Selective CO₂ Splitting by Doubly Reduced Aryl Boranes to Give CO and [CO₃ ]²

Alkali metal salts M₂ [1] (M=Li, Na) of doubly reduced 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene (1) instantaneously add the C=O bond of CO₂ across their boron centers to furnish formal [4+2]-cycloadducts M₂ [2]. If only 1 equiv of CO₂ is supplied, these products are stable. In the presence of excess CO₂ , however, C-O bond cleavage occurs and an O^2- equivalent is transferred to CO₂ to furnish CO and [CO₃ ]^2- . With M=Li, Li₂ CO₃ precipitates and the neutral 1 is liberated such that it can be reduced again to establish a catalytic cycle. With M=Na, [CO₃ ]^2- remains coordinated to both boron atoms in a bridging mode (Na₂ [4]). A mechanistic scenario is proposed, based on isolated intermediates and model reactions.