Amides as modifiable directing groups in electrophilic borylation

Amide directed C-H borylation using $two equiv. of BBr3 forms borenium cations containing a R2N(R')C] O?B(Ar)Br unit which has significant Lewis acidity at the carbonyl carbon. This enables reduction of the amide unit to an amine using hydrosilanes. This approach can be applied sequentially in a one-pot electrophilic borylation-reduction process, which for phenyl-acetylamides generates ortho borylated compounds that can be directly oxidised to the 2-(2-aminoethyl)-phenol. Other substrates amenable to the C-H borylation-reduction sequence include mono and diamino-arenes and carbazoles. This represents a simple method to make borylated molecules that would be convoluted to access otherwise (e.g. N-octyl-1-BPin-carbazole). Substituent variation is tolerated at boron as well as in the amide unit, with diarylborenium cations also amenable to reduction. This enables a double C-H borylation- reduction-hydrolysis sequence to access B,N-polycyclic aromatic hydrocarbons (PAHs), including an example where both the boron and nitrogen centres contain functionalisable handles (N-H and B-OH). This method is therefore a useful addition to the metal-free borylation toolbox for accessing useful intermediates (ArylBPin) and novel B,N-PAHs.

Automated summary

Amide directed C-H borylation using BBr3 can generate borenium cations which can be reduced to amines. This method allows for one-pot electrophilic borylation-reduction process to synthesize ortho borylated compounds and B,N-polycyclic aromatic hydrocarbons. It is a valuable addition to the metal-free borylation toolbox.