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 O2- 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.