Synthesis of Unsymmetrically Substituted [5]Helicenes by Tandem Arylation of Binaphthyl by Suzuki Coupling and C-H Activation

We report a step-efficient approach for the synthesis of unsymmetrically substituted naphtho[1,2-g]chrysenes by tandem arylation of 2,2'-diiodo-1,1'-binaphthyl with arylboronic acids, which proceeds through Suzuki coupling followed by intramolecular arylation by C-H activation. Substrate screening examining over 30 meta- and/or para-substituted arylboronic acids showed that this approach provides the desired helicene products most effectively if arylboronic acids have a small acidifying group at the meta-position. In the case of unsymmetrically substituted arylboronic acids, the regioselectivity of C-H activation is directed by the attached group(s), its effect on the C-H acidity of the adjacent position, its coordination ability, as well as steric hindrance. Interestingly, selected [5]helicenes exhibited attractive deep blue emission with fluorescence lifetimes slightly exceeding 10 ns. Tandem arylation of binaphthyl diiodide by Suzuki coupling and subsequent C-H activation affords substituted [5]helicene derivatives. C-H activation is facilitated by the presence of an ortho-activating group in the adjacent position. In the case of unsymmetrically substituted boronic acids, the regioselectivity of C-H activation is controlled by the electronic and steric effects of the substituents.image