
8
9[b]
10[b,c]
11[b,d]
--
1:7
--
1:10
a Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol), Mn2(CO)10 (0.01 mmol, 5
mol%), ligand/add. (0.04 mmol), hexane (0.2 M), 150 oC, 12 h, N2 atmosphere.
b
Yields were determined by 1H NMR. Mn2(CO)10 (10 mol%). c 1a:2a = 3:1. d
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1a:2a = 4:1.
radical scavenger, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO),
had an obvious inhibitory effect on the reaction (Scheme 4c).
Moreover, highly selective formation of hydrosilylation products
(Table 2, 3af, 3ag) was achieved by the use of dinuclear Mn2(CO)10
under UV irradiation, which was known to deliver manganese
radicals (CO)5Mn·.[10] Additionally, when vinyl cyclopropane 1u
was used, only the ring-opening product 3ua was detected
(Scheme 4d), indicating again a radical pathway. In view that
Scheme 2 Mn-catalyzed dehydrogenative silylation of alkenes. Reaction
conditions: 1 (1.5 mmol), 2f (0.5 mmol), Mn2(CO)10 (0.05 mmol), hexane
(0.2 M), 150 oC, 12 h, N2 atmosphere. Isolated yields were shown.
HMn(CO)5 Mn-III could produce (CO)5Mn·,[11]
a plausible
mechanism based on a radical pathway is proposed in Scheme 4e.
The reaction starts with the generation of HMn(CO)5 from
Mn(CO)5Br and silane 2a. The (CO)5Mn· (Mn-IV), originating from
HMn(CO)5, reacts with silane 2a affording a silyl radical and
HMn(CO)5. Next, the silyl radical attacks alkene 1 accessing a
β-silyl alkyl radical 6a, which undergoes HAT with HMn(CO)5
leading to the desired product 3 and regenerates (CO)5Mn·.
To shed light on the mechanisms of these two Mn-catalyzed
reactions of alkenes with silanes, we firstly carried out a series of
deuterium-labeling experiments (Scheme 3). Treatment of
deuterated alkenes [D3]-1e with silane 2a under hydrosilylation
conditions gave the desired product [D3]-3ea, in which the
hydrogen atom from 2a was transferred to the benzylic position
(Scheme 3a). Meanwhile, the reaction of [D3]-1e with silane 2f
under dehydrogenative silylation conditions afforded product
[D2]-4ef where one terminal deuterium was replaced by the silyl
group (Scheme 3b). When reactions of styrene 1a with deuterated
silane [D]-2f were conducted under either hydrosilylation or
dehydrogenative silylation conditions, similar phenomena were
found (Schemes 3c, d). These results indicated that there was no
H-D scrambling in the reactions.
Scheme 4 Mechanistic studies on anti-Markovnikov hydrosilylation
Scheme 3 Deuterium-labeling experiments
To obtain more information on mechanism of Mn-catalyzed
dehydrogenative silylation of olefins, a sequence of experiments
was performed.[9] Firstly, the hydrogenation byproduct,
ethylbenzene 5a, was ascertained to form in 82% yield, which was
consistent with the yield of 4af and conformed the role of styrene
1a as the hydrogen acceptor (Scheme 5a). Next, the
stoichiometric reaction of Mn-H species Mn-II with alkenes 1a
was examined at high temperature, which demonstrated the
generation of 5a (Scheme 5b vs Scheme 4a). Meanwhile, in
Scheme 5c, the ratios of H-contents in two positions of byproduct
5t indicated that the Mn-H species participated in the formation
of both vicinal C-H bonds of 5t. Consequently, a proposed
mechanism for dehydrogenative silylation is shown in Scheme 5d.
Similar to the mechanism of hydrosilylation, the early steps of the
dehydrogenative silylation also lead to the generation of the
β-silyl-alkyl radical 6b from the addition of a silyl radical to the
alkene 1a. At this stage, the recombination of the β-silyl-alkyl
radical 6b and (CO)5Mn· gave an organomanganese species Mn-V,
which releases the desired product 4af via a β-H elimination and
regenerates HMn(CO)5. Of note, the manganese-phenyl
To further probe the mechanistic details of Mn-catalyzed
hydrosilylation of olefins, we prepared Mn-Si complex Mn-I and
Mn-H complexes Mn-II and examined stoichiometric reactions of
alkene 1a, silane 2a with the above manganese complexes
(Scheme 4a).[9] While the Mn-Si species Mn-I gave negative
results, the hydrosilylation product 3aa was detected in a
quantitive yield when Mn-H species Mn-II was used. It clearly
suggested that the Mn-Si complex Mn-I wasn’t the active
intermediate, whereas the Mn-H species Mn-II played an
essential role in the hydrosilylation reaction. Furthermore, a
deuterium labeling stoichiometric reaction was carried out
(Scheme 4b), which indicated that the Mn-H species Mn-II
participated in the C-H/D bond formation of 3af. The addition of a
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π-interaction may attribute to the formation of Mn-V. Meanwhile,
the reduction of styrene 1a by HMn(CO)5 delivers the byproduct,
ethylbenzene 5a.
Acknowledgement (optional)
Financial support from the National Natural Science
Foundation of China (21472194, 21772202, 21521002) are
gratefully acknowledged.
Scheme 5 Mechanistic studies on dehydrogenative silylation, isolated
yields were shown
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In summary, a general manganese-catalyzed hydrosilylation
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Experimental
In an oven-dried Schlenk tube, a mixture of the alkene (0.5
mmol), the silane (1 mmol), Mn(CO)5Br (0.025 mmol, 5 mol%) and
hexane (2.5 mL) was stirred at 60 °C for 4 h under N2 atmosphere.
After completion of the reaction, the mixture was cooled down to
room temperature. The solvent was removed under reduced
pressure and the hydrosilylation product was isolated by column
chromatography on silica gel with EtOAc/PE.
Similarly, in an oven-dried Schlenk tube, a mixture of the
alkene (1.5 mmol), the silane (0.5 mmol), Mn2(CO)10 (0.05 mmol,
10 mol%) and hexane (2.5 mL) was stirred at 150 °C for 12 h
under N2 atmosphere. After completion of the reaction, the
mixture was cooled down to room temperature. The solvent was
removed under reduced pressure and the dehydrogenative
silylation product was isolated by column chromatography on
silica gel with EtOAc/PE.
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Diverse Fates of β-Silyl Radical under
Manganese Catalysis:
Hydrosilylation
and Dehydrogenative Silylation of Alkenes
When a radical meets manganese: A general hydrosilylation of olefins was enabled by the sole
catalysis of Mn(CO)5Br, while dehydrogenative silylation of aryl olefins was achieved by using
dinuclear Mn2(CO)10. Mechanistic studies suggested hydrogen atom tranfer (HAT) and
organometallic β-H elimination pathways from the β-silyl radical intermediate operating in the
hydrosilylation and dehydrogenative silylation of olefins respectively.
Xiaoxu Yang, Congyang Wang,*
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