The Influence of Boronate Groups on the Selectivity of the Br–Li Exchange in Model Dibromoaryl Boronates

The selectivity of the Br/Li exchange reaction of 6‐butyl‐2‐(2,5‐dibromophenyl)‐1,3,6,2‐dioxazaborocane (2,5‐Br 2 C 6 H 3 B(OCH 2 CH 2 ) 2 NBu) and the analogous anionic derivatives, 2,5‐Br 2 C 6 H 3 B(O i Pr) 3 Li and 2,5‐Br 2 C 6 H 3 BF 3 K, was investigated using n BuLi as the lithiating reagent. In the case of the former compound there was a slight preference for lithiation at the 5‐position. For 2,5‐Br 2 C 6 H 3 B(O i Pr) 3 Li, the lithiation occured exclusively at C5, but for 2,5‐Br 2 C 6 H 3 BF 3 K, 2‐lithiation was preferred. Calculations performed for the lithiation of ortho ‐ and meta ‐brominated phenyl boronates revealed that ortho ‐lithiated aryl boronates are thermodynamically more stable, but that the Br/Li exchange is generally dictated by kinetics, which accounts for the variation of selectivity depending on the type of boronate group. In addition, the successful generation of the 2,5‐dilithiophenyl boronate species 2,5‐Li 2 C 6 H 3 B(OCH 2 CH 2 ) 2 NBu by a double Br/Li interconversion is reported. The Br/Li exchange in the related 6‐butyl‐2‐[3‐(2,5‐dibromothienyl)]‐1,3,6,2‐dioxazaborocane, 2,5‐Br 2 ‐3‐ThB(OCH 2 CH 2 ) 2 NBu, occured preferentially at the 5‐position, but the product was readily transformed into the more stable 2‐lithiated isomer. The use of 2 equiv. of n BuLi resulted in the efficient formation of the dilithiated species, 2,5‐Li 2 ‐3‐ThB(OCH 2 CH 2 ) 2 NBu. The obtained lithiated aryl boronates were converted into functionalized arylboronic acids by treatment with selected electrophiles.