So I supposed right, you should use the second one:
From: ^/item1$
To: http://{siteurl}/catalog/category1/category2/item1
Consider that /item1 doesn't exist and here ReDJ redirect to existing URL. But if you get an error 404 from Apache, it means that Apache manages your request, so Joomla (and ReDJ) can't do nothing.
If you need to manage these kind of pages, you must tell to Apache to pass them to Joomla. To do this, just add this line at the end of your .htaccess file:
It should solve (remember to enable ReDJ plugin).
Luigi