Explorations into the Potential of Chiral Sulfonium Reagents to Effect Asymmetric Halonium Additions to Isolated Alkenes
摘要:
While methods for the racemic dihalogenation and halohydroxylation of alkenes have been known for decades, enantioselective variants of these processes remain elusive. Initial attempts were made to overcome this long-standing challenge by exploring the potential of chiral, crystalline, sulfur-derived halonium reagents to accomplish the asymmetric dichlorination and iodohydroxylation of 1,2-dihydronaphthalene. Asymmetric dichlorination of this substrate was achieved in 57% yield and 14% enantiomeric excess (ee), but asymmetric iodohydroxylation was much more successful, giving 67% yield and 63% ee. Thorough studies were made of these processes, including investigation of various chiral sulfide derivatives, their substrate scopes, and the reaction conditions.
Enantioselective Redox-Neutral Rh-Catalyzed Coupling of Terminal Alkynes with Carboxylic Acids Toward Branched Allylic Esters
作者:Philipp Koschker、Matthias Kähny、Bernhard Breit
DOI:10.1021/jacs.5b01131
日期:2015.3.4
We report on the first enantioselective variant of the atom-economic and redox-neutral coupling of carboxylic acids with terminal alkynes under rhodium catalysis utilizing the chiral, bidentate (R,R)-Cp-DIOP ligand. This represents the first example of this convenient asymmetric access to valuable branched allylic esters. The utility of this methodology is demonstrated by both a reaction performed on large scale and a short three-step synthesis of two naturally occurring gamma-butyrolactones. A stereochemical model explaining the observed absolute configuration of the products based on DFT calculations is given.
A Catalytic Cycle for the Asymmetric Synthesis of Epoxides Using Sulfur Ylides