One‐Electron Reductions of Organodiborane(4) Compounds: Singly Reduced Anions and Rearrangement Reactions

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, and 8 are also described.