The photochemical dehydrogenation, referred to as the Mallory photocyclization, is a pioneer reaction for formation of helicenes and other polyaromatic hydrocarbons. Four bis-Mallory photocyclization substrates have been examined from both experimental and computational (B3LYP/6-311+G(2d,p)) perspectives. Only one of the substrates showed the formation of all the three possible regioisomers at room temperature. The other three substrates only produced the two C2-symmetric products and not the C1 product. For these three reactions, it was found that a low photocyclization temperature (< 0oC) resulted in the exclusive formation of the helicene regioisomer despite the fact that the other potential isomers that could form are 11.0, 10.7, and 12.2 kcal/mol more stable. This is due to three factors: (1) the DHP (dihydrophenanthrene) precursors of these two sets of regioisomers are closer in energy (<5 kcal/mol) than the helicene products; (2) the extinction coefficients of the DHP precursors to the helicenes are smaller than that of the DHP precursors of their competitively formed regioisomers by a factor of 4.8-9.2: (3) thermal decomposition of intermediates are suppressed at low temperatures. These factors taken together allow the formation of a photostationary state dominated by the helicene DHP precursor. We have further synthesized helicenes with extended acene appendages that have been studied photophysically and show interesting chiroptical properties such as large optical rotations and higher glum values. We have also conducted calculations at the B97D/cc-pVDZ computational level for these twisted heli-acene compounds to further explore the scope of photostationary state-driven helicene formation. 

Mallory photocyclizations: Enhanced twisted heli-acene formations

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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