collisions in character not working!

hello, in my scene i have one character that is almost a cube and its a rigidbody and i need to detect where its colliding in a 2d view so i need to get bottom,left,right and top what i did first since the character is exported from 3dmax and the x,y,z are different i put it in a gameobject, and then i put 4 gameobjects in the char 1 for each different collision and inside those game objects i put a box collider and 1 script the script is the following

 public class HitDetection : MonoBehaviour 
 {
     void Update()
     {
             //this one works
             //Debug.Log("Hit!! Update!");
     }
     void OnCollisionEnter(Collision other)
     {
         Debug.Log("HIT!!1");        
     }
 }

and its not doing anything... when i use a debug.log in update it works but not this way and also the collisions are actually working because my character looks like its floating because of the box collider i put in the bottom and same for the others so any idea why this is not working?