Disable Automatic Sync on iTunes

You may want to prevent iPhone from syncing automatically when you connect iPhone to a computer other than the one you usually sync with.
Here are several options how to disable the automatic sync on iTunes that taken from iPhone user guide book.

Turn off automatic syncing for iPhone
Connect iPhone to your computer, then select iPhone in the iTunes source list (below Devices, on the left) and click the Summary tab. Deselect “Automatically sync when this iPhone is connected.”You can still sync manually by clicking the Sync button.

Prevent automatic syncing for all iPhones
In iTunes choose iTunes > Preferences (on a Mac) or Edit > Preferences (on a PC), click iPhone, and select “Disable automatic syncing for all iPhones.”
If this checkbox is selected, iPhone won’t sync automatically, even if “Automatically sync when this iPhone is connected” is selected in the Summary pane.

Prevent automatic syncing one time, without changing any settings
Open iTunes. Then, as you connect iPhone to your computer, press and hold Command-Option (on a Mac) or Shift-Control (on a PC) until you see iPhone in the iTunes source list (below Devices, on the left).

Sync manually
Select iPhone in the iTunes source list (below Devices, on the left), then click Sync in the lower-right corner of the window. Or, if you’ve changed any sync settings, click Apply.