Connect my mid 2011 macbook pro with a mini displayport to my late 2014 iMac

Use your iMac as a display with target display mode - Apple Support

The late 2014 iMac is too new (mid 2014 is the last to support Target Display Mode) and your MacBook Pro doesn't have Thunderbolt, which is needed if the iMac has Thunderbolt.

Nope.

I haven't tried it, but Turn any Mac or iPad into a second display...

https://lunadisplay.com/

Review by NAK005, you may not be happy with it…

https://discussions.apple.com/thread/251259038?answerId=252422503022#252422503022

There are no such adapters.

OK, good luck & stay safe. :)