Leave without pay for sick leave – Ontario

What is the requirement to grant leave without pay for an employee who is off on unpaid sick leave.  The specifics of the situation are that the employee has been off work for one month, has not provided a medical note to date.  We were planning on terminating the employee as no proof of illness but then the employee provided an open-ended doctor note.  We have placed the employee on unpaid leave. When the employee completes the short term disability application and it is approved the employee will be paid through STD and on the return to work we will determine if we have a role for the employee as the performance is sub standard and the employee is currently on a performance improvement plan. My questions is what if the application is denied- can we terminate the employee at that point or even before then?