Radical Reactions to Form Expanded Polyaromatics

Cascade radical transformations of acyclic precursors that incorporate alkynes open efficient, convenient, and atom-economical access to functionalized compounds. The selective sequence of cyclizations followed by controlled termination offer products of increased structural complexity. First, selectivity of initiation can be guided either by steric and geometric constraints, a tethered initiator, or through a directing group, such as an alkoxy group, placed near the desired point of attack. These methods ensure attack at the correct carbon of the central alkyne, the necessary point of initiation to result in a fully cyclized alkyne substrate. Second, reduction of geometric "defects" has been previously achieved by introducing a skipped carbon in the oligoalkyne starting material. By designing a directing group that also fragments upon the completion of the cascade, the alkyl loop left over from tethered initiation has been eliminated. Third, termination cyclizations are further optimized by the use of peri- attack at the zigzag edge of a strategically placed polyaromatic unit. When the vinyl radical present after initiation of the reaction is produced, a hexagon is will be formed as a result of the next step rather than a previously present pentagon from attack at a pendant phenyl group. However, such peri- attack approaches introduce additional complexity in that there is now stereoelectronic interactions between the directing group and Sn that were not previously necessary to consider.