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

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