- Ruthenium Lewis Acid-Catalyzed Asymmetric Diels–Alder Reactions: Reverse-Face Selectivity for α,β-Unsaturated Aldehydes and Ketones
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Acrolein, methacrolein, methyl vinyl ketone, ethyl vinyl ketone, 3-methyl-3-en-2-one, and divinyl ketone were coordinated to a cationic cyclopentadienyl ruthenium(II) Lewis acid incorporating the electron-poor bidentate BIPHOP–F ligand. Analysis by NOESY and ROESY NMR techniques allowed the determination of conformations of enals and enones present in solution in CD2Cl2. The results were compared to solid-state structures and to the facial selectivities of catalytic asymmetric Diels–Alder reactions with cyclopentadiene. X-Ray structures of four Ru-enal and Ru-enone complexes show the α,β-unsaturated C=O compounds to adopt an anti-s-trans conformation. In solution, enals assume both anti-s-trans and anti-s-cis conformations. An additional conformation, syn-s-trans, is present in enone complexes. Enantioface selectivity in the cycloaddition reactions differs for enals and enones. Reaction products indicate enals to react exclusively in the anti-s-trans conformation, whereas with enones, the major product results from the syn-s-trans conformation. The alkene in s-cis conformations, while present in solution, is shielded and cannot undergo cycloaddition. A syn-s-trans conformation is found in the solid state of the bulky 6,6-dimethyl cyclohexanone-Ru(II) complex. The X-ray structure of divinyl ketone is unique in that the Ru(II) center binds the enone via a η2bond to one of the alkene moieties. In solution, coordination to Ru–C=O oxygen is adopted. A comparison of facial preference is also made to the corresponding indenyl Lewis acids.
- Thamapipol, Sirinporn,Ludwig, Bettina,Besnard, Céline,Saudan, Christophe,Kündig, E. Peter
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p. 774 - 789
(2016/10/17)
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- Intramolecularly Sulfur-Stabilized Silicon Cations with Chiral Binaphthyl Backbones: Synthesis of Three Different Motifs and Their Application in Enantioselective Diels-Alder Reactions
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The formation and 29Si NMR spectroscopic characterization of silicon cations that are intramolecularly stabilized by a dialkyl 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.
- Rohde, Volker H. G.,Müller, Maria F.,Oestreich, Martin
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p. 3358 - 3373
(2016/01/15)
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- PROCESS FOR PRODUCING HIGH PURITY EXO-ALKENYLNORBORNENE
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Embodiments of the present invention are directed generally to methods for producing high purity exo-alkenylnorbornenes from a mixture of conformational isomers thereof.
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Page/Page column 16-17
(2009/07/02)
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