using System;

using Newtonsoft.Json;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

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

        List<Brand> brands = JsonConvert.DeserializeObject<List<Brand>>(json);

        List<string> brandIDs = brands.Select(x => x.Brand_Id).ToList();

        Console.WriteLine(JsonConvert.SerializeObject(brandIDs));

    }

}

​

public class Brand

{

    [JsonProperty("brand_id")]

    public string Brand_Id {get;set;}   

}