Place a scratching post near the couch your cat claws. Either purchase a scratching post that includes catnip in the fabric, or obtain some catnip from a pet store and periodically rub it over the scratching post. This encourages the cat to use the scratching post rather than your couch.
Cover your couch with a plastic or regular sheet. Place double-sided tape on the sheet or plastic directly over the location your cat typically scratches. The sheet will keep the tape from damaging the couch, and the tape will discourage your cat from scratching because cats do not like sticky things touching their paws.
Spray the cat with a water bottle every time it tries to claw your leather sofa. Eventually, the cat will associate the unpleasantness of getting squirted with scratching the couch and it will stop.