what is the newest iTunes I can upgrade to for 10.8.3?

What is the newest Itunes upgrade that 10.8.3 will handle? Right now I have 11.1.5 which is fine for my computer, but I just got a new iPhone 6 and it says I need 11.4 in order to sync my phone to my computer iTunes. When I click on the download link that pops up with it, nothing happens. It just takes me to the available upgrades and doesn't list iTunes as an option.