19F NMR investigation of the reaction of bis(pentafluorophenyl)borinic acid with a "proton sponge" : deprotonation, trimerization and stepwise dearylation

The reaction of Ar2BOH (1, Ar = C6F5) with the proton sponge bis(dimethylamino)naphthalene (DMAN) in CD2Cl2 solution caused a series of strictly interlaced processes, that have been investigated by variable temperature 1H and 19F NMR spectroscopy. The very slow equilibrium between the monomeric and cyclic trimeric forms of Ar2BOH was strongly accelerated by the presence of a catalytic amount of DMAN. Upon addition of stoichiometric DMAN (0.33 equivalents), all Ar2BOH was converted into a mixture of two boroxinate anions, namely [Ar6B3O3H2]– (2) and [Ar5B3O3H]– (3), in different ratios on varying the reaction conditions. The formation of the dearylated anion 3 (accompanied by equimolar pentafluorobenzene) has been attributed to a reaction process parallel to that leading to the hexa-aryl anion 2, involving base-catalyzed hydrolysis of monomeric perfluoroborinic acid to its boronic analogue, followed by aggregation and condensation. Water plays a catalytic role in this process. In the presence of a small excess of DMAN, complete and instantaneous dearylation of 2 to 3 was observed. At temperatures lower than 233 K, the penta-aryl anion 3 was in equilibrium with its conjugate base. Slow further dearylation occurred at temperatures higher than 263 K, cleanly affording the tetra-aryl anion [Ar4B3O3]– (5). Both anions 3 and 5 have been obtained in high yields also from the condensation of Ar2BOH and ArB(OH)2 (in the ratio 2:1 or 1:2, respectively), in the presence of stoichiometric DMAN. Mechanisms responsible for the stepwise dearylation from 2 to 3 and then from 3 to 5 have been discussed.