Dilithio 9,10-Diborataanthracene: Molecular Structure and 1,4-Addition Reactions

9,10-Dihydro-9,10-diboraanthracene ([1]n) and its SMe2 adduct 1(SMe2)2 are readily reduced with lithium in THF to the dianionic 9,10-diborataanthracene Li2[1]. An X-ray crystal structure analysis of (Li(thf)2)2[1] revealed monomeric inverse sandwich complexes, each of them containing two Li(thf)2 moieties coordinated to both sides of the central B2C4 ring. Compared to 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene, the four B−CAr bonds of (Li(thf)2)2[1] are shorter by 0.046(4) Å, thereby indicating an increased degree of BCAr double-bond character. Consequently, (Li(thf)2)2[1] reacts with 4,4′-dimethylbenzophenone as a BCAr−CArB diene and undergoes a [4+2] cycloaddition reaction with formation of a bicyclic product. In contrast, tert-butylacetylene reacts with (Li(thf)2)2[1] under formal 1,4-addition of its methinic C−H group instead of its CC triple bond to the two boron atoms.