One-electron reductions of organodiborane(4) compounds: Singly reduced anions and rearrangement reactions

Abstract

One-electron reductions of the tetraaryldiborane(4) compound Mes2- BBMes(Ph) (1) (Mes = 2,4,6-Me3C6H2) with KC8 to afford the singly reduced radical anions [K(DME)3][Mes2BBMes(Ph)] (2) and [K([18]crown-6)(THF)2][Mes2-BBMEs(Ph)] (3) are described. Both 2 and 3 were characterized by IR and EPR spectroscopy, and X-ray diffraction studies of 3 showed it to be a solvent-separated ion pair. Similarly, reduction of Mes(MeO)-BB(OMe)Mes with lithium in diethyl ether under controlled conditions furnished the radical anion [Li(OEt2)2]-[Mes(MeO)BB(OMe)Mes] (4), which has a contact-ion-pair structure in which lithium is solvated by oxygen atoms from ether molecules and methoxy groups. The X-ray crystallographic studies of [K([18]-crown-6)(THF)2][Mes2BBMes(Ph)] (3) and [Li(OEt2)2][Mes(MeO)BB(OMe)-Mes] (4) revealed shortened B-B bond lengths in both compounds, consistent with the presence of partial (bond order 0.5) B-B π bonds. Interestingly, the B-B distances in the singly reduced species are very similar to those in the doubly reduced dianions [R2BBR2]2, which have formal π-bond orders of unity. The synthesis and characterization of 2,6-Mes2-C6H3(MeO)BB(OMe)2 (5) and 2,6-Mes2-C6H3(MeO)BB(OMe)Mes (6) are also reported. Reduction of 6 or 5 with lithium in diethyl ether solution did not lead to multiply bonded B-B species, but to the isolation of the rearranged products [(Et2-O)Li][1-mesityl-5,7-dimethyl-9-hydro-10-{mesityl(methoxyl)boryl}-1 0-boraphen-anthrenyl] (7) and [(Et2O)2Li][1-mesityl-5,7-dimethyl-9-methoxy-9'-{methyl(methoxy)boryl}- 9-borafluorenyl] (8), respectively. Products 7 and 8 are derived from the intramolecular insertion of the boron center into C-H or C-C σ bonds. The crystal structure determinations of 5, 7, and 8 are also described.