Can you react to an Empower?

Yes. Activating an Empower ability is an activated ability, so it goes on the chain as a Pending item and creates a Closed State — giving you priority to play Reaction spells/abilities before it resolves (Rule 377.3, FAQ #11877).

Two timing notes: the Reaction resolves first (LIFO), so the unit is not yet Empowered while you're responding — an [Empowered][>] dependent ability isn't active yet. And if your reaction removes the unit from the board, the Empower does nothing when it resolves.