Show us a screen capture of the exported photo, zoomed 1:1.
Then show us another screen capture of the same photo in Lightroom, zoomed 1:1
When I export in jpg 100% the image loose the sharpness
You mean you are setting Quality to 100?
I assume that the exported image is for web use, and you should use output sharpening for screen.
Try Sharpen for screen, Amount: Standard. That will compensate for the reduced sharpness caused by downsampling.
It's alright when I opposed them in 1:1 size
Yes, I think this is caused by your retina screen.
Mac Preview, as well as web browsers will scale images to 200% on retina screens, so that their size will be similar to what it would be on a regular screen. This will affect image quality. If you import the image back into Lightroom, or view it in Photoshop at 100%, it will display correctly. On my non-retina screen the image looks perfectly all right.