Enantioselective Transformations of Carbon-Carbon Multiple Bonds Using Electrophilic Catalysts and Reagents
- Author(s): Wang, Yiming
- Advisor(s): Toste, F. Dean
- et al.
The activation of relatively unreactive carbon-carbon (C-C) multiple bonds is an important tool for the introduction of functional groups and stereochemical information in organic molecules. In recent years, the use of electrophilic cationic gold(I) complexes for the functionalization of alkynes and allenes has seen rapid development. An especially general application of gold catalysis is the nucleophilic trapping of gold-activated ð bond to give a heterocyclic compound. In the case of allenes, chiral ligands have been used to generate product with excellent enantiocontrol. In the first part of this Thesis we report studies on the development of an enantioselective cyclization using the gold-catalyzed transformation of propargyl esters to generate allenes in situ. A subsequent gold-catalyzed dynamic kinetic asymmetric cyclization of a phenol onto the allene resulted in the generation of enantioenriched cyclized chromanone derivatives from racemic starting material. The optimal catalyst for this transformation was a (biscarbene)digold(I) complex, which delivered better enantioselectivities than previously known phosphine-gold and phosphoramidite-gold complexes.
Electrophilic sources of the halogens (fluorine, chlorine, bromine, and iodine) activate C-C multiple bonds in much the same way as gold(I) complexes, but the electrophilic atom of the reagent is incorporated into the final product. Because halogen atoms are amenable to further functional group manipulation and are also present in complex natural products, the enantioselective synthesis of halofunctionalized products from alkenes is an important synthetic goal. Typically, enantioselectivity is achieved using a chiral catalyst to activate the electrophilic reagent. However, high enantioselectivities may be hampered by uncatalyzed background reactivity. The Toste research group has introduced a new approach for electrophilic functionalization (chiral anion phase transfer catalysis) by inducing ion pairing between a phosphate anion chiral source and a cationic electrophilic reagent by phase transfer. This concept was initially demonstrated for fluorination, using the cationic reagent F-TEDA-BF4 (Selectfluor®). In the second part of the Thesis, we report studies on the extension of this strategy to the heavier halogens. With the successful development of bromination and iodination reagents suitable for chiral anion phase transfer, we applied these reagents to the synthesis of halogenated benzoxazines with high levels of enantioselectivity.