Part I: Studies Towards Asymmetric α-halogenation and Mechanistic Studies of the Acrylate System with Organocatalyst and Part Ii: Synthesis of α-aryl Quaternary Carbon Centers
Date of Award
Doctor of Philosophy
Mahmun M. Hossain
Mark L. Dietz, Andrew A. Pacheco, Alan W. Schwabacher, Xiaohua Peng
Asymmetric Alpha Halogenation, Nmr Studies, Organocatalysis
Organocatalytic transformations and asymmetric α-halogenation have become an important and dynamic research topic in organic chemistry in recent years. Despite the growing research in asymmetric halogenation of carbonyl compounds, such as aldehydes and ketones, there are no current examples in the literature of asymmetric halogenation of enolic systems even though many proposed reaction mechanisms go through enolate form. The research presented is the first example of enantioselective α-chlorination and α-bromination of α-hydroxyacrylate using organocatalysis and NMR studies towards achieving asymmetric induction of the enolic system. Despite the many publications that show when an organocatalyst binds to an aldehyde or ketone, the transition state goes through the enolate tautomer, that form is not stable otherwise and conforms back into starting material when the catalyst leaves the enolate. Whereas the α-hydroxyacrylate is stable in the enolic form by itself and does not need to be bound to any catalyst. Investigation into the mechanism of the reaction with the help of NMR studies showed that organocatalysts work as a base with the acrylate system and two major species are formed, E and Z isomers. This along with the fact that the catalyst is only loosely bound to the substrate is preventing desirable asymmetric induction from occurring. α-chlorination and α-bromination were successfully achieved with 100% yield and up to 30% ee.
Shteynbuk, Maria Shevyrev, "Part I: Studies Towards Asymmetric α-halogenation and Mechanistic Studies of the Acrylate System with Organocatalyst and Part Ii: Synthesis of α-aryl Quaternary Carbon Centers" (2014). Theses and Dissertations. 542.