Trump is currently accused of asking the Ukrainian president to help investigate potential misconduct by Joe Biden's son. Are such requests a routine part of the president's interaction with foreign leaders? Are there examples of other presidents directly asking their foreign counterparts for cooperation in a specific criminal investigation?
It isn't supposed to be common, and to public knowledge, it is not.
Countries often cooperate with one another in criminal investigations. It wouldn't be unusual for the U.S. FBI to ask Ukraine for some investigatory assistance if they were pursuing a suspect of some sort. However, these cases normally are not something the president is concerned about, as the FBI and CIA each have a full complement of staff whose charge it is to take care of things like this.
It is pretty evident that Trump is elevating this issue to the highest office because it involves a potential political rival. This has drawn out whistleblower action from the rank and file, which indicates that it as at least somewhat beyond the pale for the executive branch.