The nature of Krause's adducts: The structure of the 1:1 adduct of triphenylborane with sodium metal

Triphenylborane (7) undergoes reductive dimerization with sodium metal in diethyl ether solution to form a yellow solid (3) having the empirical composition Ph3B· Na· (C2H5)2O. By 1H NMR spectroscopy, 3 has now been shown to have the structure depicted. By the chemical structure image action of heat or strongly donor ethers, 3 rearranges into 6, whose structure was likewise determined by 1H and 11B NMR spectroscopy and by deuteriodeboronation with DOAc. Prolonged reaction of sodium metal with 7 or with diphenylboron chloride (8) in DME leads to biphenyl precursors, sodium tetraphenylborate (9), and sodium phenylborohydrides. All these results can be reconciled by the formation of the radical–anion 10 from 7 and its subsequent coupling or fragmentation. The coupling of 10 to form 3 and the rearrangement of 3 to yield 6 shows that there is a striking parallel in behavior between the isoelectronic (Ph3B·)− and the triphenylmethyl radical (Ph3C·). In this light, the solution of the “hexaphenylethane riddle” is also applicable to the solution of the “hexaphenyldiborate dianion riddle,” which was posed by the discovery of Krause's adduct over 68 years ago.