The formation and Si-29 NMR spectroscopic characterization of silicon cations that are intramolecularly stabilized by a dialkeyl thioether are described. The chemical stability of the silicon sulfur Lewis pair and, hence, the viability Of the approach, were probed with a 2-[(alkylthio)methyl]phenyl-substituted hydrosilane as a proxy before three different motifs with chiral binaphthyl backbones were prepared in multistep sequences. The degree of shielding of the silicon atom in these cations was found to depend on the substitution pattern at the silicon atom and the ring size generated by the silicon sulfur interaction: These sulfur-stabilized silicon cations are sufficiently reactive to promote Diels-Alder reactions of cyclohexa-1,3-diene with various dienophiles; the same set of reactions with cyclopentadiene is also reported. One of the three chiral Lewis acids induces low, but promising, enantioselectivity, and 24% ee is the highest Value so far obtained with a cationic tetracoordinate silicon catalyst.
缺电子的硼烷催化的硅烷对CO的还原遵循一种反常理的机制,其中Si在羰基氧原子对硅原子的亲核攻击之前,H键被硼路易斯酸激活。如此形成的硼氢化物是实际的还原剂。这些步骤是通过使用硅立体异构硅烷进行阐明的,但是由于硅原子的消旋作用,将相同的技术应用于相关的CN基团的还原尚无定论。本研究现在通过我们的轴向手性硼烷催化剂和轴向手性硅烷试剂(两种对映体形式)的有意结合证明,这些氢甲硅烷基化的机理基本相同。硼烷/硅烷对的明确立体化学结果表明,两者均参与确定对映选择性的氢化物转移步骤。在发现我们的轴向手性碳原子团之后,这些实验成为可能6 F 5取代的硼烷在亚胺氢化硅烷化中诱导相当水平的对映体诱导。
缺电子的硼烷催化的硅烷对CO的还原遵循一种反常理的机制,其中Si在羰基氧原子对硅原子的亲核攻击之前,H键被硼路易斯酸激活。如此形成的硼氢化物是实际的还原剂。这些步骤是通过使用硅立体异构硅烷进行阐明的,但是由于硅原子的消旋作用,将相同的技术应用于相关的CN基团的还原尚无定论。本研究现在通过我们的轴向手性硼烷催化剂和轴向手性硅烷试剂(两种对映体形式)的有意结合证明,这些氢甲硅烷基化的机理基本相同。硼烷/硅烷对的明确立体化学结果表明,两者均参与确定对映选择性的氢化物转移步骤。在发现我们的轴向手性碳原子团之后,这些实验成为可能6 F 5取代的硼烷在亚胺氢化硅烷化中诱导相当水平的对映体诱导。
Achieving Enantioselectivity in Difficult Cyclohexa-1,3-diene Diels–Alder Reactions with Sulfur-Stabilized Silicon Cations as Lewis Acid Catalysts
作者:Polina Shaykhutdinova、Martin Oestreich
DOI:10.1021/acs.orglett.8b02945
日期:2018.11.16
A novel cationic silicon–sulfur Lewis pair with a chiral H8-binaphthyl backbone is reported. It catalyzes otherwise sluggish Diels–Alderreactions of cyclohexa-1,3-diene and chalcone derivatives in good yields and decent enantioselectivities (up to 81% ee). The enantioinduction is highest with a [1,1′-biphenyl]-4-yl substituent at the carbonyl carbon atom. This moiety can be later converted into a
Refinement of the Catalyst Backbone of Chiral Intramolecular Silicon-Sulfur Lewis Pairs: Improved Enantioselectivity in the Diels-Alder Reaction of Cyclohexa-1,3-diene and Chalcone Derivatives
Step by step. A new, structurally modified silicon cation intramolecularly stabilized by a sulfur donor catalyzes challenging Diels–Alder reactions of cyclohexa‐1,3‐diene and chalcone derivatives with improved enantiomeric excesses. The endo selectivity typically observed is inverted to exo with cyclopentadiene, and no enantioinduction is obtained.
Further Structural Modification of Sulfur-Stabilized Silicon Cations with Binaphthyl Backbones
作者:Polina Shaykhutdinova、Martin Oestreich
DOI:10.1055/s-0037-1610697
日期:2019.5
previously reported Lewis acids. Another silicon cation with a chiral spirocyclic backbone induces enantioselectivity in the same range but its synthesis is laborious. The synthesis and spectroscopic characterization of two novel cationic silicon–sulfur Lewis pairs with a chiral 4,4′-disubstituted binaphthyl silepine backbone are described. Both Lewis acids induce significant enantioselectivity in the model
Enantioselective Diels–Alder Reactions of Cyclohexa-1,3-diene and Chalcones Catalyzed by Intramolecular Silicon–Sulfur Lewis Pairs as Chiral Lewis Acids
作者:Polina Shaykhutdinova、Martin Oestreich
DOI:10.1021/acs.organomet.6b00548
日期:2016.8.22
The stereoselective preparation of diastereomeric dihydrosilepine-derived silicon cations decorated with another binaphthyl unit at the silicon atom is described. A sulfide donor attached to that additional binaphthyl substituent forms an intramolecular Lewis pair with the electron-deficient silicon atom, as verified by Si-29 NMR spectroscopy. Both chiral sulfur-stabilized silicon cations act as catalysts in the difficult Diels Alder reaction of cydohexa-1,3-diene and chalcone derivatives. Both Lewis acids induce enantioselectivity, but the S,S relative configuration is superior to the S,R configuration. With the former diastereomer, enantiomeric excesses of close to 60% are obtained. These values are the highest achieved to date in this seemingly trivial cycloaddition.
Illuminating the Mechanism of the Borane-Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane
作者:Marius Mewald、Martin Oestreich
DOI:10.1002/chem.201202693
日期:2012.10.29
mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochemical outcomes for the borane/silane pairs show that both participate in the enantioselectivity‐determining hydride‐transfer step. These experiments became possible after the discovery that our axially chiral C6F5‐substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.
缺电子的硼烷催化的硅烷对CO的还原遵循一种反常理的机制,其中Si在羰基氧原子对硅原子的亲核攻击之前,H键被硼路易斯酸激活。如此形成的硼氢化物是实际的还原剂。这些步骤是通过使用硅立体异构硅烷进行阐明的,但是由于硅原子的消旋作用,将相同的技术应用于相关的CN基团的还原尚无定论。本研究现在通过我们的轴向手性硼烷催化剂和轴向手性硅烷试剂(两种对映体形式)的有意结合证明,这些氢甲硅烷基化的机理基本相同。硼烷/硅烷对的明确立体化学结果表明,两者均参与确定对映选择性的氢化物转移步骤。在发现我们的轴向手性碳原子团之后,这些实验成为可能6 F 5取代的硼烷在亚胺氢化硅烷化中诱导相当水平的对映体诱导。