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

Abstract

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, 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.