iTunes freezes when syncing with iPhone
I would suggest you download and install a fresh copy of iTunes and report back. 

Before doing that, though, I suggest you create new user account on the iMac and see if iTunes hangs up when you plug in the phone. If there is no problem, then perhaps something is wrong with the iTunes preference files for your account.

Lastly, you might want to sign out of the iTunes Store (if you are currently logged in) and sign in again.