A Boronium Ion with Exceptional Electrophilicity

Abstract

During our studies on aromatic borylation,[1] we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 byproduct of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1,8-bis(dimethylamino)naphthalene (1), a hindered and exceptionally basic aniline that finds numerous applications as a basic catalyst or reagent due to its legendary lack of nucleophilicity.[2, 3] Strong electrophiles interact weakly, if at all, with the amine nitrogens, and very few examples are known where stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen.[2, 4-7] Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements.[5] However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known.[5a, 8] In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.[9, 10] The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.