Certified GKH Gardening Expert
Unfortunately, 2-4,D will not kill buttercup. You will need a selective herbicide, or even Glyphosate. Just make sure to follow all laws and regulation in your area, because they may be different than ours here in the US.
This article will offer more information: https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-buttercup-weeds.htm