The fundamental difference between a turboprop and a turbofan is the turbofan has an inlet, fan exit guide vanes, and an exit nozzle. Those three things all help optimize performance of the turbofan relative to the turboprop.
The big part is the inlet - as a turboprop goes faster, the air the prop sees keeps going faster. Pretty soon, the prop tips go supersonic and the efficiency goes south FAST. With an inlet, at a constant corrected fan speed, the air speed at the fan face is almost constant regardless of the freestream airspeed - the inlet almost works like a nozzle in reverse, slowing the airflow at cruise airspeed such that the air hitting the fan at max power, the airspeed at the fan face is ~ 0.4 Mach. Hence the fan can stay efficient at very high airspeeds without the fan going supersonic - unlike the turboprop.
Putting a shroud around a prop doesn't help - you need an inlet.