Selective CO 2 Splitting by Doubly Reduced Aryl Boranes to Give CO and [CO 3 ] 2 .

Alkali metal salts M-2[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 M-2[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 O2- 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 (Na-2[4]). A mechanistic scenario is proposed, based on isolated intermediates and model reactions.