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

Article abstract of DOI:10.1002/anie.201811135

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, Li₂CO₃ 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.