Hi Achduliebezeit,
Thank you for participating in the Apple Support Communities. We understand you'd like to remove the music content on your iPhone and then sync music content from your iMac. This article explains how: Sync music between your Mac and iPhone, iPad, or iPod touch.
"You can sync all or a selection of your music on your Mac to your iPhone, iPad, or iPod touch.
Important: If you subscribe to Apple Music, your music is transferred automatically to all your devices and no syncing is required. You can’t use the syncing method described here unless you turn off the Sync Library option in Apple Music settings.
See Intro to syncing your Mac and your devices.
Sync music to your device
- Connect your device to your Mac.
You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
2. In the Finder 
on your Mac, select the device in the Finder sidebar.
If you connect your device to your Mac using a USB cable and don’t see the device in the Finder sidebar, see If your device doesn’t appear in the sidebar.
3. Click Music in the button bar.
4. Select the “Sync music onto [device name]” checkbox to turn on syncing of your music.
With the checkbox selected, syncing is set to transfer all your music to your device.
5. To sync a selection of music, click “Selected playlist, artists, albums, and genres.”
6. Select the checkbox for each item you want to sync in the music list. Deselect the checkbox for any item you don’t want to sync.
Click Artists, Albums, Genres, or Playlists to see your content organized by that category.
7. Select syncing options:
- Select the “Include videos” checkbox to include videos when syncing.
- Select the “Include voice memos” checkbox to include voice memos when syncing.
- Select the “Automatically fill free space with songs” checkbox to have your Mac fill the free space on your device with songs when syncing. Use this option if you have a large music library on your Mac but don’t have room to sync all of your music to your device. This option syncs as many songs as possible. See Fill your device with a random selection of songs using Autofill.
8. When you’re ready to sync, click Apply.
You can select the “Sync only checked songs” option in the General options to create a selection of songs to your devices. Other songs in your library that aren’t checked are not synced to your Mac or devices. See Save storage space when syncing.
You can choose to sync your Mac and your device automatically whenever you connect them. See Turn automatic syncing on or off on Mac.
Before disconnecting your device from your Mac, click the Eject button 
in the Finder sidebar.
Remove automatically synced music from your iPhone, iPad, or iPod touch
To remove unwanted music from both your Mac and device, delete the music from your Mac and sync your device.
To remove music from just your device while keeping it on your Mac, follow these steps:
- Connect your device to your Mac.
- You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
- In the Finder on your Mac, select your device in the sidebar, then click Music in the button bar.
- In the Music list, deselect the checkboxes of the music you want removed from your device.
- When you’re ready to sync, click Apply.
WARNING: If you delete an automatically synced item from your Mac, the deleted item is removed from your device the next time you sync.
Before disconnecting your device from your Mac, click the Eject button in the Finder sidebar."
Take care.
