In the Dungeon Master's Guide, at pg. 271 (Chapter 9, Dungeon Master's Workshop), the Disarm optional action described appears to bypass the defenders AC: it is a contest of attack roll vs. skill. There is no mention of AC being a factor.
So could a L1 human fighter tavern brawler with a shield and no other weapon attack a defender (using the shield as an improvised weapon), declare they are attempting a disarm before damage is rolled, if successful disarm the defender, then grapple as a bonus action and finally move the defender from their just-dropped weapon by 15ft?