When vinylic boronic acids are treated with Br₂ and then with NaOH, vinylic bromides are formed with inversion of configuration, as shown in Fig. P18.89a. But when vinylic boronic acids are treated with I₂ in the presence of NaOH, vinylic iodides are formed with retention of configuration, as shown in Fig. P18.89b. Suggest mechanisms that account for these results.

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(a) 20 R H (b) R H C=C C=C Figure P18.89 H B(OH)2 H B(OH)₂ 1) Br₂ in CH₂Cl₂ 2) NaOH 12, NaOH R H R H C=C =C H Br H + Br + B(OH)4 + I + B(OH)4