A general and convenient protocol for the electrophilic borylation of aryl Grignard reagents prepd. from arylbromides by direct insertion of Mg in the presence of LiCl or by Mg/Br exchange with iPrMgCl·LiCl was developed. Various aryl boronic acids were synthesized in a straightforward manner in excellent yields at 0°. E.g., reaction of mixt. of 4-bromoanisole, Mg turnings, LiCl in THF with DIBAL-H catalyst followed by the addn. of trimethylborate at 0° gave 92% yield of 4-MeOC6H4B(OH)2.