i want to get my to cube to rotate 90 degrees on the x-axis as it jumps

Basically I want to get my to cube to rotate 90 degrees on the x-axis as it jumps, I tried slerping but that just produced strange results, any suggestions, this is the code for the movement of the cube at the moment:

 public class Movement : MonoBehaviour {
     public GameObject Box;
     public Vector3 BoxRot;
     public float BoxX;
     public float BoxY;
     public float BoxZ;
     public float JumpHeight=100f;
     public Vector3 JumpTo;
     public GameObject floor;
     public bool jumping=false;
     public float CurX;
     public string FloorTag = "Floor";
     public Rigidbody CubeRigidBody;
 
     void Jump()
     {
         CubeRigidBody.AddForce (Vector3.up * 250f);
     }
     void OnCollisionEnter(Collision Col)
     {
         if (Col.gameObject.name == "Floors")
         {
             jumping = false;
         }
     }
     void OnCollisionStay (Collision Col1)
     {
         if (Col1.gameObject.name == "Floors")
         {
             jumping = false;
         }
     }
     void OnCollisionExit (Collision Col2)
     {
         if (Col2.gameObject.name == "Floors")
         {
             jumping = true;
         }
     }
     void Sideways()
     {
         if (Input.GetKeyDown(KeyCode.LeftArrow))
         {
             CurX = Box.transform.position.x;
             Box.transform.position = Vector3.Lerp(new Vector3(CurX,Box.transform.position.y, Box.transform.position.z), new Vector3(CurX-35, Box.transform.position.y, Box.transform.position.z),2*Time.deltaTime);
         }
         if (Input.GetKeyDown(KeyCode.RightArrow))
         {
             CurX = Box.transform.position.x;
             Box.transform.position = Vector3.Lerp(new Vector3(CurX, Box.transform.position.y, Box.transform.position.z), new Vector3(CurX + 35, Box.transform.position.y, Box.transform.position.z), 2 * Time.deltaTime);
         }
 
 
     }
 
     void Update () 
     {
         Sideways();
         //Box.transform.Translate (0,0, 10 * Time.deltaTime);
         Box.transform.Translate (Vector3.forward * 10f * Time.deltaTime);
         BoxY = Box.transform.position.y;
         JumpTo = new Vector3 (0, JumpHeight, Box.transform.position.y);
         if (Input.GetKeyDown ("space") && !jumping)
         {
             //Box.transform.Translate(Vector3.up * 100 * Time.deltaTime, Space.World);
             Jump();
         }
          
     }
 
 
 
 
 }