Although the actual catalysis in this experiment involves a somewhat complicated reaction mechanism (beyond the scope of our discussion here), you can think of the net reaction as resembling an SN2-type process with a nucleophile and a leaving group. Using what you know about the reactants and products as shown in Eq. 2 below and the discussion of bond polarities given in the introduction, provide the nucleophile, the specific carbon atom where the substitution takes place, and the leaving group for the net coupling reaction.

basePd (2) aryl halide aryl boronic acid coupling product