a General Reaction Conditions: 1a(130.1 mg, 1.0 mmol, 1.0 equiv.), 2a (130.0 mg, 1.2 mmol, 1.2
equiv.), Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%),
Phosphine Ligands (0.015 mmol, 1.5 mol%), t -BuOK (4.5 mg, 0.02
mmol, 3.0 mol%), THF (3.0 mL), 25 ºC, 6 h. b Conversions of 1a and yields of 5aa were determined by
GC analysis (internal standard, 1,2,4,5-tetramethylbenzene).c r.r. was referred to as the
ratio of 5aa and 7aa , which were determined by1H NMR analysis. d b:l was referred to as the ratio of 5aa and3aa , which were determined by 1H NMR
analysis. e Isolated yield.
milarly, using 1a and 2a as model substrates,
Ni(DME)Cl2 (1.0 mol%) as precatalyst and t -BuOK
(2.0 mol%) as activator in THF under 25 oC, the
addition of PPh3 (L1 , 1.5 mol%) led to
generation of the 3,4-Markovnikov product 5aa in excellent
yield (92%) and regioselectivity (up to 98:2). To explore the role of
ligands, we tested different types of monophosphine ligands. We found
that substituents with different electronic properties on benzene rings
of ligand did not significantly affect the yield and selectivity of the
reaction (entries 2, 3), while bulky ligand L4 resulted in a
significant decrease in conversion (entry 4). These phenomena indicated
that the added phosphine ligand may occupy one coordination site to
prevent bidentate coordination of conjugated dienes with nickel center.
Moreover, alkyl phosphine (L5 ) and heteroaromatic phosphine
(L6 ) had certain negative effects on the reaction. Ultimately,
we completed the optimization of 3,4-Markovnikov hydrosilylation of
conjugated dienes by employing PPh3 (L1 ) as the
ligand.
The substrate scope of this 3,4-Markovnikov hydrosilylation was depicted
in Scheme 3. Compared with ligand-free 3,4-anti -Markovnikov
hydrosilylation, 3,4-Markovnikov hydrosilylation had a wider substrate
scope with the help of ligand, but slightly poor regioselectivities.
Electron-rich aromatic conjugated dienes could also react smoothly and
generate corresponding allylic silanes, including dienes with
electron-donating substituents (5ba-5da ), naphthalene dienes
(5ga ) and heteroaromatic dienes (5ja, 5ka ). Specially,
the electron-deficient aromatic conjugated dienes were tolerated in
current protocol to deliver allylic silanes in good yields but decreased
regioselectivities (5pa, 5ta ).
Scheme 3 Substrate scope for 3,4-Markovnikov hydrosilylation of
conjugated dienes
a General Reaction Conditions: 1 (1.0
mmol, 1.0 equiv.), 2 (1.2 mmol, 1.2 equiv.),
Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%),
PPh3 (4.0 mg, 0.015 mmol, 1.5 mol%), t -BuOK (4.5
mg, 0.02 mmol, 2.0 mol%), THF (3.0 mL), 25 ºC, 6h. The yields are
isolated yields. The value of r.r. and
b:l were determined by 1H NMR
analysis.
Scheme 4 Controlled experiments
Additionally, 1,2-disubstituted conjugated diene was applicable to
obtain 5ua in excellent yield and regioselectivity, which was
unactive in 3,4-anti -Markovnikov addition. Unfortunately, this
protocol was still unsuitable for the aliphatic conjugated diene
(5sa ), a mixture of regioisomers was generated. Furthermore,
the reactions of electron-deficient silane (5ab ), electron-rich
silane (5ac ) and alkyl silane (5ad ) all produced the
corresponding allylic silanes in high yields and regioselectivities.
Obviously, the different selectivity of above hydrosilylations reflected
different reaction processes, for which we conducted controlled
experiments (Scheme 4). We used commercially available precursor
Ni(COD)2 as the nickel(0) catalyst in both the
conditions: with t -BuOK as the activator, both reactions were
able to proceed normally to provide the consistent yields and
selectivities with standard conditions, confirming that nickel(II) was
firstly reduced to nickel(0) in both reactions; in the absence oft -BuOK,
Scheme 5 Proposed mechanisms
the starting materials could also be converted into products in both
reactions, but the reaction rate diminished, indicating that the
activation of t -BuOK to silane could significantly accelerate the
interaction between nickel center and silane.
Based on the above results and our previous
research,10 we proposed possible two kinds of
mechanisms for these different regioselective hydrosilylation of
conjugated dienes. For 3,4-anti -Markovnikov hydrosilylation at
low temperature (-30 oC): firstly, nickel(II) was
reduced to nickel(0) by activated silane, and stabilized by conjugated
diene and solvent (A ); then, the nickel(0) center tended to
form η 2-(Si−H)Ni(0) complex (B ) with
activated pentacoordinate silicate at low temperature and completed the
electrophilic activation of silane; Finally, the activated Si−H bond
directly added to the terminal alkene of conjugated diene to give the
3,4-anti -Markovnikov product 3 . For the 3,4-Markovnikov
hydrosilylation at room temperature: firstly, the nickel(II) was also
reduced to nickel(0) and stabilized by triphenylphosphine (D );
then, oxidative addition of nickel(0) to active Si−H bond occurred to
form the nickel(II) hydride
specie (E ); thirdly, conjugated diene coordinated with the
nickel(II) center (F ); next, C=C double bond migrated and
inserted into the Ni−H bond to form the alkyl nickel(II) specie
(G ), due to the coordinated stabilization of lateral alkene;
lastly, reductive elimination of G occurred to give the 3,4-Markovnikov
product 5 . The essential difference between these two selective
hydrosilylations was the high energy barrier in the oxidative addition
of the nickel(0) center to the Si-H bond, so that oxidative addition
could not take place at low temperatures and the reaction shifted to
electrophilic activation hydrosilylation. While oxidative addition could
occur normally and Chalk-Harrod
hydrosilylation would be carried out at room temperature.
Conclusions
In conclusion, we have developed a Ni-catalyzed regiodivergent
hydrosilylation of conjugated dienes. By controlling temperature and
ligands, we have successfully achieved the highly selective
3,4-anti -Markovnikov hydrosilylation and 3,4-Markovnikov
hydrosilylation respectively for the convenient synthesis of
homoallylic silanes and allylic
silanes in high yields and regioselectivities. Both protocols are
economical and operationally simple, tolerating with various types of
aromatic conjugated dienes bearing electron-donating groups, halogen and
heterocycle. Also, we have identified the origin of the differences
between the two selectivities. Under low temperature (-30oC), Ni(0) species tend to generate electrophilic
activation with active silane; while under room temperature, Ni(0)
species tend to participate in oxidative addition with active silane. We
hope that this method can provide a pragmatic route for scale
preparation of functionalized homoallylic silanes and allylic silanes.
Experimental
General procedure for
3,4-anti -Markovnikov hydrosilylation: An over-dry reaction tube
containing a magnetic stir bar was charged with
Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%) andt -BuOK (4.5 mg, 0.02 mmol, 2.0 mol%) and anhydrous THF (3.0 mL).
Then the tube was sealed with a rubber stopper and inserted into an
ethanol bath pre-cooled to -30 ºC. The mixture was
stirred for 5 min. Then the solution of conjugated diene (1 ,
1.0 mmol, 1.0 equiv.) and silane (2 , 1.2 mmol, 1.2 equiv.) was
added into the tube via syringe in seconds. The mixture was
continued to stir for 6 h. Until the reaction was finished, EtOAc (3.0
mL) was added to quench the reaction. Then the reaction solution was
filtered through a pad of silica gel to remove the catalyst and base.
EtOAc (5.0 mL × 2) was used to wash the silica gel. The combined organic
phase was concentrated under reduced pressure, the residue was purified
by column chromatography (silica
gel) to give the corresponding product3 with Petroleum ether
and EtOAc as eluent.
General procedure for 3,4-Markovnikov hydrosilylation: An over-dry
reaction tube containing a magnetic stir bar was charged with
Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%),
PPh3 (4.0 mg, 0.015 mmol, 1.5 mol%) and t -BuOK
(4.5 mg, 0.02 mmol, 2.0 mol%) and anhydrous THF (3.0 mL). Then the tube
was sealed with a rubber stopper and inserted into an oil bath at 25ºC. The mixture was stirred for 10 min. Then the
solution of conjugated diene (1 , 1.0 mmol, 1.0 equiv.) and
silane (2 , 1.2 mmol, 1.2 equiv.) was added into the tubevia syringe in seconds. The mixture was continued to stir for 6
h. Until the reaction was finished, EtOAc (3.0 mL) was added to to
quench the reaction. Then the reaction solution is filtered through a
pad of silica gel to remove the catalyst and base. EtOAc (5.0 mL × 2)
was used to wash the silica gel. The combined organic phase was
concentrated under reduced pressure, the residue was purified by column
chromatography (silica gel) to give the corresponding product 5with Petroleum ether and EtOAc as eluent.
Supporting Information
The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement
The authors acknowledge the financial support provided by the National
Natural Science Foundation of China.
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