Formal Boron–Heteroatom Addition Reactions to Alkynes
- Author(s): Issaian, Adena
- Advisor(s): Blum, Suzanne A
- et al.
Organoboron compounds and borylated heterocycles are in general versatile building blocks and precursors for organic synthesis. The common strategy for the synthesis of borylated heterocycles involves initial formation of the heterocyclic core and subsequent borylation of this unit through established methods. This dissertation describes our laboratory’s development of formal borylative heterocyclization reactions that access borylated heterocycles in one synthetic step. The dissertation starts with the development of a formal borylation reaction that makes borylated lactones (Chapter 2). This class of reactions takes advantage of bifunctional B-chlorocatecholborane (ClBcat) as a carbophilic carbon−carbon π-bond activator and eventual dealkylating agent. Our motivation in developing this new class of catalyst-free borylation reactions and subsequently expanding the formal borylation strategy is described. The thioboration of o-alkynylthioanisole substrates to form borylated benzothiophenes is presented next. Our investigations into the details of the mechanism of this formal thioboration reaction are also discussed (Chapter 3). These collaborative mechanistic studies included
experimental and computational findings that elucidated the rate-determining step and likely intermediates of the reaction. These studies further compared different boron sources as electrophiles, including those used in other known reactions, providing fundamental knowledge about the capabilities of commercially available boron reagents toward borylative heterocyclization. Our findings from these investigations next provided us with guiding principles for the design of a new class of electrophilic heterocyclization/group transfer reactions, wherein the heterocyclic core from formal borylative heterocyclization is generated and intercepted. Specifically, the cationic
heterocycle serves as a key intermediate, yet effectively blocks the pathway toward established electrophilic cyclization reactions through substrate hybridization strategies. These hybridization strategies result in a ring-opening pathway that produces
trisubstituted olefins (Chapter 4). Mechanistic studies of the group-transfer reactivity indicate that this stereo- and regiocontrolled pathway can be influenced by subtle variations in the starting material to generate different substitution patterns. The new
developments provided us with information leading toward the design of a diverse set of formal addition reactions that proceed through analogous electrophilic heterocyclization/group-transfer strategies, the preliminary results for which are presented in this dissertation (Chapter 5). This new direction from my dissertation research has generated a project now being pursued by other group members.