As discussed in the preceding sections, the catalytic activation method using HNTf2 allows efficient borylation with optimal substrates such as 22
As discussed in the preceding sections, the catalytic activation method using HNTf2 allows efficient borylation with optimal substrates such as 22. In other, less hindered aliphatic examples, the catalytic process does not go to completion, suggesting some form of product inhibition of the catalytic cycle, or a dead end decomposition pathway perhaps involving the product borenium cations. The reasons for this behavior remain unexplained in the aliphatic borylations, and the potential for retrohydroboration may be partly responsible for unknown decomposition or disproportionation events that interfere with catalyst turnover. On the other hand, aromatic borylations should be less prone to complications under catalytic conditions. The product borenium cations are somewhat more stabilized due to their “bora-benzylic” nature, involving delocalization between the aromatic π-system and the planar tricoordinate boron. Furthermore, the aromatic borylation products are incapable of undergoing retrohydroboration. Based on these considerations, we tested the catalytic activation method with the simple N,N-dimethylbenzylamine borane 1 (), and were pleased to find that >90% of the known borylation product 37 was formed after reductive quenching. Evidently, an efficient catalytic cycle is possible.
