A 1-1/2" Schedule 40 pipe is 1.90" outer diameter. When you project that up onto your 4:12 roof, you will get an ellipse, with the minor axis still 1.9", but the major axis equal to 1.9" * sqrt(12^2 + 4^2)/12 = 2.003".
If you drill a 2" hole perpendicular to the roof, you might think you'd just need to open it a little to get to 2.003" across the upslope-downslope dimension of the hole. But that doesn't account for the thickness of the roof. If you're just drilling 1/2" thick sheathing, then the thickness of the sheathing will interfere by 1/2" * 4 / sqrt(12^2 + 4^2) = 0.158". But just on one side of the hole, on the other side the edge of the hole will slope away from the pipe.
So if you just want to drill perpendicular to the roof, use a 2-1/4" or 2-1/2" hole saw, depending on how thick the roof layers you are drill through are.
An approach to give you an elliptical hole would be to cut a wedge with a 4:12 angle (18.4 degrees). Then if you tack that to the 4:12 roof in the obvious way, you'll have a level face you can drill through plumb to get an elliptical hole. Now you can use a 2" hole saw.
A quicker but trickier approach with a 2" hole saw would be to use an extra long pilot bit. Then drill the pilot hole plumb (start off perpendicular and start angling the bit towards plumb as the bit enters) and use that to run the hole saw plumb. This would require a good grip on the drill and some finesse; don't try it if you aren't comfortable.
Cheers, Wayne