Have you looked at Lufthansa ORD-FRA-IST? Given United's close relationship with LH, I have found its sometimes easier. I did DEN-FRA-IST last year was was able to upgrade. You may just call LH and see what they can do. I realize ORD-IST direct is better. However, in my experience LH is more consistent in customer service and FA service than TK. When things go well, TK can be great. But if something goes wrong with TK (weather, equipment issues, etc), its total meltdown. Just my 2 cents. I believe the ORD and JFK TK flights are handled pretty well though. They are high profile routes for TK.

I'm flying TK next month CPT-IST, only because my options were limited.

Have a great trip.