Your example is not union-closed.

"A collection, or family, of sets is considered “union-closed” if the union of any two sets in the family equals any existing set in the family."

EDIT: In your hypothetical collection of sets, the union of "2" and "3" would produce the set "2,3" which is not an enumerated set in your list.

Check out the replies to this comment [1] with the same attempted counterexample. Your set isn't union closed because {1}U{2} = {1, 2} isn't in it (same for {1}U{3} etc.).

[1] https://news.ycombinator.com/item?id=34236889#34240381