1.
There are several LGA1151 with DDR3 support but not all DDR3 can be used, only DDR3 with certain max voltage (max 1.35V, if I am not mistaken) can be used.
Such mobos are already quite hard to find these days, at least here where I live.
Example board: Gigabyte GA-Z170-HD3 DDR3
2.
I5 2500k is not compatible with any LGA1151 boards. Never!
You need I5/I7 6xxx or 7xxx.
Be carefull for 7xxx, you might need to upgrade the BIOS for 170 mobos.