magento 1 Remove Category path from product URL is not working

Question

I removed Category Path From product URL from admin area, now when Users come from google with old URL which contain (Domain.com/categorypath/Productname.html) it show 404 error instead of redirect to product page (domain.com/productname.com). I already tried Truncate core_URLrewrite table and reindex and remove all cache but did not worked. any help will be highly appreciated