The Hydronaphthalide Monoanion: Isolation of the “red transient” Birch Intermediate from liquid Ammonia

Birch reactions employing alkali metals in ammonia have been a well‐established method for the reduction and functionalisation of aromatic compounds for nearly 100 years. Speculations regarding intermediates in the reaction pathway have been discussed since the beginning. We hereby report the isolation of NMe4(HNaph) (1), a kinetically trapped intermediate of the Birch reaction of naphthalene and sodium in liquid ammonia. 1 has been fully characterised and has been shown to continue to react to 1,2/1,4‐dihydronaphthalene – the Birch product of the reduction of naphthalene. The reactivity of 1 was investigated towards activity as an electron and hydride transfer agent with both, elements and small organic molecules. 1 demonstrates a tamed reduction potential which allows for controlled reactions such as the selective formation of hexasulphide and hexaselenide anions. We present the isolation and full characterisation of a reactive intermediate from the Birch reduction which was first observed in 1939.The historically as transient described compound was isolated as an organic salt from the reaction of naphthalene, sodium, and tetramethylammonium chloride in liquid ammonia. The trapped intermediate demonstrates a tamed reduction potential enabling higher reaction control compared to stronger reducing agents.