installed new brazillian koa 3/4" hardwood floor in June 2012...tenants moved out and have a large circular stain on wood where rug was....is this normal or a chemical reaction between the wood and rug or cleaner placed on rug?

It's normal. Color change all hardwoods undergo with Tigerwood being one of the extreme ones. Will it blend in over time? Some say yes, but I've always doubted it as I had a piece of African Dossiue that was covered for two years with a laptop. Still noticing the color difference.