Terphenyl Derivatives of p-Block Elements

Terphenyl derivatives of boron were originally synthesized with the object of preparing compounds with B-B triple bonds. It was thought that these aryl ligands would be effective in the stabilization of triple bonds in a similar manner to their success in the stabilization of double bonds. For example, a B-B triple-bonded boron analogue of a substituted acetylene might be accessible by reduction of an ArBX2 or l,2-dimethoxydiborane(4) species as shown by Eq. (3). 

The B-B bonded l,2-dimethoxydiborane(4) precursor67 was selected on the basis that it could be obtained readily from (MeO)2B-B(OMe)268 by reaction with the lithium aryl. In contrast to diborane(4) tetrahalides, 

KC8 also affords a borate salt, but in this case it is uniquely dimerized with a relatively long (1.83(2) A) B-B bond, which is caused, no doubt, by the presence of adjacent negative charges and the four-coordination of the boron atoms. Reduction of 2,6-Trip2H3C6BBr2 with sodium gave the 9-borafluorenyl radical anion salt, which was characterized by EPR and elemental analysis.67 

The failure to obtain B-B multiply bonded products from these reactions led to the investigation of singly bonded B-B precursors derived from B2(OMe)4.70 However, reaction of B2(OMe)4 with 2 equiv of LiAr (Ar = -C6H3-2,6-Mes2 or -C6H3-2,6-Tips2) affords only Ar(MeO)BB(OMe)2. 

In summary, the attempted further reduction of 1,2-dimethoxydiborane compounds results, initially, in the elimination of a methoxide group. When this elimination occurs, however, there is an immediate insertion of the reactive boranide intermediate into a C-C or C-H bond before elimination of a second methoxide occurs. 

---