The synthesis of brominated-boron-doped PAHs by alkyne 1,1-bromoboration: mechanistic and functionalisation studies

The synthesis of a range of brominated-B_n-containing (n = 1, 2) polycyclic aromatic hydrocarbons (PAHs) is achieved simply by reacting BBr₃ with appropriately substituted alkynes via a bromoboration/electrophilic C-H borylation sequence. The brominated-B_n-PAHs were isolated as either the borinic acid or B-mesityl-protected derivatives, with the latter having extremely deep LUMOs for the B₂-doped PAHs (with one example having a reduction potential of E_1/2 = -0.96 V versus Fc⁺/Fc, Fc = ferrocene). Mechanistic studies revealed the reaction sequence proceeds by initial alkyne 1,1-bromoboration. 1,1-bromoboration also was applied to access a number of unprecedented 1-bromo-2,2-diaryl substituted vinylboronate esters direct from internal alkynes. Bromoboration/C-H borylation installs useful C-Br units onto the B_n-PAHs, which were utilised in Negishi coupling reactions, including for the installation of two triarylamine donor (D) groups onto a B₂-PAH. The resultant D-A-D molecule has a low optical gap with an absorption onset at 750 nm and emission centered at 810 nm in the solid state.