How to connect j leg to copper pipe

Well to begin with I thought this would be an easy fix, that was my first mistake. The house is 50 years old and all of the pipes are either cast iron or copper. The bathroom vanity was leaking so I investigated. The j leg portion of the p trap on the bathroom was leaking because of rust. The old p trap appears to be chrome platted brass or copper. When I went to remove the j leg leg it fell apart. The j leg disappeared into the wall. So I did some demo. The j leg went into some sort of brass fitting that was threaded into a 90 copper elbow. I can easily slide the new j leg into the brass fitting. How do I connect the new j leg? Can I braise the new chrome j leg to the brass fitting? Is there a rubber adapter that I can use over the elbow? The p trap that was removed was not original so I have no idea how they originally installed the vanity.

sounds like a solder bushing. I usually remove the bushing and put in a nipple then just use a new p-trap. I usually cut through the treads toward the yop of the bushing then remove it with a chisel.

thanks I will try that. it sounds better then my ides of cutting the pipe in the wall and making new connections.