There are two ways to do this. The first is when your iPhone is plugged in, look for in the File drop down menu then devices for a Transfer Purchases option. The second option is to look at your purchases and you can just download them from iTunes.
Thanks for using Apple Support Communities.
You can use this method to help you:
1. Launch the Coolmuster iPhone iPad iPod to computer transfer software and connect your iPhone/iPad to the computer with the USB cable.
2. The software will bring out all the contents in your iPhone/iPad and you can preview and select the purchased items you want to share.
3. Click the "Export" button to save the selected purchases to your computer.
You can use iStonsoft iPad/iPhone/iPod to Computer Transfer to transfer purchases from iphone to itunes,I always like to use the professional tools to solve iPhone problem, because it is very stable, can use for a long time, and the operation is simple, to be able to very quickly to sync your purchases from iphone to itunes
As for me, you can use MobiKin Assistant for iOS to transfer purchases from iPhone to iTunes directly. It is an excellent software and can sync data from iOS device to computer easily. These data including text messages, purchases, contacts, music, photos, video, tv shows, voice memos, podcasts, ringtones, books, etc. Beside, it can support almost all iOS devices, including the latest iPhone 6 plus/6, iPhone 5s/5/4s/4, iPad air/mini, iPod, etc. High compatibility make it become the one of most professional iOS tools. You can have a free try. Believe it can help you a lots.
From iphonefaq, you can follow these to transfer music from iPhone to iTunes:
1. Connect your iPhone to the computer and open iTunes.
2. Make sure that iTunes is authorized and you are logged into your account. From the top menu choose Store -> Sign In. To authorize the computer for use with your account select Store -> Authorize This Computer.
3. From the left menu under DEVICES right-click on the name of your iPhone (Windows) or Control-click (Mac) to display the shortcut menu.
4. Choose the Transfer Purchases from iPhone option.