I have performed a Miyaura borylation 5 times without problems using an aryl bromide, Pd(dppf)Cl2, B2(pin)2, KOAc in dioxane.
Now the reaction suddenly will not start. After 2 hours I have 0% conversion. All other reactions turned black after 30 min whereas these reactions remain red.
Does anyone have a suggestion to what has happened?