Does not matter. The pump would work through smaller pipe, just at a lower flow rate. Are you not able to find a 1.5" thread with a 2" pipe couple? Then you would not need adapters.
With what you have, you likely need a short 1.5" pipe into that fitting, then a 1.5"/2" reducer on the other end of the pipe, then a 2" coupler to the reducer and the other end to the pipe.