Can I use Dura-Ace / Ultegra shifters with 105 Di2 derailleurs?

Yep, you can! In fact, this is something people often do.

12-speed 105 Di2 is excellent, but the shifters don't have any extension ports or top/hood buttons. Also, there are no 105 Di2 rim brake shift levers.

This is generally not a problem unless you want to install clip-on aero bars with satellite shifters, or want to use sprint shifters or climbing shifters . Also, having hood buttons is pretty useful too.

Fortunately you can use 105 Di2 derailleurs with Ultegra or Dura-Ace shifters, no problem!

Want to build a 105 rim brake bike? Use Ultegra or Dura-Ace rim brake shifters.

Want to use satellite shifters? Use the Ultegra or Dura-Ace shifters.