It would usually be much simpler to proceed with renewal first, unless you have an urgent need to travel. Note that Canadian visa processing alone could take more than four months. Additionally, the visa and the period of stay granted to temporary residents in principle will not exceed the validity of the passport.
You are allowed to submit a different passport when your visa is approved and usually the validity should follow the new passport. But again, this introduces a complication for the processing and mistakes may happen.
If you already submitted a Canadian visa application, you can use the IRCC webform (https://secure.cic.gc.ca/enquiries-renseignements/canada-case-cas-eng.aspx) to update your file, quoting your UCI number and new passport information.
You may include a note regarding the ongoing renewal in the letter of explanation, but without the new passport information, there is not much the visa officer could do with the information.