I have a vertical pipe sleeve with a 2" pipe going thru and the pipe moves
1.75" thermally in the horizontal at 300 deg F. Any recommendations as to what kind of flexible filler I could use in the sleeve to seal the pipe and also let it move?

Rubber boots are good if they can stand the temperature. For higher temperatures, some of the expansion joint manufacturers make bellows assemblies that can be attached to a bulkhead on one end and a pipe on the other. Mineral wool is a good filler material for such things.