Connecting in the US is not really fun since you'll need to clear customs in the US, then go back through security and connect.
Here's an off the wall idea: AC just launched their new 787 on the YYZ-ZRH route next month (I'm not sure when you are travelling). Brand new plane, brand new product, plus connecting from ZRH-BOS via YYZ allows you to skip Canadian immigration, pre-clear US customs at YYZ and arrive BOS as a domestic flight.
Just a thought.