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.
References
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