I would not use any medium to large leaves as a mulch without first composting or shredding them. A single layer may be fine as it should break down quickly, but you would be better off to apply a thicker layer of composted material. Whole leaves mat down and block the movement of water and oxygen. This is why cardboard is also not recommended by plant and soil scientists (but is by many people on the internet who don't realize the problems they are causing).