Can you react to a Temporary permanent being killed?
Yes. Temporary is a triggered ability that, at the start of its controller's beginning phase, kills the permanent. When it triggers it goes on the chain and opens a closed state, so you can respond with Reaction cards and abilities before it resolves.
Cited rules
- 816.1.b It is functionally short for "At the start of this permanent's controller's Beginning Phase, before scoring, kill this."
- 383.3 When a Condition is met, a Triggered Ability behaves like an Activated Ability and is placed on the Chain.
- 309.1.a Only cards and abilities with the Reaction keyword can be played or activated in a Closed State.