Excited-state protonation and reduction enables the umpolung Birch reduction of naphthalenes

Abstract

The Birch reaction is a classical process used for the partial reduction of aromatics into non-conjugated cyclohexadienes that can be further functionalized. This strategy and its more modern variants are all based on an initial single-electron transfer event converting the arene into the corresponding radical anion for either protonation or hydrogen-atom transfer. Herein, we demonstrate an umpolung approach where the aromatic is first protonated to its corresponding carbocation and then reduced using the Lewis acid-base complex Et₃N−BH₃. This strategy requires aromatic photoexcitation so that protonation is favored by charge-transfer and driven by excited-state antiaromaticity relief. This means that aromatic excited-state basicity rather than ground-state redox potential needs to be considered when approaching reaction development. The mild conditions and the avoidance of strong reductants have enabled tolerance of functionalities generally not compatible under standard Birch conditions.