Manganese-catalysed divergent silylation of alkenes

Article abstract of DOI:10.1038/s41557-020-00589-8

Transition-metal-catalysed, redox-neutral dehydrosilylation of alkenes is a long-standing challenge in organic synthesis, with current methods suffering from low selectivity and narrow scope. In this study, we report a general and simple method for the manganese-catalysed dehydrosilylation and hydrosilylation of alkenes, with Mn₂(CO)₁₀ as a catalyst precursor, by using a ligand-tuned metalloradical reactivity strategy. This enables versatility and controllable selectivity with a 1:1 ratio of alkenes and silanes, and the synthetic robustness and practicality of this method are demonstrated using complex alkenes and light olefins. The selectivity of the reaction has been studied using density functional theory calculations, showing the use of an ⁱPrPNP ligand to favour dehydrosilylation, while a JackiePhos ligand favours hydrosilylation. The reaction is redox-neutral and atom-economical, exhibits a broad substrate scope and excellent functional group tolerance, and is suitable for various synthetic applications on a gram scale. [Figure not available: see fulltext.].

Full text of DOI:10.1038/s41557-020-00589-8

Articles
https://doi.org/10.1038/s41557-020-00589-8
Manganese-catalysed divergent silylation
of alkenes
Jie Dong1,5, Xiang-Ai Yuanꢀ ꢀ2,5, Zhongfei Yan1,3,5, Liying Mu2,5, Junyang Ma1, Chengjian Zhu1,4
1
ꢀ✉
and Jin Xieꢀ ꢀ
Transition-metal-catalysed, redox-neutral dehydrosilylation of alkenes is a long-standing challenge in organic synthesis, with
current methods suffering from low selectivity and narrow scope. In this study, we report a general and simple method for
the manganese-catalysed dehydrosilylation and hydrosilylation of alkenes, with Mn2(CO)10 as a catalyst precursor, by using a
ligand-tuned metalloradical reactivity strategy. This enables versatility and controllable selectivity with a 1:1 ratio of alkenes
and silanes, and the synthetic robustness and practicality of this method are demonstrated using complex alkenes and light
olefins. The selectivity of the reaction has been studied using density functional theory calculations, showing the use of an
iPrPNP ligand to favour dehydrosilylation, while a JackiePhos ligand favours hydrosilylation. The reaction is redox-neutral and
atom-economical, exhibits a broad substrate scope and excellent functional group tolerance, and is suitable for various syn-
thetic applications on a gram scale.
rganosilicon compounds, with their unique physical and air and moisture tolerance as well as excellent functional group
chemical characters, have been widely used in a range of compatibility26,27. The exploration of manganese-catalysed silylation
O
modern industries1,2, pharmaceuticals3, organic synthesis4 of alkenes promises to be an attractive solution to problems related
and high-performance material fields5 (Fig. 1a). Because they are to the high cost of platinum28–35. The pioneering work of Faltynek
not naturally occurring, new synthetic methodologies for the cata- in 198329 demonstrated that manganese-catalysed hydrosilylation
lytic formation of carbon–silicon (C–Si) bonds6–9 can assist silicon reactions of alkenes lead to a mixture of products, and dehydrosi-
chemistry and have gained considerable momentum in recent years. lylation is more challenged than hydrosilylation (Fig. 1c).
Selective silylation of alkenes with commercially available silanes is
Mn2(CO)10 is commercially available and possesses a weak
one of the most reliable strategies with which to produce divergent Mn–Mn bond (185kJmol−1)36. It can produce a manganese radical
organosilanes, such as alkylsilanes, vinylsilanes and allylsilanes, that can be used in a series of atom transfer reactions in organic
by hydrosilylation and dehydrosilylation10 (Fig. 1b). Although synthesis37–41. In the case of silanes and alkenes, the result of such
platinum-catalysed hydrosilylation of alkenes has already been a a reaction is a facile hydrosilylation30. Recent efforts to explore the
focus in industrial manufacturing, annually consuming 5.6 tonnes nature of silanes have been renewed, but only sterically hindered
of platinum11,12, transition-metal-catalysed dehydrosilylation, a pro- silanes have been reported to exhibit moderate dehydrosilylation
cess associated with alkene hydrosilylation, remains largely unde- selectivity (up to 10:1) with styrenes34. The ligand effect in organo-
veloped1,13–15. After several decades of efforts with precious-metal metallic chemistry prompted us to consider suitable ligands to tune
catalysts16–20, base-metal-catalysed dehydrosilylation has recently the metalloradical reactivity for divergent organic transformations,
witnessed notable progress21–23. For example, the Nakazawa group21 a subject that currently attracts little attention41,42. We realized
has successfully achieved iron-catalysed dehydrosilylation of teth- this concept through the use of a relatively electron-rich bidentate
ered dienes lacking allylic C–H bonds, and Chirik and colleagues22 iPrPNP ligand (L12), which promotes dehydrosilylation, while
have achieved an elegant cobalt-catalysed dehydrosilylation of the relatively electron-deficient JackiePhos ligand (L22) facilitates
aliphatic alkenes for the synthesis of allylsilanes. Catalytic dehy- hydrosilylation (Fig. 1d). With a developed ligand-tuned metal-
drosilylation of alkenes without external oxidants24,25, however, still loradical reactivity strategy, highly selective manganese-catalysed
faces several major challenges. These include (1) the requirement dehydrosilylation and hydrosilylation of alkenes can be controlled
for at least two equivalents of alkenes (one as a sacrificial hydro- by different ligands. This will enrich manganese-catalysed radi-
gen acceptor), (2) concomitant hydrosilylation side reactions along cal transformations and assist in the synthesis of diverse organic
with dehydrosilylation and (3) narrow scope and unpredictable silicon compounds.
stereoselectivity. Furthermore, ligand-tuned, base-metal-catalysed
controllable dehydrosilylation and hydrosilylation reactions of Results and discussion
alkenes are still elusive.
Reaction development. We began our study of the influence of
Manganese is the third most abundant transition metal in the ligands with the reaction of p-methylstyrene (1a) with triethylsi-
Earth’s crust and is relatively environmentally friendly. In general, lane (2a). As shown in Table 1, the model reaction can occur in
manganese-catalysed organic transformations exhibit satisfactory the absence of ligand, but the hydrosilylation reaction is found to
1State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center
(ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, China. 2School of Chemistry and Chemical Engineering, Qufu
Normal University, Qufu, China. 3College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, China. 4College of Chemistry and
Molecular Engineering, Zhengzhou University, Zhengzhou, China. 5These authors contributed equally: Jie Dong, Xiang-Ai Yuan, Zhongfei Yan, Liying Mu.
✉
e-mail: xie@nju.edu.cn
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a
b
R2
Si
Me
Si
Me
Si
O
Si
R
n
R
R1
R3
Transition metal
Transition metal
Developing
R
H
+
+
Si
H
Flame-retardant silicone rubber
+
R
Si
Undeveloped
R
Si
SiR3
Building blocks
SiEt3
Organosilicon
R
5
d
L
Textile finish lubricants
(CO)5Mn Mn(CO)5
LMn
Si(OEt)3
Si
Si
H
Low-rolling resistance tyres
LMn–H
iPrPNP (L12)
JackiePhos (L22)
c
H
Si
Si
Si
R
Si
Ligand control
55%
R
Si
O
Si
O
O
Si
O
MnL
H-atom transfer
Hydrosilylation
Si
H
[Mn(CO)5SiPh3]
180 °C
R
25%
3%
Si
β-H elimination
Si
R
Dehydrosilylation
+
Si
7%
R
Fig. 1 | the state of the art for divergent silylation of alkenes. a, The organosilicons are important chemicals in industry, pharmaceuticals and
high-performance material fields, as well as organic chemistry. Alkylsilanes and vinylsilanes are widely applied in textile finishing lubricants, silicone rubber,
low-rolling-resistance tyres and as versatile building blocks in synthetic chemistry. b, The state of the art for transition-metal-catalysed dehydrosilylation
and hydrosilylation. Transition metals Pt, Fe, Co, Ir, Ni, Mn and so on have been developed for hydrosilylation, and transition metals Ru, Ir, Rh, Re, Fe, Co
and so on have been applied for dehydrosilylation. Despite recent advances, base-metal-catalysed dehydrosilylation remains undeveloped and still faces
several major problems, such as low selectivity, sacrificial olefins or narrow scope. c, The seminal work of manganese-catalysed alkene hydrosilylation with
heptamethylcyclotetrasiloxane, along with dehydrosilylation and alkene isomerization, where the hydrosilylation is more favoured than dehydrosilylation at
180ꢀ°C (ref. 29). d, This work: manganese-catalysed divergent silylation of feedstock alkenes is achieved through ligand-tuned metalloradical activity, where
iPrPNP ligand L12 promotes dehydrosilylation while the JackiePhos ligand (L22) facilitates hydrosilylation.
be the predominant reaction (47% versus 10%), in accord with hydrosilylation (L17–L21). This led us to examine JackiePhos
Faltynek’s early report29. Because nitrogen-containing ligands such (L22), a relatively electron-deficient and sterically hindered biaryl
as terpyridine (L1) and bis(imino)pyridine (L2) have been applied phosphine ligand44, which with 3mol% Mn2(CO)10 as catalyst, can
in base-metal catalysis22,33,35,43, we first focused our attention on improve the yield and selectivity of hydrosilylation to 79% and pro-
screening a library of nitrogen-based ligands. It was found that duce a 3a:4a ratio of 3:79.
terpyridine (L1) cannot promote either reaction, but bis(imino)
pyridine (L2) could improve the hydrosilylation reaction, albeit Mechanistic studies. We attempted to understand the unique
i
with a lower total yield (3a:4a=1:19). Continued screening of a ligand effects of PrPNP and JackiePhos in manganese-catalysed
series of N-based ligands indicated that the concept of ligand-tuned divergent silylation. As illustrated in Fig. 2a, it was found that an
dehydrosilylation was realizable, as L6 can improve the reaction air-stable Mn–H intermediate, L12Mn(CO)3H (5), can be isolated
selectivity to 3:1 (3a:4a). We rationalized that nitrogen-containing as a yellow powder in 60% yield from a reaction mixture contain-
i
ligands (L3–L6) might deactivate the manganese catalytic reactiv- ing the PrPNP ligand (L12), Mn2(CO)10 and silanes, while other
ity. A first attempt with a traditional monophosphine ligand, tri- ligands such as L8 or L22 produced very little Mn–H complex. This
phenylphosphine (L7), supported our assumption and changed indicates that different ligands can affect the reactivity and stability
the reaction product distribution, with the hydrosilylation reaction of the manganese intermediate. In all cases, LMn(CO)3SiEt3 inter-
dominating. To achieve manganese-catalysed dehydrosilylation, mediates were not obtained. An X-ray crystallographic structure
i
one urgent challenge is thorough suppression of hydrosilylation and analysis showed that the P–Mn–P angle (β) of the PrPNPMn–H
promotion of dehydrosilylation.
intermediate L12Mn(CO)3H (5) is indeed as low as 70.1° and the
Fortunately, with L8 as the ligand, dehydrosilylation proceeds P–P distance in 5 is shorter than that in L8Mn(CO)3Br (6) (2.579
rather than hydrosilylation, increasing the selectivity of 3a:4a to versus 2.706Å).
>5:1, although the reaction is slow. Among several bidentate ligands
When the isolated Mn–H intermediate L12Mn(CO)3H (5) was
(L8–13, L15 and L16), iPrPNP (L12) is the most effective. It can fur- used as a manganese precursor, an 89% yield of 3a was obtained, but
nish the dehydrosilylation product (3a) in 85% gas chromatography the reaction of Mn(CO)5SiEt3 with the L12 combination resulted in
(GC) yield with excellent stereoselectivity (E/Z>99:1 determined only traces of product (Fig. 2b). These experimental results imply
by GC analysis). The use of electron-rich monodentate ligand that the Mn–H intermediate L12Mn(CO)3H (5) may be an impor-
L14 resulted in only a small amount of desired dehydrosilylation tant component in the catalytic cycle. Deuterium labelling experi-
product (3a) and the hydrosilylation process was inhibited. During ments (Fig. 2c) show that both β-H elimination and Si–H cleavage
ligand screening, it was also found that both the electronic and ste- may not be involved in the rate-limiting steps for dehydrosilylation,
ric effects of phosphine ligands were crucial to manganese-catalysed but Si–H cleavage (kinetic isotope effect (KIE)=4.94) would be the
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Table 1 | Discovery of reaction conditions
[Mn2(CO)10] (5 mol%)
ligand (10 mol%)
SiEt3
SiEt3
H
SiEt3
+
+
PhCF3, 140 °C, air
Me
Me
Me
1a
2a
3a
4a
Me
Ar
Me
N
No ligand
3a: 10%
4a: 47%
L4 R = H, 3a: <1%, 4a: <1%
L5 R = 2-Me, 3a: <1%, 4a: <1%
L6 R = 3-COOEt, 3a: 5%, 4a: 1.5%
N
R
N
N
N
N
N
N
Ar
N
L1
L2 Ar = 2,4,6-trimethylphenyl
3a: 1%; 4a: 19%
L3
3a: ND; 4a: ND
3a: ND; 4a: ND
PPh2
Me
Me
Ph
P
H
N
Ph2P
PPh2
Ph2P
PPh2
Ph2P
PPh2
N
Ph
Ph
PPh2
Ph2P
PPh2
L7
3a: 5%; 4a: 10%
L8
3a: 5%; 4a: <1%
L9
L10
L11
L12
3a: 6%; 4a: <1%
3a: 9%; 4a: <1%
3a: <1%; 4a: <1%
3a: 85%; 4a: ND
O
Me
Me
Ph
O
PtBu2
NMe2
PPh2
PPh2
O
O
Me
N
Ph
P
PPh2
P
O
O
Ph2P
PPh2
Me
Ph
O
L18
L13
L14
3a: 2%; 4a: ND
L15
L16
3a: <1%; 4a: 9%
L17
3a: 3%; 4a: 57%
3a: 12%a; 4a: 80%a
3a: 5%; 4a: ND
3a: 20%; 4a: 69%
3a: 3%; 4a: 35%
CF3
CF3
Me
Me
Me
Me
F3C
CF3
CF3
CF3
P
P
CF3
iPr
F3C
P
iPr
Me
Me
Me
Me
F3C
P
CF3
MeO
iPr
CF3
F3C
CF3
Me
OMe
L22
L20
L19
L21
3a: 2%b, 4a: 67%b
3a: 3%c, 4a: 79%c
3a: 5%a; 4a: 43%a
3a: 4%a; 4a: 55%a
3a: 3%b; 4a: 62%b
3a: 4%c; 4a: 59%c
Unless otherwise noted, the reaction conditions are Mn2(CO)10 (5ꢀmol%), ligand (10ꢀmol%), 1a (1.0ꢀmmol), 2a (1.0ꢀmmol), 140ꢀ°C, PhCF3 (0.2ꢀml), 24ꢀh. aMn2(CO)10 (5ꢀmol%), ligand (10ꢀmol%),
1a (1.0ꢀmmol), 2a (1.0ꢀmmol), 120ꢀ°C, PhCF3 (0.2ꢀml), 24ꢀh. bMn2(CO)10 (5ꢀmol%), ligand (5ꢀmol%), 1a (1.0ꢀmmol), 2a (1.0ꢀmmol), 120ꢀ°C, PhCF3 (0.2ꢀml), 24ꢀh. cMn2(CO)10 (3ꢀmol%), ligand (6ꢀmol%),
1a (1.0ꢀmmol), 2a (1.0ꢀmmol), 120ꢀ°C, PhCF3 (0.2ꢀml), 24ꢀh. GC yield was used during reaction conditions optimization. ND, not detected.
rate-limiting step for hydrosilylation. This also demonstrates that reagent, was subjected to electron paramagnetic resonance (EPR)
the hydrogen atom in the hydrosilylation of alkenes originates from analysis. The broad resonance signals in the resulting EPR spectra
the silanes. As shown in Fig. 2d, using different ligands leads to a may be the result of superposition of several radicals with trapped
completely different total reaction profile. For dehydrosilylation, manganese radical species (Supplementary Fig. 15). The adducts
the reaction rate in the initial phase is very fast, but then becomes of DMPO with L12(CO)3Mn• and L22(CO)4Mn• were successfully
slow, possibly due to catalyst deactivation. We speculated that the identified by high-resolution mass spectrometry (HR-MS).
fast dehydrosilylative process in the initial period could promptly
decrease the substrate concentration to ensure good dehydro- Proposed mechanism. A proposed mechanism can be derived from
silylative selectivity. The hydrosilylation rate, on the other hand, thermal cleavage of the Mn–Mn bond in ligand-coordinated car-
is relatively smooth, with ~20% yield of 4a formed after 3h. This bonyl manganese (Fig. 3a). The reactive manganese radical result-
significant difference in reaction rate further demonstrates the ing from this cleavage undergoes a hydrogen atom transfer (HAT)
important role of ligands in divergent silylation. We also investi- process with a silane to generate the corresponding LMnH species
gated both dehydrosilylation and hydrosilylation using different and a silyl radical34, which rapidly undergoes radical addition to
para-substituted styrenes. The Hammett plot (log(k/k0) versus σp) olefins to furnish a secondary alkyl radical (8). The iPrPNP-bound
shows a linear relationship, with positive slope for both dehydrosi- Mn–H species can regenerate the corresponding manganese radi-
lylation (0.5313) and hydrosilylation (0.8102) (Fig. 2d). This indi- cal and extrusion of H2 by thermal homolysis. The shuttle between
cates that the electronic effect of the alkenes may not be a vital factor L12Mn–H and L12Mn• may support a very low concentration of
for manganese-catalysed dehydrosilylation and hydrosilylation.
L12Mn•, while the concomitant silyl radical successively adds to the
On addition of 2,2,6,6-tetramethylpiperidinyloxy (TEMPO, a olefins, increasing the concentration of the alkyl radical (8). When
radical trapping reagent) into the model reaction with either L12 or the concentration of the alkyl radical becomes much higher than
L22, dehydrosilylation and hydrosilylation are both inhibited, sug- the concentration of the corresponding L12Mn•, a dynamically
gesting a likely radical pathway for both transformations (Fig. 2b). To controlled radical cross-coupling34 of alkyl radical 8 and L12Mn•,
further confirm this, a reaction mixture with L12 and L22, after the guided by a ‘persistent radical effect’, takes place45,46. In our system,
i
addition of dimethyl pyridine N-oxide (DMPO) as a spin-trapping the PrPNP-bound manganese radical would be more reactive and
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b
a
H
SiEt3
PhCF3
140 °C
(1 mmol)
Mn2(CO)10
0.1 mmol
+
Ligand + HSiEt3
LMn(CO)3SiEt3
L = L8, ND
L = L12, ND
L = L22, ND
+
LMn(CO)3H
L = L8, ND
L = L12, 60%
L = L22, ND
SiEt3
SiEt3
+
+
Ar
Me
Ar
Ar
0.2 mmol 1.0 mmol
PhCF3, 140 °C, air
Ar = (4-Me)C6H4
Me
3a
4a
7a
(i) L12Mn(CO)3H 5 (100 mol%)
89%
Trace
5%
ND
ND
Trace
Trace
Trace
Trace
α
β
(ii) Mn(CO)5SiEt3 (10 mol%), L12 (10 mol%)
(iii) Standard conditions with L12 + TEMPO (3 equiv.)
α
β
ND
(iv) Standard conditions with L22 + TEMPO (3 equiv.) Trace
Trace
5
6
L12Mn(CO)3H
L8Mn(CO)3Br
c
(92%D)
Mn2(CO)10 (5 mol%)
D
D
α = 98.3°; β = 70.1°
α = 95.1°; β = 71.0°
L12 (10 mol%)
3c-d2: 46%
D
P–P distance = 2.579 Å
P–P distance = 2.706 Å
SiEt3
Ph
+
+
+
+
Et3SiH
Et3SiH
Ph
PhCF3, 140 °C, 10 min
D
D
d
(92%D)
100
80
60
40
20
0
kH/kD = 1.30
1c-d3, 92%D
Dehydrosilylation
Hydrosilylation
Mn2(CO)10 (5 mol%)
L12 (10 mol%)
(45.5%D)
SiEt3
H(D)
1c +
1c-d3
3c/3c-d2: 68%
Ph
Ph
PhCF3, 140 °C, 10 min
(1:1)
H(D)
kH/kD = 1.20
(45.5%D)
Hammett plot
0.4
Mn2(CO)10 (5 mol%)
L12 (10 mol%)
y = 0.8102×–0.0105
R
2 = 0.9719
H
SiEt3
Ph
Et3SiD
Et3SiD
p-Cl
3c: 61%
PhCF3, 140 °C, 10 min
1c
0
kH/kD = 0.98
p-Me
y = 0.5313×–0.0411
R
2 = 0.9455
Mn2(CO)10 (3 mol%)
L22 (6 mol%)
(100%D)
SiEt3
D
p-OMe
H
Ph
4e-d1: 1.7%
–0.4
Ph
–0.4
–0.15
0.10
0.35
24
PhCF3, 120 °C, 30 min
1c
σp
20
kH/kD = 4.94
0
4
8
12
Time (h)
16
Fig. 2 | Preliminary mechanistic studies. a, Isolation and characterization of the LMn–H intermediates. The L12Mn(CO)3H complex (5) is relatively air-stable,
while L8Mn(CO)3–H and L22Mn(CO)4–H cannot be obtained. To compare the unique ligand skeleton effect in L12Mn(CO)3H (5), L8Mn(CO)3Br (6) was
prepared (Supplementary Section 7.2). The X-ray structures of 5 and 6 (hydrogen atoms are omitted for clarity) are also shown. b, Mechanistic control
experiments with GC yielded the following: (i) use of the L12Mn(CO)3H intermediate can successfully initiate dehydrosilylation, and only trace hydrogenation
by-product 7a was detected; (ii) use of Mn(CO)5SiEt3 resulted in failure; (iii) the addition of 3ꢀequiv. TEMPO significantly inhibited dehydrosilylation; (iv)
the addition of 3ꢀequiv. TEMPO suppressed hydrosilylation. The experimental results for (iii) and (iv) indicate a radical mechanism. c, Deuterium labelling
experiment. Parallel and competitive KIE experiments with D-labelled styrenes demonstrate that the β-H elimination process may not be the rate-determining
step in dehydrosilylation. Si–H cleavage is also not involved in the rate-determining step for dehydrosilylation, but it would be the rate-limiting step for
manganese-catalysed hydrosilylation. d, The total reaction profiles for the dehydrosilylation reaction (red) and hydrosilylation reaction (blue) of 1a and 2a, as
well as the Hammett plot (small window) in one single experiment. We estimate the error in the integral ratios to be <1% based on the signal-to-noise ratio
of the GC spectra. The reaction progress showcases that dehydrosilylation is faster than hydrosilylation in the initial phase. ND, not detected.
alkyl radical 8 appears to be less reactive. Because of the ligand phosphine atoms just medially distributed in the equator of 9-L12
i
effect of PrPNP, the coupled alkylmanganese(i) intermediate (9) and thus dissociation of one CO ligand might become relatively
readily undergoes CO dissociation and syn-periplanar β-H elimina- accessible to form IN3–L12 possibly due to the trans-influence
tion to afford the dehydrosilylation product (3). On the other hand, (Supplementary Fig. 26, CO dissociation energy in 9–L12: 9.9 ver-
when the JackiePhos ligand is employed, the resulting alkyl radi- sus 12.8 and 31.8kcalmol−1). The following intermediate IN3–L12
cal (8) can undergo HAT with L22MnH species to give rise to the can undergo β-H elimination via transition state TS3-L12 to pro-
hydrosilylation product (4).
duce the dehydrosilylation product 3a. Alternatively, alkyl radical 8a
can interact with L12Mn–H to undergo the HAT process via tran-
Computational selectivity studies. To better understand the origin sition state TS4-L12, which would deliver hydrosilylation product
of ligand-controlled selectivity, the key selectivity-controlled steps 4a. However, the noncovalent interactions analyses for HAT transi-
of dehydrosilylation and hydrosilylation with different ligands (L12 tion state TS4-L12 demonstrate that there is severe steric crowd-
i
and L22) were explored using density functional theory (DFT) cal- ing between the skeletons of alkyl radical 8a and PrPNP ligand
culations at the TPSSh-D3(BJ)/def2-TZVP//M06/def2-SVP level at L12. Comparing the two pathways with ligand L12, the transition
the experimental temperature (140°C for L12; 120°C for L22). The state for the formation of dehydrosilylation product 3a is lower
primary results for the reaction of radical 8a, the β-H elimination (23.9kcalmol−1 via TS3-L12) than that of hydrosilylation product
step affording dehydrosilylation product 3a and the HAT process 4a (27.9kcalmol−1 via TS4-L12), which suggests that the dehydrosi-
giving hydrosilylation product 4a are depicted in Fig. 3b (the free lylation pathway is kinetically more favourable than hydrosilylation
energy profiles are provided in Supplementary Figs. 23 and 24).
for ligand L12.
i
For the PrPNP ligand (L12), alkyl radical 8a can couple with
For the JackiePhos ligand L22, although both energy barriers
L12Mn• to give alkylmanganese intermediate 9–L12. We envisioned of the β-H elimination process and HAT process become smaller
that the small bite angle of the iPrPNP ligand would enable its two than that for L12, its HAT pathway via TS4-L22 (18.5kcalmol−1)
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a
R3Si
H
L12
L22
2
(OC)5Mn Mn(CO)5
L22Mn(CO)4
L12Mn(CO)3
R3Si
H
2
R3Si
∆
Dehydrosilylation
L = L12
Hydrosilylation
L = L22
L12Mn(CO)3H
L12Mn(CO)3
SiR3
R
R3Si
H2
8
CO
(CO)3MnL12
SiR3
SiR3
HAT
R
R3Si
R
3
(CO)2MnL12
β–H elimination
L22Mn(CO)4H
9
SiR3
H
R
R
SiR3
1
10
R
SiEt3
CO
4
b
Ar
H
Ph2
P
SiEt3
OC
OC
N
iPr
Mn
Et3Si
Ar
H
Ar
P
Ph2
P
Ph2
H
Ph2
P
CO
27.9
N
iPr
Mn
OC
OC
Et3Si
N
iPr
P
Mn
OC
Ph2
H
Ph2
P
P
TS4-L12
CO
23.9
TS3-L12
20.5
SiEt3
Ph2
Ph2
iPr
CO
N
Mn
Ar
OC
Ar
OC
P
21.4
IN5-L12
18.5
IN5-L22
P
Ph2
iPr
CO
N
Mn
P
18.5
TS4-L22
OC
TS3-L22
18.0
IN3-L12
Ph2
CO
14.5
9-L12
Ar
SiEt3
SiEt3
SiEt3
CO
OC
Ar
Ar
OC
OC
11.3
IN3-L22
OC
Mn
H
H
H
CO
OC
OC
7.7
7.2
CO
L22
Ar
OC
Mn
CO
CO
SiEt3
Mn
CO
CO
L22
L22
H
OC
Mn
Ar
SiEt3
1.5
9-L22
8a
CO
L22
Ar = 4-MeC6H4
SiEt3
Ar
OC
OC
3a
3a
Ar
–8.1
–8.3
Mn
CO
CO
–9.1 4a
SiEt3
4a
–10.0
L22
SiEt3
Ar
Dehydrosilylation
Hydrosilylation
1.88
1.57
1
2
1.80
1.60
3
9-L12
Mn-1CO
9.9
12.8
31.8
Mn-2CO
Mn-3CO
TS3-L12
TS3-L22
TS4-L12
TS4-L22
Fig. 3 | Mechanistic proposal and DFt studies. a, The proposed reaction mechanism. Ligand-coordinated carbonyl manganese is formed in situ. The β-H
elimination process and the HAT process are the key steps to control dehydrosilylation and hydrosilylation selectivity. b, The calculated free energy profile
of selectivity for different ligands at the reaction temperature (140ꢀ°C for L12; 120ꢀ°C for L22). The profiles start with 8a, which is formed from 1a, 2a and
LMn•. The full profiles for both reactions, including steps for the formation of 8a, are provided in the Supplementary Information (Supplementary Fig. 23 for
L12 and Supplementary Fig. 24 for L22). The computational results explain the difference in selectivity of the competitive reactions with different ligands;
the preferred energy barrier for iPrPNP ligand (L12) is 23.9ꢀkcalꢀmol−1 and for the JackiePhos ligand (L22) is 18.5ꢀkcalꢀmol−1. The CO dissociation free energies
of 9–L12 are given in kcalꢀmol−1 and bond distances of the transition states of ts3-L12 and ts3-L22 are given in Å. Noncovalent interaction analyses for the
HAT transition states of ts4-L12 and ts4-L22 are shown: red, blue and green surfaces represent the steric effect, strong interaction and weak interaction,
respectively. All energies are in kcalꢀmol−1.
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Table 2 | synthetic scope of ligand-controlled manganese-catalysed divergent silylation
Mn2(CO)10 (5 mol%)
Mn2(CO)10 (3 mol%)
L12 (10 mol%)
L22 (6 mol%)
SiR3
SiR3
↑
H2
R3SiH
+
+
R
R
R
PhCF3, 140 °C, air, 24 h
PhCF3, 120 °C, air, 24 h
3 (E/Z > 95:5)
4
Cl
Styrenes
R
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
O
Fe
Cl
Me
O
3h: 77%
3i: 56%
3j: 72%
3k: 84%
3l: 89%
3m: 66%
3t: 61%
3a: R = Me, 76%
3b: R = tBu, 82%
3c: R = H, 78%
3d: R = OMe, 73%
3e: R = F, 74%
Cl
SiEt3
Me
Me
Me
Me
SiEt3
N
S
SiEt3
SiEt3
O
SiEt3
SiEt3
Ph
N
N
3f: R = CN, 65%
3g: R = CF3, 75%
S
Me
SiEt3
N
Ts
Me
3n: 82%
3o: 73%
3p: 35%
Me
3q: 48%
3r: 71%
3s: 72%
Aliphatic alkenes
Me
O
Me
Et3Si
Me
SiEt3
Me
SiEt3
3
Me
SiEt3
Cl
SiEt3
5
MeO
SiEt3
Me
7
Me
3u: 58%
3v: 53%
3w: 77%
3x: 68%
3y: 70%
3z: 70%
3aa: 76%
Me
SiEt3
Ph
SiEt3 Ph
SiEt3
SiEt3
SiEt3
O
SiEt3
TMS
O
SiEt3
HO
SiEt3
O
7
2
2
3ab
3ab'
O
O
69% (3ab:3ab' = 11: 89)
3ad: 67%
3ae: 62%
3af: 60%
3ag: 71%
3ah: 50%
SiEt3
O
N
O
Et3Si
CN Et3Si
CN
SiEt3
Ph
O
SiEt3
O
PhO
3ac
77% (3ac:3ac' = 63:37)
Complex alkenes
3ac'
SiEt3
3ai: 72%
3aj: 54%
3ak: 53%
3al: 40%
(
)-3am: 67%
O
MeOOC
O
Me
SiEt3
iPr
SiEt3
O
O
SiEt3
H
MeO
O
O
O
Ph
Ph
Me
Me
N
O
H
H
SiEt3
O
Et3Si
Me
Me
3ar: 43% (from naproxen)
3an: 67% (from ibuprofen)
3ao: 45%a,b (from oestrone)
3ap: 83% (from cocal)
3aq: 56% (from oxaprozin)
O
Me
Me
Me
O
O
O
O
O
OH
Me
H
SiEt3
H
Me
H
O
O
H
H
SiEt3
O
O
H
H
O
O
H
MeO
Et3Si
Ad
3as: 39%a,b (from adapalene)
Me
Me
3av: 47%a,b (from allylestrenol)
3at: 43% (from pregnenolone)
3au: 46%b (from diacetone-D-glucose)
Et3Si
Silanes
SiMe3
Si SiMe3
SiMe3
3ax: 65%
Me
Si Ph
Ph
Me
Si
Me
Me
Si OEt
OEt
3bb: 72%
Me
Si Cy
Me
Me
Si
Et
Me
Si
tBu
Me
Si(OEt)3
Si(nPr)3
Si(nHex)3
Si(iPr)3
Bn
Et
SiR3
3aw: 79%
Me
Si noctyl
Me
3ay: 70%
Me
Si Me
Ph
3az: 51%
3ba: 57%
3bc: 85%
3bd: 86%
3be: 85%
Me
Me
Si(noctyl) 3
SiH2Ph
SiHPh2
Si
OSiMe3
Si(OiPr)3
OSiMe3
3bj: 73%
3bg: 31%
3bh: 49%
3bi: 87%
3bk: 77%
3bl: 38%
3bm: ND
3bn: ND
3bf: 87%
Hydrosilylation
O
O
SiEt3
SiEt3
SiEt3
SiEt3
O
SiEt3
4
SiEt3
4
O
S
R
4f: 50%c
h:dh = 95:5
4g: 86%
h:dh = 98:2
4h: 72%d
Me
Me
R = H, 4a: 83% (h:dh = 98:2)
R = Me, 4b: 79% (h:dh = 96:4)
R = F, 4c: 89% (h:dh = 95:5)
4d: 82%
h:dh >99:1
4e: 76%
h:dh = 98:2
h:dh > 95:5e
MeO
Ad
O
Me
Si
Me
Me
Si
Et
Me
Si
Si(nPr)3
Si
O
O
O
Si
Si
Et
Si noctyl
Me
H
H
O
Me Si
O
H
Me
Me
Si
Et3Si
Me
4i: 89%d
4j: 88%
h:dh = 95:5
4k: 85%
h:dh = 99:1
4l: 81%
h:dh > 99:1
Si
O
Si
h:dh > 95:5e
4m: 76%
Unless otherwise noted, the optimized conditions are as described in Table 1 for dehydrosilylation and hydrosilylation in 1ꢀmmol scale. The stereoselective ratio (E/Z) represents the ratio of (E)-vinylsilanes
to (Z)-vinylsilane, as determined by 1H NMR analysis of isolated products. The h:dh values represent the ratio of product 4 to 3 as determined by GC analysis. aThe reaction was performed at 160ꢀ°C.
bThe reaction time was 30–36ꢀh. cThe reaction was performed at 110ꢀ°C. dThe reaction was performed at 140ꢀ°C for 30ꢀh. eThe h:dh ratios were determined by 1H NMR analysis of isolated products. Ts,
toluenesulfonyl; Et, ethyl; Pr, propyl; Cy, cyclohexanyl; Bn, benzyl; tBu, tert-butyl; Ad, 1-adamantyl.
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a
b
Conditions A
Conditions B
Yield Selectivity
Ph
Light alkene
Ethylene
Product
Me
Yield Selectivity
Product
Ph
Ph
Ph
11: 76%
12: 61%
Me
SiEt3
99:1
40%
99:1
Et
Si Ph
Ph
Et Si Ph 90%
13: 70%
(i)
(iii)
Ph
4n
(ii)
4n
Ph
Me
Me
(iv)
Standard
nPr Si Ph 70%
1c
Ph
14: 83%
>99:1
>99:1
>99:1
Propylene
Propylene
Si Ph 40%
Ph
conditions
Me
Ph
SiEt3
Ph
4o
+
Ph
3bo
3c
(v)
Br
Ph
Et3SiH
15.26 g, 70%
Me
Si Bn
Me
3bp
Me
15: 82%
nPr Si Bn
2a
91:9
75%
51%
65%
80%
Me
Et
(vi)
Me
4p
(viii)
(vii)
O
Me
Si Bn
Me
Me
Si Bn
Me
D
O
nBu
Ph
>99:1
>99:1
Butylene
Ph
H
Ph
Me
16: 90%
(>95% D)
17: 83%
18: 70%
3bq
4q
Fig. 4 | enrichment and elaboration of products. a, Divergent silylation with light olefin. Conditions A: Mn2(CO)10 (5ꢀmol%), L12 (10ꢀmol%), alkene (0.2–
0.5ꢀMPa), silane (1–3ꢀmmol), 115–120ꢀ°C, PhCF3 (5.0ꢀml), 24ꢀh. Conditions B: Mn2(CO)10 (5ꢀmol%), L21 (5ꢀmol%), alkene (0.2–0.4ꢀMPa), silane (1–4ꢀmmol),
130ꢀ°C, PhCF3 (5ꢀml), 24ꢀh. The product selectivities for dehydrosilylation and hydrosilylation were determined by GC analysis. b, The 100ꢀmmol scaled-up
experiment under standard reaction conditions to afford product 3c and its downstream diversification. To prepare product 11: (i) Pd(OAc)2 (10ꢀmol%),
3c (0.5ꢀmmol), bromoethene (2.5ꢀmmol), tetrahydrofuran (THF), 150ꢀ°C, 24ꢀh. To prepare product 12: (ii) Pd(OAc)2 (10ꢀmol%), 3c (0.5ꢀmmol), (iodoethynyl)
benzene (1.5ꢀmmol), THF, room temperature, 24ꢀh. To prepare product 13: (iii) 3c (1ꢀmmol), Ir(ppy)2(dtbpy)PF6 (4ꢀmol%), blue LEDs (2ꢀ×ꢀ45ꢀW), CH3CN,
30ꢀh. To prepare product 14: (iv) Pd(dba)2 (10ꢀmol%), 3c (0.5ꢀmmol), Ph–I (1.5ꢀmmol), Bu4NF (1ꢀM in THF, 1.5ꢀmmol), THF, room temperature, 1ꢀh. To prepare
product 15: (v) 3c (0.5ꢀmmol), Br2 (1.5ꢀmmol), hexane, room temperature, 12ꢀh. To prepare product 16: (vi) 3c (2ꢀmmol), D–Cl (6ꢀmmol), CH3CN, 60ꢀ°C,
24ꢀh. To prepare product 17: (vii) AlCl3 (3ꢀmmol), 3c (1ꢀmmol), CH3COCl (1.5ꢀmmol), CH2Cl2, room temperature, 3ꢀh. To prepare product 18: (viii) Pd(OAc)2
(10ꢀmol%), 3c (0.5ꢀmmol), PhI(OAc)2 (0.5ꢀmmol), AcOH (2ꢀml), 120ꢀ°C, 24ꢀh. dba, bis(dibenzylideneacetone); ppy, 2-phenylpyridine; dtbpy, 2,2'-bis(4-tert-
butylpyridine).
is lower in energy compared with its β-H elimination pathway via (β-H versus β′-H elimination) is generally acceptable. A compet-
TS3-L22 (20.5kcalmol−1). This makes the hydrosilylation kineti- ing β′-H elimination occurs predominantly when aliphatic alkenes
cally more favourable than the dehydrosilylation process for ligand bear an acidic allylic C(sp3)–H bond, such as a benzylic C–H bond
L22 and thus it predominantly delivers hydrosilylation product 4a. and the α-C(sp3)–H bond of nitrile (3ab′, 3ac′). Because of the
We speculated that the small steric effect would contribute to the α-silicon effect50 in the generated alkylmanganese intermediate (10),
preferred HAT process (Supplementary Fig. 28). The preferred bar- the acidity of the β-C(sp3)–H bond adjacent to the silicon atom has
rier for iPrPNP ligand L12 is 23.9kcalmol−1 and that of JackiePhos been increased while the hydricity has been decreased compared
ligand L22 is 18.5kcalmol−1. These differences in the apparent bar- with the competing β′-C(sp3)–H bond. This may indicate that
riers might reflect the variation of reaction temperature: 140°C for manganese-mediated β-H elimination prefers to occur at the rela-
dehydrosilylation with ligand L12 and 120°C for hydrosilylation tively acidic β-C–H bonds, favouring the formation of vinylsilanes as
with ligand L22. The calculated results are in agreement with the the main product (3v–3x, 3z, 3ad–3af, 3ah–3ak, 3am, 3an, 3aq–3as,
experimental observation of the product distribution (3a versus 4a) 3av, 3bo–3bq). The regio- and stereoselectivity in aliphatic alkenes
with different ligands.
further highlights the synthetic robustness for the catalytic chem-
istry of manganese. Moreover, when the substrate contains differ-
Synthetic scope. Owing to their unique reactivity and low toxic- ent alkene units, this protocol prefers to undergo dehydrosilylation
ity, vinylsilanes are particularly attractive reagents in organic syn- at the less sterically hindered terminal double bonds (3y, 3af).
thesis4. With the optimized conditions that have been established, Conjugated 1,4-dienes can undergo manganese-catalysed dehydro-
a wide variety of structurally diverse vinylsilanes can be obtained silylation smoothly and selectively to furnish products 3t, 3y and
from commercially available feedstocks by manganese-catalysed 3aa with satisfactory yields. The alcohol-type alkenes (3ah) can still
dehydrosilylation (Table 2). The readily available substrates, broad be successfully employed for dehydrosilylation under standard con-
reaction scope, excellent functional group tolerance and good ste- ditions, albeit with 13% yield of silyl-ether by-product (RO-SiEt3).
reoselectivity (E:Z>95:5) make this a general and practical can- When terminal alkynes were employed instead of alkenes, no
didate47–49. Using 1mmol of alkene and 1mmol of triethylsilane desired dehydrosilylation products were formed (Supplementary
in an air atmosphere, all the styrenes are effective coupling part- Fig. 30). Among those alkynes tested, they gave only a trace to a
ners, in spite of the electron and substituent effects at the ortho-, small amount of Z‐selective hydrosilylation product (up to 15%
i
meta- and para-positions (3a–3k, 3n and 3s). The heteroaromatic yield). This may also indicate that the use of PrPNP ligand L12
and ferrocenyl alkenes tolerate the manganese-catalysed dehydro- would substantially change the reactivity of Mn–H species and ren-
silylation conditions well (3m, 3o–3r). The synthetic advantages der L12Mn–H unfavourable for the HAT process.
of the reaction are further illustrated by direct dehydrosilylation
Focusing on synthetic practicability and robustness, we applied
of non-activated aliphatic olefins (3u–3am), in which control of this method to dehydrosilylation of a series of complex alkenes
the regioselectivity (β-H versus β′-H elimination) remains a (3an–3av), and demonstrated its ability to retain its valuable
great challenge. For example, β′-H elimination occurs to gener- chemo-, regio- and stereoselectivity in addition to excellent func-
ate allylsilanes with cobalt catalysis22. In manganese-catalysed tional group tolerance. Importantly, the introduction of reac-
dehydrogenative silylation of aliphatic alkenes, the regioselectivity tive ketone moieties in the substrates minimally influences the
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chemoselectivity of the reaction (3ao, 3at). All the tertiary silanes References
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times. Finally, the pressure was gradually raised to the appropriate settings, and
the temperature was programmed to rise from room temperature to 130°C, which
was maintained for 24h. After the reaction was finished, the solvent was removed
under vacuum and the resultant residue was purified by column chromatography
on silica gel to afford the products.
Methods
Caution. Manganese-catalysed dehydrosilylation will produce H2 during the
reaction. Te heating of sealed Schlenk tubes may be explosive, although no
incidents have occurred in our laboratory. Appropriate safety precautions must
be taken when undertaking the manganese-catalysed dehydrosilylation reactions
reported in this Article.
Data availability
General procedure for the dehydrosilylation reaction of non-gaseous alkenes.
To an oven-dried 10-ml sealed tube, [Mn2(CO)10] (5mol%), iPrPNP ligand (L12;
10mol%, 0.1mmol) and PhCF3 (0.2ml) were added under air conditions and the
mixture was stirred at 80°C for 50min to 1h to become a deeply red homogeneous
solution. Alkene (1.0mmol) and silane (1.0mmol) were then rapidly added,
successively, into the sealed tubes with a liquid-transferring gun. The resulting
yellow reaction mixture was rigorously stirred at 140°C for 24h. Once the reaction
was finished, the solvent was removed under vacuum and the resulting residue was
purified by column chromatography on silica gel to afford the products.
All data generated or analysed during this study are included in this Article and
its Supplementary Information. Crystallographic data have been deposited at the
Cambridge Crystallographic Data Centre (CCDC) as CCDC 1937151 (5), 1937156
(6) and 1937154 (19) and can be obtained free of charge from the CCDC via www.
ccdc.cam.ac.uk/getstructures.
Acknowledgements
We thank the National Natural Science Foundation of China (grants 21971108,
21971111, 21702098, 21703118, 21732003 and 21672099), the Fundamental Research
Funds for the Central Universities (020514380214), the Natural Science Foundation of
Jiangsu Province (grant no. BK20190006), the ‘Innovation & Entrepreneurship Talents
Plan’ of Jiangsu Province, the ‘Jiangsu Six Peak Talent Project’, Shandong Provincial
Natural Science Foundation (grant no. ZR2017MB038) and start-up funds from Nanjing
University for financial support. Y. Liang, G. Wang and J. Han are acknowledged for
their helpful suggestions and discussions. X. Wu and Y. Zhao are acknowledged for their
help with H2 liberation reactions and X-ray single-crystal structure determination. We
also thank K. Liu, Z. Yan, Y. Ning and W. Li for reproducing products 3a, 3v, 4a and 4m
and C. Zhu, J. Han, Y. Pang, S. Fang and W. Li for their help with the preparation of this
manuscript. The DFT calculations were supported by the High Performance Computing
Center of Qufu Normal University.
General procedure for the dehydrosilylation reaction of light alkenes. To
an oven-dried 10-ml sealed tube, [Mn2(CO)10] (5mol%), iPrPNP ligand L12
(10mol%) and PhCF3 (5ml) were added. The resulting mixture was stirred
at 80°C for 50min to 1h to become a deeply red homogeneous solution.
The mixture was then transferred to a 50-ml autoclave, the corresponding
silane was added with a liquid-transferring gun, successively, and the gaseous
olefin was introduced to the autoclave and displaced three times. Finally, the
pressure was gradually raised to the appropriate settings, and the temperature
was programmed to rise from room temperature to 115–120°C, which was
maintained for 24h. After the reaction was finished, the solvent was removed
under vacuum and the resultant residue was purified by column chromatography
on silica gel to afford the products.
Author contributions
General procedure for the hydrosilylation reaction of non-gaseous alkenes. To
an oven-dried 10-ml sealed tube, [Mn2(CO)10] (3mol%), JackiePhos ligand L22
(6mol%) and PhCF3 (0.2ml) were added under air and the mixture was stirred
at 80°C for 50min to 1h to become a yellow homogeneous solution. Alkene
(1.0mmol) and silane (1.0mmol) were rapidly added, successively, into the sealed
tubes with a liquid-transferring gun, and the resultant yellow reaction mixture was
rigorously stirred at 120°C for 24h. After the reaction was finished, the solvent
was removed under vacuum and the resultant residue was purified by column
chromatography on silica gel to afford the products.
J.D., Z.Y. and J.X. conceived and designed the experiments. J.D., Z.Y. and J.M. performed
the experiments. J.D. and Z.Y. analysed and discussed the experimental data. X.-A.Y.
and L.M. performed DFT calculations and discussed the manuscript. J.X. wrote the
manuscript with input from all authors and discussed with C.Z. All the authors have
approved the final version of the manuscript.
Competing interests
The authors declare no competing interests.
General procedure for the hydrosilylation reaction of light alkenes. To an
oven-dried 10-ml sealed tube, [Mn2(CO)10] (5mol%), ligand L21 (5mol%) and
PhCF3 (5ml) were added under air and the mixture was stirred at 80°C for 50min
to 1h to become a yellow homogeneous solution. The mixture was then transferred
to a 50-ml autoclave, the corresponding silane was added with a liquid-transferring
gun, and the gaseous olefin was introduced to the autoclave and displaced three
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/
s41557-020-00589-8.
Correspondence and requests for materials should be addressed to J.X.
Reprints and permissions information is available at www.nature.com/reprints.
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