using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"[{'brand_id':'112'},{'brand_id':'114'},{'brand_id':'14'}]";

​

        JArray jArray = JArray.Parse(json);

        var brandIds = jArray.Select(x => x["brand_id"]);

        Console.WriteLine($"[{string.Join(",",brandIds)}]");

    }

}