19 F NMR Spectroscopic Investigation of the Reaction of Bis(pentafluorophenyl)borinic Acid with a “Proton Sponge”: Deprotonation, Trimerization and Stepwise Dearylation

The reaction of Ar 2 BOH ( 1 , Ar = C 6 F 5 ) with the proton sponge bis(dimethylamino)naphthalene (DMAN) in CD 2 Cl 2 solution caused a series of strictly interlaced processes that have been investigated by variable‐temperature 1 H and 19 F NMR spectroscopy. The very slow equilibrium between the monomeric and cyclic trimeric forms of Ar 2 BOH was strongly accelerated by the presence of a catalytic amount of DMAN. Upon addition of stoichiometric DMAN (0.33 equiv.), all Ar 2 BOH was converted into a mixture of two boroxinate anions, namely [Ar 6 B 3 O 3 H 2 ] – ( 2 ) and [Ar 5 B 3 O 3 H] – ( 3 ), in which the ratio of the two anions varied according to 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 hexaaryl 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 pentaaryl anion 3 was in equilibrium with its conjugate base. Slow further dearylation occurred at temperatures higher than 263 K, cleanly affording the tetraaryl anion [Ar 4 B 3 O 3 ] – ( 5 ). Anions 3 and 5 have also been obtained in high yields from the condensation reaction between Ar 2 BOH and ArB(OH) 2 (in the ratio 2:1 and 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)