A Boroniiim Ion with Exceptional Electrophilicity

摘要

During our studies on aromatic borylation, we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 by-product of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1,8-bis(dimethyla-mino)naphthalene (1, Scheme 1), a hindered and exceptionally basic aniline that finds numerous applications as a basiccatalyst or reagent due to its legendary lack of nucleophilicity.Strong electrophiles interact weakly, if at all, with the amine nitrogen atoms, and very few examples are known in which stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen. Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements. However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known. In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply on mixing 1 with 9-borabicyclo[3.3.1]nonane (9-BBN) bis-triflimide reagent 5 a, despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to the boron and nitrogen atoms. The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.