Is it possible to connect a MacBook Pro 2011 to the LG UltraFine 5K display using a Thunderbolt 3 to Thunderbolt 2 adapter?
No, your 2011 MacBook Pro supports a maximum resolution of 2560 x 1600 on an external display. So even if you get the cable situation sorted you will not be able to use the display on full 5K resolution.

Source: 
MacBook Pro (13-inch, Early 2011) - Technical Specifications