Nikon filed a patent in Japan for a 300mm lens with a diffractive optical element (Canon has a good DOE explanation on their website). The exact aperture was not specified, but I am pretty sure it’s f/4 (see this patent from last year). This is an interesting development because I expect Nikon to announce a new 300mm f/4 lens in the next few months. The remaining details from the patent application:
As I already mentioned above, last year Nikon filed another patent for a 300mm f/4 lens with VR and DOE and identical design.
Via Egami