Why is *ptr = (*ptr)++ Undefined Behavior

I am trying to learn how to explain the cause of(if any) of undefined behavior in the following cases(given below).

int i = 0, *ptr = &i;
i = ++i; //is this UB? If yes then why according to C++11
*ptr = (*ptr)++; //i think this is UB but i am unable to explain exactly why is this so
*ptr = ++(*ptr); //i think this is not UB but can't explain why 

I have looked at many SO posts describing UB for different pointer cases similar to the cases above, but still i am unable to explain why exactly(like using which point(s) from the standard we can prove that they will result in UB) they result in UB.

I am looking for explanations according to C++11(or C++14) but not C++17 and not Pre-C++11.