A non-symmetrical diboron reagent, B(pin)-B(dan), has been utilised in the Pd-catalysed borylation of aryl bromides and chlorides. Remarkably selective formation of aryl-B(dan) bonds is established. This represents a direct and efficient way to introduce masked boronic acids. The synthetic usefulness of this reaction is demonstrated in the preparation of boron-differentiated di- and polyboron compounds.