Concerted Reactions That Produce Diradicals and Zwitterions: Electronic, Steric, Conformational, and Kinetic Control of Cycloaromatization Processes

The diversity of conceptual approaches to controlling and directing cycloaromatization reactions was studied. They range from classic conjugation, hyperconjugation, and hybridization approaches to creative variations in strain and sterics, to transfer of orbital interactions through MO crossings, and to conformationally gated kinetic trapping. Conceptually, the methods for reaction control can be separated into reactant destabilization or TS stabilization. The first concept has been realized into practice in a variety of ways, such as alkyne bending (strained cyclic enediynes), steric assistance (ortho-effect), and hybridization effects (fluoroalkynes). In the absence of a strong stabilizing interaction for the radical, there is little incentive for the development of diradical character, and, in the parent system, the Bergman cyclization TS electronically resembles the starting material. For the formation of exocyclic radicals, the situation is different because benzylic or allylic conjugation with the developing radical can be substantial.