Coroutine with Quaternion.Angle

Im trying to rotate cube to 0 or 90. Problem is that my coroutines start to piling up and I have no idea why. This script is on cube.

 using UnityEngine;
 using System.Collections;
 
 public class RotationTest : MonoBehaviour {
     public bool isRotating = false;
     public float turningSpeed;
     public int rotationAngle = 90;
 
     // Use this for initialization
     void Start () {
     }
     
     // Update is called once per frame
     void Update () {
         if (!isRotating) {
             rotationAngle = (rotationAngle == 90) ? 0 : 90;
             StartCoroutine(RotateTo(rotationAngle));
         }
     }
     
     private IEnumerator RotateTo(int to) {
         isRotating = true;
 
         Quaternion fromPos = transform.rotation;
         Quaternion toPos = Quaternion.Euler(0, to, 0);
 
         while (Quaternion.Angle (transform.rotation, toPos) > 0.1f) {
             transform.rotation = Quaternion.Slerp (fromPos, toPos, Time.time * turningSpeed);
             yield return null;
         }
         
         isRotating = false;
     }
 }

Can someone tell me what im doing wrong there. Thanks.