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

One‐electron reductions of the tetraaryldiborane(4) compound Mes 2 ‐BBMes(Ph) ( 1 ) (Mes = 2,4,6‐Me 3 C 6 H 2 ) with KC 8 to afford the singly reduced radical anions [K(DME) 3 ][Mes 2 BBMes(Ph)] ( 2 ) and [K([18]crown‐6)(THF) 2 ][Mes 2 ‐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(OEt 2 ) 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 ][Mes 2 BBMes(Ph)] ( 3 ) and [Li(OEt 2 ) 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 [R 2 BBR 2 ] 2‐ , which have formal π‐bond orders of unity. The synthesis and characterization of 2,6‐Mes 2 ‐C 6 H 3 (MeO)BB(OMe) 2 ( 5 ) and 2,6‐Mes 2 ‐C 6 H 3 (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 [(Et 2 ‐O)Li][1‐mesityl‐5,7, and 8 are also described.