From when Turkey first invaded Syria:

Ulrike Franke, a fellow at the European Council on Foreign Relations, said it is “incredibly unlikely” that Turkey would invoke Article 5 in this situation. Asking for help would amount to an embarrassing “admission of failure” on Ankara’s part. And even if Turkey did, Article 5 does not mandate NATO members to join the Syrian war on Turkey’s side. It merely requires allies to assist with “such action as it deems necessary.”

Source.