Diborylated magnesium anthracene as precursor for B2H5--bridged 9,10-dihydroanthracene

Abstract

9,10-(Bpin)2-anthracene (3, HBpin=pinacolborane) was synthesized from 9,10-dibromoanthracene in a stepwise lithiation/borylation sequence. The reaction of 3 with highly activated magnesium furnished the diborylated magnesium anthracene 4, which was quenched in situ with ethereal HCl to yield cis-9,10-(Bpin)2-DHA (cis-5, DHA=9,10-dihydroanthracene). Compound cis-5, in turn, can be reduced with Li[AlH4] in THF to give its diborate Li2[cis-9,10-(BH3)2-DHA] (Li2[cis-6]). In the crystal lattice, the THF solvate Li2[cis-6] 3 THF establishes a dimeric structure with Li-(μ-H)-B coordination modes. Hydride abstraction from Li2[cis-6] with Me3SiCl yields the B-H-B-bridged DHA Li[7]. This product can also be viewed as a unique cyclic B2H7- derivative with a hydrocarbon backbone. Treatment of Li2[cis-6] with the stronger hydride abstracting agent Me3SiOTf (HOTf=trifluoromethanesulfonic acid) in THF affords the THF diadduct of cis-9,10-(BH(OTf))2-DHA. Bridge over DHA: cis-9,10-(Bpin)2-DHA (DHA=9,10-dihydroanthracene, HBpin=pinacolborane) readily reacts with Li[AlH4] to yield the bisborate Li2[cis-9,10-(BH3)2-DHA] (see figure), which, in turn, forms the unique B-H-B-bridged species Li[(H2B(μ-H)BH2)DHA] upon hydride abstraction with Me3SiCl.