Problem detection collider with coroutine

Hello,

I'm made ​​a Bomberman but I have a little problem with my script collision detection. My script detects the boxes without coroutine, so that when I put my coroutine it no longer detects the boxes ... Would you please solution.

 using UnityEngine;
 
 using System.Collections;
 
  
 
 public class DestroyCube : MonoBehaviour {
 
     
 
     public BoxCollider box1;
 
     public BoxCollider box2;
 
  
 
     void OnCollisionEnter(Collision collision) 
 
     {
 
         if(collision.gameObject.name == "Cube")
 
         {
 
             Debug.Log ("Destroy Cube");
 
             Destroy(collision.gameObject, 4);
 
         }
 
         else if(collision.gameObject.name == "Capsule")
 
         {
 
             Debug.Log ("Destroy Capsule");
 
             Destroy(collision.gameObject, 4);
 
         }
 
     }
 
  
 
     IEnumerator Start () {
 
         yield return new WaitForSeconds (4);
 
         box1 = gameObject.AddComponent<BoxCollider>();
 
         box2 = gameObject.AddComponent<BoxCollider>();
 
         box1.size = new Vector3 (0.1f,3,6);
 
         box2.size = new Vector3 (6,3,0.1f);
 
     }
 
 }

thanks you in advance.